Synthesis and Characterization of a Novel Heterocyclic Schiff Base and Development of a Fluorescent Sensor for Vitamin B12.

A heterocyclic Schiff base (MPDPI)was synthesized by the condensation reaction of 1-phenylisatin with 4,5-dimethylphenylene diamine. It was characterized by using spectroscopic methods including UV visible, Infrared, 1H-NMR, 13C-NMR and mass spectrometry. It acts as the fluorescent probe for the detection of Vitamin B12 (Vit.B12) which shows high selectivity over other species via dynamic quenching mechanism. It is also highly sensitive towards Vit.B12 with a detection limit of [Formula: see text]M and showed a linear concentration ranging from [Formula: see text] to [Formula: see text]. Effect of other coexisting species was also studied. The satisfactory results were also obtained in real samples.Since, there are only few reports on Vit.B12, development of selective fluorescent probes for Vit.B12 would be worthwhile.

A nanohybrid system based on covalently functionalized graphene quantum dots with dithienopyrrole derivative for the sensitive and selective fluorometric detection of Pb2+ ions.

Recently graphene quantum dot (GQD) based nanohybrid materials have received much attention. Herein, a highly fluorescent biocompatible GQD and N-functionalized dithienopyrrole (DTP-PPD) modified nanohybrid system was fabricated (DTP-PPD-fn-GQD) for the first time. Modification resulted in stable fluorescence with a quantum yield of approximately 22% and 36 nm redshift compared to unmodified GQD. The apparent bandgap tuning along with fluorescence property changes was investigated by cyclic voltammetry measurements and density functional theory studies. This water-soluble system was then applied for the sensitive and selective detection of Pb2+ ions. As a result of a specific interaction towards Pb2+ ions, the fluorescence intensity was quenched. The detection limit is found to be 1.02 nM within the linear range of 3 to 30 nM. Finally, the feasibility of the developed probe was tested with real samples of water. Chemically modified GQDs are already widely exploited for Pb2+ sensing, whereas GQD/thiophene-based nanohybrid systems are less utilized. This newly developed nanohybrid of GQD (DTP-PPD-fn-GQD) has excellent fluorescence properties and bandgap tunability. Moreover, effective fluorometric sensing of Pb2+ ions in an aqueous medium is well investigated. This gives more insight into developing GQD-based highly promising nanohybrid colorimetric as well as fluorescent sensors.

Tuning Hole and Electron Transfer from Photoexcited CdSe Quantum Dots to Phenol Derivatives: Effect of Electron-Donating and -Withdrawing Moieties.

Charge-transfer processes from photoexcited CdSe quantum dots (QDs) to phenol derivatives with electron- donating (4-methoxy) and -withdrawing (4-nitro) moieties have been demonstrated by using steady-state and time- resolved emission and femtosecond transient absorption spectroscopy. Steady-state and time-resolved emission studies suggest that in the presence of both 4-nitrophenol (4NP) and 4-methoxyphenol (4MP) CdSe QDs luminescence is quenched. Stern-Volmer analysis suggests both static and dynamic mechanisms are active for both the QD/phenol composites. Cyclic voltammetric analysis recommends that photoexcited CdSe QDs can donate electrons to 4NP and holes to 4MP. To reconfirm both electron- and hole-transfer mechanisms, CdSe/CdS quasi-type II and CdSe/CdTe type II core-shell nanocrystals were synthesized and photoluminescence quenching was monitored in the absence and presence of both 4NP and 4MP, for which hole and electron transfer were systematically restricted. Results suggest that indeed electron and hole transfer take place from photoexcited CdSe to 4NP and 4MP, respectively. To monitor the charge-transfer dynamics in both systems on an early timescale, femtosecond transient absorption spectroscopic techniques have been employed. Electron and hole transfer and charge-recombination dynamics are discussed and the effect of electron-donating and -withdrawing groups has been demonstrated.

A coumarin-modified graphene quantum dot-based luminogen for the detection of cysteine in aqueous media.

A biocompatible fluorescence sensor for cysteine detection receives wide appreciation recently, because of its importance in the medical field. Functionalized graphene quantum dots (GQDs) are recently emerging biocompatible quantum dots, which are considered as suitable candidates for biomolecule detection. Motivated by this concept, here we have developed a versatile fluorescent probe based on 3-aminocoumarin (AMC) functionalized GQDs for the detection of cysteine (Cys). Modification on GQD with AMC resulted in a stable fluorescent probe with an enhancement in quantum yield of about 84% and 40 nm redshift in emission peak compared with bare GQD. The modified GQD is then used for the sensitive and selective detection of cysteine in aqueous media. The detection of Cys within the linear range of 50 nM to 1.5 µM was achieved with a detection limit (LOD) of 0.86 nM. Here, the AMC-GQD exhibit a turn-off fluorescence sensing behavior. The quenching mechanism was also explored. The sensing process follows dynamic quenching mechanism, which is attributed to the photoinduced charge transfer from AMC-GQD to Cys. The Stern-Volmer plot, energy-level alignment obtained from cyclic voltammetry measurements and density functional theory predictions give a valid proof for this. Furthermore, the sensor was applied efficiently to the determination of Cys in real water samples.