Strategy for Regulating Surface Protrusion of Gold Nanoflowers and Their Surface-Enhanced Raman Scattering

Abstract

Mixed solutions of polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) were investigated by capillary electrophoresis, together with other technologies, to confirm the formation of PVP-bound SDS micelles through cation-bridging association, which shows the nature of polyanions and the ability to exchange Na⁺ of SDS with environmental cations. Because additional water-soluble precursors of nanomaterials are capable of entering the cation-bridging layer of the PVP-SDS pseudo-polyanions, we speculate that the PVP chain and PVP-bound SDS micelles located on both sides of the precursor may act as a two-stage soft template to synthesize nanomaterials with unique morphologies. In this study, PVP-SDS pseudo-polyanions were used as soft templates to promote the growth of gold particles into gold nanoflowers (AuNFs). Tensiometry, conductometry, capillary electrophoresis, and zeta potential measurements confirmed the formation of the new PVP-SDS-HAuCl₄ pseudo-polyanions. Transmission electron microscopy, X-ray diffraction, and UV-Vis spectroscopy analyses showed that the AuNFs synthesized in the mixed solution of PVP (50 gꞏL⁻¹)-SDS (2 mmolꞏL⁻¹)-HAuCl₄ (0.25 mmolꞏL⁻¹) possessed a face-centered cubic structure with abundant {111} crystal plane, demonstrating an average equivalent diameter of 108 nm with rich nano-protrusion of approximately 16.5 nm on the surface of AuNFs. The mechanistic study shows that PVP mainly acts as an in situ reductant for HAuCl₄ in PVP-bound SDS micelles that simultaneously act as the primary template to govern the size of the primary gold crystals. In addition to continuously reducing HAuCl₄, PVP functions as a secondary template, leading to primary gold crystals in a finite space linked by the PVP chain through preferential adsorption, stacked, and grown into mature AuNFs. Finally, a lower HAuCl₄ concentration and an adequate reduction period are favorable in the aforementioned process dominated by the soft template rather than by the crystal growth rule of gold particles; thus, the reduction rate of HAuCl₄ and nucleation-growth competition of gold particles can be regulated. Therefore, the optimal combination of low concentrations of SDS, PVP, and HAuCl₄, together with an appropriate reduction period, would result in synergism between the reduction rate of HAuCl₄, crystal growth rule of gold particles, and stacking degree of the primary gold crystals. AuNFs show strong surface-enhanced Raman scattering (SERS) activity for the Raman probe molecule of rhodamine 6G, strongly depending on the nano-protrusion morphology of AuNFs. The highest SERS enhancement factor can reach 6.71 × 10⁷, which is superior to the reported level of the similar AuNFs (10⁶). Because the particle sizes and morphologies of AuNFs can be precisely regulated, this strategy is a facile aqueous one-pot method for the synthesis of nanomaterials under normal temperature and pressure, which eliminates carrier requirements or adsorption interference from cationic surfactants, and has the potential to further enhance the SERS activity of the nanomaterials.

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