Abstract
The prospective uses of tree gum polysaccharides and their nanostructures in various aspects of food, water, energy, biotechnology, environment and medicine industries, have garnered a great deal of attention recently. In addition to extensive applications of tree gums in food, there are substantial non-food applications of these commercial gums, which have gained widespread attention due to their availability, structural diversity and remarkable properties as ‘green’ bio-based renewable materials. Tree gums are obtainable as natural polysaccharides from various tree genera possessing exceptional properties, including their renewable, biocompatible, biodegradable, and non-toxic nature and their ability to undergo easy chemical modifications. This review focuses on non-food applications of several important commercially available gums (arabic, karaya, tragacanth, ghatti and kondagogu) for the greener synthesis and stabilization of metal/metal oxide NPs, production of electrospun fibers, environmental bioremediation, bio-catalysis, biosensors, coordination complexes of metal–hydrogels, and for antimicrobial and biomedical applications. Furthermore, polysaccharides acquired from botanical, seaweed, animal, and microbial origins are briefly compared with the characteristics of tree gum exudates.
Keywords: Tree gums, Nanoparticles and nanofibers, Greener synthesis, Hydrogel, Environmental bioremediation, Biosensors, Antibacterial, Biomedical
Graphical Abstract
1. Introduction
Nanoparticles (NPs) and nanofibers (NFs) have been successfully applied in diverse fields, such as energy and the environment, water treatment, pharmaceuticals, automobiles and health care (Colvin, 2003; Fryxell and Cao, 2007; Matlack, 2010; Thavasi et al., 2008; Vigneswaran, 2009). However, the majority of the procedures presently used for the production of NPs and NFs are reliant on non-ecological materials and produce harmful waste. Green nanotechnology, the fusion of nanotechnology developments embracing green chemistry principles, is a rational approach that embraces the aspects vital for developing an ecologically sustainable society. Green chemistry is a set of principles or practices that encourages the design of products and processes with an emphasis on the use of renewable chemicals that overall reduce or eliminate the use and generation of hazardous substances (Anastas and Warner, 1998; Luque and Varma, 2012; Rickerby, 2013). Nanomaterials offer varied applications that prevent pollution by their use as nano-catalysts, which generate less waste, enable effective sensing of pollutants in water and the environment, and eliminate harmful chemicals and microorganisms (Dwivedi et al., 2015; Grassian, 2008; Sharma et al., 2015; Varma, 2016, 2014). Current greener nanotechnology practices often tend to involve the use of natural resources, non-hazardous solvents, and biodegradable and biocompatible materials via energy-reliant processes in the synthesis of NPs and NFs (Hebbalalu et al., 2013; Iravani, 2011; Pereao et al., 2016; Raveendran et al., 2003; Virkutyte and Varma, 2011).
Natural polysaccharides with their unique structure and functionality open up a new area of nanostructured material creation. Bio-templating and bio-replication of metal oxide nanostructures by polysaccharides have been shaped naturally as well as by man (Boury and Plumejeau, 2015). Tree gum polysaccharides or gum hydrocolloids are natural and versatile green materials encompassing diverse functional and structural properties to assemble varied nanostructures. Over the past few years, many new gum exudates have been introduced in the literature from various sources, with proper structures and functional characterization including specific applications. Tree gums represent a largely unexplored source of valuable natural products and contribute to diversity as an important component of global foodstuff, therapeutic, paper, fabric and other markets (Nussinovitch, 2010). Depending on their major industrial applications, plant gums may be broadly classified as ‘food’ and ‘non-food’ or ‘technological grade’ gums. Natural gum hydrocolloids are classified into various groups depending on their origin, structural-functional properties, and applications namely botanicals (locust bean gum, guar gum, and pectin), tree gums (arabic, karaya, tragacanth, ghatti, and kondagogu), seaweed polysaccharides (alginate, carrageenan, and agar), and bacterial polysaccharides (xanthan, dextran and gellan gum) (Williams et al., 2011). The surface properties of tree gums are unique due to their ability to decrease interfacial tension between different systems such as gas-liquid, liquid-liquid, and solid-liquid, which imparts stability via steric, electrostatic interaction and hydration forces. Tree gums can stabilize NPs akin to their superior use in food emulsions rivalled by other stabling agents (Adamson and Gast, 1997; Redgwell et al., 2005). Tree gum exudates have been used for food applications as an emulsifier, thickening, and stabilizing agent, but there are no comprehensive reviews on the emerging applications such as environmental, biosensor, catalytic, or biomedical uses. The present review highlights the various applications of tree gums and describes their specific applications with detailed explanations as to how these gums can be incorporated into nanostructured products (Fig. 1). Specifically, the importance of five major gums [gum arabic (GA), gum tragacanth (GT), gum karaya (GK), gum ghatti (GG), and kondagogu gum (KG)], which are among the most widely utilized gums in various applications, is summarized. A detailed comparison of sources and the functional applications of various commercial gum hydrocolloids guar gum (Das et al., 2015; Kumar et al., 2012; Pal et al., 2015; Prado et al., 2005; Prajapati et al., 2013; Pramanik et al., 2015; Vaghela et al., 2014), locust bean gum (Dufficy et al., 2015; Murali et al., 2016b; Prado et al., 2005; Prajapati et al., 2013; Tagad et al., 2014; Ventura et al., 2015), pectin (Attallah et al., 2016; Chen et al., 2015; Munarin et al., 2012; Neves et al., 2015; A. J. Wang et al., 2012; Williams et al., 2011), agar (Xuefeng Li et al., 2017; Rhein-Knudsen et al., 2015; Williams et al., 2011; Yang et al., 2014), alginates ( Davis et al., 2003; Finotelli et al., 2010; Leslie et al., 2013; Liu et al., 2015; Paul and Sharma, 2004; Ryou et al., 2013; Sari-Chmayssem et al., 2016; Wei et al., 2015; Williams et al., 2011), carrageenan (Chaudhary et al., 2016; Li et al., 2014; Nair et al., 2016; Williams et al., 2011; Xue et al., 2017), xanthan gum (Flickinger, 2010; Izawa and Kadokawa, 2010; Katzbauer, 1998; Palaniraj and Jayaraman, 2011; Petri, 2015; Qiu et al., 2016; Williams et al., 2011), and gellan gum (Bacelar et al., 2016; Dhar et al., 2012; Ferris and Panhuis, 2009; Hungerford et al., 2012; Keller et al., 2017; Stevens et al., 2016; Williams et al., 2011), including tree gums such as GA (Augustin and Hemar, 2009; Bandyopadhyaya et al., 2002; Bosnea et al., 2017; Chabot et al., 2013; Fan et al., 2012; X. Li et al., 2017; Ribeiro de Barros et al., 2016; Sanchez et al., 2017; L. Wang et al., 2012a, 2012b; Williams et al., 2011; Xhanari et al., 2017; Xu et al., 2017; Zhou et al., 2017), GG (Deshmukh et al., 2012; K. Sharma et al., 2015a, 2015b, 2014; Kashma Sharma et al., 2014; Williams et al., 2011), GK (Bie et al., 2017; Jakóbik-Kolon et al., 2017; Kumar et al., 2016; Nussinovitch, 2010; Singh and Pal, 2008; Williams et al., 2011), GT (Asghari-Varzaneh et al., 2017; López-Castejón et al., 2016; Mallakpour et al., 2018; Mostafavi et al., 2016; Niknia and Kadkhodaee, 2017; Singh and Sharma, 2017, 2014a; Williams et al., 2011) and KG (Dasari and Guttena, 2016; Nussinovitch, 2010; Puskuri et al., 2017; Rastogi et al., 2014; Rathore et al., 2016; Saravanan et al., 2012) are illustrated in Table 1.
Fig. 1:
Potential applications of NPs and NFs derived from processes based on tree gums.
Table 1:
Comparison of sources, functional properties and commercial applications of various gum hydrocolloids
| Gum Hydrocolloids | Source | Functional properties (Food applications) |
Non-food applications of gum derivatives and composites |
|---|---|---|---|
| Guar gum | Seed of guar beans | Thickener, stabilizer, emulsifier and firming agent (Prado et al., 2005; Vipul D. Prajapati et al., 2013) |
- electrochemical and biosensor (Vaghela et al., 2014) - NP stabilizing agent/antibacterial and antioxidant (Das et al., 2015) - wound healing (Pramanik et al., 2015) - nano-catalysis (Kumar et al., 2012) - hybrid nano-composite for water purification (Pal et al., 2015) |
| Locust bean gum | Carob tree seeds | Thixotropic, binder, stabilizer, gelling agent, lubricator (Prado et al., 2005; Vipul D. Prajapati et al., 2013) |
- mesoporous gel matrices (Ventura et al., 2015) - wound healing (Murali et al., 2016b) - Au NPs synthesis and ethanol sensing (Tagad et al., 2014) - binders for lithium-ion batteries (Dufficy et al., 2015) |
| Pectin | plant cell wall (citrus, apple and sugar beet) |
Thickener, stabilizer, gelling agent, emulsifier (Williams et al., 2011) |
- cell delivery (Munarin et al., 2012) - tissue engineering and regenerative medicine (Neves et al., 2015) - Cr (VI) removal from contaminated water (Chen et al., 2015) - removal of cationic and anionic dyes (Attallah et al., 2016) - photocatalytic applications (A. J. Wang et al., 2012) |
| Agar | Red seaweeds (Gelidium species) |
Thickener, stabilizer, gelling agent (Williams et al., 2011) |
- growth media for culturing bacteria and pharmaceuticals (Rhein-Knudsen et al., 2015) - electrospun composite fibers for controlled drug release (Yang et al., 2014) - hydrogels with self-healing properties and high toughness (Xuefeng Li et al., 2017) |
| Alginate | Brown seaweeds (Macrocystis, Ascophyllum, Laminaria and Ecklonia species) |
Thickening agent, stabilizer (Williams et al., 2011) |
- wound dressings, encapsulate and/or release cells and medicine (Finotelli et al., 2010; Leslie et al., 2013; Paul and Sharma, 2004) - metal adsorption (Thomas A. Davis et al., 2003) - 3D printing (Wei et al., 2015) - bio-based materials with excellent fire retardancy (Liu et al., 2015) - improved lifecycle of LiMn2O4 cathodes in lithium ion batteries (Ryou et al., 2013) - bio-based surfactants (Sari-Chmayssem et al., 2016) |
| Carrageenan | Red seaweeds (Gracilaria, Gigartina and Eucheuma species) |
Thickener, gelling agent, stabilizer, emulsifier (Williams et al., 2011) |
- drug delivery, microcapsules and microspheres (Li et al., 2014) - binders (Williams et al., 2011; Zia et al., 2017) - nanocomposite for photodegradation of dyes (Chaudhary et al., 2016) - flame-retardant (Xue et al., 2017) - hydrogel for wound healing (Nair et al., 2016) |
| Xanthan gum | Xanthomonas campestris bacterium |
Thickener, stabilizer, emulsifier and forming agent (Palaniraj and Jayaraman, 2011; Williams et al., 2011) |
- functional ionic liquid-gel and hydrogel (Izawa and Kadokawa, 2010) - metal ion removal from wastewater (Qiu et al., 2016) - agricultural chemicals (Flickinger, 2010) - water based paints (Flickinger, 2010) - textile and carpet printing (Flickinger, 2010) - adhesives (Flickinger, 2010) - paper industry(Flickinger, 2010) - ceramic glazes(Flickinger, 2010) - enhanced oil recovery(Katzbauer, 1998) - carriers for drugs and proteins and as scaffolds for cells (Petri, 2015) |
| Gellan gum | Pseudomonas elodea | Emulsifier, stabilizer, and thickener (Williams et al., 2011) |
- regenerative medicine and tissue engineering (Bacelar et al., 2016; Stevens et al., 2016) - conducting bio-materials (Ferris and in het Panhuis, 2009) - conducting hydrogels for edible electrodes (Keller et al., 2017) - silver nanoparticle dispersions and hydrogels (Dhar et al., 2012) - biosensing (Hungerford et al., 2012) |
| Gum arabic (GA) |
Acacia senegal and Acacia seyal |
Emulsifier, stabilizer, and thickener (Sanchez et al., 2017; Williams et al., 2011) |
- stabilization of single walled carbon nanotubes (Bandyopadhyaya et al., 2002; X. Li et al., 2017) - encapsulating matrix (Augustin and Hemar, 2009; Bosnea et al., 2017) - stabilizing agent for NPs (Ribeiro de Barros et al., 2016) - branched Au architectures with well-defined shapes (Wang et al., 2012a) - biocompatible gold nanoflowers (Wang et al., 2012b) - multi-functional binder for the fabrication of NiFe2O4 nanotube electrodes (Zhou et al., 2017) - binder in water-based electrode fabrication processes (Xu et al., 2017) - green corrosion inhibitors (Xhanari et al., 2017) - exfoliation and fabrication of Ag–graphene-based hybrids (Fan et al., 2012) - high yield production and purification of graphene layer (Chabot et al., 2013) |
| Gum Ghatti (GG) | Anogeissus latifolia | Thickener, stabilizer, emulsifier (Deshmukh et al., 2012; Williams et al., 2011) |
- electrically conductive biomaterials (Sharma et al., 2014) - interpenetrating network (IPN) hydrogel (Sharma et al., 2015b) - electrically conductive hydrogel (Kashma Sharma et al., 2014) - colon-specific drug delivery systems based cross-linked hydrogels (Sharma et al., 2015a) - remediation of industrial wastewater polluted by metal ions (Hemant Mittal et al., 2015b) |
| Gum karaya (GK) |
Sterculia species (urens,
villosa, setigera) |
Emulsifier, stabilizer and thickening agent (Nussinovitch, 2010; Williams et al., 2011) |
- binder for silicon-based anodes in lithium-ion batteries (Bie et al., 2017) - nanocomposite hydrogels for the removal of cationic dyes (Kumar et al., 2016) - metal ions removal (Jakóbik-Kolon et al., 2017) - wound dressing (Singh and Pal, 2008) |
| Tragacanth gum (GT) |
Astragalus species, microcephalus, gummifer kurdicus) |
Acid stable (Nussinovitch, 2010; Williams et al., 2011) Bifunctional emulsifier (Nussinovitch, 2010) Creamy texture pseudo plastic (Nussinovitch, 2010) Film former (Nussinovitch, 2010) Adhesive (Nussinovitch, 2010; Williams et al., 2011) Thickener (Nussinovitch, 2010; Williams et al., 2011) suspending agent (Nussinovitch, 2010; Williams et al., 2011) |
- tunable drug delivery system (Singh and Sharma, 2014a) - cryogel and xerogel blends (Niknia and Kadkhodaee, 2017) - hydrogels in drug delivery applications(Singh and Sharma, 2017) - microencapsulation (Asghari-Varzaneh et al., 2017) - hydrogel nanocomposite for removal of heavy metal (Mallakpour et al., 2018) - bioplastics (López-Castejón et al., 2016) - edible blend films and coatings (Mostafavi et al., 2016) |
| Kondagogu Gum (KG) |
Sterculia species (Cochlospermum gossypium) |
Emulsifier, thickening agent and stabilizer (Nussinovitch, 2010; Puskuri et al., 2017) |
- biosensor (Rastogi et al., 2017) - food and medical industries (Puskuri et al., 2017) - wound dressing (Rathore et al., 2016) - bio-sorbent for environmental contaminants (Rastogi et al., 2014; Saravanan et al., 2012) - photocatalytic, fluorescence and antimicrobial activities (Dasari and Guttena, 2016) |
Various international bodies, including the US Environmental Protection Agency (Committee on toxicology, 2001), clearly stipulate limits on the presence of heavy metals in drinking water so that they do not exceed their permissible levels. Common techniques used for the elimination of metallic impurities include adsorption, ion exchange, chemical precipitation, and reverse osmosis (Juang and Shiau, 2000; Yan and Viraraghavan, 2001; Matheickal et al., 1999). However, certain drawbacks of these techniques include partial metal elimination, generation of toxic slurry and involvement of expensive chemicals or methods. From this perspective, the advance of biological product-based entities as sustainable and inexpensive bio-sorbents is crucial. Biosorption can be a procedure that exploits cheaper bio-resources to capture toxic pollutants. In contrast to conventional methods, biosorption processes have the advantages of low investment and operational costs, with a reduction in the volumes of chemical and/or biological sludge generated. Furthermore, this process enhances effectiveness in decontaminating dilute effluents as it eradicates the problems of the toxicity of the applied materials and the loss of nutrients or cultures needed for microbial-based procedures (Davis et al., 2003; Kratochvil and Volesky, 1998). The use of natural product-based polymers is of specific economic interest as the biomaterials are essentially drop-in substitutes for synthetic adsorbents or ion exchangers, whereby embracing the philosophy of green chemistry (Klimmek et al., 2001).
Many researchers are looking for safe, active and cost-effective alternatives for the management of wastewater comprising toxic metals and other contaminants. However, many adsorbent materials bearing a variety of affixed well-designed chemical groups have been reported for this purpose (Falcaro et al., 2016; Lu and Astruc, 2018; Volesky, 2007). Naturally occurring biopolymers have been identified, which exhibit an excellent adsorption ability for multivalent metal ions. This may be exemplified by several materials, including chitin, chitosan, lignin, sea weed, wool wastes, agricultural wastes, sugarcane bagasse, biomass by-products, polysaccharides based flocculants, and algae etc., which have been employed for the exclusion of deadly contaminants from aqueous wastewater streams (Aydin and Aksoy, 2009; Davis et al., 2003; Salehizadeh et al., 2018; Varma et al., 2004). Alginic acid, present in brown seaweeds, is a biopolymer adorning carboxyl functionality capable of forming coordination complexes with metal ions (Gok and Aytas, 2009). Effective biosorption is determined in particular by active sites for the binding of the bio-sorbent, the chemical state of the binding sites and the binding strength.
2. Green chemistry and nanomaterials
One of the central problems facing human society today emanates from increasing environmental pollution due to various industrial and human activities (Wacławek et al., 2017). Many alternative sources of chemical entities available from starch, cellulose and synthetic polysaccharides or their derivatives have been developed as substitutes for food and non-food materials over the years, but tree gums have retained their uniqueness as they possess a distinct niche in terms of their properties and applications, which are much superior in comparison (Table 1 and Table 2).
Table 2:
Analytical data of important tree gums
There are many naturally occurring materials that can effectively serve as a reducing and coating mediator for the production and stabilization of NPs (Huang et al., 2015). Critical reviews on the greener assembly of nanoparticles using both living and various parts of plants materials have been summarized (Akhtar et al., 2013; Dauthal and Mukhopadhyay, 2016; Hebbalalu et al., 2013; Iravani, 2011; Mittal et al., 2013; Mohammadinejad et al., 2016). An eco-friendly assemblage characteristic for the fabrication of extensively used Ag NPs has been created, which evaluates the cost effective, biogenic and scale-up production of the NPs utilizing plant-based biomolecules (Cinelli et al., 2015). Plant extracts Lippia citriodora (Elemike et al., 2017), xerophytes—Bryophyllum sp., mesophytes—Cyperus sp. and hydrophytes—Hydrilla sp. (Jha et al., 2009), Pelargonium graveolens (Shankar et al., 2003), living plants (Aloe vera (Chandran et al., 2006), Carica papaya (Mude et al., 2009), Magnolia Kobus (Song et al., 2009),Diospyros kaki (Song et al., 2009), Medicago sativa (Lukman et al., 2011), Cymbopogon Flexuosus (Singh et al., 2006), Azadirachta indica (Shiv Shankar et al., 2004), Avena sativa (Armendariz et al., 2004), Lemongrass (Shiv Shankar et al., 2004), Pelargonium graveolens (Shankar et al., 2003), Chilopsis linearis (Rodriguez et al., 2007), Tamarindus indica (Correa et al., 2016; Singh et al., 2017), Gardenia jasminoides (Khan et al., 2014), Pinus resinosa (Coccia et al., 2012), Camellia sinensis (Ahmmad et al., 2013), and Curcuma longa (Sathishkumar et al., 2009); biopolymers {starch (El-Rafie et al., 2011; Sarma and Chattopadhyay, 2004; Vigneshwaran et al., 2006), chitosan (Long et al., 2013; Reicha et al., 2012), alginate (Yang and Pan, 2012; _S1_Reference451X. Zhao et al., 2014), cellulose (Lokanathan et al., 2014) dextran (Bankura et al., 2012), pullulan (Kanmani and Lim, 2013)}, vitamin B1 (Nadagouda et al., 2009), vitamin B2 (Nadagouda and Varma, 2008, 2006), vitamin C (Nadagouda and Varma, 2007) sugars (Nadagouda and Varma, 2007; Raveendran et al., 2006, 2003), glutathione (Baruwati et al., 2009), tea and coffee extracts (Nadagouda et al., 2008), beet juice (Kou and Varma, 2012a, 2012b), glycerol (Kou et al., 2013), red grape pomace (Baruwati and Varma, 2009), blackberry, blueberry, pomegranate, turmeric extracts (Nadagouda et al., 2014), tree-based gums {GA (Kong et al., 2014), GK (Padil and Černík, 2015; Thekkae Padil and Černík, 2013), KG (Kora et al., 2010; Padil et al., 2015c; Saravanan et al., 2012; Vinod et al., 2011a), GT (Rao et al., 2017), and GG (Kora et al., 2012; Kora and Rastogi, 2015; Kora and Sashidhar, 2015; Mittal and Mishra, 2014)} are examples of such greener alternatives, which offer chemical properties bearing useful functionalities and which have been successfully used for the production and stabilization of NPs.
3. Tree gums: Structural and functional properties
Tree gums, found in virtually every biosphere on Earth, like plants, animals and many bacteria, are hydrocolloids with complex structural features and macromolecules (Anderson and Wang, 1990; Anderson and Bridgeman, 1985; Anderson and Weiping, 1992), whose complex chemical composition depends on the source and its age. Chemically, hydrocolloids are polysaccharides with multifaceted structures encompassing glycosidic bonds with a large number of groups (-OH) repeatedly adorning the backbone of the molecule. They facilitate the networking assemblies with metal ions via templating the polymeric bonds composed and making multidimensional structures. The key chemical functionalities recognized in most of the gums are hydroxyl (OH-), ether (C-O-C), acetyl (CH3CO-), carboxyl (-COO-), aliphatic (-CH), carbonyl (-C=O) groups and those originating from protein fractions. As the name suggests, the simplest interaction of hydrocolloids is with water and this is the main reason for their use in food products. In view of the excess hydroxyl groups, water is held not only within the lattice arrangement via hydrogen bonding, but also inside the spaces creating composite molecular conformation (Anderson and McDougall, 1987; Hall, 2009; Philips and Williams, 2001). The most significant tree gum exudates that find widespread applications are GA, GK, GT, KG and GG; their morphological appearance and constituent sugar moieties are presented in Fig. 2 and their physical and chemical properties and molecular characterization are shown in Table 2. Extensive research has been conducted on various characteristics including their accessibility, molecular weight assignments, structural linkages, well-designed characteristics (rheological, hydration behavior, self-assembly, and surface properties), food and non-food applications (Anderson and Wang, 1990; Anderson et al., 1985a, 1985b, 1983; Anderson and Bridgeman, 1985; Anderson and Grant, 1988; Anderson and McDougall, 1987; Anderson and Stoddart, 1966; Anderson and Weiping, 1992; Brito et al., 2004; de Brito et al., 2005; Fauconnier et al., 2000; Hall, 2009; Janaki and Sashidhar, 2000, 1998; Kumbhare and Bhargava, 1999.; Le Cerf et al., 1990; Mahendran et al., 2008; Osman et al., 1995, 1993a, 1993b; Padala et al., 2009; Philips and Williams, 2001; Phillips and Williams, 2009, 2000, Randall et al., 1989, 1988; Singh and Sharma, 2014b; Stephen et al., 2006; Vegi et al., 2009; Verbeken et al., 2003; Vinod et al., 2008a, 2008b; Weiping, 2000; Whistler and BeMiller, 1993). Tree gums are typically grown under geographically-favorable climatic (excessive heat, shortage of moisture and at higher elevations) and soil conditions (Stephen et al., 2006; Verbeken et al., 2003). Furthermore, gum yield and quality are dictated by the time and intensity of tapping, fluctuations in temperature and rainfall during gum collection. The main countries producing GK, KG, GG, and GT are Sudan, Nigeria, Chad, Mali, and Senegal, India, Pakistan Sudan, Senegal, Mali, Turkey, Iran, Iraq, Syria, Lebanon, Afghanistan, Pakistan, and Russia. KG and GG are tree gum exudate plants that grow naturally in Indian forests (Nussinovitch, 2010; Verbeken et al., 2003; Vinod et al., 2008b).
Fig. 2:
The morphological and sugar constituents of GA, GK, GT, KG and GG.
3.1: Gum arabic (Acacia senegal) (GA)
Gum arabic (Acacia senegal) (GA) is a branched structure encompassing metal cations (calcium, magnesium, and potassium), and a complex polysaccharide. The molecular mainstay comprises 1→3-linked β-D-galactopyranosyl parts, whereas the side chain consists of two to five 1→3-linked β-D-galactopyranosyl units (as lateral chains) appended to the backbone via 1→6-linkages. The whole structure is made up of α-L-arabinofuranosyl, α-L-rhamnopyranosyl, β-D-glucuronopyranosyl and 4-O-methyl-β-D-glucuronopyranosyl segments, the last two being the end-terminal units (Anderson et al., 1985a; Fauconnier et al., 2000; Mahendran et al., 2008; Osman et al., 1995, 1993a, 1993b; Philips and Williams, 2001; Phillips and Williams, 2000; Randall et al., 1989; Verbeken et al., 2003). GA is structurally fabricated by high molecular fractions of carbohydrate masses (approx. 2.5×105 Da) chains being attached separately to a polypeptide sequence (Anderson et al., 1985a; Fauconnier et al., 2000; Osman et al., 1993a). Hence, GA is structurally made up of three molecular fractions, namely (i) arabinogalactan-peptide (AGp); (ii) arabinogalactan-protein (AGP); and (iii) glycoprotein (GP) complexes. The surface properties of GA, such as adsorption at solid-liquid interfaces, could be utilized to stabilize nanoparticles, such as carbon nanotubes, metal nanoparticles (Au, Ag, Fe), ceramics, and latex nanoparticles (Aberkane et al., 2012; Amiri et al., 2012; Batalha et al., 2010; Gashua et al., 2016; Gils et al., 2010; Kannan et al., 2012; Kattumuri et al., 2007; Ma et al., 2012; Song et al., 2011). Furthermore, GA-magnetic NPs have been explored for biomedical applications, namely as MRI contrasting agents for cell-labelling and production of Au NPs for diagnostic and therapeutic applications (Kattumuri et al., 2007; Kong et al., 2014; Palma et al., 2015). GA has a minimal Newtonian viscosity even at elevated gum concentrations, and its intercalating capability to interact with diverse molecular entities, namely proteins, minerals and polyphenols, mainly depends on its unique structural properties (carbohydrate-protein macrostructure); therefore, GA is extensively used as an additive, emulsifier, or thickener because of such remarkable properties (Sanchez et al., 2017) (Table 2). As a food additive, GA is used to prevent sugar crystallization, which assists in the beverage industry, confectionary, food emulsion, flavor encapsulation and bakery and brewing, whereas non-food applications concentrate on pharmacological, lithography, fabric, and cosmetic industries (Nussinovitch, 2010; Verbeken et al., 2003).
3.2: Gum karaya (Sterculia urens) (GK)
Gum karaya (Sterculia urens) (GK) is structurally branched and essentially comprises of a partially acetylated carbohydrate polymer secured as a magnesium and calcium salt with high molecular mass (1.6×106 Da) (Anderson et al., 1985b, 1983; Philips and Williams, 2001; Phillips and Williams, 2000). The leading structure is made up of α-D-galacturonic acid and α-L-rhamnose moieties and is connected to side chains (1,2-linkage of β-D-galactose or by 1,3-linkage of β-D-glucuronic acid), with a preponderance of rhamnose residues being linked to the core structure via 1,4-linkages of β-D-galactose (Anderson et al., 1983; Brito et al., 2004; de Brito et al., 2005; Le Cerf et al., 1990; Verbeken et al., 2003; Weiping, 2000; Whistler and BeMiller, 1993). The major sugar residues found in GK are galactose (13–26 %) and % rhamnose (15–30 %); the latter being higher than other exudate gums. Although the protein content (~1 %) is comparatively low, another compositional material is 40 % of uronic acid (β-D-glucuronic and D-galacturonic acid). Native GK is insoluble in water and its swelling characteristics are due to the presence of the high acetyl content (~ 8 %) of its structure (Janaki and Sashidhar, 2000, 1998; Kumbhare and Bhargava, n.d.). Le Cerf et al.(Le Cerf et al., 1990) differentiated three portions of GK and found that the native gum (~10 %) was solubilized in cold water, increasing to 30 % in hot water; whereas, after deacetylation via treatment with NH3 or sodium hydroxide, 90 % of GA could be solubilized in an aqueous medium (Le Cerf et al., 1990; Stephen et al., 2006; Verbeken et al., 2003). GK has recently been investigated for Si-based anodes in Li-ion batteries as a binder and also as a bio-nanocomposite for effective removal of rhodamine 6G from water (Bie et al., 2017; Kumar et al., 2016).
3.3: Kondagogu gum (Cochlospermum gossypium) (KG)
Kondagogu gum (Cochlospermum gossypium) (KG) is extracted from the Kondagogu tree, found in India. The various grades of gums and their physical and chemical, structural, solution, medical formulations and creaming properties have been described (Naidu et al., 2009; Vegi et al., 2009; Vinod et al., 2008a, 2008b). Toxicity studies have determined that the KG is non-hazardous and has a prospective use as a dietary supplement (Janaki and Sashidhar, 1998; Kumbhare and Bhargava, 2009; Puskuri et al., 2017). Various neutral sugars (glucose, arabinose, galactose, rhamnose, and mannose) and acidic sugars (galacturonic acid and glucuronic acid) comprise the structure of KG, with their complex arrangement being composed of structural linkages, namely (1 →2) β -D-Gal p, (1→6)-β-D-Gal p, (1 → 4) β-D-Glc p A, 4–0-Me-α -D-Glc p A, (1 → 2) α-L-Rha and (1 → 4) α -D-Gal p A (Vinod et al., 2008a, 2008b, 2010).
KG has shown significant differences in its properties pertaining to elemental contents, sugar composition, and amino- and fatty acid profiles compared to other well-known tree gums such as GA, GK and GT (Vinod et al., 2010). KG, a gum with a major constituent of uronic acid, differs from other natural gums such as GA, GK and GT in terms of its amino acid profile. The Zeta potential of native KG (–23.4 mv) indicates the existence of negatively charged functionalities, which may explain the biosorption mechanisms for sequestration of heavy metals involving complexation between metal ions and KG structures with other adsorption mechanisms (adsorption, surface micro-precipitation and ion-exchange etc.) (Vinod et al., 2008b).
3.4: Gum ghatti (Anogeissus latifolia) (GG)
Gum ghatti (Anogeissus latifolia) (GG) is a gum polysaccharide extracted mainly from Anogeissus latifolia (Combretaceae, Myrtales) (Davidson, 1980; Glicksman, 1982; Katayama et al., 2008). GG is available as a calcium-magnesium salt composed of D-mannose, D-xylose, D-galactose, L-arabinose, and D-glucuronic acid and less than 1 % of 6-deoxyhexose, and is used as an emulsifier, thickener, binder and stabilizer in food industries (Castellani et al., 2010; Ido et al., 2008). The gum serves as a buffer and reverts to its normal pH of approximately 4.8. GG contains an arabinogalactan-type polymeric structure consisting of (1→4) β-D-GlcA, and (1→2)-α-D-Man residues as the backbone structure. The side chain is composed of (1→3)- and (1→6)-connected galactose units, and mannose residues are present as double branch-points (Stephen et al., 2006; Tischer et al., 2002b). The structural, conformation and functional relationship of various GG fractions via size exclusion chromatography, static and dynamic light scattering methods have been examined (Kang et al., 2015a, 2015b, 2014). The structural assignments of GG using methylation analysis were established via mass spectroscopy and 2D NMR spectroscopy (COSY, TOCSY, HMQC, and HMBC). The arabinogalactan chain in GG composed of 1→4)-GlcpA(1→6)-Galp(1→ and→2)-L-Araf-(1→connections) and detailed structural assignments have been reported (Kang et al., 2011a, 2011b, 2011c).
The complex structural units and functional properties of GG can easily undergo chemical modification or grafting to create intricate molecular structures, with desirable properties for use in adsorption/complexation of metal ions, electrical conducting materials, as a sorbent for sequestration of contaminated heavy metal ions or dyes from water or industrial effluents, and biomedical applications, among others (Kaith et al., 2014; Mittal et al., 2015; Mittal et al., 2015b; K. Sharma et al., 2015b, 2014; Sharma et al., 2013).
3.5: Gum tragacanth (Astragalus gummifer) (GT)
Gum tragacanth (Astragalus gummifer) (GT) is mostly used in food and pharmaceutical fields because of its attractive features and exceptional attributes (stabilizing, thickening, emulsifying, adhesive, suspending agent and fat replacer with a long shelf life), biodegradability, non-toxicity, natural availability, and elevated resistance to microbial attacks. GT is a high molecular weight (8.4×105 Da) complex macrostructure, branched, heterogeneous and has an anionic nature consisting of magnesium, calcium and potassium salts (Singh and Sharma, 2014b; Verbeken et al., 2003; Weiping, 2000). Two portions make up the composition of the GT, namely tragacanthin and tragacanthic acid or bassorin, the latter being being insoluble in water but having a gelling capability due to swelling. GT is a typical branched arabinogalactan (tragacanthin) comprising (1→6)- and (1→3)-linked galactose and terminal (1→2)-, (1→3)-, and (1→5)-linked L- arabinose. Tragacanthic acid, a pectic type of structure, is the major constituent of GT with structural linkages such as (1→4)-linked α-D-galacturonan substituted at O-3 by β-D-xylose (Balaghi et al., 2011; Gavlighi et al., 2013a, 2013b; Singh and Sharma, 2014b; Tischer et al., 2002a; Verbeken et al., 2003). The proteinaceous materials and methoxyl groups are present in both fractions of GT, albeit in smaller amounts (Verbeken et al., 2003; Weiping, 2000). The remarkable structural and functional properties of GT have recently been exploited in many non-food applications, such as tunable drug delivery systems, hydrogels, cryogel and xerogel blends, micro-encapsulations, nanocomposites for heavy metal ions removal, bioplastics, edible blend films and coatings and grafting copolymer flocculants for destabilization of carbon nano-tubes (Asghari-Varzaneh et al., 2017; López-Castejón et al., 2016; Mallakpour et al., 2018; Mostafavi et al., 2016; Niknia and Kadkhodaee, 2017; Pal et al., 2017; Singh and Sharma, 2017, 2014a).
4. Tree gums in greener synthesis (gum–metal salt interactions) of nanoparticles (NPs)
Lately, much attention has been devoted to the use of natural products for the synthesis of nanomaterials as opposed to synthetic compounds. In addition to their application as processing materials, another part of green nanotechnology is the development of environmentally-friendly sustainable processes, e.g. exploitation of non-toxic, eco-friendly solvents; water and supercritical carbon dioxide have been investigated as alternatives to organic solvents (Ji et al., 1999; Shah et al., 2000).
The use of gums as natural bio-sorbents aimed at the sorption of toxic metals and as renewable, inexpensive and non-toxic reducing and stabilizing agents for the synthesizing of NPs has been explored (Kong et al., 2014; Kora et al., 2012, 2010; Kora and Rastogi, 2015; Mittal and Mishra, 2014; Padil et al., 2015c; Padil and Černík, 2015; Reddy et al., 2015; Saravanan et al., 2012; Padil and Černík, 2013; Vinod et al., 2011a). In general, the respective gum solutions are simply admixed with a solution of the corresponding metal salts to produce metal NPs (Fig. 3).
Fig. 3:
Schematic representation of the production of NPs using gum solutions and their characterization.
Synthesis of NPs is affected by the concentration of the precursor salts and gum, pH, temperature, basic electrolyte and contact time (Kong et al., 2014; Kora et al., 2012, 2010; Kora and Rastogi, 2015; Mittal and Mishra, 2014; Padil et al., 2015c; Padil and Černík, 2015; Reddy et al., 2015; Saravanan et al., 2012; Thekkae Padil and Černík, 2013; Vinod et al., 2011a).
Table 4 summarizes the NPs synthesized using various gums, their synthetic conditions and characterizations (particle size, shape, crystalline nature). (Table 4)
Table 4:
Summary of the literature pertaining to NPs synthesis mediated by several gum hydrocolloids
| Tree gums |
Category and composition of NPs | Method of preparation | Characterizations of NPs | References |
|---|---|---|---|---|
| GA | Ag NPs; GA (0.5 wt%); AgNO3 (0.5 wt%) | Room temperature synthesis | ~ 5 nm in size; face centered cubic structures with crystalline, |
(Mohan et al., 2007) |
| GA | Ag; GA (0.01 wt%; 2.0 mL); AgNO3 (0.1 wt%; 0.2 mL) |
γ- ray irradiation route | 3 nm; spherical shape; face centered cubic crystallinity |
(Rao et al., 2010) |
| GA | Ag NPs; 5% GA (5%); PVA (3%) and AgNO3 (1.0 mM) |
γ radiation-induced crosslinking | 10–40 nm | (Juby et al., 2012) |
| GA | Au NPs; GA (12 mg, 6 mL); NaAuCl4 (0.1 M, 0.1 mL); phosphor amino acid (0.1 M. 20 μL) |
Stirring | 15–20 nm, spherical shape | (Kattumuri et al., 2007) |
| GA | Se NPs; GA (1 mg/ml); 0.6 M selenious acid; stirring for 6 h; addition of 4.5 ml of 0.1 M ascorbic acid. |
Stirred for 0.5 h | ∼34.9 nm, spherical shape |
(Kong et al., 2014) |
| GA | Ag NPs; GA (0.3 g/L, 270 mL); AgNO3 (2 mM); NaBH4 (0.08M, 15 mL) |
Vigorous stirring | 6 nm; spherical in shape and monodispersed. |
(Cheng et al., 2011) |
| GA |
198AuNPs; GA and P(CH2NHCH(CH3)COOH)3 (THPAL) |
Spontaneous reaction | Radio labelled Au NPs | (Kannan et al., 2006) |
| GA | GA (0.01g; 5 mL); ascorbic acid (0.1M, 5mL); KAuBr4 (5mM, 5 mL) |
Sonicated for 5 s at R.T | 520mn; flower shape | (L. Wang et al., 2012b) |
| GA | ZnO NPs; GA (1g); Na acrylate (10.63 mM); N.Ni – methylene bis-acrylamide (486.47 mM); K persulfate (370.3 μM) and Zn(II), 1g) |
Sonothermal | 40– 60 nm; polar structures |
(Webb et al., 1998) |
| GK | Ag NPs; | Oxidation-reduction reaction | 7–10 nm, spherical | (Padil and Černík, 2015) |
| GK | Au NPs; GK (1 wt%); HAuCl4 (5×10−3 M, 100 μL); shaking at 75°C for 1 h. |
Stirring | 7.8± 1.8 nm, spherical, stable for 6 months |
(Padil and Černík, 2015) |
| GK | Au NPs; GK (15 mg, 1.0 mL); HAuCl4 (10 mM, 100 μL); |
Stirred for 1h at 90 oC | 20–25 nm, spherical | (Pooja et al., 2015) |
| GK | Pt NPs; GK (1 wt%); Pt (5×10−3 M, 100 μL) | Autoclaving at 103 kPa for 15 min | 5.0± 1.2 nm, spherical | (Padil and Černík, 2015) |
| GK | CuO NPs; GK (1.0 mg, 1.0 mL); CuCl2.2H2O (1mM) |
Thermal | 10.5± 2.4 nm, spherical | (Padil and Černík, 2015; Thekkae Padil and Černík, 2013) |
| GK | Fe3O4 NPs; Fe2+ and Fe3+ ions and NH4OH solution. | Co-precipitation | 18.5± 3.5nm, spherical | (Padil and Černík, 2015) |
| KG | Ag NPs; KG (100 mg, 10 mL); AgNO3 (100 μL, 10mM) | Shaking at 45°C for 1 h. | 5.5±2.5nm,spherical, face centered cubic, stable >6 months |
(Vinod et al., 2011a) |
| KG | Ag NPs; KG (0.5%); AgNO3 (1 mM) | Autoclaving at 121°C at 15 psi for 1 h. |
3 nm, spherical, highly stable |
(Rastogi et al., 2014) |
| KG | Ag NPs; Ag NPs; KG (0.5%); AgNO3 (1 mM) | Autoclaving at 121°C at 15 psi for 1 h. |
3 nm, spherical, highly stable |
(Kora et al., 2010) |
| KG | Au NPs; KG (100 mg, 10 mL); HAuCl4 (100 μL, 10mM) | Shaking at 75 °C for 1 h. | 7.8±2.3 nm, spherical, stable >6 months |
(Vinod et al., 2011a) |
| KG | Au NPs; KG (0.5%, 3 mL); HAuCl4 (1.0 mM, 1.0mL) |
Autoclaving at 15 psi, 120 oC for 10 min |
12 ± 2 nm, nano- crystalline |
(Reddy et al., 2015) |
| KG | Pd NPs; KG (0.5%); PdCl2 (1.0 mM) | Autoclaving at 121 oC and 15 psi for 30 min. |
6.5 ± 2.3 nm, crystallized in face centered cubic symmetry. |
(Rastogi et al., 2017) |
| KG | Pt NPs; GK (100mg, 10 mL); H2PtCl6 (100 μL, 10mM) |
Autoclaving at 15 psi for 15 min. |
2.4±0.7 nm, crystalline, stable for more than 6 months |
(Vinod et al., 2011a) |
| KG | Fe3 O4 NPs; KG (25 mg, 50 mL); Fe Cl2 and FeCl3
(Fe 2+/Fe 3+ ratio 2:1) |
Co-precipitation under hydrothermal conditions. |
8– 15 nm; spherical size | (Saravanan et al., 2012) |
| GT | ZnO NPs | Low-temperature plus ultrasonic |
55–80 nm, highly crystalline nature and single phase |
(Ghayempour et al., 2016) |
| GG | Ag NPs; AgNO3 (1.0 mM); GG (0.5%) | Autoclaving at 121 oC and 15 psi for 60 min. |
5.7 ± 0.2 nm, spherical NPs |
(Kora et al., 2012; Kora and Sashidhar, 2015) |
| GG | Pd NPs; GG (0.5%); PdCl2 (1.0 mM) | Autoclaving at 121 oC and 103 kPa for 30 min. |
4.8± 1.6 nm, spherical shape |
(Kora and Rastogi, 2015) |
| GG | Au NPs; GG (1% w/v; 4 mL); HAuCl4 (100 mM, 30.7 μLmL) |
Stirring and heated at 60 oC | 112.5 nm; spherical shape | (Alam et al., 2017) |
4.1. Synthesis and stabilization of NPs using gum arabic (GA)
The production of Au NPs, with varying shapes and sizes, was achieved by mixing an Au precursor (KAuBr4 or HAuCl4), reducing agents (catechol, ascorbic acid, and NaBH4) and GA (a capping agent) under sonication for 20 min. at room temperature (Wang et al., 2012a). By changing the concentration of the precursors (1.0 mM to 20 mM) and reducing agents, Au NPs could be fabricated in shapes such as flower, cauliflower, raspberry, urchin, and confeito (Fig. 4 a). GA was deemed as the best capping agent for the growth of various architectures of Au NPs via the particle-mediated aggregation model (Fang et al., 2010; Li et al., 2007; Wang and Halas, 2008) in contrast to polyvinylpyrrolidone (PVP), which afforded particles of irregular sizes.
Fig. 4:
Schematic representation of (a) Au NP formation with distinct shapes (cauliflower, raspberry, urchin, and confeito, respectively) via sonication, employing GA as a capping agent and catechol and ascorbic acid (reductants). Reproduced with the permission of ref (Wang et al., 2012), Copyright © 2012, The Royal Society of Chemistry and (b) striking color variation of the PEDOT-Au NPs prepared using GA as a stabilizer. Reprinted with the permission of ref (Rocha et al., 2014), Copyright © 2014, American Chemical Society.
An efficient strategy for the direct assembly of ‘Au nanoflowers’ was developed by sonicating the mixture of a GA solution (0.01g, 5 mL) with KAuBr4 (20 mM, 5 mL) and ascorbic acid (0.1 M, 5mL) for 5 s at room temperature. GA played a prominent role in directing the shapes and stabilization of Au nanostructures, which were stable for six months in an aqueous medium at room temperature (Wang et al., 2012b). The glycol protein, hydroxyproline, present in the GA was found to act as a steric stabilizer to protect the ensuing Au NP architectures, which can be exploited for bio-sensing and biomedical applications.
An aqueous dispersible Au nanocomposite was prepared by one-pot mechanical stirring of GA with poly (3,4-ethylenedioxythiophene) (PEDOT) and HAuCl4, where GA served as a stabilizer and a medium for dispersion of an Au- PEDOT nanocomposite. The color of the Au-PEDOT nanocomposite is directly proportional to the concentration of GA in the suspension as well as the corresponding size of the Au NPs (Fig. 4b) (Rocha et al., 2014). In addition to delineation of GA stabilization mechanism, this research established a bench mark for the rational and targeted design of novel uniform-sized nanoparticles in complex systems without aggregation.
Biocompatible Au NPs were produced via continuous mixing of an aqueous GA solution (0.2 %), with phosphine amino acid (THPAL), and NaAuCl4. GA stabilized Au NPs (GA-Au NPs), targeted as a molecular imaging contrast agent using an X-ray CT scan, displayed in vitro and in vivo durability for months in aqueous, salt and buffered solutions (Kattumuri et al., 2007).
A facile method utilizing GA has produced monodispersed Pd NPs (average size; 9.1 ±0.3 nm) at an optimum temperature (100oC). A higher concentration of GA led to smaller-sized NPs, which were stable for more than five months. Presumably, the –COO- and –OH groups of the GA structure, emanating from polysaccharide, proteins and fatty acid moieties, are responsible for encapsulating Pd NPs via steric stabilization and electrostatic repulsion (Devi et al., 2011). GA stabilized Au NPs (GA-Au NPs) with excellent stability in varying biological pHs were prepared by NaBH4 reduction of an aqueous AuCl4- solution at 25oC. The resultant NPs were found to be exceptionally stable at both highly acidic pH (pH = 1.2 or gastric) and under neutral pH (pH=6.8) in vitro stability conditions (Ribeiro de Barros et al., 2016). Furthermore, GA-Au NPs were evaluated for changes in the viability, proliferation and morphology in various cell lines. The inherent structural components of GA bearing glycoprotein and arabinogalactan-protein (AGP) with variabilities of functional groups (-OH, -COO- and –NH2) may control the size of NPs growth and enhanced colloidal stabilization via steric interaction (Cornelsen et al., 2015; Quintanilha et al., 2014; Rocha et al., 2014).
Magnetic NPs based on a GA support were synthesized by intermixing Fe2+/Fe3+ ions using an ammonia solution under hydro-thermal conditions (Banerjee and Chen, 2007a). In another variation, glucose grafted GA-magnetic NPs were prepared by coupling GA-magnetic NPs with maltose and reductive amination using NaBH3CN (Banerjee and Chen, 2007b). The GA-magnetic NPs could be used for the exclusion of Cu2+ ions from water via their complexation with –NH2 and –OH groups of GA and magnetic NPs, respectively. The glucose grafted GA-magnetic NPs were applied to bind to a glucose binding protein, concanavalin A (Con A). Con A interacted with the glucose grafted GA-magnetic NPs, wherein the -OH groups of glucose and the -NH groups or oxygens from asparagine in Con A forged the hydrogen bond.
A spontaneous reaction of Ag NPs in the presence of GA occurs without the addition of any distinctive reducing agent by mixing equal amounts of GA (0.5 wt%) and silver nitrate aqueous solutions (Mohan et al., 2007). Ag NPs (particle size ~ 5 nm) were obtained where GA served as a good stabilizer for a period of more than five months. Mechanistically, mixing of Ag+ cations with the aqueous GA solution resulted in an ion-exchange process and the carboxylate group (-COO-) of GA was converted into -COOAg. Ag+ cations attached to carboxylate groups were later converted in situ into Ag NPs and consequently stabilized by the GA.
The synthesis of translucently separable Ag NPs via the hydrothermal method using GA as a reducing and functional coating agent and various factors, such as the reaction temperature, mass ratio of GA to AgNO3 of 1:1, concentration (10 mmol/L) and reaction time, were studied, and Ag NPs with different morphological characteristics could be produced (Li et al., 2015). The temperature and GA played a vital role in the production of mono-disperse Ag NPs, due to the interaction of GA charged groups (-COOH and -NH2) with Ag+ resulting in an (Ag+-GA) complex. Furthermore, the reduction of Ag+ to Ago occurred due to easy cleavage of C-C bonds in GA, which released the electrons at elevated temperatures (> 100 oC) when the (Ag+-GA) complex was reduced to Ag0-GA. Subsequently, GA polymer chains provided Ag NPs with excellent diffusion properties via the steric effect.
Extremely stable and water dispersed, re-dispersible Ag NPs were produced by applying γ-ray radiolysis of an Ag+ solution with GA as a capping agent via steric stabilization (Fig. 5 f) (Rao et al., 2010). The major factors influencing the formation of uniform-sized Ag NPs were the radiation dosage, GA to Ag+ ratio and the ionic strength of the reaction conditions. At higher doses of radiation, face-centered cubic (FCC) crystalline NPs (XRD) with a maximum particle size of less than 3 nm (HR-TEM) were generated.
Fig. 5:
GA stabilized (a) Se NPs (TEM image); (b) HRTEM images of SeNPs; (c) and (d) aggregated Se NPs without GA as a stabilizer; (e) GA- Ag NPs formed by radiolysis; and (f) Pd NPs capped GA by thermal processes, and inset picture, selected area of electron diffraction pattern. Reprinted with the permission (a, b, c, & d) of ref. (Kong et al., 2014) and (e) ref. (Rao et al., 2010), Copyright © 2014, Academic Press Inc. Elsevier Science and (f) ref. (Devi et al., 2011), John Wiley & Sons.
The synthesis of Ag NPs, within a PVA-GA hydrogel medium, via γ-radiation tempted scaffolding has been reported (Juby et al., 2012). The work revealed that the degree of swelling of the hydrogel and the percolating of Ag NPs from the composite hydrogel depends on the GA concentration. The ensuing NPs were found to be in the range of 10–40 nm, where an interaction of hydroxyl and carboxyl groups of GA and PVA with Ag NPs was demonstrated by FTIR. TGA study revealed that the Ag- PVA-GA linkage was thermally more stable than the hydrogel without the Ag NPs, silver loading being proportional to the cross-linking. The Ag NP hydrogels showed superior antibacterial action against E. coli bacteria, which implies that the radiation-induced synthesis can be used for various biomedical applications.
A greener synthesis of Au NPs on the surface of GA-functionalized Fe3O4 NPs (Wu and Chen, 2012) has been described. A thin coating of ~2 nm Au NPs was deposited on the surface of GA-Fe3O4 NPs. The nanocomposite demonstrated good catalytic efficiencies for the reduction of C6H5NO3 (4-nitrophenol) with NaBH4 and virtuous stability lasting more than five recycles. The kinetic data suggested that this catalytic response was diffusion-controlled and the outcome bodes well for the development and application of magnetically recoverable Au nano-catalysts.
Selenium NPs (Se NPs) have been prepared by using GA as the stabilizer where the particle size, shape, aggregation behavior and in vitro antioxidant performance of the Se NPs were established by TEM (Fig. 5a), HR-TEM (Fig. 5 b), DLS, FTIR, AFM and UV-Vis spectroscopy (Kong et al., 2014). The stability of Se NPs (particle size of ~35 nm) in water was established for nearly one month. The Se NPs without GA were aggregated and presented in Fig. 5 c and d. The study illustrates how a simple food additive with a high molecular mass and branched structure bearing abundant functional groups could be used to prepare and stabilize Se NPs.
4.2. Synthesis and stabilization of NPs using gum Karaya (GK)
The synthesis of GK-stabilized Au NPs and their application in the delivery of anticancer drugs have been studied (Pooja et al., 2015). The addition of HAuCl4 (10 mM) to a GK solution (15 mg/mL) and stirring for 1 h at 90 oC produced gum-stabilized NPs (GK-Au NPs), which were found to be biocompatible during cytotoxic and hemolysis studies. The GK-Au NP conjugate showed efficacy as a drug carrier as well as enhanced colloidal stability for Au NPs, with better anticancer activity, inhibition of colony formation and ROS induction against human lung cancer cells superior to the drug, gemcitabine hydrochloride, being determined. These results also corroborated recent clinical studies with a natural gum arabic-Au NP conjugate (GA-AuNPs), which displayed bio-compatibility and non-toxicity in mammalian systems (Chanda et al., 2014). In addition, many natural polysaccharides, such as pectin (Devendiran et al., 2016), alginate (Jaouen et al., 2010; Kodiyan et al., 2012), dextran (Jang et al., 2014), β-glucan (Liu et al., 2017) and chitosan (Hortigüela et al., 2011; Jeong et al., 2011), have been conjugated with Au NPs via greener pathways to minimize the associated toxicity and hazardous nature, while being pragmatic for environmental, biological and medical applications.
A colloid-thermal synthesis of CuO NPs (Fig. 6 d) using GK (10 mg/L) with varying quantities of CuCl2.2H2O (1 mM to 3 mM) has been described at 75 °C, wherein single-phase CuO with a monoclinic structure, CuO NPs, were uniformly disseminated on the external of the gum matrix (Thekkae Padil and Černík, 2013). The ensuing smaller CuO NPs (4.8 ± 1.6 nm) were found to be exceedingly stable and displayed significantly better antibacterial proficiency on E. coli and the S. aureus relative to larger CuO NPs (7.8 ± 2.3 nm).
Fig. 6:
Representative TEM images of NPs created and stabilized by KG (a) Ag NPs; (b) Pt NPs; (c) Au NPs via hydrothermal synthesis and (d) CuO synthesized by GK (e) ZVI protected by a GK structure; (f) HRTEM images of ZVI (zerovalent iron) NPs produced by GK; (g) depiction of steric stabilization mechanism for ZVI NPs and (h) the core of ZVI surrounded by a GK network structure (Vinod et al., 2011). Reproduced with permission Copyright © 2011, Academic Press Inc. Elsevier Science and reproduced from ref. (Vinod et al., 2017) with the permission of RSC.
4.3. Synthesis and stabilization of NPs using Kondagogu gum (KG)
KG has been extensively applied for the synthesis of various NPs with the typical dual advantages of KG as a reducing and stabilizing agent (Kora et al., 2010; Kora and Sashidhar, 2014; Rastogi et al., 2017, 2015, 2014; Saravanan et al., 2012; Vinod et al., 2011a). Although the literature on tree gums-assisted nanoparticle synthesis is sparse relative to other biopolymers of a natural origin such as chitosan, alginate, hyaluronan, starch, cellulose, dextran, pectin, carrageenan, guar gum, pullulan, agar, and xanthan gum etc., recent studies have started to exploit the use of tree gums in view of their availability as an economic commodity and their distinctive structural-functional properties (Deshmukh et al., 2012; Fan et al., 2012; Riedo et al., 2013; Sanchez et al., 2017; Kashma Sharma et al., 2014; Xhanari et al., 2017).
Ag NPs (Fig. 6 a) and Au NPs (Fig. 6 c) were prepared from their salts (HAuCl4.3H2O and AgNO3; 5 × 10−3 M each) and a KG aqueous solution was produced by agitation at a selected pH and 65 °C (Ag NPs) and 75 °C (Au NPs) for 1 h (Vinod et al., 2011a). Color changes and UV-Vis spectroscopic measurements (absorbance maxima at 516 and 420 nm for Ag and Au, respectively) aided the progress of the reaction. Similarly, Pt NPs (Fig. 6 b) were synthesized from salt and a KG solution by autoclaving at 0.15 MPa for 15 min. The mean diameters of the crystalline NPs were quantitatively determined by TEM to be 2.4± 0.7 nm, 7.8± 2.3 nm and 10.5± 3.5 nm for Pt, Au and Ag NPs, respectively. In another study (Vinod et al., 2017), GK was used for the fabrication and scaffolding for zerovalent iron nanoparticles (ZVI) (Fig. 6 e and f) and the mechanism of stabilization of ZVI was further delineated (Fig. 6 g and h).
A simplistic and sustainable method by autoclaving a mixture of Ag NO3 (1.0 mM) and GK (0.5 wt %) at 121 oC and 15 psi for 60 min has been demonstrated (Kora et al., 2010). Nearly monodispersed NPs of ~ 3 nm Ag NPs ensued where the KG served as a reductant and stabilizing agent. In terms of antibacterial evaluation, the results highlighted a higher antibacterial efficiency for the Gram-positive bacteria (Staphylococcus aureus) with a larger zone of inhibition compared to Gram-negative bacterial strains (E. coli and Pseudomonas aeruginosa). The proposed mechanism of Ag NP production involved the reductive action of –OH, -CO- and –COO- functional groups present in the KG structure, which facilitated the complexation with Ag ions and utilized the –OH groups to convert Ag ions into Ag NPs at the expense of the –CO groups. Capping of the Ag NPs was performed by the polysaccharide-protein and fatty acid moieties present in the KG via steric stabilization. A similar mechanism of synthesis and the capping effect of various NPs such as Au, Ag, and Pt have been studied for GA, starch, and alginate (Devi et al., 2011; Luque et al., 2013; Vigneshwaran et al., 2006).
Ag NPs (mean size of 5.1±2.8 nm; KG-Ag NPs) have been produced by autoclaving 1 mM Ag NO3 solutions with of KG (0.5 %) at 121oC under 0.1 MPa for 60 min, and they were found to be stable at various pH (4–11) and quantity of NaCl (5–100 mM) (Rastogi et al., 2014). Their utility for the colorimetric detection of Hg2+ ions at ppb levels in water was not influenced by the common ions present in drinking water. In addition to tree gums, bio-sensing of metal ions or hazardous vapors using other gum-stabilized Ag or Au hydrocolloids, such as locust bean gum, and guar gum, has been shown (Pandey et al., 2012; Tagad et al., 2014).
KG-modified spherical magnetite (Fe3O4) NPs have been fabricated by mixing Fe2+ and Fe3+ ions in an alkaline solution in the occurrence of KG (Saravanan et al., 2012). The produced NPs (KG- Fe3O4;~ 8 to 15 nm diameters) demonstrated ferromagnetic behavior with a magnetic permeation (~60 emu/g).
4.4. Synthesis and stabilization of NPs using gum Tragacanth (GT)
The formation of Ag NPs on a cotton fabric via interaction between Ag NO3, GT, citric acid and cellulose (from cotton) has been demonstrated (Montazer et al., 2016). Mechanistically, the formation of Ag NPs occurred as abundant–OH groups from the cotton fabric and –COO- from the GT reduced the Ag+ ions to Ag NPs (particle size ~77.5 nm) on the surface of the cotton fabric. The treated cotton textile with GT-AgNPs was found to be highly effective in killing bacterial colonies, such as E. coli and S. aureus, whereby producing antibacterial cotton fabrics.
GT has been used for the preparation of urchin-like hexagonal ZnO nanorods (GT-ZnO NR) at a low-temperature using ultrasonic irradiation (Ghayempour et al., 2016). The ensuing ZnO NR (55–80 nm size) was produced with a length of 240 nm (FE-SEM, XRD, FTIR and UV-Vis analysis). GT-ZnO NR displayed good photocatalytic activity for the degradation of methylene blue. Furthermore, GT-ZnO NR showed antibacterial effectiveness against S. aureus and E. coli and antifungal action against C. albicans, respectively.
Au NPs were generated by heating a mixture of HAuCl4.3H2O with GT for 240 min. at 65 oC and their stability was examined by mixing with varying concentrations of salt, acidic and basic conditions and with human blood plasma (Rao et al., 2017). Furthermore, naringin-loaded GT-AU NPs proved to be a highly effective delivery route for naringin against both the B. subtilis and M. luteus and E. coli and P. aeruginosa bacterial strains, with superior bactericidal activity as well as loading efficacy being displayed.
4.5. Synthesis and stabilization of NPs using gum Ghatti (GG)
GG (1 % w/v) produced Au NPs upon mixing with HAuCl4 (0.1 M) and heating (70 – 90oC). The process was controlled by various independent parameters such as gum and metal salt concentrations, and temperature and dependent variables (particle size, PDI, and zeta potential), which were evaluated via a Box-Behnken-based statistical model (Alam et al., 2017). The study showed that temperature was critical for the synthesis of Au NPs, as it controlled the size, PDI, and zeta potential of the GG stabilized or capped Au NPs.
A facile greener synthesis of almost monodispersed and uniform spherical Ag NPs (5.7 ± 0.2 nm) has been reported from silver nitrate using GG, which displayed substantial antibacterial activity (Kora et al., 2012).
Nanocomposite, GG-based iron oxide magnetic NPs cross-linked with poly (acrylic acid-co-acrylamide), have been produced via free radical polymerization and exploited for the adsorption of rhodamine B (adsorption capacity = 654.8 mg/g) from water (Mittal and Mishra, 2014). The adsorptions isotherms, kinetic studies and desorption efficiency of the adsorbent were established. Other composite materials with natural biopolymers based on xanthan gum, gum arabic, guar gum, alginate and chitosan have been used for the environmental remediation of toxic heavy metal and organic dyes from water (Banerjee and Chen, 2007a; Chang and Chen, 2005; Lim et al., 2009; Sand et al., 2010; Singh et al., 2009; Yan et al., 2012).
5. Electrospun fibers from gum hydrocolloids
Electrospinning is an elegant method for producing fibers in micro- or nanometer sizes with a pronounced surface area to volume ratio (Greiner and Wendorff, 2007; Jayaraman et al., n.d.). Critical parameters of the process include solution assets (chain entanglement, molecular weight, dielectric constant, purity, solubility, concentration, viscosity, surface tension, distribution of charged groups, thickening effect, and conductivity), handling factors (applied field voltage, humidity, solution movement rate, tip-to-collector distance, and accumulator geometry) and ambient factors (moisture, movement of air and temperature) (Haider and Park, 2009; Lubambo et al., 2013; Nie et al., 2008; Mahanta et al., 2011; Toskas et al., 2011; Wang et al., 2013). The attainment of an ecologically-responsive electrospinning technology dictates the use of non-hazardous, cheaper, and more ecological solvents and spinnable resources. Compared to the present-day electrospinning method, which frequently uses harmful, corrosive and volatile organic solvents, the application of aqueous-based solvents, ionic liquids, deep eutectic solvents (DESs) or polymers make the electrospinning processes both more ecologically favorable and economical (Abbott et al., 2003; Barber et al., 2013; Freire et al., 2011; Grassian, 2008; Raveendran et al., 2003; Thavasi et al., 2008; Varma, 2016; Viswanathan et al., 2006; Zhang et al., 2012).
Electrospun fibers of natural polymers have tremendous practicability in biomedical fields (material engineering, drug carriers, biosensor and drug delivery (Sridhar et al., 2015) and encapsulation of bioactive compounds in the food industry (Liang et al., 2007; Wen et al., 2017). Numerous natural hydrocolloids including food proteins and polysaccharides (Mendes et al., 2017a), alone or blended with other natural/synthetic formulations, have been applied for electrospinning, which successfully generates uniform nanofibers (Alborzi et al., 2014, 2013; Atila et al., 2015; Bonino et al., 2011; Chang et al., 2012; Cui et al., 2017, 2016; Fu et al., 2016; Ignatova et al., 2016; Islam et al., 2012; Islam and Karim, 2010; Kong and Ziegler, 2014; Lin et al., 2013; López-Rubio et al., 2012; Lubambo et al., 2015, 2013; Mendes et al., 2017b; Nie et al., 2008; Nista et al., 2015; Padil et al., 2016, 2015a, 2015c; Padil and Černík, 2015; Qian et al., 2016; Ranjbar-Mohammadi et al., 2016c, 2013; Ranjbar-Mohammadi and Bahrami, 2016; Rezaei et al., 2016; Rockwell et al., 2014; Shalumon et al., 2011; Shekarforoush et al., 2017; Sousa et al., 2015a, 2015b; Stone et al., 2013; Tsai et al., 2015; Vashisth et al., 2017, 2016, 2014; Yang et al., 2014; Zarekhalili et al., 2017). An illustrative review of natural gum hydrocolloids, such as guar gum (Lubambo et al., 2015, 2013), alginates (Bonino et al., 2011; Chang et al., 2012; Fu et al., 2016; Islam and Karim, 2010; Nie et al., 2008; Nista et al., 2015; Shalumon et al., 2011; Stone et al., 2013), pullulan (Atila et al., 2015; Islam et al., 2012; Kong and Ziegler, 2014; López-Rubio et al., 2012; Qian et al., 2016), almond gum (Rezaei et al., 2016), pectin (Cui et al., 2017, 2016; Rockwell et al., 2014), agar (Sousa et al., 2015a, 2015b; Yang et al., 2014), carrageenan (Ignatova et al., 2016), xanthan gum (Mendes et al., 2017b; Shekarforoush et al., 2017), gellan gum (Vashisth et al., 2017, 2016, 2014), GA (Padil et al., 2016; Tsai et al., 2015), GT (Ranjbar-Mohammadi et al., 2016c, 2013; Ranjbar-Mohammadi and Bahrami, 2016; Zarekhalili et al., 2017), GK (Padil et al., 2016; Padil and Černík, 2015), and KG (Padil et al., 2016, 2015a, 2015c) and their spinning combinations (both carriers and solvents), and prospective applications are presented in Table 3.
Table 3:
Electrospinning of commercial polysaccharides and their comparison with tree gum hydrocolloids
| Commercial polysaccharides | Gum hydrocolloids with other carriers (polymers/solvents) for electrospinning |
Emerging applications | References |
|---|---|---|---|
| Guar gum | Water | Not defined | (Lubambo et al., 2013) |
| PVA/water | Biomedical | (Lubambo et al., 2015) | |
| Alginate | PVA/water | Antimicrobial wound dressings and tissue engineering |
(Fu et al., 2016) |
| Chitosan/ glycerol /acetic acid/ water |
Tissue antiadhesion barrier |
(Chang et al., 2012) | |
| Glycerol | Not defined | (Nie et al., 2008) | |
| Polyethylene oxide (PEO)/ Triton |
Not defined | (Bonino et al., 2011) | |
| Poly(vinyl alcohol) (PVA)/ water |
Not defined | (Islam and Karim, 2010) | |
| PVA / citric acid | Not defined | (Stone et al., 2013) | |
| Sodium alginate/PVA/nano ZnO |
Antibacterial wound dressings |
(Shalumon et al., 2011) | |
| Chitosan/PEO/ water/ 5% ethanol |
Not defined | (Nista et al., 2015) | |
| Pullulan | Cellulose acetate/ DMF/ DMSO |
skin or bone tissue engineering scaffolds |
(Atila et al., 2015) |
| Whey protein / phosphate-buffer |
Encapsulation of Bifidobacterium strains for functional foods |
(López-Rubio et al., 2012) | |
| DMSO | Not defined | (Kong and Ziegler, 2014) | |
| PVA/water | Distribution of bioactive compounds |
(Islam et al., 2012; Qian et al., 2016) | |
| Almond gum | PVA/water | Thermostable delivery system for vanillin |
(Rezaei et al., 2016) |
| Pectin | PEO/water | Biomedical, electrochemical devices, and drug delivery systems |
(Cui et al., 2016; Rockwell et al., 2014) |
| Chitosan/PVA/ Acetic acid/ water |
skin tissue scaffolds | (Lin et al., 2013) | |
| Alginate/PEO/NaOH solution |
Enhanced encapsulation of folic acid for functional foods |
(Alborzi et al., 2013, 2014) | |
| PEO/ water/ 1 wt% Triton X-100/ 5 wt% DMSO |
Not defined | (Cui et al., 2017) | |
| Agar | Polyacrylonitrile (PAN) |
Controlled drug release | (Yang et al., 2014) |
| PVA/water | Not defined | (Sousa et al., 2015a, 2015b) | |
| Carrageenan | Chitosan /caffeic acid–coated poly(3- hydroxybutyrate) |
Antibacterial and antioxidant activity |
(Ignatova et al., 2016) |
| Xanthan | Chitosan/ water | Drug delivery carriers | (Mendes et al., 2017b) |
| Formic acid | Not defined | (Shekarforoush et al., 2017) | |
| Gellan gum | PVA/water | Skin tissue regeneration | (Vashisth et al., 2014) |
| PVA/Water/ Ofloxacin |
Gastroretentive/mucoad hesive drug delivery |
(Vashisth et al., 2017, 2016) | |
| Gum Arabic | PVA/water | (Padil et al., 2016) | |
| Chitosan/gelatin/water | Tissue engineering | (Tsai et al., 2015) | |
| Gum Tragacanth |
Curcumin /poly(ε- caprolactone)/ acetic acid |
Biomedical | (Ranjbar-Mohammadi and Bahrami, 2016) |
| Poly lactic glycolic acid (PLGA) / tetracycline hydrochloride (TCH) |
Controlled drug release | (Ranjbar-Mohammadi et al., 2016c) | |
| PVA / water | Wound dressing | (Ranjbar-Mohammadi et al., 2013) | |
| Poly(ε-caprolactone) (PCL)/ poly(vinyl alcohol) (PVA) |
Scaffolds for skin substitutes |
(Zarekhalili et al., 2017) | |
| Gum Karaya | PVA/Water | Removal of NPs contamination from water |
(Padil et al., 2016; Padil and Černík, 2015) |
| Gum Kondagogu |
PVA/Water | Removal NPs from water |
(Padil et al., 2016, 2015c) |
| DDSA / PVA/ water | Antibacterial membrane | (Padil et al., 2015a) |
With their excellent mechanical strength and numerous physical and chemical properties, nanofibers have remarkable applications in many diverse fields including energy conversion (dye-sensitized solar cells, organic and hybrid solar and fuel cells), energy storage devices (for hydrogen storage and super capacitors), optoelectronics, transistors, personal respiratory protection from biological and chemical agents, acoustics, biotechnologies, medical fields (drug delivery, nerve and tissue engineering, wound healing), and catalysis (Frenot and Chronakis, 2003; Greiner and Wendorff, 2007; Sill and von Recum, 2008; Yoon et al., 2008). In the environmental domain, their applications include air filtration, dust capture, liquid filtration for removal of suspended solids and bacteria, heavy metal ion and organic compounds adsorption and treatment, ion exchange, membrane technologies and sensors for both liquid contaminants and gas detection (Desai et al., 2009; Homayoni et al., 2009; Lee et al., 2009; Li et al., 2011; Matsumoto and Tanioka, 2011; Minato et al., 2006; Reneker et al., 2007; Teo and Ramakrishna, 2006; Thavasi et al., 2008; Thompson et al., 2007; Vashisth et al., 2014).
The electrospinning of natural polymers and their applications in medicine has increased due to their biocompatibility, non-toxicity, biodegradability and economic profitability compared to synthetic polymers (Ramakrishna et al., 2005; Shenoy et al., 2005; Venugopal and Ramakrishna, 2005; Wang et al., 2009; Yu et al., 2006). The major areas in biomedical (scaffolding, wound dressing and drug delivery), ecological and antibacterial applications involve the use of natural electrospun fibers (Bonino et al., 2011; Matthews et al., 2002; Torres‐Giner et al., 2008; Wang et al., 2005). The choice of solvent, molecular configuration, and distribution and spinning environment, are some of the critical factors that still need to be addressed on the way to advance the electrospinning of biological polymers (silk fibroin, hyaluronic acid, chitin, chitosan, collagen, cellulose, and alginates) (Bonino et al., 2011; Elsabee et al., 2012; Lubambo et al., 2013; Matthews et al., 2002; Ranjbar-Mohammadi et al., 2013; Shenoy et al., 2005; Toskas et al., 2011; Yu et al., 2006).
Developing the electrospinning process consuming greener materials and aqueous-based solvents makes the process sustainable and opens up pathway for engineering manufacture. Natural polysaccharides (alginate, dextrose, hyaluronic acid, cellulose, chitin, and chitosan); proteins (gelatin, collagen, and silk,); DNA, and derivatives of polysaccharides (cellulose acetate, and hydroxypropyl cellulose) and composites (hydroxyapatite/ cellulose acetate and PVA/ cellulose acetate ) have all been electrospun efficaciously (Cao et al., 2013; Ji et al., 2012; Konwarh et al., 2013; Schiffman and Schauer, 2008; Sencadas et al., 2012; Singh et al., 2016). The major applications of nanofibrous biopolymers have been in medical (wound dressings and scaffolds in tissue engineering) and pharmaceutical areas (Khadka and Haynie, 2012; Kim et al., 2003; Kriegel et al., 2008; Nguyen et al., 2011; Ramakrishna et al., 2010; Zhou et al., 2013). In addition, specific applications in biotechnology, food industry, water treatment, environment protection and the energy sector have remarkably increased (Baptista et al., 2011; Ki et al., 2007; Rafique et al., 2016; Samad et al., 2013; Tian et al., 2011; Wongsasulak et al., 2010; R. Zhao et al., 2014).
Nanofibers have been fabricated via electrospinning of purified guar gum by removal of the insoluble fractions (crude fiber, proteins and ash etc.) and ethanol precipitation of the soluble fraction (Lubambo et al., 2013). Subsequent cloth filtration and removal of aggregated particles followed by membrane filtration achieves a reduction in the pore size and increases the fiber homogeneity and solubility.
Guar gum crosslinked with PVA and various concentrations of Fe3O4 NPs have been electrospun onto fibers under alkaline and non-alkaline conditions (Lubambo et al., 2015). The study revealed that the nanofibers incorporated with magnetite NPs produced under alkaline conditions was superior in the size distribution and homogeneity of the nanoparticles compared to the non-alkaline fibers, further enhancing the therapeutic efficiency of alkaline guar gum/PVA/Fe3O4 fibers.
The area of ‘greener electrospinning’ for the production of nanofibers and nanomembranes, using natural gums such as GA, GT, GK and KG blended with synthetic biocompatible polymers like poly(vinyl alcohol) (PVA), poly(ε-caprolactone) (PCL), poly(ethylene oxide) (PEO), poly(l-lactic acid) (PLLA) and poly(lactic glycolic acid) (PLGA) has been explored (Padil et al., 2016; Ranjbar-Mohammadi et al., 2016a, 2016c, 2013; Ranjbar-Mohammadi and Bahrami, 2015; Zarekhalili et al., 2017). Gums (GA, GK and KG) mixed with PVA in varying wt % of blend solutions were successfully electrospun and the resulting fibers are shown in Fig. 7. These synthetic polymers enhanced the solubility of the gum and subsequently improved the spinnability of the solution. The general understanding is that PVA can interact with the natural polymers via hydrogen bonding and disturbing the molecular configuration, resulting in improved chain entanglement and viscosity modification of the blended solution (Agarwal et al., 2008; Bhattarai et al., 2006; Bhattarai and Zhang, 2007). These efforts result in the production of smooth nanofibers with a uniform diameter and membranes with high biocompatibility and good mechanical properties. An aqueous PVA solution (12 wt%) was alternatively mixed with GA (10 wt%), GK or KG (both deacetylated; 3 wt%) in different weight proportions (PVA to gum ratios from 50/50 to 100/0) to test the spinnability and uniformity of the produced nanofibers (Padil et al., 2016). The influence of the weight proportions on the conductivity, viscosity and surface tension of the resulting blends was also determined. A proportion of PVA of 70 to 90 % represented the ideal intermingling solution in the direction of engineering to yield uniform and blobless nanofibers with a typical diameter of ~ 220 nm. PVA was found to be a better polymer partner or these three gums than PEO. PEO, which can make hydrogen bonds with gums via its oxygen groups, can interact with the gums culminating in lower chain entanglement and higher intermolecular interactions, a trait that led to the rejection of PEO as an appropriate polymer for manufacture ideal combinations with the gum. Other natural polysaccharides (alginate, gellan, green seaweed (Ulva Rigida), tragacanth, guar and chitosan) have been blended with PVA or PEO for electrospinning purposes (Bonino et al., 2011; Elsabee et al., 2012; Lubambo et al., 2013; Ranjbar-Mohammadi et al., 2013; Toskas et al., 2011; Vashisth et al., 2014).
Fig. 7:
Morphological characterizations of electrospun fibers of PVA-GA (left), PVA-GK (center) and PVA-KG (right) with varying PVA-gum proportions (50:50 to 100:0). Reproduced with the permission of ref. (Padil et al., 2016)
GA has been used for viscosity modulation to improve the electrospinning properties of gelatin-chitosan hybrid fibers fabricated in an aqueous/acetic acid solvent medium utilizing GA and PVA as copolymers (Tsai et al., 2015). GA and an acetic acid/water combination is a greener medium, which influences the electrospinning parameters and enhances the incorporation of a higher gelatin-chitosan (~16 %) content in the final electrospun product, thereby reducing the cost of production by discarding expensive and harmful solvents (trifluoroacetic acid or hexafluoroisopropanol) for the production of gelatin-chitosan fibers. The higher content of gelatin-chitosan in the electrospun fibers may also play an important role in tissue engineering applications (Sahay et al., 2012).
Electrospun fibers with a combinations of GT and two copolymers (PCL and PVA) have been produced by a co-electrospinning process, which produces nanofibers with an optimum diameter of 131.6 ± 27.5 nm (Zarekhalili et al., 2017). The antibacterial efficiency of the hybrid fibers was tested against E. coli, and S. aureus. E.coli were further vulnerable to nanofibers due the action of the sugar residues, such as L-arabinose and L-fucose, present in GT (Ranjbar-Mohammadi et al., 2013). The results determined that hybrid nanostructures can have superior proliferations and non-toxicity action towards MTT assay and NIH3T3 cells.
Aligned and random morphological nanofiber scaffolds based on GT and poly (L-lactic acid) have been produced (Ranjbar-Mohammadi et al., 2016a). Among other things, the aligned PLLA/GT (3 %; 75:25 ratio) fibers were found to display excellent physical and chemical as well as mechanical properties and could be effectively applied to nerve tissue regeneration.
In another report, GT (7 %), PCL (20 %) and varying amounts of curcumin (1 – 24 %) were mixed together in acetic acid/water (90 % v/v) and the whole solution was agitated at room temperature, and GT/PCL/curcumin hybrid fibers were subsequently formulated via electrospinning (Ranjbar-Mohammadi et al., 2016b; Ranjbar-Mohammadi and Bahrami, 2016). The antibacterial properties, curcumin release and wound healing properties were systematically studied with these GT/PCL/curcumin fibers, where the efficacy in diabetic wound healing in rat models was discerned.
Electrospun fibers of GT/PCL with excellent antibacterial properties, mechanical strength, porosity, hydrophilicity and degradation behavior have been produced (Ranjbar-Mohammadi and Bahrami, 2015). Acetic acid was used a solvent for electrospinning of GT/PCL [(3:1; 7 % (GT), 10– 20 % (PCL)] to produce an electrospun scaffold with an average diameter of 156 ± 25 nm. The results on cytotoxicity and antibacterial potencies of these GT/PCL fibers showed that this could be effective application in biomedical areas as skin and wound-healing scaffolds.
Electrospun fibers of GT/PVA have been created by mixing three concentrations of GT/PVA (3, 6 and 9 wt.%) in a various combinations (GT/PVA; 0/100 to 100/0) (Ranjbar-Mohammadi et al., 2013). Many electrospinning parameters (including processes, systems and ambient) were controlled for the smooth production of nanofibers at 40/60 ratio of GT/PVA. Excellent antibacterial efficacy of GT/PVA was found towards Gram-negative bacteria (P. aeruginosa) with superb cell proliferation and biological compatibility of the nanofibers.
An electrospun drug delivery device has been fabricated by combining GT, PLGA and TCH via blending and coaxial electrospinning (Ranjbar-Mohammadi et al., 2016c). PLGA/GT was prepared in three different formulations (PLGA: GT; 100:0, 75:25 and 50:50 wt. %) in a HEP solvent where TCH (5% w/w) was added into a polymeric mixture and stirred for 30 min. Three types of electrospun fibers, i.e. PLGA, PLGA/GT core shell, and PLGA/GT-TCH core shell, were produced. The study revealed that the higher the amount of GT in the electrospinning mixture, the smaller the diameter of the ensuing nanofibers. Nanofiber stability, TCH releasing behavior, cell culture and proliferation studies of the nanofibers were evaluated and PLGA/GT-TCH core shell nanofibers were found to be efficient for the treatment of periodontal disease.
The electrospinning of natural polysaccharides and tree exudates is a complex process, where several factors such as solution parameters (viscosity, surface tension, conductivity, and polymer concentration), polymer properties (solubility in various solvents, molecular weight, and rheological) and process considerations (electrical voltage, temperature, humidity) influence the overall success of the electrospinning process (Grein-Iankovski et al., 2016).
6. Applications for gum-grafted hydrogels, NPs and NFs
The greener synthesis of NFs and NPs and several eco-friendly applications in environmental and medical fields have been recognized. Materials such as NPs with various morphologies, functionalized NPs and NFs and membranes are extensively applied in water treatment and environmental remediation as a tool for detection and removal of hazardous gases, organic pesticides, aliphatic and aromatic volatile chemicals, adsorption/removal of heavy metals, toxic microorganisms and radioactive nuclides etc. (Das et al., 2017; Khin et al., 2012; Simeonidis et al., 2016).
6.1: Environmental applications of the gum-metal complexes, hydrogels and electrospun fibers.
Naturally occurring biopolymers have exhibited excellent adsorption abilities for multivalent metal ions. Many natural materials like chitin, chitosan, lignin, sea weed, wool wastes, agricultural wastes, sugarcane bagasse, biomass by-products, Sargassum algae etc., have been utilized for the elimination of noxious heavy metals from aqueous wastewater streams (Aydin and Aksoy, 2009; Davis et al., 2003; Varma et al., 2004). Nanofibrous membranes are potential applications of these biopolymers for heavy metal removal. A schematic representation of the adsorption/removal of toxic metals and their interaction with gum structures is shown in Fig. 8.
Fig. 8:
Schematic presentation of toxic metal adsorption onto gum structures (right) and gum electrospun membranes (left).
The simplicity of membrane construction, the assortment of materials appropriate for use, and their ensuing structures favor membranes for environmental applications such as the capture and degradation of pollutants (Fryxell and Cao, 2007; Lee et al., 2009; Mishra and Clark, 2013; Pereao et al., 2016; Ranjbar-Mohammadi et al., 2016a, 2013; Sahraei and Ghaemy, 2017; Thavasi et al., 2008). Two main benefits of electrospun nanofibers are that they have both adsorption and separation properties. From this viewpoint, electrospun fibers and membranes are emerging as truly novel green materials for an array of environmental bioremediation applications. In addition, NF membranes may offer a viable and smart means of removing toxic metals and nanoparticulate matter.
Toxic metal contamination of water bodies is a severe environmental problem often addressed by two important methods, namely adsorption and filtration. In a recent study, gums and their functional materials were successfully used for the elimination of these toxic metal contaminants from manufacturing effluents (Banerjee and Chen, 2007a; Masoumi and Ghaemy, 2014; Mittal and Mishra, 2014; Sahraei and Ghaemy, 2017; Saravanan et al., 2012; Sashidhar et al., 2015; Vinod et al., 2009; Vinod et al., 2010b; Vinod et al., 2010; Vinod et al., 2011a, 2011b).
FTIR examination of the gums determined the presence of numerous functional groups such as ether (C-O-C), acetyl (CH3CO-), aliphatic (-CH), carboxyl (R-COO-), hydroxyl (OH-), and carbonyl (C=O) groups, which collectively represent the key interacting groups accountable for metal biosorption. The results illustrated that chemisorption, ion-exchange, functional group exchanges, surface adsorption, physisorption, higher surface area and enhanced surface characteristics were all implicated in the sequestration of toxic metals by gums. Furthermore, adsorption studies were conducted to examine the consequence of process parameters like primary metal ion quantity, pH, and sorbent amount and contact time. The adsorption results closely fitted to Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich isotherm models and the kinetic data to pseudo-first order and second order kinetic models.
Among the various techniques, the morphological changes of gum structures as a result of metal biosorption have been evaluated by microscopy (SEM-EDXA mainly), functional groups involvement in metal biosorption by FTIR and XPS analysis, and the nature of the physical forms (amorphous and crystalline) by XRD.
Engineered nanoparticles pose environmental and health risks due to their ever-increasing occurrence in marketable products (Dwivedi et al., 2015; Louie et al., 2016). Currently, most nanotechnology domains are emerging rapidly in an unregulated manner, in an atmosphere that is well suited for entrepreneurship; however, a lack of disposal guidelines will will inevitably lead to potential contamination of water resources and ecosystems (Philippe and Schaumann, 2014). Studies are still ongoing in the following three areas: (i) monitoring of the fate, transport and transformation of nanoparticles/nanocomposites (Dwivedi et al., 2015) (ii) their bioavailability and (iii) exposure of humans and other living species to these potentially toxic materials (Colvin, 2003; Sharma et al., 2015).
Table 5 provides a summary of the literature pertaining to natural gum fibers, metal/metal oxide NPS functionalized gums, gum-grafted polymeric hydrogel and their electrospun fibers for the adsorption/removal of environmental contaminants (organic dyes, heavy metals, radioactive materials, and NPs) from aqueous and industrial effluents. (Table 5)
Table 5:
Natural gums and fibers for environmental bioremediation (heavy metals, organic dyes, radioactive materials and NPs)
| Gum-based Adsorbents (nanocomposite, hydrogels /NPs/fibers and membranes) |
Type of contaminants |
Adsorption capacity (mg/g), kinetic and adsorption models |
Adsorption/removal mechanisms |
References |
|---|---|---|---|---|
| GA/Fe3O4 | Cu2+ | 38.5; Langmuir isotherm and pseudo-2nd order kinetics | Complexation | (Banerjee and Chen, 2007a) |
| GK/PVA NF | Ag, Au and Pt NPs | 119 (Ag); 196 (Au) and 236 (Pt) | Functional group interactions | (Padil et al., 2015c) |
| GT/graphene oxide composite | Pd2+, Cd2+ and Ag+ | 142.50 (Pb2+), 112.50 (Cd2+) and 132.12(Ag+) pseudo-1st order kinetics, and Langmuir model. | Chemical reaction and mass transfer | (Sahraei and Ghaemy, 2017) |
| GT/Polyamidoxime nanohydrogel | Co2+, Zn2+, Cd2+ and Cr3+ | 100.0 (Co2+), 76.92 (Zn2+), 71.42 (Cr3+) and 66.67 (Cd2+); Temkin isotherm and pseudo-2nd order kinetics. | Metal- amidoxime/ GT chelating | (Masoumi and Ghaemy, 2014) |
| GT/ Fe3O4/poly(methyl methacrylate) NP | Cr 6+ | 7.64; Langmuir isotherm and pseudo-2nd order kinetics | Chemisorption | (Sadeghi et al., 2014) |
| GK | Hg 2+ | 62.5; Langmuir model and pseudo-2nd order kinetics | Micro-precipitation and ion-exchange process | (Vinod et al., 2011b) |
| GK/PAA/SiC NPs | Malachite green and rhodamine B | 757.57 (MG) and 497.51(RhB); Langmuir model and pseudo 2nd order kinetics | Physical adsorption controlled by diffusion | (Mittal et al., 2016) |
| GK/PAA-acrylamide/ SiO2 nanocomposite | Methylene blue | 1408.67; pseudo-2nd order kinetics, and Langmuir model | Physical adsorption plus electrostatic interactions | (Hemant Mittal et al., 2015a) |
| GK/PVA NF | Ag, Au, Pt, CuO and Fe3O4 NPs | 25.5 (Ag); 45.8 (Au); 51.5 (Pt); 7.5 (CuO); 5.2 (Fe3O4) |
Physisorption, interactions, complexation, | (Padil and Černík, 2015) |
| GK/PVA NF (plasma treated) |
Au, Ag, Pt, CuO and Fe3O4 NPs | 116 (Ag), 261.1 (Au), 289.8 (Pt), 66.3 (CuO) and 49.9 (Fe3O4); Langmuir model and pseudo-2nd order kinetics | Physisorption, interactions, complexation | (Padil and Černík, 2015) |
| KG | Pb2+, Cd2+, Ni2+, Cr3+, Cr6+, Fe2+, Co2+, Cu2+, Zn2+, As2+, Se2+, Hg2+ | Langmuir isotherm model and pseudo-2nd order kinetics | Physisorption, micro-precipitation and ion-exchange processes | ( Vinod et al., 2010a, 2010b; Vinod et al., 2009) |
| KG-nanocomposite | U6+ | 487; Langmuir isotherm model and pseudo-2nd order kinetics | Precipitation, ion exchange, complexation | (Sashidhar et al., 2015) |
| KG/Fe3O4 nanocomposite | Cd2+, Cu2+, Pb2+, Ni2+, Zn 2+, Hg 2+ | 106.8 (Cd2+), 56.6 (Pb2+), 85.9 (Cu2+), 49 (Ni2+), 37 (Zn2+) and 35 (Hg2+), Langmuir models | Interaction, physisorption | (Saravanan et al., 2012) |
| KG/PVA NF | Ag, Au and Pt NPs | 143.4 (Ag); 173.5 (Au) and 270.4 9 (Pt); Pseudo-2nd order kinetics | Functional group interaction | |
| GK- ZVI NPs | Cr3+ , Cr6+ and volatile organic compounds (cis-1,2-dichloroethene, perchloroethene and trichloroethene) | Complete degradation of Cr (Con. 10 ppm) and VOCs Pseudo-2nd order kinetics |
Functional group interaction | (Farooq et al., 2017) |
| GG/Pd NPs | G-250, methylene blue, methyl orange, and 4-nitrophenol | 150 μM (G-250), 62.5 μM (methylene blue), 30 μM (methyl orange) and 100 μM (4-nitrophenol) | Catalytic reduction | (Kora and Rastogi, 2015) |
| GG/PAA-co-acryl- amide /Fe3O4 NPs | Rhodamine B | 654.87; Langmuir model and pseudo-2nd order kinetics | Electrostatic attraction, and binding sites | (Mittal and Mishra, 2014) |
Significant progress has been made in the use of both gum conjugated polymeric hydrogels and functionalized electrospun fibers for environmental applications. Both GA-templated graphene (G) and Ag NPs-hybrid (GA-G-Ag) graphene have been prepared by the sonication of GA-G (0.1 mg/mL), addition of Ag NO3 (2 mg/mL) and subsequent heating of the mixture for 3h-6h at 80 oC to affect the formation of a GA-G- Ag hybrid (Fan et al., 2012). In this greener synthesis of a GA-G-Ag composite, GA serves as a reducing and stabilizing agent to provide straight phase exfoliation in an aqueous medium. In addition, the composite materials would be able to detect 4-aminothiophenol (concentration level; 10−6 M) in water using SERS (Surface Enhanced Raman Spectroscopy).
GT-based Ag NPs, graphene oxide composites, polyamidoxime nanohydrogel and poly (methyl methacrylate)/Fe3O4 NPs have all been exploited for the adsorption and elimination of metal ions (Pb2+, Cd2+, Ag+, Co2+, Zn2+, Cr3+ & Cr6+) as well as dyes (methylene blue and Congo red) from aqueous solutions (Masoumi and Ghaemy, 2014; Sadeghi et al., 2014; Sahraei and Ghaemy, 2017).
GT grafted poly (methyl methacrylate) and bentonite (GT-g-PMMA/B) have been formulated for the removal of anionic azo dyes [Congo red (CR), methyl orange (MO), and acid blue 113 (AR-113)] from water, with the maximum adsorption capacity for CR, MO and AR-113 being 900, 750 and 8.5 mg/g, respectively (Sadeghi et al., 2015). The adsorption of these anionic azo dyes onto GT-g-PMMA/B composite materials was represented as kinetic models (pseudo second-order and intra-particle diffusion) and Langmuir adsorption isotherms.
In further research by the same authors (Sahraei and Ghaemy, 2017), graphene oxide (GO) was integrated onto nanocomposite hydrogels consisting of GT-grafted 2-methylpropanesulphonicacid (AMPS). The GT-AMPS was prepared via radical polymerization using ceric ammonium nitrate and N, N1-methylene bisacrylamide (MBA) as an initiator and cross-linker, respectively. The hydrogel nanocomposite was found to be an exceedingly effective adsorbent for the exclusion of metal ions (Pb2+, Cd2+ and Ag+1) from water.
A microwave-buoyed grafting of GT with diallyldimethylammonium chloride (DADMAC) has been reported, with the latter being fashioned and applied as a flocculent for the destabilization of aqueous, multi-walled carbon nanotubes from industrial effluents (Pal et al., 2017). Natural gum-functionalized materials as carriers for the operative medium in the eradication of nanoparticle toxicity from the environment were highlighted.
The use of poly (methyl methacrylate) embedded CT and amended magnetite NPs in the selective sequestration of Cr (VI) from industrial effluents has been demonstrated (Sadeghi et al., 2014). Composites could be highly selective for particular ions and even species and applicable over a wider pH range.
A bio-composite hybrid hydrogel comprising modified GT, graphene oxide, PVA and magnetite NPs has been synthesized and utilized for the removal of heavy metals ( Pb2+, Cu2+) and both cationic (crystal violet, CV) and anionic (Congo red, CR) dyes (Sahraei et al., 2017). The adsorption kinetics and isotherms were effectively followed using pseudo-second order and Langmuir models with maximum adsorption capacities of Pb2+ (81.7 mg/g), Cu2+ (69.6 mg/g), CV (94 mg/g) and CR (101.7 mg/g), respectively.
A bio-nanocomposite encompassing GK, polyacrylamide (PAAm) and nickel sulfide (NiS/ Ni3S4) NPs has been synthesized and employed for the adsorption of Rhodamine 6 G dye (Rh 6G) from water (Kumar et al., 2016). The GK-PAAm hydrogel, prepared via graft co-polymerization and NiS/Ni3S4, was merged into the composite gel and its adsorption capability for Rh 6G (1,224.7 mg/g), which followed Langmuir isotherm behavior and pseudo-second order kinetics, was confirmed with the added advantage of its reusability for three sequential phases of the task.
The following materials viz. GK fibers; GK-functionalized NPs [GK/poly (acrylamide co-acrylic acid)/Fe3O4; GK/poly (acrylic acid)/SiC; GK/ poly (acrylic acid-acrylamide)/SiO2; GK/Fe3O4] and GK/PVA electrospun membranes have shown promise as competent adsorbents for the abstraction of heavy metal (Hg2+, Pb2+, Cr3+, Cr2O72- and Ni2+) and NPs (Ag, Au, Pt, Fe3O4 and CuO) as well as for the eradication of organic dyes (malachite green and Rhodamine B) and radioactive 32P effluent (Hemant Mittal et al., 2015a; Mittal et al., 2016; Mittal and Mishra, 2014; Padil et al., 2015c; Padil and Černík, 2015). A graft polymerization method has been demonstrated for the synthesis of nanocomposites comprising GK, poly (acrylic acid) and silicon carbide NPs, with the ensuing hydrogel composite finding application for the exclusion of dyes (Malachite Green and Rhodamine B) from wastewater (Mittal et al., 2016).
In another study (Hemant Mittal et al., 2015a), the adsorptive sequestration of MB from industrial wastewater was shown by GK-grafted poly (acrylic acid-acrylamide) with silica nanocomposite hybrid materials. A high adsorption capacity of the composite (1,408.6 mg/g) for abstraction of dyes from the aqueous environment was feasible.
An ecological hydrogel viz. GG-grafted poly (acrylamide-co-acrylonitrile), GG-g-Poly AAm-co-AN, attained via free radical polymerization has also been utilized for the exclusion or adsorption of metal ions (Pb2+ and Cu2+) from water (Hemant Mittal et al., 2015b). The adsorption was pH dependent and agreed well with the isotherm Langmuir model. The maximum adsorption capabilities of the complex hydrogels for the metal ions, Pb2+ (384.6 mg/g) and Cu2+(203.7 mg/g) were established.
The microwave-promoted production of a conducting hydrogel based on GG; N,N’-methylene-bis-acrylamide has been reported. Aniline in the form of ammonium peroxydisulfate as an initiator or cross-linking agent generated a polyacrylamide/GG/polyaniline interpenetrating network that facilitated the removal of malachite green (MG) from wastewater (Sharma et al., 2013).
Moreover, GG-stabilized Pd NPs have been studied for probing the reduction of dyes such as Coomassie Brilliant Blue G-250, methylene blue and methyl orange. The 4-nitrophenol and GG crosslinked with poly (acrylic acid-co-acrylamide) and armored with iron oxide magnetic NPs has also been employed for the exclusion of rhodamine B (Kora and Rastogi, 2015; Mittal and Mishra, 2014).
Materials based on KG, including nanocomposites and electrospun fibers, have been utilized for the remediation of heavy metals (Cd, Cu, Fe, Pb, Hg, As, Cr, Ni, Zn, Se and U), nuclear power station effluents, radioactive waste generated from 32P labelled nucleotide production and heavy metals from industrial effluents (Sashidhar et al., 2015; Vinod et al., 2009, 2010a, 2010b).
NFs have been treated with plasma methane to improve their physical and chemical properties, enhancing their adsorption capacities towards NPs (Au, Ag, Pt, CuO and Fe3O4);(Padil et al., 2016, 2015c; Padil and Černík, 2015). Membrane adsorption capacity descended in the following order Pt >Au > Ag > CuO > Fe3O4. Among other things, adsorption mechanisms pertaining to nanoparticle adsorption onto nanofibers include sorption, functional group interfaces and coordination complex reactions between NPs and various functional groups inherent in the NFs (Fig. 9). Additional adsorption mechanisms may relate to improved surface characteristics, which are mainly dictated by the hydrophilicity or hydrophobicity of the fibers, interfacial free energy and the relative superficial capacity of the plasma-modified membranes (Padil et al., 2015c; Padil and Černík, 2015).
Fig. 9:
Application of electrospun membranes for the extraction of NPs from water.
6.2: Gum-metal/metal oxide (organic - inorganic complexes) NPs and NFs for anti-bacterial applications.
NPs produced with the help of natural products and applied as mediators for the synthesis and stabilization have tremendous potential for applications in many fields. Applications in antibacterial fields have a number of potential advantages over chemogenic NPs produced by utilizing various chemical reductants (e.g. sodium borohydride, citrate or hydrazine) and/or other organic materials as capping and stabilizing agents (e.g. dodecyl sulphate, cetyltrimethylammonium chloride and polyvinylpyrrolidone).
Varieties of NPs (Ag, Au, Cu, ZnO and CuO) or organic agents like dodecenylsuccinic anhydride (DDSA) incorporated with tree gums have been used as an antibacterial mediator for biotechnology and antibacterial performance (Bajpai et al., 2016; Bajpai and Kumari, 2015; Deshmukh et al., 2012; Kattumuri et al., 2007; Kora et al., 2012, 2010; Kora and Rastogi, 2015; Kora and Sashidhar, 2015; Mittal et al., 2013; Montazer et al., 2016; Padil et al., 2015a, 2015b; Pooja et al., 2015; Ranjbar-Mohammadi and Bahrami, 2015; Sahraei and Ghaemy, 2017; Thekkae Padil and Černík, 2013).
Table 6 illustrates the antibacterial activity of metal/metal oxide NPs synthesized using many gums and the interaction of NPs of various sizes stabilized by gums and their electrospun fibers with model test strains (Gram-positive and Gram–negative bacteria). Antibacterial susceptibilities were determined mainly by the well diffusion method (zone of inhibition, ZOI) and the micro broth dilution methods (MIC and MBC) of the bacterial strains toward the actions of the NPs have also been presented, with the NPs synthesized using gum fibers and the membranes both displaying significant antibacterial action on both Gram classes of bacteria. Their promising antibacterial efficiency and surface adaptation mean that these NPs can be modified to be bio-compatible or cytotoxic in various biomedical and pharmaceutical applications. Ag NPs are the most commonly used NPs as antibacterial agents in various fields, e.g. antibacterial clothing (Table 6) (Monti et al., 2004; Navaladian et al., 2008; Thomas et al., 2007; Zhang et al., 2013).
Table 6:
Metal/metal oxide NPs synthesized by gum fibers and their electrospun functional membranes for antibacterial efficiencies.
| Composition of NPs |
Type of bacteria tested | Size and concentration of NPs |
Antibacterial susceptibility of bacterial strains towards NPs (ZOIa/MIC b/MBC c) |
References |
|---|---|---|---|---|
| PVA/GA/Ag NP | E. coli | 10–40 nm; GA (5%), PVA (3%), AgNO3 (1mM) | Not defined | (Juby et al., 2012) |
| GA/DETA microgels | E. coli, S. aureus | 5–100 μm | 5 mg/mL (E. coli), 10 mg/mL (S. aureus) |
(Farooq et al., 2017) |
| GA /poly(sodium acrylate)/Ag NPs hydrogel | E. coli | 20–30 nm; GA (1 g); Na-acrylate (10.63 mmol), AgNO3 (0.117 mmol) | Not defined | (Bajpai and Kumari, 2015) |
| GA /poly (acrylate) hydrogel/ ZnO NPs | E. coli | 40–60 nm; ZnO (2%), GA (1.0g) | 32 ± 0.07 mm | (Bajpai et al., 2016) |
| GT/ ZnO nanorod | E. coli, S. aureus, C. albicans | 55–80 nm; Zn(NO3)2·6H2O (0.1M), GT (1%) | 3.3 ± 0.1 mm (E. coli), 3.2 ± 0.1 mm (S. aureus), 3.0 ± 0.1 mm (C. albicans) | (Ghayempour et al., 2016) |
| GT/nano silver hydro citric acid and sodium hypophosphite hydrogel on cotton fabric | E. coli, S. aureus | 77.55 nm; TG (0.8%), AgNO3 (0.335%) |
Reduction number of colonies 98.0% (E. coli) and 99.69% (S. aureus) | (Montazer et al., 2016) |
| GT/PVA NF | S. aureus, P. aeruginosa | 40%(GT), 60% (PVA) | good antimicrobial properties against (P. aeruginosa) |
(Ranjbar-Mohammadi et al., 2013) |
| GK/Dodecenyl succinic anhydride (DDSA) composite |
E. coli, P. Aeruginosa, and S. aureus |
GK/DDSA (10%) | Gram-negative E. coli (14.2 ± 0.8 mm); P. aeruginosa and Gram-positive S. aureus (15.0 ± 0.8 mm), respectively for 10 wt.% concentration of DDSA/DGK | (Padil et al., 2015b) |
| GK/CuO NPs | 4.8 ± 1.6 nm; CuCl2 ⋅ 2H2O (1 mM) and GK (10 mg/mL) | S. aureus (25923) 14.5 ± 0.8 nm; E. coli (25922), 16.2 ± 0.8 mm. | (Thekkae Padil and Černík, 2013) | |
| KG / Ag NPs | Gram-negative (Escherichia coli (ATCC 25922), E. coli (ATCC 35218), Pseudomonas aeruginosa (ATCC 27853) and Gram-positive (Staphylococcus aureus (ATCC 25923) | 4.5± 3.1 nm; 0.5% KG (0.5%) and AgNO3 (1 mM) | The minimum inhibitory concentration (MIC) for S. aureus (10 μg/mL); P. aeruginosa (5 μg/mL) and E. Coli (2.0 μg/mL). The minimum bactericidal concentration (MBC) of silver NPs against test bacterial strains for both S. aureus and P. aeruginosa (12 μg/mL) each and E. coli ( 2μg/mL) | (Kora and Rastogi, 2015) |
| KG-Ag NPs |
S. aureus (ATCC 25923; ATCC 49834); E. coli (ATCC 25922) and P. aeruginosa (ATCC 27853) |
5.8 ± 2.4 nm; GK-Ag NPs (1μg/mL) |
E. coli (25922) 3.0 ug/mL; P. aeruginosa (27853) 5.0 ug/mL; S. aureus (25923) 5.0 ug/mL; S. aureus (49834) 5.0 ug/mL | (Rastogi et al., 2015) |
| KG / gelatin sponge cipro floxacin | S. aureus and E. coli | GK (1% w/v) and gelatin (3% w/v) | S. aureus (20 mm) and E. coli (25 mm) | (Rathore et al., 2016) |
| KG/ Ag NPs |
S. aureus (ATCC 25923); and Escherichia coli (ATCC 25922), E. coli (ATCC 35218) and Pseudomonas aeruginosa (ATCC 27853) |
KG (0.5%) and AgNO3 (1.0 mM) |
S. aureus (ATCC; 25923), 11 mm; E. coli (ATCC 25922), 8.0 mm; E. coli (ATCC 35218), 8.4 mm; and P. aeruginosa (ATCC 27853), 8.7 mm |
(Kora et al., 2010) |
| Dodecenyl succinic anhydride (DDSA)/ deacetylated KG fibers and membranes | Gram-negative Escherichia coli (CCM 3954) and Pseudomonas aeruginosa (CCM 3955) and Gram-positive Staphylococcus aureus (CCM 3953) | (PVA, 10 wt%) and DDSA/DGK (10 wt%) |
E. coli (8.0±0.1 mm, 6.0±0.5 mm); P. aeruginosa (7.0±0.1, 4.5±0.6); S. aureus (9.0±0.1, 7.0±0.8) |
(Padil et al., 2015a) |
| GG/ Ag NPs | Staphylococcus aureus (ATCC 25923); and Escherichia coli (ATCC 25922), E. coli (ATCC 35218), and Pseudomonas aeruginosa (ATCC 27853) |
5.7 ± 0.2 nm 0.1% GG and 1 mMAgNO3 5 μg of silver NPs |
S. aureus, 25923 (12.25 mm); E. coli 25922 (9 mm); E. coli 35218 (8 mm); P. aeruginosa 27853 (11 mm) | (Kora and Sashidhar, 2015) |
aZone of inhibition (ZOI)
bminimum inhibitory concentration (MIC)
cminimum bactericidal concentration (MBC)
A major problem confronting human health today is that most bacteria are becoming resistant to antibacterial drugs (Hajipour et al., 2012; Zheng et al., 2018). Biogenic Ag NPs exercise a synergistic effect in conjunction with varieties of antibiotics (erythromycin, chloramphenicol, ampicillin and kanamycin) towards Gram-negative and Gram positive bacteria (Devi and Joshi, 2012). Rastogi et al. (Rastogi et al., 2015) showed that suitable NPs synthesized using gums can fight against specific bacteria. Gum-assisted biogenic Ag NPs were highly stable and displayed superior antibacterial action on both Gram classes of bacteria. The ZOI values observed at a concentration of 5 μg of Ag NP were found to be higher in comparison with the corresponding values for Ag NPs synthesized by chemical agents (Pinto et al., 2010; Sadeghi et al., 2014; Thakkar et al., 2010). Furthermore, Ag NPs synthesized using KG and GT were reported to be more potent anti-bacterial agents in terms of MIC and MBC (Kora and Rastogi, 2015).
A ZnO NP integrated GA-sodium alginate hydrogel composite has been observed to kill bacterial colonies comprising Pseudomonas aeruginosa and Bacillus cereus (Raguvaran et al., 2017). The cytotoxicity effect of this hydrogel composite was much lower than ZnO NPs without any bio-polymeric (GA and sodium alginate) shield and showed higher biocompatibility than the unprotected NPs (Raguvaran et al., 2017).
GK-stabilized CuO NPs of varying sizes, 4.8 ± 1.6 nm, 5.5 ± 2.5 nm, and 7.8 ± 2.3 nm, have been prepared. The antibacterial efficiency of the smallest synthesized NPs was verified against E. coli and S. aureus. ZOI of 14.5 ± 0.8 mm and 16.2 ± 0.8 mm were perceived for the S. aureus and E. coli and cultures, respectively. Further experimental results confirmed that MBC (minimum bactericidal concentrations) depended on size and had values of 125 ± 5.5 μg/mL and 135 ± 8.8 μg/mL for E. coli and S. aureus, respectively for the smallest NPs (Thekkae Padil and Černík, 2013).
Ag NPs have been synthesized using KG, and were subsequently appraised for their elevated antibacterial and antibiofilm potencies, in combination with various antibiotics (e.g. ciprofloxacin, streptomycin and gentamicin) against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria (Rastogi et al., 2015). The combination resulted in enhanced antibacterial action with biocompatibility of up to 2.5 μg/mL as evaluated by MTT assay on a HeLa cell line. The findings suggest that KG/Ag NPs could possibly be used as ‘in vivo’ antibacterial agents in combination with antibiotics to combat the acute problem of antibiotic resistance (Rastogi et al., 2014).
KG-stabilized Ag NPs have been employed for antibacterial action via a variety of vulnerability assays (micro-broth dilution, growth kinetics, antibiofilm activity, cytoplasmic content leakage and membrane permeabilization) etc. The generation of reactive oxygen species (ROS) and cell surface damage during the interaction of bacteria with Ag NPs was demonstrated using various assays (dichlorodihydro fluorescein diacetate and N-acetylcysteine) (Kora and Rastogi, 2015).
Lyophilized forms of deacetylated GK, KG and GT were used for the preparation of DDSA/GK composite and DDSA/KG/PVA and DDSA/GT/PVA membranes (Padil et al., 2015a, 2015b; Ranjbar-Mohammadi et al., 2013). The improved antimicrobial actions of these membranes were related to two factors, i.e. the ratio of hydrophilicity/hydrophobicity and the intensified surface unevenness after chemical modification with DDSA, which facilitated an increase in the interaction between the planes of the material and the bacteria. In terms of their non-toxicity, the derivatized natural gums can be promoted as antibacterial agents, which could realistically replace CuO-based polymeric materials (along with their toxic Ag and Cu constituents) in the crucial battle against microbes (Padil et al., 2015a, 2015b; Ranjbar-Mohammadi et al., 2013).
6.3. . Biosensors for environmental and medical application
An exceedingly profound and fastidious process has been reported for the direct colorimetric detection of Hg2+ in aqueous systems by KG-Ag NPs (Rastogi et al., 2014). The size of the KG-Ag NPs was 5.07±2.8 nm, and the NPs were stable over a large pH range of 4 –11 and salt concentrations of 5 to 100 mM. More importantly, detection of Hg2+ in water was highly selective as the absorption spectra were not influenced by the presence of other common metals (Na, K, Mg, Ca, Cu, Ni, Co, As, Fe and Cd).
Stable palladium nanoparticles (Pd NPs) synthesized using KG have been reported (Rastogi et al., 2017) and their inherent peroxidase-type response was utilized for the colorimetric detection of glucose in biological samples. The peroxidase catalytic activity was observed to be dependent on the pH, temperature and quantity of the NPs.
6.4. Biomedical applications of the gum matrix- metal/metal oxide NPs coordination complexes and NFs.
The biomedical importance of gum functionalized metal/metal oxide NPs has been reported in many areas such as drug transport, diagnosis, imaging and bio-sensing. Their great water solubility, biocompatibility, biodegradability and the simplicity of furnishing chemical medications in water media make gum hydrogels and their electrospun membranes eminently attractive polymers in the synthesis of scaffolds for applications in tissue engineering. The functionalization of various natural polymeric materials including gums, mucilage and their amended hydrogel formulas for the development of drug delivery systems has also been highlighted (Das and Pal, 2015; De France et al., 2017; Forget et al., 2016; Liu et al., 2017; Mano et al., 2007; Prajapati et al., 2013; Rana et al., 2011; Shukla et al., 2016; Silva et al., 2017; Sridhar et al., 2015; Woehl et al., 2014).
Natural/synthetic polymer-supported hydrogels have been utilized in emerging applications, such as stem cell and cancer therapy research, tissue engineering, drug delivery, wound restoration, and 3D printable hydrogels etc. (Hoffman, 2002; Jungst et al., 2016; Li and Mooney, 2016; Ratheesh et al., 2017; Seliktar, 2012; Vashist et al., 2014).
6.4.1. Biomedical application of GA
A natural hydrogel based on sundew mucilage has been synthesized by a combination of GA and sodium alginate using Ca salt as a cross-linking agent and subsequently examined for drug transport and wound healing applications (Li et al., 2017).
Poly (dialdehyde) amended GA with a collagen and melatonin-based fusion scaffold (PGA/C/M) has been developed with good biocompatibility (Murali et al., 2016a). GA was converted into poly (dialdehyde) by oxidation with sodium iodate and coupled with amine functionality from a collagen forming PGA/C/M hybrid complex, and this material was applied to in vitro and in vivo tissue rejuvenation and wound remedial applications (Murali et al., 2016b; Takei et al., 2010). The high content of the poly (dialdehyde) GA was shown to have exceptional properties (high thermal stability, porosity, and swelling capacity and biodegradable and biocompatible characteristics) suitable for biomedical fields.
An oxidized form of GA has been prepared, which after conjugation with primaquine was studied for its controlled delivery in malaria or leishmaniasis (Nishi and Jayakrishnan, 2007). The complex formation, via Schiff’s base creation, was confirmed between the aldehyde (GA aldehyde) and amine groups (primaquine). The drug loading efficiency, stability of the conjugate and swelling performance etc. were evaluated, and the prepared complex gel exhibited a better performance than dextran-based drug delivery materials (Sokolsky-Papkov et al., 2006).
A magnetite-based nanocomposite material using GA and functionalized it for magnetic hyperthermia therapy has been produced (Horst et al., 2017). The GA-Fe3O4 NPs had an appropriate size, polydispersity and stability in an aqueous solution, which may be highly desirable for magnetic therapy.
The potential application of curcumin, also known as turmeric, an ancient spice-based drug, depends on its transport to the cytoplasm of Hep G2 and MCF-7 cells.(Sarika et al., 2015) GA was conjugated to enhance its solubility and stability. When dispersed in an aqueous media, the conjugate self-assembled into spherical nanomicelles (270 ± 5 nm, as determined by FE-SEM and TEM), due to the hydrophilic and hydrophobic interactions between the GA and the curcumin. This effect significantly enhanced the solubility (900x) and stability of the curcumin under physiological pH conditions. The conjugate exhibited anticancer activity, which was found to be greater in human hepatocellular carcinoma (Hep G2) cells relative to human breast carcinoma (MCF-7) cells. Due to the targeting efficiency of galactose groups present in GA, Hep G2 cells exhibit enhanced accumulation of GA/Cur NPs, thereby circumventing the obstacles previously associated with curcumin and augurs well for its usage in a promising drug delivery system.
The synthesis of hydrogels using GA and cross-linked polyacrylic acid, carbopol has been reported (Singh et al., 2013). The research highlighted the production of novel hydrogel wound bandages for the slow release of gentamicin (an antibiotic drug) and to develop the wound restorative capabilities. Various factors such as antioxidant activity, blood compatibility, antimicrobial activity, mucoadhesion, oxygen/water vapor permeability, microbial penetration and mechanical properties (burst and tensile strength, resilience, relaxation and folding endurance) of the hydrogel have been assessed. Wound healing experiments carried out on Swiss Balb C strain albino mice showed that in the case of wounds covered with hydrogel dressings, speedy wound healing coupled with the formation of well-developed fibroblasts and blood capillaries ensued, compared to open wounds. The biomedical properties indicated that hydrogel films are non-thrombogenic and non-haemolytic, while being antioxidant and mucoadhesive in character. In addition, these wound healing hydrogel films are also porous to oxygen and moisture while being resistant to microbes. The replicated fluid uptake in wounds and the discharge of antibiotics in the wound fluid indicated that hydrogel dressings could absorb high volumes of fluid. Their water absorption capacity (simulated wound fluid) is to the extent of 8.772 ± 0.184 g/g of film. Release of a gentamicin drug from the wound dressings took place courtesy of a Fickian diffusion mechanism in studies involving a simulated wound fluid. They are, therefore, appropriate for moderately to highly exuding wounds, given that they could discharge the antibiotics in a controlled manner.
Hydrogels have been prepared via free-radical polymerization of acrylamide with GA and studied for the discharge of nitrogen-containing bisphosphonate (BP) (Aderibigbe et al., 2015). They exhibited high swelling ratios at a pH of 7.4 and low at a pH of 1.2, and the release mechanism could be examined using UV-vis via complex formation with Fe(III) ions. Furthermore, these hydrogels can be used for the controlled delivery of bisphosphonate to the gastrointestinal region, wherein the presence of GA improves their swelling ability.
6.4.2. GK-based hydrogels for biomedical applications
Modification of GK deploying N, N´-methylenebisacrylamide as the crosslinker and ammonium persulfate as an initiator for the development of hydrogels has been reported (Singh and Sharma, 2008). The discharge kinetics of an anti-ulcer drug (ranitidine hydrochloride), incorporated into such hydrogels has been investigated.
Thiolated GK was prepared by reacting the GK with thioglycolic acid (~80 %) (Bahulkar et al., 2015) and it was found to exhibit longer disintegration times as well as greater swelling and mucoadhesion, with an increase in the pH of the medium that simulates the intestinal and gastric environment– in stark contrast to unmodified GK. Controlled drug release for more than 24 h by Fickian diffusion was observed with metoprolol succinate as a model drug. Furthermore, synthesized thiolated GK displayed no cytotoxicity, as determined by the Hep G2 cell line.
The preparation of pH-sensitive microparticles (MP) based on GK using the aforementioned spraying technique, employing aqueous solvents has been reported (Raizaday et al., 2015). MPs can be used for treating various conditions such as chronic hypertension, ulcerative colitis and diverticulitis.
An interpenetrating polymer network (IPN) composed of Zn-pectinate and GK for gastroretentive ziprasidone HCl delivery has been reported (Bera et al., 2015). The development of IPN beads was proficient via concurrent ionotropic gelation and covalent crosslinking. Optimized IPN beads were found to demonstrate superb mucoadhesion and buoyant properties, with a slower drug release profile. The studies suggested that the development of pectinate-based GK hydrogel beads shows enormous potential in drug delivery and tissue engineering applications. The production of GK stabilized Au NPs (GK/Au NPs) and their application as nanocarriers in the delivery of the anti-cancer drug, gemcitabine hydrochloride (GEM) has been reported (Pooja et al., 2015). Cytotoxic and hemolysis evaluations revealed their biocompatible nature, wherein GEM/GK/Au NPs showed better anti-cancer activity, colony construction and ROS stimulation against human lung cancer cells than bare GEM.
6.4.3. GT hydrogel and electrospun fibers
The addition of GT to calcium alginate (CA) generated beads that found significant application for the encapsulation of bone cells (Kulanthaivel et al., 2017). High cell proliferation, sustainability and distinction were observed in bone cells encapsulated by GT-CA beads, which may have tissue engineering applications in addition to newer drug formulations.
A magnetic surface molecularly imprinted by polymeric nanogel (MIPN) with core-shell morphology has been reported (Hemmati et al., 2016), where quercetin (QC) served as template, using Fe3O4/SiO2 NPs as a magnetic core with N-vinyl imidazole as a functional monomer and GT as cross-linker via radical polymerization. The application of the QC template as a drug carrier was affirmed by its great binding interaction, specific binding choosiness and speedy adsorption dynamic rate, compared to a non-imprinted magnetic polymeric gel.
A biocompatible GT-PVP based hydrogel has been prepared via graft polymerization of GT and PVP in the presence of N,N′-methylenebisacrylamide as a cross-linker and ammonium persulfate (APS) as an initiator (Singh and Sharma, 2014a) The drug discharge ability of the GT-PVP hydrogel was evaluated by incorporating the antifungal drug, fluconazole, which was shown to be non-thrombogenic with good blood compatibility and mucoadhesive properties.
Electrospun fibers (both random and aligned) of poly (L-lactic acid)/GT (PLLA/GT) both 3 % (w/v) and in various ratios (100:0, 75:25 and 50:50) have been produced by electrospinning (Ranjbar-Mohammadi et al., 2016a). PLLA/GT aligned nanofibres at a ratio of 75:25 displayed the well-adjusted properties and were used for an in vitro culture of nerve cells (PC-12) to assess the potential for using these scaffolds as substrates for nerve regeneration.
GT and PVA deionized water solutions at various weight ratios (from pure GT to pure PVA) were electrospun to produce nanofibers (Ranjbar-Mohammadi et al., 2013). For the ideal GT/PVA ratio (40/60), the hydrogen bonding between PVA and GT was confirmed by FTIR analysis. Human fibroblast cells were found to have adhered and proliferated well onto the GT/PVA nanofiber scaffolds. MTT assays confirmed that GT/PVA NF showed cell viability and biological compatibility. The biocompatibility and antibacterial properties of these NFs indicated that they are effective as a wound dressing because they can protect the affected area from its surroundings, so as to avoid infection and dehydration.
NFs based on GT, poly (ε-caprolactone) (GT/PCL) and acetic acid (solvent for electrospinning), were produced at various blend ratios (Ranjbar-Mohammadi and Bahrami, 2015) and were used for the in vitro cell culture of human fibroblast cell lines AGO and NIH 3T3. SEM results and MTT assays indicated that bio-composite mats heightened fibroblast adhesion and proliferation to a greater extent than the PCL scaffolds.
A drug delivery device was fabricated via electrospinning (both blend and coaxial) material comprising GT, poly lactic glycolic acid (PLGA) and tetracycline hydrochloride (TCH) as a model hydrophilic drug (in altered combinations) (Ranjbar-Mohammadi et al., 2016c). Scanning electron microscopic analysis indicated that the produced PLGA, blended GT-PLGA and core shell GT-PLGA fibers possessed smooth and beadless morphologies with diameters (180 – 460 nm).
Drug delivery studies indicated that both fractions of GT within blended NFs and core-shell structures can meaningfully regulate the TCH discharge degree from the nano-membranes. Through the integration of TCH into core-shell NFs, drug conveyance could be continued for 75 days with only 19 % of burst release within the first 120 min. This sustained drug discharge, together with the demonstrated biocompatibility as well as the mechanical and antibacterial properties, make these drug-bearing core-shell nanofibers favorable candidates for use as drug delivery systems in combating periodontal diseases.
Electrospun curcumin (Cur) loaded poly (ε-caprolactone) together with GT was tested for wound curing in diabetic rats (Ranjbar-Mohammadi et al., 2016b). These scaffolds possessing antibacterial properties against methicillin resistant Staphylococcus aureus (as gram positive) and β- lactamase (as gram negative) were used in acellular and cell-seeded forms. The pathological study revealed that the deployment of these NFs caused decidedly swift wound healing with well-formed granulation tissue governed by collagen deposition, fibroblast proliferation, formation of sweat glands and hair and complete regeneration of the epithelial layer.
Ciprofloxacin-loaded GT based hydrogels have been reported to improve the drug delivery of diverticulitis, where GT was grafted with synthetic polymers comprising acrylic acid, PVP, ammonium persulfate (initiator) and a cross-linking agent (N,N’-methylenebisacrylamide) (Singh and Sharma, 2017). The mechanical strength and network structure of the hydrogel were affected by the pH, thereby highlighting the remarkable pH-responsive and mucoadhesive assets that could be exploited for spot-specific drug delivery applications.
GT and poly (acrylic acid) based polymer hydrogels for drug delivery systems of amoxicillin were reported by Singh et al. (Singh and Sharma, 2014b) The hydrogel was prepared via aqueous radical polymerization of GT (6 % (w/v)), acrylic acid (0.874 mol/L), ammonium persulfate (0.0263 mol/L) and N,N’-methylenebisacrylamide (0.0389 mol/L) as an initiator and cross-linker, respectively. The correlations between the synthetic reaction parameters and the structural criteria were determined. A variety of models for studying release kinetics were applied to monitor the release profile of drugs from polymeric networks.
6.4.4. KG for biomedical applications
A polyelectrolyte complex (PEC) was generated via the electrostatic interaction of carboxyl groups from KG with amines on chitosan, and the ensuing complex was loaded with diclofenac sodium and its release performance on various parameters, i.e. particle size, zeta potential, complex formation, rheological properties and drug loading efficiency, was analyzed (Naidu et al., 2009). The pH dependent drug release was demonstrated when a lower release of diclofenac sodium was observed in hydrochloric acid (0.1N; pH 1.2) compared to the release in a phosphate buffer (pH 6.8). Consequently, the KG/chitosan complex has a great prospect as a medium for the controlled delivery of drugs with the added advantage of reducing the dosing frequency, thereby lowering gastric toxicity.
MW-assisted grafting of KG onto poly (acrylamide) has been conducted and subsequently employed in the in vitro release of diclofenac sodium from the matrix tablets of KG-poly (acrylamide) (Malik and Ahuja, 2011). The study highlighted a more rapid release of the drug from a grafted copolymer conjugate compared to ungrafted KG.
The carboxymethylation of KG (Kumar and Ahuja, 2012) reduced its viscosity and improved both its mucoadhesive properties and ionic gelling behavior. Prepared sustained-release bead formulations based on carboxymethyled KG showed a release of metformin (a drug used to treat people with type 2 diabetes) by zero-order kinetics.
The fabrication of sponges by blending KG with gelatin (G) and impregnating with ciprofloxacin (CP) for antibacterial and wound dressing applications has been reported (Rathore et al., 2016). The ensuing porous sponges were characterized in terms of swelling percentage and porosity together with SEM data. In vitro biocompatibility, cell adhesion and proliferation tests were carried out using NIH 3T3 fibroblast and human keratinocyte cell lines indicated that the sponges are highly biocompatible.
Gold nanoparticles synthesized and stabilized by KG (KG/Au NPs) were assessed for their anti-proliferative properties in B16F10 melanoma cells as an in vitro experimental model (Kalaignana Selvi et al., 2017). The size of the nanoparticles was assessed by TEM and varied between 4.08 and 12.73 nm. The results demonstrated that the treatment with KG/Au NPs diminished cell proliferation. The qRT-PCR analysis revealed the genes (p53, caspase-3, caspase-9, PPARa and PPARb) responsible for apoptosis to be upregulated in B16F10 cells after treating them with GK/Au NPs. These studies highlighted the fact that GK/Au NPs show antiproliferative activity through the generation of apoptosis in a B16F10 melanoma cell line, signifying that GK/Au NPs have a potential for biomedical application.
6.4.5. GG interpenetrating (IPN) network
Natural polysaccharides have been used as colon-specific drug carriers due to their exceptional characteristics. Microbial fermentation in the large intestine could assist in the delivery of drugs by physical and diffusion blockade (Khan et al., 1999; Tuovinen et al., 2002). An interpenetrating network (IPN) system of GG cross-linked with poly (acrylic acid), and polyaniline has been fabricated via radical polymerization using N, N1-methylene–bis-acrylamide (MBA) (cross-linker) and ammonium persulfate (initiator) (K. Sharma et al., 2015a). Amoxicillin trihydrate and paracetamol were incorporated and their controlled drug release potential was evaluated in the colon, and various functional groups, i.e. – OH, –CH and –NH, in the hydrogel were found to be suitable for grafting of the drugs by chemical or physical interaction.
An IPN hydrogel prepared by grafting GG with poly (methyl acrylic acid–aniline) was examined and aimed at the sustained release of amoxicillin trihydrate in altered pH environments (acidic, natural and basic) and their thermal, structural and releasing kinetics were established (K. Sharma et al., 2015b). Effective colon-specific controlled release of drug delivery in the lower gastrointestinal tract could now be possible.
7. Conclusions and future prospects
Greener pathways to the synthesis and stabilization of metal/metal oxide NPs have been successfully implemented by many researchers worldwide. This methodology mainly deployed natural materials and green solvents, while keeping the process simple and economical. The major drawbacks of this method include the use of imprecise evaluation tools for the stability, aggregation behavior, size and shape of the NPs and the subsequent systematic description of the application of the NPs as a result of their physical and chemical characteristics.
Lately, tree gums (GA, GK, GT, KG and GG) have been intensively studied for the stabilization and greener synthesis of metal/metal oxide NPs. In addition, the green electrospinning process utilizing tree gums for the production of nanofibers and membranes is in the developmental stage and much more research in this field is expected in the near future. Tree gum matrixes and NPs functionalized by gums have been produced for the biosorption of environmental pollutants such as radioactive effluents, heavy metals, volatile organic compounds and dyes. The potential applications of tree gums and their functionalized nanocomposites developed as antibacterial agents are also showing remarkable promise.
NP-functionalized gums have been demonstrated to be a highly sensitive means of biosensing environmental pollutants in water and in related applications, e.g. as a biomarker for estimating the amount of glucose levels in blood plasma. This property could lead to the development of biosensing kits for environmental pollutants in drinking water as well as for the detection of biological fluids in human systems utilizing eco-friendly and biocompatible tree gum electrospun fibers.
Many bio-derived and renewable materials, such as cellulose, starch, chitosan and collagen, have been extensively employed in biomedical applications, e.g. in tissue engineering scaffolds, drug/gene delivery, and wound healing, due to their bio-compatibility, non-toxicity, biodegradability and accessibility of chemical functionalities in their structures for chemical modifications. Tree gums should now be incorporated into this group to serve as potential candidates for biomedical research and development.
Moreover, gum-based electrospun fibers show a promising future in replacing petroleum-based plastics, a phenomenon of concern today in our oceans. The development of bioplastic from gum-based electrospun fibers would be challenging and research towards their application in the fields of food packaging, carrier bags and antibacterial pads is essential for sustainable growth. In this context, tree gums present a promising renewable and highly economical bioresource for future generations, especially for under-developed countries, to utilize the local resources for higher value applications.
Electrospun fibers and membranes based on tree gums and their nanoparticle functionalized materials should garner much future attention in filtration/separation, adsorption, catalysis, imaging, greener corrosion inhibitors, nanofiltration (nanoparticles, pesticides, pharmaceutical waste etc.), air filtration, delivery of therapeutic drugs, wound healing, tissue engineering and controlled drug delivery etc. Furthermore, enhancement of physical and chemical properties via surface modification employing various greener technologies, such as plasma, γ-ray irradiation and laser treatments, will help generate economical, sustainable and bio-organic products and processes for the future.
The future use of tree gums also relies on the development of ultralight weight, high-strength, bio-based, biodegradable, porous, and tunable, two-dimensional (2D) membranes and three-dimensional (3D) sponges with facile and easy to implement synthetic schemes. The ensuing porous functionalized structures will form an integral part of applications for sustainable development under the umbrella of green chemistry and technology, specifically in fields of biomedicine (tissue engineering and drug delivery), heterogeneous catalysis (greener catalytic remediation), extraction of environmental contaminants of various types (heavy metals, nanoparticles, toxic organic solvents, pesticides, dyes) and/or use as a safer bioplastic (as food packaging and disposable materials). Quantification of the environmental benefits of these newer materials using the Life Cycle Analysis (LCA) method is also warranted.
In view of the current studies and trends, there is no doubt that the non-food applications of tree gums and their functionalized materials will play a significant role in many important areas, such as water treatment, renewable energy, biomedical research, the food industry and biotechnology (Peng et al., 2016).
Research Highlights.
The most relevant industrial tree gums and their physical and chemical properties, structural assignments and non-food applications are discussed.
Greener synthesis and production of NPs (nanoparticles) and NFs (nanofibers) using tree gums are illustrated.
The interactions of metal species, dyes and NPs with gum structures are summarized.
Antibacterial and biomedical applications of gums with NPs, NFs and hydrogels are reviewed.
Acknowledgements
This study was supported by the project LO1201, the financial support of the Ministry of Education, Youth and Sports in the framework of the targeted support of the “National Programme for Sustainability I” and the OPR&DI project “Centre for Nanomaterials, Advanced Technologies and Innovation—CZ.1.05/ 2.1.00/01.0005”. The authors would also like to acknowledge the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic in the framework of Project No. LM2015073.
List of abbreviations
- AFM
Atomic force microscopy
- ATR
Attenuated total reflection
- AgNO3
Silver nitrate
- BP
Bisphosphonate
- COSY
correlation spectroscopy
- CR
Congo red
- C. albicans
Candida albicans
- Cur
Curcumin
- CP
Ciprofloxacin
- DLS
Dynamic light scattering
- DDSA
Dodecenylsuccinic anhydride
- E. coli
Escherichia coli
- EDXA
Electron diffraction X-ray analysis
- GA
Gum arabic
- GK
Gum karaya
- GT
Gum Tragacanth
- GG
Gum ghatti
- GEM
Gemcitabine hydrochloride
- HMBC
Heteronuclear multiple-bond correlation spectroscopy
- HMQC
Heteronuclear single-quantum correlation spectroscopy
- FE
Field emission
- FCC
Face centered cubic crystal
- FTIR
Fourier transform infrared
- KG
Kondagogu gum
- MB
Methylene blue
- MW
Microwave
- MWt
Molecular weight
- MBC
Minimum bactericidal concentration
- MIC
Minimum inhibitory concentration
- MTT
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide
- MP
Microparticles
- MIPN
Imprinted polymeric nanogel
- MG
Malachite green
- GEM
Gemcitabine hydrochloride
- HR-TEM
High resolution transmission electron microscopy
- ICP-MS
Inductively coupled plasma mass spectrometry
- IPN
Interpenetrating polymer network
- NPs
Nanoparticles
- NFs
Nanofibers
- PVA
Poly(vinyl alcohol)
- PVP
Polyvinyl pyrrolidone
- PEO
Poly(ethylene oxide)
- PCL
Poly caprolactone
- PEDOT
Poly(3,4-ethylenedioxythiophene)
- PLLA
Poly(L-lactic acid)
- PLGA
Poly lactic glycolic acid
- PEC
Polyelectrolytic complex
- P.aeruginosa
Pseudomonas aeruginos
- QC
Quercetin
- ROS
Reactive oxygen species
- RF
Radio frequency
- RhB
Rhodamione B
- SEM
Scanning electron microscopy
- S. aureus
Staphylococcus aureus
- TEM
Transmission electron microscopy
- TGA
Thermogravimetric analysis
- TCH
Tetracyclic hydrochloride
- TOCSY
Total correlation spectroscopy
- UV
Ultra violet
- Vis
Visible
- XRD
X-ray diffraction
- XPS
X-ray photon electron spectroscopy
- ZOI
Zone of inhibition
- ZVI
Zerovalent iron
Disclaimer
The views expressed in this article are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Declaration of interest: None
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