Fabrication of an Induction Heated Gold Microelectrode for SECM Applications
Authors: Dowell Timothy J , Wipf David O 
Published: February 2020

The field of electrochemistry has traditionally focused its efforts on observing and manipulating the variables of current, voltage, concentration and time as means of probing systems of interest. However, over the course of the past several decades implementation of additional parameters has resulted in the development of several new fields including thermo-, photo-, and triboelectrochemistry. Thermoelectrochemistry in particular has attracted much attention since arbitrary temperature changes can provide several key advantages over other methods. First, thermal convection leads to a micro stirring effect which in turn improves the mass transport in solution. Second, reaction kinetics are accelerated. Third, electrode surfaces can be thermally regenerated. Fourth, temperature dependant potential shifts are unique to every species which allows separation of similar analytes thereby enhancing selectivity. Lastly, the combination of these properties leads to a greatly increased signal-to-noise ratio.1

However, using a thermostated (isothermal) cell incurs problems such as destroying thermally unstable compounds, volatilizing components, and shifts in reference electrode potentials. These drawbacks can be avoided with use of electrically heated electrodes (non-isothermal) pioneered by Gründler.2 Briefly, by applying short heating pulses with a high voltage AC frequency to a wire electrode heating is limited to the solution layer directly adjacent to the electrode surface. Therefore, a single cell can house all components of the device without problems. However, these devices are shape- and size-limited, typically employing joule heating of cylindrical wires with a diameter of 25 μm.

Microelectrodes are characterized by small dimensions, relatively large diffusion areas, fast steady state achievement, and small voltage drops at fast scanning rates. These features have allowed microelectrodes to have diverse applications such as studies on corrosion, biological/cell chemistry, and kinetic studies at numerous interfaces. Microelectrodes can be used alone or incorporated into scanning electrochemical microscopes (SECM) to measure localized electrochemical systems at the liquid-liquid, liquid-gas, and liquid-solid interface.3 Fabrication of heated disk microelectrodes applicable in scanning probe microscopy techniques are limited  to those developed by Baranski.4 Incorporation of a heated microelectrode into a SECM arrangement has been described as Hot-Tip SECM or HT-SECM by Boika.5

Our goal is to develop heated-tip microelectrodes compatible with SECM and other SPM imaging. We report here the fabrication of a novel induction heated gold coated steel microelectrode having a diameter of ≥ 1 μm. Briefly, a small induction coil is placed inside the electrode housing and around the steel electrode body (work piece). The induction coil is connected in parallel with a capacitor and transformer to form an LC-tank resonator. The tank is excited by sine wave (Vrms = 2.0, frequency = 500 kHz-1 MHz) to generate a heating pulse. The heating characteristics of the electrode have been evaluated by cyclic voltammetry (CV), temperature pulse voltammetry (TPV), and open circuit potentials (OCP) using the [Fe(CN)6]3−/ [Fe(CN)6]4−and other redox couples. The electrode appears capable of rapidly heating aqueous solutions to temperatures near the boiling point. Overall the device and system represent a comparatively cost effective and simple approach to prepare heated microelectrodes.

[1] Gründler, P. In-Situ Thermoelectrochemistry: Working With Heated Electrodes; Springer: 2015

[2] Gründler, P.; Zerihun T.; Kirbs A.; Grabow H. Anal. Chim. Acta. 1995, 305, 232-240

[3] Bard, A. J. Scanning Electrochemical microscopy; Mirkin, M. V., Ed.;  Marcel Dekker: 2001

[4] Baranski, A. S. Anal. Chem. 2002, 74, 1294−1301

[5] Boika, A.  PhD. Thesis, University of Saskatchewan, 2010