Carbon Nanohorns and Their Nanohybrid/Nanocomposites as Sensing Layers for Humidity Sensors-A Review.

Carbon nanohorns (CNHs), along with their nanocomposites and nanohybrids, have shown significant potential for humidity (RH) monitoring at room temperature (RT) due to their exceptional physicochemical and electronic properties, such as high surface area, tunable porosity, and stability in nanocomposites. Resistive sensors incorporating CNHs have demonstrated superior sensitivity compared to traditional carbon nanomaterials, such as carbon nanotubes and graphene derivatives, particularly in specific RH ranges. This review highlights recent advancements in CNH-based resistive RH sensors, discussing effective synthesis methods (e.g., arc discharge and laser ablation) and functionalization strategies, such as the incorporation of hydrophilic polymers or inorganic fillers like graphene oxide (GO) and metal oxides, which enhance sensitivity and stability. The inclusion of fillers, guided by Pearson's Hard-Soft Acid-Base (HSAB) theory, enables tuning of CNH-based sensing layers for optimal interaction with water molecules. CNH-based nanocomposites exhibit competitive response and recovery times, making them strong candidates for commercial sensor applications. However, challenges remain, such as optimizing materials for operation across the full 0-100% RH range. This review concludes with proposed research directions to further enhance the adoption and utility of CNHs in sensing applications.