Effective linewidth shifts in single-molecule detection using optical whispering gallery modes

Here, we report shifts of the linewidth of a plasmon enhanced whispering gallery mode (WGM) of a glass microsphere cavity due to binding of single sub-kDa molecules. The observed linewidth of the WGM can either increase or decrease upon binding of single molecules depending on the location of their binding sites. The linewidth shifts arise due to the change in the unresolved frequency splitting of standing wave modes (SWMs). These SWMs are formed due to the scattering from the gold nanoparticles attached to the WGM. Monitoring the WGM linewidth provides a robust method for sensing single molecules over prolonged time periods as the linewidth is unaffected by changes in the host refractive index due to drifts in temperature, pressure, or change in the concentration of buffers.