Controlled synthesis of gemini surfactant-capped gold nanoparticles. Gemini structure-nanoparticle properties relationship study

Abstract

Gold nanoparticles were prepared by the method of chemical reduction utilising sodium borohydride as reducing agent and stabilised with bisammonium gemini surfactants with variable length of polymethylene spacer. Physico-chemical investigations indicated the presence of plasmon resonance peaks in the wavelength range 507–544 nm for all investigated Au nanodispersions. The nanoparticle size analysis shows the relationship between the size of gold nanoparticles and the spacer length of stabilising gemini molecules. The strong aggregation tendency of gemini surfactants with short polymethylene spacer composed of 2–4 methylene units in aqueous solutions may be responsible for the formation of clusters of gold nanoparticles which results in the observed large hydrodynamic size of nanoparticle clusters at the surfactants concentrations below the CMC. At higher surfactant-to-gold molar ratio values, clusters disintegration occurs and the decrease in size of the nanoparticles is observed. The shape of gold nanoparticles was found to be spherical and polyhedral, as evidenced by the SEM images. The size values varied between 30–60 nm and 10–15 nm for AuNPs capped with gemini molecules with short and medium/long spacer, respectively. Gold nanoparticles are strongly positively charged, with positive zeta potential values being in the interval +44 to +90 mV in the whole analysed region of molar ratio values and surfactant spacer length. The biological experiments indicate a dependence of biological activity of gold nanoparticles on the structural parameter (spacer length) of stabilising gemini molecules. Both cytotoxicity and anti-inflammatory activity of gold nanoparticles were found to increase with the increasing gemini spacer length up to 12 methylene groups in the spacer.