Controlled evaluation of silver nanoparticle dissolution using atomic force microscopy

Incorporation of silver nanoparticles (AgNPs) into an increasing number of consumer products has led to concern over the potential ecological impacts of their unintended release to the environment. Dissolution is an important environmental transformation that affects the form and concentration of AgNPs in natural waters; however, studies on AgNP dissolution kinetics are complicated by nanoparticle aggregation. Herein, nanosphere lithography (NSL) was used to fabricate uniform arrays of AgNPs immobilized on glass substrates. Nanoparticle immobilization enabled controlled evaluation of AgNP dissolution in an air-saturated phosphate buffer (pH 7.0, 25 °C) under variable NaCl concentrations in the absence of aggregation. Atomic force microscopy (AFM) was used to monitor changes in particle morphology and dissolution. Over the first day of exposure to ≥10 mM NaCl, the in-plane AgNP shape changed from triangular to circular, the sidewalls steepened, the in-plane radius decreased by 5-11 nm, and the height increased by 6-12 nm. Subsequently, particle height and in-plane radius decreased at a constant rate over a 2-week period. Dissolution rates varied linearly from 0.4 to 2.2 nm/d over the 10-550 mM NaCl concentration range tested. NaCl-catalyzed dissolution of AgNPs may play an important role in AgNP fate in saline waters and biological media. This study demonstrates the utility of NSL and AFM for the direct investigation of unaggregated AgNP dissolution.     

Dissolution of composition B detonation residuals

Composition B (Comp B) detonation residuals pose environmental concern to the U.S. Army because hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a constituent, has contaminated groundwater near training ranges. To mimic their dissolution on surface soils, we dripped water at 0.51 ml/h onto individual Comp B particles (0.1-2.0 mg) collected from the detonation of 81-mm mortars. Analyses of the effluent indicate thatthe RDX and 2,4,6-trinitrotoluene (TNT) in Comp B do not dissolve independently. Rather, the relatively slow dissolution of RDX controls dissolution of the particle as a whole by limiting the exposed area of TNT. Two dissolution models, a published steady-flow model and a drop-impingement model developed here, provide good agreementwith the data using RDX parameters for time scaling. They predict dissolution times of 6-600 rainfall days for 0.01-100 mg Comp B particles exposed to 0.55 cm/h rainfall rate. These models should bracket the flow regimes for dissolution of detonation residuals on soils, but they require additional data to validate them across the range of particle sizes and rainfall rates of interest.     

Cysteine-induced modifications of zero-valent silver nanomaterials: implications for particle surface chemistry, aggregation, dissolution, and silver speciation

The persistence of silver nanoparticles in aquatic environments and their subsequent impact on organisms depends on key transformation processes, which include aggregation, dissolution, and surface modifications by metal-complexing ligands. Here, we studied how cysteine, an amino acid representative of thiol ligands that bind monovalent silver, can alter the surface chemistry, aggregation, and dissolution of zero-valent silver nanoparticles. We compared nanoparticles synthesized with two coatings, citrate and polyvinylpirrolidone (PVP), and prepared nanoparticle suspensions (approximately 8 μM total Ag) containing an excess of cysteine (400 μM). Within 48 h, up to 47% of the silver had dissolved, as indicated by filtration of the samples with a 0.025-μm filter. Initial dissolution rates were calculated from the increase of dissolved silver concentration when particles were exposed to cysteine and normalized to the available surface area of nanoparticles in solution. In general, the rates of dissolution were almost 3 times faster for citrate-coated nanoparticles relative to PVP-coated nanoparticles. Rates tended to be slower in solutions with higher ionic strength in which the nanoparticles were aggregating. X-ray absorption spectroscopy analysis of the particles suggested that cysteine adsorbed to silver nanoparticles surfaces through the formation of Ag(+I)--sulfhydryl bonds. Overall, the results of this study highlight the importance of modifications by sulfhydryl-containing ligands that can drastically influence the long-term reactivity of silver nanoparticles in the aquatic environment and their bioavailability to exposed organisms. Our findings demonstrate the need to consider multiple interlinked transformation processes when assessing the bioavailability, environmental risks, and safety of nanoparticles, particularly in the presence of metal-binding ligands.     

Size-dependent Pb sorption to nanohematite in the presence and absence of a microbial siderophore

This study focused on the effects of particle size (40, 8.6, and 3.6 nm) and the presence of the microbial ligand desferrioxamine B (DFOB) on Pb(II) sorption to hematite, based on sorption edge experiments (i.e., sorption as a function of pH). Effects of hematite nanoparticle size on sorption edges, when plotted either as sorption density or as % Pb uptake, depended on whether the experiments were normalized to account for differences in specific surface area within the reaction vessels or postnormalized after the fact. Accounting for specific surface area within reaction vessels is needed to maintain comparable ratios of sorbate to sorbent surface sites. When normalized for BET specific surface area (A(s,BET)) within the reaction vessels, the Pb(II) sorption edge shifted ～0.5 pH units to the left for <10 nm hematite particles, but maximum sorption density (at pH ≥ 6) was unaffected by particle size. DFOB had little or no effect on Pb(II) sorption to <10 nm particles, but DFOB decreased Pb(II) sorption to the 40 nm particles at pH ≥ 6 by ～20%. Hematite (nano)particle size thus exerts subtle effects on Pb(II) sorption, but the effects may be more pronounced in the presence of a metal complexing agent.     

Secondary organic aerosol coating of synthetic metal-oxide nanoparticles

Secondary organic aerosol (SOA) from the α-pinene + ozone reaction readily coats TiO(2) and CeO(2) metal-oxide nanoparticles in smog-chamber experiments under atmospherically relevant conditions. Otherwise identical experiments compared bare nanoparticles and nanoparticles coated with poly(acrylic acid) (PAA). The PAA-coated particles result in significantly higher new-particle formation rates, suggesting that the SOA vapors coat bare metal oxide more readily than the PAA. After particles begin to grow via SOA coating, however, all particles, independent of size or the presence of a metal-oxide core, grow with a rate proportional to their surface area, modified to account for gas-phase diffusion in the transition regime between the kinetic and bulk-flow regimes. This suggests that SOA condensational growth may be modeled based on the size distribution of the condensational sink in the atmosphere.     

Size Reduction and Calcium Release of Fish Bone Particles During Nanomilling as Affected by Bone Structure

Three types of fish bones including fish skull, rib, and backbone were cooked at 120 °C for 20 min to remove tissues and impurities. The fish bones were minced and ground into microscaled particles before treated in a high-energy wet ball mill to obtain nanoparticles. The effects of bone structure on size reduction of fish bone particles during nanomilling and the calcium release properties were investigated. The results showed that fish rib has higher elasticity modulus than fish skull and fish backbone due to its highly ordered structure. Prior to nanomilling of ground fish bones, the particle size of the skull was the greatest followed by backbone and rib. The mean particle size and calcium release were not significantly different after 8-h nanomilling. However, the size reduction and calcium release rates of the nanoparticles were similar between fish skull and backbone while the fish rib had the lowest values in both categories. The kinetics of size reduction during nanomilling and the calcium release properties of the nanoparticles were well described with the first-order exponential decay function and first-order kinetic function, respectively. Correlation analysis indicated that among all mechanical properties, elasticity modulus of fish bone, to the highest degree, determined their size reduction rate and calcium release rate during nanomilling.     

Preparation and physicochemical characterization of trans-resveratrol nanoparticles by temperature-controlled antisolvent precipitation

In order to enhance the oral bioavailability, trans-resveratrol (t-RVT) nanoparticles were prepared by temperature-controlled antisolvent precipitation with the hydroxypropyl methylcellulose as the stabilizer. The mean particle size was reduced by decreasing the precipitation temperature, mainly due to the higher nucleation rate and slower growth rate. The freeze dried t-RVT nanoparticles were well reconstituted in aqueous solution maintaining a similar particle size and distribution in the nanosize range that is prior to freeze drying without the aid of cryoprotectants. The SEM images exhibited some aggregation of individual spherical nanosized particles. The FT-IR analysis confirmed that the molecular structure of t-RVT nanoparticles was not changed after the precipitation process. Furthermore, t-RVT nanoparticles showed significantly enhanced saturation solubility and dissolution rate by the decrease in particle size and degree of crystallinity when compared to the raw t-RVT. The combined results have demonstrated that this method can considerably enhance the bioavailability of t-RVT.     

Microbial reduction of Fe(III) in hematite nanoparticles by Geobacter sulfurreducens

The rates of microbial Fe(III) reduction of three sizes of hematite nanoparticles by Geobacter sulfurreducens were measured under two H2 partial pressures (0.01 and 1 atm) and three pH (7.0, 7.5, and 8.0) conditions. Hematite particles with mean primary particle sizes of 10, 30, and 50 nm were synthesized by a novel aerosol method that allows tight control of the particle size distribution. The mass-normalized reduction rates of the 10 and 30 nm particles were comparable to each other and higher than the rate for the 50 nm particles. However, the surface area-normalized rate was highest for the 30 nm particles. Consistent with a previously published model, the reduction rates are likely to be proportional to the bacteria-hematite contact area and not to the total hematite surface area. Surface area-normalized iron reduction rates were higher than those reported in previous studies, which may be due to the sequestration of Fe(II) through formation of vivianite. Similar initial reduction rates were observed under all pH and H2 conditions studied.     

Siderophore-manganese(lll) Interactions. II. Manganite dissolution promoted by desferrioxamine B

Recent laboratory and field studies suggest that Mn(lll) forms persistent aqueous complexes with high-affinity ligands. Aqueous Mn(lll) species thus may play a significant but largely unexplored role in biogeochemical processes. One formation mechanism for these species is the dissolution of Mn(lll)-bearing minerals. To investigate this mechanism, we measured the steady-state dissolution rates of manganite (gamma-MnOOH) in the presence of desferrioxamine B (DFOB), a common trihydroxamate siderophore. We find that DFOB dissolves manganite by both reductive and nonreductive reaction pathways. For pH > 6.5, a nonreductive ligand-promoted reaction is the dominant dissolution pathway, with a steady-state dissolution rate proportional to the surface concentration of DFOB. In the absence of reductants, the aqueous Mn(lIl)HDFOB+ complex resulting from dissolution is stable for at least several weeks at circumneutral to alkaline pH and at 25 degrees C. For pH < 6.5, Mn2+ is the dominant aqueous species resulting from manganite dissolution, implicating a reductive dissolution pathway. These results have important implications for the biogeochemical cycling of both manganese and siderophores--as well as Fe(lll)--in natural waters and soils.     

Can dawsonite permanently trap CO2?

Thermodynamic calculations indicate that although dawsonite (NaAlCO3(OH)2) is favored to form at the high CO2 pressures associated with carbon dioxide injection into sandstone reservoirs, this mineral will become unstable as CO2 pressure decreases following injection. To assess the degree to which dawsonite will persist following its formation in sandstone reservoirs, its dissolution rates have been measured at 80 +/- 3 degrees C as a function of pH from 3 to 10. Measured dawsonite dissolution rates normalized to their BET surface area are found to be nearly independent of pH over the range of 3.5 < pH < 8.6 at 1.58 x 10(-9) mol/(m2 x s). Use of these dissolution rates in reactive transport calculations indicate that dawsonite rapidly dissolves following the decrease of CO2 pressure out of its stability field, leading mainly to the precipitation of secondary kaolinite. This result indicates that dawsonite will provide a permanent mineral storage host only in systems that maintain high CO2 pressures, whereas dawsonite may be an ephemeral phase in dynamic settings and dissolve once high CO2 pressure dissipates either through dispersion or leakage.     

Physicochemical properties and oral bioavailability of ursolic acid nanoparticles using supercritical anti-solvent (SAS) process

The objective of the study was to prepare ursolic acid (UA) nanoparticles using the supercritical anti-solvent (SAS) process and evaluate its physicochemical properties and oral bioavailability. The effects of four process variables, pressure, temperature, drug concentration and drug solution flow rate, on drug particle formation during SAS process, were investigated. Particles with mean particle size ranging from 139.2 ± 19.7 to 1039.8 ± 65.2 nm were obtained by varying the process parameters. The UA was characterised by scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, thermal gravimetric analysis, specific surface area, dissolution test and bioavailability test. It was concluded that physicochemical properties and bioavailability of crystalline UA could be improved by physical modification, such as particle size reduction and generation of amorphous state using SAS process. Further, SAS process was a powerful methodology for improving the physicochemical properties and bioavailability of UA.     

Oxide nanoparticle uptake in human lung fibroblasts: effects of particle size, agglomeration, and diffusion at low concentrations

Quantitative studies on the uptake of nanoparticles into biological systems should consider simultaneous agglomeration, sedimentation, and diffusion at physiologically relevant concentrations to assess the corresponding risks of nanomaterials to human health. In this paper, the transport and uptake of industrially important cerium oxide nanoparticles, into human lung fibroblasts is measured in vitro after exposing thoroughly characterized particle suspensions to a fibroblast cell culture for particles of four separate size fractions and concentrations ranging from 100 ng g(-1) to 100 microg g(-1) of fluid (100 ppb to 100 ppm). The unexpected findings at such low but physiologically relevant concentrations reveal a strong dependence of the amount of incorporated ceria on particle size, while nanoparticle number density or total particle surface area are of minor importance. These findings can be explained on the basis of a purely physical model. The rapid formation of agglomerates in the liquid is strongly favored for small particles due to a high number density while larger ones stay mainly unagglomerated. Diffusion (size fraction 25-50 nm) or sedimentation (size fraction 250-500 nm) limits the transport of nanoparticles to the fibroblast cells. The biological uptake processes on the surface of the cell are faster than the physical transport to the cell at such low concentrations. Comparison of the colloid stability of a series of oxide nanoparticles reveals that untreated oxide suspensions rapidly agglomerate in biological fluids and allows the conclusion thatthe presented transport and uptake kinetics at low concentrations may be extended to other industrially relevant materials.     

PARTICLE RESIDENCE TIME DISTRIBUTIONS IN A MODEL HORIZONTAL SCRAPED SURFACE HEAT EXCHANGER1

Residence times and their distribution characteristics of potato cubes with aqueous solutions of sodium carboxymethyl cellulose (CMC), simulating non-Newtonian fluid foods, in a horizontal scraped surface heat exchanger (SSHE) were investigated. Minimum and maximum normalized particle residence times (NPRTs) and standard deviations of mean values were not significantly affected by the particle concentration while mean NPRTs of up to 10% particle concentration were signficantly lower than those of 20–40% particle concentrations (P < 0.05). Mean NPRTs were significantly influenced by process parameters including concentration of carrier, viz., viscosity, mutator speed and particle size (P< 0.001) as well as 2-way interactions among flow rate, mutator speed and particle size (P < 0.05 or 0.001). Furthermore, most of the individual particle residence time distributions in a horizontal SSHE flow could be described by either normal or gamma distribution models.     

RESIDENCE TIME DISTRIBUTION of CYLINDRICAL PARTICLES IN A CURVED SECTION of A HOLDING TUBE: the EFFECT of PARTICLE SIZE and FLOW RATE1

The residence time and the residence time distributions of cylindrical particles (density 1130 kg/m³ and particle concentration 20% v/v) in a U-bend of 22 cm radius of curvature were studied. All experiments were conducted at room temperature and with an aqueous solution of sodium carboxymethylcellulose (0.5% CMC) as the carrier medium. Two levels of particle size (11 and 20 mm) and three levels of product flow rates (0.66 × 10^?3, 0.81 × 10^?3, and 0.97 × 10^?3 m³/s) were used. the results obtained from this study showed that the mean and the standard deviation of the normalized residence time of particles decreased as either particle size or flow rate was increased. the E(θ) and F(θ) curves indicated that flow profile of particles approached the plug flow profile as the particle size or the flow rate was increased. Results obtained from regression analyses revealed that the mean and the standard deviation of the normalized residence time were significantly influenced by particle Froude number and most strongly by particle-to-tube diameter ratio.     

Normalized particle residence times as affected by process parameters in a vertical scraped surface heat exchanger

The effects of process parameters and their two-way interactions on the normalized particle residence times (NPRTs) of 1.0–2.0 cm potato cubes in 0.4–1.2% aqueous solutions of sodium carboxymethylcellulose (CMC), flowing at 453–599 mL s⁻¹ in a vertical scraped surface heat exchanger, rotating at 60–160 rpm were investigated. Minimum and maximum NPRTs and the standard deviations of the mean values were not significantly affected by the conditions studied ( P < 0.05). Mean NPRTs were significantly influenced by process parameters, including the concentration of the carrier, mutator speed and particle size, as well as two-way interactions between flow rate, mutator speed and particle size ( P < 0.001). The orientation of the heat exchanger was an important factor influencing the forces acting on the particles.     

Evidence of uranium and associated trace element mobilization and retention processes at Oklo (Gabon), a naturally radioactive site

The processes that affect the mobility of uranium and other radionuclides in the environment have been largely studied at both the laboratory and the field scales. The natural reactors found at the Oklo uranium mine in Gabon constitute a unique investigation setting as spontaneous fission reactions occurred two billion years ago. Oklo uraninites contain a large amount of other radionuclides as a result of the fission process. We have investigated the dissolution behavior of four uraninite samples from Oklo as a function of temperature (25 and 60 degrees C) and bicarbonate concentration (2.7-30 mmol/L). We have also investigated the dissolution behavior of minor components of the uraninites (i.e., Nd, Cs, Mo, Yb, and Sb) in relation to the dissolution of uranium. The results of the reported work are in good agreement with the kinetic rate laws derived from other uranium(IV) dioxide studies. Some of the minor components are found to be congruently released from the uraninite phase, while it is postulated that dissolution from segregated phases might affect the final concentrations of some of the rare earth elements, i.e., Nd and Yb. In addition, we have performed dissolution studies at 60 degrees C with two uraninites representative of different geochemical environments at Oklo, to study the uranium dissolution rates as a function of the temperature. This has allowed derivation of apparent activation energies for the bicarbonate-promoted oxidative dissolution of the Oklo uraninites. The dissolution behavior of the minor components of the uraninites at 60 degrees C was found to closely follow the behavior observed at 25 degrees C. This indicates that similar codissolution mechanisms operate in the temperature range studied. The implications for the mobility of uranium and other radionuclides in natural and anthropogenic environments are discussed.     

Influence of polyethyleneimine graftings of multi-walled carbon nanotubes on their accumulation and elimination by and toxicity to Daphnia magna

Modifications of carbon nanotubes (CNTs) for different applications may change their physicochemical properties such as surface charge. Assessments of the extent to which such modifications influence CNT ecotoxicity, accumulation, and elimination behaviors are needed to understand potential environmental risks these variously modified nanoparticles may pose. We have modified carbon-14 labeled multi-walled carbon nanotubes (MWNTs) with polyethyleneimine (PEI) surface coatings to increase their aqueous stability and to give them positive, negative, or neutral surface charges. Uptake and elimination behaviors of Daphnia magna exposed to PEI-coated and acid-modified MWNTs at concentrations of approximately 25 and 250 μg/L were quantified. PEI surface coatings did not appear to substantially impact nanotube accumulation or elimination rates. Although the PEI-modified nanotubes exhibited enhanced stability in aqueous solutions, they appeared to aggregate in the guts of D. magna in a manner similar to acid-treated nanotubes. The MWNTs were almost entirely eliminated by Daphnia fed algae during a 48 h elimination experiment, whereas elimination without feeding was typically minimal. Finally, PEI coatings increased MWNT toxicities, though this trend corresponded to the size of the PEI coatings, not their surface charges.     

Characterizing pore-scale dissolution of organic immiscible liquid in natural porous media using synchrotron X-ray microtomography

The objective of this study was to characterize the pore-scale dissolution of organic immiscible-liquid blobs residing within natural porous media. Synchrotron X-ray microtomography was used to obtain high-resolution, three-dimensional images of the aqueous, organic-liquid, and solid phases residing in columns packed with one of two porous media. Images of the packed columns were obtained after a stable, discontinuous distribution (e.g., residual saturation) of the organic liquid (trichloroethene) had been established, and three subsequent times during column flushing. These data were used to characterize the morphology of the organic-liquid blobs as a function of dissolution, and to quantify changes in total organic-liquid volume, surface area, and water-organic liquid interfacial area. The dissolution dynamics of individual blobs appeared to be influenced by the local pore configuration. In addition to dissolution-induced shrinkage, some blobs were observed to separate into multiple distinct subunits. The median blob size decreased by approximately a factor of 2 at the point where approximately 90% of the initial organic-liquid volume had been removed. The ratio of capillary associated interfacial area to total water-organic liquid interfacial area increased by 50% at the point where approximately 95% of the initial mass had been removed. A nearly linear relationship was observed between both total and capillary associated interfacial area and organic liquid volumetric fraction. Changes in the measured aqueous-phase trichloroethene effluent concentrations were well correlated with changes in the volume, surface area, and number of blobs. The effluent concentration data were adequately described by a first-order mass transfer expression employing a constant value of the mass-transfer coefficient, with values for the water-organic liquid interfacial area obtained independently from the microtomography data.     

Size effects on adsorption of hematite nanoparticles on E. coli cells

Adsorption of engineered nanoparticles (NPs) onto bacterial cells is critical for quantifying nanobio interactions as well as toxicokinetic properties of NPs. The purpose of this work was to study adsorption of hematite (α-Fe(2)O(3)) NPs onto Escherichia coli cells and to determine the particle size effects on the adsorption kinetics. Adsorption of large NPs (76 and 98 nm) on cells reached equilibrium faster (within 30-40 min) than small NPs (approximately 60-90 min). The adsorption rates in mg Fe/(L · s) decreased in the order of 98 nm > 76 nm > 53 nm > 26 nm. However, adsorption rates expressed as the number of adsorbed hematite NPs per unit cell surface area in #/(m2 · s) were faster for small NPs than those for large NPs. To interpret the size effects on adsorption kinetics, the Extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) theory was combined with interfacial force boundary layer (IFBL) theory. The computed adsorption rates for different sizes had excellent agreement with the experimental data, and they explained that that faster kinetics for smaller NPs could be attributed to faster particle mobility and lower energy barriers in the total interaction energy. This study lays the groundwork for quantifying the kinetic behavior of NPs interacting with microbial cells, and the results provide insight into adsorption processes at the nanoscale.