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In-situ synthesis of gold nanoparticles for multifunctionalization of silk fabrics
Dyes and Pigments 103 (2014) 183e190
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
In-situ synthesis of gold nanoparticles for multifunctionalization of silk fabrics Bin Tang a, b, Lu Sun a, b, Jasjeet Kaur b, Yao Yu b, Xungai Wang a, b, * a b
Ministry of Education Key Laboratory for Textile Fibers and Products, Wuhan Textile University, Wuhan 430073, China Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2013 Accepted 9 December 2013 Available online 18 December 2013
A simple in-situ synthesis route for gold nanoparticles (NPs) was developed to realize multifunctions for silk fabrics. The gold NPs were prepared in a heated solution containing white silk fabric samples. The silk fabrics were colored red and brown by the gold NPs because of their localized surface plasmon resonance (LSPR) property. Gold nanospheres on silk were obtained at a low gold content, and gold nanoplates were synthesized as the gold content increased. The silk fabrics treated with gold NPs showed good light fastness. Moreover, the gold NPs endowed silk fabrics with strong antibacterial activity, excellent UV protection property and enhanced thermal conductivity. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Coloration Silk Gold nanoparticle In-situ synthesis Antibacterial UV blocking
1. Introduction Noble metal nanoparticles (NPs) present brilliant and fascinating colors due to their localized surface plasmon resonance (LSPR) properties. LSPR is derived from the interaction of light and the metal NPs, when conductive electrons oscillate locally around the metal NPs at a certain frequency. The phenomenon of excitation of surface plasmons generated by light is termed as LSPR [1]. The LSPR property of noble metal NPs is important for applications in surface enhanced spectroscopy [2,3], sensing [4], and nanophotonic devices [5]. Noble metal NPs with bright colors have been used as decorative pigments for glass [6] and ceramics [7] since ancient times. More recently, these particles have been used for textiles including cotton [8e10] and wool [11e14]. The noble metal NPs are different from conventional dyes, in that it is the LSPR property not the chromophore that gives rise to captivating colors. This optical feature of noble metal NPs is related to particle shape, size, composition, environment, and interspaces [15e20]. The shape and size of the NPs govern their LSPR optical features, which have been illustrated already [21,22]. Therefore, the color of the noble metal NPs can be adjusted by controlling their shape and size.
* Corresponding author. Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia. Tel.: þ61 3 5227 2894. E-mail address: [email protected] (X. Wang). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.12.008
Textile products with multiple functionalities have generated great interest in recent years. Many attempts have been made to enhance functionalities of textiles, such as antibacterial [23], selfcleaning [24], and UV protection [25]. Silk as a natural protein fiber is widely used in the textile industry due to its inherently elegant sheen, excellent flexibility, environmental friendliness and good comfort. Recently, some strategies based on modification with NPs have been developed to enhanced functions of silk fabrics. For examples, Zheng et al. colored silk fabrics using gold nanorods with different aspect ratios [9]. The gold nanorod coated silk fabrics showed significant improvements on both UV protection and antibacterial functions. But the washing fastness of silk fabrics colored with gold nanorods was unsatisfactory. Li et al. modified silk fibers with TiO2 and TiO2@Ag NPs through chemical assembly technique [26]. The treated silk fabric exhibited multifunctions including UV protection, antibacterial activity, and photocatalytic capability. Besides, the anisotropic silver NPs were assembled onto the silk fibers to impart different colors and antibacterial feature to silk fabrics [27]. In the present study, an in-situ synthesis method for gold NPs was described to functionalize silk fabrics. Color and optical properties were investigated at different levels of gold concentration. Moreover, the relation between morphologies and optical features of synthesized gold NPs on silk fibers were studied. The functions of silk fabrics treated with gold NPs, including UV blocking, antibacterial activity and thermal conductivity, were investigated.
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The colorfastness properties to washing and light of gold nanoparticle (NP) modified silk were evaluated as well.
2. Experimental 2.1. Materials Tetrachloroauric(III) acid (HAuCl4$3H2O, >99%) was purchased from SigmaeAldrich. The chemicals were analytic grade reagents, and used without further purification. Crepe satin silk fabrics, with a weight of 123.4 g/m2 and a density of 51 threads/cm in the warp direction and 41 threads/cm in the weft direction, were purchased from the local Beautiful Silks store.
2.2. Instruments
2.4. Colorfastness test to washing Washing fastness tests were performed in accordance with Australian Standard AS 2001.4.15d2006. The silk fabrics with gold NPs were washed for 45 min at 50 C in the presence of ECE reference detergent by using a lab dyeing machine (Ahiba, Top Speed Nuance). The CIE Lab color coordinate values (L*, a*, and b*) for each specimen were measured before and after washing. L* represents the lightness/darkness, a* value represents the red or green chroma, and b* represents the chromaticity coordinate for yellow/blue. The color difference (DE) was obtained based on the changes in color coordinates (DL*, Da*, and Db*) with the formula: DE ¼ [(DL*)2 þ (Da*)2 þ (Db*)2]1/2. The color difference (DE) of silk fabrics before and after washing was measured by spectrophotometer to assess washing fastness of silk fabrics according to Australian Standard AS 2001.4.A05d2004.
The UVevis absorption spectra of HAuCl4 solutions were obtained using a Varian Cary 3E UV/vis spectrophotometer. The UVevis diffuse reflectance absorption spectra of silk fabrics were recorded by a Varian Cary 5000 UVeVISeNIR spectrophotometer with a diffuse reflectance accessory (DRA-2500). Scanning electron microscopy (SEM) measurements were performed with a Supra 55 VP field emission SEM. The color strength (K/S) and color difference (DE) of silk fabrics with gold NPs were obtained using a Datacolor Spectraflash SF600 Plus-CT spectrophotometer. Fourier transform infrared (FTIR) spectra were measured with a PerkinElmer Fourier transform infrared spectrometer (FTIR-1730) in attenuated total reflection (ATR) mode. Infrared thermal images were recorded by an infrared thermography video camera (H2640, NEC). Heating reaction was performed in a Stuart SBS40 shaking water bath.
2.5. Measurement of fastness to light
2.3. In-situ synthesis of gold NPs in silk
Gram negative bacteria, Escherichia coli (E. coli) (ATCC 11229), were used as test organisms. Antibacterial test was performed on untreated and treated silk fabrics. The antibacterial test was carried out according to the AATCC 100-2004 (Clause 10.2) test standard with slight modifications. 50 mL of bacteria were added on treated samples in separate flasks. After 1.0 min, 50 mL of sterile deionized water was poured into each flask, followed by vigorous shaking. Then the flasks were incubated for 18 h in a shaker oven at 120 rpm at 37 C. After that, the fabric samples were collected and the solution left in the flask was further diluted to get countable number of bacterial colonies. 103 dilution was suitable for obtaining colonies between 30 and 300. 100 mL of the 103 dilution obtained were placed on the nutrient agar plates. These plates were then incubated for 18 h in an oven at 37 C. The bacterial activity was
Pristine silk fabrics were immersed in aqueous solutions with different concentrations of HAuCl4 (1.79 104 M, 2.38 104 M, 2.99 104 M, 4.48 104 M, 5.97 104 M and 7.46 104 M). The weight ratio of HAuCl4 solution to silk is 60. The HAuCl4 solutions with silk were placed for 30 min at room temperature. The silk changed to light yellow due to absorption of chloroaurate ions (AuCl 4 ). After that, the solutions were heated at 85 C for 60 min in an oscillating water bath. The color of silk in the solutions became red or brown. The silk fabrics were taken out and rinsed with running deionized water. And then the silk fabrics with different gold content (0.21 wt%, 0.28 wt%, 35 wt%, 0.53 wt%, 0.71 wt% and 0.88 wt%) were dried at room temperature.
The gold NP treated silk fabrics (8 4 cm) with different colors were stapled on to white cardboard, and a portion (4 4 cm) of each fabric specimen was covered with cardboard and aluminum foil. The specimens were then exposed to simulated sunlight for 60, 120 and 180 h inside the Suntest instrument (SUNTEST XLSþ from Atlas Material Testing Technology LLC). Color changes were determined with the Delta E value (DE CIELAB) by a Datacolor instrument. This parameter was used to quantify the difference between two colors of the samples with and without irradiation at each exposure period. 2.6. Antibacterial test
Fig. 1. Photographs of HAuCl4 solutions (corresponding to 0.53 wt% gold content) (a) before and (b) after silk fabric was soaked in solution for 30 min, and (c) plots of absorbance intensity of HAuCl4 solution around 290 nm as a function of soaking time corresponding to different gold content after the silk fabrics were immersed in HAuCl4 solution.
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recorded by photography. The test was carried out in triplicate and the entire experiments were repeated three times. 3. Results and discussion
Absorbance (a.u.)
The HAuCl4 solution became colorless after the silk fabrics were immersed, and the silk changed to yellow due to absorption of chloroaurate ions (Fig. 1a and b). The absorption of HAuCl4 on silk fabrics was very effective. The UVevis absorbance spectra of HAuCl4 solution were recorded at different time intervals during the soaking of silk corresponding to different gold content (Fig. S1 in supporting information). The UVevis absorbance spectrum of initial HAuCl4 solution presents two absorbance bands located at around 220 nm and 290 nm, which are attributed to charge transfer between the metal and chloro ligands [28e30]. The intensities of the two absorbance bands decreased dramatically after silk fabrics were immersed in the HAuCl4 solution and almost disappeared within 30 min, which implies that the chloroaurate (AuCl 4 ) ions in solution were absorbed by silk fibers (Fig. S1 in supporting information). Plots of absorbance intensity of HAuCl4 solution around 290 nm as a function of fabric soaking time are shown in Fig. 1c. The absorbance intensity decreased with time for different concentrations of HAuCl4 solutions. In the dyeing process of textiles, electrostatic interaction and hydrogen bonding were considered to contribute greatly to absorption of dye on fiber [31]. In the present study, the pH values of HAuCl4 solutions with different concentrations were 5e6.5. The silk fibers are full of amino and carboxyl groups [31,32]. The silk fabrics carried negative charges when the pH value was above the isoelectric point of silk (around 4) [33,34]. Therefore, it is possible that combination of AuCl 4 complex ions and silk fibers was achieved through hydrogen bonding interaction of amino group and AuCl 4 complex ions [35,36]. To further investigate the interaction of silk fibers and HAuCl4, the FTIR spectra of silk fabrics before and after absorption of AuCl 4 were obtained (Fig. 2). The FTIR spectrum of silk fabrics (Curve a in Fig. 2) presents amide absorbance bands, which are assigned in detail in 1 Table 1. After absorption of AuCl assigned 4 , the band at 1626 cm 1 to amide I shifted to 1619 cm and increased obviously in intensity. Also, the intensity of the shoulder band at 1698 cm1 (amide I) decreased evidently. Meanwhile, the absorbance band at 1508 cm1 (amide II) shifted to a low frequency (Curve b in Fig. 2). In addition, the amide III bands (1261 and 1228 cm1) downshifted slightly and decreased in intensity. The differences of FTIR spectra
a b
untreated silk silk with HAuCl4
c
silk with gold NPs
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Table 1 Assignment of absorbance bands in the FTIR spectrum of silk fabric. Wavenumber (cm1)
Assignment [52e54]
1698 1626 1508 1445 1261 1228 1162 1067
CO stench, Amide I CO stench, Amide I, b-sheet Amide II, b-sheet CH plane bending vibration, eCH2, backbone NH bend, Amide III, b-sheet CN stretch and CC stretch, Amide III, Random-coil CH3 vibration, Alanine, Residue CC stretch, Peptide, b-sheet
of silk fabrics were probably due to change in bonding environments around amide of silk fibers from absorption of AuCl 4 . The FTIR analysis implies that the hydrogen bonding force could lead to absorption of AuCl 4 complex ions on silk fibers [37,38]. The silk fabrics with AuCl 4 complex ions were heated at 85 C in solution and the gold NPs were in-situ synthesized on silk fibers. It has been suggested that tyrosine groups on silk fiber reduce gold ions to gold NPs [32,39e41]. Compared with the FTIR spectrum of silk fabric with AuCl 4 , absorbance bands corresponding to the silk fabric with gold NPs did not change visibly, which indicates that the in-situ synthesis of gold NPs has no observable influence on the chemical structure of silk fibers, consistent with the previous report [39]. The silk fabrics after heating became red or brown, which indicates that the gold NPs were synthesized on silk (Fig. 3). The color of the silk fabrics darkened and changed to brown as the gold content increased (Fig. 3). The gold NPs were in-situ prepared on silk only through heating. To clarify the influence of gold content on color of silk, the color strength (K/S) curves were plotted (Fig. 3). The color strength (K/S) values of silk corresponding to 0.21 wt%, 0.28 wt%, 0.35 wt%, 0.53 wt%, 0.71 wt%, and 0.88 wt% gold content were 2.22, 2.66, 3.06, 2.27, 1.81 and 1.80, respectively. The K/S values increased with an increase in gold content in the range of 0.21 wt% w 0.35 wt%. But the K/S decreased with increasing gold content in the range of 0.53 wt% w 0.88 wt%. It was found that the K/S spectra broadened when gold content was increased to 0.53 wt %, which is consistent with the brown colors of silk fabrics. The color of silk arose from the LSPR optical properties of gold NPs on fiber surface. In order to investigate the LSPR of gold NPs on silk fabrics, the UVevis absorbance spectra of silk fabrics with in-situ gold NPs were measured (Fig. 4). The UVevis absorbance spectrum of the silk fabrics with 0.21 wt% of gold content displays one single band at 540 nm that is attributed to LSPR of spherical gold NPs [42]. When the gold content increased to 0.28 wt%, one new shoulder band appeared in UVevis absorbance spectrum (Fig. 4).
1000 1100 1200 1300 1400 1500 1600 1700 -1
Wavenumber (cm ) Fig. 2. FTIR spectra of untreated silk, silk with HAuCl4 and silk treated with gold NPs.
Fig. 3. Photographs and K/S curves of the silk fabrics with in-situ synthesized gold NPs at different weight percent.
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2.5 untreated 0.21 wt% 0.28 wt% 0.35 wt% 0.53 wt% 0.71 wt% 0.88 wt%
Absorbance (a.u.)
2.0
1.5
1.0
0.5
0.0 300
400
500
600
700
800
900
1000
Wavelength (nm) Fig. 4. UVevis absorbance spectra of silk fabrics with different gold content.
The shoulder band increased as the gold content of silk fabrics increased (Fig. 4). The appearance of shoulder bands at long wavelength reveals that the gold NPs in-situ synthesized on silk fibers were anisotropic in shape [43,44].
The morphologies of gold NPs determined the optical properties of the treated silk fabrics. SEM characterization was performed to observe the morphologies of gold NPs on fiber surface with different gold content. The gold NPs on the silk with 0.21 wt% of gold content were spherical and their size was measured to be 21.3 3.4 nm (Fig. 5a). All the gold NPs in Fig. 5a were welldistributed on the surface of silk fiber, which led to the bright red color displayed by the treated silk fabrics (Fig. 3). When the gold content of silk fabrics increased to 0.28 wt%, small triangular gold nanoplates were seen in the SEM image (Fig. 5b), indicating anisotropic growth of gold NPs, which is consistent with deduction from UVevis absorption spectroscopy. As can be seen, the size of gold NPs increased with increase in gold content (Fig. 5). At a low gold content, the density of gold NPs increased when the gold content increased. The density of gold NPs reached maximum when the gold content of silk fabric increased to 0.35 wt% (Fig. 5c), which results in the highest color strength (K/S) and intensity of main UVevis absorption band among all the above samples (Figs. 3 and 5). The size of generated platelike gold NPs increased with increasing gold content (Fig. 5d). The gold nanoplates dominated in the synthesized gold NPs when the gold content of silk fabrics was 0.71 wt% and 0.88 wt% (Fig. 5e and f). The formation of gold nanoplates resulted in the multipole plasmon resonances and decrease in intensity of band around 540 nm, changing silk fabric to brown from red. The results indicate that the LSPR properties of
Fig. 5. SEM images of the silk with in-situ synthesized gold NPs at different gold content: (a) 0.21 wt%, (b) 0.28 wt%, (c) 0.35 wt%, (d) 0.53 wt%, (e) 0.71 wt% and (f) 0.88 wt%.
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3.0 2.5 2.0
E
gold NPs on silk fabrics can be controlled by changing the gold content. Consequently, the color of silk fabrics can be adjusted. The red color strength of silk fabrics cannot be enhanced by increasing the concentration of Au ions in the range of high concentration, due to formation of gold nanoplates. To obtain brighter red color of silk with gold NPs, multi-step in-situ coloration of silk was developed. Repeated additions of a small amount of HAuCl4 solution was implemented to avoid the generation of anisotropic gold NPs. As depicted in Fig. 6a, bright red color of silk was obtained through two separate additions of HAuCl4 solution. But the silk fabric was brown when the same total amount of HAuCl4 was added in the reaction system in one step (Fig. 6a). Fig. 6b shows the color strength curve of silk fabric after two separate additions of the same HAuCl4 solution to make the gold content of silk at 0.56 wt%. The color strength value of silk after first addition is 2.66. The value increased to 6.35 after second addition of HAuCl4 (Fig. 6b). The color strength value is much higher than that from first addition of HAuCl4. But the color strength value of silk fabric is 1.33 after double amount of HAuCl4 solution was added in the reaction solution once (Fig. 6b). Fig. 6c and d presents the SEM images of silk fabrics with same gold content obtained through different addition procedures of HAuCl4 solution. Most of the gold NPs on silk fabrics after two separate additions of HAuCl4 were spherical and density of gold NPs was higher than that of gold NPs through one addition of double amount of HAuCl4. Whereas, there were many gold nanoplates presenting in the SEM image of silk fiber from one addition of double amount of HAuCl4 (Fig. 6d). The results imply that repeated additions of a small amount of HAuCl4 solution can inhibit the growth of gold nanoplates and increase monodispersion of NPs, hence enhancing color of treated silk fabrics. Colorfastness is important for textiles. The washing colorfastness of silk fabrics with gold NPs was evaluated. The silk fabric with
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1.5 1.0 0.5 0.0 1
2
3
4
5
6
7
Washing cycles Fig. 7. Evolution of the color difference (DE) of silk fabrics with in-situ synthesized gold NPs (0.35 wt%) with increasing the number of washing cycles.
0.35 wt% of gold NPs was washed for 45 min in the presence of ECE reference detergent at 50 C under each washing cycle. The color differences (DE) of the silk fabrics before and after washing are shown in Fig. 7. The average DE value of fabric was measured to be 1.1 after the first washing cycle. Though the smallest color difference (DE) which can be noticed by human eye was considered to be around 1.0 [45], it was reported that the color difference with DE value more than 2.0 could be perceived correctly 100% of the time by human observers [46]. The DE value increased to around 2.0 and remained stable with further increase in number of washing cycles.
Fig. 6. (a) Photograph of silk fabrics after two separate additions and one addition of HAuCl4 solution. (b) UVevis absorption spectra of silk fabrics corresponding to different addition procedure of HAuCl4 solution. SEM images corresponding to (c) two separate additions and (d) one addition of HAuCl4 solution.
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The gray scale rating for color change after seven washing cycles was assessed to be 4 according to DE value. These results demonstrate that the silk fabrics treated with in-situ synthesized gold NPs have good washing fastness. Many existing dyes fade severely upon exposure to sunlight for prolonged periods [47], thus, avoiding the photo-fading of the colored textiles is a critical challenge in the industry, and has been extensively investigated for nearly two centuries [48]. Color-fading is attributed mainly to the photo-degradation of the chemical structures of the dyes. Though some additives, such as UV absorbers and antioxidants, have been explored to moderate the photo-fading of the dyed fabrics [49], they did not alter the intrinsic photo-fading property. In this study, gold NPs were used to color the silk fabrics, which was different from the traditional dyeing approach. The color generating from LSPR of gold NPs is expected to be stable under sunlight. Fig. 8 shows the color differences (DE) of the different silk fabrics after they were irradiated for 60 h under simulated sunlight. The DE of untreated silk fabric was 3.26. Nevertheless, the DE values of silk fabrics colored with gold NPs (0.35 wt% and 0.53 wt%) were around 1.0, so the color difference is barely noticeable by naked eye. Compared with the gold NP colorated silk fabrics, the colors of silk fabrics colored with commercial dyes (Lanasol RED 5B and Lanasol RED GN) changed obviously, as suggested by the much higher DE values (Fig. 8). To further confirm the sunlight fastness of silk fabrics with gold NPs, the silk fabric with 0.35 wt% of gold content continued to be exposed for 180 h under simulated sunlight. The average color difference (DE) of silk fabrics was measured to be less than 2.0 (Fig. S2 in supporting information). These results indicate that the silk fabrics with insitu synthesized gold NPs have good fastness to light. Furthermore, coloration of gold NPs imparted silk fabrics with additional functions. The UV-blocking property of the silk fabrics with gold NPs was tested. Table 2 shows the transmittance values of UV light and UV protection factor (UPF) values of the untreated silk fabric and the silk fabric with gold NPs. The average transmittance values of the silk fabrics decreased obviously after gold NPs were in-situ synthesized on the silk fabrics, which demonstrated that the gold NPs enhanced prominently the ability of UV-blocking of silk fabrics. Both transmittance values in UVA (315e400 nm) and UVB (280e315 nm) regions decreased with increasing the gold content on silk fabrics (Table 2). However, the transmittance values of treated silk fabrics were the same for 0.71 wt% and 0.88 wt% gold content, suggesting that the UV-blocking ability of silk fabrics remained consistent when the gold content was increased to the
Fig. 8. Color differences (DE) of different silk fabrics after exposed for 60 h by simulated sunlight.
Table 2 UV-blocking properties of the untreated silk fabric and the silk fabric with gold NPs at different weight percentage. Sample 0.00 wt% 0.21 wt% 0.28 wt% 0.35 wt% 0.53 wt% 0.71 wt% 0.88 wt% T(UVA) 10.99% T(UVB) 2.61% UPF 21.00
1.53% 0.61% 101.09
1.30% 0.57% 109.38
0.79% 0.50% 131.67
0.57% 0.42% 160.99
0.52% 0.39% 173.24
0.52% 0.39% 172.52
threshold value. UPF presents the ability of the fabric to block UV from passing through and reaching the skin. UPF value of untreated silk fabrics was measured to be 21.00 (Table 2). In-situ coloration with gold NPs increased the UPF value of silk fabrics by 4e7 times. The results indicate that the gold NP treated silk fabrics provide excellent UV protection. For summer clothing, a high heat transfer is desirable [50,51]. In this study, the heat conductivity of untreated and gold NP treated silk fabrics was compared. Infrared thermal images of a hand covered with untreated silk and gold NP treated silk over time are presented in Fig. 9. The red region of untreated silk enlarged and deepened gradually with time. Whereas, the infrared thermal images of silk treated with gold NPs did not change obviously. The series of thermal images imply that the temperature of untreated silk increased with time and that of treated silk remained unchanged. To analyze the change process of temperature for untreated and treated silk fabrics, the temperature values of marked positions (as shown in Fig. S3 in supporting information) corresponding to untreated and treated silk fabrics were plotted as a function of time (Fig. 9). The temperature of untreated silk increased to w31 C from initial temperature of w28 C within 80 s. However, the temperature of gold NP treated silk decreased to w30 C in a short time (<10 s) from w31 C and subsequently remained almost constant. The treated silk reached thermal balance rapidly with the hand. It can be inferred that the coloration
Fig. 9. Infrared thermal images of a hand covered by untreated silk and gold NP treated silk (0.71 wt%) recorded at different time, and plots of temperature of marked positions in Fig. S3 in supporting information as a function of time.
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Fig. 10. Antibacterial activity of (a) untreated and (b) gold NP treated silk fabrics.
from gold NPs has improved the heat conductivity of silk fabrics, which is in favor of enhancing comfort of silk clothing in the summer. Additionally, it was reported that the fabrics coated by gold nanorods exhibited antibacterial activities [9]. In the present research, the antibacterial properties of gold NP treated silk fabrics were evaluated against the Gram negative bacteria, E. coli. Fig. 10 shows photographs of the bacteria on untreated and treated silk. As can be seen, there were lots of colonies of viable bacteria corresponding to the untreated silk fabrics (Fig. 10a). Whereas, there was no bacteria colonies found on agar medium of silk fabrics treated with gold NPs (Fig. 10b), which indicates that the gold NPs on silk fabrics inhibited growth of bacteria. These results demonstrate that the silk fabrics treated with in-situ synthesized gold NPs possessed significant antibacterial activity. 4. Conclusions In-situ coloration of silk fabrics was achieved through synthesis of gold nanoparticles (NPs) on fiber surface by heating. The optical property of gold NPs imparted silk with different colors, which can be adjusted by controlling gold content on silk. The SEM characterization demonstrated that the gold content influenced the morphology of gold NPs produced on silk fibers. The silk fabrics treated with gold NPs exhibited obvious antibacterial feature and remarkable UV-blocking ability. In addition, the thermal conductivity of silk fabrics increased after treatment with gold NPs. The gold NP colored silk fabrics possessed good colorfastness to washing and light irradiation, which can facilitate industrial application of in-situ coloration technique involving noble metal NPs. Acknowledgments This research was supported by the National Natural Science Foundation of China (NSFC 51273153), and the Central Research Grants Scheme and Alfred Deakin Postdoctoral Research Fellowship Scheme at Deakin University. The authors thank Dr. Jian Fang from Deakin University for help with recording infrared thermal images. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2013.12.008.
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[22] Henry AI, Bingham JM, Ringe E, Marks LD, Schatz GC, Van Duyne RP. Correlated structure and optical property studies of plasmonic nanoparticles. J Phys Chem C 2011;115:9291e305. [23] Radeti c M. Functionalization of textile materials with silver nanoparticles. J Mater Sci 2013;48:95e107. [24] Wang R, Wang X, Xin JH. Advanced visible-light-driven self-cleaning cotton by Au/TiO2/SiO2 photocatalysts. ACS Appl Mat Interfaces 2009;2:82e5. [25] Wang L, Zhang X, Li B, Sun P, Yang J, Xu H, et al. Superhydrophobic and ultraviolet-blocking cotton textiles. ACS Appl Mat Interfaces 2011;3:1277e81. [26] Li G, Liu H, Zhao H, Gao Y, Wang J, Jiang H, et al. Chemical assembly of TiO2 and TiO2@Ag nanoparticles on silk fiber to produce multifunctional fabrics. J Colloid Interface Sci 2011;358:307e15. [27] Tang B, Li J, Hou X, Afrin T, Sun L, Wang X. Colorful and antibacterial silk fiber from anisotropic silver nanoparticles. Ind Eng Chem Res 2013;52:4556e63. [28] Esumi K, Suzuki A, Yamahira A, Torigoe K. Role of poly(amidoamine) dendrimers for preparing nanoparticles of gold, platinum, and silver. Langmuir 2000;16:2604e8. [29] Wang W, Han Y, Gao M, Wang Y. Template synthesis of braided gold nanowires with gemini surfactanteHAuCl4 aggregates. J Nanopart Res 2013;15:1380. [30] Kim YG, Oh SK, Crooks RM. Preparation and characterization of 1e2 nm dendrimer-encapsulated gold nanoparticles having very narrow size distributions. Chem Mater 2004;16:167e72. [31] Tang RC, Tang H, Yang C. Adsorption isotherms and mordant dyeing properties of tea polyphenols on wool, silk, and nylon. Ind Eng Chem Res 2010;49: 8894e901. [32] Dong Q, Su H, Zhang D. In situ depositing silver nanoclusters on silk fibroin fibers supports by a novel biotemplate redox technique at room temperature. J Phys Chem B 2005;109:17429e34. [33] Chen X, Lam KF, Mak SF, Yeung KL. Precious metal recovery by selective adsorption using biosorbents. J Hazard Mater 2011;186:902e10. [34] Wongpanit P, Rujiravanit R. Combinatorial effects of charge characteristics and hydrophobicity of silk fibroin on the sorption and release of charged dyes. J Biomater Sci Polym Ed 2012;23:1199e215. [35] Cheval N, Gindy N, Flowkes C, Fahmi A. Polyamide 66 microspheres metallised with in situ synthesised gold nanoparticles for a catalytic application. Nanoscale Res Lett 2012;7:1e9. [36] Cao L, Jennings MC, Puddephatt RJ. Amine-amide equilibrium in gold(III) complexes and a gold(III)-gold(I) aurophilic bond. Inorg Chem 2007;46:1361e8. [37] Jin J, Hassanzadeh P, Perotto G, Sun W, Brenckle MA, Kaplan D, et al. A biomimetic composite from solution self-assembly of chitin nanofibers in a silk fibroin matrix. Adv Mater 2013;25:4482e7. [38] Myshakina NS, Ahmed Z, Asher SA. Dependence of amide vibrations on hydrogen bonding. J Phys Chem B 2008;112:11873e7.
[39] Kharlampieva E, Zimnitsky D, Gupta M, Bergman KN, Kaplan DL, Naik RR, et al. Redox-active ultrathin template of silk fibroin: effect of secondary structure on gold nanoparticle reduction. Chem Mater 2009;21:2696e704. [40] Zhou Y, Chen W, Itoh H, Naka K, Ni Q, Yamane H, et al. Preparation of a novel core-shell nanostructured gold colloid-silk fibroin bioconjugate by the protein in situ redox technique at room temperature. Chem Commun 2001: 2518e9. [41] Singh A, Hede S, Sastry M. Spider silk as an active scaffold in the assembly of gold nanoparticles and application of the gold-silk bioconjugate in vapor sensing. Small 2007;3:466e73. [42] Ghosh SK, Nath S, Kundu S, Esumi K, Pal T. Solvent and ligand effects on the localized surface plasmon resonance (LSPR) of gold colloids. J Phys Chem B 2004;108:13963e71. [43] Beeram SR, Zamborini FP. Purification of gold nanoplates grown directly on surfaces for enhanced localized surface plasmon resonance biosensing. ACS Nano 2010;4:3633e46. [44] Joshi GK, McClory PJ, Dolai S, Sardar R. Improved localized surface plasmon resonance biosensing sensitivity based on chemically-synthesized gold nanoprisms as plasmonic transducers. J Mater Chem 2012;22:923e31. [45] Lindsey DT, Wee AG. Perceptibility and acceptability of CIELAB color differences in computer-simulated teeth. J Dent 2007;35:593e9. [46] Seghi RR, Hewlett ER, Kim J. Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain. J Dent Res 1989;68: 1760e4. [47] Oakes J. Photofading of textile dyes. Rev Prog Color Relat Top 2001;31:21e8. [48] Batchelor SN, Carr D, Coleman CE, Fairclough L, Jarvis A. The photofading mechanism of commercial reactive dyes on cotton. Dye Pigment 2003;59: 269e75. [49] Cristea D, Vilarem G. Improving light fastness of natural dyes on cotton yarn. Dye Pigment 2006;70:238e45. [50] Stankovi c SB, Popovi c D, Popari c GB. Thermal properties of textile fabrics made of natural and regenerated cellulose fibers. Polym Test 2008;27:41e8. [51] Abbas A, Zhao Y, Wang X, Lin T. Cooling effect of MWCNT-containing composite coatings on cotton fabrics. J Text Inst 2013:1e10. [52] Luo X, Wu J, Intisar A, Geng J, Wu L, Zheng K, et al. Study on light aging of silk fabric by Fourier transform infrared spectroscopy and principal component analysis. Anal Lett 2012;45:1286e96. [53] Paquet-Mercier F, Lefevre T, Auger M, Pezolet M. Evidence by infrared spectroscopy of the presence of two types of beta-sheets in major ampullate spider silk and silkworm silk. Soft Matter 2013;9:208e15. [54] Ha SW, Tonelli AE, Hudson SM. Structural studies of Bombyx mori silk fibroin during regeneration from solutions and wet fiber spinning. Biomacromolecules 2005;6:1722e31.
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
In-situ synthesis of gold nanoparticles for multifunctionalization of silk fabrics Bin Tang a, b, Lu Sun a, b, Jasjeet Kaur b, Yao Yu b, Xungai Wang a, b, * a b
Ministry of Education Key Laboratory for Textile Fibers and Products, Wuhan Textile University, Wuhan 430073, China Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2013 Accepted 9 December 2013 Available online 18 December 2013
A simple in-situ synthesis route for gold nanoparticles (NPs) was developed to realize multifunctions for silk fabrics. The gold NPs were prepared in a heated solution containing white silk fabric samples. The silk fabrics were colored red and brown by the gold NPs because of their localized surface plasmon resonance (LSPR) property. Gold nanospheres on silk were obtained at a low gold content, and gold nanoplates were synthesized as the gold content increased. The silk fabrics treated with gold NPs showed good light fastness. Moreover, the gold NPs endowed silk fabrics with strong antibacterial activity, excellent UV protection property and enhanced thermal conductivity. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Coloration Silk Gold nanoparticle In-situ synthesis Antibacterial UV blocking
1. Introduction Noble metal nanoparticles (NPs) present brilliant and fascinating colors due to their localized surface plasmon resonance (LSPR) properties. LSPR is derived from the interaction of light and the metal NPs, when conductive electrons oscillate locally around the metal NPs at a certain frequency. The phenomenon of excitation of surface plasmons generated by light is termed as LSPR [1]. The LSPR property of noble metal NPs is important for applications in surface enhanced spectroscopy [2,3], sensing [4], and nanophotonic devices [5]. Noble metal NPs with bright colors have been used as decorative pigments for glass [6] and ceramics [7] since ancient times. More recently, these particles have been used for textiles including cotton [8e10] and wool [11e14]. The noble metal NPs are different from conventional dyes, in that it is the LSPR property not the chromophore that gives rise to captivating colors. This optical feature of noble metal NPs is related to particle shape, size, composition, environment, and interspaces [15e20]. The shape and size of the NPs govern their LSPR optical features, which have been illustrated already [21,22]. Therefore, the color of the noble metal NPs can be adjusted by controlling their shape and size.
* Corresponding author. Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia. Tel.: þ61 3 5227 2894. E-mail address: [email protected] (X. Wang). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.12.008
Textile products with multiple functionalities have generated great interest in recent years. Many attempts have been made to enhance functionalities of textiles, such as antibacterial [23], selfcleaning [24], and UV protection [25]. Silk as a natural protein fiber is widely used in the textile industry due to its inherently elegant sheen, excellent flexibility, environmental friendliness and good comfort. Recently, some strategies based on modification with NPs have been developed to enhanced functions of silk fabrics. For examples, Zheng et al. colored silk fabrics using gold nanorods with different aspect ratios [9]. The gold nanorod coated silk fabrics showed significant improvements on both UV protection and antibacterial functions. But the washing fastness of silk fabrics colored with gold nanorods was unsatisfactory. Li et al. modified silk fibers with TiO2 and TiO2@Ag NPs through chemical assembly technique [26]. The treated silk fabric exhibited multifunctions including UV protection, antibacterial activity, and photocatalytic capability. Besides, the anisotropic silver NPs were assembled onto the silk fibers to impart different colors and antibacterial feature to silk fabrics [27]. In the present study, an in-situ synthesis method for gold NPs was described to functionalize silk fabrics. Color and optical properties were investigated at different levels of gold concentration. Moreover, the relation between morphologies and optical features of synthesized gold NPs on silk fibers were studied. The functions of silk fabrics treated with gold NPs, including UV blocking, antibacterial activity and thermal conductivity, were investigated.
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The colorfastness properties to washing and light of gold nanoparticle (NP) modified silk were evaluated as well.
2. Experimental 2.1. Materials Tetrachloroauric(III) acid (HAuCl4$3H2O, >99%) was purchased from SigmaeAldrich. The chemicals were analytic grade reagents, and used without further purification. Crepe satin silk fabrics, with a weight of 123.4 g/m2 and a density of 51 threads/cm in the warp direction and 41 threads/cm in the weft direction, were purchased from the local Beautiful Silks store.
2.2. Instruments
2.4. Colorfastness test to washing Washing fastness tests were performed in accordance with Australian Standard AS 2001.4.15d2006. The silk fabrics with gold NPs were washed for 45 min at 50 C in the presence of ECE reference detergent by using a lab dyeing machine (Ahiba, Top Speed Nuance). The CIE Lab color coordinate values (L*, a*, and b*) for each specimen were measured before and after washing. L* represents the lightness/darkness, a* value represents the red or green chroma, and b* represents the chromaticity coordinate for yellow/blue. The color difference (DE) was obtained based on the changes in color coordinates (DL*, Da*, and Db*) with the formula: DE ¼ [(DL*)2 þ (Da*)2 þ (Db*)2]1/2. The color difference (DE) of silk fabrics before and after washing was measured by spectrophotometer to assess washing fastness of silk fabrics according to Australian Standard AS 2001.4.A05d2004.
The UVevis absorption spectra of HAuCl4 solutions were obtained using a Varian Cary 3E UV/vis spectrophotometer. The UVevis diffuse reflectance absorption spectra of silk fabrics were recorded by a Varian Cary 5000 UVeVISeNIR spectrophotometer with a diffuse reflectance accessory (DRA-2500). Scanning electron microscopy (SEM) measurements were performed with a Supra 55 VP field emission SEM. The color strength (K/S) and color difference (DE) of silk fabrics with gold NPs were obtained using a Datacolor Spectraflash SF600 Plus-CT spectrophotometer. Fourier transform infrared (FTIR) spectra were measured with a PerkinElmer Fourier transform infrared spectrometer (FTIR-1730) in attenuated total reflection (ATR) mode. Infrared thermal images were recorded by an infrared thermography video camera (H2640, NEC). Heating reaction was performed in a Stuart SBS40 shaking water bath.
2.5. Measurement of fastness to light
2.3. In-situ synthesis of gold NPs in silk
Gram negative bacteria, Escherichia coli (E. coli) (ATCC 11229), were used as test organisms. Antibacterial test was performed on untreated and treated silk fabrics. The antibacterial test was carried out according to the AATCC 100-2004 (Clause 10.2) test standard with slight modifications. 50 mL of bacteria were added on treated samples in separate flasks. After 1.0 min, 50 mL of sterile deionized water was poured into each flask, followed by vigorous shaking. Then the flasks were incubated for 18 h in a shaker oven at 120 rpm at 37 C. After that, the fabric samples were collected and the solution left in the flask was further diluted to get countable number of bacterial colonies. 103 dilution was suitable for obtaining colonies between 30 and 300. 100 mL of the 103 dilution obtained were placed on the nutrient agar plates. These plates were then incubated for 18 h in an oven at 37 C. The bacterial activity was
Pristine silk fabrics were immersed in aqueous solutions with different concentrations of HAuCl4 (1.79 104 M, 2.38 104 M, 2.99 104 M, 4.48 104 M, 5.97 104 M and 7.46 104 M). The weight ratio of HAuCl4 solution to silk is 60. The HAuCl4 solutions with silk were placed for 30 min at room temperature. The silk changed to light yellow due to absorption of chloroaurate ions (AuCl 4 ). After that, the solutions were heated at 85 C for 60 min in an oscillating water bath. The color of silk in the solutions became red or brown. The silk fabrics were taken out and rinsed with running deionized water. And then the silk fabrics with different gold content (0.21 wt%, 0.28 wt%, 35 wt%, 0.53 wt%, 0.71 wt% and 0.88 wt%) were dried at room temperature.
The gold NP treated silk fabrics (8 4 cm) with different colors were stapled on to white cardboard, and a portion (4 4 cm) of each fabric specimen was covered with cardboard and aluminum foil. The specimens were then exposed to simulated sunlight for 60, 120 and 180 h inside the Suntest instrument (SUNTEST XLSþ from Atlas Material Testing Technology LLC). Color changes were determined with the Delta E value (DE CIELAB) by a Datacolor instrument. This parameter was used to quantify the difference between two colors of the samples with and without irradiation at each exposure period. 2.6. Antibacterial test
Fig. 1. Photographs of HAuCl4 solutions (corresponding to 0.53 wt% gold content) (a) before and (b) after silk fabric was soaked in solution for 30 min, and (c) plots of absorbance intensity of HAuCl4 solution around 290 nm as a function of soaking time corresponding to different gold content after the silk fabrics were immersed in HAuCl4 solution.
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recorded by photography. The test was carried out in triplicate and the entire experiments were repeated three times. 3. Results and discussion
Absorbance (a.u.)
The HAuCl4 solution became colorless after the silk fabrics were immersed, and the silk changed to yellow due to absorption of chloroaurate ions (Fig. 1a and b). The absorption of HAuCl4 on silk fabrics was very effective. The UVevis absorbance spectra of HAuCl4 solution were recorded at different time intervals during the soaking of silk corresponding to different gold content (Fig. S1 in supporting information). The UVevis absorbance spectrum of initial HAuCl4 solution presents two absorbance bands located at around 220 nm and 290 nm, which are attributed to charge transfer between the metal and chloro ligands [28e30]. The intensities of the two absorbance bands decreased dramatically after silk fabrics were immersed in the HAuCl4 solution and almost disappeared within 30 min, which implies that the chloroaurate (AuCl 4 ) ions in solution were absorbed by silk fibers (Fig. S1 in supporting information). Plots of absorbance intensity of HAuCl4 solution around 290 nm as a function of fabric soaking time are shown in Fig. 1c. The absorbance intensity decreased with time for different concentrations of HAuCl4 solutions. In the dyeing process of textiles, electrostatic interaction and hydrogen bonding were considered to contribute greatly to absorption of dye on fiber [31]. In the present study, the pH values of HAuCl4 solutions with different concentrations were 5e6.5. The silk fibers are full of amino and carboxyl groups [31,32]. The silk fabrics carried negative charges when the pH value was above the isoelectric point of silk (around 4) [33,34]. Therefore, it is possible that combination of AuCl 4 complex ions and silk fibers was achieved through hydrogen bonding interaction of amino group and AuCl 4 complex ions [35,36]. To further investigate the interaction of silk fibers and HAuCl4, the FTIR spectra of silk fabrics before and after absorption of AuCl 4 were obtained (Fig. 2). The FTIR spectrum of silk fabrics (Curve a in Fig. 2) presents amide absorbance bands, which are assigned in detail in 1 Table 1. After absorption of AuCl assigned 4 , the band at 1626 cm 1 to amide I shifted to 1619 cm and increased obviously in intensity. Also, the intensity of the shoulder band at 1698 cm1 (amide I) decreased evidently. Meanwhile, the absorbance band at 1508 cm1 (amide II) shifted to a low frequency (Curve b in Fig. 2). In addition, the amide III bands (1261 and 1228 cm1) downshifted slightly and decreased in intensity. The differences of FTIR spectra
a b
untreated silk silk with HAuCl4
c
silk with gold NPs
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Table 1 Assignment of absorbance bands in the FTIR spectrum of silk fabric. Wavenumber (cm1)
Assignment [52e54]
1698 1626 1508 1445 1261 1228 1162 1067
CO stench, Amide I CO stench, Amide I, b-sheet Amide II, b-sheet CH plane bending vibration, eCH2, backbone NH bend, Amide III, b-sheet CN stretch and CC stretch, Amide III, Random-coil CH3 vibration, Alanine, Residue CC stretch, Peptide, b-sheet
of silk fabrics were probably due to change in bonding environments around amide of silk fibers from absorption of AuCl 4 . The FTIR analysis implies that the hydrogen bonding force could lead to absorption of AuCl 4 complex ions on silk fibers [37,38]. The silk fabrics with AuCl 4 complex ions were heated at 85 C in solution and the gold NPs were in-situ synthesized on silk fibers. It has been suggested that tyrosine groups on silk fiber reduce gold ions to gold NPs [32,39e41]. Compared with the FTIR spectrum of silk fabric with AuCl 4 , absorbance bands corresponding to the silk fabric with gold NPs did not change visibly, which indicates that the in-situ synthesis of gold NPs has no observable influence on the chemical structure of silk fibers, consistent with the previous report [39]. The silk fabrics after heating became red or brown, which indicates that the gold NPs were synthesized on silk (Fig. 3). The color of the silk fabrics darkened and changed to brown as the gold content increased (Fig. 3). The gold NPs were in-situ prepared on silk only through heating. To clarify the influence of gold content on color of silk, the color strength (K/S) curves were plotted (Fig. 3). The color strength (K/S) values of silk corresponding to 0.21 wt%, 0.28 wt%, 0.35 wt%, 0.53 wt%, 0.71 wt%, and 0.88 wt% gold content were 2.22, 2.66, 3.06, 2.27, 1.81 and 1.80, respectively. The K/S values increased with an increase in gold content in the range of 0.21 wt% w 0.35 wt%. But the K/S decreased with increasing gold content in the range of 0.53 wt% w 0.88 wt%. It was found that the K/S spectra broadened when gold content was increased to 0.53 wt %, which is consistent with the brown colors of silk fabrics. The color of silk arose from the LSPR optical properties of gold NPs on fiber surface. In order to investigate the LSPR of gold NPs on silk fabrics, the UVevis absorbance spectra of silk fabrics with in-situ gold NPs were measured (Fig. 4). The UVevis absorbance spectrum of the silk fabrics with 0.21 wt% of gold content displays one single band at 540 nm that is attributed to LSPR of spherical gold NPs [42]. When the gold content increased to 0.28 wt%, one new shoulder band appeared in UVevis absorbance spectrum (Fig. 4).
1000 1100 1200 1300 1400 1500 1600 1700 -1
Wavenumber (cm ) Fig. 2. FTIR spectra of untreated silk, silk with HAuCl4 and silk treated with gold NPs.
Fig. 3. Photographs and K/S curves of the silk fabrics with in-situ synthesized gold NPs at different weight percent.
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2.5 untreated 0.21 wt% 0.28 wt% 0.35 wt% 0.53 wt% 0.71 wt% 0.88 wt%
Absorbance (a.u.)
2.0
1.5
1.0
0.5
0.0 300
400
500
600
700
800
900
1000
Wavelength (nm) Fig. 4. UVevis absorbance spectra of silk fabrics with different gold content.
The shoulder band increased as the gold content of silk fabrics increased (Fig. 4). The appearance of shoulder bands at long wavelength reveals that the gold NPs in-situ synthesized on silk fibers were anisotropic in shape [43,44].
The morphologies of gold NPs determined the optical properties of the treated silk fabrics. SEM characterization was performed to observe the morphologies of gold NPs on fiber surface with different gold content. The gold NPs on the silk with 0.21 wt% of gold content were spherical and their size was measured to be 21.3 3.4 nm (Fig. 5a). All the gold NPs in Fig. 5a were welldistributed on the surface of silk fiber, which led to the bright red color displayed by the treated silk fabrics (Fig. 3). When the gold content of silk fabrics increased to 0.28 wt%, small triangular gold nanoplates were seen in the SEM image (Fig. 5b), indicating anisotropic growth of gold NPs, which is consistent with deduction from UVevis absorption spectroscopy. As can be seen, the size of gold NPs increased with increase in gold content (Fig. 5). At a low gold content, the density of gold NPs increased when the gold content increased. The density of gold NPs reached maximum when the gold content of silk fabric increased to 0.35 wt% (Fig. 5c), which results in the highest color strength (K/S) and intensity of main UVevis absorption band among all the above samples (Figs. 3 and 5). The size of generated platelike gold NPs increased with increasing gold content (Fig. 5d). The gold nanoplates dominated in the synthesized gold NPs when the gold content of silk fabrics was 0.71 wt% and 0.88 wt% (Fig. 5e and f). The formation of gold nanoplates resulted in the multipole plasmon resonances and decrease in intensity of band around 540 nm, changing silk fabric to brown from red. The results indicate that the LSPR properties of
Fig. 5. SEM images of the silk with in-situ synthesized gold NPs at different gold content: (a) 0.21 wt%, (b) 0.28 wt%, (c) 0.35 wt%, (d) 0.53 wt%, (e) 0.71 wt% and (f) 0.88 wt%.
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3.0 2.5 2.0
E
gold NPs on silk fabrics can be controlled by changing the gold content. Consequently, the color of silk fabrics can be adjusted. The red color strength of silk fabrics cannot be enhanced by increasing the concentration of Au ions in the range of high concentration, due to formation of gold nanoplates. To obtain brighter red color of silk with gold NPs, multi-step in-situ coloration of silk was developed. Repeated additions of a small amount of HAuCl4 solution was implemented to avoid the generation of anisotropic gold NPs. As depicted in Fig. 6a, bright red color of silk was obtained through two separate additions of HAuCl4 solution. But the silk fabric was brown when the same total amount of HAuCl4 was added in the reaction system in one step (Fig. 6a). Fig. 6b shows the color strength curve of silk fabric after two separate additions of the same HAuCl4 solution to make the gold content of silk at 0.56 wt%. The color strength value of silk after first addition is 2.66. The value increased to 6.35 after second addition of HAuCl4 (Fig. 6b). The color strength value is much higher than that from first addition of HAuCl4. But the color strength value of silk fabric is 1.33 after double amount of HAuCl4 solution was added in the reaction solution once (Fig. 6b). Fig. 6c and d presents the SEM images of silk fabrics with same gold content obtained through different addition procedures of HAuCl4 solution. Most of the gold NPs on silk fabrics after two separate additions of HAuCl4 were spherical and density of gold NPs was higher than that of gold NPs through one addition of double amount of HAuCl4. Whereas, there were many gold nanoplates presenting in the SEM image of silk fiber from one addition of double amount of HAuCl4 (Fig. 6d). The results imply that repeated additions of a small amount of HAuCl4 solution can inhibit the growth of gold nanoplates and increase monodispersion of NPs, hence enhancing color of treated silk fabrics. Colorfastness is important for textiles. The washing colorfastness of silk fabrics with gold NPs was evaluated. The silk fabric with
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1.5 1.0 0.5 0.0 1
2
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Washing cycles Fig. 7. Evolution of the color difference (DE) of silk fabrics with in-situ synthesized gold NPs (0.35 wt%) with increasing the number of washing cycles.
0.35 wt% of gold NPs was washed for 45 min in the presence of ECE reference detergent at 50 C under each washing cycle. The color differences (DE) of the silk fabrics before and after washing are shown in Fig. 7. The average DE value of fabric was measured to be 1.1 after the first washing cycle. Though the smallest color difference (DE) which can be noticed by human eye was considered to be around 1.0 [45], it was reported that the color difference with DE value more than 2.0 could be perceived correctly 100% of the time by human observers [46]. The DE value increased to around 2.0 and remained stable with further increase in number of washing cycles.
Fig. 6. (a) Photograph of silk fabrics after two separate additions and one addition of HAuCl4 solution. (b) UVevis absorption spectra of silk fabrics corresponding to different addition procedure of HAuCl4 solution. SEM images corresponding to (c) two separate additions and (d) one addition of HAuCl4 solution.
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The gray scale rating for color change after seven washing cycles was assessed to be 4 according to DE value. These results demonstrate that the silk fabrics treated with in-situ synthesized gold NPs have good washing fastness. Many existing dyes fade severely upon exposure to sunlight for prolonged periods [47], thus, avoiding the photo-fading of the colored textiles is a critical challenge in the industry, and has been extensively investigated for nearly two centuries [48]. Color-fading is attributed mainly to the photo-degradation of the chemical structures of the dyes. Though some additives, such as UV absorbers and antioxidants, have been explored to moderate the photo-fading of the dyed fabrics [49], they did not alter the intrinsic photo-fading property. In this study, gold NPs were used to color the silk fabrics, which was different from the traditional dyeing approach. The color generating from LSPR of gold NPs is expected to be stable under sunlight. Fig. 8 shows the color differences (DE) of the different silk fabrics after they were irradiated for 60 h under simulated sunlight. The DE of untreated silk fabric was 3.26. Nevertheless, the DE values of silk fabrics colored with gold NPs (0.35 wt% and 0.53 wt%) were around 1.0, so the color difference is barely noticeable by naked eye. Compared with the gold NP colorated silk fabrics, the colors of silk fabrics colored with commercial dyes (Lanasol RED 5B and Lanasol RED GN) changed obviously, as suggested by the much higher DE values (Fig. 8). To further confirm the sunlight fastness of silk fabrics with gold NPs, the silk fabric with 0.35 wt% of gold content continued to be exposed for 180 h under simulated sunlight. The average color difference (DE) of silk fabrics was measured to be less than 2.0 (Fig. S2 in supporting information). These results indicate that the silk fabrics with insitu synthesized gold NPs have good fastness to light. Furthermore, coloration of gold NPs imparted silk fabrics with additional functions. The UV-blocking property of the silk fabrics with gold NPs was tested. Table 2 shows the transmittance values of UV light and UV protection factor (UPF) values of the untreated silk fabric and the silk fabric with gold NPs. The average transmittance values of the silk fabrics decreased obviously after gold NPs were in-situ synthesized on the silk fabrics, which demonstrated that the gold NPs enhanced prominently the ability of UV-blocking of silk fabrics. Both transmittance values in UVA (315e400 nm) and UVB (280e315 nm) regions decreased with increasing the gold content on silk fabrics (Table 2). However, the transmittance values of treated silk fabrics were the same for 0.71 wt% and 0.88 wt% gold content, suggesting that the UV-blocking ability of silk fabrics remained consistent when the gold content was increased to the
Fig. 8. Color differences (DE) of different silk fabrics after exposed for 60 h by simulated sunlight.
Table 2 UV-blocking properties of the untreated silk fabric and the silk fabric with gold NPs at different weight percentage. Sample 0.00 wt% 0.21 wt% 0.28 wt% 0.35 wt% 0.53 wt% 0.71 wt% 0.88 wt% T(UVA) 10.99% T(UVB) 2.61% UPF 21.00
1.53% 0.61% 101.09
1.30% 0.57% 109.38
0.79% 0.50% 131.67
0.57% 0.42% 160.99
0.52% 0.39% 173.24
0.52% 0.39% 172.52
threshold value. UPF presents the ability of the fabric to block UV from passing through and reaching the skin. UPF value of untreated silk fabrics was measured to be 21.00 (Table 2). In-situ coloration with gold NPs increased the UPF value of silk fabrics by 4e7 times. The results indicate that the gold NP treated silk fabrics provide excellent UV protection. For summer clothing, a high heat transfer is desirable [50,51]. In this study, the heat conductivity of untreated and gold NP treated silk fabrics was compared. Infrared thermal images of a hand covered with untreated silk and gold NP treated silk over time are presented in Fig. 9. The red region of untreated silk enlarged and deepened gradually with time. Whereas, the infrared thermal images of silk treated with gold NPs did not change obviously. The series of thermal images imply that the temperature of untreated silk increased with time and that of treated silk remained unchanged. To analyze the change process of temperature for untreated and treated silk fabrics, the temperature values of marked positions (as shown in Fig. S3 in supporting information) corresponding to untreated and treated silk fabrics were plotted as a function of time (Fig. 9). The temperature of untreated silk increased to w31 C from initial temperature of w28 C within 80 s. However, the temperature of gold NP treated silk decreased to w30 C in a short time (<10 s) from w31 C and subsequently remained almost constant. The treated silk reached thermal balance rapidly with the hand. It can be inferred that the coloration
Fig. 9. Infrared thermal images of a hand covered by untreated silk and gold NP treated silk (0.71 wt%) recorded at different time, and plots of temperature of marked positions in Fig. S3 in supporting information as a function of time.
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Fig. 10. Antibacterial activity of (a) untreated and (b) gold NP treated silk fabrics.
from gold NPs has improved the heat conductivity of silk fabrics, which is in favor of enhancing comfort of silk clothing in the summer. Additionally, it was reported that the fabrics coated by gold nanorods exhibited antibacterial activities [9]. In the present research, the antibacterial properties of gold NP treated silk fabrics were evaluated against the Gram negative bacteria, E. coli. Fig. 10 shows photographs of the bacteria on untreated and treated silk. As can be seen, there were lots of colonies of viable bacteria corresponding to the untreated silk fabrics (Fig. 10a). Whereas, there was no bacteria colonies found on agar medium of silk fabrics treated with gold NPs (Fig. 10b), which indicates that the gold NPs on silk fabrics inhibited growth of bacteria. These results demonstrate that the silk fabrics treated with in-situ synthesized gold NPs possessed significant antibacterial activity. 4. Conclusions In-situ coloration of silk fabrics was achieved through synthesis of gold nanoparticles (NPs) on fiber surface by heating. The optical property of gold NPs imparted silk with different colors, which can be adjusted by controlling gold content on silk. The SEM characterization demonstrated that the gold content influenced the morphology of gold NPs produced on silk fibers. The silk fabrics treated with gold NPs exhibited obvious antibacterial feature and remarkable UV-blocking ability. In addition, the thermal conductivity of silk fabrics increased after treatment with gold NPs. The gold NP colored silk fabrics possessed good colorfastness to washing and light irradiation, which can facilitate industrial application of in-situ coloration technique involving noble metal NPs. Acknowledgments This research was supported by the National Natural Science Foundation of China (NSFC 51273153), and the Central Research Grants Scheme and Alfred Deakin Postdoctoral Research Fellowship Scheme at Deakin University. The authors thank Dr. Jian Fang from Deakin University for help with recording infrared thermal images. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2013.12.008.
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