Specific and Optimized PVC Membrane-Based Ion-Selective Field-Effect Transistors for Determination of Nitrate

Abstract

Nitrate (NO ${}_{{3}}^{-}$ ) monitoring is essential for precision agriculture and environmental protection, yet solid-state ion sensors often suffer from sub-Nernstian sensitivity due to unoptimized polymeric membrane formulations. In this work, we present a systematic ratio-optimization strategy for the nitrate-selective polyvinyl chloride (PVC) membrane to enhance the interfacial potential coupling efficiency of Ta₂O₅-gate ion-sensitive field-effect transistors (ISFETs). By precisely tuning the stoichiometric mass ratios of the PVC matrix, o-NPOE plasticizer, and TDDAN ionophore, the thermodynamic energy barrier for ion extraction was minimized. The optimized sensor (Formula B: 13.5 wt% TDDAN, 41.0 wt% PVC, and 45.5 wt% o-NPOE) achieves a near-Nernstian sensitivity of 59.0 mV/decade with excellent linearity (R ${}^{{2}} =0.999$ ) over a wide concentration range from $10^{-{4}}$ to $10^{-{1}}$ M, and a detection limit of ${3}.{7}{\,}{\,}\times {\,}{\,}{10} ^{-{6}}$ M. The sensor exhibits robust selectivity against common interfering anions with logarithmic selectivity coefficients ranging from −2.7 to −4.7, consistent with the classical Hofmeister series. Long-term stability tests revealed a low potential drift of 0.166 mV/h over 8 h, representing an 88.3% improvement over untreated devices. The underlying mechanism of the sensitivity enhancement is discussed in terms of the plasticizer-to-PVC ratio and its influence on ion mobility and interfacial adhesion. Real sample validation using tap water, lake water, and soil extract confirmed satisfactory accuracy with recovery rates of 93.2%–120.5%. This work provides a reliable reference for the design of high-performance solid-state ion sensors and demonstrates their potential for scalable deployment in Internet of Things (IoT)-enabled environmental monitoring networks.