Development of a portable sensor to evaluate copper speciation in natural waters

This work presents a new portable sensor to evaluate total dissolved copper and labile copper in natural waters, which is particularly valuable for the assessment of the toxicity of this metal towards aquatic biota. The sensor was constructed using graphite rod electrodes electropolymerized with 5-(4-allyloxy-phenylazo)-quinolin-8-ol (APQO). The metal preconcentration step was an electroless process performed by simply dipping the modified electrode (ME) into the sample. The polymerized electrode could be stored inside a common plastic bag at room temperature for at least 10 days, and once the Cu–APQO complex was formed, the ME was stable for up to 7 days at room temperature. These properties can greatly facilitate fieldwork. The best voltammetric response for complexed copper reduction was obtained in 0.1 mol L−1 phosphate buffer, at pH 7.3, using differential pulse (25 mV) and a scan rate of 20 mV s−1. The linear response for copper spanned nine orders of magnitude, from 10−9 to 10−1 mol L−1, with a detection limit of 0.3 × 10−9 mol L−1 and a relative standard deviation of 8.3% for copper at 1.03 × 10−6 mol L−1. Common trace metals in natural waters such as Pb, Cd, Fe, Mn, Ni, Cr, and Zn at concentrations 1000 times higher than that of copper did not interfere with the copper signal. The accuracy of the method was confirmed by the analysis of a certified river water sample and by comparison with the results obtained using adsorptive cathodic stripping voltammetry with oxine. The proposed sensor is inexpensive, stable, selective, and enables evaluation of the most bioavailable fraction of copper in natural waters.