Antimicrobial resistance has become a global issue and thus the development of natural products/biomedical materials composites with antibacterial activities is urgently needed. When acute wounds develop into chronic wounds, the wound environments become alkaline. As long as infections occur, the wound pH further increases, making the wounds difficult to heal. Besides, bacterial growth in poultry, meat, fish and seafood products is usually reflected in a marked increase of pH values. Herein, smart, stimuli responsive self-assembled multilayer and complex film were constructed through the formation of hydrogen bonds and hydrophobic interactions between hydroxypropyl methylcellulose (HPMC) and epigallocatechin-3-gallate (EGCG), thereby greatly reducing the hydrophilicity of HPMC and offering enhanced mechanical strength, superior free radical scavenging capability, and improved water vapor and light barrier properties. The EGCG/HPMC complex film was able to control EGCG release by tuning pH or temperature of the release medium. Furthermore, incorporation of CuS nanoparticles into the film allowed it to triggers EGCG release in an on-demand fashion under near-infrared (NIR) exposure. Bacterial growth in glucose-free nutrient broth medium caused pH to rise (near pH 8.0), leading to transformation of EGCG from phenol type to phenolate ion and then quinone, allowing for spontaneous generation of H2O2 to kill bacteria. The complex films changed their color in response to bacterial growth because EGCG transformed from phenol type to quinone type under alkaline condition. The green synthesized EGCG/HPMC complex films can be used as a colorimetric pH indicator and an antibacterial material for wound dressing and food packaging applications.
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