Upconversion nanoparticles excited

Yang Y, Tu LP, Zhao JW, Sun YJ, Kong XG, Zhang H (2009) Upconversion luminescence of beta-NaYp4 Yb, Er beta NaYp4 core-shell nanoparticles excitation power, density and surface dependence. J Phys Chem C 113 7164-7169... 

Figure 7 Excitation and emission wavelengths of luminescence sources are plotted on a graph. Fluorophores are helow the reflectance line because they absorb a higher energy photon and emit a lower energy photon. Upconversion nanoparticles are fundamentally different a high-energy photon is emitted following absorption of two low-energy photons. (Reproduced with permission from Ref. 68. The Optical Society, 2008.)...

Besides triplet-triplet annihilation, a further process for achieving upconversion luminescence emission under continuous wave low-energy irradiation is based oti the use of lanthanide ions, most often erbium, holmium, and thulium (III) cations. In particular, a large variety of phosphors based on an inorganic host doped by lanthanide cations have been developed. The abundance of available states in these cations opens a large variety of paths for upconversion. As an example (Scheme 7.6), upconversion nanoparticles codoped with ytterbium and erbium cations exhibit a green emission due to the transitions from Hn/2 and respectively " Sn/2 excited states to the ground state as well as a red emission from the F9/2 state [10]. 

Visibly luminescent, NIR (980 nm) excitable Er/Yb-doped nanoparticles have been used for microscopic detection in DNA microarrays [65]. The complete absence of endogenous upconversion luminescence leads to very low backgrounds and enhanced contrast. An NIR luminescent ytterbium(III) complex based on the fluorescein-isothiocyanate motif was demonstrated to be an adequate luminescent label in a sandwich-type in vitro diagnostic test [27]. 

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

Figure 12 Optical imaging of blood vessels in the mouse ear following tail vein injection of the nanoparticles (10 mg) (a) blood vessels imaged with a blue light filter, (b) upconversion image with excitation at 980 mn and a laser power density of 550 mW cm , (c) fluorescence image of the carbocyanine dye with excitation at 737 mn, and (d) merged image of the upconversion and fluorescence signals. Both the upconversion and carbocyanine fluorescence images were taken with an exposure time of 10 s. (Reproduced from Ref. 57 with permission of Tlie Royal Society of Chemistry.)...

The first two chapters of this work cover theoretical and practical aspects of the emission process, the spectroscopic techniques and the equipment used to characterize the emission. Chapter 3 introduces and reviews the property of circularly polarized emission, while Chapter 4 reviews the use of lanthanide ion complexes in bioimaging and fluorescence microscopy. Chapter 5 covers the phenomenon of two-photon absorption, its theory as well as applications in imaging, while Chapter 6 reviews the use of lanthanide ions as chemo-sensors. Chapter 7 introduces the basic principles of nanoparticle upconversion luminescence and its use for bioimaging and Chapter 8 reviews direct excitation of the lanthanide ions and the use of the excitation spectra to probe the metal ion s coordination environment in eoordination compounds and biopolymers. Finally, Chapter 9 describes the formation of heterobimetallic complexes, in whieh the lanthanide ion emission is promoted through the hetero-metal. 

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