This paper-based sensor has a wide semiquantitative detection range for Cu2+ (0.01~5.0 μg mL-1), and has been shown to be effective for the on-site visual detection of Cu2+ in red wine without any pretreatment procedure. By comparing the colors of standard and actual samples on the CdTe QDs/GCNNs paper-based sensor, the researchers were able to semiquantitatively detect Cu2+. This paper-based sensor has the characteristics of being cheap, fast, on-site, simple, and having a broad intuitive detection range, which makes it highly valuable for the determination of Cu2+ in liquid samples.