Biomedical Engineering Reference
In-Depth Information
1.3.3
Defects and Robustness of CD in Nanocrystal
Assemblies
In order to demonstrate the plasmonic CD mechanism resulting
from plasmon-plasmon interactions, a carefully designed and well-
defined helical gold NP assembly was created in the experiments
[42]. Helical chains of MNPs, mimicking twisted natural molecules,
demonstrate the best e
ciency in generating plasmonic CD among
all frame geometries investigated. However, we noticed in our
first study [18] that the sign of the CD signal may flip when the
total number of MNPs increases, as is shown in Fig. 1.6a. This
can possibly create challenges in measuring a CD signal from an
ensemble of helical MNP assemblies, as it can be that a CD signal
is averaged to zero due to this kind of randomness in a fabrication.
Then, we showed dependence of CD strength on various kinds of
parametric uncertainties in the next study [19], wherein we allow
the geometrical parameters of a helix to vary, including the pitch of
the helix, the total number of MNPs, the size and positions of MNPs,
anddefectsduetomissingMNPs.Itwasshownthatamoderatelevel
of randomness in these parameters does not diminish or change
the shape of the CD signals. In Fig. 1.7, two cases are presented
that demonstrate the consequences of randomized particle sizes
and randomized particle positions. We can see that the influence
of such disorder is weak. In nature, partially disordered molecular
complexessuchasrandom-coilproteinsstilldemonstratesignificant
CD signals. From this point of view, it is not surprising that a
helical chain of MNPs has a stable CD response. After these careful
verifications, we were confident that a plasmonic CD from helical
MNPchainswithcarefullyselectedgeometricparametersshouldbe
strong.
To explain the bisignate CD signal in the plasmon band, our
study [42] showed that it originates from the splitting between a
transverse (
xy
) mode and a longitudinal (
z
) mode. Such collective
excitations result from a plasmon-plasmon interaction in the
dipole model. When circularly polarized light propagates vertically,
the transverse (
xy
) modes in a standing helix are excited, while
longitudinal (
z
) modes will be excited in a lying helix. The strength
of an excitation will be slightly stronger if its handedness matches
