Green Synthesis of Stable Sub-Nano Silver Clusters with Potential against Cancer

Sub-nano-sized Ag-clusters represent an intermediate class between isolated atoms and nanoparticles, and they can possess new, unexpected properties differing from bulk and nano-sized particles. A simple, fast, one-step, cost-effective protocol was revealed to prepare stable sub-nano-sized Ag-clusters with green synthesis. The synthesis conditions included the low silver concentration (0.02 wt.%), excess of PVP (1.5 wt.%) as a stabilizer, pH 4, and use of water P. ruderale extract as a reducing agent. UV–visible and photoluminescence spectroscopies were used for the identification and characterization of sub-nano-sized Ag clusters, and FT-IR, SEM, STEM, and dynamic light scattering were used for the characterization of the complete particles, which included three components: the extract, the stabilizer, and Ag clusters. The sub-nano-size (<1 nm) of the synthesized Ag clusters was revealed by the photoluminescence emission band and by absorption peaks at wavelengths less than 400 nm in the UV–visible spectra. The synthesized sub-nano-sized Ag clusters were reproducible and stable for up to 30 days, which represents a significant achievement since such clusters usually tend to fast aggregation. SEM and STEM revealed the presence of complete particles (Ag clusters, the extract, and the stabilizer) with cubic and spherical morphologies with sizes of 60 nm (nanoparticles) and 200–300 nm (microparticles), respectively. Dynamic light scattering confirmed the bimodal distribution of these particles and revealed their negative charge (−10 mV). The sub-nano-sized Ag clusters showed antiproliferative activity against the MCF-7 breast cancer lying in the interval typical for the activity of different formulations of silver nanoparticles studied earlier.