SILICON-MICROFABRICATED DIFFUSION-BASED OPTICAL CHEMICAL SENSOR

A silicon-microfabricated flow structure is presented that can be used to detect chemical concentrations optically in complex sample solutio ns. The principle is exemplified by determining the pH of a sample usi ng a fluorescent pH indicator. The flow behavior of liquids in microst ructures differs significantly from that in the macroscopic world. Due to extremely small inertial forces in such structures, practically al l Bow in microstructures is laminar. This allows the movement of diffe rent layers of fluid and particles next to each other in a channel wit hout any mixing other than diffusion. On the other hand, due to the sm all lateral distances in such channels, diffusion is a powerful tool t o separate molecules and small particles according to their diffusion coefficients, which are a function of particle size. We have designed T-shaped silicon channels, in which a sample solution and a receptor s olution containing the indicator dye are joined in the T-connection. T he two streams how next to each other without turbulent mixing until t hey exit the structure. Small molecules and ions diffuse rapidly acros s the width of the channel, whereas larger molecules diffuse more slow ly. Larger particles such as blood cells shaw no significant diffusion within the time the two flow layers are in contact with each other. T hese analyte molecules diffuse into the adjacent acceptor stream with the fluorescent indicator dye. The fluorescence properties of the indi cator are a function of the concentration of the analyte molecules in the interaction zone between the two streams and can be monitored.