A minor lipid component of soy lecithin causes growth of triangular prismatic gold nanoparticles
Reduction of bromoauric acid by ascorbic acid in the presence of crude soybean lecithin produced a mixture of spherical and triangular prismatic nanoparticles while higher purity phosphatidylcholine (PC) produced only spherical nanoparticles. The triangular prismatic nanoparticles made with lecithin had an average edge length of 90 nm and a localized surface plasmon resonance between 700 and 1050 nm, depending on the synthetic conditions used. Although crude soy lecithin is composed of 75% PC and phosphatidylethanolamine, additional lipids and other small molecules are present. Preparatory gel electrophoresis was used to separate the different nanoparticle shapes and phosphatidic acid (PA) was identified as bound to the triangular prismatic nanoparticles by mass spectrometry of the gel-purified nanoprisms. Furthermore, PA proved essential to the asymmetric growth. When bromoauric acid was reduced by ascorbic acid using a mixture of pure PA and pure PC, triangular prismatic gold nanoparticles also resulted, confirming the role of PA and providing a second route to these near infrared active nanoparticles. Nanoparticles prepared in soy lecithin or PA–PC mixtures exhibited extended stability with no aggregation after months of storage, in contrast to nanoparticles prepared in pure PC. These nanoparticles were prepared without the use of the alkyl ammonium salts that have limited in vivo applications with other asymmetric gold nanoparticles. This is a versatile synthetic method providing stable, shape-controlled gold nanoparticles using a mild reducing agent and soy lecithin as an environmentally benign ligand source.