In this study, the urea dynamics inside AOT reverse micelle (RM) has been monitored without intervention of water using time-resolved fluorescence techniques from the picosecond to nanosecond time regime. It has been observed that urea dynamics inside the reverse micelle is severely retarded compared to water RM due to the formation of highly networked urea cluster inside the RM. Time-resolved fluorescence anisotropy study also confirms the existence of a confined environment around the dye at higher concentrations of urea inside the reverse micelle. The dynamics of urea-water mixtures inside AOT reverse micelle has also been monitored with increasing urea concentration to get insight about the effect of urea on the overall solvation dynamics feature. It has been observed that with the increase in urea concentration, the overall dynamics becomes slower, and it infers the presence of few water or urea molecules, those strongly associated with surrounding urea and (or) water by hydrogen bonds.
The unidirectional proton coupled electron transfer (PCET) from the excited state of Ru(II) imidazole phenanthroline complex [Ru(bpy) ipH] to 1,4-benzoquinone, was studied by steady-state (SS) and time-resolved (TR) fluorescence and transient absorption (TA) measurements. The pK (9.7) and pK * (8.6) values of the complex suggest that it behaves as a photoacid on excitation. The difference in the quenching rates obtained from SS and TR fluorescence studies indicate participation of both dynamic quenching and static quenching involving the hydrogen bonded ipH ligand of [Ru(bpy) ipH] with the 1,4-benzoquinone quencher, formed in the ground state. Within the hydrogen bonded complex, the ruthenium centre acts as the electron donor, while the ipH ligand acts as the proton donor to the hydrogen bonded 1,4-benzoquinone that acts simultaneously both as the electron and proton acceptor. It is proposed that the static quenching in the hydrogen bonded [Ru(bpy) ipH] -1,4-benzoquinone pairs occurs involving the PCET mechanism, while the dynamic quenching occurs through the simple ET mechanism, on diffusional encounter of the isolated 1,4-benzoquinone with the excited [Ru(bpy) ipH] complex. The occurrence of broad TA bands around 420-430 nm suggests formation of both 1,4-benzoquinone radical anion as well as the 1,4-benzosemiquinone radical by the interaction of excited [Ru(bpy) ipH] with 1,4-benzoquinone, thus supporting the ET process in the studied system.
In
this study, a siderophore, pyoverdine (PVD), has been isolated
from Pseudomonas sp. and used to develop a fluorescence quenching-based
sensor for efficient detection of nitrotriazolone (NTO) in aqueous
media, in contrast to other explosives such as research department
explosive (RDX), picric acid, and trinitrotoulene (TNT). The siderophore
PVD exhibited enhanced fluorescence quenching above 50% at 470 nm
for a minimal concentration (38 nM) of NTO. The limit of detection
estimated from interpolating the graph of fluorescence intensity (at
470 nm) versus NTO concentration is found to be 12 nM corresponding
to 18% quenching. The time delay fluorescence spectroscopy of the
PVD–NTO solution showed a negligible change of 0.09 ns between
the minimum and maximum NTO concentrations. The in silico absorption
at the emission peak of static fluorescence remains invariant upon
the addition of NTO. The computational studies revealed the formation
of inter- and intramolecular hydrogen-bonding interactions between
the energetically stable complexes of PVD and NTO. Although the analysis
of Stern–Volmer plots and computational studies imply that
the quenching mechanism is a combination of both dynamic and static
quenching, the latter is dominant over the earlier. The static quenching
is attributed to ground-state complex formation, as supported by the
computational analysis.
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