Ti3C2T x MXene-Enhanced Paper-Based Ion-Selective Electrodes for Sweat Sodium Monitoring

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Electrolyte balance plays a fundamental role in maintaining physiological homeostasis, as ionic fluctuations regulate cellular excitability, osmotic gradients, and neuromuscular signaling. Among electrolytes, sodium (Na+) is a critical biomarker for hydration status and several pathological conditions, including dehydration and cystic fibrosis. Here, this is the first report of Ti3C2Tx MXene integrated as a solid-contact transducer on a paper-based screen-printed electrode (MPSPE) for ion-selective sensing of Na+ in sweat. The platform combines the sustainability and intrinsic capillarity of paper substrates with the exceptional electrical conductivity and surface reactivity of Ti3C2Tx MXene. Prior to MXene deposition, oxygen plasma treatment was applied to improve surface hydrophilicity and promote homogeneous film formation. The optimized configuration, based on office paper modified with 3 mg mL–1 MXene, exhibited a near-Nernstian slope of 0.0637 V/decade and an excellent linear correlation (R2 = 0.98) over the tested Na+ range (1–150 mM). The calculated limit of detection (LOD) and limit of quantification (LOQ) were 0.35 mM and 1.16 mM, respectively, while the coefficient of variation (%CV) was found to be 7.2%, confirming outstanding repeatability. The electrode demonstrated high selectivity toward Na+ over K+, Mg2+, and Ca2+ (log KpotNa,j < −0.5). Validation with real sweat samples revealed an excellent correlation with the liquid chromatography–tandem mass spectrometry (LC–MS/MS) reference method (R2 = 0.96, mean correlation ≈ 99%). These results highlight the potential of MXene-modified paper electrodes as a low-cost, disposable, and environmentally friendly analytical platform for point-of-care electrolyte monitoring.