A. Wang, D. Koh, P. Schneider, K.W. Oh
University at Buffalo,
Keywords: electrochemical impedance spectroscopy
Summary:Abstract In this paper, a 3-D electrode configuration for electrochemical impedance spectroscopy (EIS) of a bulk solution was studied. Top- and bottom- electrode configuration, where two electrodes sandwich the sensing channel, was fabricated using the tearing the patterns from the PDMS stamp to glass substrate approach  and implementation of glass via. The device with 100 μm height sensing channel was studied with DI water and KCL (concentration of 10-3, 10-2, 10-1 M) for verifying the sensing performance. Details One of the most widely used evaluation methods of the biomolecular and cellular analysis is EIS because of its label free, non-invasive and high throughput features. The setup of electrode configuration is one of the keys to determine the sensitivity. A previous study  showed top- and bottom- electrodes have a high detection sensitivity (9.18-10-7 -1 (cells/L)) compared to co-planar electrode designs. However, few papers were reported because of the difficulty in fabrication process including the creation of the microfluidic channel membrane and establishing contact to the embedded electrode. In order to overcome these challenges, tearing the patterns from the PDMS stamp to glass substrate approach  and glass via were implemented (Fig. 1). To make the sensing channel, oxygen plasma bonded PDMS slab was teared off from the glass slide. This residue served as a sensing channel wall (Fig. 2). To fabricate the glass via, 1 mm diameter holes were drilled into the slide glasses. To cover the holes, metal was sputtered onto the front and back side allowing contact embedded electrode from backside of glass slide. Then, uncured PDMS was poured to seal the glass hole. Evaluation of the glass via was conducted using impedance analyzer in the frequency range from 40 Hz to 110 MHz, with 20 mV AC signal. As shown in Fig. 3, impedance amplitude of the glass via was lower than 100 , which was durable for impedance measurement. Next, impedance amplitude and phase for electrolyte solution was measured, where equivalent circuit model  is shown in Fig. 4. The measurement results are shown in Fig. 5. For DI water in the low frequency (below 1 kHz), double layer capacitance is dominant, while in the frequency range from 1 kHz to 100 kHz, impedance amplitudes are stable which indicates solution resistance. Above 100 kHz, dielectric capacitance of the solution becomes dominant. In the very high frequency region > 100 MHz, impedance amplitude become same order with metal, resulting in the same behavior with the glass via. Finally, linearity of solution resistance was studied against electrolyte concentration (Fig. 6). References  Lee, Hun, et al. Micromachines 7.10 (2016): 173.  Cheng, Xuanhong, et al Lab on a Chip 7.6 (2007): 746-755.  Holmes, David, et al. Lab on a Chip 9.20 (2009): 2881-2889.