Confined water in low-dimensional materials: structural, spectroscopic, and transport phenomena

Water confined in low-dimensional materials exhibits structural and dynamical behaviors that diverge fundamentally from bulk liquid water. Nanoscale confinement reshapes the hydrogen-bond network, induces molecular ordering, and alters dielectric, vibrational, and transport properties through the interplay between geometry, surface chemistry, and electrostatics. This review presents a comparative synthesis of confined water in 0D–2D environments, from single-molecule encapsulation in molecular cages and single-file flow in carbon nanotubes to layered phases trapped between atomically flat van der Waals crystals. We outline how dimensionality and surface polarity dictate hydrogen-bond rearrangement, layering, and crystallization into low-dimensional ice polymorphs. Spectroscopically, Raman, infrared, terahertz, and nonlinear optical probes reveal distinct vibrational fingerprints reflecting modified hydrogen-bond strength, dipole alignment, and collective dynamics. In the transport regime, continuum hydrodynamics breaks down, giving rise to superlubric flow, anisotropic diffusion, and quantized single-file motion. Across these systems, confinement transforms water from a fluctuating three-dimensional liquid into a tunable, ordered medium bridging molecular and solid-state physics. By unifying results across structural, spectroscopic, and transport studies, this review provides a coherent physical framework for understanding confined water in low-dimensional materials and highlights its implications for nanofluidics, energy storage, and bio-inspired systems.