Spontaneous formation and oxidation of metal-phenolic networks by zinc oxide: Expanding photoprotection and suppressing photoreactivity

Wide-bandgap metal-oxide nanoparticles are promising candidates for broad-spectrum sunscreens, yet their application is limited by photocatalytic activity and insufficient high-energy visible (HEV) light absorption. Here, we report a simple, scalable one-pot strategy for the spontaneous formation of a metal-phenolic network (MPN) on zinc oxide (ZnO) nanoparticles (ZnO/MPN NPs), utilizing intrinsic Zn²⁺ ion release from ZnO to initiate tannic acid (TA) complexation and in situ oxidation. This process forms a nanoscale MPN layer on the ZnO surface, while ZnO-mediated TA oxidation and dimerization (inspired by natural fruit browning) enhance electron delocalization, extending light absorption to the HEV region. The resulting browned MPN-coated ZnO nanoparticles (ZnO/MPN-B NPs) exhibited approximately a threefold enhancement in both sun protection factor (SPF) and UVA protection factor (UVAPF) compared to uncoated ZnO NPs. Additionally, the MPN layer effectively suppresses over 99 % of photogenerated reactive oxygen species (ROS) through its intrinsic ROS scavenging properties, significantly improving photostability. Cell viability assays further demonstrate that the MPN layer mitigates photoinduced cytotoxicity, supporting the safety and biocompatibility of these hybrids. This study suggests ZnO/MPN-B NPs as eco-friendly, high-performance candidates for next-generation sunscreen formulations, offering a scalable, efficient route to address the dual challenges of photoprotection and safety in inorganic sunscreen agents.