Insight into the photophysics of strong dual emission (blue & green) producing graphene quantum dot clusters and their application towards selective and sensitive detection of trace level Fe3+ and Cr6+ ions.

Graphene-nanostructured systems, such as graphene quantum dots (GQDs), are well known for their interesting light-emitting characteristics and are being applied to a variety of luminescence-based applications. The emission properties of GQDs are complex. Therefore, understanding the science of the photophysics of coupled quantum systems (like quantum clusters) is still challenging. In this regard, we have successfully prepared two different types of GQD clusters, and explored their photophysical properties in detail. By co-relating the structure and photophysics, it was possible to understand the emission behavior of the cluster in detail. This gave new insight into understanding the clustering effect on the emission behaviour. The results clearly indicated that although GQDs are well connected, the local discontinuity in the structure prohibits the dynamics of photoexcited charge carriers going from one domain to another. Therefore, an excitation-sensitive dual emission was possible. Emission yield values of about 18% each were recorded at the blue and green emission wavelengths at a particular excitation energy. This meant that the choice of emission color was decided by the excitation energy. Through systematic analysis, it was found that both intrinsic and extrinsic effects contributed to the blue emission, whereas only the intrinsic effect contributed to the green emission. These excitation-sensitive dual emissive GQD clusters were then used to sense Fe3+ and Cr6+ ions in the nanomolar range. While the Cr6+ ions were able to quench both blue and green emissions, the Fe3+ ions quenched blue emission only. The insensitivity of the Fe3+ ions in the quenching of the green emission was also understood through quantum chemical calculations.

Surface and morphology analyses, and voltammetry studies for electrochemical determination of cerium(iii) using a graphene nanobud-modified-carbon felt electrode in acidic buffer solution (pH 4.0 ± 0.05).

Trace determination of radioactive waste, especially Ce3+, by electrochemical methods has rarely been attempted. Ce3+ is (i) a fluorescence quencher, (ii) an antiferromagnet, and (iii) a superconductor, and it has been incorporated into fast scintillators, LED phosphors, and fluorescent lamps. Although Ce3+ has been utilized in many industries due to its specific properties, it causes severe health problems to human beings because of its toxicity. Nanomaterials with fascinating electrical properties can play a vital role in the fabrication of a sensor device to detect the analyte of interest. In the present study, surfactant-free 1,8-diaminonaphthalene (DAN)-functionalized graphene quantum dots (DAN-GQDs) with nanobud (NB) morphology were utilized for the determination of Ce3+ through electrochemical studies. The working electrode, graphene nanobud (GNB)-modified-carbon felt (CF), was developed by a simple drop-coating method for the sensitive detection of Ce3+ in acetate buffer solution (ABS, pH 4.0 ± 0.05) at a scan rate of 50 mV s-1 using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. CV and DPV studies validated the existence of distinctive peaks at approximately +0.20 and +0.93 V (vs. SCE), respectively, with a limit of detection of approximately 2.60 µM. Furthermore, electrochemical studies revealed that the GNB-modified-CF electrode was (i) stable even after fifteen cycles, (ii) reproducible, (iii) selective towards Ce3+, (iv) strongly pH-dependent, and (v) favored Ce3+ sensing only at pH 4.0 ± 0.05. Impedance spectroscopy results indicated that the GNB-modified-CF electrode was more conductive (1.38 × 10-4 S m-1) and exhibited more rapid electron transfer than bare CF, which agrees with the attained Randles equivalent circuit. Microscopy (AFM, FE-SEM, and HR-TEM), spectroscopy (XPS and Raman), XRD, and energy-dispersive X-ray (EDX) analyses of the GNB-modified-CF electrode confirmed the adsorption of Ce3+ onto the electrode surface and the size of the electrode material. Ce3+ nanobuds increased from 35-40 to 50-55 nm without changing their morphology. The obtained results provide an insight into the determination of Ce3+ to develop an electrochemical device with low sensitivity.

Graphene Nanobuds: A New Second-Generation Phosgene Sensor with Ultralow Detection Limit in Aqueous Solution.

Selective and sensitive detection of highly toxic chemicals by a suitable, fast, inexpensive, and trustworthy method is vital due to its serious health threats to humankind and breach of public security caused by unexpected terrorist attacks and industrial accidents. Phosgene or carbonyl dichloride is widely employed in many chemical industries and pharmaceuticals, and in pesticide production, which is extremely toxic by severe (short-term) inhalation exposure. Because of the non-existence of a phosgene sensor in aqueous solution and the immense emphasis gained by nanomaterials, especially carbonaceous materials, augmented attention has been given to the development of a fluorophore-functionalized carbon-based method to detect this noxious substance. In this study, surfactant free 1,8-diaminonaphthalene (DAN)-functionalized graphene quantum dots (DAN-GQDs) were prepared to detect phosgene in aqueous solution. The FESEM (field emission scanning electron microscopy) and HRTEM (high-resolution transmission electron microscopy) analyses confirm the as-prepared DAN-GQD morphology as nanobuds (NBs) with an average diameter of ca. 35-40 nm. The crystalline nature, elemental composition, and chemical state of DAN-GQDs were analyzed by standard physiochemical techniques. The edge-termination at the carboxyl functional group of GQDs with DAN was examined by XPS, Raman, FT-IR, and 1H NMR spectroscopy analyses. The aqueous solution of DAN-GQDs (4.89 × 10-9 M) exhibits a strong emission peak at 423 nm upon excitation at 328 nm. The addition of the phosgene molecule (0 → 88 µL) quenches the initial fluorescence intensity of DAN-GQDs (ΦF 53.6 → 34.6%) through the formation of a stable six-membered cyclized product. The DAN-GQDs displayed excellent selectivity and sensitivity for phosgene ( Ka = 3.84 × 102 M-1 and LoD (limit of detection) = 2.26 ppb) over other competing toxic pollutants in water. The time-resolved fluorescence analysis confirms that the quenching of DAN-GQDs follows nonradiative relaxation of excited electrons. Furthermore, bioimaging experiments of phosgene in living human breast cancer (HeLa) cells and cell viability test successfully demonstrated the practicability of DAN-GQDs.

Detoxification of cytotoxic alachlor by glutathione: characterization of conjugated adducts by electrospray ionization tandem mass spectrometry.

The most commonly used chloroacetamide herbicide, alachlor, and its conjugated adducts have been characterized by electrospray ionization mass spectrometry (ESI-MS). The reactivity of glutathione toward alachlor has been evaluated by changing experimental parameters, such as pH, temperature, and tube lens offset voltage (TLOV) in aqueous methanol, and the products were subjected to collision-induced dissociation (CID) for further characterization. In the positive mode, CID proves the formation of cyclic species by elimination of glycine and NH(3) moiety, which is similar to protonated cysteine. The results confirm that, only under basic conditions, glutathione is able to detoxify alachlor.

[1,3-Bis(diphenyl-phosphino)pentane-κP,P']tetra-carbonyl-chromium(0).

In the title compound, [Cr(C(29)H(30)P(2))(CO)(4)], the Cr atom is octa-hedrally coordinated by four carbonyl ligands and one bidentate phosphine ligand, which is bounded as a chelate in a cis position. The average Cr-P and Cr-C bond lengths are 2.377 and 1.865 Å, respectively.