The Effect of Repeated Activation on Screen-Printed Carbon Electrode Cards
Authors: Martin Candace , Grgicak Catherine 
Published: February 2020

Screen-printed carbon electrode cards have been recognized as an inexpensive and disposable sensing option with wide application. These cards are often activated to enhance the electron transfer kinetics of the working electrode surface, which typically involves holding the electrode at a high positive potential for a short period of time. This step purportedly increases the hydrophilicity (1, 2), amount of carbon-oxygen functional groups (3-5), and roughness of the electrode surface, and further removes possible surface contaminants (4, 5, 6). Signal magnitude and reversibility are improved because there is an increase in the amount of active sites on the electrode (7, 6). Despite their disposability, laboratories with high sample throughput may desire repeated use from each card.
This study examined the ability of a commercial brand of cards to be reused after activation using cyclic voltammetry (CV) and the reversible redox couple, hexaammine ruthenium. Four cards were activated in 0.05 M PBS at +1.7 V for 1 min. Cyclic voltammograms were recorded with these cards in 0.0018 M hexaammine ruthenium (in 0.02 M Tris-HCl) from -0.5 V to +0.3 V at a rate of 0.05 V/s for 10 cycles, and on four additional cards that were not activated. This process was repeated on one card from each set an additional three times over consecutive days. The card reused from the activated group was activated during each repetition.
Activation, which improves electrode performance, was shown to affect the shape of the CV curves, the transfer coefficient (α), and the real electro-active surface area. Figure 1 illustrates an increase in current with consecutive activation of the electrode card (cathodic ip RSD = 4.1%). This phenomenon is not observed on the card that was used repetitively for measuring the redox couple without the activation step (cathodic ip RSD = 1.7%, data not shown). For these carbon SPE cards the transfer coefficient is approximately 0.8 for non-activated cards, which remains stable upon reuse, and approximately 0.7 for activated cards. The transfer coefficient then decreases upon successive activation processes (data not shown). The calculated surface area is fairly consistent for the non-activated data and the between-electrodes activated data, while the within-electrode activated data exhibit an upward trend, as evident in Figure 2 (using α = 0.5, RSD = 4.1%; using experimental α, RSD = 13.6%).
A majority of the estimates of real surface area are less than the nominal surface area of 0.0314 cm2. This result would support the theory that some organic binder used in the printing process is left behind and effectively ‘insulates’ the graphite from electron transfer. The increase in calculated area observed with the reused, activated card could be explained by the removal of binder and exposure of additional electro-active sites on the graphite particles through repetitive activation. While it may be desirable to obtain a transfer coefficient closer to the theoretical 0.5 and pre-processing by repeated activation may achieve this goal, the confounding variable of increasing background current can cause issues during analysis of the peak signal. As a result, it is not recommended to reuse carbon SPE cards if repeated activation is part of the processing scheme.
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