Synthesis of NaYF4:Yb3+, Er3+ upconversion nanoparticles in normal microemulsions

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.

It is known that an important prerequisite for the applications of UCNPs is the availability of small and monodisperse nanoparticles [1]. Recently, the synthesis of various inorganic nanoparticles in normal microemulsions attracts our attention [21]. 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 NaYF 4 :Yb 3+ , Er 3+ UCNPs by this method. Therefore, we designed an oil/water interfacecontrolled reaction in normal microemulsions (water/ surfactant/n-hexane) to produce NaYF 4 :Yb 3+ , Er 3+ 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
First, the phase behavior of the system was studied to obtain the appropriate microemulsions. Figure 1 shows the empirical phase diagram of the water/ethanol/ sodium oleate (NaOA)/oleic acid (OA)/n-hexane mixtures at 298 K. Because of the complexity of the fivecomponent system, the phase diagram was simplified to a ternary phase diagram, which is composed of total OA (including the part to generate NaOA with sodium hydroxide), water plus ethanol, and n-hexane. The composition is described using volume fractions. The water/ ethanol ratio is always 1:1. The NaOA/OA molar ratio is always 2:3, and the total volume of OA is considered as the surfactant volume. The phase diagram is determined by gradual addition of n-hexane to a one-phase water/ethanol/NaOA/OA mixture with a constant volume fraction. For example, we begin from point A, and reach a critical point C where the solution starts showing a two-phase character.
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. Figure 2 shows the characterization data for the NaYF 4 :20% Yb 3+ , 2% Er 3+ sample. The TEM image (Figure 2A) demonstrates that the synthesized particles are roughly spherical, monodisperse with the size uniformity of about 20 nm in diameter. The X-ray powder diffractometer (XRD) pattern ( Figure 2B) shows well-defined peaks, indicating the high crystallinity of the synthesized material, and the peak positions and intensities from the experimental XRD pattern match closely with the calculated pattern for cubic phase of NaYF 4 (JCPDS card, No. 77-2042). From the line broadening of the diffraction peaks, the crystallite size of the sample was determined to be approximately 18 nm using the Debye-Scherrer formula, which corresponds to the particle size determined from the TEM result.
The NaYF 4 :Yb 3+ , Er 3+ UCNPs can easily be dispersed in nonpolar solvents (such as n-hexane, cyclohexane) to form homogenous colloidal solutions. Figure 3A shows images of a 1 wt.% solution of NaYF 4 :20% Yb 3+ , 2% Er 3+ UCNPs in n-hexane, demonstrating its transparency. The visible upconversion luminescence can be observed when the solution is excited at 980 nm with a power density of 1.2 kW/cm 2 ( Figure 3B). The corresponding upconversion luminescence spectrum is also shown in Figure 3C. There are three major emission bands at 520-530 nm (green light), 540-550 nm (green light), and 650-670 nm (red light), which are assigned to the 2 H 11/2 to 4 I 15/2 , 4 S 3/2 to 4 I 15/2 , and 4 F 9/2 to 4 I 15/2 transitions of Er 3+ ion, respectively. Under 980 nm excitation, the absorption of the first photon can elevate Yb 3+ ion to the 2 F 5/2 level from ground state, and then it can transfer the energy to the Er 3+ ion. This energy transfer can promote Er 3+ ion from 4 I 15/2 level to the 4 I 11/2 level and from the 4 I 11/2 level to the 4 F 7/2 by another energy transfer upconversion process (or a second 980 nm photon) if the 4 I 11/2 level is already populated. Then, the Er 3+ ion can relax nonradiatively to the 2 H 11/2 and 4 S 3/2 levels, and the green emissions occur ( 2 H 11/2 4 I 15/2 and 4 S 3/2 4 I 15/2 ). Alternatively, the ion can further relax and populate the 4 F 9/2 level leading to the red emission ( 4 F 9/2 4 I 15/2 ) [8,22]. The curve also shows that red emissions are much stronger than green  emissions, so the products present light of orange color on the whole ( Figure 3B).
It is noted that the as-prepared nanoparticles are cubic phase, whose fluorescence efficiency is at least one-order of magnitude less than that of the hexagonal phase [8]. A thermal treatment at ca. 400-600°C was reported to transform the cubic phase to the hexagonal phase, but which led to undesirable particle growth and agglomeration [2]. We carried out the annealing of the as-prepared nanoparticles under N 2 atmosphere by heating them to 600°C, and maintaining this temperature for 5 h. After annealing, the particles aggregated into larger clusters ( Figure 4A), and the XRD pattern ( Figure 4B) shows that hexagonal NaYF 4 :Yb 3+ , Er 3+ phase emerged in addition to the already existing cubic pattern (marked with asterisks), which implies that the particles transformed partially from cubic phase to hexagonal phase by annealing. In addition, upconversion luminescence emission spectrum ( Figure 5) was obtained after ultrasonic dispersion of a 1 wt.% solution of the products in nhexane, compared with the spectrum of nanoparticles before annealing, its green emission plays a dominant role, and the overall emissions are much stronger than those for cubic phase products.

Conclusions
In summary, we designed a method of normal microemulsions to prepare NaYF 4 :Yb 3+ , Er 3+ 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(NO 3 ) 3 · 6H 2 O (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(NO 3 ) 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 NaYF 4 :20% Yb 3+ , 2% Er 3+ UCNPs were extracted into the upper layer (nhexane 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 NaYF 4 :20% Yb 3+ , 2% Er 3+ UCNPs were characterized by XRD and TEM. The obtained samples were characterized by XRD using a Brucker D8-advance X-ray diffractometer with Cu Ka 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 NaYF 4 :20% Yb 3+ , 2% Er 3+ 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.