Christopher L. Kitchens

Green synthesis of robust, biocompatible silver nanoparticles using garlic extract

This paper details a facile approach for the synthesis of stable and monodisperse silver nanoparticles performed at ambient/low temperature where Allium sativum (garlic) extract functions as the silver salt reducing agent during nanoparticle synthesis as well as the post-synthesis stabilizing ligands. Varying the synthesis conditions provides control of particle size, size-distribution, and kinetics of particle formation. Infrared spectroscopy, energy dispersive x-ray chemical analysis, and high performance liquid chromatography indicated that the carbohydrates present in the garlic extract are the most likely nanoparticle stabilizing chemistry. The synthesized silver nanoparticles also demonstrate potential for biomeical applications, owing to the 1) enhanced stability in biological media, 2) resistance to oxidation by the addition of H2O2, 3) ease and scalability of synthesis, and 4) lack of harsh chemicals required for synthesis. Cytotoxicity assays indicated no decrease in cellular proliferation for vascular smooth muscle cells and 3T3 fibroblasts at a concentration of 25 μg/ml, confirming that garlic extract prepared silver nanoparticles are ideal candidates for future experimentation and implementation into biomedical applications.

Effect of jet stretch and particle load on cellulose nanocrystal-alginate nanocomposite fibers.

Alginate fibers have found many applications such as the preparation of dressings to treat exuding wounds, drug delivery, enzyme immobilization, etc.; however, their use is limited due to poor mechanical properties. Cellulose nanocrystals (CNCs) were isolated from cotton and introduced into calcium alginate fibers with the goal of improving their strength and modulus. The isolated CNCs are elongated nanoparticles of crystalline cellulose with an average length of 130 nm with a standard deviation (s) of 63 nm, an average width of 20.4 nm (s = 7.8 nm), and an average height of 6.8 nm (s = 3.3 nm). The CNCs were mixed with an aqueous sodium alginate dope solution and wet spun into a CaCl(2) bath to form fibers. It was found that if the apparent jet stretch (ratio of the fiber draw velocity to extrusion velocity) is kept constant, addition of the nanocrystals reduces the tensile strength and modulus of the material; however, a small concentration of CNCs in the dope solution increases the tensile energy to break and enables an increase in the fiber spinning apparent jet stretch ratio by nearly 2-fold at up to 25% CNCs load; the maximum ratio of 4.6 is observed at 25 wt % CNC loading as compared to a maximum of 2.4 for the native alginate. Mechanical testing showed a 38% increase in tenacity and a 123% increase in tensile modulus with 10 wt % CNCs loading and an apparent jet stretch of 4.2. The data suggest that alignment of the nanocrystals in the composites is a key factor influencing the mechanical properties. CNCs have potential to become a biocompatible, renewable, and cost-effective solution to reinforce alginate fibers.

Quantitative Measurement of Ligand Exchange on Iron Oxides via Radiolabeled Oleic Acid

Ligand exchange of hydrophilic molecules on the surface of hydrophobic iron oxide nanoparticles produced via thermal decomposition of chelated iron precursors is a common method for producing aqueous suspensions of particles for biomedical applications. Despite the wide use, relatively little is understood about the efficiency of ligand exchange on the surface of iron oxide nanoparticles and how much of the hydrophobic ligand is removed. To address this issue, we utilized a radiotracer technique to track the exchange of a radiolabeled (14)C-oleic acid ligand with hydrophilic ligands on the surface of magnetite nanoparticles. Iron oxide nanoparticles functionalized with (14)C-oleic acid were modified with poly(ethylene glycol) with terminal functional groups including, L-3,4-dihydroxyphenylalanine, a nitrated L-3,4-dihydroxyphenylalanine, carboxylic acid, a phosphonate, and an amine. Following ligand exchange, the nanoparticles and byproducts were analyzed using liquid scintillation counting and inductively coupled plasma mass spectroscopy. The labeled and unlabeled particles were further characterized by transmission electron microscopy and dynamic light scattering to determine particle size, hydrodynamic diameter, and zeta potential. The unlabeled particles were characterized via thermogravimetric analysis and vibrating sample magnetometry. Radioanalytical determination of the (14)C from (14)C-oleic acid was used to calculate the amount of oleic acid remaining on the surface of the particles after purification and ligand exchange. There was a significant loss of oleic acid on the surface of the particles after ligand exchange with amounts varying for the different functional binding groups on the poly(ethylene glycol). Nonetheless, all samples demonstrated some residual oleic acid associated with the particles. Quantification of the oleic acid remaining after ligand exchange reveals a binding hierarchy in which catechol derived anchor groups displace oleic acid on the surface of the nanoparticles better than the phosphonate, followed by the amine and carboxylic acid groups. Furthermore, the results show that these ligand exchange reactions do not necessarily occur to completion as is often assumed, thus leaving a residual amount of oleic acid on the surface of the particles. A thorough analysis of ligand exchange is required to develop nanoparticles that are suitable for their desired application.

Structural and Thermal Analysis of Lipid Vesicles Encapsulating Hydrophobic Gold Nanoparticles

The structure and stability of hybrid lipid vesicles containing bilayer-encapsulated hydrophobic nanoparticles is dependent upon lipid phase behavior. By embedding stearylamine-stabilized gold nanoparticles in dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylglycerol vesicles, we show that encapsulation at lipid to nanoparticle ratios from 10,000:1 to 5000:1 leads to bilayer thickening and hydrophobic mismatch, favoring nanoparticle inclusion in gel phase vesicles. High loadings lead to large increases in the gel to fluid melting temperature upon heating and significant hysteresis on cooling, which cannot be attributed solely to excess free ligand. This behavior is due to a cooperative effect of excess free SA ligand and nanoparticle embedment. Nanoparticle clustering was observed during lipid melting and could be reversed upon lipid freezing owing to lateral capillary forces within the bilayer. The impact of nanoparticle embedment on vesicle structure and properties at such low concentrations is reminiscent of hydrophobic proteins, suggesting that the underlying lipid biophysics between proteins and nanoparticle are similar and may provide a predictive design tool for therapeutic applications.

A spectroscopic and computational exploration of the cybotactic region of gas-expanded liquids: methanol and acetone.

Local compositions in supercritical and near-critial fluids may differ substantially from bulk compositions, and such differences have important effects on spectroscopic observations, phase equilibria, and chemical kinetics. Here, we compare such determinations around a solute probe dissolved in CO2-expanded methanol and acetone at 25 degrees C from solvatochromic experiments with molecular dynamics simulations. UV/vis and steady-state fluorescence measurements of the dye Coumarin 153 in the expanded liquid phase indicate preferential solvation in both the S0 and S1 states by the organic species. Simple dielectric continuum models are used to estimate local compositions from the spectroscopic data and are compared to molecular dynamics simulations of a single C153 molecule dissolved in the liquid phase at bubble point conditions. The simulations provide information about the local solvent structure around C153. They suggest the presence of large solvent clustering near the electron-withdrawing side of the probe. Preferential solvation exists in both the S0 and S1 states, but a large disagreement between simulation and experiment exists in the S1 state. Potential reasons for this disparity are discussed.

Discrete nanoparticles induce loss of Legionella pneumophila biofilms from surfaces

Abstract Nanoparticles (NPs) have been shown to induce dispersal events in microbial biofilms but the mechanism of the dispersal is unknown. Biofilms contaminate many man-made aquatic systems such as cooling towers, spas and dental lines. Within these biofilms, Legionella pneumophila is a primary pathogen, leading to these environments serving as sources for disease outbreaks. Here we show a reduction in biofilm bio-volume upon treatment with citrate-coated 6-nm platinum NPs, polyethylene glycol (PEG)-coated 11-nm gold NPs, and PEG-coated 8-nm iron oxide NPs. Treatment with citrate-coated 8-nm silver NPs, however, did not reduce biomass. The synthesis of NPs that remain dispersed and resist irreversible aggregation in the exposure media appears to be a key factor in the ability of NPs to induce biofilm dispersal.

Cellulose Nanocrystal Reinforced Alginate Fibers—Biomimicry Meets Polymer Processing

This research takes a biomimetic approach to the design of polymer nanocomposites and demonstrates structure-property relationships that are controllable via processing conditions. Cellulose nanocrystals (CNCs) measuring 130 nm (length) × 20.4 nm (width) × 6.8 nm (height) were isolated from cotton by sulfuric acid hydrolysis and were incorporated in an alginate fiber wet spinning dope solution. Incorporating CNCs within the alginate fiber enables a nearly two-fold increase in the apparent jet stretch (JA ), ratio of the linear draw speed to extrusion velocity. Fiber spinning at a constant JA resulted in an unexpected decrease in fiber modulus and increase in toughness. Alternatively, fiber spinning at the maximum JA resulted in modulus increases that are predicted by the Halpin-Tsai model and the Hui-Shia model. Wide-angle X-ray diffraction (WAXD) was used to elucidate the structure and orientation of cellulose nanocrystals (CNC) within the alginate nanocomposite fibers and provide correlations with mechanical property enhancements. The spread of the azimuthal intensity distribution of the CNC (2,0,0) reflection increased with higher CNC loads until the nanoparticles within the matrix spiraled around the longitudinal axis. The appearance of a spiral angle with increasing CNC load resulted in a step reduction in modulus and increase in toughness. Increased fiber stretching during spinning retarded the appearance of the spiral assembly and increased CNC alignment. This spiral orientation is also observed in native cellulose fibers as a microfibril angle and is deterministic of their mechanical properties.

Small Particle Driven Chain Disentanglements in Polymer Nanocomposites | NIST

Using neutron spin-echo spectroscopy, x-ray photon correlation spectroscopy, and bulk rheology, we studied the effect of particle size on the single-chain dynamics, particle mobility, and bulk viscosity in athermal polyethylene oxide-gold nanoparticle composites. The results reveal a ≈25% increase in the reptation tube diameter with the addition of nanoparticles smaller than the entanglement mesh size (≈5u2009u2009nm), at a volume fraction of 20%. The tube diameter remains unchanged in the composite with larger (20xa0nm) nanoparticles at the same loading. In both cases, the Rouse dynamics is insensitive to particle size. These results provide a direct experimental observation of particle-size-driven disentanglements that can cause non-Einstein-like viscosity trends often observed in polymer nanocomposites.

Physical transformations of iron oxide and silver nanoparticles from an intermediate scale field transport study

In recent years, there has been increasing concern regarding the fate and transport of engineered nanoparticles (NPs) in environmental systems and the potential impacts on human and environmental health due to the exponential increase in commercial and industrial use worldwide. To date, there have been relatively few field-scale studies or laboratory-based studies on environmentally relevant soils examining the chemical/physical behavior of the NPs following release into natural systems. The objective of this research is to demonstrate the behavior and transformations of iron oxide and silver NPs with different capping ligands within the unsaturated zone. Here, we show that NP transport within the vadose zone is minimal primarily due to heteroaggregation with soil surface coatings with results that >99xa0% of the NPs remained within 5xa0cm of the original source after 1 year in intermediate-scale field lysimeters. These results suggest that transport may be overestimated when compared to previous laboratory-scale studies on pristine soils and pure minerals and that future work must incorporate more environmentally relevant parameters.

Static light scattering of triaxial nanoparticle suspensions in the Rayleigh-Gans-Debye regime: application to cellulose nanocrystals

A simple method has been developed to measure the dimensions of triaxial nanoparticles through light scattering data. The application of Debyes expansion of the form factor has been extended beyond the second moment of the difference of distances (radius of gyration). Analytical expressions have been derived for the fourth and sixth moments that are valid for all nanoparticle shapes. The moments provide information about the aspect ratio of the nanoparticles regardless of the geometry. If a shape is assumed, simple algebraic expressions for the moments can be obtained for monodisperse and polydisperse samples, which can be used to solve for specific nanoparticle dimensions. The methods described here have been applied to static light scattering measurements of dilute aqueous suspension of triaxial cellulose nanocrystals isolated from cotton. When the polydispersity is included, the dimensions determined from light scattering data (6.4 × 28 × 152 nm) are in close agreement with atomic force microscopy measurements. This methodology can be easily applied to nanoparticles of complex shapes and extended to other scattering techniques; it would be especially useful for polymer based nanoparticles and biological macromolecules.