Tuning aggregation of microemulsion droplets and silica nanoparticles using solvent mixtures

The effect of solvent on stability of water-in-oil microemulsions has been studied with AOT (sodium bis(2-ethylhexyl)sulfosuccinate) and different solvent mixtures of n-heptane, toluene and dodecane. Dynamic light scattering DLS was used to monitor the apparent diffusion coefficient D(A) and effective microemulsion droplet diameter on changing composition of the solvent. Interdroplet attractive interactions, as indicated by variations in D(A), can be tuned by formulation of appropriate solvent mixtures using heptane, toluene, and dodecane. In extreme cases, solvent mixtures can be used to induce phase transitions in the microemulsions. Aggregation and stability of model AOT-stabilized silica nanoparticles in different solvents were also investigated to explore further these solvent effects. For both systems the state of aggregation can be correlated with the effective molecular volume of the solvent V(mol)(eff) mixture.     

The effect of analyte concentration range on measurement errors obtained by NIR spectroscopy

Near infrared spectroscopy (NIRS) was employed to quantify five compounds, ammonium, glucose, glutamate, glutamine, and lactate, in conditions similar to those obtained in animal cell cultivations over varying ranges of analyte concentrations. These components represent the primary nutrients and wastes of animal cells for which such noninvasive monitoring schemes are required for development of accurate control schemes. Ideal cultivation conditions involve maintaining concentrations of these components as low as 1 mM each, however, it is not known if measurements of these compounds can be accurately accomplished at such a low level. We have found that NIRS measurements of these analytes over narrow and low (0-1 mM) concentration ranges yield measurement errors of roughly 11% of the concentration range. By contrast, wide concentration ranges (0-30 mM) yield measurement errors of roughly 1.6% of the concentration range. Decreasing the concentration range over which an analyte is quantified in four out of five cases decreases the optimal spectral range by 100 cm(-1) for measurement by partial least squares regression analysis. There appears a similarity in the ratio of (standard error of prediction (SEP)/concentration range) which may provide an estimation of the anticipated SEP to be obtained for measurement over a new concentration range. It was found that for the five analytes evaluated here, the ratio of SEP to concentration range divided by that obtained for a second concentration range is equal to a fairly constant value of 6.6. This relationship was found to be followed reasonably well by an extensive number of measurement results reported in the literature for similar conditions.     

Methodology of analysis of unseparated mixtures: Error limits in estimating the total analyte concentration recalculated to a standard substance

In estimating the total concentration of similar analytes recalculated to a standard substance X_st, different sensitivities of their determination result in a systematic error δc, depending on the choice of X_st and the composition of the analyzed mixture. Two methods of predicting the limiting values of this error (δc _max), with and without the account of the properties of X_st are proposed and tested on model mixtures. The methods can be used if the total analytical signal linearly depends on the concentrations of analytes and is additive (spectrophotometry, conductometry, etc.). The value of δc _max is independent of the ratio of analytes concentrations in the sample. To reduce δc _max, one should equalize the sensitivities of the determination of all compounds of a given type and use analytes with medium sensitivities as X_st.     

Gold nanoparticles,capped by carboxy-calix[4]resorcinarenes: effect of structure and concentration of macrocycles on the nanoparticles size and aggregation

Small (d_core?≈?2–5 nm) well-dispersed gold nanoparticles (AuNPs) stabilized by amphiphilic octacarboxy-calix[4]resorcinarenes with different substituents on the lower rim—methyl (С₁–CR), pentyl (С₅–CR) and undecyl (С₁₁–CR)—in an aqueous solution were obtained. The nanoparticles were studied by spectrophotometry, transmission electron microscopy, FTIR-spectroscopy, dynamic light scattering, small angle X-ray scattering and X-ray powder diffraction. The influence of HAuCl₄/macrocycle ratio during the synthesis on the nanoparticles size and aggregation only for weakly associated С₁–CR and С₅–CR was achieved. The self-association effect of С₁₁–CR on the nanoparticles stabilization is found. The existence of gold in the form of crystallites and their average sizes were defined. The average nanoparticle sizes were determined and the structure of macrocyclic shells on the surface of nanoparticles in an aqueous solution was proposed. The formation of cooperative calix[4]resorcinarene associates on the AuNPs surface due to the multiple supramolecular interactions leads to the creation of functional gold nanoparticles.     

Colorimetric determination of ofloxacin using unmodified aptamers and the aggregation of gold nanoparticles

A colorimetric method is presented for the determination of the antibiotic ofloxacin (OFL) in aqueous solution. It is based on the use of an aptamer and gold nanoparticles (AuNPs). In the absence of OFL, the AuNPs are wrapped by the aptamer and maintain dispersed even at the high NaCl concentrations. The solution with colloidally dispersed AuNPs remains red and has an absorption peak at 520 nm. In the presence of OFL, it will bind to the aptamer which is then released from the AuNPs. Hence, AuNPs will aggregate in the salt solution, and color gradually turns to blue, with a new absorption peak at 650 nm. This convenient and specific colorimetric assay for OFL has a linear response in the 20 to 400 nM OFL concentration range and a 3.4 nM detection limit. The method has a large application potential for OFL detection in environmental and biological samples.

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Graphical abstract Schematic of a sensitive and simple colorimetric aptasensor for ofloxacin (OFL) detection in tap water and synthesic urine. The assay is based on the salt-induced aggregation of gold nanoparticles which results in a color change from red to purple.

     

Onset of non-linearity in zonal elution separators: the concept of effective analyte concentration

When an analyte injected in a zonal separation method (chromatography, capillary zone electrophoresis, field-flow fractionation) is not highly diluted in the carrier fluid, the retention ratio, R--or ratio of the cross-sectional average migration velocity of the analyte to that of the carrier fluid--depends on the local concentration, c, of the center of mass of the analyte zone, and the zone migration occurs in non-linear conditions. Because the zone broadens as it moves along the separator, R varies continuously from the inlet to the outlet of the separator. That concentration, c(eff), for which R(c(eff)) is equal to the length-averaged apparent retention ratio, R(app), is called effective concentration, and that distance, z(eff), from the separator inlet, for which c(z(eff)) is equal to c(eff), i.e. for which R(z(eff)) is equal to R(app), is called effective position. Assuming that near the onset the non-linear behavior, R(c), is a linear function, values of R(app), c(eff) and z(eff) have been computed in a wide range of operating conditions which are typical of situations encountered in capillary zone electrophoresis, liquid chromatography, or field-flow fractionation. Computations have been performed both in presence and in absence of the dispersion arising from the concentration dependence of the analyte migration rate (called thermodynamic dispersion in chromatography or electromigration dispersion in capillary zone electrophoresis). It is found that, whatever the range of analyte concentration covered from inlet to outlet of the separator, c(eff) is always close to two times the analyte concentration, c(out), at the outlet of the separator, and z(eff) between one-fourth and one-third of the separator length. As c(out) is easily determined from the peak recorded by a concentration-sensitive detector, a simple pragmatic expression is given for the estimation of c(eff). This effective concentration is the appropriate concentration to be used for comparing predictions of theoretical models of R(c) with experimental retention data. This is of particular interest for validating such models in field-flow fractionation.     

Separation of gas mixtures using a range of zeolite membranes: a molecular-dynamics study

Gas separation efficiencies of three zeolite membranes (Faujasite, MFI, and Chabazite) have been examined using the method of molecular dynamics. Our investigation has allowed us to study the effects of pore size and structure, state conditions, and compositions on the permeation of two binary gas mixtures, O(2)N(2) and CO(2)N(2). We have found that for the mixture components with similar sizes and adsorption characteristics, such as O(2)N(2), small-pore zeolites are not suited for separations, and this result is explicable at the molecular level. For mixture components with differing adsorption behavior, such as CO(2)N(2), separation is mainly governed by adsorption and small-pore zeolites separate such gases quite efficiently. When selective adsorption takes place, we have found that, for species with low adsorption, the permeation rate is low, even if the diffusion rate is quite high. Our results further indicate that loading (adsorption) dominates the separation of gas mixtures in small-pore zeolites, such as MFI and Chabazite. For larger-pore zeolites such as Faujasite, diffusion rates do have some effect on gas mixture separation, although adsorption continues to be important. Finally, our simulations using existing intermolecular potential models have replicated all known experimental results for these systems. This shows that molecular simulations could serve as a useful screening tool to determine the suitability of a membrane for potential separation applications.     

Colorimetric multiplexed immunoassay using specific aggregation of antigenic peptide-modified luminous nanoparticles

Two amperometric enzyme biosensor systems, based on glycerol dehydrogenase/diaphorase (GDH/DP) and glycerol kinase/glycerol-3-phosphate oxidase/peroxidase (GK/GPOx/HRP), were developed and used for estimation of glycerol content in a complex biological fluids. Enzymes were immobilized on interchangeable membranes by PCS-prepolymer technique. Buffers containing ferricyanide/NAD⁺ or ferrocyanide/ATP were used for measurements with GDH/DP and GK/GPOx/HRP biosensor, respectively. FIA assay of glycerol biosensor was characterized by a linear range of 0.01-1 or 0.01-1.5 mM glycerol, sensitivity of 6.02 or 1.42 mA/M cm² and with signal loss of 40% after 90 h or 30% after 16 h during continuous operation at a sample throughput of 10 injections/h for GDH/DP or GK/GPOx/HRP biosensors, respectively. Both biosensors were successfully used for off-line monitoring of glycerol during microbial transformation of glycerol to 1,3-propanediol using an automatized flow-through system. The results were consistent with those obtained with HPLC. The stability of described biosensor systems was sufficient for monitoring and control of fermentation process within 24 h. The storage stability of enzyme membranes was several months.     

Nanoparticle dispersion and aggregation in polymer nanocomposites: insights from molecular dynamics simulation

It is a great challenge to fully understand the microscopic dispersion and aggregation of nanoparticles (NPs) in polymer nanocomposites (PNCs) through experimental techniques. Here, coarse-grained molecular dynamics is adopted to study the dispersion and aggregation mechanisms of spherical NPs in polymer melts. By tuning the polymer-filler interaction in a wide range at both low and high filler loadings, we qualitatively sketch the phase behavior of the PNCs and structural spatial organization of the fillers mediated by the polymers, which emphasize that a homogeneous filler dispersion exists just at the intermediate interfacial interaction, in contrast with traditional viewpoints. The conclusion is in good agreement with the theoretically predicted results from Schweizer et al. Besides, to mimick the experimental coarsening process of NPs in polymer matrixes (ACS Nano 2008, 2, 1305), by grafting polymer chains on the filler surface, we obtain a good filler dispersion with a large interparticle distance. Considering the PNC system without the presence of chemical bonding between the NPs and the grafted polymer chains, the resulting good dispersion state is further used to investigate the effects of the temperature, polymer-filler interaction, and filler size on the filler aggregation process. It is found that the coarsening or aggregation process of the NPs is sensitive to the temperature, and the aggregation extent reaches the minimum in the case of moderate polymer-filler interaction, because in this case a good dispersion is obtained. That is to say, once the filler achieves a good dispersion in a polymer matrix, the properties of the PNCs will be improved significantly, because the coarsening process of the NPs will be delayed and the aging of the PNCs will be slowed.     

Identification of compounds in gasoline range mixtures using combined group-type and capillary GC separations

Summary A procedure is described to improve the identification of the hydrocarbon isomers separated on a single capillary column. Group-type PNA separation was combined with capillary GC analysis. Different paraffin and naphthene groups separated by carbon number on the 13X column of a PNA-analyzer were heart-cut and analysed off-line by capillary GC. Capillary GC chromatograms of the trapped groups compared to that of the original sample helped to identify the peaks (at least by group-type), to clear up mixed peaks and to find out the ratio of the compounds co-eluting in that particular peak. Results were used to create identification tables for different types of naphthas. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Immunoassay using surface-enhanced Raman scattering based on aggregation of reporter-labeled immunogold nanoparticles

A one-step homogenous sensitive immunoassay using surface-enhanced Raman scattering (SERS) has been developed. This strategy is based on the aggregation of Raman reporter-labeled immunogold nanoparticles induced by the immunoreaction with corresponding antigens. The aggregation of gold nanoparticles results in a SERS signal increase of the Raman reporter. Therefore, human IgG could be directly determined by measuring the Raman signal of the reporter. The process of aggregation was investigated by transmission electron microscopy (TEM) and UV-Vis absorption spectroscopy. The effects of the temperature, time, and size of gold nanoparticles on the sensitivity of the assay were examined. Using human IgG as a model protein, a wide linear dynamic range (0.1-15 microg mL(-1)) was reached with low detection limit (0.1 microg mL(-1)) under optimized assay conditions. The successful test suggests that the application of the proposed method holds promising potential for simple, fast detection of proteins in the fields of molecular biology and clinical diagnostics.     

Dependence of analytical signal on the concentration of analyte in X-ray spectral analysis

The dependence of the intensity of analytical signals on the element concentrations expressed in weight percentages I(c) and atomic percentages I(a) in X-ray spectrochemical analysis is considered. It was shown that the change-over from the dependence I(a) to the dependence I(c), which took place in the 1950s, had no theoretical grounds, was based on an incorrect application of the concepts of density and Avogadro’s constant, and was accepted because of practical convenience. In the dependence I(c), the calibration characteristics becomes more complex; it requires corrections for matrix effects, even if these effects are absent. Analytical data published in weight percentages and obtained in the analysis of various samples by conventional X-ray fluorescence analysis (XRFA), X-ray radiometric analysis with an energy-dispersive spectrometer, absorption analysis, and electron probe microanalysis are recalculated into atomic percentages. These results and the data obtained by the author in the analysis of alloys and steels demonstrate a correlation between the intensity of characteristic lines and the atomic concentration of the components. If the calibration curve is based on I(a), a linear dependence over a wide range or the entire concentration range is observed in the majority of systems, the number of the necessary reference standard materials sharply decreases (to one), the sensitivity coefficient increases, and the corrections for matrix effects decrease or the necessity for them disappears.     

Electron concentration measurement in the high voltage spark using stark broadening of Hβ emission

The applicability of Stark broadening of hydrogen atomic emission as a probe of electron concentrations in the high voltage spark discharge is critically evaluated. Instrumental limitations, broadening theory applicability and limitations due to discharge fluctuations are discussed. Comparison to literature time resolved, spatially resolved data indicates those regimes in which accurate electron concentration measurements can not be made using this technique. Suggestions are made for improvements in instrumentation which would allow faster throughput and more insight into discharge processes using Stark broadening as a diagnostic.     

How chromophore shape determines the spectroscopy of phenylene-vinylenes: origin of spectral broadening in the absence of aggregation

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support pi-electron conjugation across approximately 80 degrees curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.     

Selective recovery of micrometer particles from mixtures using a combination of selective aggregation and dissolved-air flotation

Particle–particle separation in biotechnology has gained interest over the years due to the large number of processes that yield particle mixtures. Direct isolation of the product-containing particles is a logical and efficient downstream processing route in these processes. Dissolved-air flotation is applicable for these separations when the particles that require separation have different interactions with the air bubbles and/or differ in aggregation behaviour.

In this work, model particles consisting of micrometer-sized protein-coated polystyrene particles were used to investigate the requirements for the application of dissolved-air flotation for particle–particle separation in biotechnology. These model particles have heterogeneous surfaces with surface groups (brushes) that extend out into the solution. Therefore, steric (or brush) repulsion and so-called hydrophobic interactions between the particles need to be taken into account. The flotation behaviour of the protein-coated particles was related to the size of the aggregates and the foaming behaviour of the proteins. Prediction of their aggregation behaviour was performed on the basis of calculations of the Van der Waals, electrostatic, hydrophobic and brush interactions. The brush interaction force proves to be essential for the prediction of the aggregation behaviour of the particles.     

Sustained prediction ability of net analyte preprocessing methods using reduced calibration sets. Theoretical and experimental study involving the spectrophotometric analysis of multicomponent mixtures

A newly developed multivariate method involving net analyte preprocessing (NAP) was tested using central composite calibration designs of progressively decreasing size regarding the multivariate simultaneous spectrophotometric determination of three active components (phenylephrine, diphenhydramine and naphazoline) and one excipient (methylparaben) in nasal solutions. Its performance was evaluated and compared with that of partial least-squares (PLS-1). Minimisation of the calibration predicted error sum of squares (PRESS) as a function of a moving spectral window helped to select appropriate working spectral ranges for both methods. The comparison of NAP and PLS results was carried out using two tests: (1) the elliptical joint confidence region for the slope and intercept of a predicted versus actual concentrations plot for a large validation set of samples and (2) the D-optimality criterion concerning the information content of the calibration data matrix. Extensive simulations and experimental validation showed that, unlike PLS, the NAP method is able to furnish highly satisfactory results when the calibration set is reduced from a full four-component central composite to a fractional central composite, as expected from the modelling requirements of net analyte based methods.     

Communication: Unusual dynamics of hybrid nanoparticles and their binary mixtures

We present the results on the evolution of microscopic dynamics of hybrid nanoparticles and their binary mixtures as a function of temperature and wave vector. We find unexpectedly a nonmonotonic dependence of the structural relaxation time of the nanoparticles as a function of the morphology. In binary mixtures of two of the largest nanoparticles studied, we observe re-entrant vitrification as a function of the volume fraction of the smaller nanoparticle, which is unusual for such high diameter ratio. Possible explanation for the observed behavior is provided.