Scattering from colloidal cubic silica shells: Part II, static structure factors and osmotic equation of state.

HYPOTHESIS: The shape of colloidal particles affects the structure of colloidal dispersions. The effect of the cube shape on the thermodynamics of colloidal cube dispersions has not yet been studied experimentally. Static light scattering measurements on colloidal cubic silica shells at finite concentrations allows us to measure the structure factor of colloidal cube fluids and to test theoretical predictions for the equation of state of hard convex superballs. EXPERIMENTS: Hollow silica nanocubes of varying concentrations in N,N,-dimethylformamide were studied with static light scattering. The structure factor was extracted from the scattering curves using experimental form factors. From this experimental structure factor, the specific density of the particles, and the osmotic compressibility were obtained. This osmotic compressibility was then compared to a theoretical equation of state of hard superballs. FINDINGS: The first experimental structure factors of a stable cube fluid are presented. The osmotic compressibility of the cube fluid can be described by the equation of state of a hard superball fluid, showing that silica cubes in N,N,-dimethylformamide with LiCl effectively interact as hard particles.

Scattering from colloidal cubic silica shells: Part I, particle form factors and optical contrast variation.

HYPOTHESIS: Colloidal cubic silica shells, prepared from cuprous oxide cubes, with a typical size of 100 nm are promising model particles for scattering studies on dilute, as well as concentrated fluids, of non-spherical colloids. EXPERIMENTS: Small angle X-ray scattering, and static light scattering are employed to determine form factors of cubic silica shells and silica covered cuprous oxide cubes. Contrast variation experiments are performed to assess the refractive index and optical homogeneity of the cubic silica shells, which is important for the extension of the scattering study to concentrated dispersions of cubic shells in Part II (Dekker, submitted for publication). RESULTS: The experimental form factors, which compare well to theoretical form factors, manifest cubic silica shells that are dispersed as single stable colloids with a shape intermediate between a sphere and a perfect cube. Contrast variation demonstrates that the silica shells are optically homogeneous, with a refractive index that is independent of the shell thickness. The results presented here open up the possibility to extract structure factors from light scattering measurements on concentrated cube dispersions in Part II.

Complex magnetic susceptibility setup for spectroscopy in the extremely low-frequency range.

A sensitive balanced differential transformer was built to measure complex initial parallel magnetic susceptibility spectra in the 0.01-1000 Hz range. The alternating magnetic field can be chosen sufficiently weak that the magnetic structure of the samples is only slightly perturbed and the low frequencies make it possible to study the rotational dynamics of large magnetic colloidal particles or aggregates dispersed in a liquid. The distinguishing features of the setup are the novel multilayered cylindrical coils with a large sample volume and a large number of secondary turns (55 000) to measure induced voltages with a good signal-to-noise ratio, the use of a dual channel function generator to provide an ac current to the primary coils and an amplitude- and phase-adjusted compensation voltage to the dual phase differential lock-in amplifier, and the measurement of several vector quantities at each frequency. We present the electrical impedance characteristics of the coils, and we demonstrate the performance of the setup by measurement on magnetic colloidal dispersions covering a wide range of characteristic relaxation frequencies and magnetic susceptibilities, from chi approximately -10(-5) for pure water to chi>1 for concentrated ferrofluids.

Atomic force microscopy and magnetic force microscopy study of model colloids.

Atomic force microscopy (AFM) is used to study the size, shape, and polydispersity of a variety of magnetic and nonmagnetic model colloids, previously imaged by transmission electron microscopy (TEM) only. Both height and phase images are analyzed and special attention is given to 3D morphology and softness of particles, as well as structures and presence of secondary components in the colloid, difficult to investigate with TEM. Several methods of tip characterization followed by deconvolution were applied in order to improve the accuracy of lateral diameter determination. In the case of magnetite particles dispersed in conventional ferrofluids, we explore both experimentally and theoretically the possibility of using magnetic force microscopy (MFM). We propose and discuss several models which allow to estimate the magnetic moment of a single domain superparamagnetic sphere using MFM, which cannot be done with other techniques; alternatively the tip magnetization can be determined.

A differential dielectric spectroscopy setup to measure the electric dipole moment and net charge of colloidal quantum dots.

A sensitive dielectric spectroscopy setup is built to measure the response of nanoparticles dispersed in a liquid to an alternating electric field over a frequency range from 10(-2) to 10(7) Hz. The measured complex permittivity spectrum records both the rotational dynamics due to a permanent electric dipole moment and the translational dynamics due to net charges. The setup consists of a half-transparent capacitor connected in a bridge circuit, which is balanced on pure solvent only, using a software-controlled compensating voltage. In this way, the measured signal is dominated by the contributions of the nanoparticles rather than by the solvent. We demonstrate the performance of the setup with measurements on a dispersion of colloidal CdSe quantum dots in the apolar liquid decalin.

Note: Rapid offset reduction of impedance bridges taking into account instrumental damping and phase shifting.

The sensitivity of an imperfectly balanced impedance bridge is limited by the remaining offset voltage. Here, we present a procedure for offset reduction in impedance measurements using a lock-in amplifier, by applying a complex compensating voltage external to the bridge. This procedure takes into account instrumental damping and phase shifting, which generally occur at the high end of the operational frequency range. Measurements demonstrate that the output of the circuit rapidly converges to the instrumentally limited noise at any frequency.

Simultaneous measurement of rotational and translational diffusion of anisotropic colloids with a new integrated setup for fluorescence recovery after photobleaching.

This paper describes an integrated setup for fluorescence recovery after photobleaching (FRAP) for determining translational and rotational Brownian diffusion simultaneously, ensuring that these two quantities are measured under exactly the same conditions and at the same time in dynamic experiments. The setup is based on translational-FRAP with a fringe pattern of light for both the bleaching and monitoring of fluorescently labeled particles, and rotational-FRAP, which uses the polarization of a short bleach light pulse to create a polarization anisotropy. The fringe pattern of the probe beam is modulated in conjunction with a synchronized lock-in amplifier giving a fast, sensitive, ensemble-averaged measurement compared to microscope-image based techniques. The experimental polarization geometry we used ensures that the fluorescence emission is collected without polarization bias. Therefore, only the orientation of the absorption dipole moment of the fixed dye in the particles is measured, which simplifies interpretation of the data. The polarization is modulated rapidly between two orthogonal polarization states, giving the polarization anisotropy in one, single measurement. The rotational and translational Brownian diffusion of anisotropic colloids is measured for ellipsoids of revolution. This experiment shows that in this case the rotational correlation function matches a three-exponential decay in accordance with theoretical predictions.

AFM study of the thermotropic behaviour of supported DPPC bilayers with and without the model peptide WALP23.

Temperature-controlled Atomic Force Microscopy (TC-AFM) in Contact Mode is used here to directly image the mechanisms by which melting and crystallization of supported, hydrated DPPC bilayers proceed in the presence and absence of the model peptide WALP23. Melting from the gel L(ß)' to the liquid-crystalline L(α) phase starts at pre-existing line-type packing defects (grain boundaries) in absence of the peptide. The exact transition temperature is shown to be influenced by the magnitude of the force exerted by the AFM probe on the bilayer, but is higher than the main transition temperature of non-supported DPPC vesicles in all cases due to bilayer-substrate interactions. Cooling of the fluid L(α) bilayer shows the formation of the line-type defects at the borders between different gel-phase regions that originate from different nuclei. The number of these defects depends directly on the rate of cooling through the transition, as predicted by classical nucleation theory. The presence of the transmembrane, synthetic model peptide WALP23 is known to give rise to heterogeneity in the bilayer as microdomains with a striped appearance are formed in the DPPC bilayer. This striated phase consists of alternating lines of lipids and peptide. It is shown here that melting starts with the peptide-associated lipids in the domains, whose melting temperature is lowered by 0.8-2.0°C compared to the remaining, peptide-free parts of the bilayer. The stabilization of the fluid phase is ascribed to adaptations of the lipids to the shorter peptide. The lipids not associated with the peptide melt at the same temperature as those in the pure DPPC supported bilayer.

Magnetic-field-induced nematic-nematic phase separation and droplet formation in colloidal goethite.

We demonstrate the suitability of polarization microscopy to study the recently discovered (parallel) nematic-(perpendicular) nematic phase separation. This novel type of phase transition is induced by applying an external magnetic field to a nematic liquid crystal of boardlike colloidal goethite and is due to an interplay between the intrinsic magnetic properties of goethite and the collective effect of liquid crystal formation. It is shown that the intense ochre colour of goethite does not preclude the use of polarization microscopy and interference colours, and that dichroism can give valuable qualitative information on the nature of the phases, their anchoring and their sedimentation and order parameter profiles. We also apply these techniques to study 'nematic-nematic tactoids': nematic droplets sedimenting within a nematic medium with mutually perpendicular orientations.

Fluorescent monodisperse silica ellipsoids for optical rotational diffusion studies.

We report on the preparation of monodisperse, fluorescent hematite-silica core-shell ellipsoids, with adjustable shapes ranging from spindles to nearly spheres, that are suitable for optical rotational diffusion studies. Hematite cores are grafted with poly(vinylpyrrolidone) which ensures colloidal stability during the silica coating provided by the base-catalyzed hydrolysis and polymerization of tetraethoxysilane. Using tetramethylammonium hydroxide as base instead of the volatile ammonia facilitates continuous seeded growth of silica to colloids with a desired aspect ratio. A convenient feature of the hematite-silica particles is the rapid dissolution of the iron oxide core by acid, producing hollow silica ellipsoids that can be optically matched to near transparency. The control of shape and size of the silica ellipsoids, their optical properties, and the fairly high yield in comparison to other preparation methods for nonspherical model colloids make the ellipsoids very suitable for quantitative studies. As a case in point, we have measured the rotational diffusion coefficient of fluorescent ellipsoids with rotational fluorescence recovery after photobleaching. Dye-labeled ellipsoids can be imaged with confocal microscopy.

Rotational dynamics of colloidal spheres probed with fluorescence recovery after photobleaching.

We report a polarized fluorescence recovery after photobleaching (pFRAP) method to measure the rotational dynamics of fluorescent colloids over a wide dynamic range. The method is based on the polarization anisotropy in the fluorescence intensity, generated by bleaching of fluorescently labeled particles with an intense pulse of linearly polarized laser light. The rotational mobilities of the fluorescent particles can be extracted from the relaxation kinetics of the postbleach fluorescence polarization anisotropy. Our pFRAP setup has access to correlation times over a range of time scales from tens of microseconds to tens of seconds, and is highly sensitive, so very low concentrations of labeled particles can be probed. We present a detailed description of the theoretical background of pFRAP. The performance of the equipment is demonstrated for fluorescent colloidal silica spheres, dispersed in pure solvents as well as in fd-virus suspensions.