Stable iridium-modified boron-doped diamond electrode for the application in electrochemical detection of arsenic (III)
Graphical abstract
Illustration of the preparation of iridium-modified BDD electrode for electrochemical detection of arsenic (III).
Introduction
Electrochemical techniques provide accurate and quick sensing in the laboratory and have the potential to be developed as a mobile technique. In addition, its lower cost compared to other methods is also beneficial [1]. Meanwhile, boron-doped diamond (BDD) is a well-established electrode material due to its excellent properties, specifically the very low background currents and wide potential windows as well as good electrical conductivity and high stability [[2], [3], [4], [5]]. Applications of BDD for the determination of trace metals have been previously reported, however, some metals, such as arsenic species, cannot be oxidized nor reduced on the surface of unmodified BDD [6]. Therefore, modification of the BDD surface is required. Gold and iridium are known to have high a catalytic activity and have been developed for the detection of arsenic (III) species [[7], [8], [9], [10]]. Arsenic detection using iridium oxide-modified BDD has been reported with cyclic voltammetry as a technique to modify the BDD surface [7]. The modified electrode shows an excellent catalytic activity, yet improvement for stability is required due to sp3 coordination of the BDD structure [6]. Previously, our group developed a modified BDD with iridium method using ion implantation, which exhibited excellent catalytic activity and great stability with relative standard deviations (RSDs) of ~1.58 and 3.26%, respectively [11]. However, the electrode preparation with ion implantation is not easy and expensive. Further, Gao et al. [12] reported a new method to deposit platinum on the BDD surface with great stability with only less than 1% of Pt particles removed after tested using ultrasound removal and produced homogeneous particles on the diamond surface. This method comprises of three deposition steps, including wet chemical seeding and two times electrochemical deposition steps before and after thermal annealing [12,13].
In this study, this method was employed with some modifications to deposit iridium particles on BDD surface. To the best of our knowledge, this is the first time that iridium was deposited on BDD surface with the method explained in this report. Different from platinum, potential reduction of Ir(0) is very high, therefore, it is not easy to deposit iridium at the surface of BDD by electrochemical method. A step of chemical seeds of iridium particles at BDD surface was expected to be a driving force to deposit more iridium at the BDD surface. The Ir-modified BDD was then applied for arsenic detection, especially the arsenic (III) species, which is the most toxic among other arsenic species [14]. The modified electrode from each step was characterized using Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), Raman, X-Ray Photoelectron Spectroscopy (XPS), and cyclic voltammetry. Application for the measurement of arsenic (III)-added in tap water and lake water samples showed that the method is promising for the preparation of electrodes as arsenic sensors.
Section snippets
Materials
K2IrCl6, NaBH4, H3PO4, 2-propanol was obtained from Wako Inc. KH2PO4, K2HPO4. NaOH, NaAsO2 was obtained from Merck, while Pt spiral was obtained from Nilaco and Ag/AgCl reference electrode system as supplied by Bioanalytical Systems (BAS) Inc. In this study, all reagents were CAS-certified reagent grade and used without any pre-treatment or further purification. Pure water for this study was supplied by Simply-Lab water system (Direct-Q UV3, Millipore).
Electrode preparation
BDD (B/C 0.1%) was prepared by deposition
Electrode preparation and characterization
Wet chemical seeding was carried out to seed the metal particles onto the BDD surface by reducing metal precursors with a reducing agent. This method was applied, as a conventional electrodeposition method generates inhomogeneous deposited particles, resulting from non-uniform electronic properties of diamond electrodes [16].
BDD surfaces with H termination will absorb BH4− due to the electrostatic interaction [17]. The complex solution of K2IrCl6 was added and reduced by BH4− on the electrode
Conclusion
Electrodeposition of iridium on BDD surface was completed with a wet chemical seeding step and a rapid thermal annealing step showed a better dispersion as well as size distribution of iridium particles on the BDD surface. Raman spectra showed no sp3 damage during the annealing process, while XPS spectra indicated that iridium oxide was deposited on the BDD surface. Furthermore, the prepared electrode with the complete step showed the best performance for arsenic (III) detection at optimum pH 3
Declaration of competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by Hibah Q1Q2 Universitas Indonesia, Contract No. NKB-271/UN2.R3.1/HKP.05.00/2019.
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