Elastic scattering of evanescent electromagnetic waves

UV emitting glass: A promising strategy for biofilm inhibition on transparent surfaces

Marine biofouling causes serious environmental problems and has adverse impacts on the maritime industry. Biofouling on windows and optical equipment reduces surface transparency, limiting their application for on-site monitoring or continuous measurement. This work illustrates that UV emitting glasses (UEGs) can prevent the establishment and growth of biofilm on the illuminated surfaces. Specifically, this paper describes how UEGs are enabled by innovatively modifying the surfaces of the glass with light scattering particles. Modification of glass surface with silica nanoparticles at a concentration 26.5 μg/cm2 resulted in over ten-fold increase in UV irradiance, while maintaining satisfactory visible and IR transparency metrics of over 99 %. The UEG reduced visible biological growth by 98 % and resulted in a decrease of 1.79 log in detected colony forming units when compared to the control during a 20 day submersion at Port Canaveral, Florida, United States. These findings serve as strong evidence that UV emitting glass should be explored as a promising approach for biofilm inhibition on transparent surfaces.

Label-free quantification of protein binding to lipid vesicles using transparent waveguide evanescent-field scattering microscopy with liquid control

Recent innovations in microscopy techniques are paving the way for label-free studies of single nanoscopic biological entities such as viruses, lipid-nanoparticle drug carriers, and even proteins. One such technique is waveguide evanescent-field microscopy, which offers a relatively simple, yet sensitive, way of achieving label-free light scattering-based imaging of nanoparticles on surfaces. Herein, we extend the application of this technique by incorporating microfluidic liquid control and adapting the design for use with inverted microscopes by fabricating a waveguide on a transparent substrate. We furthermore formulate analytical models describing scattering and fluorescence intensities from single spherical and shell-like objects interacting with evanescent fields. The models are then applied to analyze scattering and fluorescence intensities from adsorbed polystyrene beads and to temporally resolve cholera-toxin B (CTB) binding to individual surface-immobilized glycosphingolipid GM1 containing vesicles. We also propose a self-consistent means to quantify the thickness of the CTB layer, revealing that protein-binding to individual vesicles can be characterized with sub-nm precision in a time-resolved manner.

Blood Protein Exclusion from Polymer Brushes

We report interactions between adsorbed copolymers of poly(ethylene glycol) (PEG) in the presence of two abundant blood proteins, serum albumin and an immunoglobulin G, up to physiological blood concentrations. We directly and nonintrusively measure interactions between PEG triblock copolymers (PEG-PPO-PEG) adsorbed to hydrophobic colloids and surfaces using Total Internal Reflection Microscopy, which provides kT- and nanometer-scale resolution of interaction potentials (energy vs separation). In the absence of protein, adsorbed PEG copolymer repulsion is consistent with dimensions and architectures of PEG brushes on both colloids and surfaces. In the presence of proteins, we observe concentration dependent depletion attraction and no change to brush repulsion, indicating protein exclusion from PEG brushes. Because positive and negative protein adsorption are mutually exclusive, our observations of concentration dependent depletion attraction with no change to brush repulsion unambiguously indicate the absence of protein coronas at physiological protein concentrations. These findings demonstrate a direct sensitive approach to determine interactions between proteins and particle/surface coatings important to diverse biotechnology applications.

Effect of photon counting shot noise on total internal reflection microscopy

Total internal reflection microscopy (TIRM) measures changes in the distance between a colloidal particle and a transparent substrate by measuring the scattering intensity of the particle illuminated by an evanescent wave. From the distribution of the recorded separation distances, the height-dependent effective potential φ(z) between the colloidal particle and the substrate can be measured. In this work, we show that spatial resolution with which TIRM can measure φ(z) is limited by the photon counting statistics of the scattered laser light. We develop a model to evaluate the effect of photon counting statistics on different potential profiles using Brownian dynamics simulations and experiments. Our results show that the effect of photon counting statistics depends on spatial gradients ∂φ/∂z of the potential, with the result that sharp features tend to be significantly blurred. We further establish the critical role of photon counting statistics and the intensity integration time τ in TIRM measurements, which is a trade-off between narrowing the width of the photon counting distribution and capturing the instantaneous position of the probe particle.

Light scattering in TIRF microscopy: A theoretical study of the limits to surface selectivity

In TIRF microscopy, the sample resides near a surface in an evanescent optical field that, ideally, decreases in intensity with distance from the surface in a pure exponential fashion. In practice, multiple surfaces and imperfections in the optical system and refractive index (RI) inhomogeneities in the sample (often living cells) produce propagating scattered light that degrades the exponential purity. RI inhomogeneities cannot easily be avoided. How severe is the consequent optical degradation? Starting from Maxwell's equations, we derive a first-order perturbative approximation of the electric field strength of light scattered by sample RI inhomogeneities of several types under coherent evanescent field illumination. The approximation provides an expression for the scattering field of any arbitrary RI inhomogeneity pattern. The scattering is not all propagating; some is evanescent and remains near the scattering centers. The results presented here are only a first-order approximation, and they ignore multiple scattering and reflections off the total internal reflection (TIR) surface. For simplicity, we assume that the RI variations in the z direction are insignificant within the depth of the evanescent field and consider only scattering of excitation light, not fluorescence emission light. The general conclusion of most significance from this study is that TIR scattering from a sample with RI variations typical of those on a cell culture alters the effective thickness of the illumination to only ∼50% greater than it would be without scattering. The qualitative surface selectivity of TIR fluorescence is largely retained even in the presence of scattering. Quantitatively, however, scattering will cause a deviation from the incident exponential decay at shorter distances, adding a slower decaying background. Calculations that assume a pure exponential decay will be approximations, and scattering should be taken into account. TIR scattering is only slightly dependent on polarization but is strongly reduced for the highest accessible incidence angles.

Integrated evanescent field detector for ultrafine particles-theory and concept

Recent studies on ultrafine particles (UFP), which are smaller than 100 nm, emphasized their hazardous potential to the human organism. They are comparable in size to typical nano-organisms such as viruses and can penetrate physiological barriers in a similar way. Currently, there are no low-cost and miniaturized detectors for UFP available. In our first experiments with an integrated evanescent field particle detector, we could already successfully detect single 200 nm polystyrene latex (PSL) spheres, although the implemented waveguide geometry was only rudimentary optimized with costly 3D simulations. We developed a fast and structured optimization model for waveguide geometry and operation wavelength of an integrated evanescent field particle detector in order to exploit its full potential for the detection of discrete analytes in the UFP size range. The optimization model is based on a modified formulation of Mie theory and its computational effort is reduced by a factor of 100 compared to 3D simulations. The optimization potential of the sensor response signal is demonstrated for several waveguide geometries that can be produced with established semiconductor fabrication technology at high production volumes and low costs. An optimized silicon nitride waveguide features sensor response signals that are about one order of magnitude higher compared to previous experiments, which pushes the limit of detection even further down to particle sizes below 100 nm. A small integrated evanescent field particle detector based on this optimized waveguide will be used for the first low-cost and miniaturized devices that can monitor the personal exposure to UFP.

Synergistic Polymer-Surfactant-Complex Mediated Colloidal Interactions and Deposition

Total internal reflection microscopy (TIRM) is used to directly, sensitively, and simultaneously measure colloidal interactions, dynamics, and deposition for a broad range of polymer-surfactant compositions. A deposition state diagram containing comprehensive information about particle interactions, trajectories, and deposition behavior is obtained for polymer-surfactant compositions covering four decades in both polymer and surfactant concentrations. Bulk polymer-surfactant phase behavior and surface properties are characterized to provide additional information to interpret mechanisms. Materials investigated include cationic acrylamide-acrylamidopropyltrimonium copolymer (AAC), sodium lauryl ether sulfate (SLES) surfactant, silica colloids, and glass microscope slides. Measured colloid-substrate interaction potentials and deposition behavior show nonmonotonic trends vs polymer-surfactant composition and appear to be synergistic in the sense that they are not easily explained as the superposition of single-component-mediated interactions. Broad findings show that at some compositions polymer-surfactant complexes mediate bridging and depletion attractions that promote colloidal deposition, whereas other compositions produce electrosteric repulsion that deters colloidal deposition. These findings illustrate mechanisms underlying colloid-surface interactions in polymer-surfactant mixtures, which are important to controlling selective colloidal deposition in multicomponent formulation applications.

Nanoparticle and Transparent Polymer Coatings Enable UV-C Side-Emission Optical Fibers for Inactivation of Escherichia coli in Water

Pathogenic bacteria pose a health threat and operational challenge in drinking water. UV-C light-emitting diodes (UV-C LEDs) are becoming a competitive disinfection technology but are limited by their small irradiation area. Side-emitting optical fibers (SEOFs) can serve as a UV-C LED light delivery technology for reactors or tubing. Modifying the surfaces of conventional optical fibers with scattering centers allows for side emission of 265 nm radiation from an LED for microbial inactivation in water. Solid-material absorbance and flux measurements differentiated light absorption from scattering for all materials. Silica spheres >200 nm in diameter achieved higher scattering than smaller silica. A critical discovery was that treating the silica-coated optical fiber in a solution of high ionic strength increased UV-C side emission by greater than 6-fold. Additionally, the cladding polymer Cytop had negligible absorbance at 265 nm wavelength. A scalable four-step treatment process was developed to fabricate the novel SEOF. Attached to a 265 nm LED, the side-emitting optical fiber achieved 2.9 log inactivation of Escherichia coli at a delivery dose of 15 mJ/cm². The results illustrate proof of concept that UV-C SEOFs can inactivate E. coli and should be further explored for delivering LED light into water.

Near-surface microrheology reveals dynamics and viscoelasticity of soft matter

The development of experimental techniques able to probe the microscale viscoelastic properties of soft matter over a broad time scale is essential to uncover the physics that govern their behavior. Herein, we report the development of a microrheology technique that can determine the near-surface dynamics and viscoelastic behaviors of soft matter like polymer solution/gels and colloidal dispersions. Our approach combines a magnetic-field-induced stimulator with total internal reflection microscopy (TIRM) to apply mechanical loading (∼pN) to a micro-sized probe particle and capture its axial displacement near the surface with nano-scaled sensitivity. We demonstrate the use of this technique to measure the detachment of a colloid to a solid substrate and identify three quantitatively different regimes of mechanical coupling that differ in colloid-surface separation and interaction: exclusion, aging, and non-exclusion. We also apply it to study a physical gelation process of a volume-phase transition in thermosensitive microgels and a chemically cross-linked sol-gel transition of 4-arm star polymers by monitoring the evolution of complex modulus near solid surface with frequency, time, and separation distance. In contrast to passive microrheology techniques that rely on particle tracking, we can probe the viscoelastic behavior over five orders of magnitude in stiffness, from 10-3 to 102 Pa, providing excellent coverage for dynamics and heterogeneous samples. We expect this technique will stimulate the development of new experimental methods to explore the complex microscale rheology of macromolecular networks, soft materials, and living cytoplasm.

Direct Measurements of kT-Scale Capsule-Substrate Interactions and Deposition Versus Surfactants and Polymer Additives

We report a novel approach to directly measure the interactions and deposition behavior of functional capsule delivery systems on glass substrates versus the concentration of an anionic surfactant sodium lauryl ether sulfate (SLES) and a cationic acrylamide-acrylamidopropyltrimonium copolymer (AAC). Analyses of three-dimensional optical microscopy trajectories were used to quantify lateral diffusive dynamics, deposition lifetimes, and potentials of mean force for different solution conditions. In the absence of additives, negatively charged capsule surfaces yield electrostatic repulsion with the negatively charged substrate, which inhibits deposition. With an increasing SLES concentration below the critical micelle concentration (CMC), capsule-substrate electrostatic repulsion is mediated by the charged surfactant solution that decreases the Debye length. Above the SLES CMC, depletion attraction causes enhanced deposition until eventually depletion repulsion inhibits deposition at concentrations ∼10 wt %. Addition of an ACC causes deposition via capsule-substrate bridging at all concentrations; the weakest deposition occurs at intermediate AAC concentrations from a competition of steric repulsion and attraction via a few extended bridges. The novel measurements and models of capsule interactions and deposition on substrates in this work provide a basis to fundamentally understand and rationally design complex rinse-off cleansing formulations with optimal characteristics.

Refractive Index Imaging of Cells with Variable-Angle Near-Total Internal Reflection (TIR) Microscopy

The refractive index in the interior of single cells affects the evanescent field depth in quantitative studies using total internal reflection (TIR) fluorescence, but often that index is not well known. We here present method to measure and spatially map the absolute index of refraction in a microscopic sample, by imaging a collimated light beam reflected from the substrate/buffer/cell interference at variable angles of incidence. Above the TIR critical angle (which is a strong function of refractive index), the reflection is 100%, but in the immediate sub-critical angle zone, the reflection intensity is a very strong ascending function of incidence angle. By analyzing the angular position of that edge at each location in the field of view, the local refractive index can be estimated. In addition, by analyzing the steepness of the edge, the distance-to-substrate can be determined. We apply the technique to liquid calibration samples, silica beads, cultured Chinese hamster ovary cells, and primary culture chromaffin cells. The optical technique suffers from decremented lateral resolution, scattering, and interference artifacts. However, it still provides reasonable results for both refractive index (~1.38) and for distance-to-substrate (~150 nm) for the cells, as well as a lateral resolution to about 1 µm.

Three-dimensional all-dielectric metamaterial solid immersion lens for subwavelength imaging at visible frequencies

Although all-dielectric metamaterials offer a low-loss alternative to current metal-based metamaterials to manipulate light at the nanoscale and may have important applications, very few have been reported to date owing to the current nanofabrication technologies. We develop a new "nano-solid-fluid assembly" method using 15-nm TiO2 nanoparticles as building blocks to fabricate the first three-dimensional (3D) all-dielectric metamaterial at visible frequencies. Because of its optical transparency, high refractive index, and deep-subwavelength structures, this 3D all-dielectric metamaterial-based solid immersion lens (mSIL) can produce a sharp image with a super-resolution of at least 45 nm under a white-light optical microscope, significantly exceeding the classical diffraction limit and previous near-field imaging techniques. Theoretical analysis reveals that electric field enhancement can be formed between contacting TiO2 nanoparticles, which causes effective confinement and propagation of visible light at the deep-subwavelength scale. This endows the mSIL with unusual abilities to illuminate object surfaces with large-area nanoscale near-field evanescent spots and to collect and convert the evanescent information into propagating waves. Our all-dielectric metamaterial design strategy demonstrates the potential to develop low-loss nanophotonic devices at visible frequencies.

Digital design of multimaterial photonic particles

Scattering of light from dielectric particles whose size is on the order of an optical wavelength underlies a plethora of visual phenomena in nature and is a foundation for optical coatings and paints. Tailoring the internal nanoscale geometry of such "photonic particles" allows tuning their optical scattering characteristics beyond those afforded by their constitutive materials-however, flexible yet scalable processing approaches to produce such particles are lacking. Here, we show that a thermally induced in-fiber fluid instability permits the "digital design" of multimaterial photonic particles: the precise allocation of high refractive-index contrast materials at independently addressable radial and azimuthal coordinates within its 3D architecture. Exploiting this unique capability in all-dielectric systems, we tune the scattering cross-section of equisized particles via radial structuring and induce polarization-sensitive scattering from spherical particles with broken internal rotational symmetry. The scalability of this fabrication strategy promises a generation of optical coatings in which sophisticated functionality is realized at the level of the individual particles.

Orientation-and polarization-dependent optical properties of the single Ag nanowire/glass substrate system excited by the evanescent wave

As an important plasmon one-dimensional material, orientation- and polarization-dependent properties of single Ag nanowires/glass substrate system are investigated by a powerful platform consisting of evanescent wave excitation, near-/far-field detection and a micromanipulator. In the case of the nanowire perpendicular or parallel to the incident plane and p- ors-polarized evanescent excitation respectively, optical properties of the nanowire is measured both in far-field and near-field. For the perpendicular situation, scattering light from the nanowire shows strong dependence on the polarization of incident light, and period patterns along the nanowire are observed both in the near- and far-field. The chain of dipole model is used to explain the origin of this pattern. The discrepancy of the period patterns observed in the near- and far-field is due to the different resolution of the near- and far-field detection. For the parallel case, light intensity from the output end also depends on the incident polarization. Both experimental and calculation results show that the polarization dependence effect results from the surface plasmon excitation. These results on the orientation- and polarization-dependent properties of the Ag nanowires detected by the combination of near- and far-field methods would be helpful to understand interactions of one-dimensional plasmonic nanostructures with light.

Near-Field Effects in Mesoscopic Light Transport

In dense multiple scattering media, optical fields evolve through both homogeneous and evanescent waves. New regimes of light transport emerge because of the near-field coupling between individual scattering centers at mesoscopic scales. We present a novel propagation model that is developed in terms of measurable far- and near-field scattering cross sections. Our quantitative description explains the increase of total transmission in dense scattering media and its accuracy is established through both full-scale numerical calculations and enhanced backscattering experiments.

A colloid model system for interfacial sorption kinetics

Particle adsorption to an interface may be a complicated affair, motivating detailed measurements of various processes involved, to discover better understanding of the role of particle characteristics and solution conditions on adsorption coverage and rate. Here we use micron size colloids with a weak interfacial interaction potential as a model system to track particle motion and measure the rates of desorption and adsorption. The colloid-interface interaction strength is tuned to be less than 10 kBT so that it is comparable to many nanoscale systems of interest such as proteins at interfaces. The tuning is accomplished using a combination of depletion, electrostatic, and gravitational forces. The colloids transition between an entropically trapped adsorbed state and a desorbed state through Brownian motion. Observations are made using an light-emitting diode (LED)-based total internal reflection microscopy (TIRM) setup. The observed adsorption and desorption rates are compared to theoretical predictions based on the measured interaction potential and near-wall particle diffusivity. The results demonstrate that diffusion dynamics play a significant role when the barrier energy is small. This experimental system will allow for the future study of more complex dynamics such as nonspherical colloids and collective effects at higher concentrations.

Tuning the particle-surface interactions in aqueous solutions by soft microgel particles

Due to the softness and deformability, interaction between colloidal surfaces induced by soft particles varies in a more complex way than for solid particles and thus has attracted much attention in recent years. In the present study, we use total internal reflection microscopy (TIRM) to directly measure the interaction between polystyrene (PS) microparticles and a flat glass surface in a poly(N-isopropylacrylamide) (PNIPAM) microgel dispersion with concentration varying from dilute (0.1 wt %) to highly concentrated regime (7.5 wt %). Our result shows that the PS particle-surface interactions mediated by the soft microgels are greatly affected by the particle concentration, the configuration of those microgels adsorbed on the surfaces, and the structure and packing of microgels in bulk solution. With increasing the microgel concentration (Cmicrogel), the interaction between the PS particle and surface turned from bridging attraction to steric repulsion, and then depletion attraction, which were mainly governed by the adsorption amount and configuration of microgels on the two surfaces. By further increasing Cmicrogel to condensed situation, structural force with oscillated energy wells was detected. The variation of interactions induced by the soft microgels was further confirmed by optical imaging. Crystallization of the PS microparticles appeared at moderate Cmicrogel; however, crystallization was hindered at higher Cmicrogel where the microgels are highly packed in the bulk solution. Furthermore, using TIRM, microgel packing with local energy well (0.1-1.0 kBT) at the highly condensed state (7.5 wt %) was resolved from the interaction profiles. Therefore, the shear force and modulus generated by such microgel packing can be determined as ∼0.2 pN and tens of mPa, respectively, which are much weaker than data given by conventional active methods.

Eliminating unwanted far-field excitation in objective-type TIRF. Part I. identifying sources of nonevanescent excitation light

Total internal reflection fluorescence microscopy (TIRFM) achieves subdiffraction axial sectioning by confining fluorophore excitation to a thin layer close to the cell/substrate boundary. However, it is often unknown how thin this light sheet actually is. Particularly in objective-type TIRFM, large deviations from the exponential intensity decay expected for pure evanescence have been reported. Nonevanescent excitation light diminishes the optical sectioning effect, reduces contrast, and renders TIRFM-image quantification uncertain. To identify the sources of this unwanted fluorescence excitation in deeper sample layers, we here combine azimuthal and polar beam scanning (spinning TIRF), atomic force microscopy, and wavefront analysis of beams passing through the objective periphery. Using a variety of intracellular fluorescent labels as well as negative staining experiments to measure cell-induced scattering, we find that azimuthal beam spinning produces TIRFM images that more accurately portray the real fluorophore distribution, but these images are still hampered by far-field excitation. Furthermore, although clearly measureable, cell-induced scattering is not the dominant source of far-field excitation light in objective-type TIRF, at least for most types of weakly scattering cells. It is the microscope illumination optical path that produces a large cell- and beam-angle invariant stray excitation that is insensitive to beam scanning. This instrument-induced glare is produced far from the sample plane, inside the microscope illumination optical path. We identify stray reflections and high-numerical aperture aberrations of the TIRF objective as one important source. This work is accompanied by a companion paper (Pt.2/2).

Direct measurements of particle–surface interactions in aqueous solutions with total internal reflection microscopy

This feature article reviews the experimental studies of the interactions between designed colloidal surfaces in the presence or absence of macromolecules/nanoparticles including depletion attraction, steric repulsion, bridging flocculation, and specific interactions by using Total Internal Reflection Microscopy.

Microplate-compatible total internal reflection fluorescence microscopy for receptor pharmacology

We report the use of total internal reflection fluorescence (TIRF) microscopy for analyzing receptor pharmacology and the development of a microplate-compatible TIRF imaging system. Using stably expressed green fluorescence protein tagged β₂-adrenergic receptor as the reporter, we found that the activation of different receptors results in distinct kinetic signatures of the TIRF intensity of cells. These TIRF signatures closely resemble the characteristics of their respective label-free dynamic mass redistribution signals in the same cells. This suggests that TIRF in microplate can be used for profiling and screening drugs.

Mie scattering and optical forces from evanescent fields: a complex-angle approach

Mie theory is one of the main tools describing scattering of propagating electromagnetic waves by spherical particles. Evanescent optical fields are also scattered by particles and exert radiation forces which can be used for optical near-field manipulations. We show that the Mie theory can be naturally adopted for the scattering of evanescent waves via rotation of its standard solutions by a complex angle. This offers a simple and powerful tool for calculations of the scattered fields and radiation forces. Comparison with other, more cumbersome, approaches shows perfect agreement, thereby validating our theory. As examples of its application, we calculate angular distributions of the scattered far-field irradiance and radiation forces acting on dielectric and conducting particles immersed in an evanescent field.

An active one-particle microrheometer: incorporating magnetic tweezers to total internal reflection microscopy

We present a novel microrheometer by incorporating magnetic tweezers in the total internal reflection microscopy (TIRM) that enables measuring of viscoelastic properties of materials near solid surface. An evanescent wave generated by a solid∕liquid interface in the TIRM is used as the incident light source in the microrheometer. When a probe particle (of a few micrometers diameter) moves near the interface, it can interact with the evanescent field and reflect its position with respect to the interface by the scattered light intensity. The exponential distance dependence of the evanescent field, on the one hand, makes this technique extremely sensitive to small changes from z-fluctuations of the probe (with a resolution of several nanometers), and on the other, it does not require imaging of the probe with high lateral resolution. Another distinct advantage is the high sensitivity in determining the z position of the probe in the absence of any labeling. The incorporated magnetic tweezers enable us to effectively manipulate the distance of the embedded particle from the interface either by a constant or an oscillatory force. The force ramp is easy to implement through a coil current ramp. In this way, the local viscous and elastic properties of a given system under different confinements can therefore be measured by resolving the near-surface particle motion. To test the feasibility of applying this microrheology to soft materials, we measured the viscoelastic properties of sucrose and poly(ethylene glycol) solutions and compared the results to bulk rheometry. In addition, we applied this technique in monitoring the structure and properties of deformable microgel particles near the flat surface.

Diffusing colloidal probes of protein-carbohydrate interactions

We present diffusing colloidal probe measurements of weak, multivalent, specific protein-polysaccharide interactions mediated by a competing monosaccharide. Specifically, we used integrated evanescent wave and video microscopy methods to monitor the three-dimensional Brownian excursions of conconavilin A (ConA) decorated colloids interacting with dextran-functionalized surfaces in the presence of glucose. Particle trajectories were interpreted as binding lifetime histograms, binding isotherms, and potentials of mean force. Binding lifetimes and isotherms showed clear trends of decreasing ConA-dextran-specific binding with increasing glucose concentration, consistent with expectations. Net potentials were accurately captured by superposition of a short-range, glucose-independent ConA-dextran repulsion and a longer-range, glucose-dependent dextran bridging attraction modeled as a harmonic potential. For glucose concentrations greater than 100 mM, the net ConA-dextran potential was found to have only a nonspecific repulsion, similar to that of bovine serum albumin (BSA) decorated colloids over dextran determined in control experiments. Our results demonstrate the first use of optical microscopy methods to quantify the connections between potentials of mean force and the binding behavior of ConA-decorated colloids on dextran-functionalized surfaces.

Directed assembly of optically bound matter

We present a study of optically bound matter formation in a counter-propagating evanescent field, exploiting total internal reflection on a prism surface. Small ensembles of silica microspheres are assembled in a controlled manner using optical tweezers. The structures and dynamics of the resulting optically bound chains are interpreted using a simulation implementing generalized Lorentz-Mie theory. In particular, we observe enhancement of the scattering force along the propagation direction of the optically bound colloidal chains leading to a microscopic analogue of a driven pendulum which, at least superficially, resembles Newton's cradle.

Imaging carbon nanotube interactions, diffusion, and stability in nanopores

We report optical microscopy measurements of three-dimensional trajectories of individual multiwalled carbon nanotubes (MWCNTs) in nanoscale silica slit pores. Trajectories are analyzed to nonintrusively measure MWCNT interactions, diffusion, and stability as a function of pH and ionic strength. Evanescent wave scattering is used to track MWCNT positions normal to pore walls with nanometer-scale resolution, and video microscopy is used to track lateral positions with spatial resolution comparable to the diffraction limit. Analysis of MWCNT excursions normal to pore walls yields particle-wall potentials that agree with theoretical electrostatic and van der Waals potentials assuming a rotationally averaged potential of mean force. MWCNT lateral mean square displacements are used to quantify translational diffusivities, which are comparable to predictions based on the best available theories. Finally, measured MWCNT pH and ionic strength dependent stabilities are in excellent agreement with predictions. Our findings demonstrate novel measurement and modeling tools to understand the behavior of confined MWCNTs relevant to a broad range of applications.

Novel perspectives for the application of total internal reflection microscopy

Total Internal Reflection Microscopy (TIRM) is a sensitive non-invasive technique to measure the interaction potentials between a colloidal particle and a wall with femtonewton resolution. The equilibrium distribution of the particle-wall separation distance z is sampled monitoring the intensity I scattered by the Brownian particle under evanescent illumination. Central to the data analysis is the knowledge of the relation between I and the corresponding z, which typically must be known a priori. This poses considerable constraints to the experimental conditions where TIRM can be applied (short penetration depth of the evanescent wave, transparent surfaces). Here, we introduce a method to experimentally determine I(z) by relying only on the distance-dependent particle-wall hydrodynamic interactions. We demonstrate that this method largely extends the range of conditions accessible with TIRM, and even allows measurements on highly reflecting gold surfaces where multiple reflections lead to a complex (z).

Fluorescence enhancements of fiber-optic biosensor with metallic nanoparticles

The mechanism of fluorescence enhancements of fiber-optic biosensor with metallic nanoparticles is studied using scattering theory of evanescent waves by a metallic nanoparticle in dilute solution approximation. High local-field enhancement in the vicinity of metallic nanoparticles resulting from localized surface plasmon excitation and the fluorescence enhancement is estimated by calculating averaged local-field enhancement and radiative-rate enhancement of fluorophores in the presence of metallic anoparticles. The metallic nanoparticles not only provide strong local field to enhance the fluorescence signal of fluorophores, but also help to scatter the fluorescence signal and to increase the far-field detectable signals of the fiber-optic biosensor. The effects of the radius of gold nanoparticles, fluorophore-particle separation, and fiber-particle separation on the fluorescence enhancement are discussed in detail.

Resonant effects in evanescent wave scattering of polydisperse colloids

Measurements and predictions are reported to understand large variations in evanescent wave (EW) scattering intensities between different particles from the same batch of single mode, polydisperse colloids. Measured EW scattering intensity distributions are obtained for three different micrometer sized latex particles irreversibly deposited onto glass surfaces. Predicted EW scattering intensity distributions are obtained using measured particle size distributions as input in a Mie theory for the three-dimensional scattering of a sphere under EW illumination. Good agreement is observed between measured and predicted EW scattering intensity distributions using no adjustable parameters. Our results indicate how finite polydispersity together with resonant effects produce large, nonlinear intensity variations between particles that appear to be physically and chemically uniform. Our findings allow such resonant effects to be understood and exploited in EW based particle-surface characterization techniques (e.g., using total internal reflections, surface plasmons) and chemical and biomolecular sensing applications (e.g., using whispering gallery modes).

A programmable light engine for quantitative single molecule TIRF and HILO imaging

We report on a simple yet powerful implementation of objective-type total internal reflection fluorescence (TIRF) and highly inclined and laminated optical sheet (HILO, a type of dark-field) illumination. Instead of focusing the illuminating laser beam to a single spot close to the edge of the microscope objective, we are scanning during the acquisition of a fluorescence image the focused spot in a circular orbit, thereby illuminating the sample from various directions. We measure parameters relevant for quantitative image analysis during fluorescence image acquisition by capturing an image of the excitation light distribution in an equivalent objective backfocal plane (BFP). Operating at scan rates above 1 MHz, our programmable light engine allows directional averaging by circular spinning the spot even for sub-millisecond exposure times. We show that restoring the symmetry of TIRF/HILO illumination reduces scattering and produces an evenly lit field-of-view that affords on-line analysis of evanescnt-field excited fluorescence without pre-processing. Utilizing crossed acousto-optical deflectors, our device generates arbitrary intensity profiles in BFP, permitting variable-angle, multi-color illumination, or objective lenses to be rapidly exchanged.

Interaction potential and near wall dynamics of spherical colloids in suspensions of rod-like fd-virus

Averaged diffusivities of spherical colloids in solutions of rod-like particles, i.e. fd-virus, were measured with EWDLS and TIRM. While the experimentally observed near wall dynamics of the spheres in the absence of fd are well described by standard hydrodynamic theories, there are significant deviations at finite fd concentrations. Both experimental methods yield data which are significantly smaller than the theoretical predictions.

Even illumination in total internal reflection fluorescence microscopy using laser light

In modern fluorescence microscopy, lasers are a widely used source of light, both for imaging in total internal reflection and epi-illumination modes. In wide-field imaging, scattering of highly coherent laser light due to imperfections in the light path typically leads to nonuniform illumination of the specimen, compromising image analysis. We report the design and construction of an objective-launch total internal reflection fluorescence microscopy system with excellent evenness of specimen illumination achieved by azimuthal rotation of the incoming illuminating laser beam. The system allows quick and precise changes of the incidence angle of the laser beam and thus can also be used in an epifluorescence mode.

Electrostatically confined nanoparticle interactions and dynamics

We report integrated evanescent wave and video microscopy measurements of three-dimensional trajectories of 50, 100, and 250 nm gold nanoparticles electrostatically confined between parallel planar glass surfaces separated by 350 and 600 nm silica colloid spacers. Equilibrium analyses of single and ensemble particle height distributions normal to the confining walls produce net electrostatic potentials in excellent agreement with theoretical predictions. Dynamic analyses indicate lateral particle diffusion coefficients approximately 30-50% smaller than expected from predictions including the effects of the equilibrium particle distribution within the gap and multibody hydrodynamic interactions with the confining walls. Consistent analyses of equilibrium and dynamic information in each measurement do not indicate any roles for particle heating or hydrodynamic slip at the particle or wall surfaces, which would both increase diffusivities. Instead, lower than expected diffusivities are speculated to arise from electroviscous effects enhanced by the relative extent (kappaa approximately 1-3) and overlap (kappah approximately 2-4) of electrostatic double layers on the particle and wall surfaces. These results demonstrate direct, quantitative measurements and a consistent interpretation of metal nanoparticle electrostatic interactions and dynamics in a confined geometry, which provides a basis for future similar measurements involving other colloidal forces and specific biomolecular interactions.

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.

Localized surface plasmon coupled fluorescence fiber-optic biosensor with gold nanoparticles

A novel fiber-optic biosensor based on a localized surface plasmon coupled fluorescence (LSPCF) system is proposed and developed. This biosensor consists of a biomolecular complex in a sandwich format of <antibody/antigen/Cy5-antibody-gold nanoparticle (GNP)>. It is immobilized on the surface of an optical fiber where a <Cy5-antibody-GNP> complex forms the fluorescence probe and is produced by mixing Cy5-labeled antibody and protein A conjugated gold nanoparticles (Au-PA). The LSPCF is excited by localized surface plasmon on the GNP surface where the evanescent field is applied near the core surface of the optical fiber. At the same time, the fluorescence signal is detected by a photomultiplier tube located beside the unclad optical fiber with high collection efficiency. Experimentally, this novel LSPCF biosensor is able to detect mouse immunoglobulin G (IgG) at a minimum concentration of 1 pg/mL (7 fM) during the biomolecular interaction of the IgG with anti-mouse IgG. The analysis is expanded by a discussion of the amplification of the LSPCF intensity by GNP coupling, and overall, this LSPCF biosensor is confirmed experimentally as a biosensor with very high sensitivity.

Direct measurement of interaction forces between a single bacterium and a flat plate

A technique for precisely measuring the equilibrium and viscous interaction forces between a single bacterium and a flat surface as functions of separation distance is described. A single-beam gradient optical trap was used to micromanipulate the bacterium against a flat surface while evanescent wave light scattering was used to measure separation distances. Calibrating the optical trap far from the surface allowed the trapped bacterium to be used as a force probe. Equilibrium force-distance profiles were determined by measuring the deflection of the cell from the center of the optical trap at various trap positions. Simultaneously, viscous forces were determined by measuring the relaxation time for the fluctuating bacterium. Absolute distances were determined using a best-fit approximation to the theoretical prediction for the hindered mobility of a diffusing sphere near a wall. Using this approach, forces in the range from 0.01 to 4 pN were measured at near-nanometer resolution between Staphylococcus aureus and glass that was bare or coated with adsorbed protein.

Imaging and sizing of diamond nanoparticles

Typical disturbances of biological environment such as background scatter and refractive index variations have little effect on the size-dependent scattering property of highly refractive nanocrystals, which are potentially attractive optical labels. We report on what is to our knowledge the first investigation of these scattering optical labels, and their sizing, in particular, by imaging at subvideo frame rates and analyzing samples of diamond nanocrystals deposited on a glass substrate in air and in a matrix of weakly scattering polymer. The brightness of a diffraction-limited spot appears to serve as a reliable measure of the particle size in the Rayleigh scattering limit.

Refractive index of thin, aqueous films between hydrophobic surfaces studied using evanescent wave atomic force microscopy

We have studied the refractive index of a thin aqueous film between microscopic hydrophobic surfaces using evanescent wave atomic force microscopy (EW-AFM). An evanescent wave, generated at a solid-liquid interface, is scattered by AFM tips or glass particles attached to AFM cantilevers. The scattering of this wave is used to determine the refractive index as a function of separation between these surfaces. Measurements were performed on surfaces that were rendered hydrophobic with octadecyltrichlorosilane, which produces solid-water contact angles in excess of 90 degrees. For AFM tips, the average refractive index in the thin film was always equal to that of water when the film was thicker than approximately 100 nm. At smaller separations, the refractive index was always greater than or equal to that of water. This is inconsistent with the formation of air or vapor films and consistent with a small amount of organic material between the surfaces. For colloidal spheres (R approximately 10 microm), we were not able to detect changes in the refractive index of the thin film between the sphere and plate.

Measurement and interpretation of particle-particle and particle-wall interactions in levitated colloidal ensembles

This paper reports measurements of particle-wall and particle-particle interactions in levitated colloidal ensembles using integrated total internal reflection microscopy (TIRM) and video microscopy (VM) techniques. In levitated colloidal ensembles with area fractions of phiA = 0.03-0.25, ensemble TIRM measured height distribution functions are used to interpret particle-wall interactions, and VM measured pair distribution functions are used to interpret particle-particle interactions using inverse Ornstein-Zernike (OZ) and three-dimensional inverse Monte Carlo (MC) analyses. An inconsistent finding is the observation of an anomalous long-range particle-particle attraction and recovery of the expected Derjaguin-Landau-Verwey-Overbeek (DLVO) particle-wall interactions for all concentrations examined. Because particle-wall and particle-particle potentials are expected to be consistent in several respects, the analytical and experimental methods employed in this investigation are examined for possible sources of error. Comparison of inverse OZ and three-dimensional inverse MC analyses are used to address uncertainties related to dimensionality, effects of particle concentration, and assumptions of the OZ theory and closure relations. The possible influence of charge heterogeneity and particle size polydispersity on measured distribution functions is discussed with regard to inconsistent particle-wall and particle-particle potentials. Ultimately, achieving a consistent understanding of particle-wall and particle-particle interactions in interfacial and confined colloidal systems is essential to numerous complex fluid and advanced material technologies.

Direct measurement of single and ensemble average particle-surface potential energy profiles

This work involves the development of a novel technique that integrates total internal reflection and video microscopy methods to simultaneously measure single particle and ensemble average particle-surface interactions. For the 2 mum silica colloids and glass coverslip used in this study, particle size polydispersity is found to be a dominant factor in determining the distribution of single particle profiles about ensemble average profiles. In conjunction with this observation, chemical and physical nonuniformity are not evident in any of our measurements even with sensitivity to interactions on the order of kT. One advantage of using ensemble averaging in conjunction with time averaging is the ability to dramatically decrease the time required to measure average particle-wall interactions which scales inversely with interfacial particle concentration. A number of experimental issues are addressed in the development of this technique including (1) combining single particle distribution functions, (2) statistical sampling of distribution functions using both time and ensemble averaging, and (3) correcting overlapping scattering signals between adjacent particles. The capabilities of the ensemble averaging technique are also demonstrated to provide unique measurements of particle-surface interactions in metastable systems by selecting only height excursions of levitated particles when calculating potentials. Ultimately, this new technique provides several important advantages over single particle measurements, which provides a foundation for measuring interactions in increasingly complex interfacial systems.

Evanescent wave scattering and local electric field enhancement at ellipsoidal silver particles in the vicinity of a glass surface

We report on the numerical analysis of the local electric field enhancement of nanosized silver ellipsoids placed in the evanescent field near a glass surface. Across the visible spectrum the enhancement factor for silver particles varies by more than one order of magnitude because of surface-plasmon resonance. Because of the spatially inhomogeneous excitation, higher-order modes additionally contribute and modify the spectral dependence of the electric field compared with plane-wave excitation. Moving the metal particle toward the glass surface increases the field enhancement and shifts the plasmon resonance, which in addition splits between both ends of the particle. Besides the near-field properties of such a probe we also discuss to what extent these local properties can be measured in the far field.

Calibration method for measurement of linear nanometric distances by scattered total internal reflection

Scattered total internal reflection of visible light is used to measure linear nanometric distance to as small as 10 nm. Specifically, we measure the height of magnetic transducer heads above a rotating glass disk. A breakthrough in the approach to calibration, based on combining the second derivative of the transmittance of the scattered light and parameter fitting, substantially improves the quality of the measurement relative to previous demonstrations of this method. The results agree to 1 nm with an industry-standard three-color interferometer to and including the lowest values measured. The technique in principle remains robust to as low as the zero height. Furthermore the calibration point can be as low as 10 nm, which is especially attractive in practice.