Anodic Niobium–Titanium Oxide Crossbar Memristor Arrays for pH Sensing in Liquids

Anodic Nb–Ti oxide crossbar memristor arrays are successfully produced on Si substrates. Their use as pH sensors is tested by analyzing their current–voltage (I–U) sweeps in Ringer´s solutions with different pH values. A gradual evolution of the memristive hysteretic curve is observed upon pH variation. A sensor calibration showing a linear trend with least square error close to 1 is performed and the performance of the pH sensing is assessed. The limit of detection (LOD), limit of quantification (LOQ), and the sensitivity (S) are determined. The calculated LOD is 0.4 pH, and the LOQ is 1.2 pH. The S is calculated as 0.5 pH corresponding to a concentration of c(OH−) = 3.5 mM. The reproducibility and repeatability of the measurements are calculated as 3.8% and 1.4%, respectively. An X-ray photoelectron spectroscopy survey allows the study of the oxide composition providing information about its pH-sensing mechanism. It is found that the OH− species diffused into the mixed anodic oxide are responsible for the pH-induced high- to low-resistance state changes. A conduction mechanism analysis is carried out and Schottky emission is identified as predominant.