摘要
以聚乙烯吡咯烷酮(PVP)与十二烷基硫酸钠(SDS)通过阳离子架桥形成的拟聚阴离子为软模板,通过改变PVP、SDS和纳米材料前驱体氯金酸(HAuCl4)浓度以及反应时间等因素,调控还原产物金纳米花形貌及粒径.表面张力、电导率、毛细管电泳及Zeta电位等实验结果表明PVP-SDS-HAuCl4形成新的拟聚阴离子,透射电子显微镜和X射线衍射结果表明SDS、PVP和HAuCl4的较低浓度组合更易获得表面凸起丰富的金纳米花.PVP-SDS拟聚阴离子发挥了二级软模板作用,在PVP(50 g∙L-1)-SDS(2 mmol∙L-1)-HAuCl4(0.25 mmol∙L-1)溶液中调控合成的金纳米花为{111}晶面为主的面心立方结构,其平均等效粒径为108 nm,且表面上密集分布约16.5 nm的凸起.该金纳米花有较强的表面增强拉曼散射(SERS)活性,探针分子罗丹明6G的SERS信号强度依赖于金纳米花的表面凸起形貌.该研究中金纳米花的SERS增强因子最高达6.71×107,优于同类金纳米花的文献报道水平(106);尽管低于石墨负载的金纳米粒子(1×108)或阳离子软模板合成的金纳米棒(5×109),但成功避免了基质干扰或阳离子强吸附使应用受限.
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 g·L-1)-SDS(2 mmol·L-1)-HAuCl4(0.25 mmol·L-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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