Performance evaluation of fast Fourier-transform continuous cyclic-voltammetry pesticide biosensor

In this work, a method for the fast monitoring of OPs in flow-injection systems was evaluated. The fast Fourier transform continuous cyclic-voltammetry (FFTCCV) at the carbon-paste electrode in a flowing solution system was used for determination of OPs. In this method the S/N ratio is enhanced by using of fast Fourier transform of the analyte and signal integration. FFTCCV can be considered as a new sensitive, accurate and fast method for determination of drugs and some pesticides. However, in order to obtain better sensitivity for a specific target, experimental parameters should be optimized. Response surface methodology (RSM) was applied to optimize three effective parameters (enzyme activity, multiwall carbon nanotube quantity and acidic sol–gel quantity). The optimum values for the tested parameters were enzyme amount H0.169 U cm⁻², multiwall carbon nanotube (MWCNT) 0.607 mL and acidic sol–gel 1.012 mL. The optimum feed pH, feed flow rate, ATChCl concentration and sweeping-rate were found to be 7.4, 0.34 mL min⁻¹, 0.750 mM and 10 V s⁻¹, respectively. The long-term stability of this flow-through system was 80% of its initial response after 120 days. Based on an incubation time of 12 min, it was found that the detection limit for paraoxon was equal to 1.7 × 10⁻⁷ mg L⁻¹ (6.2 × 10⁻¹³ M). The developed biosensor exhibited good repeatability and reproducibility. This study provides a new, modern, sensitive tool for the analysis of organophosphate pesticides.     

The Laplacian of electron density versus NICSzz scan: measuring magnetic aromaticity among molecules with different atom types

The electron density versus NICS(zz) (the out-of-plane component of nucleus-independent chemical shifts (NICS)) scan for assessing magnetic aromaticity among similar molecules with different ring sizes is improved by scanning the Laplacian of electron density versus NICS(zz) to include molecules containing different types of atoms. It is demonstrated that the new approach not only reproduces the results of the previous method but also surpasses that in the case of species with different atom types. The relative positions of curves in the plots of the Laplacian of electron density versus NICS(zz) correlate well with the ring current intensities of these molecules both near and far from the ring planes of the considered molecules. Accordingly, relative magnetic aromaticity of a number of planar hydrocarbons and a group of double aromatic metallic/semimetallic species are studied and discussed.     

First Principles Calculations of Electric Field Effect on the (6,0) Zigzag Single-Walled Silicon Carbide Nanotube for use in Nano-Electronic Circuits

Structural, electronic, and electrical responses of the H-capped (6,0) zigzag single-walled silicon carbide nanotube (SiCNT) was studied under the parallel and transverse electric fields with strengths 0–140 × 10^?4 a.u. by using density functional calculations. Analysis of the structural parameters indicates that resistance of the nanotube against the applied parallel electric field is more than resistance of the nanotube against the applied transverse electric field. The dipole moments, atomic charge variations, and total energy of the (6,0) zigzag SiCNT show increases with any increase in the applied external electric field strengths. The length, tip diameters, electronic spatial extent, and molecular volume of the nanotube do not change significantly with any increasing in the electric field strength. The energy gap of the nanotube increases with any increases in the electric field strength and its reactivity is decreased. Increase of the ionization potential, electron affinity, chemical potential, and HOMO and LOMO in the nanotube with increase of the applied external electric field strengths indicates that the properties of SiCNTs can be controlled by the proper external electric field for use in nano-electronic circuits.     

New computational method for solving some 2-dimensional nonlinear Volterra integro-differential equations

The aim of this paper is to present an efficient numerical procedure for solving the two-dimensional nonlinear Volterra integro-differential equations (2-DNVIDE) by two-dimensional differential transform method (2-DDTM). The technique that we used is the differential transform method, which is based on Taylor series expansion. Using the differential transform, 2-DNVIDE can be transformed to algebraic equations, and the resulting algebraic equations are called iterative equations. New theorems for the transformation of integrals and partial differential equations are introduced and proved. The reliability and efficiency of the proposed scheme are demonstrated by some numerical experiments.     

A differential quadrature analysis of unsteady open channel flow

A rapid, convergent and accurate differential quadrature method (DQM) is employed for numerical simulation of unsteady open channel flow. To the best of authors’ knowledge, this is the first attempt to use the DQM in open channel hydraulics. The Saint-Venant equations and the related nonhomogenous, time dependent boundary conditions are discretized in spatial and temporal domain by DQ rules. The unknowns in the entire domain are computed by satisfying governing equations, boundary and initial conditions simultaneously. By employing DQM, accurate results can be obtained using dramatically less grid points in spatial and time domain. The stability of DQM solution is not sensitive to choosing time step or Courant number unlike other methods. Although numerical problems such as instability, oscillation and underestimation near critical depth can be seen by using other methods but DQM solution is smooth and accurate in this case. The results are sensitive to grid distribution in time domain. In light of this, Chebyshev–Gauss–Lobatto distribution performance is excellent. To validate the DQM solutions, the obtained results are compared with those of the characteristic method. In conclusion, DQM is a potential powerful method with minimum computational effort for unsteady flow simulation.     

Selective and non-selective determination of heavy metal ions in flowing solutions by fast stripping cyclic voltammetry.

A simple and fast stripping voltammetric detection method has been designed for selective and non-selective measurements of heavy metal ions in a flow-injection system. A special computer numerical method is introduced for calculating the analyte signal and noise reduction, where the signal is calculated based on the partial and total charge exchange at electrode surface. For a selective determination, the currents are integrated in the range of the oxidation and reduction of the analyte. For non-selective measurements, the integration range is set for the whole potential scan range (including oxidation and reduction of the Au surface). The time for stripping has been shown to be less than 300 ms. The main advantages of the detection method are as follows: first, removal of oxygen from the measured solution is not required; second, it is sufficiently fast for the determination of heavy metal ions in various chromatographic analysis methods. The limit of detection for tested ions was between 3 x 10(-9) and 6 x 10(-10) M, and the relative standard deviation at 50 ppb Pb2+ was 4.7% for 10 runs.     

Stable 10 ns, kilowatt peak-power pulse generation from a gain-switched Tm-doped fiber laser

We report what we believe to be the first demonstration of stable short pulse (10 ns) generation from a gain-switched Tm-doped fiber laser in the 2 microm spectral region. A modulated 1.55 microm pump laser was used for fast gain switching to regulate the conventionally chaotic spiking in the gain-switched fiber laser. From a 6.3 microm core all-fiber oscillator, over a kilowatt of peak power was obtained at the slope efficiency of 50% and freely varying repetition rate up to 500 kHz.     

Investigation of the Antifouling Properties of Phenyl Phosphorylcholine‐Based Modified Gold Surfaces

Low impedance, antifouling coatings on gold electrodes based on three new zwitterionic phenyl phosphorylcholine (PPC)‐based layers namely 1) reductively adsorbed PPC diazonium salt, 2) dithiocarbamate PPC SAM and 3) lipoamide PPC SAM (PPC coupled to α‐lipoic acid) were evaluated. The layers were assessed for their ability to limit nonspecific adsorption of proteins to electrode surface with some significant differences observed compared with previously studied PPC diazonium salts reductively adsorbed on glassy carbon. Fluorescence microscopy and electrochemical impedance spectroscopy results suggest that protein adsorption is sensitive to the difference in the structure of the PPC molecules and the charge neutrality of the layers. The lipoamide PPC SAM was shown to be the most effective at resisting nonspecific protein adsorption and this layer was as effective as the ‘gold standard’ of oligo(ethylene oxide) SAMs on gold and PPC diazonium salts reductively adsorbed on glassy carbon.     

On a class of logarithmic integrals

In this note we give closed forms for a class of logarithmic integrals in terms of Bernoulli polynomials. This provides a method for unifying a large class of definite integrals.