Refractometric sensing with fluorescent-core microcapillaries.

Capillaries present a promising structure for microfluidic refractive index sensors. We demonstrate a capillary-type fluorescent core microcavity sensor based on whispering gallery mode (WGM) resonances. The device consists of a microcapillary having a layer of fluorescent silicon quantum dots (QDs) coated on the channel surface. The high effective index of the QD layer confines the electric field near the capillary channel and causes the development of WGM resonances in the fluorescence spectrum. Solutions consisting of sucrose dissolved in water were pumped through the capillary while the fluorescence WGMs were measured with a spectrometer. The device showed a refractometric sensitivity of 9.8 nm/RIU (up to 13.8 nm/RIU for higher solution refractive index) and a maximum detection limit of ~7.2 x 10(-3) RIU. Modeling the field inside the capillary structure, which is analogous to a layered hollow ring resonator, shows that sensitivities as high as 100 nm/RIU and detection limits as low as ~10(-5) RIU may be achievable by optimizing the QD film thickness.