Sunlight-Induced Coloration of Silk
© The Author(s). 2016
Received: 1 April 2016
Accepted: 30 May 2016
Published: 14 June 2016
Silk fabrics were colored by gold nanoparticles (NPs) that were in situ synthesized through the induction of sunlight. Owing to the localized surface plasmon resonance (LSPR) of gold NPs, the treated silk fabrics presented vivid colors. The photo-induced synthesis of gold NPs was also realized on wet silk through adsorbing gold ions out of solution, which provides a water-saving coloration method for textiles. Besides, the patterning of silk was feasible using this simple sunlight-induced coloration approach. The key factors of coloration including gold ion concentration, pH value, and irradiation time were investigated. Moreover, it was demonstrated that either ultraviolet (UV) light or visible light could induce the generation of gold NPs on silk fabrics. The silk fabrics with gold NPs exhibited high light resistance including great UV-blocking property and excellent fastness to sunlight.
Noble metal nanoparticles (NPs), including gold and silver NPs, have attracted extensive attention due to their localized surface plasmon resonance (LSPR) [1–4]. Conduction electrons around metal NPs locally oscillate at a certain frequency when light interacts with these NPs. The excitation of surface plasmons by light is known as LSPR . The features of LSPR can be controlled by the particle size , shape , composition , external environment , and inter-particle spacing . Many strategies have been used to synthesize the noble metal NPs . The particular optical properties from noble metal NPs can produce brilliant and vivid colors, which can be used for textile coloration [11, 12]. In our previous research, various natural fibers including wool, bamboo, ramie, and cotton fabrics were treated with silver and gold NPs to render fibrous materials’ different functions [11–14]. This study focused on treating silk fabrics with noble metal NPs.
Silk fibers have some prominent properties such as good hand feeling, great drapability, and bright luster , which distinguish silk from the other fibers. Silk fabrics have been dyed with many traditional dyes, such as acid dyes , disperse dyes , and reactive dyes . Most studies on dyeing of silk have been focused on washing fastness , water consumption , and dye sources . However, color fading and high water and energy consumption remain critical issues facing the conventional dyeing methods. We have developed novel coloration methods for silk based on noble metal NPs [22, 23]. Anisotropic silver NPs were prepared and assembled onto silk fibers through electrostatic interaction. In addition, gold NPs were in situ synthesized on silk by heating, realizing the coloration of silk fabrics. Different from the previous works, the present coloration strategy used sunlight as a green energy resource and a water-saving concept to achieve vivid colors on silk fabrics.
Herein, in situ synthesis of gold NPs on the silk fabrics was achieved through a sunlight-induced process. The as-prepared gold NPs endow fabrics with bright colors and excellent photo-stability due to the LSPR properties, as well as strong ultraviolet (UV) resistance. The key factors including gold ion concentration, pH value, irradiation time, and light source were investigated. The morphology and structure of the treated silk fibers were also observed. The properties of the treated silk fabrics including light fastness, washing fastness, and rubbing fastness were examined. Moreover, water-saving coloration method based on sunlight induction was exploited.
Tetrachloroauric(III) acid (HAuCl4·3H2O, >99 %) was purchased from Sigma-Aldrich. All chemicals were analytic grade reagents and used without further purification. Plain weave silk fabrics, with a weight of 67 g/m2 and a density of 59 threads/cm in the weft direction and 107 threads/cm in the warp direction, were purchased from a local retailer. They were used without pretreatment.
Ultraviolet-visible (UV-vis) reflectance absorption spectra were obtained from a Varian Cary 5000UV-VIS-NIR spectrophotometer with a diffuse reflectance accessory (DRA-2500). The morphologies of fibers were observed on a Supra 55 VP field emission scanning electron microscope (SEM). A Datacolor Spectraflash SF600 Plus-CT spectrophotometer was used for the measurement of color strength (K/S). A YG902 Fangyuan UV measurement system was used for testing the UV-blocking properties. The light irradiation test was conducted with a Suntest instrument (SUNTEST XLS+ from Atlas Material Testing Technology LLC). The temperature of the coloration process was monitored by an instrument equipped with needle-type temperature probes (from ICT SFM, a sap flow meter produced by ICT international Pty. Ltd.). X-ray diffraction (XRD) data were collected from fabrics on a Panalytical X’Pert Powder instrument using a CuKa radiation source (λ = 1.54181 Å). Rubbing color fastness was tested by a Gellowen G238BB electronic crockmeter instrument.
In Situ Synthesis of Gold NPs on Silk Fabrics
Silk fabrics were washed with deionized water and immersed in HAuCl4 aqueous solution with different concentrations (0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 mM). The mass ratio of HAuCl4 solution to silk fabric was 100. The solutions containing silk fabrics were shaken for 15 min at room temperature before they were placed under the Suntest instrument for irradiation. After irradiated for 2 h, the silk fabrics were taken out from reaction solution and rinsed with running deionized water. Subsequently, the treated fabrics were dried at room temperature. The treated silk fabrics corresponding to 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 mM of HAuCl4 were denoted as GS-02, GS-03, GS-04, GS-05, GS-06, and GS-07. Unless otherwise stated, the irradiation time and the light power for different samples were 2 h and 250 W, respectively.
Silk fabrics were stuck to glass slides for XRD characterization. XRD analysis of silk fabrics was performed on the Panalytical X’Pert Powder instrument at a voltage of 45 kV and a current of 20 mA. XRD data were collected with 2θ range of 10°–80° and a scanning rate of 2°/min at a step of 0.01°.
Color Fastness Test to Sunlight
The gold NP-treated silk fabrics (8 × 4 cm) corresponding to different concentrations of HAuCl4 were stapled with cardboard, and a part (4 × 4 cm) of the sample was covered with cardboard and aluminum foil. After that, the samples were exposed under simulated sunlight in the Suntest instrument for 65, 130, and 195 h at 600 W. The color differences were measured using the Datacolor Spectraflash SF600 Plus-CT spectrophotometer.
Color Fastness Test to Washing
The Australian Standard AS 2001.4.15—2006 was followed in tests of the washing fastness of the silk fabrics treated with gold NPs. The washing process lasted for 45 min at 50 °C with the ECE reference detergent in a dyeing machine (Ahiba IR Pro Dyeing Machine). The silk fabrics after washing were dried for 15 min at 50 °C in an oven. The color difference (ΔE) of silk fabrics was measured by a spectrophotometer before and after washing. Gray scale was assessed using ΔE according to the Australian Standard AS 2001.4.A05—2004. A small ΔE value means small change in color.
Color Fastness to Rubbing
The dry and wet rubbing color fastnesses of treated silk fabrics were evaluated according to the Australian Standard AS 2001.4.3—1995. The fabrics colored with gold NPs were rubbed using an undyed cotton cloth. The staining of the cotton cloths were assessed using the standard gray scale for staining. Both dry and wet rubbing fastness tests were performed.
Results and Discussion
Coloration of Silk Fabrics via In Situ Synthesis of Gold NPs
To check the effect of natural sunlight on the synthesis of gold NPs, the irradiation of samples was implemented outdoor under natural sunlight. Similar to the case of simulated sunlight, the photo-induction of natural sunlight produced bright colors including red and brownish red on silk fabrics (Additional file 1: Figure S1), which reveals that the photo-induced coloration of silk with gold NPs can be realized using natural sunlight.
Influences of Irradiation Time and pH Value
Characterization of Silk Fabrics Treated with Gold NPs
Properties of Gold NP-Colored Silk Fabrics
Fabrics are often rubbed against other surfaces, and color change during the abrasion process should be minimized. In the present study, the color fastness to rubbing of the samples GS-04 and GS-06 was tested. The dry rubbing color fastness of the gold NP-treated silk fabrics was rated as 5 for both GS-04 and GS-06. And the rating of the wet color fastness of the fabrics with gold NPs was assessed to be 5. The staining of the cotton cloths was assessed to be 4–5 for both GS-04 and GS-06 under wet and dry conditions. The results demonstrate that the silk fabrics colored with gold NPs exhibited good color fastness to rubbing (dry or wet).
Investigation of Light Source and Water Consumption
It has been reported that the gold NPs could be in situ synthesized on silk fabrics by heating . Is it possible for the heat effect from sunlight to result in the synthesis of gold NPs during the photo-induced process? To clarify this point, the time-resolved temperature of reaction solution during the simulated sunlight-induced process was recorded with a thermal sensor. The curve of temperature as a function of irradiation time was plotted in Additional file 1: Figure S3. The temperature of reaction solution increased to ~31.5 from 23.3 °C (room temperature) within 90 min and kept almost unchanged during the irradiation process. In order to further inspect the effect of heat from sunlight, the solution containing silk fabric (0.2 g) and HAuCl4 solution (0.5 mM, 40 mL) was heated for 2 h at 40 °C (higher than 31.5). The color of silk fabric after heating did not change observably. Besides, the flask containing silk fabrics and HAuCl4 solution was wrapped with an aluminum foil to shade the light and subsequently placed in the Suntest instrument. No visible color differences of the silk fabric were found even though the flask was irradiated for 5 h under simulated sunlight at 250 W. These results verify that the heat from sunlight induction cannot have gold NPs in situ synthesized on silk fabrics.
The effect of light source on the synthesis of gold NPs on silk was also investigated. An UV lamp was employed to irradiate reaction system to inspect the role of UV light on the in situ synthesis of gold NPs. The emission spectrum of the UV lamp is shown in Additional file 1: Figure S4, with two main emission bands in the range of 340–450 nm. Additional file 1: Figure S5 displays optical images of silk fabrics after being irradiated under UV lamp for 2 h in the presence of 0.4 and 0.5 mM HAuCl4. The colors of fabrics turned out to be red after UV light irradiation, which suggests that in situ synthesis of gold NPs can be realized by UV light. Furthermore, different optical filters were used to cover the reaction solution during irradiation process to find out the influences from the different parts of sunlight. The transmission spectra of the optical filters are shown in Additional file 1: Figure S6. Red silk fabric was obtained when it was irradiated by simulated sunlight, covered with the optical filter (Additional file 1: Figure S6a) that blocks the light with wavelength less than 385 nm. The cutoff of most UV light from simulated sunlight did not prevent the in situ synthesis of gold NPs on silk fabrics. The filter in Additional file 1: Figure S6b only allows the passage of light in the region of 550–630 nm. The color of silk fabric irradiated with the filter in Additional file 1: Figure S6b was light red, implying that the visible light at long wavelength with low energy can still induce the synthesis of gold NPs on silk, although the amount of synthesized gold NPs was low. It is inferred that not only UV light but also visible light could induce the synthesis of gold NPs on silk.
Patterning Application of Silk Fabrics Treated with In Situ Synthesized Gold NPs
The in situ synthesis of gold nanoparticles (NPs) on silk fabrics was realized by the induction of sunlight. The as-synthesized gold NPs endowed silk fabrics with bright colors due to localized surface plasmon resonance (LSPR) of metal NPs. The K/S of the treated silk fabrics could be tuned by controlling the concentration of gold ions in reaction system. Acid condition favored the sunlight-induced synthesis of gold NPs. The colored silk fabrics exhibited great color fastness to sunlight. Significantly, the treatment with gold NPs rendered excellent UV protection ability to silk fabrics. The photo-induced coloration based on gold NPs was also feasible under a humid condition without large water consumption, which not only provides a novel water-saving dyeing technology but also offers an effective route to achieve pattern dyeing of fabrics. Combination of sunlight-induced coloration method and surface modification of fibers will facilitate the development of multi-functional textiles.
This research was supported by the National Natural Science Foundation of China (No. 51403162, 51273153), as well as the MoE Innovation Team Project in Biological Fibers Advanced Textile Processing and Clean Production. The authors also thank Mr Xing Jin from Deakin University for the help with monitoring the time-resolved temperature.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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