Reactive dyes in the fabrication of gold nanoparticles as wool textile colorants

Abstract

Gold nanoparticles make an appealing candidate as textile colorants due to the range of attractive colors they can produce and their exceptional stability compared to organic dyes. However, ensuring a strong interaction between the fabric and the nanoparticles remains a challenge. We have investigated the use of commercial reactive dyes to (1) reduce gold ions and form nanoparticles and (2) form a chemical link between the nanoparticles and wool fibers. For the first time, gold nanoparticles have been produced using organic reactive dyes as reducing agents. As part of this process, the chromophore of the dye has been chemically altered such that it does not contribute any color, and the red shades displayed result entirely from gold nanoparticles. These nanoparticles have then been tested for their ability to dye wool yarn samples. Careful analysis has indicated that a weak bond has formed between the particles and the fibers due to degradation of the dye structure during reduction. The mechanism of degradation has been studied, providing insight into the type of chemical structure that could be used in the future to achieve wash-fast gold nanoparticle textiles.

Keywords

Gold nanoparticles reactive dye reduction wool chemical link

For many years gold nanoparticles have been utilized for the attractive colors they display. Dated to the fourth century, the famous Lycurgus cup appears green in reflected light but red in transmitted light due to alloyed gold–silver–copper nanoparticles present within its glass.¹ Later in the 17th century, the commercial production of gold nanoparticle glass was instituted by Kunckel, who used Purple of Cassius to obtain the popular ruby red color.² More recently, gold nanoparticles have been applied as an attractive dye to different textiles, including wool,³ cotton,^4–6 and silk.^4,7,8 The use of gold nanoparticles as a dye is appealing because the color they display originates from localized surface plasmon resonance (LSPR) effects, which means a range of attractive colors can be achieved by changing the size and shape of the nanoparticles.^9,10 For example, spherical gold nanoparticles with diameters of 20 nm undergo LSPR at approximately 520 nm, which gives a red color. Increasing the size of the particles results in a red-shift in the LSPR absorption and a change in color from red to purple to grey. In addition, gold nanoparticle color is fade resistant and stable under ultraviolet (UV) light, which is a significant advantage over traditional organic dyes.

Both in situ and ex situ gold nanoparticle syntheses have been utilized to produce these composite materials, and both approaches have distinct advantages and disadvantages. In situ nanoparticle synthesis produces intensely colored, stable textiles that show good wash-fastness; however it utilizes large amounts of the expensive gold precursor. The more commercially feasible ex situ nanoparticle synthesis has a problem with wash-fastness due to the nature of the bond between the gold and the textile fiber. It is desirable to increase the strength of this bond to improve the composite’s durability to washing. Attempts at this have been made by decorating the gold nanoparticles with reactive groups that will react covalently with moieties present in the fiber.^11,12 These, however, involve complicated multi-step nanoparticle syntheses and have shown only moderate success.

The treatment of wool textiles with organic reactive dyes results in superior wash-fastness due to the formation of covalent bonds between the wool fiber and dye molecules. For example, Lanasol dyes (Ciba) contain α-bromoacrylamide or α,β-dibromopropionylamide groups, which bind to certain amino acid side chains in the wool via nucleophilic substitution and/or nucleophilic addition.¹³ Similarly, Remazol dyes (Hoechst) contain one or more β-sulfoethylsulfone groups, which bind to the wool fiber via nucleophilic addition.^14,15 Because these reactive groups have been proven to bind strongly and effectively to wool fibers, we propose that the installation of these groups on the surface of gold nanoparticles would produce a strong covalent nanoparticle–wool bond and a resulting composite product with good wash-fastness properties. To minimize the number of steps and complexity of the chemistry, we have hypothesized that the reactive dye molecule itself could be used to reduce Au³⁺ ions and form gold nanoparticles. Although reactive dyes have never before been used to form gold nanoparticles, many contain moieties, such as amines, which are well known to act as a reducing agent for gold.^16–19 This approach is summarized in Figure 1.

Figure 1.
Schematic diagram of the proposed approach used to chemically bind gold nanoparticles to wool fibers.

We present the use of commercially available reactive dyes to reduce Au³⁺ ions and form stable gold nanoparticles. The chromophore component of the dye is destroyed during this process, meaning that the color displayed results from the gold nanoparticles only. The resulting particles and their color have been analyzed for their ability to be used as a dye for wool textiles.

Experimental details

Materials

Hydrogen tetrachloroaurate (HAuCl₄ · 3H₂O) was sourced from Sigma-Aldrich. Wool yarn was supplied by Grentex & Co., Pvt Ltd (Mumbai, India). Four different reactive dyes were kindly provided by Tararua Yarns Ltd (Levin, New Zealand): Lanasol Red 6G (C.I. Reactive Red 84), Lanasol Blue 3G (C.I. Reactive Blue 69), Lanasol Blue 3R (C.I. Reactive Blue 50), and Remazol Brilliant Blue R (C.I. Reactive Blue 19). The structures of each of these dyes are given in Figure 2. Distilled water was used throughout this research.

Figure 2.
Structures of the four reactive dyes used in this research.

Gold nanoparticle synthesis

All glassware was cleaned with aqua regia prior to synthesis. Distilled water (10 mL) was brought to the boil in a glass vial, then HAuCl₄ solution (10 µL, 0.25 mol L⁻¹) followed by a dye solution (various volumes, 1 wt%) were added. The resulting colloid was allowed to boil for a further 15 min. Experiments were run with each of the four dyes, varying the volume of dye added in each test (50–400 µL).

Dyeing of wool yarn

Colloids synthesized with dyes C.I. Reactive Blue 69, C.I. Reactive Blue 50, and C.I. Reactive Blue 19 were used to dye wool yarn samples (C.I. Reactive Red 84 was not used in this section of research). For each dye type, the synthesis methodology outlined above was repeated twice using 200 µL of dye each time, producing approximately 20 mL of colloid (2.9 × 10⁻⁶ moles of gold (Au)).

Dyeing then proceeded as per the standard method outlined in the literature with few changes,¹⁴ as detailed in the dyeing curve shown in Figure 3 and described here. The colloid was brought to 50°C and the pH set to 4.5 before 2 g of yarn was added. The temperature was then increased to 70°C at a rate of 1°C min⁻¹, held at 70°C for 10 min, then increased to 98°C at a rate of 1°C min⁻¹. A temperature of 98°C was maintained for 60 min, then decreased to 30°C before the yarn was removed from the dye bath, thoroughly rinsed with water, and allowed to dry.

Figure 3.
Dyeing curve describing the method used to dye yarn samples with gold nanoparticles in this study, based on the literature method.¹⁴

Wash-fastness tests

Composite samples (1 g) were submerged in detergent solutions (Kindness wool detergent, 100 mL, 1 wt%), and agitated on a shaker table at 250 rpm for 45 min.

Characterization techniques

Gold nanoparticle characterization

Extinction spectra of colloids were measured using a Shimadzu UV-2600 spectrophotometer (note that extinction spectra are equivalent to absorbance spectra when scattering is negligible). Transmission electron microscopy (TEM) images were obtained with a JEOL 2010 microscope operated at 200 kV, and samples were plasma treated with a JEOL EC-52000IC ion cleaner for 15 min prior to imaging. Fourier transform infrared spectroscopy (FTIR) measurements were achieved with a Bruker Alpha II spectrometer. Gold colloid samples were drop-cast onto KBr disks and allowed to dry for analysis. Unreacted dye samples were measured directly with an attenuated total reflectance crystal. 1H spectra were recorded on a JEOL JNM-ECZ500S 500MHz spectrometer equipped with a ROYAL digital auto tune probe S, operating at 500 MHz. Chemical shifts were referenced to the dimethylsulfoxide (DMSO) solvent peak δ 2.50, and spectra were recorded at 298 K.

Composite gold nanoparticle–yarn characterization

Reflectance spectra of yarn were measured using a Shimadzu UV-2600 spectrophotometer with an ISR-2600Plus integrating sphere attachment. Reflectance spectra were submitted to Kubelka–Munk transformation using Shimadzu software for comparison to colloidal spectra. A Hunterlab ColorQuest spectrophotometer was used to obtain the CIELAB L, a, and b* color space values of washed and unwashed yarn samples, which were then used to calculate the total tone difference ΔE* as per Equations (1)–(4).
$Δ E^{} = \sqrt{(Δ L^{})^{2} + (Δ a^{})^{2} + (Δ b^{})^{2}}$
(1)

$Δ L^{} = L_{Washed}^{} - L_{Unwashed}^{}$
(2)

$Δ a^{} = a_{Washed}^{} - a_{Unwashed}^{}$
(3)

$Δ b^{} = b_{Washed}^{} - b_{Unwashed}^{}$
(4)

Results and discussion

Formation of stable gold nanoparticles using reactive dyes

Due to the presence of an amine group in each of the dyes examined, it was hypothesized that these molecules would be capable of reducing Au³⁺ to Au⁰ to effectively form gold nanoparticles. The presented research has confirmed this, and importantly it has shown that for selected dyes the chemistry of the chromophore has been altered such that it no longer contributes any color.

The volume of dye added to a fixed concentration of HAuCl₄ was altered to determine the most appropriate concentration. Sufficient dye molecules were needed such that there was complete reduction of the gold ions and a stable colloid with a narrow LSPR peak was produced, but if too much dye was added there would be excess molecules present with intact chromophores contributing to the color. For example, consider the addition of C.I. Reactive Blue 69 dye to the gold ion solution (Figure 4). When 50 µL of dye was added there was evidence of nanoparticle formation in the visible spectrum as a peak with a λ_max value of 540 nm formed; however, the peak was very broad, indicating significant particle aggregation or the formation of large particles and, as a result, the colloid was grey in color. Clearly, the concentration of dye was insufficient for it to act effectively as a stabilizing agent in addition to as a reducing agent. Increasing the volume of dye resulted in the visible spectrum peak blue-shifting and becoming narrower, and the addition of 150 µL of dye gave a narrow peak centered at 531 nm and a clear, red colored colloid. However, increasing the volume of dye above 200 µL produced peaks/shoulders at approximately 605 nm, which lined up with the peak in the visible spectrum of the dye itself. In this case there were excess dye molecules present in the colloid that had not reacted with the gold, and therefore have intact chromophores that contribute to the final color. Similar trends were observed for C.I. Reactive Blue 50 and C.I. Reactive Blue 19 dyes, with 200 µL proving to be the most appropriate volume for each dye (Figure 5).

Figure 4.
Extinction spectra and photographs of the nanoparticle colloids formed when various volumes of C.I. Reactive Blue 69 dye (50–300 µL) were reacted with HAuCl₄, compared to the extinction spectrum of C.I. Reactive Blue 69 dye in water (labeled “Dye”).

Figure 5.
Extinction spectra and photographs of the nanoparticle colloids formed when various volumes of C.I. Reactive Blue 50 dye (50–400 µL) and C.I. Reactive Blue 19 (50–400 µL) were reacted with HAuCl₄, compared to the extinction spectra of C.I. Reactive Blue 50 dye and C.I. Reactive Blue 19 dye in water (labeled “Dye”): (a) C.I. Reactive Blue 50 spectra and (b) C.I. Reactive Blue 19 spectra.

However, due to overlap between the C.I. Reactive Red 84 dye chromophore peak and the gold nanoparticle peak, the conclusion for this dye was not as easily reached (Figure 6(a)). For the addition of volumes of dye of 100 µL or less, there was aggregation occurring or large particles forming, as evidenced by a broad LSPR peak with high extinction at high wavelengths. At 200 µL of dye, there was less evidence of aggregation and a peak at 539 nm suggests that gold nanoparticles have formed. However, it is unclear whether or not the dye chromophore was still contributing color to this sample, as the chromophore peak may have been concealed within the LSPR peak. The high extinction or shoulder that formed between 450 and 500 nm (where the dye chromophore peak also lies) suggests that this may be the case. This colloid was therefore submitted to centrifugation to separate the nanoparticles (in the pellet) from any remaining dye molecules not bound to the nanoparticles (in the supernatant) (Figure 6(b)). Visible spectroscopy of the supernatant gave a peak consistent with the peak of the unreacted dye, indicating that the reaction between the dye and the gold ions has not completely destroyed the dye chromophore in this sample. The reason why the chromophore was completely destroyed for the other dye molecules studied but not for C.I. Reactive Red 84 could be related to a lower reactivity between gold and the azo (N=N) or sulfone (SO₂) linker groups, as compared to that between gold and the amine (NH₂) linker groups present in the other dye molecules.

Figure 6.
(a) Extinction spectra and photographs of the nanoparticle colloids formed when various volumes of C.I. Reactive Red 84 dye (50–400 µL) were reacted with HAuCl₄, compared to the extinction spectrum of C.I. Reactive Red 84 dye in water (labeled “Dye”) and (b) Extinction spectra of the supernatant and pellet (redispersed in water) following centrifugation of the nanoparticle colloid synthesized with 200 µL of C.I. Reactive Red 84 dye.

Gold nanoparticle characterization

The three gold nanoparticle colloids produced using 150 µL of C.I. Reactive Blue 69 (labeled sample RB69-Au), 200 µL of C.I. Reactive Blue 50 (RB50-Au), and 200 µL of C.I. Reactive Blue 19 (RB19-Au) displayed clear, bright, and attractive shades of red. They have shown good long-term stability with no or very little change in the visible spectrum of each sample over two months (Figure 7). TEM analysis has been utilized to characterize the size and shape of the gold nanoparticles formed in each of these three samples (Figure 8). The majority of each sample consisted of spherical or quasispherical nanoparticles with average diameters of 39 nm ± 6 nm, 29 nm ± 6 nm, and 24 nm ± 6 nm for RB69-Au, RB50-Au, and Rb19-Au samples, respectively. The RB69-Au and RB50-Au samples also had a small number of plates and other shaped particles present (5% and 4% of the particles analyzed for RB69-Au and RB50-Au, respectively). However, the RB19-Au sample contained a significant 21% of non-spherical particles, including plates and pyramids, which explains the broadness of its visible spectrum peak compared to the other two samples.

Figure 7:
Changes over time in the extinction spectra of samples RB69-Au (a), RB50-Au (b), and RB19-Au (c). Samples were stored in sealed containers at 4°C. RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19.

Figure 8.
Representative transmission electron micrographs and corresponding histograms of the particle diameter distributions for the nanoparticles in samples (a) RB69-Au, (b) RB50-Au, and (c) RB19-Au. RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19.

To better understand the role of the C.I. Reactive Red 84 dye in gold nanoparticle formation, two colloids formed with this dye were also examined via TEM. When 100 µL of dye was used, it appeared that there was little contribution of dye chromophore color to the overall color of the sample; however, visible spectroscopy indicated that significant particle aggregation had occurred or very much larger particles were present. TEM revealed that this sample contained a number of quasispherical particles of various sizes, but was dominated by very large plates of gold (Figure 9). The plates grew in an uninhibited fashion, which created unusual unsymmetrical shapes, suggesting that the small volume of dye added was insufficient to control the growth of the particles.

Figure 9.
Representative transmission electron micrographs of the particles in the colloid synthesized with 100 µL of C.I. Reactive Red 84.

When 300 µL of C.I. Reactive Red 84 dye was used, the LSPR peak in the visible spectrum became much narrower, suggesting that more regular particles had formed (although the dye chromophore remained intact). TEM has shown spherical/quasispherical particles that were larger and had a greater size distribution (average diameter of 54 nm ± 20 nm, Figure 10) than those produced by the other dyes. In addition, 16% of the particles observed were non-spherical, including a large number of plate-shaped particles. For these reasons, and because the chromophore was not sufficiently destroyed, C.I. Reactive Red 84 was not studied further in this research program.

Figure 10.
Representative transmission electron micrographs and corresponding histogram of the particle diameter distribution for the nanoparticles in the colloid synthesized with 300 µL of C.I. Reactive Red 84.

Dyeing of yarn with gold nanoparticles

The gold nanoparticle colloids synthesized using C.I. Reactive Blue 69 (RB69-Au), C.I. Reactive Blue 50 (RB50-Au), and C.I. Reactive Blue 19 (RB19-Au) were tested for their ability to dye wool yarn samples. The resulting composite samples have been labeled RB69-Au-wool, RB50-Au-wool, and RB19-Au-wool, respectively. The dyeing conditions were chosen such that a reaction between the reactive group and the wool fiber amino acid groups could proceed, by replicating the reaction conditions used to dye wool with the reactive dyes (without any gold).¹⁴ After complete uptake, leaving behind a clear and colorless solution, the three yarn samples displayed three distinctly different shades of color. Reflectance spectroscopy carried out on yarn samples RB69-Au-wool and RB50-Au-wool gave absorption peaks that were red-shifted compared to the peaks of their respective colloids, indicating that nanoparticle aggregation had occurred upon dyeing (Figures 11(a) and (b)). However the red-shift observed for C.I. Reactive Blue 69 (from 531 to 568 nm) was much larger than that for C.I. Reactive Blue 50 (from 526 to 537 nm), suggesting a greater level of aggregation has occurred for C.I. Reactive Blue 69, which resulted in larger particles/aggregates and a purple/grey shade of yarn. The significantly smaller amount of aggregation occurring for the C.I. Reactive Blue 50 colloid gave a pink colored yarn sample. In contrast, the absorption peak λ_max for yarn sample RB19-Au-wool blue-shifted compared to the colloid RB19-Au (from 524 to 498 nm) (Figure 11(c)). However, the peak is very broad and extends significantly out into the high wavelength region. This explains the slightly “dirty” shade of pink displayed by the yarn, compared to the cleaner colors of the other samples (which gave narrower absorption peaks). The colors shown by the yarn samples have further been characterized via their CIELAB color space coordinates, given in Table 1. Despite the aggregation that occurred at some level for each sample upon dyeing, this process produced attractive shades of dyed yarn where the color resulted entirely from gold.

Figure 11.
Kubelka–Munk transformed reflectance spectra and photographs of composite yarn samples: (a) RB69-Au-wool; (b) RB50-Au-wool and (c) RB19-Au-wool. Corresponding extinction spectra (in red) and photographs of the gold nanoparticle colloids used to dye the yarn samples (RB69-Au, RB50-Au, and RB19-Au) are included for comparison. RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19. (Color online only.)

Table 1.
CIELAB color space coordinates for composite yarn samples

RB69-Au-wool RB50-Au-wool RB19-Au-wool

L 56.25 57.72 55.29

a* 0.77 4.93 2.93

b* –0.06 2.15 3.46

RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19.

Wash-fastness of gold nanoparticle yarn samples

As an initial test of the strength of the bonding between the gold nanoparticles and wool fibers, the three dyed yarn samples were submitted to wash-fastness trials. For comparison, control yarn samples (with no gold) were also produced using 150 µL of C.I. Reactive Blue 69 (dyed yarn sample labeled RB69-wool), 150 µL of C.I. Reactive Blue 50 (RB50-wool), and 200 µL of C.I. Reactive Blue 19 (RB19-wool), and submitted to the same wash-fastness trials. It was proposed that if the bond formed between the gold nanoparticles and the wool was of the same nature as the bond formed between the reactive dyes and the wool (in the control samples), then they would display similar wash-fastness properties.

Two techniques were utilized to measure wash-fastness. Firstly, the CIELAB L, a, and b* color space values were obtained for both the washed and unwashed yarn samples and used to calculate the total tone difference ΔE, which describes the magnitude of the overall change in color. In addition, Kubelka–Munk transformed reflectance spectra were obtained for both the washed and unwashed samples (Figure 12), and the decrease in the height of the peak following washing (expressed as a percent) has been used as an indicator of wash-fastness. Both approaches have indicated that the wash-fastness of the gold colloid sample was significantly less than that of the control dye sample, for each of the dyes examined (Figure 13). There was also a visible change in color or fading observed in the yarn following washing. It should be noted that the gold nanoparticle samples produced in this work demonstrated similar wash-fastness properties to other fabrics dyed with gold nanoparticles studied in the literature. For example, gold nanorods on cotton and silk were reported to give ΔE values of 5.23 and 11.05, respectively, upon washing,⁴ and in a separate study gold nanoparticles on cotton at different concentrations gave ΔE* values of 0.7 and 2.6.⁶ Although these literature values are comparable in magnitude to those obtained in this study, because the gold nanoparticle samples did not perform as well as the reactive dye samples, this indicates that the strength of the bond between the gold nanoparticles and the wool fibers was considerably weaker than the bond between the dye molecules and the wool fibers.

Figure 12.
Kubelka–Munk transformed reflectance spectra of the composite gold–yarn samples RB69-Au-wool (a), RB50-Au-wool (b), and RB19-Au-wool (c), compared to reactive dye (no gold) samples RB69-wool (a), RB50-wool (b), and RB19-wool (c), before and after wash-fastness tests. RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19.

Figure 13.
Analysis of the wash-fastness of the composite gold–yarn samples: (a) RB69-Au-wool; (b) RB50-Au-wool; (c) RB19-Au-wool, compared to the reactive dye (no gold) samples: (a) RB69-wool; (b) RB50-wool and (c) RB19-wool. Wash-fastness is measured by both the total tone difference (left) and the change in height of the Kubelka–Munk transformed reflectance peak (right). RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19.

Mechanism of gold nanoparticle formation and dye degradation

To gain more insight into the interaction between the wool and the gold nanoparticles, FTIR (Figure 14) and nuclear magnetic resonance (NMR; Figure 15) spectra have been obtained for the unreacted dye molecules and the gold nanoparticle samples (RB69-Au, RB50-Au, and RB19-Au). This has allowed analysis of what chemical changes have occurred to the dye molecule upon reaction with gold. A number of significant differences have been observed.

Figure 14.
Fourier transform infrared (FTIR) spectra of samples (a) RB69-Au, (b) RB50-Au, and (c) RB19-Au (red) compared to the FTIR spectra of reactive dyes as received (blue). RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19. (Color online only.)

Figure 15.
Nuclear magnetic resonance spectra of samples (a) RB69-Au, (b) RB50-Au, and (c) RB19-Au (red) compared to the Four transform infrared spectra of reactive dyes as received (blue). RB69: C.I. Reactive Blue 69; RB50: C.I. Reactive Blue 50; RB19: C.I. Reactive Blue 19. (Color online only.)

Compared to the FTIR spectra of the dye molecules, the spectra of the gold nanoparticle samples have shown a substantial increase in the intensity of the signal at 3165 cm⁻¹ (or 3194 cm ⁻¹ for RB19-Au). This signal will contain some contribution from N-H stretches due to the presence of primary and secondary amines in the molecules; however, there is also evidence that a carboxylic acid O-H stretch has contributed to this peak, in that the signal at ∼1401 cm⁻¹ (corresponding to the O-H bend) has also increased in intensity. The change observed has been attributed to the formation of carboxylic acid rather than to environmental water due to the position of the O-H stretch signal, and also because the KBr disks used to run these spectra were stored in a desiccator, and the blank disk used to run the background spectra was submitted to the same conditions as the sample disks.

In the FTIR spectra of the dye molecules before reaction, the signal corresponding to the C=O stretch was very weak due to conjugation with the aromatic rings in the dye structure. However, upon reaction with gold this signal has increased significantly in intensity, indicating that this conjugation has been broken.

The C=O signal has also shifted to a higher wavenumber for RB69-Au and RB50-Au, again consistent with broken conjugation.

The aromatic C=C stretches in the 1580–1480 cm ⁻¹ region also appear to have shifted to a higher wavenumber upon reaction, further suggesting a decrease in conjugation.

Finally, NMR spectra have clearly shown the generation of ammonia upon reaction of each dye with gold.

The decrease in conjugation observed suggests that the anthraquinone segment of the dye structure has been broken, and an increase in the O-H stretches and bends signals suggests that alcohol or carboxylic acid moieties have formed. This is consistent with the formation of phthalic acid, which is a known degradation product of similar anthraquinone-based dyes.^20,21 The generation of ammonia observed via NMR also supports this, as ammonia is known to be liberated when the dye structure is oxidized and phthalic acid is produced.²¹ Phthalic acid has been shown to be an effective stabilizing molecule for silver nanoparticles,²² so is a reasonable candidate for gold nanoparticle stabilization in this work. The proposed mechanism of gold reduction by the dye structures, as deduced by this work, is summarized by Equations (5) and (6)

The evidence indicates that the dye molecules have degraded upon reduction of gold, and consequently the dye reactive group moiety is not associated with the gold nanoparticles. Therefore, no covalent link is formed between the nanoparticles and the wool fibers and the particles are removed upon washing, explaining the poor wash-fastness observed. To obtain a covalent link, the reactive group should be installed onto the gold nanoparticles in a more controlled fashion, despite the additional expense and time that this will involve. We propose that a good candidate for this future work might involve the reactive group bound directly to the anthraquinone structure, such that upon reduction and degradation the reactive group is bound to a phthalic acid group, as this work suggests that phthalic acid moieties interact with gold nanoparticles.

Conclusions

For the first time, stable gold nanoparticles have been successfully synthesized using commercial reactive dyes. The dye behaved as a combined reducing and stabilizing agent, and the dye chromophore has been altered and its color destroyed as part of this process. The resulting gold nanoparticles are monodisperse and stable, and have displayed clean and clear shades of red. They have then been used as an effective colorant for wool yarn, imparting purple and pink shades to the fibers. However, significant degradation of the dye molecule has resulted in a weak interaction between the gold nanoparticles and the wool fiber, resulting in poor wash-fastness.

Despite the advantages of gold nanoparticles as a textile colorant, the formation of a strong metal–fiber bond remains a challenge. Producing a material that is wash-fast over time is an obstacle that must be overcome in order for this technology to enjoy widespread use. This can only be achieved through further research, such as that presented in this study.

	RB69-Au-wool	RB50-Au-wool	RB19-Au-wool
L*	56.25	57.72	55.29
a*	0.77	4.93	2.93
b*	–0.06	2.15	3.46

Footnotes

Acknowledgement

We are grateful to Tararua Yarns Ltd (Levin, New Zealand) for providing the reactive dyes used in this research.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Emma G Wrigglesworth

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