A simple method based on screen-printed electrodes and a handheld potentiostatic device is reported for the detection of water soluble CdS quantum dots modified with glutathione. The detection method is based on the stripping of electrochemically reduced cadmium at pH7.0 by using square wave voltammetry. Various parameters that affect the sensitivity of the method are optimized. QD suspension volumes of 20νl and a number of around 2 × 1011 CdS quantum dots have been able to be detected. The proposed method should be of special interest for bioanalytical assays, where CdS quantum dots can be used as electrochemical tracers. © IOP Publishing Ltd.