Novel chelating ligands are very significant for preparing nanocrystals with different morphologies and applications. In this paper, we directly introduced amine groups onto UCNPs by choosing a new chelating ligand tetraethylene pentamine (TEPA) to synthesis
Upconversion (UC) means that the materials absorb long-waved photons to emit short-waved photons. Rare earth compounds have evoked much attention due to their novel electronic, optical, and chemical characteristics. The special 4f shell electron structure of the rare earth elements makes the upconverting nanoparticles (UCNPs) capable of converting two or more near-infrared (NIR) photons to one visible light photon via continuing electronic excitation and energy transfer processes [
Generally, the synthesis methods of UCNPs can be divided into two categories. One was thermolysis method, using long chain organic ligands as solvent and chelating ligand under very high temperature (300–340°C). Because of the high temperature of the thermolysis synthesis, few chelating ligands can be choosen. The most frequently used chelating ligand was oleic acid (OA) together with a noncoordinating solvent octadecene (ODE), which controlled the nucleation and growth as well as stabilization of the crystals in a thermolysis procedure. Other chelating ligands such as oleylamine [
Herein, we directly introduced amine groups onto UCNPs by choosing a new chelating ligand tetraethylene pentamine (TEPA) to synthesis NaYF4:Yb, Er through hydrothermal method. Tetraethylene pentamine’s coordination groups are amine whose coordination ability is relatively week, while rod
All the chemicals were used directly without further purification. Tetraethylene pentamine (chemical grade) and Y (NO3)3
In a typical synthesis of hexagonal phase NaYF4:Yb, Er nanocrystals, 4 mL 0.25 M 78% Y(NO3)3, 20% Yb(NO3)3, 2% Er(NO3)3 aqueous solution, 16 mL deionized water, and 20 mL ethanol were mixed with stirring at room temperature, after then, 0.2 mL TEPA was added into above solution, forming a A solution. 504 mg NaF was dissolved in 10 mL deionized water and 10 mL ethanol to form solution B. After 0.5 h stirring of solution A, B solution was added into A solution drop by drop under vigorous stirring. After aging for 30 min, the mixture was transferred to a 76 mL Teflon-lined autoclave, sealed, and heated at 200°C for 6 h. As the autoclave was cooled to room temperature naturally, collected the precipitates in the bottle, washed with ethanol and deionized water in sequence, gathered the products by centrifugation, then dried in air at 70°C for 10 h.
X-ray powder diffraction patterns were measured on a Rigaku D/MAX-2400 with Cu-Ka radiation. Upconversion fluorescence spectra were recorded on Hitachi F-4500 fluorescence spectrophotometer under the excitation of a 980 nm diode laser. The morphologies of the samples were observed by using a Nova Nanosem 450 field emission scanning electron microscopy (FE-SEM).
The synthetic conditions of NaYF4:Yb, Er via the hydrothermal method were investigated in detail. The influences of rare earth concentration, solvent composition, the ratio of RE/TEPA, and reaction time were all found to have effect on the luminescence properties and morphology of NaYF4:Yb, Er nanocrystals.
It is know that chelating agent is very important to the crystal’s growth and aggregation. To prove the effect of TEPA on NaYF4:Yb, Er nanocrystals, 0 mL, 0.1 mL, 0.2 mL, and 0.4 mL TEPA had been used to prepare NaYF4:Yb, Er samples, that is, the ratio of the RE/TEPA was 1 : 0, 2 : 1, 1 : 1, and 1 : 2, respectively. Figure
XRD patterns of NaYF4:Yb, Er nanocrystals prepared under different ratio of RE/TEPA (
Upconverted fluorescence spectra of NaYF4:Yb, Er nanocrystals prepared under different ratio of RE/TEPA (
FE-SEM images of NaYF4:Yb, Er nanocrystals prepared under different ratio of RE/TEPA: (a) RE/TEPA = 1 : 0; (b) RE/TEPA = 2 : 1; (c) RE/TEPA = 1 : 1; (d) RE/TEPA = 1 : 2 (
Different reaction time was investigated to clarify that time has a effect on NaYF4:Yb, Er nanocrystal’s size and fluorescence intensity. 2 h; 4 h; 6 h; 8 h; 10 h were chosen to study. Figure
XRD patterns of NaYF4:Yb, Er nanocrystals prepared under different reaction time (RE/TEPA = 1 : 1; water/ethanol = 1 : 1; 16.7 mmol/L).
Upconverted fluorescence spectra of NaYF4:Yb, Er nanocrystals prepared under different reaction time (RE/TEPA = 1 : 1; water/ethanol = 1 : 1; 16.7 mmol/L).
FE-SEM images of NaYF4:Yb, Er nanocrystals prepared under different reaction times: (a) 2 h; (b) 4 h; (c) 6 h; (d) 8 h; (e) 10 h (RE/TEPA = 1 : 1; water/ethanol = 1 : 1; 16.7 mmol/L).
Water and ethanol were used as solvent in this study. The composition of the water/ethanol was taken as 2 : 1; 1 : 1; 1 : 2 with identical other parameters. The XRD of the as-prepared products is showed in Figure
XRD patterns of NaYF4:Yb, Er nanocrystals prepared under different solvent composition (RE/TEPA = 1 : 1;
Upconverted fluorescence spectra of NaYF4:Yb, Er nanocrystals prepared under different solvent composition (RE/TEPA = 1 : 1;
FE-SEM images of NaYF4:Yb, Er nanocrystals prepared under different solvent composition: (a) water/ethanol = 2 : 1; (b) water/ethanol = 1 : 1; (c) water/ethanol = 1 : 2 (RE/TEPA = 1 : 1;
As the volume of the Teflon-lined autoclave and total liquid is constant (60 mL), we changed moles of the rare earth to investigate the effect of the rare earth concentration on the fluorescence intensity and morphology of the as-prepared NaYF4:Yb, Er. The concentration of the rare earth was taken as 8.33, 16.7, and 33.3 mmol/L, respectively, the other parameters were constant. Figure
XRD patterns of NaYF4:Yb, Er nanocrystals prepared under different rare earth concentration (RE/TEPA = 1 : 1;
Upconverted fluorescence spectra of NaYF4:Yb, Er prepared under different rare earth concentration (RE/TEPA = 1 : 1;
FE-SEM images of NaYF4:Yb, Er nanocrystals prepared under different rare earth concentration: (a) 8.33 mmol/L; (b) 16.7 mmol/L; (c) 33.3 mmol/L (RE/TEPA = 1 : 1;
In summary, we have directly introduced amine groups onto UCNPs using TEPA as a new chelating ligand. A series of hexagonal phase NaYF4:Yb, Er nanoparticles was obtained through hydrothermal method. The influences of rare earth concentration, the ratio of rare earth to TEPA, reaction time, and solvent composition on the morphology and fluorescence intensity of the as-prepared NaYF4:Yb, Er samples were investigated. It was found the lower the rare earth concentration, the stronger the fluorescence intensity of the as-prepared NaYF4:Yb, Er. When the ratio of RE/TEPA decreased, the size of the as-prepared nanocrystals increased, while when RE/TEPA reduced to 1 : 2, the fluorescence intensity of the product decreased. Long reaction time was beneficial to the growth of the nanocrystal, when the time extended to 6 h, prolonged reaction time, nanocrystal’s size had no evidence change, but the fluorescence intensity still enhanced. The solvent in different composition of water and ethanol was also discussed; with the increase of ethanol in the solvent, the size of the particle decreases, and when water/ethanol is 1 : 1, the fluorescence intensity of the as-prepared samples is strongest. The paper demonstrates that amine can be covered by nanocrystals directly without any other surface modification that makes the synthesis of bioprobe and bioimaging more easy and effective.
This work was supported by the National Natural Science Foundation of China (21076038, 20836001, 20923006), the Fundamental Research Funds for the Central University (DUT11LK30), National Key Technology Support Program 2011BAE07B01, and the Doctoral Fund of Ministry of Education of China (20100041120024).