Relationship between the Different Surface Morphologies of Diamond Electrodes and Their Electroanalytical Responses to Cadmium Detection

The heavy metal ions monitoring, such as cadmium, in water is essential for human health and safety. In this way, the conventional electroanalytical determination of metals usually involves mercury-based electrodes. Thus, the boron doped diamond electrodes have been extensively studied due to their attractive electrochemical properties, favoring their use as electrodes to detect heavy metals, substituting the mercury electrodes in analytical techniques [1]. One of the most sensitive techniques is the square-wave voltammetry, which is a powerful electrochemical method that may be applied to both electrokinetic and analytical measurements. In this paper the cadmium detection was investigated by Square Wave Anodic Stripping Voltammetry (SWASV) measurements applied on diamond electrodes with different morphologies of MBDD/NBDD (Microcristalline Boron Doped Diamond/ Nanocristalline Boron Doped Diamond). MBDD films were grown in a hot filament reactor by chemical vapor deposition technique on Si substrate using 200 sccm total flow (standard centimeter cubic per minute) and the flow part H₂/CH₄ 198 sccm and 2 sccm, respectively, and the films NBDD were also grown with total flow of 200 sccm, and the flow part were  3, 2, 160 sccm of H₂/CH₄/Ar, respectively. Boron was obtained from an additional H₂ line passing 35 sccm through a bubbler containing trimethylborate [(CH₃O)₃B] dissolved in methanol with dissolution of 20000 ppm of B/C. The analysis of 1 mg L^-1 Cd²⁺ were performed using SWASV technique in 4 x 10^-4 mol L^-1  ammonium acetate buffer pH 4.2 after purging nitrogen for 1800 s. The peak currents around -0.7 V vs. Ag/AgCl were measured for Cd²⁺ concentration ranged from 1 to 20 ppb. The measurements were taken considering the optimized parameters among the peak current, the pulse frequency, the amplitude, and the potential increment. From Scanning electron microscopy images significant changes in diamond grain sizes may be observed as a function of argon addition of in the gas mixture, as expected. The crystal sizes were reduced from the microcrystalline scale to nanocrystalline scale, and the shape of the grain diamond also changed. This change in grain size and roughness of the films affect how cadmium is deposited on the surface of the electrodes and, consequently, the electroanalytical response for the SWV measurements. For all optimized conditions and settings, both MBDD and NBDD electrodes provided cadmium detection limits (DL) around 2 ppb showing that diamond electrodes are a suitable mercury-free method to determine cadmium trace levels in water. This DL value is lower than that of 5 ppb required for drinking water by the Ministry of Health in Brazil [2].

 Acknowledgments: This work was supported by CNPq, CAPES and FAPESP.

 [1] A. Manivannan, R. Kawasaki, D.A. Tryk, A. Fujishima, Electroch. Acta,  49 (2004) 3313-3318

 [2] Portaria nº2914 de 12 de Dezembro de 2011. In <http://bvsms.saude.gov.br/bvs/saudelegis/gm/2011/prt2914_12_12_2011.html>. 26/02/2013.