Photoresponsive antioxidant-based gels incorporating natural polyphenols and ZnO nanoparticles: a synergistic approach to UV protection and photostability.

Excessive exposure to ultraviolet (UV) radiation accelerates skin aging, induces oxidative stress, and increases the risk of skin cancer. Conventional sunscreens often contain synthetic filters with potential environmental and dermatological concerns, highlighting the need for biocompatible, multifunctional alternatives. In this work, photoresponsive gel formulations were developed using natural polyphenolic antioxidants (green tea and pomegranate peel extracts), photochromic agents (dihydroxyflavone and spiropyran), and zinc oxide (ZnO) nanoparticles. The formulations followed green chemistry principles and were optimized for enhanced sun protection factor (SPF), photostability, and physicochemical performance. A full 2⁴ factorial design was employed to evaluate synergistic effects among active components. The optimized gel achieved an SPF of 16.32 with improved photostability (ΔE = 4.5) after UV-A/UV-B exposure, comparable to historically established low-SPF reference formulations, while offering enhanced photostability, antioxidant functionality, and sustainability. Spectroscopic analysis confirmed strong UVB absorption, and statistical tests demonstrated significant interactions, particularly between polyphenols and dihydroxyflavone. Rheological, pH, and emulsion stability tests validated the formulation's robustness. While the achieved SPF remains within the low-to-moderate range compared to modern commercial sunscreens, the formulation demonstrates enhanced photostability, antioxidant functionality, and sustainability, highlighting its potential as a complementary or research-stage photoprotective platform. These findings suggest that combining natural antioxidants and ZnO nanoparticles in photochromic gels provides a sustainable, effective strategy for next-generation UV-protective skincare.