Very small noble metal nanoparticles are in fact molecules with definite molecular formulae and structures, which have been determined by mass spectrometry and single-crystal x-ray diffraction, respectively. We have found that current models are far too simple to account for the chemical and physical properties of these complex and structurally diverse materials. Contemporary thinking is that molecular structures are determined by the details of the metal core, however we have found that the ligand can play a determinant role. For example, families of molecular silver nanoparticles are produced with aliphatic ligands, such as glutathione, captopril, or cysteine, as demonstrated by gel electrophoresis and mass spectrometry. In contrast, phenyl-containing ligands have been found to produce only a 44-atom Ag nanoparticle that is extremely stable and that is not observed with aliphatic ligands. The unprecedented stability of this nanoparticle resulted in (i) a truly single-sized molecular product, (ii) ~140 g of pure product from a single batch, and (iii) an opportunity to crystallize the nanoparticle and understand its protective ligand shell, atomic and electronic structure, and thereby its stability. Notably, this has provided key insights into the stability of silver nanoparticles, which has been a major obstacle. Further, the nanoparticle molecular crystal has a framework structure that is predicted to have very unusual mechanical properties, with both structure and response being controlled by the ligands. From these results it is evident that ligands can have a very significant effect on the materials synthesized and in fact, with judicious choices, they be used as multiscale structure-directing synthons.

Role of ligands in the structure and properties of molecular silver nanoparticles and their assemblies

249th National Meeting (2015)

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