Collective circular dichroism by chiral plasmonic nanoparticles

Molecular chirality refers to the geometric property of molecules with broken mirror symmetry. Characterizing molecular chirality and understanding their role in physiochemical situations has been important in broad research areas such as biology, chemistry and pharmacy.

Generally molecular chirality can be analyzed using circular dichroism (CD) spectroscopy, which measures the absorption difference of left and right circularly polarized light (LCP and RCP). However, the signal or change resulting from the interaction is too low due to the mismatch of light (several hundred nanometers) and molecules (few nanometers).

Although molecular CD can be enhanced by localized surface plasmon resonance (LSPR), which limits the electromagnetic field to the molecular scale, it is still difficult to detect molecular chirality at very low concentrations.

A research team from Seoul National University and Korea University used a novel CD mode in regularly assembled chiral plasmonic nanoparticles for ultrasensitive, in-situ quantification of molecular chirality. The study is published in the journal Nature.

The new chiral plasmonic nanoparticles (180 nm), which are four-, three-, two-fold rotational symmetry without any mirror symmetry, are arranged with a periodicity of 400 nm using a hexagonal patterned polymer template. At the specific angle of incidence and wavelength of CPL, a strong CD response can be generated in addition to the CD response of single nanoparticle LSPR.

Although the additional plasmonic resonance for the LSPR of a single nanoparticle has been previously demonstrated in an achiral nanoparticle array, there is no CD response. Thus, coupling to chiral molecules cannot occur across the surface and only a slight signal enhancement can be expected from the coupling to chiral molecules and LSPR.

The research team found the physical origin of strong CD (i.e. collective CD) in the collective resonance and spinning of induced electric dipoles on each nanoparticle across the array. The CPL and periodic regulation of nanoparticles induces a wave of free electrons in each nanoparticle (i.e. effective electric dipole) that interact collectively along the surface. Here, the circular polarization of light causes each dipole to rotate in the same direction.

They also found that the collective spinning of dipoles generates uniform and chiral electromagnetic field throughout the series. As a result, the chiral interaction between molecules and this field is significantly enhanced to alter the CD response in different ways depending on molecular handedness (i.e., opposite spectral shift for left- and right-handed molecules).

The current study clarified how the chiral molecules affected the CD response and achieved ultra-sensitivity in situ (limit of detection: 10^-4 M) Detection of molecular chirality. The integration of arrays in proof-of-concept devices, such as polarization-resolved colorimetric sensor and fluidic chip, proved the versatility of the underlying principle presented in this study for the enantioselective monitoring of DNA/RNA hybridization and structural change in protein at extremely low concentrations. It shows the potential of this detection principle to be applied in investigating membrane proteins, by integrating two-dimensional membranes onto the array and monitoring chirality changes in their structures and folding.