School of Physics and Beijing Key Laboratory of Nanophotonics & Ultrafine Optoelectronic Systems, Beijing Institute of Technology, Beijing, 100081, China.
Nanoscale. 2017 Apr 20;9(16):5110-5118. doi: 10.1039/c6nr09419h.
Recently, there has been great interest in studying ultrasensitive detection and characterization of biomolecules using plasmonic particles, because they are of considerable importance in biomedical science and pharmaceutics. So far, all the theories on plasmon-induced circular dichroism (CD) have been based on the dipole approximation; the electric quadrupolar contribution is generally considered to be relatively small and neglected. Here we demonstrate that the electric quadrupolar contribution not only cannot be ignored, but it also plays a key role in many cases. Particularly, for the chiral medium that possesses preferential molecular orientations and is located at the hotspot of plasmonic nanostructures, the plasmonic CD strength contributed by molecular electric quadrupoles (EQs) can be two orders of magnitude higher than that contributed by molecular electric/magnetic dipoles. Unlike the case of the dipole approximation, molecular EQ associated plasmonic CD activity appears mainly at the plasmonic resonance absorptions that facilitate the optically enhanced near-field with steep electric field gradients, and is correlated with the boosted emission rate of a molecular EQ. Based on such physical understandings, we can design nanostructures to realize a giant chiroptical effect using the EQ contribution according to the requirements, which provide a new strategy for ultrasensitive detection and quantification of molecular chirality.
最近，人们对使用等离子体粒子进行生物分子的超灵敏检测和特性研究产生了浓厚的兴趣，因为它们在生物医学科学和药物学中具有相当重要的意义。到目前为止，所有关于等离子体诱导圆二色性（CD）的理论都是基于偶极近似的；电四极贡献通常被认为相对较小，可以忽略不计。在这里，我们证明了电四极贡献不仅不能被忽略，而且在许多情况下还起着关键作用。特别是对于具有优先分子取向且位于等离子体纳米结构热点位置的手性介质，分子电四极矩（EQs）贡献的等离子体 CD 强度可比分子电/磁偶极矩贡献的高出两个数量级。与偶极近似的情况不同，分子 EQ 相关的等离子体 CD 活性主要出现在等离子体共振吸收处，这些吸收有助于具有陡峭电场梯度的光学增强近场，并且与分子 EQ 的增强发射速率相关。基于这种物理理解，我们可以根据需要设计纳米结构来利用 EQ 贡献实现巨大的手性效应，这为超灵敏检测和定量分子手性提供了一种新策略。
