Luminescent Silver Clusters with Covalent Functionalization of Graphene

Brightly luminescent, atomically precise subnanometer clusters of silver protected by glutathione were covalently functionalized by solution phase thiolated graphene using ligand exchange. A hybrid molecule was formed, which was obtained in a nearly pure form by phase transfer from the aqueous to the organic phase. The resulting hybrid exhibited properties of both of these nanoscale materials. UV−vis, luminescence, XPS, EDAX, TEM, and Raman spectroscopy were used to characterize the hybrid structure, which showed the spectroscopic characteristics of both the constituents. The luminescence of the cluster was retained in the hybrid, along with features characteristic of graphene. Atomically precise clusters with covalently linked graphene will be useful for new applications in the areas of drug delivery, sensors, and catalysis. ■ INTRODUCTION Graphene, the two-dimensional analogue of carbon, has been the most fascinating material of the recent past. Quantum clusters (QCs) are the emerging category of noble metal based materials with atomically precise cores and well-defined features in optical absorption and emission. QCs contain only a few atoms in their core, which leads to discrete energy levels in them rather than continuous density of states, making them brightly luminescent with distinct emission features. The fascinating properties of both of these materials have made them important candidates for applications in diverse areas ranging from chemistry to biology. The large surface areas of graphenic materials allow them to capture active molecules and clusters. Noble metal QCs, due to their luminescence, are excellent reporters of chemical and physical changes of the medium. As a result, creation of a covalently bonded hybrid of the two would enhance the properties of both these nanoscale materials. Analogous structures have been made in the case of large plasmonic nanoparticles to create new reporter species. In this article, we report the first synthesis of such a QC−graphene hybrid molecule exhibiting the characteristic features of both the constituents. Functionalization of QCs with diverse molecules has been achieved to impart additional properties such as fluorescence resonance energy transfer (FRET), receptor mediated transfer and antigen−antibody interaction. This functionalization chemistry, extending to graphene and such other carbon forms will extend the scope of cluster research. Ag and Au atoms have been well-known to have an affinity toward sulfur, and hence, solution phase graphene, which typically contains the carboxylic acid group, was functionalized to form thiolated graphene. Thiol protected Ag QCs were bound to this by ligand exchange of thiol functionalities to form a covalently bound graphene cluster hybrid molecule. The hybrid was characterized in the aqueous phase and was later phase transferred to toluene to separate unbound graphenes