Calibration of Polarization-Sensitive and Dual-Angle Laser Light Scattering Methods Using Standard Latex Particles

A polarization-sensitive laser light scattering (PSLLS) method and a dual-angle laser light scattering (DALLS) method have been studied for in situ measurement of submicrometer hydrosol and aerosol particles. By using standard monodisperse polystyrene latex particles suspended in water and air as test particles, calibration of systems built based on the above methods have been performed. The effects of light scattered by agglomerated aerosol particles (multiplets) were corrected by considering the fraction of multiplets as determined with an aerosol measurement technique using a differential mobility analyzer. The change in the measured intensities of scattered light with particle diameter was then determined by calculations based on Mie theory. It was shown that the PSLLS system can determine particle diameters as small as approximately 60 nm for the test hydrosol particles and approximately 100 nm for test aerosol particles, respectively. The DALLS system can determine smaller diameters than the PSLLS system for test particles with no light absorption. The change in scattered light intensities with particle diameter was also investigated by theoretical calculations with various refractive indexes and scattering angles. The PSLLS and DALLS systems promise to become routine measurement tools for absorbing and nonabsorbing particles, respectively. Copyright 2001 Academic Press.     

Measurement of Colloidal Stability in Solutions of Simple, Nonadsorbing Polyelectrolytes

The stability of a solution of charged polystyrene particles in the presence of nonadsorbing polyelectrolyte macromolecules is measured using optical light scattering. The particles were negatively charged polystyrene latex spheres (0.5–1 μm diameter) while the macromolecules were simulated using negatively charged colloidal silica spheres (5–7 nm diameter). Because of the electrostatic repulsion between the particles, the solution is found to be stable against primary flocculation (irreversible flocculation into a primary energy minima). However, because of long-range attractive depletion forces, reversible secondary flocculation of the particles occurs into a local potential energy minimum. As observed with uncharged macromolecules, the polyelectrolyte first induces flocculation at a critical flocculation concentration (v*), but later restabilizes the system at a critical restabilization concentration (v**). These critical concentrations are found to decrease with decreasing macromolecule size and increasing particle size. The restabilized solutions are found to remain suspended for periods greater than 20 days. Comparison of the measured flocculation and restabilization results to predictions made using a recently developed force-balance model show qualitative agreement.     

Elastic light scattering from nanoparticles by monochromatic vacuum-ultraviolet radiation

Elastic light scattering is reported using monochromatic vacuum-ultraviolet radiation to study free, spherical silica nanoparticles prepared by approaches from colloidal chemistry, with diameters between 100 and 240 nm. The colloidal nanoparticles of defined size are transferred from an aqueous solution into the gas phase using a particle beam experiment. After focusing of the particle beam by an aerodynamic lens, the scattered light from monochromatic synchrotron radiation is measured. Angle-resolved elastically scattered light is detected, showing a strong forward-scattering component. Additional evidence for the detection of elastically scattered light comes from plotting the scattered light intensity as a function of the dimensionless parameter qR, where q is the magnitude of the scattering wave vector and R is the particle radius. This yields different power-law regimes that are assigned to scattering from the surface and the bulk of the nanoparticles. Furthermore, there is evidence for modulations in the scattered light intensity as a function of scattering angle, which is clearly distinguished from the forward-scattering component. The experimental results are compared to Mie scattering simulations for isolated particles, yielding general agreement with the experimental results. Deviations from Mie simulations are observed for samples consisting of significant amounts of aggregates. The present results indicate that the optical properties of free nanoparticles are sensitively probed by vacuum-ultraviolet radiation.     

Laser-induced breakdown detection combined with asymmetrical flow field-flow fractionation: application to iron oxi/hydroxide colloid characterization

The combination of asymmetrical flow field-flow fractionation (AsFlFFF) with the laser-induced breakdown detection (LIBD) is presented as a powerful tool for the determination of colloid size distribution at trace particle concentrations. Detection limits (D1) of 1, 4, and 20 microg/L have been determined for a mixture of polystyrene reference particles with 20, 50, and 100 nm in size, respectively. This corresponds to injected masses of 1, 4, and 20 pg, which is lower than found in a previous study with the symmetrical FlFFF (SyFlFFF). The improvement is mainly due to the lower colloid background discharged from the AsFlFFF channel. The combined method of AsFlFFF-LIBD is then applied to the analysis of iron oxi/hydroxide colloids being considered as potential carriers for the radionuclide migration from a nuclear waste repository. Our LIBD arrangement is less sensitive for iron colloid detection as compared to reference polystyrene particles which results in a detection limit of approximately 240 microg/L FeOOH for the AsFlFFF-LIBD analysis. This is superior to the detection via UV-Vis absorbance and comparable to ICP-MS detection. Size information (mean size 11-18 nm) for different iron oxi/hydroxide colloids supplied by the present method is comparable to that obtained by sequential ultrafiltration and dynamic light scattering. A combined on-line ICP-MS detection is used to gain insight into the colloid-borne main and trace elements.     

石英纳米通道单磷脂囊泡的电阻-脉冲分析方法

本文报告了用于检测单囊泡及其粒径分析的石英纳米通道电阻-脉冲分析方法. 采用圆柱形石英纳米通道可检测粒径为100~300 nm的单磷脂囊泡和直径为170~400 nm的聚苯乙烯纳米颗粒. 单囊泡和纳米颗粒的迁移可通过检测各自产生的方波电流脉冲信号, 并由此确定颗粒尺寸. 结果表明,采用石英纳米通道电阻-脉冲分析方法得到的颗粒/囊泡粒径与采用动态光散射法和扫描电子显微法得到的结果完全一致. 这种基于电子的分析方法具有快速简单的特点,所用的自制微传感器廉价耐用. 石英通道的应用还可与其它分析方法如电流分析法和荧光显微法联用,以获得生物囊泡及人工囊泡更完全的信息.     

Photochemical interaction of polystyrene nanospheres with 193 nm pulsed laser light

The photochemical interaction of 193 nm light with polystyrene nanospheres is used to produce particles with a controlled size and morphology. Laser fluences from 0 to 0.14 J/cm2 at 10 and 50 Hz photofragment nearly monodisperse 110 nm spherical polystyrene particles. The size distributions before and after irradiation are measured with a scanning mobility particle sizer (SMPS), and the morphology of the irradiated particles is examined with a transmission electron microscope (TEM). The results show that the irradiated particles have a smaller mean diameter ( approximately 25 nm) and a number concentration more than an order of magnitude higher than nonirradiated particles. The particles are formed by nucleation of gas-phase species produced by photolytic decomposition of nanospheres. A nondimensional parameter, the photon-to-atom ratio (PAR), is used to interpret the laser-particle interaction energetics.     

Hollow silica capsules with well-defined asymmetric windows in the shell

A straightforward and effective approach to fabricate porous silica capsules with well-defined asymmetric windows in the shell using raspberry-like templates has been developed. This process begins with the formation of a hierarchical template by chemically coupling a large polystyrene sphere to an ensemble of small, polystyrene latex spheres. The hierarchical template in conjunction with a hard templating method and spin-coating leads to silica capsules with well-defined, asymmetric pores (windows) in the outer shell. Proof-of-principle of this approach has been demonstrated using a 1500/110 nm hierarchical template. The silica capsules thus produced were characterized with scanning electron microscopy and STEM. The diameter of the capsules was ~1400 nm, and the outer opening of the windows was ~100 nm in size, consistent with the diameters of the core and satellite spheres considering the shrinkage due to the calcination. The inner opening was ~30 nm, which gives rise to an asymmetry factor, defined as the diameter of the outer window to the diameter of the inner window, of ~3. In another example, surface-bound capsules with an asymmetry factor of ~1 were made. Collectively, these windows can provide efficient pathways to connect the inside of the capsule to the outside and have potential for asymmetric diffusion and rectification.     

Contamination by larger particles of two almost-uniform latices: analysis by combined dynamic light scattering and turbidimetry

Multiangle dynamic light scattering (MDLS) and turbidimetry (T) were applied (both individually and combined) for determining the contamination by larger particles of two almost-uniform polystyrene (PS) latices. Latex 1 was synthesized in our laboratories, and it contained a main population diameter of 340 nm together with a small fraction of larger particles. This latex was used as the base material for producing an immunoassay kit. Latex 2 was obtained by a simple blend of two uniform PS standards. The proposed data treatment calculates the diameter and number fraction of the large particles contamination assuming that the PSDs are bimodal. The calculation involves minimizing the errors between the measurements and their theoretical predictions. When analyzed by combined MDLS-T, the contamination of Latex 1 involved number fraction 0.6% and particle diameter 865 nm. The T average diameter is a function of the measurement wavelength, and the highest deviations of this average to an increasing contamination by large particles were always observed at the higher wavelengths. The DLS average diameter is a function of the measurement angle, but in this case it is impossible to determine a priori the angle of observation that provides the largest deviation of this average diameter to an increasing contamination.     

Determination of individual microsphere properties by capillary electrophoresis with laser-induced fluorescence detection

Capillary electrophoresis with postcolumn laser-induced fluorescence detection was used to individually detect 6.0, 1.0, 0.5, and 0.2 num diameter polystyrene microspheres and individually measure their electrophoretic mobility. The analysis of a nanoliter-size volume from a microsphere suspension results in an electropherogram characterized by several narrow spikes in a well-defined migration time window. Each spike is associated with one microsphere because, when one single microsphere is introduced into the capillary by micromanipulation, the electropherogram has only one spike in the same migration time window. The distributions of individual measurements resulting from an electropherogram were used to evaluate the reproducibility from run to run, observe the effect of sodium dodecyl sulfate (SDS) added to the running buffer, and to investigate the origin of electrophoretic dispersion. As expected from the interactions between microspheres and SDS, the addition of this surfactant to the running buffer narrowed the range and shifted the average electrophoretic mobility to more negative values. After evaluating common sources of broadening in capillary electrophoresis, electrophoretic dispersion was attributed to microsphere heterogeneity. Unlike electropherograms displaying Gaussian-like profiles, the two-dimensional representations of the individual measurements provide a new alternative to evaluate and study electrophoretic-related properties of microspheres.     

Force measurements between sub-100 nm colloidal particles

Interparticle forces have been measured between polystyrene latex particles as small as 85 nm in diameter in KCl solutions. A variant of the differential electrophoresis technique, called particle force light scattering (PFLS), was used to measure forces between Brownian, nearly touching particles for diameters from 4500 nm down to 85 nm. The forces, some less than 0.1 pN, matched to within a factor of 2 with predictions from depletion and DLVO theory.     

Measurements of extinction by aerosol particles in the near-infrared using continuous wave cavity ring-down spectroscopy

Cavity ring-down spectroscopy using a fiber-coupled continuous wave distributed feedback laser at a wavelength of 1520 nm has been used to measure extinction of light by samples of nearly monodisperse aerosol particles <1 μm in diameter. A model is tested for the analysis of the sample extinction that is based on the Poisson statistics of the number of particles within the intracavity laser beam: variances of measured extinction are used to derive values of the scattering cross section for size-selected aerosol particles, without need for knowledge of the particle number density or sample length. Experimental parameters that influence the performance of the CRD system and the application and limitations of the statistical model are examined in detail. Determinations are reported of the scattering cross sections for polystyrene spheres (PSSs), sodium chloride, and ammonium sulfate, and, for particles greater than 500 nm in diameter, are shown to be in agreement with the corresponding values calculated using Mie theory or Discrete Dipole Approximation methods. For smaller particles, the experimentally derived values of the scattering cross section are larger than the theoretical predictions, and transmission of a small fraction of larger particles into the cavity is argued to be responsible for this discrepancy. The effects of cubic structure on the determination of optical extinction efficiencies of sodium chloride aerosol particles are examined. Values are reported for the real components of the refractive indices at 1520 nm of PSS, sodium chloride, and ammonium sulfate aerosol particles.     

基质与分析物的摩尔比和激光能量对分析物离子信号的影响

A method of aerosol introduction for matrix-assisted laser desorption/ionization (MALDI) is described. The aerosol particles containing matrix and analyte enter directly into the aerosol time-of-flight mass spectrometer (ATOFMS) at atmospheric pressure. The scattered light signals from the aerosol particles are collected by a photomultiplier tube (PMT) and are passed on to an external electronic timing circuit, which determines particle size and is used to trigger a 266 nm pulsed Nd:YAG laser. The aerosol MALDI mass spectra and aerodynamic diameter of single particles can be obtained in real-time. Compared with other methods of liquid sample introduction, this method realizes detection of single particles and, more importantly, the sample consumption is lower. The effects of matrix-to-analyte ratio and laser pulse energy on analyte ion yield are examined. The optimal matrix-to-analyte ratio and laser energy are 50-110:1 and 200-400 μJ respectively.     

Method for viscosity measurements at high pressure

The technique of quasielastic light scattering is utilized to determine the viscosity of fluids subjected to large hydrostatic pressures. In this method, a small concentration of insoluble polystyrene spheres of known diameter, 0.109 μm, is added to the fluid. The high-pressure cell in which the fluid is contained has three optical windows. The laser light scattered from the suspended spheres is analyzed to determine the diffusion coefficient of the spheres. Since, by the Stokes law, the viscosity of the fluid is determined from the measured diffusion coefficient and known particle radius r. A small correction is required for the contraction of the spheres and for refractive-index change of the water under pressure. Results on the viscosity of water at 25°C up to pressures of 2.5 kbar are reported and compared with published results. This method is applicable to lubricants and polymer solutions where the viscosity is many orders of magnitude greater than that of water.     

Correcting for a density distribution: particle size analysis of core-shell nanocomposite particles using disk centrifuge photosedimentometry

Many types of colloidal particles possess a core-shell morphology. In this Article, we show that, if the core and shell densities differ, this morphology leads to an inherent density distribution for particles of finite polydispersity. If the shell is denser than the core, this density distribution implies an artificial narrowing of the particle size distribution as determined by disk centrifuge photosedimentometry (DCP). In the specific case of polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, we demonstrate that the particle density distribution can be determined by analytical ultracentrifugation and introduce a mathematical method to account for this density distribution by reanalyzing the raw DCP data. Using the mean silica packing density calculated from small-angle X-ray scattering, the real particle density can be calculated for each data point. The corrected DCP particle size distribution is both broader and more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction, and dynamic light scattering. Artifactual narrowing of the size distribution is also likely to occur for many other polymer/inorganic nanocomposite particles comprising a low-density core of variable dimensions coated with a high-density shell of constant thickness, or for core-shell latexes where the shell is continuous rather than particulate in nature.     

Determination of colloid size by 2-D optical detection of laser induced plasma

A new method is presented for the size determination of aquatic colloids, which is based on the selective plasma generation (breakdown) on individual particles by a focused laser pulse. For this purpose, the breakdown events of particles are monitored by 2-D optical detection instead of commonly used photoacoustic detection. The length of the breakdown volume determined for the 2-D projection of spatially distributed plasma events generated on colloidal particles is proportional to the particle size. Based on this fact, an average diameter of a given colloidal dispersion can be determined. The method is calibrated by well-characterised hard-sphere polystyrene reference particles of different diameters.     

Integrated particle detection chip for environmental monitoring

This paper reports an integrated particle detection chip for low-cost and point-of-interest environmental monitoring; it consists of a micro virtual impactor and a micro corona discharger. With this system, airborne particles are introduced into the micro virtual impactor of the chip where they are classified according to their aerodynamic diameters. The particles are then charged and their number-concentration is detected in the micro corona discharger from the electrical current carried by the charged particles. The characteristics of each component were first analyzed, and the components were then integrated into a single chip. The micro virtual impactor was designed to have a cut-off diameter of 600 nm or 1.0 microm. Its classification characteristics were examined by classifying polydisperse particles-dioctyl sebacate particles ranging in diameter from 100 to 600 nm and carbon particles ranging in diameter from 0.6 to 10 microm. From the classification results, the cut-off diameter of the micro virtual impactor was measured to be either 550 nm or 1.1 microm. The micro corona discharger was fabricated based on a sharp silicon tip and a planar electrode and charged particles at 1.3 kV. Using the integrated particle detection chip comprising the micro virtual impactor and the micro corona discharger, the sensitivity for monodisperse particles-500 nm dioctyl sebacate in diameter-was measured to be 8 x 10(-7) pA/(particle cm(-3)).     

Using light scattering to evaluate the separation of polydisperse nanoparticles

The analysis of natural and otherwise complex samples is challenging and yields uncertainty about the accuracy and precision of measurements. Here we present a practical tool to assess relative accuracy among separation protocols for techniques using light scattering detection. Due to the highly non-linear relationship between particle size and the intensity of scattered light, a few large particles may obfuscate greater numbers of small particles. Therefore, insufficiently separated mixtures may result in an overestimate of the average measured particle size. Complete separation of complex samples is needed to mitigate this challenge. A separation protocol can be considered improved if the average measured size is smaller than a previous separation protocol. Further, the protocol resulting in the smallest average measured particle size yields the best separation among those explored. If the differential in average measured size between protocols is less than the measurement uncertainty, then the selected protocols are of equivalent precision. As a demonstration, this assessment metric is applied to optimization of cross flow (V_x) protocols in asymmetric flow field flow fractionation (AF⁴) separation interfaced with online quasi-elastic light scattering (QELS) detection using mixtures of polystyrene beads spanning a large size range. Using this assessment metric, the V_x parameter was modulated to improve separation until the average measured size of the mixture was in statistical agreement with the calculated average size of particles in the mixture. While we demonstrate this metric by improving AF⁴V_x protocols, it can be applied to any given separation parameters for separation techniques that employ dynamic light scattering detectors.     

Comparison of nanoparticle size and electrophoretic mobility measurements using a carbon-nanotube-based coulter counter, dynamic light scattering, transmission electron microscopy, and phase analysis light scattering

The precision and accuracy of measurements of the diameter and electrophoretic mobility (mu) of polymeric nanoparticles is compared using four different analytical approaches: carbon-nanotube-based Coulter counting, dynamic light scattering (DLS), transmission electron microscopy (TEM), and phase analysis light scattering (PALS). Carbon-nanotube-based Coulter counters (CNCCs) use a 132 nm diameter channel to simultaneously determine the diameter (28-90 nm) and mu value for individual nanoparticles. These measurements are made without calibration of the CNCC and without labeling the sample. Moreover, because CNCCs measure the properties of individual particles, they provide true averages and polydispersities that are not convoluted into the intrinsic instrumental response function of the CNCC. CNCCs can be used to measure the size of individual nanoparticles dispersed in aqueous solutions, which contrasts with the TEM-measured size of individual dehydrated particles and the ensemble size averages of dispersed particles provided by DLS. CNCCs provide more precise values of mu than PALS.     

Particle formation by infrared laser ablation of glycerol: implications for ion formation

The quantity and size distribution of micrometer-sized particles ejected from thin films of glycerol were measured using light scattering particle sizing. Thin glycerol films were irradiated at atmospheric pressure with an infrared optical parametric oscillator at wavelengths between 2.95 and 3.1 microm. Particulate material resulting from the ablation was sampled directly into a particle-sizing instrument and particles with diameters greater than 500 nm were detected and sized by light scattering. The fluence threshold for particle formation was between 2000 and 3000 J/m2 for all laser wavelengths. At threshold, fewer than 100 particles/cm3 were detected and this value increased to several thousand particles/cm3 at twice the threshold fluence. The average size of the coarse particles ranged from 900 nm to 1.6 microm at threshold and decreased by 10-20% at twice the threshold fluence. The coarse particle formation observations were compared with ion formation behavior in matrix-assisted laser desorption ionization and interpreted in terms of a photomechanical mechanism for material ablation and ion formation.     

Sensitive light-scattering detection–magnetophoretic acceleration mass analysis of single microparticles in an atmosphere

Optically detected magnetophoretic acceleration mass analysis of an individual micro-particle in an atmosphere has been remarkably improved in sensitivity by using a reflective microscope objective, by which forward scattered light from a particle could be effectively collected. From the light-scattering simulation, the detection limit for the radius of a micro-particle was estimated to be smaller than 0.4 μm, and about 60 times intensity enhancement was observed for a polystyrene particle with a radius of 2.8 μm. For both paramagnetic and diamagnetic micro-particles, the mass magnetic susceptibility and the relaxation time could be determined without knowing any parameters of the particles. From the relaxation time, the mass of a particle was obtained if the radius or the density of the particle was known. For a test sample silica particles were used to adsorb paramagnetic dysprosium(III), the surface concentration of dysprosium(III) on a single particle could be successfully determined by use of this method.