Selective sensing of isoprene by Ti-doped ZnO for breath diagnostics

Exhaled isoprene could enable non-invasive monitoring of cholesterol-lowering therapies. Here, we report an isoprene-selective sensor at high relative humidity (RH) for the first time (to our knowledge). It is made of nanostructured, chemo-resistive Ti-doped ZnO particles (10-20 nm crystal size) produced by flame spray pyrolysis (FSP) and directly deposited in one step onto compact sensor substrates forming highly porous films. The constituent particles consist of stable Ti-doped ZnO solid solutions for Ti levels up to 10 mol% apparently by substitutional incorporation of Ti⁴⁺ into the ZnO wurtzite lattice and dominant presence at the particle surface. These Ti⁴⁺ point defects strongly enhance the isoprene sensitivity (>15 times higher than pure ZnO) and turn ZnO isoprene-selective, while also improving its thermal stability. In situ infrared spectroscopy confirms that Ti⁴⁺ intensifies the surface interaction of Ti-doped ZnO with isoprene by providing additional sites for chemisorbed hydroxyl species. In fact, at an optimal Ti content of 2.5 mol%, this sensor shows superior isoprene responses compared to acetone, NH₃ and ethanol at 90% RH. Most notably, breath-relevant isoprene concentrations can be detected accurately down to 5 ppb with high (>10) signal-to-noise ratio. As a result, an inexpensive isoprene detector has been developed that could be easily incorporated into a portable breath analyzer for non-invasive monitoring of metabolic disorders (e.g. cholesterol).