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 (microAPD) 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-microm 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 microAPD array, was embedded in a thick slab (1 cm) of PDMS. A thin (80 microm) colored polycarbonate filter was placed on the top of the embedded microAPD to absorb scattered excitation light before it reached the detector. The microAPD was placed below the microchannel and orthogonal to the axis of the optical fiber. The close proximity (approximately 200 microm) of the microAPD to the microchannel made it unnecessary to incorporate transfer optics; the pixel size of the microAPD (30 microm) matched the dimensions of the channels (50 microm). A blue light-emitting diode was used for fluorescence excitation. The microAPD was operated in Geiger mode to detect the fluorescence. The detection limit of the prototype (approximately 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.

Figuring Miniature Aspherics-lon Polishing.

A technique is described for accurately polishing extremely small areas by ion machining. A graphite mask is prepared with a pulsed laser cutting tool. Prepared under computer control, this mask defines the contour of the cavity to be formed in a glass tube. Two methods for measuring final contour of the tube are described.

Laser Raman spectrometer for process control.

An interference filter laser Raman spectrometer has been assembled and tested. Sensitivity and linearity of response have been demonstrated for a number of gases, volatile liquids, and mixtures thereof. It has been demonstrated that the bread-board device will function with less than 1 W of laser power, with a consequent tradeoff in sensitivity. It is felt that this device has demonstrated the principles required for implementation of the design and construction of a prototype process control instrument.