Application of silica gel as an effective modifier for the voltammetric determination of dopamine in the presence of ascorbic acid and uric acid

Amorphous silica gel modified carbon paste electrode (CPE) offers substantial improvements in voltammetric sensitivity and selectivity towards determination of dopamine (DA). Cyclic voltammetry of Fe(CN)₆^3−/4− as a negatively charged probe revealed that the surface of the silica gel modified carbon paste electrode had a high density of negative charge at pH 8.0. Therefore, the modified electrode adsorbed DA (pK_a = 8.9) and enhanced its voltammetric response while repulsed ascorbic acid (AA) (pK_a = 4.2) and uric acid (UA) (pK_a = 5.4) and inhibited their interfering effects. The influence of various experimental parameters including percent of silica gel in the CPE, pH of solution, and accumulation time and potentials, on the voltammetric response of DA was investigated. At the optimum conditions, the analytical curve was linear for dopamine concentrations from 2.0 × 10⁻⁷ to 1.0 × 10⁻⁶ mol L⁻¹ and 2.0 × 10⁻⁶ to 1.5 × 10⁻⁴ mol L⁻¹ with a detection limit (3σ) of 4.8 × 10⁻⁸ mol L⁻¹. The prepared electrode was used for determination of DA spiked into DA injection and human serum samples, and very good recovery results were obtained over a wide concentration range of DA.     

Zinc(II) complex of terpyridine-crown macrocycle: A new motif in fluorescence sensing of zwitterionic amino acids

A new fluorescence macrocyclic receptor based on the Zn(II) complex of a C₂-terpyridine and a crown ether has been developed for molecular recognition of zwitterionic amino acids in water/DMF solution with remarkable selectivity towards l-aspartate (K = 4.5 × 10⁴ M⁻¹) and l-cysteine (K = 2.5 × 10⁴ M⁻¹).     

Low thermal expansion porous SiC–WC composite ceramics

Low thermal expansion porous SiC–WC composite ceramics were prepared by solid state reaction of Si and WC at 1560 °C, with NH₄HCO₃ as a pore generating agent. Phase composition, thermal expansion, flexural strength, and microstructure of the carbide ceramics were examined. Presence of the SiC, WC and WC_1−X phases were detected in the carbide ceramics. As Si content increased from 2 to 14 wt%, the coefficient of thermal expansion first decreased and then increased, with a minimum of 4.11 × 10⁻⁶ °C at 8 wt% Si, whereas the flexural strength decreased gradually, from 143.9 to 82.7 MPa. Pores of SiC–WC ceramics were less than 2 μm in diameter, because of the stacking interstice of carbide particles and volatilization of silicon. However in the presence of NH₄HCO₃, pores of SiC–WC ceramics were bimodally distributed, the stacking interstice of carbide particles loosened from 1 to 4 μm and pores larger than 5 μm were also formed.     

Sawdust: A green and economical sorbent for thallium removal

Removal/preconcentration of thallium(I) ions from aqueous solution by sawdust; a waste material derived from the commercial processing of Cedrus Deodar wood for furniture production was investigated. A simple and low-cost modification results in increasing the sorption capacity of raw sawdust from 2.71 to 13.18 mg g⁻¹. Sorption was found to be rapid (98% within 8 min). The binding of metal ions was found to be pH dependent, optimal sorption accruing at around pH 6–9. Potentiometeric titrations of sawdust revealed two distinct pK_a values, the first having the value similar to carboxylic groups (3.3–4.8) and second comparable with that of amines (8.53–10.2) with the surface site densities of 1.99 × 10⁻⁴ and 7.94 × 10⁻⁵ mol g⁻¹, respectively. Retained Tl(I) ions were eluted with 5 ml 0.1 mol l⁻¹ HCl. Detection limit of 0.0125 μg ml⁻¹ was achieved with an enrichment factor of 160. Recovery was quantitative using sample volume of 800 ml. The Langmuir, Freundlich and D–R isotherm equations were used to describe partitioning behavior for the system at different temperatures. Kinetic and thermodynamic behavior of sawdust for Tl(I) ions removal was also studied.     

Spatial variability of transport parameters in the Boom Clay

Several transport parameters (as hydraulic conductivity K, apparent diffusion coefficient D_p and diffusion accessible porosity η of HTO and iodide) have been intensively measured in the laboratory on high-quality cores taken at the Mol-1 borehole of the Mol site, in Belgium. The borehole was cored in 1997 from about 145 to 325 m depth, including the whole thickness of the Boom Formation, a Tertiary clay situated between 186 and 288 m depth (ground level), and part of the surrounding layers.The hydraulic conductivity measurements confirm the low permeability of the Boom Clay. An upper 90-m-thick clay layer within this formation can be considered as homogeneous with respect to the hydraulic conductivity. The vertical hydraulic conductivity K_v (i.e. K perpendicular to the bedding) is in the order of magnitude of 10⁻¹² m s⁻¹ and the average is 2.3×10⁻¹² m s⁻¹. This layer comprises from top to bottom the “Transition Zone”, the Putte Member, the Terhagen Member and the top of the Belsele-Waas Member of the Boom Formation. The 12 m at the base of the Formation, which corresponds to the lower part of the Belsele-Waas Member is characterised by larger K_v values (ranging between 10⁻¹¹ and 9×10⁻¹¹ m s⁻¹).The same thick clay layer can also be considered as homogeneous, regarding the values of the apparent diffusion coefficient and the diffusion accessible porosity η of tritiated water (HTO) and iodide. The average value of the diffusion accessible porosity is 0.37±0.03 for HTO and 0.16±0.02 for iodide. The apparent diffusion coefficient varies from 1.1×10⁻¹⁰ to 5.5×10⁻¹⁰ m² s⁻¹ for HTO and from 9.1×10⁻¹¹ to 5.2×10⁻¹⁰ m² s⁻¹ for iodide.     

Novel coated graphite electrode for the selective determination of Gd(III) in rocks and waste water samples

A novel gadolinium selective coated graphite electrode based on 2,6-bis-[1-{N-cyanopropyl,N-(2-methylpridyl)}aminoethyl]pyridine [P] is described. The best performance was exhibited by the electrode having membrane composition P:NaTPB:PVC:NPOE as 8:4:30:58 (%, w/w). The electrode demonstrates excellent potentiometric characteristics towards gadolinium ion over several interfering ions. The electrode exhibited a Nernstian response to Gd³⁺ ion over a wide concentration range 2.8 × 10⁻⁷ to 5.0 × 10⁻² M with a detection limit (6.3 ± 0.1) × 10⁻⁸ M and slope 19.6 ± 0.1 mV decade⁻¹ of a_Gd³⁺. Furthermore, it showed a fast response time (12 s) and can be used for 2.5 months without significant divergence in its characteristics. Noticeably, the electrode can tolerate the concentration of different surfactants up to 1.0 × 10⁻⁴ M and can be used successfully in 30% (v/v) ethanol media and 10% (v/v) methanol and acetonitrile water mixture. The useful pH range of this sensor is 2.0 to 8.0. It is sufficiently selective and can be used for the determination of Gd³⁺ ions in waste water and rock samples. It also serves as a good indicator in the potentiometric titration of GdCl₃ with EDTA.     

Multi-walled carbon nanotubes (MWCNTs) and room temperature ionic liquids (RTILs) carbon paste Er(III) sensor based on a new derivative of dansyl chloride

Solution studies showed the strong interaction of [5-(dimethylamino) naphthalene-1-sulfonyl 4-phenylsemicarbazide] (NSP) with Er(III) ions. NSP was used as a sensing material during construction of carbon paste Er(III) sensors. The electrodes were modified with 1-n-butyl-3-methylimidazolium tetrafluoroborate, [bmim]BF₄, as room temperature ionic liquid (RTIL) and multi-walled carbon nanotube (MWCNT). Potentiometric sensors constructed with [bmim]BF₄ and MWCNTs show better sensitivity, selectivity, response time, and response stability compared to Er(III) carbon paste sensors. The best performance for the modified sensor was obtained with an electrode composition of 20% [bmim]BF₄, 20% NSP, 45% graphite powder and 15% MWCNT. This particular sensor formulation exhibits a Nernstian response (19.8 ± 0.3 mV decade⁻¹) toward Er(III) ions in the range of 1.0 × 10⁻⁷ to 1.0 × 10⁻¹ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁸ mol L⁻¹. The proposed modified Er(III) sensor can be used over the pH range from 3.5 to 9.0.     

Incorporation of tin in different types of pores in SBA-15: Synthesis, characterization and catalytic activity

Mesoporous Sn-SBA-15 has been synthesized by three different methods such as conventional hydrothermal route, using cocatalyst NH₄F and in the presence of organosilane precursor. All the materials are thoroughly characterized by powder X-ray diffraction (XRD), SEM, TEM, N₂ sorption and surface area measurements, diffuse-reflectance UV–visible and FTIR spectroscopy, TG–DTA and elemental analysis through ICP. Nitrogen adsorption data, XRD patterns, and TEM observations suggests that the textural properties are retained during the isomorphous substitution of silicon by tin. ICP chemical analysis indicates that tin can be substituted in the range of Si/Sn = 69–162. UV–visible spectra of samples synthesized by the cocatalytic approach exhibit unique absorption band at 213 nm characteristics of tin atom substituted in the smaller pores (2–3 nm) located inside the walls of mesopores. Further, an additional band at 224 nm can be assigned to Sn atoms located in the distorted tetrahedral position along the primary mesopores. In contrary, only one absorption band centered at 224 nm is observed for all the samples synthesized by conventional hydrothermal as well as in the presence of organosilane precursor. ¹⁹F NMR spectra confirmed (no signal) the absence of occluded F⁻ ions in the samples made with NH₄F. Observed high catalytic activity in Baeyer–Villiger oxidation and Meerwin–Pondorf–Verly reduction under the liquid-phase conditions suggest the incorporation of a portion of tin in the smaller pores for the Sn-SBA-15 materials synthesized through cocatalyst method.     

Study of LiNi0.5Mn1.5O4 synthesized via a chloride-ammonia co-precipitation method: Electrochemical performance, diffusion coefficient and capacity loss mechanism

LiNi_0.5Mn_1.5O₄ powders are prepared via a new co-precipitation method. In this method, chloride salts are used as precursors and ammonia as a precipitator. The impurity of chlorine can be removed via a thermal decomposition of NH₄Cl in the subsequent calcination. X-ray diffraction pattern reveals that the final product is a pure spinel phase of LiNi_0.5Mn_1.5O₄. Scanning electron microscopy shows that the powders have an octahedron shape with a particle size of about 2 μm. Electrochemical test shows that the LiNi_0.5Mn_1.5O₄ powders exhibit an excellent cycling performance and after 300 cycles, the capacity retention is 83%. The lithium diffusion coefficient is measured to be 5.94 × 10⁻¹¹ cm² s⁻¹ at 4.1 V, 4.35 × 10⁻¹⁰ cm² s⁻¹ at 4.75 V and 7.0 × 10⁻¹⁰ cm² s⁻¹ at 4.86 V. The mechanism of capacity loss is also explored. After 300 cycles, the cell parameter ‘a’ decreases by 0.54% for the quenched sample (LiNi_0.5Mn_1.5O_4−δ) and by 0.42% for the annealed sample (LiNi_0.5Mn_1.5O₄). Besides, it is the first time to identify experimentally that the Ni and Mn ions dissolved in the electrolyte can be further deposited on the surface of anode.     

Poly (acridine orange) film modified electrode for the determination 1-naphthol in the presence of 2-naphthol

The voltammetric behavior of 1-naphthol was studied with a poly (acridine orange) (PAO) film modified glass carbon electrode (GCE). The electrooxidation of 1-naphthol was an irreversible process with its oxidation overpotential at the PAO electrode 180 mV lower than that on the GCE. PAO electrode demonstrated electrocatalytic activity to the electrooxidation of 1-naphthol giving a greatly improved detection limit down to 8 × 10⁻⁸ mol L⁻¹(S/N = 3). At the optimal experimental condition, the oxidation peak current from the PAO electrode was linearly proportional to the concentration of 1-naphthol in the range of 2 × 10⁻⁷ to 3.2 × 10⁻⁶ mol L⁻¹ and 5.2 × 10⁻⁶ to 1.2 × 10⁻⁴ mol L⁻¹. The differences of the oxidation peak potentials between 1-naphthol and the coexisted 2-naphthol was 170 mV allowing the selective detection of 1-naphthol in a mixed solution with 2-naphthol. The detection of 1-naphthol in tap water and river water was carried out with satisfactory results.     

Synthesis of proton conductive membranes based on inorganic–organic hybrid structure bound with phosphonic acid

Proton conductive inorganic–organic hybrid membranes were synthesized from styrene derivatives of alkoxysilane and ethyl 2-[3-(dihydroxyphosphonyl)-2-oxopropyl] acrylate (EPA) through copolymerization followed by sol–gel reaction. Self-standing, homogeneous and transparent hybrid membranes with chemically bound phosphonic acid groups were synthesized. FT-IR analysis exhibited the hybrid membranes included phosphonic acid groups. ¹³C and ²⁹Si NMR studies indicated that alkoxysilyl functionalized styrene derivatives were not only copolymerized with EPA but also condensed yielding Si–O–Si linkages by sol–gel reaction. TG–DTA analysis revealed that these membranes were thermally stable up to 200 °C in dry O₂. The proton conductivities of the hybrid membranes increased with phosphonic acid content and temperature. The P/Si ratio of the membrane was dependent on the number of alkoxy group in the starting alkoxysilane. The hybrid membrane from (dimethylmethoxysilylmethyl)styrene (DMMSMS(M))/EPA = 1/6 revealed proton conductivities of 6.3 × 10⁻³ and 2.4 × 10⁻⁴ S cm⁻¹ at 68.0% relative humidity and 18.8% relative humidity, respectively, at 140 °C.     

Solubilities of actinides in a domestic groundwater and a bentonite porewater calculated by using PHREEQC

This study presents the solubilities and speciations of actinides, calculated by the PHREEQC (V.2) code in a granitic groundwater and a Ca-bentonite porewater under a reducing condition. The respective solubilities for the amorphous U, Am, Th, Np and Pu compounds in the groundwater were 2.2 × 10⁻⁵, 1.2 × 10⁻⁷, 3.1 × 10⁻⁹, 3.4 × 10⁻⁹ and 6.3 × 10⁻¹¹ mole/L, and these values are comparable to the results calculated by the MUGREM and EQ3/6 codes. The major dissolved species for U, Am, Th, Np and Pu were UO₂(OH)₃⁻, Am(OH)₂⁺, Th(OH)₄(aq), Np(OH)₃CO₃⁻ and Pu(OH)₃CO₃⁻, respectively. However, carbonate complex ions were anticipated as the major species in the porewater except for thorium due to an increase of the carbonate concentration and a decrease of the pH.     

Urea release rate from a scoop of coated pure urea beads: Unified extreme analysis

Urea release from a scoop of coated beads in a given volume of a well stirred liquid has been investigated analytically and experimentally. A method for determining the fractional cumulative release and fractional release rate curves for the scoop without knowing particle number and radii is presented. The representative D/K_b for a scoop of urea beads spray coated with ethyl cellulose is near 3.5 × 10⁻⁸ cm²/s, and that with cellulose acetate phthalate is near 7 × 10⁻⁸ cm²/s.     

Functionalized sepiolite for heavy metal ions adsorption

Surface modification of clays has become increasingly important due to the practical applications of clays such as fillers and adsorbents. The surface modification of sepiolite with [3-(2-aminoethylamino)propyl]trimethoxysilane has been employed. The modified sepiolite surface was investigated by FTIR, XRD and DTA/TG analysis. It was found that the chemical bonding takes place between the hydroxyl groups and/or oxygen atoms within the structure of sepiolite and methoxy groups of [3-(2-aminoethylamino)propyl]trimethoxysilane. The changes on electrokinetic properties of modified sepiolite particles were studied by measuring the zeta potential of particle as a function of metal concentration and equilibrium pH of solution. It was found that the zeta potential of the clay particles was always negative independent of the metal concentration in solution. This study also provides some evidence for the adsorption of metal ions on modified sepiolite. The adsorption of metal ions onto modified sepiolite has varied with the type of metal cations. The available basic nitrogen centers covalently bonded to the sepiolite skeleton were studied for Co(II), Cu(II), Mn(II), Zn(II), Fe(III) and Cd(II) adsorption from aqueous solutions. It was found that the amount of metal ion adsorbed onto modified sepiolite increases with increase in solution equilibrium pH and temperature, whereas it generally decreases with the ionic strength. The experimental data were correlated reasonably well by the Langmuir adsorption isotherm and the isotherm parameters (q_m and K) were calculated. The ability to adsorb the cations gave a capacity order of Zn(II) > Cu(II) ~ Co(II) > Fe(III) > Mn(II) > Cd(II) with affinities of 2.167×10⁻⁴, 1.870×10⁻⁴, 1.865×10⁻⁴, 1.193×10⁻⁴, 0.979×10⁻⁴ and 0.445×10⁻⁴ mol g⁻¹, respectively.     

Development and applications of quaternary ammonium (QA) membrane electrodes in pharmaceutical preparation and in bioavailability of Prostaglandin E1 and Deoxycholate

The two novel ion-pairs (PB-TPB and NB-TPB) of quaternary ammonium drugs; propantheline bromide (PB), N,N-Diisopropyl-N-methyl-N-[2-(xanthen-9ylcarbonyloxy)ethyl] ammonium bromide and neostigmine bromide (NB), 3-(dimethylcarbamoyloxy) phenyl]-trimethylazanium have been synthesized, respectively and incorporated in poly (vinyl chloride)-based membrane electrodes for the quantification of propantheline bromide and neostigmine bromide in different pharmaceutical preparations. The influences of membrane compositions on the potentiometric responses of membrane electrodes have been found to substantially improve the performance characteristics. The best performance was reported with membranes having composition (w/w) of PB-TPB or NB-TPB (6%): PVC (34%): o-NPOE (60%). The proposed electrodes exhibit nernstian response in the concentration ranges of 2.1 × 10⁻⁷ M to 1.0 × 10⁻² M and 4.4 × 10⁻⁷ M to 1.0 × 10⁻² M with detection limit of 1.5 × 10⁻⁷ M and 3.3 × 10⁻⁷ M, respectively. Both the membrane electrodes perform satisfactorily over pH ranges of (3.5–7.5 and 4.0–7.0) with fast response times (11 s and 13 s), respectively. These drugs (PB and NB) were further utilized as different ion-pairs of Prostaglandin E₁ (PGE₁) and Deoxycholate (DOC) in poly (vinyl chloride)-based membrane electrodes for the determination of bioavailability of Prostaglandin E₁ and Deoxycholate in plasma of different patients.     

Electrochemical behaviour of N-acetyl-l-cysteine on gold electrode—A tentative reaction mechanism

The electrochemical behaviour of N-acetyl-l-cysteine (NAC) has been investigated by linear and cyclic voltammetry on gold electrode at room temperature. The results showed two oxidation peaks under acid and neutral conditions and only one in basic medium. For each oxidation, as many electron was exchanged as proton. The influence of both the concentration and the potential scan rate on the peak currents highlighted a diffusion-controlled phenomenon for the first peak and an adsorption-limited reaction rate for the second one. The diffusion coefficient of NAC in solution and the surface concentration of the adsorbed species at pH 3 and 7 were close to 2 × 10⁻⁴ to 2 × 10⁻⁵ cm² s⁻¹ and 6 × 10⁻⁹ to 6 × 10⁻¹⁰ mol cm⁻², respectively. Film transfer experiments resulted in an irreversible adsorption of NAC on gold electrode, and the formation of a self-assembled monolayer (SAM).     

Amperometric glucose biosensor with high sensitivity based on self-assembled Prussian Blue modified electrode

A glucose biosensor, which was based on self-assembled Prussian Blue (PB) modified electrode with glucose oxidase (GOD) immobilized in cross-linked glutaraldehyde matrix, was developed. Fourier-transform infrared spectroscopy shows that the immobilized GOD retains its native conformation. Cyclic voltammetry was used to examine the electrocatalytic property of the enzyme electrode. The prepared glucose biosensor exhibits fast response (<4 s) and low detection limit of 5 × 10⁻⁶ M. The calculated apparent Michaelis constant K_M was 6.3 ± 1.2 mM, indicating a high affinity between the GOD and glucose. The effects of glutaraldehyde concentration and GOD loading on the sensitivity of the glucose biosensor have also been investigated. Under the optimal conditions, the biosensor shows a high sensitivity of about 80 mA M⁻¹ cm⁻² in a concentration range up to 1 × 10⁻³ M. The relative standard deviation (RSD) for intra-electrode and inter-electrode were 4% and 5%, respectively. In addition, the anti-interferent ability and stability of the biosensor were also discussed.     

Magnetic properties of CNX whiskers

Carbon–nitrogen whiskers have been prepared by pyrolysis of 1,2-diaminopropane at 950 °C or of allylamine at 900 °C followed by quenching. They are scrolls of carbon film typically 250 nm thick and up to 1 mm long with about five layers in a structure like a “cigare russe” or “brandy snap”, about 50 μm in diameter. Approximately 8 wt% of nitrogen is incorporated into the carbon films, which are practically amorphous, exhibiting a broad diffraction peak at d = 0.34 nm. The whiskers are on the border of metallic conductivity with a resistivity of about 10⁻⁶ Ωm, and they may show either a positive or a negative temperature coefficient of resistance. The pyrolysis produces either whiskers, soot or both. Magnetization measurements of the whiskers made from 1,2-diaminopropane reveal a large diamagnetic susceptibility of χ = −170 × 10⁻⁹ m³  kg⁻¹ and a small ferromagnetic component of unknown origin with σ_S of up to 0.2 A m² kg⁻¹, whereas the soot shows a purely diamagnetic signal, with χ ≈ −40 × 10⁻⁹ m³ kg⁻¹.     

Photochemical production of nitric oxide from a nitrosyl phthalocyanine ruthenium complex by irradiation with light in the phototherapeutic window

The photochemical behavior of [Ru(NO)(NO)₂pc] (pc = phthalocyanine) is reported in this paper. In addition to ligand localized absorption bands (λ < 300 nm), the electronic spectrum of this complex in dichloromethane solution was dominated by an intense absorption at 640 nm characterized as Q-bands. Irradiation of [Ru(NO)(NO)₂pc] at 366 and 660 nm led to the production of nitric oxide (NO) as detected by a NO-sensor. NO production by light irradiation at high energy involved excitation of d_π–π^* transition, while a photoinduced electron transfer occurred at long wavelength irradiation. The NO quantum yields varied from 1.4 × 10⁻³ to 2.3 × 10⁻² mol einstein⁻¹, depending on oxygen concentration.     

Highly sensitive, reusable electrochemical aptasensor for adenosine

A reusable electrochemical aptasensor for highly sensitive detection of small molecules had been developed using adenosine as a model molecule. The sensing interface was fabricated by self-assembling the part DNA duplex hybridized by 5′-thiolated part complementary strand (TPCS) and 3′-ferrocene(Fc)-labeled adenosine-binding aptamer strand (FABA) through S–Au bonding on a gold electrode surface. When the modified electrode was incubated in the adenosine solutions, the aptamer made structure switching to bind adenosine. As a result, Fc-labeled adenosine-binding aptamer strand was taken off from the sensing interface, resulting in a decrease of the redox current. The aptasensor was characterized electrochemically by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV of the resulting aptasensor showed a linear response to the increase of the adenosine concentration in the range of 5.0 × 10⁻⁸–1.5 × 10⁻⁵ M with a linear correlation of r = 0.9898 and a detection limit of 1.65 × 10⁻⁸ M. Moreover, the aptasensor exhibited several excellent characteristics, such as high sensitivity, selectivity, good stability, and reproducibility.