Importance of Volume Ratio in Photonic Effects of Lanthanide-Doped LaPO4 Nanocrystals.

Experimental variation of the volume ratio (filling factor: i.e., volume of nanoparticles (NPs) compared with that of medium) of nanocomposite materials with doped lanthanide ions demonstrates that it has a significant affect upon local field effects. Lanthanum orthophosphate NPs are doped with Eu3+ and/or Tb3+ and immersed in organic solvents and lead borate glasses for Tb3+ 5 D4 lifetime measurements. For media with a refractive index (nmed ) less than that of LaPO4 (nnp = 1.79), the 5 D4 emission decay rate increases with increasing volume ratio of the NPs, whereas for nmed > 1.79, the decay rate decreases with increasing volume ratio. Fitting with the model of Pukhov provides an estimation of the radiative lifetime of 5 D4 and the quantum yield. Energy transfer (ET) from Tb3+ to Eu3+ occurs in co-doped LaPO4 NPs with excitation into a Tb3+ absorption band. The ET rate is independent on nmed and the energy transfer efficiency decreases with an increase in nmed . The behavior of ET rate with regard to the local field is consistent with the Dexter, but not Förster, equation for ET rate involving the electric dipole-electric dipole mechanism. This has consequences when using the spectroscopic ruler approach to measure distances between donor-acceptor chromophores.

Energy Transfer between Tb3+ and Eu3+ in LaPO4: Pulsed versus Switched-off Continuous Wave Excitation.

The energy transfer (ET) between Tb3+ and Eu3+ is investigated experimentally and with available theoretical models in the regime of high Tb3+ concentrations in ≈30 nm LaPO4 nanoparticles at room temperature. The ET efficiency approaches 100% even for lightly Eu3+-doped materials. The major conclusion from the use of pulsed laser excitation and switched-off continuous wave laser diode excitation is that the energy migration between Tb3+ ions, situated on La3+ sites with ≈4 Å separation, is not fast. The quenching of Tb3+ emission in singly doped LaPO4 only reduces the luminescence lifetime by ≈50% in heavily doped samples. Various theoretical models are applied to simulate the luminescence decays of Tb3+ and Tb3+, Eu3+-doped LaPO4 samples of various concentrations and the transfer mechanism is identified as forced electric dipole at each ion.

In vitro cell imaging using multifunctional small sized KGdF4:Yb3+,Er3+ upconverting nanoparticles synthesized by a one-pot solvothermal process.

Multifunctional KGdF4:18%Yb(3+),2%Er(3+) nanoparticles with upconversion fluorescence and paramagnetism are synthesized. The average sizes of the nanoparticles capped with branched polyethyleneimine (PEI) and 6-aminocaproic acid (6AA) are ~14 and ~13 nm, respectively. Our KGdF4 host does not exhibit any phase change with the decrease of particle size, which can prevent the detrimental significant decrease in upconversion luminescence caused by this effect observed in the well-known NaYF4 host. The branched PEI and 6AA capping ligands endow our nanoparticles with water-dispersibility and biocompatibility, which can favor internalization of our nanoparticles into the cytoplasm of HeLa cells and relatively high cell viability. The strong upconversion luminescence detected at the cytoplasm of HeLa cells incubated with the branched PEI-capped nanoparticles is probably attributed to the reported high efficiency of cellular uptake. The magnetic mass susceptibility of our nanoparticle is 8.62 × 10(-5) emu g(-1) Oe(-1). This is the highest value ever reported in trivalent rare-earth ion-doped KGdF4 nanoparticles of small size (≤14 nm), and is very close to that of nanoparticles used as T1 contrast agents in magnetic resonance imaging. These suggest the potential of our KGdF4:Yb(3+),Er(3+) nanoparticles as small-sized multifunctional bioprobes.

Towards pure near-infrared to near-infrared upconversion of multifunctional GdF(3):Yb(3+),Tm(3+) nanoparticles.

Nearly pure near-infrared to near-infrared (NIR-to-NIR) upconversion in GdF(3) host with 23% Yb(3+) and 1% Tm(3+) under 980 nm excitation is firstly reported. The ratio of the intensity of the emission at 807 nm to that at 478 nm can reach to 105, and the intensity of the emission at 807 nm is preserved. Moreover, the excitation and the emission at 980 and 807 nm are away from the visible region. These are beneficial to deeper tissue penetration and reduced autofluorescence. Raman spectroscopy measurements suggest the high probability of NIR emission in GdF(3) host. Our results indicate that the reported multifunctional nanoparticles are promising in bio-imaging and bio-separation.