Measuring Liquid Volumes in Sub-nanoliter Wells

We are developing a method for high-throughput screening using arrays of “nanowells” built into a silicon substrate. These wells can serve as bioreactors for studying a variety of biochemical reactions such as the enzymatic activity that occurs in yeast metabolism. For a variety of studies it is important to know the volume of liquid that has been deposited in a given well and/or to monitor the evaporation of the liquid. Using silicon as our substrate means that we can take advantage of the ability to build microelectronics into the wells in order to develop “smart” wells. The wells are manufactured on silicon wafers using conventional photolithography and etching techniques and typical wells measure 200 × 200 × 20 μm 3 which is a volume of 800 pl. Aluminum electrodes are patterned on the floor of the wells. The floor as well as the electrodes are then covered by an electrical insulation layer. The complex impedance measured through the electrodes is then related to the volume of liquid in the wells. Using fluorescence microscopy as well as interference microscopy to calibrate our system, we can measure liquid volumes with an accuracy of about 5% and a resolution better than 1 pl. Real-time monitoring of fluid volumes in a collection of wells is possible by additional on-chip microelectronics which permits multiplexing the measurements over the bioreactor array.