Cation-controlled laser synthesis of surfactant-free gold nanoclusters with tuneable ion-specific electronic and photoemission properties

Abstract

Fully inorganic gold nanoclusters (Au NCs) synthesised by pulsed laser fragmentation in liquids (LFL) constitute an underexplored class of nanomaterials, characterised by high colloidal stability at salt concentrations of 0.4–1 mM due to high surface charge densities and unique photoluminescence properties. In this work, we systematically analyse how these organic-ligand-free Au NCs interact with their ionic nano-environment, focusing on the impacts of cation valency and cation hydration enthalpy. Thereto, micromolar concentrations of Na+, Ca2+, Al3+, and Ce3+ are added during synthesis (in situ) and corresponding millimolar concentrations to pre-formed aqueous Au NCs (ex situ) at alkaline pH, characterising particle size distributions and crystal structure by analytical ultracentrifugation and HR-TEM, surface charge density by zeta-potential measurements, as well as optical properties via absorbance, emission, fluorescence lifetime, and quantum yield. Our findings reveal a pronounced influence of Al3+ ions on the particle formation process, yielding unique bimodal size distributions charge screening and inversion of the surface charge from negative to positive values. This is attributed to element-specific cation adsorption to Au-O surface sites, while the impact of Na+ and Ca2+, as well as pH, was negligible. Furthermore, we find a pronounced blue shift of the emission wavelengths, indicating that the adsorption of the highly charged Al3+ can directly affect the electronic structure of the Au NCs and their HOMO–LUMO bandgap energy. Ex situ experiments further show cation-specific changes in emission wavelength and intensity, with blue shifts for Al3+ and red shifts for Ce3+, suggesting the potential of ligand-free Au NCs as candidate materials for ion-specific sensors, which may be useful in environmental analytics, where trivalent ions like Al3+ are of particular relevance.