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 omega-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 P-31 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.