We report a solution NMR-based analysis of (16-mercaptohexadecyl)­trimethylammonium bromide (MTAB) self-assembled monolayers on colloidal gold nanospheres (AuNSs) with diameters from 1.2 to 25 nm and gold nanorods (AuNRs) with aspect ratios from 1.4 to 3.9. The chemical shift analysis of the proton signals from the solvent-exposed headgroups of bound ligands suggests that the headgroups are saturated on the ligand shell as the sizes of the nanoparticles increase beyond ∼10 nm. Quantitative NMR shows that the ligand density of MTAB-AuNSs is size-dependent. Ligand density ranges from ∼3 molecules per nm2 for 25 nm particles to up to 5−6 molecules per nm2 in ∼10 nm and smaller particles for in situ measurements of bound ligands; after I2/I– treatment to etch away the gold cores, ligand density ranges from ∼2 molecules per nm2 for 25 nm particles to up to 4−5 molecules per nm2 in ∼10 nm and smaller particles. T 2 relaxation analysis shows greater hydrocarbon chain ordering and less headgroup motion as the diameter of the particles increases from 1.2 nm to ∼13 nm. Molecular dynamics simulations of 4, 6, and 8 nm (11-mercaptoundecyl)­trimethylammonium bromide-capped AuNSs confirm greater hydrophobic chain packing order and saturation of charged headgroups within the same spherical ligand shell at larger nanoparticle sizes and higher ligand densities. Combining the NMR studies and MD simulations, we suggest that the headgroup packing limits the ligand density, rather than the sulfur packing on the nanoparticle surface, for ∼10 nm and larger particles. For MTAB-AuNRs, no chemical shift data nor ligand density data suggest that two populations of ligands that might correspond to side-ligands and end-ligands exist; yet T 2 relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.

Formation of a protein corona around nanoparticles when immersed into biological fluids is well-known; less studied is the formation of lipid coronas around nanoparticles. In many cases, the identity of a nanoparticle-acquired corona determines nanoparticle fate within a biological system and its interactions with cells and organisms. This work systematically explores the impact of nanoparticle surface chemistry and lipid character on the formation of lipid coronas for 3 different nanoparticle surface chemistries (2 cationic, 1 anionic) on 14 nm gold nanoparticles exposed to a series of lipid vesicles of 4 different compositions. Qualitative (plasmon band shifting, ζpotential analysis, dynamic light scattering on the part of the nanoparticles) and quantitative (lipid liquid chromatography/ mass spectrometry) methods are developed with a "pull-down" scheme to assess the degree of lipid corona formation in these systems. In general, cationic nanoparticles extract 60−95% of the lipids available in vesicles under the described experimental conditions, while anionic nanoparticles extract almost none. While electrostatics apparently dominate the lipid−nanoparticle interactions, primary amine polymer surfaces extract more lipids than quaternary ammonium surfaces. Free cationic species can act as lipid-binding competitors in solution.

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T h er e a c t i o no f[ R u (η 6-cymene)Cl 2 ] 2 and PPh 2 Cl in the ratio 1:2 gives a stable [Ru(η 6-cymene) Cl 2 (PPh 2 Cl)] complex. Attempts to make the cationic [Ru(η 6-cymene)Cl(PPh 2 Cl) 2 ]Cl with excess PPh 2 Cl and higher temperatures led to adventitious hydrolysis and formation of [Ru(η 6-cymene)Cl 2 (PPh 2 OH)]. Attempts to make a phosphinite complex by reacting [Ru(η 6-cymene)Cl 2 ] 2 with PPh 2 Cl in the presence of an alcohol results in the reduction of PPh 2 Cl to give [Ru(η 6-cymene)Cl 2 (PPh 2 H)] and the expected phosphinite. The yield of the hydride complex is highest when the alcohol is 1-phenyl-ethane-1,2-diol. All three half-sandwich complexes are characterized by X-ray crystallography. Surprisingly, the conversion of chlorodiphenylphosphine to diphenylphosphine is mediated by 1-phenyl-ethane-1,2-diol even in the absence of the ruthenium half-sandwich precursor.