Simultaneous absolute determination of particle size and effective density of submicron colloids by disc centrifuge photosedimentometry

Quantitative evaluation of particle-binder interactions in ceramic slurries via differential centrifugal sedimentation

In diverse materials science spanning from fine ceramics to lithium-ion batteries and fuel cells, the particle-binder interactions in slurries play a crucial role in governing the ultimate performance. Despite numerous efforts to date, quantitatively elucidating these hidden interactions has remained a longstanding challenge. Here, we demonstrate a dynamic approach to evaluate adsorptive interactions between ceramic particles and polymeric binders entangled in a slurry utilizing differential centrifugal sedimentation (DCS). Particles settling under a centrifugal force field impart significant viscous resistance on the adsorbed binder, leading to its detachment, influenced by particle size and density. This behaviour directly reflects the particle-binder interactions, and detailed DCS spectrum analysis enables the quantitative assessment of nano-Newton-order adsorption forces. An important finding is the strong correlation of these forces with the mechanical properties of the moulded products. Our results provide insight that forming a flexible network structure with appropriate interactions is essential for desirable formability.

Laser scattering centrifugal liquid sedimentation method for the accurate quantitative analysis of mass-based size distributions of colloidal silica

This paper proposes a laser scattering centrifugal liquid sedimentation (LS-CLS) method for the accurate quantitative analysis of the mass-based size distributions of colloidal silica. The optics comprised a laser diode light source and multi-pixel photon-counting detector for detecting scattered light intensity. The unique optics can only detect the light scattered by a sample through the interception of irradiated light. The developed centrifugal liquid sedimentation (CLS) method comprised a light-emitting diode and silicon photodiode detector for detecting transmittance light attenuation. The CLS apparatus could not accurately measure quantitative volume- or mass-based size distribution of poly-dispersed suspensions, such as colloidal silica, because the detecting signal includes both transmitted and scattered light. The LS-CLS method exhibited improved quantitative performance. Moreover, the LS-CLS system allowed the injection of samples with concentrations higher than that permitted by other particle size distribution measurement systems with particle size classification units using size-exclusion chromatography or centrifugal field-flow fractionation. The proposed LS-CLS method achieved an accurate quantitative analysis of the mass-based size distribution using both centrifugal classification and laser scattering optics. In particular, the system could measure the mass-based size distribution of approximately 20 mg mL^-1 poly-dispersed colloidal silica samples, such as in a mixture of the four mono-dispersed colloidal silica, with high resolution and precision, thereby demonstrating high quantitative performance. The measured size distributions were compared with those observed through transmission electron microscopy. The proposed system can be used in practical setups to achieve a reasonable degree of consistency for determining particle size distribution in industrial applications.

Direct Measurement of Sedimentation Coefficient Distributions in Multimodal Nanoparticle Mixtures

Differential centrifugal sedimentation (DCS) is based on physical separation of nanoparticles in a centrifugal field prior to their analysis. It is suitable for resolving particle populations, which only slightly differ in size or density. Agglomeration presents a common problem in many natural and engineered processes. Reliable data on the agglomeration state are also crucial for hazard and risk assessment of nanomaterials and for grouping and read-across of nanoforms. Agglomeration results in polydisperse mixtures of nanoparticle clusters with multimodal distributions in size, density, and shape. These key parameters affect the sedimentation coefficient, which is the actual physical quantity measured in DCS, although the method is better known for particle sizing. The conversion into a particle size distribution is, however, based on the assumption of spherical shapes. The latter disregards the influence of the actual shape on the sedimentation rate. Sizes obtained in this way refer to equivalent diameters of spheres that sediment at the same velocity. This problem can be circumvented by focusing on the sedimentation coefficient distribution of complex nanoparticle mixtures. Knowledge of the latter is essential to implement and optimize preparative centrifugal routines, enabling precise and efficient sorting of complex nanoparticle mixtures. The determination of sedimentation coefficient distributions by DCS is demonstrated based on supracolloidal assemblies, which are often referred to as "colloidal molecules". The DCS results are compared with sedimentation coefficients obtained from hydrodynamic bead-shell modeling. Furthermore, the practical implementation of the analytical findings into preparative centrifugal separations is explored.

Avoiding the Spherical Particle Assumption: Fractal Particle Density, Size, and Structure Characterization through Combined Sedimentation and Viscometry Measurements

Physically meaningful characterization of irregularly shaped particles continues to present substantial challenges to the experimentalist. "Equivalent diameters" based on experimental techniques such as static and dynamic light scattering or sedimentation have proliferated to the point that they are often no longer recognized as equivalent. This study demonstrates the use of dual-fluid disk centrifuge photosedimentometry coupled with rheological measurements of viscosity to provide direct insights into both the average mass of a structured particle size distribution and the average hydrodynamic diameter.

Validation of a Homogeneous Incremental Centrifugal Liquid Sedimentation Method for Size Analysis of Silica (Nano)particles

Silica nanoparticles display many unique physicochemical properties that make them desirable for use in a wide variety of consumer products and composite materials. Accurately measuring the size of these nanoparticles is important for achieving the desired nanoscale functionality of the final product and for regulatory compliances. This study covers the validation of a centrifugal liquid sedimentation method for accurate measurement of the Stokes diameter of silica particles with a near-spherical shape and dimensions in the nanometer and sub-nanometer scale range. The validated method provided unbiased results in the range of 50 nm to 200 nm, with a lower limit of detection of ≤20 nm. The relative standard uncertainties for precision, quantified in terms of repeatability and day-to-day variation, ranged from 0.2% to 1.0% and from <0.1% to 0.5%, respectively. The standard uncertainty for trueness was assessed at 4.6%. Within its working range, the method was found robust with respect to the type of cuvette, light factor, operator, and for defining the meniscus of the sample suspension. Finally, a relative expanded measurement uncertainty of 10% confirmed the satisfactory performance of the method.

Characterization methods for studying protein adsorption on nano-polystyrene beads

This work is dealing with the use of polystyrene (PS) nanoparticles as substrates for bioanalytical specific interactions. Different techniques were used for the accurate characterization of the PS nanoparticles of 100 nm and 196 nm before coating them with a layer of antibodies against immunoglobulins of type E (aIgE), giving to the particle a specific functionality. The formation of the aIgE adsorbed layer was monitored using centrifugal particle separation (CPS) and centrifugal field flow fractionation (CF3) experiments, which allowed to determine the size changes and the adsorbed mass. Particle sizes were also measured with DLS, used both as stand-alone instrument and coupled to CF3 (CF3-DLS). The complementary information obtained from the CPS and CF3-DLS measurements allowed the estimation of the density of the aIgE shell. The proteins immobilized at the surface fully retained their activity, as proven by the reactions between the functionalized PS-aIgE particles and immunoglobulins of type E (IgE) dispersed in suspensions prepared on purpose.

Establishing SI-Traceability of Nanoparticle Size Values Measured with Line-Start Incremental Centrifugal Liquid Sedimentation

Line-start incremental centrifugal liquid sedimentation (disc-CLS) is a powerful technique to determine particle size based on the principles of Stokes’ law. As most input quantities of the Stokes equation cannot be easily determined for typical instruments used for this method, an alternative method which depends on calibrating the sedimentation time scale with reference particles has become common practice. Unfortunately, most of these calibration materials (calibrants) come with limited information regarding their metrological reliability (e.g., lack of measurement uncertainties and traceability statements, incomplete measurand definitions). As a consequence, routine particle size results obtained by disc-CLS are mostly only traceable to the calibrant used, and effective comparisons can only be made for those results originating from measurements performed with the same types of calibrants. In this study, we discuss the concept of metrological traceability and demonstrate that particle size results obtained by disc-CLS can be traceable to the ultimate metrological reference, i.e., the unit of length in the International System of Units (SI), the meter. Using the example of two colloidal silica certified reference materials, we describe how laboratories can realize metrological traceability to the SI by simplifying complex traceability networks.

Improved Metrological Traceability of Particle Size Values Measured with Line-Start Incremental Centrifugal Liquid Sedimentation

Line-start incremental centrifugal liquid sedimentation (disc-CLS) is a powerful method to determine particle size based on the principles of Stokes' law. Because several of the input quantities of the Stokes equation cannot be easily determined for this case of a rotating disc, the disc-CLS approach relies on calibrating the sedimentation time scale with reference particles. To use these calibrant particles for establishing metrological traceability, they must fulfill the same requirements as those imposed on a certified reference material, i.e., their certified Stokes diameter and density value must come with a realistic measurement uncertainty and with a traceability statement. As is the case for several other techniques, the calibrants do not always come with uncertainties for the assigned modal diameter and effective particle density. The lack of such information and the absence of a traceability statement make it difficult for the end-user to estimate the uncertainty of the measurement results and to compare them with results obtained by others. We present the results of a collaborative study that aimed at demonstrating the traceability of particle size results obtained with disc-CLS. For this purpose, the particle size and effective particle density of polyvinyl chloride calibrants were measured using different validated methods, and measurement uncertainties were estimated according to the Guide to the Expression of Uncertainty in Measurement. The results indicate that the modal Stokes diameter and effective particle density that are assigned to the calibrants are accurate within 5% and 3.5%, respectively, and that they can be used to establish traceability of particle size results obtained with disc-CLS. This conclusion has a great impact on the traceability statement of certified particle size reference materials, for which the traceability is limited to the size and density values of the calibrant particles.

Quantification of the cellular dose and characterization of nanoparticle transport during in vitro testing

BACKGROUND

The constant increase of the use of nanomaterials in consumer products is making increasingly urgent that standardized and reliable in vitro test methods for toxicity screening be made available to the scientific community. For this purpose, the determination of the cellular dose, i.e. the amount of nanomaterials effectively in contact with the cells is fundamental for a trustworthy determination of nanomaterial dose responses. This has often been overlooked in the literature making it difficult to undertake a comparison of datasets from different studies. Characterization of the mechanisms involved in nanomaterial transport and the determination of the cellular dose is essential for the development of predictive numerical models and reliable in vitro screening methods.

RESULTS

This work aims to relate key physico-chemical properties of gold nanoparticles (NPs) to the kinetics of their deposition on the cellular monolayer. Firstly, an extensive characterization of NPs in complete culture cell medium was performed to determine the diameter and the apparent mass density of the formed NP-serum protein complexes. Subsequently, the kinetics of deposition were studied by UV-vis absorbance measurements in the presence or absence of cells. The fraction of NPs deposited on the cellular layer was found to be highly dependent on NP size and apparent density because these two parameters influence the NP transport. The NP deposition occurred in two phases: phase 1, which consists of cellular uptake driven by the NP-cell affinity, and phase 2 consisting mainly of NP deposition onto the cellular membrane.

CONCLUSION

The fraction of deposited NPs is very different from the initial concentration applied in the in vitro assay, and is highly dependent of the size and density of the NPs, on the associated transport rate and on the exposure duration. This study shows that an accurate characterization is needed and suitable experimental conditions such as initial concentration of NPs and liquid height in the wells has to be considered since they strongly influence the cellular dose and the nature of interactions of NPs with the cells.

Determining the Effective Density and Stabilizer Layer Thickness of Sterically Stabilized Nanoparticles

A series of model sterically stabilized diblock copolymer nanoparticles has been designed to aid the development of analytical protocols in order to determine two key parameters: the effective particle density and the steric stabilizer layer thickness. The former parameter is essential for high resolution particle size analysis based on analytical (ultra)centrifugation techniques (e.g., disk centrifuge photosedimentometry, DCP), whereas the latter parameter is of fundamental importance in determining the effectiveness of steric stabilization as a colloid stability mechanism. The diblock copolymer nanoparticles were prepared via polymerization-induced self-assembly (PISA) using RAFT aqueous emulsion polymerization: this approach affords relatively narrow particle size distributions and enables the mean particle diameter and the stabilizer layer thickness to be adjusted independently via systematic variation of the mean degree of polymerization of the hydrophobic and hydrophilic blocks, respectively. The hydrophobic core-forming block was poly(2,2,2-trifluoroethyl methacrylate) [PTFEMA], which was selected for its relatively high density. The hydrophilic stabilizer block was poly(glycerol monomethacrylate) [PGMA], which is a well-known non-ionic polymer that remains water-soluble over a wide range of temperatures. Four series of PGMA x -PTFEMA y nanoparticles were prepared (x = 28, 43, 63, and 98, y = 100-1400) and characterized via transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). It was found that the degree of polymerization of both the PGMA stabilizer and core-forming PTFEMA had a strong influence on the mean particle diameter, which ranged from 20 to 250 nm. Furthermore, SAXS was used to determine radii of gyration of 1.46 to 2.69 nm for the solvated PGMA stabilizer blocks. Thus, the mean effective density of these sterically stabilized particles was calculated and determined to lie between 1.19 g cm-3 for the smaller particles and 1.41 g cm-3 for the larger particles; these values are significantly lower than the solid-state density of PTFEMA (1.47 g cm-3). Since analytical centrifugation requires the density difference between the particles and the aqueous phase, determining the effective particle density is clearly vital for obtaining reliable particle size distributions. Furthermore, selected DCP data were recalculated by taking into account the inherent density distribution superimposed on the particle size distribution. Consequently, the true particle size distributions were found to be somewhat narrower than those calculated using an erroneous single density value, with smaller particles being particularly sensitive to this artifact.

Sedimentation coefficient distributions of large particles

The spatial and temporal evolution of concentration boundaries in sedimentation velocity analytical ultracentrifugation reports on the size distribution of particles with high hydrodynamic resolution. For large particles such as large protein complexes, fibrils, viral particles, or nanoparticles, sedimentation conditions usually allow migration from diffusion to be neglected relative to sedimentation. In this case, the shape of the sedimentation boundaries of polydisperse mixtures relates directly to the underlying size-distributions. Integral and derivative methods for calculating sedimentation coefficient distributions g*(s) of large particles from experimental boundary profiles have been developed previously, and are recapitulated here in a common theoretical framework. This leads to a previously unrecognized relationship between g*(s) and the time-derivative of concentration profiles. Of closed analytical form, it is analogous to the well-known Bridgman relationship for the radial derivative. It provides a quantitative description of the effect of substituting the time-derivative by scan differences with finite time intervals, which appears as a skewed box average of the true distribution. This helps to theoretically clarify the differences between results from time-derivative method and the approach of directly fitting the integral definition of g*(s) to the entirety of experimental boundary data.

A uniform measurement expression for cross method comparison of nanoparticle aggregate size distributions

Measurement methods produce incomparable results when applied to aggregated nanoparticles.

Monodisperse PEGylated spheres: an aqueous colloidal model system

Fluorinated core–shell spheres have been synthesized using a novel semibatch emulsion polymerization protocol employing slow feeding of the initiator. The synthesis results in aqueous dispersions of highly monodisperse spheres bearing a well-defined poly(ethylene glycol) graft (PEGylation). Measurements are consistent with the synthesis achieving a high grafting density that moreover consists of a single PEG layer with the polymer significantly elongated beyond its radius of gyration in bulk. The fluorination of the core of the particles confers a low index of refraction such that the particles can be refractive index matched in water through addition of relatively small amounts of a cosolvent, which enables the use of optical and laser-based methods for studies of concentrated systems. The systems exhibit an extreme stability in NaCl solutions, but attractions among particles can be introduced by addition of other salts, in which case aggregation is shown to be reversible. The PEGylated sphere dispersions are expected to be ideally suited as model systems for studies of the effect of PEG-mediated interactions on, for instance, structure, dynamics, phase behavior, and rheology.