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Concept
Upconverting nanoparticles
Summary
Upconverting nanoparticles (UCNPs) are nanoscale particles (diameter 1–100 nm) that exhibit photon upconversion. In photon upconversion, two or more incident photons of relatively low energy are absorbed and converted into one emitted photon with higher energy. Generally, absorption occurs in the infrared, while emission occurs in the visible or ultraviolet regions of the electromagnetic spectrum. UCNPs are usually composed of rare-earth based lanthanide- or actinide-doped transition metals and are of particular interest for their applications in in vivo bio-imaging, bio-sensing, and nanomedicine because of their highly efficient cellular uptake and high optical penetrating power with little background noise in the deep tissue level. They also have potential applications in photovoltaics and security, such as infrared detection of hazardous materials.
Before 1959, the anti-Stokes shift was believed to describe all situations in which emitted photons have higher energies than the corresponding incident photons. An anti-Stokes shift occurs when a thermally excited ground state is electronically excited, leading to a shift of only a few kBT, where kB is the Boltzmann constant, and T is temperature. At room temperature, kBT is 25.7 meV. In 1959, Nicolaas Bloembergen proposed an energy diagram for crystals containing ionic impurities. Bloembergen described the system as having excited-state emissions with energy differences much greater than kBT, in contrast to the anti-Stokes shift.
Advances in laser technology in the 1960s allowed the observation of non-linear optical effects such as upconversion. This led to the experimental discovery of photon upconversion in 1966 by François Auzel. Auzel showed that a photon of infrared light could be upconverted into a photon of visible light in ytterbium–erbium and ytterbium–thulium systems. In a transition-metal lattice doped with rare-earth metals, an excited-state charge transfer exists between two excited ions.
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