Self-assembly and accurate preparation of Au nanoparticles in the aqueous solution of a peptide A6D and a zwitterionic C14DMAO

Abstract

Self-assembled structures such as nanotubes, nanobelts, nanofibers, and nanovesicles of surfactant-like peptides were obtained in water because of electrostatic interactions, hydrogen bonding, hydrophobic interactions and/or other weak interactions between peptide molecules. In this paper, we firstly prepared self-assembled nanotubes with very large aspect ratio and unfolded nanobelts by combining a peptide Ac-AAAAAAD-NH₂ (A₆D) with a zwitterionic surfactant, tetradecyldimethylaminoxide (C₁₄DMAO), in aqueous solution at room temperature. The self-assembled nanotubes and nanobelts were characterized by negative-stained transmission electron microscopy (TEM), field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and cryo-TEM in detail. The optimal concentration ratio, R = c_A6D/c_C14DMAO, of nanotube formation was obtained to be 1 : 8. β-Sheet and random coil structures were proved to co-exist in both nanotubes and nanobelts by Fourier transform infrared (FT-IR) and circular dichroism (CD) measurements. Based on these results, we proposed the formation mechanism of the self-assembled nanostructures. Particularly, Au nanoparticles with diameter of 6.5 ± 1.1 nm from nanobelts of 8 mmol L⁻¹ A₆D/32 mmol L⁻¹ C₁₄DMAO and 6.8 ± 1.5 nm from nanotubes of 8 mmol L⁻¹ A₆D/64 mmol L⁻¹ C₁₄DMAO were synthesized, and mainly observed on the nanobelt surfaces or the nanotube walls. Moreover, the Au nanoparticles/nanotubes and Au nanoparticles/nanobelts were proved via UV-vis measurements by the characteristic peaks at around 520 nm.