Application of the independent molecule model to elucidate the dynamics of structure I methane hydrate

Raman spectroscopy has exhibited the C–H stretch (A₁ mode) frequency ν₁ of hydrated methanes at 2915 cm⁻¹ for the 5¹² cage and 2905 cm⁻¹ for the 5¹²6² cage. These values are lower than the frequency of 2916.5 cm⁻¹ in gaseous methane. In this paper, we theoretically examine the Raman spectra observed in methane hydrate by normal mode analysis using the independent molecule model. By a breakdown of the symmetry, the four frequencies in modes A₁, E, T₂ and T₂ observed in gaseous methane are separated into nine frequencies in the hydrate. It is necessary to consider the anharmonic potential energy within methane and hydrogen bonding between methane hydrogen and water oxygen in order to get a result in qualitative agreement with experiment. The frequency in the 5¹²6² cage is shifted downward in comparison to the one in 5¹², and the frequencies in the both cages are also shifted downward compared with the frequencies in gas. Calculations are also reported for the isotopic methane (CD₄, ¹³CH₄) hydrates.     

Application of the independent molecule model to elucidate the dynamics of structure I methane hydrate: A second report

Two model systems of methane hydrate are constructed. One has a small cage surrounded by 12 large cages. The other has a large cage surrounded by four small cages and ten large cages. Three different H-bonding network patterns between waters are formed, and three random configurations of methane in each cage are chosen. A new method called the surface water fixed model is presented in which the energy minimum conformations for both model systems are preserved close to the X-ray crystallized structure. With normal mode analysis, we calculated frequencies of 2916.6 cm⁻¹ for a small cage at a centre, 2915.9 cm⁻¹ not at a centre, and 2911.7 cm⁻¹ for a large cage at a centre, and 2911.3 cm⁻¹ not at a centre. These frequencies are in moderate agreement with the corresponding Raman spectra, though not adequate. With our new method, however, it should be possible to improve agreement with the Raman spectra, if a model system vastly larger than the present model systems were constructed.     

Gasochromic effect of palladium doped peroxopolytungstic acid films prepared by the sol-gel route

Gasochromic palladium doped peroxopolytungstic acid (Pd:P-PTA) films have been prepared using dip-coating deposition from peroxopolytungstic acid (P-PTA) sols into which PdCl₂ was added in molar ratios Pd:W=1:125, 1:100, 1:53, and 1:40. These films exhibit reversible colouring/bleaching changes when exposed to hydrogen or hydrogen/argon mixture (4%) and air, alternatively. Gasochromically coloured and bleached films were characterised using in-situ Fourier transform infrared (FT-IR) spectroscopy. The vibrational modes of as-deposited, coloured and bleached films were assigned and the polaron absorption, which characterises the IR spectra of coloured films, was detected. Colouring/bleaching kinetics of films exposed to H₂ and H₂/Ar mixture as a function of the concentration of the catalyst and temperature of heat-treatment is reported. Proton (σ_pr) and electronic (σ_el) conductivities determined from impedance spectra revealed an increase in σ_el from 10⁻⁵ S cm⁻¹ in bleached state, to 10⁻³ S cm⁻¹ in coloured state, while σ_pr remained constant (10⁻² S cm⁻¹).     

Application of the independent molecule model to elucidate the dynamics of structure I methane hydrate

Raman spectroscopy has exhibited the C–H stretch (A₁ mode) frequency ν₁ of hydrated methanes at 2915 cm⁻¹ for the 5¹² cage and 2905 cm⁻¹ for the 5¹²6² cage. These values are lower than the frequency of 2916.5 cm⁻¹ in gaseous methane. In this paper, we theoretically examine the Raman spectra observed in methane hydrate by normal mode analysis using the independent molecule model. By a breakdown of the symmetry, the four frequencies in modes A₁, E, T₂ and T₂ observed in gaseous methane are separated into nine frequencies in the hydrate. It is necessary to consider the anharmonic potential energy within methane and hydrogen bonding between methane hydrogen and water oxygen in order to get a result in qualitative agreement with experiment. The frequency in the 5¹²6² cage is shifted downward in comparison to the one in 5¹², and the frequencies in the both cages are also shifted downward compared with the frequencies in gas. Calculations are also reported for the isotopic methane (CD₄, ¹³CH₄) hydrates.     

Direct electrochemistry and electrocatalysis of myoglobin immobilized in zirconium phosphate nanosheets film

Myoglobin (Mb) is incorporated on a novel matrix—zirconium phosphate nanosheets (ZrPNS) and immobilized at a glassy carbon electrode surface. UV–vis spectra and electrochemical measurements show that the matrix is well biocompatible and can retain the bioactivity of immobilized Mb. The direct electron transfer between Mb and electrode exhibits a couple of well-defined redox peaks. The cathodic and anodic peaks are located at −0.340 and −0.280 V vs. Ag/AgCl, respectively. The ZrPNS can improve the electron transfer between Mb and electrode with an electron transfer constant of 5.6 s⁻¹. Meanwhile, the catalytic ability of the protein toward the reduction of H₂O₂, O₂, NaNO₂, trichloroacetic acid (TCA) is also studied and a third-generation biosensor is subsequently fabricated. The linear range of biosensor to H₂O₂ is from 8 × 10⁻⁷ to 1.28 × 10⁻⁵ M with the limit detection of 1.4 × 10⁻⁷ M. The small apparent Michaelis–Menten constant (34 μM) suggests that Mb/ZrPNS film performs good affinity with H₂O₂. The biosensor also exhibits acceptable stability and reproducibility. This work paves a way to develop other biologic active materials in this kind of nanosheets for constructing novel biosensors.     

Simultaneous determination of lead and cadmium at a glassy carbon electrode modified with Langmuir–Blodgett film of p-tert-butylthiacalix[4]arene

A glassy carbon electrode (GCE) modified with a Langmuir–Blodgett (LB) film of p-tert-butylthiacalix[4]arene (TCA) has been investigated as a disposable sensor for measuring the trace levels of lead and cadmium. The possibility of determining lead and cadmium at trace levels was examined with differential pulse stripping voltammetry in the measurement step. The electrochemical response was characterized with respect to supporting electrolyte, pH of solution, accumulation time, accumulation potential, layers of the LB films, and possible interferences. Calibration plots were found to be linear in the range 2 × 10⁻⁷ to 5 × 10⁻⁵ mol l⁻¹ (Cd²⁺) and 1 × 10⁻⁷ to 2.5 × 10⁻⁵ mol l⁻¹ (Pb²⁺); the detection limits were 2 × 10⁻⁸ mol l⁻¹ (Cd²⁺) and 8 × 10⁻⁹ mol l⁻¹ (Pb²⁺). Possible recognition mechanism was also discussed. From the analysis of real samples (river, lake and tap water) it can be concluded that the method is sensitive and reproducible in determining of these elements and can be used in the analysis of natural water samples.     

Determination of reduced glutathione using an amperometric carbon paste electrode chemically modified with TTF–TCNQ

The development of an amperometric sensor for the determination of reduced glutathione (GSH) is described. The sensor is based on tetrathiafulvalene–tetracyanoquinodimethane (TTF–TCNQ) incorporated into the graphite powder/Nujol oil matrix. The electrooxidation of GSH was monitored amperometrically at 200 mV versus SCE (saturated calomel electrode). The amperometric response of the sensor was linearly proportional to the GSH concentration between 20 and 300 μmol l⁻¹, in 0.1 mol l⁻¹ phosphate buffer (pH 8.0), containing 0.1 mol l⁻¹ KCl and 0.5 mmol l⁻¹ Na₂H₂EDTA, as supporting electrolyte.

The detection limit, considering signal/noise ratio equal three, was 4.2 μmol l⁻¹ for GSH and the repeatability obtained as relative standard deviation was of 5.1% for a series of 10 successive measurements.     

Adsorptive voltammetry of Hg(II) ions at a glassy carbon electrode coated with electropolymerized methyl-red film

The glassy carbon electrode coated with electropolymerized methyl-red film, 1.2 × 10⁻⁶ m in thickness, (PMRE) showed high sensitivity towards Hg(II) ions. PMREs were adopted to accumulate and detect Hg(II) ions in a pH 2.56 Britton–Robinson buffer solution. Cyclic voltammogram of the accumulated Hg species on PMREs exhibited an anodic wave at 0.64 V and a cathodic wave at 0.13 V, due to the oxidation of accumulated Hg species on PMREs and the reduction of Hg(II) ions in the solution, respectively. For this heterogeneous adsorption of Hg(II) ions onto PMREs, the maximum surface concentration, adsorption equilibrium, and Gibbs energy change were evaluated to be 5.12 × 10⁻⁶ mol m⁻², 3.7 × 10⁵ l mol⁻¹, and −30.1 kJ mol⁻¹, respectively. The anodic peak current at 0.64 V was linear with the concentration of Hg(II) ions in the range of 1.1 × 10⁻¹⁰ to 1.1 × 10⁻⁷ M with a detection limit of 4.4 × 10⁻¹¹ M. The proposed method was utilized successfully for the detection of Hg(II) ions in the lake water.     

Effects of microstructure of films on CVD diamond X-ray detectors

Although the unique properties of chemical vapor deposition (CVD) diamond films have made it a candidate material for radiation detectors, the polycrystalline nature of the films has severely limited the development of CVD diamond detectors. In this work, three CVD diamond films with different microstructure were grown by using a hot-filament chemical vapor deposition (HFCVD) technique and were fabricated as CVD diamond detectors. The electric contact is good ohmic for bias voltage up to 150 V. 5.9 keV ⁵⁵Fe X-ray was used to measure the photocurrent and the pulse height distribution (PHD). For the detector based on the best quality film, the dark-current of 16.0 nA and the net photocurrent of 15.9 nA are obtained at an electric field of 50 kV cm⁻¹. The PHD peak is well separated from the noise pedestal, indicating a high counting efficiency and a low detection limit.     

A hydrogen peroxide biosensor based on direct electrochemistry of hemoglobin in Hb–Ag sol films

Hemoglobin (Hb) was used as a template to fabricate hemoglobin–silver (Hb–Ag) sol in which the hemoglobin showed direct electrochemistry on a glass carbon (GC) electrode. Ultraviolet–visible (UV–vis) spectra and reflectance absorption infrared (RAIR) spectra suggested that hemoglobin in Hb–Ag sol retained its native secondary structure. Scanning electron microscopy (SEM) demonstrated that the morphology of the Hb film was much different from the Hb–Ag sol film. The Hb–Ag film proved to exhibit a good electrocatalytic activity for the reduction of hydrogen peroxide. Based on this, a novel amperometric hydrogen peroxide biosensor was developed, which showed a sensitive response to the reduction of H₂O₂ without any electron mediator. Under optimum conditions, the biosensor responded linearly to H₂O₂ in the concentration range of 1 × 10⁻⁶ to 2.5 × 10⁻² M with detection limit of 1 × 10⁻⁷ M at 3σ. Moreover, the studied biosensor exhibited high sensibility, good reproducibility, and long-term stability.     

Correlation between carrier porous structure and gas-sensitive properties of thermocatalytic gas sensors

In order to determine the correlation between the gas-sensitive properties of the thermocatalytic gas sensor (TGS) and the porous structure of the carrier, two types of carriers have been investigated. Their sorptional parameters at different calcination temperatures have been studied. The parameters of the porous structure and the gas-sensitive properties of the TGS are characterized by measurements of benzene vapour adsorption and methane oxidation using methane/air mixtures, respectively. The obtained data indicate that the sensitivity of the TGS is related in some way with the structural parameters of the carrier (pore size and diameter distribution). Carriers must have a sorption capacity no less than 0.30 cm³ g⁻¹, surface area of 100–140 m² g⁻¹ and possess biporous pore-volume size distribution.     

Perchlorate selective membrane electrodes based on synthesized platinum(II) complexes for low-level concentration measurements

Three synthesized platinum(II) complexes, [PtR₂(NN)] (R = Me, p-MeC₆H₄ and p-MeOC₆H₄; NN = 2,2′-bipyridyl), were studied to characterize their ability as an anion carrier in a PVC membrane electrode. The polymeric membrane electrodes (PME) and also coated glassy carbon electrodes (CGCE) prepared with [Pt(p-MeOC₆H₄)₂(NN)] showed excellent response characteristics to perchlorate ions. The electrodes exhibited Nernstian responses to ClO₄⁻ ions over a wide concentration range from 5 × 10⁻⁷ to 4.0 × 10⁻¹ M for PME and 1.5 × 10⁻⁷ to 2.7 × 10⁻¹ M for CGCE with low detection limits (4.0 × 10⁻⁷ M for PME and 1.0 × 10⁻⁷ M for CGCE). The electrodes possess fast response time, satisfactory reproducibility, appropriate lifetime and, most importantly, good selectivity toward ClO₄⁻ relative to a variety of other common anions. The potentiometric response of the electrodes is independent of the pH of the test solution in the pH range 2.5–9.5. The proposed sensors were used in potentiometric determination of perchlorate ions in mineral water, urine samples and also samples containing interfering anions. The interaction of the ionophore with perchlorate ions was shown by UV–vis spectroscopy.     

Development of a highly sensitive and selective optical chemical sensor for batch and flow-through determination of mercury ion

A very sensitive, highly selective and reversible optical chemical sensor (optode) for mercury ion is described. The sensor is based on the interaction of Hg²⁺ with 2-mercapto-2-thiazoline (MTZ) in plasticized PVC membrane incorporating a proton-selective chromoionophore (ETH5294) and lipophilic anionic sites (sodium tetraphenylborate, NaTPB). The membranes were cast onto glass substrates and used for the determination of mercury ion in aqueous solutions by batch and flow-through methods. The sensor could be used in the range 2.0 × 10⁻¹⁰ to 1.5 × 10⁻⁵ M (0.04 ng mL⁻¹ to 3 μg mL⁻¹) Hg²⁺ with a detection limit of 5.0 × 10⁻¹¹ M and a response time of <40 s. It can be easily and completely regenerated by dilute nitric acid solution. The sensor has been incorporated into a home-made flow-through cell for determination of mercury ion in flowing streams with improved sensitivity, precision and detection limit. The sensor showed excellent selectivity for Hg²⁺ with respect to several common alkali, alkaline earth and transition metal ions. The results obtained for the determination of mercury ion in river water samples using the proposed optode was found to be comparable with the well-established cold-vapor atomic absorption method.     

Perchlorate-selective membrane sensors based on two nickel-hexaazamacrocycle complexes

The perchlorate salts of nickel(II) complexes of 1,3,5,8,10,13-hexaazacyclotetradecane (1) and 1,8-tert-butyl-1,3,5,8,10,13-hexaazacyclotetradecane (2) were used in construction of PVC based membrane electrodes. These sensors show very good selectivity for ClO₄⁻ ions over a wide variety of anions. These electrodes exhibit Nernstian behavior with the slopes of 59.5 and 59.3 mV per decade for (1) and (2), respectively. The working concentration ranges of the sensors are 1.0 × 10⁻¹–9.0 × 10⁻⁷ M (1) and 1.0 × 10⁻¹–5.0 × 10⁻⁷ M (2) with the detection limits of 6.0 × 10⁻⁷ and 2.0 × 10⁻⁷ M, respectively. The response time of the both sensors is very fast, and can be used for 2 (I) and 12 (II) weeks in a pH range of 3.0–11.0. These electrodes were applied to the determination of perchlorate ions in wastewater and cattle urine samples.     

Electrocatalytic oxidation of methanol and other short chain aliphatic alcohols on glassy carbon electrodes modified with conductive films derived from Ni-(N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminobenzene)

Complexes of nickel(II) with the ligand N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminobenzene (Ni^II-DHS) can be electropolymerized onto glassy carbon surfaces in alkaline solution to give electroactive films strongly adhered on the electrode surface. In alkaline solution, these poly-[Ni^II-DHS]/GC films present the typical voltammetric response of a surface-immobilized redox couple, as can be anticipated for the Ni²⁺/Ni³⁺ transitions into the film. In addition, the films exhibit a potent and persistent electrocatalytic activity towards the oxidation of methanol. The electrocatalytic currents are, at least, 80 times higher than those obtained for the oxidation of methanol at electrodes modified with nickel hydroxide films in alkaline solutions. In addition, the current is proportional to the concentration of methanol from 0.050 to 0.30 μM. The detection limit and the sensitivity were found to be 26 ± 2 nM and 7.4 × 10⁻² ± 6 × 10⁻³ A cm² mol⁻¹ M⁻¹, respectively. Electrodes modified with poly-[Ni^II-DHS]/GC films show a moderate electrocatalytic activity towards the oxidation of other aliphatic short chain alcohols, such as: ethanol, 1-propanol, 2-propanol and n-butanol. In all cases the catalytic currents present linear dependences with the concentration of alcohol in alkaline solution. The analytical properties of these potential alcohol sensors have also been studied.     

A selective modified bentonite–porphyrin carbon paste electrode for determination of Mn(II) by using anodic stripping voltammetry

A novel voltammetric sensor based on chemically modified bentonite–porphyrin carbon paste electrode (MBPCE) has been introduced for the determination of trace amount of Mn(II) in wheat flour, wheat rice and vegetables. In this method Mn(II) gives well-defined voltammetric peak at the pH range of 3.5–7.5. For the preliminary screening purpose, the catalyst was prepared by modification of bentonite with porphyrin and characterized by thermogravimetric method (TG) and UV–vis spectroscopy. The detection limit (three times signal-to-noise) with 4 min accumulation is 1.07 × 10⁻⁷ mol L⁻¹ Mn(II). The peak currents increases linearly with Mn(II) concentration over the range of 6.0 × 10⁻⁷ to 5.0 × 10⁻⁴ mol L⁻¹ (r² = 0.9959). Statistical treatment of the results gave a relative standard deviation lower than 2.30%. The chemical and instrumental parameters have been optimized and the results showed that 1000-fold excess of the additive ions had not interferences on the determination of Mn(II).     

Evaluation of the selective detection of 4,6-dinitro-o-cresol by a molecularly imprinted polymer based microsensor electrosynthesized in a semiorganic media

A carbon fiber microelectrode (CFME) was coated with a polymeric film, in order to synthesize an MIP based voltammetric microsensor for the selective determination of DNOC. The polymeric synthesis was carried out by electrocopolymerization of aniline and o-phenylenediamine (o-PD) in a water:methanol (1:1) media. The response of the MIP-sensor synthesized to the target analyte, was linear in a range from 8 × 10⁻⁷ to 10⁻⁴ M with a sensitivity of 1.6 × 10³ nA M⁻¹ and good stability and repeatability (<14%). Different rebinding experiments were carried out in order to evaluate the binding properties of the MIP-sensor. The experimental adsorption isotherms were fitted to Langmuir, Bi-Langmuir and Freundlich–Langmuir isotherms and the Langmuir model was chosen as the best fitting model. Under the optimized experimental conditions, the voltammetric microsensor was able to differentiate between DNOC and other closely related compounds such as other dinitrophenolic pesticides like binapacryl or dinobuton.     

Manufacture and characterization of sol–gel V1−x−yWxSiyO2 films for uncooled thermal detectors

V_1−x−yW_xSi_yO₂ films for uncooled thermal detectors were coated on sodium-free glass slides with sol–gel process, followed by the calcination under a reducing atmosphere (Ar/H₂ 5%). The V_1−x−yW_xSi_yO₂ films as prepared inherit various phase transition temperatures ranging from 20 to 70 °C depending on the dopant concentrations and the fabrication conditions. Compared to the hysteresis loop of plain VO₂ films, a rather steep loop was obtained with the addition of tungsten components, while a relaxed hysteresis loop with the tight bandwidth was contributed by Si dopants. Furthermore, the films with switching temperature close to room temperature were fabricated to one-element bolometers to characterize their figures of merit. Results showed that the V_0.905W_0.02Si_0.075O₂ film presented a satisfactory responsivity of 2600 V/W and detectivity of 9 × 10⁶ cm  Hz^1/2/W with chopper frequencies ranging from 30 to 60 Hz at room temperature. It was proposed that with appropriate amount of silicon and tungsten dopants mixed in the VO₂, the film would characterize both a relaxed hysteresis loop and a fair TCR value, which effectively reduced the magnitude of noise equivalent power without compromising its performance in detectivity and responsivity.     

Evaluation of three gas collection devices

Traditionally, the mouthpiece, nose-clip, and headgear arrangement (MN) used in indirect calorimetry is uncomfortable for some subjects, interferes with communications, and often interrupts tests if the nose-clip is lost. Gas collection masks (MASK) offer the possibility of alleviating the problems of MN but leakage and increased air dead space have been problematic. The present study compares the use of MN with two MASK designs (with nose-dam which prevents nasal breathing (MASKnd), and without nose-dam (MASKwo), from light to peak work rates in 20 well-trained subjects (M=12, F=8). Ventilation rate (Ve), oxygen uptake (VO₂), respiratory exchange ratio (RER) and heart rate (HR) were measured and comfort was assessed with a questionnaire. At the highest work rate, subjects achieved higher (as determined by repeated measures ANOVA) Ve for MASKnd(114.0±27 L min⁻¹) and MASKwo=(115.0±29 L min⁻¹) than for MN (108±28 L min⁻¹). Likewise, VO₂ in both MASKnd (55.0±10 ml kg min⁻¹) and MASKwo (56.2±11 ml kg min⁻¹) were higher than in the MN (53.7±12 ml kg min⁻¹)(p<0.05). Both MASKnd and MASKwo MASKs resulted in greater subject comfort than that for MASKmn. These data suggest that true maximal VO₂ can be more often attained with a MASK than an MN, since Ve and VO₂ were higher in MASK compared to that in MN.

Relevance to industry

This study will provide an evaluation of gas masks as an alternative to the traditional but uncomfortable mouth-piece and nose-clip used in indirect calorimetry.     

An organically modified sol–gel membrane for detection of lead ion by using 2-hydroxy-1-naphthaldehydene-8-aminoquinoline as fluorescence probe

A fluorescent reagent, 2-hydroxy-1-naphthaldehydene-8-aminoquinoline (HNAAQ) was synthesized, and an organically modified sol–gel membrane for detection of lead ion by using HNAAQ as fluorescence probe was fabricated. Under the optimum conditions, by a coplanar effect and the degree of molecular conjugation due to the complexation of Pb²⁺ with HNAAQ the relative fluorescence intensity I₁₀₀/I₀ of the sensing membrane is linearly increased over the Pb²⁺ concentration range of 1.9 × 10⁻⁷ to 1.9 × 10⁻⁴ mol/L with the detection limit of 8.3 × 10⁻⁸ mol/L. The preparation of this organically modified sol–gel membrane and its characteristics were investigated in detail.