Abhisek Gupta

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.

Removal of toxic Cr(VI) by UV-active functionalized graphene oxide for water purification

Extraction of hazardous heavy metals like As, Hg, Cd, Cr(VI), etc. for water purification is a great challenge. Exploiting the large surface area of graphene, in the present work, we have synthesized a UV-active 2,6-diamino pyridine–reduced graphene oxide (DAP–RGO) composite to remove Cr(VI) from acidic water solution. Here, the presence of an extra pyridinic-nitrogen lone pair facilitates the removal efficiency of excess Cr(VI) [500 mg L−1 in 3 h only] over reported results so far. In addition to that, the unique advantage of this UV-active material is the enhancement of removal efficiency by 18% at a higher pH value. We believe that this study will bring forth a new class of UV-active graphene based adsorbents with remarkably high removal efficiency for toxic heavy metals from waste-water in future.

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.

Antiferro quadrupolar ordering in Fe intercalated few layers graphene

The π electron cloud above and below the honeycomb structure of graphene causes each carbon atom to carry a permanent electric quadrupole moment which can attach any cation to impart interesting physical properties. We have synthesized Fe intercalated graphene structures to investigate tunable magnetic properties as a result of this chemical modification. An interesting antiferro quadrupolar ordering is observed which arises due to a coupling between magnetic dipole moment of Fe and electric quadrupole moment on graphene surface. In contrast to antiferromagnetic Neel temperature (TN), here the ordering temperature (TQ) increases from 35.5 K to 47.5 K as the magnetic field is raised upto 1 Tesla.

1‐Aza‐15‐Crown‐5 Functionalized Graphene Oxide for 2D Graphene‐Based Li+‐ion Conductor

Attachment of Li(+) ion on graphene surface to realize Li(+)-ion conductor is a real challenge because of the weak interaction between the ions and the functional groups of graphene oxide; although, a large number of theoretical results are already available in the literature. To overcome this problem, graphene oxide is functionalized by 1-aza-15-crown-5, the cage-like structure containing four oxygens that can bind Li(+) ion through electrostatic interaction. Li(+) migration on graphene surface has been investigated using ac relaxation mechanism. Perfect Debye-type relaxation behavior with β (relaxation exponent) value ≈1 resulting from single ion is observed. The activation energy of Li(+) migration arising due to cation-π interaction is found to be 0.37 eV, which agrees well with recently reported theoretical value. It is believed that this study will help to design isolated ion conductors for Li(+)-ion battery.

Trap induced tunable unusual dielectric properties in transition metal doped reduced graphene oxide

Graphene being an excellent electronic material has poor dielectric properties. In addition to unusual dielectric response (permittivity increases with frequency) due to trap induced capacitance, here we have tuned the trap states to achieve a giant value of permittivity (e ∼ 2214) and remarkably high magneto-dielectric effect (23%) in nickel doped reduced graphene oxide (RGO). The current–voltage characteristics in the space charge limited conduction give quantitative information about these trap states. We estimate an average trap density of 1.92 × 1022 m−3 at room temperature. We believe that this transition metal doped RGO with tunable dielectrics has potential applications in electrical storage devices.

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.

Photoluminescence study in diaminobenzene functionalized graphene oxide

Being an excellent electronic material graphene is a very poor candidate for optoelectronic applications. One of the major strategies to develop the optical property in GO is the functionalization of graphene oxide (GO). In the present work GO sheets are functionalized by o-phenylenediamine to achieve diaminobenzene functionalized GO composite (DAB-GO). Formation of DAB-GO composite is further characterized by FTIR, UV, Raman studies. Excellent photoluminescence is observed in DAB-GO composite via passivation of the surface reactive sites by ring-opening amination of epoxides of GO.