Multituning of Structural Color by Protonation and Conjugate Bases

The pH (acidity/alkalinity) and type of acid are crucial parameters affecting the thermodynamics, kinetics, and reaction mechanism of acid–base systems. However, most reported pH-responsive materials can only determine the pH and cannot identify the acid type. Herein, we report the selective swelling of polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP) photonic crystals, which produces different morphologies with a change in pH or type of acid, based on Henderson–Hasselbalch principle and Coulomb coupling between pyridiniums and the conjugate bases. The swelling behavior in different acidic environments was modeled by the transfer matrix method (TMM), and the results were compared with the experimental reflectometry results. The thickness and refractive index of P2VP layers change with a change in H+ concentration or the hydration energy of the conjugate bases. Since the different morphologies produce distinct structural colors, the pH and type of acid can be directly identified visually. Furthermore, the structural color is affected by not only the molecular weight of PS-b-P2VP but also the cross-linking of the P2VP layers, which can shift the structural color wavelength, thereby changing the sensitivity to pH. Our findings can become a driving force to transcend the frontiers of conventional optical diagnostics or sensing technology.