Recent developments in stimuli responsive nanomaterials and their bionanotechnology applications

Abstract

Bionanotechnology is an ever-expanding field as innovations in nanotechnology continue to be developed based on biological systems or to be applied to address unmet needs in biology, biomedicine, etc., including various sensor and drug delivery solutions. Amidst the wide range of bionanomaterials that have been developed, stimuli responsive bionanomaterials are of particular interest and are thus emphasized within this review. Here, we have highlighted the most recent advances for stimuli responsive bionanomaterials with focus on those possessing responses based on activation, expansion/contraction and self-assembly/disassembly. The aim of this review is to bring attention to some of the most current bionanotechnology research and the interesting applications within this field.

Introduction

Since its explosive growth in the late 20th century, nanotechnology has piqued the interest of researchers across the world and has led to the design and development of unique nanoscale materials, known as nanomaterials [1]. Any material that has at least one dimension less than 100 nm in size is typically regarded as a nano-material, and at this size range, properties of the materials are often quite different from their bulk counterparts. Unique materials can be formed by carefully tuning the properties of these nanomaterials [2]. There is a long history of gaining inspiration from and attempting to mimic the efficient systems found in Mother Nature for practical application and biomimicry eventually trickled down to nanoscale processes where humans have utilized existing small-scale systems in nature to form novel biomimetic nanomaterials. Nanotechnology has drawn encouragement from living systems to develop nanomaterials with biological functionalities and has thus offered a more insightful way to study the human body at a scale which was previously, as of just a few decades ago, unimaginable. With each new advancement, bionanotechnology continues to mature into a discipline that encompasses a multitude of fascinating materials designed to enhance and elevate our lives [3,4].

Bionanotechnology is a subset of nanotechnology where concepts from physics, chemistry, material science, engineering and biology converge at a nanoscale level to develop and better understand materials specifically intended for use in biological applications [5]. Nanotechnology inspires nanocomposite synthesis with tunable functionalities, and these useful bionanomaterials have been extensively studied due to their growing importance in fields of disease treatment. For example, gold, silver, silica and iron oxide nanoparticles have been used extensively as bionanomaterials for applications ranging from drug delivery to sensing and beyond [6–9]. These nanoparticles act as carrier molecules on which different types of bio-friendly functional molecules are attached based on the specific application. In their review, Peppas et al., talk about the growing importance of nanoscale hydrogels as bionanomaterials and discuss their therapeutic applications [10]. Biological molecules, such as proteins, are also being developed for use in various nanomaterials [7] as well as assorted shapes and sizes of carbon-based nanomaterials (nanotubes, nanofibers, nanospheres, nanohelices, etc.) which are applied extensively in the field of biomedicine [11].

Bionanotechnology is already a vast field of study and yet continues to grow day by day with each new discovery. The ultimate objective of this review is to highlight interesting and promising stimuli responsive bionanomaterial research that has been reported in last five years. Stimuli responsive bionanomaterials have the ability to respond to various triggers and over the past couple decades extensive research on stimuli responsive materials has been completed, especially as nanomaterials. These materials respond molecularly to specific stimuli triggers such as pH change [12], temperature change [13], presence of external chemical agents [14], ionic strength [15], application of electrical and/or magnetic fields [16], and irradiation of UV/visible light [17]. Physical material responses that result can include structural change, morphological change, assembly/disassembly, size change, activation, motion, etc. [17,18,19••,20].

Due to their unique stimulus-specific responsive nature, these nanomaterials have been applied to various fields including drug delivery [21], sensing, self-healing [22], and environmental applications [23]. Existing reviews regarding stimuli responsive materials typically categorize systems-based on their final application, synthesis method, physical response of the nanomaterial due to the applied stimulus or the type of stimuli used [17,23–25]. Characterization of stimuli responsive bionanomaterials can be a challenge as materials often can fit into multiple catergories since these systems can possess multiple responses to the same stimulus or a similar response to different stimuli. With that in mind, the main goal of this review was to categorize the highlighted materials based on the physical response when a stimulus is applied. Examples of responses inlcude activation, expansion/contraction, self-assembly/disassembly, gatekeeping, autonomous motion, and morphological changes [17]. In this review, we have focused on activation, expansion/contraction, and self-assembly/disassembly (illustrated in Figure 1) as there are a wide range of exciting examples of stimuli responsive nanomaterials with these responses to highlight.

Figure 1.

Conceptual design of stimuli responsive nanomaterials representing three different pathways of response to an external stimulus; activation pathway depicts the (a) release of drug attached to the network and (b) local temperature variation response to an external stimulus, expansion/contraction pathway presents (c) the release of drug through contraction of the network and self-assembly/disassembly pathway shows (d) micellar assembly of a polymer and (e) monolayer assembly over a substrate due to application of the stimulus.

Activation

In some cases, stimuli can activate surface transformations that lead to drug release, generation of chemically reactive species, etc. In fact, a significant amount of stimuli-responsive nanomaterial research involves direct activation with a focus on approaches that utilize exogenous triggers such as light and electromagnetism to drive the desired surface transformation [24]. The research highlighted within this section represent nanomaterials that elicit a response when exposured to either visible light, alternating magnetic fields (AMF), or laser irradiation, leading to disruption of surface interactions or enhancement of surface reactivities. Specifically, direct activation techniques have recently been popular for generation of reactive oxygen species (ROS) [20,26], and in a groundbreaking report, Bagchi et al. utilized modified nanoscale metal–organic frameworks (NMOFs) to combat multidrug resistance bacteria [27••]. A post-synthetic approach was used to encapsulate the hydrophobic photosensitizer drug, squaraine, in a zeolitic-based framework (ZIF8-SQ) in order to enhance antimicrobial photodynamic therapy efficacy against drug-resistant planktonic bacteria, specifically methicillin-resistant Staphylococcus aureus (MRSA). The resulting ZIF8-SQ nanohybrid exhibited red-light responsive and pH-responsive behavior which, upon excitation in response to red-light and/or acidic environment exposure, could result in significant ROS generation, thus resulting in decreased MRSA growth as shown in Figure 2. The enormous antimicrobial photodynamic therapy effect of ZIF8-SQ towards MRSA, even at distinctly low concentrations, with no apparent human cell toxicity, suggest the exceptional and highly applicable nature of this nanomaterial.

Figure 2.

Growth curves of MRSA in the presence of varying ZIF8-SQ concentrations (a) in the dark (b) with 30 min of red-light illumination. Pictures of MRSA culture plates with 100 nm ZIF8-SQ grown under (c) dark conditions (d) red-light irradiation. Reprinted (adapted) with permission from Ref. [27••]. Copyright 2019 American Chemical Society.

A few researchers have reported the use of an AMF to trigger ROS generation at the surface of magnetic nanoparticles. One such example was reported by Wydra et al., where after iron oxide nanoparticles (IONP) were exposed to an AMF, a significant increase in methylene blue decolorization was observed, indicating a significant increase in ROS generation [28]. The findings from this research have the potential to impact advances in intracellular treatment processes and advanced oxidation processes. Alternately, Mai and Hilt reported on the inhibition of IONP ROS generation, where IONPs were functionalized with small molecules such as citric acid, sodium phosphate, amino silane and dopamine in order study their effect on ROS generation and surface reactivity under AMF exposure [29]. The reported results indicated that small molecules inhibit IONP’s ability to generate ROS, which is an important factor when designing such systems for use in therapeutic applications. Additional studies on the importance of IONP ROS generation and their biomedical applications have also been explored [30–33].

In another example of actuated response, a remote activated nanomaterial developed by Li et al., produced cancer theranostic agents for multimodal imaging and chemo-photothermal combination therapy [34]. Bovine serum albumin (BSA) modified bismuth nanoraspberries (Bi-BSA NRs) were synthesized in aqueous phase via a facile reduction method. The resulting NRs exhibited a porous nanostructure capable of high drug loading capacities and controllable drug release behavior activated through laser irradiation and/or pH manipulation. Accelerated drug release was due to increased protonation and hydrophilia of DOX molecules under acidic conditions. Additionally, laser irradiation enhanced thermal vibration of molecular bonds via rapid local temperature increase and weakened DOX-NR binding forces, thereby accelerating DOX release. These successful nanoplatforms demonstrated a promising route for the development of more effective and precise anti-cancer treatments.

Expansion/contraction

Expansile nanomaterials have the unique ability to swell or contract in response to specific stimuli. Temperature and pH are the most commonly used triggers to invoke swelling behavior which can then enable release of loaded materials such as anti-microbial agents or drugs. For example, pH shifts can affect protonation states of functional moieties such as carboxyl or amino groups, thereby altering the hydrophobicity and/or ionic interactions of the nanomaterial leading to a structural change which can be tremendously useful in various applications, such as controlled release of loaded medicine to targeted areas in the body [35,36]. Thermoresponsive materials can exhibit a change in structure due to temperature shifts, such as precipitation/solubilization for non-crosslinked polymers or swelling for crosslinked polymers resulting from critical temperature transitions. Polymers that exhibit this type of behavior in water can change from hydrophilic to hydrophobic because of hydrogen bonds present in the surrounding water molecules and the polymer itself [37]. Ma et al. developed very interesting natural polymeric surfactants that exhibit strong pH responsiveness during emulsion preparations and consequently show very favorable potential for use in applications such as the controlled release of pesticide formulations and organic pollution remediation [38]. A bio-based poly(glycyrrhizic acid) (PGly) homopolymer was synthesized via reversible addition fragmentation chain transfer polymerization. The resulting polymer contains two carboxylic groups on the side chain of PGly, giving rise to a multiple pH-responsive system. As pH values decreased from 5.0 to 2.0, a significantly more amount of intra and interactions among PGly chains were generated due to weakened electrostatic interactions thus transforming the structure to a coil state where visible insoluble particles formed. Ultimately, the emulsion stability of PGly was proven to be carefully controlled through manipulation of pH values [38].

In order to mimic the self-cleaning properties of Gecko feet, Du et al. modified poly(acrylic acid) gel micro-brushes with stimuli responsive polymer nanobrushes [39]. Inspiration was drawn from the underwater superoleophobic surface of Gecko feet in order to develop biomaterials that can be used in applications such as oil contamination remediation and marine antifouling. Gel micro-brushes are formed via free radical polymerization and subsequently modified with stimuli responsive polymer nanobrushes through addition of thermoresponsive N-isopropylacrylamide (NIPAAm) or poly 3-sulfopropyl methacrylate potassium salt (PSPMA). The ‘smart’ properties exhibited by the poly(N-isopropylacrylamide) (PNIPAAm) modified micro-brushes is namely due to molecular conformation transitions around the polymer’s lower critical solution temperature (LCST). At 20°C (below LCST), intermolecular hydrogen bonding interactions between PNIPAAm and water results in a super hydrated and swollen state thereby preventing oil adhesion. At 60°C (above LCST), intramolecular hydrogen bonding forms between the carboxide and secondary amine groups in PNIPAAm, resulting in a collapsed and dehydrated state around the brushes, ultimately increasing oil adhesion properties. Additionally, the PSPMA modified brushes exhibited increased under water oil adhesion as NaCl concentration is raised. These interesting and bio-inspired materials provide eco-friendly anti-fouling materials that can be safely used in marine environments.

Yahia-Ammar et al. developed positively charged pH-dependent nanoparticles (NPs) from gold nanoclusters (Au NC) [40••]. Polyelectrolyte poly(allyl amine hydrochloride) (PAH) was used with glutathione coated Au NCs (Au-GSH) to form spherical self-assembled nanoparticles (Au-GSH-PAH) with sizes around 100 nm. The charged NPs exhibited pH-dependent swelling behavior most likely due to Columbic interactions between Au-GSH and PAH. These electrostatic interactions caused strong fluorescence activity (i.e. a fourfold enhancement) to be observed when pH was manipulated due to aggregation-induced emission (AIE), as illustrated in Figure 3. Demonstration of the AIE effect involving electrostatic interaction inside the Au-NCs was confirmed by controlling particle expansion and followed by steady-state and time-resolved fluorescence detection. Drug loading and delivery capability were investigated due to the highly applicable nature of pH-mediated size-controllable NPs. Peptides and antibodies were reported to be easily loaded into the NPs using a simple one-pot synthesis process and mediated release through manipulation of swelling behavior could potentially be realized resulting in well-designed new drug delivery systems.

Figure 3.

Fluorescence and pH-dependent swelling behavior exhibited by polyelectrolyte crosslinked gold nanoclusters: (a) fluorescence emission under UV illumination (λ_exc = 366 nm) increases fourfold when the GSH stabilized Au NPs are crosslinked with PAH to form spherical self assembled nanoparticles (Au-GSH-PAH); (b) schematic representation of pH-dependent swelling behavior and enhanced fluorescence activity exhibited by the nanoclusters when crosslinked with PAH. Reprinted (adapted) from Ref. [40••]. Copyright 2016 American Chemical Society.

An innovative application for thermoresponsive nanomaterials was presented by Bashari et al. [41•]. The group reported on the concept of smart textiles through introduction of thermoresponsive poly(N-isopropylacrylamide)/chitosan nanohydrogels containing cinnamon oil (PNCS) on cotton fabric in order to promote bio-antibacterial activity at different temperatures. The antibacterial activity of treated cotton fabrics resulted from the release of bioactive compound at elevated temperatures (40°C), which was reported to be much higher than the number of bioactive compounds released at 25°C. This occurrence can be explained by the contraction of the hydrogel network above the LCST of the temperature-responsive part of PNCS nanohydrogels, which suggested an increase on the mechanical stress of the β-cyclodextrin rings, thereby accelerating release of antibacterial agents from the polymer framework.

Self-assembly/disassembly

Autonomous organization of structures on all scales is governed by the fundamental principle of self-assembly [42]. For nanoscale materials, various properties like shape, size, charge etc. can be tuned through self-assembly [42]. Self-assembly utilizes non-covalent interactions like ionic, hydrogen bonds, van der waals, and hydrophobic interactions to organize and instantaneously form well-defined structures at molecular level [43]. These interactions are singularly weak but collectively can form a flexible yet stable structures. In some cases, such assemblies can be manipulated using a stimuli which changes the morphology of the structures leading to stimuli responsive materials with unique properties [44]. A few of such exciting stimuli triggered assembly/dissambly materials have beendisussed below.

In their report, Zhang and Zhang synthesized photo-controlled, reversible and recyclable 2D self-assembled nanosheets[45•]. The sheets were loaded with doxorubicin (DOX) as a model drug and in the presence of UV irradiation, the sheets disassembled and decomposed into irregular fragments which led to controlled discharge of the drug. Exposure to visible light, the structures returned to their original shape, thus illustrating the reversible assembly of the nanosheet. The drug release behaviour as a function of light was studied and the authors were able to demonstrate the instantaneous release in the presence of UV light as well as controlled on and off release with the alternation of turning on and off of the UV irradiation. Sun etal. developed stimuli responsive bio-nanogels specifically designed for enhanced tumor-targeting cancer therapy [46]. Through hydrogen bonding and oxidation reactions, keratin–hyaluronic acid nanogels (KHA–NGs) were synthesized as targeted anti-cancer drug carriers with stimuli responsiveness. The resulting crosslinked polymer structure allowed for high doxorubicin hydrochloride (DOX) loading efficiency (54.1%), and release was successfully demonstrated through disassembly of the structure with the help of manipulation of intracellular tumor microenvironment(pH or trypsin or GSH concentrations). Both in vitro and in vivo results verified that the nanodrug carriers can successfully suppress tumor growth with milder side effects as compared to free DOX.

Amphiphilic molecules form a large family of self-assembly systems due to their ability to form variety of structures like micelles, nanotubes, vesicles, lamellae in presence of water and therefore such materials are widely studied in bionanotechnology as well [47]. Ke et al. synthesized a polyethylene glycol (PEG) based amphiphilic polymer containing azobenzene and ferrocene moieties [48]. In aqueous solution, the polymer can reversibly self-assemble into nanostructures and disassemble either slowly via UV irradiation or rapidly via redox reactions. Because of this controlled disassembly of the nanostructure, the authors were able to control the release of drug rhodamine 6G (R6G). Upon exposure to different stimuli (light irradiation or redox reaction), polymer hydrophilicity was controllably enhanced and resulted in R6G release. Gao etal. Presented a preparation of cholesterol end capped polymer of poly (ethylene glycol methyl ether methacrylate) [49]. This amphiphilic polymer was applied to form quercetin loaded micelles through self-assembly in aqueous solution and drug release was exhibited by altering the self-assembly through variation in pH and presence of cyclodextrins in the released medium.

Exploiting the interactions present in self-associating molecules and nanoparticles has led to the development of novel self-healing materials which have recently garnered great interest. Li et al. presented a self-healable hydrogel with graphene nanoparticles [50•]. The hydrogel was prepared via Schiff base linkage which composed of chondroitin sulfate multialdehyde (CSMA), branched polyethylenimine (BPEI) and BPEI conjugated graphene (BPEI-GO). Schematics of the synthesis as well as application are shown in Figure 4. The hydrogel possessed exceptional self-healing capabilities and improved mechanical properties due to the presence of imine bonds in the network which provided sustained drug delivery of DOX in vitro. The authors were able to conclude that the hydrogel was an effective tool for in vitro cancer cell inhibition and in vivo prevention of postoperative breast cancer recurrence via combined chemo-photothermal therapy. Appel et al. synthesized self-assembled hydrogels with shear thinning and self-healing properties based on hydrophobic interactions between cellulose derivatives and nanoparticles (NPs) [44]. These interactions govern self-assembly processes of the hydrogels and help them to flow under applied stress while recovering as soon as the stress is relaxed. The researchers also developed a biocompatible hydrogel-based on poly(ethylene glycol)-block-poly(-lactic acid) (PEG-b-PLA) NPs to enable dual loading of hydrophobic and hydrophilic molecules. Impressively, the subsequent hydrogels were able to release multiple loaded model therapeutics from a single material.

Figure 4.

Schematic representation of (a) CSMA/BPEI/BPEI hydrogel and (b) its application in prevention of recurrent breast cancer (Li et al.). Reprinted (adapted) with permission from Ref. [50•]. Copyright 2019 American Chemical Society.

Conclusions

This review has examined nanomaterials that undergo physical responses induced by various stimuli in order to facilitate a multitude of potential processes. The responses categorized within this review focused on activation, expansion/contraction, and assembly/disassembly. The scope and range of applications that benefit from advances in stimuli-responsive nanomaterials are diverse. Not only do these nanoscale materials shed light on new possibilities for therapeutic delivery systems and targeted bio-imaging, they also significantly enhance environmental remediation efforts and beyond. Careful selection of monomers and composite materials can give way to nanomaterials that possess stimuli responsive properties. Additionally, nanomaterials gain many advantages and applications through addition of surface functionalization, stimuli responsiveness and reversibly self-associating materials. Taking into account the desired application, the responsiveness of these nanomaterials can be directed through introduction of either an environmental shift or an external stimulus. Obstacles must be considered, however, in order to achieve clinical success of these materials. Potential shortcomings need to be addressed when focusing on nanosystems that elicit a response through activation from external agents such as visible light or magnetic field. Use of these external sources may lead to higher treatment costs due to use of specialized instruments to stimulate response. However, these systems also have the potential to provide more selective treatment as opposed to those that depend on bulk environmental properites to drive the desired process. For example, expansile/contractile systems used as drug delivery agents, which are commonly subject to triggering agents such as pH or temperature change, are typically limited in the extent to which the response and action can be localized. Thus, research exploring more specific biomarkers as triggers for swelling/deswelling materials is of high interest for cases where a more localized or targeted response is desired. The research highlighted in this review represents recent and forward-thinking work done within the field of bionanotechnology. The need for continued research toward truly selective and response specific bionanomaterials and future projects like them cannot be overstated.