Water-soluble gold nanoparticles stabilized with cationic phosphonium thiolate ligands

Attachment of cationic groups to the surface of gold nanoparticles (AuNPs) is an attractive proposition for facilitating mitochondria-targeted therapeutics and diagnostics. With this in mind we have prepared and characterised AuNPs functionalised with phosphonium groups derived from either triarylphosphoniopropylthiosulfate zwitterions or ω-thioacetylpropyl(triphenyl)phosphonium salts; organophosphonium cations display remarkable lipophilicity and are readily taken up by cells and are concentrated in the mitochondria. The phosphonium-functionalised AuNPs can be dispersed in water and biological media. Transmission Electron Microscopy reveals the formation of spherical particles with diameters in the range 3–5 nm. The presence of the phosphonioalkylthiolate ligands on the surface of the AuNPs is confirmed by XPS, LDI-TOF-MS, TOF-SIMS and 31P NMR spectroscopy. The phosphonium-AuNPs display excellent stability and preliminary studies indicate that the phosphonioalkylthiolate ligands are slowly oxidised over a period of months to the corresponding phosphonioalkylsulfonate species with a concomitant increase in the particle size, and particle size distribution, of the AuNPs.