Size Matters: Influence of Gold‐to‐Ligand Ratio and Sulfur‐Sulfur Distance of Linear Thioether Heptamers on the Size of Gold Nanoparticles

Abstract

A systematic investigation of two parameters steering the size of linear octadentate heptamer‐coated gold nanoparticles (AuNPs) is presented, being i) the chemical structure (sulfur‐sulfur distance) of the coating thioether heptamer ligand and ii) the ratio of ligand to tetrachloroauric acid (HAuCl4) reduced during the formation of the AuNPs. For this purpose, a novel terphenyl‐based thioether heptamer (Ter) is synthesized via an end‐capping oligomerization strategy, comprising an increased distance between neighboring sulfur atoms in the ligand backbone compared to the meta‐xylene‐ (Xyl) and tetraphenylmethane‐ (TPM) based heptamers. While for both investigated parameters a clear trend to various‐sized NPs is shown, a stronger influence in the resulting sizes is observed by alteration of ligand to gold‐ratio. Remarkable processability‐ and long‐term stability‐features were observed for AuNPs stabilized by the bulky tetraphenylmethane‐based heptamer (TPM).

Gold Nanoparticles Stabilized by Single Tripodal Ligands

“…AuNP syntheses from all five ligands 1 – 5 were carried out following an already implemented protocol based on a variation of the AuNP synthesis proposed by Brust et al., but also considering the dimensional differences of the various ligands. In the aqueous phase of a biphasic system, one equivalent of gold salt (HAuCl 4 ) for each sulfur atom in the ligand, dissolved in the organic phase (DCM), was added.…”

“…The limited availability of gold salt during the particles' syntheses is one of the factors defining the AuNP size and it might be that the dimensions of AuNPs being stabilized by a particular ligand structure are not fully explored with this restricted protocol. In order to explore the size limits of potentially stabilized AuNPs by 4 and 5 , the amount of HAuCl 4 used for the particle synthesis was doubled systematically . Thus the AuNP passivation features of 4 were also investigated with particles formed with 12 and 24 equivalents of chloroauric acid.…”

“…While rather complex and synthetically demanding dentritic ligands already displayed their ability to stabilize an entire AuNP, a more recent ligand design showed that also linear oligomers are able to stabilize an entire particle when bulky enough to sterically protect the surface of the ligand‐coated AuNP . With a series of linear oligomers of various sulfur–sulfur distances, a correlation between the size of the stabilized AuNP and both parameters, the distance between coordination points of these multivalent ligand structures and the equivalents of gold salt used has been observed . The study, however, also unraveled some limitations of the approach, namely an increasing loss over the control of the NP's size going along with a decrease in the stability of these AuNPs with increasing size.…”

“…In the past decade, the variety of ligand designs has been enlarged toward benzylic sulfide‐based oligomers due to their reversible binding to gold surfaces, enabling supramolecular approaches toward AuNP stabilization. The flexibility of the benzyl sulfide linkers enables favorable conformations of the ligands upon passivation of the AuNP and various ligand designs have been investigated, ranging from oligomeric, tripodal, and dendrimeric to even cage‐like thioether‐based ligands, most of them yielding AuNPs with narrow size distributions. Cages controlling the size of the nanoparticles were also developed for other noble metals like palladium, displaying catalytic activity .…”

An organic cage controlling the dimension and stability of gold nanoparticles

“…28,29 Based on those seminal works, Pankau et al proposed the use of benzylic thioether-based structures, 30,31 benefiting from the quasi-reversible character of the dispersion force-type bond between thioether and gold, 29,32 allowing the ligand to adopt its most favorable conformation upon Au NP formation. This ligand design allowed the synthesis of more complex sulfur-based ligands such as linear, [33][34][35][36] dendritic [37][38][39] and branched 40,41 oligomers that allow the synthesis of Au NPs passivated by a small, discrete number of ligands. While these oligomers were optimized to cover and thereby passivate a well-defined area of the particle's surface, the particle itself acted as template, shaping and arranging the multidentate oligomeric ligand by the attractive interaction between the ligand's thioether and the particle's gold surface.…”