Au paste electrode to increase the intensity of cathodic electrochemiluminescence
Authors: Endo Ayane , Kubota Koki , Ito-sasaki Takahiro , Inoue Kumi Y. 
Published: December 2024

Bipolar electrochemical microscopy (BEM) is a novel electrochemical imaging platform using a vertical closed bipolar electrode (cBPE) array as an imaging element. In BEM, the electrochemical reaction generated at one end of the BPE is converted to electrochemiluminescence (ECL) at the other end of the BPE. Unlike the conventional electrode array method, which requires lead wires for each electrode, BEM can operate without wires to each electrode. Therefore, imaging with both high-temporal and high-spatial resolution is possible, from which advantage, BEM is expected to be a tool for the analysis of biological phenomena that occur in a very short period in a local area on the scale of a single cell. As a simple method for fabricating vertical cBPE arrays, Iwama et al. previously proposed a method of filling conducting carbon pastes (CPs) into pores of a glass capillary plate.1 As another direction of BEM development research, we are conducting dopamine (DA) detection using N,N’-dimethyl-3,4,9,10-perylenetetracarboxylicdiimide (PDI-CH3) as a cathodic luminophore for ECL.2 We are currently investigating the use of a PDI-CH3 mixed paste electrodes as BEM detection elements for DA imaging, taking advantage of the flexibility and ease of modification of the paste electrode. In the process, we found a phenomenon in which ECL is enhanced using Au paste (AuP).3 In this study, we compared PDI-CH3 mixed CP and AuP electrode by three-electrode system and cBPE-ECL detection system.
CP was prepared by mixing acetylene carbon black (particle size 42 nm), and liquid paraffin at a weight ratio of 1:1. AuRoFUSE (particle size 0.2–0.4 µm) was used for AuP. The PDI-CPs were obtained by wet mixing of PDI-CH3 and CP at weight ratios from 4:1 to 1:20 in an alumina mortar with ethanol. PDI-AuPs were obtained in a similar way by wet mixing of PDI-CH3 and AuP at weight ratios from 1:5 to 1:20. Each paste was filled into a paste electrode holder (1.6 mm in diameter). PDI-CH3/CP 1:1 by weight and PDI-CH3/AuP 1:20 by weight contain the same volume of PDI-CH3 in a given volume of each paste, because the specific gravities of CP and AuP are speculated 0.95 g/cm3 and 19.3 g/cm3, respectively. Measurements using three-electrode system was conducted with 35 mm dish filled with 0.2 M Na2S2O8 (containing phosphate-buffered saline; PBS), a working electrode (WE; 1.6 mm in diameter), a counter electrode (CE; platinum plate), and a reference electrode (RE; Ag/AgCl). Linear sweep voltammograms (LSVs) and ECL intensities were obtained at a scan rate of 50 mV/s with 500 ms exposure time of a CMOS camera. In the cBPE-ECL detection system, two 35 mm dishes filled with 0.2 M Na2S2O8 (containing PBS) and 0 or 1 mM DA (containing PBS) were used as a detection cell and a sample cell, respectively. A paste electrode to be evaluated in the detection cell and a glassy carbon electrode (1.6 mm in diameter) in the sample cell were connected to each other to be a cBPE. As driving electrodes, a platinum plate in the detection cell, an Ag/AgCl electrode and a second platinum plate in the sample cell were connected to the WE, RE, and CE terminals of a potentiostat, respectively. LSVs and ECL intensities were obtained at a scan rate of 20 mV/s with 5 s exposure time of the CMOS camera.
At the three-electrode system, LSVs of PDI-CH3/CP 1:1 and PDI-CH3/AuP 1:20 in weight ratio, containing the same volumes of PDI-CH3 in the pastes, produced similar maximum values of the first current peak related to ECL reaction, while the peak ECL intensities were 1.75 times higher in case using PDI-AuP than in case using PDI-CP. This result suggests that the ECL intensity can be enhanced using AuP. This ECL enhancement may be due to the surface plasmon field-enhancement effect by Au particles on the electrode surface, given that the AuRoFUSE particle size is 0.2 to 0.4 µm. The figure shows the images obtained by CMOS camera at 1.2 V using cBPE-ECL detection system. The ECL intensities peaks originating from DA oxidation in the sample cell were 9.5 times higher using PDI-AuP rather than PDI-CP, even though the peak current values obtained using PDI-AuP and PDI-CP were almost the same. By using AuP instead of CP, imaging such as DA is expected be with high sensitivity in BEM system.
References:

T. Iwama et al., Anal. Chem., 94, 8857 (2022).

T. Iwama et al., available at SSRN, http://dx.doi.org/10.2139/ssrn.4632693.

A. Endo et al., Electrochemistry, 92, 022013 (2024).

Figure 1