Positively charged, surfactant-free gold nanoparticles for nucleic acid delivery

Positively charged, surfactant-free gold nanoparticles (Au NPs) with diameters ranging between 2-200 nm have been synthesised in water via a seed-mediated growth method, involving the reduction of gold(III) chloride (AuCl3) by hydroxylamine hydrochloride (NH2OH center dot HCl) in the presence of L-cysteine methyl ester hydrochloride (HSCH2CH(NH2)COOCH3 center dot HCl) as a capping agent. The mercapto group(-SH) on the capping ligand has a high affinity for Au, anchoring the cysteine group to the nanoparticles, whilst the ammonium group (-NH3+), formed by the presence of an amine group in slightly acidic media (pH similar to 4.5-5), resulted in positively charged colloidal nanoparticles (zeta-potential +33 to +49 mV), which was key to their electrostatic stability. Data from cytotoxicity studies performed on a range of different cell types (human and murine), including human prostate cancer cells (PC3), showed that the positively charged Au-L-cysteine-cysteine nanoparticles were less cytotoxic than positively charged Au NPs produced using commonly employed surfactant cetyl trimethyl ammonium bromide (CTAB) under similar conditions. In addition, the positively charged Au NPs could be successfully complexed with small interfering RNA (siRNA). At the cellular level, the uptake of fluorescein siRNA from the charged nanoparticles was comparable to uptake from the commercial carrier INTERFRin (TM), implying the potential application of these novel vectors for nucleic acid delivery.