Emerging Frontiers of Upconversion Nanoparticles

Different from organic dyes and semiconductor quantum dots that generally produce downshifting luminescence, upconversion nanoparticles (UCNPs) are a class of optical transducers that convert long-wavelength excitation to shorter-wavelength emission by taking advantage of long-lived intermediate energy states of lanthanide ions. A good number of strategies have become available to enhance optical emission in UCNPs, which has largely expanded the scope of upconversion research. UCNPs with intense and tunable emissions have enabled many advanced applications such as upconversion lasing, super-resolution imaging, sensing, and patterning. Lanthanide-doped upconversion nanoparticles capable of converting near-infrared excitation into UV and visible emissions have attracted considerable research interests in recent years. Low-density near-infrared light needed for excitation of these nanoparticles is nondestructive and highly penetrating, offering significant technological advantages for biological and photonic applications such as remote neuron modulation, super-resolution imaging, and short-wavelength lasing. In this review, we describe several emerging classes of upconversion nanoparticles that have been developed to promote these technological applications by providing boosted emission intensities. We explain how heavy doping, thermal stimulating, and hybridizing can be harnessed to maximize absorption of excitation light and minimize quenching processes in these nanomaterials. Lanthanide-doped upconversion nanoparticles capable of converting near-infrared excitation into UV and visible emissions have attracted considerable research interests in recent years. Low-density near-infrared light needed for excitation of these nanoparticles is nondestructive and highly penetrating, offering significant technological advantages for biological and photonic applications such as remote neuron modulation, super-resolution imaging, and short-wavelength lasing. In this review, we describe several emerging classes of upconversion nanoparticles that have been developed to promote these technological applications by providing boosted emission intensities. We explain how heavy doping, thermal stimulating, and hybridizing can be harnessed to maximize absorption of excitation light and minimize quenching processes in these nanomaterials. a photon conversion process in which the emitted photon features a shorter wavelength than the absorbed photon. an energy exchange in which one optical center initially in a high-lying excited state donates part of its energy to an optical center initially in the ground state, resulting in both centers occupying a low-lying excited state. a thermally activated process, in which the electrons at the excited energy level jump over the crossing point between the excited state and ground state parabola by overcoming an energy barrier and then return to ground state nonradiatively. a parameter referring to the ability to absorb incident light by a medium. a nonradiative electron transition process between two electronic states with different spin multiplicity. the number of electromagnetic modes, per unit frequency and per unit volume, available for a luminescent center to radiate into. an unusual physicochemical process in which some materials contract upon heating, rather than expand as most other materials do. a biological technique that involves the use of light to control neuron activities by activating or inhibiting light-sensitive ion channels. a nonresonant energy transfer process assisted by phonons, which may come from endogenous lattice vibration or exogenous environment. a parameter in condensed matter physics describing the number of states that are available to be occupied by phonons at each level of energy. radiative decay of an excited luminescent center induced by a radiation field. a method of thermal analysis in which the mass of a sample is measured over time as the temperature changes.