Biomedical Engineering Reference
In-Depth Information
several experimental papers showing successful fabrications of
opticallyactivechiralnanostructuresandnano-assemblieswithnew
plasmonic CD signals.
1.1 Introduction
Chirality can also be referred to as handedness. Chiral objects are
not superimposable onto their mirror images. Looking around us,
our hands, snail shells, screws, and so on, are all chiral. It often
provides a smooth, e
cient, and strong coupling between objects
with the same chirality, such as the tightening between a bolt and a
nut or a firm handshake using both right hands. Chirality also plays
animportantroleintheoriginoflife.Forexample,allnaturalamino
acid monomers have
L
-configuration, while sugars are
D
-isomers.
If two chiral molecules are mirror images of each other, we call
them “enantiomers.” The chemical properties may be dramatically
different for each enantiomer of the same molecule. In the human
body, for example, sometimes biological stereoselectivity only
allows a particular type of enantiomers in a racemic drug to react
with the targeted biomolecules [12]. Therefore, chirality is of great
interestinthepharmaceuticalindustry.Inordertonotonlyimprove
the safety of drugs but also to reduce their production costs, efforts
have been made into manufacturing drugs with only one kind of
enantiomers, as the other kind of enantiomers may be less active,
inactive, or even responsible for adverse effects in treatments, a
distressing examplebeing the sedative drug thalidomide.
Circular dichroism (CD) refers to the difference in absorption
of left- and right-circularly polarized (LCP and RCP) light in chiral
medium. It has been shown that CD spectroscopy is a powerful tool
to study chirality of materials. Most of the homochiral materials
had been found created in nature, until CD was demonstrated in
the lab from a ligand-protected metal nanocluster system more
than a decade ago [60, 61]. The interest in such artificial chirality
grows exponentially. It was summarized [24, 53] that the CD from
a nanocluster can be induced by intrinsically chiral metal core;
chiral environment; or an intermediate mechanism called footprint
effect. It was also shown that a mixture of these mechanisms is
