Upconverting nanoparticles

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 lanthanide- or actinide-doped transition metals and are of particular interest for their applications in bio-imaging and bio-sensing at the deep tissue level. They also have potential applications in photovoltaics and security, such as infrared detection of hazardous materials. 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 lanthanide- or actinide-doped transition metals and are of particular interest for their applications in bio-imaging and bio-sensing at 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. Auzel observed that this charge transfer allows an emission of photon with much higher energy than the corresponding absorbed photon. Thus, upconversion can occur through a stable and real excited state, supporting Bloembergen's earlier work. This result catapulted upconversion research in lattices doped with rare-earth metals. One of the first examples of efficient lanthanide doping, the Yb/Er-doped fluoride lattice, was achieved in 1972 by Menyuk et al. Photon upconversion belongs to a larger class of processes by which light incident on a material induces anti-Stokes emission. Multiple quanta of energy such as photons or phonons are absorbed, and a single photon with the summed energy is emitted. It is important to make the distinction between photon upconversion, where real metastable excited states allow for sequential absorption, and other nonlinear processes like second-harmonic generation or two-photon excited fluorescence which involve virtual intermediate states such as the 'simultaneous' absorption of two or more photons. It is also distinct from more weakly anti-Stokes processes like thermoluminescence or anti-Stokes Raman emission, which are due to initial thermal population of low-lying excited states and consequently show emission energies only a few kBT above the excitation. Photon upconversion is distinctly characterized by emission-excitation differences of 10–100 kBT. Photon upconversion relies on metastable states to facilitate sequential energy absorption. Therefore, a necessary condition for upconverting systems is the existence of optically active long-lived excited states. This role is traditionally filled by lanthanide metal ions embedded in an insulating host lattice. Generally in the +3 oxidation state, these ions have 4fn electronic configurations and typically exhibit f-f transitions. These 4f orbitals allow for complex electronic structures and a large number of possible electronic excited states with similar energies. When embedded in bulk crystals or nanostructures, the energies of these excited states will further split under crystal field, generating a series of states with many closely spaced energies. The 4f shell is localized near the core of the ion and is therefore non-bonding, while the 5s and 5p shells provide further shielding from the exterior crystal field. Thus, the coupling of electronic excited states to the surrounding lattice is weak, leading to long excited state lifetimes and sharp optical lineshapes. The physical processes responsible for upconversion in nanoparticles are the same as those in bulk crystals on the microscopic level, although total efficiency and other ensemble effects will have unique considerations in the nanoparticle case. The processes contributing to upconversion may be grouped according to the number of ions involved. The two most common processes by which upconversion can occur in lanthanide-doped nanoscale materials are excited state absorption (ESA) and energy transfer upconversion (ETU). A single ion in the lattice sequentially absorbs two photons and emits a photon of higher energy as it returns to the ground state. ESA is most common when dopant concentrations are low and energy-transfer is not probable. Since ESA is a process where two photons must be absorbed at a single lattice site, coherent pumping and high intensity are much more important (but not necessarily required) than for ETU. Because of its single-ion nature, ESA does not depend on the lanthanide ion concentration. Two-ion processes are usually dominated by energy transfer upconversion (ETU). This is characterized by the successive transfer of energy from singly excited ions (sensitizers/donors), to the ion which eventually emits (activators/acceptors). This process is commonly portrayed as the optical excitation of the activator followed by further excitation to the final fluorescing state due to energy transfer from a sensitizer. While this depiction is valid, the more strongly contributing process is the sequential excitation of the activator by two or more different sensitizer ions.