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Abstract

Cotton fabric with improved antibacterial properties is always invited for effective use in wound dressing and healing applications. In this study, poly(N-isopropyl acrylamide) network has been produced in situ in porous cotton cellulose fabric by photo polymerization using UV-radiation. The thermoresponsiveness of the resulting fabric has been used to entrap silver nanoparticles with the fabric. The presence of silver nanoparticles in the fabric has been characterized by X-ray diffraction, transmission electron microscopy, and dynamic light scattering analysis. The fabrics have been found to possess fair mechanical properties. The individual and aggregation particle size of the silver nanoparticles was found to be in the range 13–20 nm and 140–220 nm. This aggregation phenomenon was also confirmed by dynamic light scattering measurement, i.e. 256 nm. The silver nanoparticles exhibits polydispersity index 0.18 and zeta potential −2.86 mV. The fabric exhibited fair biocidal action against Escherichia coli and Staphylococcus aureus, thus indicating its possible utility in medicinal application.

Keywords

Silver nanoparticles cotton fabric photo polymerization antibacterial action thermosensitive polymer

Introduction

The development of new clothing products, based on the immobilization of nanophased materials on textile fibers, has received growing interest from both academic and industrial sectors [1]. There have been several reports in which the cotton/wool fabrics have been loaded with metal nanoparticles to impart antibacterial properties [2 –5]. In most of these strategies, the immersion of fabric in solution of metal nanoparticles is commonly followed [1,4]. The major drawback with such antibacterial textiles is that there are not much controlling factors to regulate the quantity of nanostructure loaded in to the fabric [4]. Additionally, the release of metal ions from those fabrics cannot be controlled. Considering all these drawbacks in mind, recently we developed a new approach in which a suitable polymer layer was grafted onto cotton fabric (CF) via Ce(IV)-induced graft co-polymerization [6] followed by entrapment of metal nanoparticles into grafted fabric [7]. However, such extensive approach involves use of HNO₃ as catalyst which ultimately reduce the mechanical strength of final product [8]. These studies advice any such process dealing to generate a good antibacterial fabric must avoid or minimize use of such harsh chemical or conditions which damage fabric.

In order to maintain the strength and other physico-chemical properties of fabrics, various strategies have been followed in recent past to develop antibacterial fabrics [6,8]. For instance, the silica–silver core–shell particles were synthesized by simple one pot chemical method and were employed on the CF as an antibacterial agent [9]. Extremely small (1–2 nm) silver nanoparticles (AgNPs) were attached on silica core particles of average 270 nm size. The optimum density of the nanosilver particles was sufficient to show good antibacterial activity as well as the suppression in their surface plasmon resonance responsible for the color of the core–shell particle for antibacterial textile applications. Thanh and Phong [10] have proposed a simple approach to synthesize AgNPs employing polyol process with microwave heating and incorporated on CF surfaces. The antibacterial performance of the antibacterial CF was tested for different concentrations of silver colloid, contact time with germs, and washing times. It was found that antibacterial activity increased with increasing concentrations of silver colloid. The antibacterial fabric with 758 mg/kg of AgNPs on surface cotton was highly effective in killing test bacteria and had excellent water-resisting property. A composite antibacterial binder containing styrene–butadiene acrylate and 12.5 nm AgNPs grafting promotes extensively their antibacterial activity against Escherichia coli [11].

Poly(N-isorpoyl acrylamide) (PNIPAm) is a temperature sensitive polymer which exhibits a lower critical solution temperature (LCST) (32℃) where the hydrophilic chains of polymers are dominated by hydrophobic chains [12]. This polymer has been proposed for tissue engineering, controlled drug delivery, and implant applications [12 –14]. In this report, thermosensitive property of PNIPAm has been explored to develop a unique approach to embed AgNPs in ultrafine polymer grafts on CF. Furthermore, we propose that if such fabrics applied for wound healing application, entrapped AgNPs may exhibit superior antimicrobial properties.

Experimental

Materials and methods

N,N′-methylene bisacrylamide (MBAm, cross-linker), benzophenone (BP, photoinitiator), dimethylformamide (DMF, solvent), silver nitrate, and sodium borohydride (NaBH₄, SBH) were obtained from Hi Media Chemicals (Mumbai, India) and were used as received. N-isopropyl acrylamide (NIPAm) was obtained from Aldrich (St Louis, MO, USA) and was recrystallized from mixture of benzene/hexane (1 : 3) to remove inhibitor. The CF was obtained from Indore Textiles (Indore, India) and its weight per square centimeter was nearly 0.026 g. The double distilled water was used throughout the investigations.

Preparation of PNIPAm-CF composite

A CF piece of known dimension (length 5 cm, width 2 cm, and weight ∼0.40 g) was immersed in a solution which contained 4.418 mM of monomer NIPAm, 0.324 mM of cross-linker MBAm, 1.097 mM of photoinitiator BP; all dissolved in DMF with a total volume of 5 mL. Then, it was passed through a padding mangle, which was running at a speed of 10 m/min with a pressure of 15 kg·f/cm² to give a wet pick up of 100% (owing of weight of fabric). Immediately after padding, the fabric sample was hung in air for 10 min at 30℃ and then exposed to UV-radiation (UVITEC, Germany, equipped with three lamps; wavelength 365 nm; intensity 1250 μW/cm²) for a period of 1 h. Then, the fabric sample was washed in ethanol : water (50 : 50) mixture to remove un-grafted or bound homopolymer. The fabric sample was dried in air chamber at 40℃. The amount of grafted polymer in composite was calculated using the following expression

%Weight gain = (final weight − initial weight)/initial weight × 100.

Temperature-dependent swelling

A completely dried and pre-weighed polymer/fabric was placed in 250 mL of distilled water at 30 ± 0.2℃ and its water uptake was measured periodically by removing the CF composite from water, blotting the surface water with filter paper and then weighing with an electronic balance write model (Mttler Toledo, Germany) with a precision of 0.0001 g. The mass measurements were continued till the CF composite attained a constant weight. For deswelling (shrinking) study, the completely swollen fabric was placed in distilled water at 30 ± 0.2℃, and its mass measurement was carried out periodically till the attainment of constant weight. The mass swelling percentage (% Ms) was determined using the following expression

% Ms = (W_t − W_o)/W_o × 100

where W_t and W_o are the weights of gel at time t and in dry state, respectively. All the experiments were carried out in triplicate and average values have been given in the data.

Preparation of AgNPs loaded composite

Loading of AgNPs is achieved by entrapment method [15]. But in this study, we employed cotton fiber instead of hydrogel to load AgNPs. A piece of PNIPAm-CF composite was first placed in aqueous silver nitrate (100 mg in 100 mL H₂O) solution at 27℃ for a period of 4 h. The Ag^(I) ions loaded composite fabric was then immersed in aqueous solution of 25 mL sodium borohydride (1% w/v) pre-maintained at 33℃ for 4 h. The dark brown color of fabric indicated formation of AgNPs within the composite fabric.

Characterization

X-ray diffraction analysis

The X-ray diffraction (XRD) method was used to identify AgNPs in the CF nanocomposite. These measurements were carried out on a Rigaku diffractometer (Cu radiation = 0.1546 nm) running at 40 kV and 40 mA. Cotton fiber before and after modification were imaged at 200× using an Olympus® 1X71 microscope equipped with a DP71 camera (Olympus Corporation, Tokyo, Japan).

Transmission electron microscope analysis

The transmission electron microscopy (TEM) was used to find out the size of AgNPs inside the fabric/polymer nanocomposite. To image the AgNPs, the fabric was cut into a number of very small pieces and dispersed in 5 mL of distilled water; 5 days of incubation at 27℃ discreet most of the AgNPs come out from polymer network into the aqueous phase. A few drops of this aqueous solution were dropped on a copper grid and allowed to dry at room temperature. The copper grid was inserted into JEOL-1210 TEM (JEOL, Tokyo, Japan) (operating at 60 kV) operating at an acceleration voltage of 80 kV.

Mechanical properties

The mechanical properties of the fabrics were determined according to the procedure reported elsewhere [16]. They were cut into strips (38 × 5.8 mm²) and their tensile strength (TS) and percent elongation at break (E) were measured using an Instron Universal Testing Instrument (Model 1011). The initial grip separation and crosshead speed were set to 50 and 300 mm/min, respectively. Data represents from five measurements of each film. TS was calculated by dividing the maximum load (F) on the film before failure by the initial cross-sectional area (S), that is, TS = F/S.

Antimicrobial experiment

The antimicrobial activity of AgNPs loaded PNIPAm-CF composite was tested qualitatively by ‘zone of inhibition’ method [15], using E. coli and Staphylococcus aureus as model bacteria. In the zone inhibition method, 100 µL of the inoculum solution was added to 5 mL of soft agar, which was sprayed onto hard agar plates. The circular piece of PNIPAm-CF composite was placed at the center on the bacterial lawn. The plates were incubated for 48 h at 37℃ in the appropriate aerobic incubation chamber. The plates were visually examined for zones of inhibition around the sample powder and diameter of the zone was measured at two cross-sectional points and the average was taken as the inhibition zone. The composite fabric without AgNPs was used as control.

Result and discussion

Preparation of PNIPAm-CF composite

As mentioned in ‘Introduction’, the Ce(IV)-induced graft polymerization of CF causes a severe loss in the mechanical strength of fabric due to cleavage of the C₂–C₃ bond of cellulose chain. In the current strategy, the PNIPAm chains are formed in situ within the ultrafine network of cotton fiber, thus without disrupting cellulose chains. The overall scheme for formation of PNIPAm within the cotton cellulose fabric network may be explained as follows: when CF is allowed to soak in the reaction mixture, the monomer (NIPAm), cross-linker (MBAm), and photoinitiator (BP) are absorbed into the ultrafine networks within the CF (Figure 1). This fabric is exposed to UV-radiation which causes the photo polymerization reaction take place. Therefore, cross-linked PNIPAm network is formed within the ultrafine CF network.

Figure 1.

Preparation of AgNPs loaded poly(N-isopropyl acrylamide) CF (AgNPs–PNIPAm-CF) composite: (A) schematic representation of AgNPs–PNIPAm-CF composite preparative route in two steps. Step 1 UV-radiation/photo polymerization of NIPAm monomer in the presence of cross-linker and initiator on CF and Step 2 Silver nitrate entrapment using thermosensitive property of PNIPAm and reduced with sodium borohydrate to embedded AgNPs on the CF via PNIPAm chain attachment. (B and C) Photographs and optical microscope images of CF, PNIPAm-CF composite, and AgNPs–PNIPAm-CF composite, respectively.

Swelling–deswelling cycles of PNIPAm-CF composite

The temperature-dependent volume-phase transition of PNIPAM-CF composite was investigated. The results, as shown in Figure 2, reveal that composite exhibits a total present swelling of nearly 392 in 100 min in presence of distilled water as swelling medium at 27℃. On transferring the swollen composite into distilled water at 33℃, it undergoes a sharp deswelling, expelling out the entrapped water and attains a minimum swelling of 135% in nearly 90 min. This swelling–deswelling process was repeated so many times by immersing the sample in distilled water at 27℃ and 33℃ successively. The three such swelling–deswelling cycles are shown. Here, it is also to be noticed that the maximum swelling obtained in first cycle (i.e. 392%) is slightly greater than these obtained in next cycle (i.e. ≈355%), which may probably be attributed to the fact that some un-reacted salts and impurity might be present in the PNIPAm-CF composite which increased the osmotic swelling pressure, thus enhancing the water uptake of the composite fabric. However, after leaching out of them during the first swelling–deswelling cycle, the maximum swelling obtained in following cycles was almost the same.

Figure 2.

Thermosensitive swelling and deswelling behavior of PNIPAm-CF composite.

Preparation of AgNPs-loaded PNIPAm-CF composite

A number of different approaches exist to achieve AgNPs either by grafting, embedding, and entrapped in fiber or textiles [17 –20]. However, our study reports for the time, the temperature-dependent swelling property of PNIPAm has been exploited for entrapment of AgNPs into the composite. PNIPAm shows temperature-dependent swelling undergoing drastic volume-phase transition at its LCST [21,22]. Below its LCST (i.e. 32℃), it is almost transparent in due to balance of hydrophilic and hydrophobic polymer chains (swollen state) while as the temperature exceeds LCST, polymer chains are collapsed due to hydrophobic polymer chain dominated and becomes opaque. This sharp volume-phase transition along LCST has been used to incorporate Ag^(I) ions into the composite material. When the PNIPAm-CF composite is immersed in aqueous solution of AgNO₃ for a period of 4 h at 27℃, the composite undergoes appreciable swelling. The temperature of swelling medium is quite below the CST of N-isopropyl acrylamide, thus allowing Ag^(I) ions to enter into the swollen network. The electron rich moieties like N and O of composite matrix serve as templates for these incoming Ag^(I) ions. This insures almost uniform distribution of Ag^(I) ions within the composite. Now, this Ag^(I)-containing swollen composite is placed in 1% (w/v) solution of sodium borohydride, pre-maintained at 34℃. This causes a drastic volume-phase transition in the composite as the temperature of medium is now above the LCST of PNIPAm. Thus, the gel shrinks, letting entrapped water out of the composite matrix. The entrapped Ag^(I) ions are now reduced to AgNPs due to incoming borohydride ions. In this way, an almost uniform distribution of AgNPs is obtained within the composite. The PNIPAm-CF and AgNPs-loaded PNIPAm-CF composite is shown in Figure 1. The dark brown color of composite indicates formation of AgNPs within the PNIPAm-CF composite. A clear variation was observed between CF, PNIPAm-CF composite, and AgNPs–PNIPAm-CF composite in their bulk fabric (Figure 1(B)) and microscopic images (Figure 1(C)).

Characterization of AgNPs–PNIPAm-CF composite

The XRD pattern of AgNPs-loaded fabric is shown in Figure 3. The PNIPAm network does not show any characteristic peaks, indicating the amorphous nature of the network. However, a little broad peak at 20–22° is indicative of overlapping of two small peaks usually obtained at 20° and 22° due to reflection at (1 1 0) and (2 0 0) planes of cellulose. In addition to this, the characteristics peaks are observed at 2θ values of 38.1°, 44.2°, and 64.4° which corresponds to reflections at (1 1 1), (2 0 0), and (2 2 0) planes of face-centered cubic structure of silver nanostructures, respectively. These characteristic peak values are in consistent to our previous reports [6,7,15,16].

Figure 3.

XRD patterns of AgNPs–PNIPAm-CF composite.

The mechanical properties of the fabrics were found to be improved appreciably due to grafting of the polymer. All the properties are given in Table 1. The observed increase in the mechanical properties of fabric may simply be attributed to the presence of micrometer-sized polymeric layer within the ultrafine pores of CF. Here, we would like to mention that in situ formation of polymeric layer within the CFs has this advantage over the grafting of polymer on to cotton cellulosic backbone that in the former case the mechanical strength of the fabric is maintained, or even improved as observed in the present case.

Table 1.

Mechanical properties of plain and PNIPAm-CF composite for antibacterial applications.

Mechanical properties	Plain fabric	PNIPAm-CF fabric
Maximum force (N)	10 ± 1.84	14.70 ± 1.26
Stress (MPa)	18.03 ± 3.10	22.01 ± 2.78
Elongation at break (%)	4.47 ± 2.12	2.23 ± 1.88
Elongation at maximum force (%)	3.89 ± 1.12	3.55 ± 1.48

PNIPAm: poly(N-isorpoyl acrylamide); CF: cotton fabric.

Values reported as average of five replicates ± SD.

The TEM images of AgNPs are shown in Figure 4. The majority of particles appear to be quite spherical with an average particle size lying in the range 13–20 nm. However, nanoparticle clusters (group of nanoparticles) are also appeared in the image, having diameter size range 140–220 nm. The aggregation of AgNPs can be attributed to the absence of any stabilizer which is usually employed to prevent their aggregation. To further establish the size and particle size distribution of AgNPs, dynamic light scattering (DLS) analysis was also carried out, as shown in Figure 5. This data shows average particle size of 256 nm which represents aggregation of nanoparticles in PNIPAm-CF composite layers in water. The AgNPs exhibits polydispersity index 0.18 and zeta potential −2.86 mV.

Figure 4.

TEM images of AgNPs extracted from AgNPs–PNIPAm-CF composite.

Figure 5.

DLS particle size analysis of AgNPs extracted from AgNPs–PNIPAm-CF composite.

There have been several reports describing the antibacterial action of AgNPs (alone or on immobilization) [23 –26]. It was confirmed that on contact with moisture, Ag⁺ ions are released from AgNPs and they bind both to microbial DNA, preventing bacterial replication, and to the sulfhydryl groups of metabolic enzymes of the bacteria electron transport chain, that causing their inactivation. Excessive reactive oxygen species generation would also leads to kill most of the bacteria [23].

Antibacterial activity

In order to test antibacterial properties, AgNPs loaded polymer-fabric composites was prepared by immersing the PNIPAm-CF composite in silver nitrate solutions of three different concentrations, namely 25, 40, and 55 mg AgNO₃ per 25 mL of water followed by deswelling-cum-reduction in borohydride solution. The resulting fabric composites were designated as AgNPs–PNIPAm-CF (x) where the number x within the parenthesis denotes the amount of silver nitrate in mg present in 25 mL loading solution. Therefore, the samples were represented AgNPs–PNIPAm-CF (25), AgNPs–PNIPAm-CF (40), and AgNPs–PNIPAm-CF (55), respectively. The results of antibacterial experiment are shown in Figure 6. It can be seen that petri plates containing the plain polymer/CF composite shows dense population of bacterial colonies, while petri plates supplemented with AgNPs–PNIPAm-CF (25), AgNPs–PNIPAm-CF (40), and AgNPs–PNIPAm-CF (55), show clear zone of inhibition with their average radii as 1.8, 2.0, and 2.4 cm for E. coli, 2.1, 2.6, and 3.5 for S. aureus, respectively. This indicates that the antibacterial activity of fabric composite increases with the amount of Ag^(I) present in the loading solution that are responsible to generate AgNPs.

Figure 6.

Antibacterial activity of AgNPs–PNIPAm-CF composites against E. coli.

Overall, this study confirms the simplicity of preparation of CF composite with fair antibacterial property. Our future studies would deal to embed more amounts of AgNPs into the CF to improve further antibacterial properties.

Conclusion

This study reports that temperature-dependent swelling–shrinking property of PNIPAm-CF composite can be conveniently used for the in situ preparation of AgNPs within the composite matrix. The fabric composite thus obtained shows fair antibacterial action against E. coli as model bacteria.

Footnotes

Acknowledgement

Authors are thankful to Dr OP Sharma, Head of the Department, Government Model Science College, for providing facilities.

Funding

Authors are thankful to the Bhabha Atomic Research Center (BARC), Mumbai, for providing financial support via project no: (2008/37/22/BRNS/1484).

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Silver nanoparticles loaded thermosensitive cotton fabric for antibacterial application

Abstract

Keywords

Introduction

Experimental

Materials and methods

Preparation of PNIPAm-CF composite

Temperature-dependent swelling

Preparation of AgNPs loaded composite

Characterization

X-ray diffraction analysis

Transmission electron microscope analysis

Mechanical properties

Antimicrobial experiment

Result and discussion

Preparation of PNIPAm-CF composite

Swelling–deswelling cycles of PNIPAm-CF composite

Preparation of AgNPs-loaded PNIPAm-CF composite

Characterization of AgNPs–PNIPAm-CF composite

Antibacterial activity

Conclusion

Footnotes

Acknowledgement

Funding

References