Photon Correlation Spectroscopy Measurements Research Articles

Aggregation behavior of amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution studied by fluorescence correlation spectroscopy

The diffusional behavior of amphiphilic poly(2-oxazoline) diblock copolymers in aqueous solution is studied using photon correlation spectroscopy (PCS) and fluorescence correlation spectroscopy (FCS). The polymers were synthesized by living cationic polymerization and were fluorescence-labeled with tetramethyl rhodamine isothiocyanate either at the end of the hydrophilic or the hydrophobic block. Temperature-resolved PCS showed that, at room temperature, large metastable aggregates are present along with unimers and micelles. An annealing above ~40 °C resulted in stable equilibrium micellar solutions. By means of FCS, the hydrodynamic radii of the unimers and the micelles were measured simultaneously in a broad concentration range, and the critical micelle concentration could be determined. Comparison of the results from conventional PCS measurements with this first FCS study showed excellent agreement and the high potential of the FCS technique.

An investigation of a sol-gel/melt transition: the poly(ethylene oxide)/methanol/LiClO4 system.

The crossover behavior of 50 000 molar mass poly(ethylene oxide)/methanol solutions from dilute solution to the melt/gel was examined. At first this behavior was investigated without LiClO(4) and then reexamined with LiClO(4). To better understand this behavior, the dependencies of dynamic light scattering (specifically, photon correlation spectroscopy) measurement results on polymer concentration, on the scattering wave vector and on temperature, and the dependence of static light scattering results on the scattering wave vector were studied. This study produced interesting and important results about network structure and behavior in poly(ethylene oxide) solutions and melts generally and about the effects of LiClO(4) on this structure and behavior more particularly.

SolEmuls ®—novel technology for the formulation of i.v. emulsions with poorly soluble drugs

Intravenously injectable o/w emulsions of drugs being poorly soluble in water and simultaneously in oils need to be produced by locating the drug in the interfacial lecithin layer, e.g. amphotericin B. For achieving this, up to now organic solvents were required. The objective was to develop a solvent-free production method for such emulsions. Drug and the pre-formed parenteral emulsion Lipofundin ® were mixed and subjected to high pressure homogenisation. Drug powder and emulsions were characterised regarding size and physical stability by photon correlation spectroscopy (PCS), laser diffractometry (LD) and zeta potential measurements. Drug incorporation was studied using light microscopy, electron microscopy (EM) and a centrifugation test to separate non-dissolved drug. Amphotericin B and carbamazepine were used as model drugs. The high streaming velocities lead to accelerated drug dissolution and partitioning into the interfacial layer (so-called “solubilisation by emulsification”, SolEmuls ® Technology). The interfacial layer could incorporate (solubilise) a certain amount of drug, revealed by EM pictures. Exceeding this concentration, hybrid dispersions were formed consisting of drug-loaded oil droplets and drug nanocrystals of similar size (approximately 200 nm). Both dispersion types are i.v. injectable opening the opportunity to deliver the drug in a concentrated form at desired low injection volume, e.g. 10 mg/ml.

Surface and bulk elasticity determined fluctuation regimes in smectic membranes.

We report combined x-ray photon correlation spectroscopy (XPCS) and neutron spin echo (NSE) measurements of the layer-displacement fluctuations in smectic liquid-crystal membranes in the range from 10 ns to 10 micros. NSE reveals a new regime, determined by bulk elasticity, in which relaxation times decrease with the wave vector of the fluctuations. XPCS probes slower surface-tension-dominated relaxation times, independent of the wave vector. XPCS gives a difference in correlation times at specular and off-specular positions that can be related to different detection schemes.

Characterization and Modification of Low Molecular Water-Soluble Chitosan for Pharmaceutical Application

The low molecular water-soluble chitosan nanoparticles (LMWSC-NPs) were prepared, which was modified with hydrophilic and hydrophobic moieties to evaluate the potential for pharmaceutics application. The synthesis of LMWSC-NPs was identified by FT-IR and 'H-NMR spectra. Also, we measured the photon correlation spectroscopy (PCS), transmission electron microscope (TEM) and atomic force microscope (AFM) to investigate the characteristics and morphology of the LMWSC-NPs. At the PCS measurement, the more increase the number of substitutive group, the more decrease the positive charge of LMWSC-NP surface. From the results of TEM and AFM, spherical morphologies were observed, and their sizes were 30-150 nm. Resultantly, LMWSC-NPs prepared in this experiment will be expected as a suitable device for the drug targeting system.

Shell-Restricted Swelling and Core Compression in Poly(N-isopropylacrylamide) Core−Shell Microgels

We report on an investigation of the structural relationship between the core and shell components in stimuli-sensitive core/shell microgels. Photon correlation spectroscopy measurements indicate that the presence of the shell alters the extent of core swelling. Core particles composed of poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) undergo volume changes as a function of temperature and pH, where deprotonation of the acidic sites results in a volume increase (particle swelling). The addition of a poly(N-isopropylacrylamide) (pNIPAm) shell restricts the core from swelling to its native volume both above and below the pKa of acrylic acid. Furthermore, the pH responsive core undergoes only small temperature-induced volume changes above the characteristic lower critical solution temperature (LCST) of pNIPAm (31 °C) at pH values where the acid groups are fully deprotonated. The addition of a pNIPAm shell results in compression of the core and induces a large volume change at the pNIPAm LCST. Th...

Dendriplexes and their characterisation

The interaction of DNA with partial dendrimers (dendritic polylysine containing seven lysines and eight terminal amino groups with or without a lipidic core) was studied. Compact complexes were formed which we term “dendriplexes”. Agarose gel electrophoresis and exclusion of ethidium bromide confirmed the interaction. All the dendrons formed compact complexes above a 2:1 (+/−) charge ratio in water and HBSS. Photon correlation spectroscopy, electron microscopy and zeta potential measurements were used to determine, respectively, the particle size, shape and surface charge of the dendriplexes. The z-average diameter of the dendriplexes were found to be 60–70 nm irrespective of the dendron used and the zeta potential varied from 10 to 35 mV at a 3:1 (+/−) charge ratio depending on the dendron. The protection of the DNA component of these dendriplexes from nuclease degradation was confirmed by DNase protection assays.

Dynamics of Buried Polymer -polymer Interfaces in Thin Films

ABSTRACTWe have performed x-ray photon correlation spectroscopy (XPCS) measurements on a polymer-bilayer system comprised of 100 nm polystyrene film on top of an 80 nm polybromostyrene film, supported on a Si substrate. In order to distinguish the dynamics at the top interface from that at the polymer-polymer interface we have performed the measurement at grazing incidence. In this geometry, a standing wave is set up in the film. We derive a relation for the intensity of the standing wave and the resulting diffuse scattering. This model is compared with the measured diffuse scattering from which we extract a value of 0.7±0.4 dyne/cm for the surface tension between PS and PBrS at 180C. XPCS was then measured in each of two standing wave conditions, first where diffuse scattering only occurs at the polymer-vacuum interface and then where it only occurs at the interior polymer-polymer interface. The measured time correlation functions for each of the two interfaces show clear differences, with the polymer-polymer interface exhibiting much slower dynamics.

Influence of conformational changes on diffusion properties of bovine serum albumin: a holographic interferometry study

The effect of various stresses (heating, or salt excess) applied to a protein solution (BSA) have been evidenced via an efficient dynamic process: mass diffusion. The measurements were performed with a holographic interferometry technique, which is particularly interesting for the study of transport processes. The diffusion coefficient has been found to be quite independent of the BSA concentration but strongly dependent on the ionic strength of the solvent and also on heating, both processes leading to a protein conformational change as demonstrated by photon correlation spectroscopy (PCS) measurements. In all cases, the increase of the protein size leads to the decrease of the diffusion coefficient value. An application of these first experiments has been the eventual evidence of conformational changes of proteins in a solution used as a lubricating fluid for the tribological measurements of prosthetic joints. Up to 0.5 million constraint cycles performed in a hip-joint simulator, the protein solutions showed a stable or very slight increase of the diffusion coefficient. The bacteriological contamination being absent, this experiment proves that this kind of friction test has no or very slight effect on protein conformation thus showing the reliability of such trials on prosthetic elements.

Influence of pH and Time on the Stability of Aqueous Alumina Suspensions Containing Sodium Polyacrylates: A Revisited Process

ABSTRACT The action of polyacrilates as stabilizing agents in alumina aqueous suspensions has been investigated using viscometry, zeta potential, and photon correlation spectroscopy (PCS) measurements. The present work emphasizes that the study of stability of dispersions using polyelectrolytes must necessarily include several techniques in order to evaluate different physicochemical properties of colloidal suspensions, and thus attain a wide comprehension of the system under analysis. In our point of view, this is necessary because, for instance, while viscosity measurements suggest that there is no advantage in the use of the polyelectrolyte at some pH values, particle size measurement as a function of time shows that the presence of PANa at that pH values allows a longer handling time in suspension processing. Low molecular sodium polyacrylate presented a good performance as a deflocculating agent, because it leads to dispersions of low viscosity, stable at a wide range of pH, and also low variation in particle size as a function of time.

Investigation of surface dynamics on micro- and nanometer scales

A kinematical scattering theory for the interpretation of X-ray photon correlation spectroscopy (XPCS) measurements from fluctuating surfaces is discussed in detail. First a general formulation without approximations is given. Then as an example XPCS measurements of capillary wave dynamics on glycerol surfaces will be presented and compared to the calculations applying the usual approximations. It turns out that these approximations work well if the perpendicular wave-vector transfer is small. The measurements confirm on micrometer length scales the prediction from classical hydrodynamics that all waves are overdamped due to the high viscosity of glycerol. In addition, the measurements show that thermally activated capillary wave motion is slowed down exponentially when the sample is cooled below 273 K. Further, the extension of XPCS measurements down to nanometer scales is discussed.

Flash synthesis of zirconia nanoparticles by microwave forced hydrolysis

Forced hydrolysis preparation of zirconia sols and powders by microwave heating of zirconium tetrachloride solutions at temperatures equal to 180 °C leads in a few minutes to monodispersed nanoscale zirconia particles. Synthesis was performed in a microwave reactor called the RAMO system. This microwave reactor was designed by the authors. This flash-synthesis process combines the advantages of forced hydrolysis (homogeneous precipitation) and microwave heating (very fast heating rates). The sols and powders were characterized by x-ray diffraction,photon correlation spectroscopy (PCS), small-angle x-ray scattering, and transmission electron microscopy. Sols are colloidally stable, which means that after 6 months no sedimentation is observed and the size distribution given by PCS measurements has not changed. For all synthesis conditions (with or without HCl, zirconium salt concentration, and synthesis time), zirconia polycrystalline particles were produced. According to the different analyses, these zirconia polycrystalline particles were constituted of aggregates of small primary clusters.

Simulations and experiments on number fluctuations in photon-correlation spectroscopy at low particle concentrations

At low particle concentrations number fluctuations in photon-correlation spectroscopy measurements lead to a slow decay in the autocorrelation function, which contains information about the concentration. The characteristic time tau(c) of this number-fluctuation decay depends on the size of both the particle and the scattering volume. For determination of the concentration a measurement should last for at least a time of 100tau(c). Using Brownian dynamics simulations and measurements, we have studied the effects of the measurement time on the decay of the number fluctuations.

The degree of aggregation in solution controls the adsorbed amount of mussel adhesive proteins on a hydrophilic surface

The adsorption behaviour of the mussel adhesive protein Mefp-1 on a hydrophilic surface was studied by Surface Plasmon Resonance (SPR) at pH values of 4.5 and 6.5 under aerobic conditions and at an ion strength of 0.1 M NaCl. In this environment Mefp-1 molecules aggregate by crosslinking, likely via Dopa–Dopa quinone charge transfer interactions. The initial rate of aggregation increases with increasing pH, as could be derived from Photon Correlation Spectroscopy measurements. The degree of aggregation determines the adsorption plateau value of Mefp-1. Step-like adsorption curves have been found at pH 6.5, as well as at pH 4.5, which can be interpreted as the adsorption of an ad-layer of Mefp-1 aggregates onto the initially adsorbed Mefp-1 layer on the surface. The rate of formation of this second layer increases with increasing pH and Mefp-1 concentration. The affinity of the ad-layer for the first adsorbed layer appears to be much smaller than the affinity of the first layer for the surface (Poly Vinyl Alcohol). Probably, also the ad-layer formation proceeds by the establishment of specific crosslinks with the first layer of adsorbed Mefp-1.

Rotational and translational dynamics of lysozyme in water–glycerol solution

In this paper, we report a study of the effect of solvent viscosity on both translational and rotational dynamics of a simple model protein: the egg white lysozyme. For this, we investigated the dynamical properties of lysozyme in mixtures water–glycerol by means of parallel measurements of photon correlation spectroscopy (PCS) and dielectric spectroscopy at radiofrequencies (DS). In the framework of the Debye–Stokes–Einstein theory, the translational and rotational coefficients allow an estimation of hydrodynamic radius of the protein. A decoupling between translational and rotational dynamics, observed as a different estimation of hydrodynamic radius, is reported in the literature for some systems. In order to ascertain if this effect is present also in our sample, we performed PCS and DS measurements on lysozyme–water–glycerol solutions. The content of glycerol was in the range of 0–70% w/w, with a solvent viscosity from 0.9 to about 10 cpoise, and the protein concentration was up to 20 mg ml −1. The average sizes of lysozyme, obtained by the two methods, are remarkably different at high protein concentrations. However, the values of hydrodynamic radius extrapolated to infinite dilution are coincident and independent of glycerol. These results indicate that the diffusive behavior of lysozyme in the water–glycerol mixture is coherent with the Debye–Stokes–Einstein hydrodynamic model.

PHOTON CORRELATION SPECTROSCOPY USING SINGLE-MODE OPTICAL FIBERS

Obtaining true beautiful data from any photon correlation experiment demands on serious attention to improve the signal-to-noise ratio. Single-mode optical fibers provide the ideal receiver optics for photon correlation spectroscopy measurements. The analysis shows that a single-mode fiber can achieve a high collection efficiency and improve the signal-to-noise ratio effectively in photon correlation experiments. All these features are also demonstrated in experiments.

Short-Range Interactions of Globular Proteins at High Ionic Strengths

We compare the interactions of betalactoglobulin and lysozyme under different salt concentrations of ammonium sulfate by means of photon correlation spectroscopy measurements. The diffusion coefficient depends linearly upon protein concentration (1-25 g/L), and no evident aggregation occurs even at the highest salt concentrations. The slope of the diffusion coefficient is larger for lysozyme than for betalactoglobulin, especially at high salt concentration. The analysis of the interaction coefficients by means of the DLVO theory does not offer an unified picture of the protein interactions for the two systems. We discuss the possibility that specific (hydrogen bonding, salt bridges, etc.) or nonspecific (hydration forces) interactions, that go beyond the DLVO approximation, might be the origin of the larger van der Waals attraction shown by lysozyme. The data are consistent with a contribution of repulsive hydration forces that are smaller for lysozyme than for betalactoglobulin. This effect is in agreement with the difference in the protein size and charge.

Polyvinylpyrrolidone Adsorption on Na-Montmorillonite. Effect of the Polymer Interfacial Conformation on the Colloidal Behavior and Binding of Chemicals

Adsorption behavior of polyvinylpyrrolidone (PVP) on sodium montmorillonite, a swelling clay mineral, was investigated in diluted solutions at an ionic strength of 0.01 M, representative of environmental soil solution conditions. Physicochemical methods such as microcalorimetry, light scattering absorbance, photon correlation spectroscopy, microelectrophoresis, X-ray diffraction, and sedimentation volume measurements were used to characterize the colloidal behavior of montmorillonite particles with various PVP loadings. The extent and properties of the adsorbed PVP train, loop, and end fractions in the particle-stabilizing process have been specified. The interaction study of chemicals such as anionic surfactant sodium dodecyl sulfate and nonionized 4-monochlorophenol as well as their mixture also underlines the importance of the PVP interfacial conformations for the adsorbent capacity of this organoclay.

Particle size analysis of concentrated phospholipid microemulsions II. Photon correlation spectroscopy.

The solvated droplet size of concentrated water-in-oil (w/o) microemulsions prepared from egg and soy lecithin/water/isopropyl myristate and containing short-chain alcohol cosurfactants has been determined using photon correlation spectroscopy (PCS). The effect of increasing the water volume fraction (from 0.04 to 0.26) on the solvated size of the w/o droplets at 298 K has been investigated at 4 different surfactant/cosurfactant weight ratios (Km of 1:1, 1.5:1, 1.77:1, and 1.94:1); in all cases the total surfactant/cosurfactant concentration was kept constant at 25% w/w. In the case of the microemulsions prepared from egg lecthin, the diffusion coefficients obtained from PCS measurements were corrected for interparticulate interactions using a hard-sphere model that necessitated estimation of the droplet volume fractions, which in the present study were obtained from earlier total intensity light-scattering (TILS) studies performed on the same systems. Once corrected for hard-sphere interactions, the diffusion coefficients were converted to solvated radii using the Stokes-Einstein equation assuming spherical microemulsion droplets. For both egg and soy lecithin systems, no microemulsion droplets were detected at water concentrations less than 9 wt% regardless of the alcohol and Km used, suggesting that at low concentrations of added water, cosolvent systems were formed. At higher water concentrations, however, microemulsion droplets were observed. The changes in droplet size followed the expected trend in that for a fixed Km the size of the microemulsion droplets increased with increasing volume fraction of water. At constant water concentration, droplet size decreased slightly upon increasing Km. Interestingly, only small differences in size were seen upon changing the type of alcohol used. The application of the hard-sphere model to account for interparticulate interactions for the egg lecithin systems indicated that the uncorrected diffusion coefficients underestimated particle size by a factor of slightly less than 2. Reassuringly, the corrected droplet sizes agreed very well with those obtained from our earlier TILS study.

Area detector based photon correlation in the regime of short data batches: Data reduction for dynamic x-ray scattering

A method for reducing time sequences of raw scattering images to intensity time-autocorrelation functions is presented. The procedure is based on the use of a charge coupled device (CCD) area detector, and optimized for operating in the regime of short data batches. Its application to x-ray photon correlation spectroscopy (XPCS) measurements is described in detail. Using a slow-scan CCD, we explain how to achieve data acquisition on a 30 ms or faster time scale, while simultaneously acquiring data from many coherence areas in parallel. The statistical uncertainties of the acquired XPCS data are quantified experimentally, and compared to the theoretically expected noise levels of the correlation functions.