The ability to control the properties of monolayer protected gold nanoparticles (MPNPs) discloses unrevealed features stemming from collective properties of the ligands forming the monolayer and presents opportunities to design new materials. To date, the influence of ligand end-group size and capacity to form hydrogen bonds on structure and hydration of small MPNPs (<5 nm) has been poorly studied. Here, we show that both features determine ligands order, solvent accessibility, capacity to host hydrophobic compounds and interfacial properties of MPNPs. The polarity perceived by a radical probe and its binding constant with the monolayer investigated by electron spin resonance is rationalized by molecular dynamics simulations, which suggest that larger space-filling groups - trimethylammonium, zwitterionic and short polyethylene glycol - favor a radial organization of the thiolates, whereas smaller groups - as sulfonate - promote the formation of bundles. Zwitterionic ligands create a surface network of hydrogen bonds, which affects nanoparticle hydrophobicity and maximize the partition equilibrium constant of the probe. This study discloses the role of the chemistry of the end-group on monolayer features with effects that span from molecular- to nano-scale and opens the door to a shift in the conception of new MPNPs exploiting the end-group as a novel design motif.

Department of Chemical and Pharmaceutical Sciences and INSTM Trieste Research Unit University of Trieste 34127 Trieste

Department of Chemistry G Ciamician University of Bologna 1 40126 Bologna

Department of Engineering and Architecture University of Trieste 34127 Trieste

Department of Physics University of Trieste 34127 Trieste

Elettra Sincrotrone Trieste S C p A 34149 Trieste

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Spotting Local Environments in Self-Assembled Monolayer-Protected Gold Nanoparticles