Synthesis of Ag‐NPs impregnated cellulose composite material: its possible role in wound healing and photocatalysis

Abstract

Cellulose is the natural biopolymer normally used as supporting agent with enhanced applicability and properties. In present study, cellulose isolated from citrus waste is used for silver nanoparticles (Ag‐NPs) impregnation by a simple and reproducible method. The Ag‐NPs fabricated cellulose (Ag‐Cel) was characterised by powder X‐rays diffraction, Fortier transform infrared spectroscopy and scanning electron microscopy. The thermal stability was studied by thermo‐gravimetric analysis. The antibacterial activity performed by disc diffusion assay reveals good zone of inhibition against Staphylococcus aureus and Escherichia coli by Ag‐Cel as compared Ag‐NPs. The discs also displayed more than 90% reduction of S. aureus culture in broth within 150 min. The Ag‐Cel discs also demonstrated minor 2,2‐diphenyl 1‐picryl‐hydrazyl radical scavenging activity and total reducing power ability while moderate total antioxidant potential was observed. Ag‐Cel effectively degrades methylene‐blue dye up to 63.16% under sunlight irradiation in limited exposure time of 60 min. The Ag‐NPs impregnated cellulose can be effectively used in wound dressing to prevent bacterial attack and scavenger of free radicals at wound site, and also as filters for bioremediation and wastewater purification.

1 Introduction

Cellulose is the most abundant natural biopolymer with a fascinating structural behaviour that has shown vast applications due to high specific strength and stiffness [1]. Owing to the fibrous material, remarkable physical properties, special surface chemistry, low toxicity, biodegradability and biocompatibility [2], cellulose can be used as adsorbent substrates for retention of metal ions and as a matrix for the production of nanocomposites [3, 4] which can be used in different applications. Cellulose and its derivatives are widely used in pharmaceutics, wound dressing, cosmetics, coatings, water filtration and in various hybrids [1, 3, 5]. Though, cellulose itself has no antimicrobial activity and degradation potential to prevent wound infection and water treatment, respectively [6, 7] but the composite of cellulose and nanoparticles (NPs) may fascinate these applications.

Ag‐NPs have diverse physio‐chemical properties such as chemical stability, high electrical and thermal conductivity, optical behaviour, surface‐enhanced Raman scattering and catalytic activity [8, 9]. These properties make them industrially valuable, i.e. inks [10, 11], microelectronics [12] and medical imaging [13]. Low‐cost manufacturing and antibacterial properties [8] also make Ag‐NPs demanding in different consumers materials such as plastics, soaps, pastes, metals and textiles [9]. Though Ag is considered relatively harmless to humans. However, Ag's bactericidal properties have been exploited by certain groups commercialising colloidal Ag suspensions as ‘health supplements’. Besides this Ag ions (Ag⁺) released from Ag NPs and others in the environment are highly toxic to prokaryotes, freshwater and marine invertebrates and fish [14].

The mutual use of cellulose and metallic NPs is of great interest to develop bio‐sensing platform and molecular detection. Cellulosic matrices isolated from natural sources combined with metallic NPs appear more sensitive as compared with the other analogues [15, 3]. Chemically synthesised Ag‐NPs have recently been applied as antimicrobial agents in various commercial products and degradation of dye molecules from waste stream under sunlight irradiation [16, 17, 18]. Though literature describes many methods to prepare metallic NPs attached to the cellulose extracted from various natural resources [7, 19, 20] but NPs appended on the surface have chances of release into the environment. By taking advantage of the characteristic porosity of cellulosic fibres, availability of binding groups; metallic NPs can be impregnated on it [7]. The high oxygen density of cellulose fibres act as supporting and binding material for metallic NPs. The binding of Ag‐NPs with supporting material may change chemistry of the surrounding environment that ultimately will affect association of Ag with biota in terms of bioavailability and toxicity.

In this paper, initially cellulose was isolated from agriculture and food industry citrus waste. An easy chemical reduction method to fabricate the Ag‐NPs impregnated cellulosic paper (Ag‐Cel) is reported. The synthesised paper is characterised by Fortier transform infrared spectroscopy (FTIR), powder X‐ray diffraction (XRD) and field‐emission scanning electron microscope (SEM) analysis to reveal the structure. The antibacterial, antioxidative response and photocatalytic action of prepared paper is also studied.

2 Materials and methods

2.1 Materials

All solvents and chemicals used were of analytical grade. For preparations of all solutions millipore water was used. Ethanol, (analytical grade) hydrogen peroxide (Merck 35%), toluene (high‐performance liquid chromatography grade) and sodium hydroxide (NaOH) were purchased from Sigma‐Aldrich. Dimethyl sulphoxide (DMSO) and Ag nitrate (AgNO₃) were procured from Merck. The peel of citrus (Citrus reticulate) fruit of locally named as Kinnow was dried and grounded to use initially as raw material for cellulose extraction.

2.2 Methodology

2.2.1 Extraction of cellulose from citrus peel

The citrus peel was dried under sunlight and grounded in household grinder. The powder was passed through sieve of 60 mm mesh size and used for cellulose extraction following the method described by Naz et al. [18]. About 10 g powder was soaked overnight in toluene and ethanol 2:1. The mixture was filtered thereafter and the residue was dried at room temperature. The dried material was bleached by treating with hydrogen peroxide and heated at 80°C in water‐bath. This process was repeated until a clear white product was obtained. The whitish material was filtered and treated again overnight with 6% NaOH at room temperature. The mixture was filtered and the residue was re‐treated with 6% NaOH in water‐bath for 2 h and neutralised with distilled water thereafter. The mixture was sonicated for 45–50 min in ice cold water and filtered. This filtrate pasty material was characterised as extracted cellulose. Finally, obtained material was thoroughly washed with distilled water until the cellulose pasty material became neutral and then immersed in the distilled water prior to use.

2.2.2 Impregnation of Ag‐NPs into cellulose

The cellulose obtained was used as medium for impregnation of Ag‐NPs following the methodology described by Maneerung et al. [21] with some modifications. Citrus cellulose pasty material was sonicated for 15 min prior to use. The cellulose was initially treated with 1 mM AgNO₃ (aqueous) for 1 h with occasional shaking, followed by rinsing with ethanol for 2–3 min. The Ag⁺ treated cellulose material was then reduced by 0.01 M NaOH (aqueous) for 10 min at 50 rpm shaking. The mixture was filtered and thoroughly washed with ultra‐pure water to remove the excess chemical. The viscid material was placed overnight under vacuum drying at room temperature until a partially dried pasty material obtained. This relatively dried material containing cellulose‐impregnated Ag‐NPs was then designed to form sheet by pressing it with a steel gauge having flat and smooth surface. Finally, different pieces of paper of cellulose with impregnated Ag‐NPs (Ag‐Cel) were obtained having the size 1–5 cm² in range.

2.2.3 Synthesis of Ag‐NPs

Ag‐NPs were synthesised following the same procedure described above, except in absence of cellulose material. For synthesis of Ag‐NPs, 1 mM AgNO₃ in aqueous was treated with 0.01 M NaOH for 10 min during constant shaking (50 rpm) at room temperature under dark condition. The solution turned greyish black due to formation of Ag‐NPs. The NPs were separated at 13,000 rpm for 10 min which were dried in vacuum at room temperature.

2.2.4 Characterisation of Ag‐NP‐impregnated cellulose

FTIR measurements were performed at transmission mode between 4000–400 range and 4 cm⁻¹ resolution maintaining a total of 50 scans for each sample analysed. The formation of Ag‐NPs was identified by XRD (PANalyticalX'pert PRO Origin, The Netherlands). Sample was scanned from 2θ  = 30° to 80° at a scanning rate of 5°×2θ min⁻¹. The morphology of Ag‐NPs impregnated on citrus cellulose was observed by JEOL JSM‐5200 SEM operating at 15 kV on different magnifications. Thermo‐gravimetric analysis (TGA) was carried out with a Mettler Tory thermo‐gravimetric analyser.

2.2.5 Antioxidant activities

For antioxidative analysis, Ag‐NPs were suspended in DMSO at 4 mg/ml and the mixture was sonicated for 15 min before use. The cellulose paper and Ag‐impregnated cellulose were weighed as 200 µg for a reaction mixture of 1 ml and the paper was suspended in the reaction mixture.

2,2‐Diphenyl 1‐picryl‐hydrazyl (DPPH) free radical scavenging: Standard DPPH method was adopted for free radical scavenging potential of samples [22]. For Ag‐NPs; solution of 10 µl in DMSO (final concentration 200 µg/ml) was combined with DPPH solution of 190 µl (in methanol). While for cellulose and Ag‐impregnated cellulose, the samples were directly treated with DPPH solution at 200 µg/ml. After the incubation for 15 min in dark at 37°C, papers were removed from the reaction mixture and absorbance was measured at 517 nm using spectrophotometer.

Total antioxidant capacity (TAC) assay: TAC was determined by phosphor‐molybdenum method [23]. The reagent solution of 1 ml (4 mM ammonium molybdate, 28 mM sodium phosphate and 0.6 M sulphuric acid) was added with 0.1 ml of sample in case of Ag‐NPs. In case of paper (cellulose and Ag‐Cel), 200 µg paper was suspended in the reaction mixture. The mixture was incubated at 95°C for 90 min and then cooled to room temperature. Sample TAC was expressed as ascorbic acid equivalent and the absorbance was measured at 695 nm.

Total reducing power (TRP) assay: The Ag‐Cel and cellulose 200 µg, each were used for TRP while Ag‐NPs were suspended in DMSO at 4 mg/ml [22]. Briefly, samples were mixed with 200 μl phosphate buffer (0.2 mol/l, pH 6.6) and 250 μl potassium ferricyanide (1%). The mixture was then incubated at 50°C for 20 min and then 10% trichloro‐acetic acid (200 μl) was added. At room temperature, the mixture was centrifuged at 3000 rpm for 10 min. The upper layer of solution (150 μl) was then mixed with FeCl₃ (50 μl, 0.1%). Finally, the absorbance was measured at 630 nm on spectrophotometer. The enhanced absorbance of the reaction mixture showed better reducing power. Blank was prepared by adding 400 μl of DMSO to the above‐mentioned reaction mixture instead of the sample. The reducing power of samples was expressed as ascorbic acid (vitamin C) equivalent.

2.2.6 Antimicrobial activity studies

Antimicrobial activities of samples (cellulose, Ag‐NPs and Ag‐Cel) have been investigated against Escherichia coli as a model Gram‐negative bacteria and Staphylococcus aureus as a model Gram‐positive bacteria. The antimicrobial activities were carried out by two methods.

The disc diffusion method: This method was carried out in Luria‐Bertani (LB) agar medium. In short, 24 h old cultures of E. coli and S. aureus were evenly spread on LB agar plates. About 6 mm cellulose and Ag‐impregnated cellulose discs were placed on media surface. The plates were incubated for 24 h at 37°C and inhibition zones were measured thereafter.

The colony forming count method: The cellulose and Ag‐Cel were cut into small pieces. Experiment was performed in test tubes containing 2 ml autoclaved nutrient broth. The tubes were seeded with S. aureus as 10⁴ colony forming units per millilitre (cfu/ml). The Ag‐NPs, cellulose and Ag‐Cel were put as 1, 5 and 10 mg/ml in respective test tubes and placed on shaker incubator at 37°C. After each 30 min, 10 µl was taken and spread on pre‐prepared nutrient agar plate surface. The plates were incubated at 37°C for 24 h and colonies were counted under colony counter. The percentage reduction in bacterial count was calculated by the formula [24]

$$\frac{\left(\mathrm{viable}\mathrm{count}\mathrm{at}0\min -\mathrm{viable}\mathrm{count}\mathrm{at}150\min \right)\times 100\text{\%}}{\mathrm{viable}\mathrm{count}\mathrm{at}0\min }$$

2.2.7 Photocatalytic activity

The photocatalytic activity of Ag‐NPs, cellulose and Ag‐Cel was studied by degradation of methylene‐blue under sunlight irradiation. Two stock solutions of 5 and 10 ppm methylene‐blue dye were separately prepared in 500 ml deionised water. The samples were weighed as 2 mg and added to 60 ml dye solution (of 5 and 10 ppm). The mixture was then stirred on orbital shaker for 15 min in dark before exposing to sunlight. A control methylene‐blue solution was prepared (without sample addition) and kept under the similar condition for comparing any change in colour. The colloidal suspension was then kept under sunlight irradiation with constant stirring. Dispersion was monitored on regular intervals (every 10 min) from 0 to 90 and 0 to 60 min for 10 and 5 ppm dye solutions, respectively. The average temperature of the ambience during the experiment was around 18°C with 4 h mean shine duration (Islamabad: 33°43′ N 73°04′ E; date: 31st December 2015). At frequent intervals (every 10 min), 1 ml suspension was taken from the colloidal mixture and centrifuged at 5000 rpm for 15 min to obtain clean supernatant. The absorbance spectrum of the supernatant was subsequently measured at 350–850 nm wavelength using the Perkin Elmer ultraviolet–visible (UV–vis) spectroscope (USA). Concentration of dye during degradation was calculated by the absorbance value at 660 nm. Percentage of dye degradation was estimated by the following formula:

$$\text{\%}\mathrm{degradation}=100\times \frac{\left(C^0-C\right)}{C^0}$$

where C ⁰ is the initial concentration of dye solution and C is the concentration of dye solution after photocatalytic degradation at specific interval of time.

2.2.8 Statistical analysis

All the assays were performed in triplicate and results are presented mean with standard deviation. Furthermore, the mean were statistically analysed using least significant difference (LSD) analysis at significance level 0.05.

3 Results and discussion

3.1 Morphology of citrus cellulose

The organic and alkaline treatment disrupts the cross‐linking of lignin and lignocellulose of citrus peel powder resulted in white fibrils with varying size but cross‐linked. The cellulose fibres were ∼10–25 µm in length. The process adopted to purify cellulose from plant source does not much favour conversion of long cellulose fibre into smaller lengths but repeated treatment with alkali and the mesh size are important steps for micron structured cellulose. The ultra‐SEM analysis reveals cellulose fibre hollow tube like structure (Figs. 1 a and b). The pasty material and SEM confirmation derives it as elastic, high strength and conformability.

Fig. 1.

SEM micrographs of cellulose and impregnated cellulose

(a), (b) Cellulose isolated from citrus peel after alkali treatment, (c), (d) Impregnation of Ag‐NPs on cellulose fibre

3.2 Impregnation of Ag‐NPs on cellulose

Most of the methodologies for impregnation of metallic NPs on cellulose describe mixing of prepared NPs with cellulose fibre. Those most probably interact by trapping between the cellulose fibres and somehow weak interaction between free OH• group of cellulose and NPs. The direct synthesis of NPs over the surface of fibre, as adopted in current methodology, has more advantages due to strong interaction between glucose OH• group and metal ion; strong bonding capability; weak chances of NPs release; and facial synthesis of NPs intra and over surface of fibres [25]. The entrapment of Ag⁺ in cellulose fibres followed by their reduction with an external reducing agent makes the NPs promptly bind. The absorbed Ag⁺ bind to cellulose micro‐fibrils possibly through electrostatic interactions (Fig. 2), since the electron‐rich oxygen atoms of polar hydroxyl groups on surface and inside of cellulose tubes are expected to interact with electro‐positive transition metal cations. The unbounded cations washed with ethanol rinse. Ultimately, Ag⁺ were reduced to form Ag‐NPs after the reduction in aqueous NaOH (Fig. 2). This resulted change of colourless cellulose to grey black (Fig. 3) that may be due to the typical absorption of Ag‐NPs on cellulosic fibres [26]. First, the Ag‐NPs impregnated cellulosic material was dried by the vacuum drying to sustain the inventive structure of cellulose. The formation of Ag‐NPs on the cellulose surface was further observed by SEM analysis (Figs. 1 c and d). At different magnifications, some of the impregnated Ag‐NPs are observed of different shapes and sizes (rod and spherical). The cellulose hollow tube must contain Ag‐NPs due to available OH• groups but they were hard to recognise. It can also be proposed that Ag⁺ interact with OH• group of two cellulose strands, making them densely packed fibrils.

Fig. 2.

Representation of the chemical structure of cellulose. A linear polymer made up of β‐D‐gluco‐pyranose units covalently linked with (1–4) glycosidic bonds and the production of Ag‐NPs on citrus cellulose nano‐fibrillar structure through chemical reduction method

Fig. 3.

Ag‐NPs impregnated citrus cellulosic paper (Ag‐Cel) pieces of 1–5 cm²

The surface chemistry of Ag‐impregnated cellulose was investigated using FTIR spectroscopy. All the FTIR spectra of cellulose impregnated with Ag‐NPs display the typical bands of cellulose [27, 28, 29]. For the sample Ag‐Cel, the band centred at around 3331 cm⁻¹ can be attributed to the stretching vibration of hydroxyl group. It is important to note that the broad peak at around 3331 cm⁻¹ became broader in sample Ag‐Cel comparatively with that of cellulose (Fig. 4). A similar phenomenon in cellulose‐based NPs has been reported for cellulose–calcium silicate nanocomposites [30, 31] and cellulose–Fe₂ O₃ nanocomposites and many others [32, 33, 34].

Fig. 4.

FTIR spectra of citrus extracted cellulose (above) and cellulose impregnated with Ag‐NPs (below)

The XRD was used to confirm the formation of Ag‐NPs. The XRD pattern of Ag‐NPs impregnated citrus cellulosic paper (Ag‐Cel) (Fig. 5) showed characteristic peaks at 2θ  = 23.1°, and 34.9° corresponding to the planes of face centred cubic structure of the metallic Ag‐NPs [35, 36].

Fig. 5.

XRD pattern of

(a) Citrus cellulose material, (b) Ag‐NPs impregnated citrus cellulosic paper (Ag‐Cel) prepared from the NaOH:AgNO₃

The effect of Ag‐NPs on cellulose was investigated with TGA. The presence of Ag is identified as a TGA residue up to 10.6 mass% after heating to 800°C; neither non‐impregnated nor polymer‐impregnated cellulose fibres leave a residue at this temperature. Density measurements support this statement as the density increased with the increase of Ag content. TGA curves of both samples show that the main reduction in mass occurs at 300–400°C due to the decomposition of Ag‐Cel. From the TGA information (Fig. 6), it has been concluded that these samples had the same component but different contents. The weight loss between 300 and 800°C was attributed to the decomposition of the grafted polymer and cellulose fibre matrix. The residues remaining after pyrolysis were assumed to be a mixture of Ag‐NPs and cellulose fibre matrix. It might be due to the higher impregnating efficiency for Ag particles derived from the higher overlaying cellulose/Ag ratio.

Fig. 6.

TGA curves of the synthesised Ag‐NPs impregnated cellulose (Ag‐Cel) and citrus cellulose

3.3 Antioxidative and reducing power potential

DPPH is a stable and well‐characterised synthetic radical for evaluation of antioxidant potential. The DPPH reduces on accepting the hydrogen or electron and extent is observed by change in colour from purple to yellow. Ag‐NPs impregnated cellulosic paper (Ag‐Cel) showed 10.1% activity, slightly higher than cellulose powder (CP) and Ag‐NPs (Table 1) at the same concentration. Ag like other metals also acts as a good oxidant that can easily lose electrons. In recent studies, DPPH scavenging activity was found to be increased with increase in concentrations of NPs [37]. The minor activity by cellulose might be due to OH• group of glucose unit that on change in pH may release H⁺ ion. However, contrary to the rest of the chain, this polysaccharide has at its right extremity a free reducing aldehyde group [38]. Barua et al. [39] reported that copper (Cu) oxide NPs impregnated cellulose bears good DPPH activity; however, the activity is also based on concentration of Cu NPs bounded over cellulose. The free radical chain reaction is widely accepted as a common mechanism of lipid peroxidation. Radical scavengers may directly react and quench peroxide radicals to terminate the peroxidation chain reactions [40] which are important in the pathogenesis of various diseases. The present results suggest that the Ag‐Cel and Ag are moderate free radical scavengers and might have the ability to inhibit autoxidation of lipids; making them beneficial in the treatment of various diseases where lipid peroxidation is an important mechanism for pathogenesis [41].

Table 1.

DPPH free radical scavenging activities, TAC, TRP, and antibacterial activity of synthesised cellulose, Ag‐NPs and Ag‐Cel

				Antibacterial, mm
Sample	DPPH assay, %	TAC, µg AAE/mg	TRP, µg AAE/mg	E. coli	S. aureus
cellulose	3.51 ± 0.84c	11.33 ± 1.9c	4.30 ± 1.0bc	nil	nil
Ag‐NPs	6.29 ± 1.2b	15.30 ± 1.1b	5.51 ± 1.5b	13 ± 0.5b	14 ± 0.5b
Ag‐Cel	10.10 ± 1.3a	18.15 ± 1.3a	10.66 ± 1.9a	15 ± 1.0a	19 ± 0.5a

Small letters marked on values shows significant difference among means at probability 0.05% through LSD.

The sample paper (Ag‐Cel), CP and Ag‐NPs showed a wide range of TAC values. Ag‐Cel displayed maximum TAC with 18.15 µg Ascorbic Acid Equivalent (AAE)/mg significantly different from Ag and cellulose. The sample Ag‐Cel showed highest reducing power with 10.66 µg AAE/mg followed by Ag‐NPs and cellulose (Table 1). These results may be due to lesser reducing efficiency of particles as reported by Subramanian et al. [42] who proposed a positive correlation between Ag‐NPs to their antioxidant character. The electron donating capacity and reflection of reducing power are associated with antioxidant activity. The redox reaction takes place simultaneously by the antioxidant and reductants [40]. Antioxidant capacity of composites is correlated to their ability of defending a biological system against the potentially harmful effects of reactive oxygen and nitrogen species [43]. The antioxidant character is attributed to the different mechanisms such as prevention of chain initiation, reducing capacity, radical scavenging and decomposition of peroxides [44]. The results predict that Ag‐Cel has strong activity to scavenge free radicals. Though free radicals (oxidants) function as cellular messenger at micromolar level, however, excess of oxidants may deplete antioxidants in the wound that may also lead to tissue damage, extended inflammatory phase, less angiogenesis stage and decreased production of collagen and fibroblast [45, 46]. This antioxidant can also protect collagen and glycosaminoglycans from oxidation, which may speed the rate of wound closure [47].

3.4 Antibacterial activity

The antibacterial activity against human pathogens such as E. coli and S. aureus determined through disc diffusion method exhibited an inhibition zone 15 and 19 mm, respectively. Ag‐Nps showed less activity while cellulose paper did not show any activity (Table 1). Ag‐NPs act against both Gram‐negative and Gram‐positive species of bacteria [48]. Though the mechanism associated with microbial growth inhibition by Ag is not clear; however, it is postulated that Ag⁺ interact with thiol (–SH) containing proteins inside the cellular boundary or by surface molecules; inactivating the proteins or decreasing membrane permeability, respectively [49]. The difference in activities against Gram negative and positive strains might be associated with differential expression of molecular moieties at the surface of the respective groups [39].

Colony forming unit (CFU) method: Time kill assay was performed to measure Ag‐NPs and Ag‐Cel potential to kill S. aureus with time scale. In the presence of cellulose CFU increased with time. During initial 60 min, CFU value was at lag phase followed by log phase; steep increase in growth. However, application of Ag‐Cel NPs decreased the viable cell count that was also concentration dependent (Fig. 7; Table 2). As concentration of Ag‐Cel increased in the culture media, steady slope was observed in reduction of bacterial population. On the other hand, reduction in viable population of bacteria was less in the presence of Ag‐NPs. The more inhibition by Ag‐Cel might be due to thick attachment of NPs with cellulose and also the bacteria that come in contact with fibre or trap in fibre face high concentration of NPs; lethal for them.

Fig. 7.

(a) Time kill assay of cellulose, (b) Ag‐NPs impregnated cellulose, (c) Ag‐NPs

Table 2.

Per cent reduction of S. aureus within 150 min in the presence of cellulose, Ag‐NPs and Ag‐Cel

Sample	Concentration, mg	Reduction, %
cellulose	1	−86.41
	5	−158.79
	10	−219.01
Ag‐Cel	1	48.75
	5	92.31
	10	94.23
Ag‐NPs	1	30.82
	5	35.30
	10	48.75

3.5 Photocatalytic degradation of dye

Methylene‐blue degradation was visually observed by gradual change in the colour of the dye solution from deep blue to light blue with time. The degradation of dye in the presence of Ag‐Cel and Ag‐NPs was studied by measuring the peak intensity at 660 nm for 60 and 90 min of exposure for 5 and 10 ppm dye solutions, respectively, under sunlight (Fig. 8). Ag‐Cel showed degradation efficiency of 63.16% against 5 ppm dye solution while the Ag‐NPs exhibited only 44.12% degradation. Similarly, the 10 ppm dye solution slightly more degraded (48.05%) by the sample Ag‐Cel as compared with Ag‐NPs. It was found that the efficiency of dye degradation decreased with an increase in concentration of dye and time (Table 3). The possible reason of lesser dye degradation may be due to the intervention of by‐products formed during the degradation of mother dye molecules. At high dye concentrations, the active sites are covered with dye ions; subsequently, the production of •OHC and •OH₂ C radicals on the surface of catalyst get reduced [50, 51]; therefore, the greater amount of dye reduces the efficiency of catalytic reaction. As reported by Gupta; low concentrations of methylene blue and optimum pH 7 significantly enhance dye adsorption percentages [52, 53, 54, 55]. During exposure in sunlight, photons hit the nanomaterials present in the composite mixture, exciting the electrons at the particle surface [56, 57]. The dissolved oxygen molecules in the reacting medium accept the excited electrons to form hydroxide/oxide radicals. These radicals break the organic dye into simpler organic moieties [58, 59].

Fig. 8.

UV–vis absorbance spectra of methylene‐blue photo‐degraded by Ag‐Cel and Ag‐NPs under UV irradiation versus time

Table 3.

Per cent degradation of methylene blue by Ag‐NPs and Ag‐Cel

Time, min	10 ppm		5 ppm
	Ag‐NPs	Ag‐Cel	Ag‐NPs	Ag‐Cel
0	0.00	0	0	0.0
10	4.92	1.41	8.82	28.1
20	9.84	8.29	17.64	36.8
30	16.39	12.17	26.47	43.9
40	26.23	21.46	35.29	50.9
50	29.51	26.62	41.17	57.9
60	32.79	34.91	44.11	63.2
70	37.70	38.25
80	42.62	42.43
90	45.90	48.70

4 Conclusion

A paper‐shaped sample (Ag‐Cel) comprising of Ag‐NPs impregnated on the surface of isolated citrus cellulose from citrus waste is demonstrated. Their combination was highly beneficial due to the characteristics of both constituents and synergistic effects. The cellulose with well impregnated, dispersed and consistent NPs could be prepared under the optimised conditions of the reaction mixture. The preparative procedure (chemical reduction method) is surprisingly simple. It can provide a facile approach toward manufacturing of metallic nanocomposites, antimicrobial materials, low‐temperature catalysts and other useful materials. These outcomes have essential technological implications since they can open a new avenue to generate flexible catalytic mantles along with multifunctional fabrics. Thus, it could be evidently anticipated that the methods allowing chemical modifications of cellulose matrices and metal NPs are appeared to be a very promising field of research to develop new functional materials because it is less exploited strategy to formulate multifunctional nanocomposites. Finally, the health and environmental impacts of such nanocomposites may be an issue in the schema of the scientific community; however, their significance will be augmented because of the commercialisation of products based on these materials.