Air-coupled micro-disk resonator for robust reproducible sensing of trace phosphate ions in the tap water and biological samples

A micro-disk resonator (MDR) sensor was developed and rigorously characterized for the rapid and robust detection of ultra-trace phosphate ions (PO₄³⁻) in both tap water and complex biological matrices. Operating in the whispering gallery mode (WGM), the MDR is precision-engineered from a high-purity magnesium fluoride (MgF₂) crystal by ultra-precision machining to achieve nanometre-scale surface smoothness. Notably, the device has its coupling region in air and its sensing region immersed in liquid, resulting in high-quality factor performance and enhanced system stability, which is a significant improvement over conventional resonator architectures in which both the coupling and sensing regions are immersed in liquid. The surface of the MDR was functionalized with ferritin immobilized via reduced graphene oxide (rGO) nanoparticles, enabling high selectivity for PO₄³ ⁻. The sensor exhibited a wide linear detection range from 5 × 10⁻⁶ to 1 × 10⁻² mg/L, with a limit of detection (LoD) as low as 4.30 × 10⁻⁶ mg/L, demonstrating competitive sensitivity and offering a distinct air-coupled WGM sensing architecture. The assay could be completed within 10 min and exhibited high selectivity in the presence of competing anions. The sensor was applied to real samples, including tap water and biological fluids, with spiked recovery rates ranging from 96% to 104%, demonstrating its practicality.