Sara M. Hashmi

Impact of Surface Functionalization on Bacterial Cytotoxicity of Single-Walled Carbon Nanotubes

The addition of surface functional groups to single-walled carbon nanotubes (SWNTs) is realized as an opportunity to achieve enhanced functionality in the intended application. At the same time, several functionalized SWNTs (fSWNTs), compared to SWNTs, have been shown to exhibit decreased cytotoxicity. Therefore, this unique class of emerging nanomaterials offers the potential enhancement of SWNT applications and potentially simultaneous reduction of their negative human health and environmental impacts depending on the specific functionalization. Here, the percent cell viability loss of Escherichia coli K12 resulting from the interaction with nine fSWNTs, n-propylamine, phenylhydrazine, hydroxyl, phenydicarboxy, phenyl, sulfonic acid, n-butyl, diphenylcyclopropyl, and hydrazine SWNT, is presented. The functional groups range in molecular size, chemical composition, and physicochemical properties. While physiochemical characteristics of the fSWNTs did not correlate, either singularly or in combination, with the observed trend in cell viability, results from combined light scattering techniques (both dynamic and static) elucidate that the percent loss of cell viability can be correlated to fSWNT aggregate size distribution, or dispersity, as well as morphology. Specifically, when the aggregate size polydispersity, quantified as the width of the distribution curve, and the aggregate compactness, quantified by the fractal dimension, are taken together, we find that highly compact and narrowly distributed aggregate size are characteristics of fSWNTs that result in reduced cytotoxicity. The results presented here suggest that surface functionalization has an indirect effect on the bacterial cytotoxicity of SWNTs through the impact on aggregation state, both dispersity and morphology.

Electrochemical Activation of Cp* Iridium Complexes for Electrode-Driven Water-Oxidation Catalysis

Organometallic iridium complexes bearing oxidatively stable chelate ligands are precursors for efficient homogeneous water-oxidation catalysts (WOCs), but their activity in oxygen evolution has so far been studied almost exclusively with sacrificial chemical oxidants. In this report, we study the electrochemical activation of Cp*Ir complexes and demonstrate true electrode-driven water oxidation catalyzed by a homogeneous iridium species in solution. Whereas the Cp* precursors exhibit no measurable O2-evolution activity, the molecular species formed after their oxidative activation are highly active homogeneous WOCs, capable of electrode-driven O2 evolution with high Faradaic efficiency. We have ruled out the formation of heterogeneous iridium oxides, either as colloids in solution or as deposits on the surface of the electrode, and found indication that the conversion of the precursor to the active molecular species occurs by a similar process whether carried out by chemical or electrochemical methods. This work makes these WOCs more practical for application in photoelectrochemical dyads for light-driven water splitting.

Mechanical properties of individual microgel particles through the deswelling transition

Microgels are important materials for both basic science and engineering and have wide applications from the study of phase transitions to the delivery of drugs. These micron and sub-micron particles, made of hydrogel materials, respond to solvent conditions. The most common microgels are environmentally sensitive, responding to temperature and pH. Our material of interest, poly(N-isopropylacrylamide) or NIPAM, undergoes a deswelling transition above a critical temperature. The deswelling behavior of this polymeric material has been thoroughly studied in ensemble microgel systems as well as in bulk hydrogel samples. We present measurements of the elastic properties of single microgel particles through the deswelling transition using atomic force microscopy. While the fully collapsed particle (E = 123 kPa) is ten times stiffer than a fully swollen one (E = 13 kPa), we observe a dramatic softening of the particle near the transition (E = 3 kPa).

Aggregation rate and fractal dimension of fullerene nanoparticles via simultaneous multiangle static and dynamic light scattering measurement.

The time-evolutions of nanoparticle hydrodynamic radius and aggregate fractal dimension during the aggregation of fullerene (C(60)) nanoparticles (FNPs) were measured via simultaneous multiangle static and dynamic light scattering. The FNP aggregation behavior was determined as a function of monovalent (NaCl) and divalent (CaCl(2)) electrolyte concentration, and the impact of addition of dissolved natural organic matter (humic acid) to the solution was also investigated. In the absence of humic acid, the fractal dimension decreased over time with monovalent and divalent salts, suggesting that aggregates become slightly more open and less compact as they grow. Although the aggregates become slightly more open, the magnitude of the fractal dimension suggests intermediate aggregation between the diffusion- and reaction-limited regimes. We observed different aggregation behavior with monovalent and divalent salts upon the addition of humic acid to the solution. For NaCl-induced aggregation, the introduction of humic acid significantly suppressed the aggregation rate of FNPs at NaCl concentrations lower than 150mM. In this case, the aggregation was intermediate or reaction-limited even at NaCl concentrations as high as 500mM, giving rise to aggregates with a fractal dimension of 2.0. For CaCl(2)-induced aggregation, the introduction of humic acid enhanced the aggregation of FNPs at CaCl(2) concentrations greater than about 5mM due to calcium complexation and bridging effects. Humic acid also had an impact on the FNP aggregate structure in the presence of CaCl(2), resulting in a fractal dimension of 1.6 for the diffusion-limited aggregation regime. Our results with CaCl(2) indicate that in the presence of humic acid, FNP aggregates have a more open and loose structure than in the absence of humic acid. The aggregation results presented in this paper have important implications for the transport, chemical reactivity, and toxicity of engineered nanoparticles in aquatic environments.

Carbon nanotube-based antimicrobial biomaterials formed via layer-by-layer assembly with polypeptides.

Biomaterials capable of suppressing microbial infection are of clear importance in various health care applications, e.g. implantable devices. In this study, we investigate the antimicrobial activity of single-walled carbon nanotubes (SWNT) layer-by-layer (LbL) assembled with the polyelectrolytes poly(L-lysine) (PLL) and poly(L-glutamic acid) (PGA). SWNT dispersion in aqueous solution is achieved through the biocompatible nonionic surfactant polyoxyethylene(20) sorbitan monolaurate (Tween 20), and the amphiphilic polymer phospholipid-poly(ethylene glycol) (PL-PEG). Absorbance spectroscopy and transmission electron microscopy (TEM) show SWNT with either Tween 20 or PL-PEG in aqueous solution to be well dispersed, at about the level of SWNT in chloroform. Quartz crystal microgravimetry with dissipation (QCMD) measurements show both SWNT-Tween and SWNT-PL-PEG to LbL assemble with PLL and PGA into multilayer films, with the PL-PEG system yielding the greater final SWNT content. Escherichia coli and Staphylococcus epidermidis inactivation rates are significantly higher (up to 90%) upon 24h incubation with SWNT containing films, compared to control films (ca. 20%). This study demonstrates the potential usefulness of SWNT/PLL/PGA thin films as antimicrobial biomaterials.

Effect of dispersant on asphaltene suspension dynamics: aggregation and sedimentation.

When oil is mixed with light alkanes, asphaltenes can precipitate out of oil solutions in a multistep process that involves the formation of nano and colloidal scale particles, the aggregation of asphaltene colloids, and their eventual sedimentation. Amphiphilic dispersants can greatly affect this process. The mechanism of the dispersant action in colloidal asphaltene suspensions in heptane has been shown through previous work to be due in part to a reduction in the size of the colloidal asphaltene particles with the addition of dispersant. However, previous studies of the sedimentation behavior revealed evidence of aggregation processes in the colloidal asphaltenes in heptane that has yet to be investigated fully. We investigate the effect of dispersants on this aggregation behavior through the use of dynamic light scattering, showing that both the amount of dispersant and the amount of heptane dilution can slow the onset of aggregation in colloidal asphaltene suspensions. An effective dispersant acts by suppressing colloidal aggregation in asphaltene suspensions, as shown by light scattering, and therefore also slows separation from the bulk, as revealed through macroscopic sedimentation experiments.

Self-assembly of resins and asphaltenes facilitates asphaltene dissolution by an organic acid

Asphaltene precipitation occurs in petroleum fluids under certain unfavorable conditions, but can be controlled by tuning composition. Aromatic solvents in large quantities can prevent precipitation entirely and can dissolve already precipitated asphaltenes. Some polymeric surfactants can dissolve asphaltenes when added at much lower concentrations than required by aromatic solvents. Other dispersants can truncate asphaltene precipitation at the sub-micron length scale, creating stable colloidal asphaltene dispersants. One particular asphaltene dispersant, dodecylbenzene sulfonic acid (DBSA), can do both, namely: (1) stabilize asphaltene colloids and (2) dissolve asphaltenes to the molecular scale. Acid-base interactions are responsible for the efficiency of DBSA in dissolving asphaltenes compared to aromatic solvents. However, many details remain to be quantified regarding the action of DBSA on asphaltenes, including the effect of petroleum fluid composition. For instance, resins, naturally amphiphilic components of petroleum fluids, can associate with asphaltenes, but it is unknown whether they cooperate or compete with DBSA. Similarly, the presence of metals is known to hinder asphaltene dissolution by DBSA, but its effect on colloidal asphaltene stabilization has yet to be considered. We introduce the concepts of cooperativity and competition between petroleum fluid components and DBSA in stabilizing and dissolving asphaltenes. Notably, we find that resins cooperatively interact with DBSA in dissolving asphaltenes. We use UV-vis spectroscopy to investigate the interactions responsible for the phase transitions between unstable suspensions, stable suspensions, and molecular solutions of asphaltenes.

Acid–base chemistry enables reversible colloid-to-solution transition of asphaltenes in non-polar systems

The conjugated π-bonding in asphaltenes, a naturally occurring member of the polyaromatic hydrocarbon family, provides a unique platform for investigating electrostatics and electronics in non-polar systems, but at the same time causes asphaltenes to be insoluble in all except aromatic liquids. Asphaltenes precipitate from petroleum fluids under a variety of conditions, including depressurization and compositional changes, plaguing both recovery operations and remediation in the case of equipment failure. Aromatic solvents like toluene dissolve asphaltenes, but only at very high concentrations, nearly 50% by weight. Polymeric dispersants can stabilize asphaltene colloids, and in some cases can inhibit asphaltene precipitation entirely. Strong organic acids such as dodecyl benzene sulfonic acid (DBSA) can dissolve precipitated asphaltenes when introduced in concentrations as little as 1 percent by weight. Here we demonstrate for the first time that DBSA enables a reversible transition from unstable to stable colloidal-scale asphaltene suspensions to molecularly stable solutions. A continuum of acid–base reactions explains the apparent dual-action of DBSA. The suspension–solution transition occurs through the protonation of heteroatomic asphaltene components and subsequent strong ion pairing with DBSA sulfonate ions, effectively forming DBSA-doped asphaltene complexes with a solvation shell.

Polymeric dispersants delay sedimentation in colloidal asphaltene suspensions.

Asphaltenes, among the heaviest components of crude oil, can become unstable under a variety of conditions and precipitate and sediment out of solution. In this report, we present sedimentation measurements for a system of colloidal scale asphaltene particles suspended in heptane. Adding dispersants to the suspension can improve the stability of the system and can mediate the transition from a power-law collapse in the sedimentation front to a rising front. Additional dispersant beyond a crossover concentration can cause a significant delay in the dynamics. Dynamic light scattering measurements suggest that the stabilization provided by the dispersants may occur through a reduction of both the size and polydispersity of the asphaltene particles in suspension.

Tuning size and electrostatics in non-polar colloidal asphaltene suspensions by polymeric adsorption

The destabilization of asphaltenes adversely affects many aspects of the petroleum energy industry. Although polymeric dispersants have been shown to stabilize asphaltene colloids in non-polar media, the mechanism by which they prevent aggregation is not well-understood. We use a variety of techniques to investigate systems of colloidal asphaltenes stabilized in heptane by an effective dispersant. Phase analysis light scattering (PALS) measurements reveal an increase in the electrophoretic mobility as a function of dispersant concentration, suggesting electrostatic repulsion as the primary stabilizing force. Dynamic light scattering (DLS) measurements indicate that the increase in mobility corresponds to a decrease in particle size. A simple scaling argument suggests that the dispersant adsorbs to the surface of the asphaltene colloids. UV-visible spectroscopy and static light scattering (SLS) measurements corroborate this proposal. Interestingly, the colloidal asphaltene properties change below the critical micelle concentration (cmc) of the dispersants used. The nature of the asphaltenes themselves plays an important role in allowing for this tunability of their properties. Contrary to currently accepted views of non-polar colloidal suspensions, our results indicate that isolated dispersant molecules, not inverse micelles, can lead to charge-stabilization of asphaltene colloids.