An Integrated Fluorescence Detection System in Poly(dimethylsiloxane) for Microfluidic Applications

This paper describes a prototype of an integrated fluorescence detection system that uses a microavalanche photodiode ([micro]APD) as the photodetector for microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The prototype device consisted of a reusable detection system and a disposable microfluidic system that was fabricated using rapid prototyping. The first step of the procedure was the fabrication of microfluidic channels in PDMS and the encapsulation of a multimode optical fiber (100-[micro]m core diameter) in the PDMS; the tip of the fiber was placed next to the side wall of one of the channels. The optical fiber was used to couple light into the microchannel for the excitation of fluorescent analytes. The photodetector, a prototype solid-state [micro]APD array, was embedded in a thick slab (1 cm) of PDMS. A thin (80 [micro]m) colored polycarbonate filter was placed on the top of the embedded [micro]APD to absorb scattered excitation light before it reached the detector. The [micro]APD was placed below the microchannel and orthogonal to the axis of the optical fiber. The close proximity (~200 [micro]m) of the [micro]APD to the microchannel made it unnecessary to incorporate transfer optics; the pixel size of the [micro]APD (30 [micro]m) matched the dimensions of the channels (50 [micro]m). A blue light-emitting diode was used for fluorescence excitation. The [micro]APD was operated in Geiger mode to detect the fluorescence. The detection limit of the prototype (~25 nM) was determined by finding the minimum detectable concentration of a solution of fluorescein. The device was used to detect the separation of a mixture of proteins and small molecules by capillary electrophoresis; the separation illustrated the suitability of this integrated fluorescence detection system for bioanalytical applications.