Theoretical Investigation On Interfacial Properties Of Gold Nanomaterials

Gold(Au)nanomaterials are easy to be prepared and have biocompatility,thus Au nanomaterials present potential applications in multiple fields such as biosensor as well as biotherapy.To extend its potential applications to various fields,the modification through diverse functional groups is necessary.Understanding the influences from different ligands on the interfacial structure and properties of Au nanomaterials requires the knowledgement of the structure of different sized Au nanomaterials.In this thesis,we investigated the nucleation/growth mechanism of thiolate-protected Au nanoclusters and the light-driven switching processes of azobenzene self-assembly monolayers(SAMs)on Au nanomaterials.The size-evolution trend of the structure and properties of Au nanomaterials are concluded,which provied theoretical insight into the design of artificial Au nanomaterials.Our results are summarized as follows:1.The nucleation and growth mechanism of thiolated-protected Au nanocluster.The synthesis of monodisperse clusters with varying sizes on a large scale and the determination of precise atomic structures as well as properties challenge the chemists for many years,due to the lack of size-evolution information on the intermediates with different sizes.Based on density functional theory(DFT)calculations,we performed a systematical global minimum search on a series of Aum(SR)n with m and n ranging from 5 to 12.The preliminary nucleation and growth processes from homoleptic Au(I)-SR to core-stacked structure are revealed,following core-growth,core-sulotion and staple-motif-growth rules,respectively.The metastble isomers are also as important as the global minima in the growth process.By comparing these"intermediates" with the experimentally resolved clusters,the predicted inner Au4,Au6 and Au7 cores are found to serve as the building blocks.The unstable and reactive intermediates prone to participate in the further growth,which generates diverse product through diverse assembly pattern.The systematic study rationalizes the initial kinetxcally controlled "reduction-growth" and the thermodynamically controlled size-focusing stages observed in experiment.This work advances the knowledge of fundemental nucleation and growth mechanism of the Au nanoclusters,which is useful for designing new synthetic routes in the future.2.Tuning the collective switching behavior of azobenzene/Au hybrid materials.The combination of photo-responsive azobenzene(AB)and biocompatible Au nanomaterials possesses potential applications in diverse fields such as biosensor and thermotherapy.To explore the influence of azobenzene moities and Au substrates on the collective switching behavior,two different azobenzene derivatives(rigid biphenyl-controlled versus flexible alkoxyl chain-linked)and different Au substrates(planar Au(111)surface versus highly curved Au25 cluster)were chosen to form four AB@Au combinations.A reactive molecular dynamics(RMD)model considering both C-N-NN-C torsion and C-N=N kversion path was implemented to simulate the cis-to-trans switching process on Au substrates,and further investigate the collective effect of AB moities in monolayers.The major driving force for isomerization is demonstrated to be the torsion of the C-N=N-C dihedral angle with minor contribution from an inversion-assisted torsion pathway.The isomerzation process can be decomposed into the preliminary conformation switching stage and the latter relaxation stage.A gradual self-organization is observed during 40 ps.The Au substrate mainly affects the packing structure of AB monolayer,whilst the choice of different ABs tunes the intermolecular interaction in the monolayer.Flexible alkoxyl-linked AB turns out to achieve much faster conversion on Au cluster.For rigid biphenyl-based AB anchored on Au nanoparticle(AuNP),a competitive torsion between the biphenyl and C-N=N-C dihedral may delay the C-N=N-C dihedral torsion and the following isomerization process.After the AB molecules being anchored on Au(111)surface,the strong ?-? stacking between biphenyl units of neighboring AB chains accelerates the collective isomerization process of AB monolayer.The cooperation between functional AB monolayers and Au substrate determines the collective switching behavior of AB@Au materials.During the switching process,the energy difference between cis and trans states is calculated to be around 15 kcal/mol for a single azobenzene molecule.With the excellent biocompatibility of Au nanomaterial and energy storage/release of AB molecule,the new AB@Au hybrid material is considered as a candidate in thermotherapy of tumor.These results can guide rational design of AB@Au hybrid materials for different usages.