###### Abstract  
Diclofenac (DCF) is a nonsteroidal anti-inflammatory drug to treat pain and inflammatory diseases. The high consumption of the drug leads to a significant change in the ecosystem. With the aim of optimizing a fast screening analysis for DCF detection on many samples with a sensitive and cheap procedure, we considered electrochemical methods using carbon-based electrodes as sensors. The electrochemical behavior of the DCF was studied on glassy carbon electrodes (GCE) and on screen-printed carbon electrodes (SPCEs) from two different suppliers after an anodic activation. The surface of the SPCEs was analyzed by scanning electron microscope (SEM) and Energy Dispersive Spectrometry (EDS). On all the activated electrodes, the voltammetric procedure (Differential Pulse Voltammetry) for the determination of DCF was optimized by the experimental Design method, and the linearity range of the response, as well as the calibration and limit parameters (limits of detection--LoD; limit of quantification--LoQ), were defined. Analyses on SPCEs were performed both by immersing the electrode in the solution and by deposing a drop of solution on the electrode. DCF signals are stabilized by the polishing process and enhanced by the anodic activation and acid pH. The electrochemical response of DCF is not reversible, and its by-products tend to be adsorbed on the surfaces, particularly on GCE. The lowest limit parameters were obtained using the GCE (LoD = 1.6 \(\mu\)g L\({}^{-1}\)) and the SPCE, having the smallest surface, immersed in solution (LoD = 7 \(\mu\)g L\({}^{-1}\)). diclofenac; glassy carbon electrode; screen printed electrodes; voltammetry; electrochemical activation; natural surface waters +
Footnote †: journal: Applied Sciences  
published in 2018 by the European Joint Research Centre [3] showed a detection frequency of DCF above 50% and a mean concentration value of the drug equal to 66 ng L\({}^{-1}\). These values were estimated by analyzing 6698 samples of surface water collected at 607 sites across 25 European Countries. Moreover, a direct eco-toxicological effect of DCF residues in wildlife was documented [4; 5] and, based on the results collected during the monitoring program of the EU [3], the estimated Predicted No Effect Concentration (PNEC) for DCF was lowered from 0.1 to 0.05 ng L\({}^{-1}\). The fast detection of DCF in the effluent of WWTPs, surface water, or in any type of natural water sample would allow for an estimation of the drug contamination level, which could help immediate decision-making. The suggested analytical method to detect DCF in water samples is Liquid Chromatography coupled with Mass Spectrometry after the treatment of the sample with Solid Phase Extraction (SPE-LC-MS-MS) [6; 7]. These techniques are expensive and time-consuming; moreover, they need highly qualified staff and qualified maintenance service of the instruments and can neither be performed in the field after sampling nor directly in the stream. In order to allow a fast screening analysis on many samples with a sensitive and cheap procedure, these techniques are unsuitable, whereas electrochemistry could be a good solution thanks to its low cost, high sensitivity, and potential portability. Several research papers report on the application of electrochemical methods for the quantification of DCF using, in particular, carbon-based electrodes [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] (see Table 1). Most of these applications use carbon electrodes modified by the application of materials that enhance electron transfer, such as ionic liquids, carbon nanotubes, and silica nanoparticles. More simply, in this work, an electro-activation method was tested. The procedure was applied in previous works to increase the signal of paracetamol on glassy carbon electrodes (GCEs) and required the application of an anodic potential for a defined time, with the electrode surface immersed in a borate-phosphate buffer solution at pH 9 [20; 21].