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 high emission efficiency of poly-Au5. To further evidence the inference above, we successfully used PAMAM to realize the self-assembly of presynthesized, separated, and red-emitting Au-GSH NCs through electrostatic interaction between negative charges of carboxylic groups in Au-GSH NCs and positive charges of amine groups in PAMAM. As expected, the emission efficiency of Au-GSH NCs was obviously enhanced by PAMAM-mediated self-assembly. Moreover, the as-synthesized poly-Au5 assemblies exhibited excellent cell permeability and great biostability against various metal ions, high REDOX stress, and complex intracellular environments. By virtue of MnO4– as an intermediary agent, poly-Au5 was successfully used for sensitive and stable intracellular fluorescent imaging of endogenous GSH. This study lights up the emission origin of dendrimer-hosted Au NCs with strong emission and implies their huge applications in biomedical sensing and imaging.