Photothermal Performance of Metal-Phenolic Networks and Its pH-Dependent Coordination Regulation

Fe³⁺-polyphenol coordination complexes have attracted growing interest for photothermal applications due to their tunable chemistry and good biocompatibility. However, how pH and the metal-to-ligand ratio collectively affect their photothermal performance remains poorly understood. In this work, we synthesized Fe³⁺-gallic acid (GA) metal-phenolic networks (MPNs) under a wide range of pH conditions and different mixing ratios. The materials were then characterized through electron microscopy, infrared spectroscopy, UV-vis absorption, and photothermal testing. Our results show that a near-neutral pH (around 7) is critical for forming an effective ligand-to-metal charge transfer complex, which appears as a distinct absorption band near 560 nm. Acidic or strongly alkaline environments severely disrupt coordination and weaken light absorption. Among all formulations, the sample prepared at pH 7 with a suitable Fe³⁺/GA ratio gave the best photothermal conversion, reaching a temperature rise of 42.8 °C and a photothermal conversion efficiency of 32.67%. We also found that photothermal heating increases steadily with GA concentration and peaks sharply at neutral pH. These findings demonstrate that optimal photothermal efficiency requires both neutral pH and a well-balanced metal-to-ligand ratio. This work provides a simple and practical set of conditions for developing high-performance Fe³⁺-GA MPNs for applications such as local heating, antibacterial surfaces, and light-triggered drug release.