Feasibility of a Fluorescent-Core Microcapillary for Biosensing Applications

Optical fluidic microcavities exhibit many attractive properties for the development of a sensitive, label-free biosensor. They can be inexpensive, non-toxic, reusable, and sensitive to low analyte concentrations, making them candidates for integration into Lab-on-a-Chip (LoC) devices. Here, a fluorescent-core microcapillary (FCM) sensor is demonstrated. It utilizes the whispering gallery mode (WGM) resonances in the fluorescence spectrum of a silicon quantum-dot-coated microcapillary channel. Well-defined WGMs with a visibility of 0.37 and Q-factors up to 700 were observed. The FCMs showed refractometric sensitivities of 10-20 nm/RIU for methanol, ethanol, and sucrose solutions, and detection limits approaching 10(-4) RIU. The FCMs were then functionalized for biotin and streptavidin detection, in order to demonstrate the feasibility of this microfluidic device for biosensing applications. Biotin molecules were immobilized on the inner surface of the FCMs using an aminopropyltrimethoxysilane linker. Buffer solutions were then pumped through the capillary channel and the fluorescence spectrum was monitored. Resonance shifts on the order of tens of pm were detected after the binding of biotin, and after the subsequent attachment of streptavidin molecules to the biotinylated surface. Nonspecific streptavidin binding remains a significant problem.