Mercury nanodroplets supported at biochar for electrochemical determination of zinc ions using a carbon paste electrode Paulo Roberto de Oliveira a , Alyne Cristina Lamy-Mendes a , Jeferson Luiz Gogola a , Antonio Salvio Mangrich b, c , Luiz Humberto Marcolino Junior a , Márcio F. Bergamini a, * a Laboratório de Sensores Eletroquímicos (LabSensE), Departamento de Química, Universidade Federal do Paraná (UFPR), CEP 81.531-980, Curitiba-PR, Brazil b Laboratório de Química de Húmus e Fertilizantes, Departamento de Química, Universidade Federal do Paraná (UFPR), CEP 81.531-980, Curitiba-PR, Brazil c Instituto Nacional de Ciência e Tecnologia de Energia e Ambiente (INCT E&A/CNPq), Brazil A R T I C L E I N F O Article history: Received 22 September 2014 Received in revised form 7 November 2014 Accepted 10 November 2014 Available online 11 November 2014 Keywords: Mercury nanodroplets Biochar Carbon Paste Electrode Zinc determination A B S T R A C T In the present work a modified carbon paste electrode (MCPE) was prepared using mercury ions (Hg 2+ ) supported at a biochar (bio-charcoal) surface obtained from castor oil cake and used for electrode preparation. The proposed electrode was evaluated for preconcentration and determination of zinc(II) ions present in commercial samples (collyrium and ointment) by differential pulse anodic stripping voltammetry (DPASV). The procedure is based on spontaneous interactions between the highly functionalized biochar surface and zinc(II) ions followed by reduction of ions into mercury droplets which promote a high localized zinc concentration and improvement on the stripping anodic current obtained under differential pulse voltammetric conditions. SEM images revealed dimensions of mercury droplets in the range from 16 nm to 38 nm. Parameters involved in the mercury incorporation, preconcentration of zinc ions and voltammetric stripping step were studied and optimized. Using the best set of experimental conditions a linear response for zinc(II) ions was observed for concentration range (LDR) of 5.0  10 7 to 3.0  10 5 mol L 1 with limits of detection (LOD) and quantification (LOQ) of 1.7  10 7 mol L 1 and 5.8  10 7 mol L 1 , respectively, and sensibility of 5.2 mA L mmol 1 . The proposed methodology was applied for quantification of zinc-containing commercial samples, whose results were compared with those given by inductively coupled plasma optical emission spectroscopy (ICP-OES) and complexometric titration. Results produced by the methods are in agreement at a 95% of confidence level. ã 2014 Elsevier Ltd. All rights reserved. 1. Introduction The polarographic technique, together with the mercury drop electrode itself, is widespread and has been used for decades [1]. This electrode combined with stripping procedures presents numerous operational advantages over other electrochemical conventional methods including low background current, accura- cy, high signal/noise ratio and extensive working range in the cathodic region [2], high sensitivity and low limits of detection and quantification [3]. However, the use of mercury drop electrodes presents some limitations, mainly reported with practicality of the system and environmental aspects, due to the known potential toxicity of mercury. In order to use the advantages of electrochemical character- istics of the mercury and diminish its drawbacks cited above, the development of electrodes based on films and other forms of mercury became highly desirable, since these devices employ a considerably lower quantity of mercury. A decrease in the amount of mercury on the electrode surface also results in an increased sensitivity when stripping techniques have been used, because the signal generated by the species of interest is directly proportional to the analyte concentration into mercury layer [4]. In this sense, mercury films (prepared ex-situ or in-situ from solutions containing mercuric ions) [5] and solid amalgam (mainly formed with silver) [6] have been used to improve the analytical performance of these devices. Song et al. [7] developed a printed carbon electrode modified with mercury nanodroplets supported by an array of multi-walled carbon nanotubes/chitosan for simultaneous determination of cadmium(II), lead(II) and copper (II). Using a similar approach, several different materials has been used to adsorb metal ions, such as mercury(II), and used as support for chemically modified electrodes (CME). An interesting and alternative material is biochar (biomass + charcoal), a carbonaceous material richly functionalized * Corresponding author. Tel. : +55 41 3361 3177; fax: number: +55 41 3361 3186. E-mail address: bergamini@ufpr.br (M.F. Bergamini). http://dx.doi.org/10.1016/j.electacta.2014.11.057 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved. Electrochimica Acta 151 (2015) 525–530 Contents lists available at ScienceDirect Electrochimica Acta journa l home page : www.e lsevier.com/loca te/ele cta cta