Stimuli-responsive Azo-QPS-containing Methacrylate Monomers/Polymers
Objectives: Stimuli-responsive materials are vital for the success of targeted treatments and on-site drug delivery. The objective is to develop pH-responsive monomers and polymers capable of switching physicochemical properties in response to pH variations in oral environments.
Methods: Four structurally different monomers entailing a polymerizable methacrylate functional group and an azobenzene quaternary pyridinium salt (Azo-QPS) as a pH-responsive molecule were synthesized and characterized. Furthermore, Azo-QPS-containing methacrylate polymers were synthesized via two-step reactions. Firstly, bromo-functionalized methacrylates were polymerized, and then the bromide groups were replaced with Azo-QPS functionalities via Menshutkin reaction. Various stoichiometric ratios of Azo-QPS relative to bromide groups were utilized in the reaction to alter the total number of Azo-QPS functionalities. The structure/composition of monomers and polymers was confirmed by nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). Their pH-responsive characteristics were analyzed via UV/Vis and NMR spectroscopies in variant buffers with different pH and in DMSO with the addition of triethylamine (TEA) or trifluoroacetic acid (TFA).
Results: Within the pH range 4-8, all the Azo-QPS monomers presented similar pH-sensitive color change between orange and purple and reversible physical/chemical properties as observed in NMR and UV/Vis spectrometers. Polymers consisting of different amounts of Azo-QPS groups, e.g., 5%, 50%, and 99% substitution with Azo-QPS, exhibited reversible pH-responsive properties. Specifically, NMR spectroscopy detected gradual peak broadening and peak intensity decreasing in the aromatic region of both Azo-QPS monomers and polymers upon additions of various molar equivalents of TEA. Concurrently, red shifts from approximately 340 nm to 560 nm were observed in the UV/Vis spectra. These changes were reversible over multiple cycles by alternating additions of acid and base.
Conclusions: pH-responsive Azo-QPS-containing monomers and polymers were successfully prepared and characterized. These materials have great potential in dental applications as a new class of stimuli-responsive restoratives.