  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, we show 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. We utilize antibody-functionalized microlasers
to selectively detect protein binding. Different stages of the multilayer
surface modification can be resolved, and protein binding is demonstrated for
two different proteins, IgG and CRP. Moreover, by continuously monitoring
single lasers, we demonstrate the possibility of real-time monitoring of
binding dynamics between antigens in solution phase and the immobilized
antibodies. 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. We envision 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
heterogenous cell populations.
