pH-Responsive Capsules Fabricated by Interfacial Photo-Cross-Linking Utilizing the Photoreactivity and pH-Responsiveness of Thymine

pH-responsive capsules are useful materials for various applications, such as drug delivery systems and nano-/microreactors. Herein, we developed pH-responsive capsules through interfacial photo-cross-linking with thymine-functionalized parent polymer particles, where thymine functioned as a photoreactive and pH-responsive group. A sufficient hydrophilicity of the capsule polymers was necessary to achieve the controlled release of encapsulated molecules by utilizing the pH-responsiveness of thymine groups. Based on a series of solubility tests for various polymers derived from 4-vinylbenzyl thymine (VBT), styrene (St), 2-vinylpyridine (2VP), and 4-vinylpyridine (4VP), P(St-2VP-VBT) and P(St-VBT) were suitable for preparing capsule particles by interfacial photo-cross-linking. Release tests revealed that P(St-2VP-VBT), but not P(St-VBT), could release its cargo under basic conditions, where thymine transforms from the nonionic to anionic state. This behavior indicates that the increased hydrophilicity of the polymer shell layer obtained by copolymerization with 2VP is critical for cargo release induced by thymine deprotonation. Furthermore, P(St-2VP-VBT) also responded to acidic pH owing to protonation of the pyridine groups. Finally, we successfully created acidic and alkaline dual pH-responsive capsules via interfacial photo-cross-linking using P(St-2VP-VBT) particles.