A portable optical device for quantitative detection of lithium in blood plasma†

Lithium ion (Li⁺) monitoring is critical for managing bipolar disorder therapy, yet conventional detection methods rely on bulky instruments and are not suitable for point-of-care diagnostics (POCD). Here, we present a portable biosensor for rapid and accurate Li⁺ quantification in human blood plasma. The device operates based on absorbance changes at 505–525 nm induced by Li⁺-specific complexation with a porphyrinoid reagent. A nonlinear logistic calibration curve (R² = 0.999) is established across 0–2.0 mM Li⁺, demonstrating high repeatability. In addition, validation using independent spiked samples (0.2–1.8 mM) shows an excellent agreement between measured and actual concentrations (R² = 0.995). Moreover, specificity testing confirms robust anti-interference capability against a range of common cations. The system features automated light intensity stabilization and user-friendly operation, achieving sample-to-result within 2.0 min using 3.0 μL plasma. The device is compact and handheld, with a per-device cost of only $34.8 and a per-test cost of only $1.62. This low-cost portable device addresses the urgent need for decentralized lithium therapeutic drug monitoring, particularly in resource-limited settings.