Study of Water-Soluble p-MBA-Protected Gold Nanoclusters and Their Superstructures

Abstract

The development of gold nanoclusters has made remarkable progress enriching the research field of nanomaterial science. The breakthroughs in synthetic chemistry enable the preparation of functional materials using gold nanoclusters. The assembly of gold nanoclusters into well-defined structures creates a system with new optical and electronic properties and functionalities, often sensitive to single atom or single electron in the system. Assembly of superstructures with molecular precision requires not only mastering the size and structural uniformities of building blocks but also understanding reaction kinetics and the interplay between competing reaction mechanisms. In the light of this, a robust synthesis of covalently linked nanoscale constructs from gold nanoclusters with atomically precise structures was developed, using common thiolation chemistry. The generality of the synthesis was proven by using various molecules connecting the particles, producing in high yield a large number of geometrical structures of linked gold clusters. The successful gel electrophoresis separation of dimers, trimers, and tetramers allowed detailed characterization and systematic studies of reaction mechanism of the created systems. Multimers of linked structures exhibit additional transitions in their UV-vis spectrum at 630 nm and 810 nm, indicating the presence of hybridized localized surface plasmon resonance (LSPR) modes. Nearly constant distances observed in transmission electron microscope (TEM) between singular gold clusters and the analysis of the oligomer yields supported with MD simulations suggest that gold nanoclusters are covalently bound by formation of singular molecular bridges. The molecular bridge is most likely formed by formation of an S-S bond between the terminal sulfurs of two (or more) dithiols connecting the particles. The successful disulfide bonding was also observed spectroscopically by X-ray photoelectron (XPS) and electron energy-loss spectroscopy (EELS). The scanning electron microscope (STEM) combined with EELS experiment gives spatial insight into the linked structures. The measurements confirm the molecular bridge between clusters by producing images with chemical contrast indicating the presence of molecules between particles. This achievement of making well-defined metallic nanoscale superstructures allows detailed engineering of the materials with atomic precision.