Simultaneous dyeing and functionalization of silk with three natural yellow dyes

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Simultaneous dyeing and functionalization of silk with three natural yellow dyes Yuyang Zhou a , Jue Zhang a , Ren-Cheng Tang a,∗ , Jun Zhang b a b

National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, 199 Renai Road, Suzhou 215123, China Suzhou Institute of Trade and Commerce, 287 Xuefu Road, Suzhou 215009, China

a r t i c l e

i n f o

Article history: Received 10 June 2014 Received in revised form 17 September 2014 Accepted 21 September 2014 Available online xxx Keywords: Rheum emodi Gardenia yellow Curcumin Silk Antioxidant activity Antibacterial activity.

a b s t r a c t Three natural yellow dyes, namely Rheum emodi, Gardenia yellow and curcumin, were applied to the simultaneous dyeing and functionalization of silk. Their dyeing properties and functionalities as well as the effects of post-mordanting on the hue, color fastness and functionalities of dyeings were compared, and the correlations between the chemical structures and application characteristics of dyes were revealed. The three dyes exhibited large variations in dyeing and mordanting properties, and functionalities. Curcumin displayed the greatest coloring power. The uptake of Gardenia yellow was most sensitive to the pH of dyebath. Gardenia yellow and curcumin had much higher building-up ability than R. emodi with anthraquinone structures. Gardenia yellow possessed the highest fastness ratings, while curcumin and R. emodi showed poor wet rub fastness and wash fastness for staining. Curcumin imparted the highest antioxidant activity to silk because of its high adsorption and two phenolic hydroxyl groups in its structure and also gave the highest UV protection ability. Curcumin and R. emodi provided higher antibacterial activities than Gardenia yellow. The post-mordanting with ferrous and ferric salts exerted great influence on the color parameters, color fastness and UV protection ability of dyeings. This study points out that the common dyeing process of three natural yellow dyes can impart the color and functional properties to silk. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Silk fiber has been historically referred to as the “queen of textiles”, and used as a natural source of textile materials for thousands of years owing to its excellent performance such as softness, smoothness, luster, comfortableness, breath ability, hygroscopicity and so on. Silk has also found a wide range of applications as a biomaterial in the medical field for sutures and scaffolds due to its remarkable mechanical performance, biocompatibility, and controlled degradability (Li et al., 2012). With all its advantages, silk fiber is by no means without its limitations. Silk suffers from some shortcomings such as wrinkling, photooxidation, deterioration, yellowing, poor UV protection capability, poor antioxidant and antimicrobial activities, etc. (Baltova and Vassileva, 1998; Jiang et al., 2009; Li et al., 2011, 2012). These defects inevitably restrict the application of silk fiber; hence some measures must be taken to overcome the disadvantages and enhance the functionalities of silk. In particular, if silk fabric is used for the purposes of medical

∗ Corresponding author. Tel.: +86 512 6716 4993; fax: +86 512 6724 6786. E-mail address: [email protected] (R.-C. Tang).

and healthy clothing, and bioactive dressings, its biological activities such as antimicrobial and antioxidant activities should be upgraded. Antioxidant activity is one of the most important properties of bioactive textiles, and the radical scavenging textiles can deactivate highly reactive and harmful species such as active oxygen radicals. However, up to now, much less attention has been paid to the antioxidant activity of silk textiles. The adsorption treatment with oleuropein and rutin extracted from olive leaf extract was found to enhance the antioxidant capacity of silk fibroin powder (Bayc¸in et al., 2007). The simultaneous coloration and functionalization of the tyrosinase-catalyzed oxidation products of caffeic acid was able to significantly increase the antioxidant activity of silk fabric (Sun et al., 2013). Silk fiber allows microbial growth and multiplication by providing good environments such as nutrient (protein), moisture, etc. Microbial growth and proliferation on silk fiber lead to generation of foul odors, discoloration, mildew formation, fiber degradation, dermal infection, allergic responses and other related diseases (Shahid et al., 2013). In recent years, a lot of reports have focused on the antimicrobial functionalization of silk by the treatment with quaternary ammonium compounds (Koller et al., 2007), quaternary

http://dx.doi.org/10.1016/j.indcrop.2014.09.041 0926-6690/© 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: Zhou, Y., et al., Simultaneous dyeing and functionalization of silk with three natural yellow dyes. Ind. Crops Prod. (2014), http://dx.doi.org/10.1016/j.indcrop.2014.09.041

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ammonium organosilanes (Higgins et al., 2010), chlorinated compounds (Dickerson et al., 2012), inorganic materials (Jiang et al., 2009; Li et al., 2011; Tang et al., 2014), chitosan (Periolatto et al., 2012), antimicrobial peptides (Bai et al., 2008) and bactericidal enzyme lysozyme (Dickerson et al., 2011). In addition, some synthetic dyes, e.g., metallic dyes (Tsukada et al., 2002) and quinazolinone-based monoazo reactive dyes (Patel and Patel, 2011), were able to show good antimicrobial properties when applied to the dyeing of silk fiber, suggesting that the dyeing and antimicrobial finishing of silk are simultaneously accomplished. The one-step dyeing and finishing achieved by choosing specific dyes possesses the potential to save time and energy, reduce cost, increase production and efficiency, and reduce effluent load. In this regard, natural dyes as naturally derived colorants and antimicrobial agents have more advantages over synthetic dyes in that they exhibit good biodegradability and compatibility with the environment (Shahid et al., 2013), and most of them possess a wide range of functional properties such as antimicrobial activity (Ghoreishian et al., 2013; Han and Yang, 2005; Hong et al., 2012; Khan et al., 2012; Koh and Hong, 2014; Mirjalili and Karimi, 2013; Shahid et al., 2013; Silva et al., 2011; Singh et al., 2005; Sousa et al., 2009) and antioxidant activity (Hong et al., 2012; Koh and Hong, 2014; Liu et al., 2013; Silva et al., 2011; Sousa et al., 2009). Natural dyes are obtained mainly from plants, producing different colors like yellow, red, blue, brown, black and a combination of these colors. The sources for yellow dyes are enormous (Bechtold and Mussak, 2009), and the plants which yield yellow dyes outnumber those yielding other colors. The chromophores of natural yellow dyes include flavonoids, carotenoids, hydroxylanthraquinones, and bis-␣,␤-unsaturated diketone polyphenols (Bechtold and Mussak, 2009; Ferreira et al., 2004). Rheum emodi, Curcuma longa L., and Gardenia jasminoides Ellis are the vital sources of natural yellow dyes. R. emodi or Rhubarb is a valuable medicinal herb and distributed in the temperate and sub-tropical regions of Asian countries. A large number of anthraquinone derivatives are present in the roots of the plant and also used in the coloring of food stuffs and textiles (Khan et al., 2012). Curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6heptadiene-3,5-dione) is extracted from the ground roots of C. longa L., a plant growing abundantly in the East Indies and China, and well known both in traditional and in modern medicine due to its antioxidant, anti-inflammatory, antifungal, and anticancer properties (Barzegar, 2012). Curcumin is often used as spice, cosmetic ingredient, natural medicine, food preservative, food colorant, and textile colorant. The yellow colorant extracted from gardenia fruits, G. jasminoides Ellis, is well known as Gardenia yellow in China. Gardenia yellow is a typical plant carotenoid, and its major constituents are crocins and crocetin. It is utilized as herb medicine and food colorant on account of its water solubility (Chen et al., 2008). The structures shown in Fig. 1 reflect the chemical components present in these dyes (Chen et al., 2008; Han and Yang, 2005; Khan et al., 2012). R. emodi has been used to the dyeing of wool, silk, and cotton fibers with and without the treatment of metal mordants (Das et al., 2008; Khan et al., 2012; Vankar et al., 2007), and the good antimicrobial activity of dyed wool was found (Khan et al., 2012). Curcumin has been successfully utilized to the antibacterial dyeing of wool, silk, nylon fibers (Ghoreishian et al., 2013; Han and Yang, 2005; Mirjalili and Karimi, 2013), and applied to the dyeing of polyester and modified acrylic fibers (El-Shishtawy et al., 2009; Kerkeni et al., 2012). Gardenia yellow widely used as a dye in middle China in ancient times is being investigated for its application in the dyeing of various fibers (Liu and Tang, 2013; Shen et al., 2014), and it was found that the chitosan fiber dyed with it had the good antioxidant and deodorant properties (Liu and Tang, 2013). In spite of the above investigations, the comparison of the dyeing properties of R. emodi, Gardenia yellow and curcumin for

silk has not been reported, and little attention has been paid to the antibacterial and antioxidant activities of silk dyed with these dyes. In this study, the important dyeing properties of R. emodi, Gardenia yellow, and curcumin applied to silk fabric were compared in terms of pH dependence, building-up capability, color fastness, and post-mordanting properties; and what is more, the antibacterial, antioxidant, and UV protection properties of the dyed silk without and with post-mordanting were evaluated. This research is expected to provide the basis for developing healthy and hygienic silk textiles and materials, and the assistance for better understanding the relationships between the chemical structures and functionalities of natural yellow dyes. 2. Materials and methods 2.1. Materials The scoured silk fabric of crepe de Chine fabric was supplied by Wujiang Zhiyuan Textile Co. Ltd., China. R. emodi dye with a purity of 95% was purchased from Xi’an Qing Yue Biotechnology Co. Ltd., China; Gardenia yellow with a color value of 550, and curcumin with a purity of 90% were obtained from Yunnan Tonghai Yang Natural Products Co. Ltd., China. 2,2 -Azino-bis(3ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS) was bought from Sigma-Aldrich (Shanghai) Trading Co. Ltd. Citric acid, disodium hydrogen phosphate, potassium dihydrogen phosphate, potassium persulfate, ethanol, aluminum sulfate, ferrous sulfate, and ferric sulfate were of analytical reagent grade. Nutrient agar and nutrient broth were obtained from Sinopharm Chemical Reagent Co. Ltd., China, and Shanghai Sincere Biotech Co. Ltd., China, respectively. 2.2. Dyeing experiments All the experiments were carried out in the sealed and conical flasks housed in a XW–ZDR low-noise oscillated dyeing machine (Jingjiang Xinwang Dyeing and Finishing Machinery Factory, China). The liquor ratio was 50:1. The dyeing temperature was started at 30 ◦ C, and raised to 90 ◦ C at a rate of 2 ◦ C/min with a holding time of 60 min. At the end of dyeing, the fabrics were washed in tap water and then dried in the open air. To assess the dependence of the uptake of natural yellow dyes on the pH value of dyebath, the silk fabrics were dyed with 3% owf (on the weight of fabric) dyes at the pH values approximately ranging from 2.3 to 7.4; McIlvaine buffers (citric acid and disodium hydrogen phosphate mixture) were added to adjust pH. To estimate the building-up properties of natural yellow dyes on silk, the dye concentration of 0.5–8% owf were used; the pH values of R. emodi, Gardenia yellow, and curcumin solutions were adjusted by McIlvaine buffers to 5.18, 2.75, and 2.32, respectively. 2.3. Post-mordanting treatment The fabrics dyed with 3% owf dyes were mordanted using three metallic salts (aluminum sulfate, ferrous sulfate, and ferric sulfate). The concentrations of the mordants ranged from 2 to 8% owf. The dyed fabrics were immersed in the mordant solutions at 70 ◦ C for 30 min. After post-mordanting, the fabrics were rinsed thoroughly in tap water and allowed to dry in the open air. 2.4. Measurements 2.4.1. Uptake of dyes by silk The absorption spectra and absorbance of the solutions were measured using a Shimadzu UV-1800 UV–Vis spectrophotometer. Because R. emodi and curcumin have poor solubility in water, the

Please cite this article in press as: Zhou, Y., et al., Simultaneous dyeing and functionalization of silk with three natural yellow dyes. Ind. Crops Prod. (2014), http://dx.doi.org/10.1016/j.indcrop.2014.09.041

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Fig. 1. Chemical structures of the components present in (a) R. emodi, (b) Gardenia yellow, and (c) curcumin.

dye solution which was composed of ethanol and water (80/20, v/v) was used in the experiments of spectroscopic analyses for the purpose of their concentration determination. After reference to the respective calibration curves of the dyes using Lambert–Beer’s law, the quantities of dyes in solution were able to be calculated, and the percentage of exhaustion (%E) was determined using Eq. (1), where m0 and m1 are the quantities of dyes before and after dyeing. The quantity of dyes on silk was calculated by the difference in the initial and final concentrations of dyes in solution as well as the weight of the dried fabric. %E =

m0 − m1 × 100 m0

(1)

2.4.2. Color characteristics The L*, a* and b* color coordinates (color strengths [K/S], lightness [L*], redness-greenness value [a*], and yellowness-blueness value [b*]) of dyed silk fabric were evaluated using a HunterLab UltraScan PRO reflectance spectrophotometer (illuminant D65; 10◦ standard observer). Each sample was folded twice so as to give a thickness of four layers.

2.4.3. Color fastness The wash fastness of the fabrics dyed with 3% owf dyes was carried out by a WashTec-P fastness tester (Roaches International, England), using the standard test method ISO 105-C06. The rubbing fastness was measured on a Model 670 crockmaster (James H. Heal, England) according to ISO 105-X12. The color fastness to light was evaluated using an Atlas XenoTest Alpha (SDL Atlas, USA) light fastness tester according to GB/T 8427-2008, and the fabrics were exposed to xenon arc lamp for 10 h in a standard testing condition.

2.4.4. Antioxidant property The antioxidant activity was determined by the ABTS radical decolorization assay according to a previously reported method (Re et al., 1999). ABTS was dissolved in water to a concentration of 7 mM. The ABTS radical cation (ABTS˙ + ) was produced by reacting ABTS stock solution with 2.45 mM potassium persulfate (final concentration), and allowing the mixture to be kept in the dark at room temperature for 12-16 h before use. The radical was stable in this form for more than two days when stored in the dark at room temperature. Before usage, the ABTS˙ + solution was diluted with a phosphate buffer (0.1 M, pH 7.4) to reach an absorbance of 0.700 ± 0.025 at 734 nm. Then, 10 mg of dyed silk was added to 10 ml of ABTS˙ + solution. After 30 min, the scavenging capability of ABTS˙ + at 734 nm was calculated using Eq. (2): Antioxidant activity (%) =

Acontrol − Asample Acontrol

× 100

(2)

where Acontrol is the initial absorbance of the ABTS˙ + and Asample is the absorbance of the remaining ABTS˙ + in the presence of fiber sample. 2.4.5. Antibacterial activity The antibacterial activity of dyed silk was evaluated according to GB/T 20944.3-2008. Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were adopted in this test. The specimens of the tested material fragments (0.75 g) were immersed in the conical flasks with bacteria housed in a water bath shaker, and treated for 24 h; the treatment temperatures were 30 ◦ C for E. coli solution, and 24 ◦ C for S. aureus solution, respectively. Then the bacteria solution was diluted to 1000 times with sterilizing phosphoric buffer solution so as to obtain a test bacteria solution. The diluted bacteria solutions were inoculated onto agar plates, and incubated at 37 ◦ C for 24 h for E. coli solution, and for 48 h for S. aureus solution. Finally,

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Fig. 2. UV–Vis absorption spectra of R. emodi and curcumin in ethanol as well as Gardenia yellow in water (dye concentration: 0.010 g/L).

the quantity of the visually bacterial colonies on agar plates was counted, and the antibacterial activity was evaluated based on Eq. (3): Antibacterial activity (%) =

Ncontrol − Nsample Ncontrol

× 100

(3)

where Ncontrol and Nsample are the quantities of the visual bacterial colonies of standard cotton fabric and tested silk fabric, respectively. 2.4.6. Ultraviolet protection ability The ultraviolet protection factor (UPF) and the UV transmittance of dyed silk fabric were determined in a Labsphere UV-1000F ultraviolet transmittance analyzer (Labsphere Inc., USA). Each sample was tested four times at different positions, and the average of the data was used. 3. Results and discussion 3.1. UV–Vis absorption spectra of natural yellow dyes The UV–Vis absorption spectra of R. emodi and curcumin in ethanol as well as Gardenia yellow in water are depicted in Fig. 2. Three dyes exhibited a large variation in spectra due to their difference in chemical structures. In the spectrum of R. emodi, the maximum absorptions were observed at 220, 252, 265, 290, and 437 nm, being suggestive of a hydroxyanthraquinone structure (Matsuda et al., 2001). Curcumin had two characteristic absorption bands at 249 and 421 nm due to the presence of phenolic groups and conjugated ␲-bond system (Kim et al., 2013); curcumin exists in an enol form, and the presence of the diketone moiety is responsible for its enol form with trans geometry (Wang et al., 1997). Gardenia yellow exhibited a maximum absorption peak at 441 nm and a shoulder peak at 324 nm, corresponding to the absorptions of its conjugated ␲-bond system and a trace of chlorogenic acid (Wang et al., 2014), respectively. Among three dyes, curcumin showed the shortest maximum absorption wavelength in the visible light region, and the strongest absorption intensity which indicates its great coloring power. 3.2. Dyeing properties of natural yellow dyes 3.2.1. Effect of pH on the uptake of dyes The pH of dye solution is an important dyeing process parameter because it can exert great impact on the stability of natural dyes, the charge nature of silk fiber, and the exhaustion of dye bath. The exhaustion of three natural yellow dyes under different pH values is shown in Fig. 3. The uptake of Gardenia yellow was found to most sensitive to pH and increased with decreasing pH. The main components of Gardenia yellow are crocins and crocetin (Chen et al., 2008)

Fig. 3. Effect of pH on the uptake of three natural yellow dyes by silk.

as shown in Fig. 1. Crocin 3 and crocetin possess one and two carboxyl groups, respectively, and they can be adsorbed by silk fiber by means of electrostatic interaction. Low pH can increase the ionization extent of amino groups in silk fiber, and consequently enhance the uptake of Gardenia yellow. In view of the chemical structures of R. emodi and curcumin, their affinities to silk fiber are predominantly attributed to physical rather than chemical interactions. R. emodi is sparingly soluble in water and bears a close resemblance to disperse dyes. The uptake of R. emodi by silk fiber was less affected by pH within a wide pH range of 3.2–6.1, being consistent with the previously reported result (Das et al., 2008). At a high pH value, the solubility of R. emodi increases due to the increased ionization of phenolic hydroxyl groups in its structure; as a result, its uptake exhibited a tendency toward a low level. Curcumin is a bis-␣,␤-unsaturated diketone and in solution exists in equilibrium with the corresponding enol tautomer. Under acidic and neutral conditions the bis-keto form predominates, whereas above pH 8 the enol tautomer is favored (Boga et al., 2013). Curcumin has poor solubility at acidic and neutral pH values (Boga et al., 2013). In addition to this, curcumin is unstable in aqueous solution and undergoes hydrolysis followed by molecular fragmentation at various pH values. Wang et al. (1997) found that 90% of curcumin degraded within 30 min in phosphate buffer at pH 7.4 into various products and that the first-order degradation rate constant of curcumin increased with increasing pH in a range of pH values from 3 to 8. In the light of the pH dependence of the keto–enol tautomerism and stability of curcumin, the rational conclusions can be drawn that the bis-keto form is predominant in the uptake of curcumin over the pH ranges used, and the uptake of curcumin decreases with increasing pH as demonstrated by the data in Fig. 3. The similar result about the pH dependence of the uptake of curcumin was also found in the dyeing of hair (Boga et al., 2013). 3.2.2. Building-up and color characteristics of dyes on silk The building-up properties of dyes are of great importance for the selection of dyes applied to the dark shade dyeing of textiles. The dye with a good building-up ability can impart a darker shade to the dyed textile. The building-up properties of dyes generally depend on dye structures, dye affinities to fibers, and dyeing conditions. The building-up properties of three natural yellow dyes expressed by the quantity of dyes adsorbed by silk (Cf ), and the apparent color depth of dyed silk are depicted in Fig. 4. Gardenia yellow showed the same trend in the apparent color depth and its adsorption quantity, both of which increased with its increasing initial concentration, indicating a good building-up capability. R. emodi and curcumin behaved differently from Gardenia yellow and curcumin. R. emodi had a poor building-up capability demonstrated by no increase in its adsorption when its initial concentration exceeded 4% owf, and in the color depth of dyed silk within its

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Table 1 Color fastness of the dyed silk fabrics without and with post-mordanting. Dyes

R. emodi Unmordanted Fe2+ mordanted Fe3+ mordanted Al3+ mordanted Gardenia yellow Unmordanted Fe2+ mordanted Fe3+ mordanted Al3+ mordanted Curcumin Unmordanted Fe2+ mordanted Fe3+ mordanted Al3+ mordanted

Washing fastness

Rubbing fastness

Color change

Dry

Wet

Staining

Light fastness

Silk

Cotton

3 4 4 4

3 3–4 3 3

2 3 2–3 4

4 4 4–5 4–5

3–4 4 4 4

2 3 4 3

5 5 5 5

4 4–5 4–5 4

3 3–4 3 3

5 5 5 5

4 4–5 4–5 4–5

4 4 3 4

4–5 4–5 4–5 4–5

2–3 3–4 3 2–3

2 2–3 2–3 2

3 4 4 4

2–3 3–4 3–4 4

2 3 3 3

i.e., their shades had a little shift towards yellow-coordinate in the yellow-red zone of color space, and became greenish yellow. This observation is in line with the UV–Vis absorption spectra of three yellow dyes in solution, where curcumin has a shorter maximum absorption wavelength in the visible light region as seen in Fig. 2.

Fig. 4. Building-up properties and color characteristics of three natural yellow dyes on silk: (a) the exhaustion and adsorption quantity of dyes, (b) the color depth of dyed silk, and (c) the colorimetric data of dyed silk.

concentration range used. As for curcumin, its adsorption quantity increased with increasing initial concentration, but the color depth of dyed silk had no augmentation when its initial concentration exceeded 3% owf. Overall, curcumin also had a relatively good building-up capability. The building-up capability of three dyes had the following order: Gardenia yellow > curcumin > R. emodi. On the other hand, Fig. 4 also reveals that in the case of the initial dye concentration above 3% owf, the uptake of curcumin and Gardenia yellow by silk displayed the high extent of exhaustion than that of R. emodi, and the exhaustion was higher than 60% in the concentration range used, indicating that the two dyes has a high utilization rate when applied. From the colorimetric data (a*/b* plots) depicted in Fig. 4c, it can be seen that the silk samples dyed with three natural yellow dyes showed different color characteristics. The yellow shade from R. emodi produced low color saturation with low a* and b* values due to its low adsorption by silk, whereas the application of curcumin and Gardenia yellow exhibited high color saturation. The samples dyed with curcumin showed more yellow colors with high b* values compared with those dyed with Gardenia yellow and R. emodi,

3.2.3. Color fastness of dyes on silk Silk textiles are subjected to frequent washing, rubbing, and lighting during their usage. In practical applications, the textiles for different purposes have varied requirements for their resistance to these conditions. It is well known that natural dyeings have relatively inferior color fastness properties which can be improved by the post-treatment of metallic mordants (Shahid et al., 2013). The color fastness of the dyed silk fabrics was evaluated, and the results are given in Table 1. Table 1 reveals the highest fastness ratings in the case of the dyeing of Gardenia yellow. Curcumin showed poor rub and light fastness as well as low wash fastness rating for staining although it had the high wash fastness ratings for color change. R. emodi had the characteristics of good rub fastness, fair wash fastness for color change, poor wash fastness for staining, and poor light fastness. Due to its hydrophobic nature, curcumin molecule is prone to aggregate in solution and on fibers, thus its aggregates existing inside the void space between silk fibers possibly lead to the poor rub fastness of dyed silk. Although R. emodi molecule is also hydrophobic in nature, it has the low extent of adsorption on silk as described above, and the corresponding low degree of aggregation on silk, which make its rub fastness rating higher than that of curcumin. The varied light fastness of three natural yellow dyes should be related to their chemical structures and quantities of adsorption on silk as well as their distribution states on silk. 3.3. Functionalities of silk fabrics dyed with natural yellow dyes 3.3.1. Antioxidant activity The bioactive fibers with good antioxidant activity can deactivate highly reactive and harmful species such as active oxygen radicals. Because silk is usually in direct contact with skin when in use, its antioxidant activity is of great significance for developing healthy and hygienic silk textiles and materials. Silk can be endowed with antioxidant activity by means of the adsorption of antioxidant compounds. Several investigations reported the antioxidant activities of the anthraquinone derivatives from R. emodi rhizomes, the extracts from G. jasminoides Ellis and curcumin (Barzegar, 2012; Barclay et al., 2000; Chen et al., 2008; Mishra

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(a)

(b)

Fig. 5. Antioxidant activity of silk fabrics dyed with three natural yellow dyes.

et al., 2014; Pham et al., 2000). Thus, the antioxidant activity was expected on the silk fabrics dyed with the three natural yellow dyes. Original silk had an antioxidant activity value of 34%, revealing its relatively poor radical scavenging ability. As can be seen in Fig. 5, the silk fabrics dyed with three dyes exhibited obviously enhanced antioxidant activity which increased with increasing initial dye concentrations. Curcumin endowed silk with the highest antioxidant activity which originates from its highest extent of adsorption on silk and the good inhibition ability of radical scavenging due to two hydrogen-donating phenolic hydroxyl groups existing in both sides of its chemical structure (Barclay et al., 2000). The hydrogen donors of the anthraquinone derivatives from R. emodi contributed to the antioxidant activity of dyed silk. Gardenia yellow imparted the antioxidant activity to silk which was attributable to its main active constituents–crocins; crocins were found to possess antioxidant activity (Chen et al., 2008; Pham et al., 2000), and sugars attached to the crocetin moiety were reported to be beneficial for the antioxidant activity of crocins (Chen et al., 2008), although information about the antioxidant mechanism of crocins has not yet been well established (Pham et al., 2000). 3.3.2. Antibacterial activities A number of reports are now available on natural dyes for imparting antimicrobial properties to textiles (Ghoreishian et al., 2013; Han and Yang, 2005; Hong et al., 2012; Khan et al., 2012; Koh and Hong, 2014; Mirjalili and Karimi, 2013; Shahid et al., 2013; Silva et al., 2011; Singh et al., 2005; Sousa et al., 2009). The antimicrobial activities of natural dyes are dependent on their chromophores and the functional groups present in their molecules (Shahid et al., 2013). Khan et al. (2012) reported the antimicrobial activities of wool yarns dyed with R. emodi against two bacterial (E. coli and S. aureus) and two fungal (Candida albicans and C. tropicalis) species. Several researchers studied the antibacterial of curcumin on wool and polyamide fabrics against E. coli and S. aureus (Han and Yang, 2005; Mirjalili and Karimi, 2013), as well as the antibacterial activity of silk fabric dyed with curcumin and mordanted with metallic mordants (Ghoreishian et al., 2013). To the best of our knowledge, the antibacterial ability of the textiles dyed with Gardenia yellow has not been reported. However, the antibacterial activity of Gardenia yellow present in solution against E. coli had been ascertained (Boo et al., 2012). In the present study, the antibacterial activities of the silk fabrics dyed with R. emodi, Gardenia yellow, and curcumin against E. coli and S. aureus were checked and compared, and the results are presented in Fig. 6. Apparently, natural dyeing remarkably increased the antibacterial activities of silk fabrics against E. coli and S. aureus. Increasing initial dye concentration was capable to enhance the antibacterial activities, but the increment in antibacterial activities was not high enough when initial dye concentration increased from 2 to 6%. At a dye concentration of 2%, all the dyed silk fabrics obtained the high

Fig. 6. Antibacterial activities of silk fabrics dyed with three natural yellow dyes against (a) E. coli and (b) S. aureus.

antibacterial activities more than 80% against both S. aureus and E. coli except that the antibacterial activity of the silk fabric dyed with Gardenia yellow against E. coli was 65%. The silk fabrics dyed with Gardenia yellow were found to be active against S. aureus, but relatively inactive against E. coli. 3.3.3. UV protection performance All dyes, regardless of natural or synthetic, can impart a certain degree of UV protection effect to the dyed textiles due to their UV absorption characteristics caused by the conjugated systems in their molecules. As expected, the silk fabrics dyed with three natural yellow dyes exhibited obvious improvement in UPF. It is clear from Fig. 7 that the UPF values of the silk fabrics dyed with curcumin and Gardenia yellow increased as the initial dye concentrations increased. Curcumin resulted in a higher increase in UPF compared with Gardenia yellow because of its higher UV absorption capability as shown in Fig. 2. Although R. emodi had the strongest UV absorption intensity in the UV absorption spectrum of its solution (Fig. 2) among three yellow dyes, it gave the lowest increase in the UPF values of dyed silk as a result of its lowest uptake by silk. 3.4. Influence of post-mordanting on the color parameters, color fastness, and functionalities of dyed silk fabrics 3.4.1. Color and fastness changes The color changes of the dyed silk fabrics mordanted with three metallic salts at the concentrations ranging from 2 to 8% owf are illustrated by the a* vs b* plots depicted in Fig. 8. Compared with the original white fabric, the undyed fabrics mordanted with ferrous and ferric salts displayed a pale and dull yellow color. Fig. 8 shows that the effect of post-mordanting on the colors of the dyed

Fig. 7. UPF values of silk fabrics dyed with three natural yellow dyes.

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Fig. 8. Effect of post-mordanting on the colorimetric data of dye silk fabrics.

fabrics depended greatly on metallic mordants and dyes. The colors of the fabrics dyed with three natural yellow dyes were little affected by treatment with aluminum sulfate as marginal changes were observed in the a* and b* values for the mordanted samples in comparison to the unmordanted ones. After post-mordanting with ferrous and ferric salts, the fabrics dyed with R. emodi and Gardenia yellow became fairly dull, as evidenced by the decreased a* and b* values. The changes in the colors of the curcumin dyed fabrics imparted by treatment with ferrous and ferric salts were clearly apparent from a comparison of the a* and b* values obtained for the unmordanted and mordanted samples; the fabrics showed a dark brown color after mordanting. These observations suggest that ferrous and ferric ions can interact strongly with the silk fiber and the curcumin dye adsorbed on silk by virtue of coordination bonds. In essence, the diketone and corresponding enol tautomer of curcumin provide great possibility for the coordination reaction of curcumin with ferrous and ferric ions which can yield a six-membered ring complex; such a reaction had been verified by some researches (Bernabé-Pineda et al., 2004; Borsari et al., 2002). The main components present in R. emodi are 1,8dihydroxyanthraquinone derivatives whose carbonyl and hydroxyl groups have the ability to form complexes with metal ions (Khan et al., 2012). Crocins and crocetin as the main constituents of Gardenia yellow have significantly less ability to coordinate with metal ions because of lack of the functional groups which can form a stable five or six-membered chelate ring. The color fastness of the silk fabrics dyed with 3% owf dyes and subsequently mordanted with 2% owf mordants is also listed in Table 1. The fabrics dyed with Gardenia yellow had relatively minor changes in color fastness imparted by post-mordanting because Gardenia yellow lacks the ability to form complexes with metal ions. For the fabrics dyed with R. emodi and curcumin, post-mordanting increased wash fastness for staining on adjacent fabrics as well as rub and light fastness, and improved the wash fastness of the dyeings of R. emodi for color change.

3.4.2. Functionality changes The antioxidant and antibacterial activities of the dyed silk fabrics dyed with 3% owf dyes and subsequently mordanted with 2% owf mordants were measured. The effects of post-mordanting on antioxidant and antibacterial activities were assessed using the percentage of variation which is a ratio of the difference between

Fig. 9. Effect of post-mordanting on the percent variation in the antioxidant activity of dye silk fabrics.

the activity values of the mordanted and unmordanted fabrics to the value of the unmordanted fabric. The changes in the antioxidant and antibacterial activities are shown in Figs. 9 and 10, respectively. Fig. 9 shows that post-mordanting gave rise to minor changes in the antioxidant activity of dyed fabrics, and the variation percentage was below 9%. This variation is preliminarily thought to originate from the impact of dye–mordant complex and metal ions on silk on the ABTS radical decolorization during the process of antioxidant activity analysis. Some metallic compounds based on metals like copper, nickel, chromium ion, tin, aluminum, and silver had been proved to be

(a)

(b)

Fig. 10. Effect of post-mordanting on the percent variation in the antibacterial activities of dye silk fabrics against (a) E. coli and (b) S. aureus.

Please cite this article in press as: Zhou, Y., et al., Simultaneous dyeing and functionalization of silk with three natural yellow dyes. Ind. Crops Prod. (2014), http://dx.doi.org/10.1016/j.indcrop.2014.09.041

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Fig. 11. Effect of post-mordanting on the UV protection capability of dye silk fabrics.

capable of imparting antibacterial activity to textiles (Ghoranneviss and Shahidi, 2012; Haji, 2012). In our study, the antibacterial activities of the undye silk fabrics mordanted with three metallic salts against E. coli and S. aureus ranged from 24 to 40%, whereas those of the undyed fabrics were 19 and 17%, respectively. Several researchers noticed the changes in the antibacterial activity of natural dyeings imparted by pre-mordanting. The increase in antibacterial activity was induced by pre-mordanting for some natural dyeings (Ghaheh et al., 2014; Haji, 2012; Mirjalili and Karimi, 2013), while the decrease occurred for other natural dyeings (Khan et al., 2011, 2012; Yusuf et al., 2012). In our work, post-mordanting was applied for the treatment of dyed fabrics. Fig. 10 shows that the antibacterial activities of dye fabrics were more or less improved by means of post-mordanting. It is worth noting that the fabrics dyed with Gardenia yellow exhibited a higher increase in the antibacterial activity against E. coli after the post-mordanting with ferrous and ferric salts. This increment should be a consequence of the contribution of ferrous and ferric ions adsorbed on silk to antibacterial activity. The UPF values of the silk fabrics dyed with 3% owf dyes and subsequently mordanted with three metallic salts at the concentrations ranging from 2 to 8% owf were evaluated, and the results are shown in Fig. 11. For comparative purposes, the UV protection performance of the undyed and mordanted fabrics was measured. The UPF values of the undyed fabrics were much less affected by the mordanting with aluminum sulfate, but increased remarkably with increasing amounts of ferrous and ferric salts. Similar phenomena were also found for the dyed and mordanted fabrics. In addition, the mordanting with ferrous and ferric salts exhibited greater influence on the UPF values of the fabrics dyed with curcumin and R. emodi than on those of the fabrics dyed with Gardenia yellow, which should be related to the greater ability of the complexation of the first two dyes with ferrous and ferric ions, and the inherent contribution of ferrous and ferric ions on silk fiber to UV protection.

the chemical structures and application characteristics of dyes. The results showed that a conventional dyeing process with no need of additional chemical finishing was able to successfully achieve the simultaneous dyeing and functionalization effects. The dyeing and mordanting properties and functionalities of three natural yellow dyes were found to correlate with dye structures. Curcumin with the diketone moiety and two phenolic hydroxyl groups had the greatest coloring power and good building-up ability and imparted high antibacterial activities and the highest antioxidant activity to silk. R. emodi with anthraquinone structures showed poor building-up ability. Gardenia yellow, whose major components are crocins and crocetin, displayed the highest building-up ability and pH-sensitive uptake characteristic. Both R. emodi and Gardenia yellow provided silk with high antibacterial activities and enhanced antioxidant activity. Whereas Gardenia yellow had the highest fastness ratings, curcumin and R. emodi had disadvantages in wet rub fastness, wash fastness for staining, and light fastness. The post-mordanting with ferrous and ferric salts was able to increase the color fastness and UV protection ability of dyed silk, and obviously change the color of curcumin dyed silk. Overall, R. emodi, Gardenia yellow, and curcumin can be used suitably for developing healthy and hygienic silk textiles and materials. Acknowledgments This study was funded by Jiangsu Provincial Natural Science Foundation of China (No. BK20130344), Suzhou Research Program of Application Foundation (SYG201333), 2014 Qinglan Project of Qinglan Project Jiangsu Education Commission, and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. References

4. Conclusion The study compared the dyeing and functional properties of three natural yellow dyes (R. emodi, Gardenia yellow, and curcumin) applied to silk and attempted to reveal the correlations between

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Please cite this article in press as: Zhou, Y., et al., Simultaneous dyeing and functionalization of silk with three natural yellow dyes. Ind. Crops Prod. (2014), http://dx.doi.org/10.1016/j.indcrop.2014.09.041