Core-shell upconversion nanoparticles of type NaGdF.sub.4:Yb,Er@NaGdF.sub.4:Nd,Yb and sensitized with a NIR dye are a viable probe for luminescence determination of the fraction of water in organic solvents

Lanthanide-doped core-shell upconversion nanoparticles (UCNPs) of type NaGdF.sub.4:Yb,Er@NaGdF.sub.4:Yb,Nd were prepared by the co-precipitation method. The luminescence intensity was further enhanced by adding the sensitizer dye IR-808. If water is added to organic solvents [such as N,N-dimethylformamide (DMF), dimethyl sulfoxide, methanol, acetone, acetonitrile, and ethanol] containing the probe, its luminescence intensity peaking at 545 nm is reduced. The decrease is linearly related to the percentage of water in the respective organic solvent. Water fractions ranging from 0.05% to 10% (volume %) can be sensitively detected, and the detection limit is 0.018% of water in DMF. The detection scheme is mainly attributed to the fact that the transfer of energy from the near-infrared light (NIR) dye to the UCNPs is strongly reduced in the presence of traces of water.