Effect of the Geometry on the Mass Transfer Enhancement at Conical W/WO₂ Ultramicroelectrodes

Electrodes with at least one critical dimension lower than 25 µm, also known as ultramicroelectrodes (UMEs), have been used in several different applications over the last four decades, ranging from probing chemical homogeneous and heterogeneous reactions [1,2] to high resolution imaging [3]. This is due to the unique characteristics of UMEs, which include high rate of mass transfer, low capacitive and resistive effects, and low time constant [4,5]. Due to these properties, scientists are continuously trying to unravel the fundamentals that govern the kinetics and the thermodynamics of electrochemical reactions happening at UMEs. Conical UMEs with high aspect ratio are of especial interest because they possess geometrical features that may allow the study of electrochemical reactions occurring in the internal compartments of living cells and organelles. However, there is a lack of experimental studies with individual unshielded conical electrodes aiming at quantifying the impact of the geometry and dimensions on their electrochemical response. In this work, W / WO2 conical UMEs with aspect ratios ranging from 6.6 to 22 and apexes with nm-size dimensions were fabricated by electrochemical etching of tungsten wires through an induced dynamic meniscus regime[6]. An example is shown in Figure 1a. The electrodes were characterized by scanning electron microscopy and by cyclic voltammetry in 5 mM [Fe (CN)6]3- / 5 mM [Fe (CN)6]4- in 0.5 M KCl as a function of the depth of the UME immersed in the solution (Figure 1b). Computational fluid dynamics simulations were used to investigate the mass transfer of the electroactive species at the vicinity of the electrodes (Inset Figure 1c). Analytical expressions to predict the steady-state current of conical electrodes with aspect ratios from 3 to 22 and radius of curvature below 110 nm were also derived. Experiments showed that the ratio electrochemical surface area / geometric area rapidly increases when the depth of the UME’s in solution is lower than 15 μm, in agreement with a rapid increase of the magnitude of the total flux towards the UMEs apex (Figure 1c). Both experimental and simulation studies point to the radius of curvature as the most important parameter determining the rate of the oxidation / reduction of the [Fe (CN)6]3-/ [Fe (CN)6]4- species at non-insulated conical UMEs with high aspect ratio[6]. References [1] M.M. Collinson, P.J. Zambrano, H. Wang, J.S. Taussig, Langmuir 15 (1999) 662–668. [2] R.M. Wightman, Anal. Chem. 53 (1981) 1125A-1134A. [3] T.E. Lin, S. Rapino, H.H. Girault, A. Lesch, Chem. Sci. 9 (2018) 4546–4554. [4] S.M. Oja, Y. Fan, C.M. Armstrong, P. Defnet, B. Zhang, Anal. Chem. 88 (2016) 414–430. [5] C.G. Zoski, M. V. Mirkin, Anal. Chem. 74 (2002) 1986–1992. [6] U. Bruno-Mota, I.N. Rodriguez-Hernández, R. Doostkam, P. Soucy, F. Navarro-Pardo, G. Orozco, A. Yurtsever, A.C. Tavares, Electrochim. Acta 402 (2021) 139524. Figure 1