CdTe QDs and GCNNs were physically mixed together to create the sensor. Under a UV lamp, the fluorescent intensity of traffic yellow CdTe QDs is linearly quenched by Cu2+ (used as the detection signal), while blue GCNNs remains unchanged (used as the reference). This results in a distinguishable color change from pink yellow to blue. The limit of detection (LOD) for Cu2+ using this sensor is as low as 0.47 ng mL-1 with 1.4% RSD. The researchers successfully applied the established method to detect Cu2+ in various drinks with satisfactory results. In addition, the authors developed a paper-based sensor by soaking cellulose acetate membrane in CdTe QDs/GCNNs sensor solution.