One-Pot Synthesis of Core-Shell Silver-Gold Nanoparticle Solutions and Their Interaction with Methylene Blue Dye

A one-pot modified Turkevich synthesis was used to synthesise a range of colloids from pure Ag, through Ag/Au coreshell mixtures to pure Au by the thermal co-addition of trisodium citrate to auric acid and silver nitrate mixtures. The colloids were analysed by means of XRD and wide-beam energy dispersive X-ray (EDX) analysis. The UV/Vis spectroscopy showed a non-linear variation in the surface plasmon resonance band with the Au/Ag ratio, consistent with core-shell formation. High resolution transmission electron microscopy (HRTEM) imaging, in conjunction with thin-beam EDX line analysis, confirmed the presence of predominantly Ag cores and Au exteriors in the Au/Ag mixture. The argon ion sputtering with X-ray photoelectron spectroscopy (XPS) measurements of the particles also indicated a Ag-Au core-shell formation. Ag-0, Au-0 and significant levels of Au-I states were observed, which correlate with a Ag-Au coreshell model where the gold shell becomes partially oxidised at the surface. The subsequent titration of the colloids with methylene blue (MB) dye showed strong positive increases in the extinction coefficient at the absorption maximum. At the optimum levels of dye addition, a linear relationship was found between the average nanoparticle size and the number of dye solvation shells. The pure Au and Ag colloids showed the greatest propensity for an increase in the extinction coefficient of MB due to an enhanced transfer of the surface plasmons to the localised dye molecules and demonstrated the potential for increased functionality as light-activated agents in the lethal photosensitisation of bacteria.