On the on-line maintenance scheduling problem

A machine instantly serves requests but needs to undergo maintenance after serving a maximum of L requests. We want to maximize the number of requests served. In the on-line version, we prove that serving L requests before placing a maintenance is 0.5-competitive and is best possible for deterministic algorithms. We describe a 0.585-competitive randomized algorithm and show an upper bound of \(2L/(3L-1)\). We also analyze the empirical performance of various on-line algorithms on specific arrival distributions.     

Surfactant-assisted synthesis of polyaniline nanofibres without shaking and stirring: effect of conditions on morphology and conductivity

Polyaniline (PANI) nanofibres were synthesised by the chemical oxidative polymerisation method using ammonium peroxydisulphate (APS) as an oxidant/initiator. In this work, a surfactant-assisted method without shaking and stirring was used for the synthesis of PANI nanofibres. The effect was investigated of various parameters such as monomer/oxidant ratio, polymerisation temperature, and the presence of surfactant (Triton X-100 as a non-ionic surfactant) on the morphology and electrical conductivity of nanofibres. The morphology of PANI nanofibres was characterised by scanning electron microscopy and transmission electron microscopy. The results demonstrate that the morphology of PANI nanofibres was significantly influenced by the aniline/APS mole ratio, polymerisation temperature and presence of the surfactant during synthesis. The results showed that more regular and consistent nanofibres were obtained using a monomer/oxidant ratio of 4 at ambient temperature of polymerisation. PANI nanofibres with diameters in the range of 10?C100 nm and length up to several ??m were obtained. PANI nanofibres were also characterised using FTIR and UV-VIS absorption spectroscopy. The electrochemical behaviour of PANI nanofibres was studied by cyclic voltammetry. It was found that the electrical conductivity of PANI nanofibres increased with the increasing monomer/oxidant ratio and decreasing polymerisation temperature, respectively.     

Electrospun terpolymeric nanofiber membrane for micro solid‐phase extraction of diazinon and chlorpyrifos from aqueous samples

The present study deals with the synthesis and electrospining of a new terpolymer nanofiber in order to determine the amount of diazinon and chlorpyrifos in water and fruit juice samples. The synthesized terpolymer and the prepared nanofiber were characterized using ¹H NMR spectroscopy, FTIR spectroscopy, scanning electron microscopy, and gel permeation chromatography. The performance of terpolymer nanofiber, prepared as a sorbent for micro solid phase extraction was investigated for the extraction of diazinon and chlorpyrifos from aquaeous media. Then, the target analytes were desorbed from the coating with an organic solvent and analyzed by gas chromatography with flame ionization detector. Extraction efficiencies were significant (>90%) under the optimum condition. The proposed method also demonstrated good linear dynamic ranges for diazinon and chlorpyrifos (3–250 and 5–200 µg/L), and low limit of detections (0.5 and 0.7 µg/L) respectively. Moreover, under optimum condition for extraction of diazinon and chlorpyrifos, square of correlation coefficients (R²) of 0.9978 and 0.9953 and relative standard deviations of 4.6 and 5.1% were achieved, respectively. The recoveries for diazinon and chlorpyrifos were in the range of 85–97%.     

Quantitative determination of trace phenazopyridine in human urine samples by hyphenation of dispersive solid‐phase extraction and liquid‐phase microextraction followed by gas chromatography/mass spectrometry analysis

Magnetic dispersive solid‐phase extraction followed by dispersive liquid?liquid microextraction coupled with gas chromatography/mass spectrometry was applied for the quantitative analysis of phenazopyridine in urinary samples. Magnetic dispersive solid‐phase extraction was carried out using magnetic graphene oxide nanoparticles modified by poly(thiophene‐pyrrole) copolymer. The eluting solvent of this step was used as the disperser solvent for the dispersive liquid?liquid microextraction procedure. To reach the maximum efficiency of the method, effective parameters including sorbent amount, adsorption time, type and volume of disperser and extraction solvents, pH of the sample solution, and ionic strength as well as desorption time, and approach were optimized, separately. Characterization of the synthesized sorbent was studied by utilizing infrared spectroscopy, scanning electron microscopy, and energy‐dispersive X‐ray analysis. Calibration curve was linear in the range of 0.5?250 ng/mL (R² = 0.9988) with limits of detection and quantification of 0.1 and 0.5 ng/mL, respectively. Intra‐ and interday precisions (RSD%, n = 3) of the method were in the range of 4.6?5.4% and 4.0?5.5%, respectively, at three different concentration levels. Under the optimal condition, this method was successfully applied for the determination of phenazopyridine in human urine samples. The relative recoveries were obtained in the range of 85.0?89.0%.     

Using ONIOM calculations to investigate the abilities of simple and nitrogen,boron, sulfur-doped carbon nanotubes in sensing of carbon monoxide

In this work, geometries, stabilities, and electronic properties of the carbon monoxide (CO) molecule as an adsorbent in a simple carbon nanotube (CNT) and nitrogen (N), boron (B), sulfur (S)-doped CNTs (NCNT, BCNT, and SCNT) with parallel and perpendicular configurations are fully considered using ONIOM, natural bond orbital, and quantum theory of atom in molecule (QTAIM) calculations. The adsorption energies (E_ad) demonstrate that a CO molecule could be adsorbed on the surface of the simple CNT with parallel configuration and N-doped CNT with perpendicular configuration in an exothermic process. QTAIM calculations showed the close-shell (noncovalent) interactions between the CO molecule and CNT or N, B, S-doped CNTs. In addition, the energy gap (E_g) values between the highest occupied molecular orbital and the lowest unoccupied molecular orbital are calculated. In accordance with the results of energy gap, simple and N-doped CNTs could be used as CO sensors.     

Vinyl Triphenylphosphonium Salt–Mediated Synthesis of Functionalized Alkenes

Deoxybenzoin undergoes a smooth reaction with acetylenic esters in the presence of triphenylphosphine to produce phosphorus ylides. Only the tert-butyl phosphorus ylide undergoes an intramolecular Wittig reaction to produce highly strained cyclobutenes in boiling toluene, which spontaneously undergoes ring-opening reactions to produce highly functionalized 1,3-dienes. The unstable ylides undergo a 1,2-proton transfer and the loss of PPh₃ to produce functionalized alkenes.     

Determination of free formaldehyde in vaccines and biological samples using solid‐phase microextraction coupled to GC–MS

A headspace solid‐phase microextraction method coupled to GC–MS was successfully developed for the trace determination of formaldehyde in veterinary bacterial and human vaccines, and diphtheria–tetanus antigen. The formaldehyde was derivatized by means of the Hantzsch reaction prior to extraction and subsequent determination. Three different types of solid‐phase microextraction fibers, polar, and nonpolar poly(dimethylsiloxane) and polyethylene glycol were prepared by using a sol–gel technique. The effects of different parameters such as type of fiber coating, extraction time and temperature, desorption conditions, agitation rate, and salt effect were investigated. Under the optimized conditions, the detection limit of the method was 979 ng/L using the selected ion‐monitoring mode. The interday and intraday precisions of the developed method under the optimized conditions were below 13%, and the method shows linearity in the range of 1.75–800 μg/L with a correlation coefficient of 0.9963. The optimized method was applied to the determination of formaldehyde from some biological products. The results were satisfactory compared to the standard method.     

Characterization of peptide immobilization on an acetylene terminated surface via click chemistry

Peptide (A-A-A-A-G-G-G-E-R-G-D)¹ conjugated surfaces were prepared on silicon surfaces through click chemistry. The amino acid sequence RGD is the cellular attachment site of a large number of extracellular matrices such as blood and cell surface proteins. Recent research has focused on developing RGD peptides which mimic cell adhesion proteins and integrins [1], [2].The steps involved the formation of an alkyne-terminated monolayer on Si(111), followed by linking the peptide to 4-azidophenyl isothiocyanate via a specific and gentle reaction. This was followed by the attachment of the azido peptide to the surface-bound alkynes using the Cu (I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction. The surface structures of the alkyne terminated monolayer and the attached peptide were characterized using high resolution impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared (ATR-FTIR) Spectroscopy. EIS characterization revealed the alkyne layer and the hydrophobic and polar regions of the attached peptide. XPS analysis showed a high surface coverage of the peptide on the silicon substrates and this was confirmed by FTIR.Our results confirmed a specific covalent attachment of the peptide on the silicon surfaces. This approach offers a versatile, experimentally simple, method for the specific attachment of peptide ligands. This approach would have applications for cell attachment and biosensors.     

Ionic‐liquid‐based surfactant‐emulsified microextraction procedure accelerated by ultrasound radiation followed by high‐performance liquid chromatography for the simultaneous determination of antidepressant and antipsychotic drugs

In the present work, an efficient and environmental friendly method of ionic‐liquid‐based emulsified microextraction procedure accelerated by ultrasound radiation has been developed. Subsequently, its performance was compared with dispersive liquid–liquid microextraction and ultrasound‐assisted surfactant‐based emulsification microextraction methods. The optimization of experimental conditions was carried out by combination of central composite design and response surface methodology. The optimum conditions of variables were set as follows: 50 μL of 1‐hexyl‐3‐methylimidazolium hexafluorophosphate (extracting solvent), 10 min ultrasound time, and 10 min vortex time for agitating 6 mL sample solution in pH 3 in the presence of 4 mg sodium dodecyl sulfate without addition of salt and 200 μL of methanol as diluent solvent. Under these conditions, the responses are linear for doxepin and perphenazine in the range of 0.3–1000 and 5–1000 μg/L, respectively. The limits of detection were 0.1 μg/L for doxepin and 1 μg/L for perphenazine. Relative standard deviations were lower than 3.5 for the determination of both species. Finally, the method was used for the preconcentration and determination of doxepin and perphenazine in urine sample with relative recoveries in the range of 89–98%.     

Sensitive determination of atorvastatin in human plasma by dispersive liquid–liquid microextraction and solidification of floating organic drop followed by high‐performance liquid chromatography

A novel and sensitive dispersive liquid–liquid microextraction method based on the solidification of the floating organic drop combined with high‐performance liquid chromatography and ultraviolet detection was used for the determination of atorvastatine in blood serum samples. The chromatographic separation of atorvastatin was carried out using methanol as the mobile phase organic modifier. Various parameters affecting the extraction efficiency were optimized, such as the kind and volume of extraction solvent (1‐undecanol) and disperser solvent (acetonitrile), pH, and the extraction time. The calibration curve was linear in the range of 0.2–6000 μg/L of atorvastatin (r² = 0.995) with a limit of detection of 0.07 μg/L. The relative standard deviation for 100 μg/L of atorvastatin in human plasma was 8.4% (n = 4). The recoveries of plasma samples spiked with atorvastatin were in the range of 98.8–113.8%. The obtained results showed that the proposed method is fast, simple, and reliable for the determination of very low concentrations of atorvastatin in human plasma samples.