Semiconductor-Type Gas Sensors Based on Surface-Modified Iron Oxide Nanoparticles for ppb-Level Detection of Acetone

Volatile organic compounds (VOCs) at parts-per-billion (ppb) concentrations in biological gases, such as human breath and skin emissions, are recognized as potential disease biomarkers. As a result, biological gas analysis has attracted considerable attention as a noninvasive approach for disease screening. The purpose of this study was to propose a method to prepare a gas sensor that is capable of detecting VOCs at ppb levels by realizing the high specific surface area of a gas-sensitive layer. Iron oxide nanoparticles modified with 3,4-dihydroxyphenylacetic acid were drop-cast onto an alumina substrate with platinum electrodes to prepare the gas-sensitive layer of the gas sensor. The resistivity of the gas-sensitive layer was measured at 230 °C under dry air with and without VOC gases. The response value was calculated as Ra/Rg, where Ra is the resistance in dry air and Rg is the resistance with VOC gases. The modified nanoparticles exhibited response values of 7.8 and 9.0 for 100 ppb of acetone and acetaldehyde, respectively, substantially higher than those of unmodified iron oxide nanoparticles, which showed responses of 3.0 and 3.1. Scanning electron microscopy and nitrogen adsorption–desorption isotherm analyses revealed that the surface-modified nanoparticles possessed a high specific surface area with numerous pores possibly because the increased interparticle affinity between iron oxide nanoparticles was modified with bifunctional organic molecules. These structural features likely enhanced gas interaction, contributing to the improved sensor response compared to the unmodified counterparts. This study would contribute to screening for diseases such as diabetes with the same ease as breath alcohol testing.