Highly oxidation-resistant imidazolium-modified catechol for stable wet adhesion under alkaline conditions

Premature oxidation of catechol drastically reduces its adhesion strength and utility. This study chemically linked catechol to an imidazolium group, which functioned as an intrinsic protecting group that retarded catechol oxidation and enabled stable wet adhesion under basic conditions. Johnson-Kendall-Roberts (JKR) contact mechanical testing was performed to determine the work of adhesion (W _adh) of imidazolium-modified catechol (VIMCAT), which remained unchanged for up to 48 hours when incubated at pH 7.4. Contrastingly, W _adh values of unprotected catechol were quickly reduced by over 25% within 30 minutes. VIMCAT still retained its initial adhesion strength even after incubation at pH 9 for over 2 hours. Electron spin resonance (ESR) and ultraviolet-visible (UV-vis) spectra confirmed that VIMCAT resisted conversion to quinone under alkaline conditions. VIMCAT is intrinsically more resistant to oxidation as demonstrated by a higher anodic potential observed from the cyclic voltammetry (CV) experiment and a lower highest occupied molecular orbital (HOMO) energy level calculated using density functional theory (DFT) analysis when compared to unmodified catechol. The direct conjugation of imidazolium with catechol offers a metal-free and intrinsically stabilizing molecular design for a high-performance adhesive polymer, capable of long-term operation in alkaline environments.