Enhancement of Chiroptical Signals by Circular Differential Mie Scattering of Nanoparticles.

Seokjae Yoo,Q-Han Park

doi:10.1038/srep14463

Abstract

We enhance the weak optical signals of small chiral molecules via circular differential Mie scattering (CDMS) of nanoparticles immersed in them. CDMS is the preferential Mie scattering of left- and right-handed circularly polarized light by nanoparticles whose sizes are about the same as the wavelength of light. Solving the Mie scattering theory for chiral media, we find that the CDMS signal of the particle is linearly proportional to the chirality parameter κ of the molecules. This linear amplitude enhancement by CDMS of the particle holds, even for large particles, which have a retardation effect. We also demonstrate that the CDMS of a nanoparticle is sensitive to changes of molecular concentration, and that the nanoparticle can be utilized as a chiroptical biosensor detecting the concentration of analyte. We expect that the enhancement of molecular chiroptical signals by CDMS will pave the way for novel chiroptical spectroscopy using nanostructures.

Highlights

Chirality, which is a property of objects that cannot be superimposed on their mirror images, is a common feature of life’s building blocks such as actin, myosin, proteins, lipids, amino acids and sugars1
The chirality parameter κ of the chiral molecule medium represents the chiroptical signals of the chiral molecules
Electromagnetic fields propagating through chiral media display circular birefringence12; left- and right-handed circularly polarized light experience different speeds through the medium according to wavevectors kL,R = (n ± κ)k0 with the conventional refractive index of the medium n and vacuum wavevector k0 = 2π/λ

Summary

Scattering of Nanoparticles

We enhance the weak optical signals of small chiral molecules via circular differential Mie scattering (CDMS) of nanoparticles immersed in them. Measurement of the chiroptical signals of small molecules is limited to samples in microgram quantities3 To overcome this drawback of chiroptical signals as indicators of stereochemical information, it has been noted that nanostructures can amplify the differential absorption by molecules of oppositely polarized circular light via the molecule-plasmon Coulomb interaction and optical chirality enhancement. To overcome this drawback of chiroptical signals as indicators of stereochemical information, it has been noted that nanostructures can amplify the differential absorption by molecules of oppositely polarized circular light via the molecule-plasmon Coulomb interaction and optical chirality enhancement8 These approaches are still in the Rayleigh regime, in that the molecules are still small compared with the wavelength λ of the absorbed or scattered light. Our work enables the real-time and local measurement of molecular dynamics, such as protein binding kinetics, in the vicinity of the nanoparticles using CDMS

Results

The circular bases for the scattered fields Es are given by

Discussion

This is consistent with

Additional Information