Luminescent gold nanoclusters as biocompatible probes for optical imaging and theranostics

Optical bioimaging is a powerful tool for investigating living organisms in real time and space, both in-vivo and in-vitro. As an advantage, luminescence based techniques are poorly invasive and highly sensitive when compared to other diagnostic approaches. In order to exploit these attractive features, the search of highly biocompatible contrast agents with spectral controllable signatures and good photostability is needed. Gold nanoclusters (GNCs) based probes promise to merge all these characteristics, being their emission tunable from the visible (VIS) to the near infrared region (NIR), a spectral window ideal for photo-activation and detection in-vivo. In this review article, examples of the most recent applications of GNCs to in-vitro and in-vivo imaging are discussed, also considering the development and the use of multimodal and multifunctional probes, able to combine diagnostic and therapeutic (theranostic) actions. Brightness, photostability and toxicity of GNCs in biological systems are critically analyzed. For a complete overview, the origin of the optical properties of GNCs, as well as their preparation, is briefly discussed.