Ratiometric fluorescent nanosensors for ultra-sensitive detection of mercury ions based on AuNCs/MOFs

Ratiometric fluorescent nanosensors were developed to detect mercury ions (Hg2+) using enhanced dual emissions from glutathione stabilized gold nanoclusters/indium-based metal–organic frameworks modified with cysteine (AuNCs/MIL-68(In)-NH2/Cys). The nanosensors exhibited bright pink fluorescence with AuNCs evenly distributed on MIL-68(In)-NH2. Under 370 nm excitation, the obtained sensor presented double fluorescence emission around 438 nm and 668 nm, which was attributed to MIL-68(In)-NH2 and GSH-AuNCs, respectively. The fluorescence emission was remarkably enhanced after modification with Cys. In the presence of Hg2+, the red fluorescence peak at 668 nm was quenched, while the blue fluorescence peak at 438 nm was slightly altered. The prepared AuNCs/MIL-68(In)-NH2/Cys nanosensors exhibited two linear ranges for the detection of Hg2+, namely from 20 pM to 0.2 μM and 0.2 μM to 60 μM, with a detection limit of 6.7 pM. They also presented high selectivity towards other ions and good performance in real water samples. Moreover, a radial star-shaped microfluidic paper-based analytical device (μPAD), as a straightforward and convenient platform, was successfully fabricated for the visual detection of Hg2+ with a wide detection range from 5 nM to 50 μM.