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Ion Spectroscopy Using Microfluidic FlowFETs

ABSTRACT FlowFETs consisting of drain-source contacts to liquids in microfluidic channels and an isolated gate electrode are shown to generate and control the electro-osmotic fluid flow (EOF) [1,2]. Here we use flowFETs to analyze the ionic content of channel fluids by performing flowFET gate conductance spectroscopy and drain-source current measurements at different gate voltages. The field effect modulates both the ion mobility and density in the active part of the channel enabling the flowFET to differentiate between different ions through their contributions to the drain current. Compared to ion-selective electrodes, a flowFET is faster and does not have any ion-selective “fixed” membranes. In a sense, the gate region in the flowFETs can be viewed as a programmable membrane with its selectivity programmed by the gate potential. Here, for the first time we discuss the operation principles of flowFETs as general-purpose electro-chemical microfluidic sensor. Figure 1 shows the flowFET used in our studies. Its fabrication process and operation characteristics are given in [1]. The device has a 20-μm high, 200-μm wide main channel. The gate electrode was formed by electroplating nickel through a 20-μm thick photoresist mold. The mold had 10 openings, 10-μm wide, 100-pin long, separated by 10-μm. The structure was over electroplated so that grown pillars with mushroom structures forming closed channels forming 20-μm high, 10-μm wide 10 parallel channels. The electroplated channel array structure is conformally coated with 0.5-μm thick parylene [1]. The measured gate capacitance was around 2pF at 100 KHz. The gate voltage modulates the zeta potential of the fluid in the microchannel. But not all ions in the fluid are affected in a similar manner. This forms the operation principle of using the flowFET as a ion-sensitive device. Figure 2 shows the IDS-VDS of the flowFET with DI water, different concentrations of NaCl, CuSO4 and AgNO3. We will describe the theory of operation and sensitivity, selectivity, response of flowFET sensors.