  Recently, there is a 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 plasmoninduced circular dichroism (CD) are 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 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 (EQ) can be two orders of magnitude higher than that contributed by
molecular electric/magnetic dipoles. Unlike the case of 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 filed gradients, and is correlated with the boosted emission
rate of a molecular EQ. Based on such physical understandings, we can design
nanostructures to realize giant chiroptical effect using the EQ contribution
according to the requirements,2 which provide a new strategy for ultrasensitive
detection and quantification of molecular chirality.
