Bikash Kumar Shaw

Highly Selective Detection of Trinitrophenol by Luminescent Functionalized Reduced Graphene Oxide through FRET Mechanism

Among different nitro compounds, trinitrophenol (TNP) is the most common constituent to prepare powerful explosives all over the world. A few works on the detection of nitro explosives have already been reported in the past few years; however, selectivity is still in its infant stage. As all the nitroexplosives are highly electron deficient in nature, it is very difficult to separate one from a mixture of different nitro compounds by the usual photoinduced electron transfer (PET) mechanism. In the present work, we have used a bright luminescent, 2,6-diamino pyridine functionalized graphene oxide (DAP-RGO) for selective detection of TNP in the presence of other nitro compounds. The major advantage of using this material over other reported materials is not only to achieve very high fluorescence quenching of ∼96% but also superior selectivity >80% in the detection of TNP in aqueous medium via both fluorescence resonance energy transfer and PET mechanisms. Density functional theory calculations also suggest the occurrence of an effective proton transfer mechanism from TNP to DAP-RGO, resulting in this tremendous fluorescence quenching compared to other nitro compounds. We believe this graphene based composite will emerge a new class of materials that could be potentially useful for selective detection, even for trace amounts of nitro explosives in water.

Thymine Functionalized Graphene Oxide for Fluorescence “Turn-off-on” Sensing of Hg2+ and I– in Aqueous Medium

Selective detection of either mercury (Hg2+) or iodide (I-) ion using fluorescence turn-on or turn-off processes is an important area of research. In spite of intensive research, simultaneous detection of both mercury and iodide using fluorescence turn-off-on processes, high sensitivity and theoretical support concerning the mechanisms are still lacking. In the present work, graphene oxide is functionalized by thymine to realize simultaneous detection of both Hg2+ and I- selectively using fluorescence turn-off-on mechanism. Ultra high sensitivity to the extent of ppb level exploiting large surface area of graphene is achieved. DFT calculations also assist to realize the detailed mechanisms involving this PL quenching and also its regain during sensing of these ions in aqueous solution.

Field-induced ferromagnetism and multiferroic behavior in end-on pseudohalide-bridged dinuclear copper(II) complexes with tridentate Schiff base blocking ligands.

Four new end-on pseudohalide-bridged dinuclear copper(II) complexes, [Cu2(L(1))2(N3)2]·DMF (1), [Cu2(L(2))2(N3)2] (2), [Cu2(L(3))2(NCS)2] (3), and [Cu2(L(4))2(N3)2] (4) {where HL(1), HL(2), HL(3), and HL(4) are tridentate N2O donor Schiff bases}, are synthesized and characterized. Complexes 1, 2, and 3 possess π···π stacking interactions, while in addition hydrogen-bonding interactions are present in 1 and 3. However, by contrast, complex 4 contains neither type of interaction. Field-induced long-range ferromagnetic ordering beyond 0.9 T is observed in complexes 1 and 2 due to π···π stacking interactions, while ferroelectric ordering is observed in complexes 1 and 3 due to hydrogen-bonding interactions. Most interestingly, complex 1, which contains both π···π stacking and hydrogen-bonding interactions, shows multiferroic behavior as a result of coupling between the dielectric and magnetic fields with 8% change in the magneto-dielectric effect at room temperature. We believe that from this study will emerge a new class of multiferroic materials.

Unusual dielectric response in cobalt doped reduced graphene oxide

Intensive research on cobalt doped reduced graphene oxide (Co-RGO) to investigate the modification in graphene magnetism and spin polarization due to presence of transition metal atom has been carried out, however, its dielectric spectroscopy, particularly, how capacitance changes with impurity levels in graphene is relatively unexplored. In the present work, dielectric spectroscopy along with magneto-dielectric effect are investigated in Co-RGO. Contrary to other materials, here permittivity increases abruptly with frequency in the low frequency region and continues to increase till 107 Hz. This unusual behavior is explained on the basis of trap induced capacitance created due to impurity levels.

Highly luminescent N-doped carbon quantum dots from lemon juice with porphyrin-like structures surrounded by graphitic network for sensing applications

Here we demonstrate a simple, low cost, and green synthetic approach to synthesizing water-soluble, nitrogen-doped, fluorescent carbon quantum dots (NCQDs) from lemon juice and ammonia by hydrothermal treatment. Chemical characterizations and low temperature photoluminescence and photoconductivity results show interesting structural features of the as-prepared NCQDs. These new NCQDs consist of a ring type moiety (porphyrin/chlorin) in the centre surrounded by the graphitic network and serve as an efficient fluorescent probe for label-free, sensitive, and selective detection of Fe3+ with a detection limit of 140 ppb (2.5 μM), which is remarkably lower than the earlier reports on CQDs-based sensing systems. DFT calculations are carried out to optimise the structural aspects for selective detection of Fe3+. This extremely low detection limit (140 ppb) arises due to static quenching in addition to dynamic quenching which generally occurs in most cases.

Photoluminescence study of optically active diaminopyridine intercalated graphene oxide

Diaminopyridine (DAP) ligand is successfully intercalated in GO layers to achieve a layered-type structure with interlayer separation ∼1.03 nm. Density functional theory (DFT) was applied to investigate the stability of the modified structure along with its interlayer separation, and agrees well with the experimental results. As-synthesized diaminopyridine functionalized GO (DAP–fGO) composites show better photoluminescent (PL) property compared with GO via surface passivation. Experimental observation of excitation-dependent PL spectra of DAP–fGO composite was further verified by DFT calculations of HOMO–LUMO band gaps. We believe that this study will help to design different GO-based nanomaterials with potential physical, chemical and optical applications.

Asymmetric bis-(μ1,1-azido) bridged dinuclear copper(II) complex with N2O donor Schiff base: synthesis, structure and magnetic study

A copper(II) complex, [Cu2(L)2(N3)2] [where HL = 2-((3-(methylamino)propylimino)methyl)-6-methoxyphenol] has been synthesized and characterized by elemental analysis, IR, UV–vis and fluorescence spectroscopy, and single-crystal X-ray diffraction studies. The complex crystallizes in the trigonal space group R. The deprotonated tridentate Schiff base occupies three coordination sites of copper(II). The fourth coordination site is occupied by an azide. A symmetry-related azide from a different molecule coordinates with the fifth site of copper(II), thereby forming a double end-on azide-bridged centrosymmetric dimer. Variable temperature solid–state magnetic studies between 2 and 300 K were carried out and the data indicate predominant antiferromagnetic exchange interactions with 2J = –0.45 cm−1. The magnetic field-dependent magnetization study (M − H) reveals existence of antiferromagnetic ordering at a lower temperature (2 K) with a very small coercive field (~20 Oe) suggesting soft magnet behavior of the complex. A double end-on azide-bridged centrosymmetric dinuclear copper(II) complex, [Cu2(L)2(N3)2], has been synthesized and characterized. X-ray crystal structure analysis has confirmed the structure; variable temperature (2–300 K) magnetic susceptibility measurement indicates antiferromagnetic exchange interactions with 2J = –0.45 cm−1.

Mononuclear manganese(III) complexes of bidentate NO donor Schiff base ligands: synthesis, structural characterization, magnetic and catecholase studies

We have synthesized four mononuclear manganese(III) complexes (1–4) of four closely related bidentate NO donor Schiff-base ligands, out of which three (2–4) were structurally characterized. Crystal structure determination reveals that all these complexes are in octahedral geometries. Magnetic studies have been carried out on complexes 2, 3 and 4 in the temperature range 2–300 K under a magnetic field of 0.1 T which yielded negative ZFS parameters of −2.96, –3.51 and −3.72 cm−1 respectively. The catecholase activities of complexes 1–4 have been investigated following the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) with molecular oxygen in DCM at 25 °C, which were found to follow the Michalis–Menton type relation giving the highest TON (Kcat) for the so far reported Mn(III) complexes.

Field-induced ferromagnetism due to magneto-striction in 1-D helical chains

Two homochiral copper(II) helices, [Cu(μ1,3-N3)(L1)]n (1) and [Cu(μ1,3-NCO)(L2)]n (2), with end-to-end pseudohalide bridges, were synthesized using two N2O donor achiral Schiff bases via spontaneous chiral resolution. Field-induced ferromagnetic ordering due to magneto-striction in homochiral 1-D helix [Cu(μ1,3-N3)(L1)]n (1) is reported for the first time. At temperatures below 5.5 K, under a magnetic field of 1 T, orthogonality between the magnetic orbitals of copper centres increases significantly due to the contraction of lattice parameters, giving rise to long-range ferromagnetic ordering in the helical chain. The magneto-dielectric results are also indicative of the observed magneto-striction effect in the present complex 1. We believe that this study will uncover a new way to invoke long-range ferromagnetic ordering in 1-D helical chains.

Observation of the predicted potential barrier at the graphene/transition metal interface

To verify the prediction of a potential barrier (~0.4 eV) as a result of charge transfer from a transition metal (TM) to graphene at the graphene/TM interface, in the present work Fe atoms are intercalated in between the few-layer graphene. A diode-like behavior with cut-in voltage (0.41 V–0.65 V) and rectification ratio 11 at 1.7 V is observed depending on the Fe concentration. The cut-in voltage decreases with an increase in both temperature and magnetic field because of better charge sharing across the covalent bond formed between Fe and graphene. This tuning of charge sharing under a magnetic field exhibits a remarkable effect of the magneto-dielectric property with a 2597% change in dielectric permittivity at 1.8 T. We believe that the material exhibiting diode-like behavior with a large rectification ratio and a giant magneto-dielectric effect has potential applications in magneto-electronic devices.