Towards Ultra-Bright Gold Nanoclusters

Fluorescence bioimaging is a non-invasive technique that permits to investigate living organism in real time with high tridimensional resolution. Properly engineered fluorescent (or photoluminescent) nanoparticles promise to surpass conventional fluorescent molecular probes as contrast agent. Photoluminescent semiconductor quantum dots show, for example, enhanced brightness and photostability. Concerns arising from the toxic metal content of quantum dots prompted the search for alternative inorganic nanoparticles with similar properties but less hazardous. Gold is almost unanimously considered to be highly tolerated by living organism and the environment. Nevertheless, although photoluminescence of gold nanocluster is known by decades, brightness of these nanoparticles has typically reported to be low, mostly because of the poor photoluminescence quantum yield (< 1%). Different approaches have been proposed to enhance this poor quantum yield aiming to achieve ultra-bright gold nanoparticles with features superior to conventional dyes. Here most relevant and recent results in this direction are summarized and critically discussed.