Browsing by Author "Sima Umrao"

Now showing 1 - 15 of 15

2D SnS2 Nanostructure-Derived Photocatalytic Degradation of Organic Pollutants Under Visible Light
(Frontiers Media S.A., 2021) Rohit Ranjan Srivastava; Pramod Kumar Vishwakarma; Umakant Yadav; Suyash Rai; Sima Umrao; Rajiv Giri; Preeti Suman Saxena; Anchal Srivastava
Wastewater produced by the textile industry contains various dyes and organic compounds that directly or indirectly affect surface water or groundwater pollution. Visible-light-driven semiconductor photocatalysis is the leading pathway for the degradation of environmental pollutants. Herein we report the bottom-up hydrothermal growth of 2D tin disulfide nanostructures (SnS2 NSs) for the efficient photodegradation of organic pollutants such as Rhodamine B (Rh.B) and Methyl Violet (M.V) in an aqueous medium under visible light (λ > 400 nm) irradiation. The as-synthesized SnS2 NSs were characterized by various structural, morphological, and optical techniques such as XRD, RAMAN, TEM, UV–Vis, Brunauer–Emmett–Teller, etc. Furthermore, the low bandgap (∼1.6 eV), the high surface area (56 m2/g), and the anionic nature of SnS2 NSs attribute to it as an efficient photocatalyst for photocatalytic applications. The photocatalytic properties of SnS2 NSs showed good degradation efficiency of 94 and 99.6% for Rh. B and M.V, respectively, in 25 min. The kinetic rate constant of these dyes was estimated by using the Langmuir–Hinshelwood model. Here we also performed the recyclability test of the photocatalyst and discussed the plausible mechanism for the photocatalytic degradation of organic pollutants. The XPS spectra of SnS2 NSs were studied before and after the photodegradation of Rh.B and M.V, indicating the high stability of the photocatalyst. Moreover, in vitro cytotoxicity was also evaluated against human cervical cancer cell lines (HeLa cells) with different concentrations (0–1,000 μg/ml) of as-synthesized SnS2 NSs. This intended work provides a possible treatment for the degradation of organic pollutants under visible light to balance the aquatic ecosystems. Copyright © 2021 Srivastava, Kumar Vishwakarma, Yadav, Rai, Umrao, Giri, Saxena and Srivastava.
A novel Raman spectroscopic approach to identify polymorphism in leflunomide: A combined experimental and theoretical study
(John Wiley and Sons Ltd, 2016) Poornima Sharma; Debraj Gangopadhyay; Sima Umrao; Shiv Kumar; A.K. Ghosh; P.C. Mishra; Ranjan K. Singh
Polymorphism is an important characteristic which affects the activity, solubility and other physical properties of a compound and can be induced by varying temperature, pressure and solvent. The presence and conversion of α to β polymorphic forms of an anti-rheumatic drug leflunomide have been studied by temperature-dependent and in situ Raman observations. Both α and β polymorphs were found to co-exist in the temperature interval 367-372 K. The α form alone exists below 367 K and the β form alone above 373 K. The C=O stretching band clearly demonstrates the α → β conversion because of breaking of N-H···O bond and formation of N-H···N bond. On cooling the Raman spectra suggest the irreversibility of this conversion. Thermodynamic stability, crystal parameters and surface morphology of both forms in the leflunomide powder used for the present study have been verified by differential scanning calorimetry, X-ray powder diffraction and scanning electron microscopy. © 2015 John Wiley & Sons, Ltd.
A possible mechanism for the emergence of an additional band gap due to a Ti-O-C bond in the TiO2-graphene hybrid system for enhanced photodegradation of methylene blue under visible light
(Royal Society of Chemistry, 2014) Sima Umrao; Shiju Abraham; Frank Theil; Shobhit Pandey; Valerian Ciobota; P.K. Shukla; Caroline J. Rupp; Sudip Chakraborty; Rajeev Ahuja; Jürgen Popp; Benjamin Dietzek; Anchal Srivastava
Here we report the experimental and theoretical study of two TiO2-graphene oxide (TG) and TiO2-reduced graphene oxide (TR) composites synthesized by a facile and ecological route, for enhanced visible light (∼470 nm) photocatalytic degradation of Methylene Blue (MB) (99% efficiency), with high rate constant values (1800% over bare TiO2). TG couples TiO2 nanopowder with Graphene Oxide (GO) while TR couples it with reduced graphene oxide (RGO). The present study, unlike previous reports, discusses never-before-reported double absorption edges obtained for both TG (3.51 eV and 2.51 eV) and TR (3.42 eV and 2.39 eV) composites, which represents the reason behind feasible visible light (2.56 eV) induced photocatalysis. TiO2 domains in the composites dominate the higher band edge, while GO/RGO domains explain the lower band edge. Formation of Ti-O-C bonds in both TG and TR drives the shifting upwards of the valence band edge and reduction in band gap. Further, these bonds provide a conductive pathway for charge carriers from TiO2 nanopowder to the degraded species via the GO/RGO matrix, resulting in decreased charge carrier recombination in TiO2 and enhanced efficiency. To attest that the developed theory is correct, density function theory (DFT) calculations were performed. DFT obtained energetics and electronic structures support experimental findings by demonstrating the role of the Ti-O-C bond, which results in double band edge phenomenon in composites. Finally, the mechanism behind MB degradation is discussed comprehensively and the effect of the weight percent of GO/RGO in the composite on the rate constant and photodegradation efficiency has been studied experimentally and explained by developing analytical equations. © The Royal Society of Chemistry 2014.
Eu:Y2O3 highly dispersed fluorescent PVA film as turn off luminescent probe for enzyme free detection of H2O2
(Elsevier B.V., 2017) Dhananjay Kumar; Sima Umrao; Himanshu Mishra; Rohit Ranjan Srivastava; Monika Srivastava; Anchal Srivastava; S.K. Srivastava
In this work, a novel sensing scaffold consisting highly dispersed Eu:Y2O3 fluorescent flexible film of poly vinyl alcohol (EYCP) was synthesized by simple mixing method and applied to facilitate non-enzymatic detection of hydrogen peroxide (H2O2) by turn off probe of fluorescence. The fluorescence spectra of EYCP consisting number of emission sharp transitions from excited, 5D0→7Fj (j = 0, 1, 2, 3, 4) energy levels of doped Eu3+ ion in Y2O3 host. In the presence of H2O2, the fluorescence intensity of the EYCP film was quenched due to the reduction of electron–hole pair recombination in Eu centers by electron transfer from Eu–O excited state to H2O2 energy level rather than 5D0 state of Eu3+, which reduces the number of electron in 5D0 state. The EYCP film shows excellent fluorescence quenching in presence of H2O2 by significantly increasing concentration of H2O2 and completely quenched at ∼150 μM. A linear relationship is observed between 0.0 and 60 μM with a correlation coefficient of 0.989. H2O2 sensing is also compared with the EYC nanoparticles. This study is expected to have a significant impact on further study of the Eu:Y2O3 fluorescent flexible film for wide range applications. © 2017 Elsevier B.V.
Facile, rapid and upscaled synthesis of green luminescent functional graphene quantum dots for bioimaging
(Royal Society of Chemistry, 2014) Vinod Kumar; Vimal Singh; Sima Umrao; Vyom Parashar; Shiju Abraham; Anand K. Singh; Gopal Nath; Preeti S. Saxena; Anchal Srivastava
We report here the upscaled synthesis of green luminescent functionalized graphene quantum dots (FGQDs) by using an inexpensive and commonly occurring natural precursor viz. graphite powder. We observed in our sample that photoluminescence increases for excitation wavelengths of 300 nm to 350 nm and then decreases when excited at 375 to 425 nm for FGQDs at neutral pH. We found that the synthesized FGQDs do not show a drastic change in emission properties when kept under different pH conditions, which makes them a potential candidate for in vivo imaging, where the pH of the culture media plays a crucial role in the maintenance of the fluorescence. Water solubility, and excellent photostability along with low cytotoxicity of FGQDs are manifested as a remarkable bioimaging material. This journal is © the Partner Organisations 2014.
Graphene Oxide-Based Biosensor for Food Toxin Detection
(Humana Press Inc., 2014) Saurabh Srivastava; Md Azahar Ali; Sima Umrao; Upendra Kumar Parashar; Anchal Srivastava; Gajjala Sumana; B.D. Malhotra; Shyam Sudhir Pandey; Shuji Hayase
We report results of the studies relating to the fabrication of a highly sensitive label free biosensor based on graphene oxide (GO) platform for the detection of aflatoxin B1 (AFB1) which is most toxic and predominant food toxin, using electrochemical impedance spectroscopy. The structural and optical characterization of GO/Au and anti-AFB1/GO/Au has been done by electron microscopy, Raman, X-ray diffraction (XRD), UV–vis and electrochemical impedance spectroscopy (EIS). The impedimetric sensing response of immunoelectrode as a function of AFB1 concentration reveals wider linear detection range (0.5–5 ng/ml), high sensitivity (639 Ω ng−1 ml), improved detection limit (0.23 ng ml−1) and good stability (5 weeks) for the label-free detection. Association constant (ka) for antigen–antibody interaction obtained as 0.46 ng ml−1 indicates high affinity. © 2014, Springer Science+Business Media New York.
Green luminescence of quasi-molecular level in graphene quantum dots fabricated by microwave bottom-up strategy
(Institute of Electrical and Electronics Engineers Inc., 2016) Min-Ho Jang; Sima Umrao; Jung-Hwan Jung; Anchal Srivastva; Il-Kwon Oh; Yong-Hoon Cho
Green photoluminescent graphene quantum dots were synthesized by one step microwave assisted method using organic solvent acetylacetone, which have two different light emissions at 460 and 505 nm irradiated by 370 and 470 nm of monochromatic light from Xenon lamp, respectively. © 2015 IEEE.
In Situ Functionalized Fluorescent WS2-QDs as Sensitive and Selective Probe for Fe3+ and a Detailed Study of Its Fluorescence Quenching
(American Chemical Society, 2019) Vijay K. Singh; Himanshu Mishra; Rashid Ali; Sima Umrao; Rajesh Srivastava; Shiju Abraham; Arvind Misra; Vidya Nand Singh; Hirdyesh Mishra; R.S. Tiwari; Anchal Srivastava
Most of the reports suggest that liquid exfoliated WS2-QDs are unstable; therefore the need of present day is to develop a novel synthesis route for producing long-term stable WS2-QDs. Herein, we report a bottom-up single-step hydrothermal growth of in situ functionalized blue fluorescent WS2-QDs with stable fluorescence in aqueous media without subsequent treatments. Presence of various functional groups over the surface of f-WS2-QDs provides high solubility and stability to f-WS2-QDs in aqueous media preserving its fluorescence. Further, photoluminescence property of f-WS2-QDs has been employed to devise an optical sensor with a high sensitivity (KD ∼104 M-1) and selectivity for ferric (Fe3+) ions. Under the optimal condition, response of the sensor is found to be linear in the range of 0-55 μM with a limit of detection (LOD) of 1.32 μM, which is within the maximum permissible level of Fe3+ (∼5.4 μM) in human drinking water by the USEPA. Further, we have also carried out a detailed evaluation on fluorescence quenching kinetics of f-WS2-QDs. Nonlinear behavior of S-V plot and TRPL measurements suggest that quenching is a mixed phenomenon of dynamic as well as static processes. Finally we have proposed a mechanism for fluorescence quenching of f-WS2-QDs in the presence of Fe3+ © 2018 American Chemical Society.
Microwave bottom-up route for size-tunable and switchable photoluminescent graphene quantum dots using acetylacetone: New platform for enzyme-free detection of hydrogen peroxide
(Elsevier Ltd, 2015) Sima Umrao; Min-Ho Jang; Jung-Hwan Oh; Guntae Kim; Sumanta Sahoo; Yong-Hoon Cho; Anchal Srivastva; Il-Kwon Oh
We report a microwave sequential bottom-up route to produce green and blue luminescent graphene quantum dots (g-GQDs and b-GQDs) with size-tunable and switchable functionalities by tailoring the diameter size and functional groups via microwave carbonization and aromatization processes from acetylacetone as a starting organic solvent. The b-GQDs as the final product show only one emission peak at 433 nm and pH-independent blue luminescence, because two-step microwave irradiation could reduce the size and the oxygen functional groups of the g-GQDs as an intermediate product. Also, the b-GQDs provide an exemplar enzyme-free platform for hydrogen peroxide detection through the electrochemical sensing due to much higher electron density and electron donating properties. In contrast, the g-GQDs show two different switchable photoluminescent emissions at ∼460 nm (P1) and ∼500 nm (P2): the P1 emission with sky-blue fluorescence originates from randomly conjugated oxygen-functional groups on the basal plane and/or edge of the g-GQDs and the P2 emission with green fluorescence results from quasi-molecular fluorophores formed by the electronic coupling of carboxylic acid groups. © 2014 Elsevier Ltd. All rights reserved.
Microwave-assisted boron and nitrogen co-doped reduced graphene oxide as a transparent conductive electrode
(American Institute of Physics Inc., 2017) Sima Umrao; Himanshu Mishra; Anchal Srivastava; Sungjoo Lee
A crystalline Boron (B)- and Nitrogen (N)-co-doped microwave-assisted reduced graphene oxide (BNMRGO) film was investigated as a potential transparent conducting electrode (TCE) material. X-ray diffraction results revealed the good crystallinity of the BNMRGO film, and the presence of a (0004) reflection plane indicated the formation of a few small domains of hexagonal boron nitride in the microwave assisted reduced graphene oxide (MRGO) sheets under the co-doping process. Raman and X-ray photoelectron spectroscopic results indicated a reduction of sp3 carbon centers upon co-doping. The ID/IG ratio decreased after co-doping from 0.89 to 0.24, indicating a low average defect density of ∼1.01 × 1010 cm-2. Optoelectronic characterization of the BNMRGO film on a glass substrate revealed a high optical transparency of 82% at 550 nm and a low sheet resistance (Rsh) of 355 Ω/sq, which was lower than that observed from the MRGO sheets (Rsh = 719 Ω/sq). BNMRGO provided a ratio between the direct conductivity (σdc) to the optical conductivity (σoc), that is, the figure of merit of a TCE material, of 5.96. Overall, this work paves the way toward developing a manufacturable TCE. © 2017 Author(s).
Microwave-Assisted Synthesis of Boron and Nitrogen co-doped Reduced Graphene Oxide for the Protection of Electromagnetic Radiation in Ku-Band
(American Chemical Society, 2015) Sima Umrao; Tejendra K. Gupta; Shiv Kumar; Vijay K. Singh; Manish K. Sultania; Jung Hwan Jung; Il-Kwon Oh; Anchal Srivastava
The electromagnetic interference (EMI) shielding of reduced graphene oxide (MRG), B-doped MRG (B-MRG), N-doped MRG (N-MRG), and B-N co-doped MRG (B-N-MRG) have been studied in the Ku-band frequency range (12.8-18 GHz). We have developed a green, fast, and cost-effective microwave assisted route for synthesis of doped MRG. B-N-MRG shows high electrical conductivity in comparison to MRG, B-MRG and N-MRG, which results better electromagnetic interference (EMI) shielding ability. The co-doping of B and N significantly enhances the electrical conductivity of MRG from 21.4 to 124.4 Sm-1 because N introduces electrons and B provides holes in the system and may form a nanojunction inside the material. Their temperature-dependent electrical conductivity follows 2D-variable range hopping (2D-VRH) and Efros-Shklovskii-VRH (ES-VRH) conduction model in a low temperature range (T < 50 K). The spatial configuration of MRG after doping of B and N enhances the space charge polarization, natural resonance, dielectric polarization, and trapping of EM waves by internal reflection leading to a high EMI shielding of -42 dB (∼99.99% attenuation) compared to undoped MRG (γ28 dB) at a critical thickness of 1.2 mm. Results suggest that the B-N-MRG has great potential as a candidate for a new type of EMI shielding material useful in aircraft, defense industries, communication systems, and stealth technology. © 2015 American Chemical Society.
Multi-layered graphene quantum dots derived photodegradation mechanism of methylene blue
(Royal Society of Chemistry, 2015) Sima Umrao; Poornima Sharma; Anushka Bansal; Ranjna Sinha; Ranjan K. Singh; Anchal Srivastava
The photocatalytic degradation of methylene blue (MB) under visible light in the presence of a multi-layered graphene quantum dots (MLGQDs) photocatalyst has been investigated in aqueous heterogeneous solution. The photodegradation mechanism, following pseudo first order kinetics, was examined for the effects of the existing monomer (MB+)-dimer {(MB+)2} equilibrium in MB solution. The photocatalytic degradation efficiency of MB+ achieved 93.3% with a rate constant (k) of 0.056 min-1 after 60 min irradiation with green light, while less degradation ∼ 89.44% with k ∼ 0.024 min-1, was achieved for (MB+)2. MB+ is perhaps a short-lived species and favors the photodegradation of MB in comparison to the (MB+)2 species. Similar trends have been found under blue light irradiation. MB+ species easily pulls a proton from the functional groups of MLGQDs, resulting in an intermediate product Luco-methylene blue (LMB). Finally, all MB species and intermediate products degrade into an environmental benign product via highly reactive OH radicals. In addition, our ab initio theoretical results reveals that monomers abstract a proton from hydroxyl groups of MLGQDs and formation of LMB takes place, which is weakly bonded with MLGQDs by hydrogen bonds. © The Royal Society of Chemistry.
Nanostructured palladium-reduced graphene oxide platform for high sensitive, label free detection of a cancer biomarker
(2014) Vinod Kumar; Saurabh Srivastava; Sima Umrao; Ram Kumar; Gopal Nath; Gajjala Sumana; Preeti S. Saxena; Anchal Srivastava
We report the results of studies related to the fabrication of a palladium nanoparticle decorated-reduced graphene oxide (Pd@rGO) based electrochemical immunosensor for the label free ultrasensitive detection of the prostate-specific antigen (PSA), a prostate cancer biomarker. The synergistic electrochemical activities of Pd and rGO result in an enhanced electron transfer used for the development of an ultrasensitive immunosensor. A facile approach was developed for the in situ synthesis of Pd@rGO using ascorbic acid as the reducing agent which enables the simultaneous reduction of both Pd+2 and GO into Pd nanoparticles and rGO, respectively. XRD, FTIR, SEM and TEM investigations were carried out to characterize the Pd@rGO material. A thin film of nanostructured Pd@rGO was electrophoretically deposited on an ITO coated glass electrode that was subsequently functionalized with anti-PSA antibodies. The electrochemical sensing results of the proposed immunosensor showed a high sensitivity {28.96 μA ml ng-1 cm-2}. The immunosensor is able to detect PSA at concentrations as low as 10 pg ml-1. The simple fabrication method, high sensitivity, good reproducibility and long term stability with acceptable accuracy in human serum samples are the main advantages of this immunosensor. © 2014 The Royal Society of Chemistry.
pH Dependent Optical Switching and Fluorescence Modulation of Molybdenum Sulfide Quantum Dots
(Wiley-VCH Verlag, 2017) Himanshu Mishra; Sima Umrao; Jai Singh; Rajesh Kumar Srivastava; Rashid Ali; Arvind Misra; Anchal Srivastava
Investigations on fluorescent molecules with fluorescence switching behavior as function of pH, temperature, ion concentration, etc. is highly desirable in the field of sensor device fabrication. In this study, the authors report an easy and eco-friendly hydrothermal route for the synthesis of pH dependent fluorescent MoS2 quantum dots (MoS2-QDs). The average size of the synthesized QDs is found to be ≈7 nm which was confirmed with transmission electron microscopy and atomic force microscopy. These MoS2-QDs show the pH dependent fluorescence switching behavior. Under pH ≈ 1 the fluorescence intensity of the MoS2-QDs is quenched while it shows ≈200 times enhancement under pH ≈ 13. This florescence ON/OFF switching is mainly due to the surface adsorbed functional groups (NH2, SO4 2−, OH−, etc.) and is a combined effect viz. protonation–deprotonation process, acid etching, quantum confined Stark effect and particle agglomeration. A plausible mechanism for this pH dependent ON/OFF switching behavior is also discussed in this study. The band gap calculation under different pH environment is also in good agreement to the hypothesis. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Synthesis, Characterization, and Tribological Evaluation of TiO2-Reinforced Boron and Nitrogen co-Doped Reduced Graphene Oxide Based Hybrid Nanomaterials as Efficient Antiwear Lubricant Additives
(American Chemical Society, 2016) Vinay Jaiswal; Kalyani; Sima Umrao; Rashmi B. Rastogi; Rajesh Kumar; Anchal Srivastava
The microwave-synthesized reduced graphene oxide (MRG), boron-doped reduced graphene oxide (B-MRG), nitrogen-doped reduced graphene oxide (N-MRG), boron-nitrogen-co-doped reduced graphene oxide (B-N-MRG), and TiO2-reinforced B-N-MRG (TiO2-B-N-MRG) nanomaterials have been synthesized and characterized by various state-of-the-art techniques, like Raman spectroscopy, powder X-ray diffraction, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Furthermore, the tribological properties of prepared nanomaterials as antiwear additives in neutral paraffin oil have been evaluated using a four-ball machine at an optimized additive concentration (0.15% w/v). The tribological parameters, like mean wear scar diameter, coefficient of friction, and wear rates, revealed that these nanomaterials have potential to be developed as environmentally friendly sulfated-ash-, phosphorus-, and sulfur-free antiwear lubricant additives. The friction- and wear-reducing behavior of MRG increased upon successive doping of nitrogen, boron, and both nitrogen and boron. Among these additives, B-N-co-doped MRG shows superior tribological behavior in paraffin base oil. Besides this, the load-carrying properties of B-N-co-doped MRG have significantly improved after its reinforcement with TiO2 nanoparticles. A comparative study of the surface morphology of a lubricated track in the presence of various additives has been assessed by SEM and contact-mode atomic force microscopy. The X-ray photoelectron spectroscopy studies have proved that the excellent lubrication properties of TiO2-B-N-MRG are due to the in situ formation of a tribofilm composed of boron nitride, adsorbed graphene layers, and tribosintered TiO2 nanoparticles during the tribocontact. Being sulfur-, halogen-, and phosphorus-free, these graphene-based nanomaterials act as green antiwear additives, protecting interacting surfaces significantly from wear and tear. © 2016 American Chemical Society.