Antibacterial nanocomposite hydrogels for superior biomedical applications: a Facile eco-friendly approach

Abstract

In this scientific paper, we report a facile and eco-friendly fabrication of antibacterial nanocomposite hydrogels of Au-core Ag-shell nanoparticles, embedded within Carbopol® 980 NF/Noveon® AA-1 polycarbophil acrylic acid polymeric matrix. The aim of the study was to investigate whether these nanocomposite hydrogels have the potential to be used for bacterial inactivation applications. The key feature was that, unlike the use of chemical reductants, auxiliary stabilizers and specialized expensive equipment, the Au-core Ag-shell nanoparticles (∼15 ± 3 nm) were synthesized utilizing aqueous mint leaf extracts. The developed hydrogels were characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy/energy-dispersive spectroscopy and thermogravimetric analysis. Swelling studies were performed in phosphate buffered saline (pH 7.4) solution. A sustained antibacterial study against E. coli (G−) and B. subtilis (G+) showed their excellent antibacterial efficiency, which suggested that the developed hydrogels are potential candidates for a wide range of biomedical applications.

---

Introduction

The rapid growth of materials science has provided many types of functional materials for biomedical applications. One such type of functional material is hydrogel.^1–6 Hydrogels are three-dimensional (3D) hydrophilic polymeric networks, which are capable of imbibing large amounts of water or biological fluids,^7,8 resemble natural living biological tissue more than any other class of synthetic biomaterials, due to their high water content, and can provide a better feeling on the skin in comparison to conventional ointments and patches.⁹ These well-defined characteristic properties have prompted researchers to utilize hydrogels for various biomedical applications, which include production of wound-dressing materials, transdermal systems, drug delivery carriers, sanitary pads, disposable diapers, dental materials, implants, injectable polymeric systems, ophthalmic applications and hybrid-type organs (encapsulated living cells).^10–14

In general, hydrogels are synthesized from raw materials of either natural or synthetic origin. Over the last two decades, significant modifications have revolutionized the comprehensive application of hydrogels in almost all fields. These have gradually replaced ‘natural-based hydrogels’ by ‘synthetic-based hydrogels’.¹⁵ The present research work is one such modification, aiming to fabricate antibacterial hydrogels from commercially available acrylic acid polymers: Carbopol® 980 NF (acrylic acid polymers crosslinked with allyl ethers of pentaerythritol) and Noveon® AA-1 polycarbophil (acrylic acid polymer crosslinked with divinyl glycol).^16,17 The available literature pertaining to these materials is very small. Among the available literature Carbopol® 980 and Noveon® AA-1 polycarbophil have been known to be used mainly for drug delivery applications. Hence, through this article, nanocomposites of Carbopol® 980 and Noveon® AA-1 polycarbophil hydrogel are being brought into light as effective antibacterial hydrogels. These materials were particularly chosen because of their great relevance in pharmaceutical and biomedical applications, in the formulation of buccal, vaginal, nasal, ophthalmic and rectal bioadhesive products.^16,18,19

Recently, metal nanocomposite hydrogels for antibacterial applications have been developed by impregnation of nanoparticles of either monometallic or bimetallic (as an alloy) type.^6,20–23 However, in the present investigation bimetallic Au-core Ag-shell nanoparticles have been impregnated. Gold and silver were specifically chosen because of their attracting renewed attention for combating the threat of bacterial infection. Their additional advantage is that, unlike antibiotics, metal nanoparticles (either Ag or Au) do not act via cell receptors to kill microorganisms.²⁴ So an immune response in microbes, to develop resistance against these nanoparticles, is not possible. Hence, the problem of disease transmission/contamination through various microorganisms could be largely eradicated without causing any resistance in microorganisms.²⁵

In general, bimetallic core–shell nanoparticles have been synthesized through seed-mediated growth, template synthesis, chemical reduction and laser ablation. These methods utilize not only chemical reductants but also auxiliary stabilizers, often involve specialized expensive equipment and are even hazardous to the environment.²⁶ Hence, in the present investigation, extracts of naturally available mint leaves were utilized for synthesizing core–shell nanoparticles through an eco-friendly process.^27–29 This is the most appropriate and cost-effective method, which utilizes ambient conditions for nucleation of core–shell nanoparticles.

Overall, the present scientific research work involved the fabrication of Au-core Ag-shell nanocomposite hydrogels, separately, from commercially available Carbopol® 980 NF and Noveon® AA-1 polycarbophil. The core–shell nanoparticles were synthesized from extracts of mint leaves by an eco-friendly process. The developed nanocomposite hydrogels showed excellent swelling patterns in phosphate buffered saline (PBS) (pH 7.4) solution. Antibacterial studies confirmed their excellent antibacterial efficiency. Overall, the results indicated that the novel nanocomposite hydrogels, developed by an eco-friendly process, can be utilized for superior biomedical applications. The experimental findings pertaining to these inorganic Au-core Ag-shell nanocomposite hydrogels are presented here.

Experimental

Materials

Carbopol® 980 NF (CP) and Noveon® AA-1 polycarbophil (NP) were obtained as gift samples from Lubrizol Advanced Materials, Europe. Acrylamide (AAm), N,N′-methylenebisacrylamide (MBA), ammonium persulfate (APS), gold chloride (HAuCl₄·xH₂O) and silver nitrate (AgNO₃) were purchased from S.D. Fine-Chem Ltd, Mumbai, India. All chemicals were used without further purification. Double-distilled water was used throughout the experimentation.

Preparation of the mint leaf extract

Mint leaf extract was prepared by following the standard procedure, similar to the methods described in our earlier study.^1,2 In brief, fresh mint leaves were collected and thoroughly washed with double-distilled water. Mint leaf broth was prepared by placing 2.5 g thoroughly washed and finely cut mint leaves in a 500 mL Erlenmeyer flask with 100 mL sterile double-distilled water. The contents of the leaves were extracted by heating the solution at 100 °C for 2 min, cooling to room temperature and filtering through a 0.45 μm PVDF Millex filter using a 50 mL syringe. The extracted solution was preserved at 4 °C and utilized for nucleation of Au³⁺ and Ag⁺ ions.

Fabrication of hydrogels

A set of Carbopol® 980 NF hydrogels (P(CP–AAm)_x, x = 1–3) and Noveon® AA-1 polycarbophil hydrogels (P(NP–AAm)_x, x = 1–3) were separately fabricated by dissolving AAm (14.06 mM) and various ratios (0.05–0.15 g) of acrylic acid polymers (CP/NP) in 3 mL distilled water, stirring at 300 rpm for 2 h at 25 °C. To this aqueous solution, MBA (0.648 mM) and APS (2.191 mM) were added and the temperature was raised to 50 °C for 15 min to initiate a free-radical polymerization reaction. The reaction was maintained at ambient conditions for 4 h. During the reaction period, gelation occurred, leading to the formation of hydrogels. The obtained hydrogels were immersed in distilled water at room temperature for 24 h to remove unreacted materials present in the hydrogel network. Finally, various formulations of CP and NP hydrogels were dried out at ambient room temperature for 48 h. The feed compositions of the various formulated CP and NP hydrogels are presented in Table 1.

Table 1 Feed compositions of various formulated CP and NP hydrogels

Hydrogel code	AAm (mM)	CP (g)	NP (g)	MBA (mM)	APS (mM)
P(AAm)	14.06	0.0	0.0	0.648	2.191
P(CP–AAm)1	14.06	0.05	0.0	0.648	2.191
P(CP–AAm)2	14.06	0.10	0.0	0.648	2.191
P(CP–AAm)3	14.06	0.15	0.0	0.648	2.191
P(NP–AAm)1	14.06	0.00	0.05	0.648	2.191
P(NP–AAm)2	14.06	0.00	0.10	0.648	2.191
P(NP–AAm)3	14.06	0.00	0.15	0.648	2.191

---

Fabrication of Au-core Ag-shell nanocomposite hydrogels

Approximately 500 mg dry hydrogels were allowed to swell in distilled water for 48 h to reach equilibrium swelling. The swollen hydrogels were transferred to a 50 mL glass beaker containing 20 mL aqueous silver nitrate (5 mM) and 10 mL aqueous gold(III) chloride (5 mM) solutions for 24 h in order to permit equilibration. During this equilibrium stage, Ag⁺ and Au³⁺ ions were being exchanged from aqueous solution to the hydrogel networks. Finally, the ion-loaded hydrogels were immersed into the mint leaf extract for up to 6 h at room temperature in order to nucleate Au³⁺ and Ag⁺ ions to form Au-core Ag-shell nanoparticles. Subsequently, the hydrogels with nucleated Au-core Ag-shell nanoparticles (P(CP–AAm)_x + Ag⁰ + Au⁰, x = 1–3 and P(NP–AAm)_x + Ag⁰ + Au⁰, x = 1–3) were dried out at ambient room temperature for 48 h and crushed for characterizations.

Characterizations

The core–shell nanocomposites were studied through morphological studies (TEM, SEM), elemental analysis (EDS), spectral analysis (FTIR), thermal analysis, swelling behavior and antibacterial tests.

Transmission electron microscopy (TEM) was conducted on a JEOL JEM-1200EX (Tokyo, Japan). A TEM sample was prepared by dispersing two to three drops (1 mg/1 mL) of finely ground core–shell nanocomposite hydrogel solution on a 3 mm copper grid and drying at ambient temperature.

Scanning electron microscopy/energy-dispersive spectroscopy analysis was performed using a JEOL JEM-7500F (Tokyo, Japan), operated at an accelerating voltage of 2 kV. All SEM-EDS samples were gold-coated prior to examination.

The required Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer FTIR spectrometer (model: Impact 410, Wisconsin, MI, USA). Samples were examined between 500 and 4000 cm⁻¹.

Thermogravimetric analysis (TGA) of the samples was carried out using an SDT Q 600 DSC instrument (T.A. Instruments-Waters LLC, New Castle, DE 19720, USA) at a heating rate of 20 °C min⁻¹ under a constant nitrogen flow (100 mL min⁻¹).

Swelling studies

The swelling ratio (S_g/g) of the hydrogels was determined by a gravimetric method by allowing the hydrogels to swell in phosphate buffered saline (PBS) (pH 7.4) for 24 h at 37 °C. The value of the swelling ratio was calculated using the equation S_g/g = W_s/W_d where W_s and W_d denote the weight of the swollen hydrogel at equilibrium and the weight of the dry hydrogel, respectively.^21,22 The data provided was an average value of 3 individual readings of the samples.

Antibacterial test

The antibacterial activity was tested against E. coli (G−) and B. subtilis (G+). The method followed was the ‘disc diffusion method’, as described in the literature. The required nutrient agar medium was prepared by mixing peptone (5.0 g), beef extract (3.0 g), sodium chloride (5.0 g) and agar (15.0 g) in 1000 mL distilled water, and the pH was adjusted to 7.0. The agar medium was sterilized in a conical flask at a pressure of 15 lbs in⁻² for 30 min and transferred into sterilized Petri dishes in a laminar air flow chamber (Microfilt Laminar Flow Ultra Clean Air Unit, Mumbai, India) for solidification. Later, 50 μL microbial culture was uniformly streaked over the solid surface. Into this inoculated Petri dish pre-impregnated discs with a standard gel concentration (20 mg/10 mL distilled water) were placed and incubated at 37 °C for 48 h to obtain inhibition zones. Finally, the formed inhibition zones were measured and photographed.

Results and discussions

In the present investigation, Carbopol® 980 NF (CP) and Noveon® AA-1 polycarbophil (NP) were successfully utilized to fabricate hydrogels. During a typical polymerization process, gelation occurs through linking acrylamide (AAm) units to initially form polydisperse soluble branched polymers of acrylamide, called a ‘sol’. The process of linking causes the branched acrylamide polymer to increase its size to form an insoluble infinite polymer with interpenetrating CP/NP molecular chains, called a ‘gel’ or ‘network’. The transition of a system from finite branched polymers to an infinite polymer is called the ‘sol–gel transition’ (or ‘gelation’) and the critical point where a gel first appears is called the ‘gel point’.³⁰ Once a hydrogel was formed, bimetallic Au-core Ag-shell nanoparticles were impregnated into the gel network by a swelling method.² Core–shell metal nanoparticles that are produced using chemical reducing agents are usually associated with environmental toxicity or biological hazards. From this perspective, extracts of naturally occurring mint leaves were chosen for nucleation of core–shell nanoparticles. The successful fabrication of inorganic Au-core Ag-shell nanocomposite hydrogels via an eco-friendly green process is pictorially presented in Scheme 1.

Scheme 1 Schematic representation of a facile eco-friendly fabrication of antibacterial nanocomposite hydrogels.

Mechanism of Au-core Ag-shell formation

Mint leaf extracts, which contain hydroxyl groups (–OH) associated with menthol (the major component present in mint extracts), offer sufficient reduction capacity for the reduction of metal ions (Ag⁺ and Au³⁺).³¹ In a competitive process involving both silver ions and gold ions, reduction of gold ions occurs first, then that of silver ions.²⁸ The comparatively slower reduction rate of silver ions relative to that of gold ions is due to the difference in their reduction potentials. The redox potential being considerably lower for Ag⁺ to Ag⁰ (E⁰ = +0.80 V) than for Au³⁺ to Au⁰ (E⁰ = +1.50 V) allows gold ions to be reduced first, then silver ions (eqn. (1) and (2)).²⁸ Once the gold ions are reduced, the surrounding silver ions are eventually reduced and deposited on the reduced Au³⁺ (i.e. Au⁰) in the form of a layer, giving Au-core and Ag-shell nanoparticles.²⁸ Furthermore, size reduction to the nano-dimension increases not only the surface energy but also the number of binding sites between Au and Ag atoms. This stabilizes the total energy of each individual core–shell nanoparticle.³² Hence, the formation of bimetallic Au-core and Ag-shell nanoparticles might be explained by a combination of these factors, involving a balance between the binding energy and the surface energy.

Ag⁺ + ē → Ag⁰, E⁰ = +0.80 V (slower reduction) (1)

Au³⁺ + 3ē → Au⁰, E⁰ = +1.50 V (faster reduction) (2)

Evidence for the successful formation of core–shell nanoparticles was given by transmission electron microscopy (TEM) analysis. TEM images of the core–shell nanoparticles are shown in Fig. 1. Similarly to the observations of many research groups, a boundary between Au and Ag elements can be distinguished by bright/dark contrast, where the dark region is assumed to be the core (Au) and the bright region is assumed to be the shell (Ag).³³ The additional information revealed by TEM data implied that the nanoparticles formed were of the nano-dimension at ∼15 ± 3 nm, and without any aggregation. It may be presumed that the core–shell nanoparticles were highly stabilized through the available hydrophilic functional groups of the hydrogel network.²¹ Hence, core–shell nanoparticles without aggregation were formed. The formation of the core–shell structure was supported by scanning electron microscopy/energy-dispersive spectroscopy (SEM-EDS) analysis. SEM-EDS images of the core–shell nanocomposite hydrogels, P(CP–AAm)₃ + Ag⁰ + Au⁰ (Fig. 2A(c)) and P(NP–AAm)₃ + Ag⁰ + Au⁰ (Fig. 2B(c)), show signals corresponding to Au and Ag in the spectra, indicating the core–shell structure was composed of Au and Ag.^29,34 Furthermore, morphological studies through SEM examination show a clear surface pattern for pure P(CP–AAm)₃ hydrogel (Fig. 2A(a)) and P(NP–AAm)₃ hydrogel (Fig. 2B(a)), and a nanoparticle distribution pattern for the core–shell nanocomposite hydrogels P(CP–AAm)₃ + Ag⁰ + Au⁰ (Fig. 2A(b)) and P(NP–AAm)₃ + Ag⁰ + Au⁰ (Fig 2B(b)). This supports the presence of core–shell nanoparticles in the hydrogel network.

Fig. 1 TEM images of core–shell nanoparticles present in hydrogel nanocomposites of (A) P(CP–AAm)₃; (B) P(NP–AAm)₃ with (a) lower magnification and (b) higher magnification.

Fig. 2 SEM images of (A) (a) pure P(CP–AAm)₃ hydrogel, (b) P(CP–AAm)₃ + Ag⁰ + Au⁰ hydrogel; (B) (a) pure P(NP–AAm)₃ hydrogel, (b) P(NP–AAm)₃ + Ag⁰ + Au⁰ hydrogel and SEM-EDS images of (A) (c) P(CP–AAm)₃ + Ag⁰ + Au⁰ hydrogel; (B) (c) P(NP–AAm)₃ + Ag⁰ + Au⁰ hydrogel.

Evidence for the successful preparation of a Au-core Ag-shell nanocomposite hydrogel was provided through hydrogel–core–shell nanoparticle interaction from FTIR spectral data, as shown in Fig. 3. FTIR spectra of pure P(CP–AAm)₃ and the core–shell nanocomposite P(CP–AAm)₃ + Ag⁰ + Au⁰ are presented in Fig. 3A. The spectrum of the P(CP–AAm)₃ hydrogel shows absorption bands at 3335 cm⁻¹ and 3190 cm⁻¹ corresponding to –OH and –NH group stretching vibrations, respectively.^3,4 The other important bands at 2924 cm⁻¹, 1643 cm⁻¹, 1184 cm⁻¹ and 1114 cm⁻¹ were assigned to stretching vibrations of –C–H, carbonyl and –C–O groups, respectively, present in the P(CP–AAm)₃ hydrogel.^22,35 These peaks were shifted in the case of P(CP–AAm)₃ + Ag⁰ + Au⁰ due to interactions of the core–shell nanoparticles with the functional groups of the hydrogel. FTIR spectra of pure P(NP–AAm)₃ and the core–shell nanocomposite P(NP–AAm)₃ + Ag⁰ + Au⁰ are presented in Fig. 3B. For pure P(NP–AAm)₃, the absorption bands at 3330 cm⁻¹ and 3182 cm⁻¹ correspond to –OH and –NH group stretching vibrations, respectively.^3,4 The other important bands at 2932 cm⁻¹, 1644 cm⁻¹, 1181 cm⁻¹ and 1119 cm⁻¹ were assigned to stretching vibrations of –C–H, carbonyl and –C–O groups, respectively, present in the P(NP–AAm)₃ hydrogel.^22,35 These peaks were shifted in the case of P(NP–AAm)₃ + Ag⁰ + Au⁰, as noted in the earlier case. Overall, the shifting of the peaks confirms the interactions of the core–shell nanoparticles with the functional groups of the hydrogel.

Fig. 3 : FTIR spectra of (A) pure P(CP–AAm)₃ hydrogel, P(CP–AAm)₃ + Ag⁰ + Au⁰ hydrogel; (B) pure P(NP–AAm)₃ hydrogel, P(NP–AAm)₃ + Ag⁰ + Au⁰ hydrogel.

Thermogravimetric analysis (TGA), another piece of evidence, indicates simultaneously the existence of core–shell nanoparticles within the three-dimensional (3D) networks of the hydrogels and the thermal stability of the hydrogels. As shown in the TGA curves (Fig. 4A), at 700 °C pure P(CP–AAm)₃ (Fig. 4A(a)) and pure P(NP–AAm)₃ (Fig. 4A(b)) degraded almost completely, but the core–shell nanocomposite hydrogels remained with residual mass. The residual mass left over from the nanocomposites, P(CP–AAm)₁ (1.86%), P(CP–AAm)₃ (4.77%), P(NP–AAm)₁ (2.58%) and P(NP–AAm)₃ (6.58%), clearly indicates the existence of inorganic Au-core Ag-shell nanoparticles internally in the 3D networks of the hydrogels. Thermal studies led to the conclusion that with the increase in the percentage (%) of CP and NP, the residual mass left over increased. This was due to the rise in the number of nanoparticles, which occurred due to the strong bonding characteristics which developed between the nanoparticles and the acrylic acid polymers with the increase in CP and NP concentrations. This phenomenon is comparable with Ag- or Au-impregnated nanocomposite systems.^1,2,36 Overall, the TGA data demonstrates that both the nanocomposite hydrogels and the pure hydrogels show an almost similar pattern in their curves, indicating no significant difference in their thermal stability. The residual mass of the nanocomposite left over at a temperature over 700 °C is actually the mass of the core–shell nanoparticles fabricated in the hydrogels.

Fig. 4 (A) Thermogravimetric analysis of (a) pure P(CP–AAm)₃ hydrogel, P(CP–AAm)₃ + Ag⁰ + Au⁰ hydrogel and P(CP–AAm)₁ + Ag⁰ + Au⁰ hydrogel; (b) pure P(NP–AAm)₃ hydrogel, P(NP–AAm)₃ + Ag⁰ + Au⁰ hydrogel and P(NP–AAm)₁ + Ag⁰ + Au⁰ hydrogel; (B) swelling ratio of pure and core–shell nanocomposite hydrogels of (a) P(CP–AAm), (b) P(NP–AAm).

Swelling studies

Swelling studies is a characteristic analysis used to determine the ‘swelling property’ or ‘swelling ratio’ of developed hydrogels. The ‘swelling property’ or ‘swelling ratio’ is a characteristic parameter, which indicates the absorption capacity of a functional material for blood, bodily fluids, secretions and exudates from injuries and wounds.

In the present investigation, it was noted that the value of the swelling ratio of the developed nanocomposite hydrogels ranges from 12 to 22 (g g⁻¹), indicating that they can absorb blood or any other bodily fluid to an extent of 1200–2200% of their dry weight (shown in Fig. 4B(a) and (b)). Furthermore, the value of the swelling ratio is proportional to the concentration of CP/NP. From the swelling ratio data, it is also evident that, with an increase in CP/NP concentration, the value of the swelling ratio also increases. This observed phenomenon was due to the hydrophilic nature of the developed CP/NP hydrogel.¹ Furthermore, the core–shell nanocomposite hydrogels show a higher swelling ratio than the respective pure P(CP–AAm) hydrogels (Fig. 4B(a)) and pure P(NP–AAm) hydrogels (Fig. 4B(b)). The reason being that, when the Au³⁺ and Ag⁺ ion-loaded hydrogels were treated with mint leaf extract, nucleation of the ions occurred, resulting in the formation of core–shell nanoparticles. This allowed the hydrogel to expand its 3D networks and promote a higher water molecule uptake capacity.^1,2 One more reason for the expansion of the 3D networks is the ‘size factor’. When nanoparticles are formed with varying nano-dimensions, this results in their acquisition of different charges over their surfaces and causes absolute expansion of the networks.⁴

The overall swelling data significantly confirmed that the developed hydrogels were good absorbents for blood, bodily fluids, secretions and exudates from injuries, wounds or other body parts.

Antibacterial activity

Destruction of bacteria is the key parameter that determines the utility of the developed hydrogels for various applications in bacteria-prone areas. The efficiency of the developed nanocomposite hydrogels against bacteria was determined by conducting an antibacterial activity assay against E. coli (G−) and B. subtilis (G+), as shown in Fig. 5. The results revealed a strong reduction in the number of bacterial colonies around core–shell nanocomposite samples. The mechanism of the growth-inhibitory effect of the nanoparticles against microorganisms is not yet clear.³⁷ However, among the various possible mechanisms proposed by many authors, ‘inhibition by formation of pits’ was considered to be suitable here. It was assumed that interaction of nanoparticles with bacteria results in the formation of pits in the cell wall or the bacterial membrane. These pits cause the leakage of biologically important lipopolysaccharide molecules and membrane proteins, leading to microbial death.^38–41 Although the shell of the core–shell nanoparticles that interact with the cell wall or the bacterial membrane comprises silver atoms, their efficiency in bacterial inhibition cannot be compared with that of silver nanoparticles because Au-core Ag-shell nanoparticles show enhanced antibacterial activity. This was proved by Banerjee et al.⁴² The enhanced antibacterial properties of Au-core Ag-shell nanoparticles were possibly due to higher activity of the silver atoms in the shell surrounding the Au-core, due to high surface free energy of the surface silver atoms, owing to thinness of the shell in the bimetallic nanoparticle structure.

Fig. 5 Antibacterial activity of (a) P(CP–AAm)₃ + Ag⁰ + Au⁰ hydrogel, (b) P(NP–AAm)₃ + Ag⁰ + Au⁰ hydrogel, (c) pure P(CP–AAm)₃ + pure P(NP–AAm)₃ against: (A) E. coli; (B) B. subtilis.

The diameter of the inhibition zone exhibited by the optimized nanocomposite hydrogel samples against E. coli and B. subtilis was found to be in the range 14.2–17 mm. The diameters of the inhibition zones of P(CP–AAm)₃ + Ag⁰ + Au⁰ and P(NP–AAm)₃ + Ag⁰ + Au⁰ against E. coli were 15.8 mm and 17.0 mm, and against B. subtilis 14.2 mm and 15.1 mm, respectively. These results are quite as expected and seemed to be in accordance with the nanoparticle content in the hydrogel matrices. It is evident from the TGA analysis that the percentage nanoparticle content was found to be higher for P(NP–AAm)₃ + Ag⁰ + Au⁰ (6.58%) than for P(CP–AAm)₃ + Ag⁰ + Au⁰ (4.77%). Hence, the inhibition zones were observed to be larger for P(NP–AAm)₃ + Ag⁰ + Au⁰ than for P(CP–AAm)₃ + Ag⁰ + Au⁰.³⁶ Furthermore, according to the standard antibacterial test “SNV 195920–1992”, specimens showing more than a 1 mm zone of microbial inhibition can be considered good antibacterial agents.⁴³ Hence, the novel inorganic Au-core Ag-shell nanocomposite hydrogels developed via an eco-friendly green process can be considered good antibacterial agents, effective at killing bacteria.

Conclusion

Antibacterial nanocomposite hydrogels of Au-core Ag-shell structure were fabricated from medically important Carbopol® 980 NF (CP) and Noveon® AA-1 polycarbophil (NP) acrylic acid polymeric matrices. The process adopted was eco-friendly, where a natural mint leaf extract, free from toxic chemicals, was successfully utilized to synthesize Au-core Ag-shell nanoparticles. The developed nanocomposite hydrogels showed excellent antibacterial activity against E. coli (G−) and B. subtilis (G+), and also exhibited pronounced swelling properties. Hence, from the point of view of applications, the developed hydrogels may be utilized for superior biomedical applications, ranging from designing wound-dressing materials to sanitary appliances like incontinence articles, tampons, nappy pants, nappy liners and sanitary napkins.

Acknowledgements

The author (IF 110192) wishes to acknowledge the Department of Science & Technology (DST, INDIA) and the Ministry of Science & Technology for providing the financial assistance through the Innovation in Science Pursuit for Inspired Research programme (INSPIRE). The author KVP wishes to acknowledge the CIPA, CONICYT Regional, and GORE BIO-BIO R08C1002 and PAI proyecto 781302011 Conicyt, Chile.

References

1. T. Jayaramudu, G. M. Raghavendra, K. Varaprasad, R. Sadiku and K. M. Raju, Carbohydr. Polym., 2013, 92, 2193–2200.
2. T. Jayaramudu, G. M. Raghavendra, K. Varaprasad, R. Sadiku, K. Ramam and K. M. Raju, Carbohydr. Polym., 2013, 95, 188–194.
3. N. N. Reddy, Y. M. Mohan, K. Varaprasad, S. Ravindra, P. A. Joy and K. M. Raju, J. Appl. Polym. Sci., 2011, 122, 1364–1375.
4. N. Narayana Reddy, K. Varaprasada, S. Ravindra, G. V. Subba Reddy, K. M. S. Reddy, K. M. M. Reddy and K. M. Raju, Colloids Surf., A, 2011, 385, 20–27.
5. N. N. Reddy, Y. M. Mohan, K. Varaprasad, S. Ravindra, K. Vimala, P. A. Joy and K. M. Raju, J. Polym. Res., 2011, 18, 2285–2294.
6. K. Varaprasad, Y. M. Mohan, S. Ravindra, N. N. Reddy, K. Vimala, K. Monika, B. Sreedhar and K. Mohana Raju, J. Appl. Polym. Sci., 2010, 115, 1199–1207.
7. N. A. Peppas and A. G. Mikos, Hydrogels in Medicine and Pharmacy, CRC Press, Boca Raton, FL, 1986, vol. 1, pp. 1–27.
8. L. Brannon-Peppas, Absorbent Polymer Technology, Elsevier, Amsterdam, 1990, pp. 45–66.
9. N. A. Peppas, P. Bures, W. Leobandung and H. Ichikawa, Eur. J. Pharm. Biopharm., 2000, 50, 27–46.
10. S. Benamer, M. Mahlous, A. Boukrif, B. Mansouri and S. L. Youcef, Nucl. Instrum. Methods Phys. Res., Sect. B, 2006, 248, 284–290.
11. Y. C. Nho, S. E. Park, H. I. Kim and T. S. Hwang, Nucl. Instrum. Methods Phys. Res., Sect. B, 2005, 236, 283–288.
12. J. M. Rosiak, P. Ulanski and A. Rzeinicki, Nucl. Instrum. Methods Phys. Res., Sect. B, 1995, 105, 335–339.
13. J. M. Rosiak and F. Yoshii, Nucl. Instrum. Methods Phys. Res., Sect. B, 1999, 151, 56–64.
14. G. C. Maity, J. Phys. Sci., 2008, 12, 173–186.
15. S. Q. Liu, R. Tay, M. Khan, P. L. R. Ee, J. L. Hedrickc and Y. Y. Yang, Soft Matter, 2010, 6, 67–81.
16. S. Gupta, M. K. Samanta and A. M. Raichur, AAPS PharmSciTech, 2010, 11, 322–335.
17. M. Kerec, M. Bogataj, B. Mugerle, M. Gasperlin and A. Mrhar, Int. J. Pharm., 2002, 241, 135–143.
18. N. Aggarwal, S. Goindi and S. D. Mehta, AAPS PharmSciTech, 2012, 13, 67–74.
19. M. T. Islam, N. Rodríguez-Hornedo, S. Ciotti and C. Ackermann, AAPS J., 2004, 6, 35.
20. Y. M. Mohan, K. Vimala, V. Thomas, K. Varaprasad, B. Sreedhar, S. K. Bajpai and K. M. Raju, J. Colloid Interface Sci., 2010, 342, 73–82.
21. P. S. K. Murthy, Y. M. Mohan, K. Varaprasad, B. Sreedhar and K. M. Raju, J. Colloid Interface Sci., 2008, 318, 217–224.
22. K. Varaprasad, Y. Murali Mohan, K. Vimala and K. M. Raju, J. Appl. Polym. Sci., 2011, 121, 784–796.
23. P. R. Reddy, K. Varaprasad, N. Narayana Reddy, K. M. Raju and N. S. Reddy, J. Appl. Polym. Sci., 2012, 125, 1357–1362.
24. N. S. Z. Haider, U. Kiran, I. M. Ali, A. Hameed, S. Ahmed and N. Ali, Int. J. Nanomed., 2013, 8, 3187–3195.
25. T. Ristic, L. Z. Zemljic, M. Novak, M. K. Kuncic, S. Sonjak, N. G. Cimerman and S. Strnad, Science against microbial pathogens: communicating current research and technological advances, ed. A. Méndez-Vilas, 2011, pp. 36–51.
26. G. Zhan, J. Huang, M. Du, I. Abdul-Rauf, Y. Ma and Q. Li, Mater. Lett., 2011, 65, 2989–2991.
27. D. S. Shenya, J. Mathewa and D. Philip, Spectrochim. Acta, Part A, 2011, 79, 254–262.
28. S. S. Shankar, A. Rai, A. Ahmad and M. Sastry, J. Colloid Interface Sci., 2004, 275, 496–502.
29. A. Rai, M. Chaudhary, A. Ahmad, S. Bhargava and M. Sastry, Mater. Res. Bull., 2007, 42, 1212–1220.
30. M. Rubinstein and R. H. Colby, Polymer Physics, Oxford University Press, Oxford, 2003.
31. S. Ravindra, Y. M. Mohan, N. N. Reddy and K. M. Raju, Colloids Surf., A, 2010, 367, 31–40.
32. K. Hirakawa, in Smart Nanoparticles Technology, ed. A. Hashim, 2012, ISBN: 978-953-51-0500-8.
33. S. Pyne, P. Sarkar, S. Basu, G. P. Sahoo, D. K. Bhui, H. Bar and A. Misra, J. Nanopart., 2011, 13, 1759–1767.
34. T. Li, S. Chattopadhyay, T. Shibata, R. E. Cook, J. T. Miller, N. Suthiwangcharoen, S. Lee, R. E. Winansa and B. Lee, J. Mater. Chem., 2012, 22, 14458–14464.
35. K. Vimala, Y. Murali Mohan, K. Samba Sivudu, K. Varaprasad, S. Ravindra, N. Narayana Reddy, Y. Padma, B. Sreedhar and K. M. Raju, Colloids Surf., B, 2010, 76, 248–258.
36. G. M. Raghavendra, T. Jayaramudu, K. Varaprasad, R. Sadiku, S. Sinha Ray and K. M. Raju, Carbohydr. Polym., 2013, 93, 553–560.
37. H. H. Lara, E. N. Garza-Treviño, L. Ixtepan-Turrent and K. S. Dinesh, J. Nanobiotechnol., 2011, 9, 30.
38. N. A. Amro, L. P. Kotra, K. Wadu-Mesthrige, A. Bulychev, S. Mobashery and G. Liu, Langmuir, 2000, 16, 2789–2796.
39. A. Nasrollahi, K. h. Pourshamsian and P. Mansourkiaee, Int. J. Nano Dimens., 2011, 1, 233–239.
40. M. F. Zawrah and S. I. A. El-Moez, J. Life Sci., 2011, 8, 37–44.
41. M. Singh, S. Singh, S. Prasada and I. S. Gambhir, Dig. J. Nanomater. and Bios., 2008, 3, 115–122.
42. M. Banerjee, S. Sharma, A. Chattopadhyay and S. S. Ghosh, Nanoscale, 2011, 3, 5120–5125.
43. G. M. Raghavendra, T. Jayaramudu, K. Varaprasad, S. Ramesh and K. M. Raju, RSC Adv., 2014, 4, 3494–3501.

---