Direct electrochemistry of cytochrome c and biosensing for hydrogen peroxide on polyaniline grafted multi-walled carbon nanotube electrode

Cytochrome c (cyt c) was immobilized into a matrix consisting of polyaniline (PANI) and multi-walled carbon nanotubes (MWNT) by a new strategy. First, PANI chains were grafted onto MWNT through electropolymerization. Second, the amine groups in PANI chains were oxidized at an applied potential of +0.80 V to acquire positive charges that would effectively immobilize negatively charged cyt C. The ITO/MWNT-g-PANI(O)/cyt c electrode exhibited a pair of redox peaks with a peak potential separation (anodic to cathodic) of 0.25 V (vs Ag/AgCl) in 0.1 M phosphate buffer (pH 7.0). The results demonstrated that ITO/MWNT-g-PANI(O)/cyt c promoted direct electron transfer between cyt c and electrode with a high electron transfer rate constant (17 s⁻¹). The ITO/MWNT-g-PANI(O)/cyt c electrode catalyzes the reduction of H₂O₂. The ITO/MWNT-g-PANI(O)/cyt c biosensor displays an amperometric response to H₂O₂ with a linear concentration range from 0.5 μM to 1.5 mM (r = 0.99, n = 12), a high sensitivity (32.2 μAm M⁻¹) and fast response (9 s) and detection limit of 0.3 μM (S/N = 3).     

Direct electrochemistry of cytochrome P450 6A1 in mimic bio-membrane and its application for pesticides sensing

The direct electrochemistry of house fly cytochrome P4506A1 (CYP6A1) confined in dioctadecyl dimethyl ammonium bromide (DDAB) film was achieved. The immobilized CYP6A1 displayed a pair of redox peaks with a formal potential of −0.36 mV in pH 7.0 O₂-free phosphate buffers at scan rate of 1 V s⁻¹ and the direct electron transfer of CYP6A1 was characterized by voltammetry. The CYP6A1 in the DDAB film retained its bioactivity and could catalyze the reduction of dissolved oxygen. Upon addition of its substrate aldrin or heptachlor to the air-saturated solution, the reduction peak current of dissolved oxygen increased, which indicates the catalytic behavior of CYP6A1 to its substrates. By amperometry a calibration linear range was obtained to be 9.08 × 10⁻⁶-4.54 × 10⁻⁵ mol L⁻¹ with a sensitivity of 80 μA mM⁻¹ for aldrin or 8.91 × 10⁻⁶-4.46 × 10⁻⁵ mol L⁻¹ with a sensitivity of 66 μA mM⁻¹ for heptachlor. The apparent Michaelis-Menten constant for the electrocatalytic activity of CYP6A1 was found to be 7.468 × 10⁻⁵ mol L⁻¹ for aldrin and 4.316 × 10⁻⁵ mol L⁻¹ for heptachlor. The bioelectrocatalytic products were analysed using gas chromatography (GC) and electron ionization-mass spectrometry (EI-MS). The results confirmed that epoxidation was the main pathways of CYP6A1-mediated organochlorine pesticides oxidation.     

Conformational flexibility decreased due to Y67F and F82H mutations in cytochrome c: Molecular dynamics simulation studies

Cytochrome c (cyt c), a mitochondrial protein, has dual functions in controlling both cellular energetic metabolism and apoptosis (programmed cell death). During apoptosis, cyt c (Fe³⁺) released into the cytosol initiates caspase activation leading to apoptosis. Since, X-ray crystallography gives only the static structure, we report here the dynamic behavior of holo and apo wild type (WT), Y67F and F82H mutant cyt c's (Fe³⁺) in their apoptotic states. Four nanosecond MD simulations were run for holo WT, Y67F and F82H cyt c's with and without FeS (Met-80) bond and also for apo WT and mutated cyt c's (Y67F and F82H) in water using GROMOS96 force field. Mutations of Y67F and F82H resulted in the decrease of backbone and Cα RMSDs, and radii of gyration (backbone and protein) in both the holo and apo forms. MD and ED results revealed that the flexibility of mutated holo cyt c's decreased perhaps affecting their ability to take part in mitochondrial electron/proton transfer process. Without FeS bond, the backbone and Cα RMSD increased in holo cyt c's perhaps resulting in enhanced peroxidase activity. ED revealed that four to six eigenvectors involved in over all motions of holo cyt c's without FeS bond, and six to eight eigenvectors in apo cyt c's in comparison to three to four eigenvectors for holo cyt c's with FeS bond.     

Label-free sensitive optical detection of polychlorinated biphenyl (PCB) in an aqueous solution based on surface plasmon resonance measurements

A label-free, ultrasensitive method for the optical detection of polychlorinated biphenyl (PCB) is described. The detection mechanism for PCB is based on PCB-induced conformational changes of immobilized Cytochrome c (Cyt c) on an Au thin film altering the local dielectric function of the supported Cyt c, which can be detectable by surface plasmon resonance (SPR) spectroscopy. Results showed that the confirmations of Cyt c changed in a very sensitive way depend on PCB concentrations as confirmed by circular dichroism (CD) measurements and atomic force microscope (AFM) observations. Based on the results, we investigated the PCB-sensitive detection performance of the self-assembling Cyt c monolayer surface by SPR. On exposure to PCB, the reflectance R at the SPR angle of the supported Cyt c showed a sensitive and systematic increase with increasing PCB concentrations. The limit of detection was as low as 0.1 ppb and the responses completed within 10 min.     

A novel microbial biosensor based on Circinella sp. modified carbon paste electrode and its voltammetric application

Microbial biosensors have been developed for voltammetric determination of various substances. This paper describes the development of a new biosorption based microbial biosensor for determination of Cu²⁺. The developed biosensor is based on carbon paste electrode consisting of whole cells of Circinella sp. Cu²⁺ was preconcentrated on the electrode surface at open circuit and then cathodically detected with the reduction of Cu²⁺. The voltammetric responses were evaluated with respect to percentage cell loading in the carbon paste, preconcentration time, pH of preconcentration solution, scan rate and interferences. The optimum response was realized by biosensor constructed using 5 mg of dry cell weight per 100 mg of carbon paste in pH 5.5 preconcentration solution. Under the optimum experimental conditions, the developed microbial biosensor exhibited an excellent current response to Cu²⁺ over a linear range from 5.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M (r² = 0.9938) with a detection limit of 5.4 × 10⁻⁸ M (S/N = 3). The microbial biosensor had good sensitivity and reproducibility (R.S.D. 4.3%, n = 6). Finally, the applicability of the proposed microbial biosensor to voltammetric determination of Cu²⁺ in real sample was also demonstrated and validated with atomic absorption spectrophotometric (AAS) method.     

Synthesis of Ag2Se nanomaterial by electrodeposition and its application as cataluminescence gas sensor material for carbon tetrachloride

In this paper, we presented a carbon tetrachloride gas sensor with strong cataluminescence response based on Ag₂Se nanomaterial, which was synthesized via the electrodeposition on the surface of Al foil by directly using a non-aqueous dimethyl sulfoxide (DMSO) solution with CH₃COOAg and SeCl₄. The deposited Ag₂Se material was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Then, the prepared Ag₂Se material along with the Al foil substrate was employed to design the carbon tetrachloride gas sensor. Under the optimized conditions, the present gas sensor exhibited a broad linear range of 0.9-228 μg mL⁻¹, with a limit of detection of 0.3 μg mL⁻¹ (S/N = 3). The proposed gas sensor showed good characteristics with high selectivity, fast response and long lifetime.     

Synthesis of ZnO nanorods and its application in NO2 sensors

One-dimensional (1D) ZnO nanorods with pencil-like shape and high aspect ratio were successfully synthesized using a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal process at 90 °C. The surface morphology and structure of nanocrystals were characterized by FE-SEM, XRD and XPS analysis. Experimental results show that the surfactant and base concentration play important roles in the formation and growth orientation of ZnO nanorods. The ZnO nanorods synthesized exhibits high response and selectivity to NO₂, the highest response to 40 ppm NO₂ reached 206 and the selectivity with respect to CO and CH₄ at same concentration reached 10.3 and 30 times, respectively. The effects of synthesis method, surfactant and calcination condition on sensing properties were systematically investigated. The results indicate that the CTAB-assisted low temperature hydrothermal process is a potentially facile method for synthesis of 1D ZnO nanorods and excellent potential candidates as gas sensing materials.     

Separate assessment of chemoresistivity and Schottky-type gas sensitivity in M–metal oxide–M′ structures

Maximum response levels reported for chemoresistive sensors span from less than 10 to over 10⁸. These differences are attributed to either the different micro- and nano-structured oxide pallets used or the properties of the metal–metal oxide junctions provided. Here, we report separate measurements and model-based estimations of the chemical responses arising from these different origins. The results quantitatively connect the observed responses to the parameters of the metal and metal oxide components of the device. It is shown that while the peak chemoresistive response is microstructure-dependent, the highest attainable Schottky-type gas sensitivity is almost microstructure-independent and is determined by the intrinsic properties of the materials involved. Measurements carried out on different Ag–TiO₂–Ti, Au–TiO₂–Ti and Ti–TiO₂–Ti structures verified the estimations: While chemoresistive responses in TiO₂ can hardly rise over 10², atmosphere-sensitive noble metal–TiO₂ junctions can cause responses as high as ∼10⁷.     

Estimating aboveground carbon in a catchment of the Siberian forest tundra: Combining satellite imagery and field inventory

This study was part of an interdisciplinary research project on soil carbon and phytomass dynamics of boreal and arctic permafrost landscapes. The 45 ha study area was a catchment located in the forest tundra in northern Siberia, approximately 100 km north of the Arctic Circle.The objective of this study was to estimate aboveground carbon (AGC) and assess and model its spatial variability. We combined multi-spectral high resolution remote sensing imagery and sample based field inventory data by means of the k-nearest neighbor (k-NN) technique and linear regression.Field data was collected by stratified systematic sampling in August 2006 with a total sample size of n = 31 circular nested sample plots of 154 m² for trees and shrubs and 1 m² for ground vegetation. Destructive biomass samples were taken on a sub-sample for fresh weight and moisture content. Species-specific allometric biomass models were constructed to predict dry biomass from diameter at breast height (dbh) for trees and from elliptic projection areas for shrubs.Quickbird data (standard imagery product), acquired shortly before the field campaign and archived ASTER data (Level-1B product) of 2001 were geo-referenced, converted to calibrated radiances at sensor and used as carrier data. Spectral information of the pixels which were located in the inventory plots were extracted and analyzed as reference set. Stepwise multiple linear regression was applied to identify suitable predictors from the set of variables of the original satellite bands, vegetation indices and texture metrics. To produce thematic carbon maps, carbon values were predicted for all pixels of the investigated satellite scenes. For this prediction, we compared the kNN distance-weighted classifier and multiple linear regression with respect to their predictions.The estimated mean value of aboveground carbon from stratified sampling in the field is 15.3 t/ha (standard error SE = 1.50 t/ha, SE% = 9.8%). Zonal prediction from the k-NN method for the Quickbird image as carrier is 14.7 t/ha with a root mean square error RMSE = 6.42 t/ha, RMSE_r = 44%) resulting from leave-one-out cross-validation. The k-NN-approach allows mapping and analysis of the spatial variability of AGC. The results show high spatial variability with AGC predictions ranging from 4.3 t/ha to 28.8 t/ha, reflecting the highly heterogeneous conditions in those permafrost-influenced landscapes. The means and totals of linear regression and k-NN predictions revealed only small differences but some regional distinctions were recognized in the maps.