A microchip electrophoresis system with integrated in-plane electrodes for contactless conductivity detection

Abstract

We present a new approach for contactless conductivity detection for microchip-based capillary electrophoresis (CE). The detector integrates easily with well-known microfabrication techniques for glass-based microfluidic devices. Platinum electrodes are structured in recesses in-plane with the microchannel network after glass etching, which allows precise positioning and batch fabrication of the electrodes. A thin glass wall of 10-15 microm separates the electrodes and the buffer electrolyte in the separation channel to achieve the electrical insulation necessary for contactless operation. The effective separation length is 34 mm, with a channel width of 50 microm and depth of 12 microm. Microchip CE devices with conductivity detection were characterized in terms of sensitivity and linearity of response, and were tested using samples containing up to three small cations. The limit of detection for K+ (18 microM) is good, though an order of magnitude higher than for comparable capillary-based systems and one recently reported example of contactless conductivity on chip. However, an integrated field-amplified stacking step could be employed prior to CE to preconcentrate the sample ions by a factor of four.

A three-layer PMMA electrophoresis microchip with Pt microelectrodes insulated by a thin film for contactless conductivity detection

“…Lichtenberg structured Pt microelectrodes into a recess, and a 10 mm-thick glass wall isolated the recess from the separation channel. 12 Although these two methods can be used to achieve very thin insulating layers, the fabrication of microchips is complicated.…”

Thinly Insulated Wire Cells – A New Device for Sensitive Contactless Conductivity Detection in Flow Analyses

“…All the cell designs employ tubular or semitubular electrodes placed around the capillaries, or planar electrodes created on the chips and thus the thickness of the dielectric -the wall of the capillary or the polymer lid covering the separation channel -is quite large, several hundred of mm; only in special designs for chip electrophoresis (see, e.g., [6]) is the dielectric thickness smaller, several tens of mm. This, of course, places limitations on the measuring sensitivity and the limits of detection attained.…”

Lab-on-a-Chip Microdevice with Contactless Conductivity Detector

“…There are many possible implementations of non-contact measurement of conductivity, mainly due to the way of the location of the electrodes relative to the channel. The electrodes of a contactless detector may be placed under a microchannel [26], around a microchannel [27] and in the plane of a microchannel [28]. Also interesting is the variety of materials used to construct the electrodes.…”