Browsing by Author "Shiju Abraham"

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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.
Carbon nanostructure (0-3 dimensional) supported isolated gold nanoparticles as an effective SERS substrate
(Elsevier B.V., 2018) Shiju Abraham; Matthias König; Sunil K. Srivastava; Vinod Kumar; Bernd Walkenfort; Anchal Srivastava
The present work reports a comparative surface enhanced Raman scattering (SERS) study of nanohybrids of spatially isolated gold nanoparticles (Au NPs) with different carbon nanostructures (CNS). For a meticulous comparative analysis, different CNS covering from zero dimensional to three dimensions were used. SERS activities of developed nanohybrids platforms were evaluated with a Raman marker i.e. Mercaptobenzoic acid (4-MBA, 1 μM). Synergistic Raman signal enhancement from each Au NP (electromagnetic enhancement) and underlying CNS matrix (chemical enhancement) was observed in nanohybrid enabling detection of very small concentration of 4-MBA i.e. 1 μM convincingly. Among the different underlying matrix, GQDs-Au NPs based SERS platform have shown highest SERS enhancement by a factor of 107, followed by GO-Au NPs combination (∼ 3 × 106) and others. Enhanced SERS activity of GQDs -Au NPs -4-MBA nanohybrid platform is attributed to the D and G bands of GQDs, overpowering the strong fluorescent background of GQDs alone. Reported nanohybrids; specially GQDs-Au NPs offers numerous possibilities to be used as sensitive and reproducible SERS platform for the detection of other biologically/chemically important analytes, showing potential for standardization in near future. © 2018 Elsevier B.V.
Colorimetric detection of cholesterol based on highly efficient peroxidase mimetic activity of graphene quantum dots
(Elsevier, 2015) Narsingh R. Nirala; Shiju Abraham; Vinod Kumar; Anushka Bansal; Anchal Srivastava; Preeti S. Saxena
In the present study, we report graphene quantum dots (GQDs), an enzyme mimetic of horse radish peroxidase (HRP), for unprecedented detection of free cholesterol. Synthesized directly from graphite using simple and quick one step wet chemical method, these GQDs in the presence of H2O2 exhibit highly efficient catalytic activity toward the oxidation of peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) to produce a blue colored product. The proposed detection system based on GQDs allows wide range (0.02-0.6 mM) of cholesterol sensing with a detection limit as low as 0.006 mM. Further, higher Vmax (7.3 × 10-6 M s-1) along with lower Km (0.01 mM) attest enhanced peroxidase like catalytic activity and better binding affinity of cholesterol oxidase (ChOx) to cholesterol resulting in good biosensor stability and resistance to environmental interferences. The proposed method without the use of sophisticated instruments perceives the cholesterol using naked eye with blue color compound formation. The potential of the method to be applied on field is shown by the proposed cholesterol measuring color wheel, where the shades of color are related to actual levels of cholesterol in the sample. © 2015 Published by Elsevier B.V.
Enhanced electrochemical biosensing efficiency of silica particles supported on partially reduced graphene oxide for sensitive detection of cholesterol
(Elsevier B.V., 2015) Shiju Abraham; Saurabh Srivastava; Vinod Kumar; Shobhit Pandey; Pankaj Kumar Rastogi; Narsingh R. Nirala; Sunayana Kashyap; Sunil K. Srivastava; Vidya Nand Singh; Vellaichamy Ganesan; Preeti S. Saxena; Anchal Srivastava
The present work introduces partially reduced graphene oxide (pRGO)-silica (SiO2) particles hybrid system (pRGOSHs) for sensitive and cost effective free cholesterol detection. Fabricated out of thin layers of pRGOSHs, these proposed ChOx/pRGOSHs/ITO based biosensors have a detection range of 2.6-15.5 mM with an appreciable detection limit of 1.3 mM and sensitivity of 11.1 μA/mM/cm2. Low Michaelis-Menten constant (Km) (4.9 × 10- 4 mM) and high diffusivity constant (D) (3.2 × 10- 10 cm2/s) values clearly indicate enhanced immobilization of enzyme over the substrate. Additionally, electrochemical impedance studies indicate that the synergistic combination of SiO2 and pRGO also results in much lower impedance values (40% and 18% decrease in comparison to SiO2 and pRGO respectively) for an overall enhanced sensing performance. These results are further corroborated by the density functional theory based theoretical simulations indicating enhanced electron density (theoretically) in case of the proposed hybrid system. Finally, the present work also highlights the importance of Si-OH bonds formation in the proposed pRGOSHs composite system for attaining such enhanced biosensing ability. © 2015 Elsevier B.V.
Excellent storage stability and sensitive detection of neurotoxin quinolinic acid
(Elsevier Ltd, 2017) Ranjana Singh; Sunayana Kashyap; Suveen Kumar; Shiju Abraham; Tejendra K. Gupta; Arvind M. Kayastha; Bansi D. Malhotra; Preeti Suman Saxena; Anchal Srivastava; Ranjan K. Singh
Quinolinic acid (QA) is a metabolite of tryptophan degradation obtained through kynurenine pathway, produced naturally in the mammalian brain as well as in the human cerebrospinal fluid. The presence of QA ~10–40 µM is a clear indicator of many neurological disorders as well as deficiency of vitamin B6 in human being. In the present work; rapid, sensitive and cost-effective bio-electrodes were prepared to detect the trace amount of endogenous neurotoxin (QA). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies were carried out to measure the electrochemical response of the fabricated bio-electrodes as a function of QA concentrations. These devices were found to exhibit desirable sensitivity of ~7.86 mA μM−1 cm−2 in wide concentration range (6.5 μM-65 mM). The lower detection limit of this device is as low as 6.5 μM and it has excellent storage stability of ~30 days. The capability of the proposed electrochemical bio-sensor was also checked to detect QA in the real samples (human serum). These results reveal that the use of this electrochemical bio-sensor may provide a potential platform for the detection of QA in the real samples for the prior detection of many diseases. © 2016
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.
Functional graphene-gold nanoparticle hybrid system for enhanced electrochemical biosensing of free cholesterol
(Royal Society of Chemistry, 2015) Shiju Abraham; Narsingh R. Nirala; Shobhit Pandey; Monika Srivastava; Sunil Srivastava; Bernd Walkenfort; Anchal Srivastava
Realizing the unavailability of fast and reliable diagnostic techniques, especially for cholesterol measurement, the present work reports the development of cost effective bioelectrodes based on a reduced graphene oxide-functionalized gold nanoparticle (∼25 nm) hybrid system (RGO-Fn Au NPs). The electrodes fabricated by the electrophoretic deposition technique attest a synergistically enhanced electrochemical sensing ability of 193.4 μA mM-1 cm-2 for free cholesterol detection, which is much higher than that of the traditional RGO system. The electrochemical impedance studies (EIS) show low charge transfer resistance, RCT, for the hybrid system which is 57% and 60% lower than those of RGO and Au NPs respectively. Also higher loading capacity and enhanced kinetics have been realized for the hybrid system, owing to lower Km value (0.005 mM) and enhanced rate constant (3.8 × 10-4 cm s-1) in comparison with RGO and Au NPs. Moreover, the RGO-Fn Au NP platform promises a wider range of cholesterol detection (0.65-12.93 mM), while simultaneously being capable of detecting as low as 0.34 mM of free cholesterol. Apart from better sensitivity, loading capacity, kinetics and detection range, the system also has appreciable selectivity and stability. This supports its potential to be brought on field in the near future for cost effective and reliable detection from the complex system of human serum. This journal is © The Royal Society of Chemistry 2015.
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.
Mesoporous silica particle embedded functional graphene oxide as an efficient platform for urea biosensing
(Royal Society of Chemistry, 2014) Shiju Abraham; Valerian Ciobota; Saurabh Srivastava; Sunil K. Srivastava; Rajesh K. Singh; Jan Dellith; B.D. Malhotra; Michael Schmitt; Jürgen Popp; Anchal Srivastava
The mesoporous silica particle embedded graphene oxide (GO) hybrid system is a promising platform for electrochemical biosensing owing to its large 2-dimensional structure, fast electron transfer kinetics, improved hydrophilic nature and surplus functional groups. Here, GO sheets were synthesized by Hummer's improved method and sub-micron sized homogeneous mesoporous silica (SiO2) particles were prepared by Stober's method. The SiO 2 particles were embedded on the GO surfaces and were optimized with different concentrations for better applicability and hydrophilicity. Micro-structural and spectroscopic characterization of as-synthesized materials was carried out to confirm the successful synthesis as well as the functionalities required for biosensing. Scanning electron microscopy investigations suggest that the average size of the SiO2 particles decorated on the GO surface is ∼500 nm. Raman investigation provides information regarding the increase in defects and disorder on the GO surface with the increase in the SiO2 content. The optimized GO-SiO 2 (GOS) composite electrode was prepared by the electrophoretic deposition technique and was used for the attachment of urease and glutamate dehydrogenase enzymes for urea detection employing the cyclic voltammetry method. The reproducibility, specificity and stability of the fabricated biosensor were found to be excellent for the urea sensing. Such an easy and cost effective material based GOS urea sensor showed a high sensitivity (2.6 μA mM-1 cm-2) and a good detection limit (14 mg dL -1). This journal is © the Partner Organisations 2014.
Partially reduced graphene oxide-gold nanorods composite based bioelectrode of improved sensing performance
(Elsevier B.V., 2015) Narsingh R. Nirala; Shiju Abraham; Vinod Kumar; Shobhit A. Pandey; Umakant Yadav; Monika Srivastava; S.K. Srivastava; Vidya Nand Singh; Arvind M. Kayastha; Anchal Srivastava; Preeti S. Saxena
Abstract The present work proposes partially reduced graphene oxide-gold nanorods supported by chitosan (CH-prGO-AuNRs) as a potential bioelectrode material for enhanced glucose sensing. Developed on ITO substrate by immobilizing glucose oxidase on CH-prGO-AuNRs composite, these CH-prGO-AuNRs/ITO bioelectrodes demonstrate high sensitivity of 3.2 μA/(mg/dL)/cm2 and linear range of 25-200 mg/dL with an ability to detect as low as 14.5 mg/dL. Further, these CH-prGO-AuNRs/ITO based electrodes attest synergistiacally enhanced sensing properties when compared to simple graphene oxide based CH-GO/ITO electrode. This is evident from one order higher electron transfer rate constant (Ks) value in case of CH-prGO-AuNRs modified electrode (12.4×10-2 cm/s), in contrast to CH-GO/ITO electrode (6×10-3 cm/s). Additionally, very low Km value [15.4 mg/dL(0.85 mM)] ensures better binding affinity of enzyme to substrate which is desirable for good biosensor stability and resistance to environmental interferences. Hence, with better loading capacity, kinetics and stability, the proposed CH-prGO-AuNRs composite shows tremendous potential to detect several bio-analytes in the coming future. © 2015 Elsevier B.V.
Protein conjugated carboxylated gold@reduced graphene oxide for aflatoxin B1 detection
(Royal Society of Chemistry, 2015) Saurabh Srivastava; Shiju Abraham; Chandan Singh; Md. Azahar Ali; Anchal Srivastava; Gajjala Sumana; Bansi D. Malhotra
A sensitive, reproducible, stable and label-free immunosensor has been prepared via simultaneous reduction of graphene oxide and gold(III) salt using an eco-friendly and non-toxic reducing agent sodium citrate resulting in uniformly distributed gold nanoparticles on reduced graphene oxide (rGO) sheets. The in situ grown gold@carboxylated reduced graphene oxide (Au@rGO) surface has been used for bioconjugation with monoclonal antibodies of aflatoxin B1 using EDC-NHS chemistry. The in situ growth of AuNPs (gold nanoparticles) onto the rGO sheet results in improved electrocatalytic activity and loading of the antibodies due to the enhanced surface area. The monodispersion of the Au nanoparticles on the rGO sheets yields heterogeneous electron transfer (2.85 × 10-4 cm s-1) resulting in improved biosensor efficacy compared to that based on the rGO electrode. This immunosensor is sensitive to detect as low as 0.1 ng mL-1 concentration of aflatoxin compared to the reported ELISA (enzyme-linked immunosorbent assay) standard method. The Au@rGO based immunosensor exhibits high sensitivity (182.4 μA (ng mL-1)-1 cm-2) in a wide linear detection range of 0.1-12 ng mL-1. Results of the studies related to this immunosensor reveal that the Au@rGO nanocomposite is a suitable platform for the development of a compact biosensing device for food toxin monitoring. © The Royal Society of Chemistry 2015.