Developing efficient ways to control the nanocluster properties and synthesize atomically precise metal nanoclusters are the foremost goals in the field of metal nanocluster research. In this article, we demonstrate that the direct synthesis of atomically precise, hydrophilic metal nanoclusters as well as tuning of their properties can be achieved by an appropriate selection of reactants, binding ligand, and their proportions. Thus, a facile, single-step method has been developed for the direct synthesis of Au(SCHCOH) nanocluster in an aqueous medium under ambient conditions. The synthesis does not require any pH or temperature control and postsynthesis size-separation step. The use of a hydrophilic, bifunctional short carbon-chain capping ligand, HSCHCOH, allows tuning of cluster properties such as the photoluminescence and stability in an aqueous medium via the variation of pH of the cluster solution. By using a phase transfer catalyst, the nanoclusters can also be transferred into toluene solvent, which further enhances the nanocluster photoluminescence. The formation, composition, and purity of the product clusters have been characterized by using a number of methods such as the polyacrylamide gel electrophoresis, UV-visible and FTIR spectroscopies, transmission electron microscopy, energy dispersive X-ray analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy, and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Gold nanoclusters with properties such as water solubility, water-to-organic phase-transfer ability, and tunable stability and photoluminescence are promising for various studies and applications. The work reveals a few principles that can be helpful in the development of a general toolbox for the rational design of size-selective synthesis and properties tuning of the metal nanoclusters.
Studies of metal nanoclusters can provide valuable information on how various properties evolve in the nanoscale materials. Previously often mixed‐sized products have been separated in order to study their size‐related properties. We present here a one‐pot aqueous synthesis protocol that allows direct observations of the evolutions of size‐dependent optical and electrochemical properties of the gold nanoclusters while the clusters grow from a few‐atom, ultra‐small size to the plasmonic nanoparticle stage. Our selection of a mixed ligand system comprising of methionine and CTAB for the aqueous synthesis of gold nanoclusters by the chemical reduction of gold ions led to the slow growth of the nanoclusters making direct observations of the cluster growth and concomitant evolution of properties possible. UV‐visible spectrophotometric and dynamic light scattering studies demonstrated the size‐dependent transition from the molecule‐like, non‐plasmonic to plasmonic properties in the evolving nanoclusters. Differential pulse voltammetry studies demonstrated how the electrochemical properties changed from the molecule‐like redox behavior to a quantized double‐layer charging type with the growth of the nanocluster with time. Thermogravimetric analysis, MALDI‐TOF mass spectrometry, etc. were employed to further characterize the nanocluster systems. This report will inspire further studies in understanding the evolution of the size‐dependent properties of the nanoscale materials.
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