Fluorometric determination of copper(II) using CdTe quantum dots coated with 1-(2-thiazolylazo)-2-naphthol and an ionic liquid

Abstract

The sulfur-containing reagents dithizone, 2,3-dimercapto-1-propanol, 2-thenoyltrifluoroacetone, 1,3,4-thiadiazole-2,5-dithiol, and 1-(2-thiazolylazo)-2-naphthol (TAN) were first examined with respect to their stabilizing effect on CdTe nanocrystals. The indicator TAN was found to best passivate the surface of CdTe NCs if added during synthesis of the NCs at 100 °C. These NCs (referred to as NC-1) have an emission that peaks at 430 nm, and a quantum yield (QY) of 25 %. If, however, TAN-capped particles are prepared in presence of an ionic liquid at room temperature, the QY of such nanocrystals (referred to as NC-2) is increased by 80 % and the emission peak is shifted to 460 nm. The NC-1 particles bind Ag(I), Pb(II), Hg(II), Fe(III), Cr(III), and Cu(II) in phosphate buffer of pH 6.5 while NC-2 particles bind Cu(II) only. Binding of metal ions is associated with a drop in fluorescence intensity. In case of NC-1, quenching is observed in the 23.3 nM to 23.3 μM Cu(II) concentration range with detection limit of 0.5 nM. In case of NC-2, quenching occurs in the 233 nM to 233 μM Cu(II) concentration range with detection limit of 5.0 nM. The method was applied to the determination of Cu(II) in spiked biosamples, and recoveries of Cu(II) (at a 233 nM level) in BSA and fetal bovine serum using NC-2 are 95 % and 92 %. This is better than the recoveries from the same media with NC-1 (70 % and 32 %). These findings demonstrate that the NC-2 material is superior in terms of selectivity and stability but inferior in terms of detection limit.