A morpholinium surfactant crystallization induced formation of Au nanoparticle sheet-like assemblies with uniform SERS activity

Crystallization of the N-methyl-N-hexadecylmorpholinium bromide (C16MMB) surfactant in Au nanoparticles (Au NPs) sol could induce a large-scale production of uniform rectangular Au@C16MMB crystal sheets with uniform surface enhanced Raman scattering (SERS) activity. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopic measurements have been adopted to characterize the crystal sheet morphology and structure. The island-like Au NPs aggregates were observed to distribute uniformly over the surface of C16MMB crystal, which provide sufficient SERS-active sites. Results from zeta potential measurement and molecular dynamic simulation indicated that the C16MMB molecules were electrostatically adsorbed on Au NPs to reverse the nanoparticle surface charge to be positive. The concentration and crystallization capability of C16MMB in solution play a key role to induce the formation of the self-organized Au@C16MMB crystal sheets. A surfactant crystallization induced assembling mechanism has been proposed. Raman tests of Au@C16MMB as an ideal SERS substrate exhibit the high enhancement factor and good uniformity. It is believed that this approach could provide a unique way to fabricate uniform noble metal nanoparticle-based SERS substrates.