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J Buffle

Publications and source records attributed to J Buffle.

12 recordsLinked to original sources
PubMed2001

Applications of fluorescence correlation spectroscopy: measurement of size-mass relationship of native and denatured schizophyllan.

Diffusion dynamics of a polysaccharide, schizophyllan has been studied by fluorescence correlation spectroscopy (FCS). Several different sizes of nondenatured and denatured schizophyllan have been labeled with rhodamine 6G in borate buffer. The length of the nondenatured schizophyllan was calculated from FCS data by using the Broersma's relationship for rod-like macromolecules. The obtained length was close to that obtained by atomic force microscopy (AFM) measurements. Denatured schizophyllan possesses a random coil conformation. Its hydrodynamic radius R(h) was measured by FCS. The relationship between R(h) and the molecular mass M has been studied and the scaling relationship R(h)--M(0.59) has been obtained, which is in agreement with the random coil model with excluded volume effect. The persistence length q(denat) of the denatured schizophyllan was determined by Hearst's relationship, to be equal to 5.16 +/- 0.75 (nm). The work demonstrates the utility of FCS method for dynamics investigations of biopolymers especially in diluted regime (concentration lower than 10(-8)M could be measured) where other techniques could not be used.

Biopolymers↗
PubMed2001

Individually addressable gel-integrated voltammetric microelectrode array for high-resolution measurement of concentration profiles at interfaces.

The application of a novel voltammetric probe, based on an individually addressable gel-integrated microelectrode array (IA-GIME), for real-time, high-spatial resolution concentration profile measurements at interfaces is described. Reliability and validity of steep metal concentration gradients obtained with this novel system have been demonstrated by performing systematic tests at well-controlled liquid-liquid and liquid-solid interfaces. The liquid-liquid interface was formed by two layers of aqueous solutions with different components; only one layer contained trace metal ions (Pb(II) and Cd(II)); the individually addressable microelectrode array was placed at the interface of the liquid-liquid system; the concentration profiles were recorded as function of time; and the effective diffusion coefficients were calculated. The liquid-"solid" interface was formed from an aqueous solution layer overlying a bed of silica particles saturated with an aqueous solution. The sensor array has been used to monitor the diffusion processes of Tl(I) or Pb(II) from the liquid phase to the "solid" phase. The influences of porosity, geometry of the porous media, and complexation between metal ion and silica, on the diffusion processes, have been studied. All these results show that correct diffusion profiles of metal ions at interfaces can be obtained with 200-microm resolution with the IA-GIME. They also demonstrate that, for measurements in "solid" phase, the aforementioned factors must be considered carefully for correct calibration of any electrodes and the gel-integrated microelectrodes are unique tools to enable calibration of the sensors with synthetic solutions.

Journal Article↗
PubMed2001

Noise on Fluorescence Correlation Spectroscopy.

The time dependence of the noise and the signal-to-noise (SN) ratio of the fluorescence correlation spectroscopy (FCS) autocorrelation function is obtained from replica measurements of standard dextran solutions. The noise dependence on the delay time is fitted by a hyperbolic function with two fitting parameters. The dependence of these parameters on concentration, fluorescence intensity, and accumulation time is obtained experimentally. The behavior of SN at zero delay time agrees well with the theoretical predictions reported in the literature. The obtained data are useful for the quantitative evaluation of the FCS data fits, as well as for simulation of the FCS autocorrelation functions. Copyright 2001 Academic Press.

Journal Article↗
PubMed2000

Permeation liquid membrane metal transport: studies of complex stoichiometries and reactions in Cu(II) extraction with the mixture 22DD-laurate in toluene/phenylhexane

The role of lauric acid (LAH) in the transport of copper(II) through a permeation liquid membrane (PLM) comprising 1,10-didecyldiaza-18-crown-6 (22DD) and lauric acid (ratio 1:1) in 1:1 v/v toluene/phenylhexane has been investigated by determining the stoichiometry of metal extraction and of the metal complex formed in the organic phase by performing 1H NMR and liquid/liquid and liquid/membrane extraction measurements. In the absence of copper(II), the 1H NMR data suggest that there is a strong interaction between the proton of LAH and the nitrogen of the 22DD macrocycle but no interaction between the aliphatic long chains of LAH and 22DD. Thus, in the organic solution, the two compounds are associated as (22DD-H)(+)-LA-, the laurate being away from (22DD-H)+. The signal intensity of the acidic proton was found to decrease when the metal Pb(II) was incorporated by the carrier after its extraction from the aqueous phase. Additionally, liquid/liquid as well as liquid/membrane extraction results reveal that Cu(II) extraction proceeds via the loss of two protons from the organic phase. The Cu(II) is found to be located in the 22DD cavity and the stoichiometry of the complex in the organic phase is (22DD-Cu)(2+)-2LA-. Metal extraction is governed by 22DD and laurate acts only as counteranion. An unexpected feature was observed in the liquid/liquid extraction which was that, at low 22DD and LAH concentrations, the slope for log(Kp) = f(pH) was 2 whereas it was much lower at high carrier concentration. This unexpected result seems to stem from impurities present in 22DD: only 0.1 mol% of impurity can indeed influence the exchange ratio of Cu(II) and H+. This type of anomaly, however, is not found in the normal procedure of liquid/membrane extraction possibly due to the lower carrier/metal molar ratio which is used in the classical PLM conditions.

Journal Article↗
PubMed2000

Voltammetric and reference microelectrodes with integrated microchannels for flow through microvoltammetry. 1. The microcell

This paper describes the construction of 2 microsensor units for on-line voltammetric detection inside a cylindrical microcell (a working microsensor unit and a reference and auxiliary microsensor unit), for application to heavy-metal analysis in complex media such as natural waters. Both microsensor units include a channel for the solution renewal in the microcell after analysis. The working microsensor, a Hg-plated Ir microelectrode, is protected against fouling with an agarose gel including a hydrophobic chromatographic phase (C18). The fabrication steps and the quality tests related to long-term use and reliability, as well as to precision, are described. The application of the protective gel layer against fouling by hydrophobic or surface active small molecules is of general application, reliable, and very efficient. The reference and auxiliary unit is composed by an iridium oxide based mini reference electrode, an auxiliary Pt electrode, and a circulation channel. It is built to enable its use inside a 700-microm-diameter tubing connected to a hollow fiber supported liquid membrane lumen (volume, 5-10 microL) for heavy-metal analysis. However, it can be used in any other microanalytical system. The reference electrode is sufficiently stable for voltammetric applications (1-2 mV drift/day), and its lifetime is more than one year. The Ti/IrO2 core is immersed in a pH-buffered agarose gel, to guarantee potential stability even when the electrode is immersed in variable pH solutions.

Journal Article↗
PubMed2000

Voltammetric and reference microelectrodes with integrated microchannels for flow through microvoltammetry. 2. Coupling the microcell to a supported liquid membrane preconcentration technique

The paper describes the assembly and functioning of the microvoltammetric cell described in Part 1, with a hollow Fiber Supported Liquid Membrane (HFSLM), for trace metal analysis. Membrane stability, working-electrode behavior, mercury-film lifetime inside the HFSLM, hydrodynamic conditions, as well as transport kinetics of the metal through the SLM have been studied in detail. System calibrations have been performed in the range 5-120 nM Pb(II). The reproducibility and sensitivity of the whole microsystem is discussed as well as limitations and possible improvements.

Journal Article↗
PubMed2000

Simultaneous determination and speciation of zinc, cadmium, lead, and copper in natural water with minimum handling and artifacts, by voltammetry on a gel-integrated microelectrode array.

The paper reports a new approach based on a gel-integrated Hg-plated-Ir-based microelectrode array (GIME), for measuring Cu, Pb, Cd, and Zn speciation in natural waters. This paper focuses on the quantitative discrimination between mobile and colloidal metal species (size limit of a few nanometers), for which most classical separation techniques present severe drawbacks. Previous papers have shown qualitatively that GIME combined with square wave anodic stripping voltammetry (SWASV) has the basic characteristics required to discriminate between these two fractions directly on the unperturbed sample. In addition, because of the large sensitivity provided by GIME, complexation parameters (equilibrium constants and site concentrations) can be determined in little-perturbed samples, particularly without metal addition or with small addition compared with natural concentrations. The advantages of this procedure are exemplified and the possible artifacts occurring when titrating the sample with metals, in particular intermetallic compound formation and other problems, are discussed in detail. The present paper shows that the characteristics of GIME make it a unique tool to get quantitative information on metal speciation at nanomolar or even subnanomolar levels, with only minor sample handling.

Cadmium↗
PubMed1999

Applications of Fluorescence Correlation Spectroscopy: Polydispersity Measurements.

The method of histograms is applied to the determination of polydispersity of particles and molecules in solution from fluorescence correlation spectroscopy (FCS) data. This is an ill-posed problem, which can be overcome by using a common strategy for imposed regularization and constraint conditions. The method developed for evaluating the polydispersity is tested on both computer-generated correlation curves and real FCS data. The results obtained show that FCS measurements can be successfully used for the determination of polydispersity of suspensions, with an efficiency comparable to that of photon correlation spectroscopy (PCS). The advantage of FCS, however, is its better sensitivity to small particles (size <50 nm) and molecules in dilute solutions, as well as its better selectivity. The usefulness of FCS for environmental chemistry is discussed with regard to the obtained results. Copyright 1999 Academic Press.

Journal Article↗
PubMed1998

Flocculation of Hematite with Polyacrylic Acid: Fractal Structures in the Reaction- and Diffusion-Limited Aggregation Regimes.

The structure of hematite aggregates in the presence of fairly monodisperse polyacrylic acid (PAA) with two different molecular weights (Mw = 1.36 x 10(6), Mw/Mn = 1.53; Mw = 3.69 x 10(4), Mw/Mn = 1.60) was studied using static light scattering (SLS). The fractal dimensions were calculated from the scattering exponents, after taking into account the finite size of aggregates, using exponential and Gaussian cutoff functions. Three flocculation regimes, namely, pre-DLA, DLA (diffusion-limited aggregation), and post-DLA, were defined based on the polymer concentration. In the DLA regime, fractal dimension values, Df = 1.84 +/- 0.02 and 1.73 +/- 0.02, were obtained using exponential and Gaussian cutoff functions, respectively. A fractal dimension of approximately 2.0 was found, as expected, in the pre-DLA regime (at PAA concentrations lower than the optimal dosage for a DLA regime) where the flocculation rate was reaction limited. In contrast, in the post-DLA regime, the flocculation was slow but the structure of aggregates was as tenuous as in the DLA regime with a fractal dimension Df approximately 1.8. Moreover, for all three regimes, the Df values were independent of the molecular weights of PAA. The lower fractal dimension in post-DLA was probably due to the increased concentration of polymer chains between adjacent particles in aggregates. The steric hindrance favored tip-to-tip aggregation, leading to a more tenuous structure. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

Computer Simulation of Flocculation Processes: The Roles of Chain Conformation and Chain/Colloid Concentration Ratio in the Aggregate Structures.

The flocculation of colloidal particles in the presence of adsorbing polymers is a key process in colloid science, as well as in the chemical and biological regulation of aquatic systems. Polymers can influence important physical properties of colloidal aggregates such as their densities and settling velocities, as well as their chemical properties, affecting the probability that two colloidal particles will stick together when they collide. The presence of polymers usually makes more difficult the application of a coagulation theory to colloidal suspensions and the interpretation of experimental observations. Knowledge of floc structures is a key factor in the understanding of flocculation processes, and simulation may provide useful insights required to interpret the results of experimental studies and elaborate new theoretical models. Although modeling leaves much room for more progress, researchers now find it indispensible from a fundamental point of view and for environmental applications. In this paper, we report a computer simulation study of a two- and three- dimensional model for bridging flocculation between large linear polymer chains and comparatively small colloidal particles. The floc structures are investigated as a function of chain/particle concentration ratio, chain conformation, and space dimension. The values of the sticking probabilities are chosen to emphasize colloid-chain interactions compared to colloid-colloid or chain-chain interactions. The results suggest that the floc morphology is strongly dependent on the chain conformation and to a slight extent on the chain/particle concentration ratio. In particular, colloid interactions with linear rods result in a network characterized by fractal dimensions significantly higher than those obtained on the basis of the Cluster-Cluster Aggregation models of colloids only, or by flocculation of colloids with coiled chains. Copyright 1998 Academic Press.

Journal Article↗