Local Sensing of Absolute Refractive Index During Protein‐Binding using Microlasers with Spectral Encoding

Multiplexed, specific, and sensitive detection of antigens is critical for the rapid and accurate diagnosis of disease and the informed development of personalized treatment plans. Here, it is shown that polymer microsphere lasers can be used as photonic sensors to monitor and quantify direct surface binding of biomolecules via changes in the refractive index. The unique spectral signature of each individual laser can be used to find their size and effective refractive index which adds a new encoding dimension when compared to conventional fluorescent beads. Antibody‐functionalized microlasers selectively detect protein binding, as demonstrated for Immunoglobulin G and C‐reactive protein, and have the ability to resolve different stages of the multilayer surface modification. Moreover, by continuously monitoring single lasers, the possibility of real‐time monitoring of binding dynamics between antigens in solution phase and the immobilized antibodies is demonstrated. For multiplexed detection, the microlasers are employed in a flow cytometer configuration, with fast spectral detection and identification of microlasers with and without antigen binding. It is envisioned that by combining microlasers with well‐established surface modification chemistries and flow geometries, the multiplexing ability of microbead immunoassays can be strongly increased while also opening avenues for single‐cell profiling within heterogeneous cell populations. Polymer whispering gallery mode microlasers are used for real‐time detection of specific proteins. The spectral characteristics of the laser emission are used to monitor absolute changes in local refractive index, revealing protein surface binding with very high sensitivity. Antibodies and corresponding antigens are detected and their binding dynamics are monitored. High throughput is achieved with on‐chip microfluidics.