JOURNAL OF RARE EARTHS, Vol. 29, No. 12, Dec. 2011, P. 1137 Foundation item: Project supported by Vietnam's National Foundation for Science and Technology Development (NAFOSTED) (103.06.46.09) and part of Viet- nam Basic Research Programming for Application (2/2/742/2009/ HĈ-ĈTĈL) Corresponding author: Tran Thu Huong (E-mail: huongtt@ims.vast.ac.vn, thuhuongims@yahoo.com; Tel.: +84 466 599 000) DOI: 10.1016/S1002-0721(10)60612-6 Fabrication and characterization of YVO 4 :Eu 3+ nanomaterials by the micro- wave technique Tran Thu Huong 1 , Vu Duc Tu 2 , Tran Kim Anh 1 , Le Thi Vinh 3 , Le Quoc Minh 1,2 (1.Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay Distr., Hanoi, Vietnam; 2. University of Engi- neering and Technology, National University Hanoi, Hanoi, Vietnam; 3. Departments of Chemistry, Hanoi University of Mining and Geology, Hanoi, Vietnam) Received 15 August 2011; revised 1 September 2011 Abstract: Fabrication and characterization of YVO 4 :Eu 3+ nanophosphors prepared by microwave (MW) irradiation assisted soft template synthesis were reported. The effects of synthesis conditions such as different powers of MW irradiation, pH values and concentration of reac- tion materials on properties of nanophosphor were also investigated to obtain the controllable size, morphology and high luminescence effi- ciency. Morphology, crystalline structure, and optical properties were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and fluorescence spectroscopy, respectively. The results showed that YVO 4 :Eu 3+ nanophosphors were obtained by using diethyleneglycol (DEG) as soft template, with pH values in the range of 4 to 12, upon microwave irradiation from 300 to 900 W, at temperature of 80 ºC. The high fluorescent YVO 4 :Eu 3+ nanocrystals obtained with size from 15 nm down to 8 nm are more effec- tive to develop an ultrahigh sensitive fluorescent label for biomolecule, cell and tissue. Keywords: microwave synthesis method; DEG soft template; YVO 4 :Eu 3+ nanophosphor; luminescence; rare earths Recently fluorescent nanophosphor become a novel bio- medical analysis tool for their potential to develop a labeling immunoassay with high sensitivity, photochemical stability and the increasing ratio of fluorescent agents to biomolecu- lar-protein (F/P ratio) [1–6] . Especially, YVO 4 :Eu 3+ nano- phosphor have a high quantum efficiency of red emission under UV light irradiation. It is expected that YVO 4 :Eu 3+ nanophosphor can be used in some fields such as lighting, security, the agrobiology and biomedicine. YVO 4 :Eu 3+ nanophosphors have been prepared by hydrothermal treat- ment [7] , and by coprecipitation [8] . However, most of nano- phosphors reported in past immunoassays are from 30 to 15 nm in diameter. The particles sizes over 30 nm are proven to bring obviously prolonged equilibration time and enhanced nonspecific adsorption [9] . Therefore nanophosphors have a smaller size, for which their kinetic properties could be im- proved and non-specific adsorption decreased meanwhile. Moreover, the nanophosphors should be large enough to bind several proteins on the surface, which is expected to in- crease the immunological affinity [10] . We have successfully synthesized by hydrothermal method at 200 ºC in autoclave YVO 4 :Eu 3+ nanophosphors with the size around 18 nm which possessed high fluorescent efficiency. We have de- veloped from this nanophosphor a nanolabel for security and newly for fluorescent label in biomedicine [11–15] . In the case of the MW irradiation or heating synthesis there are reports on synthesis of YVO 4 nanoparticle in MW irradiation of re- action fluid [16] or fabrication of YVO 4 :Eu 3+ phosphors by MW heated solid state reaction at very high temperature [17] . However, according to our knowledge, there are rather few reports on the chemical synthesis by MW irradiation at low temperature (below 100 ºC) to obtain YVO 4 :Eu 3+ nano- phosphor with the size below 15 nm. In this paper, we report new results about synthesis and characterization of YVO 4 :Eu 3+ nanophosphors with size from 15 to 8 nm by microwave assisted soft template syn- thesis at 80 ºC. The effects of synthesis conditions such as powers of MW irradiation, pH values and concentration of soft template agents on properties of nanomaterials were also investigated to obtain the controllable size, expected mor- phology and high luminescence. 1 Experimental 1.1 Synthesis The microwave synthesis used a MAS-II digital control- ling microwave synthesis extraction workstation, Sino Co. The workstation provided an exact control on temperature from room one to several hundreds, powers of MW irradia- tion from 200 to 1000 W, reaction time from second to min- ute scale, and magnetic or mechanical stirring. The YVO 4 :Eu 3+ nanophosphors were prepared by micro- wave irradiation of an aqueous solution of yttrium(III) nitrate, europium(III) nitrate and Na 3 VO 4 , in an open reflux system. The synthesis reactions were implemented at temperature of