Upconversion particles

In this study we use the dye Coumarin 343 (C343) adsorbed on the surfaces of ZrC>2 nano particles in aqueous solution to study the solvation dynamics close to these surfaces. Zr02 is, in many respects, very similar to Ti02 and serves as a suitable model substance since, due to its higher band gap energy, electron injection from adsorbed dyes does not occur. To measure the time resolved Stokes shift, we used femtosecond frequency-resolved upconversion. 

Luminescence has lead to many more applications than those discussed in the previous four chapters. In this chapter some of these will be discussed shortly. For this purpose we selected the following topics upconversion, the luminescent center as a probe, luminescence immuno-assay, electroluminescence, optical fibers, and small particles. 

Figure 2 shows a high-resolution transmission electron micrograph (HRTEM) of core/shell LaP04 Yb,Er nanocrystals. The cores of these particles have been doped with 1% of Er " and the shells with approximately 1% of Yb ". The particles did crystallise in a monochnic monazite structure. Core and shell can be clearly distinguished in the HRTEM. It was observed that the upconversion luminescence intensity of core/shell nanoparticles is larger as compared with homogeneous mies. 

Fig. 16.21 Room temperature upconversion emission spectra of a NaYF4 Yb/Er (18/2 mol%), b NaYF4 Yb/Tm (20/0.2 mol%), c NaYF4 Yb/Er (25-60/2 mol%), and d NaYF4 Yb/Tm/Er (20/0.2/0.2-1.5 mol%) particles in ethanol solutions (10 mM). The spectra in (c) and (d) were normalized to Er 650-nm and Tm 480-nm emissions, respectively. Compiled luminescent photos showing corresponding colloidal solutions of e NaYF4 Yb/Tm (20/0.2 mol%), f-j NaYF4 Yb/Tm/Er (20/0.2/0.2-1.5 mol%), and k-n NaYF4 Yb/Er (18-60/2 mol%). The samples were excited at 980 nm with a 600 mW diode laser. The photographs were taken with exposure times of 3.2 s for e-l and 10 s for m and n Reprinted from Ref. [52], with permission from Springer Science + Business Media...

Proper synthetic approaches should be selected depending on the scope of applications. For bioimaging, it is recommended to use UCNPs with an adequate upconversion luminescence efficiency, a small size (preferably at sub-10 mn level), a narrow size distribution, and availability/ease of surface modifications. However, in general, small-sized UCNPs have weak upconversion luminescence, and some modification methods would adversely affect or quench the luminescence. Therefore, compromising the luminescence intensity with particle size and functionalization becomes inevitable. 

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