Synthesis of NaYF4:Yb3+, Er3+ upconversion nanoparticles in normal microemulsions
© Shan et al; licensee Springer. 2011
Received: 9 April 2011
Accepted: 3 October 2011
Published: 3 October 2011
An interface-controlled reaction in normal microemulsions (water/ethanol/sodium oleate/oleic acid/n-hexane) was designed to prepare NaYF4:Yb3+, Er3+ upconversion nanoparticles. The phase diagram of the system was first studied to obtain the appropriate oil-in-water microemulsions. Transmission electron microscopy and X-ray powder diffractometer measurements revealed that the as-prepared nanoparticles were spherical, monodisperse with a uniform size of 20 nm, and of cubic phase with good crystallinity. Furthermore, these nanoparticles have good dispersibility in nonpolar organic solvents and exhibit visible upconversion luminescence of orange color under continuous excitation at 980 nm. Then, a thermal treatment for the products was found to enhance the luminescence intensity. In addition, because of its inherent merit in high yielding and being economical, this synthetic method could be utilized for preparation of the UCNPs on a large scale.
The synthesis and spectroscopy of NaYF4:Yb3+, Er3+ upconversion nanoparticles (UCNPs) have attracted a tremendous amount of attention because of their potential use in bioanalysis and medical imaging recently [1–5]. Upconversion was first recognized and formulated by Auzel in the mid-1960s , which is a process where low energy light, usually near-infrared (NIR) or infrared (IR), is converted to higher energies, ultraviolet (UV) or visible, via multiple absorptions or energy transfers. Up to now, several synthetic paths have been reported to obtain UCNPs, such as co-precipitation , hydrothermal, or solvothermal processing [7–11], liquid-solid two-phase approach , co-thermolysis of trifluoroacetate [13–17], decomposition of carbonate , diffusion-limited growth , and ionic liquid-assisted technique .
It is known that an important prerequisite for the applications of UCNPs is the availability of small and monodisperse nanoparticles . Recently, the synthesis of various inorganic nanoparticles in normal microemulsions attracts our attention . In the normal microemulsions, reactions are taking place at the interface of the normal micelles. Owing to the polarity inverse caused by the neutralization, the particles can be transferred from water phase to the oil phase. However, to the best of our knowledge, there is no study about the synthesis of NaYF4:Yb3+, Er3+ UCNPs by this method. Therefore, we designed an oil/water interface-controlled reaction in normal microemulsions (water/surfactant/n-hexane) to produce NaYF4:Yb3+, Er3+ UCNPs. The products are small, monodisperse, and high-yielding. They show good dispersibility in nonpolar organic solvents and emit visible upconversion luminescence under 980 nm excitation. Moreover, this synthetic strategy is very facile and less costly, which could be applied to mass-production.
Results and discussion
The result shows that the one-phase/two-phase envelope extends from the point at 100% water plus ethanol to the point at 26.23% water plus ethanol, 20.45% OA, and 53.32% n-hexane, and the two-phase part is located in the lower OA region. Obviously, with an increase of the ratio of OA/(water plus ethanol), more n-hexane can be dissolved into their mixtures to form a stable system. The actual point (point B) we used is located in the right-bottom region, where the oil-in-water microemulsions are formed.
In summary, we designed a method of normal microemulsions to prepare NaYF4:Yb3+, Er3+ UCNPs, which are small, monodisperse, and have good dispersibility in nonpolar organic solvents. Besides, the products exhibited visible upconversion luminescence under 980 nm excitation and a thermal treatment was proved to be able to strengthen the luminescence intensity. This method has its inherent merit in high yielding and being economical. Further study is currently underway to functionalize these synthesized UCNPs for their applications in biolabel and medical imaging.
Materials and methods
All reagents used in this study, including sodium hydroxide, oleic acid, ethanol, n-hexane, sodium fluoride, and Ln(NO3)3 · 6H2O (Ln = Y, Yb, and Er, 99.99%) salt, were of analytical grade from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). These chemicals were used without further purification. Water used in the experiment was double distilled.
In a typical synthetic route, sodium hydroxide (400 mg) was dissolved in a mixture of water (20 mL) and ethanol (30 mL), followed by the addition of oleic acid (7.4 mL) and n-hexane (4 mL); this formed a bright yellow transparent solution. Then, two separate aqueous solutions (5 mL) of Ln(NO3)3 (0.8 mmol, Y:Yb:Er = 78:20:2) and sodium fluoride (3.2 mmol) were added to the above microemulsions one after the other with vigorous stirring. Then, the solution was transferred to a Teflon-lined stainless steel autoclave and heated at 180°C for 6 h. When the autoclave was cooled down to room temperature, the products were found deposited at the bottom. Then, n-hexane (30 mL) was added to destroy the one-phase solution and form a two-phase mixture, so the hydrophobic colloidal NaYF4:20% Yb3+, 2% Er3+ UCNPs were extracted into the upper layer (n-hexane region). With precipitation by additional ethanol, and highspeed centrifugation, the white products (yield: 85%) were re-dispersed in n-hexane to bring out a transparent colloidal solution.
The structure and morphology of NaYF4:20% Yb3+, 2% Er3+ UCNPs were characterized by XRD and TEM. The obtained samples were characterized by XRD using a Brucker D8-advance X-ray diffractometer with CuKa radiation (λ = 1.5418 Å). The low- and high-resolution transmission electron microscopy (HRTEM) was performed on a JEOL JEM-3010 electron microscope operated at 300 kV. The upconversion emission spectra of NaYF4:20% Yb3+, 2% Er3+ UCNPs were acquired using a Jobin-Yvon Fluorolog-3 fluorescence spectrometer system equipped with an external 0-1300 mW adjustable laser (980 nm, Beijing Hi-Tech Optoelectronic Co., China) as the excitation source, instead of the Xenon source in the spectrophotometer, and with an optic fiber accessory.
This study was supported by the Program for New Century Excellent Talents in University (NCET-08-0897), the National 973 Project (No.2010CB933901), the Shanghai Education Committee (09SG43,09zz137, S30406), and the SHNU (SK201101, DZL806).
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