Metalless electrodes for capacitively coupled contactless conductivity detection on electrophoresis microchips

Abstract

This paper describes the use of ionic solutions as sensing electrodes for capacitively coupled contactless conductivity detection on electrophoresis microchips. Initially, two channels were engraved in a PMMA holder by using a CO2 laser system and sealed with a thin adhesive membrane. PDMS electrophoresis chips were fabricated by soft lithography and reversibly sealed against the polymer membrane. Different ionic solutions were investigated as metalless electrodes. The electrode channels were filled with KCl solutions prepared in conductivity values from approximately 10 to 40 S/m. The best analytical response was achieved using the KCl solution with 21.9 S/m conductivity (2 mol/L). Besides KCl, we also tested NaCl and LiCl solutions for actuating as detection electrodes. Taking into account the same electrolyte concentration (2 mol/L), the best response was recorded with KCl solution due to its higher ionic conductivity. The optimum operating frequency (400 kHz) and the best sensing electrode (2 mol/L KCl) were used to monitor electrophoretic separations of a mixture containing K(+) , Na(+) , and Li(+) . The use of liquid solutions as sensing electrodes for capacitively coupled contactless conductivity detection measurements has revealed great performance to monitor separations on chip-based devices, avoiding complicated fabrication schemes to include metal deposition and encapsulation of electrodes. The LOD values were estimated to be 28, 40, and 58 μmol/L for K(+) , Na(+) , and Li(+) , respectively, what is comparable to that of conventional metal electrodes. When compared to the use metal electrodes, the proposed approach offers advantages regarding the easiness of fabrication, simplicity, and lower cost per device.

Hand drawing of pencil electrodes on paper platforms for contactless conductivity detection of inorganic cations in human tear samples using electrophoresis chips

“…In the proposed approach, we have made use of just paper and pencil to produce sensing electrodes for C 4 D. When compared to metal (Au, Pt, Cu, Al, etc.) electrodes fabricated by sputtering , polymeric electrodes prepared by laser cutting or even low cost electrodes produced with metal adhesive tapes , aluminum foil , printed circuit boards , alloys , and electrolytic solutions , the production of PDE is simpler, faster, and cheaper. Furthermore, the proposed electrodes for C 4 D can be drawn in different geometries.…”

Application of capacitively coupled contactless conductivity as an external detector for zone electrophoresis in poly(dimethylsiloxane) chips

“…In order to obtain better separation resolution the detection gap of C 4 D should be shortened. The detection gap of the applied C 4 D was 500 μm (the usual gap sizes are ranged from 500 μm to 2 mm, ). For the ∼100 μm length plug of sample (typical for chip electrophoresis, where the effective length is 6–10 cm), the optimal detection gap could be around 20–100 μm.…”

“…Nevertheless, a large number of lab‐made microchips are manufactured from PDMS because this material is a flexible, inexpensive, optically transparent polymer for optical detection, offering a relative ease of fabrication of a complex microfluidic system without sophisticated instrumentation . However, only a few papers could be found about the use of a PDMS chip with C 4 D detection. Guijt reported a reusable, reversibly bonded PDMS‐hybrid device using a dry photoresist insulating layer with the electrodes prepared by printed circuit board technology .…”

“…Ding and Rogers used PDMS chips with attached triangle‐shaped electrodes made from aluminium foil strips for the detection of haloacetic acids in water . Very recently, different ionic solutions have been applied in PMMA/PDMS chip as metalless sensing electrodes for C 4 D or by simply adding extra “electrode” channels to the microchip design, low melting point metals (e.g. Ga) are injected to a channel serving as electrodes .…”

Contactless conductivity detection for analytical techniques— Developments from 2014 to 2016

“…In previous microchip designs, metallic C 4 D electrodes were usually embedded into the microchannel structure and were covered by a thin layer of insulating material for good transmission of the ac signal from the function generator into the separation channel and from the separation channel into the current‐to‐voltage processing circuitry. Coltro and coworkers demonstrated that the material for C 4 D electrodes can be formed by conductive solutions embedded directly underneath the separation microchannel. Two electrode channels (1 × 1 mm) were engraved in a PMMA substrate, sealed with a thin adhesive membrane (40 μm) and bonded with a lithographically fabricated PDMS microchip.…”