Gold and silver nanoparticle monomers are non-SERS-active: a negative experimental study with silica-encapsulated Raman-reporter-coated metal colloids.
Gold and silver nanoparticle monomers are non-SERS-active: a negative experimental study with silica-encapsulated Raman-reporter-coated metal colloids.
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DOI：
10.1039/c4cp05073h
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发表时间：
2015-08
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影响因子：
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通讯作者：
Yuying Zhang;Bernd Walkenfort;J. Yoon;S. Schlücker;W. Xie
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文献类型：
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作者：
Yuying Zhang;Bernd Walkenfort;J. Yoon;S. Schlücker;W. Xie
贵金属纳米粒子是表面增强拉曼散射（SERS）实验中最常用的等离子体基板。计算机模拟表明，银和金纳米晶体的单体（“球形”NPs）没有表现出显著的等离子体增强，即它们本质上是非sers活性的。然而，在实验中，球形NP胶体增强SERS的报道屡见不鲜。这意味着单体没有很强的SERS活性，但可检测到的增强应该或多或少存在。由于理论与实践之间的差距，重要的是通过实验证明金属胶体实际上是如何具有SERS活性的，并且在观察到SERS信号的情况下，它来自何处。特别是，由于洗涤步骤中的高离心力或由于悬浮介质的恶劣离子环境而引起的胶体聚集，应该得到控制，因为它是NP簇非常高的SERS活性，支配着胶体的总体SERS信号。本文报道了80 nm Au和Ag NP单体SERS活性的实验评价。我们提出了如何获得负实验数据的方法，而不是展示花哨的纳米结构和超级SERS增强。在这种方法中，当NPs被小心地封装在厚厚的二氧化硅外壳中时，金属表面上具有拉曼报告的胶体没有获得SERS信号。如果没有二氧化硅封装，如果洗涤步骤使用非常低的离心速度，即使在非常高的NP浓度下，也只能检测到可忽略不计的SERS信号。相反，当NPs悬浮在酸性溶液中时，可以检测到强烈的SERS信号。这些结果表明，Au和Ag NP单体基本上没有实际相关的SERS活性。
Noble metal nanoparticles (NPs) are the most commonly employed plasmonic substrates in surface-enhanced Raman scattering (SERS) experiments. Computer simulations show that monomers of Ag and Au nanocrystals ("spherical" NPs) do not exhibit a notable plasmonic enhancement, i.e., they are essentially non-SERS-active. However, in experiments, SERS enhanced by spherical NP colloids has been frequently reported. This implies that the monomers do not have strong SERS activity, but detectable enhancement should more or less be there. Because of the gap between theory and practice, it is important to demonstrate experimentally how SERS-active the metal colloid actually is and, in case a SERS signal is observed, where it originates from. In particular the aggregation of the colloid, induced by high centrifugal forces in washing steps or due to a harsh ionic environment of the suspension medium, should be controlled since it is the very high SERS activity of NP clusters which dominates the overall SERS signal of the colloid. We report here the experimental evaluation of the SERS activity of 80 nm Au and Ag NP monomers. Instead of showing fancy nanostructures and super SERS enhancement, we present the method on how to obtain negative experimental data. In this approach, no SERS signal was obtained from the colloid with a Raman reporter on the metal surface when the NPs were encapsulated carefully within a thick silica shell. Without silica encapsulation, if a very low centrifugation speed is used for the washing steps, only a negligible SERS signal can be detected even at very high NP concentrations. In contrast, strong SERS signals can be detected when the NPs are suspended in acidic solutions. These results indicate that Au and Ag NP monomers essentially exhibit no SERS activity of practical relevance.