Abstract
Small; lithographically-defined and closely-spaced metallic features of dimensions and separation in the micrometer range are of strong interest as working and counter electrodes in compact electrochemical sensing devices. Such micro-electrode systems can be integrated with microfluidics and optical biosensors, such as surface plasmon waveguide biosensors, to enable multi-modal sensing strategies. We investigate lithographically-defined gold and platinum micro-electrodes experimentally, via cyclic voltammetry (CV) measurements obtained at various scan rates and concentrations of potassium ferricyanide as the redox species, in potassium nitrate as the supporting electrolyte. The magnitude of the double-layer capacitance is estimated using the voltammograms. Concentration curves for potassium ferricyanide are extracted from our CV measurements as a function of scan rate, and could be used as calibration curves from which an unknown concentration of potassium ferricyanide in the range of 0.5–5 mM can be determined. A blind test was done to confirm the validity of the calibration curve. The diffusion coefficient of potassium ferricyanide is also extracted from our CV measurements by fitting to the Randles–Sevcik equation (D = 4.18 × 10−10 m2/s). Our CV measurements were compared with measurements obtained using macroscopic commercial electrodes, yielding good agreement and verifying that the shape of our CV curves do not depend on micro-electrode geometry (only on area). We also compare our CV measurements with theoretical curves computed using the Butler–Volmer equation, achieving essentially perfect agreement while extracting the rate constant at zero potential for our redox species (ko = 10−6 m/s). Finally, we demonstrate the importance of burn-in to stabilize electrodes from the effects of electromigration and grain reorganization before use in CV measurements, by comparing with results obtained with as-deposited electrodes. Burn-in (or equivalently, annealing) of lithographic microelectrodes before use is of general importance to electrochemical sensing devices
Highlights
This article is an open access articleSmall, closely spaced metallic features of dimensions and separation in the micrometer range are of interest in electrochemical device applications or for integration with optical biosensors, e.g., based on surface plasmon-polaritons (SPPs) [1,2,3]
Motivated by the high sensitivity of Au stripe long-range surface plasmon-polariton (LRSPP) waveguide biosensors [4,5,6,7], along with their suitability for high levels of integration, we investigate microelectrode systems suitable for integration with such optical biosensors, to enable multi-modal sensing strategies, of strong interest in, e.g., disease detection problems
[35] cyclic voltammetry (CV) experiments were carried out over the potential range of 0–0.5 V vs. the Ag/AgCl reference electrode using a triangular waveform at scan rates of 5, 10, 20, 50, and 100 mV/s Applying such a voltage to the system leads to the reversible reaction of potassium ferricyanide (K3 [ Fe(CN )6 ]) to potassium ferrocyanide (K4 [ Fe(CN )6 ]) as a redox couple in a 1-electron transfer process: K4 [ Fe(CN )6 ] ↔ K3 [ Fe(CN )6 ] +e−
Summary
Closely spaced metallic features of dimensions and separation in the micrometer range are of interest in electrochemical device applications or for integration with optical biosensors, e.g., based on surface plasmon-polaritons (SPPs) [1,2,3]. The drive towards compactness and high levels of integration has motivated electrochemical SPR sensing approaches based on Au micro- and nano-structures [16,17]. Other approaches to integrating optical and electrochemical sensing include a goldcoated graded-index optical waveguide sensor, where changes in optical transmittance were measured while cyclic voltammetry was performed in sulfuric or perchloric acid solutions [21]. Motivated by the high sensitivity of Au stripe LRSPP waveguide biosensors [4,5,6,7], along with their suitability for high levels of integration, we investigate microelectrode systems suitable for integration with such optical biosensors, to enable multi-modal sensing strategies, of strong interest in, e.g., disease detection problems. Electrochemical measurements are performed in the form of CV for different scan rates and concentrations of the redox species (potassium ferricyanide). The rate constant at zero potential of our redox species
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