Evaluation of the dynamic properties of polymer latex particles interacting with quartz interface by evanescent wave dynamic light scattering

The diffusion behavior of polymer latex particles in dispersion near the quartz interface has been estimated by evanescent wave dynamic light scattering (EVDLS) technique. The diffusion coefficient of the particles was measured as a function of the distance between the particle and interface. The apparent diffusion coefficient estimated by EVDLS was small for particles near the interface and increased upon increasing the distance from the interface, and then saturated at a certain value which is close to the value expected for free-motion. The range of the distance over which diffusion was affected by interaction with the interface depended on the added salt concentration. This means that the diffusion of the particle is influenced by an electrostatic interaction between the particle and quartz interface in addition to the hydrodynamic effect near the wall. This range was found to be more than 800?nm at 0?M salt condition but about 400?nm at 10^-4 and 10^-3?M salt conditions. Hence it is appropriate to say that the hydrodynamic effect reaches up to 400?nm and the electrostatic effect is longer ranged, more than 800?nm, for the system studied here. The EVDLS technique is a very powerful tool for quantitative estimations of the dynamic behavior of the particle near the interface and for estimation of the range where the wall effect is dominant. EVDLS will give us an answer to the question of “where is the ‘interface’ and where is the ‘bulk’?”.     

On the Possibility of Evanescent Wave Excitation Distal from a Solid-Liquid Interface Using Light Quenching

Abstract— Evanescent wave illumination with total internal reflection is often used to provide excitation near a quartz-water interface. We now show that evanescent illumination at one wavelength and incident angle, coupled with light quenching at a second wavelength and incident angle, can be used for selective excitation of fluorophores located up to 5000 Å into the aqueous phase. The displacement of the fluorophore population from the solid-liquid interface depends on the angles of incidence of the excitation and quenching beams and the optical power of the quenching beam. Light quenching with an evanescent wave was demonstrated to be experimentally possible using Pyridinez and a light-quenching wavelength of 736 nm. The use of combined evanescent wave excitation and evanescent wave quenching could provide selective excitation of fluorophores in the cytoplasmic region of cells and may provide improved response times for optical sensors based on evanescent excitation.     

Anisotropic diffusion in confined colloidal dispersions: the evanescent diffusivity

We employ an analogy to traditional dynamic light scattering to describe the inhomogeneous and anisotropic diffusion of colloid particles near a solid boundary measured via evanescent wave dynamic light scattering. Following this approach, we generate new expressions for the short-time self- and collective diffusivities of colloidal dispersions with arbitrary volume fraction. We use these expressions in combination with accelerated Stokesian dynamics simulations to calculate the diffusivities in the limit of large and small scattering wave numbers for evanescent penetration depths ranging from four particle radii to one-fifth of a particle radius and volume fractions from 10% to 40%. We show that at high volume fractions, and larger penetration depths, the boundaries have little effect on the dynamics of the suspension parallel to the wall since, to a first approximation, the boundary acts hydrodynamically much as another nearby particle. However, near and normal to the wall, the diffusivity shows a strong dependence on penetration depth for all volume fractions. This is due to the lubrication interactions between the particles and the boundary as the particle moves relative to the wall. These results are novel and comprehensive with respect to the range of penetration depth and volume fraction and provide a complete determination of the effect of hydrodynamic interactions on colloidal diffusion adjacent to a rigid boundary.     

Colloidal dynamics near a wall studied by evanescent wave light scattering: experimental and theoretical improvements and methodological limitations

The dynamics of colloidal spheres near to a wall is studied with an evanescent wave scattering setup that allows for an independent variation of the components of the scattering wave vector normal and parallel to the wall. The correlation functions obtained with this novel instrumentation are interpreted on the basis of an expression for their short time behavior that includes hydrodynamic interactions between the colloidal spheres and the wall. The combination of the evanescent wave scattering setup and the exact expression for the short time behavior of correlation functions allows for an unambiguous measurement of the particle mobility parallel and normal to the wall by means of light scattering. It is possible to measure the viscous wall drag effect on the dynamics of particles with radii as small as 27 nm, where, however, the method reaches its limits due to the low scattering intensities of such small particles.     

Brownian diffusion close to a polymer brush

In an effort to control particle diffusion near surfaces, we have studied the dynamics of colloidal hard spheres and soft compliant star copolymers on surfaces coated with polymer brushes using evanescent wave dynamic light scattering. The same experiments provide information on the brush structure and confined particle motion. The penetration into dense polydisperse brushes is size- and solvent-dependent.     

Direct measurement of forces between a colloidal particle and a phospholipid bilayer

Colloidal interaction forces between a silica particle and a solid-supported Langmuir-Schaefer phospholipid bilayer were directly measured using a gradient optical trap and evanescent wave light scattering. A small custom-built Langmuir trough was integrated with an optical trapping microscope to allow force measurements on a single particle within the subphase of the trough after the dip of the substrate was completed. The novel method allows the force measurements to be conducted without transferring the substratum across an air/water interface. The fluctuating particle position near the bilayer was tracked by evanescent wave light scattering to determine the deflection due to surface forces, and the relaxation time of particle fluctuations was measured to simultaneously determine the viscous forces. Measured equilibrium and viscous force-distance profiles of silica microspheres with diameters of 1 and 5 microm on bilayers of dipalmitoyl phosphatidyl choline (DPPC) were markedly different than force-distance on bare mica and DPPC monolayers under the same electrolyte conditions.     

Direct estimation of the electrostatic interaction between colloidal particle and chemically modified glass surface by the evanescent wave light scattering microscope method

The profile of the interaction potential between polystyrene latex particle and chemically modified glass surface was estimated directly by the evanescent wave light scattering microscope (EVLSM) method; this enables us to measure the distance between particle and surface as a function of time in the order of less than a millisecond. The minimum of the potential profile, which is the result of an electrostatic repulsion and an apparent attraction by gravity between the particle and surface, was clearly observed. To change the electrostatic nature, the glass surface was chemically modified by treatment with a silanization reagent and a vinyl monomer with a sulfonate group. As the absolute value of the zeta potential of the glass surface became larger, the position of the potential minimum on the interaction potential profile shifted away from the glass surface, reflecting an increase of electrostatic repulsion between the particle and the wall. The ionic strength dependence of the potential profile was also clearly observed. In conclusion, EVLSM is a powerful tool for the quantitative estimation of particle-wall interactions. Received: 3rd March 1998 Accepted in revised form: 26 August 1998     

Direct estimation of dynamic characteristics and interaction potential of latex particles interacting with a glass surface by evanescent wave light-scattering microscope method

The dynamic and static characteristics of a polystyrene latex particle in dispersion interacting with a glass surface were studied by the evanescent wave light-scattering microscope (EVLSM) technique originally proposed by Prieve et al. for static studies. The dynamic behavior of the thermal vibration of the particle in a potential well created by electrostatic interaction between the particle and glass and gravity was clearly and quantitatively estimated, in addition to the estimation of the potential profile itself. The potential minimum became shallower with increasing added salt concentration. It was also clearly observed that the vibrational motion of the particle in the well became large in amplitude and the probability of the occurrence of the large vibration became large with increasing salt concentration. Such information on the dynamics is essential for the correct under-standing of the interaction potential. The EVLSM method is shown to be a very powerful technique for the estimation of not only the potential profile but also dynamic characteristics.     

Dynamic Light Scattering Based Microelectrophoresis: Main Prospects and Limitations

Microelectrophoresis based on the dynamic light scattering (DLS) effect has been a major tool for assessing and controlling the conditions for stability of colloidal systems. However, both the DLS methods for characterization of the hydrodynamic size of dispersed submicron particles and the theory behind the electrokinetic phenomena are associated with fundamental and practical approximations that limit their sensitivity and information output. Some of these fundamental limitations, including the spherical approximation of DLS measurements and an inability of microelectrophoretic analyses of colloidal systems to detect discrete charges and differ between differently charged particle surfaces due to rotational diffusion and particle orientation averaging, are revisited in this work. Along with that, the main prospects of these two analytical methods are mentioned. A detailed review of the role of zeta potential in processes of biochemical nature is given too. It is argued that although zeta potential has been used as one of the main parameters in controlling the stability of colloidal dispersions, its application potentials are much broader. Manipulating surface charges of interacting species in designing complex soft matter morphologies using the concept of zeta potential, intensively investigated recently, is given as one of the examples. Branching out from the field of colloid chemistry, DLS and zeta potential analyses are now increasingly finding application in drug delivery, biotechnologies, physical chemistry of nanoscale phenomena and other research fields that stand on the frontier of the contemporary science. Coupling the DLS-based microelectrophoretic systems with complementary characterization methods is mentioned as one of the prosperous paths for increasing the information output of these two analytical techniques.     

动态光散射法测量颗粒粒径的溯源性

分别对动态光散射粒径测量仪的入射波长、散射角度、测量池温度进行校准,并对影响测量结果准确性各因素的不确定度分量进行了评价,校准后动态光散射仪的测量结果可溯源至国家计量标准。为消除多重散射、颗粒间相互作用、颗粒粒径分布对动态光散射测量结果的影响,建立了动态光散射测量结果修正方法。其中为消除多重散射及颗粒间相互作用的影响,需采用多浓度测量或线性回归的方法得到特定浓度下的颗粒粒径;为修正颗粒粒径分布对动态光散射测量结果的影响,需先采用SEM方法准确测量颗粒粒径分布,然后根据光强加权动力学平均粒径和数量平均粒径的理论公式,得到二者之间的差异。     

Atomic force microscopy colloid-probe measurements with explicit measurement of particle-solid separation

We describe the use of evanescent wave scattering to measure the separation between the surface of a solid and a particle that is attached to an atomic force microscope (AFM) cantilever. Termed evanescent wave atomic force microscopy, our approach involves measuring the intensity of the light scattered from an evanescent field formed by the total internal reflection of a laser beam at a solid/fluid interface. In a conventional AFM "colloid probe" measurement, this separation must be inferred from an examination of the surface forces. Direct measurement of this separation with an evanescent wave atomic force microscope (EW-AFM) removes some ambiguity in the surface force measurement and, in addition, allows new types of measurements. For example, the force can be monitored at a constant separation. Our evanescent scattering apparatus is essentially identical to that used in total internal reflection microscopy (TIRM), except that we collect the light that scatters back into the incident medium, because the AFM partly obscures the forward scattered light (i.e., light scattered into the transmitted region). Compared to a conventional TIRM measurement, where the particle moves freely, attaching the particle to the cantilever in an EW-AFM gives much greater control of the particle position.     

Diffusing Wave Spectroscopy of aggregating and gelling systems

Diffusing Wave Spectroscopy is a relatively recent but rather under-utilized branch of light scattering. It has the advantage that it can be used in highly-scattering suspensions to measure the motions of the scattering particles. This means that it can be used to study food colloidal systems in their normal state, with no requirement for dilution of the samples. The motions of the particles in a suspension depends on their state (free or limited diffusion, gelation, phase separation, etc.). Therefore, analysis of the light scattering behaviour can give information not only on particle sizes, but on the mechanisms by which they aggregate. In addition to the dynamics of the particles, it is also possible to gather information about the interactions of the particles from the overall turbidity of the suspensions. This review describes the application of the DWS technique to the study of a number of food-related suspensions, and shows how the results can be interpreted for the different systems.     

Evanescent wave excited luminescence from levitated quantum dot modified colloids

Evanescent wave excited luminescence of quantum dot modified polystyrene (QDPS) colloids is investigated to measure potential energy profiles of QDPS colloids electrostatically levitated above a planar glass surface. Luminescence is characterized for three different-sized PS colloids modified with three different-sized QDs using confocal microscopy, emission spectra, flow cytometry, and temporal measurements of levitated and deposited colloids. Colloid-surface potential energy profiles constructed from scattering and luminescence intensity data display excellent agreement with each other, theoretical predictions, and independently measured parameters. QDPS luminescence intensity is indirectly confirmed to have an exponential dependence on height similar to conventional colloidal evanescent wave scattering. Our findings indicate that evanescent wave excited QDPS luminescence could enable total internal reflection microscopy measurements of index-matched hard spheres, multiple specific biomolecular interactions via spectral multiplexing, enhanced morphology-dependent resonance modes, and integrated evanescent wave-video-confocal microscopy experiments not possible with scattering.     

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.     

生化芯片上的光纤消逝波吸收光度检测

生化芯片上探测器的总体性能将影响整个芯片分析系统的检出限、检测速度和适用范围等指标,是芯片分析系统的一个关键部分.针对生化芯片检测区的检测体积小、样品剂量少以及与生化芯片集成等特殊要求,设计了易于集成在芯片内的,用分光光度法对混合后液体的消逝波吸收光谱探测的方法.     

Relationship between scattered intensity and separation for particles in an evanescent field

We describe measurements of the scattering of visible light from an evanescent field by both spherical particles (R = 1-10 mum) that are glued to atomic force microscopy (AFM) cantilevers, and by sharp tips (R < 60 nm) that were incorporated onto the cantilevers during manufacture. The evanescent wave was generated at the interface between a flat plate and an aqueous solution, and an atomic force microscope was used to accurately control the separation, h, between the particle and the flat plate. We find that, for sharp tips, the intensity of scattered light decays exponentially with separation between the tip and the plate all the way down to h approximately 0. The measured decay length of scattered intensity, delta, is the same as the theoretical decay length of the evanescent intensity in the absence of the sharp tip. For borosilicate particles, (R = 1-10 mum), the scattering also decays exponentially with separation at large separations. However, when the separation is less than roughly 3delta, the measured scattering intensity is smaller in magnitude than that which would be predicted by extrapolating the exponential decay observed at large separations. For these particles, the scattering approximately fits the sum of two exponentials. The magnitude of the deviation from exponential at contact was roughly 10-15% for R = 1 mum particles and about 30% for larger particles and is larger for s-polarized light. Preliminary experiments on polystyrene particles shows that the scattering is also smaller than exponential at small separations but that the deviation from exponential is larger for p-polarized light. In evanescent wave AFM (EW-AFM) the scattering-separation can be calibrated for situations where the scattering is not exponential. We discuss possible errors that could be introduced by assuming that exponential decay of scattering continues down to h = 0.     

Experimental verification of an exact evanescent light scattering model for TIRM

Total internal reflection microscopy (TIRM) is a method for the precise measurement of interaction potentials between a spherical colloidal particle and a wall. The method is based on single-particle evanescent wave light scattering. The well-established model used to interpret TIRM data is based on an exponential relation between scattering intensity and particle wall distance. However, applying this model for a certain range of experimental parameters leads to significant distortions of the measured potentials. Using a TIRM setup based on a two-wavelength illumination technique, we were able to directly measure the intensity distance relation revealing deviations from an exponential decay. The intensity-distance relations could be compared to scattering simulations taking into account exact experimental parameters and multiple reflections between a particle and the wall. Converging simulation results were independently obtained by the T-matrix method and the discrete sources method (DSM) and show excellent agreement with experiments. Using the new scattering model for data evaluation, we could reconstruct the correct potential shape for distorted interaction potentials as we demonstrate. The comparison of simulations to experiment intrinsically yields a new method to determine absolute particle-wall distances, a highly desired quantity in TIRM experiments.     

Measuring colloidal forces using evanescent wave scattering

The evanescent wave scattering technique total internal reflection microscopy has enabled the direct measurement of the mean potential energy of interaction between a Brownian particle and a flat surface. With a distance resolution of 1 nm and a force resolution of 10 fN, this technique has successfully measured a variety of colloidal forces. Recent measurements of van der Waals interactions have given rise to new theories for the effect of surface roughness on the interaction. In addition, recent measurements of depletion interactions have shown that energetic as well as entropic effects must be considered when computing the interaction potential.     

Light Scattering Probes of Viscoelastic Fluids and Solids

In this article, we discuss recent advances in static and dynamic light scattering applied to soft materials. Special emphasis is given to light scattering methods that allow access to turbid and solid‐like systems, such as colloidal suspensions, emulsions, glasses, or gels. Based on a combination of single‐ and multispeckle detection schemes, it is now possible to cover an extended range of relaxation times from a few nanoseconds to minutes or hours and length scales below 1 nm up to several microns. The corresponding elastic properties of viscoelastic fluids or solid materials range roughly from below 1 Pa to several 100 kPa. Different applications are discussed such as light scattering from suspensions of highly charged colloidal particles, colloid and protein gels, as well as dense surfactant solutions.