Formation of a Au/Au₉Ga₄ Alloy Nanoshell on a Bacterial Surface through Galvanic Displacement Reaction for High-Contrast Imaging

The spontaneous electron transfer between GaAs and ionic gold through the galvanic displacement reaction results in the formation of gold nanoparticles and a Au₉Ga₄ alloy. We investigated this process for decorating Legionella pneumophila and Escherichia coli, aiming at enhanced imaging of these bacteria. The surface of bacteria was modified with gold ions through the electrostatic linkage of ionic liquids with phosphate units of the bacterial cell wall. The modified bacteria were further incubated with an antibody-functionalized GaAs substrate. Due to a large gap in the reduction potential of gold and gallium ions, the induced reaction involving bacteria resulted in a reduction of the gold ions to gold nanoparticles and oxidation of GaAs to Ga₂O₃ and a Au₉Ga₄ alloy. The bacteria covered with a Au/AuGa nanoshell, if excited at 377 nm, show a bright emission at 447 nm originating from Au/Au₉Ga₄. This approach offers a simple and potentially less expensive method for high-contrast imaging of bacteria in comparison to the conventional methods of staining with different dyes or by conjugating green fluorescent proteins.