Abstract
Metal nanoclusters featuring tunable luminescence and high biocompatibility are receiving attention as fluorescent markers for cellular imaging. The recently discovered ability of gold clusters to scavenge cytotoxic reactive oxygen species (ROS) from the intracellular environment extends their applicability to biomedical theranostics and provides a novel platform for realizing multifunctional luminescent probes with engineered anti-cytotoxic activity for applications in bio-diagnostics and conceivably cellular therapy. This goal could be achieved by using clusters of strongly reactive metals such as silver, provided that strategies are found to enhance their luminescence while simultaneously enabling direct interaction between the metal atoms and the chemical surroundings. In this work, we demonstrate a synergic approach for realizing multifunctional metal clusters combining enhanced luminescence with strong and lasting ROS scavenging activity, based on the fabrication and in situ protection of Ag nanoclusters with a supramolecular mantle of thiolated-Au atoms (Ag/Au-t). Confocal imaging and viability measurements highlight the biocompatibility of Ag/Au-t and their suitability as fluorescent bio-markers. ROS concentration tests reveal the remarkable scavenging activity of Ag-based clusters. Proliferation tests of cells in artificially stressed culture conditions point out their prolonged anti-cytotoxic effect with respect to gold systems, ensuring positive cell proliferation rates even for long incubation time.
Highlights
When overexpressed in stressed conditions, reactive oxygen species (ROS) lead to accelerated cell ageing and, in extreme cases, to premature cellular death[27]
The Ag/Au-t capped with 16-mercaptohexadecanoic acid (MHDA)-tetrabutylammonium (TBA) salt were prepared according to the bottom-up route shown in Fig. 1a, which is based on the chemical reduction of the Ag precursor (AgNO3) in the presence of HAuCl4, mercapto-palmitic acids and tetrabutyl ammonium salts[48,49,50]
The spectral position of the absorption maximum remains unchanged at all stages of the synthesis, which indicates the formation of clusters with identical dimensions according to the typical size-focused growth of metal clusters[51,52,53]
Summary
When overexpressed in stressed conditions (i.e. chemical intoxication, UV exposure, overheating), ROS lead to accelerated cell ageing and, in extreme cases, to premature cellular death[27]. One possible strategy for achieving higher ROS harvesting performances would be to use clusters of more reactive metals than gold, such as silver. In order to simultaneously achieve enhanced optical and anti-cytotoxic performances, it is paramount to develop synergistic passivation strategies that protect the excited states of the clusters while ensuring accessibility of ROS-sensitive surface metal core and sulfur functionalitity of the respecive thiol ligands to the intracellular environment. A core-shell approach, where a shell of fluorescent Ag2 and Ag3 clusters was added to Au clusters, has been recently used for obtaining highly fluorescent core–shell particles[47] Taking inspiration from these pioneering studies, in this work, we develop a one-pot aqueous route for the synthesis and in situ protection of Ag clusters with Au-thiolate complexes, resulting in enhanced and spectrally pure blue emission and strong ROS scavenging activity. Proliferation tests of cells with artificially accelerated metabolic activity highlight the prolonged ROS buffering effect of Ag and Ag/Au-t clusters with respect to gold systems, ensuring positive and high cell proliferation rate even after 96 hours of incubation in stressed condition
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