Role of different phospholipids in the synthesis of pearl-necklace-type gold-silver bimetallic nanoparticles as bioconjugate materials

A seed-growth (S-G) method has been used to synthesize gold (Au) and Au-silver (Ag) bimetallic pear-necklace-type nanoparticles (NP) as bioconjugate materials by using a series of phospho-glycerol (PG) and phospho-choline (PC) lipids as capping agents. All PG lipids produce a fine pear-necklace arrangement of Au NP with dimensions between 10 and 20 nm. Addition of Ag converts this arrangement into Au-Ag bimetallic NP with a diameter essentially close to that of Au NP. Use of PC lipids does not show this arrangement but promotes a significant anisotropic growth especially in the presence of Ag. This difference has been attributed to a difference in the capping ability of PG and PC lipids because of their anionic and zwitterionic nature, respectively. XPS results have demonstrated the presence of adsorbed PG and PC lipids on An or Au-Ag bimetallic surfaces in their respective samples. The results also indicate a decrease in the capping amount of a lipid with an increase in the growth of Au-Ag bimetallic NP. The growth of Au-Ag bimetallic NP from Au NP has been ascribed to the nucleation of Ag atoms at the {111} facets of Au NP in the presence of PG lipids, while anisotropic growth is occurring mainly at all other planes of fee crystal geometry of Au-Ag bimetallic NP in the presence of PC lipids. It has been concluded that in order to get fine Au-Ag bimetallic bioconjugate materials capped with PG lipids, one has to use ascorbic acid (AA) as a weak reducing agent at the end of the S-G reaction sequence so as to give the required time for lipid molecules to adsorb at the liquid-solid interface.