A Polydimethylsiloxane Electrophoresis Microchip For Contactless Conductivity Detection

Contactless conductivity detection (CCD) is a simple and universal detection method, and has been applied more and more in electrophoresis microchips. In order to achieve the contactless mode on microchips, the detecting electrode is isolated from the solution by an insulating layer. It has been proved that the thickness of this insulating layer was one of the important factors that affected the sensitivity of the CCD, and methods for decreasing the thickness of the insulating layer have been widely studied in this field.A polydimethylsiloxane (PDMS) electrophoresis microchip with a thickness controllable insulating layer is presented. The microchip consists of a glass slide with integrated Pt microelectrodes, a PDMS insulating layer and a PDMS channel plate (containing the microchannel). First, Pt microelectrodes were fabricated on a glass slide by using the "lift-off" process; second, a PDMS film was spin-coated on the electrode plate to be the insulating layer; third, the PDMS channel plate was fabricated by the cast molding technique; finally, the PDMS channel plate and the PDMS film were permanently bonded together after the oxygen plasma surface treatment. The thickness of the PDMS film can be precisely controlled down to submicrometers by using spin-coating techniques.The spin coating technique of PDMS films was studied. To decrease the thickness of PDMS films, PDMS prepolymer was diluted by toluene. The effects of the ratio of dilution and the spin-coating speed on the thickness of the PDMS film were studied, and a wide range of thicknesses (60μm-0.6μm) was achieved. Furthermore, in most cases, the relative standard deviations (RSDs) of the thickness of different PDMS films were within5%, which makes the thickness of the insulating layer precisely controllable, and thus the whole microchip repeatable.With the microchip with a0.6μm-thick insulating layer, the limit of detection (LOD), separation efficiencies and reproducibilities were studied. First, the excitation frequency and voltage of CCD were optimized, and the optimized excitation frequency and voltage were120kHz and30Vpp (peak-to-peak), respectively. Second, under the optimized excitation frequency and voltage, a LOD of0.07μM for Na+was obtained, which is lower than all previous results obtained for microchip CCD. To further demonstrate the effect of the thickness of the insulating layer on the sensitivity of the CCD, another two microchips with the same design but different insulating layer thicknesses (50μm and15μm) were fabricated and tested. The LODs for Na+were3μM and1μM respectively, which are almost two orders of magnitude higher.Third, the separation efficiency of the microchip was evaluated, and theoretical plates of49000and41000plates/m for K+and Na+were obtained. These values are greater than those results reported by using hybrid chips. Finally, the excellent reproducibilities both from a single microchip and from different microchips were also demonstrated. RSDs of migration time for K+and Na+were lower than1%, and RSDs of peak height for K+and Na+were lower than3%.