Near Infrared YVO4:Nd,Yb Nano Phosphor Synthesized By Microreaction Method for In Vivo Bioimaging
Authors: Inagaki Toru , Ishigaki Tadashi , Sakata Ryousuke , Uehara Wataru , Ohmi Koutoku 
Published: February 2020

Over past few years, nano phosphors have been attracting attention as candidate materials for in vivo biological imaging, and several types of phosphor material have been investigated. Organic phosphor materials have an excellent luminescent efficiency, however their protein has a discoloration problem during the observation. Semiconductor quantum dot phosphors also have a problem in practice, that is, it contains harmful elements in a human body. Therefore, the development of highly stable and harmless phosphors are desired.[1]An ultraviolet (UV) light, which is usually used for exciting both organic fluorescent materials and semiconductor quantum dot phosphors, damages the human body. In addition, it is hard to observe deep part of a human body by using a UV light. To solve these problems, it is effective to use a high bio-permeability near-infrared region (so called biological window).[2] Therefore, the phosphors having the excitation and emission band in a near-infrared region must be developed. The particle size of 30 - 200 nm is also needed for drug delivery system. [3]We previously reported the synthesis of the nano phosphor materials synthesized by the microreaction (MR) method, which is one of the solution synthesis methods. [4] The MR method has an advantage to perform a homogeneous reaction and to precisely control the reaction conditions such as temperature and pH. A higher quantum efficiency has been obtained by MR method compared to the traditional solution synthesis. [5] In this study, we pay attention to YVO4:Nd,Yb as the vivo bioimaging nano phosphor. The pH value and temperature during the MR synthesis have been varied, and the relation between these reaction parameters and luminescent properties has been investigated.The precursor of YVO4:Nd,Yb was prepared by a citric-acid-gel method using the MR system. The MR system consists of liquid sources, syringe pumps, a mixer cell, a mixing/reaction tube, and an auto-sampler. The 1-mm-width T-branch channel was used as the mixer cell. The cell was prepared by a sandblasting method with glass substrates. A pH sensor was placed at the outlet of the mixer cell. Y(CH3COO)3·4H2O, Nd(CH3COO)3·H2O, Yb(CH3COO)3·4H2O and C6H5Na3O7·2H2O was used as the acid source (pH=5). Na3VO4and NaOH were used as the basic solution (pH=12) sources. These two sources were mixed at the T-branch mixer, and the mixture solution was passed through a Teflon tube (1000μm diameter) in an ultrasonic bath maintained at 85 ºC. The in-situ pH monitor was performed during the synthesis.The obtained precursor was poured into the autoclave. The autoclave treatment was carried out at 130 ºC for 6 h. The synthesized nano phosphor particles were collected by centrifugation and freeze-drying. The photoluminescent (PL) characteristics in the near-infrared region were measured using a spectrometer (JASCO FP-8700).
Figure 1 shows the X-ray diffraction (XRD) pattern of the synthesized samples. Only diffraction peaks due to the tetragonal zircon structure of YVO4 are observed. The grain size (D50) of the sample is about 50 nm as shown in Fig. 2. From the scanning electron microscope (SEM) measurements, it was confirmed that the spherical polycrystal grain consists of smaller crystallites of about 10 nm. Figure 3 shows the typical PL result of the obtained YVO4:Nd, Yb nano phosphor sample. Three PL peaks at about 905, 1070, and 1350 nm are due to the 4F3/2→4I9/2, 4I11/2, and 4I13/2 electron transitions in Nd3+ centers, respectively. That at 985nm is due to the 2F5/2→2F7/2 transition in Yb3+ centers. The PL excitation spectrum (PLE) monitored at 1070 nm is shown in Fig. 4. The excitation bands peaking at about 435, 480, 530, 598, 685, 756 and 810 nm are attributed to the transitions of 4I9/2→4G11/2, 4G9/2, 4G7/2, 4G5/2,4F9/2, 4F7/2 and 4F5/2 in Nd3+ centers, respectively. The board excitation band located at 300-350 nm is due to the YVO4 host absorption. It was confirmed that the proper temperature and pH values are 50 ℃ and pH 8.5, respectively. It concludes that YVO4: Nd, Yb nano phosphor synthesized by MR method is suitable for in vivo bioimaging.
References
[1] T. Zako, et al., Biomateral. Sci., 3, p. 59 (2015).[2] A. M. Smith, et al., Nat. Nanotech., 4, p. 170 (2009).[3] M Gaumet, et al., Eur J PHARM BIOPHARM p.69 (2008).[4] H. Okura et al., Proc. IDW’10, p. 1097 (2010).[5] T. Ishigaki et al., ECS Abstract, p. 2156 (2014).

Figure 1