Microjet ring electrode (MJRE): Development, modelling and experimental characterisation

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Abstract

A novel type of hydrodynamic ultramicroelectrode (UME) is described, which employs a ring UME coupled to a high speed perpendicular impinging microjet. Two types of ring UME have been fabricated, both based on an optical fibre, coated in a thin metal film, which is then sealed using either epoxy resin or glass. After polishing, a thin ring UME (≈300–1000 nm) is obtained. When employed in the impinging microjet system, both UMEs show an increase in mass-transport-limited current with flow rate, for simple redox processes such as the reduction of Ru(NH3)63+ or methyl viologen dication in aqueous solution. However, the mass-transport rates observed are significantly lower than predicted by solving the Navier–Stokes and diffusion equations for an idealised coplanar UME. Characterisation of the UMEs with microscopy reveals imperfections on a 10 nm–1 μm length scale which impact mass-transport significantly. When these imperfections are included in the simulations, it is possible to account for the transport-limiting currents observed experimentally. A general implication of the studies in this paper is that even small perturbations in electrode structure can dramatically influence high-speed convective flow across small-scale UMEs, such that thorough geometric characterisation of UMEs employed in fast-flow systems is important.

Introduction

Hydrodynamic electrodes use forced convection of solution to provide well-defined and reproducible mass-transport rates under steady-state conditions [1]. The deployment of ultramicroelectrodes (UMEs) in convective systems has been shown to greatly enhance mass-transport rates, compared to UMEs in quiescent solution, leading to advantages for kinetic and analytical studies [2].

Two classes of hydrodynamic UMEs are generally available. First, those in which solution moves with respect to a stationary electrode, such as the microjet electrode (MJE) (miniaturised wall-tube) [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], the high speed channel electrode [14], [15], [16], [17], [18], [19], and the radial flow microring electrode (RFMRE) [20], [21]. In the second type, the electrode is moved mechanically in the solution, such as with vibrating microband electrodes [22], [23]. Modulated versions of the MJE [24] and RFMRE [25] employ both forced flow and mechanical movement of the electrode.

In the MJE, solution containing the electroactive species of interest is fired at high velocities through a fine capillary nozzle (typically 25–120 μm diameter) usually onto a disc-shaped UME (25 μm diameter or less) [3], [4], [5], [7], [8], [10]. Mercury hemisphere UMEs, which are formed by depositing mercury onto conventional disc UMEs have also been introduced in the MJE arrangement [6]. The well-defined and variable mass transfer rates obtained by using the microjet configuration led to the construction of the RFMRE, where a ring UME was introduced onto the fine capillary nozzle, insulated, and finely polished to produce a hydrodynamic system that yielded mass-transport rates comparable with the high-speed channel electrode [26], [27], [28], [29].

Ring UMEs have interesting mass-transport properties compared to other UME geometries. In particular, as the ring thickness approaches the nanometer scale, high mass-transport rates are predicted [30], [31], [32], [33], [34], [35], [36], [37], [38]. The inner and outer edge produce high current density improving analytical detection limits and allowing the study of fast electrode reactions [20], [21], [39], [40], [41].

Russell and co-workers [42] were among the first to introduce a gold-ring UME to measure heterogeneous rate constants for several fast one-electron-transfer reactions under steady-state diffusion conditions. A hydrodynamic thin-ring UME was used by Symanski and Bruckenstein [43], in which the ring electrode was rotated. However, negative deviations of the experimental mass-transport behaviour from theory were evident, due to difficulties in constructing the electrode coplanar with the insulating material.

In 1990, Kuhn et al. [44] introduced a combination of optical and electrochemical methods, in which an optical fibre was coated with a thin metal and then an insulator to produce a micro-optical ring electrode. This was developed further by both Smyrl et al. [45], [46] for imaging purposes and Boxall and O’Hare [47], [48] for kinetic studies of photochemical processes. More recently, Lee and Bard [49] combined scanning electrochemical microscopy (SECM) and optical microscopy (OM) using a ring UME that acted as both an optical and electrochemical probe for imaging microstructures. When a constant shear-force (tuning fork) mode was introduced, simultaneous topographical, electrochemical, and optical images of an interdigitated array electrode were obtained [49].

To support the practical development of hydrodynamic UMEs, recent theoretical studies [50] have used numerical methods to provide detailed information on mass transport, for realistic cell geometries. These approaches have improved upon analytical expressions [51], [52] which make assumptions that have been shown to unrealistic in practical experimental systems of interest [8], [50].

In this paper we describe simple procedures for making ring UMEs using metal-coated optical fibres, insulated with two different methods. Characterisation of these probes reveals small geometric imperfections which do not significantly affect studies in quiescent solutions, but have a major impact when the electrodes are introduced into a hydrodynamic system such as the impinging microjet. Nonetheless, by characterising these electrodes and developing simulations that reflect the true electrode geometry, it is possible to fully account for the experimental mass-transport behaviour.

Section snippets

Ring electrode fabrication

The methods for fabricating the ring UMEs involved sputter-coating gold onto an optical fibre, and introducing a surrounding insulating sheath, fabricated from epoxy resin or glass. First, an all-silica optical fibre (F-MCC-T, core diameter 200 ± 5 μm, Newport Corp., US) was stripped from its polyimide coating and was sputter-coated using an Edwards E306 vacuum evaporator (Moorfield, UK) fitted with a minibox conversion to configure the system into a true multitechnique vacuum deposition system.

Simulations

The finite element method [54] was used to simulate velocity profiles in real space as a consequence of flow from a microcapillary nozzle impinging on a finite solid ring electrode. The incompressible Navier–Stokes equations [55] for momentum balance (Eq. 1) and continuity (Eq. 2) were solved in axisymmetric cylindrical coordinates (under steady-state conditions).ρV·V=-p+η2V·V=0where ρ is the density of water (1.00 g cm−3 [56]), V is the velocity vector (with components u and v in the r and z

Voltammetry at ring UMEs in quiescent solution

Fig. 8 shows typical cyclic voltammograms for the reduction of 1 mM Ru(NH3)63+ at (a) an epoxy-sealed electrode; and (b) a glass-sealed Au sputtered fibre. The electrode comprised the same fibre dimensions but different ring thicknesses, as a longer sputtering time and higher power was used for the epoxy-coated electrode. Furthermore, the epoxy-sealed electrode included a Cr underlayer (∼50 Å) used to promote better adhesion between the gold and the plasma cleaned quartz fibre.

The voltammetry of

Conclusions

This paper has considered the deployment of a thin ring UME in an impinging jet system for the first time. The inner diameter of the ring electrode is about twice that of the nozzle from which solution flows, so as to optimise mass transport [4], [8]. Simulations of mass transport, by solving the Navier–Stokes and diffusion equations, have revealed that mass transport to coplanar electrodes should be greatly enhanced in this configuration. However, smaller enhancements were observed

Acknowledgements

This research was supported by the EPSRC (E.B.) and University of Warwick Postgraduate Research Fellowship Scheme (N.C.R.). We thank Dr. Neil R. Wilson and Sophie Martin for producing the AFM images. We would also like to express our appreciation to Dr. Julie Macpherson (University of Warwick), Mr. Martin Edwards (University of Warwick), and Dr. Sabine Szunerits (Domaine Universitaire, Grenoble) for helpful discussions and advice.

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