Fabrication of a versatile chitosan nanocomposite hydrogel impregnated with biosynthesized silver nanoparticles using Sapindus mukorossi: characterization and applications

Abstract

A biological and eco-friendly method has been adopted for the synthesis of silver nanoparticles. Biosynthesized silver nanoparticles (BSN) were obtained when AgNO₃ was mixed with Sapindus mukorossi extract (SME) and subjected to microwave irradiation. The SME not only reduced the Ag⁺ to Ag⁰, but also stabilized the silver nanoparticles (AgNPs). The biosynthesis of the silver nanoparticles with a particle size between 35–45 nm has been confirmed by UV-Vis spectroscopy, XRD analysis and HRTEM images with EDS showing the presence of elemental silver at 3 keV. Optimization of the parameters was carried out to get higher amounts of the BSN. For eco-friendly utilization, the BSN was impregnated into chitosan and the resulting nanocomposite hydrogel films (BSNC) were cast by physically blending varying concentrations of the BSN stabilized by the SME and fixed amounts of 2% w/v chitosan solution, sodium bicarbonate as the porogen and glutaraldehyde as the crosslinker. The BSNC-15 nanocomposite hydrogel film which exhibited good water swelling properties was characterized for chemical composition by FTIR spectroscopy; thermal behavior by thermogravimetric analysis and differential thermogravimetric curves; the surface morphology by AFM and SEM analysis. The versatility of the BSNC film has been assessed for its adsorbing and reducing nature towards Cr(IV), photocatalytic nature which allows it to degrade methyl orange dye, and its antibacterial activity against Staphylococcus aureus and Escherichia coli. Thus cost effective, easily processed BSNC nanocomposite films can be used to degrade Cr(IV) and methyl orange dye from the effluents and in the pharmaceutical field as a wound dressing material owing to its hydrogel and antibacterial nature.

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Introduction

Natural polymers like cellulose, chitin, chitosan, rubber, starch, collagen, gelatin, fibrin etc. are being used to prepare biomaterials.¹ Out of the above mentioned polymers it is the cellulose based polymers that are abundant in nature and can be utilized to make the biomaterials of our choice.

Chitosan is a cellulose based biopolymer found in crustaceans, a very cheap and easily available natural resource. Innumerable biomaterials especially for therapeutic applications have been prepared using chitosan,² owing to its biodegradable, biocompatible and antimicrobial activity.³ Recently, a focus on using them for other applications like as catalysts for synthesis,⁴ and biosensors,⁵ and in electronic appliances⁶ and electrical devices⁷ etc. has been increasing. Chitosan is used as a heterogeneous catalyst^8,9 for synthesizing many chemical compounds, especially organic compounds, because of its insolubility in most solvents and because it can be recycled after use. In water based reactions its hydrogel properties are an additional feature that enhance its efficiency.¹⁰

Recently, nanometal-impregnated chitosan hydrogels have been studied for their catalytic activity,¹¹ as swollen hydrogels provide a large free space between crosslinked networks that can act as matrices or protecting agents against the aggregation of nanoparticles, and as nanoreactors for the nucleation and growth of nanoparticles.¹²

Among metals, noble metals like gold and silver are preferred and a lot of work is being done using them. Despite being cost effective there are some limitations associated with the use of silver nanoparticles. Silver nanoparticles in the free state with a size less than 50 nm can cause toxic effects on human health and the environment. The silver nanoparticles have an inherent ability to agglomerate in solution with time, due to the high active surface area. The above mentioned limitations can be overcome if silver nanoparticle containing composites are developed.¹³ Nanocomposites of polymer/silver nanoparticles have attracted great attention because of their potential applications in the fields of catalysis, bioengineering, photonics and electronics.¹⁴ The best way to synthesise metal nanoparticles is through biosynthesis. This has been done using naturally available materials like extracts of leaves, barks, and seeds etc. which are rich in steroids, terpenoids and alkaloids. The biosynthesis process is purely natural and the extract used for synthesis acts as a capping, and stabilizing agent.¹⁵

The Sapindus mukorossi plant is said to possess many pharmacological properties and some of them have been reported including antifungal,¹⁶ antimicrobial,¹⁷ hepatoprotective,¹⁸ spermicidal¹⁹ and anticancer activity.²⁰ An extract of the Sapindus mukorossi (soapnut shells) fruit pericarp, mainly composed of saponins (natural surfactants) and flavonoids,²¹ is utilized for the biosynthesis of noble metals. The biosynthesis of gold and silver nanoparticles assisted by Sapindus mukorossi and their catalytic applications have been studied and it has been reported that the aqueous extract of Sapindus mukorossi has reduced Ag⁺ ions of AgNO₃ into Ag⁰ nanoparticles, hence the extract acts as a capping, reducing as well as stabilizing agent.²²

In the present study the silver nanoparticles have been prepared through a biosynthesis method, employing microwave irradiation using a domestic microwave oven, for which the extract of Sapindus mukorossi is being utilized. The biostabilized silver nanoparticles are mixed with 2% chitosan solution in 0.5 M acetic acid, 0.1 mL of 25% glutaraldehyde as the crosslinker, and 0.5 M sodium bicarbonate which acts as a porogen. Thus biosynthesized silver nanoparticle-impregnated chitosan (BSNC) hydrogels with varying silver concentration are cast into films. The films are characterized for their water absorption capacity, chemical composition, thermal behaviour and surface morphology. The versatility of the BSNC hydrogel film is proven by evaluating its capacity to remove hexavalent chromium present in aqueous solution, degrade methyl orange dye through photocatalytic activity, and inhibit the growth of bacteria through antimicrobial activity against Staphylococcus aureus and Escherichia coli.

Experimental

Reagents and instruments

Sapindus mukorossi (soap nut) was purchased from grocery stores in Chennai, India. Chitosan (MMW) was purchased from Aldrich (CAS 44-8869) with a deacetylation percentage of 75–85%, and with a Brookfield viscosity of 20 cps, and used as received. Acetic acid (glacial, 99–100%), silver nitrate (AgNO₃), glutaraldehyde 25% (GA), sodium bicarbonate, potassium dichromate and methyl orange dye were purchased from Merck (India) and used without further purification. Double-distilled water was used for the preparation of all solutions throughout the study. A UV-visible spectrophotometer (Cary 60, Agilent) was used to measure absorbance in the range of 200–800 nm. The attenuated total reflection (ATR) infrared spectra of the BSNC hydrogel films were taken with an FTIR (Bruker) instrument. The thermal properties of the hydrogels were determined with the instrument TGA Q500 V20.10 Build 36 and the analysis was carried out under a N₂ atmosphere. The surface morphological images were taken using a Tescan Vega3 SEM instrument under high vacuum and atomic force microscopy (AFM) analyses (non-contact mode) using an XE-100 scanning probe microscope (Park systems, South Korea). The elemental composition was obtained by EDX using X-Flash Detector 410 M with Bruker ESPRIT QUANTAX EDS analyzing software. XRD analysis to find the structure of the AgNps was performed using an Ultima III Rigaku X-ray diffractometer with a scanning rate of 0.02° min⁻¹. Samples were scanned in the range 10° < 2θ < 80° at a wavelength of 1.540 Å.

Preparation of Sapindus mukorossi extract (SME)

The pericarp of Sapindus mukorossi was dried under sunlight and ground into a fine powder of 40 mesh size using the laboratory mill. 20% w/v of an aqueous solution of the Sapindus mukorossi was prepared by stirring overnight. The extract was filtered through Whatman no. 1 filter paper to remove all of the unextractable matter. The solution obtained was finally centrifuged at about 10 000 rpm to get the clear extract. The percentage of the water soluble matter in the extract was estimated by gravimetric methods. The pH of the SME solution remained the same before and after the extraction (pH = 7.2) and it was found to contain 8% solid. It was stored at 5 °C before use and used without any further purification.

Biosynthesis of silver nanoparticles (BSN)

The silver nanoparticles were synthesized by mixing 8% Sapindus mukorossi extract (SME) with silver nitrate solution and subjecting the resulting mixture to microwave radiation. A household microwave oven (manufacturer Kenstar) was used for this purpose. A microwave-irradiation power of 120 W was employed for a given period of time. After cooling to room temperature, the resulting solution was centrifuged at about 10 000 rpm. The supernatant solution containing the BSN stabilized by the SME was confirmed by UV-Vis spectroscopy. The reaction parameters were varied as follows: 8% w/v aqueous SME volume, 1–3 mL; time of microwave exposure, 15–60 s and concentration of aqueous AgNO₃ solution, 5–35 mM. The pH of the reaction remained neutral throughout. In each experiment one of the parameters was varied and the others were maintained at a constant level; the total reaction volume was made up to 25 mL with double distilled water. Fig. 1 gives the BSN suspension (photochromic in nature) after being prepared by varying the microwave exposure time. Table 1 gives the various preparations of the BSN suspension by varying the AgNO₃ concentration while maintaining a constant SME volume and microwave exposure time.

Fig. 1 BSN suspensions prepared by varying the microwave exposure time from 15–60 s, while keeping the other parameters constant with an SME volume of 2mL, AgNO₃ solution at a concentration of 5 mM and total reaction volume made up to 25 mL (with double distilled water).

Table 1 Preparation of the BSN suspension by varying the AgNO₃ concentration while maintaining a constant SME volume and microwave exposure time

Sl. no.	BSN suspension (25 mL)	Concentration of AgNO3 solution in mM	Volume of SME (mL)	Microwave exposure time in s
1	BSN0	0	2.5	45
2	BSN5	5	2.5	45
3	BSN10	10	2.5	45
4	BSN15	15	2.5	45
5	BSN20	20	2.5	45
6	BSN25	25	2.5	45
7	BSN30	30	2.5	45
8	BSN35	35	2.5	45

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Preparation of BSNC hydrogel films

A homogeneous solution of 2% w/v of chitosan in 0.5 M aqueous acetic acid was prepared by stirring the solution at 65 °C for 16 h. The BSNC hydrogels were prepared by stirring 15 mL of chitosan solution, 0.1 mL of 25% glutaraldehyde, 1 mL of 0.5 M sodium bicarbonate and varying the amount of BSN solution between 0–25 mM. The contents were mixed, cast into films and were dried using a vacuum desiccator. The photographs of the BSNC films are shown in Fig. 2. Scheme 1 shows the BSNC hydrogel films prepared by varying the BSN volume. Table 2 gives data on the preparation of the various BSNC hydrogels.

Fig. 2 The photographs of (a) BSNC-0 and (b) BSNC-15.

Scheme 1 Preparation of the BSNC hydrogel film.

Table 2 Preparation of the BSNC hydrogel films

Sl. no.	BSNC films	BSN	Volume of BSN (mL)	Volume of 2% chitosan (mL)	Volume of 25% glutaraldehyde (mL)
1	BSNC-0	BSN0	5	15	0.1
2	BSNC-5	BSN5	5	15	0.1
3	BSNC-10	BSN10	5	15	0.1
4	BSNC-15	BSN15	5	15	0.1
5	BSNC-20	BSN20	5	15	0.1
6	BSNC-25	BSN25	5	15	0.1

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Equilibrium water content of BSNC hydrogels

The equilibrium water content (EWC) of the BSNC hydrogels was measured using the weight difference between the swollen hydrogel and the dehydrated polymer as described previously.²³

This is expressed as

Cr(IV) adsorption studies

An aqueous solution of 50 ppm of Cr(IV) was prepared using K₂Cr₂O₇. The hydrogel films with and without BSNs with 1 cm × 1 cm dimensions were dropped into chromium solutions separately in two containers. It was occasionally shaken and after a time interval, the amount of chromium left unadsorbed was analyzed using UV-Vis spectroscopy.

Photocatalytic degradation

The photocatalytic degradation of methyl orange (MO) dye by BSNC was evaluated. All the experiments were performed outdoors with sunlight as the main source of light. To a 5 × 10⁻⁴ M methyl orange solution 1 cm × 1 cm of the BSNC hydrogel film was added with constant stirring for about 30 minutes in the dark to maintain equilibrium. During the experiment, the entire set up was kept under sunlight within a Pyrex glass beaker and stirred constantly. The mean temperature was found to be 30 °C with 10 hours shine duration. An aliquot of the MO solution was taken periodically to measure the absorption spectrum using a UV-visible spectrophotometer to determine the degradation of the same.²⁴

Antibacterial activity

Well diffusion assay²⁵. Nutrient agar was prepared and poured into the sterile Petri dishes and allowed to solidify. 24 h growing bacterial cultures (Staphylococcus aureus and Escherichia coli) were swabbed onto it. Then, the test samples were placed onto the nutrient agar plate using sterile forceps. Chloramphenicol was used as standard. The plates were then incubated at 37 °C for 24 h. After incubation the inhibition diameter was measured.

Results and discussion

The biological method provides a wide range of resources for the synthesis of Ag-NPs, and this method can be considered as an environmentally friendly approach and also a cost effective technique. The rate of reduction of the metal ions using biological agents is found to be much faster under ambient temperature and pressure conditions. Currently many researchers are focusing their research on biogenic silver nanoparticle-impregnated chitosan composites.²⁶ In our present study the effect of Sapindus mukorossi, the reducing agent, which is also a biosurfactant,²⁷ on the biosynthesized silver nanoparticle-impregnated chitosan hydrogels has been analyzed.

Effect of variables on synthesis of biosynthesised silver nanoparticles (BSN)

Optimization of the parameters to get a higher concentration of Biosynthesised Silver Nanoparticles (BSN) was carried out by measuring the surface plasmon resonance (SPR) of AgNPs,²⁸ which gives a characteristic band at ∼450 nm in the UV-visible spectrum because the concentration of the BSN can be expressed in terms of absorbance, as absorbance is directly related to the amount of BSN present in the system. The optimum conditions determined to get higher amounts of the BSN are given as follows: 45 s of microwave exposure, 2.5 mL of the SME and 20 mM of AgNO₃.

The effect of microwave exposure time from 15–60 s on the concentration of the BSN in terms of absorbance is given in Fig. 3a. When the microwave exposure time is varied, keeping all other parameters constant, the concentration of the BSN obtained is at a maximum for 45 s exposure. The yield slowly increased from 15 s (60 °C), reached a maximum for 45 s (70 °C) and then decreased for 60 s (85 °C). When the microwave exposure time increases, the temperature associated with it also increases. Hence when the temperature increases, natural substances like SMEs may lose their reducing capability due to the thermolabile nature of saponin,²⁹ the major constituent of the SME.

Fig. 3 (a) Effect of varying microwave exposure time on the concentration of the BSN. (b) Effect of varying the SME volume on the BSN yield. (c) Effect of varying AgNO₃ concentration on the BSN yield. (d) The XRD pattern of biosynthesized silver nanoparticles (BSN). (e) HRTEM image of the BSN with 50 nm magnification. (f) HRTEM of a single BSN particle. (g) EDX of the BSN. (h) The average particle size of the BSN.

The effect of varying the SME volume from 1–3 mL on the concentration of the BSN in terms of absorbance is given in Fig. 3b. The concentration of the BSN increases up until 2.5 mL of SME had been added, which can be inferred from the intensity of the absorbance peak. But the absorbance decreases when 3 mL of the SME is added indicating a lower concentration of the BSN. The fact that, with an increase of capping agent, especially if it is a surfactant, the rate of synthesis of the AgNPs decreases with retention of the morphology has been proved.³⁰

The effect of varying precursor AgNO₃ concentration from 5–35 mM on the concentration of the BSN in terms of absorbance is given in Fig. 3c. The peak intensity increases linearly with an increase in the concentration of AgNO₃ from 5 to 20 mM, and then starts decreasing from 25–35 mM. An aqueous extract of soapnut shell has a high content of saponins and flavonoids. The hydroxyl groups in these biocompounds could participate in silver bioreduction.³¹ The bioreduction of silver(I) ions to silver(0) nanoparticles might be occurring through oxidation of the hydroxyl to the carbonyl group of saponin, the major component of the SME, as represented in Scheme 2. But for higher concentrations of AgNO₃, there is a decrease in absorbance, which may be due to agglomeration of the AgNPs.³²

Scheme 2 Synthesis of the AgNP by saponin of the SME.

Characterization

Biosynthesised silver nanoparticles

The XRD pattern of the BSN is given in Fig. 3d. Peaks were obtained at 2θ values of 38°, 43.9°, 64.9° and 77°, which are in perfect agreement to those of the JCPDS card no. 89–3722.³³ The X-ray diffraction results clearly show that the BSN formed by the reduction of Ag⁺ ions by the Sapindus mukorossi extract are crystalline with fcc structure.

The HRTEM images of varying magnification of the BSN are given in Fig. 3. Fig. 3e shows that the synthesized BSNs are well dispersed and are mostly spherical structures with decahedron and icosahedron shapes. Fig. 3f gives a single nanoparticle with a decahedron shape. The EDX spectrum of the BSN is given in Fig. 3g. The peak corresponding to silver appears exactly at 3 keV.³⁴ The particle size distribution of the BSN was calculated using image analysis software. The average particle size of the BSN was 35–45 nm which is obvious from Fig. 3h.

BSNC hydrogel films

Swelling behaviour of BSNC hydrogel films. The swelling capacity of a hydrogel is important in deciding on the efficacy of the material for various applications. Most of the aqueous based reactions taking place inside the hydrogel depend on its swelling behavior. In hydrogels, water acts as a plasticizer and a transport medium within the polymer matrix.³⁵ Hence, the EWC is the most important property of any hydrogel to be studied first.

The % equilibrium water content of various BSNC hydrogel films that were prepared are given in Fig. 4. The film with 0 mM has no biosynthesized silver nanoparticles and exhibited a lower swelling capacity whereas the other BSNC hydrogel films exhibited higher swelling capacities due to the presence of the BSN, which when present will increase the overall porosity of the film and hence the water imbibing capacity will also increase.³⁶ On comparing the hydrogels, it is the hydrogel that has 15 mM of the BSNC that exhibits the highest swelling capacity. On further increasing the concentration of the BSN, the swelling capacity decreases. This lowering in the swelling capacity is attributed to the binding of AgNPs with the electrons of “O” and “N” atoms of the hydroxyl and amine groups present in chitosan.³⁷ This produces additional crosslinks within the chain network. Higher crosslinking within the films restricts the penetration of water for swelling.³⁸

Fig. 4 Water absorbing capacity of the various BSNC hydrogels.

FTIR analysis

FTIR measurements were carried out to identify possible interactions between the silver nanoparticles, chitosan, SME and crosslinker glutaraldehyde molecules. The FTIR spectrum of pristine chitosan is shown in Fig. 5a. It shows a broad absorption band in the range of 3000 to 3500 cm⁻¹, which are the overlapped stretching frequencies of –OH and –NH. The stretching frequency of a methylene group is observed at 2900–2880 cm⁻¹. The characteristic amide I peak of –C=O (N-acetyl group) and the amide II peak of –NH₂ bending (after deacetylation) are observed at 1634 cm⁻¹ and 1548 cm⁻¹ respectively. The peaks at 1406 cm⁻¹ and at 1064 cm⁻¹ correspond to the –CH₂OH and –C–O–C– groups of chitosan respectively.³⁹

Fig. 5 The FTIR spectrum of (a) pristine chitosan, (b) pristine Sapindus mukorossi, (c) glutaraldehyde crosslinked chitosan–SME matrix and (d) BSNC-15 hydrogel film.

The FTIR spectrum of pristine Sapindus mukorossi is given in Fig. 5b. The broad peak at 3334 cm⁻¹ is due to the –OH groups and the one at 2931 cm⁻¹ is that of the –CH group. The presence of a carbonyl group is evident from the peak at 1724 cm⁻¹ whereas the adjacent peak at 1610 cm⁻¹ is due to a –C=C– group. The sugar and non-sugar components are linked through an ether linkage.⁴⁰

The reaction, when glutaraldehyde is added to chitosan and SME, can be understood from the FTIR spectrum given in Fig. 5c. The interaction between chitosan and glutaraldehyde, while crosslinking to give a Schiff base with imine (–C=N) formation, can be proven by a reduction in the intensity as well as the shifting of the amide II band of chitosan from 1548 to 1563 cm⁻¹ and a similar shifting of the amide I peak from 1634 to 1642 cm⁻¹. There are no major changes in the SME bands except for the hydrogen bonding between saponin and chitosan.⁴¹

The BSNC-15 hydrogel containing the BSN when analysed revealed a change in the relative intensities of the various bands, which could be observed in Fig. 5d. The intensity ratio I₁₅₅₀/I₁₆₄₄ depends on the molecular weight of the polymer, but it is always lower for silver loaded samples and decreases with silver loading.⁴² Similar results have been obtained in Fig. 5d, where silver nanoparticles have decreased the intensity ratio of I₁₅₆₃/I₁₆₄₂ corresponding to the amide and also the imine formed during crosslinking with glutaraldehyde. According to Wei and Qian⁴³ the attachment of silver to the nitrogen atoms in chitosan–silver samples would reduce the vibrational intensity of the N–H bonds due to an increase in the molecular weight of the complex which becomes heavier after Ag bonding. The decrease in the intensity of the band, related to the –NH groups in the primary amines of the BSNC-15 hydrogel, would also support the hypothesis that silver, even as a metallic nanoparticle, is interacting with nitrogen. Support for this idea is shown as the intensity of the –CH₂OH peak of chitosan at 1405 cm⁻¹ has decreased and it is proportional to the increase in adsorbed Ag nanoparticles.⁴⁴ The presence of silver not only reduced the intensity, it has also shifted the –OH and –NH stretching towards a lower frequency from 3284 cm⁻¹ to 3268 cm⁻¹ indicating a weakening of the intermolecular hydrogen bonds due to the presence of the silver nanoparticles.⁴⁵

Thermal analysis of BSNC hydrogel films

The TGA of BSNC-0 and BSNC-15 was carried out to study the thermal behaviors of the hydrogels with and without silver nanoparticles. The BSNC-0 hydrogel without silver nanoparticles exhibits lower thermal stability when compared with the BSNC-15 hydrogel with silver nanoparticles. The first stage degradation between 30 to 150 °C is due to water evaporation and between 150 to 400 °C is due to the degradation of chitosan in both BSNC-0 and BSNC-15. 50% degradation was observed at 360 °C for BSNC-0 whereas it was 410 °C for BSNC-15. So, this shows that silver nanoparticles increase the thermal stability of the films.⁴² The same property can be inferred by comparing the DTG of BSNC-0 and BSNC-15. The degradation of chitosan is at a maximum at 270 °C for BSNC-0 whereas it is around 285 °C for BSNC-15. There is a marked difference in the degradation temperature for the chitosan present in the hydrogels with and without silver nanoparticles. Fig. 6a and b represents the TGA and DTG of BSNC-0 and BSNC-15 respectively. The broad peak observed at 590 °C in the DTG of BSNC-15 indicates the presence of silver nanoparticles in the hydrogel.⁴⁶ The total weight loss of BSNC-0 at 600 °C was found to be 42 wt%, while that of BSNC-15 was 38 wt% indicating that the weight difference is due to the presence of silver.

Fig. 6 (a) TGA and DTG of the BSNC-0 hydrogel and (b) TGA and DTG of the BSNC-15 hydrogel.

AFM and SEM of BSNC hydrogel films

Atomic force microscopy (AFM) has been applied to determine the morphology of nanohybrids and is considered as a powerful tool for imaging the topography of the surfaces because of its high spatial and vertical resolution. The three dimensional morphology of the BSNC-0 (without AgNps) and BSNC-15 hydrogel films with Ag nanoparticles embedded into the chitosan–SME matrix is shown in Fig. 7a and b respectively. A uniform distribution of the silver nanoparticles can be observed from Fig. 7b. The average size distribution of the particles is in the range of 40-50 nm. The SEM image of BSNC-15, Fig. 7c, gives a clear morphological image with the BSN embedded throughout the film. The porous nature of the film is clearly visible when compared with that of BSNC-0 (Fig. 7b), with no silver particles. The EDX spectrum of the BSNC (inset of Fig. 7b and c) proves the presence of silver nanoparticles embedded into the film.

Fig. 7 AFM images of (a) BSNC-0 (b) BSNC-15 and SEM images of (c) BSNC-0 (d) BSNC-15 (EDX are shown as insets).

Cr(IV) adsorption studies

Physical examination showed that both the BSNC-0 and BSNC-15 hydrogel films, after being immersed into potassium dichromate solution, turned dark brown in colour, leaving the solution colourless (Fig. 8a and b). This shows that the films have adsorbed the total amount of chromium from the solution. After complete adsorption, the pH of the solution, along with the film, was made acidic. Now interestingly the colour of the BSNC-15 film turned green within 15 minutes (Fig. 8a), while that of BSNC-0 took one hour for the colour to change (Fig. 8b). Adsorption of chromium onto both the BSNC hydrogel films is enhanced due to the hydrophilic nature of chitosan and amphiphilic nature of SME, the capping agent, by increasing the mobility of water within the system. When acid is added the Cr(IV) adsorbed on the film gets reduced to Cr(III). The rate of the reduction increases in the case of BSNC-15, due to the synergistic action of both the chitosan and silver nanoparticles. It has been proved that chitosan, after chromium adsorption upon acidification, reduced Cr(IV) to Cr(III).⁴⁷ Similarly Ag nanoparticles along with capping agents can reduce Cr(IV) to Cr(III) and this property has been used to design a probe for the rapid detection of Cr(IV).⁴⁸ Fig. 8c gives the SEM images of the Cr(IV) adsorbed onto the BSNC-15 hydrogel film. The chromium deposition can be very clearly seen with salt like depositions, which are further confirmed by the EDX spectrum giving Cr at 43.83 wt%, with the rest being silicon, present in the grid.

Fig. 8 (a) Acidified BSNC-0 film after 15 min, (b) acidified BSNC-15 film after 15 min and (c) SEM image of Cr(IV) adsorbed onto BSNC-15.

Photocatalytic degradation of methyl orange dye

The characteristic absorption peak of the methyl orange solution is around 450 nm. The photographs showing MO (control) and MO degradation by BSNC-15 after 30 min are given in Fig. 9a and b. Fig. 9c gives the absorbance of methyl orange measured after regular intervals of time under the influence of the BSNC-15 hydrogel film under sunlight. Degradation is complete within 30 minutes, which is clear from the peak and the colour change of the film from yellow to pinkish orange. It has been reported that solar light was found to be faster in decolorizing methyl orange in the presence of a metal catalyst compared to other irradiation techniques.⁴⁹ Degradation of MO takes place due to the excitation of the SPR of the silver nanoparticles upon exposure to sunlight which is nothing but an oscillation of the charge density that can propagate at the interface between the metal and dielectric medium.⁵⁰ The rate of degradation is higher and it can be attributed to the hydrogel nature of the BSNC composite, enabling water mobility and thereby enhancing the degradation.

Fig. 9 Photographs showing (a) MO (control), (b) MO degradation by BSNC-15 after 30 min and (c) UV-visible spectra showing the degradation of MO with time.

Antibacterial activity of BSNC hydrogels

The zone of inhibition was evaluated experimentally to compare the antibacterial activity of chitosan, BSNC-0 and BSNC-15, using chloramphenicol as standard, which is represented graphically in Fig. 10a and b. Two bacterial strains Staphylococcus aureus (Gram positive) and Escherichia coli (Gram negative) were used for this in vitro evaluation. The antibacterial property of chitosan is being used in innumerable therapeutic products.⁵¹ Similarly the potential of chitosan-saponin (CS-SP) nanoparticles for biological applications as DNA delivery systems has been reported.⁵² Sapindus mukorossi, a biosurfactant, amphiphilic in nature, exhibits antimicrobial properties and has been used as an adjunct in the medical field.⁵³ The third component of the composite is biosynthesized silver nanoparticles whose antibacterial property is being explored in many ways.⁵⁴ The BSNC hydrogel composite, having all the above mentioned components being combined in a proper manner, i.e., uniformly dispersed BSN particles bound physically to a chitosan–SME matrix, will be a better choice for an antibacterial material. The photographs of the zones of inhibition are given in Fig. 10c and d. BSNC-15 has a well-defined boundary with a largest ZOI of 20 mm, against S. aureus (Fig. 10c) and 25 mm against E. coli (Fig. 10d). The ZOI of BSNC-0 is also larger when compared to chitosan for both of the bacterial species; this proves the antibacterial nature of Sapindus mukorossi. These BSNC hydrogels can be an excellent wound dressing material with such antibacterial activity and can also be used as a drug carrier.

Fig. 10 The graphical representation of the antibacterial activity of BSNC-15 against (a) Staphylococcus aureus and (b) Escherichia coli. The photographs showing the zone of inhibition of BSNC-15 against (c) Staphylococcus aureus and (d) Escherichia coli.

Conclusion

The Sapindus mukorossi extract was used to prepare biogenic silver nanoparticles under microwave irradiation. The reaction parameters that would allow a maximum yield of the BSN were optimized. The synthesis of spherical BSNs with an fcc structure in the range of 35–45 nm was confirmed by UV-Vis spectrophotometry, HRTEM images and XRD analysis. The BSNC films with varying concentrations of the BSN exhibited water swelling capacities, with BSNC-15 exhibiting higher values. The addition of the BSN improved the thermal properties of the chitosan film and the morphology of the film was studied using SEM and AFM images. The prepared BSNC hydrogel is an eco-friendly biomaterial which exhibited a Cr(IV) adsorbing and reducing nature, can degrade methyl orange through photocatalysis and can also inhibit the growth of bacteria through its antibacterial properties. Thus cost effective BSNC hydrogels has been proved to be a versatile nanocomposite which can be recycled and will be a value added material even after usage.

Acknowledgements

The authors would like to thank the DST (Department of Science and Technology, India) for the financial support that was extended to carry out this study.

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