Synthesis of Gold Nanoparticles Using Tannin-Rich Extract and Coating onto Cotton Textiles for Catalytic Degradation of Congo Red

Gold nanoparticles (AuNPs) were synthesized under ambient conditions from chloroauric acid in aqueous solution at pH 4. Tannin-rich extract fromXylocarpus granatum bark was used as both reducing and capping agent, rapidly converting Au (I) salt to AuNPs. Transmission electron microscopy showed the as-prepared AuNPs to be predominantly spherical, with an average diameter of 17 nm. (e AuNPs were tested for catalytic reduction of Congo red (CR), a carcinogenic azo dye, in aqueous sodium borohydride solution. Cotton samples were coated with the AuNPs, taking on a reddish-purple color. (e samples showed significantly reduced tearing strength after coating, though tensile strength was unaffected. UV-visible spectroscopy was used to determine the dye concentration in the water. CR degradation was observed only when AuNPs were present, and the efficiency of degradation was strongly linked to the AuNP loading. (e AuNP-coated fabrics left only a 4.7% CR concentration in the solution after 24 h and therefore promise as a heterogeneous catalyst for degradation of CR in aqueous solution.


Introduction
Nanostructured materials are known to be promising candidates for conversion or removal of toxic pollutants. Nanosorbents, nanoparticles, and nanocatalytic membrane systems have demonstrated effectiveness in low-energy treatment of wastewater [1]. Gold nanoparticles (AuNPs) offer a large surface-to-volume ratio, low toxicity, good biocompatibility, and unique optoelectronic properties [2]. ey are increasingly used as sensory probes as well as in drug delivery, catalysis, and electronic devices [3][4][5][6]. e color of spherical AuNPs depends on their dimensions and may be brown, orange, red, or purple [7]. AuNPs have been used for the catalytic reduction of organic compounds including 4-nitrophenol [8], nitrobenzene, and Congo red [9].
Congo red (CR) is an azo dye. It is carcinogenic and toxic to many organisms. Its use in catalytic degradation has been investigated in many research studies [10,11]. Naseem et al. [12] present an overview of the use of nanocatalysts for reduction of CR in industrial wastewater. In brief, the reaction between CR and NaBH 4 is thermodynamically possible but not kinetically practical, given the very low reaction rate. Interaction between borohydride ions and CR molecules requires a catalyst surface to which they can attach. e large surface area of nanocatalysts makes them suitable as an electron transfer mediator, promoting electron transfer from the borohydride ions (electron donor) to dye molecules (electron acceptor). If the correct metal nanoparticles are used, the reduction process then becomes kinetically feasible. A range of nanocatalysts have been employed in CR reduction, and rapid catalytic degradation of the dye has been demonstrated. To avoid overlapping between the surface plasmon resonance band of the nanoparticle and the absorption band of the dye, very small amounts of metal nanoparticles must be used. During reduction, the azo bonds of CR are cleaved to form aromatic amines [12]. Wastewater degradation of CR by AuNPs with sodium borohydride (NaBH 4 ) has also been demonstrated [13]. Cleavage at the azo bond releases products that have lower toxicity and are less environmentally damaging [12]. Figure 1 shows the decomposition products [13].
Nanomaterials can be derived from biological sources, including nanoparticles, wires, flowers, and tubes. Biological entities including plants and microorganisms may act as reducing or stabilizing agents in the formation of nanostructures. e use of organisms in the formation of bio(nano)materials dates back to the 1980 paper of Beveridge et al., [14] who reported that enzymes excreted by Bacillus subtilis are capable of converting metal ions, including Au, to their elemental form, whereas physicochemical methods may require the use of toxic chemicals, whose toxicity may persist in the nanoparticles obtained; biosynthesis is more biocompatible as naturally-occurring compounds are deposited onto the nanoparticle surface [15][16][17]. is makes the biologically-stabilized nanoparticles more suitable in medical, cosmetic, or food applications [17].
As plant extracts are abundant and easily processed, their use in nanostructure synthesis has attracted considerable attention. Phyto-assisted synthesis is both energy-efficient and cost-effective. It requires no additional reducing agent, surfactant template, organic solvents, or use of hazardous materials [18][19][20][21]. However, biosynthesis often produces nonuniform nanoparticles at low yields. is requires finetuning of synthesis parameters such as salt concentration, ratio of biological extract to metal salt, pH, temperature, incubation time, and aeration [19,21].
Many natural products have been used in the synthesis of AuNPs from aurous ion sources including chloroauric acid (HAuCl 4 ) [22]. e use of plant extracts has been widely reported, including extracts from leaves, fruit, flowers, roots, seeds, and bark [23]. e morphology of the nanoparticles obtained may include spheres, quasispheres, triangles, cubics, pentagons, hexagons, rods, and plates, though spheres have been most frequently reported [20]. Particle size ranges from less than ten nanometers to a few hundred. Salmalia malabarica gum from Bombax ceiba has served as a reducing and capping agent in the synthesis of AuNPs from chloroauric acid [24]. e authors concluded that hydroxyl groups play a key role in the reduction of aurous ions. e nanoparticles were predominantly spherical with an average size of 12 nm. Bacteria and fungi, as well as their products, have also been used in the synthesis of gold nanoparticles [25].
In this work, AuNPs were produced using bio-assisted synthesis. Extract from Xylocarpus granatum bark was chosen as the reducing agent, as it is known to have good water solubility and low toxicity [26], and is an abundant tannin-rich resource [27]. e tropical tree Xylocarpus granatum is commonly known as the cannonball mangrove. e catalytic reduction of CR in water was carried out both using the as-prepared AuNPs and AuNP-coated cotton. Cotton was selected to facilitate the handling of the nanoparticles and their removal from water after use. e mechanical properties of the coated fabrics were also evaluated.

Materials.
Chloroauric acid (HAuCl 4 ), Congo red (C 32 H 22 N 6 Na 2 O 6 S 2 ), sodium borohydride (NaBH 4 ), hydrochloric acid (HCl), and sodium hydroxide (NaOH) were of analytical grade and were used without further purification. Whey protein isolate of 90% purity was purchased. Deionized water was used throughout. Tannin was extracted from Xylocarpus granatum by heating the sundried bark in water at 80°C for 1 h (water to bark mass ratio of 5 : 1) followed by spray drying to yield a reddish-brown dye powder with condensed tannin content of 74.0%.

Tannin-Mediated Synthesis of AuNPs for Direct Use in the Catalytic Reduction of Congo Red (CR).
A specified amount of HAuCl 4 was dissolved in DI water at room temperature under vigorous stirring. Tannin powder (0.080 g) was then added. e total solution volume was 500 ml. e pH was adjusted to 4 and the mixture was stirred for a few minutes until the color became reddish-purple. HAuCl 4 concentrations of 0.010 g/L, 0.03 g/L, and 0.050 g/L were used. e colloidal solutions obtained were coded AuNP-1, AuNP-3, and AuNP-5.

Tannin-Mediated Synthesis of AuNPs and Coating of
Cotton Fabrics. Whey protein isolate (WPI) was mixed with DI water at a concentration of 0.2 g/L. Woven cotton samples of 10 cm × 15 cm were coated with the WPI solution using a padding mangle at a nip pressure of 1 kg/cm 2 to obtain 100% wet pick-up. e coated fabrics were dried at ambient temperature and pressure. e fabrics were coated a second time with colloidal solutions containing AuNPs, prepared using HAuCl 4 at concentrations of 0.1, 0.6, 0.8, and 1.0 g/L. ese were padded onto cotton samples, which were coded Cot-01, Cot-06, Cot-08, and Cot-10. e samples were air-dried under ambient conditions. Excess AuNPs were removed by soaping with 2 g/L AATCC1993 detergent at 60°C for 20 min. Samples were then air-dried.

Catalytic Reduction of CR.
A solution of CR, NaBH 4 , and DI water at pH 4 was used in evaluating the catalytic activity of AuNPs. In the first experiment, the as-prepared colloidal solution containing AuNPs was used to directly degrade 0.100 g/L CR. In the second experiment, the AuNP-coated cotton samples were immersed in the 0.020 g/L CR solution. A lower dye concentration was used in the latter experiment because the decomposition efficiency of the AuNP-coated fabrics was significantly lower than that achieved when making direct use of as-prepared nanoparticles.

Characterization.
e morphology of the as-prepared AuNPs was examined using transmission electron microscopy (JEOL, JEM-2100/HR). A scanning electron microscope (JEOL, JSM-5410 LV) was used to study the morphology of the AuNP-coated fabrics. e tensile resistance of the fabrics was tested using a tensile tester (Instron 5567), following ASTM D5035. Tear resistance was tested using a tear tester (ElmaTear 655), following ASTM D1424. Color values were reported as CIELAB coordinates and measured using spectrophotometry (GretagMacbeth Color i5). e concentrations of the CR solutions were determined using UV-Visible spectrophotometry (Perki-nElmer Lambda 25).

Synthesis of AuNPs.
Successful tannin-assisted synthesis of AuNPs at pH 4 was confirmed as the solution rapidly changed color from yellow to reddish-purple (see Figure 2).
is mirrored an earlier report of AuNP synthesis using extract from banana peel at a similar range of pH values (2-5) [28]. is was expected as banana peel also contains phenolic compounds and tannin [29]. Under alkaline conditions (pH > 7), the reaction mixture did not change color within 24 h, suggesting that AuNPs had not formed. (Figure 3) showed the predominant formation of spherical gold nanoparticles with an average diameter of 17 nm, measured using ImageJ software. e reddish-purple color was explained by the heterogeneity of the particles. Due to the range of nanoparticle sizes, the solution comprised red and blue particles, which mixed to yield an overall purple color. e smaller nanoparticles absorbed wavelengths in the blue-green spectrum and reflected red wavelengths. e larger particles absorbed red and reflected blue. e transition from red to blue in gold nanoparticles has been attributed to aggregation [30]. Figure 4(a) shows scanning electron micrographs of the pristine cotton and Figure 4(b) shows micrographs of Cot-10. e EDX mapping shown in Figure 4(c) confirmed that elemental gold was uniformly distributed across the fiber surface.

Color Properties of the AuNP-Coated Cotton Fabrics.
As the AuNPs were reddish-purple, they imparted this color to the white cotton samples. e color values are reported in Table 1. A stronger fabric color (higher K/S) is associated with a greater AuNP loading. A higher a * value indicates a redder shade, while a lower b * value indicates a bluer shade. A lower L * value indicates a darker shade and was usually associated with a higher K/S value. e color parameters accurately represented the appearance of the coated samples and confirmed they took on a more reddish-purple shade in the presence of an AuNP coating.

Catalytic Reduction of CR.
Both the as-prepared AuNPs and the AuNP-coated cotton fabrics were effective in catalytic reduction of CR in water, using sodium borohydride (NaBH 4 ) as a reducing agent. Figure 5 shows that the CR concentration decreased with contact time, until the curves plateaued. No decomposition of CR was observed under ambient conditions, in the absence of either AuNPs or NaBH 4 . is confirmed that, in this system, AuNPs were required to catalyze the NaBH 4 reaction. A greater AuNP loading was expected to increase dye decomposition. is was confirmed as the CR degradation time decreased from 6 h when using AuNP-3 to only 4 h when using AuNP-5. AuNP-1 and AuNP-3 were added to the CR solutions, yielding residual CR concentrations of 15.9% and 4.6% after 24 h. e AuNP-1 curve took almost 24 h to plateau. As can be seen from Figure 6, the AuNP-coated fabric showed more gradual decomposition of CR than the use of AuNPs directly. In both treatments, a higher AuNP loading on the coated cotton fabrics enhanced CR degradation. CR degradation in the Cot-01 sample was comparable with that of untreated cotton. Over a 24 h period, the concentration of residual dye decreased by 52.4% for Cot-06 and 4.7% for Cot-10. In contrast, increasing the NaBH 4 concentration from 1.00 g/L to 2.00 g/L had no significant effect on decomposition (data not shown). is was attributed to the 1.00 g/L NaBH 4 concentration being sufficient to completely disintegrate the CR molecules in the system. As the cotton exhibited strong affinity to CR, absorption of CR would add to the catalytic degradation attributable to AuNPs and NaBH 4 . is is evident from Figure 6, which shows the CR AuNPs + Figure 1: Catalytic reduction of CR by NaBH 4 and AuNPs [13]. concentration also decreases mildly with contact time in the sample that was not treated with AuNPs. Partial leaching of AuNPs from the coated fabrics was also observed.

Mechanical
Properties. e WPI-coated fabrics showed significant declines in tearing strength in both warp and weft directions ( Table 2). is was attributed to the WPI restricting yarn and fiber movement. is would in turn disrupt stress distribution, lower the tearing strength, and result in early breaking. WPI was necessary to prevent particle agglomeration and increase nanoparticle stability. In a previous study, we prepared silver nanoparticles (AgNPs) in a similar way, using WPI-assisted synthesis with a silver nitrate precursor. Tannin extract was subsequently introduced to promote adhesion between the WPI-capped AgNPs and cotton samples [26]. Adding AuNPs to the WPI-coated cotton samples produced no further decrease in tearing resistance, with all coated samples showing a reduction in tearing strength of approximately 40% in both warp and weft directions. e tensile properties of the AuNP-coated fabrics are presented in Table 3. Because the tensile and tearing forces operate in different modes, different effects result from fabric coating. All samples, coated and uncoated, had comparable tensile properties. In contrast with our previous study [31], we observed a marginal decrease in tensile strength after protein coating. However, in this work, the solution had a much lower WPI solid content (0.20 g/L or 0.02% solid content, compared with 1.4% in the previous work). is may have masked any negative effects of WPI on the tensile properties.   Cot-08 Cot-10 Figure 6: Catalytic reduction of CR using AuNPs-coated cotton fabrics (C 0 is the initial dye concentration, and C is the dye concentration at time t).

Conclusions
We demonstrated successful synthesis of spherical gold nanoparticles from chloroauric acid and tannin in pH 4 solution under ambient conditions. e particles had a nonuniform size distribution, with an average diameter of 17 nm. e nanoparticles were coated onto cotton samples pretreated with whey protein isolate, via pad-drying. e coating imparted a reddish-purple color to the white fabric. Catalysis of Congo red reduction by both the as-prepared nanoparticles and the nanoparticle-coated samples was demonstrated, though the latter were significantly less efficient. e efficiency of dye decomposition was strongly linked to the nanoparticle concentration. e treatment significantly reduced tearing strength, while tensile strength was unaffected. e successful degradation of Congo red suggests that gold nanoparticle-coated cotton is a promising heterogeneous catalyst for treatment of dye-contaminated water.

Data Availability
e data used to support the finding of this study are included within the article.

Disclosure
is study was presented in part at the 59th Kasetsart University Annual Conference, Bangkok, ailand (March, 2021).

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.