Structure–Property Engineering of Imogolite-Type Nanotubes: A Critical Review of Synthesis Control, Chemical Modification, Functional Properties, and Emerging Applications

Imogolite-type nanotubes (INTs) are one-dimensional inorganic nanotubes distinguished by chemically asymmetric inner and outer surfaces, composition-dependent curvature, tunable porosity, and theoretically predicted radial wall polarization. This critical review examines how synthesis conditions, compositional substitution, and surface functionalization govern INT structure, physicochemical properties, and application performance. The review integrates foundational studies and subsequent developments in atomic-structure modeling, hydrothermal and microwave-assisted synthesis, Si-to-Ge substitution, Fe and Ti modification, inner-wall functionalization, and interfacial engineering. The review organizes evidence within a processing–structure–property–function framework and distinguishes material-property demonstrations from proof-of-concept, functional validation, and translation-oriented evidence levels. Across the literature, several design principles emerge: tetrahedral-site chemistry primarily controls curvature and diameter, processing conditions govern axial growth and higher-order organization, accessible rather than nominal surface area determines interfacial performance, and functional substitution is effective only within a structural tolerance window. Experimentally validated applications are strongest in adsorption and environmental remediation, membrane separation, polymer nanocomposites, organized fibers, and selected catalytic or photocatalytic systems, whereas advanced biomedical uses remain comparatively speculative. Safety, environmental fate, reproducibility, regeneration, scale-up, and life-cycle impacts remain major barriers to translation. Future progress will require standardized material descriptors, application-relevant testing, long-term stability assessment, and safe-and-sustainable-by-design strategies linking processing, structure, property, function, and manufacturability.