AgNP/Alginate Nanocomposite hydrogel for antimicrobial and antibiofilm applications.

The synthesis and specific surface functionalization of antimicrobial silver nanoparticles (AgNPs) and their incorporation into an alginate hydrogel is described. Divalent cation-mediated ionic crosslinking was used to disperse the AgNPs throughout the gel, made possible by -COO- cross-linking sites provided by the surface-enhanced nanoparticles, inspired by the classic egg-box model crosslinking of calcium alginate. An AgNP concentration, 10-20 µg g-1 increased hygrogel elasticity, viscosity, and shear resistance by 45, 30, and 31% respectively. Cryo-TEM revealed evenly distributed AgNP assemblies of discrete AgNPs throughout the gel matrices. FTIR-ATR indicated AgNPs were involved in alginate carboxylate-Ca2+-COO-AgNP crossbridging, which was not achieved through mixing of AgNPs into preformed gels. Live/dead fluorometric assays determined a minimal bactericidal concentration of 25 µg g-1 Ag for 6 microorganisms. Anti-biofilm assays showed species-dependent cell death of 44 -61%, with limited silver ion release of 0.41% and 1.1% after 7 days for Gram positive and negative bacteria, respectively.

Towards Dual-Functionality Spin-Crossover Complexes.

The multistep synthesis of a versatile new 4-substituted 3,5-bis(2-pyridyl)-1,2,4-triazole (Rdpt) ligand, 4-[4-(2-aminomethyl)phenyl]-3,5-bis(2-pyridyl)-4 H-1,2,4-triazole (apdpt), is reported, which features a reactive aminomethyl para-substituent on the phenyl group that points "out of the back" of the triazole. This enables further functionalisation under mild conditions by using a range of esters to form an amide link. Specifically, this proof of principle study demonstrates the synthesis of apdpt successfully appended with gold-binding thioctic acid (tpdpt), graphene-binding/emissive pyrene/propylpyrene (prdpt/pbdpt), and a Langmuir-Blodgett film-forming polyethylene glycol (PEG) tail (pgdpt). These ligands are subsequently reacted with [Fe(pyridine)4 (NCBH3 )2 ] to give the mononuclear iron(II) complexes [Fe(Rdpt)2 (NCBH3 )2 ]⋅solvent, in which Rdpt/solvent is tpdpt/2.5 H2 O (1), prdpt/0.5 CHCl3 ⋅H2 O (2), and pbdpt/0.5 CHCl3 ⋅2 H2 O (3), as red powders. Magnetic studies on these powders indicate that the complexes undergo only very gradual and incomplete spin crossover, from completely or mostly high spin at 300 K, to half or three-quarters high spin at 50 K. Gold nanoparticles are successfully functionalised with the thioctic acid tpdpt ligand to give tpdpt@Au with an average diameter (as determined by TEM) of (3.1±0.7) nm. Preliminary studies on the two pyrene systems in dimethylformamide show that upon excitation at λ=345 nm the blue fluorescence observed for the free ligands is retained, essentially unaffected, in the respective complexes.

Proof of Principle: Immobilisation of Robust CuII3 TbIII -Macrocycles on Small, Suitably Pre-functionalised Gold Nanoparticles.

In a proof-of-principle study, a soluble macrocyclic single-molecule magnet (SMM) containing a CuII3 TbIII magnetic core was covalently grafted onto small gold nanoparticles pre-functionalised with carboxylate-terminated tethers. A modified microemulsion method allowed production of the small and monodisperse nanoparticles (approximately 3.5 nm in diameter) for the chemisorption of a large amount of intact macrocyclic complexes in the hybrid system.

Size selectable nanoparticle assemblies with magnetic anisotropy tunable across the superparamagnetic to ferromagnetic range.

We present a novel approach for the preparation of magnetic nanoparticle clusters of controlled size and selectable magnetic anisotropy, which provides materials with properties selectable for biomedical applications and as components in magnetically responsive nanocomposites. The assembly process is based on a ligand desorption strategy and allows selection of nanoparticle size and temporal control over final cluster size. Detailed NMR analysis of the suspensions pinpoints the role of particle size in controlling the interparticle interactions, within the clusters, which effectively determine the anisotropy. Colloidal interaction modelling confirms this interpretation and provides a means to predict both colloidal stability and magnetic anisotropy.

Insights into the Partitioning Behavior of Secondary Surfactants in a Microemulsion-Based Synthesis of Metal Nanoparticles: A DLS and 2D NMR Spectroscopic Investigation.

Diffusion-ordered NMR spectroscopy (DOSY) and nuclear Overhauser effect spectroscopy (NOESY) have been used to explore the diffusion and partitioning behavior of secondary surfactants added to suspensions of reverse micelles (RMs) containing either silver or gold nanoparticles (NPs), with an aim of advancing our understanding of the mechanism of metal NP extraction and/or surface functionalization with specific capping agents as performed during a microemulsion-based synthesis. We have coupled these NMR techniques with corresponding dynamic light scattering (DLS) measurements of RMs, with and without encapsulated metal NPs, upon addition of secondary surfactants. Using oleylamine (OAm), oleic acid (OA), dodecylamine (DDAm), and dodecanethiol (DDT), we show that all four secondary surfactants can rapidly diffuse into/out of the RM environment with their head groups in close proximity to the RM interior and encapsulated water molecules; however, surfactant molecules containing a terminal -NH2 or -COOH group undergo a persistent association with the molecules of the RMs, thus solubilizing and partially sequestering a portion of the total concentration of these secondary agents within the RM interface for a lengthened period of time (in relation to the time frame of the DOSY experiments) and slowing their rate of exchange with freely diffusing molecules in the bulk solvent. The extraction of Ag or Au NPs from RMs into organic phase was determined to be critically dependent on the type and concentration of secondary surfactant added to the system, with DDT proving to be most efficient for the extraction of Ag NPs, while OA was shown to be most efficient for Au NPs. Consideration of the results obtained from this particular combination of techniques has provided new knowledge with respect to dynamic metal NP-containing microemulsion systems.

The effect of cholesterol on membrane dynamics on different timescales in lipid bilayers from fast field-cycling NMR relaxometry studies of unilamellar vesicles.

The general applicability of fast field-cycling nuclear magnetic resonance relaxometry in the study of dynamics in lipid bilayers is demonstrated through analysis of binary unilamellar liposomes composed of 1,2-dioleoyl-sn-glycero-3-posphocholine (DOPC) and cholesterol. We extend an evidence-based method to simulating the NMR relaxation response, previously validated for single-component membranes, to evaluate the effect of the sterol molecule on local ordering and dynamics over multiple timescales. The relaxometric results are found to be most consistent with the partitioning of the lipid molecules into affected and unaffected portions, rather than a single averaged phase. Our analysis suggests that up to 25 mol%, each cholesterol molecule orders three DOPC molecules, providing experimental backup to the findings of many molecular dynamics studies. A methodology is established for studying dynamics on multiple timescales in unilamellar membranes of more complex compositions.

Temperature and size-dependence of membrane molecular dynamics in unilamellar vesicles by fast field-cycling NMR relaxometry.

New methods to study dynamics in lipid bilayers are of interest particularly where they may bridge the gap between conventional experimental techniques and molecular dynamics simulations. Fast field cycling nuclear magnetic resonance relaxometry can provide valuable information as it is sensitive to dynamic processes that occur over a broad time scale. By analysis of data recorded for large unilamellar liposomes composed of 1,2-dimyristoyl-sn-glycero-3-posphocholine (DMPC) or 1,2-dioleoyl-sn-glycero-3-posphocholine (DOPC) at different temperatures and sizes, we validate an evidence-based approach to studying dynamics by relaxometry. Specifically, the number and form of the spectral density contributions from a range of dynamic processes are determined. This success of the approach strongly suggests its general applicability for the study of dynamics in membranes of more complex composition and for parameterizing molecular dynamics simulations.

Hierarchical gold-decorated magnetic nanoparticle clusters with controlled size.

We present a new route to stable magnetic-plasmonic nanocomposite materials with exceptional control over composite size and very high monodispersity. The method involves the assembly of magnetic iron oxide nanoparticles, of any size in the superparamagnetic size range, whose steric repulsion is gradually reduced by competitive stabilizer desorption arising from the presence of a tertiary silica phase. Subsequent addition of gold nanoparticles results in hierarchical assemblies in the form of gold-decorated magnetic nanoparticle clusters, in a range of possible sizes from 20 to 150 nm, selected by the timing of the addition. This approach adds plasmonic and chemical functionality to the magnetic clusters and improves the physical robustness and processability of the suspensions. Most critically, detailed NMR relaxation analysis demonstrates that the effect of the gold NPs on the interaction between bulk solvent and the magnetic moments of the cluster is minimal and that the clusters remain superparamagnetic in nature. These advantages enhance the potential of the materials as size-selected contrast agents for magnetic resonance imaging. The possibility of generalizing the approach for the production of hierarchical assemblies of variable composition is also demonstrated.

Interpretation of molecular dynamics on different time scales in unilamellar vesicles using field-cycling NMR relaxometry.

Fast field-cycling (FFC) and rotating-frame nuclear magnetic resonance relaxometry were used to study molecular and collective dynamics in unilamellar liposome systems. Relaxation data for liposomes of diameter about 100 nm composed of 1,2-dimyristoyl-sn-glycero-3-posphocholine (DMPC) or 1,2-dioleoyl-sn-glycero-3-posphocholine (DOPC) were obtained. The Larmor frequency dependence of the spin-lattice relaxation rates was interpreted in terms of clearly defined relaxation mechanisms associated with the underlying molecular dynamics. The physical parameters obtained from the analysis are consistent with values available in the literature obtained from a range of experimental techniques. This work establishes the potential of our approach to study dynamics in liposomal samples of more complex lipid composition.

Nonaqueous magnetic nanoparticle suspensions with controlled particle size and nuclear magnetic resonance properties.

We report the preparation of monodisperse maghemite (gamma-Fe2O3) nanoparticle suspensions in heptane, by thermal decomposition of iron(III) acetylacetonate in the presence of oleic acid and oleylamine surfactants. By varying the surfactant/Fe precursor mole ratio during synthesis, control was exerted both over the nanocrystal core size, in the range from 3 to 6 nm, and over the magnetic properties of the resulting nanoparticle dispersions. We report field-cycling 1H NMR relaxation analysis of the superparamagnetic relaxation rate enhancement of nonaqueous suspensions for the first time. This approach permits measurement of the relaxivity and provides information on the saturation magnetization and magnetic anisotropy energy of the suspended particles. The saturation magnetization was found to be in the expected range for maghemite particles of this size. The anisotropy energy was found to increase significantly with decreasing particle size, which we attribute to increased shape anisotropy. This study can be used as a guide for the synthesis of maghemite nanoparticles with selected magnetic properties for a given application.

Linear assemblies of magnetic nanoparticles as MRI contrast agents.

Using a one-step procedure we have prepared magnetic fluids comprising of polyelectrolyte stabilized magnetite nanoparticles. These nanocomposites are comprised of linear, chain-like assemblies of magnetic nanoparticles, which can be aligned in parallel arrays by an external magnetic field. We have shown the potential use of these materials as contrast agents by measuring their MR response in live rats. The new magnetic fluids have demonstrated good biocompatibility and potential for in vivo MRI diagnostics.