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 solutions. The principle is exemplified by determining the pH of a sample using a fluorescent pH indicator. The flow behavior of liquids in microstructures differs significantly from that in the macroscopic world. Due to extremely small inertial forces in such structures, practically all Bow in microstructures is laminar. This allows the movement of different layers of fluid and particles next to each other in a channel without any mixing other than diffusion. On the other hand, due to the small lateral distances in such channels, diffusion is a powerful tool to 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 solution containing the indicator dye are joined in the T-connection. The two streams how next to each other without turbulent mixing until they exit the structure. Small molecules and ions diffuse rapidly across the width of the channel, whereas larger molecules diffuse more slowly. Larger particles such as blood cells shaw no significant diffusion within the time the two flow layers are in contact with each other. These analyte molecules diffuse into the adjacent acceptor stream with the fluorescent indicator dye. The fluorescence properties of the indicator are a function of the concentration of the analyte molecules in the interaction zone between the two streams and can be monitored.