Swept-source optical
coherence tomography (SSOCT) is a noncontact,
noninvasive bioimaging tool that uses various probes to obtain high
contrast images. Rare-earth based upconversion particles (UCPs) provide
a unique platform to enhance the capabilities of bioimaging techniques.
The UCPs can act as contrast enhancers for the SSOCT with additional
optical functionality, and hence in this work upconversion emission
optimized polyvinylpyrrolidone (PVP)/NaGdF4:Er3+/Yb3+ UCPs were synthesized and then injected into chicken
breast tissues to enhance the SSOCT images. As-synthesized samples
have shown good biocompatibility toward HEK293, HeLa cell lines and
showed apparent enhancement in contrast to SSOCT images. Additionally,
compared to the bare NaGdF4:Er3+/Yb3+ UCPs the polyvinylpyrrolidone-stabilized UCPs have shown 3-fold
enhancement in the frequency of upconverted green and red light intensity
under 976 nm diode laser excitation (26 W/cm2).
Phase pure nanoparticles of NaCe(WO 4 ) 2 green phosphor were synthesized via a template-free solvothermal method using water (NaCeW-a) and ethylene glycol (NaCeWb) as reaction solvents. Rietveld refinements using powder neutron diffraction (NPD) data confirms that both NaCeW-a and NaCeWb crystallize in the Scheelite-like tetragonal I4 1 /a space group. However, local structure analysis using total scattering atomic pair distribution function (PDF) refinements in a shorter 'r' range (1.5 to 10 Å) established the noncentrosymmetric I4̅ space group where Na/CeO 8 polyhedra are slightly distorted, distributed in 2b and 2d sites. The interesting observation of noncentrosymmetry locally in both NaCeW-a and NaCeW-b is supported by optical nonlinear second harmonic generation (SHG) measurements. Closer inspection of the scanning electron micrograms showed a distinct difference in the particle morphology as a function of reaction solvent. Observation of the +3 valence state of the cerium ion in NaCe(WO 4 ) 2 via X-ray photoelectron spectroscopy measurements was further corroborated by magnetic (VSM) measurements. In addition, both the samples emitted bright narrow green emission upon UV excitation with no considerable change in emission intensity with respect to solvent. Hence, the sharp green light emission and SHG properties make the NaCe(WO 4 ) 2 a potential candidate for solid state display and nonlinear optical applications.
Investigation
of the red luminescence in NaEu(WO4)2 nanoparticles
synthesized using water and ethylene glycol
and its biocompatibility on HeLa cells, Escherichia coli, Staphylococcus aureus, and Candida albicans has been evaluated for the first time. The Scheelite-like pure tetragonal
NaEu(WO4)2 nanoparticles have been synthesized
by a simple solvothermal method by using water (NaEuW I) and ethylene
glycol (NaEuW II) as a reaction medium. Detailed particle size and
crystal structural analysis of both samples were carried out using
high resolution transmission electron microscopy and Synchrotron powder
diffraction. Enhanced red fluorescence was observed on increase in
calcination temperature in both samples, signifying their greater
stability and increased crystallinity on thermal treatment. The blue
shift of charge transfer (CT) band in NaEuW II nanoparticles is explained
by the nature of the Eu–O bond lengths obtained by Rietveld
refinement using Synchrotron diffraction data. Chromaticity diagrams
revealed the possibility of tuning the red emission of as-prepared
and annealed samples against particle size, choice of solvent, and
calcination temperature. The calculated Commission Internationale
de l’Eclairage (CIE) coordinates of NaEuW II at 600 °C
matches the CIE values of the commercial red phosphor Y2O3S:Eu3+ standards of the National Standards
of Television Commission (NSTC). This indicates the potential applications
of nano-NaEuW [NaEu(WO4)2] to generate red emission.
Investigation of solvent effect on particle size dependent biocompatibility
of NaEu(WO4)2 via in vitro cytotoxicity
studies showed no significant toxicity toward HeLa cells, E. coli, S. aureus, and C. albicans. More interestingly, bioimaging studies show excellent biodistribution
and localization of luminescent nano-NaEuW II. Hence, our preliminary
studies could demonstrate a new strategy to design nanoluminescent
NaEu(WO4)2 for theranostic applications.
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