Ratiometric fluorescence detection of Cd2+ and Pb2+ by inner filter-based upconversion nanoparticle-dithizone nanosystem

This paper reports a fluorescence sensor based on inner filter effect (IFE) between upconversion nanoparticles (UCNPs) and dithizone for the highly selective and sensitive detection of cadmium ion (Cd2+) and lead ion (Pb2+) in black tea. The fluorescence at 546 nm, 657 nm, 758 nm and 812 nm were obtained and applied as signal indicator upon upconversion nanoparticles excitation at the single wavelength of 980 nm. With the formation of UCNPs-dithizone mixed system pH at 8, the dithizone-Cd2+ complex increases with increasing in concentration of Cd2+, which cause the bathochromic shifting in absorption bands and an upconversion fluorescence (UCF) quenching at 546 nm; at pH 6, the absorption band of dithizone shows a blue shift with addition of Pb2+, leading an upconversion fluorescence recovering at 657 nm. However, in the presence/absence of Cd2+/Pb2+, the fluorescence at 758 nm and 812 nm were not influenced. This implies the Cd2+ and Pb2+ concentration could be monitored based on the fluorescence ratio I546/I758 and I657/I758 respectively. Under optimal condition, the fluorescence show a good linear within the ranges of 0.01 μM-1.0 μM for Cd2+; and 0.025 μM-1.0 μM for Pb2+, with a detection limit of 3.7 and 8.4 nM achieved. The method was applied for Cd2+ and Pb2+ in real sample (black tea and tap water) with recoveries of 99.6% to 108% and RSD value in the range of 0.96 to 1.23 for Cd2+, with recoveries of 96% to 103.2% and RSD value in the range of 0.98 to 1.27 for Pb2+.