Carbon Nano-Onion-Based Sensors for Relative Humidity, Gas, and Temperature Monitoring: A Review

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.