Characterization of mRNA Lipid Nanoparticles by Electron Density Mapping Reconstruction: X-ray Scattering with Density from Solution Scattering (DENSS) Algorithm

PURPOSE

This study aimed to test the feasibility of using Small Angle X-ray Scattering (SAXS) coupled with Density from Solution Scattering (DENSS) algorithm to characterize the internal architecture of messenger RNA-containing lipid nanoparticles (mRNA-LNPs).

METHODS

The DENSS algorithm was employed to construct a three-dimensional model of average individual mRNA-LNP. The reconstructed models were cross validated with cryogenic transmission electron microscopy (cryo-TEM), and dynamic light scattering (DLS) to assess size, morphology, and internal structure.

RESULTS

Cryo-TEM and DLS complemented SAXS, revealed a core-shell mRNA-LNP structure with electron-rich mRNA-rich region at the core, surrounded by lipids. The reconstructed model, utilizing the DENSS algorithm, effectively distinguishes mRNA and lipids via electron density mapping. Notably, DENSS accurately models the morphology of the mRNA-LNPs as an ellipsoidal shape with a "bleb" architecture or a two-compartment structure with contrasting electron densities, corresponding to mRNA-filled and empty lipid compartments, respectively. Finally, subtle changes in the LNP structure after three freeze-thaw cycles were detected by SAXS, demonstrating an increase in radius of gyration (Rg) associated with mRNA leakage.

CONCLUSION

Analyzing SAXS profiles based on DENSS algorithm to yield a reconstructed electron density based three-dimensional model can be a useful physicochemical characterization method in the toolbox to study mRNA-LNPs and facilitate their development.

Antibody-Conjugated Polymersomes with Encapsulated Indocyanine Green J-Aggregates and High Near-Infrared Absorption for Molecular Photoacoustic Cancer Imaging

Imaging plays a critical role in all stages of cancer care from early detection to diagnosis, prognosis, and therapy monitoring. Recently, photoacoustic imaging (PAI) has started to emerge into the clinical realm due to its high sensitivity and ability to penetrate tissues up to several centimeters deep. Herein, we encapsulated indocyanine green J (ICGJ) aggregate, one of the only FDA-approved organic exogenous contrast agents that absorbs in the near-infrared range, at high loadings up to ∼40% w/w within biodegradable polymersomes (ICGJ-Ps) composed of poly(lactide-co-glycolide-b-polyethylene glycol) (PLGA-b-PEG). The small Ps hydrodynamic diameter of 80 nm is advantageous for in vivo applications, while directional conjugation with epidermal growth factor receptor (EGFR) targeting cetuximab antibodies renders molecular specificity. Even when exposed to serum, the ∼11 nm-thick membrane of the Ps prevents dissociation of the encapsulated ICGJ for at least 48 h with a high ratio of ICGJ to monomeric ICG absorbances (i.e., I895/I780 ratio) of approximately 5.0 that enables generation of a strong NIR photoacoustic (PA) signal. The PA signal of polymersome-labeled breast cancer cells is proportional to the level of cellular EGFR expression, indicating the feasibility of molecular PAI with antibody-conjugated ICGJ-Ps. Furthermore, the labeled cells were successfully detected with PAI in highly turbid tissue-mimicking phantoms up to a depth of 5 mm with the PA signal proportional to the amount of cells. These data show the potential of molecular PAI with ICGJ-Ps for clinical applications such as tumor margin detection, evaluation of lymph nodes for the presence of micrometastasis, and laparoscopic imaging procedures.

Characterizing Protein-Protein Interactions and Viscosity of a Monoclonal Antibody from Low to High Concentration Using Small-Angle X-ray Scattering and Molecular Dynamics Simulations

Understanding protein-protein interactions and formation of reversible oligomers (clusters) in concentrated monoclonal antibody (mAb) solutions is necessary for designing stable, low viscosity (η) concentrated formulations for processing and subcutaneous injection. Here we characterize the strength (K) of short-range anisotropic attractions (SRA) for 75-200 mg/mL mAb2 solutions at different pH and cosolute conditions by analyzing structure factors (Seff(q)) from small-angle X-ray scattering (SAXS) using coarse-grained molecular dynamics simulations. Best fit simulations additionally provide cluster size distributions, fractal dimensions, cluster occluded volume, and mAb coordination numbers. These equilibrium properties are utilized in a model to account for increases in viscosity caused by occluded volume in the clusters (packing effects) and dissipation of stress across lubricated fractal clusters. Seff(q) is highly sensitive to K at 75 mg/mL where mAbs can mutually align to form SRA contacts but becomes less sensitive at 200 mg/mL as steric repulsion due to packing becomes dominant. In contrast, η at 200 mg/mL is highly sensitive to SRA and the average cluster size from SAXS/simulation, which is observed to track the cluster relaxation time from shear thinning. By analyzing the distribution of sub-bead hot spots on the 3D mAb surface, we identify a strongly attractive hydrophobic patch in the complementarity determining region (CDR) at pH 4.5 that contributes to the high K and consequently large cluster sizes and high η. Adding NaCl screens electrostatic interactions and increases the impact of hydrophobic attraction on cluster size and raises η, whereas nonspecific binding of Arg attenuates all SRA, reducing η. The hydrophobic patch is absent at higher pH values, leading to smaller K, smaller clusters, and lower η. This work constitutes a first attempt to use SAXS and CG modeling to link both structural and rheological properties of concentrated mAb solutions to the energetics of specific hydrophobic patches on mAb surfaces. As such, our work opens an avenue for future research, including the possibility of designing coarse-grained models with physically meaningful interacting hot spots.

Subclass Effects on Self-Association and Viscosity of Monoclonal Antibodies at High Concentrations

The effects of a subclass of monoclonal antibodies (mAbs) on protein-protein interactions, formation of reversible oligomers (clusters), and viscosity (η) are not well understood at high concentrations. Herein, we quantify a short-range anisotropic attraction between the complementarity-determining region (CDR) and CH3 domains (K_CDR-CH3) for vedolizumab IgG1, IgG2, or IgG4 subclasses by fitting small-angle X-ray scattering (SAXS) structure factor S_eff(q) data with an extensive library of 12-bead coarse-grained (CG) molecular dynamics simulations. The K_CDR-CH3 bead attraction strength was isolated from the strength of long-range electrostatic repulsion for the full mAb, which was determined from the theoretical net charge and a scaling parameter ψ to account for solvent accessibility and ion pairing. At low ionic strength (IS), the strongest short-range attraction (K_CDR-CH3) and consequently the largest clusters and highest η were observed with IgG1, the subclass with the most positively charged CH3 domain. Furthermore, the trend in K_CDR-CH3 with the subclass followed the electrostatic interaction energy between the CDR and CH3 regions calculated with the BioLuminate software using the 3D mAb structure and molecular interaction potentials. Whereas the equilibrium cluster size distributions and fractal dimensions were determined from fits of SAXS with the MD simulations, the degree of cluster rigidity under flow was estimated from the experimental η with a phenomenological model. For the systems with the largest clusters, especially IgG1, the inefficient packing of mAbs in the clusters played the largest role in increasing η, whereas for other systems, the relative contribution from stress produced by the clusters was more significant. The ability to relate η to short-range attraction from SAXS measurements at high concentrations and to theoretical characterization of electrostatic patches on the 3D surface is not only of fundamental interest but also of practical value for mAb discovery, processing, formulation, and subcutaneous delivery.

Characterizing Experimental Monoclonal Antibody Interactions and Clustering Using a Coarse-Grained Simulation Library and a Viscosity Model

Attractive protein-protein interactions in concentrated monoclonal antibody (mAb) solutions may lead to the formation of clusters that increase viscosity. Here, we propose an analytical model that relates mAb solution viscosity to clustering by accounting for the contributions of suboptimal mAb packing within a cluster and cluster fractal dimension. The influence of short-range, anisotropic attractions and long-range Coulombic repulsion on cluster properties is investigated by analyzing the cluster-size distributions, cluster fractal dimensions, radial distribution functions, and static structure factors from a library of coarse-grained molecular dynamics simulations. The library spans a vast range of mAb charges and attractive interactions in solutions of varying ionic strength. We present a framework for combining the viscosity model and simulation library to successfully characterize the attraction, repulsion, and clustering of an experimental mAb in three different pH and cosolute conditions by fitting the measured viscosity or structure factor from small-angle X-ray scattering. At low ionic strength, the cluster-size distribution is impacted by strong charges, and both the viscosity and net charge or structure factor and net charge must be considered to deconvolute the effects of short-range attraction and long-range repulsion.

Ultrastable N2/Water Foams Stabilized by Dilute Nanoparticles and a Surfactant at High Salinity and High Pressure

The rapid development of unconventional oil and gas resources presents challenges for foam flooding for reservoirs with high salinity and high heterogeneity at elevated temperatures. In this study, hydrophilic anionic sulfonate-modified nanoparticles (NPs) exhibited a synergistic effect with a cationic surfactant in stabilizing N₂/water foam in the presence of concentrated divalent ions from ambient temperature up to 70 °C. With low concentrations of both the sulfonated NPs (SNPs) and cationic surfactant, the foams remained stable for 4 days at 50 °C and atmospheric pressure, while the surfactant-stabilized foams collapsed completely in 1 day. This stability mechanism of foams by the SNPs and cationic surfactant is described in terms of phase behavior, bulk shear rheology of the aqueous phase, and the dilational modulus of the gas-brine interface. The high surface elastic dilational modulus E' observed upon addition of the SNP provided stability against coarsening according to the Gibbs criteria. The cryo-SEM images also showed the compact bubble structure of foams provided by the SNPs. Consequently, very minor changes in the foam bubble size were observed at 208 bar (3000 psi) and 50 °C for up to 48 h with only 0.1 wt % or 0.3 wt % SNPs and 0.01 wt % Arquad 12-50, indicating excellent foam stability. The ability of the surfactant and NPs to stabilize foams at low concentrations broadens the application of foams in subsurface reservoirs at high temperatures and salinities.

Elastic gas/water interface for highly stable foams with modified anionic silica nanoparticles and a like-charged surfactant

HYPOTHESIS

Surface active anionic nanoparticles (NPs) with strategically designed covalent ligands may be combined with a liked-charged surfactant to form a highly elastic gas-water interface leading to highly stable gas/water foams.

EXPERIMENTS

The colloidal stability of the NPs was determined by dynamic light scattering, and the surface elastic dilational modulus E' of the interface by sinusoidal oscillation of a pendant droplet at 0.1 Hz, which was superimposed on large-amplitude compression-expansion cycles. The foam stability was measured with optical microscopy of the bubble size distribution and from the macroscopic foam height.

FINDINGS

The NPs played the key role the formation of a highly elastic air-water interface with a high E' despite a surfactant level well above the critical micelle concentration. Unlike the case for most previous studies, the NP amphiphilicity was essentially independent of the surfactant given the very low adsorption of the surfactant on the like-charged NP surfaces. With high E' values, both coalescence and coarsening were reduced leading to highly foam up to 80 °C. However, the surfactant facilitated foam generation at much lower shear rates than with NPs alone. The tuning of NP surfaces with ligands for colloidal stability in brine and simultaneously high amphiphilicity at the gas-water interface, over a wide range in surfactant concentration, is of broad interest for enabling the design of highly stable foams.

Highly Elastic Interconnected Porous Hydrogels through Self-Assembled Templating for Solar Water Purification

Interfacial evaporation using porous hydrogels has demonstrated highly effective solar evaporation performance under natural sunlight to ensure an affordable clean water supply. However, it remains challenging to realize scalable and ready-to-use hydrogel materials with durable mechanical properties. Here, self-assembled templating (SAT) is developed as a simple yet effective method to fabricate large-scale elastic hydrogel evaporators with excellent desalination performance. The highly interconnected porous structure of the hydrogels with low tortuosity and tunable pore size enables high level of tunability on the water transport rate. With superior elasticity, the porous hydrogels are easy to process with a rapid shape recovery after being rolled, folded, and twisted over hundred times, and exhibit highly effective and stable evaporation with an evaporation rate of ≈2.8 kg m^-2  h^-1 and ≈90 % solar-to-vapor efficiency. It is anticipated that this SAT strategy, without the typical need for freeze-drying, will accelerate the industrialization of hydrogel solar evaporators for practical applications.

Molecular Engineering of Hydrogels for Rapid Water Disinfection and Sustainable Solar Vapor Generation

Consumption of unsafe water is a major cause of morbidity and mortality in developing regions. Pasteurizing or boiling water to remove pathogens is energy-intensive and often impractical to off-grid communities. Therefore, low capital cost, rapid and energy-efficient water disinfection methods are urgently required to address global challenges of safe water access. Here, anti-bacterial hydrogels (ABHs) with catechol-enabled molecular-level hydrogen peroxide generators and quinone-anchored activated carbon particles are designed for effective water treatment. The bactericidal effect is attributed to the synergy of hydrogen peroxide and quinone groups to attack essential cell components and disturb bacterial metabolism. ABHs can be directly used as tablets to achieve >99.999% water disinfection efficiency within 60 min without energy input. No harmful byproducts are formed during the treatment process, after which the ABH tablets can be easily removed without residues. Taking advantage of their excellent photothermal and biofouling-resistant properties, ABHs can also be applied as solar evaporators to achieve stable water purification under sunlight (≤1 kW m^-2 ) after months of storage and operation in bacteria-containing river water. The ABH platform offers reduced energy and chemical demands for point-of-use water treatment technologies in remote areas and emergency rescue applications.

Tuning Surface Chemistry and Ionic Strength to Control Nanoparticle Adsorption and Elastic Dilational Modulus at Air-Brine Interface

The relationship between the interfacial rheology of nanoparticle (NP) laden air-brine interfaces and NP adsorption and interparticle interactions is not well understood, particularly as a function of the surface chemistry and salinity. Herein, a nonionic ether diol on the surface of silica NPs provides steric stabilization in bulk brine and at the air-brine interface, whereas a second smaller underlying hydrophobic ligand raises the hydrophobicity to promote NP adsorption. The level of NPs adsorption at steady state is sufficient to produce an interface with a relatively strong elastic dilational modulus E' = dγ/d ln A. However, the interface is ductile with a relatively slow change in E' as the interfacial area is varied over a wide range during compression and expansion. In contrast, for silica NPs stabilized with only a single hydrophobic ligand, the interfaces are often more fragile and may fracture with small changes in area. The presence of concentrated divalent cations improves E' and ductility by screening electrostatic dipolar repulsion and strengthening the attractive forces between nanoparticles. The ability to tune the interfacial rheology with NP surface chemistry is of great interest for designing more stable gas/brine foams.

Tuning Nanoparticle Surface Chemistry and Interfacial Properties for Highly Stable Nitrogen-In-Brine Foams

The design of surface chemistries on nanoparticles (NPs) to stabilize gas/brine foams with concentrated electrolytes, especially with divalent ions, has been elusive. Herein, we tune the surface of 20 nm silica NPs by grafting a hydrophilic and a hydrophobic ligand to achieve two seemingly contradictory goals of colloidal stability in brine and high NP adsorption to yield a viscoelastic gas-brine interface. Highly stable nitrogen/water (N₂/brine) foams are formed with CaCl₂ concentrations up to 2% from 25 to 90 °C. The viscoelastic gas-brine interface retards drainage of the lamellae, and the high dilational elasticity arrests coarsening (Ostwald ripening) with no observable change in foam bubble size over 48 h. The ability to design NP-laden viscoelastic interfaces for highly stable foams, even with high divalent ion concentrations, is of fundamental mechanistic interest for a broad range of foam applications and in particular foams for CO₂ sequestration and enhanced oil recovery.

Effect of surface chemistry of silica nanoparticles on contact angle of oil on calcite surfaces in concentrated brine with divalent ions

HYPOTHESIS

For an oil droplet on calcite with an intervening brine film, the water contact angle θ_w may be reduced markedly (greater water wetness) with surface modified silica nanoparticles (NP). Modification with cationic, anionic, and nonionic ligands may be used to control the nanoparticle adsorption and interactions at the oil-brine and brine-calcite interfaces to influence the rate and degree of reduction in θ_w.

EXPERIMENTS

The colloidal stability at 25 °C was determined in concentrated divalent brine (8 wt% NaCl and 2 wt% CaCl₂) with dynamic light scattering, and the NP adsorption was determined on calcite. The NP adsorption at the oil-brine interface was characterized with the elastic dilational modulus. θ_w was measured for model decane-stearic acid droplets and crude oil droplets on calcite from 25 to 80 °C.

FINDINGS

The fastest rate and greatest extent of reduction in θ_w for grafted ligands followed the order: cationic quaternary trimethylamine > sulfonate > methyl phosphonate > gluconamide. New mechanisms for reduction in θ_w were demonstrated on the basis of changes in interactions from NP adsorption at each interface. The greatest efficacy for the cationic NPs results from the weakest adsorption on calcite, steric repulsion at the three-phase contact line and the greatest desorption of carboxylate surfactants from the calcite.

Coarse-Grained Molecular Dynamics Simulations for Understanding the Impact of Short-Range Anisotropic Attractions on Structure and Viscosity of Concentrated Monoclonal Antibody Solutions

Understanding protein-protein interactions in concentrated therapeutic monoclonal antibody (mAb) solutions is desirable for improved drug discovery, processing, and administration. Here, we deduce both the net protein charge and the magnitude and geometry of short-ranged, anisotropic attractions of a mAb across multiple concentrations and cosolute conditions by comparing structure factors S(q) obtained from small-angle X-ray scattering experiments with those from molecular dynamics (MD) simulations. The simulations, which utilize coarse-grained 12-bead models exhibiting a uniform van der Waals attraction, uniform electrostatic repulsion, and short-range attractions between specific beads, are versatile enough to fit S(q) of a wide range of protein concentrations and ionic strength with the same charge on each bead and a single anisotropic short-range attraction strength. Cluster size distributions (CSDs) obtained from best fit simulations reveal that the experimental structure is consistent with small reversible oligomers in even low viscosity systems and help quantify the impact of these clusters on viscosity. The ability to systematically use experimental S(q) data together with MD simulations to discriminate between different possible protein-protein interactions, as well as to predict viscosities from protein CSDs, is beneficial for designing mAbs and developing formulation strategies that avoid high viscosities and aggregation at high concentration.

Indocyanine Green J Aggregates in Polymersomes for Near-Infrared Photoacoustic Imaging

Clinical translation of photoacoustic imaging (PAI) has been limited by the lack of near-infrared (NIR) contrast agents with low toxicity required for regulatory approval. Herein, J aggregates of indocyanine green (ICG) with strong NIR absorbance were encapsulated at high loadings within small 77 nm polymersomes (nanocapsules) composed of poly(lactide-co-glycolide-b-poly(ethylene glycol)) (PLGA-b-PEG) bilayers, thus enabling PAI of of breast and ovarian cancer cells with high specificity and a sensitivity at the level of ∼100 total cells. All of the major components of the polymersomes are FDA approved and used in the clinic. During formation of polymersomes with a water-in-oil-in-water double emulsion process, loss of ICG from the ICG J aggregates was minimized by coating them with a layer of branched polyethylenimine and by providing excess "sacrificial" ICG to adsorb at the oil-water interfaces. The encapsulated J aggregates were protected against dissociation by the polymersome shell for 24 h in 100% fetal bovine serum, after which the polymersomes biodegraded and the J aggregates dissociated to ICG monomers.

Self-diffusion of a highly concentrated monoclonal antibody by fluorescence correlation spectroscopy: insight into protein-protein interactions and self-association

The dynamic behavior of monoclonal antibodies (mAbs) at high concentration provides insight into protein microstructure and protein-protein interactions (PPI) that influence solution viscosity and protein stability. At high concentration, interpretation of the collective-diffusion coefficient Dc, as determined by dynamic light scattering (DLS), is highly challenging given the complex hydrodynamics and PPI at close spacings. In contrast, self-diffusion of a tracer particle by Brownian motion is simpler to understand. Herein, we develop fluorescence correlation spectroscopy (FCS) for the measurement of the long-time self-diffusion of mAb2 over a wide range of concentrations and viscosities in multiple co-solute formulations with varying PPI. The normalized self-diffusion coefficient D0/Ds (equal to the microscopic relative viscosity ηeff/η0) was found to be smaller than η/η0. Smaller ratios of the microscopic to macroscopic viscosity (ηeff/η) are attributed to a combination of weaker PPI and less self-association. The interaction parameters extracted from fits of D0/Ds with a length scale dependent viscosity model agree with previous measurements of PPI by SLS and SAXS. Trends in the degree of self-association, estimated from ηeff/η with a microviscosity model, are consistent with oligomer sizes measured by SLS. Finally, measurements of collective diffusion and osmotic compressibility were combined with FCS data to demonstrate that the changes in self-diffusion between formulations are due primarily to changes in the protein-protein friction in these systems, and not to protein-solvent friction. Thus, FCS is a robust and accessible technique for measuring mAb self-diffusion, and, by extension, microviscosity, PPI and self-association that govern mAb solution dynamics.

Decoupling the roles of carbon and metal oxides on the electrocatalytic reduction of oxygen on La1-xSrxCoO3-δ perovskite composite electrodes

Perovskite oxides are active room-temperature bifunctional oxygen electrocatalysts in alkaline media, capable of performing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with lower combined overpotentials relative to their precious metal counterparts. However, their semiconducting nature necessitates the use of activated carbons as conductive supports to generate applicably relevant current densities. In efforts to advance the performance and theory of oxide electrocatalysts, the chemical and physical properties of the oxide material often take precedence over contributions from the conductive additive. In this work, we find that carbon plays an important synergistic role in improving the performance of La1-xSrxCoO3-δ (0 ≤ x ≤ 1) electrocatalysts through the activation of O2 and spillover of radical oxygen intermediates, HO2- and O2-, which is further reduced through chemical decomposition of HO2- on the perovskite surface. Through a combination of thin-film rotating disk electrochemical characterization of the hydrogen peroxide intermediate reactions (hydrogen peroxide reduction reaction (HPRR), hydrogen peroxide oxidation reaction (HPOR)) and oxygen reduction reaction (ORR), surface chemical analysis, HR-TEM, and microkinetic modeling on La1-xSrxCoO3-δ (0 ≤ x ≤ 1)/carbon (with nitrogen and non-nitrogen doped carbons) composite electrocatalysts, we deconvolute the mechanistic aspects and contributions to reactivity of the oxide and carbon support.

Anion-Based Pseudocapacitance of the Perovskite Library La1- xSr xBO3-δ (B = Fe, Mn, Co)

We have synthesized a library of perovskite oxides with the composition La_{1- x}Sr _xBO_3-δ ( x = 0-1; B = Fe, Mn, Co) to systematically study anion-based pseudocapacitance. The electrochemical capacitance of these materials was evaluated by cyclic voltammetry and galvanostatic charging/discharging in 1 M KOH. We find that greater oxygen vacancy content (δ) upon systematic incorporation of Sr²⁺ linearly increases the surface-normalized capacity with a slope controlled by the B-site element. La_0.2Sr_0.8MnO_2.7 exhibited the highest specific capacitance of 492 F g^-1 at 5 mV s^-1 relative to the Fe and Co oxides. In addition, the first all-perovskite asymmetric pseudocapacitor has been successfully constructed and characterized in neutral and alkaline aqueous electrolytes. We demonstrate that the asymmetric pseudocapacitor cell voltage can be increased by widening the difference between the B-site transition metal redox potentials in each electrode resulting in a maximum voltage window of 2.0 V in 1 M KOH. Among the three pairs of asymmetric pseudocapacitors constructed from SrCoO_2.7, La_0.2Sr_0.8MnO_2.7, and brownmillerite (BM)-Sr₂Fe₂O₅, the BM-Sr₂Fe₂O₅//SrCoO_2.7 combination performed the best with a high energy density of 31 Wh kg^-1 at 450 W kg^-1 and power density of 10 000 W kg^-1 at 28 Wh kg^-1.

Protein-Protein Interactions of Highly Concentrated Monoclonal Antibody Solutions via Static Light Scattering and Influence on the Viscosity

The ability to design and formulate mAbs to minimize attractive interactions at high concentrations is important for protein processing, stability, and administration, particularly in subcutaneous delivery, where high viscosities are often challenging. The strength of protein-protein interactions (PPIs) of an IgG1 and IgG4 monoclonal antibody (mAb) from low to high concentration was determined by static light scattering (SLS) and used to understand viscosity data. The PPI were tuned using NaCl and five organic ionic co-solutes. The PPI strength was quantified by the normalized structure factor S(0)/ S(0)_HS and Kirkwood-Buff integral G₂₂/ G_22,HS (HS = hard sphere) determined from the SLS data and also by fits with (1) a spherical Yukawa potential and (2) an interacting hard sphere (IHS) model, which describes attraction in terms of hypothetical oligomers. The IHS model was better able to capture the scattering behavior of the more strongly interacting systems (mAb and/or co-solute) than the spherical Yukawa potential. For each descriptor of PPI, linear correlations were obtained between the viscosity at high concentration (200 mg/mL) and the interaction strengths evaluated both at low (20 mg/mL) and high concentrations (200 mg/mL) for a given mAb. However, the only parameter that provided a correlation across both mAbs was the oligomer mass ratio ( m_oligomer/ m_{monomer+dimer}) from the IHS model, indicating the importance of self-association (in addition to the direct influence of the attractive PPI) on the viscosity.

Exceptional electrocatalytic oxygen evolution via tunable charge transfer interactions in La0.5Sr1.5Ni1-xFexO4±δ Ruddlesden-Popper oxides

The electrolysis of water is of global importance to store renewable energy and the methodical design of next-generation oxygen evolution catalysts requires a greater understanding of the structural and electronic contributions that give rise to increased activities. Herein, we report a series of Ruddlesden–Popper La_0.5Sr_1.5Ni_1−xFe_xO_4±δ oxides that promote charge transfer via cross-gap hybridization to enhance electrocatalytic water splitting. Using selective substitution of lanthanum with strontium and nickel with iron to tune the extent to which transition metal and oxygen valence bands hybridize, we demonstrate remarkable catalytic activity of 10 mA cm⁻² at a 360 mV overpotential and mass activity of 1930 mA mg⁻¹_ox at 1.63 V via a mechanism that utilizes lattice oxygen. This work demonstrates that Ruddlesden–Popper materials can be utilized as active catalysts for oxygen evolution through rational design of structural and electronic configurations that are unattainable in many other crystalline metal oxide phases.

Water electrolysis provides a potential means to large-scale renewable fuel generation, although sluggish oxygen evolution kinetics challenges progress. Here, authors report on Ruddlesden–Popper oxides as active oxygen evolution electrocatalysts that provide impetus for overcoming kinetic barriers.

Aqueous Superparamagnetic Magnetite Dispersions with Ultrahigh Initial Magnetic Susceptibilities

Superparamagnetic nanoparticles with a high initial magnetic susceptibility χ_o are of great interest in a wide variety of chemical, biomedical, electronic, and subsurface energy applications. In order to achieve the theoretically predicted increase in χ_o with the cube of the magnetic diameter, new synthetic techniques are needed to control the crystal structure, particularly for magnetite nanoparticles larger than 10 nm. Aqueous magnetite dispersions (Fe₃O₄) with a χ_o of 3.3 (dimensionless SI units) at 1.9 vol %, over 3- to 5-fold greater than those reported previously, were produced in a one-pot synthesis at 210 °C and ambient pressure via thermal decomposition of Fe(II) acetate in triethylene glycol (TEG). The rapid nucleation and focused growth with an unusually high precursor-to-solvent molar ratio of 1:12 led to primary particles with a volume average diameter of 16 nm and low polydispersity according to TEM. The morphology was a mixture of stoichiometric and substoichiometric magnetite according to X-ray diffraction (XRD) and Mössbauer spectroscopy. The increase in χ_o with the cube of magnetic diameter as well as a saturation magnetization approaching the theoretical limit may be attributed to the highly crystalline structure and very small nonmagnetic layer (∼1 nm) with disordered spin orientation on the surface.

Reversible Self-Assembly of Glutathione-Coated Gold Nanoparticle Clusters via pH-Tunable Interactions

Nanoparticle (NP) clusters with diameters ranging from 20 to 100 nm are reversibly assembled from 5 nm gold (Au) primary particles coated with glutathione (GSH) in aqueous solution as a function of pH in the range of 5.4 to 3.8. As the pH is lowered, the GSH surface ligands become partially zwitterionic and form interparticle hydrogen bonds that drive the self-limited assembly of metastable clusters in <1 min. Whereas clusters up to 20 nm in size are stable against cluster-cluster aggregation for up to 1 day, clusters up to 80 nm in size can be stabilized over this period via the addition of citrate to the solution in equal molarity with GSH molecules. The cluster diameter may be cycled reversibly by tuning pH to manipulate the colloidal interactions; however, modest background cluster-cluster aggregation occurs during cycling. Cluster sizes can be stabilized for at least 1 month via the addition of PEG-thiol as a grafted steric stabilizer, where PEG-grafted clusters dissociate back to starting primary NPs at pH 7 in fewer than 3 days. Whereas the presence of excess citrate has little effect on the initial size of the metastable clusters, it is necessary for both the cycling and dissociation to mediate the GSH-GSH hydrogen bonds. In summary, these metastable clusters exhibit significant characteristics of equilibrium self-limited assembly between primary particles and clusters on time scales where cluster-cluster aggregation is not present.

Charge Shielding Prevents Aggregation of Supercharged GFP Variants at High Protein Concentration

Understanding protein stability is central to combatting protein aggregation diseases and developing new protein therapeutics. At the high concentrations often present in biological systems, purified proteins can exhibit undesirable high solution viscosities and poor solubilities mediated by short-range electrostatic and hydrophobic protein-protein interactions. The interplay between protein amino acid sequence, protein structure, and solvent conditions to minimize protein-protein interactions is key to designing well-behaved pharmaceutical proteins. However, theoretical approaches have yet to yield a general framework to address these problems. Here, we analyzed the high concentration behavior of superfolder GFP (sfGFP) and two supercharged sfGFP variants engineered to have formal charges of -18 or +15. Under low cosolute conditions, sfGFP and the -18 variant formed a gel or phase separated at ∼10 mg/mL. Under conditions that screen surface charges, including formulations with high histidine or high NaCl concentrations, all three variants attained concentrations up to 250 mg/mL with moderate viscosities. Moreover, all three variants exhibited very similar viscosity-concentration profiles over this range. This effect was not mimicked by high sugar concentrations that exert excluded-volume effects without shielding charge. Collectively, these data demonstrate that charge shielding neutralizes not only long-range electrostatic interactions but also, surprisingly, short-range electrostatic effects due to surface charge anisotropy. This work shows that supercharged sfGFP behavior under high ionic strength is largely determined by particle geometry, a conclusion that is supported by colloid models and may be applicable to pharmaceutically relevant proteins.

Control of Primary Particle Spacing in Gold Nanoparticle Clusters for Both High NIR Extinction and Full Reversibility

Reversible NIR-active nanoparticle clusters with controlled size from 20 to 100 nm were assembled from 5 nm gold nanoparticles (Au NP), with either citrate (CIT) or various binary ligands on the surface, by tuning the electrostatic repulsion and the hydrogen bonding via pH. The nanoclusters were bound together by vdW forces between the cores and the hydrogen bonds between the surface ligands and dissociated to primary nanoparticles over a period of 20 days at pH 5 and at pH 7. When high levels of citrate ligands were used on the primary particle surfaces, the large particle spacings in the nanoclusters led to only modest NIR extinction. However, a NIR extinction (E_1000/525) ratio of up to ∼0.4 was obtained for nanoclusters with binary ligand mixtures composed of citrate and either cysteine (CYS), glutathione (GSH), or thioctic acid zwitterion (TAZ) while maintaining full reversibility to primary particles. The optimum ligand ratio for both an E_1000/525 of ∼0.4 and full reversibility decreased with increasing length of the secondary ligand (1.5/1 for CYS/CIT, 0.75/1 for GSH/CIT, and 0.5/1 for TAZ/CIT) because a longer secondary ligand maintains a sufficient interparticle spacing required for dissociation more effectively. Interestingly, the zeta potential and the first-order rate constant for nanocluster dissociation were similar for all three systems at the optimum ligand ratios. After incubation in 10 mM GSH solution (intracellular concentration), only the TAZ/CIT primary nanoparticles were resistant to protein opsonization in 100% fetal bovine serum, as the bidentate binding and zwitterion tips of TAZ resisted GSH exchange and protein opsonization, respectively.

Static Adsorption of an Ethoxylated Nonionic Surfactant on Carbonate Minerals

The static adsorption of C_12-14E₂₂, which is a highly ethoxylated nonionic surfactant, was studied on different minerals using high-performance liquid chromatography (HPLC) combined with an evaporative light scattering detector (ELSD). Of particular interest is the surfactant adsorption in the presence of CO₂ because it can be used for foam flooding in enhanced oil recovery applications. The effects of the mineral type, impurities, salinity, and temperature were investigated. The adsorption of C_12-14E₂₂ on pure calcite was as low as 0.01 mg/m² but higher on dolomite depending on the silica and clay content in the mineral. The adsorption remained unchanged when the experiments were performed using a brine solution or 0.101 MPa (1 atm) CO₂, which indicates that electrostatic force is not the governing factor that drives the adsorption. The adsorption of C_12-14E₂₂ on silica may be due to hydrogen bonding between the oxygen in the ethoxy groups of the surfactant and the hydroxyl groups on the mineral surface. Additionally, thermal decomposition of the surfactant was severe at 80 °C but can be inhibited by operating in a reducing environment. Under reducing conditions, adsorption of C_12-14E₂₂ increased at higher temperatures.

Steric stabilization of nanoparticles with grafted low molecular weight ligands in highly concentrated brines including divalent ions

Whereas numerous studies of stabilization of nanoparticles (NPs) in electrolytes have examined biological fluids, the interest has grown recently in media with much higher ionic strengths including seawater and brines relevant to environmental science and subsurface oil and gas reservoirs. Given that electrostatic repulsion is limited at extremely high ionic strengths due to charge screening, we have identified ligands that are well solvated in concentrated brine containing divalent cations and thus provide steric stabilization of silica nanoparticles. Specifically, the hydrodynamic diameter of silica nanoparticles with grafted low molecular weight ligands, a diol ether, [3-(2,3-dihydroxypropoxy)propyl]-trimethoxysilane, and a zwitterionic sulfobetaine, 3-([dimethyl(3-trimethoxysilyl)propyl]ammonio)propane-1-sulfonate, is shown with dynamic light scattering to remain essentially constant, indicating lack of aggregation, at room temperature and up to 80 °C for over 30 days. An extended DLVO model signifies that steric stabilization is strongly dominant against van der Waals attraction for ∼10 nm particles given that these ligands are well solvated even in highly concentrated brine. In contrast, polyethylene glycol oligomers do not provide steric stabilization at elevated temperatures, even at conditions where the ligands are soluble, indicating complicating factors including bridging of the ether oxygens by divalent cations.

Formation of Small Gold Nanoparticle Chains with High NIR Extinction through Bridging with Calcium Ions

The self-assembly of citrate-capped Au nanoparticles (5 nm) resulted in branched nanochains by adding CaCl2 versus spherical nanoclusters for NaCl. These assemblies were formed between 1 s to 30 min by tuning the electrostatic repulsion and the interparticle bridging attraction between the cations and citrate ligands as a function of electrolyte concentration. For dilute Ca(2+), strong interparticle bridging favored particle attachment at chain ends. This resulted in the formation of small, branched chains with lengths as short as 20 nm, due to the large Debye length for the diffuse counterions. Furthermore, the bridging produced very small interparticle spacings and sintering, as evident in high-resolution TEM despite the low temperature. This morphology produced a large red shift in the surface plasmon resonance, as characterized by a broad extinction peak with NIR absorption out to 1000 nm, which is unusual for such small particles. Whereas these properties were seen for primary particles with partial citrate monolayers, the degrees of sintering and NIR extinction were small in the case of citrate multilayers. The ability to design the size and shape of nanoparticle clusters as well as the interparticle spacing by tuning bridging and electrostatic interactions may be expected to be quite general and of broad applicability in materials synthesis.

Ultradry Carbon Dioxide-in-Water Foams with Viscoelastic Aqueous Phases

For foams with ultra low water contents, the capillary pressure is very large and induces rapid drainage that destabilizes the aqueous lamellae between the gas bubbles. However, we show that high-pressure CO2-in-water foams can be stabilized with a viscoelastic aqueous phase composed of entangled wormlike micelles, even for extremely high CO2 volume fractions ϕ of 0.95 to 0.98; the viscosity of these ultradry foams increased by up to 3-4-fold, reaching more than 100 cP relative to foams formed with conventional low viscosity aqueous phases. The foam morphology consisted of fine ∼20 μm polyhedral-shaped CO2 bubbles that were stable for hours. The wormlike micelles were formed by mixing anionic sodium lauryl ether sulfate (SLES) with salt and a protonated cationic surfactant, as shown by cryogenic transmission electron microscopy (cryo-TEM) and large values of the zero-shear viscosity and the dynamic storage and loss moduli. With the highly viscous continuous aqueous phases, the foam lamella drainage rates were low, as corroborated by confocal microscopy. The preservation of viscous thick lamellae resulted in lower rates of Ostwald ripening relative to conventional foams as shown by high-pressure optical microscopy. The ability to stabilize viscous ultra high internal phase foams is expected to find utility in various practical applications, including nearly "waterless" fracturing fluids for recovery of oil and gas in shale, offering the possibility of a massive reduction in the amount of wastewater.

Origin and detection of microstructural clustering in fluids with spatial-range competitive interactions

Fluids with competing short-range attractions and long-range repulsions mimic dispersions of charge-stabilized colloids that can display equilibrium structures with intermediate-range order (IRO), including particle clusters. Using simulations and analytical theory, we demonstrate how to detect cluster formation in such systems from the static structure factor and elucidate links to macrophase separation in purely attractive reference fluids. We find that clusters emerge when the thermal correlation length encoded in the IRO peak of the structure factor exceeds the characteristic length scale of interparticle repulsions. We also identify qualitative differences between the dynamics of systems that form amorphous versus microcrystalline clusters.