Mahabul Shaik

Electrochemical immunosensor for botulinum neurotoxin type-E using covalently ordered graphene nanosheets modified electrodes and gold nanoparticles-enzyme conjugate.

In this work, a novel electrochemical immunosensor was developed for the detection of botulinum neurotoxin-E (BoNT/E). This method relied on graphene nanosheets-aryldiazonium salt modified glassy carbon electrodes (GCE) as sensing platform and enzyme induced silver nanoparticles (AgNPs) deposited on gold nanoparticles (AuNPs) as signal amplifier. Herein, a GCE was electrografted with mixed monolayer of phenyl and aminophenyl (Ph-PhNH2/GCE) by diazotization reaction. Further, graphene nanosheets (GNS) were covalently attached on electrode surface (GNS/Ph-PhNH2/GCE). Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were employed to characterize synthesized graphene oxide and modified electrode surfaces. In the sandwich immunoassay format, the sensitivity was amplified using rabbit anti-mouse IgG-alkaline phosphatase (RαMIgG-ALP) functionalized with gold nanoparticles (RαMIgG-ALP/AuNPs). In order to study the immunosensing performance of GNS/Ph-PhNH2/GCE, first the capturing antibody (rabbit-anti BoNT/E antibody) was covalently immobilized via EDC/NHS chemistry. Further, the electrode was sequentially subjected to sample containing spiked BoNT/E, revealing antibody (mouse-anti BoNT/E) followed by RαMIgG-ALP/AuNPs. 3-indoxyl phosphate (3-IP) was used as substrate which finally reduces the silver ions. The deposited AgNPs on electrode surface were determined by linear sweep voltammetry (LSV). The developed electrochemical immunosensor could detect BoNT/E with linear range from 10pg/ml to 10ng/ml with the minimum detection limit of 5.0pg/ml and total analysis time of 65min. In addition, the immunosensor was successfully evaluated against food samples (orange juice and milk).

Chemiresistive gas sensor for the sensitive detection of nitrogen dioxide based on nitrogen doped graphene nanosheets

In this paper, we report on the development of a chemiresistive sensor for the detection of nitrogen dioxide (NO2) gas at room temperature using nitrogen-doped graphene nanosheets (NGS). The substitution of the nitrogen atoms in the honey-comb structure of graphene enhances the adsorption sites for gas molecules and thereby the sensitivity of the detection of the adsorbed gas molecules increases. Graphene nanosheets (GS) and NGS were prepared by hydrothermal treatment of graphene oxide in the absence and presence of nitrogen precursor respectively. The sensing materials were characterized by FESEM, TEM, XRD, XPS and elemental analysis. The nitrogen content in as-prepared NGS is at around 10%. The thin films of GS and NGS on pre-patterned gold interdigitated electrodes (IDEs) were obtained by the drop-drying method. The NGS coated sensor showed good response for sensing NO2 in comparison to that of GS at room temperature. The recovery of the sensor was greatly accelerated by ultra-violet light illumination. The proposed sensor showed excellent characteristics such as a low detection limit of 120 ppb (at S/N = 3). The effect of humidity on sensor performance was also studied. The proposed sensor also showed excellent selectivity with respect to various common interfering gases.

Relative efficiency of zinc sulfide (ZnS) quantum dots (QDs) based electrochemical and fluorescence immunoassay for the detection of Staphylococcal enterotoxin B (SEB)

In this paper an attempt was made to detect Staphylococcal enterotoxin B (SEB) both by electrochemical and fluorescence immunoassay methods using zinc sulphide (ZnS) QDs. Wet-chemical method was adopted for the preparation of fluorescent ZnS QDs (diameter ∼ 5–10 nm). These QDs were bioconjugated with monoclonal antibodies and then characterized by various method. A detection limit of 0.02 ng mL−1 by fluorescence assay and 1.0 ng mL−1 by electrochemical assay for SEB was achieved. While by sandwich ELISA it is possible to detect 0.24 ng mL−1 only. The sensitivity of all techniques is very good, since the LD50 of SEB is 20 ng kg−1. Electrochemical assay is faster, need low-cost instrument, independent to the size of QDs and found to be one of the best alternative methods as compared to the other existing methods studied herein. The presented method could be expanded to the development of electrochemical and fluorescence biosensors for various agents for field and laboratory use.

Sensitive detection of staphylococcal enterotoxin B (SEB) using quantum dots by various methods with special emphasis on an electrochemical immunoassay approach

Staphylococcal enterotoxin B (SEB) is a potent foodborne pathogen and categorized as a class B type of biological warfare agent. In this research work, SEB is detected by various sensitive analytical methods such as enzyme linked immunosorbent assay (ELISA), quantum dots-based fluorescence linked immunosorbent assay (QDs-FLISA) and square-wave voltammetry (SWV). The obtained results were compared in terms of sensitivity, ease of experimentation and analysis time. For the QD-based detection, fluorescent lead sulfide (PbS) QDs were prepared by a bottom-up approach and characterized by various techniques. Highly specific antibodies against SEB were conjugated with the prepared PbS QDs and were used as revealing antibodies. For the electrochemical detection of SEB, rabbit anti-SEB polyclonal antibodies (primary antibodies) were immobilized on screen-printed electrodes (SPEs) followed by the addition of various concentrations of SEB antigen. These electrodes were further incubated with revealing antibodies. Finally, 1 M HCl solution was added to the SPE to dissolve the PbS QDs which were captured in a sandwiched immunoassay, and resulting Pb2+ ions were determined by the SWV method using a glassy-carbon electrode. The obtained peak current is proportional to the amount of Pb2+ ions which indirectly depends on the SEB concentration. Linearity was observed in the concentration range of 1 ng mL−1 to 1 μg mL−1 of SEB antigen. The limit of detection was found to be 0.01 ng mL−1 for SEB. The results reveal that electrochemical SWV sensing is much easier, faster and provides high sensitivity as compared to the other methods. It is found that the detection limits achieved for sandwich ELISA and QDs-FLISA were 0.24 ng mL−1 and 0.03 ng mL−1 respectively. In addition, the developed SWV method can be implemented for the on-site detection of SEB particularly for civil and defense applications where security is of prime importance.

Enzyme free detection of staphylococcal enterotoxin B (SEB) using ferrocene carboxylic acid labeled monoclonal antibodies: an electrochemical approach

Staphylococcal enterotoxin B (SEB) is one of the most potent toxin threats in bioterrorism and can be produced as an offensive biological warfare (BW) agent (Type B). This article reports on an electrochemical detection of SEB. In order to study the detection performance, a composite containing graphene–chitosan–gold nanoparticles–capturing antibodies (GR–Ch–AuNPs–CAb) was used to modify a glassy carbon electrode (GCE). Similarly, a multi-walled carbon nanotubes–chitosan–gold nanoparticles–capturing antibody (MWCNTs–Ch–AuNPs–CAb) composite was used for comparison. Further, this bionanocomposite film modified GCE was subjected to SEB, which is detected in an electrochemical sandwich immunoassay format using ferrocene labeled-mice anti-SEB IgG (Fc–MAb) as the detection antibodies and we compared the sensitivity. The limit of detection is found to be 5 ng mL−1 and 10 ng mL−1 for GR–Ch–AuNPs and MWCNTs–Ch–Au NPs modified bionanocomposite films, respectively, and the total analysis time was 35 min. The signal amplification strategy proposed here is quite promising and can be extended for the monitoring of other biorecognition events. In addition, this is a reagentless and non-enzymatic method of electrochemical detection and is suitable for field use.

Controllable gold nanoparticle deposition on carbon nanotubes and their application in immunosensing

A CNT–AuNPs hybrid nanocomposite platform was prepared from nanodisperse AuNPs in N-[3-(trimethoxysilyl)propyl]ethylenediamine (EDAS) sol–gel matrices with purified MWCNT. EDAS, an amine group-containing sol–gel solution, was utilized for its ability to stabilize the nanoparticles in solution. The developed model system was based on immobilized rabbit anti-mouse IgG-HRP (horseradish peroxidase) for reagentless detection of mouse IgG. The immunosensing platform was prepared by using Nafion for the immobilization of rabbit anti-mouse IgG-HRP and CNT–AuNPs hybrid nanocomposite on a glassy carbon electrode used for the detection of mouse IgG which provides a biocompatible microenvironment. The resulting CNT–AuNPs nanocomposite brings new capabilities for electrochemical devices by using the synergistic action of its electrocatalytic activity. The CNT–AuNPs were characterized using SEM, TEM, EIS, and AFM, and the analytical performance was monitored by differential pulse voltammetry. The detection limit of mouse IgG is 0.5 ng mL−1 (S/N ratio = 3). In addition, the immunosensor efficiently allowed a specific electrochemical analysis of mouse IgG and easy discrimination of goat IgG, chicken IgG, and rabbit IgG.

p-Hexafluoroisopropanol phenyl functionalized graphene for QCM based detection of dimethyl methylphosphonate, a simulant of the nerve agent sarin

The detection of DMMP (dimethyl methylphosphonate, a simulant of nerve agent sarin) was performed by using p-hexafluoroisopropanol phenyl (HFIPP) functionalized graphene (GR) via hydrogen bond interactions. For this, the HFIPP moiety was covalently functionalized on the surface of GR by a diazo reaction. The HFIPP-GR film-modified QCM electrodes were fabricated and their sensing characteristics towards DMMP were investigated. The proposed sensor showed good response towards sensing DMMP vapor at room temperature. In order to see the effect of HFIPP derivatives on DMMP vapor sensing, a comparative study was also conducted with unfunctionalized graphene. The sensitivity and detection limit of the HFIPP-GR sensor against DMMP vapors were 12.24 Hz ppm−1 and 150 ppb respectively. The HFIPP-GR coated sensors showed good selectivity towards sensing DMMP vapors when compared with common organic vapors.