DOI: 10.1002/elan.201400213 Single Oil Drop Electrochemistry on a Screen-Printed Electrode Surface Etienne S. Oliveira, [a] Pablo A. Fiorito, [a] and H. B. Suffredini* [a] Oil/water-interface electrochemistry is a promising and growing area, especially in the fabrication of sensors and biosensors [1, 2] and in the simulation of biological struc- tures [3–6]. Charge- and mass-transfer processes have also been studied with respect to the interface between immiscible liquids [7–10]. Important advances in interfacial electrochemistry have been made in systems known as “ITIES” (Interfaces be- tween Two Immiscible Electrolyte Solutions) since the pioneering work published by Koryta [11]. The literature also contains descriptions of the interface between an ionic liquid and water (see, for example, the work of Kohno et al. [12]). Electrochemical studies related to the detection of ferrocene have been performed using a conta- minated nitrobenzene drop in an extensive research pro- gram conducted by Tasakorn et al., which was published in 2002 [13]. The use of the single-oil-drop technique described in this paper to analyse hydrophobic molecules is advanta- geous compared to the use of classic electrochemical sys- tems An extensive review was published by Banks et al showing the use of interfacial electrochemistry in conven- tional electrodes [14] and a text book addressing the oil- drop electrochemistry was published by Scholz et al. [15]. When the analyte is hydrophobic, several pre-treatment techniques are necessary for classic electrochemical sys- tems because electroanalytical studies cannot be per- formed directly in oil. Another possible way to analyze hydrophobic molecules involves the use of emulsions or micelles to conduct electrochemical studies; however, such studies are sometimes complicated. Liquid/liquid interfaces also present certain disadvan- tages, such as high resistivity compared to that encoun- tered in classical support-electrolyte analysis and the need to stabilise the partition between the analyte and the immiscible liquids. In this context, an interesting tool for observing the charge-transfer resistance is electro- chemical impedance spectroscopy (EIS), which has been extensively used to investigate biosensors, as an example [16] To enable this study, a promising methodology was used in which a single drop of limonene oil contaminated with 4-NP was placed directly on the surface of a screen- printed electrode. This compound was chosen as a model since it represents an extensively studied molecule in standard electrochemical systems, as the paper published by Pedrosa et al. [17]. This oil drop and the three electro- des were covered with a conventional supporting electro- lyte solution, thereby creating a very simple electrochemi- cal cell, which we describe in detail. Experimental The oil phase consisted of R-(+)-limonene, some samples of which were contaminated with 4-NP For this purpose, a 10 À3 mol L À1 solution of 4-NP was prepared directly in limonene oil. Tetrabutylammonium chloride (TBAC) was added to the oil phase in some experiments to increase its conductivity. The aqueous phase consisted of a conventional sup- porting electrolyte: a 0.04 mol L À1 BrittonÀRobinson (BR) buffer adjusted to pH 6.5 (optimised value). A conventional DS-110 screen-printed electrode (DropSens) was used in this study. The DS-110 was com- posed of carbon (as the working and counter electrodes) Abstract : The use of a contaminated single oil drop on a screen-printed carbon electrode is described for the first time here. The simple methodology developed herein opens the possibility of conducting such measurements. R-(+)-limonene oil, some samples of which were conta- minated with 4-nitrophenol (4-NP), was used as the oil phase, and BrittonÀRobinson (BR) buffer was used as the aqueous phase. An oxidation peak at approximately 0.8 V vs. Ag was obtained when the system comprised an oil/water interface. The charge transfer resistance de- creased by a factor of approximately 7.1 when an interfa- cial system composed of two immiscible liquids was used as an electrochemical tool. Keywords: Single oil drop · Oil/water interface · Screen-printed electrode · Liposoluble contaminant [a] E. S. Oliveira, P. A. Fiorito, H. B. Suffredini Universidade Federal do ABC Avenida dos Estados, 5001 – Bangu, Santo AndrØ, SP, Brazil CEP 09210-580 tel: + 55 11 4437-0170 hugo.suffredini@ ufabc.edu.br www.electroanalysis.wiley-vch.de # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2014, 26, 1660 – 1663 1660 Short Communication