Looking at the interface of nanoparticles with solution NMR spectroscopy (Invited)
Résumé
Nanoparticles are one of the main pillars on which nanosciences and nanotechnology have grown over the last decades. Their high surface to mass ratio and properties that arise from their small sizes are strong motivations for their study. Usually selected for the properties of their core (ex. photoluminescence, plasmon band, …), many nanoparticles also possess a corona of surface ligands which plays a central role in their synthesis and mediates their interactions with the surrounding medium. These ligands can indeed make the nanoparticles compatible with solvents of different polarity, introduce functional motifs that bring new properties, or promote binding to specific targets. Characterizing this shell of ligands is therefore of prime importance to fully describe nanoparticles, better understand their behavior and tailor their design.
Solution NMR spectroscopy, is a powerful analysis technique well known and abundantly utilize in molecular chemistry. However, it is often blurred or even totally blind for ligands grafted on nanoparticles, especially when one is interested by the sites close to the surface, which are the more informative on the binding. An elegant way to prove that ligands are anchored onto nanoparticles is pulsed field gradient spin echo (PGSE) experiments, which give access to diffusion coefficients. These measures allow one to distinguish bound and free ligands on the basis of their translational mobility.
For nanoparticles with mixed ligands, the composition of their corona might even be more important to finely tune their properties. This composition can be conveniently studied with a reaction able to release the ligands in solution coupled to quantification by solution NMR. Such approach allows to estimate the relative affinity of the ligands for a given surface and draw a binding scale for competing ligands.
Examples on gold nanoparticles or quantum dots will be presented to illustrate these various ways of using solution NMR spectroscopy to study the ligands grafted on their surface.