9-Ethynylphenanthrene (EPT) bound to highly monodispersed Ruthenium (Ru) nanocluster (Ru:EPT) with mean diameter of 1.5 ± 0.2 nm and mol wt. of ∼8600 Da was synthesized via a facile and high yield biphasic ligand exchange protocol using similar sized ethylene glycol (EG)-stabilized Ru clusters (Ru:EG) as precursor. The synthesized organometallic nanocluster was meticulously analyzed to understand its size distribution, oxidation state, crystallinity, optical and luminescence behavior and metal-ligand interfacial structure. Contrary to the extensive quenching of ligand emission by metalcore as usually observed, the ruthenium core here acts as a conductor, which conjugates surface ligands with strong emission property courtesy to an unusual vinylidene-binding motif. Thus, the synthesized nanocluster shows good luminescence property (φ = ∼7%) originated from the ligand skeleton and the spherical metal core restricts lateral overlap of phenanthrene moiety to cause any excimer emission. This nanocluster showed high sensitivity for solution phase detection of nitroaromatic explosives through luminescence quenching method (K SV up to 4.98 × 10 4 M −1) and mimic the mechanism like conjugated organic polymer. We propose that dynamic π − π interaction between Ru bound phenanthrene moiety and nitroaromatic compounds followed by photoinduced electron transfer (PET), as well as Förster Resonance Energy Transfer (FRET), are the possible mechanisms behind this luminescence quenching.
By
attaching the1-pyreneiodide ancillary ligand to the surface
of polyvinylpyrrolidone-stabilized gold (Au:PVP) cluster or the cetyltrimethyl
ammonium bromide-stabilized gold (Au:CTAB) nanorod, a new class of
luminescent mixed ligand-stabilized gold nanostructures is synthesized.
This postsynthetic surface modification method followed by us is a
comparatively easier and hassle-free technique to acquire surface-active
luminescent “functional nanomaterials”.
Careful analyses of transmission electron microscopy images revealed
that the sizes of these Au-clusters or Au-nanorods remain unchanged
without any noticeable aggregation in the medium. Owing to the formation
of an excimer within the neighboring pyrenes mounted on the surface
of core nanostructures (i.e., Au:PVP nanocluster and Au:CTAB nanorod),
the resulting pyrene-grafted nanocomposites exhibit strong emission
characteristics. The strong excimer emission is significantly quenched
in the presence of electron-deficient chemical inputs, and this phenomenon can be used for analytical
purposes. Using these luminescent Au-nanomaterials, we demonstrate
a selective detection and sensing of trace-level nitroaromatic explosives
(e.g., trinitrotoluene, trinitrophenol (TNP), dinitrotoluene, 4-nitrotoluene,
etc.). It was observed that the Py-Au:PVP nanocluster is equally effective
for explosive detection in both solution and solid phases with the
limit of detection up to 10 nanomolar. A high Stern–Volmer
constant of up to 3.88 × 106 M–1 was seen in the case of TNP in anhydrous methanol at 298 K. The
deactivation pathway operating within the Py-Au:PVP nanocluster and
the analytes is thought to be a result of a predominating static quenching
process, where a nonfluorescent D–A supramolecular adduct is
formed in the medium. Py-Au:PVP has also been successfully used to
develop latent fingerprints from nonporous surfaces under an exposure
of 365 nm UV light. The results suggest that these new composite materials
could behave as potential “functional nanomaterials”, which might be a promising alternative for on-the-spot
detection of explosive traces as well as for easy visualization of
latent fingerprints.
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