Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA.
Nano Lett. 2010 Apr 14;10(4):1374-82. doi: 10.1021/nl100010v.
Our calculations show that a nonchiral nanocrystal is able to dramatically change the circular dichroism (CD) of a chiral molecule when the nanocrystal and molecule form a complex and couple via dipole and multipole Coulomb interactions. Plasmon resonances of metal nanocrystals in the nanocrystal-molecule complex result in both the resonant enhancement of CD signals of molecules and the appearance of new spectral structures. Two mechanisms, in which a nanocrystal can influence the CD effect, have been identified. The first mechanism is the plasmon-induced change in the electromagnetic field inside the chiral molecule. The second is the optical absorption of the nanocrystal-molecule complex due to the chiral currents inside the metal nanocrystal induced by the dipole of the chiral molecule. The first mechanism creates a change in the angle between the effective electric and magnetic dipoles of the molecule. This mechanism can lead to symmetry breaking and to a plasmon-induced CD signal of the nonchiral molecule. Both mechanisms create interesting Fano-like shapes in the CD spectra. Importantly, the second mechanism gives the main contribution to the CD signal at the plasmon frequency when the absorption band of the chiral molecule is far from the plasmon resonance. This may happen in many cases since many biomolecules are optically active in the UV range, whereas plasmon resonances in commonly used nanometals are found at longer wavelengths. As concrete examples, the paper describes alpha-helix and calixarene ligand molecules coupled with metal nanocrystals. The above results are also applied to complexes incorporating semiconductor nanocrystals. The results obtained here can be used to design a variety of hybrid nanostructures with enhanced and tailored optical chirality in the visible wavelength range.
我们的计算表明，当纳米晶体和分子形成复合物并通过偶极子和多极子库仑相互作用耦合时，非手性纳米晶体能够显著改变手性分子的圆二色性（CD）。纳米晶体-分子复合物中金属纳米晶体的等离子体共振导致分子的 CD 信号的共振增强以及新的光谱结构的出现。已经确定了纳米晶体可以影响 CD 效应的两种机制。第一种机制是手性分子内部的电磁场的等离子体诱导变化。第二种机制是由于手性分子的偶极子引起的金属纳米晶体内部的手性电流的光学吸收。第一种机制在手性分子的有效电偶极子和磁偶极子之间产生角度变化。这种机制可能导致对称破缺并在手性分子的非手性分子中产生等离子体诱导的 CD 信号。两种机制在手性分子的吸收带远离等离子体共振时，在 CD 光谱中都产生了有趣的类 Fano 形状。重要的是，当手性分子的吸收带远离等离子体共振时，第二种机制在手晶的等离子体频率下对 CD 信号的主要贡献。由于许多生物分子在 UV 范围内具有光学活性，而通常使用的纳米金属中的等离子体共振位于较长的波长处，因此在许多情况下可能会发生这种情况。作为具体示例，本文描述了与金属纳米晶体耦合的 alpha-螺旋和杯芳烃配体分子。上述结果也适用于包含半导体纳米晶体的复合物。这里获得的结果可用于设计具有增强和定制的光学手性的各种混合纳米结构，其范围在可见波长范围内。
