Influence Of TGA Modification On Upconversion Luminescence Of Hexagonal-Phase NaYF₄: Yb, Er Nanoparticles

Recently, fluorescence-based techniques have gained considerable attention due to their current and potential applications in the field of biology. Currently, the commonly used fluorescent nanoprobes are organic dyes and quantum dots (QDs), because they can be very small in size and have high aqueous solubility. However, both of them are down-conversion fluorescent materials. The biological tissue can be photodamaged after long-term ultraviolet (UV) irradiation. Besides, organic dyes and quantum dots (QDs) have their intrinsic disadvantages such as photobleaching, broad emission band, chemical instability, background noise, and toxic to living organisms.The rapid development of material science has brought us some newer and striking lanthanide (III)-doped upconverted fluorescent labels, which have unique antistokes optical property with the excitation of noninvasive 980-nm near infrared (NIR) light. The intrinsic properties of the lanthanide (III)-doped upconversion (UC) NCs, such as high chemical stability, high quantum yield, and low background noise, can overcome many problems existed in the earlier classes of fluorescent nanoprobes. Among all the lanthanide (III)-doped upconverted NCs, NaYF₄ is a perfect host material for green and blue phosphors owing to its low lattice phonon energy thereby can minimize nonradiative loss and maximize the radiative emission. Moreover, crystal structure of the fluoride plays an important role to control the optical properties. Due to site symmetry and multisite character of the structure of hexagonal phase, the UCL intensity of hexagonal-phase NaYF₄:Yb,Er NCs is at least one order of magnitude more than the cubic-phase NCs. These considerations have lead to intensive research on the synthesis of hexagonal-phase NaYF₄ NCs with free carboxylic acid groups or amino groups on their surfaces which allow conjugation with biomolecules for further biomedical applications.Many efforts have been devoted to the synthesis of hexagonal-phase NaYF₄ NCs with controllable size and morphology in recent years. Fluorescent nanoparticles used for biomedical applications should be small with narrow size distribution, and water soluble with high luminescent efficiency. In this paper, uniform hexagonal-phase NaYF₄:20%Yb,2%Er nanocrystals (NCs) were synthesized in high boiling solvent oleylamine (OM) at 320℃in an efficient and user friendly method in which rare earth nitrates were used instead of trifluoroacetates. In our present work, we fixed the ratio of Re3+/F- as a constant of 1/4 to ensure that there were no excessive fluoride reactants. Because at high temperatures, the excessive fluoride reactants were easily hydrolyzed to produce HF gas which may raise safety concerns.These hydrophobic NCs were further rendered hydrophilic using the ligand thioglycollic acid (TGA). TGA has been successfully used as the stabilizer in synthesizing water soluble semiconductor quantum dots (QDs). To the best of our knowledge, it has not been used for the surface modifications of lanthanide ion doped nanoparticles. Our goal was to utilize its sulfhydryl group (-SH) coordinated with lanthanide ions, and its carboxylic acid group (-COOH) stretched out, which facilitated further conjugation with bimolecular such as antibodies and antigens.And more, previous studies mostly concentrated on the preparation and charatererization of such compositions. As to the influence of surface modification on UC processes, was rarely studied. Here, we systemically discussed the influence of TGA modification on UCL properties of NaYF₄:Yb3+, Er3+ NCs and their origins. After surface modification, the UCL revealed not only an enhancement of the red (4F9/2–4I15/2) emission with respect to the green (2H11/2, 4S3/2–4I15/2) one, but also an increase of the blue (2H9/2–4I15/2) due to the involving of surface vibration bonds–COOH, while the overall intensity had rarely change. UCL dynamics were also studied, which indicate that the decay time constants of the green and the red emissions had only a little decrease, while the rising time constants of them decreased considerably. The rate equations for the red and green emissions were set up and solved to analyze the luminescent dynamics. The present results demonstrate that the modification of TGA can both make the NaYF₄ NCs realize bio-functionality and efficient UCL.