Upconversion Fluorescence Emission Research Articles

Dissolution Mechanism of Upconverting AYF4:Yb,Tm (A = Na or K) Nanoparticles in Aqueous Media.

The dissolution of upconverting AYF4:Yb,Tm (A = Na or K) nanoparticles (UCNPs) in aqueous media was systematically studied. UCNPs with a cubic structure and sizes of between 10 and 33 nm were synthesized solvothermally in ethylene glycol at 200 °C. The UCNPs of both compositions showed an upconversion fluorescence emission characteristic of Tm(3+). The effects of the A cation, the particle size, the temperature, the pH, and the composition of the aqueous medium on the dissolution of the UCNPs were evaluated. The degree of dissolution was determined from the fraction of dissolved fluoride (F(-)) using potentiometry. Unexpectedly, the composition of aqueous media had the most significant effect on the dissolution of the UCNPs. The highest degree of dissolution and rate were measured for the phosphate-buffered saline (PBS), which can be explained by the formation of stable lanthanide compounds with phosphates. The degree of dissolution was much lower in water and in the phthalate buffer, which was attributed to the release of F(-) as a result of the hydrolysis of the UCNPs' surfaces.

Yb3+ sensitized Er3+ doped Gd2SiO5 powders prepared by combustion synthesis: Up-conversion fluorescence emission at visible from near-infrared

The up-conversion (UC) fluorescence performance of Er3+: Gd2SiO5 and Er3+: Yb3+: Gd2SiO5 powders prepared by combustion synthesis was investigated under near-infrared (λ=975nm) continuous wave laser excitation. The surface morphology and structure of the powders were studied with scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray powder diffraction (XRPD). UC fluorescence spectra of the samples show bands centered at~525,~560 and~660nm, corresponding respectively to 4f–4f transitions from upper levels 2H11/2, 4S3/2 and 4F9/2 to the ground state level 4I15/2 of Er3+. The dynamics of the UC fluorescence signal obtained in the co-doped Er3+: Yb3+: Gd2SiO5 sample was used along with a simple rate equation model for the Er3+ and Yb3+ populations to estimate the Yb3+→Er3+ energy transfer rates.

In vivo targeted magnetic resonance imaging and visualized photodynamic therapy in deep-tissue cancers using folic acid-functionalized superparamagnetic-upconversion nanocomposites.

Multifunctional nanoprobes used in magnetic resonance imaging (MRI) and photodynamic therapy (PDT) also have potential applications in diagnosis and visualized therapy of cancers, and hence it is important to investigate the active-targeting ability and in vivo reliability of these nanoprobes. In this work, folic acid (FA)-targeted, photosensitizer (PS)-loaded Fe3O4@NaYF4:Yb/Er (FA-NPs-PS) nanocomposites were synthesized for in vivo T2-weighted MRI and visualized PDT of cancers by modeling MCF-7 tumor-bearing nude mice. By measuring the upconversion luminescence (UCL) and fluorescence emission spectra, the as-prepared FA-NPs-PS nanocomposites showed near-infrared (NIR)-triggered PDT performance due to the production of a singlet oxygen species. Moreover, by tracing PS fluorescence in MCF-7, HeLa cells and in MCF-7 tumors, the FA-targeted nanocomposites demonstrated good targeting ability both in vitro and in vivo. Under the irradiation of a 980 nm laser, the viabilities of MCF-7 and HeLa cells incubated with FA-NPs-PS nanocomposites could decrease to about 18.4% and 30.7%, respectively, and the inhibition of MCF-7 tumors could reach about 94.9%. The transverse MR relaxivity of 63.79 mM(-1) s(-1) (r2 value) and in vivo MR imaging of MCF-7 tumors indicated an excellent T2-weighted MR performance. This work demonstrated that FA-targeted MRI/PDT nanoprobes are effective for in vivo diagnosis and visualized therapy of breast cancers.

Growth and Characterization of Er$lt;sup$gt;3+$lt;/sup$gt;-doped Relaxor-based Ferroelectric Crystal PMNT

According to the molar ratio of 0.71Pb(Mg1/3Nb2/3)O3-0.26PbTiO3-0.03Pb(Er1/2Nb1/2)O3, Er-doped PMNT polycrystalline material was prepared by two-step solid-state synthesis at elevated temperature. Er-doped PMNT relaxor-based ferroelectric crystal with size of φ25 mm × 100 mm was grown from the stoichiometric melts by vertical Bridgman method. In the ferroelectric crystal with perovskite structure, Er ions were doped into the crystal lattice via one composition of the ternary solid solution compound. The dielectric, piezoelectric and ferroelectric properties and up-conversion emission performance of Er-doped PMNT crystal wafers were investigated systematically. It is verified that Er-doped PMNT crystal presents the similar electrical properties to those of undoped PMNT crystal. Under the excitation of 980 nm, the crystal also exhibits a strong up-conversion fluorescence emission characteristic to Er ions doped in the medium, and the emission intensity will be enhanced after being poled.

Simultaneous morphology control and upconversion fluorescence enhancement of NaYF4:Yb,Er crystals through alkali ions doping

We report a facile oleic acid-assisted hydrothermal route to synthesize NaYF4:Yb/Er crystals doped by alkali ions (Li+ or K+ ions), and the effects of alkali ion doping on the crystal structure and upconversion (UC) fluorescence emission of the NaYF4:Yb,Er crystals are investigated in detail. With the increasing dosage of Li+ ions in the NaYF4:Yb/Er crystals, their morphologies are changed from rod to disk and finally to polyhedron. However, the morphologies of NaYF4:Yb/Er crystals doped with K+ ions almost keep the rod-like shape. In addition, the phase transition of LixNa1−xYF4 crystals initiates when x is equal to or larger than 0.5, but the phase of KyNa1−yYF4 remains unchanged even if y reaches 0.85. Furthermore, the UC emission intensity of NaYF4:Yb/Er crystals reduces with the doping of Li+ ions. On the contrary, the UC emission intensity of NaYF4:Yb/Er crystals increases as K+ ions are introduced. It is interesting that the green and red UC emissions of the K0.7Na0.3YF4:Yb,Er crystal are high 8 and 7 times, respectively.

Kinetics of Photon Upconversion in Ionic Liquids: Energy Transfer between Sensitizer and Emitter Molecules

The efficiency of triplet-sensitized photon upconversion in ionic liquids was previously found to be dependent on the type of ionic liquid employed. The properties of the intermolecular energy transfer need to be understood in order to improve the upconversion efficiency. Here, we investigate the kinetics of the triplet energy transfer from the triplet sensitizing molecule to the emitter molecule where the latter is responsible for delayed upconversion fluorescence emission. The collision kinetics between the sensitizer and emitter molecules in imidazolium ionic liquids are investigated by systematically changing the alkyl chain length of the ionic liquid cation. Stern-Volmer analysis reveals unique diffusion behavior of the solute molecules in ionic liquids, and this observation is attributed to the microheterogeneity of the ionic liquids. Through time-resolved transient absorption measurements and determination of the triplet-triplet absorption coefficient of the sensitizer molecule used, we find that the quantum efficiency of the triplet energy transfer in the present system is sufficiently high (ca. 0.75) and independent of the type of ionic liquid. These findings show that the ionic liquid dependence of the upconversion efficiency arises from the later processes pertaining to the emitter molecule rather than the triplet energy transfer process.

Triple and Double Photons Absorption Process and Down-Conversion Laser Emitting Investigation of Er-Ion Doped Microsphere

We report a readily and cheap method to build taper optical fiber-Er3+ doped microsphere platform to investigate upconversion fluorescence emission and down-conversion laser oscillation with low threshold pump power. We demonstrate to dope Er3+ into silica microsphere surface by dipping a single-taper optical fiber into a certain concentration of erbium nitrate solution (Er(NO3)) , then dry it and use the electrical-arc of the optical fiber splicer to melt the tip of taper fiber to form the Er3+-doped silica microsphere due to surface tension induced. We also present a HF acid etching setup to fabricate low loss biconical optical taper fibers. We demonstrate the Er3+ doped silica microsphere triple photons and dual photons absorption process of up-conversion fluorescence emission and down-conversion laser oscillation spectra by using the optical tapered fiber to couple 976 nm/1534 nm pump light source.

Synthesis of tetragonal-phased LiY(Yb)F<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup>crystal particles and the upconversion luminescence spectrum

Tetragonal-phased LiYF₄:Yb³⁺/Er³⁺and LiYbF₄:Yb³⁺/Er³⁺crystal particles with octagonal morphology are prepared by hydrothermal method and characterized with XRD and SEM. The up-conversion fluorescence was studied with spectroscopic method. The results showed that crystallization and particle size was adjusted by introducing EDTA (ethylenediaminetetraacetic acid), which finally introduced the change of fluorescence intensity. Green fluorescence emissions from ⁴S_3/2→⁴I_15/2and ²H_11/2→⁴I_15/2transitions, violet emission from ²H_9/2→⁴I_15/2transition, and red emission from ⁴F_9/2→⁴I_15/2were observed when the samples were excited with 980 nm IR laser. Meanwhile, the mechanism and the influence of the host matrix on the upconversion fluorescence emission were discussed.

Fluorescence upconversion properties of a chiral polybinaphthyl induced by two‐photon absorption

AbstractThe fluorescence properties of a chiral polymer based on optically active polybinaphthyls were studied in tetrahydrofuran solution. One‐photon excited fluorescence of the polymer was located in the range of ∼ 596 nm and the corresponding lifetime was ∼ 4.38 ns. From the excitation spectra and emission spectra excited at 800 nm, the upconversion fluorescence emission was observed. When excited by using 800 nm fs laser pulses with different input irradiances, the peak fluorescence intensity of the solution provides square dependence with the input irradiance power, giving an evidence for two‐photon excited fluorescence. Furthermore, open aperture Z‐scan measurements were performed at different irradiation intensities to confirm two‐photon absorption property of the solution at 800 nm excitation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Tuning of the Structure and Emission Spectra of Upconversion Nanocrystals by Alkali Ion Doping

Recently, lanthanide based nanocrystals with upconversion fluorescence emission have attracted a lot of interest and the nanocrystals have been used for bioimaging, biodetection, and therapeutic applications. Use of the nanocrystals for multiplexed detection has also been explored; however, nanocrystals with multicolor emission are required. Some efforts have been made to tune the emission spectra of the nanocrystals based on manipulation of upconverting lanthanide ions doped in the crystals or creation of core/shell structures. In this work, alkali ions with an ionic radius slightly larger or smaller than Na such as Li and K were doped into NaYF(4):Yb,Er nanocrystals and their effect on the crystal structure and subsequently the upconversion emission spectra were studied. It was found that the phase transition occurs in the nanocrystals when a different amount of Li and K was doped. Furthermore, the intensity ratios between the blue, green, and red emission peaks changed accordingly, and make it possible to tune the upconversion fluorescence of the nanocrystals by Li and K doping.

NIR to visible upconversion in Er 3+/Yb 3+ co-doped CaYAl 3O 7 phosphor obtained by solution combustion process

Using the combustion synthesis, CaYAl 3O 7:Er 3+ phosphor powders co-doped with Yb 3+ have been prepared at low temperatures (550 °C) in a few minutes. Formation of the compound was confirmed by X-ray powder diffraction. Near-infrared to visible upconversion fluorescence emission in the Er 3+ doped CaYAl 3O 7 phosphor powder has been observed. The effect of co-doping with triply ionized ytterbium in the CaYAl 3O 7:Er 3+ phosphor has been studied and the process involved is discussed.

Intense red upconversion fluorescence emission in NIR-excited erbium–ytterbium doped laponite-derived phosphor

In this report the optical properties and energy-transfer frequency upconversion luminescence of Er 3+/Yb 3+-codoped laponite-derived powders under 975 nm infrared excitation is investigated. The 75%(laponite):25%(PbF 2) samples doped with erbium and ytterbium ions, generated high intensity red emission around 660 nm and lower intensity green emission around 525, and 545 nm. The observed emission signals were examined as a function of the excitation power and annealing temperature. The results indicate that energy-transfer, and excited-state absorption are the major upconversion excitation mechanism for the erbium excited-state red emitting level. The precursor glass samples were also heat treated at annealing temperatures of 300 °C, 400 °C, 500 °C, and 600 °C, for a 2 h period. The dependence of the visible upconversion luminescence emission upon the annealing temperature indicated the existence of an optimum temperature which leads to the generation of the most intense and spectrally pure red emission signal.

Optical temperature sensor based on up-conversion fluorescence emission in Yb3+:Er3+ co-doped ceramics glass

Yb3+:Er3+ co-doped oxy-fluoride ceramics glass has been prepared. The mechanism of up-conversion emissions about Er3+ was discussed, and the temperature properties of green up-conversion fluorescence between 303 and 823 K were investigated. The results show that the sensitivity of this sample reaches its maximum value, about 0.0047 K−1, when the temperature is 383 K, indicating that this kind of sample can be used as high temperature and high sensitivity optical temperature sensor.

Improvement on the up-conversion fluorescence emission in Tm3+ doped optical materials by adjusting phonon distribution

The influence of phonon distribution on the frequency upconverted fluorescence emission is studied with laser spectroscopy in different matrices and at different ambient temperatures. The spectrum of phonon energy can be adjusted by varying the content of SiO2 in the oxyfluoride glass and glass ceramics, which affects the upconverted fluorescence emission of doped ions. Since the phonon energy depends on the sample temperature, the variation of the temperature can also influence the population rate of 3H4 level in Tm3+ doped low-phonon-energy LaF3 crystal, resulting in a change of the fluorescence upconversion.

Thermal effect on up-conversion in Er3+/Yb3+ co-doped silicate glass

Er3+–Yb3+ co-doped silicate glasses with high up-conversion efficiency were fabricated. A fivefold enhancement of the up-conversion fluorescence emission at 525 and 550nm with a temperature rising in them, excited at 970nm, is carried out. A model is proposed to describe the experimental phenomena based on the temperature dependent multiphonon-assisted Stokes excitation and non-radiative transition theories. The thermometry range, relying on the fluorescence intensity ration method, is excessively extended due to the high vitreous transition temperature in silicate glass.

Ho 3+:TeO 2 glass, a probe for temperature measurements

Considerable effort has recently been devoted to the development of optical fiber based temperature monitoring device owing to their advantage over conventional thermometric techniques. Optical thermometry based upon near infrared excited upconversion fluorescence emission in Ho 3+ activited tellurite glasses excited at 890 nm is presented. Temperature sensing in the range 265–440 K within 2 K accuracy using excitation powers readily obtainable from commercially available Ti–sapphire laser have been achieved. This temperature sensing approach is independent of fluctuations in excitation intensity, environment, transmission and requires a simple and low cost signal detection and processing system. Our results also indicate that the glassy host material also plays a major role in the process of the temperature sensing capacity.

Phonon-assisted mechanisms and concentration dependence of Tm3+ blue upconversion luminescence in codoped NaY(WO4)2 crystals

Optical parameters of NaY(WO4)2 crystals were calculated from absorption spectra with Judd–Ofelt theory. Intensive infrared-to-visible frequency upconversion has been discovered and investigated under a 974 nm CW laser diode excitation at room temperature. The bright blue upconversion fluorescence emission is analysed in detail. The optimal concentration of Tm3+, which is about 1 at% for a Yb3+ concentration of 10 at% for the blue emission in the crystals, is much higher than that in other hosts. The experimental results and theoretical analysis suggest that NaY(WO4)2 with high phonon energy is most probably an even more promising crystal than fluoride.

Enhanced Operational Stability of the Up‐Conversion Fluorescence in Films of Palladium–Porphyrin End‐Capped Poly(pentaphenylene)

The phenomenon of up-conversion, that is, the generation of higher-energy photons by simultaneous or sequential absorption of two or more photons, with lower energy, is mostly associated with the use of relatively high light intensities available from pulsed lasers. In organic systems, up-conversion fluorescence has also been described as a product of bimolecular processes in which the mechanism of triplet–triplet annihilation plays a prominent role. In our previous work, we demonstrated photon up-conversion in films containing two active components—metallated porphyrin macrocycles (MOEP) in a matrix of a polyfluorene (PF). The so-excited up-conversion fluorescence emission of the matrix polymer occurs at relatively low intensity and under continuous wave (cw) optical excitation. The method was successfully extended to a series of blue-emitting polymer matrices. A general advantage of the systems with more than one active component is the possibility to optimise each one of them independently. The major challenge is the optimisation and stability of the component distribution in the system. The up-conversion efficiency and operational stability of the emitting system is limited by both exand intrinsic factors. The extrinsic factors include the concentration of oxygen—either adsorbed in the film during the spin-coating process or permeated through the film surface over the entire period of optical testing of the sample. Oxygen decreases drastically the life time of the excited triplet state in the dopant molecule and hence the efficiency of the up-conversion process. Preparation and encapsulation of the sample in an inert atmosphere can control the concentration of oxygen. However, the control of the intrinsic parameters that decrease the up-conversion efficiency of the sample during operation is more challenging. In samples blended with dopant molecules, phase separation between the active polymer matrix and the dopant molecules occurs during the operation mostly because of the local in-

Three-photon-absorption induced fluorescence and optical limiting properties of a new organic compound

Three-photon-absorption-induced frequency up-conversion fluorescence emission has been observed in a solution of 4-2-(7-(4-amido styrene)-9,9-di(2-ethyl hexyl)-9H-fluorene-2-) ethylene) aniline (BASF) in N,N-dimethylformamide(DMF), pumped with 38ps Q-switched 1064nm laser pulses. The spectral peak of the observed fluorescence emission is located in the 456—775nm range, and the intensity dependence of the visible emission on the IR excitation obeys the cubic law. At higher solute concentration and moderate IR excitation intensity levels, obvious optical limitation behavior has been observed due to three-photon-absorption. The measured nonlinear absorption coefficient and the molecular three-photon-absorption cross section are 4.34×10-20cm3/Ｗ2 and 2.4×10-39cm6/Ｗ2, respectively.

Three-photon absorption enhancement in a symmetrical charge transfer fluorene derivative

In this letter we report the three-photon absorption-induced upconversion fluorescence emission and the three-photon absorption cross-section of two fluorene derivatives with D–π–D (9,9-didecyl-2,7-bis-( N, N-diphenylamino)fluorene) and D–π–A ((7-benzothiazol-2-yl-9,9-didecylfluoren-2-yl)diphenylamine) structural motifs. The three-photon absorption cross-section of the D–π–D analog ( σ 3 ′=82×10 −78 cm 6 s 2 photon −2) is 2.2 times greater than that of its D–π–A counterpart ( σ 3 ′=37×10 −78 cm 6 s 2 photon −2), showing that symmetric charge transfer enhances 3PA. The three-photon-excitation of these two compounds in hexane solution (9.8×10 −3 M) was accomplished with a tunable OPG pumped by picosecond laser pulses. The three-photon absorption coefficients were measured using an open aperture Z-scan technique.