An "off-on" colorimetric and fluorometric assay for Cu(II) based on the use of NaYF4:Yb(III),Er(III) upconversion nanoparticles functionalized with branched polyethylenimine.

Abstract

The authors describe an "off-on" colorimetric and fluorometric assay for the determination of Cu(II). It is based on the use of upconversion nanoparticles (UCNPs) of type NaYF4:Yb(III),Er(III) that were functionalized with branched polyethylenimine (BPEI). A color change from colorless to blue occurs within 2s after addition of Cu(II) to a solution of the modified UCNPs. The color change can be visually detected at Cu(II) concentrations down to 80μM. The upconversion fluorescence of the modified UCNPs, measured at excitation wavelength of 980nm, is reduced due to the predominant inner filter effect caused by the formation of the BPEI-Cu(II) complex. Normalized fluorescence intensity drops linearly in the 50nM to 10μM Cu(II) concentration range, and the fluorometric detection limit is 45nM. Both the color and the fluorescence are recovered on addition of EDTA. Excellent selectivity over other metal ions and anions is achieved. Graphical abstract Upconversion nanoparticles of type NaYF4:Yb,Er werefunctionalized with branched polyethylenimine (UCNP/BPEI) and used in an "off-on" colorimetric and fluorometric assay for Cu(II). The upconversion fluorescence is selectively quenched on addition of Cu(II), and this isaccompanied by a rapid colorless-to-blue color switch. The colorimetric changes and quenched fluorescence canbe reversed by adding EDTA.