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Investigation of inorganic, organic and hybrid nanoparticles by diffusion ordered NMR spectroscopy

woensdag, 29 april, 2009 - 16:30
Campus: Brussels Humanities, Sciences & Engineering campus
K
auditorium P. Janssens
Luk Van Lokeren
doctoraatsverdediging

In diffusion ordered NMR spectroscopy (DOSY NMR) translational motions of nuclear spins are traced, which gives rise to a two-dimensional so-called DOSY spectrum combining the chemical shift dimension with a diffusion dimension, thus discriminating between chemical compounds both on the basis of their chemical shifts and diffusion coefficients. The development and progresses of DOSY NMR opened doors to industrial, technological and fundamental applications, not only to investigate molecular translations, but also to estimate the molecular size and mass of (macro)molecules, to study complexes and aggregates, and to probe chemical reactions, interactions and exchange in chemical systems. The diffusion coefficients sampled cover structural units from free molecules to particles on the nanometric scale, so-called nanoparticles. Such particles can be micelles or vesicles, polymers, rigid organic or inorganic structures or even hybrid − functionalized − moieties.

Their large surface-to-volume ratio makes them extremely useful for a wide range of industrial applications like catalysts, nanofillers, nanomembranes,… Hence, a fine-tuned understanding of their surface chemistry is essential since functionalizing such nanoparticles by organic complexing ligands deeply modifies their chemical features. The surface can be functionalized in order, to prevent aggregation, but also to introduce a hydrophobic protective shell, to promote selective binding or to optimize transferability into organic solvents or polymers.
In a first part of this work, DOSY NMR was implemented to reveal exchange processes between inorganic nanoclusters and free molecules but also to demonstrate the aggregation of several nanoclusters into so-called nanobuilding blocks.

In a second part, hybrid nanoparticles, i.e. organically functionalized inorganic nanoparticles, were systematically investigated starting from model solutions over blank solutions to nanoparticle dispersions in order to discriminate between solvation and interaction phenomena also present in model solutions and association/dissociation phenomena resulting from the presence of nanoparticles. The results illustrated the ability of DOSY NMR to analyze thermodynamic and kinetic features of interaction and exchange processes in such hybrid nanoparticle systems.

In a third part, DOSY NMR was implemented to investigate organic supramolecular structures, micelles, generated upon self-aggregation of surface-active compounds - surfactants - in solution above the critical micelle concentration. Combining DOSY with NOESY NMR allowed not only to characterize size and shape of the micelles formed, but also to elucidate the spatial arrangement of surfactant molecules in pure micelles, and, more important, in multi-component micellar mixtures containing a polymer and a salt.