Microfluidic Three-Electrode Cell Array for Low-Current Electrochemical Detection

Abstract

This paper reports the implementation and calibration of a microscopic three-electrode electrochemical sensor integrated with a polydimethylsiloxane (PDMS) microchannel to form a rapid prototype chip technology that is used to develop sensing modules for biomolecular signals. The microfluidic/microelectronic fabrication process yields identical, highly uniform, and geometrically well-defined microelectrodes embedded in a microchannel network. Each three-microelectrode system consists of a Au working electrode with a nominal surface area of 9 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma-treated Ag/AgCl reference electrode, and a Au counter electrode. The patterned electrodes on the glass substrate are aligned and irreversibly bonded with a PDMS microchannel network giving a channel volume of 72 nL. The electrokinetic properties and the diffusion profile of the microchannels are investigated under electrokinetic flow and pressure-driven flow conditions. Cyclic voltammetry of 10 mM K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 </sub> Fe(CN) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> in 1 M KNO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> demonstrates that the electrode responses in the cell are characterized by linear diffusion. The voltammograms show that the system is a quasi-reversible redox process, with heterogeneous rate constants ranging from 3.11 to 4.94times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> cm/s for scan rates of 0.1-1 V/s. The current response in the cell is affected by the adsorption of the electroactive species on the electrode surface. In a low-current DNA hybridization detection experiment, the electrode cell is modified with single-stranded thiolated DNA. The electrocatalytic reduction of 27 muM Ru(NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> in a solution containing 2 mM Fe(CN) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-</sup> is measured before and after the exposure of the electrode cell to a 500-nM target DNA sample. The preliminary result showing an increase in the peak current response demonstrates the hybridization-based detection of a complementary target DNA sequence