Size specific emission in peptide capped gold quantum clusters with tunable photoswitching behavior

Three different types of fluorescent gold clusters (namely blue, green and red emitting) have been prepared from a gold precursor (chloroauric acid) under moderate conditions in aqueous medium. A cysteine containing dipeptide has been used for the formation of these quantum clusters as this peptide molecule contains a thiol group in the side chain to cap these nascently formed clusters and the free amino and carboxylic moieties assist in water solubility. Thus, the clusters are also environmentally friendly as the capped peptide is made up of only naturally occurring protein amino acids. These clusters have been well characterized by using UV-visible, fluorescence, X-ray photoelectron spectroscopy (XPS)spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and ultrahigh resolution field emission gun-transmission electron microscopy (UHR-FEG-TEM). Arrangements of gold atoms and their interaction with the corresponding ligands in three different fluorescent clusters have been predicted computationally. The excited state behavior of three different clusters has also been studied using time dependent density functional theory (TD-DFT). Time correlated single photon counting (TCSPC) and computational studies suggest intersystem crossing (S₁ → T₁) in the case of red-emitting Au₂₃ clusters. Interestingly, these gold clusters exhibit semiconducting and photoswitching properties (I_on/I_off), which are shown to be controlled by varying the size of these clusters. This holds future promise of using these gold cluster based nanomaterials for optoelectronic applications.