posted on 2012-10-04, 00:00authored byAlexander Kyrychenko, Galina V. Karpushina, Denis Svechkarev, Dmitry Kolodezny, Sergey I. Bogatyrenko, Alexander P. Kryshtal, Andrey O. Doroshenko
Understanding the interaction of fluorescent dyes with
monolayer-protected
gold nanoparticles (AuNPs) is of fundamental importance in designing
new fluorescent nanomaterials. Among a variety of molecular sensors
and reporters, fluorescent probes based on a 3-hydroxychromone (3HC)
skeleton appear to be very promising. They exhibit the phenomenon
of dual band emission, resulting from excited-state intramolecular
proton transfer (ESIPT), known to be highly sensitive to a nature
of microenvironment surrounding a fluorophore. In this study, dodecanethiol-protected
gold nanoparticles were synthesized, and, owing to the transmission
electron micrograph imaging, their average diameter was found to be
∼1.4 nm. Fluorescence titrations of the 3HC ESIPT probes with
AuNPs in toluene solutions demonstrate significant changes in the
intensity ratio of their normal and tautomeric emission bands, suggesting
that the probe molecules become noncovalently bound to a dodecanethiol
layer of AuNPs. Despite expected fluorescence quenching induced by
close proximity to the metal surface, no fluorescence lifetime decrease
was observed, indicating that a bound-fluorophore is shielded from
a nanoparticle core. Further spectral analysis revealed that the ratiometric
fluorescence changes could be interpreted as a consequence of intermolecular
hydrogen bonding between a probe and residual ethanol molecules, trapped
into the dodecanethiol shell of AuNPs during the synthesis. Evidences
for residual traces of ethanol in the ligand shell of the nanoparticles
were also observed in NMR spectra, suggesting that alkylthiol-coated
gold nanoparticles may not be as hydrophobic as one could expect.
To elucidate structural features of dodecanethiol-stabilized gold
nanoparticles at the supramolecular level, a molecular dynamics (MD)
model of AuNP was developed. The model was based on the all-atom CHARMM27
force field parameters and parametrized according to available experimental
data of the synthesized AuNPs. Our MD simulations show that in bulk
toluene the 3HC probe molecule becomes weakly bound to a dodecanethiol
monolayer, so that a fluorophore favors residence in an outer shell
of AuNP. In addition, MD simulations of transfer of AuNP from bulk
ethanol to toluene demonstrate that a small population of ethanol
molecules are able to penetrate deeply into the dodecanethiol layer
and may indeed be trapped into the ligand shell of alkylthiol-functionalized
gold nanoparticles. The results of our fluorescence experiments and
molecular dynamics simulation suggest that 3-hydroxychromones can
be used as a noncovalent fluorescent labeling agent for alkylthiol-stabilized
noble metal nanoparticles.