Frequency-Domain Method for Characterization of Upconversion Luminescence Kinetics.

The frequency-domain (FD) method provides an alternative to the commonly used time-domain (TD) approach in characterizing the luminescence kinetics of luminophores, with its own strengths, e.g., the capability to decouple multiple lifetime components with higher reliability and accuracy. While extensively explored for characterizing luminophores with down-shifted emission, this method has not been investigated for studying nonlinear luminescent materials such as lanthanide-doped upconversion nanoparticles (UCNPs), featuring more complicated kinetics. In this work, employing a simplified rate-equation model representing a standard two-photon energy-transfer upconversion process, we thoroughly analyzed the response of the luminescence of UCNPs in the FD method. We found that the FD method can potentially obtain from a single experiment the effective decay rates of three critical energy states of the sensitizer/activator ions involved in the upconversion process. The validity of the FD method is demonstrated by experimental data, agreeing reasonably well with the results obtained by TD methods.

Power-dependent upconversion quantum yield of NaYF4:Yb3+,Er3+ nano- and micrometer-sized particles - measurements and simulations.

Photophysical studies of nonlinear lanthanide-doped photon upconverting nanoparticles (UCNPs) increasingly used in biophotonics and photovoltaics require absolute measurements of the excitation power density (P)-dependent upconversion luminescence (UCL) and luminescence quantum yields (ΦUC) for quantifying the material performance, UCL deactivation pathways, and possible enhancement factors. We present here the P-dependence of the UCL spectra, ΦUC, and slope factors of the different emission bands of representative 25 nm-sized oleate-capped ß-NaYF4:17% Yb3+, 3% Er3+ UCNPs dispersed in toluene and as powder as well as ΦUC of 3 µm-sized upconversion particles (UCµP), all measured with a newly designed integrating sphere setup, enabling controlled variation of P over four orders of magnitude. This includes quantifying the influence of the beam shape on the measured ΦUC and comparison of experimental ΦUC with simulations utilizing the balancing power density model of the Andersson-Engels group and the simulated ΦUC of UCµP from the Berry group, underpinned by closely matching decay kinetics of our UC material. We obtained a maximum ΦUC of 10.5% for UCµP and a ΦUC of 0.6% and 2.1% for solid and dispersed UCNPs, respectively. Our results suggest an overestimation of the contribution of the purple and an underestimation of that of the red emission of ß-NaYF4:Yb3+,Er3+: microparticles by the simulations of the Berry group. Moreover, our measurements can be used as a guideline to the absolute determination of UCL and ΦUC.

Environmental and Excitation Power Effects on the Ratiometric Upconversion Luminescence Based Temperature Sensing Using Nanocrystalline NaYF4 :Yb3+ ,Er3.

The luminescence intensity ratio (LIR) of the green emissions of the near-infrared excited NaYF4 :Yb3+ ,Er3+ nanocrystals is a promising method for temperature sensing. Here, the influence of excitation power density, excitation pulse length, excitation wavelength, silica shell, and solvent on the LIR and its temperature response is reported. The primary objective is to study the LIR mechanism and the impact of measurement and environmental parameters on the calibration and precision of the LIR. The LIR value is demonstrated to be unaffected by the excitation intensity in the studied range. This result is essential, considering the application feasibility of the LIR method as temperature sensor, where the effective excitation power density depends on the sample matrix and the distance excitation light travels in the sample. The pulsed excitation, however, results in an increase in the LIR value upon short pulse width. Silanization of bare nanocrystals has no effect on the LIR values, but the local warming of H2 O samples under laser exposure results in slightly increased LIR values compared to other solvents; D2 O, oleic acid, and dimethyl sulfoxide. The thermal quenching of luminescence lifetimes of Er3+ emission is proved to be too weak for sensing applications.

Mechanisms of Tenebrescence and Persistent Luminescence in Synthetic Hackmanite Na8Al6Si6O24(Cl,S)2.

Synthetic hackmanites, Na8Al6Si6O24(Cl,S)2, showing efficient purple tenebrescence and blue/white persistent luminescence were studied using different spectroscopic techniques to obtain a quantified view on the storage and release of optical energy in these materials. The persistent luminescence emitter was identified as impurity Ti(3+) originating from the precursor materials used in the synthesis, and the energy storage for persistent luminescence was postulated to take place in oxygen vacancies within the aluminosilicate framework. Tenebrescence, on the other hand, was observed to function within the Na4(Cl,S) entities located in the cavities of the aluminosilicate framework. The mechanism of persistent luminescence and tenebrescence in hackmanite is presented for the first time.

Quenching of the upconversion luminescence of NaYF4:Yb³âº,Er³âº and NaYF4:Yb³âº,Tm³âº nanophosphors by water: the role of the sensitizer Yb³âº in non-radiative relaxation.

We have studied the mechanisms of water-based quenching of the upconversion photoluminescence of upconverting nanophosphors (UCNPs) via luminescence decay measurements for a better understanding of the non-radiative deactivation pathways responsible for the relatively low upconversion luminescence efficiency in aqueous solutions. This included both upconversion luminescence measurements and the direct excitation of emissive energy states of Er(3+) and Yb(3+) dopants in NaYF4:Yb(3+),Er(3+) UCNPs by measuring the decays at 550 and 655 nm upon 380 nm excitation and at 980 nm upon 930 nm excitation, respectively. The luminescence intensities and decays were measured from both bare and silanized NaYF4:Yb(3+),Er(3+) and NaYF4:Yb(3+),Tm(3+) UCNPs in H2O and D2O. The measurements revealed up to 99.9% quenching of the upconversion photoluminescence intensity of both Er(3+) and Tm(3+) doped bare nanophosphors by water. Instead of the multiphonon relaxation of excited energy levels of the activators, the main mechanism of quenching was found to be the multiphonon deactivation of the Yb(3+) sensitizer ion caused by OH-vibrations on the surface of the nanophosphor. Due to the nonlinear nature of upconversion, the quenching of Yb(3+) has a higher order effect on the upconversion emission intensity with the efficient Yb-Yb energy migration in the ∼35 nm nanocrystals making the whole nanophosphor volume susceptible to surface quenching effects. The study underlines the need of efficient surface passivation for the use of UCNPs as labels in bioanalytical applications performed in aqueous solutions.

Ligand enabling visible wavelength excitation of europium(III) for fluoroimmunoassays in aqueous micellar solutions.

Fluorescent reporters based on lanthanide ions, such as europium chelates, enable highly sensitive detection in immunoassays and other ligand binding assays. Unfortunately they normally require UV-excitation produced by a xenon flash or nitrogen laser light source. In order to use modern solid state excitation sources such as light emitting diodes (LEDs), these reporters need to be excited at wavelengths longer than 365 nm, where high-powered ultraviolet LEDs are available. A novel ligand, 9-ethyl-3,6-bis(5',5',5',4',4'-pentafluoro-1',3'-dioxopentyl)carbazole (bdc), was synthesized to efficiently excite europium(III) at wavelengths up to 450 nm in micellar solutions, and its performance was compared to a commercially available DELFIA enhancement solution. The detection limit of Eu(III) with the bdc-ligand using 365 nm excitation was determined to be 63 fM, which is 3 times lower than with the DELFIA solution. The bdc-ligand enabled sensitive detection of europium(III) ions in solution using 365 nm excitation and displayed similar sensitivity and functionality as commercially available DELFIA enhancement solution. Therefore, this novel enhancement solution might be a feasible alternative in producing time-resolved fluorescence under LED-excitation.

Distance and temperature dependency in nonoverlapping and conventional Förster resonance energy-transfer.

Förster resonance energy-transfer (FRET) is a powerful and widely applied bioanalytical tool. According to the definition of FRET by Förster, for energy-transfer to take place, a substantial spectral overlap between the donor emission and acceptor excitation spectra is required. Recently also a phenomenon termed nonoverlapping FRET (nFRET) has been reported. The nFRET phenomenon is based on energy-transfer between a lanthanide chelate donor and a spectrally nonoverlapping acceptor and thus obviously differs from the conventional FRET, but the mechanism of nFRET and resulting implications to assay design have not been thoroughly examined. In this work, a homogeneous DNA-hybridization assay was constructed to study the distance and temperature dependency of both nFRET and conventional FRET. Capture oligonucleotides were labeled at the 5'-end with a Eu(III)-chelate, and these conjugates hybridized to complementary tracer oligonucleotides labeled with an organic fluorophore at various distances from the 3'-end. The distance dependency was studied with a fluorometer utilizing time-resolution, and the temperature dependency was studied using a frequency-domain (FD) luminometer. Results demonstrated a difference in both the distance and temperature dependency between conventional FRET and nFRET. On the basis of our measurements, we propose that in nFRET thermal excitation occurs from the lowest radiative state of the ion to a higher excited state that is either ionic or associated with a ligand-to-metal charge-transfer state.

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.

Frequency-domain measurement of luminescent lanthanide chelates.

The sinusoidal modulation of excitation intensity and phase-sensitive detection of emission is ideally suitable for the accurate determination of the lifetime and intensity of lanthanide luminescence. In this work we elaborate on the general mathematical and instrumental techniques of the frequency-domain (FD) measurements in the low-frequency domain below 100 kHz. A modular FD luminometer is constructed by using a UV-LED as the excitation source, proper light filters in the excitation and emission paths, a photomultiplier with a fast preamplifier, and a conventional dual-phase lock-in amplifier. Starting from the set of linear differential equations governing the excited-state processes of the lanthanide chelates, an equation linking the luminescence intensity to the general form of the excitation modulation was derived. Application to the sinusoidal modulation in the Euler's exponential form gives the expression for the in-phase and out-of-phase signals of a dual-phase lock-in amplifier. It is shown that by using a relatively large number of logarithmically equidistant modulation frequencies it is possible to use the Kramers-Kronig relation for checking the compatibility of the out-of-phase and in-phase signals. As an example, the emission from two different europium(III) chelates were measured by using 200 modulation frequencies between 10 Hz and 100 kHz. In addition to the conventional transition between (5)D(0) and (7)F(2) levels emitting at 615 nm, also the emission from the transition between (5)D(1) and (7)F(1) levels at ca. 540 nm was measured. The latter emission was also measured at different temperatures, yielding the energy difference between the (5)D(1) and (5)D(0) levels. The relatively large number of modulation frequencies allows also an accurate determination of lifetimes and corresponding amplitudes by using an appropriate nonlinear regression method. Comparison of the time-domain and frequency-domain methods shows that the weighting of data is different and both methods have application areas of their own.

Preparation and characterization of nanocrystalline ZrO2:Yb3+,Er3+ up-conversion phosphors.

Nanocrystalline up-converting phosphors with zirconium oxide (ZrO(2)) as the host lattice were prepared with combustion and sol-gel methods. Impurities were analyzed with Fourier transform infrared (FT-IR) spectroscopy. Yb(3+) absorption was studied in the wave number region 10,000-11,500 cm(-1) at room temperature and at 10 K. The whole-blood absorption was measured in the region 9100-41,600 cm(-1) at room temperature. Up-conversion luminescence was excited at room temperature with an IR-laser at 977 nm. The up-conversion luminescence spectra showed red (650-685 nm) and green emission (520-560 nm) due to the (4)F(9/2) --> (4)I(15/2) and ((2)H(11/2), (4)S(3/2)) --> (4)I(15/2) transitions of Er(3+), respectively. The materials prepared with combustion synthesis were found to yield the most efficient up-conversion intensity and the longest luminescence decay.

Defect structure and up-conversion luminescence properties of ZrO2:Yb3+,Er3+ nanomaterials.

The up-converting ZrO2:Yb3+,Er3+ nanomaterials were prepared with the combustion and sol-gel methods. FT-IR spectroscopy was used for analyzing the impurities. The crystal structures were characterized with X-ray powder diffraction and the mean crystallite sizes were estimated with the Scherrer formula. Up-conversion luminescence measurements were made at room temperature with IR-laser excitation at 977 nm. The IR spectra revealed the conventional NO3- and OH- impurities for the combustion synthesis products. The structure of the ZrO2:Yb3+, Er3+ nanomaterials was cubic except for the minor monoclinic and tetragonal impurities obtained with the sol-gel method. The materials showed red (650-700 nm) and green (520-560 nm) up-conversion luminescence due to the 4F9/2-->4I15/2 and (2H11/2, 4S3/2)-->4I15/2 transitions of Er3+, respectively. The products obtained with the combustion synthesis exhibited the most intense luminescence intensity and showed considerable afterglow.