A Paper-Based Reagent Storage Approach for Point-of-Care Electrochemical Sensing
We fabricated the three-electrode electrochemical sensor on a flexible polyethylene terephthalate (PET) film by evaporation of metal (Au or Cu) followed by standard photolithography and wet etching. Our electrochemical sensor consists of two shorted interdigitated array (IDA) working electrode (WE) with finger width/spacing of 10µm/5µm, an auxiliary electrode (AE) and a reference electrode (RE) (Fig. 1a). We arranged the electrode layout into USB style for convenient connection with a potentiostat. We fabricated paper-based reagent-storage pads by shaping Whatman No.1 filter paper with a paper puncher or a pair of scissors. We designed paper pads with a size of 12mm×10mm to cover the entire area of the three electrodes (Fig. 1b).
We first demonstrate cyclic voltammetry by rehydrating a paper pad containing dry ferricyanide on gold IDA sensor. A 10mM ferricyanide solution was first dried on a paper pad. After contacting with the sensor surface, 50µL DI water was dispensed on the pad followed by cyclic voltammetry (CV) scanning from -0.5V to 0.3V with a scan rate of 100mV/s. As shown in Fig. 2a, the CV spectra exhibites two redox peaks as expected, while the CV spectra using a control paper pad without ferricyanide is flat. This indicates that the dry-form of ferricyanide in the paper pad can be dissolved into aqueous solution during re-hydration process supporting electrochemical reaction on the sensor surface.
The paper-based dry-reagent platform can be used in anodic stripping voltammetry (ASV) for detection of lead (Pd) . Reagents containing pH 5.5 acetate buffer with 50, 100 and 200ppb Pd were dispensed and dried on three different paper pads. After aligning a paper pad with a Cu-based sensor , 50µL liquid sample was introduced on the paper pad followed by ASV scanning. The experimental parameters were selected after optimization: -0.8 V and 300 s for preconcentration; 50 ms period, 50 mV amplitude, and 8 mV increment for square wave. As shown in Fig. 2b, the peak amplitude of ASV spectra increases with the concentration of Pd within the paper pad.
This approach can provide supporting electrolyte for electrochemical impedimetric sensing. We first dried 10mM ferricyanide on a paper pad. After aligning the paper pad with a gold IDA sensor and rehydrating with PBS, electrochemical impedance scanning was conducted from 0.1Hz to 1MHz under a DC bias of -0.15V with an AC bias of 10mV. The Nyquist plot (Fig. 2c) appears as a semicircle indicating the background charge transfer resistance through bare Au electrode surface to be ~4000ohm. This result suggests the paper pad with dry ferricyanide can provide Fe2+/Fe3+to the sensor surface for electrochemical impedance spectroscopy without disrupting the measurement.
In conclusion, we developed a paper-based platform capable of storing dry reagents for electrochemical sensing applications. Paper pads containing dry reagents can be easily re-hydrated to provide adequate aqueous environment for different types of electrochemical sensing techniques. We envision this platform can be valuable for low-cost, point-of-care electrochemical sensing applications.
 W. Kang, X. Pei, A. Bange, E. Haynes, W. R. Heineman, and I. Papautsky, Anal. Chem. 2014, in press.
 X. Pei, W. Kang, W. Yue, A. Bange, W. R. Heineman, and I. Papautsky, Anal. Chem. 2014, 86, 4893-4900.
This work was supported in part by funds provided by the NIEHS award R01ES022933.
Fig. 1 (a) Photography of IDA sensor fabricated on a flexible PET film. (b) Aligning a paper pad containing dry reagent on sensor surface. (c) Conventional way of dispensing samples for EC detection. (d) Operation of dry-reagent pad including (1) dispensing reagent on a paper pad, (2) drying reagent, (3) Aligning reagent pad with sensor, and (4) rehydrate dry pad with aqueous sample for EC measurements.