Yield enhancement in whispering gallery mode biosensors: microfluidics and optical forces

We analyze the transport of 25nm polystyrene nanoparticles through a microfluidic geometry housing a microtoroidal whispering gallery mode (WGM) sensor, taking into account diffusion, convection and optical forces. In particular, we look at the effects of tightly confining the flow of nanoparticles near the microtoroid's equatorial rim, and calculate the average nanoparticle detection time as well as the total yield for passage of a 1L sample. Compared to purely diffusive transport, flow rates on the order of 2nLmin give yield enhancements of up to 15 , with nanoparticles being detected every 12 s. Even shorter detection times (1.2 s) can be achieved, but at the expense of lower yield. The sensor's transport properties are further improved due to gradient optical forces exerted on the nanoparticles.