Stimuli-responsive polymeric photocatalysts: smart, controllable, and recyclable materials for photocatalytic reactions
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Abstract
The emergence of visible-light photocatalysis has provided a sustainable and promising approach to enable chemical transformations under mild conditions. Inspired by natural photosynthesis, various visible-light photocatalytic materials have been developed and investigated, such as molecular photocatalysts and polymeric photocatalysts. These materials have facilitated a great number of chemical reactions, including aqueous pollutant degradation, water splitting, photodynamic therapy, and organic synthesis. However, rapid separation and recovery of photocatalytic materials with controllable photocatalytic reactivity remains challenging for novel designs of photocatalytic systems. This work aims to design smart photocatalytic materials with switchable reactivity and efficient recyclability, enabling sustainable applications as well as potential cancer therapy. Here, several stimuli-responsive polymeric photocatalytic systems have been developed to address specific application occasions.
Firstly, the realization of enhanced recyclability and efficiency of photocatalytic materials has been addressed. Here, the photoactive diphenyl benzothiadiazole (Ph2BT) moiety has been modified with a polymerizable methacrylate functional group, resulting in a photocatalytic monomer. This photocatalytic monomer can be copolymerized with pH-responsive and classical monomers in a controlled manner, producing the final pH-responsive photocatalytic nanoparticles. These pH-responsive polymeric photocatalytic nanoparticles have been applied for photocatalytic reduction, oxidation, and redox reactions, where high conversion and selectivity was obtained. Furthermore, these particles can be easily recovered from the reaction medium by altering the pH value and reused for repeated cycles without losing their efficiency, demonstrating a stable, switchable, and recyclable photocatalytic system.
Secondly, the production of recoverable photocatalytic materials that can respond to an orthogonal magnetic stimulus has been investigated. Although pH alternation has offered an efficient approach to recycling photocatalytic materials, an external energy supply for centrifugation is still required for the recycling procedure. Therefore, we have implanted magnetic nanoparticles with photocatalytic polymer chains to produce recoverable hybrid materials excluding the centrifugation process. Here, magnetic nanoparticles have been encapsulated into photocatalytic polymers that have facilitated photocatalytic oxidation reactions and can be easily reused for repeated cycles. This project highlighted the importance of creating straightforward photocatalytic platforms that can be easily recovered in response to an orthogonal stimulus to enable sustainable applications.
Lastly, the pH switchability of the aforementioned system has encouraged us to further explore pH-sensitive photocatalytic systems to facilitate tumor microenvironment targeted therapeutic applications. Solid tumors are ubiquitously featured by the dysregulated pH, where the extracellular microenvironment (pHe 6.5-6.9) is slightly lower than normal tissues (pH 7.2-7.4). Therefore, we have designed a photocatalytic nanoparticle system that can specifically respond to this subtle pH change, allowing the targeted activation of photocatalytic moieties at the tumor site. Upon visible light irradiation, reactive oxygen species (ROS) are generated that can either directly kill the cancer cells or further activate prodrugs with ROS-sensitive linkers to generate active anti-cancer therapeutics. Here, these photocatalytic nanoparticles can respond to an extremely subtle pH change from 6.8-6.5, resulting in the disassembly of nanoparticles (Dh ~100 nm, pH 6.8) into free polymer chains or oligomers (Dh ~10 nm, 6.5). These free photocatalytic polymers or oligomers can further activate several prodrug model compounds with different ROS-sensitive linkages, demonstrating a versatile platform for targeted therapeutic applications.
Overall, this thesis intends to create smart, controllable, and recyclable stimuli-responsive photocatalytic systems that can be switched on/off and recycled in response to external stimuli.