Rebecca A. Zangmeister

Characterization of polydopamine thin films deposited at short times by autoxidation of dopamine.

Current interest in melanin films derived from the autoxidation of dopamine stems from their use as a universal adhesion layer. Here we report chemical and physical characterization of polydopamine films deposited on gold surfaces from stirred basic solutions at times ranging from 2 to 60 min, with a focus on times ≤10 min. Data from Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical methods suggest the presence of starting (dopamine) and intermediate (C=N-containing tautomers of quinone and indole) species in the polydopamine films at all deposition times. A uniform overlayer analysis of the XPS data indicates that film thickness increased linearly at short deposition times of ≤10 min. At deposition times ≥10 min, the films appeared largely continuous with surface roughness ≈ ≤ 2 nm, as determined by atomic force microscopy (AFM). Pinhole-free films, as determined by anionic redox probe measurements, required deposition times of 60 min or greater.

Aggregation kinetics of colloidal particles measured by gas-phase differential mobility analysis.

We demonstrate the utility of electrospray gas-phase ion-mobility analysis as a new method to investigate nanoparticle flocculation, or aggregation. Au nanoparticle (Au-NP) solutions were sampled via electrospray (ES), followed by differential ion-mobility analysis (DMA) to determine the particle mobility distribution. Multimodal size distributions obtained with ES-DMA indicated the presence of single Au-NPs (monomer) as well as larger Au-NP clusters such as dimers, trimers, and tetramers under specific solution conditions. The fraction of each aggregate species as a function of time was quantitatively characterized, from which the degree of aggregation, aggregation rate, and stability ratio at different ionic strengths were determined. The latter enabled the extraction of a surface potential (or surface charge density) of 64 +/- 2 mV for 10 nm Au-NPs, which is in good agreement with values obtained from other methods, thus validating our approach. Our results show that ES-DMA is a valuable tool for quantitatively probing the early stages of colloidal aggregation or as a preparatory tool for the size election of aggregates.

Packing and Size Determination of Colloidal Nanoclusters

Here we demonstrate a rapid and quantitative means to characterize the size and packing structure of small clusters of nanoparticles in colloidal suspension. Clustering and aggregation play important roles in a wide variety of phenomena of both scientific and technical importance, yet characterizing the packing of nanoparticles within small clusters and predicting their aerodynamic size remains challenging because available techniques can lack adequate resolution and sensitivity for clusters smaller than 100 nm (optical techniques), perturb the packing arrangement (electron microscopies), or provide only an ensemble average (light scattering techniques). In this article, we use electrospray-differential mobility analysis (ES-DMA), a technique that exerts electrical and drag forces on the clusters, to determine the size and packing of small clusters. We provide an analytical model to determine the mobility size of various packing geometries based on the projected area of the clusters. Data for clusters aggregated from nominally 10 nm gold particles and nonenveloped viruses of various sizes show good agreement between measured and predicted cluster sizes for close-packed spheres.

Method for determining the absolute number concentration of nanoparticles from electrospray sources.

We have developed a simple, fast, and accurate method to measure the absolute number concentration of nanoparticles in solution. The method combines electrospray differential mobility analysis (ES-DMA) with a statistical analysis of droplet-induced oligomer formation. A key feature of the method is that it allows determination of the absolute number concentration of particles by knowing only the droplet size generated from a particular ES source, thereby eliminating the need for sample-specific calibration standards or detailed analysis of transport losses. The approach was validated by comparing the total number concentration of monodispersed Au nanoparticles determined by ES-DMA with UV/vis measurements. We also show that this approach is valid for protein molecules by quantifying the absolute number concentration of Rituxan monoclonal antibody in solution. The methodology is applicable for quantification of any electrospray process coupled to an analytical tool that can distinguish monomers from higher order oligomers. The only requirement is that the droplet size distribution be evaluated. For users only interested in implementation of the theory, we provide a section that summarizes the relevant formulas. This method eliminates the need for sample-specific calibration standards or detailed analysis of transport losses.

Length Distribution of Single-Walled Carbon Nanotubes in Aqueous Suspension Measured by Electrospray Differential Mobility Analysis

The first characterization of the length distribution of single-walled carbon nanotubes (SWCNT) dispersed in a liquid by electrospray differential mobility analysis (ES-DMA) is presented. Although an understanding of geometric properties of SWCNTs, including length, diameter, aspect ratio, and chirality, is essential for commercial applications, rapid characterization of nanotube length distributions remains challenging. Here the use of ES-DMA to obtain length distributions of DNA-wrapped SWCNTs dispersed in aqueous solutions is demonstrated. Lengths measured by ES-DMA compare favorably with those obtained from multiangle light scattering, dynamic light scattering, field flow fractionation with UV/vis detection, and atomic force microscopy, validating ES-DMA as a technique to measure SWCNTs of <250 nm in length. The nanotubes are previously purified and dispersed by wrapping with oligomeric DNA in aqueous solution and centrifuging to remove bundles and amorphous carbon. These dispersions are particularly attractive due to their amenability to bulk processing, ease of storage, high concentration, compatibility with biological and high-throughput manufacturing environments, and for their potential applications ranging from electronics and hydrogen-storage vessels to anticancer agents.

Quantification and Compensation of Nonspecific Analyte Aggregation in Electrospray Sampling

Electrospray (ES) sources are commonly used to introduce nonvolatile materials (e.g., nanoparticles, proteins, etc.) to the gas phase for characterization by mass spectrometry or ion mobility. Recent studies in our group using ES ion mobility to characterize protein aggregation in solution have raised the question as to whether the ES itself induces aggregation and thus corrupts the results. In this article, we develop a statistical model to determine the extent to which the ES process induces the formation of dimers and higher-order aggregates. The model is validated through ES differential mobility experiments using gold nanoparticles. The results show that the extent of droplet-induced aggregation is quite severe and previously reported cutoff criterion is inadequate. We use the model in conjunction with experiment to show the true dimer concentration in a protein solution as a function of concentration. The model is extendable to any ES source analytical system and to higher aggregation states. For users only interested in implementation of the theory, we provide a section that summarizes the relevant formulas.

Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities

The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir–Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10 -6 cm 2 ·V -1 ·s -1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10 -4 cm 2 ·V -1 ·s -1 . Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad.

A lectin-based gold nanoparticle assay for probing glycosylation of glycoproteins.

We report a glycoanalysis method in which lectins are used to probe the glycans of therapeutic glycoproteins that are adsorbed on gold nanoparticles. A model mannose‐presenting glycoprotein, ribonuclease B (RNase B), and the therapeutic monoclonal antibody (mAb) rituximab, were found to adsorb spontaneously and non‐specifically to bare gold nanoparticles such that glycans were accessible for lectin binding. Addition of a multivalent binding lectin, such as concanavalin A (Con A), to a solution of the modified gold nanoparticles resulted in cross‐linking of the nanoparticles. This phenomenon was evidenced within 1 min by a change in the hydrodynamic diameter, DH, measured by dynamic light scattering (DLS) and a shift and increase in absorbance of the plasmon resonance band of the gold nanoparticles. By combining the sugar‐binding specificity and the cross‐linking capabilities of lectins, the non‐specific adsorption of glycoproteins to gold surfaces, and the unique optical reporting properties of gold nanoparticles, a glycosylation pattern of rituximab could be generated. This assay provides advantages over currently used glycoanalysis methods in terms of short analysis time, simplicity of the conjugation method, convenience of simple spectroscopic detection, and feasibility of providing glycan characterization of the protein drug product by using a variety of binding lectins. Biotechnol. Bioeng. 2012;109: 2240–2249.

Equilibrium electrostatics of responsive polyelectrolyte monolayers.

The physical behavior of polyelectrolytes at solid-liquid interfaces presents challenges both in measurement and in interpretation. An informative, yet often overlooked, property that characterizes the equilibrium organization of these systems is their membrane or rest potential. Here a general classification scheme is presented of the relationship between the rest potential and structural response of polyelectrolyte films to salt concentration. A numerical lattice theory, adapted from the polymer community, is used to analyze the rest potential response of end-tethered polyelectrolyte layers in which electrostatics and short-range contact interactions conspire to bring about different structural states. As an experimental quantity the rest potential is a readily accessible, nonperturbing metric of the equilibrium structure of a polyelectrolyte layer. A first set of measurements is reported on monolayers of end-tethered, single-stranded DNA in monovalent (NaCl) and divalent (MgCl(2)) counterion environments. Intriguingly, in NaCl electrolyte at least two different mechanisms appear by which the DNA layers can structurally relax in response to changing salt conditions. In MgCl(2) the layers appear to collapse. The possible molecular mechanisms behind these behaviors are discussed. These studies provide insight into phenomena more generally underlying polyelectrolyte applications in the chemical, environmental, and biotechnological fields.

Recent Advances in Electrochemical Glycobiosensing

Biosensors based on electrochemical transduction mechanisms have recently made advances into the field of glycan analysis. These glyco-biosensors offer simple, rapid, sensitive, and economical approaches to the measurement need for rapid glycan analysis for biomarker detection, cancer and disease diagnostics, and bioprocess monitoring of therapeutic glycoproteins. Although the prevalent methods of glycan analysis (high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy) provide detailed identification and structural analysis of glycan species, there are significantly few low-cost, rapid glycan assays available for diagnostic and screening applications. Here we review instances in which glyco-biosensors have been used for glycan analysis using a variety of electrochemical transduction mechanisms (e.g., amperometric, potentiometric, impedimetric, and voltammetric), selective binding agents (e.g., lectins and antibodies), and redox species (e.g., enzyme substrates, inorganic, and nanomaterial).