Microfluidic All-Solid-State Electrodes for Real-Time Monitoring of Salivary Cations (Na⁺, K⁺, and Ca^{2 +})

Saliva is a valuable biofluid for noninvasive diagnostics, offering insights into oral and systemic health through ion composition analysis. Key electrolytes such as calcium (Ca^{2 +}), sodium (Na⁺), and potassium (K⁺) are critical for tooth enamel integrity, remineralization, and caries resistance. Conventional detection methods such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) are impractical for real-time or point-of-care applications. Here, we report the fabrication and integration of all-solid-state ion-selective electrodes (ISEs) into a microfluidic flow device for real-time, low-volume, and wireless salivary ion monitoring. The platform enables precise fluid handling, automated processing, and simultaneous multi-ion detection. The fabricated ISEs exhibited near-Nernstian responses for Ca^{2 +} and Na⁺, while the K⁺ sensor showed a slightly sub-Nernstian behavior due to membrane deposition effects. Importantly, validation with real human saliva confirmed concentration-dependent responses, stable operation in complex biological matrices, and reproducibility across independent samples. The Na⁺ sensor demonstrated high selectivity (log₁₀ K^Pot _Na,K = -1.63 and log₁₀ K^Pot _Na,Ca = -4.63), while response times of 3-5 min supported near real-time monitoring. These results demonstrate that integrating solid-state ISEs with microfluidics provides a robust and noninvasive diagnostic platform for oral and systemic health assessment. The system represents a step toward miniaturized, cost-effective, and user-friendly point-of-care tools.