Synthesis Of NaYF₄:Yb/Er/Gd Up-conversional Nanoparticles And Their Application In Luminescence Resonance Energy Transfer

Rare-earth (RE) doped upconversion luminescent nanoparticles (UCNPs), which can convert a longer wavelength radiation to a shorter wavelength luminescence via a two-photon or multi-photon mechanism, have emerged as an important class of luminescent materials. UCNPs show prominent advantages such as low toxicity, good stability, high luminous intensity, and large Stokes shifts, etc. In addition, its excitation light is IR radiation, leading to an excellent signal-to-noise ratio due to the absence of autofluorescence and reduction of light scattering. So, UCNPs used as biolabels for biological detections have garnered a tremendous amount of attention due to their unique optical properties. The controllable synthesis and surface functionalization of UCNPs with high-quality and strong fluorescent performance are the precondition for their applications in biological detections. In this work, a systemic study on the synthesis, surface modification and application in luminescence resonance energy transfer of NaYF4:Yb/Er/Gd UCNPs was carried out.A kind of excellent UCNPs were synthesized via a solvothermal method by using of rare-earth (Y3+, Yb3+, Er3+and Gd3+) stearate and NaF as precursor as well as ethanol, water and oleic acid as solvent. The doped Gd3+ions can promote the crystal transfer of NaYF4from cubic to hexagonal and shorten the reaction time meanwhile the luminescence intensity stronger than those NaYF4:Yb/Er UCNPs obtained in the identical experiment condition. The reaction temperature was rather lower and any poisonous reagent was not used in the process. X-Ray diffraction (XRD), fourier transformation infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and luminescence spectroscopy were applied to characterize the prepared NaYF4:Yb/Er/Gd UCNPs. The nanoparticles showed small size and excellent green up-conversion photoluminescence properties which made them suitable for biological application.After the surface of NaYF4:Yb/Er/Gd UCNPs was modified with a typical Stober method, the amino-modified NaYF4:Yb/Er/Gd UCNPs were first conjugated with phycoerythrin (R-PE) which has many carboxyl groups. Then a luminescence resonance energy transfer (LRET) system was developed by using NaYF4:Yb/Er/Gd UCNPs as an energy donor and R-PE as an energy acceptor. In the LRET, there are two linear ralationships, one is between the Napierian Logarithm of the UCNP’s fluorescent intensity at543nm (In Iuc) and the concentration of R-PE (C) while the other is between the Napierian Logarithm of the R-PE’s fluorescent intensity (In I) and the Napierian Logarithm of the R-PE’s concentration (In C). A detection limit (3a) of R-PE of0.5μg/mL was obtained by the LRET system with a RSD of2.0%(n=11,21.9μg/mL). This approach is expected to be extended to detect other protein, biological molecules and so on.