Tunable optical and UV-blocking properties of eco-friendly CMC/ZnO nanocomposite films

As environmental sustainability standards become more stringent, there is a growing need for biodegradable and renewable UV-protective films. Thus, this study looked at how zinc oxide nanoparticles (ZnO-NPs) affect the optical properties of carboxymethyl cellulose sodium (CMC) using a standard method called solution-casting. Fourier-transform infrared (FTIR) spectroscopy revealed that the carboxyl groups of CMC were ionized, forming hydrogen bonds with ZnO-NPs. X-ray diffraction (XRD) analysis confirmed the formation of CMC/ZnO nanocomposites through the characteristic diffraction peaks of ZnO-NPs, while the crystallite size of the nanocomposites decreased with increasing ZnO-NP concentration. The optical analysis found that as the amount of ZnO increased, the HOMO/LUMO band gap decreased from 5.21 eV to 5.10, 4.67, 4.43, and 3.98 eV, which was linked to the smaller crystallite size. Moreover, the refractive index of CMC increased from 1.78 to 1.84, 2.23, 2.51, and 2.60 due to the addition of 2, 4, 6, and 8  wt.% of ZnO-NPs, indicating modified optical properties suitable for various applications. The CMC/ZnO nanocomposite films showed strong UV-blocking performance, with the film containing 8 wt% ZnO-NPs blocking 94% of UVC (200-280 nm), 93.5% of UVB (280-320), and 93% of UVA (320-400 nm) radiation. Thus, based on the known biodegradable nature of the CMC matrix, CMC/ZnO nanocomposites can be considered as promising candidates for biodegradable UV-protective materials. The long-term stability of the optical properties was evaluated by re-measuring the absorbance and transmittance of pure CMC films and those doped with 8 wt% ZnO-NPs after more than 6 months of storage under normal conditions. Despite minor variations in film thickness, the optical measurements remained highly consistent, demonstrating that both pure and doped CMC films retain their optical performance over extended periods. A preliminary test using green chillies was conducted to evaluate the moisture-retention performance of the prepared films under UVA light, providing an initial indication of their ability to reduce water loss and maintain firmness during storage.