Enhanced upconversion fluorescent probe of single NaYF 4 :Yb 3+ /Er 3+ /Zn 2+ nanoparticles for copper ion detection.

Surface modified NaYF 4 :Yb 3+ /Er 3+ /Zn 2+ upconversion nanoparticles were obtained by using branched polyethylenimine (PEI). Strong fluorescence emission was observed and the influence of copper ions on the fluorescence emission of the PEI-modified NaYF 4 :Yb 3+ /Er 3+ /Zn 2+ nanoparticles was investigated. It was found that the fluorescence emission can be quenched through luminescence resonance energy transfer from the particle to the copper ions. The results show that the PEI modified NaYF 4 :Yb 3+ /Er 3+ /Zn 2+ nanoparticle can be used as a fluorescent probe for highly sensitive and selective detection of copper ions.