PEDOT:PSS/EG-based solid-state ion-selective electrochemical sensors for multiplexed Na⁺ and K⁺ detection in saliva, serum, and interstitial fluid

Accurate monitoring of sodium (Na +) and potassium (K +) ions in biological fluids is critical for assessing electrolyte balance and health status in calves and is strongly associated with major diseases such as bovine respiratory disease (BRD) and neonatal diarrhoea (scours). Alterations in Na + and K + concentrations provide key insight into dehydration, metabolic disturbance, and disease progression; however, routine electrolyte monitoring in veterinary settings remains challenging due to matrix complexity, sensor drift, and limited stability of solid-state ion-selective electrodes. Here, we report a multiplexed solid-state ion-selective sensor platform based on microfabricated interdigitated gold electrodes modified with a poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) solid-state layer and drop-cast Na +-and K +-selective membranes. Ethylene glycol (EG) was incorporated into the PEDOT:PSS layer to enhance electrical conductivity and interfacial charge transport, and the solid-state thickness and EG concentration were systematically optimized. The fabricated sensors exhibited near-Nernstian responses with sensitivities of 54 mV.dec-1 for Na + over a linear range of 10-170 mM and 51 mV.dec-1 for K + over 1-40 mM. Stable open-circuit potential responses were obtained with rapid equilibration (~10 s), low drift, high selectivity, minimal hysteresis, and excellent long-term stability. Sensor performance was validated in complex biofluids, including artificial saliva (AS), fetal bovine serum (FBS), and simulated interstitial fluid (ISF), using a standard addition approach. Quantitative comparison with commercial ionselective meters yielded recovery values within acceptable ranges. Overall, this work demonstrates a robust and scalable electrolyte sensing platform with strong potential for onfarm diagnostics, point-of-care testing, and future wearable systems for animal health monitoring.