Condensable Vapor Sorption by Low Charge State Protein Ions

Measurement of the gas-phase ion mobility of proteins provides a means to quantitatively assess the relative sizes of charged proteins. However, protein ion mobility measurements are typically singular values. Here, we apply tandem mobility analysis to low charge state protein ions (+1 and +2 ions) introduced into the gas phase by nanodroplet nebulization. We first determine protein ion mobilities in dry air and subsequently examine shifts in mobilities brought about by the clustering of vapor molecules. Tandem mobility analysis yields mobility-vapor concentration curves for each protein ion, expanding the information obtained from mobility analysis. This experimental procedure and analysis is extended to bovine serum albumin, transferrin, immunoglobulin G, and apoferritin with water, 1-butanol, and nonane. All protein ions appear to adsorb vapor molecules, with mobility "diameter" shifts of up to 6-7% at conditions just below vapor saturation. We parametrize results using κ-Köhler theory, where the term κ quantifies the extent of uptake beyond Köhler model expectations. For 1-butanol and nonane, κ decreases with increasing protein ion size, while it increases with increasing protein ion size for water. For the systems probed, the extent of mobility shift for the organic vapors is unaffected by the nebulized solution pH, while shifts with water are sensitive to pH.

Multidimensional Nanoparticle Characterization through Ion Mobility-Mass Spectrometry

Multidimensional techniques that combine fully or partially orthogonal characterization methods in a single setup often provide a more comprehensive description of analytes. When applied to nanoparticles, they have the potential to reveal particle properties not accessible to more conventional 1D techniques. Herein, we apply recently developed 2D characterization techniques to nanoparticles using atmospheric-pressure ion mobility-mass spectrometry (IM-MS), and we demonstrate the analytical capability of this approach using ultraporous mesostructured silica nanoparticles (UMNs). We show that IM-MS yields a 2D particle size-mass distribution function, which in turn can be used to calculate not only important 1D distributions, i.e. particle size distributions, but also nanoparticle structural property distributions not accessible by other methods, including size-dependent particle porosity and the specific pore volume distribution function. IM-MS measurement accuracy was confirmed by measurement of NIST-certified polystyrene latex particle standards. For UMNs, comparison of IM-MS results with TEM and N₂ physisorption yields quantitative agreement in particle size and qualitative agreement in average specific pore volume. IM-MS uniquely shows how within a single UMN population, porosity increases with increasing particle size, consistent with the proposed UMN growth mechanism. In total, we demonstrate the potential of IM-MS as a standard approach for the characterization of structurally complex nanoparticle populations, as it yields size-specific structural distribution functions.

Nanoparticle Characterization: What to Measure?

What to measure? is a key question in nanoscience, and it is not straightforward to address as different physicochemical properties define a nanoparticle sample. Most prominent among these properties are size, shape, surface charge, and porosity. Today researchers have an unprecedented variety of measurement techniques at their disposal to assign precise numerical values to those parameters. However, methods based on different physical principles probe different aspects, not only of the particles themselves, but also of their preparation history and their environment at the time of measurement. Understanding these connections can be of great value for interpreting characterization results and ultimately controlling the nanoparticle structure-function relationship. Here, the current techniques that enable the precise measurement of these fundamental nanoparticle properties are presented and their practical advantages and disadvantages are discussed. Some recommendations of how the physicochemical parameters of nanoparticles should be investigated and how to fully characterize these properties in different environments according to the intended nanoparticle use are proposed. The intention is to improve comparability of nanoparticle properties and performance to ensure the successful transfer of scientific knowledge to industrial real-world applications.

Ultrasonic Nebulization for TEM Sample Preparation on Single-Layer Graphene Grids

Spray-coating using ultrasonic nebulization is reported for depositing nanoparticles on a TEM grid without many of the drying artifacts that are often associated with drop-casting. Spray-coating is suitable for preparing TEM samples on fragile support materials, such as suspended single-layer graphene, that rupture when samples are prepared by drop-casting. Additionally, because ultrasonic nebulization produces uniform droplets, nanoparticles deposited by spray-coating occur on the TEM grid in clusters, whose size is dependent on the concentration of the nanoparticle dispersion, which may allow the concentration of nanoparticle dispersions to be estimated using TEM.

Determination of the transfer function of an atmospheric pressure drift tube ion mobility spectrometer for nanoparticle measurements

A new method is introduced to determine the transfer/transmission function of a drift tube ion mobility spectrometer.

Ion-Mobility-Based Quantification of Surface-Coating-Dependent Binding of Serum Albumin to Superparamagnetic Iron Oxide Nanoparticles

Protein binding and protein-induced nanoparticle aggregation are known to occur for a variety of nanomaterials, with the extent of binding and aggregation highly dependent on nanoparticle surface properties. However, often lacking are techniques that enable quantification of the extent of protein binding and aggregation, particularly for nanoparticles with polydisperse size distributions. In this study, we adapt ion mobility spectrometry (IMS) to examine the binding of bovine serum albumin to commercially available anionic-surfactant-coated superparamagnetic iron oxide nanoparticles (SPIONs), which are initially ∼21 nm in mean mobility diameter and have a polydisperse size distribution function (geometric standard deviation near 1.4). IMS, carried out with a hydrosol-to-aerosol converting nebulizer, a differential mobility analyzer, and a condensation particle counter, enables measurements of SPION size distribution functions for varying BSA/SPION number concentration ratios. IMS measurements suggest that initially (at BSA concentrations below 50 nM) BSA binds reversibly to SPION surfaces with a binding site density in the 0.05-0.08 nm(-2) range. However, at higher BSA concentrations, BSA induces SPION-SPION aggregation, evidenced by larger shifts in SPION size distribution functions (mean diameters beyond 40 nm for BSA concentrations near 100 nM) and geometric standard deviations (near 1.3) consistent with self-preserving aggregation theories. The onset of BSA aggregation is correlated with a modest but statistically significant decrease in the specific absorption rate (SAR) of SPIONs placed within an alternating magnetic field. The coating of SPIONs with mesoporous silica (MS-SPIONs) as well as PEGylation (MS-SPIONs-PEG) is found to completely mitigate BSA binding and BSA-induced aggregation; IMS-inferred size distribution functions are insensitive to BSA concentration for MS-SPIONs and MS-SPIONs-PEG. The SARs of MS-SPIONs are additionally insensitive to BSA concentration, confirming the SAR decrease is linked to BSA-induced aggregation.