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M. Foquet, S.W. Turner, A. Lopez, H.G.Craighead.

Introduction

Fabrication techniques of microfluidic systems are being developed for use in laser induced fluorescence studies of macromolecules. Devices with submicrometer size capillaries have been fabricated on glass substrates for the study of electrophoretic motion of biopolymers. The motion of individual DNA molecules can be observed and their speed estimated. Other devices integrating both optical waveguides and capillaries have been fabricated. The waveguides are created to perform fluorescence using laser as light source, allowing for the excitation of very small volume combined with a very high intensity. Gratings defined by electron-beam lithography are used for the coupling of light into the waveguide. The same fabrication process can readily be used to fabricate capillaries with dimensions down to 0.1 mm. Light has been coupled into the waveguide and the patterns of scattered light have been recorded. Excitation of fluorescent solution in the capillaries can be observed. We are now in the process of characterizing the waveguide/capillary system.

Motivation

Fluorescence is one of the main tools biologists and biochemists have used to understand the processes of life. It is routinely used in laboratories to monitor the progress of biochemical reactions in vitro or in vivo, measure the produced/consumed/available quantity of important biological agent (proteins, DNA,...) and study the dynamics of living cell at a molecular scale (concentrations, reactions rates, pH, diffusion constants,...).

Fluorescence has several advantages as a probe for biochemical processes. Extremely small amounts of dye can yield a large signal, minimizing the perturbation of the natural process. Some dye exhibit drastic change in fluorescence quantum yield or in emission spectrum on binding giving a high specificity of the signal on the environment. It is more available and practical than radioactive markers. Finally, improvements in the detection apparatus have shown that fluorescence can reach the highest sensitivity possible, that is temporally resolved single molecule detection.

The optics used for the detection of fluorescence have pushed the probe volume to lower and lower sizes with the use of lasers as light sources and methods like confocal microscopy and two-photon excitation. The small probe volumes have allowed for a higher sensitivity due to the decrease in the raman background from the solvent. It has also brought a wealth of new information on the dynamics of the systems through the study of the signal correlation functions. In parallel to that evolution, the amount of reagent that biochemist have used for their experiment has been decreasing steadily. Smaller and smaller liquid handling systems have been fabricated to handle the new needs.

Ultimately, processes and methods from the semiconductors industry have been used to create the evolved structures necessary to control the small amounts of fluid. However, it appears that the techniques used for the detection process have not followed the same path. In this work, we are showing that not only the liquid handling system can be miniaturized, but that part of the detection system can be readily integrated on a single chip using the same type of technology.

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Last modified 14 January 1999
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