Publications

2014
K. P. Johnston, Mazuski, M. A., Engstrom, J., and Rodrigues, M. A., “Low viscosity highly concentrated suspensions”. US Patent 8,779,094, 2014. Publisher's VersionAbstract
The present invention also provides a high concentration low viscosity suspension of an pharmaceutically acceptable solvent with one or more sub-micron or micron-sized non-crystalline particles comprising one or more proteins or peptides. Optionally one or more additives in the pharmaceutically acceptable solvent to form a high concentration low viscosity suspension with a concentration of at least 20 mg/ml and a solution viscosity of between 2 and 100 centipoise that is suspendable upon shaking or agitation, wherein upon delivery the one or more sub-micron or micron-sized peptides dissolves and do not form peptide aggregates syringeable through a 21 to 27-gauge needle.
J. Dong, Worthen, A. J., Foster, L. M., Chen, Y., Cornell, K. A., Bryant, S. L., Truskett, T. M., Bielawski, C. W., and Johnston, K. P., “Modified montmorillonite clay microparticles for stable oil-in-seawater emulsions,” ACS applied materials & interfaces, vol. 6, no. 14, pp. 11502-11513, 2014. Publisher's VersionAbstract
Environmentally benign clay particles are of great interest for the stabilization of Pickering emulsions. Dodecane-in-synthetic seawater (SSW) emulsions formed with montmorillonite (MMT) clay microparticles modified with bis(2-hydroxyethyl)oleylamine were stable against coalescence, even at clay concentrations down to 0.1% w/v. Remarkably, as little as 0.001% w/v surfactant lowered the hydrophilicity of the clay to a sufficient level for stabilization of oil-in-SSW emulsions. The favorable effect of SSW on droplet size reduction and emulsion stability enhancement is hypothesized to be due to reduced electrostatic repulsion between adsorbed clay particles and a consequent increase in the continuous phase (an aqueous clay suspension) viscosity. Water/oil (W/O) emulsions were inverted to O/W either by decreasing the mass ratio of surfactant-to-clay (transitional inversion) or by increasing the water volume fraction (catastrophic inversion). For both types of emulsions, coalescence was minimal and the sedimentation or creaming was highly correlated with the droplet size. For catastrophic inversions, the droplet size of the emulsions was smaller in the case of the preferred curvature. Suspensions of concentrated clay in oil dispersions in the presence of surfactant were stable against settling. The mass transfer pathways during emulsification of oil containing the clay particles were analyzed on the droplet size/stability phase diagrams to provide insight for the design of dispersant systems for remediating surface and subsurface oceanic oil spills
R. Stover, Murthy, A., Gourisankar, S., Nie, G., Martinez, M., Truskett, T., Sokolov, K., and Johnston, K., “Plasmonic biodegradable gold nanoclusters with high NIR-absorbance for biomedical imaging,” SPIE BiOS. International Society for Optics and Photonics, pp. 89550T-89550T-7, 2014. Publisher's VersionAbstract
Gold plasmonic nanoparticles are receiving attention for a variety of types of NIR optical biomedical imaging including photoacoustic imaging. Herein we present a novel method to assemble equilibrium gold nanoclusters from 5 nm primary gold nanospheres, which exhibit high near-infrared (NIR) absorbance and subsequently fully dissociate back to primary particles, which has the potential to enable renal clearance. The nanoparticle assembly is manipulated via controlling colloidal interactions, specifically electrostatic repulsion and depletion attraction. The charge on the primary ~5 nm gold nanospheres is tailored via place exchange reactions with a variety of biocompatible ligands such as citrate, lysine and cysteine. The primary particles form clusters upon addition of a biodegradable polymer, PLA(1k)-b- PEG(10k)-b-PLA(1k), followed by controlled solvent evaporation. The cluster size may be tuned from 20-40 nm in diameter by manipulating the gold and polymer concentrations along with the solvent evaporation extent. Salt is also added to increase the NIR absorbance and reduce the nanocluster size by reducing polymer adsorption. The adsorption of the polymer onto the Au surfaces effectively quenches the nanoclusters. High NIR absorption facilitates photoacoustic imaging, even for the small cluster sizes. In response to acidic cellular pH environments, the polymer degrades and the clusters dissociate back to primary particle on the order of 5 nm, which are small enough for renal clearance. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only
R. J. Stover, Murthy, A. K., Nie, G. D., Gourisankar, S., Dear, B. J., Truskett, T. M., Sokolov, K. V., and Johnston, K. P., “Quenched assembly of NIR-active gold nanoclusters capped with strongly bound ligands by tuning particle charge via pH and salinity,” The Journal of Physical Chemistry C, vol. 118, no. 26, pp. 14291-14298, 2014. Publisher's VersionAbstract
Gold nanospheres coated with a binary monolayer of bound citrate and cysteine ligands were assembled into nanoclusters, in which the size and near-infrared (NIR) extinction were tuned by varying the pH and concentration of added NaCl. During full evaporation of an aqueous dispersion of 4.5 ± 1.8 nm Au primary particles, the nanoclusters were formed and quenched by the triblock copolymer polylactic acid (PLA)(1K)-b-poly(ethylene glycol) (PEG)(10K)-b-PLA(1K), which also provided steric stabilization. The short-ranged depletion and van der Waals attractive forces were balanced against longer ranged electrostatic repulsion to tune the nanocluster diameter and NIR extinction. Upon lowering the pH from 7 to 5 at a given salinity, the magnitude of the charge on the primary particles decreased, such that the weaker electrostatic repulsion increased the hydrodynamic diameter and, consequently, NIR extinction of the clusters. At a given pH, as the concentration of NaCl was increased, the NIR extinction decreased monotonically. Furthermore, the greater screening of the charges on the nanoclusters weakened the interactions with PLA(1K)-b-PEG(10K)-b-PLA(1K) and thus lowered the amount of adsorbed polymer on the nanocluster surface. The generalization of the concept of self-assembly of small NIR-active nanoclusters to include a strongly bound thiol and the manipulation of the morphologies and NIR extinction by variation of pH and salinity not only is of fundamental interest but also is important for optical biomedical imaging and therapy
A. S. Elhag, Chen, Y., Chen, H., Reddy, P. P., Cui, L., Worthen, A. J., Ma, K., Hirasaki, G. J., Nguyen, Q. P., and Biswal, S. L., “Switchable amine surfactants for stable CO2/brine foams in high temperature, high salinity reservoirs,” SPE Improved Oil Recovery Symposium. Society of Petroleum Engineers, 2014. Publisher's VersionAbstract
Description of the material. Stable CO2/water (C/W) foams at high temperatures and salinities have been achieved with substituted amines in limestone, sandstone and glass bead packs with permeabilities from 1 to 78 Darcy. Foams were formed upon injection of the CO2 soluble surfactant in the CO2 phase and would be beneficial for improving sweep efficiency in EOR process.Application. Despite significant interest in CO2 foams for EOR, very few studies have reported stable foams at high temperatures (120 °C) and high salinities, which are often encountered in the Middle East and elsewhere. The foams provide mobility control and stabilize the displacement front in CO2flooded zones to improve sweep efficiency.Results, Observations, Conclusions. The amine surfactants are switchable between the nonionic and cationic states with pH or the nature of the solvent. They exhibit nonionic behavior when introduced in the CO2 phase, which favors injectivity, and cationic in the presence of concentrated brine with dissolved CO2. The hydrophilic/lipophilic balance of the amines was tuned by modification of the amine head group or tail length to design strong foams. It was important to increase the basicity of surfactants to enhance the solvation in the aqueous phase over a pH range of 4 to 7. These surfactants were effective in lowering the interfacial tension between water and CO2 at high temperature and salinity. They generated viscous C/W foams in limestone, sandstone and glass bead packs at 120 °C in the presence of 22% TDS brine when surfactants were injected from either the aqueous or CO2 phase. At pH below 6, these surfactants exhibited low oil/water partition coefficients on the order of 0.1 which suggests that these surfactants will have minimal retardation due to partitioning into oil in the EOR process.Significance of Subject Matter. These surfactants stabilized C/W foam at high temperature and salinity, and partitioned to the water phase over dodecane phase for efficient surfactant utilization. The high solubility in CO2 is beneficial for the surfactant to be available along CO2 flow pathways in a reservoir to minimize viscous fingering and gravity override.
A. S. Elhag, Chen, Y., Reddy, P. P., Noguera, J. A., Ou, A. M., Hirasaki, G. J., Nguyen, Q. P., Biswal, S. L., and Johnston, K. P., “Switchable diamine surfactants for CO2 mobility control in enhanced oil recovery and sequestration,” Energy Procedia, vol. 63, pp. 7709-7716, 2014. Publisher's VersionAbstract
The design of switchable amine surfactants for CO2 EOR in carbonate reservoirs at high temperatures is challenging because of the increase in the pH due to dissolution of calcium carbonate at acidic conditions. The increased pH hinders the protonation of the surfactant and its aqueous solubility. In this work, the addition of a second amine headgroup ensured that C16-18 N(EO) C3N(EO)2 is soluble in 22%TDS brine at neutral pH conditions. Also, captive bubble tensiometry measurements confirmed the activity of the surfactant at the C-W interface by large reduction in the interfacial tension coupled with high adsorption. Also, the surfactant generated viscous foam that can stabilize the displacement front in CO2 EOR processes and decrease the mobility of CO2 for enhanced CO2 sequestration
Y. Chen, Elhag, A. S., Poon, B. M., Cui, L., Ma, K., Liao, S. Y., Reddy, P. P., Worthen, A. J., Hirasaki, G. J., and Nguyen, Q. P., “Switchable nonionic to cationic ethoxylated amine surfactants for CO2 enhanced oil recovery in high-temperature, high-salinity carbonate reservoirs,” SPE Journal, vol. 19, no. 02, pp. 249-259, 2014. Publisher's VersionAbstract
To improve sweep efficiency for carbon dioxide (CO2) enhanced oil recovery (EOR) up to 120°C in the presence of high-salinity brine (182 g/L NaCl), novel CO2/water (C/W) foams have been formed with surfactants composed of ethoxylated amine headgroups with cocoalkyl tails. These surfactants are switchable from the nonionic (unprotonated amine) state in dry CO2 to cationic (protonated amine) in the presence of an aqueous phase with a pH less than 6. The high hydrophilicity in the protonated cationic state was evident in the high cloudpoint temperature up to 120°C. The high cloudpoint facilitated stabilization of lamellae between bubbles in CO2/water foams. In the nonionic form, the surfactant was soluble in CO2 at 120°C, and 3,300 psia at a concentration of 0.2% (w/w). C/W foams were produced by injecting the surfactant into either the CO2 phase or the brine phase, which indicated good contact between phases for transport of surfactant to the interface. Solubility of the surfactant in CO2 and a favorable C/W partition coefficient are beneficial for transport of surfactant with CO2-flow pathways in the reservoir, to minimize viscous fingering and gravity override. The ethoxylated cocoamine with two ethylene oxide (EO) groups was shown to stabilize C/W foams in a 30-darcy sandpack with NaCl concentrations up to 182 g/L at 120°C, 3,400 psia, and foam qualities from 50 to 95%. The foam produces an apparent viscosity of 6.2 cp in the sandpack and 6.3 cp in a 762-µm-inner-diameter capillary tube (downstream of the sandpack) in contrast with values well below 1 cp without surfactant present. Moreover, the cationic headgroup reduces the adsorption of ethoxylated alkyl amines on calcite, which is also positively charged in the presence of CO2 dissolved in brine. The surfactant partition coefficients (0 to 0.04) favored the water phase over the oil phase, which is beneficial for minimizing losses of surfactant to the oil phase for efficient surfactant usage. Furthermore, the surfactant was used to form C/W foams, without forming stable/viscous oil/water (O/W) emulsions. This selectivity is desirable for mobility control whereby CO2 will have low mobility in regions in which oil is not present and high contact with oil at the displacement front. In summary, the switchable ethoxylated alkyl amine surfactants provide both high cloudpoints in brine and high interfacial activities of ionic surfactants in water for foam generation, as well as significant solubilities in CO2 in the nonionic dry state for surfactant injection
A. J. Worthen, Foster, L. M., Dong, J., Bollinger, J. A., Peterman, A. H., Pastora, L. E., Bryant, S. L., Truskett, T. M., Bielawski, C. W., and Johnston, K. P., “Synergistic formation and stabilization of oil-in-water emulsions by a weakly interacting mixture of zwitterionic surfactant and silica nanoparticles,” Langmuir, vol. 30, no. 4, pp. 984-994, 2014. Publisher's VersionAbstract
Oil-in-water emulsions were formed and stabilized at low amphiphile concentrations by combining hydrophilic nanoparticles (NPs) (i.e., bare colloidal silica) with a weakly interacting zwitterionic surfactant, caprylamidopropyl betaine, to generate a high hydrophilic–lipophilic balance. The weak interaction of the NPs with surfactant was quantified with contact angle measurements. Emulsions were characterized by static light scattering to determine the droplet size distributions, optical photography to quantify phase separation due to creaming, and both optical and electron microscopy to determine emulsion microstructure. The NPs and surfactant acted synergistically to produce finer emulsions with a greater stability to coalescence relative to the behavior with either NPs or surfactant alone. As a consequence of the weak adsorption of the highly hydrophilic surfactant on the anionic NPs along with the high critical micelle concentration, an unusually large surfactant concentration was available to adsorb at the oil–water interface and lower the interfacial tension. The synergy for emulsion formation and stabilization for the two amphiphiles was even greater in the case of a high-salinity synthetic seawater aqueous phase. Here, higher NP adsorption at the oil–water interface was caused by electrostatic screening of interactions between (1) NPs and the anionic oil–water interface and (2) between the NPs. This greater adsorption as well as partial flocculation of the NPs provided a more efficient barrier to droplet coalescence
H. Yu, Yoon, K. Y., Neilson, B. M., Bagaria, H. G., Worthen, A. J., Lee, J. H., Cheng, V., Bielawski, C. W., Johnston, K. P., and Bryant, S. L., “Transport and retention of aqueous dispersions of superparamagnetic nanoparticles in sandstone,” Journal of Petroleum Science and Engineering, vol. 116, pp. 115-123, 2014. Publisher's VersionAbstract
We evaluate the transport of surface-treated superparamagnetic iron-oxide nanoparticles in Boise-sandstone rocks by injecting aqueous dispersions of the particles into core plugs. Several different surface treatments yield stable dispersions of these particles, but provide very different transport characteristics. Effluent concentration histories are measured to obtain the particle retention in the rock. The results are used to optimize the particle surface coating so that the reservoir application requirements for the functional nanoparticles can be achieved. The application of interest here requires the nanoparticles to adsorb to oil/water interfaces. Our earlier experiments (Yu et al., 2010) showed that the paramagnetic nanoparticles stabilized with small negatively-charged citrate ligands have little retention in sedimentary rocks, but their preferred adsorption at the oil/water interfaces in rock pores was not achieved. A major improvement in surface coating optimization is achieved by creating a crosslinked polymer film that wraps around the nanoparticle so that it does not detach from the particle surface even under the harsh reservoir conditions. To fine-tune the coating to satisfy the reservoir application requirements, co-polymers and ter-polymers with different constituent monomers are employed. Nanoparticles stabilized with (poly-styrene sulfonate–alt-maleic acid) coating show a good adsorption tendency at the oil/water interfaces, while with very low adsorption at rock surface (~0.02 mg/m2). The dispersion also has long-term stability even at high salinity (8 wt% NaCl). Other polymers, such as (polyacrylic acid–r-butyl acrylate), (polyacrylic acid–b-styrene sulfonic acid), and (polyacrylic acid–r-butyl acrylate–b-styrene sulfonic acid), were also tested. The coating with the last polymer (PAA–PBA–PSS) provides a very low retention of particles in the rock, but only marginal preferred adsorption at oil/water interfaces. Graphical abstract pSS-alt-pMA co-polymer coating to paramagnetic nanoclusters (TEM image) provides good reservoir transportability (blue effluent curve) and preferential adsorption of nanoclusters to oil/water interfaces (red curve).
W. G. Hardin, Mefford, T. J., Slanac, D. A., Patel, B. B., Wang, X., Dai, S., Zhao, X., Ruoff, R. S., Johnston, K. P., and Stevenson, K. J., “Tuning the electrocatalytic activity of perovskites through active site variation and support interactions,” Chemistry of Materials, vol. 26, no. 11, pp. 3368-3376, 2014. Publisher's VersionAbstract
We present a series of perovskite electrocatalysts that are highly active for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in an aqueous alkaline electrolyte. Lanthanum-based perovskites containing different transition metal active sites (LaBO3, B = Ni, Ni0.75Fe0.25, Co, Mn) are synthesized by a general colloidal method, yielding phase pure catalysts of homogeneous morphology and surface area (8–14 m2/g). Each perovskite’s ability to catalyze the OER and ORR is examined using thin film rotating disk electrochemistry (RDE). LaCoO3 supported on nitrogen-doped carbon is shown to be ∼3 times more active for the OER than high-surface-area IrO2. Furthermore, LaCoO3 is demonstrated to be highly bifunctional by having a lower total overpotential between the OER and ORR (ΔE = 1.00 V) than Pt (ΔE = 1.16) and Ru (ΔE = 1.01). The OER and ORR pathways are perturbed by the introduction of peroxide disproportionation functionality via support interactions and selective doping of the catalyst. LaNi0.75Fe0.25O3’s ability to disproportionate peroxide is hypothesized to be responsible for the ∼50% improvement over LaNiO3 in catalytic activity toward the ORR, despite similar electronic structure. These results allow us to examine the pathways for OER and ORR in context of support interactions, transition metal redox processes, and catalytic bifunctionality.
2013
A. K. Murthy, Stover, R. J., Hardin, W. G., Schramm, R., Nie, G. D., Gourisankar, S., Truskett, T. M., Sokolov, K. V., and Johnston, K. P., “Charged Gold Nanoparticles with Essentially Zero Serum Protein Adsorption in Undiluted Fetal Bovine Serum,” Journal of the American Chemical Society, vol. 135, pp. 7799-7802, 2013. Publisher's VersionAbstract
The adsorption of even a single serum protein molecule on a gold nanosphere used in biomedical imaging may increase the size too much for renal clearance. In this work, we designed charged similar to 5 nm Au, nanospheres coated with binary mixed-charge ligand monolayers that do not change in size upon incubation in pure fetal bovine serum (FBS). This lack of protein adsorption was unexpected in view of the fact that the Au surface was moderately charged. The mixed-charge monolayers were composed of anionic citrate ligands modified by place exchange with naturally occurring amino acids: either cationic lysine or zwitterionic cysteine ligands. The zwitterionic tips of either the lysine or cysteine ligands interact weakly with the proteins and furthermore increase the distance between the "buried" charges closer to the Au surface and the interacting sites on the protein surface. The nm nanospheres were assembled into similar to 20 nm diameter nanoclusters with strong near-IR absorbance (of interest in biomedical imaging and therapy) with a biodegradable polymer, PLA(1k)-b-PEG(10k)-b-PLA(1k). Upon biodegradation of the polymer in acidic solution, the nanoclusters dissociated into primary similar to 5 nm Au nanospheres, which also did not adsorb any detectable serum protein in undiluted FBS.
H. G. Bagaria, Neilson, B. M., Worthen, A. J., Xue, Z., Yoon, K. Y., Cheng, V., Lee, J. H., Velagala, S., Huh, C., Bryant, S. L., Bielawski, C. W., and Johnston, K. P., “Adsorption of iron oxide nanoclusters stabilized with sulfonated copolymers on silica in concentrated NaCl and CaCl2 brine,” Journal of Colloid and Interface Science, vol. 398, pp. 217-226, 2013. Publisher's VersionAbstract
Transport of metal oxide nanoparticles in porous rock is of interest for imaging and oil recovery in subsurface reservoirs, which often contain concentrated brine. Various copolymers composed of acrylic acid and either 2-acrylamido-2-methylpropanesulfonate or styrenesulfonate were synthesized and adsorbed on iron oxide nanoclusters to provide colloidal stability and to achieve low adsorption on silica in high salinity brine composed of 8% wt. NaCl + 2% wt. CaCl2. Furthermore, the degree of adsorption of the nanoparticles on silica was controlled by modifying the acrylic acid groups in the copolymers with a series of diamines and triamines to add hydrophobicity. The adsorption on colloidal silica microparticles ranged from <1 mg/m(2) for highly charged hydrophilic surfaces on the iron oxide nanoparticles to 22 mg/m(2) for the most hydrophobic amine-modified surfaces, corresponding to monolayer coverages that ranged from 0.2% to 11.5%, respectively. The specific adsorption (mg-IO/m(2)-silica), monolayer coverage, and parameters for Langmuir isotherms were evaluated for various 10 nanoclusters as a function of the properties of the copolymers on their surfaces. (C) 2013 Elsevier Inc. All rights reserved.
A. B. Watts, Wang, Y. B., Johnston, K. P., and Williams, R. O., “Respirable Low-Density Microparticles Formed In Situ from Aerosolized Brittle Matrices,” Pharmaceutical Research, vol. 30, pp. 813-825, 2013. Publisher's VersionAbstract
Inhalation of low-density porous particles enables deep lung delivery with less dependence on device design and patient inspiration. The purpose of this study was to implement Thin Film Freezing (TFF) to investigate a novel approach to dry powder inhalation. Powders produced by TFF were evaluated for aerodynamic and geometric particle size by cascade impaction and laser light scattering, respectively. Density measurements were conducted according to USP methods and calculated using data from particle size measurements. Excipient inclusion and its effect on moisture sorption was measured by Dynamic Vapor Sorption (DVS). TFF-produced brittle matrix powders were sheared apart into respirable microparticles using a passive DPI device, producing fine particle fractions (FPF) up to 69% and mass median aerodynamic diameters (MMAD) as low as 2.6 mu m. Particles had a mean geometric diameter ranging from 25 mu m to 50 mu m and mass densities of approximately 0.01 g/cm(3). Powders were susceptible to moisture-induced matrix collapse, capillary forces and electrostatic charging; although formulations containing mannitol or no sugar excipient proved to be more robust. Aerosolized brittle matrices produced by TFF may prove to be a useful platform for highly efficient pulmonary delivery of thermally labile, highly potent, and poorly soluble drugs.
H. G. Bagaria, Yoon, K. Y., Neilson, B. M., Cheng, V., Lee, J. H., Worthen, A. J., Xue, Z., Huh, C., Bryant, S. L., Bielawski, C. W., and Johnston, K. P., “Stabilization of Iron Oxide Nanoparticles in High Sodium and Calcium Brine at High Temperatures with Adsorbed Sulfonated Copolymers,” Langmuir, vol. 29, pp. 3195-3206, 2013. Publisher's VersionAbstract
A series of sulfonated random and block copolymers were adsorbed on the surface of similar to 100 nm iron oxide (IO) nanoparticles (NPs) to provide colloidal stability in extremely concentrated brine composed of 8% wt NaCl + 2% wt CaCl2 (API brine; 1.4 M NaCl + 0.2 M CaCl2) at 90 degrees C. A combinatorial materials chemistry approach, which employed Ca2+-mediated adsorption of anionic acrylic acid-containing sulfonated polymers to preformed citrate-stabilized IO nanoclusters, enabled the investigation of a large number of polymer coatings. Initially a series of poly(2-methyl-2-acrylamidopropanesulfonate-co-acrylic acid) (poly(AMPS-co-AA)) (1:8 to 1:1 mcl:mol), poly(styrenesulfonate-block-acrylic acid) (2.4:1 mol:mol), and poly(styrenesulfonate-alt-maleic acid) (3:1 mol:mol) copolymers were screened for solubility in API brine at 90 degrees C. The ratio of AMPS to AA groups was varied to balance the requirement of colloid dispersibility at high salinity (provided by AMPS) against the need for anchoring of the polymers to the iron oxide surface (via the AA). Steric stabilization of IO NPs coated with poly(AMPS-co-AA) (1:1 mol:mol) provided colloidal stability in API brine at room temperature and 90 degrees C for up to 1 month. The particles were characterized before and after coating at ambient and elevated temperatures by a variety of techniques including colloidal stability experiments, dynamic light scattering, zeta potential, and thermogravimetric analysis.
A. K. Murthy, Stover, R. J., Borwankar, A. U., Nie, G. D., Gourisankar, S., Truskett, T. M., Sokolov, K. V., and Johnston, K. P., “Equilibrium Gold Nanoclusters Quenched with Biodegradable Polymers,” Acs Nano, vol. 7, pp. 239-251, 2013. Publisher's VersionAbstract
Although sub-100 nm nanoclusters of metal nanoparticles are of interest in many fields including biomedical imaging, sensors, and catalysis, it has been challenging to control their morphologies and chemical properties. Herein, a new concept is presented to assemble equilibrium Au nanoclusters of controlled size by tuning the colloidal interactions with a polymeric stabilizer, PLA(1k)-b-PEG(10k)-b-PLA(1k). The nanoclusters form upon mixing a dispersion of similar to 5 nm Au nanospheres with a polymer solution followed by partial solvent evaporation. A weakly adsorbed polymer quenches the equilibrium nanocluster size and provides steric stabilization. Nanocluster size is tuned from similar to 20 to similar to 40 nm by experimentally varying the final Au nanoparticle concentration and the polymer/Au ratio, along with the charge on the initial Au nanoparticle surface. Upon biodegradation of the quencher, the nanoclusters reversibly and fully dissociate to individual similar to 5 nm primary particles. Equilibrium cluster size is predicted semiquantitatively with a free energy model that balances short-ranged depletion and van der Waals attractions with longer-ranged electrostatic repulsion, as a function of the Au and polymer concentrations. The close spacings of the Au nanoparticles In the clusters produce strong NIR extinction over a broad range of wavelengths from 650 to 900 nm, which is of practical interest in biomedical imaging.
L. L. Ma, Borwankar, A. U., Willsey, B. W., Yoon, K. Y., Tam, J. O., Sokolov, K. V., Feldman, M. D., Milner, T. E., and Johnston, K. P., “Growth of textured thin Au coatings on iron oxide nanoparticles with near infrared absorbance,” Nanotechnology, vol. 24, 2013. Publisher's VersionAbstract
A homologous series of Au coated iron oxide nanoparticles with hydrodynamic diameters smaller than 60 nm was synthesized with very low Au-to-iron mass ratios, as low as 0.15. The hydrodynamic diameter was determined by dynamic light scattering and the composition by atomic absorption spectroscopy and energy dispersive x-ray spectroscopy. Unusually low Au precursor supersaturation levels were utilized to nucleate and grow Au coatings on iron oxide relative to the formation of pure Au nanoparticles. This approach produced unusually thin coatings by lowering autocatalytic growth of Au on Au, as shown by transmission electron microscopy. Nearly all of the nanoparticles were attracted by a magnet, indicating a minimal number of pure Au particles. The coatings were sufficiently thin to shift the surface plasmon resonance to the near infrared with large extinction coefficients, despite the small particle hydrodynamic diameters observed from dynamic light scattering to be less than 60 nm.
A. J. Worthen, Bagaria, H. G., Chen, Y. S., Bryant, S. L., Huh, C., and Johnston, K. P., “Nanoparticle-stabilized carbon dioxide-in-water foams with fine texture,” Journal of Colloid and Interface Science, vol. 391, pp. 142-151, 2013. Publisher's VersionAbstract
The concept of hydrophilic/CO2-philic balance (HCB) was extended to describe stabilization of carbon dioxide-in-water (C/W) foams (also called emulsions) with silica nanoparticles adsorbed at the CO2-water interface. Opaque, white C/W foams (bubble diameter <100 mu m) were generated with either PEG-coated silica or methylsilyl modified silica nanoparticles in a beadpack with CO2 densities between 0.2 and 0.9 g mL(-1). For methylsilyl modified silica nanoparticles, 50% SiOH modification provided an optimal HCB for generation and stabilization of viscous C/W foams with high stability. The apparent viscosity measured with a capillary tube viscometer reached 120-fold that of a CO2-water mixture without nanoparticles, a consequence of the small bubble size and the energy required to deform a high density of aqueous lamellae between CO2 bubbles. Air-in-water (A/W) foams stabilized with nanoparticles were used to gain insight into the relationship between nanoparticle surface properties and adsorption of the nanoparticles at various types of interfaces. With suitable nanoparticles, A/W foams were stable for at least 7 days and C/W foams were stable for at least 23 h. The ability to achieve long term stability for nanoparticle stabilized C/W foams could offer an alternative to conventional surfactants, which are known to have much lower adsorption energies. (C) 2012 Elsevier Inc. All rights reserved.
K. Y. Yoon, An, S. J., Chen, Y. S., Lee, J. H., Bryant, S. L., Ruoff, R. S., Huh, C., and Johnston, K. P., “Graphene oxide nanoplatelet dispersions in concentrated NaCl and stabilization of oil/water emulsions,” Journal of Colloid and Interface Science, vol. 403, pp. 1-6, 2013. Publisher's VersionAbstract
Stable dispersions of graphene oxide nanoplatelets were formed in water at pH 2-10 even with 5 wt% NaCl. For these conditions, oil-in-water emulsions stabilized with graphene oxide nanoplatelets remained partially stable for 1 year. The droplet sizes were as small as similar to 1 mu m with a low nanoplatelet concentration of 0.2 wt%. The emulsions were stable even for nanoplatelet concentrations down to 0.001 wt%. The stabilities of the emulsions even at high salinity may be attributed to the high anion density at the graphene oxide nanoplatelet edges which protrude into the water phase. Furthermore, the graphene oxide nanoplatelets are shown to adsorb on the surfaces of the oil droplets. The conceptual picture of graphene oxide nanoplatelets adsorbed to a greater extent on the water side of the oil/water interface, along with the high density of anions on their edges, cause the oil/water interface to curve about the oil phase, resulting in oil-in-water emulsion droplets. The dispersion stability with a very small amount of graphene oxide-based stabilizer, offers an intriguing opportunity for applications including CO2 sequestration and enhanced oil recovery in deep subsurface formations, which generally contain high-salinity brines. (C) 2013 Elsevier Inc. All rights reserved.
W. G. Hardin, Slanac, D. A., Wang, X., Dai, S., Johnston, K. P., and Stevenson, K. J., “Highly Active, Nonprecious Metal Perovskite Electrocatalysts for Bifunctional Metal-Air Battery Electrodes,” Journal of Physical Chemistry Letters, vol. 4, pp. 1254-1259, 2013. Publisher's VersionAbstract
Perovskites are of great interest as replacements for precious metals and oxides used in bifunctional air electrodes involving the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, we report the synthesis and activity of a phase-pure nanocrystal perovskite catalyst that is highly active for the OER and ORR. The OER mass activity of LaNiO3, synthesized by the calcination of a rapidly dried nanoparticle dispersion and supported on nitrogen-doped carbon, is demonstrated to be nearly 3-fold that of 6 nm IrO2 and exhibits no hysteresis during oxygen evolution. Moreover, strong OER/ORR bifimctionality is shown by the low total overpotential (1.02 V) between the reactions, on par or better than that of noble metal catalysts such as Pt (1.16 V) and Ir (0.92 V). These results are examined in the context of surface hydroxylation, and a new OER cycle is proposed that unifies theory and the unique surface properties of LaNiO3.
H. G. Bagaria, Xue, Z., Neilson, B. M., Worthen, A. J., Yoon, K. Y., Nayak, S., Cheng, V., Lee, J. H., Bielawski, C. W., and Johnston, K. P., “Iron Oxide Nanoparticles Grafted with Sulfonated Copolymers are Stable in Concentrated Brine at Elevated Temperatures and Weakly Adsorb on Silica,” Acs Applied Materials & Interfaces, vol. 5, pp. 3329-3339, 2013. Publisher's VersionAbstract
Magnetic nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery, carbon dioxide sequestration, and electromagnetic imaging. Herein we report a rare example of steric stabilization of iron oxide (10) nanoparticles (NPs) grafted with poly(2-acrylamido-2-methylpropanesulfonate-co-acrylic acid) (poly-(AMPS-co-AM) that not only display colloidal stability in standard American Petroleum Institute (API) brine (8% NaCI + 2% CaCl2 by weight) at 90 C for 1 month but also resist undesirable adsorption on silica surfaces (0.4% monolayer NPs). Because the AMPS groups interacted weakly with Ca2+, they were sufficiently well solvated to provide steric stabilization. The PAA groups, in contrast, enabled covalent grafting of the poly(AMPS-co-AA) chains to amine-functionalized 10 NPs via formation of amide bonds and prevented polymer desorption even after a 40000-fold dilution. The aforementioned methodology may be readily adapted to stabilize a variety of other functional inorganic and organic NPs at high salinities and temperatures.

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