Identifying Eigen-Like Hydrated Protons at Negatively Charged Interfaces

Despite the importance of the hydrogen ion in a wide range of biological, chemical, and physical processes, its molecular structure in solution remains lively debated. Progress has been primarily hampered by the extreme diffuse nature of the vibrational signatures of hydrated protons in bulk solution. Using the inherently surface-specific vibrational sum frequency spectroscopy, we show that at selected negatively charged interfaces, a resolved spectral feature directly linked to the H₃O⁺ core in an Eigen-like species can be readily identified in a biologically compatible pH range. The results offer a new molecular perspective for tracking and understanding the behaviour of hydrated protons at interfaces.