Dressing up a Magnetic Nanoparticle at Atomic Resolution: Molecular Simulation of Full Carrier Grafting by Self-Assembled Monolayers

We present an all-atom model mimicking superparamagnetic iron oxide nanoparticles functionalized by alkyl phosphonic acids and demonstrate the prediction of grafting density, X-ray diffraction profiles, and solvent-dependent particle radii. The inherent complexity of this nanosystem calls for careful model preparation to avoid bias from human intuition. We suggest a cascade of simulated annealing steps for (i) providing reasonable starting points for the iron oxide particle and (ii) grafting by self-assembled monolayers. The latter was performed as a function of the number of phosphonic acid molecules deposited, thus offering an unbiased assessment of the grafting density and structural alignment of the tail groups that define the outer shell of the nanoparticle. The overall protocol is widely transferable and implies moderate computational costs as compared to the explicit modeling of phosphonic acid association from solution. In turn, solvent effects on the surface structure of the nanoparticle model initially prepared in the gas phase were considered using hexane, propanol, and water, respectively. Voronoi analyses clearly demonstrated the solvent-dependent bundling of the terminal alkyl groups of the phosphonic acids grafted onto iron oxide nanoparticles. To this end, the nanoparticle-solvent interface gives rise to nanometer-scale patterns of differently oriented monolayer structures.