Small. 2016 Dec; 12(48):6698-6702

Weakening Circular Dichroism of Plasmonic Nanospirals Induced by Surface Grafting with Alkyl Ligands

Wai-Fung Lau, Lin Yang, Fan Bai and Zhifeng Huang

From the Department of Physics, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong SAR, China

To whom correspondence should be addressed: Prof. Z. F. Huang, Department of Physics, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong SAR, China, Telephone: (852) 3411 5886; FAX: (852) 3411 5813; E-mail: zfhuang@hkbu.edu.hk

 

Abstract

Chirality is a natural feature of, especially, biological systems, and molecular chirality substantially determines biological functions. Plasmonic nanomaterials possessing chiral structures, such as in the helical, will produce nanoplasmons with chiroptical activity in the UV-visible region, under optical excitation. Analogous to surface plasmon resonance, plasmons with chiroptical activity would pave the way for developing crucial chirality-related bio-applications, given the understanding of chirality-associated plasmon-molecule interactions. However, little is known about the interaction. Given the chirality-related complexity of the interaction of chiral molecules and chiroptically active nanoplasmons, it is necessary to study the interaction with achiral molecules.

This work devises a quantitative method to study the interaction of achiral molecules and chiroptically active nanoplasmons, by monitoring an alkyl grafting-induced change in chiroptical activity (characterized by circular dichroism, i.e., CD) of silver nanospirals (AgNSs) as a function of bond energy of ligand-Ag contacts. The grafting of alkyl ligands on the surfaces of AgNSs quenches the CD signals of the transverse (T) and longitudinal (L) modes, and the CD deterioration is exacerbated with increasing bond energy of the Ag-ligand contacts, which is attributed to the increase of medium dielectric constant and the enhancement of electron withdrawal from the AgNSs towards the grafted ligands. Proposing such a simple approach to quantitatively investigate the molecule-plasmon interactions paves the way to explore a wide range of chirality-related bio-applications, in the areas of pharmaceutical and agricultural production, food quality control, disease diagnosis and treatment, and environmental protection.

KEYWORDS: chiroptical activity; silver nanospirals; alkyl ligands; circular dichroism; glancing angle deposition.

DOI:  10.1002/smll.201602236

 

Supplement:

We study the interaction of alkyl ligands (CH3(CH2)6CH2-G or C8-G, where G is –H, –COOH, –NH2 and –SH) with AgNSs, using the experimental procedures shown in Figure 1.

 

 

 

Figure 1.Experimental procedures: 1. CD spectrum of a pristine AgNS array was recorded; 2. C8-G was self-assembled on the AgNS array; 3. CD spectrum of the alkyl ligand-grafted AgNS array was recorded, which was compared to the CD spectrum monitored in the procedure 1.

 

A close-packed array of AgNSs generated by glancing angle deposition exhibits strong chiroptical activity composed of a plasmonic T-mode at a wavelength of ~370 nm and broadband L-mode in the visible region centered at a wavelength of ~570 nm, with CD sign opposite one another. The grafting of C8-G on the AgNSs quenches the two plasmonic CD modes, and the T-mode is weakened more seriously than the L-mode. The CD deterioration is exacerbated with increasing bond energy of the Ag-G contacts, which is ascribed to the increase of medium dielectric constant and the enhancement of electron withdrawal from the AgNSs toward C8-G. This work devises a simple method to quantitatively study the chirality-associated interaction of nanoplasmons and molecules. The interaction substantially stems from the formation of the metal-ligand contacts, including the ligands terminated with -NH2 and -COOH that amino acids typically contain. Proteins are composed of 20 kinds of amino acids that all have homochirality of L-configuration. The homochirality leads to the chirality-dependent interaction of biological systems and chiral drugs/pesticides. Given a quick development of drug carriers based on the plasmonic nanoparticles, this work provides an additional degree of freedom in terms of chiroptical activity to study the biological functions of chiral drugs and pesticides, paving the way to develop a wide range of chirality-related applications in the areas of pharmaceutical and agricultural production, food quality control, disease diagnosis and treatment, and environmental protection.