Ag/AgCl microelectrodes with improved stability for microfluidics

Abstract

A method for fabricating Ag/AgCl planar microelectrodes for microfluidic applications is presented. Micro-reference electrodes enable accurate potentiometric measurements with miniaturized chemical sensors, but such electrodes often exhibit very limited lifetimes. Our goal is to construct Ag/AgCl microelectrodes reliably with improved potential stability that are compatible with surface mounted microfluidic channels. Electrodes with geometric surface areas greater than or equal to 100 μm² were fabricated individually and in an array format by electroplating silver, greater than 1 μm thickness, onto photolithographically patterned thin-film metal electrodes. The surface of the electroplated silver was chemically oxidized to silver chloride to form Ag/AgCl micro-reference electrodes. Characterization results showed that Ag/AgCl microelectrodes produced by this fabrication method exhibit increased stability compared with many devices previously reported. Electrochemical impedance spectroscopy allowed device specific parameters to be extracted from an equivalent circuit model, and these parameters were used to describe the performance of the microelectrodes in a microfluidic channel. Thus, stable Ag/AgCl microelectrodes, fabricated with a combination of photolithographic techniques and electroplating, were demonstrated to have utility for electrochemical analysis within microfluidic systems.

Introduction

While the theory and practice of reference electrodes for chemical analysis have been established for decades [1], [2], [3], there remains much current interest in the practical application of micro-reference electrodes to miniaturized chemical sensors [4], [5], [6], [7], [8]. Particularly desirable are fabrication techniques for Ag/AgCl electrodes, which can be compatible with complementary metal-oxide-semiconductor (CMOS) microelectronic circuit fabrication [9], [10], [11], [12]. Also of interest are electrodes compatible with microfluidics for separations in lab-on-a-chip systems [7], [13], [14].

A vexing problem associated with most reported thin-film Ag/AgCl electrodes has been their poor stability. Open circuit potentials are typically observed to be stable for a time period in the range of a few minutes to a few hours [5], [8], [13], [15]. Instability has generally been attributed to dissolution of the very thin layer (few nm) of sparingly soluble AgCl and the subsequently developed mixed potentials at the electrode/solution interface. Silver chloride has a solubility product constant, K_sp, of 1.8 × 10⁻¹⁰, which implies, from a thermodynamic point of view, that about 1.9 mg of AgCl will dissolve in a liter of water at room temperature. On micro-scale terms, a rectangular piece of silver, 100 μm × 100 μm × 1 μm on a side, could be converted entirely to silver chloride and the resulting AgCl could be dissolved in less than 75 μL of water.

A commonly used method of improving stability has been to coat the Ag/AgCl electrode with gel or polymer materials, such as agar or polyurethane [15], [16]. The coatings create a diffusion barrier to slow down the rate at which the AgCl dissolves and simultaneously provide a relatively constant concentration of chloride ion at the Ag/AgCl surface. A buffer layer of Ni between an adhesion layer of Ti and an Ag top coat was recently reported to improve stability [17]. However, the central problem remains; the AgCl coating will dissolve relatively rapidly. In conventional macro-sized electrodes confined to an isolated solution compartment, a small amount of AgCl dissolution from a large wire actually increases potential stability with fluctuating temperatures by saturating the filling solution with AgCl. Such dissolution is fatal to the performance of thin-film electrodes because the very small quantity of AgCl available may entirely dissolve away.

A simple solution to this problem, which does not require polymer coatings is to increase the amount of silver chloride on the microelectrode surface. The typical methods of forming AgCl on Ag include anodization in chloride containing solutions, chemical oxidation and thermal or plasma treatment in chlorine containing atmospheres [11], [12], [14], [15], [18], [19], [20]. Any of these treatments can produce similar quality AgCl but, applied to a thin layer of silver (∼10² nm thick), they can only result in a thin-film of silver chloride. Therefore, the mass of silver available must be increased. A facile technique to add mass is to electroplate an additional quantity of silver (≈10³ nm) onto evaporated thin-film electrodes of either gold or silver. The electroplated silver forms a quasi-bulk phase, which allows for subsequent formation of a much thicker layer of silver chloride. Furthermore, the electroplated silver has a much rougher surface than the evaporated layer, providing a larger electrochemically active surface area in the same geometric footprint of the original microelectrode. Taken together, the large quantity of AgCl and the increased surface area formed by this technique enable stabilization of Ag/AgCl microelectrodes, as demonstrated in this report.

An additional advantage of electroplating silver onto thin-film electrodes is that individual electrodes in an array may be selected for conversion to Ag/AgCl, while leaving others unaltered. To achieve the same type of configurability using either metal evaporation or electroless deposition would require extra mask steps.

Our objective is to create micrometer-sized quasi-reference electrodes and electrode arrays with improved stabilities and lifetimes, which can be used in microfluidic applications. This work discusses practical aspects of the fabrication and characterization of stable Ag/AgCl microelectrodes, both individually and in an array format. The resultant quasi-reference electrodes were characterized with scanning electron microscopy and open circuit potential (OCP) measurements versus an aged commercially available Ag/AgCl macro-reference electrode. The utility of an array of the Ag/AgCl microelectrodes was demonstrated by monitoring potential in a microfluidic channel with applied electric field. Finally, electrochemical impedance spectroscopy was conducted on the microelectrode array in a microfluidic channel, and equivalent circuit modeling helped identify the sources of impedance in the system.