The study presents findings on the relative humidity (R.H.) sensing capabilities of a resistive sensor. This sensor utilizes sensing layers composed of a ternary nanohybrid, consisting of holey carbon nanohorn (CNHox), potassium chloride (KCl), and polyvinylpyrrolidone (PVP), with mass ratios of 7/1/2, 6.5/1.5/2, and 6/2/2 (w/w/w). The sensing structure comprises a silicon substrate, a SiO₂ layer, and interdigitated transducer (IDT) electrodes. The sensing film is deposited on the sensing structure via the drop-casting method. The sensing layers’ morphology and composition are investigated through Scanning Electron Microscopy (SEM) and RAMAN spectroscopy. The resistance of thin-film sensors based on ternary hybrids increased with exposure to a range of relative humidity (R.H.) levels, from 0% to 100%. The newly designed devices demonstrated a comparable response at room temperature to that of commercial capacitive R.H. sensors, boasting excellent linearity, swift response times, and heightened sensitivity. Notably, the studied sensors outperform others employing CNHox-based sensing layers in terms of sensitivity, as observed through manufacturing and testing processes. It elucidates the sensing mechanisms of each constituent within the ternary hybrid nanocomposites, delving into their chemical and physical properties, electronic characteristics, and affinity for water molecules. Various alternative sensing mechanisms are considered and discussed, including the reduction in holes within CNHox upon interaction with water molecules, proton conduction, and PVP swelling.

We report the relative humidity (R.H.) sensing response of a resistive sensor, employing sensing layers based on a ternary nanohybrid comprising holey carbon nanohorns (CNHox), potassium chloride (KCl), and polyvinylpyrrolidone (PVP) at mass ratios 7/1/2, 6.5/1.5/2, and 6/2/2 (w/w/w/w). The sensing structure comprises a silicon substrate, a SiO2 layer, and interdigitated transducer (IDT) electrodes. The sensing film is deposited on the sensing structure via the drop-casting method. The sensing layers' morphology and composition are investigated through Scanning Electron Microscopy (S.E.M.) and RAMAN spectroscopy. The ternary hybrid-based thin film resistance increases when the sensors are exposed to R.H., ranging from 0% to 100%. The manufactured devices show a room temperature response comparable to a commercial capacitive R.H. sensor and are characterized by excellent linearity, rapid response time, and good sensitivity. The presented sensors exhibit superior performance in terms of sensitivity compared to other similarly manufactured and tested sensors that employ a CNHox-based sensing layer. We explain the sensing role of each constituent of the ternary hybrid nanocomposites based on their chemical and physical properties, electronic properties, and affinity for water molecules. Different alternative sensing mechanisms are considered and discussed, such as the decreasing number of holes in the holey CNHox at the interaction with water molecules, proton conduction, and PVP swelling.

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.

In recent years, carbon nano-onions (CNOs), together with their functionalized derivatives, nanocomposites, and nanohybrids, have attracted increasing attention as sensing materials for monitoring relative humidity (RH), gases, and temperature. Their concentric graphitic structure provides good electrical conductivity, chemical and thermal stability, accessible surface sites, tunable surface chemistry, and compatibility with polymer matrices and flexible substrates. This review highlights recent advances in the synthesis and functionalization of CNOs and examines their integration into chemiresistive, surface acoustic wave, flexible, and printed sensing platforms. Particular attention is devoted to pristine and oxidized CNOs, heteroatom-doped materials, and composites incorporating hydrophilic or conducting polymers, metal oxides, and other functional fillers. CNOs-based sensing layers demonstrate room-temperature (RT) detection of RH, hydrogen, ammonia, acetone, ethanol, isopropanol, carbon dioxide, hydrogen sulfide, and other volatile organic compounds. In addition, CNOs and CNOs–polymer films exhibit significant temperature-dependent variations in resistance, supporting their potential use in flexible and wearable temperature sensors. Although several CNOs-based devices show superior performance in sensitivity, response, recovery characteristics, mechanical flexibility, and low-power operation, the studies on CNOs available in the literature remain limited compared with those on carbon nanotubes, graphene derivatives, and other carbonaceous materials. Further progress on CNOs-based structures requires reproducible, large-scale synthesis; improved film uniformity and selectivity; standardized testing; compensation for temperature–humidity cross-sensitivity; and long-term stability studies. This review concludes by highlighting research directions to bridge the gap between laboratory prototypes and commercially viable CNOs-based sensing devices.

Carbon nanohorns (CNHs), their nanocomposites, and nanohybrids have demonstrated significant potential for relative humidity (RH) monitoring at room temperature (RT) due to their exceptional physicochemical and electronic properties. At the same time, over the last few decades, resistive sensors have been extensively designed and manufactured due to their simple design, small size, low cost, robustness, quick response times, and wide RH measurement ranges. Recently, resistive sensors with CNHs as key sensing elements have been reported for use in RH monitoring. We summarize in this review the recent progress on resistive RT RH sensors based on CNHs. The most effective strategies to synthesize CNHs and approaches for functionalization, as well as the most relevant physicochemical and electronic properties of nanohorns, are presented in the first two sections of this review. The design of various RH resistive sensors, employing carbon nanohorn-based materials as sensing films, and the synthesis and performance of several CNH-based sensing materials in RH monitoring within the context of resistive sensor design are presented in the following two parts of this review. Pristine and functionalized carbon nanohorns, nanocomposites with different hydrophilic polymers, and nanohybrids with several semiconducting metal oxides are compared in terms of sensitivity, response time, and recovery time. The fifth part of this review presents several sensing mechanisms that are involved in RH monitoring. Finally, in the sixth part of this review, the authors aim to explain which are the most important advantages of using CNHs- based sensing layers in resistive RH detection and why these nanocarbonic materials are less used, at least for the moment, than graphene, graphene oxide, reduced graphene oxide, single and double- walled carbon nanotubes.

In recent years, carbon nano-onions (CNOs), together with their functionalized derivatives, nanocomposites, and nanohybrids, have attracted increasing attention as sensing materials for monitoring relative humidity (RH), gases, and temperature. Their concentric graphitic structure provides good electrical conductivity, chemical and thermal stability, accessible surface sites, tunable surface chemistry, and compatibility with polymer matrices and flexible substrates. This review highlights recent advances in the synthesis and functionalization of CNOs and examines their integration into chemiresistive, surface acoustic wave, flexible, and printed sensing platforms. Particular attention is devoted to pristine and oxidized CNOs, heteroatom-doped materials, and composites incorporating hydrophilic or conducting polymers, metal oxides, and other functional fillers. CNO-based sensing layers demonstrate room-temperature (RT) detection of RH, hydrogen, ammonia, acetone, ethanol, isopropanol, carbon dioxide, hydrogen sulfide, and other volatile organic compounds. In addition, CNOs and CNO–polymer films exhibit significant temperature-dependent resistance variations, supporting their potential use in flexible and wearable temperature sensors. Although several CNO-based devices show superior performance in sensitivity, response, recovery characteristics, mechanical flexibility, and low-power operation, the studies on CNOs available in the literature remain limited compared with those on carbon nanotubes, graphene derivatives, and other carbonaceous materials. Further progress on CNO-based structures requires reproducible, large-scale synthesis; improved film uniformity and selectivity; standardized testing; compensation for temperature–humidity cross-sensitivity; and long-term stability studies. This review concludes by highlighting research directions to bridge the gap between laboratory prototypes and commercially viable CNO-based sensing devices.