Novel photoelectrocatalytic approach aiming at the digestion of water samples, estimation of organic matter content and stripping analysis of metals in a special UV-LED irradiated cell with a TiO 2 -modified gold electrode Alexandre L.B. Baccaro, Ivano G.R. Gutz ⁎ Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000 São Paulo, SP, Brazil abstract article info Article history: Received 22 February 2013 Accepted 1 March 2013 Available online 6 March 2013 Keywords: UV-LED photoelectrochemical cell Gold–TiO 2 electrode Cu(II)-EDTA photovoltammetry Trace metal stripping voltammetry EDTA photooxidation Photoelectrocatalytic sample treatment Fast photoelectrocatalytic degradation of organic matter is proposed as a means to estimate the organic load and in situ sample preparation for concomitant trace metal determination by voltammetry. To prove this con- cept, a thin-layer type flow-through cell comprising a UV-LED focused on a gold disc working electrode, mod- ified with TiO 2 nanoparticles (P25, mainly anatase) embedded into an electrodeposited gold film, was developed and evaluated with the model system Cu(II)-EDTA plus EDTA in excess. Consecutive cyclic voltammograms (-0.3 to +0.7 V vs. Ag/AgCl) exhibited no peaks in the dark, whereas under irradiation (370 nm, ≈150 mW) unchelated copper ions were reduced and reoxidised as usual in anodic stripping voltammetry (ASV), denoting the degradation of EDTA near the interface by photogenerated valence-band free holes or indirectly by •OH radicals. The photocurrent due to conduction-band electrons withdrawal, ad- ditively superimposed on the voltammograms, increased with the electrode potential and with the EDTA concentration, thus allowing an estimation of the organic load in the samples. Full depletion of EDTA from the solution layer was found to be unnecessary and the high ‘baseline’ provided by the photocurrent could be avoided by performing ASV immediately after switching off the LED. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Ultraviolet light irradiation, a widely used process for the disinfec- tion and degradation of low loads of organic matter in waters, is also valuable as a pre-treatment step in inorganic analysis. For example, environmental water samples are exposed to a mercury lamp in quartz vials for 1 h or so to get rid of the interference of complexing organic compounds before the determination of total concentrations of metals by voltammetry or other techniques [1]. Enhancement of the photooxidation process by heterogeneous photocatalysis at semi- conductor oxides, especially TiO 2 , has been thoroughly investigated for decades [2], promoting interest in sectors such as wastewater treatment by using either sunlight or UV lamps to carry electrons from the valence to the conduction band of the semiconductor. The photoholes left in the valence band might oxidise organic compounds stepwise by different mechanisms, including generation and indirect action of free radicals, especially •OH [3]. In analytical chemistry, heterogeneous photocatalysis was first proposed as an aid to UV sample digestion for trace metal analysis in 2001 by our group [4]. An automated voltammetric FIA system was developed to add TiO 2 in suspension to sample plugs (150 μL); the mixture was then irradiated with a mercury vapour lamp in a tubular Teflon® photoreactor and displaced into the voltammetric flow cell. The Cd(II) reduction process and, consequently, the anodic stripping voltammetry (ASV) peak was completely suppressed by EDTA in excess, but fully recovered after 3 min of irradiation [4]. In 2007, the group presented a microfluidic version of the photocatalytic reactor for sample digestion in which a gold film with embedded TiO 2 (Au–TiO 2 ) was irradiated by a 365 nm 20 mA LED to treat some 19 nL of sample and after 4 min, the Cu(II)/Cu 0 voltammetric wave was re- covered [5]. This UV-LED is effective because the band gap of TiO 2 in the form of anatase is 3.23 eV, equivalent to 384 nm. A further reduc- tion in the treatment time by positively biasing the Au–TiO 2 electrode to minimise the recombination of photoholes with electrons was an- nounced by the group for the microfluidic photoreactor [6]. Meanwhile, high intensity 365 nm LEDs (e.g., 3W per die) entered mass production, extending research possibilities on photoelectrocatalytic sample treatment to conventional-size flow- through thin-layer cells, as will be shown. Available from various suppliers for electrochemical detection in HPLC and FIA, such cells are demountable, need no special micropumps and valves and oper- ate with spacers in the range of 0.1–1.0 mm, instead of b 0.02 mm in the microfluidic cell. A thicker solution layer relieves the iR poten- tial drop but introduces a non-negligible flux of organic matter to the electrode at steady state by radial diffusion. Besides the conversion of such cells to photoelectrocatalytic operation, a novel approach is proposed here, intending to estimate the organic matter load during Electrochemistry Communications 31 (2013) 28–30 ⁎ Corresponding author. Tel./fax: +55 11 30912150. E-mail address: gutz@iq.usp.br (I.G.R. Gutz). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.03.001 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom