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.

Oxidative Layer-By-Layer Multilayers Based on Metal Coordination: Influence of Intervening Graphene Oxide Layers.

Layer-by-layer (LbL) fabricated oxidative multilayers consisting of successive layers of inorganic polyphosphate (PP) and Ce(IV) can electrolessly form thin conducting polymer films on their surface. We describe the effect of substituting every second PP layer in the (PP/Ce) multilayers for graphene oxide (GO) as a means of modifying the structure and mechanical properties of these (GO/Ce/PP/Ce) films and enhancing their growth. Both types of LbL films are based on reversible coordinative bonding between the metal ions and the oxygen-bearing groups in PP and GO, instead of purely electrostatic interactions. The GO incorporation leads to the doubling of the areal mass density and to a dry film thickness close to 300 nm after 4 (GO/Ce/PP/Ce) tetralayers. The film roughness increases significantly with thickness. The (PP/Ce) films are soft materials with approximately equal shear storage and loss moduli, but the incorporation of GO doubles the storage modulus. PP displays a marked terminating layer effect and practically eliminates mechanical losses, making the (GO/Ce/PP/Ce) films almost pure soft elastomers. The smoothness of the (PP/Ce) films and the PP-termination effects are attributed to the reversible coordinative bonding. The (GO/Ce/PP/Ce) films oxidize pyrrole and 3,4-ethylenedioxythiophene (EDOT) and form polypyrrole and PEDOT films on their surfaces. These polymer films are considerably thicker than those formed using the (PP/Ce) multilayers with the same nominal amount of cerium layers. The GO sheets interfere with the polymerization reaction and make its kinetics biphasic. The (GO/Ce) multilayers without PP are brittle and thin.

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.

Influence of synthetic polyelectrolytes on the growth and properties of hyaluronan-chitosan multilayers.

Both hyaluronan (HA) and chitosan (CHI) are biocompatible polysaccharide electrolytes. The multilayers formed by these polyelectrolytes alone are known to be rather soft and strongly viscoelastic. In this work we study multilayers formed by incorporating synthetic nonsaccharide polyelectrolytes such as polyallylamine (PAH) and poly(acrylic acid) (PAA) in various proportions into the HA/CHI layers. The buildup was followed on a quartz crystal resonator. Surface acoustic impedance recorded in these measurements, in suitable conditions, gives a spiral when plotted in the complex plane. The shape of this spiral depends on the viscoelasticity of the layer material and regularity of the growth process. We found that poly(acrylic acid) destroys the soft diffuse matrix formed by hyaluronan. It forms diffusion barriers when deposited sparsely between the layers. If its proportion is higher, the film growth adopts a linear buildup in the layer-by-layer process. The linear buildup of CHI/PAA reveals that the buildup regime of a multilayer film does not determine the viscoelastic properties of the film. Linearly and exponentially growing films may have very similar mechanical properties. Polyacrylic acid forms a kind of scaffold inside the film giving the natively soft hyaluronan/chitosan film more mechanical strength. The optimal combination gave more than 100-fold increase in the shear modulus.

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.

Modeling the growth processes of polyelectrolyte multilayers using a quartz crystal resonator.

The layer-by-layer buildup of chitosan/hyaluronan (CH/HA) and poly(l-lysine)/hyaluronan (PLL/HA) multilayers was followed on a quartz crystal resonator (QCR) in different ionic strengths and at different temperatures. These polyelectrolytes were chosen to demonstrate the method whereby useful information is retrieved from acoustically thick polymer layers during their buildup. Surface acoustic impedance recorded in these measurements gives a single or double spiral when plotted in the complex plane. The shape of this spiral depends on the viscoelasticity of the layer material and regularity of the growth process. The polymer layer is assumed to consist of one or two zones. A mathematical model was devised to represent the separation of the layer to two zones with different viscoelastic properties. Viscoelastic quantities of the layer material and the mode and parameters of the growth process were acquired by fitting a spiral to the experimental data. In all the cases the growth process was mainly exponential as a function of deposition cycles, the growth exponent being between 0.250 and 0.275.

Aqueous dispersion, surface thiolation, and direct self-assembly of carbon nanotubes on gold.

We report the efficient aqueous dispersion of pristine HiPco single-walled carbon nanotubes (SWNTs) with ionic liquid (IL)-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), the thiolation of nanotube sidewalls with 2, and the controlled self-assembly of positively charged SWNT-1,2 composites on gold. Optical absorption spectra and resonance Raman (RR) data of obtained aqueous SWNT-1,2 dispersions are consistent with debundled and noncovalently functionalized nanotubes whose electronic properties have not been disturbed. Additionally, the dispersion of pristine nanotube material with surfactants 1 and 2 leads to a high degree of purification from carbonaceous particles. The chiralities of the 14 smallest semiconducting HiPco SWNTs in resonance with Raman excitation at 1064 nm (1.165 eV) were determined in SWNT-2 aqueous dispersion using UV-vis-NIR and RR spectra. X-ray photoelectron spectroscopy (XPS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopy of SWNT-2 submonolayers on gold verified the encapsulation of individualized SWNTs with IL surfactants, the cleavage of S-S disulfide bonds formed in aqueous SWNT-2 suspensions, and the direct chemisorption of the SWNT-2 composite on bare gold via the Au-S bond. Aqueous dispersions of SWNTs with IL-based surfactants add biofunctionality to carbon nanotubes by imparting the positive surface charge necessary for interactions with cell membranes. Our technique, which purifies pristine nanotube material and produces water-soluble, positively charged nanotubes with pendent surface-active thiol groups, may also be translated to other carbon nanotubes and carbon nanostructures. Self-assembled, positively charged submonolayers of SWNTs can be further used for applications in cell biology and sensor technology.

Structure of self-assembled multilayers prepared from water-soluble polythiophenes.

We have studied the structure and morphology of self-assembled polyelectrolyte multilayers prepared using poly(styrenesulfonate) (PSS) and four different cationic poly(alkoxythiophene) derivatives bearing methylimidazolium-terminated ionic side chain at the 3-position of the thiophene ring: poly(1-methyl-3-[3-[3-thienyloxy]-propyl]-1H-imidazolium) (P3TOPIM), poly(1-methyl-3-[6-[3-thienyloxy]-hexyl]-1H-imidazolium) (P3TOHIM), poly(1-methyl-3-[2-[(4-methyl-3-thienyl)oxy]-ethyl]-1H-imidazolium ) (P4Me-3TOEIM), and poly(1-methyl-3-[6-[(4-methyl-3-thienyl)oxy]-hexyl]-1H-imidazolium ) (P4Me-3TOHIM). All the multilayers exhibited regular growth. The thickness of the multilayers was measured with ellipsometry, their layer-by-layer growth was followed by polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and ellipsometry, and the morphology of the films was studied by atomic force microscopy (AFM). The length of the methylimidazolium-terminated side chain (C(n), n = 2, 3, 6) and the substituent (H or Me) at the 4-position of the thiophene ring were varied. All multilayers were inhomogeneous in the sub-micrometer scale and contained aggregates of two kinds. The large ones with a low and constant surface number density were attributed to PSS, whereas the small aggregates were polythiophene-based. The surface density of these organic semiconducting nanoparticles greatly depended on the structure of polythiophene, being favored by polymer regioregularity and the length of the side chain. The side chains remained disordered in all the multilayers, but with polythiophenes having hexyl chains both the imidazolium and thiophene rings tended to orient themselves more perpendicular to the surface than in films containing shorter chains (C2 or C3). The relative water content of the multilayers (at 7.1% relative humidity) did not depend on the film thickness and was the lowest for P4Me-3TOHIM. As the number of bilayers increased the methylimidazolium-sulfonate ion pairs gradually weakened and became more individually hydrated.

Method for measuring the losses and loading of a quartz crystal microbalance.

A novel, experimentally simple, and highly sensitive method for measuring the loading of a quartz crystal resonator was developed. The method is based on the use of double-sideband suppressed-carrier modulated high-frequency signal, which is swept through the resonance range of the resonator. Induced current in the resonator is passed through a capacitor, and the voltage over the capacitor is demodulated on an analog multiplier. The phase and amplitude information is carried to the frequency-doubled modulation signal and measured on a conventional low-frequency two-phase lock-in amplifier. A complex dimensionless loading parameter is obtained from the experimental data by nonlinear model fitting. The real and complex parts of this loading parameter have a simple relationship with other parameters commonly used for characterizing the resonator loading. The performance of the method was demonstrated by measuring a series of different glycerol-water mixtures ranging from 0 up to 100% glycerol. The results were close to the shear acoustic impedance of these mixtures measured and calculated from their viscosities and densities.

Effect of temperature on the buildup of polyelectrolyte multilayers.

The effect of temperature on the buildup of polyelectrolyte multilayers consisting of poly(styrenesulfonate) (PSS), poly(diallyldimethylammonium) (PDADMA), and poly(allylamine) (PAH) was studied by using a quartz crystal microbalance. The increase of temperature in the deposition process was shown to have a considerable effect on the rate of the layer-by-layer buildup. The effect of temperature on the PDADMA/PSS deposition was found to be stronger than on the PAH/PSS deposition. The increasing temperature was found to extend the exponential buildup regime in all of the studied systems. A buildup model was created to simulate the buildup and to explain the effect of temperature. The model is based on the assumption that each deposition step leads to a quasi-equilibrium between the concentration of the polymer repeating unit in solution and the composition of the layer. According to the model, the layer-by-layer buildup is inherently exponential, becoming linear whenever diffusion is not fast enough to carry the polymer within the entire thickness of the film. This buildup model is discussed jointly with the earlier published three-zone model of the polyelectrolyte multilayers. The rate of the buildup is characterized by growth exponent beta. The temperature dependence of the growth exponent is discussed in connection with the thermodynamic parameters of the deposition.

Large apparent interfacial slippage at polyelectrolyte-perfluorocarbon interfaces on a quartz crystal resonator.

The apparent negative areal mass densities obtained for a polyelectrolyte multilayer on a quartz crystal resonator in contact with four different perfluorocarbon liquids are explained by the interfacial slippage between the multilayer and the liquids. It is shown that the zone of interfacial slipping can be conveniently treated as a separate layer with distinct physical parameters. Three models of slippage were taken into a closer study. In the first model, the so-called de Gennes model, a very thin gas-filled cavity is formed between the moving phase and the stationary phase. The second model is based on the slipping layer consisting of water. In the third model, the so-called "true slipping" model, it is assumed that the particle velocity has a discontinuity at the interface. In each case, the slipping admittances and slippage lengths as well as the corrected areal mass densities were calculated from the experimental data. Although no unambiguous experimental evidence was found to favor strongly any of these three models, the slightly smaller variation in the slipping admittance and the areal mass density seems to give more credibility to the de Gennes model.

The Hofmeister anion effect and the growth of polyelectrolyte multilayers.

The influence of a variety of counteranions on the properties of polyelectrolyte multilayers deposited by layer-by-layer technique is studied by using ellipsometry and AFM. We found out that in thin dry multilayers (20-90 nm) ofpoly(4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA), the thickness follows reasonably well the position of the counteranion in the Hofmeister series. The polyelectrolyte-counteranion interaction is studied by means of viscosity measurements of semidilute solutions of PDADMA in the presence of different anions. The dynamic viscosities follow the Hofmeister series of anions and correlate with the thickness of multilayers. Two parameters describing the interaction of ions with water, the Jones-Dole viscosity B coefficient and the hydration entropy, are used to explain the anion effect on the developing multilayer thickness. Reasonably smooth and monotonic functional dependence is observed between the layer thickness and these two parameters.

Effect of polyelectrolyte multilayers on the response of a quartz crystal microbalance.

The effect of a polyelectrolyte (PE) multilayers made by a layer-by-layer technique on the response of a quartz crystal microbalance (QCM) is studied by using novel mathematical methods based on the Möbius transformations and their matrix representations in the complex plane. In the first method, the basic properties of the Möbius transformation are used for obtaining the PE bilayer matrix from the QCM impedance measurements taken at four different numbers of layers. In the second method, nonlinear fitting with concomitant error estimation is used for obtaining the elements of the bilayer matrix. The methods are applied to a multilayer composed of 150 bilayers of poly(sodium 4-styrenesulfonate) and poly(diallyldimethylammonium) chloride on a quartz crystal resonator. The structure of the system is discussed, and the bulk acoustic impedance and areal mass density of the bilayer are calculated from the layer matrix.