Comparison of concentration dependence of relative fluorescence quantum yield and brightness in first biological window of wavelengths for aqueous colloidal solutions of Nd3+: LaF3 and Nd3+: KY3F10 nanocrystals synthesized by microwave-hydrothermal treatment

We set simple criteria to choose crystal host for Nd3+ doping and a value of doping concentration for synthesis of aqueous colloidal solutions of the fluoride nanocrystals (NCs) to use them as fluorescence probes for bioimaging in the first biological window of wavelengths (750–950 nm). It is a large Ω4/Ω6 ratio of intensity parameters, used in Judd-Ofelt theory, which increases a fluorescence branching ratio β to the 4F3/2 → 4I9/2 transition lying in the first biological window; and a low value of intensity parameters Ω6, which makes weaker the Nd-Nd fluorescence self-quenching and quenching by OH- molecular groups positioned in the volume of the NCs. Hypothesis is checked on the concentration series of highly dispersible aqueous colloidal solutions of the Nd3+: LaF3 and Nd3+: KY3F10 NCs synthesized by hydrothermal microwave (HTMW) treatment with PVP as biocompatible surfactant. To do this, the dependences of relative fluorescence quantum yield (FQY) and fluorescence brightness (FB) on Nd3+ concentration connected with nonradiative energy transfer to the quenching acceptors are found from the measured fluorescence kinetics. We observed smooth decrease in FQY and pronounced maximum in FB with increasing neodymium concentration in both systems. However, for Nd3+: LaF3 NCs the fall of fluorescence brightness begins at a concentration four times higher than for Nd3+: KY3F10, which is due to a weaker energy transfer to the quenching acceptors in the former. At the same time the fluorescence branching ratios to the first and the second biological windows are almost equal for the Nd3+: LaF3, while for Nd3+: KY3F10 it is higher to the second biological window. The results are in line with twice larger the Ω4/Ω6 ratio and three times smaller Ω6 intensity parameter in Nd3+: LaF3. These criteria must be accounted together with a choice of a strategy for synthesis of the NPs with high crystallinity [ChemistrySelect 2 (2017) 4874–4881] and high water dispersibility during selection of a crystal host for Nd3+ doping to use it for bioimaging in the first biological window of wavelengths.