Coating thickness and coverage effects on the forces between silica nanoparticles in water

The structure and interactions of coated silica nanoparticles have been studied in water using molecular dynamics simulations. For 5 nm diameter amorphous silica nanoparticles we studied the effects of varying the chain length and grafting density of polyethylene oxide (PEO) on the nanoparticle coating's shape and on nanoparticle-nanoparticle effective forces. For short ligands of length $n=6$ and $n=20$ repeat units, the coatings are radially symmetric while for longer chains ($n=100$) the coatings are highly anisotropic. This anisotropy appears to be governed primarily by chain length, with coverage playing a secondary role. For the largest chain lengths considered, the strongly anisotropic shape makes fitting to a simple radial force model impossible. For shorter ligands, where the coatings are isotropic, we found that the force between pairs of nanoparticles is purely repulsive and can be fit to the form $(R/2r_\text{core}-1)^{-b}$ where $R$ is the separation between the center of the nanoparticles, $r_\text{core}$ is the radius of the silica core, and $b$ is measured to be between 2.3 and 4.1.Comment: 20 pages, 6 figure

Quantifying nanoparticle dispersion: application of the Delaunay network for objective analysis of sample micrographs

Measuring quantitatively the nanoparticle dispersion of a composite material requires more than choosing a particular parameter and determining its correspondence to good and bad dispersion. It additionally requires anticipation of the measure’s behaviour towards imperfect experimental data, such as that which can be obtained from a limited number of samples. It should be recognised that different samples from a common parent population can give statistically different responses due to sample variation alone and a measure of the likelihood of this occurring allows a decision on the dispersion to be made. It is also important to factor into the analysis the quality of the data in the micrograph with it: (a) being incomplete because some of the particles present in the micrograph are indistinguishable or go unseen; (b) including additional responses which are false. With the use of our preferred method, this article investigates the effects on the measured dispersion quality of nanoparticles of the micrograph’s magnification settings, the role of the fraction of nanoparticles visible and the number of micrographs used. It is demonstrated that the best choice of magnification, which gives the clearest indication of dispersion type, is dependent on the type of nanoparticle structure present. Furthermore, it is found that the measured dispersion can be modified by particle loss, through the limitations of micrograph construction, and material/microscope imperfections such as cut marks and optical aberrations which could lead to the wrong conclusions being drawn. The article finishes by showing the versatility of the dispersion measure by characterising various different spatial features. <br/

Analysis of fullerene-C 60 and kinetic measurements for its accumulation and depuration in Daphnia magna

A simple method for analyzing masses of water suspended fullerenes (nC 60 ) in Daphnia magna by extracting to toluene and measuring by ultraviolet-vis spectrophotometry was developed. This method was used to assess bioaccumulation and depuration rates by daphnia after nC 60 exposure in artificial freshwater. Accumulation was rapid during the first few hours, and based on accumulation modeling, 90% of the steady-state concentration was reached in 21 h. After exposure for 24 h to a 2 mg/L fullerene solution, the daphnia accumulated 4.5 ± 0.7 g/kg wet weight, or 0.45% of the organism wet mass. Daphnids exposed to 2 mg/L fullerenes for 24 h eliminated 46 and 74% of the accumulated fullerenes after depuration in clean water for 24 and 48 h, respectively. Transmission electron microscopy revealed that the majority of the fullerenes present in the gut of daphnids were large agglomerates. The significant fullerene uptake and relatively slow depuration suggest that D. magna may play a role as a carrier of fullerene from one trophic level to another. Additionally, D. magna may impact the fate of suspended fullerene particles in aquatic ecosystems by their ability to pack fullerene agglomerates into larger particles than were found in the exposure water, and then excrete agglomerates that are not stable in water, causing them to settle out of solution. This process decreases fullerene exposure to other aquatic organisms in the water column but may increase exposure to benthic organisms in the sediment. Environ. Toxicol. Chem. 2010;29:1072–1078. © 2010 SETACPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71370/1/124_ftp.pd

Elucidating the Importance of Structure, Surfaces, and Interfaces in Polymer Nanoparticles and Nanocomposites

This dissertation details research conducted to elucidate the importance of structure, surfaces, and interfaces in both polymeric nanoparticles and polymer nanocomposites. The fundamental understanding that is garnered in these studies provides a foundation to rationally develop nanocomposites tailored for unique functionalities, performance and applications. Soft polymeric nanoparticles, have shown to imbue non-traditional diffusive properties, the strength of which decreases with crosslinking density of the nanoparticle. The crosslinking dependent morphology of these nanoparticles is first characterized in a dilute solution of good solvent (Chapter 2). The scattering results revealed that the structure ranges from a swollen polymer in good solvent (0% XL), to a collapsed polymer in theta solvent (0.4%XL), to unequivocally particle-like ( ≥ 0.8%XL). The transition to a particle-like morphology hinges on the clear presence of a measurable surface. To better understand the mechanisms behind their non-traditional diffusive properties, the internal dynamics of these nanoparticles were probed (Chapter 3). While globally exhibiting Zimm-like dynamics, the nanoparticles showed a heterogeneity of local internal dynamics, exhibiting significantly slowed dynamics on length scales that contained crosslinks and linear-like dynamics over length scales where crosslinks were mostly absent. The clear separation of internal dynamics magnifies the importance of the nanoparticle’s core-shell structure. The consequences of surfaces and interfaces within polymer nanocomposites is also explored. The permanency of a bound polymer layer is characterized by monitoring the time evolution of the volume fraction profile of an adsorbed polymer layer (Chapter 4). While the total thickness of the layer remained unchanged, the composition varied indicating that individual chains are not necessarily “irreversibly” adsorbed. Additionally, the molecular weight dependence on the kinetics of chain desorption are studied, finding that desorption in the melt transitions from diffusion limited to a combination of diffusion and surface detachment limited with increasing molecular weight. Finally interfaces between a fire-retardant small molecule and polymer is compatibilized using a polymeric dispersant (Chapter 5). The average and homogeneity of particle size is improved in melt mix blends of the three components as the polymer dispersant can disrupt the intramolecular hydrogen bonding within the flame-retardant with intermolecular interactions

Effective Thermal Conductivity of Carbon Nanotube-Based Cryogenic Nanofluids

Nanofluids consist of nanometer-sized particles or fibers in colloidal suspension within a host fluid. They have been studied extensively since their creation due to their often times anomalous and unique thermal transport characteristics. They have also proven to be quite valuable in terms of the scientific knowledge gained from their study and their nearly unlimited industrial and commercial applications. This research has expanded the science of nanofluids into a previously unexplored field, that of cryogenic nanofluids. Cryogenic nanofluids are similar to traditional nanofluids in that they utilize nanometer-sized inclusion particles; however, they use cryogenic fluids as their host liquids. Cryogenic nanofluids are of great interest due to the fact that they combine the extreme temperatures inherent to cryogenics with the customizable thermal transport properties of nanofluids, thus creating the potential for next generation cryogenic fluids with enhanced thermophysical properties. This research demonstrates that by combining liquid oxygen (LOX) with Multi-Walled Carbon Nanotube (MWCNT) inclusion particles, effective thermal conductivity enhancements of greater than 30% are possible with nanoparticle volume fractions below 0.1%. Three distinct cryogenic nanofluids were created for the purposes of this research, each of which varied by inclusion particle type. The MWCNT\u27s used in this research varied in a number of physical characteristics, the most obvious of which are length and diameter. Lengths vary from 0.5 to 90 microns and diameters from 8 to 40 nanometers. The effective thermal conductivity of the various cryogenic nanofluids created for this research were experimentally determined by a custom made Transient Hot Wire (THW) system, and compared to each other and to more traditional nanofluids as they vary by type and particle volume fraction. This work also details the extensive theoretical, experimental, and numerical aspects of this research, including a rather detailed literature review of many of the salient sciences involved in the study of cryogenic nanofluids. Finally, a selection of the leading theories, models, and predictive equations is presented along with a review of some of the potential future work in the newly budding field of cryogenic nanofluids

7Be-recoil radiolabelling of industrially manufactured silica nanoparticles

Radiolabelling of industrially manufactured nanoparticles is useful for nanoparticle dosimetry in biodistribution or cellular uptake studies for hazard and risk assessment. Ideally for such purposes any chemical processing post production should be avoided as it may change the physico-chemical characteristics of the industrially manufactured species. In many cases proton irradiation of nanoparticles allows radiolabelling by transmutation of a tiny fraction of their constituent atoms into radionuclides. However, not all types of nanoparticles offer nuclear reactions leading to radionuclides with adequate radiotracer properties. We describe here a process whereby in such cases nanoparticles can be labelled with 7Be, which exhibits a physical halflife of 53.29 days and emits γ-rays of 478 keV energy and is suitable for most radiotracer studies. 7Be is produced via the proton-induced nuclear reaction 7Li(p,n)7Be in a fine-grained lithium compound with which the nanoparticles are mixed. The high recoil energy of 7Be-atoms gives them a range that allows the 7Be-recoils to be transferred from the lithium compound into the nanoparticles by recoil implantation. The nanoparticles can be recovered from the mixture by dissolving the lithium compound and subsequent filtration or centrifugation. The method has been applied to radiolabel industrially manufactured SiO2 nanoparticles. The process can be controlled in such a way that no alterations of the 7Be-labelled nanoparticles are detectable by dynamic light scattering, X-ray diffraction and electron microscopy. Moreover, cyclotrons with maximum proton energies of 17 to 18 MeV that are available in most medical research centres could be used for this purpose.JRC.I.4-Nanobioscience