Fluorescent Gold Nanocluster Inside a Live Breast Cell: Etching and Higher Uptake in Cancer Cell

Time-resolved confocal microscopy is applied to compare fluorescence properties of gold nanocluster (Au-NC) inside human breast cells with those in bulk water. In bulk water, Au-NC, coated with bovine serum albumin (BSA), displays a major emission peak at similar to 640 nm, a minor peak at 460 nm and a very weak peak at 500 nm. The major peak is ascribed to an Au-25 cluster with an icosahedral Au-13 core, surrounded by six thiol (from BSA) mediated Au-2 staples. Inside the live cells, emission maximum of Au-NC exhibits a dramatic blue shift to 530 nm in normal breast cell (MCF10A) and 510 nm in breast cancer cell (MCF7). The 510-530 nm emission peak corresponds to an icosahedral Au-13 cluster. It appears that inside the cell, glutathione competes with and replaces BSA as a ligand of the Au-NC. This leads to etching of the Au-NC to Au-13. Confocal images indicate that the Au-NCs localize in the membrane of the normal breast cell, MCF10A. In the case of breast cancer cell MCF7, the Au-NCs localize in a much larger volume encompassing the cell membrane and the cytoplasm. This demonstrates higher uptake of Au-NCs by the cancer cell. Fluorescence correlation spectroscopy (FCS) is applied to measure viscosity inside the live cells, using Au-NC as a probe. For the cancer cell, the cytoplasmic viscosity is found to be 7 cP. The FCS data for the membrane is fitted to two-dimensional (2D) diffusion. From this the surface viscosity is obtained using Saffman-Stokes-Einstein theory. The surface viscosity in the cancer cell is similar to 9-times higher than that in the normal cell.