Impacts of pH and Intermolecular Interactions on Surface-Enhanced Raman Scattering Chemical Enhancements
Impacts of pH and Intermolecular Interactions on Surface-Enhanced Raman Scattering Chemical Enhancements
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DOI：
10.1021/acs.jpcc.8b04019
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发表时间：
2018-07-05
影响因子：
3.7
通讯作者：
Haes, Amanda J.
中科院分区：
文献类型：
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作者：
Phan, Hoa T.;Haes, Amanda J.
表面增强拉曼散射（Sers）是一种表面敏感技术，它揭示了纳米颗粒表面分子吸附驱动力的信息。虽然Sers基底的等离子体性质提供了最大的信号增强，但化学增强机制对分子吸附和分子间相互作用更敏感。在此，稳定在微孔二氧化硅膜内的金涂覆的银纳米颗粒用于监测短程化学增强效应。首先，二氧化硅膜提供等离子体稳定性，同时2还促进动力学测量，使得可以识别分子质子化、分子-分子相互作用、分子-二氧化硅相互作用和分子-Au相互作用的影响。为了做到这一点，4-巯基苯甲酸（4-MBA）的振动频率作为时间和pH值的函数进行监测。应用菲克第二定律与时间相关的响应表明，分子通量随pH值的增加而降低。Sers光谱表明，这种现象的动力学依赖于4-MBA的质子化状态，因此，分子通过带负电荷的二氧化硅膜所需的能量。也就是说，去质子化分子（R-COO-）和二氧化硅壳之间的排斥静电相互作用增加了传输所需的能量，这随后降低了分子通过二氧化硅壳的通量和随后吸附到金属表面。随着pH接近中性条件，去质子化的4-MBA的分数增加。与质子化的分子相比，这些分子在芳环中具有更高的电子密度，有利于选择性地化学增强不对称与对称的C-C伸缩模式。此外，吸附分子之间分子间相互作用的增加促进电子从芳香环离域到4-MBA的羧酸基团。这种响应导致羧酸盐的pK（a）从4.8（在溶液中）逐渐增加到7.7（在纳米颗粒表面上）。因此，Sers信号的这种分子可以理解相对于分子质子化状态，通量，和分子间的相互作用，使用这些电磁稳定的等离子体纳米结构。
Surface-enhanced Raman scattering (SERS) is a surface sensitive technique that reveals information regarding molecular adsorption driving forces at nanoparticles surfaces. While the plasmonic properties of SERS substrates provide the largest signal enhancements, chemical enhancement mechanisms are more sensitive to molecular adsorption and intermolecular interactions. Herein, gold coated silver nano particles that are stabilized inside microporous silica membranes are used for monitoring short-range chemical enhancement effects. First, the silica membrane provides plasmonic stability while 2 also facilitating kinetic measurements so that impacts of molecular protonation, molecule-molecule interactions, molecule-silica interactions, and molecule-Au interactions can be identified. To do this, the vibrational frequencies of 4-mercaptobenzoic acid (4-MBA) are monitored as a function of time and pH. Applying Fick's second law to time-dependent responses reveals that molecular flux decreases with increasing pH. SERS spectra suggest that the kinetics of this phenomenon depend on the protonation state of 4-MBA and, hence, the energy required for the molecules to pass through the negatively charged silica membrane. Namely, repulsive electrostatic interactions between deprotonated molecules (R-COO-) and the silica shell increase the energy required for transport, which subsequently decreases the flux of molecules through the silica shell and subsequent adsorption to the metal surface. As pH approaches neutral conditions, the fraction of deprotonated 4-MBA increases. These molecules, which have a higher electron density in the aromatic rings versus protonated ones, favor selective chemical enhancement of the asymmetric versus symmetric C-C stretching modes. In addition, increasing intermolecular interactions between adsorbed molecules promote electron delocalization from aromatic rings to the carboxylate groups of 4-MBA. This response causes the pK(a) of the carboxylate to gradually increase from 4.8 (in solution) to 7.7 (on nanoparticle surfaces). Consequently, SERS signals for this molecule can be understood with respect to molecular protonation state, flux, and intermolecular interactions using these electromagnetically stable plasmonic nanostructures.