Rajshahi Silk” Using Green Silver Nanoparticles

fibers

Article Surface Functionalization of “Rajshahi Silk” Using Green Silver Nanoparticles

Sakil Mahmud 1,2,*, Mst. Zakia Sultana 1,3, Md. Nahid Pervez 1,4 ID , Md. Ahsan Habib 1,5 and Hui-Hong Liu 1,* ID

1 School of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430200, China; [email protected] (M.Z.S.); [email protected] (M.N.P); [email protected] (M.A.H.) 2 Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China 3 NUS Graduate School for Integrative Science & Engineering, National University of Singapore, 28 Medical Drive, Singapore 117456, Singapore 4 Research Institute of Flexible Materials, School of Textiles & Design, Heriot-Watt University, Galashiels TD1 3HF, UK 5 Institutes of Chemistry, Chinese Academy of Sciences, Beijing 100190, China * Correspondence: [email protected] (S.M.); [email protected] (H.-H.L.); Tel.: +86-131-1665-5691 (S.M.); +86-150-7136-6519 (H.-H.L.)

Academic Editor: Stephen C. Bondy Received: 14 August 2017; Accepted: 25 August 2017; Published: 18 September 2017

Abstract: In this study, a novel functionalization approach has been addressed by using sodium alginate (Na-Alg) assisted green silver nanoparticles (AgNPs) on traditional “Rajshahi silk” fabric via an exhaustive method. The synthesized nanoparticles and coated silk fabrics were characterized by different techniques, including ultraviolet–visible spectroscopy (UV–vis spectra), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FT-IR), which demonstrated that AgNPs with an average size of 6–10 nm were consistently deposited in the fabric surface under optimized conditions (i.e., pH 4, temperature 40 ◦C, and time 40 min). The silk fabrics treated with AgNPs showed improved colorimetric values and color fastness properties. Moreover, the UV-protection ability and antibacterial activity, as well as other physical properties—including tensile properties, the crease recovery angle, bending behavior, the yellowness index, and wettability (surface contact angle) of the AgNPs-coated silk were distinctly augmented. Therefore, green AgNPs-coated traditional silk with multifunctional properties has high potential in the textile industry.

Keywords: silver nanoparticles; silk; functional ﬁnishing; wettability; surface modiﬁcation

1. Introduction In recent years, nanoparticles have generated great interest in different ﬁelds, including chemistry, physics, materials science, life sciences, and engineering due to their superior properties; for example, optical, magnetic, electronic and catalytic properties [1]. Among metallic nanoparticles (NPs), the size of AgNPs ranges between 1 nm and 100 nm, making them one of the best candidates for several applications including biosensing, antibacterial, antiviral, and antifungal activities, drug delivery, catalysis, electrochemical, conductivity [2,3], and so on, with exponential accretion production. Generally, a number of physical and chemical methods are available for the preparation of metal NPs [4,5], which are not environmentally friendly. Recently, the ever-increasing interest has been ampliﬁed by requests for “green” synthesis methods [6]. Therefore, knowledge of current

Fibers 2017, 5, 35; doi:10.3390/fib5030035 www.mdpi.com/journal/fibers Fibers 2017, 5, 35 2 of 18 environmental issues is considered to be the present motivation for the green synthesis of nanoparticles. Ideally, the synthesis of nanoparticles from a green chemistry perspective is associated with three main points: the choice of the solvent medium, use of an environmentally benign reducing agent, and a source of nontoxic material for the stabilization of the nanoparticles. Some researchers have reported the green preparation of nanoparticles through natural polymers like chitosan, soluble starch, polypeptide, heparin, and hyaluronan used as reducing and stabilizing agents [7]. Recently, polysaccharide-based materials have been associated with the synthesis of green AgNPs through the use of an eco-friendly benign solvent; i.e., using water and polysaccharides as capping agents [8]. Sodium alginate is a type of polysaccharide polymer that is isolated from marine algae and consists of β-D-mannuronic (M) and its stereoisomer α-L-guluronic (G) acid that forms a kind of linear block copolymer of branched chains. It is ideal for use due to its excellent cytocompatibility and biocompatibility, biodegradation, sol–gel transition properties, and chemical versatility that makes it more viable in terms of making modifications to tailor its properties [9,10]. A number of free hydroxyl and carboxyl groups from its backbone allow it to dissolve well in water due to a negatively-charged carboxyl group. In the synthesis of sodium alginate (Na-Alg)-assisted AgNPs, the reaction between Na-Alg and Ag+ possibly leads to the formation of a Na-Alg complex [Ag (Na-Alg)]+, which is responsible for the formation of AgNPs by silver ion reduction in the presence of alginic acid regeneration. Darroudi et al. reported that using glucose can reduce silver ions to metallic silver to form AgNPs, and through this process it is oxidized to glucolic acid [11]. The better stability of alginates compared to chitosan has been well documented due to their noteworthy properties, such as being water-soluble that covers gel formation in the absence of heating or cooling, and also playing a pivotal role in trapping molecules—which remain free to migrate by diffusion, depending on their size—through capillary forces. These features make alginates ideal for the stabilization of AgNPs in order to fulfill the demand of the eco-friendly or green synthesis approach. Nowadays, the modification of silk material with AgNPs has attracted a good deal of interest for diverse applications in the clinical, safety, and production engineering, water technology, clothing, lightweight creation, and automotive industries [12]. Rajshahi silk is the name given to the silk produced in Rajshahi, Bangladesh. It is a famous name in the clothing and textiles sector. Although the situation of the Bangladesh textile market is presently dominated by synthetic products, the charming features of Rajshahi silk allow it to maintain its celebrated position [13]. Rajshahi silk is an aerial and bendable fiber produced from the cocoons of silkworms, and is covered with a protein known as sericin that has been commonly used in textiles for thousands of years due to its inherent luster, notable flexibility, environmental friendliness, and exquisite mechanical strength [14]. Analysis of the chemical structure of silk reveals that the sericin protein is composed of 18 amino acids that—most importantly—have sturdy polar moieties which include hydroxyl, carboxyl, and amino groups that are sufficient to bind charged functional institution of natural or inorganic substances. As a natural protein fiber, silk possesses a structure similar to human skin with easy, breathable, tender, non-itchy, and antistatic characteristics. The occurrence of these distinct properties makes silk an ideal material for the selective adhering of metal ions [15]. However, research around the advancement of “Rajshahi silk” fiber properties in order to retain their traditional position through a functionalization approach is limited. To the best of our knowledge, this is the first report of the surface treatment on Rajshahi silk fabric with green-synthesized AgNPs by using Na-Alg as both a reducing and stabilizing agent. Data from the fabrics treated with this method have been noted in detail in this study.

2. Materials and Methods

2.1. Materials The mulberry silk fabric was collected from the Bangladesh Sericulture Research and Training Institute (BSR and TI), Rajshahi, Bangladesh. The materials used to synthesize AgNPs were Fibers 2017, 5, 35 3 of 18 Fibers 2017, 5, 35 3 of 18

QingdaoAgNO3 (ShanghaiYingfei Chemical Zhanyun Co., Chemical Ltd., Qingdao, Co., Ltd., Chin Shanghai,a), and sodium China), hydroxide sodium alginate (Sinopharm (Mw ~ Chemical 68,000 Da. ReagentQingdao Co., Yingfei Ltd., Chemical Beijing, Co.,China). Ltd., Nonionic Qingdao, co China),mmercial and detergents sodium hydroxide were used (Sinopharm to wash Chemicalthe silk sample.Reagent These Co., materials Ltd., Beijing, were China).used throughout Nonionic the commercial experiment detergents without further were usedpurification to wash bythe using silk deionizedsample. Thesewater. materials were used throughout the experiment without further purification by using deionized water. 2.2. Green Synthesis of Silver Nanoparticles 2.2. Green Synthesis of Silver Nanoparticles A typical synthesis of green AgNPs was accomplished by using an indigenous protocol put A typical synthesis of green AgNPs was accomplished by using an indigenous protocol put together in our laboratory. In brief, 1 mm of AgNO3 was prepared from silver nitrate salt and 1% w/v together in our laboratory. In brief, 1 mm of AgNO was prepared from silver nitrate salt and 1% w/v of Na-Alg was prepared with deionized water. NaOH3 solution was prepared by dissolving NaOH of Na-Alg was prepared with deionized water. NaOH solution was prepared by dissolving NaOH (0.399 g) in deionized water (100 mL). Typically, 1 mL of Na-Alg (1% w/v) solution was dropped in 4 (0.399 g) in deionized water (100 mL). Typically, 1 mL of Na-Alg (1% w/v) solution was dropped in mL of silver nitrate (0.001 m) solution with the addition of NaOH solution (approximately 1 mL of 4 mL of silver nitrate (0.001 m) solution with the addition of NaOH solution (approximately 1 mL 0.01 m) to maintain a pH of 11 in order to accelerate the reaction. These mixtures were stirred for 1 of 0.01 m) to maintain a pH of 11 in order to accelerate the reaction. These mixtures were stirred for min. Then, the mixture was kept in a heat bath (60 °C) for 40 min. The transparent colorless solution 1 min. Then, the mixture was kept in a heat bath (60 ◦C) for 40 min. The transparent colorless solution became pale yellow and then brownish-red in color, indicating the formation of AgNPs, which was became pale yellow and then brownish-red in color, indicating the formation of AgNPs, which was further confirmed by using UV–vis spectra. further confirmed by using UV–vis spectra. 2.3. Application of AgNPs to Rajshahi Silk 2.3. Application of AgNPs to Rajshahi Silk The degumming of the raw silk through the removal of sericin from the fibroin was carried out The degumming of the raw silk through the removal of sericin from the fibroin was carried according to [16] with minor modifications. Typically, the degumming process was done three times out according to [16] with minor modifications. Typically, the degumming process was done three with 0.05% Na2CO3 solution at boiling point for 30 min. Then, the resulting samples were washed times with 0.05% Na2CO3 solution at boiling point for 30 min. Then, the resulting samples were with distilled water and dried at room temperature in order to prepare for the subsequent step. washed with distilled water and dried at room temperature in order to prepare for the subsequent Then, the silk fabric was functionalized with the prepared AgNPs through use of an exhaustive step. Then, the silk fabric was functionalized with the prepared AgNPs through use of an exhaustive method [17]. The important parameters (pH, temperature, and time) of the AgNPs application to silk method [17]. The important parameters (pH, temperature, and time) of the AgNPs application to were observed. The beakers containing silver solutions were kept in a shaker bath. Since the AgNPs’ silk were observed. The beakers containing silver solutions were kept in a shaker bath. Since the pH is naturally alkaline, the pH was adjusted by adding weak acid, and the temperature was AgNPs’ pH is naturally alkaline, the pH was adjusted by adding weak acid, and the temperature maintained using the thermostat control on the shaker machine. When the optimum conditions (i.e., was maintained using the thermostat control on the shaker machine. When the optimum conditions the pH and temperature of the AgNPs in the bath) were reached, the silk fabric was immersed and (i.e., the pH and temperature of the AgNPs in the bath) were reached, the silk fabric was immersed treated at 40 °C for 40 min. Finally, the obtained AgNPs-treated samples were placed for drying at and treated at 40 ◦C for 40 min. Finally, the obtained AgNPs-treated samples were placed for drying at room temperature without rinsing. The whole functionalization process is shown in Scheme 1. room temperature without rinsing. The whole functionalization process is shown in Scheme1.

Scheme 1. Schematic diagram of silver nanoparticles coating on silk fabric. Scheme 1. Schematic diagram of silver nanoparticles coating on silk fabric.

2.4. Measurements and Characterization The UV–vis spectra of silk fabrics with AgNPs were recorded using a UV-2600 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The morphologies of samples were observed by using a scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan) after gold coating. During the SEM test, an energy dispersive X-ray spectroscopy (EDS) (JEOL Ltd., Tokyo, Japan) spectrum was collected to analyze the composition of chemical elements of the prepared samples. Transmission electron microscopy (TEM) (Hitachi H-7600, Tokyo, Japan) was used to study the particle size distribution of the AgNPs. The crystal behavior of samples was obtained using an X-ray diffractometer (XRD) (Bruker D8 ADVANCE, Karlsruhe, Germany). Fourier transform infrared (FT-IR) measurements were taken with a (Bruker Corporation, Tensor 27, Karlsruhe, Germany) set to a normal transmission mode. Analysis of the thermal behavior of samples was performed in a thermo-gravimetric analyzer (TGA) (Mettler-Toledo Corp., Greifensee, Switzerland) at a heating rate of 10 ◦C/min in a nitrogen atmosphere. The color strength (K/S) and CIELAB color coordinates (under illuminate D65 and 10◦ standard observer) values of samples were measured with the help of a Macbeth Color Eye 7000A spectrophotometer (Gretag Macbeth Ltd., Regensdorf, Switzerland). The color fastnesses to light, ISO 105-BO5, and color fastness to washing, ISO 105 E04, were evaluated. The UV protective characteristics of the original and the silver-coated fabrics were determined in accordance with the Australian/New Zealand Standard AS/NZS 4399:1996 by using a UV–visible spectrophotometer. The antibacterial efﬁciency of the green-synthesized AgNPs in the silk fabrics against Staphylococcus aureus ATCC 6538 as Gram-positive and Escherichia coli ATCC 8739 as Gram-negative bacteria [18] was determined. The tensile properties of the silk fabrics were tested before and after AgNPs treatment. The crease recovery angle of the samples was determined as per AATCC Test Method 66-2003 using a Sasmira crease recovery tester (SASMIRA, Mumbai, India). The stiffness in terms of the bending length of untreated and AgNPs-treated samples was tested as per AATCC Test Method 115-2005 using a Sasmira stiffness tester (SASMIRA, Mumbai, India). Static contact angles were measured with a Kruss contact angle instrument (DSA 100, Kruss GmbH, Hamburg, Germany) using deionized water droplets (4 mL) at 25 ◦C. The moisture regained by each sample was measured in standard atmospheric conditions using the ASTMD 2495 test standard.

3. Results and Discussion

3.1. Characterization of AgNPs In the present investigation, AgNPs were synthesized via a “green” method using a natural biopolymer alginate assisted by heating an aqueous solution of Na-Alg and AgNO3. The preliminary afﬁrmation of nanoparticles formation was ascertained by recording the absorbance of the colloidal suspension with the use of UV–visible spectrophotometer (Shimadzu Corporation, Kyoto, Japan) in the range of 200–700 nm and simple observation for any color change. The solution turned reddish brown in color, indicating the reduction of silver ions and hence acknowledging the synthesis of the desired AgNPs, as shown in (Figure1A inset). The distinctive colors of AgNPs become apparent due to a phenomenon known as plasmon absorbance. Incident light creates oscillations in the conduction electrons on the surface of the nanoparticles, and electromagnetic radiation is scattered [19]. Figure1A shows the UV–vis spectra of pure silver nitrate (AgNO3), SA (sodium alginate), and AgNPs. In the presence of Na-Alg, the band at ~301 nm exhibited by AgNO3 in aqueous solution disappears, indicating possible chelation of Ag+ by the OH and COOH groups from the alginate. The Ag+ chelate produces Ag◦ on heating. The intensity and role of the shoulder of the alginate spectrum at approximately 290 nm barely shifted, conﬁrming that there were interactions between the polymer and the metallic precursor. The UV–vis spectrum of the Ag-NPs showed a characteristic peak at about 400 nm, assigned to a strong surface plasmon resonance which has been properly documented for various metal nanoparticles with sizes ranging from 2 to 100 nm [20]. Fibers 2017, 5, 35 5 of 18

For the synthesis of AgNPs, the typically accepted mechanism shows a two-step process; i.e., atom formation and then polymerization of the atoms. In the first step, a portion of metal ions in the solution is reduced by the available reducing groups. The atoms produced which act as nucleation centers and used to catalyze the remaining metal ions in the bulk solution. Compared with other water-soluble polymers, Na-Alg is an anionic polymer with a high charge density; the negatively-charged alginate allows the positively-charged of silver ions to attach themselves in the polymeric chains, which were reduced in presence of reducing groups. The resulting surface negative charge of alginate fragments containing carboxylic groups stabilizes nanoparticles against coalescing with each other due to electrostatic repulsion and steric outcomes [21]. The FT-IR measurements were executed to analyze the functional groups of the material. Figure1C shows the pure Na-Alg spectra presenting two primary peaks at about 3435 cm−1 (corresponding to the absorption of stretching of the OH groups) and at −1 2025 cm (corresponding to the C–H stretching of the CH2 groups). The CO asymmetrical stretching at a maximum of 1609 cm−1 along with a weaker symmetrical stretching band at 1414 cm−1 [22] were empiric due to the salt nature of carboxylic acid groups in pure Na-Alg. An accelerated band at 1083 cm−1 reflects the stretching of C–O–C group; the peaks at 601 cm−1 belong to C–O–C glycosidic linkage (ring breathing) [23]. They are accompanying its saccharide structure [24]. In the case of − −1 −1 AgNPs, the band of CO2 shifted to 1613 cm , while the peaks of 3435 and 1414 cm shifted to 3422 and 1384 cm−1, respectively, due to ring stretching of metal groups and indicating that Na-Alg − was doped by NO3 because of the stabilization of AgNPs. The comparison between the Na-Alg and Na-Alg/AgNPs FT-IR spectra only showed minor changes in position as well as the absorption bands. Therefore, the FT-IR spectrum confirms that AgNPs have been capped with the aid of the lone pair electrons around the oxygen atoms in organic compound in Na-Alg with Van der Waals forces [25]. Figure1B presents an SEM image of Na-Alg-loaded AgNPs. It is evident that green-synthesized AgNPs can cautiously be admired as nanoparticles with a spherical nature. Due to the interactions of hydrogen bonds and electrostatic interactions between the bioorganic capping molecules bound to the Ag-NPs expose larger as a result of the Ag-NPs agglomeration was revealed [26]. The TEM images with the corresponding particle size distribution (PSD) of the prepared AgNPs are shown in Figure1D,E. This demonstrates the formation of AgNPs and offers us a clear view of the shape, length, and distribution of the particles in nanoscale. From the image, it can be seen that the located AgNPs are round in form and nicely separated in the aqueous medium, which covers them with a layer. This layer might be the phytoconstituents of Na-Alg. In addition, the aggregation is lower because AgNPs collide less frequently. From the particle size distribution curve, it can be seen that the most of the particles sizes ranging from 6 nm to 10 nm. . The reductive properties of Na-Alg are notably more desirable owing to the base hydrolysis with the formation of low molecular weight reducing fragments, consequently reflecting the twin function of Na-Alg as a stabilizing and reducing agent in an alkaline medium [27]. The additional support of the reduction of Ag+ ions to elemental silver confirmed by EDS analysis is shown in Figure1F. The optical absorption peak is observed at approximately 3 keV, which is typical for the absorption of metallic silver nanocrystals due to surface plasmon resonance [28], confirming the presence of nanocrystalline elemental silver. An XRD pattern of Na-AgNPs was completed to verify that the crystal phase of the prepared AgNPs fell within the range of 30◦–80◦. Figure1G illustrates the typical XRD pattern of the alginate–AgNPs prepared with several distinct diffraction peaks at 38.1◦, 44.2◦, 64.3◦, and 77.4◦, which are assigned to reflections from the (111), (200), (220), and (311) planes of the silver crystal, respectively. This confirms the existence of silver; furthermore, they can be indexed as the face-centered-cubic (FCC) structure of silver. These peaks occur due to the crystalline and amorphous natural stages that accompany crystallized AgNPs. In addition to the Bragg peaks representative of FCC silver nanocrystals, additional, and yet unassigned peaks were also observed due to amoebic compounds that were responsible for silver ion reduction and the stabilization of the resultant nanoparticles [29]. Fibers 2017, 5, 35 6 of 18 Fibers 2017, 5, 35 6 of 18

Figure 1. (A) UV–vis spectra; (B) SEM; (C) Fourier transform infrared (FT-IR) spectra; (D) TEM; (E) particle Figure 1. (A) UV–vis spectra; (B) SEM; (C) Fourier transform infrared (FT-IR) spectra; (D) TEM; (E) size distribution (PSD); (F) energy dispersive X-ray spectroscopy (EDS); and (G) XRD of AgNPs. particle size distribution (PSD); (F) energy dispersive X-ray spectroscopy (EDS); and (G) XRD of AgNPs. 3.2. Optimization of Silk Treatment Conditions 3.2. OptimizationThe inherent of natureSilk Treatment of protein-based Conditions Bombyx mori silk is amphoteric; an important reason behindThe this inherent remarkable nature property of protein- is demonstratedbased Bombyx by attributingmori silk is ionizable amphoteric; groups an ofimportant the side chainreason of behindvarious this amino remarkable acids that property dissociated is demonstrated equilibrium state by attributing is inﬂuenced ionizable by pH conditionsgroups of the [30 side]. As chain a result, of various amino acids that dissociated equilibrium state is influenced by pH conditions [30]. As a Fibers 2017, 5, 35 7 of 18 Fibers 2017, 5, 35 7 of 18 result, this property of silk is responsible for attracting and binding the charged metal ions [31]. When silk was immersed in an aqueous solution of metal salts, it showed the propensity to engross metalthis property cations, ofwhile silk the is responsible rate and extent for attracting of the upta andke bindingdepended the on charged several metal factors, ions such [31]. as When the type silk ofwas metal immersed and its in valence an aqueous state; solution the pH of of metal the a salts,pplication it showed medium; the propensity the time toof engrossexhaustion; metal and cations, the temperaturewhile the rate of andthe extentapplication of the bath. uptake However, depended the on effect several of the factors, application such as medium the type pH of metal(keeping and otherits valence parameters state; theconstant pH of for the 40 application min at a temperat medium;ure the of time 40 of°C) exhaustion; on the AgNPs-treated and the temperature fabric was of the application bath. However, the effect of the application medium pH (keeping other parameters investigated. On the absorption of AgNPs,◦ the silk fabric acquired a pale-yellow shade that can be computedconstant for by 40 the min K/S at value a temperature of the treated of 40 silkC) fabric. on the Fr AgNPs-treatedom Figure 2a, it fabric is clear was that investigated. the K/S values On theof theabsorption treated silk of AgNPs, with 35 the ppm silk and fabric 70 acquiredppm AgNPs a pale-yellow were 0.4465 shade and that 0.6848, can berespectively, computed byat pH the K/S4.0, indicatingvalue of the an treated excellent silk uptake fabric. Fromof AgNPs Figure by2a, th ite issilk. clear When that thethe K/SAgNPs values application of the treated medium silk withwas more35 ppm acidic and (pH 70 ppmabout AgNPs 3.0), the were K/S 0.4465value of and the 0.6848, treated respectively, fabric was comparatively at pH 4.0, indicating less—about an excellent 0.3235 anduptake 0.4952 of AgNPsfor 35 ppm by the and silk. 70 Whenppm AgNPs, the AgNPs respectively. application However, medium in was a more more alkaline acidic (pH medium about (pH 3.0), 10),the the K/S K/S value values of the came treated to 0.2701 fabric and was 0.2857 comparatively for silk fabric less—about treated with 0.3235 35 andppm 0.4952 and 70 for ppm 35 ppm AgNPs, and respectively,70 ppm AgNPs, which respectively. is very near However, to the K/S ina value more of alkaline the untreated medium silk, (pH re 10),presenting the K/S valuesa much came lower to uptake0.2701 andof AgNPs. 0.2857 Some for silk studies fabric have treated reported with 35 that ppm the and AgNPs 70 ppm produce AgNPs, a negative respectively, zeta potential which is ( very−35 mV)near around to the K/S the valuesurface of thewhen untreated dispersed silk, in representing water [32]. aTherefore, much lower the uptake low uptake of AgNPs. of the Some AgNPs studies by − thehave silk reported fabric thatat athe higher AgNPs pH produce could be a negative explained zeta by potential negative–negative ( 35 mV) around repulsion the surfacebecause when the amphotericdispersed in nature water [of32 ].the Therefore, silk fabric the lowdeveloped uptake ofa thestronger AgNPs negative by the silkcharge fabric on at aits higher surface. pH Furthermore,could be explained due to bythis negative–negative very amphoteric nature, repulsion the because silk fabric the becomes amphoteric more nature positively of the charged silk fabric in adeveloped lower pH a and stronger hence negative more easily charge attracts on its surface. the negatively-charged Furthermore, due AgNPs; to this very this amphotericresults in higher nature, uptake.the silk fabric becomes more positively charged in a lower pH and hence more easily attracts the negatively-charged AgNPs; this results in higher uptake.

◦ FigureFigure 2. 2. EffectEffect of ofthe the (a) (pHa) pH(time (time 40 min, 40 min, temp temp 40 °C); 40 (bC);) temperature (b) temperature (pH 4, (pHtime 4,40time min); 40and min); (c) ◦ effectand ( cof) effectthe application of the application time (pH time 4, temp (pH 4,40 temp°C) on 40 theC) particle on the particleuptake in uptake terms in of terms K/S value. of K/S value. FibersFibers 20172017,, 55,, 3535 8 of 18

The effect of temperature (keeping other parameters constant for 40 min at pH 4) on the AgNPs uptakeThe was effect evaluated of temperature in terms (keeping of the K/S other values parameters of the treated constant silk forfabrics; 40 min these at pHare 4)shown on the in AgNPs Figure uptake2b. From was Figure evaluated 2b, it can in terms be observed of the K/Sthat valuesthe K/S of value the treateddecreased silk dramatically fabrics; these with are increasing shown in Figuretemperature,2b. From demonstrating Figure2b, it can a bedecrease observed in thatthe AgNP the K/Ss uptake. value decreased The absorption dramatically of AgNPs with increasingby the silk temperature,sample might demonstrating be an exothermic a decrease interaction, in the AgNPs and uptake.hereafter The any absorption increase ofin AgNPs the application by the silk sampletemperature might results be an exothermic in lower AgNPs interaction, uptake. and hereafterExperiments any increasewere repeated in the application several times temperature at each resultstemperature in lower to AgNPsstudy the uptake. effect Experimentsof temperature. were Ther repeatedefore, severalit cannot times be accidental at each temperature that there to was study an theimportant effect of difference temperature. in AgNPs Therefore, uptake it cannot at each be temperature accidental thatlevel. there was an important difference in AgNPsThe uptake effects at of each exhaustion temperature time (keeping level. other parameters constant; at pH 4 and a temperature of 40 °C)The of effectsAgNPs of on exhaustion the silk fabric time (keepingin terms otherof the parameters K/S value can constant; be seen at pHin Figure 4 and a2c. temperature The figure ◦ ofreveals 40 C) that of AgNPsthe K/S onvalue the increased silk fabric between in terms 10 of theand K/S40 min. value This can is be due seen to inthe Figure changing2c. The pattern figure of revealsthe silk that materials the K/S and value the increasedzeta potential between of the 10 andAgNPs. 40 min. The This silk is surface due to thehad changing a positive pattern charge of and the silkAgNPs materials surface and had the zetaa negative potential charge, of the AgNPs.which Theresulted silk surfacein a strong had a positiveattraction charge between and AgNPs them. surfaceTherefore, had the a negative rate of Ag-NPs charge, whichuptake resulted was very in ahigh strong at 40 attraction min and between beyond them. this time Therefore, no significant the rate ofchange Ag-NPs was uptake appeared was in very the highcurve at and 40 min can andbe considered beyond this as time asymptotic no significant behavior change up to was the appeared80 min. in the curve and can be considered as asymptotic behavior up to the 80 min. 3.3. Absorption Characteristics 3.3. Absorption Characteristics The UV–vis absorption spectrum of the solution of AgNPs before and after coating is presented in FigureTheUV–vis 3A. It can absorption be noticed spectrum that the of absorbance the solution units of AgNPs of the before70 ppm and AgNPs after coatingsolution is was presented higher inthan Figure the 335A. ppm It can AgNPs be noticed solution that theafter absorbance coating. This units could of the be 70 due ppm to AgNPsthe high solution affinity was of the higher silk, thanwhich the is 35sufficient ppm AgNPs to adsorb solution nearly after all coating.Ag+ and ThisAgNPs could present. be due This to theshows high that affinity silk fabric of the has silk, a whichbetter absorption is sufficient capability to adsorb for nearly AgNPs all Ag+in an and alka AgNPsline environment. present. This This shows can thatbe explained silk fabric by has the a betterdecreased absorption repulsion capability forces between for AgNPs the negatively-cha in an alkalinerged environment. AgNPs and This the cansilk befibers. explained The colors by the of decreasedthe treated repulsion fabrics forcesthat betweenemerged thewere negatively-charged associated with AgNPs the surface and the plasmon silk fibers. resonance The colors of(SPR) the treatedproperties fabrics of green-synthesized that emerged were AgNPs. associated In order with to the analyze surface the plasmon SPR of resonance AgNPs on (SPR) the treated properties fabric, of green-synthesizedthe absorption traits AgNPs. of the In order 35 and to analyze 70 ppm the SPRAgNPs-treated of AgNPs on fabrics the treated were fabric, evaluated the absorption using a traitsspectrophotometer of the 35 and 70within ppm the AgNPs-treated range of 200 fabricsand 800 were nm, evaluatedas shown usingin Figure a spectrophotometer 3B. From this figure, within it is theclear range that ofthe 200 control and 800 fabric nm, had as shown no absorption in Figure 3spB.ectra. From On this the figure, contrary, it is clear the thatAgNPs-treated the control fabricfabric hadexhibited no absorption an absorption spectra. band On located the contrary, in the range the AgNPs-treated of 410–470 nm, fabric which exhibited indicates an the absorption presence of band the locatedsurface inplasmon the range absorption of 410–470 band nm, which of AgNPs; indicates th theese presenceafter-effects of the enhanced surface plasmonthe depositing absorption of bandgreen-synthesized of AgNPs; these AgNPs after-effects on the silk. enhanced the depositing of green-synthesized AgNPs on the silk.

Figure 3.3. UV–vis spectra:spectra:( A(A)) for for residual residual Ag Ag NPs NPs solution solution with with before before and and after after treatment; treatment; and and (B) the(B) untreatedthe untreated and and AgNPs-treated AgNPs-treated fabrics. fabrics.

3.4. FT-IR and XRDXRD AnalysisAnalysis FT-IR spectral analysis analysis was was performed performed to to observ observee the the growth growth of ofpolymer polymer and and AgNPs AgNPs on the on silk the silkfabrics. fabrics. Figure Figure 4A,4 (a),A, (a),(b), (b),and and (c) show (c) show the theFT-IR FT-IR spectra spectra of both of both untreated untreated and and AgNPs-treated AgNPs-treated (35 ppm and 70 ppm, respectively) fabric samples. Figure 4A (a) shows the FT-IR spectrum of silk that is Fibers 2017, 5, 35 9 of 18

Fibers(35 ppm 2017, 5 and, 35 70 ppm, respectively) fabric samples. Figure4A (a) shows the FT-IR spectrum of9 of silk 18 that is characterized by strong bands over 3000 cm−1 (due to OH and NH stretching vibrations) −1 characterizedand various amideby strong bands bands at 1550,over 3000 1250, cm and (due 650 cmto OH−1 (from and NH stretching stretching vibrations vibrations) of CNand andvarious NH −1 amidedeformation bands vibrations),at 1550, 1250, which and are 650 typical cm of(from polypeptides stretching and vibrations proteins andof CN allow and their NH conformational deformation vibrations),characterization which [33 ].are Compared typical of with polypeptides the FT-IR spectrum and proteins of the untreatedand allow silk their fabric, conformational the spectrum characterizationof the silver-coated [33]. silk Compared fabric also with has the these FT-IR characteristic spectrum of peaks, the untreated making itsilk possible fabric, tothe observe spectrum the of the silver-coated silk fabric also has these characteristic peaks, making it possible to observe the chelation between silk and silver. Subfigures (b) and (c) from Figure4A show the –NH 2 group at chelation3200 cm− between1 (stretching silk frequencyand silver. of Subfigures the C=O) after(b) and the (c) modification from Figure of 4A AgNPs, show which the –NH demonstrates2 group at −1 3200that itcm successfully(stretching bound frequency to the of fabric. the C=O) This shiftafter occurs the modification because of theof AgNPs, reduction which of the demonstrates double bond thatcharacter it successfully of the C=O, bound which to the is fabric. due to This the synchronizationshift occurs because of oxygen of the reduction to the metal of the center double and bond is in characteraccordance of withthe C=O, the effects which caused is due by to thethe added synchronization complexes definedof oxygen previously to the metal [34]. center and is in accordanceSilk fibroin with isthe known effects to caused be crystalline by the added and accelerates complexes the defined X-ray diffractionpreviously pattern.[34]. Figure4B (a), (b), andSilk (c) fibroin show is the known XRD spectra to be crystalline of untreated and and accelerates Ag nanoparticles the X-ray (35 diffraction ppm and 70 pattern. ppm, respectively) Figure 4B (a),treated (b), fabricand (c) samples. shows the Figure XRD4B, spectra (a) provides of untreated the XRD and spectra Ag nanoparticles of pristine silk (35 fabricppm thatand shows70 ppm, a respectively)strong diffraction treated peak fabric (2θ =samples. 20.7◦), a Figure weak diffraction 4B, (a) provides peak (2 theθ = 24.5XRD◦ ),spectra and two of shoulderpristine silk diffraction fabric thatpeaks shows (2θ = a 28.8 strong◦ and diffraction 40.0◦). These peak XRD (2θ diffraction = 20.7°), a peaks weak arediffraction attributed peak to the (2θβ -sheet= 24.5°), structure and two of shouldernative silk diffraction [35], indicating peaks the(2θ high= 28.8° crystallinity and 40.0°). of These pristine XRD silk diffra fabric.ction Compared peaks are with attributed this, the to XRD the βpattern-sheet structure of the treated of native silkexhibited silk [35], indicating five new XRD the high peaks crystallinity with 2θ = 38.4 of pristine◦, 44.6◦, silk 64.8 fabric.◦, 77.8 ◦Compared, and 81.9◦ with(Figure this,4B), the which XRD canpattern be ascribed of the treated to (111), silk (200), exhibited (220), five (311), new and XRD (222) peaks planes with of the2θ face-centered= 38.4°, 44.6°, 64.8°,cubic 77.8°, (FCC) and lattice 81.9° of (Figure silver (JCPDS 4B), which card. can No. be 4-0783) ascribed [36 ].to The(111), peaks (200), are (220), broadened (311), and due (222) to the planes small ofsize the of face-centered NPs. Finally, fromcubic the (FCC) XRD lattice spectra of it issilv possibleer (JCPDS to conclude card. No. that 4-0783) the crystalline [36]. The nature peaks of are the broadeneduntreated anddue treated to the small samples size was of NPs. not altered Finally, by from AgNPs, the XRD which spectra could it be is clarified possible further to conclude by the that fact thethat crystalline the Bragg’s nature angle of is the the untreated same for both and samples.treated samples was not altered by AgNPs, which could be clarified further by the fact that the Bragg’s angle is the same for both samples.

FigureFigure 4. 4. ((A) FT-IR spectraspectra andand ( B(B)) XRD XRD spectra spectra of of the the silk; silk; (a) ( untreated,a) untreated, and and treated treated with with (b) 35 (b ppm) 35 ppmand (andc) 70 (c ppm) 70 ppm AgNPs AgNPs at optimized at optimized condition. condition.

3.5. Thermal Properties The thermal behavior of the control and AgNPs-treated fabrics was tested by thermo-gravimetric analysis (TGA); the results are shown in Figure5A. The degradation temperature—known as a standardFibers 2017, of5, 35 thermal degradation—was measured based on the TGA curves. Figure5A, (a), (b), and10 of (c) 18 illustrate the TGA curves of the control (untreated) and the Ag nanoparticles (35 ppm and 70 ppm, respectively)nanoparticles treated (35 ppm silk and samples, 70 ppm, with respectively) two peak temperatures treated silk samples, and three with successive two peak stages temperatures of weight loss.and three The initial successive weight stages loss ofof theweight control loss. and The AgNPs-treated initial weight samplesloss of the came control to about and AgNPs-treated 5.5% and 4.5%, respectively,samples came until to about about 5.5% 198 ◦ C.and As 4.5%, stated respectively, previously, theseuntil about weight 198 changes °C. As could stated be previously, explained by these the removalweight changes of adsorbed could water be explained [37]. Due toby the the hydrophilic removal of nature adsorbed of silk, water it was [37]. subjected Due to tothe a dehydrationhydrophilic process,nature of in silk, which it was absorbed subjected or crystal to a waterdehydration was removed, process, and in which the color absorbed turned yellow.or crystal A weightwater was loss ofremoved, about 50% and and the 57%color was turned observed yellow. in theA weight control loss silk of and about the AgNPs-treated50% and 57% was sample observed respectively, in the incontrol the temperature silk and the range AgNPs-treated of 290–500 sample◦C, which respec cantively, be associated in the temperature with the breakdown range of 290–500 of theside °C, chainwhich groups can be of associated the silk structure with the [38 breakdown]. Another substantialof the side weightchain groups loss of aboutof the 23% silk occurredstructure in [38]. the 500–650Another◦ substantialC range for weight both samples, loss of andabout can 23% be attributedoccurred in to the breakdown500–650 °C ofrange the mainfor both chain samples, groups ofand silk can ﬁbroin, be attributed as reported to previously the breakdown [39]. The of massthe lossmain in chain the ﬁrst groups stage ofof the silk TG fibroin, curves (200–300as reported◦C) ispreviously due to the [39]. moisture The mass evaporating loss in the from first thestage silk. of the The TG moisture curves content(200–300 (%) °C) of is the due silk to the treated moisture with nanoparticlesevaporating from is shown the silk. in FigureThe moisture5B. It can content be observed (%) of the that silk loading treated the with material nanoparticles with nanoparticles is shown in ledFigure to an 5B. increase It can inbe theobserved moisture that content loading of thethe silkmaterial since thewith silk nanoparticles treated with led AgNPs to an (70 increase ppm AgNPs) in the presentedmoisture content the highest of the moisture silk since content the silk (7%). treated This with results AgNPs are (70 in goodppm AgNPs) agreement presented with the the results highest of silvermoisture content content is shown (7%). This in physical results propertiesare in good of agr samples.eement with the results of silver content is shown in physical properties of samples.

(A) (B)

Figure 5. ((A)) Thermo-gravimetricThermo-gravimetric analysis analysis (TGA) (TGA) curve: curve: (a )( controla) control silk; silk; and and treated treated silk withsilk with(b) 35 (b ppm) 35 AgNPsppm AgNPs and (c and) 70 ppm(c) 70 AgNPs. ppm AgNPs. (B) Moisture (B) Moisture content content (%). (%).

3.6. Scanning Electron Microscope (SEM) 3.6. Scanning Electron Microscope (SEM) The surface morphologies of the treated fabrics were observed by SEM in order to observe the The surface morphologies of the treated fabrics were observed by SEM in order to observe the assembly of AgNPs on the fabric surface. The SEM micrographs are shown in Figure 6. Figure 6a assembly of AgNPs on the fabric surface. The SEM micrographs are shown in Figure6. Figure6a shows the pristine fabric morphology with a typically smooth longitudinal fibril surface structure. shows the pristine fabric morphology with a typically smooth longitudinal fibril surface structure. The morphologies of fabrics with AgNPs incorporated are shown in Figure 6 (b), (c). It can be seen The morphologies of fabrics with AgNPs incorporated are shown in Figure6b,c. It can be seen that a that a uniform layer of nano-sized Ag was only deposited on both fabrics’ surfaces, which indicates uniform layer of nano-sized Ag was only deposited on both fabrics’ surfaces, which indicates that the that the morphologies of treated fabrics were not damaged during the functionalization process of morphologies of treated fabrics were not damaged during the functionalization process of the AgNPs, the AgNPs, and the coloration of silk was influenced by this behavior of AgNPs on the fiber surface. and the coloration of silk was influenced by this behavior of AgNPs on the fiber surface.

Figure 6. SEM micrographs of (a) control silk and silk treated at (b) 35 ppm and (c) 70 ppm AgNPs. Fibers 2017, 5, 35 10 of 18

nanoparticles (35 ppm and 70 ppm, respectively) treated silk samples, with two peak temperatures and three successive stages of weight loss. The initial weight loss of the control and AgNPs-treated samples came to about 5.5% and 4.5%, respectively, until about 198 °C. As stated previously, these weight changes could be explained by the removal of adsorbed water [37]. Due to the hydrophilic nature of silk, it was subjected to a dehydration process, in which absorbed or crystal water was removed, and the color turned yellow. A weight loss of about 50% and 57% was observed in the control silk and the AgNPs-treated sample respectively, in the temperature range of 290–500 °C, which can be associated with the breakdown of the side chain groups of the silk structure [38]. Another substantial weight loss of about 23% occurred in the 500–650 °C range for both samples, and can be attributed to the breakdown of the main chain groups of silk fibroin, as reported previously [39]. The mass loss in the first stage of the TG curves (200–300 °C) is due to the moisture evaporating from the silk. The moisture content (%) of the silk treated with nanoparticles is shown in Figure 5B. It can be observed that loading the material with nanoparticles led to an increase in the moisture content of the silk since the silk treated with AgNPs (70 ppm AgNPs) presented the highest moisture content (7%). This results are in good agreement with the results of silver content is shown in physical properties of samples.

(A) (B)

Figure 5. (A) Thermo-gravimetric analysis (TGA) curve: (a) control silk; and treated silk with (b) 35 ppm AgNPs and (c) 70 ppm AgNPs. (B) Moisture content (%).

3.6. Scanning Electron Microscope (SEM) The surface morphologies of the treated fabrics were observed by SEM in order to observe the assembly of AgNPs on the fabric surface. The SEM micrographs are shown in Figure 6. Figure 6a shows the pristine fabric morphology with a typically smooth longitudinal fibril surface structure. The morphologies of fabrics with AgNPs incorporated are shown in Figure 6 (b), (c). It can be seen that a uniform layer of nano-sized Ag was only deposited on both fabrics’ surfaces, which indicates Fibersthat2017 the, 5 ,morphologies 35 of treated fabrics were not damaged during the functionalization process11 of of 18 the AgNPs, and the coloration of silk was influenced by this behavior of AgNPs on the fiber surface.

FigureFigure 6. 6.SEM SEM micrographs micrographs of of ( a(a)) control control silksilk andand silk treated treated at at ( (bb) )35 35 ppm ppm and and (c ()c 70) 70 ppm ppm AgNPs. AgNPs. Fibers 2017, 5, 35 11 of 18 3.7. Color Measurements and Fastness 3.7. Color Measurements and Fastness TheThe color color characteristics characteristics (i.e., (i.e. K/S K/S values, values, L*,L*, a*,a*, andand b*)b*) ofof thethe silksilk coatedcoated withwith AgNPsAgNPs areare listedlisted in Tablein 1Table. Typically, 1. Typically, by measuring by measuring color color strength strength (K/S) (K/S) values, values, the the amount amount of dyeof dye as as a percentage a percentage of of the coloredthe colored fabric canfabric be determined,can be determined, where awhere higher a K/Shigher value K/S shows value greatershows colorgreater intensity, color intensity, indicating a greaterindicating amount a greater of AgNPs amount incorporated of AgNPs incorporated in the fabric. in The theresults fabric. revealThe results that allreveal K/S that values all ofK/S the silkvalues samples of the with silk AgNPs samples included with AgNPs were included notably were higher notably than higher the untreated than the untreated ones; this ones; is due this to is the formationdue to the of AgNPsformation that of cause AgNPs coloration that cause trait colorati of the material.on trait of The the lightness material. (L*)The of lightness silver-coated (L*) of silk samplessilver-coated was lower silk thansamples that was of the lower original than silk, that whileof the a* original and b* silk, values while increased a* and dramatically.b* values increased Positive a*dramatically. and b* values Positive were obtained a* and b* forvalues all treatedwere obtained fabric samples; for all treated this isfabric because samples; the modifiedthis is because fabrics appearedthe modified yellowish-brown fabrics appeared in color yellowish-brown (Figure7). The in differentcolor (Figure color 7). of The the different treated fabricscolor of could the treated be due to thefabrics formation could be of largedue to discrete the formation particles of and large the discrete aggregation particles of small and particles.the aggregation The absorption of small of AgNPsparticles. relies The significantly absorption on of the AgNPs form, relies length, signif andicantly application on the media form, andlength, varying and synthesisapplication conditions media canand influence varying these synthesis characteristics conditions [40 can]. Therefore, influence increasingthese characteristics the concentration [40]. Therefore, of AgNPs increasing can affect the the absorptionconcentration features of AgNPs of the loadedcan affect AgNPs, the absorption but might features not confirm of the any loaded correlation AgNPs, between but might the not color strengthconfirm of any the treatedcorrelation silk andbetween the Ag the content. color strength It was observedof the treated that thesilk K/S and value the Ag can content. be a measure It was of observed that the K/S value can be a measure of the concentration of deposited AgNPs on the fabric the concentration of deposited AgNPs on the fabric due to the unique optical properties of AgNPs [41]. due to the unique optical properties of AgNPs [41]. In Table 1, it is possible to see that washing- and In Table1, it is possible to see that washing- and light-fastness properties range from good to very light-fastness properties range from good to very good. It was observed that due to the presence of good. It was observed that due to the presence of chemical linkage (i.e., Van der Waals forces) in the chemical linkage (i.e., Van der Waals forces) in the Ag NPs-treated fabrics, they showed good Ag NPs-treated fabrics, they showed good fastness properties. fastness properties.

Table 1. Color coordinate data (CIE Lab) and K/S values of untreated and AgNPs-treated silk. Table 1. Color coordinate data (CIE Lab) and K/S values of untreated and AgNPs-treated silk.

SampleSample L* L* A* A* B* B* K/S K/S CFL1 1 CFWCFW2 2 UntreatedUntreated silk silk 89.97 89.97 −0.07−0.07 8.54 8.54 0.2603 0.2603 - - - 35 35ppm ppm AgNPs AgNPs 83.68 83.68 1.72 1.72 15.43 15.43 0.4465 0.4465 6 4–5 4–5 70 ppm AgNPs 81.93 0.77 22.23 0.6848 6 4–5 70 ppm AgNPs 81.93 0.77 22.23 0.6848 6 4–5 1 Colorfastness to light; 2 Colorfastness to wash. 1 Colorfastness to light; 2 Colorfastness to wash.

FigureFigure 7. 7.Photographs Photographs of of (a ()a) control control silk, silk, andand AgNPs-treatedAgNPs-treated fabric fabric with with (b (b) )35 35 ppm ppm and and (c ()c 70) 70 ppm. ppm.

3.8. UV Protection The ultraviolet radiation (UVR) band consists of three regions: the UV-A band (320–400 nm); the UV-B band (290–320 nm); and the UV-C band (200–290 nm). The damage to human skin from UV radiation occurs in the range of UV-A. Recently, metal nanoparticles and metal salts have been used as UV protecting agents [42]. Therefore, the effect of green-based AgNPs on the optical properties of fabrics—specifically, the UV transmittance of untreated and treated silk samples—was measured using UV transmittance spectroscopy at the wavelength range between 200 and 440 nm (Figure8a). The transmittance spectra of all samples demonstrated obvious differences. Typically, the untreated Fiberssilk showed2017, 5, 35the highest percentage of UV transmittance compared to those treated, which suggests12 of 18 that silk fabric had no significant effect on UV protection properties. Once the silk was functionalized decreased to about 46% in comparison with the raw silk fabric. It was found that the incorporation with 35 ppm AgNPs, the corresponding spectrum of AgNPs-treated silk decreased to about 46% in of 70 ppm AgNPs on fabric-treated clearly reduced UV transmittance to 84%, suggesting that comparison with the raw silk fabric. It was found that the incorporation of 70 ppm AgNPs on AgNPs offer high protection from UV rays. These results indicate the ability of the AgNPs-treated fabric-treated clearly reduced UV transmittance to 84%, suggesting that AgNPs offer high protection fabrics to block UV rays from passing through and reaching the skin. The high UV protection levels from UV rays. These results indicate the ability of the AgNPs-treated fabrics to block UV rays from of the finished fabrics with incorporated AgNPs could come about due to the large refractive index passing through and reaching the skin. The high UV protection levels of the finished fabrics with of AgNPs, resulting in very efficient UV scattering [43]. Previous studies have revealed that AgNPs incorporated AgNPs could come about due to the large refractive index of AgNPs, resulting in very enhance the UV protection property of fabric due to the plasmon structure of AgNPs [44]. efficient UV scattering [43]. Previous studies have revealed that AgNPs enhance the UV protection Therefore, our results indicate that green-based AgNPs absorb UV rays, which is consistent with property of fabric due to the plasmon structure of AgNPs [44]. Therefore, our results indicate that previous findings. green-based AgNPs absorb UV rays, which is consistent with previous findings.

FigureFigure 8. (a )8. UV (a transmission) UV transmission spectra andspectra (b) stress–strain and (b) stress–strain curve for the untreatedcurve for and the AgNPs-treated untreated and fabrics. AgNPs-treated fabrics. 3.9. Antibacterial Activity 3.9. Antibacterial Activity The bacterial inhibition efﬁciency of AgNPs-treated fabrics was investigated using the zone of inhibitionThe bacterial test, inhibition which is a efficien qualitativecy of analysisAgNPs-treated involving fabrics an agarwas diffusioninvestigated test. using To examine the zone the of inhibitionantibacterial test, efﬁcacy which of is the a qualitative samples, bacteria-inoculated analysis involving agar an platesagar diffusion were used test. as media,To examine as shown the antibacterialin Figure9. Bacterial efficacy growthof the samples, was noted bacteria-inoculated by evaluating the agar size plates of the diameterwere used of as zones media, of inhibitionas shown inaround Figure silk 9. Bacterial treated with growth a commercial was noted antibacterial by evaluating agent, the size and of fabric the diameter treated with of zones green-synthesized of inhibition aroundAgNPs silk (Table treated2), thus with investigating a commercial the antibacteria inhibitory effectl agent, of and green-based fabric treated AgNPs. with The green-synthesized inhibition zone AgNPstest results (Table in 2), Figure thus9 investigatingclearly show the that inhibitory the commercial effect of antibacterial green-based agentAgNPs. displayed The inhibition the largest zone testinhibition results zones in Figure for E. 9 coli clearly(24 ± show1.04) andthatS. the aureus commercial(29 ± 0.52). antibacterial The AgNPs-treated agent displayed fabrics also the showedlargest inhibitionvery obvious zones inhibition for E. zonescoli (24 around ± 1.04) the and specimen, S. aureus and (29 the ± 0.52). diameter The of AgNPs-treated each transparent fabrics zone also was showedclose to very the commercial obvious inhibition sample. zones The concentrations around the specimen, of AgNPs and played the diameter a vital role of each in certifying transparent the zonezone was of inhibition close to results.the commercial As shown sample. in Table The2, thereconcentrations were more of silver AgNPs ions played when thea vital fabric role was in certifyingfunctionalized the zone using of 70inhibition ppm AgNPs, results. yielding As shown a sample in Table which 2, there exhibited were more a better silver inhibitory ions when effect the fabricagainst was both functionalized microorganisms using with 70 a zoneppm ofAgNPs, inhibition yielding up to a 23 sample± 0.38 mmwhich in diameterexhibited fora betterE. coli inhibitoryand 26 ± 0.46effect mm against in diameter both microorganisms for S. aureus; whereas with a thezone sample of inhibition with 35 ppmup to AgNPs 23 ± 0.38 established mm in diameterzones of inhibitionfor E. coli and upto 26 22 ± ±0.460.44 mm mm in indiameter diameter for for S. E.aureus coli;and whereas 24 ± the0.66 sample mm in with diameter 35 ppm for AgNPs established zones of inhibition up to 22 ± 0.44 mm in diameter for E. coli and 24 ± 0.66 mm in diameter for S. aureus. This phenomenon was attributed to the mechanism of biocidal action of the fabric enhanced by the leaching of Ag+ ions [45]. In addition, AgNPs are extremely reactive with proteins because they adversely affect cellular metabolism electron transfer systems and strongly inhibit the substrate transportation into cell membranes when these nanoparticles come in to contact with bacteria and fungi responsible for infection, odor, itchiness, and sores. A quantitative assessment of the antibacterial activity of functionalized silk with synthesized green-based AgNPs was also performed according to the percentage reduction method (AATCC 100) shown in Table 2. AgNPs have a significant effect on bacterial reduction; both concentrations of AgNPs caused the inhibition rate to increase to above 90%. When the concentration of AgNPs increased, antibacterial activity against E. coli and S. aureus improved slowly and stably. This can be described in terms of a Fibers 2017, 5, 35 13 of 18

certain degree of a sterilizing effect due to the presence of metallic ions, as well as metallic compounds. Perhaps a segment of atmospheric oxygen in water or air is transformed into active oxygen due to the catalysis of metallic ion, thereby dissolving the organic substance to enhance sterilizing action [46]. Furthermore, AgNPs are able to increase their contact with microbes because of their extremely large relative surface area, which results in the enhancement of antimicrobial success. The antibacterial performance demonstrated in the above experiments is perfectly in agreement with the broad spectrum of antimicrobial activity of AgNPs, showing that this could be a good competitor for conventional antibacterial agents.

Table 2. Antibacterial activity of silk fabric coated with green-based AgNPs.

Bacterial Reduction (%) Zone of Inhibition (mm) Samples E. coli S. aureus E. coli S. aureus Commercial 95 ± 2.2 97 ± 4.2 24 ± 1.04 29 ± 0.52 35 ppm AgNPs 88 ± 1.10 91 ± 2.08 22 ± 0.44 24 ± 0.66 70 ppm AgNPs 90 ± 2.12 94 ± 4.06 23 ± 0.38 26 ± 0.46 Values represent means ± standard deviations from three experiments.

Fibers 2017, 5, 35 13 of 18 3.10. Surface Wettability The wettability of all fabric samples with water was measured by taking obvious contact angle S.measurements. aureus. This phenomenon As the thermodynamic was attributed property to the mechanismthat characterizes of biocidal the wettability action of the of fabric solid enhancedsurfaces, + bythe thecontact leaching angle of Agis ofions utmost [45]. importance In addition, in AgNPs modern are technological extremely reactive applications with proteins and materials because theyscience. adversely Contact affect angles cellular are metabolismsensitive to electron many transferfactors, systemssuch as and surface strongly geometry, inhibit the roughness, substrate transportationcontamination, intodeformation, cell membranes etc. Such when sensitivity these nanoparticles enables the detection come in of to very contact small-scale with bacteria outcomes and fungivia this responsible macroscopic for infection,measurement odor, [47]. itchiness, When and the sores.water Acontact quantitative angle is assessment smaller than of the90°, antibacterial the surface activityis identified of functionalized as hydrophilic; silk withif the synthesized water contact green-based angle is AgNPs larger wasthan also 90°, performed the surface according is called to thehydrophobic. percentage The reduction data on method the contact (AATCC angle 100) meas shownurements in Table is2. shown AgNPs in have Figure a signiﬁcant 10. These effect results on bacterialdepict the reduction; evolution both of water concentrations droplets on of the AgNPs surface caused of the the silk inhibition samples rateover totime, increase indicating to above that 90%. the Whenwater thedroplets concentration on untreated of AgNPs silk disappeared increased, antibacterial rapidly with activity respect against to theE. behavior coli and S. of aureus waterimproved droplets slowlyon the treated and stably. sample. This The can contact be described angle inof termsthe untrea of a certainted silk degree fabric ofwas a sterilizing0° after 10 effects. Similarly, due to the presencewater droplets of metallic on the ions, surface as well of as AgNPs-treated metallic compounds. silk fabric Perhaps disappeared a segment within of atmospheric 20 s. This oxygen clearly in watershows or that air AgNPs-coating is transformed intosignificantly active oxygen increased due to th thee surface catalysis contact of metallic angle ion,of the thereby silk fabric. dissolving The thegreater organic water substance contact angle to enhance exhibited sterilizing by the actionAgNPs-treated [46]. Furthermore, samples implies AgNPs that are the able hydrophobic to increase theirproperty contact of silk with fabric microbes was becauseimproved. of theirThis can extremely be attributed large relative to the surfaceformation area, of whichstable resultschemical in thestructures enhancement on the fabric of antimicrobial surface after success. AgNPs The coating. antibacterial Compared performance with untreated demonstrated silk fabric, in the the slower above experimentschange rate of is perfectlywater droplets in agreement on the withAgNPs-tr the broadeated spectrum silk suggests of antimicrobial that the assembly activity of AgNPs,AgNPs showingincreased that the thishydrophobicity could be a good of the competitor silk. for conventional antibacterial agents.

Figure 9.9. AntibacterialAntibacterialactivity activity of of (1 ()1 the) the commercial commercial and and AgNPs-treated AgNPs-treated fabrics fabrics (2) 35(2 ppm,) 35 ppm, (3) 70 (3 ppm) 70 onppm (a )onE. ( colia) E.and coli ( band) S. ( aureusb) S. aureus. .

Table 2. Antibacterial activity of silk fabric coated with green-based AgNPs.

3.10. Surface Wettability The wettability of all fabric samples with water was measured by taking obvious contact angle measurements. As the thermodynamic property that characterizes the wettability of solid surfaces, the contact angle is of utmost importance in modern technological applications and materials science. Contact angles are sensitive to many factors, such as surface geometry, roughness, contamination, deformation, etc. Such sensitivity enables the detection of very small-scale outcomes via this macroscopic measurement [47]. When the water contact angle is smaller than 90◦, the surface is identiﬁed as hydrophilic; if the water contact angle is larger than 90◦, the surface is called hydrophobic. The data on the contact angle measurements is shown in Figure 10. These results depict the evolution of water droplets on the surface of the silk samples over time, indicating that the water droplets on Fibers 2017, 5, 35 14 of 18 untreated silk disappeared rapidly with respect to the behavior of water droplets on the treated sample. The contact angle of the untreated silk fabric was 0◦ after 10 s. Similarly, the water droplets on the surface of AgNPs-treated silk fabric disappeared within 20 s. This clearly shows that AgNPs-coating signiﬁcantly increased the surface contact angle of the silk fabric. The greater water contact angle exhibited by the AgNPs-treated samples implies that the hydrophobic property of silk fabric was improved. This can be attributed to the formation of stable chemical structures on the fabric surface after AgNPs coating. Compared with untreated silk fabric, the slower change rate of water droplets on Fibersthe AgNPs-treated 2017, 5, 35 silk suggests that the assembly of AgNPs increased the hydrophobicity of the15 of silk. 18

FigureFigure 10. 10. WaterWater contact contact angles on the silk fabr fabricic surface, before and after washing.

4.3.11. Conclusions Mechanical Properties TheFigure present8b demonstrates paper proposed the tensilea novel behavior approach of to silk the samples functionalization treated with of “Rajshahi AgNPs of silk different fabric” byconcentrations. an exhaustive The method analysis through of the the stress–strain green synthe curvesis of indicates AgNPs thatfor the the first AgNPs-treated time. The FT-IR silk samples spectra confirmedexhibitedthe the same presence elasticity of functional and then continuously groups, while hardened TEM image until finaland failure.XRD data The reveal diagrams that show the synthesizedthat the tensile AgNPs force possess for the silkgood sample crystalline treated stru withctures. AgNPs SEM wasstudies up toshowed 2.50% that more the than AgNPs that ofwere the well-depositeduntreated one. Basedon the onfiber this surface, result, itwhile can be the concluded residue weight that the observed Na-Alg-stabilized in the TGA AgNPs-treated experiment furthersilk sample proved improved the successful the mechanical synthesis properties of AgNPs ofon the the fabric. silk fiber The surface. reason forThe this effects may of be pH, that time, the andintercalation temperature nature were of silveralso studied helps to while disperse AgNPs nanoparticles were applied in the on silk silk, fiber where which optimized undergoes conditions slippage wereunder determined tensile loading at pH [48 4,]. and Another 40 °C explanationfor 40 min. AgNPs could be treatment attributed enhanced to the fact the that color the strength particle of size silk is andin the also nano improved range, allowing the fastness the AgNPs towards to enter light in and between washing, the polymer which structuresuggests andthis thusmethod possibly can overcomefunctioning the as limitations a filler or crosslinkingof traditional agent, dyeing further processes. contributing The treatment to the loadwith shearing AgNPs improved phenomenon the fabric’sduring thetensile application properties of load and tocrease the material. recovery This angl resulte, with is in almost agreement no effect with on previous the rigidity research of [the49]. material.The loss ofThe strength results due of the to thebacterial treatment test isconfirm arounded 20%, that and the asproposed such, the method observed is highly loss in effective strength liesfor antibacterialwell within the action, reported as the values antibacterial and is acceptedactivity in increased the industry. with increasing concentration of AgNPs on the fiber surface. 3.12. Physical Properties Acknowledgments: The authors wish to thanks the Runhe Chemical Industry, China & Color Root (Hubei) Table3 gives the data on the various physical properties of the treated fabric at varying Technology Limited, China, for providing the technical support and the School of Chemistry & Chemical Engineering,concentrations Wuhan of AgNPs. Textile University, There was China, slight for improvement providing chemicals in crease and recovery all measurements. angle of AgNPs-treated silk fabric with a small increase in bending length. This phenomenon confirms that the particles Author Contributions: S.M. and H.-H.L conceived the paper; S.M. designed and performed the experiments; M.Z.S. performed the wettability experiments; M.N.P. performed the XRD experiments; M.A.H. performed the TGA experiments; S.M. and M.N.P. wrote the paper; and all the authors reviewed and edited the final paper.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Dubey, S.P.; Lahtinen, M.; Särkkä, H.; Sillanpää, M. Bioprospective of sorbus aucuparia leaf extract in development of silver and gold nanocolloids. Colloids Surf. B 2010, 80, 26–33. 2. Chen, W.; Yan, J.; Song, N.; Li, Q.; Yang, B.; Dai, Y. Study on silver nanopowder prepared by chemical reduction with an organic reductant. Guijinshu (Pre. Met.) 2006, 27, 14–17. Fibers 2017, 5, 35 15 of 18 slipping between the polymer molecules do not contribute much to the overall polymer ﬂexibility. The behavior is thus devoid of harshness to the materials. No distinct change could be felt by hand between untreated and AgNPs-treated silk; all the silk samples felt equally soft and smooth. To further observe the inﬂuences of the assembly of AgNPs on the handling of the silk, bending lengths were measured (Table3). Since fabric bending length is related to fabric stiffness or handling, these results suggest that the assembly of AgNPs did not change the handling of the silk samples, as the size of AgNPs lies in nano range. The effects of the AgNPs on the yellowness of silk samples treated with AgNPs are also described in Table3. It is clear that the green-synthesized AgNPs caused a slight yellowing compared with the untreated silk fabric. The untreated silk fabric sample had the lowest yellowness, while the fabric sample treated with the highest AgNPs concentration also had the highest yellowness index value (82 for 70 ppm AgNPs). This might result from pyrolysis and oxidation of the surface components [50].

Table 3. Physical properties of samples.

Samples Ag Content (mg/kg) CRA (deg) Bending Length (cm) Yellowness Index Untreated Silk - 124 2.30 22 35 ppm AgNPs-treated Silk 7125 ± 15 132 (±6.02) 2.46 (±6.00) 65 70 ppm AgNPs-treated Silk 8362 ± 23 135 (±5.55) 2.53 (±5.02) 82

4. Conclusions The present paper proposed a novel approach to the functionalization of “Rajshahi silk fabric” by an exhaustive method through the green synthesis of AgNPs for the first time. The FT-IR spectra confirmed the presence of functional groups, while TEM image and XRD data reveal that the synthesized AgNPs possess good crystalline structures. SEM studies showed that the AgNPs were well-deposited on the fiber surface, while the residue weight observed in the TGA experiment further proved the successful synthesis of AgNPs on the silk fiber surface. The effects of pH, time, and temperature were also studied while AgNPs were applied on silk, where optimized conditions were determined at pH 4, and 40 ◦C for 40 min. AgNPs treatment enhanced the color strength of silk and also improved the fastness towards light and washing, which suggests this method can overcome the limitations of traditional dyeing processes. The treatment with AgNPs improved the fabric’s tensile properties and crease recovery angle, with almost no effect on the rigidity of the material. The results of the bacterial test confirmed that the proposed method is highly effective for antibacterial action, as the antibacterial activity increased with increasing concentration of AgNPs on the fiber surface.

Acknowledgments: The authors wish to thanks the Runhe Chemical Industry, China & Color Root (Hubei) Technology Limited, China, for providing the technical support and the School of Chemistry & Chemical Engineering, Wuhan Textile University, China, for providing chemicals and all measurements. Author Contributions: S.M. and H.-H.L. conceived the paper; S.M. designed and performed the experiments; M.Z.S. performed the wettability experiments; M.N.P. performed the XRD experiments; M.A.H. performed the TGA experiments; S.M. and M.N.P. wrote the paper; and all the authors reviewed and edited the final paper. Conflicts of Interest: The authors declare no conflict of interest.

References

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