Advanced Immunoassay Detection Using Microwave Whispering Gallery Mode Resonators

This study introduces a novel immunoassay technique that utilizes a microwave Whispering Gallery Mode (WGM) silicon resonator, illuminated by near-infrared (NIR) light, for highly sensitive detection of antigen-antibody interactions. The WGM resonator, integrated with a microstrip transmission line, acts as the primary sensing element for detecting specific binding events within the NIR spectrum. Antigen-antibody complexes are immobilized at the bottom of a transparent plexiglas container, allowing NIR light from a light-emitting diode (LED) to illuminate the resonator. This interaction modifies the resonator's conductivity and alters the transmission coefficient (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{21}$</tex>) at the resonant frequency, with changes in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{21}$</tex> directly linked to antigen-antibody interactions, enabling precise detection. The study validates this label-free detection method for small molecules, exemplified by 25 -hydroxyvitamin D3 (calcidiol), and emphasizes its simplicity, cost-effectiveness, and portability, indicating its potential applications in virus detection and cancer diagnostics.