Quantification of Heteroaggregation between Citrate-Stabilized Gold Nanoparticles and Hematite Colloids.

Collisions with and attachment to natural colloids (heteroaggregation) is likely to influence significantly the fate, transport, and toxicity of engineered nanoparticles (ENPs). This study investigated heteroaggregation between hematite (α-Fe2O3) colloids and citrate-capped gold nanoparticles (Cit-AuNPs) using a novel approach involving time-resolved dynamic light scattering and parallel experiments designed to quantify nanoparticle attachment and heteroaggregate surface charge. Experiments were performed in low ionic strength synthetic water at environmentally relevant pH in the presence and absence of Suwannee River Natural Organic Matter (SRNOM). In the absence of SRNOM at pH values where Cit-AuNPs and hematite are oppositely charged, attachment efficiencies are high and Cit-AuNPs are capable of destabilizing hematite following an "electrostatic patch" mechanism. Furthermore, maximum observed surface coverages were far below those predicted by geometry alone, a fact predicted by the random sequential adsorption (RSA) model that has significant implications for the estimation of heteroaggregate attachment efficiencies. At pH values where both particles are negative or in the presence of small amounts of SRNOM, attachment was minimal. Calculated attachment efficiencies using the measured surface coverages corroborate these findings. The calculation of attachment efficiencies and the identification of mechanisms governing heteroaggregation represents an important step toward predicting the transport, fate, and toxicity of ENPs in the environment.

Calcium Chloride in Neonatal Parenteral Nutrition Solutions with and without Added Cysteine: Compatibility Studies Using Laser and Micro-Flow Imaging Methodology.

BACKGROUND: Previous studies of compatibility of calcium chloride (CaCl2) and phosphates have not included particle counts in the range specified by the United States Pharmacopeia. Micro-flow imaging techniques have been shown to be comparable to light obscuration when determining particle count and size in pharmaceutical solutions. OBJECTIVE: The purpose of this study was to do compatibility testing for parenteral nutrition (PN) solutions containing CaCl2 using dynamic light scattering and micro-flow imaging techniques. METHODS: Solutions containing TrophAmine (Braun Medical Inc, Irvine, CA), CaCl2, and sodium phosphate (NaPhos) were compounded with and without cysteine. All solutions contained standard additives to neonatal PN solutions including dextrose, trace metals, and electrolytes. Control solutions contained no calcium or phosphate. Solutions were analyzed for particle size and particle count. Means of Z-average particle size and particle counts of controls were determined. Study solutions were compared to controls and United States Pharmacopeia (USP) Chapter 788 guidelines. The maximum amount of Phos that was compatible in solutions that contained at least 10 mmol/L of Ca in 2.5% amino acids (AA) was determined. Compatibility of these solutions was verified by performing analyses of 5 repeats of these solutions. Microscopic analyses of the repeats were also performed. RESULTS: Amounts of CaCl2 and NaPhos that were compatible in solutions containing 1.5%, 2%, 2.5%, and 3% AA were determined. The maximum amount of NaPhos that could be added to TrophAmine solutions of > = 2.5% AA containing at least 10 mmol/L of CaCl2 was 7.5 mmol/L. Adding 50 mg/dL of cysteine increased the amount of NaPhos that could be added to solutions containing 10 mmol/L of CaCl2 to 10 mmol/L. CONCLUSION: Calcium chloride can be added to neonatal PN solutions containing NaPhos in concentrations that can potentially provide an intravenous intake of adequate amounts of calcium and phosphorus.

Determination of copper speciation in highway stormwater runoff using competitive ligand exchange - Adsorptive cathodic stripping voltammetry.

Low concentrations of dissolved copper have been shown to adversely affect the olfactory system of salmonid species, impairing their ability to avoid predators and likely increasing mortality. These studies have resulted in increased regulatory scrutiny of stormwater discharges to surface waters inhabited by threatened and endangered salmonid species. Because it is primarily the free ionic (Cu(2+)) and weakly complexed forms of copper that are bioavailable, it is critical to understand the speciation of copper in stormwater. This paper reports on the characterization of copper binding ligands and copper speciation in composite samples of highway stormwater runoff collected at four sites in Oregon, USA using competitive ligand exchange - adsorptive cathodic stripping voltammetry (CLE-ACSV). Although the concentration and strength of copper binding ligands in stormwater varied considerable between sites and storms, the vast majority (>99.9%) of the total dissolved copper in composite samples was complexed by organic ligands in stormwater. Although total dissolved copper concentrations range from 2 to 20 µg/L, the analytically determined free ionic copper concentrations did not exceed 10(-10) M (6.3 ng/L) in any of the fully characterized samples, suggesting that much of the copper in highway stormwater is not bioavailable. Analytically determined free ionic copper concentrations were compared with those predicted by a readily available chemical equilibrium models and found to be in reasonable agreement.

Effects of natural organic matter type and concentration on the aggregation of citrate-stabilized gold nanoparticles.

The aggregation of 12 nm citrate-stabilized gold nanoparticles (cit-AuNPs) in the presence of four different natural organic matter (NOM) isolates and a monovalent electrolyte (KCl) was evaluated using time-resolved dynamic light scattering. All four NOM isolates stabilized the cit-AuNPs with respect to aggregation. However, specific effects varied among the different NOM isolates. At pH = 6 in 80 mM KCl, low concentrations (<0.25 mg C per L) of large molecular weight Suwannee River Humic Acid (SRHA) was required to stabilize cit-AuNPs, while larger concentrations (>2 mg C per L) of smaller Suwannee River Fulvic Acid (SRFA) were necessary at the same ionic strength. Suwannee River NOM (SRNOM) which contains both SRHA and SRFA behaved in a manner intermediate between the two. Pony Lake Fulvic Acid (PLFA), an autochthonous NOM isolate, provided substantial stability at low concentrations, yet aggregation was induced at NOM concentrations > 2 mg C per L, a trend that is hypothesized to be the result of favourable hydrophobic interactions between coated particles induced at increased surface coverage. For all NOM isolates, it appears that NOM adsorption or conformational changes at the AuNP surfaces result in significant increases in the hydrodynamic diameter that aren't attributable to NP-NP aggregation.

Influence of liberated silver from silver nanoparticles on nitrification inhibition of Nitrosomonas europaea.

The ecotoxicity of silver nanoparticles (Ag-NPs) to wastewater biota, including ammonia oxidizing bacteria (AOB), is gaining increasing interest as the number of products containing Ag-NPs continues to rise exponentially and they are expected to accumulate in wastewater treatment plants. This research demonstrated that the addition order of Ag-NP and the media constituents had a profound influence on the stability of the Ag-NP suspension and the corresponding repeatability of results and sensitivity of Nitrosomonas europaea. N. europaea, a model AOB, was found to be extremely sensitive to ionic silver (Ag(+)) and two sizes of Ag-NPs (20 and 80 nm). Ag(+) exposures resulted in the highest level of toxicity with smaller Ag-NPs (20 nm) being more toxic than larger Ag-NPs (80 nm). The increased sensitivity of N. europaea to smaller Ag-NPs was caused by their higher rates of dissolved silver (dAg) release, via dissolution, due to a greater surface area to volume ratio. dAg was shown to be responsible for the vast majority of the observed Ag-NP toxicity, as determined by abiotic Ag-NP dissolution tests. For the sizes of Ag-NP studied (20 and 80 nm), there appears to be a negligible nanoparticle-specific toxicity. This was further supported by similarities in inhibition mechanisms between Ag(+) and Ag-NP, with both causing decreases in AMO activity and destabilization of the outer-membrane of N. europaea. Finally, equal concentrations of total silver were found to be tightly associated to both Ag(+) and Ag-NP-exposed cells despite Ag-NP concentrations being five times greater, by mass, than Ag(+) concentrations.

Interactions between natural organic matter and gold nanoparticles stabilized with different organic capping agents.

The adsorption of natural organic matter (NOM) to the surfaces of natural colloids and engineered nanoparticles is known to strongly influence, and in some cases control, their surface properties and aggregation behavior. As a result, the understanding of nanoparticle fate, transport, and toxicity in natural systems must include a fundamental framework for predicting such behavior. Using a suite of gold nanoparticles (AuNPs) with different capping agents, the impact of surface functionality, presence of natural organic matter, and aqueous chemical composition (pH, ionic strength, and background electrolytes) on the surface charge and colloidal stability of each AuNP type was investigated. Capping agents used in this study were as follows: anionic (citrate and tannic acid), neutral (2,2,2-[mercaptoethoxy(ethoxy)]ethanol and polyvinylpyrrolidone), and cationic (mercaptopentyl(trimethylammonium)). Each AuNP type appeared to adsorb Suwannee River Humic Acid (SRHA) as evidenced by measurable decreases in zeta potential in the presence of 5 mg C L(-1) SRHA. It was found that 5 mg C L(-1) SRHA provided a stabilizing effect at low ionic strength and in the presence of only monovalent ions while elevated concentrations of divalent cations lead to enhanced aggregation. The colloidal stability of the NPs in the absence of NOM is a function of capping agent, pH, ionic strength, and electrolyte valence. In the presence of NOM at the conditions examined in this study, the capping agent is a less important determinant of stability, and the adsorption of NOM is a controlling factor.

Differential stability of lead sulfide nanoparticles influences biological responses in embryonic zebrafish.

As the number of nanoparticle-based products increase in the marketplace, there will be increased potential for human exposures to these engineered materials throughout the product life cycle. We currently lack sufficient data to understand or predict the inherent nanomaterial characteristics that drive nanomaterial-biological interactions and responses. In this study, we utilized the embryonic zebrafish (Danio rerio) model to investigate the importance of nanoparticle (NP) surface functionalization, in particular as it pertains to nanoparticle stability, on in vivo biological responses. This is a comparative study where two lead sulfide nanoparticles (PbS-NPs) with nearly identical core sizes, but functionalized with either sodium 3-mercaptopropanesulfonate (MT) or sodium 2,3-dimercaptopropanesulfonate (DT) ligand, were used. Developmental exposures and assessments revealed differential biological responses to these engineered nanoparticles. Exposures beginning at 6 h post fertilization (hpf) to MT-functionalized nanoparticles (PbS-MT) led to 100% mortality by 120 hpf while exposure to DT-functionalized nanoparticles (PbS-DT) produced less than a 5% incident in mortality at the same concentration. Exposure to the MT and DT ligands themselves did not produce adverse developmental effects when not coupled to the NP core. Following exposure, we confirmed that the embryos took up both PbS-MT and PbS-DT material using inductively coupled plasma-mass spectrometry (ICP-MS). The stability of the nanoparticles in the aqueous solution was also characterized. The nanoparticles decompose and precipitate upon exposure to air. Soluble lead ions were observed following nanoparticle precipitation and in greater concentration for the PbS-MT sample compared to the PbS-DT sample. These studies demonstrate that in vivo assessments can be effectively used to characterize the role of NP surface functionalization in predicting biological responses.

Particle size distribution dynamics during precipitative softening: declining solution composition.

Particle removal is a critical step in the treatment of surface water for potable use, and the majority of drinking water treatment plants employ precipitative coagulation processes such as alum and iron "sweep-floc" coagulation or lime softening for particle pre-treatment. Unfortunately, little is quantitatively known about how particle size distributions are shaped by simultaneous precipitation and flocculation. In an earlier paper, we demonstrated the effects of the saturation ratio, the mixing intensity and the seed concentration on the rates of homogeneous nucleation, precipitative growth and flocculation during precipitation of calcium carbonate at constant solution composition using electronic particle counting techniques. In this work, we extend those findings to systems more closely emulating the conditions in actual softening processes (i.e., declining solution composition). Key findings include the strong dependence of the rate of flocculation on the initial saturation ratio and demonstration of the benefits of seeding precipitative softening from the perspective of optimizing the effluent particle size distribution. The mixing intensity during precipitation was also shown to strongly influence the final particle size distribution. Implications of the findings with respect to softening practice are discussed.

Particle size distribution dynamics during precipitative softening: constant solution composition.

In the treatment of surface water for potable use, precipitative coagulation (e.g., lime softening, alum or iron sweep coagulation) is widely utilized prior to particle removal processes. The particle size distribution (PSD) formed during such processes is a prime determinant of the removal efficiency for suspended and dissolved contaminants, but little is known quantitatively about how PSDs change by simultaneous precipitation and flocculation. Using precipitative softening as an example, detailed measurements of the PSD (using electronic particle counting) were made during precipitation of CaCO(3) under conditions of constant solution composition. Examination of the time-varying PSDs revealed dramatic changes resulting from nucleation, crystal growth, and flocculation. The influence of the saturation ratio, seed concentration, and mixing intensity on those processes was quantified. Implications with respect to the design and operation of water treatment facilities are discussed.

Influence of surface charge distributions and particle size distributions on particle attachment in granular media filtration.

Filtration experiments were performed with a laboratory-scale filter using spherical glass beads with 0.55 mm diameter as collectors. Suspensions were made with Min-U-Sil 5 particles, and two different methods (pH control and polymer dosing) were used for destabilization. In the pH control experiments, all particles had negative surface charge, and those with lower (absolute value) charge were selectively attached to the collectors, especially during the early stage of filtration. This selective attachment of the lower charged particles caused the zeta potential distribution (ZPD) of the effluent to move to a more negative range. However, the ZPD of the effluent did not continue moving to more negative values during the later stages of filtration, and this result was attributed to two reasons: ripening effects and detachment of flocs. In the polymer experiments, substantial differences were found between experiments performed with negatively charged particles (underdosing) and those with positively charged particles (overdosing). With under-dosing, the results were similar to the pH control experiments (which also had negatively charged particles), but with overdosing, the effluent's ZPDs in the early stages did not overlap with those of the influent and more highly charged particles were removed more efficiently than lesser-charged particles. It is hypothesized that, despite a substantial period of pre-equilibration of media and coagulant, this equilibrium shifted when particles were also added. It was assumed that coagulant molecules previously adsorbed to the particles desorbed and subsequently attached to the filter media because of surface area differences in the particle and filter media.

Integral water treatment plant modeling: improvements for particle processes.

An update of research on particle behavior in water treatment plants first performed 25 years ago under the direction of Charles O'Melia is provided. The earlier work involved mathematical modeling of the changes in particle size distributions in the flocculation and sedimentation processes in water treatment plants. The current model includes corrections for short-range interactions between particles as they approach one another. These corrections severely reduce the expected collision frequency between particles that are very different in size and, therefore, substantially change the model predictions. Both experimental and field measurements of particle size distributions are provided; such measurements were unavailable in the earlier work and represent a touchstone to reality for the modeling efforts. The short-range model successfully fits experimental results for flocculation when the mechanism of particle destabilization is charge neutralization. However, the model does not account for the creation of new solids by precipitation either when hydrolyzing salts of aluminum or iron are added for particle destabilization by "sweep floc" destabilization or lime is added to remove calcium and magnesium as calcium carbonate and magnesium hydroxide in softening. The flocculent sedimentation model yields results that are in strong qualitative agreement with typical field measurements.