A new colorimetric sensor for visible detection of Cu(II) based on photoreductive ability of quantum dots

Quantum dots (QDs) have been widely used to develop analytical methods due to their unique fluorescent and photoelectrochemical characteristics. However, analytical strategies based on their catalytic property are rarely reported. Here, we demonstrate that the photoreductive ability of QDs can be utilized to develop a sensitive and selective colorimetric sensor for Cu(II). By using UV irradiation, 3,3',5,5'-tetramethylbenzidine (TMB) is oxidized to its oxidation product, which can be reduced back to TMB by QDs. The presence of Cu(II) decreases the reductive activity of QDs and results in a color change in a signal-on format. The proposed sensor can be activated using a portable UV lamp or direct sunlight. Under optimized conditions, the detection limit (S/N = 3) is 10 nM via spectrophotometry and 0.1 μM by visible color change. This sensing system was applied to quantify Cu(II) in real samples, such as river water and tap water. Results were in agreement with those obtained using inductively coupled plasma spectrometry (ICP). This study may provide new ideas for the development of QD-based sensors.