Chiroptical Activity in Silver Cholate Nanostructures
Induced by the Formation of Nanoparticle Assemblies
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Abstract
We report on an interesting mechanism
of inducing chiroptical response
at plasmonic silver nanoparticles (NPs) through the formation of plasmonic
hot spots in small metal-NP–chiral-surfactant assemblies. Circular
dichroism (CD) was measured at the surface plasmon resonance of cholate-coated
silver nanostructures (AgCT) in the visible region of the spectrum.
Low temperature cryogenic transmission electron microscopy (cryo-TEM)
micrographs of the AgCT nanostructures in solution reveal small assemblies
of silver NPs. Upon pH increase these assemblies are separated into
individual NPs, and the induced plasmonic CD vanishes. This process
was monitored via spectroscopy (CD and absorption), cryo-TEM, small-angle
X-ray scattering (SAXS), and dynamic light scattering (DLS) measurements.
The synthesis of well-separated AgCT NPs, which was performed with
a large excess of sodium cholate (NaCT), also did not show any chiroptical
effects. We interpret and model the formation of strong CD signals
in the visible range in terms of the molecule–plasmon interaction
in plasmonic hot spots formed in nanoparticle aggregates. Importantly,
this study of the chiral induction, transfer to the visible range,
and local field enhancement offers very attractive possibilities for
sensing and detection of chirality of small amounts of molecules using
visible light