A smartphone-assisted ratiometric fluorescence sensor based on metal nanoclusters for visual detection of Cu²

A smartphone-assisted ratiometric fluorescence sensor based on green emissive Cu nanoclusters (Cu NCs) and red emissive Ag nanoclusters (Ag NCs) is constructed, in which the Cu NCs and Ag NCs bind with each other through electrostatic interaction, leading to fluorescence resonance energy transfer from Cu NCs to Ag NCs. The copper ions (Cu²⁺) can quench the red fluorescence of the Cu NCs -Ag NCs system, while the green fluorescence remains essentially unchanged, thus enabling the fluorescence ratio detection of Cu²⁺. This method demonstrates high selectivity for Cu²⁺ and a wide detection range (1.0 to 60.0 μM). The limit of detection is as low as 0.36 μM, which is significantly lower than the maximum tolerable concentration in drinking water (~ 20 μM). In addition, the fluorescence color can be converted into RGB values by a smartphone, indicating great potential for point-of-care testing of Cu²⁺ in real water samples.