Thermochemical, volumetric and spectroscopic properties of lysozyme–poly(ethylene) glycol system

Investigations of lysozyme–polyethylene glycol system were made by differential scanning calorimetry, fluorescence and density techniques. The values of unfolding enthalpies, ΔH_N^U, unfolding temperatures, T_m, excess molar heat capacities, ΔC_p, and apparent molar volumes, V_Φ , were determined as functions of PEG concentration. The three PEGs of average molecular mass (MW) 6000, 10000, 20000 were used as macromolecular crowding agents. The concentration of polymers was changed in the range 0–30% mass per volume (w/v). The values of ΔH_N^U remained constant with no dependence on PEG concentration, while PEG addition to buffered lysozyme solutions caused linear decrease of T_m. The values of ΔC_p and V_Φ of lysozyme dramatically changed in the range of 8–10% of PEG concentration. The fluorescence spectroscopy was used in order to investigate the polymer influence on possible solvent–lysozyme interactions. The electrical properties of polymer–water and polymer–buffer systems, the dielectric constants of solutions were determined with use of impedance spectroscopy.     

Enantioselective piezoelectric quartz crystal sensor for d-methamphetamine based on a molecularly imprinted polymer

A piezoelectric quartz crystal (PQC) sensor based on a molecularly imprinted polymer (MIP) has been developed for enantioselective and quantitative analysis of d-(+)-methamphetamine (d(+)-MA). The sensor was produced by bulk polymerization and the resulting MIP was then coated on the gold electrode of an AT-cut quartz crystal. Conditions such as volume of polymer coating, curing time, type of PQC, baseline solvent, pH, and buffer type were found to affect the sensor response and were therefore optimized. The PQC-MIP gave a stable response to different concentrations of d(+)-MA standard solutions (response time = 10 to 100 s) with good repeatability (RSD = 0.03 to 3.09%; n = 3), good reproducibility (RSD = 3.55%; n = 5), and good reversibility (RSD = 0.36%; n = 3). The linear range of the sensor covered five orders of magnitude of analyte concentration, ranging from 10⁻⁵ to 10⁻¹ μg mL⁻¹, and the limit of detection was calculated as 11.9 pg d(+)-MA mL⁻¹ . The sensor had a highly enantioselective response to d(+)-MA compared with its response to l(−)-MA, racemic MA, and phentermine. The developed sensor was validated by applying it to human urine samples from drug-free individuals spiked with standard d(+)-MA and from a confirmed MA user. Use of the standard addition method (SAM) and samples spiked with d(+)-MA at levels ranging from 1 × 10⁻³ to 1 × 10⁻² μg mL⁻¹ showed recovery was good (95.3 to 110.9%).     

Determination of the chemical oxygen demand (COD) using a copper electrode: a clean alternative method

A clean alternative method for the determination of chemical oxygen demand (COD) was developed using a copper electrode as an electrocatalytic sensor. The measuring principle is based on oxidation current of organic compounds in the wastewater. The effects of important experimental conditions, such as electrolytic solution concentration and potential scan rate, on analytical performance have been investigated. Analytical linear range of 53.0–2,801.4 mg l⁻¹ COD with detection limit of 20.3 mg l⁻¹ COD was achieved. The procedure was successfully applied to the COD determination in wastewater from soft industries. The results obtained using the proposed method were in good agreement with those obtained using the conventional (i.e., dichromate) COD method. In this fashion, the COD value of a sample can be determined in a simple, rapid, accurate manner, and the end products do not contain toxic metals.     

Novel surface acoustic wave-interdigitated array electrode gas sensor for dissolved ammonia

A novel surface acoustic wave-interdigitated array electrode (SAW-IDA) ammonia gas sensor is proposed. A gas-permeable membrane is employed to separate the buffer solution in the inner cell of the gas-sensing probe from the sample solution in the detection cell. The response of the IDA conductive electrodes is based on the impedance change of the buffer solution during ammonia adsorption. Therefore, this gas sensor overcomes the influence of water vapour in the conventional film-coated SAW gas sensor and can be used for the detection of gases in aqueous solutions. The ammonia sensor exhibits a favourable frequency response to 5 × 10⁻⁷−1 × 10⁻³ mol/1 ammonia. The optimal buffer composition and probe parameters have been determined. Dynamic range, response time, selectivity, and temperature drift are discussed. The ammonia sensor was also applied to the determination of serum ammonia. Results were in good agreement with those from the conventional enzymatic-spectro-photometric method.     

A sensitive nicotine sensor based on molecularly imprinted electropolymer of o-aminophenol

A sensitive nicotine sensor based on a molecularly imprinted electropolymer of o-aminophenol is proposed and its configuration and performance are studied in detail. On the condition of weak acidity, the sensitive layer was prepared by electropolymerization of o-aminophenol on a gold electrode in the presence of the template (nicotine). The sensor exhibits good selectivity and sensitivity to nicotine. The determination limit is 2.0×10⁻⁷ mol/L and a linear relationship between the current and concentration is found in the range of 4.0×10⁻⁷ ∼ 3.3 ×10⁻⁵ mol/L. The sensor has also been applied to the analysis of nicotine in tobacco samples with recovery rates ranging from 99.0% to 102%. __________ Translated from Journal of Hunan university (Nature Science), 2005, 32(3) (in Chinese)     

The influence of ZnO-Sensor modification by porphyrins on to the character of sensor response to volatile organic compounds

The influence of modification of semi conductive ZnO and SnO₂ sensors by porphyrins and their metal complexes on the parameters of sensor response to volatile organic compounds, ethanol, acetone and benzene, was analyzed. The concentration of volatile organic compounds (c _i ) varied in the range of 1–200 ppm. The sensor response was characterized by specific sensitivity γ _i = 100ΔR _i /R ₀ c _i , where ΔR _i = R _i − R ₀, R _i and R ₀ are measurable and initial resistance of the sensitive sensor layer correspondingly. It was established that the modification of sensors by porphyrins caused changes in sensor response and first of all decrease of ZnO sensor temperature at which the threshold of sensitivity is achieved (as used at this method c _i ^min = 0.1 ppm) from 300 to 100°C. It also caused a change in sing of the parameter γ, which was of importance for creating “electronic nose” sensor systems. It was shown also, that the modification of ZnO sensor by metal complexes of porphyrin did not change sensor response parameters.     

A deuterium–palladium electrode as a new sensor in non-aqueous solutions: potentiometric titration of weak acids in N,N-dimethylformamide and N-methylpyrrolidone

A deuterium–palladium electrode was employed as a new indicator electrode for the titration of weak acids in N,N-dimethylformamide and methylpyrrolidone. The investigated electrode showed a linear dynamic response for p-toluenesulfonic acid in the concentration range from 0.1 to 0.001 M, with a nernstian slope of 78.0 mV in N,N-dimethylformamide and of 64 mV per decade in N-methylpyrrolidone. Sodium methylate, potassium hydroxide, and tetrabutylammonium hydroxide proved to be very suitable titrating agents for these titrations. The potential in the course of the titration and at the titration end point was rapidly established. The response time was less than 10–11 s, and the lifetime of the electrode is long. The experimental results obtained for the proposed electrochemical sensor and a conventional glass electrode were in good agreement. The advantages of the electrode are log-term stability, fast response, reproducibility, and easy preparation.     

An optode membrane for determination of gold using a simple light-emitting diode-based device

Abstract  

A selective optode based on immobilization of 5-(p-dimethylaminobenzylidene)rhodanine on a triacetylcellulose membrane was developed for quantitative determination of Au(III). The determination procedure was performed using a simple light-emitting diode (LED)-based device as a new effort to overcome low reproducibility and repeatability problems which usually accompany optode-based determinations. The results obtained were compared with those of conventional spectrophotometric methods. The response characteristics of the sensor including dynamic range, reproducibility, response time, and lifetime are discussed in detail. This sensor was used for the determination of Au(III) in ore and electroplating liquid effluent samples with satisfactory results in comparison with flame atomic absorption spectroscopy as standard method. Under the optimum conditions, the calibration plot was linear in the concentration range of 0.3–6.0 μg cm⁻³. The relative standard deviations for five replicate determinations of 1.0 μg cm⁻³ Au(III) and the corresponding limits of detection were 1.76% and 0.12 μg cm⁻³, respectively.     

Revisiting the Solubility Concept of Pharmaceutical Compounds

Summary.  The kinetic and thermodynamic solubilities of Roche (Ro) pharmaceutical compounds were determined by HPLC, titrimetry, and UV/Vis spectroscopy in aqueous buffers and in non-buffered systems. For kinetic solubility, a turbidimetric method that allows the rapid determination of solubilities using small amounts of compounds (5–50 mg) was used. Two types of precipitation were observed during the kinetic solubility determinations: i) a disperse precipitation where the solution became foggy with very small particles uniformly distributed in the solution, and ii) discrete precipitation characterized by formation of crystals that rapidly sediment. The thermodynamic solubility was determined by shake flask and titrimetrically using a pH-STAT. The pH-STAT titrimetric method for the pH-thermodynamic solubility profile determination eliminates the buffer species and represents a new way to approach the solubility characterization of pharmaceutical compounds. The strengths of the turbidimetric method for determining the kinetic solubility are its rapidity, minimal compound requirements, and suitability for high throughput screening. The limitations are that the maximum solubility is limited to less than 100 mg · cm⁻³, and the precipitation of trace impurities cannot be distinguished from precipitation of the analyte. The pH-STAT titrimetric approach for the thermodynamic solubility has a lower throughput and is suitable for the characterization of the lead candidate. It is not limited in its solubility range and provides a common basis for the comparison of the solubility values at different pH values in contrast to traditional buffered systems. Received August 21, 2000. Accepted (revised) February 5, 2001     

Novel surface acoustic wave-interdigitated array electrode gas sensor for dissolved ammonia

 A novel surface acoustic wave-interdigitated array electrode (SAW-IDA) ammonia gas sensor is proposed. A gas-permeable membrane is employed to separate the buffer solution in the inner cell of the gas-sensing probe from the sample solution in the detection cell. The response of the IDA conductive electrodes is based on the impedance change of the buffer solution during ammonia adsorption. Therefore, this gas sensor overcomes the influence of water vapour in the conventional film-coated SAW gas sensor and can be used for the detection of gases in aqueous solutions. The ammonia sensor exhibits a favourable frequency response to 5×10^-7–1×10^-3 mol/l ammonia. The optimal buffer composition and probe parameters have been determined. Dynamic range, response time, selectivity, and temperature drift are discussed. The ammonia sensor was also applied to the determination of serum ammonia. Results were in good agreement with those from the conventional enzymatic-spectrophotometric method. Received: 1 December 1995/Revised: 9 April 1996/Accepted: 14 April 1996     

Effect of Temperature and Composition on the Transport and Thermodynamic Properties of Binary Mixtures of Ionic Liquid <Emphasis Type="Italic">N</Emphasis>-Butyl-<Emphasis Type="Italic">N</Emphasis>-methylpyrrolidinium bis(Trifluoromethanesulfonyl)imide and Propylene Carbonate

The thermodynamic and transport proprieties have been determined for the whole concentration range of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P₁₄) and propylene carbonate (PC) binary mixtures in the temperature range 293.15–318.15 K. Strong effects of the mole fraction and temperature on these properties have been observed. The isobaric expansivities and excess properties have been evaluated. Their corresponding coefficients were positive but the excess volume had both negative and positive values. The viscosity parameter was found to be temperature-dependant and followed the Arrhenius law. The variation of activation energies for viscous flow E _a,η versus the ILs concentration exhibited a change in the medium structure. The conductivity-temperature relationship was found to have a better fitting on the Vogel-Tammann-Fulcher model than on the Arrhenius law. In addition, the variation of conductivity with the molar fraction was well described by the Casteel-Amis equation. Finally, the Walden product showed significant dependence of the conductivity on the viscosity of the medium. The results allowed good characterisation of both the ruling interactions and the medium structure.     

Experimental Study of the Effect of Temperature,Pressure and Concentration on the Viscosity of Aqueous NaBr Solutions

The viscosity of 10 (0.049, 0.205, 0.464, 0.564, 0.820, 1.105, 1.496, 2.007, 2.382, and 2.961 mol ċ kg⁻¹) binary aqueous NaBr solutions has been measured with a capillary-flow technique. Measurements were made at pressures up to 40 MPa. The range of temperature was 288–595 K. The total uncertainty of viscosity, pressure, temperature and composition measurements were estimated to be less than 1.6%, 0.05%, 15 mK, and 0.02%, respectively. The effect of temperature, pressure, and concentration on viscosity of binary aqueous NaBr solutions were studied. The measured values of the viscosity of NaBr(aq) were compared with data, predictions and correlations reported in the literature. The temperature and pressure coefficients of viscosity of NaBr(aq) were studied as a function of concentration and temperature. The viscosity data have been interpreted in terms of the extended Jones–Dole equation for the relative viscosity (η/η₀) to calculate accurately the values of viscosity A- and B-coefficients as a function of temperature. The derived values of the viscosity A- and B-coefficients were compared with the results predicted by the Falkenhagen–Dole theory of electrolyte solutions and calculated with the ionic B-coefficient data. The physical meaning parameters V and E in the absolute rate theory of the viscosity and hydrodynamic molar volume V k were calculated using the present experimental viscosity data. The TTG model has been used to compare predicted values of the viscosity of NaBr(aq) solutions with experimental values at high pressures.     

Tripodal chelating ligand-based sensor for selective determination of Zn(II) in biological and environmental samples

Potassium hydrotris(N-tert-butyl-2-thioimidazolyl)borate [KTt^t-Bu] and potassium hydrotris(3-tert-butyl-5-isopropyl-l-pyrazolyl)borate [KTp^t-Bu,i-Pr] have been synthesized and evaluated as ionophores for preparation of a poly(vinyl chloride) (PVC) membrane sensor for Zn(II) ions. The effect of different plasticizers, viz. benzyl acetate (BA), dioctyl phthalate (DOP), dibutyl phthalate (DBP), tributyl phosphate (TBP), and o-nitrophenyl octyl ether (o-NPOE), and the anion excluders sodium tetraphenylborate (NaTPB), potassium tetrakis(p-chlorophenyl)borate (KTpClPB), and oleic acid (OA) were studied to improve the performance of the membrane sensor. The best performance was obtained from a sensor with a of [KTt^t-Bu] membrane of composition (mg): [KTt^t-Bu] (15), PVC (150), DBP (275), and NaTPB (4). This sensor had a Nernstian response (slope, 29.4 ± 0.2 mV decade of activity) for Zn²⁺ ions over a wide concentration range (1.4 × 10⁻⁷ to 1.0 × 10⁻¹ mol L⁻¹) with a limit of detection of 9.5 × 10⁻⁸ mol L⁻¹. It had a relatively fast response time (12 s) and could be used for 3 months without substantial change of the potential. The membrane sensor had very good selectivity for Zn²⁺ ions over a wide variety of other cations and could be used in a working pH range of 3.5–7.8. The sensor was also found to work satisfactorily in partially non-aqueous media and could be successfully used for estimation of zinc at trace levels in biological and environmental samples. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Dopamine sensor based on molecularly imprinted electrosynthesized polymers

Molecularly imprinted polymers (MIPs) have been applied as molecular recognition elements to chemical sensors. In this paper, we combined the use of MIPs and electropolymerization to produce a sensor which was capable of detecting dopamine (DA). The MIP electrode was obtained by electrocopolymerization of o-phenylenediamine and resorcinol in the presence of the template molecular DA. The MIP electrode exhibited a much higher current response compared with the non-imprinted electrode. The response of the imprinted sensor to DA was linearly proportional to its concentration over the range 5.0 × 10⁻⁷-4.0 × 10⁻⁵ M. The detection limit of DA is 0.13 μM (S/N = 3). Moreover, the proposed method could discriminate between DA and its analogs, such as ascorbic acid and uric acid. This method was successfully applied to the determination of DA in dopamine hydrochloride injection and healthy human blood serum. These results revealed that such a sensor fulfilled the selectivity, sensitivity, sped, and simplicity requirements for DA detection and provided possibilities of clinical application in physiological fields.     

The Physical Properties of Aqueous Solutions of the Ionic Liquid [BMIM][BF4]

We report here the systematic study of the effect of concentration on the physical properties of aqueous solutions of the room-temperature ionic liquid [BMIM][BF₄]. The measurements of density, ρ, refractive index △n, viscosity η, specific conductivity κ and surface tension, γ, were made over the whole concentration range. The equivalent conductance Λ _m was calculated. The observed linear variations of density and refractive index with the molar concentration are established as those of an ideal solution. The surface tension varied most rapidly in the dilute region whereas the viscosity changed much more rapidly in the concentrated region. Two regions with different composition dependences were found after the analyses of the relationship between the conductivity and the concentration of [BMIM][BF₄]. A proposed model for a structural change in the mixtures was described. The physical origin of the observed concentration dependence of these properties is discussed. The physical properties of the solutions vary with changes of association between anions and cations and the interaction between [BMIM][BF₄] and water.     

Intermolecular Forces of Sugars in Water

Summary. Apparent molar volume and viscosities of fructose, glucose, mannose, and sucrose have been measured in dilute aqueous solution, concentration range 0.028–0.336 M at 293 K. The viscosity coefficient B and A were calculated from the viscosity data using the Jones-Dole equation for all the studied sugars. The data were also analysed for Stauarding equation. The structure making behavior was obtained for all the sugars. A modified Jone-Dole equation was proposed by using ratio of mole fractions of solute and solvent in place of concentrations of solute.     

Intermolecular Forces of Sugars in Water

Apparent molar volume and viscosities of fructose, glucose, mannose, and sucrose have been measured in dilute aqueous solution, concentration range 0.028–0.336 M at 293 K. The viscosity coefficient B and A were calculated from the viscosity data using the Jones-Dole equation for all the studied sugars. The data were also analysed for Stauarding equation. The structure making behavior was obtained for all the sugars. A modified Jone-Dole equation was proposed by using ratio of mole fractions of solute and solvent in place of concentrations of solute.     

Kinetic analysis of interaction between lipopolysaccharide and biomolecules

Lipopolysaccharide (LPS) is a major component of the outer membrane of all gram-negative bacteria. It interacts with some biomolecules and triggers a toxic reaction. In this paper, we studied the interaction between LPS from Salmonella Minnesota and some biomolecules using a surface plasmon resonance (SPR) biosensor. Biomolecules were immobilized on a CM5 sensor chip using the amino coupling method and LPS was injected over the immobilized surfaces. The affinity constant K_A of LPS with serum albumin, hemoglobin, chitosan and lysozyme was 2.36 × 10⁷, 2.03 × 10⁸, 7.58 × 10⁶, 2.82 × 10⁴ L·mol⁻¹, respectively. However, LPS could not interact with ferritin. __________ Translated from Chinese Journal of Analytical Chemistry, 2007, 35(5): 677–680 [译自: 分析化学]     

Azide-selective sensor based on tripodal iron complex for direct azide determination in aqueous samples

A potentiometric azide-selective sensor based on the use of iron(III) hydrotris(3,5-dimethylpyrazolyl)borate acetylacetonate chloride [Tp^Me2Fe(acac)Cl] as a neutral carrier for an azide-selective electrode is reported. Effect of various plasticizers, viz. o-nitrophenyloctyl ether (o-NPOE), dioctylphthalate (DOP), dibutylphthalate (DBP), and benzylacetate (BA), and an anion excluder, hexadecyltrimethylammonium bromide (HTAB), with [Tp^Me2Fe(acac)Cl] complex in poly(vinyl chloride) (PVC) were studied. The best performance was obtained with a membrane composition of [Tp^Me2Fe(acac)Cl]/HTAB/DOP/PVC in a ratio of 5:2:190:100 (w/w). The sensor exhibits significantly enhanced selectivity toward azide ions over the concentration range 6.3 × 10⁻⁷ to 1.0 × 10⁻² M with a lower detection limit of 3.8 × 10^–7 M and a Nernstian slope of 59.4 ± 1.1 mV decade⁻¹. Influences of the membrane composition, pH and possible interfering anions were investigated on the response properties of the electrode. Fast and stable response, good reproducibility, long-term stability and applicability over a wide pH range (3.5–9.0) are demonstrated. The sensor has a response time of 14 s and can be used for at least 45 days without any considerable divergence in the potential response. The proposed electrode shows fairly good discrimination of azide from several inorganic and organic anions. It was successfully applied to the direct determination of azide in orange juice, tea extracts and human urine samples.     

A microchip sensor for calcium determination

A newly designed glass-PDMS microchip-based sensor for use in the determination of Ca²⁺ ions has been developed, utilizing reflectance measurements from arsenazo III (1,8-dihydroxynaphthalene-3,6-disulfonic acid-2,7-bis[(azo-2)-phenyl arsenic acid]) immobilized on the surface of polymer beads. The beads, produced from cross-linked poly(p-chloromethylstyrene) (PCMS), were covalently modified with polyethylenimine (PEI) to which the Arsenazo III could be adsorbed. The maximum amount of Arsenazo III which could be immobilized onto the PEI-attached PCMS beads was found to be 373.71 mg g⁻¹ polymer at pH 1. Once fabricated, the beads were utilized at the detection point of the microfluidic sensor device with a fiber optic assembly for reflectance measurements. Samples were mobilized past the detection point in the sensor where they interact with the immobilized dye. The sensor could be regenerated and re-used by rinsing with HCl solution. The pH, voltage, linear range, and the effect of interfering ions were evaluated for Ca²⁺ determination using this microchip sensor. At the optimum potential, 0.8 kV, and pH 9.0, the linear range of the microchip sensor was 3.57 × 10⁻⁵ – 5.71 × 10⁻⁴ M Ca²⁺, with a limit of detection (LOD) of 2.68 × 10⁻⁵ M. The microchip biosensor was then applied for clinical analysis of calcium ions in serum with good results. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.