This work develops a refractive-index sensor based on whispering gallery modes (WGMs) in glass microcapillaries. The capillary channel is coated with a layer of fluorescent silicon quantum dots (QDs), which provides a fluorescence source that also supports the WGMs. When different fluids are pumped into the channel, the fluorescence spectrum responds as the resonances shift to different frequencies. A study of the WGM spectral shift analysis techniques improved the detection limits to ∼10−4 refractive index units, and permitted the development of sensorgram-type analyses in which the channel fluid is probed continuously in time. The feasibility of the device as a microfluidic biosensor was demonstrated by first functionalizing the silica surface and then detecting the binding of biotin and streptavidin to the capillary channel. These structures could be attractive as microfluidic biological sensors, since they are easy to fabricate, mechanically robust, and relatively inexpensive compared to other technologies.
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