Surfactant selective synthesis of gold nanowires by using a DPPC-surfactant mixture as a capping agent at ambient conditions

Gold (Au) nanowires (NW) have been synthesized by using mixtures of L-alpha-dipalmitoylphosphatidylcholine (DPPC) and various conventional surfactants (CS) of different polarities. The capping ability of the surfactant mixture has been quantitatively evaluated by using UV-vis, TEM, XPS, FTIR, and XRD studies. The synthesis has been carried out by using a total [DPPC+CS] in the range of 0.125 to 0.5 mM. The results clearly demonstrate a surfactant selective synthesis that significantly depends on the polarity of the CS head group. An anionic surfactant like sodium dodecylsulfate (SDS) along with DPPC leads to a network of NW formation with approximate to 50 nm thickness at different total surfactant concentrations while cationic surfactant, cetyltrimethylammonium bromide (CTAB), predominantly controls the spherical Au nanoparticle (NP) formation at high concentration. At low [DPPC+CTAB], the NW is achieved by end-to-end fusion of spindle shaped Au NP with an aspect ratio of approximate to 2.15. The chemical composition of the adsorbed capping surfactant on the Au surface and the alignment of the molecules were confirmed from XPS and FTIR studies. XPS results indicate that a small amount of DPPC+SDS caps the Au NW in comparison to that of DPPC+CTAB. FTIR results further support this and indicate even a smaller amount of DPPC is adsorbed in comparison to that of SDS, which is attributed to a stable micellar phase formation in the case of the DPPC+SDS mixture in comparison to DPPC+CTAB. This leaves a small amount of surfactant for capping action in the former case and causes anisotropic growth of Au NP to NW formation. Interestingly, no Au NW formation is observed if no DPPC is used. All pure CS lead to the formation of spherical Au NP of different dimensions. This fact suggests that one can control the morphology of Au NP by using DPPC along with different CS and thus one can design biofriendly nanomaterials for bioengineering applications.