A simple, sensitive and time-saving differential-pulse adsorptive stripping voltammetric (DPAdSV) procedure using a screen-printed carbon electrode modified with carboxyl functionalized multiwalled carbon nanotubes (SPCE/MWCNTs-COOH) for the determination of diclofenac (DF) is presented. The sensor was characterized using optical profilometry, SEM, and cyclic voltammetry (CV). The use of carboxyl functionalized MWCNTs as a SPCE modifier improved the electron transfer process and the active surface area of sensor. Under optimum conditions, very sensitive results were obtained with a linear range of 0.1-10.0 nmol L −1 and a limit of detection value of 0.028 nmol L −1 . The SPCE/MWCNTs-COOH also exhibited satisfactory repeatability, reproducibility, and selectivity towards potential interferences. Moreover, for the first time, the electrochemical sensor allows determining the real concentrations of DF in environmental water samples without sample pretreatment steps.Keywords: screen-printed carbon electrode modified with carboxyl functionalized multiwalled carbon nanotubes; diclofenac; differential-pulse adsorptive stripping voltammetry; environmental water samples; HPLC with photo-diode array detection Materials 2020, 13, 781 2 of 15 to ground and surface water, causing their pollution. Low water temperatures and short days hinder the process of photolysis and the biodegradation of pharmaceutical preparations, which can cause adverse and unpredictable effects in the ecosystem. The main sources of pollution are factories and production plants, as well as hospitals, health centers, and practically every household. DF is one of the most commonly found ingredients in water, and its concentration is from 3.7 × 10 −11 to 1.4 × 10 −8 mol L −1 [2,3].Although the likelihood of any form of short-term human health risk after DF release into the environment is low, a study links the catastrophic decline of Gyps vulture populations across the Indian subcontinent to DF [4,5]. Generally, NSAIDs such as DF and ibuprofen, for example, have Log Kow values greater than three and may have the capacity to bioaccumulate in the tissues of organisms [6]. DF exhibits acute hepatotoxicity, causes changes in kidneys and gills in rainbow trout (O. mykiss) and exhibits acute toxicity to phytoplankton and zooplankton. Moreover, the possibility of synergetic effects with other pharmaceuticals or chemicals in the aquatic environment increases the environmental risk as well [7].There are many methods in the literature that allow for the determination of DF at various concentration levels. The most popular methods are spectrophotometry (determined DF concentrations: 6.8 × 10 −7 -8.4 × 10 −2 mol L −1 ), spectrofluorimetry (determined DF concentrations: 4.2 × 10 −7 -1.7 × 10 −4 mol L −1 ), calorimetry (determined DF concentrations: 4.6 × 10 −6 -2.7 × 10 −4 mol L −1 ), high-performance thin-layer chromatography (determined DF concentrations: 6.8 × 10 −7 -2.7 × 10 −6 mol L −1 ), and HPLC (determined DF concentrations: 1.7 × 10 −8 -1.4 × 10 −5 mol L −1 )...
