Among various metal oxides, titanium dioxide (TiO2) has received considerable interest as a gas-sensing material owing to its high reliability at high operating temperatures. Nonetheless, TiO2 generally has low sensitivity to target gases. In particular, TiO2-based sensors have difficulty in sensitively detecting benzene, toluene, and xylene (referred to as BTX). Moreover, the reported TiO2-based sensors have not simultaneously satisfied the demand for tens of ppb BTX detection and operation with low power consumption. This work proposes a BTX sensor using cobalt porphyrin (CoPP)-functionalized TiO2 nanoparticles as a sensing material on a suspended microheater fabricated by bulk micromachining for low power consumption. TiO2 nanoparticles show an enhanced sensitivity (245%) to 10 ppm toluene with CoPP functionalization. The proposed sensor exhibits high sensitivity to BTX at concentrations ranging from 10 ppm down to several ppb. The high reliability of the sensor is also explored through the long-time operation with repeated exposure to 10 ppm toluene for 14 h.

A volatile organic compound (VOC) sensor array based on metal oxide nanoparticles (MOX NPs) functionalized by metalloporphyrins (MPPs) was demonstrated. The VOC sensor array was composed of four single sensors based on SnO 2 NPs/cobalt-porphyrin, SnO 2 NPs/zinc-porphyrin, SnO 2 NPs/nickel-porphyrin and ZnO NPs/cobalt-porphyrin. The MOX NP/MPP-based sensors were fabricated by drop-casting the MOX NPs dispersion and MPPs solution onto a MEMS platform. The fabricated sensor successfully detected toluene at a concentration as low as 20 ppb, which is below the limit detection concentration of previously reported porphyrin-based VOC sensor arrays. We also confirmed the selectivity between benzene, toluene, ethylbenzene, and xylene (BTEX) by using principal component analysis in contrast to previous studies on MOX/MPP-based sensor. BTEX was classified from 1 to 9 ppm at a resolution of 2 ppm, and the sensor array showed stable performance even after considerable impact.

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In this study, the sensing characteristics of tin oxide-based gas sensors deposit with different amounts of metalloporphyrin, which is a functionalization substance, are evaluated. The mass of metalloporphyrin deposited is 3, 10, 20, 30, and 40 mg for 5 different sensors prepared. The deposition of 3 mg of metalloporphyrin result in an island form of functionalization instead of a thin film; meanwhile, thin films with thicknesses of 25, 35, 74, and 92 nm are formed for the other four cases. As the deposition amount of metalloporphyrin increase, the performance of the sensor deteriorate. The samples are prepared by subdividing the amount of metalloporphyrin source to determine the optimized deposition amount. A sample is prepared with deposition amounts ranging between 1 to 10 mg. The sensors deposit with 3–5 mg metalloporphyrin has excellent response, response, and recovery time characteristics.

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