Potentiometric Biosensing of Ascorbic Acid, Uric Acid, and Cysteine in Microliter Volumes Using Miniaturized Nanoporous Gold Electrodes.

Christopher J Freeman,Maryanne M Collinson,Md Shafiul Islam,Borkat Ullah

doi:10.3390/bios11010010

Christopher J Freeman, Maryanne M Collinson + Show 2 more

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https://doi.org/10.3390/bios11010010

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Abstract

Potentiometric redox sensing is a relatively inexpensive and passive approach to evaluate the overall redox state of complex biological and environmental solutions. The ability to make such measurements in ultra-small volumes using high surface area, nanoporous electrodes is of particular importance as such electrodes can improve the rates of electron transfer and reduce the effects of biofouling on the electrochemical signal. This work focuses on the fabrication of miniaturized nanoporous gold (NPG) electrodes with a high surface area and a small footprint for the potentiometric redox sensing of three biologically relevant redox molecules (ascorbic acid, uric acid, and cysteine) in microliter volumes. The NPG electrodes were inexpensively made by attaching a nanoporous gold leaf prepared by dealloying 12K gold in nitric acid to a modified glass capillary (1.5 mm id) and establishing an electrode connection with copper tape. The surface area of the electrodes was ~1.5 cm2, providing a roughness factor of ~16 relative to the geometric area of 0.09 cm2. Scanning electron microscopy confirmed the nanoporous framework. A linear dependence between the open-circuit potential (OCP) and the logarithm of concentration (e.g., Nernstian-like behavior) was obtained for all three redox molecules in 100 μL buffered solutions. As a first step towards understanding a real system, the response associated with changing the concentration of one redox species in the presence of the other two was examined. These results show that at NPG, the redox potential of a solution containing biologically relevant concentrations of ascorbic acid, uric acid, and cysteine is strongly influenced by ascorbic acid. Such information is important for the measurement of redox potentials in complex biological solutions.

Highlights

Potentiometry is an important electroanalytical technique routinely used to measure the concentration of small ions (e.g., H+, K+, F− ) in solution [1]
As a first step toward understanding the potentiometric response in complex solutions and the measurement of mixed potentials, in general, we report on the potentiometric sensing of three small, biologically important redox-active molecules as well as a mixture of all three using nanoporous gold (NPG) electrodes
The results demonstrate that in this mixture of analytes, the experimental open-circuit potential (OCP) is controlled by ascorbic acid, the bioreagent that is more oxidized

Summary

Introduction

Potentiometry is an important electroanalytical technique routinely used to measure the concentration (activity) of small ions (e.g., H+ , K+ , F− ) in solution [1]. Redox potentiometry using metallic redox electrodes, albeit less studied, has been shown to be a useful tool to evaluate the redox properties of complex environmental or biological samples and determine the concentration of small molecules [1,7,8,9]. In this experiment, the open-circuit potential (OCP) or zero-current potential of an indicating, redox electrode (EInd ) (i.e., inert electrode such as gold or platinum) is measured with respect to a reference (Eref ) electrode using a high-impedance voltmeter, such that

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