Reproducible Fabrication Method for Micrometer-Sized Au Disk UMEs

Jayol Kim; Taek Dong Chung

doi:10.33961/jecst.2025.00045

Abstract

The reproducible fabrication of Au disk ultramicroelectrode (UME) is a significant challenge. Particularly, for Au UMEs with sizes in the range of several micrometers, the common fabrication method involves inserting a gold wire into a glass capillary and pulling it with annealing. However, this method suffers from poor reproducibility. Herein, we propose a novel method for fabricating Au UMEs by first creating a glass tip using a puller, then producing a hollow carbon UME through pyrolysis, electroplating gold onto it, and finally milling it using focused ion beam (FIB). This method not only allows the electrode area to be adjusted to the desired size using FIB but also enables control over the RG ratio by modifying the glass capillary fabrication process with the puller.

Keywords: Gold ultramicroelectrode, Carbon pyrolysis, Electrochemical gold deposition, Reproducible, Micrometer-size

INTRODUCTION

UME refers to an electrode with at least one dimension smaller than 25 μm [1]. UME exhibits several advantages as an electrochemical tool, distinct from conventional electrodes. Key advantages include high spatial resolution resulting from its small size [2] and unique electrochemical characteristics not commonly observed in macroelectrodes. Specifically, UME offers high sensitivity due to low ohmic drop, minimal double-layer charging, and high mass transport rates [3]. These advantages make UME widely applicable in various fields, such as imaging and measurements using scanning electrochemical microscopy (SECM) [4–8], analyzing the electrochemical behavior of single molecules or nanoparticles [9–11], investigating chemical substances within cells [12,13], studying electron transfer kinetics [14,15], and utilizing nano- and microbiosensors [16].

Gold (Au) is widely used as an electrode material due to its high stability in body fluids [17] and the ease of surface modification through Au-thiol interactions [18,19], making it highly suitable for electrochemical analysis. Most micro-sized Au UMEs are fabricated using micron-level Au wires and laser-based micropullers [20]. However, the significant difference in melting points between Au (1064°C) and quartz (1710°C) or borosilicate glass (1640°C) makes it challenging to fabricate Au UMEs with precise dimensions. As an alternative, methods involving Au electroplating to produce small-sized Au UMEs have been proposed. For instance, Lai et al. fabricated nanometer-sized Au UMEs by electroplating Au onto recessed Pt UMEs, which were prepared by electrochemically etching nanometer-sized Pt UMEs fabricated with a laser puller [21]. In another approach, Zhang et al. used laser-pulled quartz nanopipette as templates. The taper side of fabricated nanopipette was immersed in a liquid gallium-indium alloy electrode, onto which Au was deposited. The resulting Au nanoelectrodes were electrically connected using conducting polymers, enabling their use as nanometer-sized Au UMEs [22]. However, these Au deposition methods involve extremely small-scale contacts, leading to significantly prolonged plating times. This issue becomes even more pronounced when fabricating micro-scale electrodes. To address this, Sen et al. proposed a method using carbon fibers. Electrically connected carbon fibers were heated, elongated, and then inserted into glass nanopipettes for subsequent Au plating, resulting in Au micro UMEs [23]. While this approach allows for the fabrication of various forms of Au UMEs depending on the carbon fiber preparation and plating methods, a reproducible process for fabricating micro-scale Au disk UMEs with precise dimensions has not yet been well established.

In this paper, we introduce a reproducible method for fabricating micro-sized Au disk UMEs through a novel approach utilizing hollowed carbon UME. We first fabricate a carbon UME by first producing a quartz glass nanopipette using a laser puller, then injecting butane gas into the pipette and carrying out pyrolysis. By modifying the section where heating was applied, we obtained a hollowed carbon UME with an empty tapered region at the front of glass nanopipette. Then, using FIB to form the access point to the hollowed carbon UME and introduced a Au plating solution into the empty space for Au deposition. Finally, the resulting Au UME was cut again with FIB to achieve the desired size, thereby reproducibly fabricating a micro-scale Au disk UME.

EXPERIMENTAL

Materials

Quartz glass capillary with filaments (O.D./I.D.: 1/0.5 mm and O.D./I.D.: 1.2/0.9 mm) was purchased from Sutter Instruments (USA), chloroauric acid (HAuCl₄·xH₂O) (Sigma Aldrich, USA), sulfuric acid (1N H₂SO₄) (Samchun, Korea), Potassium hexacyanoferrate(II) trihydrate (K₄Fe(CN)₆·3H₂O) (Sigma Aldrich, USA), Potassium nitrate (KNO₃) (Sigma Aldrich, USA) were of reagent grade quality or better and used without further purification. The carbon powder (Sigma Aldrich, USA) used to contact with a Cu-wire (0.3 mm diameter) was purchased from Goodfellow (England).

Apparatus

A laser puller (P-2000; Sutter Instruments, USA) was used for the pulling of the glass nanopipette. Electrode-position and cyclic voltammetry (CV) was performed using a potentiostat (CHI 750A, CH Instruments, USA). Optical images obtained by Nikon YS100 (Nikon Instruments, Japan). Focused Ion Beam (FIB) and scanning electron microscopy (SEM) was performed by Helios NanoLab™ 650 (FEI company, USA).

UME Preparation

A quartz glass nanopipette was fabricated from a quartz capillary (O.D./I.D.: 1/0.5 mm) using a laser puller with the following parameters: “Heat: 700, Filament: 4, Velocity: 40, Delay: 128, Pull: 80” (Fig. 1b). After fabrication, butane gas was introduced into the nanopipette through tygon tubing, and the tapered part of the nanopipette was then placed into another quartz capillary (O.D./I.D.: 1.2/0.9 mm) filled with argon gas. A butane jet flame torch was used to deposit pyrolytic carbon (Fig. 1c) [24]. By applying heat slightly front the taper end of the nanopipette, a hollowed carbon UME with an empty tapered front was created (Fig. 1d). Since the entrance to the taper became blocked from the heat during the process, the front part of the hollowed carbon UME was partially milled using FIB to create an opening for introducing solution. The milling was performed employing a current of 77 pA at the acceleration voltage of 30 kV. The hollowed carbon UME was then placed into a solution of 2 mM chloroauric acid and 1 N H₂SO₄, and Au electrodeposition was carried out by pulsed potential. Using an Ag/AgCl electrode as the reference electrode, the potential was alternated between 0.85 V for 5 s and 0.7 V for 1 s. This sequence was repeated continuously until the maximum value of the resulting current increase (Fig. 1e). Finally, the UME was milled using FIB to expose the Au surface (Fig. 1f).

UME characterization

The size of the fabricated UME was confirmed using SEM. Next, the electrochemical behavior of this probe was examined via CV in a solution of 10 mM potassium hexacyanoferrate(II) trihydrate (K₄Fe(CN)₆·3H₂O) and 250 mM KNO₃. CV curves were obtained by sweeping the potential of the working electrode between 0 and +0.5 V (vs. Ag/AgCl) at a scan rate of 20 mV/s.

RESULTS AND DISCUSSION

Hollowed carbon UME fabrication

The micro-size Au disk UME fabrication process is schematically illustrated in Fig. 1. Establishing a reproducible fabrication protocol requires consistent production of hollowed carbon UMEs. In particular, it is crucial that the hollowed carbon layer should be formed at an appropriate distance from the tip. If carbon is deposited all the way to the taper tip, obtaining a clean Au disk UME after Au plating and subsequent milling becomes difficult. Conversely, if carbon is deposited too far from the taper, the time required for Au plating increases exponentially. To address these issues, a glass nanopipette with a very small diameter in taper end was employed. Small opening of a nanopipette makes the air layer inside the pipette cannot easily escape from the tip taper even under high-pressure butane gas. By applying heat for about 30 seconds slightly ahead of the nanopipette’s taper, the entrance at the tip becomes sealed. As a result, the original air layer remains at a very narrow portion near the tip end, followed by a layer of butane gas. Pyrolyzing this butane gas layer allows the formation of a hollowed carbon UME with a very small empty region at the front (~30 μm) (Fig. 2). Since the tip end became sealed by the heat applied during the fabrication of the hollowed carbon UME, we needed to use FIB milling to create an opening on the tapered end for solution access and thus allow the plating solution to enter the hollow space.

Au electrodeposition

Au was electrodeposited to fill the empty space of the fabricated hollowed carbon UME. In previous studies involving Au electrodeposition, Au deposition was conducted by applying a constant potential at which the Au precursor (2mM chloroauric acid) could be reduced [19–21] (Fig. 3a-1). However, this often resulted in dendritic growth of Au on the carbon electrode, leaving unfilled parts of the tip’s hollow space (Fig. 3a-2). To address this issue and achieve a more uniformly filled Au electrode, we utilized pulsed potential method instead of a constant-potential method (Fig. 3b-1). By employing very short deposition times and longer resting times, it is possible to facilitate uniform mass transport of Au precursors across the electrode surface and prevent dendritic electrodeposition (Fig. 3b-2). The thoroughly filled Au electrode can then be milled using FIB to fabricate an Au disk UME.

Characterization of Au Disk UMEs Across Various Dimensions

The fabrication procedure was verified by the high-quality production of Au disk UMEs in various dimensions. In particular, by FIB milling, it was possible to create UMEs of different electrode areas. Moreover, various forms of nanopipettes with different RG ratios were fabricated by altering the micropuller’s pulling conditions [20], the types of glass capillaries used, and the pyrolysis process. The UMEs were characterized using SEM imaging and CV. From side-view SEM images (tilted at 52°), it was confirmed that the UMEs were insulated by a glass sheath without any bubbles or voids (Fig. 4A). The fabricated UMEs ranged in diameter from about 1 to 5 μm and exhibited various RG ratios. To characterize the electrochemical behavior of these UMEs, CV was conducted to determine the steady-state current for each UME (Fig. 4B). The steady-state current (i_ss) is governed by the flux of the redox species in solution to the electrode surface and is given by [25]:

(1)

iss=knFDaC*β

Where k is a geometric constant, 4 for a disk UME, n is the number of electrons involved in the reaction, F is the Faraday constant (96485 Cmol^-1), a is the radius of each disk UME, D is the diffusion coefficient of the redox species (D_Ferro =7.1× 10^-6 cm² s^-1) [26], C* is the concentration of the redox species in solution, and  is a term accounting for the influence of the RG ratio. If  is assumed to be 1, the radius of each UME can be estimated from its steady-state current (Table 1). Overall, the calculated radii correlate well with the SEM measurements, with minor discrepancies likely due to the increase in the limiting current resulting from the enhanced mass transport caused by the thin insulating layer [27] or due to microscopic cracks that cannot be detected by SEM. An additional noteworthy observation is that UMEs with lower RG ratios exhibit significantly higher charging currents, presumably because a thinner insulating layer leads to increased capacitance (Fig. 4B-d). In addition, we obtained cyclic voltammogram of another Au disk UME using different redox species, Ru(NH₃)₆Cl₃, which gives reversible behavior with a reasonable steady-state current (See supporting information, Fig. S1). This result shows that the lack of reversibility observed in some of the cyclic voltammograms in Fig. 4B is not due to surface dimensions of the Au disk UMEs, but rather to surface sensitive electrokinetics of K₄Fe(CN)₆, which is sensitive to the Au surface condition.

CONCLUSIONS

Au UMEs are of great importance due to their suitability for biological applications and the ease of surface modification via Au-thiol interactions. Nevertheless, reproducible fabrication method for micro-sized Au disk UMEs was not well established. Herein, we report a general approach for the successful fabrication of microsized Au disk UMEs with various electrode areas and RG ratios. By electroplating Au onto hollowed carbon UMEs using pulsed potential, we produced electrodes with diameters ranging from 1 to 5 μm with a various RG ratios. A key advantage of the proposed method is the reproducible fabrication of controlled electrode structures. We anticipate that these electrodes will exhibit strong potential as electrochemical sensors when coupled with various surface modification strategies.

Notes

ACKNOWLEDGEMENTS

This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (nos. RS-2021-NR060082 and RS-2022-NR070547).

SUPPORTING INFORMATION

jecst-2025-00045-Supplementary-Fig-S1.pdf

Fig. 1.

Fabrication of Au disk UME. a) Quartz glass capillary, b) glass nanopipette fabricated by puller, c) hollowed carbon UME by carbon deposition, d) FIB treatment to hollowed carbon UME, e) Au deposition on hollowed carbon UME, f) Fabricated Au disk UME by FIB milling.

Fig. 2.

Fabrication of hollowed carbon UME, a) Carbon UME image, b) hollowed carbon UME image.

Fig. 3.

Au electrodeposition methods, a-1) constant potential i–t curve, b-1) last part of i–t curve applying pulsed potential (inlet: first 4 cycles of curve, ■ : current, ▲: potential), a-2) SEM image of deposited electrode by constant potential, b-2) SEM image of deposited electrode by pulsed potential method, a-3) Optical image of deposited electrode by constant potential, b-3) Optical image of deposited electrode by pulsed potential method.

Fig. 4.

Characterization of Au disk UME, A: SEM image, B: Cyclic Voltammogram.

Table 1.

diameter estimated from SEM and CV (μm)

	SEM image	Estimated radius form CV
a)	1.1	1.0
b)	2.1	1.6
c)	4.8	3.8
d)	5.7	5.0

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