An electro-osmotic micro-pump based on monolithic silica for micro-flow analyses and electro-sprays

A high-pressure electro-osmotic micro-pump fabricated by a sol–gel process is reported as a fluid-driving unit in a flow-injection analysis (FIA) system. The micro FIA system consists of a monolithic micro-pump on a glass slide (2.5×7.5 cm), a micro-injector, and a micro-sensor (2.5×1.5 cm). The monolithic silica matrix has a continuous skeleton morphology with micrometer-sized through-pores. The micrometer-size pores with a large negative surface charge density build up a large pressure under a DC electric field to drive fluid through the downstream units. A novel Nafion joint for the downstream cathode eliminates flow into the electrode reservoir and further enhances pressure build-up. The measured pump-pressure curve indicated a maximum pressure of 0.4 MPa at flow rate of 0.4 L min^–1 at 6 kV. Despite the large voltage, the small current transmission area through the monolith produced a negligible current (less than 100 A) that did not generate bubbles or ion contaminants. The flow rate can be precisely controlled in the range 200 nL to 2.5 L min^–1 by varying the voltage from 1 to 6 kV. The high pump pressure and the large current-free DC field also enabled the pump to act as an electro-spray interface with a downstream analytical instrument.