Aggregation induced emission of surface ligand controlled gold nanoclusters employing imidazolium surface active ionic liquid and pH sensitivity

Abstract

Gold nanoclusters (AuNCs) are one of the emerging materials due to their luminescence property, but their fluorescence intensity significantly gets quenched in aqueous medium due to the intramolecular vibration and rotation of the capping ligands. Herein, we have prepared long carbon chain containing mercaptan acids capped AuNCs, using 16-mercaptohexadecanoic acid (MHDA) and 11-mercaptoundecanoic acid (MUA) respectively, in aqueous medium with quantum yields (QYs) 3.01% and 6.27% respectively. These AuNCs are negatively charged particles confirmed by zeta potential measurements. After addition of positively charged long hydrocarbon chain containing imidazolium surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium chloride (C16mimCl) in these aqueous AuNCs solutions, respective fluorescence intensities increase due to the formation of rigid aggregates with AuNC. The intramolecular vibration and rotation of these capping ligands (MHDA and MUA) are reduced due to the formation of these rigid aggregates and therefore, the QYs of these MHDA and MUA capped AuNCs increase to 5.78% and 6.87% respectively. Now the aggregation induced emission is strongly dependent on the concentration of added C16mimCl with the presence of three distinct regions (first turbid, precipitate and second turbid). The aggregation induced emission enhancement in case of MHDA capped AuNC becomes more prominent than MUA capped AuNC because of more hydrophobic interaction. Moreover, with decreasing the pH of the solution, the carboxylate (–COO-) groups of the capping ligands are protonated and therefore, the electrostatic interaction between AuNCs and C16mimCl no longer exist. This results the gradual decrement in emission intensity and this produces a linear relationship against pH of the solution within pH range 7.5 to 1.5.