Photofading of Natural Indigo Dye in Cotton Coated with Zinc Oxide Nanoparticles Synthesized by Precipitation Method

Blue dye (indigo) from Indigofera tinctoria leaves is a popular natural dye used worldwide. The lower light fastness of natural indigo dyes compared to that of synthetic blue dyes is one of the drawbacks of the former, limiting its utilization in the textile industry. In this study, zinc oxide nanoparticles (ZnONPs) were synthesized and characterized, and their effect on the photofading of cotton fabric dyed with natural indigo was investigated. ZnO was produced by simple precipitation. Fourier transform infrared spectroscopy (FTIR), X-ray powder diffractometry (XRD), and Brunauer–Emmett–Teller analysis were employed to characterize the composition, shape, crystallinity, size, and surface area of the resulting NPs. The optical characteristics and bandgap energy of the ZnONPs were also determined using a UV-Vis spectrophotometer. XRD and scanning electron microscopy (SEM) confirmed the synthesis of ZnONPs. The ZnONPs were applied to cotton fabrics via the dip-coating method. The transmittance of cotton coated with ZnONPs was lower than that of the uncoated sample. Photofading tests with UV-A irradiation were conducted, and the fading rate of natural indigo dye in cotton showed first-order kinetics. Overall, the synthesized ZnONPs provided excellent UV protection to reduce the photofading of cotton dyed with natural indigo.

Indigofera tinctoria; Natural Dye; Photofading; UV-protection; Zinc Oxide Nanoparticle

Ecofriendly natural dyes with minimal impacts on the environment have gained increased research attention. Natural dyes present the advantages of high renewability, biodegradability, and nontoxicity (Rahayuningsih et al., 2019). Indigo is a popular natural dye that can be extracted from various plants, such as Indigofera tinctoria, Indigofera arecta, Strobilanthes flaccidifollus, Isatis tinctoria, and other indigoid plants. The indicans and isatans present in these plants are converted to colorless indoxyl and glucose by enzymatic hydrolysis or fermentation. Indoxyl forms indigotin pigment under alkali conditions by oxidation. Indigotin must be reduced into a soluble leuco form that bonds with cloth fibers and oxidizes back to its original insoluble form to develop color (Degani et al., 2015). Because this insoluble pigment is trapped within fibers, indigo has better wash fastness compared with most other natural dyes. Unfortunately, compared with different dyes, the light fastness of natural indigo dye is poor or moderate. According to ISO standard method, where grade 1 represents poor fastness and 5 represents excellent fastness, the light fastness values of natural indigo dye in cotton is rated Grade 3 to 3/4  (Comlekcioglu et al., 2015). Figure 1 shows the chemical structure of indigotin.

Figure1 Chemical structure of indigotin in natural indigo dye

Exposure to visible and UV light can promote the photofading of color. Because shorter wavelengths of light have higher energy, UV light is generally more harmful than visible light. The quantum energy of UV light is similar to the bond energies of organic molecules. Thus, UV light can cause undesirable degradation. Dye fading occurs because of the loss of conjugation of double bonds in whole molecules. The initial step of indigo fading may involve C=C double bond cleavage because the central C=C double bond is highly reactive (Iuga et al., 2012).

The addition of a UV protection agent is a suitable approach to reduce the photofading of colors on the fabric. UV protection agents are usually composed of organic or inorganic compounds with strong absorption in the UV range (i.e., wavelengths below 400 nm). Organic UV protection agents absorb UV rays throughout the spectra and dissipate the absorbed energy to avoid color degradation (Yang & Naarani, 2006; Latif et al., 2015). However, these agents are prone to reductions in efficiency over time. Compared with organic agents, inorganic UV protection agents are generally preferred because the latter are nontoxic, stable under UV exposure and high temperature, and insoluble in neutral pH. The mechanism of inorganic UV protection agents involves  the absorption, reflection and scattering of UV rays through their high refractive index (Fajzulin et al., 2015).
    Nanosized zinc oxide (ZnO) has recently received great attention for its application as  a UV protection finish in textile. Nanosized ZnO has a wide energy band gap of approximately 3.3 eV, which means it can absorb UV rays with wavelengths below 375 nm (Li et al., 2012). The material has a high surface-area-to-volume ratio, which endows it with excellent effectiveness in blocking UV radiation when compared with the bulk material.   Nanoparticles tend to have a better affinity to fabric surfaces and, thus, provide greater durability compared with bulk materials (Yadav et al., 2006). Additionally, ZnO is generally recognized as safe by the Food and Drug Administration. Also, ZnO is commonly applied in cosmetics as UV sunscreen (Smijs and Pavel, 2011). In vitro and in vivo studies of sunscreen with ZnONPs applied on UV-B damaged porcine skin showed that ZnO did not enter into the viable epidermis and remained on the skin surface (Monteiro-Riviere et al., 2011). Therefore, the application of ZnONPs in textile is considered safe. ZnONPs are also inexpensive, making them suitable for use in the textile industry (Karthik et al., 2017).
    On the other hand, nanoscale ZnO is known to possess the photocatalytic ability. Several studies show that ZnONP enhanced the degradation of dye solution in wastewater (Zafar et al., 2019; Agustina et al., 2020). Following light absorption, electrons in the valence band of ZnO nanoparticles (ZnONPs) are promoted to the conduction band, which produces a positive hole in the valence band. Photogenerated holes and electrons induce  oxidation-reduction reactions with water and oxygen on the ZnONP surface and form hydroxyl radicals and ROS (Das et al., 2019). These radicals and ROS can promote the oxidative degradation of organic compounds. Hence, there is a concern that, applying ZnONPs in dyed fabrics, especially in naturally dyed fabrics might accelerates photofading of dye molecules.
    Many researchers utilized ZnONP as UV protection in fabric; however, they focus on investigating ZNONPs as anti-UV as skin protection (Román et al., 2019; Tania & Ali, 2020). Moreover, ZnONPs were applied in undyed fabric. Kathirvelu et al., (2009) studied the synthesis of ZnONPs and their application on cotton and polyester/cotton dyed with reactive dye. The result shows that the UV absorbing activity of ZnO-treated fabrics was significantly improved. However, the effect of ZnONPs on dye fastness was not discussed. To the best of our knowledge, no report on the application of ZnONPs as inorganic anti-UV to protect dyed fabrics is yet available.  Investigations that address the photofading kinetics of natural dye are also rare. This study aims to study the effect of ZnONPs on photofading characteristics of natural indigo-dyed cotton coated with ZnONPs.
    In the present work, ZnONPs were synthesized via a simple precipitation method using zinc nitrate (Zn(NO₃)₂) and sodium hydroxide (NaOH) according to the reactions in Equations 1 and 2. Precipitation is a cost-effective, scalable, repeatable, and highly controllable method that has been used to synthesize a wide variety of ZnO nanostructures (Raoufi & Raou, 2013).

                                      Zn(NO₃)₂ + 2NaOH àZn(OH)₂ + 2NaNO₃                                           (1)

                                                     Zn(OH)₂ à ZnO + H₂O                                                           (2)

In summary, ZnONPs with a crystallite size of 23.33 nm were successfully synthesized via a simple precipitation method. Incorporation of the obtained ZnONPs into cotton dyed with natural indigo decreased the sample's average transmittance of UV light up to 9.3%, which means the NPS confer excellent UV protection to the fabric. The photofading of natural indigo-dyed cotton with and without ZnONPs showed first-order kinetics with a rate constant of 6.35 × 10^-4 h^-1 and 8.84 × 10^-4 h^-1. The lower fading rate indicates significant improvements in color protection after coating with the ZnONPs. The facile production of ZnONPs indicates their potential applicability to the textile industry, including in small-scale industries. However, research on methods to enhance the durability of ZnONP coatings on cotton fabric remains necessary.

    The authors are grateful to Lembaga Pengelola Dana Pendidikan Kementerian Keuangan Republik Indonesia for providing financial support via the Beasiswa Unggulan Dosen Indonesia-Dalam Negeri scholarship.

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