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

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Abstract

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 ∼640 nm, a minor peak at 460 nm and a very weak peak at 500 nm. The major peak is ascribed to an Au25 cluster with an icosahedral Au13 core, surrounded by six thiol (from BSA) mediated Au2 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 Au13 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 Au13. 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 ∼9-times higher than that in the normal cell.

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