Itoh Tamitake, Hashimoto Kazuhiro, Biju Vasudevanpillai, Ishikawa Mitsuru, Wood Bayden R, Ozaki Yukihiro
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan.
J Phys Chem B. 2006 May 18;110(19):9579-85. doi: 10.1021/jp0609939.
We have observed simultaneously temporal fluctuation of surface-enhanced resonance Raman scattering (SERRS) and its background-light emission from single Ag nanoaggregates that were adsorbed with metal-free tetraphenylporphine (H(2)TPP) molecules. We found that temporally stable SERRS spectra showed clearly a SERRS band that is attributed to a stretching mode of a chemical bond between a carbon atom and a non-hydrogenated nitrogen atom (C(alpha)-N). This stretching mode was not observed in regular resonance Raman spectra which are free from surface enhancement. On the other hand, we also found that temporally unstable SERRS spectra did not clearly show a C(alpha)-N stretching mode in SERRS bands. Furthermore, temporally stable SERRS spectra were accompanied by temporally stable background-light emission. Kobayashi et al. [J. Phys. Chem. 1985, 89, 5174] reported that formation of an Ag-N bond between surface Ag atoms and non-hydrogenated N atoms in a pyrrole ring enhances the intensity of a C(alpha)-N stretching mode. Thus, the observed relationship between clear appearance of a C(alpha)-N stretching mode and temporal stability of SERRS plus background-light emission strongly suggests that formation of a stable Ag-N bond suppresses fluctuation of both SERRS and background-light emission. Furthermore, the observed relationship implies that chemical contribution to SERRS is stabilization of H(2)TPP molecules that are adsorbed on SERRS-active sites by formation of Ag-N bonds. Additionally, we attributed background-light emission to luminescence of complexes between H(2)TPP molecules and surface Ag atoms considering possible formation of Ag-N bonds, synchronized SERRS intensity with background-light emission intensity, blue-shifted background-light emission maxima from normal fluorescence maxima, and previous reports related to electronic structures of H(2)TPP molecules on Ag surfaces.
我们同时观察到了表面增强共振拉曼散射（SERRS）及其背景光发射的时间波动，这些现象来自吸附有无金属四苯基卟啉（H₂TPP）分子的单个银纳米聚集体。我们发现，时间上稳定的SERRS光谱清晰地显示出一个SERRS带，该带归因于碳原子与非氢化氮原子（Cα-N）之间化学键的拉伸模式。这种拉伸模式在没有表面增强的常规共振拉曼光谱中未被观察到。另一方面，我们还发现，时间上不稳定的SERRS光谱在SERRS带中没有清晰地显示出Cα-N拉伸模式。此外，时间上稳定的SERRS光谱伴随着时间上稳定的背景光发射。小林等人[《物理化学杂志》1985年，89卷，5174页]报道，吡咯环中表面银原子与非氢化氮原子之间形成的Ag-N键增强了Cα-N拉伸模式的强度。因此，观察到的Cα-N拉伸模式的清晰出现与SERRS加背景光发射的时间稳定性之间的关系强烈表明，稳定的Ag-N键的形成抑制了SERRS和背景光发射的波动。此外，观察到的这种关系意味着对SERRS的化学贡献是通过形成Ag-N键来稳定吸附在SERRS活性位点上的H₂TPP分子。此外，考虑到可能形成的Ag-N键、SERRS强度与背景光发射强度的同步、背景光发射最大值相对于正常荧光最大值的蓝移以及之前与Ag表面上H₂TPP分子电子结构相关的报道，我们将背景光发射归因于H₂TPP分子与表面银原子之间配合物的发光。
