An agar hydrogel-CuNPs/N@CQDs dual-mode colorimetric/fluorescent indicator for non-destructive monitoring of banana ripening.

A colorimetric-fluorescence ripeness indicator, based on copper nanoparticles and carbon quantum dots doped with nitrogen (CuNPs/N@CQD) immobilized in agar hydrogel (AGH-CuNPs/N@CQDs) for ethylene gas detection, was developed for detecting the ripening of bananas. Ethylene could reduce the fluorescence intensity of CuNPs/N@CQDs and cause a red shift of the LSPR peak. The prepared AGH-CuNPs/N@CQDs indicator had an irreversible response to ethylene with LOD and LOQ of 9.94 and 30.12 μM, respectively. The results of XPS and FTIR confirmed the formation of Cu-OH bonds. The color of the AGH-CuNPs/N@CQDs indicator changed from pale green to yellow and ultimately to brown during banana ripening. There was a strong correlation between the decrease in firmness of banana tissue, the increase in reducing sugars, and the change in the indicator's color. Therefore, the AGH-CuNPs/N@CQDs indicator is capable of simple, low-cost, high-accuracy, and naked-eye identification of the fruit's ripening in intelligent packaging.