Ligand-induced
chirality in semiconducting nanocrystals has been
the subject of extensive study in the past few years and shows potential
applications in optics and biology. Yet, the origin of the chiroptical
effect in semiconductor nanoparticles is still not fully understood.
Here, we examine the effect of the interaction with amino acids on
both the fluorescence and the optical activity of chiral semiconductor
quantum dots (QDs). A significant fluorescence enhancement is observed
for
l/d
-Cys-CdTe QDs upon interaction with all the tested
amino acids, indicating suppression of nonradiative pathways as well
as the passivation of surface trap sites brought
via
the interaction of the amino group with the CdTe QDs’ surface.
Heterochiral amino acids are shown to weaken the circular dichroism
(CD) signal, which may be attributed to a different binding configuration
of cysteine molecules on the QDs’ surface. Furthermore, a red
shift of both CD and fluorescence signals in
l
/
d
-Cys-CdTe QDs is only observed upon adding cysteine, while other
tested amino acids do not exhibit such an effect. We speculate that
the thiol group induces orbital hybridization of the highest occupied
molecular orbital (HOMOs) of cysteine and the valence band of CdTe
QDs, leading to the decrease of the energy band gap and a concomitant
red shift of CD and fluorescence spectra. This is further verified
by density functional theory calculations. Both the experimental and
theoretical findings indicate that the addition of ligands that do
not “directly” interact with the valence band (VB) of
the QD (noncysteine moieties) changes the QD photophysical properties,
as it probably modifies the way cysteine is bound to the surface.
Hence, we conclude that it is not only the chemistry of the amino
acid ligand that affects both CD and PL but also the exact geometry
of binding that modifies these properties. Understanding the relationship
between the QD’s surface and chiral amino acid thus provides
an additional perspective on the fundamental origin of induced chiroptical
effects in semiconductor nanoparticles, potentially enabling us to
optimize the design of chiral semiconductor QDs for chiroptic applications.
Many organic ligands were synthesized to recognize G-quadruplexes. However, different kinds of G-quadruplexes (G4s) possess different structures and functions. Therefore, selective recognition of certain types of G4s is important for the study of G4s. In this paper, a novel cyanine dye, 3-(2-(4-vinylpyridine))-6-(2-((1-(4-sulfobutyl))-3,3-dimethyl-2-vinylbenz[e]indole)-9-ethyl-carbazole (9E PBIC), composed of benzindole and carbazole was designed and synthesised. The studies on UV-vis and fluorescence properties of the dye with different DNA forms showed that the dye exhibits almost no fluorescence under aqueous buffer conditions, but it increased over 100 fold in the presence of c-myc G4 and 10-30 fold in the presence of other G4s, while little in the presence of single/double-stranded DNA, indicating that it has excellent selectivity to c-myc 2345 G4. For the binding studies the dye is interacted with the c-myc 2345 G-quadruplex by using the end-stack binding model. It can be said that the dye is an excellent targeting fluorescent probe for c-myc G-quadruplexes.
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