Paradigms and challenges for bioapplication of rare earth upconversion luminescent nanoparticles: small size and tunable emission/excitation spectra.

Rare earth (RE) materials, which are excited in the ultraviolet and emit in the visible light spectrum, are widely used as phosphors for lamps and displays. In the 1960's, researchers reported an abnormal emission phenomenon where photons emitted from a RE element carried more energy than those absorbed, owing to the sequential energy transfer between two RE ions--Yb(3+)-sensitized Er(3+) or Tm(3+)--in the solid state. After further study, researchers named this abnormal emission phenomenon upconversion (UC) emission. More recent approaches take advantage of solution-based synthesis, which allows creation of homogenous RE nanoparticles (NPs) with controlled size and structure that are capable of UC emission. Such nanoparticles are useful for many applications, especially in biology. For these applications, researchers seek small NPs with high upconversion emission intensity. These UCNPs have the potential to have multicolor and tunable emissions via various activators. A vast potential for future development remains by developing molecular antennas and energy transfer within RE ions. We expect UCNPs with optimized spectra behavior to meet the increasing demand of potential applications in bioimaging, biological detection, and light conversion. This Account focuses on efforts to control the size and modulate the spectra of UCNPs. We first review efforts in size control. One method is careful control of the synthesis conditions to manipulate particle nucleation and growth, but more recently researchers have learned that the doping conditions can affect the size of UCNPs. In addition, constructing homogeneous core/shell structures can control nanoparticle size by adjusting the shell thickness. After reviewing size control, we consider how diverse applications impose different requirements on excitation and/or emission photons and review recent developments on tuning of UC spectral profiles, especially the extension of excitation/emission wavelengths and the adjustment and purification of emission colors. We describe strategies that employ various dopants and others that build rationally designed nanostructures and nanocomposites to meet these goals. As the understanding of the energy transfer in the UC process has improved, core/shell structures have been proved useful for simultaneous tuning of excitation and emission wavelengths. Finally, we present a number of typical examples to highlight the upconverted emission in various applications, including imaging, detection, and sensing. We believe that with deeper understanding of emission phenomena and the ability to tune spectral profiles, UCNPs could play an important role in light conversion studies and applications.