Self-Nucleation and Self-Assembly of Highly Fluorescent Au5 Nanoclusters for Bioimaging

Fluorescent Au nanoclusters (NCs) are new excellent nanomaterials for biomedical applications but plagued by the problems of limited emission efficiency, unclear emission mechanism, and poor biological application ability. Herein, a novel strategy was developed to facilely synthesize poly(amidoamine) (PAMAM) dendrimer-hosted Au5 NCs (poly-Au5) with a high fluorescence quantum yield of 25%. Most importantly, a two-stage growth process of poly-Au5 was demonstrated through in situ time-course experiments. Stage I was a simultaneous self-nucleation and self-assembly with a rapid rate of fluorescence increase; stage II was a sole self-assembly after the end of reduction with a relatively slower rate of fluorescence increase but contributed 30% to the overall emission intensity of end products. In both stages, enhanced aurophilic interactions promoted the excited state relaxation dynamics; enhanced rigid structures reduced the level of nonradiative relaxition of excited states, and these two factors ensured hig...