Proline stabilizes amphiphilic gold nanoparticles via hydrophobic interactions

Colloidal dispersions are key in many fields of science and technology. Recently, we have shown that small molecules can stabilize dispersions of nanoscale objects, such as proteins and nanoparticles by screening their net attractive interactions. This new effect is essentially the opposite of the well-known salt screening of electrostatic interaction. Here we show that small molecule stabilization of nanoparticles is a phenomenon strongly linked to the hydrophobic content of the particles as well as to the strength of their hydrophobic attraction. We compare the effect of proline on gold nanoparticles coated with 11-mercaptoundecane sulfonate (MUS) at varying percentages of the hydrophobic ligand octanethiol (OT). We show that the larger the percentage of OT, the larger the proline stabilization effect is. We also compare the effect of proline on water dispersions of nanoparticles with that on heavy water dispersions. In the latter, the hydrophobic effect plays a bigger role. We find that in D₂O, proline stabilization is larger. We also compare the effect of proline on the same MUS:OT gold nanoparticles before and after an annealing process that is known to render the particle more hydrophilic. Proline is more effective on the particles before annealing. Finally, we study the effect of proline on non-aggregating allMUS nanoparticles. We find that proline stabilization of these particles is mainly due to a reduction in the long-range attraction coefficient. Overall, we show that proline stabilizes nanoparticle dispersions more effectively as the hydrophobic attraction between nanoparticles increases.