Multiplexed potentiometric sensing of nitrite in environmental waters

Nitrite is a key water quality indicator whose monitoring is essential for environmental and human safety protection, yet conventional methods such as chromatography and spectrophotometry require expensive instrumentation and centralised laboratory infrastructure. Potentiometric solid-contact ion-selective electrodes, particularly in screen-printed formats, offer a promising cost-effective and miniaturised alternative, but their use in real scenarios still demands versatile operation modes and affordable multiplexed readout to increase throughput and statistical robustness. In this context, a potentiometric nitrite sensor based on commercial screen-printed electrodes (SPEs) modified with graphene oxide and functionalised with an optimised polymeric membrane containing Nitrite Ionophore VI, was developed and characterised for batch, drop, and flow analysis using a custom open-source multiplexer that extends a single-channel EmStat4M potentiostat (PalmSens BV) to eight pseudo-parallel channels. The NO₂-SPEs exhibited a Nernstian response over the linear range from 3.0 × 10^-6 to 1.0 × 10^-2 M under batch, drop, and flow analytical modes, covering nitrite's regulated contaminant level in freshwater. The potentiometric response was additionally characterised by a fast response time (<40 s), good reversibility, reproducibility (RSD <4.3% for slope between identical sensor units, n = 16), and potential stability (medium-term drift of 32.8 ± 15.3 μV min^-1) under batch mode. The proposed potentiometric systems were applied to nitrite determination in spiked river and groundwater samples, yielding recovery values from 83.5 to 108.9% across all three operational modes. Beyond these analytical figures of merit, the combined sensor-multiplexer platform provides a practical route toward decentralised measurements, enabling sensing automation, increased statistical sample size, and multianalyte detection capabilities.