Molecular states and interfacial interactions at solid-liquid interfaces: advances and challenges in multimodal characterization.

Solid-liquid interfaces govern key functions in biosensing, catalysis, energy conversion, drug delivery, and organic electronic devices because molecular adsorption, hydration, charge regulation, and structural reorganization occur within an ultrathin interfacial region. However, these phenomena are intrinsically multiscale and multidimensional, involving strongly coupled variables such as mass, interfacial potential, molecular orientation, viscoelasticity, and local nanoscale heterogeneity. As a result, no single measurement technique can fully describe interfacial states or their dynamics. This review summarizes recent advances in multimodal characterization strategies designed to overcome these limitations. We first outline the conceptual framework of molecular interactions at solid-liquid interfaces, focusing on electrostatic interactions, van der Waals forces, hydration structures, hydrophobic effects, specific adsorption, and protein conformational changes across wide temporal and spatial scales. We then compare the observation windows, defined here as the combined range of spatiotemporal resolution and accessible physical observables, and limitations of major stand-alone methods, including optical, electrochemical, acoustic, and scanning probe techniques. Building on this basis, we discuss the recent development of multimodal platforms that integrate optics with electrochemistry, optics with quartz crystal microbalance measurements, and local probes with optical or electrochemical readouts, highlighting how these combinations reveal correlations among dry and wet mass, charge state, molecular vibration, hydration, and local structure. Finally, we discuss remaining challenges, including causal ambiguity among coupled interfacial variables, the trade-off between spatial and temporal resolution, and the difficulty of quantitatively linking experimental data with theoretical models. Multimodal characterization is expected to provide a foundation for the rational design of next-generation biointerfaces, delivery systems, and organic devices.