Veerappan Mani

Direct electrochemistry of glucose oxidase at electrochemically reduced graphene oxide-multiwalled carbon nanotubes hybrid material modified electrode for glucose biosensor.

Direct electrochemistry of glucose oxidase (GOx) at an electrochemically reduced graphene oxide-multiwalled carbon nanotubes hybrid (ERGO-MWCNT) modified glassy carbon electrode (GCE) has been reported. The π-π stacking interaction operating between the MWCNT and graphene oxide (GO) has been revealed by UV-Vis absorption spectroscopy. GOx was well immobilized onto the ERGO-MWCNT hybrid film, as a result direct electrochemistry of GOx has been achieved. Compared with pristine MWCNT, 2.1 fold higher peak current and very low peak to peak separation (ΔE(p)) of 26 mV were observed at the hybrid film, demonstrating faster electron transfer between GOx and the modified electrode surface. Moreover, the modified film exhibited high electrocatalytic activity towards glucose via reductive detection of oxygen consumption and in the presence of mediator. The proposed biosensor exhibits low detection limit of 4.7 μM with wide linear range of 0.01-6.5mM and acquires excellent storage and operational stabilities. The accurate glucose determination in human blood serum and good recoveries achieved in spiked urine samples revealed their great potential in the practical applications.

Direct electrochemistry of myoglobin at reduced graphene oxide-multiwalled carbon nanotubes-platinum nanoparticles nanocomposite and biosensing towards hydrogen peroxide and nitrite

We described the preparation of a novel nanobiocomposite, reduced graphene oxide- multiwalled carbon nanotubes-platinum nanoparticles/myoglobin (RGO-MWCNT-Pt/Mb) for the direct electrochemistry of myoglobin and its application towards determination of hydrogen peroxide (H2O2) and nitrite (NO2(-)). RGO-MWCNT-Pt nanocomposite has been prepared by simple solution based approach and its structure was characterized. RGO-MWCNT-Pt/Mb nanobiocomposite was prepared and attained the direct electrochemistry of Mb with pair of well-defined redox peaks with the formal potential of -0.33 V and peak to peak separation of 22 mV. Amount of electroactive protein (Г) and heterogeneous electron transfer rate constant (ks) were calculated to be 1.04 × 10 (-9) mol cm(-2) and 9.47 s(-1). The sensor displayed lowest detection limit (LOD) of 6 pM which is the lowest LOD ever achieved for the detection of H2O2. Two linear ranges were observed for the detection of H2O2: (1) 10 pM-0.19 nM with sensitivity of 1.99 (± 0.058) µA pM(-1)cm(-2) and (2) 0.25 nM-2.24 µM with sensitivity of 0.037 (± 0.081) µA nM(-1)cm(-2). In addition, the biosensor offered good analytical parameters towards determination of NO2(-) with wide linear range of 1 µM to 12 mM and high sensitivity of 0.1651 (± 0.026) µA µM(-1) cm(-2). The sensor acquires good selectivity, repeatability, reproducibility and stability. The practical feasibility of the sensor has been addressed.

Simultaneous electrochemical determination of dopamine and paracetamol on multiwalled carbon nanotubes/graphene oxide nanocomposite-modified glassy carbon electrode.

In the present study, multiwalled carbon nanotubes (MWCNT)/graphene oxide (GO) nanocomposite was prepared by homogenous dispersion of MWCNT and GO and used for the simultaneous voltammetric determination of dopamine (DA) and paracetamol (PA). The TEM results confirmed that MWCNT walls were wrapped well with GO sheets. The MWCNT/GO nanocomposite showed superior electrocatalytic activity towards the oxidation of DA and PA, when compared with either pristine MWCNT or GO. The major reason for the efficient simultaneous detection of DA and PA at nanocomposite was the synergistic effect between MWCNT and GO. The electrochemical oxidation of DA and PA was investigated by cyclic voltammetry, differential pulse voltammetry and amperometry. The nanocomposite modified electrode showed electrocatalytic oxidation of DA and PA in the linear response range from 0.2 to 400 µmol L(-1) and 0.5 to 400 µmol L(-1) with the detection limit of 22 nmol L(-1) and 47 nmol L(-1) respectively. The proposed sensor displayed good selectivity, sensitivity, stability with appreciable consistency and precision.

Highly selective amperometric sensor for the trace level detection of hydrazine at bismuth nanoparticles decorated graphene nanosheets modified electrode

A highly selective amperometric sensor was developed for the trace level determination of hydrazine at bismuth nanoparticles (Bi) decorated graphene nanosheets (GR) composite film modified glassy carbon electrode (GCE). GR-Bi nanocomposite has been successfully prepared via simple and facile chemical reduction approach and its structure was characterized by various techniques. Surface morphological and X-ray diffraction studies revealed the formation and high loading of Bi nanoparticles on graphene sheets. GR-Bi nanocomposite modified GCE exhibited greatly enhanced electrocatalytic performance towards electro-oxidation of hydrazine in terms of decrease in overpotential and increase in oxidation peak current (Ip). The kinetic parameters such as electron transfer coefficient (α) and diffusion coefficient (Do) of the hydrazine oxidation were determined to be 0.70 and 2.65×10(-5) cm(2) s(-1), respectively. An amperometric sensor has been fabricated which detects trace level concentration of hydrazine. The sensor exhibited a wide linear range from 20 nM to 0.28 mM and a very low detection limit (LOD) of 5 nM. Remarkably, this is the lowest LOD achieved for the determination of hydrazine in neutral pH among other reported electrochemical hydrazine sensors. In addition, the sensor selectively detects hydrazine even in the presence of 1000 fold excess quantity of common interferrants. The practical feasibility of the sensor has been assessed in water and urine samples with good recoveries. Furthermore, the sensor exhibited appreciable stability, repeatability and reproducibility results.

Rapid microwave assisted synthesis of graphene nanosheets/polyethyleneimine/gold nanoparticle composite and its application to the selective electrochemical determination of dopamine

In this study, a simple and fast microwave assisted chemical reduction method for the preparation of graphene nanosheet/polyethyleneimine/gold nanoparticle (GNS/PEI/AuNP) composite was developed. PEI, a cationic polymer, was used both as a non-covalent functionalizing agent for the graphene oxide nanosheets (GONSs) through electrostatic interactions in the aqueous medium and also as a stabilizing agent for the formation of AuNPs on PEI wrapped GNSs. This preparation method involves a simple mixing step followed by a simultaneous microwave assisted chemical reduction of the GONSs and gold ions. The prepared composite exhibits the dispersion of high density AuNPs which were densely decorated on the large surface area of the PEI wrapped GNS. X-ray photoelectron spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, field-emission scanning electron microscopy with energy dispersive X-ray spectroscopy, and thermo-gravimetric analysis, were used to characterize the properties of the resultant composite. The prepared GNS/PEI/AuNP composite film exhibited excellent electrocatalytical activity towards the selective determination of dopamine in the presence of ascorbic acid, which showed potential application in electrochemical sensors. The applicability of the presented sensor was also demonstrated for the determination of dopamine in human urine samples.

Glucose biosensor based on glucose oxidase immobilized at gold nanoparticles decorated graphene-carbon nanotubes

Biopolymer pectin stabilized gold nanoparticles were prepared at graphene and multiwalled carbon nanotubes (GR-MWNTs/AuNPs) and employed for the determination of glucose. The formation of GR-MWNTs/AuNPs was confirmed by scanning electron microscopy, X-ray diffraction, UV-vis and FTIR spectroscopy methods. Glucose oxidase (GOx) was successfully immobilized on GR-MWNTs/AuNPs film and direct electron transfer of GOx was investigated. GOx exhibits highly enhanced redox peaks with formal potential of -0.40 V (vs. Ag/AgCl). The amount of electroactive GOx and electron transfer rate constant were found to be 10.5 × 10(-10) mol cm(-2) and 3.36 s(-1), respectively, which were significantly larger than the previous reports. The fabricated amperometric glucose biosensor sensitively detects glucose and showed two linear ranges: (1) 10 μM - 2 mM with LOD of 4.1 μM, (2) 2 mM - 5.2 mM with LOD of 0.95 mM. The comparison of the biosensor performance with reported sensors reveals the significant improvement in overall sensor performance. Moreover, the biosensor exhibited appreciable stability, repeatability, reproducibility and practicality. The other advantages of the fabricated biosensor are simple and green fabrication approach, roughed and stable electrode surface, fast in sensing and highly reproducible.

Direct electrochemistry of cytochrome c immobilized on a graphene oxide–carbon nanotube composite for picomolar detection of hydrogen peroxide

We describe the fabrication of an amperometric biosensor based on cytochrome c (Cyt c) immobilized graphene oxide–multiwalled carbon nanotube (GO–MWCNT) composite on a nano Au modified glassy carbon electrode for trace level detection of H2O2. Morphology and surface characterization of the nanocomposite reveal the successful formation of a highly conducting MWCNT network on the GO surface. Electrochemical impedance studies indicate a lower charge transfer resistance compared to the bare electrode. Cyclic voltammetry studies clearly demonstrate an enhanced direct electrochemistry of Cyt c with a high electron transfer rate constant (ks) value of 3.4 s−1. An amperometric H2O2 biosensor has been fabricated with an excellent current sensitivity of 0.533 μA pM−1 cm−2 and a very low detection limit of 27.7 pM. The fabricated sensor shows exceptional selectivity to H2O2 in the presence of a high concentration of some likely interferents. Moreover, the sensor exhibits high stability with appreciable repeatability and reproducibility.

Immobilization of glucose oxidase on graphene and cobalt phthalocyanine composite and its application for the determination of glucose.

We described a simple and facile chemical reduction strategy for the preparation of graphene (GR)-cobalt phthalocyanine (CoPc) composite and explored it for the enzymatic determination of glucose. CoPc is an active mediator and electrocatalysts for the immobilization of GOx and determination of glucose. However, it is not stable on the electrode surface and also suffers from lack of conductivity. Here, we have employed GR as the suitable support to stabilize CoPc through simple chemical reduction method and the resulting composite has been used for the glucose biosensor application. Scanning electron microscopy, X-ray diffraction and Energy-dispersive X-ray spectroscopy studies confirmed the successful formation of composite. Direct electron transfer of glucose oxidase (GOx) was observed with well defined redox peaks at the formal potential of -0.44 V. The amount of electroactive GOx (Г) and electron transfer rate constant (ks) were calculated to be 3.77×10(-10) mol cm(-2) and 3.57 s(-1), respectively. The fabricated amperometric biosensor detects glucose in wide linear concentration range from 10 μM to 14.8 mM with high sensitivity of 5.0 9μA mM(-1) cm(-2). The sensor offered very low detection limit (LOD) of 1.6 μM. In addition, practical feasibility of the sensor has been explored in screen printing carbon electrode with accurate determination of glucose present in human blood serum and urine samples. Furthermore, the sensor exhibited appreciable stability, repeatability and reproducibility results.

Molybdenum disulfide nanosheets coated multiwalled carbon nanotubes composite for highly sensitive determination of chloramphenicol in food samples milk, honey and powdered milk

We have described a hybrid material that consists of molybdenum disulfide nanosheets (MoS2) coated on functionalized multiwalled carbon nanotubes (f-MWCNTs) for sensitive and selective determination of chloramphenicol (CAP). The MoS2/f-MWCNTs nanocomposite was successfully prepared through a hydrothermal process and its structure was characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy and cyclic voltammetry. The MoS2/f-MWCNTs nanocomposite holds excellent electrochemical properties and it displays excellent electrocatalytic ability to CAP. Under optimized working conditions, the nanocomposite film modified electrode responds linearly to CAP in the concentration range of 0.08-1392μM. The detection limit was obtained as 0.015μM (±0.003). The electrode has high level of selectivity in presence of large excess concentrations of interfering species. In addition, the modified electrode offers satisfactory repeatability, reproducibility and stability. The practical applicability of the electrode was demonstrated in food samples such as, milk, powdered milk and honey samples and the recoveries are agreeable which clearly revealed its practical feasibility in food analysis.

An electrochemical synthesis strategy for composite based ZnO microspheres–Au nanoparticles on reduced graphene oxide for the sensitive detection of hydrazine in water samples

An electrochemical synthesis strategy has been developed to prepare a novel composite viz. reduced graphene oxide nanosheets/ZnO microspheres (∼0.6 μm)–Au nanoparticles (∼50 nm) modified glassy carbon electrode (GCE/RGO/ZnO–Au) for the trace level detection of hydrazine. Scanning Electron Microscopy (SEM) along with Energy Dispersive X-ray (EDX) analysis, confirming the presence of Au nanoparticles along with globular ZnO microspheres embedded over the entire surface of graphene nanosheets. The electrochemical detection of hydrazine is performed by cyclic voltammetry and chronoamperometry methods. Fascinatingly, the oxidation peak current of hydrazine at RGO/ZnO–Au modified GCE is 4.1 fold higher than that of RGO–Au modified GCE and 2.4 fold higher than ZnO/Au-modified GCE in addition to a favorable lower overpotential at 0.1 V. The chronoamperometric hydrazine sensor shows a very low detection limit of 18 nM with a high sensitivity of 5.54 μA μM−1 cm−2. The excellent analytical parameters of the RGO/ZnO–Au modified electrode over the various related modified electrodes suggest that the electrode can be advantageous for use in trace level detection of hydrazine in several industrial applications with low cost, ease of preparation, repeatability and long-term stability.