Yoon Jun Hee, Park Jung Shin, Yoon Sangwoon
Department of Chemistry, Center for Photofunctional Energy Materials, Dankook University, 126 Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, Korea.
Langmuir. 2009 Nov 3;25(21):12475-80. doi: 10.1021/la9031865.
We report the time- and symmetry-dependent surface-enhanced Raman scattering (SERS) of gold nanoparticle (AuNP) aggregates. The addition of p-aminothiophenol (p-ATP) instantly induces the aggregation of AuNPs, confirmed by large absorption in the near-IR region. Dynamic light scattering measurements show that the addition of p-ATP immediately assembles the AuNPs (13 nm) to form aggregates with a mean diameter of approximately 200 nm, which then further grow to a size of approximately 300 nm. Raman spectra acquired via time lapse show that the a(1)-symmetry bands of p-ATP are enhanced simultaneously with the formation of the aggregates, indicating that the electromagnetic enhancement largely contributes to the SERS of the AuNP aggregates. In contrast, the enhancement of the b(2)-symmetry bands occurs approximately 10 h after the formation of the aggregates and slowly progresses. The enhancement of the b(2) mode is attributed to the charge transfer between AuNPs and adsorbates, rather than the reorientation of the adsorbates because thiophenol and p-methylthiophenol that have surface structures and intermolecular interactions similar to those of p-ATP do not exhibit a symmetry-specific Raman enhancement pattern. To elucidate the disparity in the timescale between the charge-transfer resonance and the formation of the aggregates, we propose two models. A further close approach of the AuNPs constituting the aggregates causes the additional adsorption of the initially adsorbed p-ATP onto neighboring AuNPs, tuning the charge transfer state to be in resonance with the Raman excitation laser. Density functional theory calculations confirm the resonance charge-transfer tunneling through the bridging p-ATP in the AuNP-p-ATP-AuNP structures. Alternatively, the gradual continuing adsorption of p-ATP increases the local Fermi level of AuNPs into the region of resonant charge transfer from the Fermi level to the LUMO of the adsorbates. This model is corroborated by the faster appearance of b(2)-mode enhancement for the AuNPs with initially higher zeta potentials.
我们报道了金纳米颗粒（AuNP）聚集体的时间和对称性相关的表面增强拉曼散射（SERS）。对氨基硫酚（p-ATP）的加入立即诱导了AuNP的聚集，这通过近红外区域的大量吸收得以证实。动态光散射测量表明，p-ATP的加入立即使AuNP（13纳米）聚集形成平均直径约为200纳米的聚集体，然后进一步生长到约300纳米的尺寸。通过延时采集的拉曼光谱表明，p-ATP的a(1)对称带在聚集体形成的同时增强，这表明电磁增强在很大程度上促成了AuNP聚集体的SERS。相比之下，b(2)对称带的增强在聚集体形成后约10小时出现，并缓慢进展。b(2)模式的增强归因于AuNP与吸附质之间的电荷转移，而不是吸附质的重新取向，因为具有与p-ATP相似的表面结构和分子间相互作用的硫酚和对甲基硫酚没有表现出对称特异性的拉曼增强模式。为了阐明电荷转移共振与聚集体形成之间时间尺度上的差异，我们提出了两个模型。构成聚集体的AuNP进一步靠近会导致最初吸附的p-ATP额外吸附到相邻的AuNP上，将电荷转移状态调整为与拉曼激发激光共振。密度泛函理论计算证实了通过AuNP-p-ATP-AuNP结构中桥连的p-ATP的共振电荷转移隧穿。或者，p-ATP的逐渐持续吸附将AuNP的局部费米能级提高到从费米能级到吸附质LUMO的共振电荷转移区域。对于最初具有较高zeta电位的AuNP，b(2)模式增强更快出现这一现象证实了该模型。
