Semiquantitative naked-eye detection of Cu(II) with a standard colorimetric card via a hydrogel-coated paper sensor

A new type of hydrogel-coated paper sensor was developed for the semiquantitative and naked-eye detection of Cu2+ ions with high selectivity and good regenerability. The paper sensor showed a continuous color change from colorless to blue, resulting in semiquantitative naked-eye detection. Furthermore, similar to the classical pH test paper, a standard colorimetric card was used in the visual detection of Cu2+ ions by identifying the RGB value of the hydrogel-coated paper sensor. The application of the standard colorimetric card has greatly shortened the detection time and reduced the cost of detection, providing a new strategy for the naked-eye and semiquantitative detection of heavy-metal ions in water samples.