Instant room temperature synthesis of self-assembled emission-tunable gold nanoclusters: million-fold emission enhancement and fluorimetric detection of Zn2+

Facile synthesis of luminescent metal nanoclusters (NCs) accompanied by emission color tuning is currently an active area of research. In this work we describe a rapid (1 s) room temperature synthesis of luminescent Au NCs from completely nonluminescent NCs through the incorporation of Zn2+. The nanoclusters are initially stabilized by mercaptopropionate, and the coordination of Zn2+ with the carboxylate groups of the ligands rigidifies the Au(I) thiolates restricting the intramolecular rotation-vibrational motion. This significantly reduces the nonradiative relaxation of the excited state to produce yellow luminescent NCs (lambda(em) = 580 nm, QY: 6%, tau = 0.2 ms) with almost a million-fold emission enhancement. The enhanced luminescence is due to the self-assembly mediated aggregation induced emission (AIE) of NCs. These NCs on aging for 24 hours transform to highly ordered green emitting NCs (lambda(em) = 500 nm, QY: 20%, tau = 20 ns). The blue shift in emission is due to the dominance of inter Au(I)-Au(I) interaction and inter-NC Zn2+ interaction over the intra modes. TEM images show this distinct transition, a decrease in inter NC distance with increased self-assembly. Excited state relaxation dynamics associated with Au(I) thiolate shell dynamics in yellow and green emitting NCs is explained based on the time resolved fluorescence study. The rapid formation of luminescent NCs from nl-NCs has been used for efficient visual and fluorimetric detection of Zn2+