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Lanthanide-doped nanoparticles exhibit unique luminescent properties,including a large Stokes shift,a sharp bandwidth of emission,high resistance to optical blinking,and photobleaching.Uniquely,they can also convert long-wavelength stimulation into shortwavelength emission.These attributes offer the opportunity to develop alternative luminescent labels to organic fluorophores and quantum dots.In recent years,researchers have taken advantage of spectral-conversion nanocrystals in many important biological applications,such as highly sensitive molecular detection and autofluorescence-free cell imaging.With significant progress made over the past several years,we can now design and fabricate nanoparticles that display tailorable optical properties.In particular,we can generate a wealth of color output under single-wavelength excitation by rational control of different combinations of dopants and dopant concentration.We can obtain unprecedented single-band emissions by careful selection of host matrices.By incorporating a set of lanthanide ions at defined concentrations into different layers of a core-shell structure,we have expanded the emission spectra of the particles to cover almost the entire visible region,a feat hardly accessible by conventional bulk phosphors.Importantly,we demonstrate that an inert-shell coating provides the particles with stable emission against perturbation in surrounding environments,paving the way for their applications in biological systems.