Up-/Downconversion Fluorescence Dual-Channel Probe Based on NaYF₄: Yb/Er/Eu Nanoparticles for the Determination of Cu(II)

Copper ions [Cu(II)] are essential in many biological processes. Accurate determination of Cu(II) is of great significance for environment monitoring and diseases diagnosis. A dual-channel probe (DCP) with up/downconversion fluorescence is developed using a one-pot solvothermal method in this work. Yb(III), Er(III), and Eu(III) are doped into NaYF4 capped with ethylene imine polymer (PEI), that is, PEI-capped NaYF4: Yb(III), Er(III), and Eu(III) NPs (marked as PEI-capped NYF: Yb/Er/Eu NPs), forming the DCP to determine Cu(II) through the up- and downconversion fluorescence variations. The DCP is designed aiming to minimize mutual interference between the fluorescent signals. Under the optimized conditions, the up-/downconversion fluorescence performances of DCP are excellent, including a linear range for 30 nM–10 μM Cu(II) with the detection limits of 29.30, 18.23, and 26.04 nM (monitoring wavelengths of 545, 592, and 616 nm) as well as excellent selectivity to Cu(II). The mechanism of Cu(II) determination could be attributed to the quenching of up-/downconversion fluorescence because of the inner filter effect triggered by PEI–Cu(II) complexes. To illustrate the potential application of up-/downconversion fluorescence DCP, the concentration of Cu(II) in tap water is detected, and satisfactory results are acquired, indicating that the up-/downconversion fluorescence DCP has promising applications in the fields of food safety and environmental detection. More importantly, the proposed design idea is universal regarding the construction of other dual-channel probes for the detection of metal ions.