Facile synthesis of near-infrared-excited NaYF:Yb, Tm nanoparticles for label-free detection of dopamine in biological fluids.

Rare-earth-doped upconversion nanoparticles (UCNPs) behaved advantages such as non-autofluorescence, environment-friendly synthetic route, low cost and simple detection procedures. In addition, the UCNPs show unique upconvering optical property of converting low-energy infrared photons to high-energy visible ones. Here in our work, water and ethylene glycol at the ratio of 1/1 (v/v) were chosen as solvent. Polyethyleneimine (PEI) was chosen as surface additives to synthesize the NaYF:Yb, Tm UCNPs by one-pot route at 200°C. The as-prepared UCNPs exhibited a strong emission at 473nm under the continuous excitation at 980nm. Dopamine (DA), one of the important catecholamine neurotransmitters, shows a crucial role to regulate the memory and attention in brain, renal and hormonal systems. DA molecules could be gently oxidized to quinone structure (ox-DA) in air-containing buffer solution under alkaline condition. In this study, a label-free luminescent probe based on the luminescence energy transfer (LET) between the NaYF:Yb, Tm UCNPs and ox-DA was developed for highly sensitive and selective detection of DA. The luminescent response ratios (I/I) increased linearly with the concentration of DA increasing in the ranges of 0.25 - 150µM. The relative standard deviation (RSD) for 11 replicate detections of 25µM DA was 0.48%, and the limit of detection was 124nM. The developed method was applied for detection of DA in biological fluids with the quantitative spike recoveries from 94.8% to 103.6%.