Potential of Sorghum Husk Extracts as a Natural Functional Dye for

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Research Article pubs.acs.org/journal/ascecg

Potential of Sorghum Husk Extracts as a Natural Functional Dye for Wool Fabrics Xiuliang Hou,*,† Fangfang Fang,† Xueling Guo,† Jakpa Wizi,† Bomou Ma,† Yongying Tao,‡ and Yiqi Yang*,§,∥ †

Key Laboratory of Science & Technology of Eco-Textiles, Ministry of Education, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, China ‡ Luolai Lifestyle Technology Co. Ltd., 1699 Xinghu Road, Nantong, Jiangsu 226009, China § Department of Textiles, Merchandising & Fashion Design, University of Nebraska−Lincoln, 234, HECO Building, Lincoln, Nebraska 68583-0802, United States ∥ Department of Biological Systems Engineering, University of Nebraska−Lincoln, 234, HECO Building, Lincoln, Nebraska 68583-0802, United States ABSTRACT: We report a novel application of sorghum husk extracts (SHE) as a natural functional dye for wool fabrics. Sorghum husk is an abundant, cheap and readily available agricultural byproduct. A small proportion of sorghum husk has been used to extract food colorants. In order to add value to sorghum and decrease environmental pollution, a number of investigations need to be undertaken to explore newer application for the husk. This paper investigated the stability of SHE, the colorfastness, UV-protection and fluorescence properties of dyed wool fabrics with SHE by different dyeing methods. SHE had good thermal and pH stability suitable for the dyeing and finishing processes of textiles. Wool fabrics dyed directly or with Al3+ and Fe2+ mordant demonstrated good colorfastness to washing, to rubbing, to wet ironing and acceptable colorfastness to light. The dyed wool fabrics showed good UV-protection and fluorescence properties. After 30 home laundering cycles, the UV-Protection Factor (UPF) and fluorescence intensity of wool fabrics dyed with SHE were still remarkably higher than those of wool fabrics dyed with mixed synthetic dyes with similar shade and depth and undyed fabric. SHE would be a feasible alternative for some synthetic dyes and functional finishing agent. KEYWORDS: Sorghum husk extracts, Natural dye, Wool fabric, Color fastness, UV-Protection property, Fluorescence

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madder, turmeric, saffron petals, henna, and Chinese gall.7 The wool fabrics dyed with indicaxanthin had interesting fluorescence properties.8 Terminalia arjuna had both antimicrobial and fluorescent properties.9 Acacia nilotica exhibited fluorescence property in the solution phase as well as after application on wool fiber.10 A number of researchers are geared toward exploring natural resources for their functional properties and improved color stability for natural dyes.2 Sorghum is the fifth leading crop in the world after wheat, maize, rice, and barley and is grown in tropical, subtropical, and arid regions. Annual sorghum production is about 57 million tons globally.11 Sorghum is a rich resource for various phytochemicals including tannins, phenolic acids, and anthocyanins. These phytochemicals have the potential to combat common nutrition-related diseases including cancer, cardiovas-

INTRODUCTION With the consumer’s enhanced awareness of eco-safety, there has been an increasing tendency toward the use of sustainable and environmentally friendly materials. Considerable attention has been given to natural textile dyes and functional finishing agents in recent years.1−3 Using functional dyes in textiles integrates the dyeing and finishing process and results in a more efficient technique in terms of less water and energy usage. Some natural dyes are known to have functional effects, thus they can be considered as green eco-friendly substitute to synthetic dyes. Orange peel has potential to be utilized as a natural textile dyestuff that could impart textiles with UVprotection properties.4 Rheum emodi, Gardenia yellow, and curcumin are natural dye sources that can impart color yellow and functional properties such as UV-protection and antibacterial properties to silk.5 Pomegranate peel extracts are rich in tannins and have been used for their medicinal properties and dyeing.6 Antibacterial function was imparted on wool fabrics from natural dyes obtained from green tea, © 2017 American Chemical Society

Received: December 7, 2016 Revised: May 3, 2017 Published: May 4, 2017 4589

DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

Research Article

ACS Sustainable Chemistry & Engineering

Center, Shandong, China. Gallic acid Standard with HPLC purity above 99% was obtained from Sinopharm Chemical Reagent Co., Ltd., China. Apigeninidin chloride standard with a HPLC purity above 97% (Extrasynthese, France) was purchased from Shanghai Zzbio Co., Ltd., China. Magic Amah laundry detergent was produced by Magic Amah Daily-Use Commodity Co. Ltd. (Suzhou, China). Extraction Method of Sorghum Husk. According to the optimized extraction conditions for food colorants from sorghum brans,15 sorghum husk was pulverized with a high-speed universal disintegrator FW 100 (Tianjin Taisite Instrument, China) and passed through a 0.8 mm mesh. The powder was added to 70% (v/v) ethanol/water at a liquid-to-solid ratio of 50:1 and was extracted at 80 °C for 120 min using a thermostated water bath shaker (Rapid Precision Machinery Co., Ltd., Xiamen, China). The extracted liquid was filtered with a stainless steel sieve (200 meshes per square inch) and cooled to room temperature. The filtrate was centrifuged at 8000 rpm for 60 min with a high-speed refrigerated centrifuge Avanti J-E (Beckman Coulter). The supernatant was concentrated under reduced pressure using a rotary evaporator and dried at 80 °C in oven to obtain SHE. SHE had a yield of 6.15 ± 0.27% based on weight of powdered sorghum husk and was used without further purification. Measurement of the Chemical Composition of SHE. The FTIR spectrum was measured with a Fourier-transform infrared spectrophotometer (Nicolet iS10). SHE sample, SHE-Al3+, and SHEFe2+ was dried in a vacuum oven at the temperature of 50 °C for 12 h and then recorded from 4000 to 500 cm−1 at a resolution of 4 cm−1. The total phenolic content in SHE was measured according to the modified Folin-Ciocalteu method using gallic acid as the standard.14,15,17 The total flavonoid content was determined according to the aluminum chloride colorimetric method using rutin as the standard.23 The total phenolic and flavonoid contents were tested 3 times, and the mean value ± standard error was calculated. Mass spectra (MS) of SHE and apigeninidin chloride standard were obtained by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI SYNAPT Q-TOF MS, Waters Corporation). The main conditions for MS were electrospray ionization (ESI +), collision energy 6 V, and mass to charge ratio 50−1000 m/z. Separation was at C18 50 mm × 2.1 mm 1.7 μm column (Waters). Flow rate was 0.3 mL/min, injection volume was 1 μL, and column temperature was 45 °C. The mobile phase was (A) 100% acetonitrile and (B) 10% formic acid in water. The gradient was as follows: 10% A, 5 min; 50% A, 7 min; 100% A, 11 min; 10% A. Identification of apigeninidin components was by matching peak retention times and MS characteristics with that of standard. Stability Analysis of SHE. SHE solution was heated to different temperatures from 60 to 90 °C, respectively, with a thermostatic water bath shaker (Rapid Precision Machinery Co. Ltd., Xiamen, China) and from 100 to 130 °C, respectively, with a high-pressure steam sterilizer OPS-40IS (Shanghai Ouster Industrial, China) for 2 h. SHE solution was also heated to 100 °C with varying time from 1 to 5 h. The solutions after different heat-treatments were all diluted to 5 L with distilled water. The same SHE solution which was set at room temperature for 2 and 5 h, respectively, was diluted in the same way and liquor served as the control. After dilution for 30 min, the UV−vis absorbance spectra were measured and compared with UV−vis spectrophotometer TU-1901 (Purkinje General Instrument Co. Ltd., Beijing, China) using a quartz cell with a path length of 1 cm. The absorbance measurements were made in the 200−800 nm range with a 1 nm step. Thermogravimetric analysis of SHE powder was performed on thermogravimetric analyzer TGA Q 500 (Waters) in an air atmosphere. The samples were heated from 30 to 500 °C at a heating rate of 10 °C/min. The mass of the samples ranged from 5 to 10 mg. SHE solution was adjusted to different pH values from 3 and 10. The pH values of SHE solutions were measured using pH meter EL20 (Mettler Toledo, Switzerland). Absorption spectra at different pH values were recorded from 200 to 800 nm after samples were equilibrated for 30 min at room temperature. Dyeing Methods and Conditions. According to our previous research,22 the wool fabrics were dyed at SHE dosage of 1.2% on

cular disease, and obesity and significantly impact human health.11−14 Therefore, there is an increased interest in growing sorghum.12 Pigmented sorghum pericarps have high levels of 3Deoxyanthocyanins, which are stable to change in pH and have high antioxidant activity.11,12 These 3-deoxyanthocyanins are potentially valuable resources for natural food colorants, which can be utilized in the food industry in preparing cream products, caramels, fruit starch jellies, and beverages with high antioxidant properties and excellent stability to light, heat, and change in pH.15,16 There were some significant correlation between pericarp color and antioxidant activity. Darker grains contain higher levels of phenolic compounds and antioxidant activity.12,17 Further research will enhance the extraction techniques of phenolic compounds from sorghum and the interest in growing sorghum. As a byproduct of sorghum grain, sorghum husk is an abundant, safe, cheap, and readily available biomass from the increased production of sorghum. The investigation proved that colorants from sorghum husk were nontoxic and did not cause health issues.18 A small proportion of sorghum husk has been used to extract food colorants. The colorants from sorghum husk also have been utilized as food additives for several years in China. In order to effectively utilize the sorghum husk, many investigations have been done to explore its new application. Waghmare et al. used sorghum husk as substrates of white rot fungus Phanerochaete chrysosporium to produce cellulase.19 Saratale et al. demonstrated that a cost-effective hydrogen production was possible with sorghum husk as a lignocellulosic feedstock.20 Sorghum bicolor L. biomass could be an effective adsorbent for removing toxic metals, including Pb(II), Cd(II), and Cu(II) from wastewater owing to the fine adsorption capacity.21 In order to further effectively utilize the abundant sorghum husk, a number of investigations need to be undertaken to explore its new applications. Colorants from sorghum husk are friendly to humans and the environment if they are used as dyestuff. The utilization of sorghum husk extracts (SHE) as dye for wool fabrics was investigated earlier by our group.22 We studied the dyeing kinetics and thermodynamics of SHE onto wool fabrics and reported that the adsorption process could be well described by the pseudo-second-order kinetic model. This paper investigated the stability of SHE, the color fastness, UV-protection, and fluorescence properties of the SHE dyed wool fabrics, which have not been reported based on our best knowledge.

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EXPERIMENTAL SECTION

Materials and Chemicals. Worsted wool fabrics were purchased from Wuxi Xiexin Textile Co. Ltd., China. With the following characteristics: Gabardine, warp and weft 17 × 2 tex, warp density 402 threads per 10 cm, weft density 225 threads per 10 cm, weight 237 g/ m2. The dried sorghum husk was obtained from Changling, Jilin Province, China. Synthetic dyes including Neolan Yellow P, Red P, and Blue P (metal complex dyes) were from Huntsman Textile Dyestuffs & Chemicals Co. Ltd. (China). Acetic acid, hydrochloric acid, sodium carbonate, and sodium hydroxide were for pH adjustment, aluminum potassium sulfate (KAl(SO4)2·12H2O), and ferrous sulfate (FeSO4·7H2O) were used as mordants and were all AR grades from Sinopharm Chemical Reagent Co., Ltd., China. The two mordants are generally used in dyeing processes of natural dye and both are benign to human health and the environment.2 Rutin with a HPLC purity above 98% was purchased from Traditional Chinese Medicine Reference Standards Engineering Technology Research 4590

DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

Research Article

ACS Sustainable Chemistry & Engineering weight fabric (o.w.f.), 98 °C, pH 6.5 and a bath ratio of 30:1 for 90 min in a thermostatic water bath shaker (Rapid Precision Machinery Co. Ltd., Xiamen, China). The dyed fabrics were rinsed in water at 40 °C for 30 min with Magic Amah laundry detergent and dried at ambient temperature. Direct dyeing (without mordant) and mordant dyeing methods (premorda,nt, one-bath and postmordant) were used to dye wool fabrics with SHE. For mordant dyeing methods, the initial dosage of mordant KAl(SO4)2·12H2O and FeSO4·7H2O was 6% (o.w.f.) and 3% (o.w.f.), respectively. In Al3+ premordant or postmordant process, the samples were mordanted at 80 °C for 60 min.4 In Fe2+ premordant or postmordant process, the fabrics were mordanted at 60 °C for 45 min.4 Wool fabrics were also dyed with mixed synthetic dyes including Neolan Yellow P, Red P, and Blue P at pH 3, with a bath ratio 30:1, at 98 °C for 90 min. Initial concentrations of the dyeing auxiliaries were sodium sulfate 0.6 g/L, formic acid 0.4 g/L, Albegal plus 0.3 g/L, and sulfuric acid 0.05 g/L. In all the dyeing processes, the dyeing temperature was increased from 60 to 98 °C at a heating rate of 1 °C/ min. Evaluation of Color and Colorfastness of the Dyed Wool Fabrics. After being equilibrated in standard conditions (20 ± 2 °C and 65 ± 3% relative humidity) for 24 h, color characteristic values, L*, a*, b*, c*, h°, and K/S of the dyed wool fabrics were evaluated by a ColorEye7000A spectrophotometer (Gemini BV, Gretag Macbeth Company) using an illuminant D65 and 10° standard observer. L*, a*, b*, c*, h°, and K/S are lightness, redness-greenness, yellownessblueness, saturation, hue, and color depth, respectively. ΔE is the CIELAB color difference between the tested sample and the control. Colorfastness to washing with soap was evaluated according to ISO 105-C10:2006. The dyed wool fabrics were stitched between cotton and wool fabrics and washed in a soap solution of 5 g/L at 40 °C for 30 min. The extent of stain on cotton or wool fabric and color change of the dyed wool fabric were determined. Colorfastness to rubbing was evaluated following ISO105-X12:2001. Colorfastness to wet ironing was evaluated according to ISO 105-X11-1994. The dyed wool fabrics covered with wet cotton fabrics (100 wt % of moisture content) were hot pressed at 200 °C under 4 kPa for 15 s. The extent of dye that stained on cotton fabrics and color change of the dyed wool fabric samples were determined. Colorfastness to light was evaluated following ISO105-B02:2000. The dyed wool fabrics and blue wool reference materials were exposed for 9 or 20 h in Atlas 150S lightfastness device (ATLAS Ltd., German) equipped with a xenon arc lamp. Light-fastness rating was determined according to the color change of the tested fabrics and blue wool reference materials. Evaluation of UV-Protective and Fluorescence Properties of Wool Fabrics Dyed with SHE. The UV-protection properties of the fabrics were measured according to the widely adopted Australian/ New Zealand standard AS/NZS 4399:1996 Sun Protective ClothingEvaluation and Classification.4,24 Ultraviolet transmittance rates of the undyed and dyed fabrics were determined using Varian Cary 50 fabric UV protection measurement system (Agilent Technologies Ltd.). The UV-Protection Factor (UPF) and ultraviolet transmittance rates TUVA, TUVB were recorded. TUVA is the transmittance rate of UVA at wavelength between 315 and 380 nm; TUVB is the transmittance rate of UVB at wavelength between 280 and 315 nm. UPF, TUVA, TUVB were calculated, respectively, according to the equations in the reference.4 Each sample was measured three times. Lower ultraviolet transmittance rate and higher UPF value means better UV-protection property of the fabric. The excitation and emission spectra of SHE solution were measured with a fluorescence spectrophotometer (F-7000 FL, Hitachi HighTechnologies Corporation, Japan) with a photomultiplier tube voltage of 600 V, 150 W Xe lamp as an excitation source and a scan speed of 30 000 nm/min. The fluorescent photomicrographs of the wool fabrics before and after dyeing were taken using an inverted fluorescence microscope Ti− S (Nikon Eclipse Inc., Japan) with a high-pressure mercury lamp as excitation source. The filter sets of FITC (excitation filter 465−495 nm; barrier filter 515−555 nm) were used for fluorescence

measurement. The microscope was combined with a Nikon digital sight DS-Filch. All fluorescent images were taken at room temperature in the dark. In order to measure the durability of UV-protective and fluorescence properties, the dyed wool fabrics with SHE were laundered with Magic Amah laundry detergent, which is commonly used in home laundering in China. One laundering cycle was carried out for 10 min, in a detergent concentration of 3 g/L with bath ratio of 30:1 at room temperature. Fabrics were washed for 5, 10, 15, 20, 25, and 30 laundering cycles. The fabrics were then dried in an ambient temperature and equilibrated in a conditioning room at standard air conditions (20 ± 2 °C and 65 ± 3% relative humidity) for 24 h. The UPF and fluorescence were measured and compared after washing. Characterization of Dispersing of Medal Mordants on Wool Fabric Surface. A scanning electron microscopy/energy dispersive Xray spectrometer (SEM-EDX) (model S4800, Hitachi Corporation, Japan) was used to examine the loading and dispersing of mordant Al or Fe element on the surfaces of the dyed wool fabrics.

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RESULTS AND DISCUSSION Composition of SHE. The total phenolic content was 574 ± 13 mg of gallic acid equivalents per gram of dry SHE. The total flavonoid content was 462 ± 16 mg of rutin equivalents per gram of dry SHE. As shown in Figure 1, the FTIR spectrum

Figure 1. FTIR spectrum of SHE.

of SHE presented intense absorption peaks at 3200 cm−1, 2920 cm−1, 1600 cm−1, and 1050 cm−1. The peak at 3200 cm−1 is the stretching vibration of O−H, whereas the peak at 2920 cm−1 is the stretching vibration of C−H. The peak at 1600 cm−1 corresponded to the stretching vibration of CC, which is a characteristic absorption peak of the phenyl. The peak at 1050 cm−1 corresponded to the stretching vibration of C−O. These four intense peaks indicated that SHE had phenolic and flavonoid compounds. Figure 2 shows the total ion chromatograms (TIC) and mass spectra (MS) of SHE and apigeninidin chloride standard. As shown in Figure 2, the TIC for SHE and apigeninidin chloride standard revealed a peak at t = 3.96 and t = 3.88, respectively, which had the same peak of protonated molecule [M + 1] (m/ z, 255) and similar fragment ion peaks as shown in MS. Therefore, SHE had apigeninidin component. According to the references,11,25,26 pigmented sorghum pericarps also had an apigeninidin component. Stability of SHE. Figure 3 shows the UV−vis absorbance curves of SHE solutions after different hydrothermal treatments. As shown in Figure 3, SHE solutions had strong absorbance in the ultraviolet region (200−380 nm) with the same absorbance peaks at 268 nm for both pre and post 4591

DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

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ACS Sustainable Chemistry & Engineering

Figure 2. Total ion chromatograms (TIC) and mass spectra (MS) of SHE and apigeninidin chloride standard.

the color of the wool fabric dyed without mordant (direct dyeing) was different from those dyed with Al3+ or Fe2+ mordants. Direct dyeing produced light brown. Mordant dyeing with Al3+ (premordant, one-bath and postmordant) produced redder and brighter shades, indicated by higher a* and L*. Mordant dyeing with Fe2+ (premordant and postmordant) yielded dark brown shades, indicated by higher K/S, lower L*, lower a*, and higher h° value. Compared with premordant or postmordant dyeing, one-bath mordant dyeing could produce brighter color as indicated by higher c*. The chelation between the metal ion and SHE can be identified by Figure 7. Figure 7 shows the color changes and precipitation of SHE solution due to the addition of the metal ions, indicating the chelation between SHE and the metal ions.27,28 Figure 7 also shows the fine changes in the wavenumbers from 1118 to 1021 cm−1 for FTIR spectra of the precipitated SHE-Al3+ complex and SHE- Fe2+ complex compared with SHE. These changes also indicated the chelation between SHE and the metal ions. Possible interaction between Al3+ or Fe2+ and SHE and wool are presented in Figure 8. Al3+ and Fe2+ can form complexes with carboxyl and amino in wool fiber and with the two adjacent phenolic hydroxyl groups in the phenolic colorants of SHE. Such coordination could change the shades of the dyed wool fabrics. Figure 9 shows the uniform distribution of metal elements on the surface of wool fabrics. The content of elements Al and Fe were 1.89−2.03% and 0.85−1.53%, respectively, and there was little element K. Comparatively one-bath mordant dyeing is a one-step dyeing process which saves time, water, and energy. Therefore, direct dyeing and onebath mordant dyeing with Al3+ or Fe2+ are preferable for dyeing wool fabrics with SHE. The functionalities of the dyed wool fabrics by these three dyeing methods were subsequently studied. Colorfastness Properties of the Wool Fabrics Dyed with SHE. Table 2 shows that wool fabrics dyed with SHE using different dyeing methods have excellent colorfastness to rubbing, wet ironing (no less than rating of 4), washing with soap, including staining on cotton and wool fabrics, and color

hydrothermal treatments. Compared with SHE solution set at room temperature as control, SHE solutions after hydrothermal treatments at different temperatures from 60 to 90 °C for 2 h (Figure 3a) and at 100 °C for different times from 1 to 5 h (Figure 3c) presented similar shapes of the absorbance curves. The absorbance in visible region (380−550 nm) and a slightly higher absorbance in ultraviolet region (250−380 nm) indicated that the colors of SHE solutions hardly changed. For hydrothermal treatments at different temperatures from 100 to 130 °C for 2 h, SHE solutions presented higher absorbance in the visible region (380−420 nm) and in the ultraviolet region (250−380 nm) (Figure 3b), which might result from some depolymerized condensed tannins, SHE solution showing darker shades. If the dyed fabrics with SHE are treated at above 100 °C, they will show a little bit darker shades. Figure 4 shows the thermogravimetric (TG) and differential thermogravimetric (DTG) curves for SHE. As shown in Figure 4, the weight loss of SHE at 100 °C was 5.6%, which resulted from the evaporation of water in SHE. With increasing the temperature from 100 to 200 °C, the weight of SHE slightly decreased and the weight loss was 7.9%. DTG curve presented the first decomposition peak at 222 °C. Usually, the dyeing and finishing temperatures for textiles are below 180 °C. Figure 5 shows the UV−vis absorbance curves for SHE solutions after hydrothermal treatments at 100 °C for 2 h, with different pH values from 3 to 10. As shown in Figure 5, the shapes of the UV−vis absorbance curves of SHE solutions are almost the same in the pH range of 3−10. With increasing the pH value from 3 to 10, there was a slightly higher absorbance in the visible range (380−600 nm) and a higher absorbance in the ultraviolet region (200−380 nm), indicating slightly deeper colors of SHE solutions. Normally, the pH values for dyeing and finishing of wool fabrics range from 3 to 10. In general, SHE had good thermal and pH stability suitable for the dyeing and finishing processes of wool fabrics. Dyeing Properties of SHE. Effect of Dyeing Methods and Mordants on the Color of Dyed Wool Fabrics. Figure 6 and Table 1 shows the effect of dyeing methods on the color change on wool fabrics dyed with SHE. It could be seen that 4592

DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

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ACS Sustainable Chemistry & Engineering

Figure 5. UV−vis absorbance curves for SHE solutions after hydrothermal treatments at 100 °C, with different pH values from 3 to 10 for 2 h.

Figure 6. Wool fabrics dyed with SHE using different dyeing methods: (a) direct dyeing without mordant, (b) premordant dyeing with Al3+, (c) one-bath mordant dyeing with Al3+, (d) postmordant dyeing with Al3+, (e) premordant dyeing with Fe2+, (f) one-bath mordant dyeing with Fe2+, (g) postmordant dyeing with Fe2+. Dyeing conditions: initial dosage of SHE 1.2% (o.w.f.), 98 °C, 90 min, pH 6.5, a bath ratio of 30:1. Main conditions of mordant dyeing: initial dosage of KAl(SO4)2· 12H2O 6% (o.w.f.), initial dosage of FeSO4·7H2O (o.w.f) 3% (o.w.f.), a bath ratio of 30:1. Figure 3. UV−vis absorbance curves of SHE solutions after hydrothermal treatments (a) at 60−90 °C for 2 h, (b) at 100−130 °C for 2 h, and (c) at 100 °C for varying times from 1 to 5 h.

Table 1. Color Characteristic Values of the Wool Fabrics Dyed with SHE by Different Dyeing Methodsa color characteristic values the dyed wool fabric Figure Figure Figure Figure Figure Figure Figure a

6a 6b 6c 6d 6e 6f 6g

K/S

L*

a*

b*

c*

h°

11.0 9.6 4.9 16.7 19.4 11.5 22.1

43.3 50.5 57.7 43.5 39.7 48.0 36.2

14.9 18.6 20.8 17.7 11.5 14.4 9.7

19.3 20.6 19.8 21.5 19.6 23.4 17.3

24.3 27.8 28.7 27.8 22.7 27.5 19.9

52.2 48.0 43.7 50.6 59.7 58.4 60.6

Dyeing conditions were the same with that in Figure 6.

change (above rating of 3). The excellent colorfastness to wet ironing resulted from the excellent thermal stability of SHE. The colorfastness to light of wool fabrics dyed by direct dyeing and mordant dyeing with Al3+ rated 3, which was the lowest grade of fastness for application as a textile dye.29 The dyed wool fabrics using mordant dyeing with Fe2+ had

Figure 4. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves for SHE.

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DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

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ACS Sustainable Chemistry & Engineering

Figure 7. Changes of SHE due to the addition of Al3+ and Fe2+.

Figure 9. Uniform distribution and content of metal elements on the surfaces of the dyed fabrics by one-bath mordant dyeing.

Figure 8. Possible complexation reaction of (a) SHE-Al3+-wool and (b) SHE- Fe2+-wool.

Actually, most natural dyestuffs have poor colorfastness to light in comparison with synthetic dyes.2 For example, turmeric, weld, madder, and woad had a poor light colorfastness rating of 1−2.2 The good light colorfastness of SHE as natural dyestuffs indicated that SHE has a great potential to be used as a natural dyestuff in the textile industry. Functionality of the Wool Fabrics Dyed with SHE. UVProtection Property. Figure 10 displays wool fabrics dyed with SHE and with mixed synthetic dyes. Table 3 presents the color characteristic values, color difference ΔE and UV-protection properties. Figure 10 shows a set of three pair of dyed wool

increased colorfastness to light (above rating of 3) than those with direct dyeing and mordant dyeing with Al3+. The colorfastness of SHE chelated with Fe2+ improved more than that with Al3+, probably because the chelates with Fe2+ are more stable than that with Al3+. Cristea also concluded that the mordant was more important than the dye itself in determining the light fastness of colored textiles.30 Use of Fe2+ mordant resulted in significantly less fading than when Al3+ was used. Another possible reason was that Fe2+ may have a negative catalytic effect on the photochemical degradation of SHE.30 4594

DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

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ACS Sustainable Chemistry & Engineering Table 2. Colorfastness Rating of the Wool Fabrics Dyed with SHE colorfastness ratinga to washing with soap Figure Figure Figure Figure Figure Figure Figure a

6a 6b 6c 6d 6e 6f 6g

to rubbing

to wet ironing

to light

staining on cotton

staining on wool

color change

dry

wet

staining on cotton

color change

4 4−5 4−5 4−5 4 4−5 4

4−5 4−5 4−5 4−5 4−5 4−5 4−5

3−4 3−4 3−4 3−4 4−5 5 4−5

4−5 4−5 5 4−5 4 4−5 4−5

4−5 4 4−5 4−5 4−5 4−5 4−5

5 5 5 5 5 5 5

4−5 4−5 4−5 5 5 4−5 5

3 3 3 3 3−4 4 4

Higher colorfastness rating means better colorfastness. Dyeing conditions were the same with that in Figure 6.

synthetic dyes with similar shade and depth of shade, respectively (Figure 10b,d,f). Compared with the undyed wool fabric, the wool fabrics dyed with SHE and mixed synthetic dyes all had a higher UPF value and lower TUVA and TUVB, which indicated that the color of the wool fabrics could increase the UV-protection properties. The excellent UVprotection properties of the wool fabrics dyed with SHE resulted from the colored phenolic components in SHE that had strong absorbability in the UV area as shown in Figure 3 and Figure 5. The researchers also reported that phenolic components had strong absorbability to ultraviolet rays.4,5,24,31 Therefore, SHE can provide not only shades but also protection from UV for wool fabrics. Table 4 shows the effect of repeated home laundering cycles on the UV-protective properties of the dyed wool fabrics with

Figure 10. Wool fabrics dyed (a, c, e) with SHE and (b, d, f) with mixed synthetic dyes: (a) direct dyeing with SHE 1.20% (o.w.f.); (b) Neolan Yellow P 0.69%, Red P 0.52%, and Blue P 0.27% (o.w.f.); (c) one-bath Al3+ mordant dyeing with SHE; (d) Neolan Yellow P 0.29%, Red P 0.31%, and Blue P 0.06% (o.w.f.); (e) one-bath Fe2+ mordant dyeing with SHE; and (f) Neolan Yellow P 0.87%, Red P 0.46%, and Blue P 0.18% (o.w.f.).

Table 4. Effect of Repeated Home Laundering Cycles on UPF of Wool Fabrics Dyed with SHE and Synthetic Neolan Dyesa UPF after repeated home laundering cycles

fabrics, three were SHE dyed (Figure 10a,c,e) and the other three were dyed with mixed synthetic dyes (Figure 10b,d,f) with varying concentration and were paired as Figure 10a,b; c,d; and e,f. Both SHE and mixed synthetic sorted dyed fabrics had similar color characteristic values (including K/S, L*, a*, b*, c*, and h°) and less ΔE values of 0.37, 0.32, and 0.54 for each pair, respectively, indicating that each pair of dyed wool fabrics almost had the same color. However, the UPF values for the wool fabrics dyed with SHE (Figure 10a,c,e) were approximately 7−10 times of the wool fabrics dyed with mixed

wool fabric Figure Figure Figure Figure Figure Figure a

10a 10b 10c 10d 10e 10f

0

5

10

15

20

25

30

4428 652 3939 393 4460 488

4418 672 3983 399 4471 491

3982 613 3651 362 4012 471

3422 597 3188 313 3601 454

3219 568 2621 298 3128 422

2829 499 2361 270 2981 392

2817 390 2285 264 2822 387

The dyeing conditions were the same with those in Figure 10.

Table 3. Comparison of the Color Characteristic Values and the UV-Protection Properties of Wool Fabrics Dyed with SHE and Neolan Synthetic Dyesa color characteristic values wool fabric

K/S

L*

a*

b*

c*

h

ΔE

UPF

TUVA(%)

TUVB(%)

Figure 10a Figure 10b

6.4 6.8

49.2 48.6

11.4 11.2

18.0 17.8

21.3 21.5

57.5 57.2

0.37 control

4428 652

0.9 1.8

0.008 0.033

Figure 10c Figure 10d

3.1 3.2

60.8 60.4

16.6 16.5

20.4 20.1

26.3 26.6

50.9 50.9

0.32 control

3939 396

1.1 2.6

0.002 0.100

Figure 10e Figure 10f

6.5 7.0

51.0 50.4

13.8 13.3

24.8 24.8

28.4 28.5

60.9 58.9

0.54 control

4460 488

1.2 2.3

0.001 0.028

186

5.3

0.099

undyed a

UV-protection properties o

The dyeing conditions were the same with those in Figure 10. 4595

DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

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ACS Sustainable Chemistry & Engineering

fabric (Figure 12A,g) all presented very weak fluorescence. The research reported that the emission spectrum of the undyed wool fabric did not illustrate any peak.8 Figure 12B shows SHE dyed wool fabrics by different methods had strong green fluorescence after 30 repeated home laundering cycles. SHE is not a single compound, and there are many molecules in SHE with the structures shown in Figure 13. This structure is

SHE and synthetic dyes. It can be seen that the UPF of the dyed fabrics decreased with the increasing of home laundering cycles. However, the UPF values for wool fabrics dyed with SHE were still above 2200 after 30 home laundering cycles, which were 7 times above UPF of the dyed wool fabrics with synthetic dyes. The SHE dyed wool fabrics had obviously higher UV-protective properties after 30 repeated home laundering cycles. Fluorescence Property. The excitation and emission spectra of SHE solution in Figure 11 showed the excitation and

Figure 13. Possible fluorophore in SHE.

very similar to coumarin, a known fluorophore.32,33 These are the fluorophores of SHE. The fluorophores in SHE are only part of the molecular structures of SHE. The whole SHE molecules have affinity to wool, especially after chelated with mordants; therefore, they are retained in wool after repeated launderings. Some of the SHE molecules have both fluorophores and chromophores. These molecules emit visible light under normal daylight but only emit fluorescent light under UV light. The organic fluorescent compound is an interesting and active area in organic chemistry. Fluorescent dyes have wide application in biological labels, cellular dyeing, and immunofluorescence.

Figure 11. Excitation and emission spectra of SHE solution.

emission wavelengths 390 and 535 nm, respectively, indicating that SHE could emit fluorescence under UV lights. Figure 12A demonstrates that the three fabrics dyed with SHE by three different dyeing methods (Figure 12A,a,c,e) all emitted strong green fluorescence (515−555 nm) when absorbing the excitation light (465−495 nm), whereas the three fabrics dyed with synthetic dyes (Figure 12A,b,d,f) and the undyed

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CONCLUSIONS The abundant agricultural byproduct sorghum husk could be a promising resource for natural functional dye. The total phenolic content was 574 mg of gallic acid equivalents per gram of dry SHE, and one component of the phenolic compounds was apigeninidin. SHE had good thermal and pH stabilities and would be suitable for the dyeing and finishing processes of textiles. The SHE dyed wool fabrics by direct dyeing and mordant dyeing with Al3+ or Fe2+ yielded different colors and had good colorfastness to washing with soap (above rating of 3), to rubbing, to wet ironing (above rating of 4), and an acceptable colorfastness to light, especially Fe2+ mordant dyeing could increase the light colorfastness rating of 0.5−1.5. All dyed wool fabrics with SHE had strong UV-protection properties and fluorescence properties. After 30 repeated home laundering cycles, the UPF of the wool fabrics dyed with SHE were 7 times above those of wool fabrics dyed with mixed synthetic dyes with similar depth of shades. Their fluorescence intensities were obviously stronger than the wool fabrics dyed with mixed synthetic dyes and undyed fabric. In general, SHE have a great potential to be utilized as a natural brown dye, which simultaneously imparted textiles with good colorfastness, excellent UV-protection properties, and strong fluorescence intensity. SHE could be a feasible alternative to some synthetic dyes and functional finishing agents.

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Figure 12. (A) Comparison of green fluorescence images of the dyed wool fabrics with SHE and with synthetic dyes and the undyed fabric. (B) Green fluorescence images of the dyed wool fabrics with SHE after 30 repeated home laundering cycles Wool fabrics marked a−f were the same with that in Figure 10a−f, respectively; Wool fabric marked g was undyed.

AUTHOR INFORMATION

Corresponding Authors

*Phone: +0086-510-85912007. E-mail: [email protected]. *Phone: +001 402-472-5197. Fax: + 001 402-472-0640. E-mail: [email protected]. 4596

DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597

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Xiuliang Hou: 0000-0001-5763-1979 Yiqi Yang: 0000-0001-8153-4159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by Special Fund for the Transformation of Scientific & Technological Achievements in Jiangsu Province (Grant BA2016117), Science & Technology Guidance Project by China Textile Industry Association (Grant 2015063), Fundamental Research Funds for the Central Universities (Grants JUSRP51723B and JUSRP51505), and the 111 projects (Grant No. 17021). It was also supported by USDA-National Institute of Food and Agriculture (Hatch Act, Multistate Research Project S-1054, NEB 37-037) and the Agricultural Research Division at the University of NebraskaLincoln, USA.

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DOI: 10.1021/acssuschemeng.6b02969 ACS Sustainable Chem. Eng. 2017, 5, 4589−4597