Upconversion luminescence properties of monodisperse spherical Y 2 O 2 S:Yb,Ho nanocrystals
- PDF / 768,633 Bytes
- 5 Pages / 584.957 x 782.986 pts Page_size
- 65 Downloads / 202 Views
Monodisperse spherical Y2O2S:Yb,Ho nanocrystals with particle size about 50 nm were prepared by a modified homogeneous precipitation method combined with lowtemperature sulfurization process. The Y2O2S:Yb,Ho nanocrystals exhibit an intense green emission assigned to the Ho3+ ions 5F4, 5S2 ! 4I15/2 transition under 980 nm infrared pump. The upconversion luminescence brightness of Y2O2S:Yb,Ho is 526 Cd/m2 under 4.63 W/cm2 pump density, indicating that the as-prepared Y2O2S:Yb,Ho nanocrystals can meet the requirement of in vivo imaging. The formation mechanism of monodisperse spherical oxysulfide nanoparticles is also discussed. I. INTRODUCTION
The recently introduced fluorescent probe based on upconversion particles (FPUP) represents the new generation of biological detection systems because of its advantages, including large anti-Stokes shifts, no autofluorescence background, and no photobleaching. The detection sensitivity depends on the upconversion luminescence (UPL) intensity, and higher UPL intensity will lead to better detection sensitivity. On the other hand, the application depends on the particle size, for example, 200 to 400 nm particles can penetrate the cellular membranes of the alveoli through the lung to enter the flood circulatory system; particles small than 100 nm can enter bone marrow. So it is generally most preferred that the luminescence particle is as small as possible for FPUP. Because of the nature of UPL, its efficiency (5%) is quite low compared with downconversion luminescence (100%). Furthermore, the UPL efficiency of nanoparticles is rather worse than the counterpart bulk materials. For example, the UPL efficiency decreases dramatically when the particle size is below 30 nm scale.1–3 Thus, a higher pump density is required to obtain needed brightness. To avoid possible thermal decomposition to living organisms, however, the irradiation intensity should be not higher than the order of 10 W/cm2 magnitude for continuous-wave (cw) laser diodes (LD). Therefore, 50 nm UPL nanocrystals may be an optimal choice between sensitivity and application.4,5 So it is essential for in vivo to choose UPL material with higher UPL efficiency. b-NaYF4:Yb,Er is reported as one of the most efficient upconversion luminescence materials known today. The UPL efficiency of b-NaYF4:Yb,Er is a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0208
1756
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 12 Mar 2015
30% higher than that of Y2O2S:Yb,Er under relatively low irradiation intensity.6 Our previous result demonstrated that the UPL output of Y2O2S:Yb,Ho is 2.2 times that of Y2O2S:Yb,Er.7,8 Furthermore, the LD50 for lanthanoides oxides (Ln2O3) is on the order of 1000 mg/ kg,9 lanthanoides oxysulfide (Ln2O2S) presents better chemical stability with respect to that of Ln2O3. So Ln2O2S is not likely to be toxic. As a consequence, Ln2O2S:Yb,Ho is a suitable FPUP. Preparation of monodisperse spherical nanometersized particles with hig
Data Loading...
