Helmut Hinterwirth, Stefanie Kappel, Thomas Waitz, Thomas Prohaska, Wolfgang Lindner, and Michael Lämmerhofer ACS Nano Article ASAP
These researchers develop a method to quantify thiol ligand density on gold nanoparticles by employing inductively coupled plasma mass spectrometry (ICP-MS). By comparing the Au/S ratio (measured using ICP-MS) against the nanoparticle diameteter (measured by TEM), they were able to assess ligand density with uncertainties less than 10%. An advantage of this method is that it does not depend on the amount of sample prepared which is often affected by washing and other factors. This study goes on to compare the densities measured for various ligand spacer lengths with the conclusion that longer linkers assemble less densely than shorter linkers. This is in opposition to several studies which measure increased density with increased linker spacing, a result they atribute to van-der-walls interactions. They defend their contrary findings with the fact that lipophilic and hydrophilic linkers assemble very similarly according to their methods.