Upconversion and Anti-Stokes Processes with f and d Ions in Solids

Controllable synthesis of NaYF(4) : Yb,Er upconversion nanophosphors and their application to in vivo imaging of Caenorhabditis elegans

β-NaYF(4) : Yb,Er upconversion nanoparticles (UCNPs) can emit bright green fluorescence under near-infrared (NIR) light excitation which is safe to the body and can penetrate deeply into tissues. The application of UCNPs in biolabeling and imaging has received great attention recently. In this work, β-NaYF(4) : Yb,Er UCNPs with an average size of 35 nm, uniformly spherical shape, and surface modified with amino groups were synthesized by a one-step green solvothermal approach through the use of room-temperature ionic liquids as the reactant, co-solvent and template. The as-prepared UCNPs were introduced into Caenorhabditis elegans (C. elegans) to achieve successful in vivo imaging. We found that longer incubation time, higher UCNP concentration and smaller UCNP size can make the in vivo fluorescence of C. elegans much brighter and more continuous along their body. The worms have no apparent selectivity on ingestion of the UCNPs capped with different capping ligands while having similar size and shape. The next generation of worms did not show fluorescence under excitation. In addition, low toxicity of the nanoparticles was demonstrated by investigating the survival rates of the worms in the presence of the UCNPs. Our work demonstrates the potential application of the UCNPs in studying the biological behavior of organisms, and lays the foundation for further development of the UCNPs in the detection and diagnosis of diseases.

Preferential suppression of high-energy upconverted emissions of Tm3+ by Dy3+ ions in Tm3+/Dy3+/Yb3+-doped LiYF4 colloidal nanocrystals

The intensity of high energy UV and blue upconverted emissions of Tm(3+) ions in Tm(3+)/Yb(3+) co-doped LiYF(4) colloidal nanocrystals was selectively reduced compared to the NIR emission at 802 nm. This was achieved by doping a small amount of Dy(3+) ions into the host matrix.

Highly spectral dependent enhancement of upconversion emission with sputtered gold island films

We report a five-fold overall enhancement of upconversion emission in NaYF(4) : Yb/Er nanocrystals when coupled with gold island films. Spectroscopic studies show that the enhancement factors are highly dependent on the exact spectral positions and excitation power density, with a largest enhancement factor of more than 12 observed at selected spectral positions, which may be attributed to different upconversion processes involved.

Luminescent multifunctional lanthanides-based metal-organic frameworks

Metal-organic frameworks based on trivalent lanthanides (LnMOFs) are a very promising class of materials for addressing the challenges in engineering of luminescent centres. Lanthanide-bearing phosphors find numerous applications in lighting, optical communications, photonics and biomedical devices. In this critical review we discuss the potential of LnMOFs as multifunctional systems, which combine light emission with properties such as microporosity, magnetism, chirality, molecule and ion sensing, catalysis and activity as multimodal imaging contrast agents. We argue that these materials present a unique chance of observing synergy between several of these properties, such as the coupling between photoluminescence and magnetism. Moreover, an integrated approach towards the design of efficient, stable, cheap, environmentally-friendly and multifunctional luminescent LnMOFs is still missing. Although research into LnMOFs is at its early stage and much basic knowledge is still needed, the field is ripe for new ideas, which will enable sensor devices and photonic prototypes to become a commercial reality (81 references).

Small upconverting fluorescent nanoparticles for biomedical applications

Fluorescent labels have been widely used for biological applications, primarily in imaging and assays. Traditional fluorophores such as fluorescent dyes are mainly based on downconversion fluorescence, which have several drawbacks such as photobleaching, high background noise from autofluorescence, and considerable photodamage to biological materials. Upconverting fluorescent nanoparticles emit detectable photons of higher energy in the near-infrared (NIR) or visible range upon irradiation with an NIR light in a process termed 'upconversion.' They overcome some of the disadvantages faced by conventional downconversion labels, thus making them an ideal fluorescent label for biological applications. This review looks at the development of these particles, critically examines the reported applications, and discusses their future in biomedicine.

Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shape-directed self-assembly

We report a one-pot chemical approach for the synthesis of highly monodisperse colloidal nanophosphors displaying bright upconversion luminescence under 980 nm excitation. This general method optimizes the synthesis with initial heating rates up to 100 °C/minute generating a rich family of nanoscale building blocks with distinct morphologies (spheres, rods, hexagonal prisms, and plates) and upconversion emission tunable through the choice of rare earth dopants. Furthermore, we employ an interfacial assembly strategy to organize these nanocrystals (NCs) into superlattices over multiple length scales facilitating the NC characterization and enabling systematic studies of shape-directed assembly. The global and local ordering of these superstructures is programmed by the precise engineering of individual NC's size and shape. This dramatically improved nanophosphor synthesis together with insights from shape-directed assembly will advance the investigation of an array of emerging biological and energy-related nanophosphor applications.

Magnetic and upconverted luminescent properties of multifunctional lanthanide doped cubic KGdF4 nanocrystals

Lanthanide (Ln3+) doped KGdF4 (Ln=Yb3+, Er3+, Ho3+, Tm3+) nanocrystals with a mean diameter of approximately 12 nm were synthesized by a hydrothermal method using oleic acid as a stabilizing agent at 180 °C. The nanocrystals crystallize in the cubic phase as α-NaGdF4. When excited by a 980 nm laser, these Ln3+ doped nanocrystals exhibit multicolor up-conversion (UC) emissions in red, yellow, blue and white. The calculated color coordinates demonstrate that white UC emission (CIE-X=0.352, CIE-Y=0.347) can be obtained by varying the dopant concentrations in the Yb3+/Ho3+/Tm3+ triply-doped nanocrystals to yield different RGB emission intensities. The measured field dependence of magnetization (M-H curves) of the KGdF4 nanocrystals shows their paramagnetic characteristics that can be ascribed to the non-interacting localized nature of the magnetic moment of Gd3+ ions. Moreover, low temperature thermal treatment can enhance UC properties, magnetization and magnetic mass susceptibility of Ln3+ doped KGdF4 nanocrystals. The multifunctional Ln3+ doped KGdF4 nanocrystals have potential applications in color displays, bioseparation, and optical-magnetic dual modal nanoprobes in biomedical imaging.

Enhancing the photoelectrical performance of dye-sensitized solar cells using TiO2:Eu3+ nanorods

TiO(2):Eu(3+) nanorods are hydrothermally grown and used to fabricate a bilayer film electrode in a dye-sensitized solar cell. A light-to-electrical energy conversion efficiency of 8.0% and a quantum efficiency of 93.7% (at 575 nm) is achieved in this solar cell. The high efficiency is due to the improvement of ultraviolet light harvesting via a down-conversion luminescence process by the Eu(3+) ion and the increase of light scattering by one-dimensional TiO(2).

Optical ammonia sensor based on upconverting luminescent nanoparticles

The sensor exploits the phenomenon of upconversion luminescence and is based on (a) the use of upconverting nanoparticles (UCNPs) of the NaYF(4):Yb,Er type that can be excited with 980 nm laser light to give a green and red luminescence and (b) the pH probe phenol red immobilized in a polystyrene matrix. Exposure of the sensor film to ammonia causes a strong increase in the 560 nm absorption of the pH probe which, in turn, causes the green emission of the UCNPs to be screened off. The red emission of the UCNPs, in contrast, remains unaffected by ammonia and can serve as a reference signal. Due to the use of 980 nm as the excitation light source, the optical signal obtained is completely free of background visible luminescence of the sample and of scattered light. This is highly advantageous in the case of sensing ammonia in complex matrixes.

In vivo near-infrared fluorescence imaging of cancer with nanoparticle-based probes

The use of in vivo near-infrared fluorescence (NIRF) imaging techniques for sensitive cancer early detection is highly desirable, because biological tissues show very low absorption and autofluorescence in the NIR spectrum window. Cancer NIRF molecular imaging relies greatly on stable, highly specific and sensitive molecular probes. Nanoparticle-based NIRF probes have overcome some of the limitations of the conventional NIRF organic dyes, such as poor hydrophilicity and photostability, low quantum yield, insufficient stability in biological system, low detection sensitivity, etc. Therefore, a lot of efforts have been made to actively develop novel NIRF nanoparticles for in vivo cancer molecular imaging. The main focus of this article is to provide a brief overview of the synthesis, surface modification, and in vivo cancer imaging applications of nanoparticle-based NIRF probes, including dye-containing nanoparticles, NIRF quantum dots, and upconversion nanoparticles.

Synthesis and upconversion luminescence of BaY2F8:Yb3+/Er3+ nanobelts

BaY(2)F(8):Yb(3+)/Er(3+) nanobelts were prepared with oleic acid as capping ligand. Under 980 nm excitation, the (2)H(9/2)→(4)I(15/2), (4)F(7/2)→(4)I(15/2), (2)H(11/2)→(4)I(15/2), (4)S(3/2)→(4)I(15/2), and (4)F(9/2)→(4)I(15/2) transitions were observed. The intensity ratio of (2)H(11/2)/(4)S(3/2)→(4)I(15/2) to (4)F(9/2)→(4)I(15/2) and (2)H(11/2)→(4)I(15/2) to (4)S(3/2)→(4)I(15/2) increased with increasing excitation power.

Upconverting luminescent nanoparticles for use in bioconjugation and bioimaging

Upconverting luminescent nanoparticles (UCNPs) display the unique property of emitting visible light following photoexcitation with near-infrared laser light. This results in features such as virtually zero autofluorescence of (biological) matter and easy separation of the emission peaks from stray light. Other features include rather narrow emission bands, very high chemical stability, the lack of bleaching, and the absence of blinking effects. This article reviews the work performed in the past few years with UCNPs in terms of surface modifications, bioconjugation, and optical (cellular) imaging.

Bimodal magnetic-fluorescent probes for bioimaging

Fluorescent optical probes have been intensively used in the area of bio-imaging. In this review article, we describe the recent advancements in the synthesis and application of bimodal magnetic-fluorescent probes for bioimaging. The bimodal probes consist of fluorescent [semiconducting quantum dots (e.g., CdSe/ZnS) or rare-earth doped (e.g., NaYF(4) :Yb,Er)] nanoparticles (NPs) and magnetic (iron oxide or gadolinium based) NPs for optical and magnetic resonance (MR) imaging.

Use of nonlinear upconverting nanoparticles provides increased spatial resolution in fluorescence diffuse imaging

Fluorescence diffuse imaging (FDI) suffers from limited spatial resolution. In this Letter, we report a scanning imaging approach to increase the resolution of FDI using nonlinear fluorophores. The resolution of a linear fluorophore was compared with nonlinear upconverting nanoparticles (NaYF(4):Yb(3+)/Tm(3+)) in a tissue phantom. A resolution improvement of a factor of 1.3 was found experimentally. Simulations suggested a maximum resolution improvement of a factor of 1.45. Usage of nonlinear fluorophores is a promising method for increasing the spatial resolution in FDI.

Multicolored redox active upconverter cerium oxide nanoparticle for bio-imaging and therapeutics

Cytocompatible, co-doped cerium oxide nanoparticles exhibited strong upconversion properties that were found to kill lung cancer cells by inducing apoptosis thereby demonstrating the potential to be used as clinical contrast agents for imaging and as therapeutic agents for treatment of cancer.

Highly sensitive and selective label-free optical detection of mercuric ions using photon upconverting nanoparticles

We demonstrate a fluorescence-based, label-free detection scheme that reports the presence of Hg(II) ion using photon upconverting nanoparticles. A single-stranded DNA containing a number of thymine bases to be used as the Hg(2+)-capturing element is covalently attached to the photon upconverting NaYF(4):Yb(3+),Tm(3+) nanoparticles. Under the illumination of 980 nm laser, energy transfer takes place between the NaYF(4):Yb(3+),Tm(3+) nanoparticles as the donor and SYBR Green I, a DNA intercalating dye, as the acceptor. By monitoring the ratio of the acceptor emission to the donor emission, we can quantitatively detect the presence of the mercuric ions with a directly observed detection limit of 0.06 nM. The remarkably high signal-to-noise ratio of photon upconverting particles leads to very high sensitivity and specificity without the need of fluorophore labeling. The sensor also does not suffer from photobleaching.

Luminescence resonance energy transfer from an upconverting nanoparticle to a fluorescent phycobiliprotein

Water dispersible upconverting polyethylenimine (PEI)-capped NaYF(4) nanoparticles co-doped with trivalent erbium (Er(3+)) and ytterbium (Yb(3+)) were prepared via solvothermal synthesis with an 18 nm average particle diameter. These upconverting nanoparticles can be used to sensitize a light-harvesting phycobiliprotein (R-Phycoerythrin) via luminescence resonance energy transfer (LRET).

Multifunctional nanocomposites of superparamagnetic (Fe3O4) and NIR-responsive rare earth-doped up-conversion fluorescent (NaYF4 : Yb,Er) nanoparticles and their applications in biolabeling and fluorescent imaging of cancer cells

A new kind of magnetic/luminescent multifunctional nanoparticles was synthesized by covalently linking multiple carboxyl-functionalized superparamagnetic Fe(3)O(4) nanoparticles and individual amino-functionalized silica-coated fluorescent NaYF(4) : Yb,Er up-conversion nanoparticles (UCNPs). The resultant nanocomposites bear active carboxylic and amino groups on the surface that were proved to be chemically active and useful for further facile bioconjugation with biomolecules. The UCNPs in the nanocomposite particles can emit visible light in response to the irradiation by near infrared (NIR) light, enabling the application of the nanocomposites in bioimaging. X-Ray diffraction, infrared spectroscopy, transmission electron microscopy, luminescence spectroscopy, and magnetometry were applied to characterize the multifunctional nanocomposites. The nanocomposites exhibited good superparamagnetic and excellent green up-conversion photoluminescent properties that can be exploited in magnetic separation and guiding as well as bioimaging. Due to the presence of active functional groups on the nanocomposite surface, the Fe(3)O(4)/NaYF(4) : Yb,Er magnetic/luminescent nanocomposites were successfully conjugated with a protein called transferrin, which specifically recognizes the transferrin receptors overexpressed on HeLa cells, and can be employed for biolabeling and fluorescent imaging of HeLa cells. Because NIR light can penetrate biological samples with good depth without damaging them and can avoid autofluorescence from them, the presence of both NIR-responsive UCNPs and superparamagnetic nanoparticles in the nanocomposite particles will enable the practical application of the nanocomposites in bioimaging and separation.

Ultrasmall monodisperse NaYF(4):Yb(3+)/Tm(3+) nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence

Photoluminescent NaYF(4):Yb(3+)/Tm(3+) nanocrystals are ideally suited for in vitro and in vivo photoluminescence (PL) bioimaging due to their virtue of near-infrared to near-infrared (NIR-to-NIR) upconversion (UC); they display PL with a peak at approximately 800 nm if excited at approximately 980 nm. Here, we report the synthesis of monodisperse NaYF(4):Yb(3+)/Tm(3+) nanocrystals of ultrasmall size (7-10 nm) with high UC efficiency. The intensity of their NIR UC emission was demonstrated to increase by up to 43 times along with an increase in the relative content of Yb(3+) ions from 20 to 100%, with a corresponding decrease in the Y(3+) content from 80 to 0%. The achieved ultrasmall NaYbF(4):2% Tm(3+) nanocrystals manifest NIR PL emission, which is 3.6 times more intense than that from 25-30 nm sized NaYF(4):20% Yb(3+)/2% Tm(3+) nanocrystals, previously synthesized and used for in vitro and in vivo bioimaging. An optimization of both size and UC PL efficiency of NIR-to-NIR nanocrystals provides us with highly efficient optical imaging probes for bioapplications.

Temperature sensing using fluorescent nanothermometers

Acquiring the temperature of a single living cell is not a trivial task. In this paper, we devise a novel nanothermometer, capable of accurately determining the temperature of solutions as well as biological systems such as HeLa cancer cells. The nanothermometer is based on the temperature-sensitive fluorescence of NaYF(4):Er(3+),Yb(3+) nanoparticles, where the intensity ratio of the green fluorescence bands of the Er(3+) dopant ions ((2)H(11/2) --> (4)I(15/2) and (4)S(3/2) --> (4)I(15/2)) changes with temperature. The nanothermometers were first used to obtain thermal profiles created when heating a colloidal solution of NaYF(4):Er(3+),Yb(3+) nanoparticles in water using a pump-probe experiment. Following incubation of the nanoparticles with HeLa cervical cancer cells and their subsequent uptake, the fluorescent nanothermometers measured the internal temperature of the living cell from 25 degrees C to its thermally induced death at 45 degrees C.

Mesoporous silica encapsulating upconversion luminescence rare-earth fluoride nanorods for secondary excitation

Mesoporous silica encapsulating upconversion luminescence NaYF(4) nanorods with uniform core-shell structures have been successfully synthesized by the surfactant-assistant sol-gel process. The thickness of ordered mesoporous silica shells can be adjusted from 50 to 95 nm by varying the amount of hydrolyzed silicate oligomer precursors from tetraethyl orthosilicate (TEOS), which further influences the BET surface area, pore volume, and the luminescence intensity. After coated with mesoporous silica shells, the hydrophobic nanorods is rendered to hydropholic simultaneously. The obtained beta-NaYF(4)@SiO(2)@mSiO(2) core-shell nanorods possess high surface area (71.2-196 m(2) g(-1)), pore volume (0.07-0.17 cm(3) g(-1)), uniform pore size distribution (2.3 nm), and accessible channels. Furthermore, the uniform core-shell nanorods show strong upconversion luminescence property similar to the hexagonal upconversion cores. The open mesopores can not only provide convenient transmission channels but also offer the huge location for accommodation of large molecules, such as fluorescent dyes and quantum dots. The secondary-excitation fluorescence of Rhodamine B is generated from the upconversion rare-earth fluoride nanorods cores to the fluorescent dyes loaded in the mesoporous silica shells.

Colloidal synthesis and blue based multicolor upconversion emissions of size and composition controlled monodisperse hexagonal NaYF4:Yb,Tm nanocrystals

Monodisperse beta-NaYF4:Yb,Tm nanocrystals with controlled size (25-150 nm), shape (sphere, hexagonal prism, and hexagonal plate), and composition (Yb: 20-40%, Tm: 0.2-5%) were synthesized from the thermolysis of metal trifluoroacetates in hot surfactant solutions. The upconversion (UC) of near-infrared light (980 nm) to ultra-violet (360 nm), blue (450 and 475 nm), red (650 and 695 nm) and infrared (800 nm) light in the beta-NaYF4:Yb,Tm nanocrystals has been studied by UC spectroscopy. Both the total intensity of UC emissions and the relative intensities of emissions at different wavelengths have shown a strong dependence on different particle sizes and different Tm3+ and Yb3+ concentrations. As a result, different overall output colors of UC emissions can be achieved by altering sizes and Yb3+/Tm3+ doping concentrations of the beta-NaYF4:Yb,Tm nanocrystals. The intensity-power curves of a series of samples have proved that emissions at 360 and 450 nm can be ascribed to four-photon process (1D2 to 3H6 and 1D2 to 3H4, respectively), while emissions at 475 and 650 nm are three-photon processes (1G4 to 3H6 and 1G4 to 3H4, respectively) and emissions at 695 and 800 nm are two-photon ones (3F2 to 3H6 and 3F4 to 3H6, respectively). A UC saturation effect would occur under a certain excitation intensity of the 980 nm CW diode laser for the as-obtained beta-NaYF4:Yb,Tm nanocrystals, leading to the decrease of the slopes of the I-P curves. The results of our study also revealed that the successive transfer model instead of the cooperative sensitization model can be applied to explain the UC behaviors of the beta-NaYF4:Yb,Tm nanocrystals. Further, an unexpected stronger emissions of four-photon process at 360 and 450 nm for approximately 50 nm beta-NaYF4:Yb,Tm nanocrystals than those for the bigger (approximately 150 nm) nanocrystals was observed and explained in terms of the effects of crystallite size, surface-to-volume ratio and homogeneity of the doping cations.

Up-conversion luminescence of the NaRF4-NaR'F4 (R: Y, Yb, Er) core-shell nanomaterials

Up-converting NaRF(4)-NaR'F(4) (R: Y, Yb, Er) nanomaterials with different core-shell combinations were prepared with the co-precipitation method. The X-ray powder diffraction (XPD) measurements revealed the presence of both the cubic and hexagonal NaRF(4) phases. The crystallite sizes calculated with the Scherrer formula were 100 and 150 nm for the cubic and hexagonal phases, respectively. The FT-IR spectra showed water impurities. The up-conversion luminescence and luminescence decays were studied with NIR laser excitation at 970 nm. The up-conversion luminescence spectra showed strong red (640-685 nm) ((4)F(9/2) → (4)I(15/2)) and moderate green (515-560 nm) ((2)H(11/2,) (4)S(3/2) → (4)I(15/2)) Er(3+) luminescence. The strongest up-conversion luminescence and longest red luminescence decay was obtained from the Na(Y,Yb)F(4)-NaErF(4) core-shell combination.