Advanced Immunoassay Detection Using Microwave Whispering Gallery Mode Resonators

This study presents a novel immunoassay technique leveraging the high sensitivity of a Whispering Gallery Mode (WGM) silicon resonator illuminated by near-infrared (NIR) light. The WGM resonator, integrated with a microstrip transmission line, serves as the primary sensing element for detecting specific antigen-antibody binding events within the NIR spectrum. Antigen-antibody complexes are immobilized at the base of a transparent Plexiglas container, through which NIR light from a light-emitting diode passes to illuminate the silicon WGM resonator. This interaction modulates the resonator's conductivity and alters the transmission coefficient (S21) at the resonant frequency. The observed changes in S21 are directly correlated with antigen-antibody interactions, enabling highly sensitive detection. The study demonstrates the effectiveness of this label-free detection method for small molecules, using 25-hydroxyvitamin D3 as a model compound. The simplicity, affordability, and portability of this technique highlight its potential for diverse applications, including virus detection and blood analysis for cancer diagnostics.