Sub-20 nm core-shell and water-soluble SrF2:Yb,Tm@CaF2:Gd heteronanoparticles with both upconversion luminescence (UCL) and magnetic resonance imaging (MRI) capabilities were designed and synthesized via a two-step hydrothermal method. In the design of the heteronanoparticles, SrF2:Yb,Tm nanoparticles with high UCL efficiency are chosen as the core material for strong UCL output; and by epitaxially coating the SrF2:Yb,Tm core particles with inert and biocompatible shells of CaF2:Gd, the core-shell heteronanoparticles are endowed with a magnetic capability (longitudinal relaxivity of 2.4 mM(-1) s(-1)) for MRI, as well as an enhancement of the near infrared (NIR) UCL by 9.2 times. The aqueous dispersion of SrF2:Yb,Tm@CaF2:Gd heteronanoparticles with a concentration of 2.6 wt% can emit NIR UCL so as to be easily detected with a fiber optical spectrometer under illumination of a 975 nm laser diode with a power density of 8.8 W cm(-2). Such a dispersion with a Gd(3+) concentration of 0.0143 mM in the shell region of the heteronanoparticles can also generate the detectable quickening of longitudinal relaxation. The results promise the strong potential of this nanomaterial for applications in bioimaging as a dual-functional probe.

Metallic nanostructures are often used to enhance photoluminescence of nanomaterials based on local field enhancement with plasmons at metal surfaces. Here upconversion luminescence (UCL) enhancement of submicrometer-size NaGd0.3 Yb0.7 F4 :Er particles in cap-like metal cavities, formed by deposition of a silver film on the particles dispersed on glass substrates, is studied. UCL of the particles is shown to be influenced by not only the plasmon-enhanced local field but also the cavity modes. By varying the cavity size and location of the particles in the cavities, fluctuant variations of the UCL enhancement and electronic depopulation rate are observed in experiments. Typically, a maximum of 12-fold enhancement of the UCL intensity is obtained. Combining the results with numerical simulations, the phenomenon is ascribed to effects of metal quenching, plasmonic field enhancement, and the cavity modes for the excitation and emission photons. Finally it is verified that, for the cap-like submicrometer metal cavities, allocating the particles at the open mouths of the cavities is more advantageous to obtaining stronger enhancements of the particles' UCL. And the demonstrated structure is also convenient to fabricate for applications, e.g., in solar cells.

In solvothermal synthesis of Er 3? -doped