MP33:Liquid Electrodes for Contactless Conductivity Detection in Microchip

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Liquid Electrodes for Contactless Conductivity Detection in Microchip

Thiago P. Segato
University of São Paulo
São Carlos, SP Brazil

Renato S. Lima, Nathan V. de Castro, José A. F. da Silva, Emanuel Carrilho

The aim of this research was to integrate the Ga-In eutectic (EGaIn, > 99.99%) as electrodes for contactless conductivity detection (C4D) in microchip, obtaining an innovative platform to replace the use of irreproducible tapes, or expensive vapor-deposited films as electrodes for analysis in miniaturized systems with C4D. Conversely, the proposed platform provides inherent advantages of the two methods mentioned, namely: (i) ease of microfabrication (present in cases that use tapes as electrodes), and (ii) satisfactory reproducibility (obtained when vapor deposited films are used as electrodes). Microdevices used in this study consisted of two layers of PDMS with channels fabricated by soft lithography. The channels for the electrodes were filled with EGaIn using a simple syringe, resulting in a continuous filling of channels by the liquid metal. Analytical signals associated to a KCl solution (deionized water was used as blank) were monitored under a flow analysis procedure. Analysis were conducted by simply switching between two syringe-pumps containing KCl solution and deionized water, both operating at a flow rate of 100 μL min-1. The first study was performed to select the optimal excitation signal parameters (frequency and peak-to-peak amplitude) for detection in two different designs (aligned and non-aligned electrodes). The best results were obtained when high amplitude potential (10 V) and low frequencies (50 and 40 kHz, for the aligned and non-aligned designs, respectively) were chosen. These parameters were used to perform the sensitivity studies. Measurements performed under flow analysis using different concentration of KCl solutions provided calibration curves and the limits of detection (LOD) for both electrode configurations. The following experimental conditions were adopted: 100 μL min-1 flow rates with 15 s sampling time. The values of LOD for the aligned and non-aligned designs, calculated using parameters of standard curves, were 3230.0 and 20.7 μmol L-1. The non-aligned electrodes showed significantly lower LODs (about three orders of magnitude) compared to those obtained with the aligned electrodes, which can be attributed to the lower stray capacitance between the detection electrodes. Additional experiments are being carried out to predict the influent of the position of one electrode relative to the other, and the distance between the solution in the analytical channel and the electrode. The new fabrication procedure proposed here provides a simple alternative to integrate the detection electrodes to microchip-C4D in a reproducible and inexpensive fashion.

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