The extraction of Cr(III) from aqueous thiocyanate solutions and its separation from Co(II) and Ni(II)

For the system liquid anion-exchanger—Cr(III)−NCS⁻, an investigation has been made of the dependence of the percentage extraction of Cr(III) on parameters such as standing time of the Cr(III)−NCS⁻ solution, temperature, pH and type of exchanger. Quantitative extraction of e.g. 4·10⁻⁴ M Cr(III) by 0.1M Aliquat in CCl₄ is easily achieved at room temperature, using 4.75M KNCS−0.05N HCl as aqueous phase. At high Cr(III) concentrations, the complex anion present in the organic phase is Cr(NCS) ₆ ³⁻ ; when working with dilute metal ion solutions, the species extracted is Cr(NCS)₄ (H₂O) ₂ ⁻ . Separations of mixtures containing 10⁻²−10⁻⁴ M Co(II), Ni(II) and Cr(III) have successfully been accomplished.     

Solvent extraction of hafnium(IV)

^{175, 181}Hafnium(IV) was extracted by HDBP in 2-ethylhexanol from 1–10M solutions of HClO₄, HCl and HNO₃, and 1–8M H₂SO₄. As with low polar organic phase diluents, the acidity dependence of the distribution ratio of Hf, D, passes through a minimum for HClO₄, HCl, and H₂SO₄ whereas only an increase of D can be observed with increasing HNO₃ concentration. From the slope analysis the following complexes were found to be extracted (HDBP=HA): HfA₄ at <4M HClO₄ and <5M HCl, lg K_extr=9, HfX₄(HA)₄ (X=ClO ₄ ⁻ , Cl⁻ or NO ₃ ⁻ ) at >5M HClO₄, >7M HCl and 1–10M HNO₃, Hf(SO₄)A₂(HA)_3–4 at <3M H₂SO₄, and Hf(SO₄)₂ (HA)₄ at >6M H₂SO₄. Coextraction of sulphate with hafnium from H₂SO₄ solutions was evidenced in experiments with macro concentrations of Hf(IV) and³⁵SO ₄ ²⁻ . Part XX: Coll. Czech. Chem. Commun., 40 (1975) 3617.     

Synthesis of pyridyl derivatives for the future functionalization of biomolecules labeled with the fac-[<Superscript>188</Superscript>Re(CO)<Subscript>3</Subscript>(H<Subscript>2</Subscript>O)<Subscript>3</Subscript>]<Superscript>+</Superscript> precursor

In this paper, we investigated three ligand systems, symmetric and asymmetric pyridyl-containing tridentate ligands (L¹NH₂ = (bis(2-pyridylmethyl)-amino)-ethylamine, L²H = (bis(2-pyridylmethyl)-amino)-acetic acid, L³NH₂ = [(6-amino-hexyl)-pyridyl-2-methyl-amino]-acetic acid) as bifunctional chelating agents for labeling biomolecules. These ligands reacted with the precursor fac-[¹⁸⁸Re(CO)₃(H₂O)₃]⁺ and yielded the radioactive complexes fac-[¹⁸⁸Re(CO)₃L] (L = three ligands), which were identified by RP-HPLC. The corresponding stable rhenium tricarbonyl complexes (1–3) were allowed for macroscopic identification of the radiochemical compounds. ¹⁸⁸Re tricarbonyl complexes, with log P _o/w values ranging from −1.36 to −0.32, were obtained with yields of ≥90% using ligand concentrations within the 10⁻⁶−10⁻⁴M range. Challenge studies with cysteine and histidine revealed the high stability properties of these radioactive complexes, and biodistribution studies in normal mice indicated a fast rate of blood clearance and high rate of total radioactivity excretion, primarily through the renal-urinary pathway. In summary, these asymmetric and symmetric pyridyl-containing tridentate ligands are potent bifunctional chelators for the future biomolecules labeling of fac-[¹⁸⁸Re(CO)₃(H₂O)₃]⁺.     

Mixed-ligand complexation of calcium and magnesium ions with heparin and glycine

Chemical equilibria in dilute aqueous solutions containing high-molecular-weight heparin (Na₄hep) and Glycine (HGly), as well as in solutions of the MCl₂-Na₄hep-HGly-H₂O-NaCl system (M = Ca²⁺, Mg²⁺) against the background of 0.15 M NaCl at 37°C, have been studied by mathematical modeling of chemical equilibria on the basis of pH-metric titration data. The model of equilibria of the Na₄hep-HGly-H₂O-NaCl system for the range 2.30 ≤ pH ≤ 10.50 at different ratios of initial heparin and glycine concentrations showed that, in the pH range of blood plasma stability (pH 6.80–7.40), the protonated H H₃hepGly₃⁴⁻ species prevailed. This was supported by UV absorption spectra of heparin and glycine solutions in the presence of 0.15 M NaCl and absorbance dynamics for solutions containing heparin and glycine. The results of modeling equilibria in the five-component MCl₂-Na₄hep-HGly-H₂O-NaCl systems (M = Ca²⁺, Mg²⁺) showed that the Ca²⁺ and Mg²⁺ ions form with heparin and glycine stable protonated mixed-ligand complexes M H₃hepGly₃²⁻. The formation constants of these species are one order of magnitude higher than the formation constants of the homoligand calcium and magnesium with heparin. In the pH range 6.80–7.40, the calcium content decreases depending on the ratio of the initial concentrations of Na₄hep, HGly, and CaCl₂: at the 1 : 3 : 1 ratio, it decreases by a factor of 5.7 owing to the formation of the predominant species CaH₃hepGly₃²⁻, and at equimolar amounts of the reagents (1 : 1 : 1), the calcium content decreases by a factor of 3.5 (the CaH₃hepGly₃²⁻ concentration is three time as low as the NaCahep⁻ concentration).     

Synthesis and biological activity of novel heavy metal complexes of 5-amino-1, 10-phenanthroline and 1,10-phenanthroline

Novel heavy metal complexes: Sr(5-NH₂-phen)₄(NO₃)(OH)(H₂O)₂ (1) (synthesized via a static self-assembly process) and Sn(phen)(NO₃)(OH)(H₂O) (2), Sn(5-NH₂-phen)(OH)(Cl)(H₂O) (3), Pb(5-NH₂-phen)(NO₃)₂(H₂O) (4) (obtained via metal competitive reactions under mild conditions) were reported. The coordination compounds were characterized by elemental analysis, FTIR-spectroscopy and FAB-mass spectrometry. Their cytotoxicity was measured by MTS-test towards human tumour (MDA-MB-231, HT-29, HeLa, HepG2) and non-tumour diploid (Lep-3) cell lines. The most pronounced cytotoxic effect on all cancer lines showed 1 and 4 at their high concentrations as well as 1 at its lower ones (≤ 4×10⁻² mg). Therefore, strontium complex of 5-amino-o-phenanthroline (1) exhibited the widest antitumour spectrum activity, having no toxicity to non-tumour cells at quantities ≤ 4×10⁻² mg. The computed EC₅₀ values of 1–4 against MDA-MB-231, HT-29, HeLa, HepG2 varied from 1.40×10⁻³ to 6.31×10⁻⁶ M. Towards Lep-3 substances 2–4 showed IC₅₀ 7.52×10⁻⁴ − 0.44 M. Substance 1 possess EC₅₀=1.26×10⁻⁷ M to the non-tumour cells.      

Thermodynamic Model for ThO<Subscript>2</Subscript>(am) Solubility in Alkaline Silica Solutions

No thermodynamic data for Th complexes with aqueous Si are available. To obtain such data, extensive studies on ThO₂(am) solubility were carried out as functions of: (1) a wide range of aqueous silica concentrations (0.0004 to 0.14 mol⋅L⁻¹) at fixed pH values of about 10, 11, 12, and 13; and (2) and variable pH (ranging from 10 to 13.3) at fixed aqueous Si concentrations of about 0.006 mol⋅L⁻¹ or 0.018 mol⋅L⁻¹. The samples were equilibrated over long periods (ranging up to 487 days), and the data showed that steady-state concentrations were reached in < 29 days. X-ray diffraction, FTIR, and Raman analyses of the equilibrated solid phases showed that the Th solids were amorphous ThO₂(am) containing some adsorbed Si. The solubility of ThO₂(am) at pH values ranging from 10 to 13.3 at fixed 0.018 mol⋅L⁻¹ aqueous Si concentrations decreases rapidly with an increase in pH, and increases dramatically with an increase in Si concentrations beyond about 0.003 mol⋅L⁻¹ at fixed pH values > 10. The data were interpreted using both the Pitzer and SIT models, and required only the inclusion of one mixed-hydroxy-silica complex of Th [Th(OH)₃(H₃SiO₄)₃²⁻]. Both models provided similar complexation constant values for the formation of this species. Density functional theory calculations predict complexes of this stoichiometry, having six-fold coordination of the Th cation, to be structurally stable. Predictions based on the fitted value of log ₁₀ K ⁰=−18.5±0.7 for the ThO₂(am) solubility reaction involving Th(OH)₃(H₃SiO₄)₃²⁻[ThO₂(am)+3H₄SiO₄+H₂O↔Th(OH)₃(H₃SiO₄)₃²⁻+2H⁺], along with the thermodynamic data for aqueous Si species reported in the literature, agreed closely with the extensive experimental data and showed that under alkaline conditions aqueous Si makes very strong complexes with Th.     

Amperometric hydrogen peroxide sensor based on a sol-gel-derived ceramic carbon composite electrode with toluidine blue covalently immobilized using 3-aminopropyltrimethoxysilane

A carbon composite amperometric hydrogen peroxide sensor has been developed using a sol-gel technique. Toluidine blue (TB), which acts as the redox mediator, was covalently immobilized via glutaraldehyde crosslinking with an organically modified silane, namely 3-aminopropyltrimethoxysilane (APTMOS). Methyltrimethoxysilane (MTMOS) was used as the additional monomer; this controls the hydrophobicity of the electrode surface, thus limiting the wettability. The immobilization of TB within the sol-gel matrix was confirmed with FTIR studies. The sol-gel mixture containing TB immobilized in APTMOS and MTMOS was mixed with graphite powder in order to prepare the carbon composite electrode. The electrode was characterized using voltammetric techniques and its electrocatalytic activity for the reduction of hydrogen peroxide was also studied. The carbon composite electrode has the advantage of sensing H₂O₂ at a lower potential and with a higher sensitivity, and interferences due to ascorbic acid, uric acid and acetaminophen were greatly minimized. The linear range for the determination of H₂O₂ extends from 5.37 × 10⁻⁶ to 6.15 × 10⁻³ M, with a correlation coefficient of 0.9981. The detection limit was found to be 2.15 × 10⁻⁶ M. The covalent immobilization of TB effectively prevents the leakage of the water-soluble mediator during measurements. The modified electrode, aside from electrocatalyzing the reduction of H₂O₂, exhibits distinct advantages in terms of surface renewal in the event of surface fouling, as well as simple preparation, good chemical and mechanical stability, and good reproducibility. Figure Amperometric hydrogen peroxide sensor based on sol-gel-derived ceramic carbon composite electrode with toluidine blue covalently immobilized using 3-aminopropyltrimethoxysilane Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Synthesis, in vitro and in vivo behavior of 188Re(I)-tricarbonyl complexes for the future functionalization of biomolecules

Radiolabeling of biologically active molecules with fac-[¹⁸⁸Re(CO)₃(H₂O)₃]⁺ unit has been of primary interest in recent years. Therefore, we herein report ligands L¹−L⁴ (L¹=histidine, L²=nitrilotriacetic acid, L³=2-picolylamine-N,N-diacetic acid, L⁴=bis(2-pyridymethy)amine) that have been evaluated by radiochemical reactions with fac-[¹⁸⁸Re(CO)₃(H₂O)₃]⁺. These reactions yielded the radioactive complexes of fac-[¹⁸⁸Re(CO)₃L] (L = L¹−L⁴, ¹⁸⁸Re tricarbonyl complexes 1–4), which were identified by HPLC. Complexes 1–4, with log P _o/w values ranging from −2.23 to 2.18, were obtained with yields of ≥95% using ligand concentrations within 10⁻⁶–10⁻⁴M range. Thus, specific activities of 220 GBq/μmol could be achieved. Challenge studies with cysteine and histidine revealed high stability for all of these radioactive complexes, and biodistribution studies in mice indicated a fast rate of blood clearance and high rate of total radioactivity excretion occurring primarily through the renal-urinary pathway. In summary, the ligands L¹–L⁴ are potent chelators for the future functionalization of biomolecules labeling with fac-[¹⁸⁸Re(CO)₃(H₂O)₃]⁺.     

Simultaneous determination of silicic acid, Ca, Mg and Al in mineral water and composite tablets by Ion chromatography

Summary A simple, selective and sensitive ion-chromatography method was investigated for simultaneously determining silicic acid, Ca²⁺, Mg²⁺, Al³⁺ and anions (Cl⁻ and NO ₃ ⁻ ) in real samples. It involved a single-column ion-chromatograph with sodium hydroxide-methanol-water eluent and conductometric detection. Cations were converted to complex anions by adding EDTA to the sample solution. A set of well-defined peaks of silicic acid, Ca²⁺, Mg²⁺, Al³⁺, Cl⁻ and NO ₃ ⁻ were obtained. Detection limits using 3.3σ (σ=standard deviation of blank solution) were 1.25×10⁻⁶ M for H₃SiO ₄ ⁻ , 1.32×10⁻⁶ M for Ca²⁺, 1.28×10⁻⁶ M for Mg²⁺, 1.33×10⁻⁶ M for Al³⁺, 1.31×10⁻⁶ M for Cl⁻ and 1.24×10⁻⁶ M for NO ₃ ⁻ . The method was successfully applied to analysis of mineral water and composite tablets.     

Surface charge of detonation nanodiamond particles in aqueous solutions of simple 1 : 1 Electrolytes

Adsorption isotherms of potential-determining H⁺ and OH⁻ ions and the pH dependences of the specific surface charge of detonation nanodiamond (DND) particles are obtained in a pH range of 3–10 by the acid-base titration of their hydrosols containing 0.001–1 M LiCl, NaCl, KCl, NaNO₃, KNO₃, and NaClO₄ as background electrolytes. The data obtained attest to the chemical nonuniformity (heterogeneity) of a DND surface and different degrees of binding of background electrolyte cations and anions with ionized groups. It is revealed that the adsorption of OH-anions diminishes in the lyotropic series of cations Na⁺ > K⁺ > Li⁺ and increases with a decrease in the adsorbability of anions in the following series: NO₃⁻ ≊ ClO₄⁻ > Cl⁻. The adsorption of potential-determining H⁺ and OH⁻ ions on a DND surface containing two types of functional groups, i.e., acidic carboxyl and amphoteric hydroxyl groups, is simulated by the Protofit software package. The optimal surface densities and ionization constants that correspond to minimal deviations of model adsorption isotherms from the experimental curves are found for these groups.     

Crystal and molecular structure of diphenyliodonium triiodide

A new salt diphenyliodonium triiodide (C₁₂H₁₀I₄) was obtained. The [C₁₂H₁₀I⁺][I₃⁻] compound was isolated as red brown crystals and studied by single-crystal X-ray diffraction. The structure of diphenyliodonium triiodide consists of separate, virtually linear I₃⁻ anions and C₁₂H₁₀I⁺ cations. Strong intermolecular anion-anion (I₃⁻…I₃⁻) and anion-cation (I₃⁻…I⁺) interactions in the crystal structure leads to a change in the symmetry of triiodide ions. The complex formation in the system organic cation iodide-elementary iodine was studied by spectrophotometry. The complex composition was found (1: 1), and the stability constant of the complex in chloroform was determined (loggB = 3.91).     

A new amperometric H<Subscript>2</Subscript>O<Subscript>2</Subscript> biosensor based on nanocomposite films of chitosan–MWNTs,hemoglobin, and silver nanoparticles

A new H₂O₂ biosensor was fabricated on the basis of nanocomposite films of hemoglobin (Hb), silver nanoparticles (AgNPs), and multiwalled carbon nanotubes (MWNTs)–chitosan (Chit) dispersed solution immobilized on glassy carbon electrode (GCE). The immobilized Hb displayed a pair of well-defined and reversible redox peaks with a formal potential (E ^θ′) of −22.5 mV in 0.1 M pH 7.0 phosphate buffer solution. The apparent heterogeneous electron transfer rate constants (k _s) in the Chit–MWNTs film was evaluated as 2.58 s⁻¹ according to Laviron’s equation. The surface concentration (Γ*) of the electroactive Hb in the Chit–MWNTs film was estimated to be (2.48 ± 0.25) × 10⁻⁹ mol cm⁻². Meanwhile, the Chit–MWNTs/Hb/AgNPs/GCE demonstrated excellently electrocatalytical ability to H₂O₂. Its apparent Michaelis–Menten constant (K _M^app) for H₂O₂ was 0.0032 mM, showing a good affinity. Under optimal conditions, the biosensors could be used for the determination of H₂O₂ ranging from 6.25 × 10⁻⁶ to 9.30 × 10⁻⁵ mol L⁻¹ with a detection limit of 3.47 × 10⁻⁷ mol L⁻¹ (S/N = 3). Furthermore, the biosensor possessed rapid response to H₂O₂ and good stability, selectivity, and reproducibility.     

Amperometric determination of xanthine in tea,coffee, and fish meat with graphite rod bound xanthine oxidase

A method is described for construction of an amperometric xanthine biosensor based on graphite rod modified through adsorption of xanthine oxidase. Enzymatically produced H₂O₂ from xanthine was split into 2H⁺ + O₂ + 2^e− at 0.6 V and the current was measured, which was directly proportional to xanthine concentration ranging from 1 ° 10⁻⁷ to 6 ° 10⁻⁷ M with a detection limit of 1 ° 10⁻⁷ M. The biosensor exhibited optimum response within 35 sec at pH 7.0 and 35°C. It was employed for determination of xanthine in tea leaves (0.9 ° 10⁻⁵−2.5 ° 10⁻⁵ mmol/g), coffee powder (3.2 μmol/g) and fish meat (90 mmol/g). The content of xanthine in fish meat increased 6.5 times with its storage at room temperature during 15 days. The enzyme electrode could be reused 200 times during the span of 30 days, when stored in reaction buffer at 4°C.     

Nickel,copper and manganese hexacyanoferrate with poly(3,4-ethylenedioxythiophene) hybrid film modified electrode for selectively determination of ascorbic acid

The NiHCF-PEDOT, CuHCF-PEDOT and MnHCF-PEDOT films were prepared on glassy carbon electrode (GCE) by multiple scan cyclic voltammetry and characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) techniques. The advantages of these films are demonstrated for selectivity detection of ascorbic acid using cyclic voltammetry and amperometric method. Interestingly, the NiHCF-PEDOT and CuHCF-PEDOT modified electrodes exhibited a wide linear response range (5 × 10⁻⁶−3 × 10⁻⁴ M, R ² = 0.9973 and 1.8 × 10⁻³−1.8 × 10⁻² M, R ² = 0.9924). The electrochemical sensors facilitated the oxidation of AA but not responded to other electroactive biomolecules such as dopamine, uric acid, H₂O₂, glucose. The difference is MnHCF-PEDOT/GCE that no response to AA. In addition, the NiHCF-PEDOT and CuHCF-PEDOT modified electrodes exhibited a distinct advantage of simple preparation, specificity, stability and reproducibility.     

Direct coating of quantum dots with silica shell

This paper describes a method for direct coating of fluorescent semiconductor nanoparticles with silica shell. The fluorescent semiconductor nanoparticles used were CdSe _x Te_1–x nanoparticles coated with ZnS and succeedingly surface-modified with carboxyl groups, or quantum dots (Q-dots). The Q-dots were silica-coated by performing sol–gel reaction of tetraethyl orthosilicate (TEOS) using NaOH as a catalyst in the presence of the Q-dots. Quasi-perfect Q-dots/silica core-shell particles were formed at 5.0 M H₂O and 4.0 × 10⁻⁴ M NaOH. Under these concentrations of H₂O and NaOH, the particle size of Q-dots/silica particles could be varied from 20.1 to 38.1 nm as the TEOS concentration increased from 2.5 × 10⁻⁴ to 50 × 10⁻⁴ M. The Q-dots/silica particles showed fluorescence as well as the uncoated Q-dots.     

Aggregation and sedimentation stability of microcrystalline cellulose suspensions in aqueous aluminum nitrate solutions

The aggregation and sedimentation stability of dilute suspensions of microcrystalline cellulose in aqueous solutions of Al(NO₃)₃(2 × 10⁻⁵–2 × 10⁻³ mol/l) is studied by the photometric method at pH 2–11. It is found that, in the absence of Al(NO₃)₃, microcrystalline cellulose suspensions are stable with respect to aggregation throughout the pH range in question. The addition of Al(NO₃)₃ induces the coagulation and accelerates the sedimentation of microcrystalline cellulose aggregates. At all concentrations of Al(NO₃)₃, the maximum loss in stability is observed at pH 7–9. Original Russian Text ? P.M. Mosur, A.V. Lorentsson, Yu.M. Chernoberezhskii, 2009, published in Kolloidnyi Zhurnal, 2009, Vol. 71, No. 4, pp. 566–568.     

Electrogeneration of polyluminol and chemiluminescence for new disposable reagentless optical sensors

A performant reagentless electrochemiluminescent (ECL) detection system for H₂O₂ is presented, based on an electropolymerized polyluminol film prepared under near-neutral conditions. Such an original polyluminol electrodeposition is reported for the first time and on a screen-printed electrode (SPE) surface. Electropolymerized luminol acts as an active luminophore of the electrochemiluminescent reaction, as the monomer does. Polymerization conditions have been optimized in order to obtain the best ECL responses to H₂O₂. By performing electrodeposition in a potentiostatic mode, at 425 mV vs. Ag|AgCl, in 0.1 mol L⁻¹ phosphate/0.1 mol L⁻¹ KCl pH 6 and 1 mmol L⁻¹ luminol, with a total charge of 0.5 mC, the linear range for H₂O₂ detection extends from 7.9 × 10⁻⁸ mol L⁻¹ to 1.3 × 10⁻³ mol L⁻¹. Such performant disposable reagentless easy-to-use miniaturized systems based on SPEs should be applicable to the electrochemiluminescent detection of many oxidase-substrate compounds. Figure An original polyluminol electrodeposition process on a screen-printed electrode surface is reported for the first time. The polymeric structure is demonstrated to behave as an electrochemiluminescent luminophore, allowing disposable reagentless easy-to-use optical sensors for hydrogen peroxide detection to be designed.     

Estimation of the specific content and nature of sorption sites of iron(<Emphasis Type="SmallCaps">III</Emphasis>) and zirconium(<Emphasis Type="SmallCaps">IV</Emphasis>) oxyhydroxides

The sorption of anions H₂PO₄ ⁻, HPO₄ ²⁻, PO₄ ³⁻, [Fe(CN)₆]³⁻, and [Fe(CN)₆]⁴⁻ from aqueous solutions on the surface of Fe^III and Zr^IV oxyhydroxide hydrogels freshly precipitated at pH 4–13 was studied. The region of sorbate concentrations was from 0.00025 to 0.06 mol L⁻¹. The plots of the anion uptakes vs. their equilibrium concentrations are represented by isotherms of the first type, which are well described by the Langmuir equation if the quantity of the amount adsorbed is expressed as mol-site g⁻¹. The maximum uptakes and constants of the Langmuir equation were calculated. The phosphate anions occupy the same number of sorption sites on the sorbents precipitated at different pH. The average specific content of sorption sites for the ferro- and zirconogels in the metastability period is independent of the pH of their precipitation, being 3.1·10⁻³ and 3.2·10⁻³ mol-site g⁻¹, respectively. The [Fe(CN)₆]³⁻ and [Fe(CN)₆]⁴⁻ anions are sorbed only on the positively charged sites of the hydrogels and occupy not more than 2·10 mol-site g⁻¹ in the studied interval of pH of precipitation. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1736—1741, August, 2005.     

Quartz Crystal Microbalance for Selenite Sensing Based on Growth of Cadmium Selenite Crystals Immobilized on a Monolayer of Phosphorylated 11-Mercapto-1-Undecanol

 A quartz crystal microbalance (QCM) sensor for selenite ions in aqueous solution was constructed based on crystal formation of cadmium selenite, immobilized with a self-assembly monolayer (SAM) of phosphorylated 11-mercapto-1-undecanol (MUD) on a QCM gold electrode surface. The mass change caused by the selective adsorption of selenite ions on the cadmium selenite crystals at the solid/solution interface was detected by the QCM. The response (−ΔF) of the modified QCM oscillator increased with increasing selenite ion concentrations in sample solutions, ranging from 9.7×10⁻⁵ to 9.0×10⁻⁴ M at pH 7.4. The synthetic process of anchoring cadmium selenite crystals on the phosphorylated MUD organic film was also followed by using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The atomic concentrations measured by XPS confirmed the crystal growth of cadmium selenite on the phosphorylated MUD SAM at the QCM gold electrode surface. From the AFM images, changes in surface topographic features were followed: the MUD SAM and phosphorylated MUD on the QCM gold electrode had similar surface roughness; however the difference for the cadmium selenite film on the phosphorylated MUD SAM was clearly seen. The observed QCM frequency change of the modified QCM oscillator per unit time was found to be proportional to the square of the supersaturation of cadmium selenite, indicating the crystal growth of cadmium selenite at the solid/solution interface. The modified QCM oscillator exhibited selectively strong QCM response to SeO₃ ²⁻ ion. In contrast, the responses to tested interfering anions were almost negligible. The order of anion selectivities of the present modified QCM sensor was SeO₃ ²⁻≫CO₃ ²⁻>SeO²⁻ ₄, SO₄ ²⁻, Br⁻, I⁻, NO₃ ⁻. These selectivities were basically attributable to the differences in solubility products and solubilities for the salts of each anion with cadmium (II) ion. Received May 12, 1998. Revision December 29, 1998.     

An amperometric hydrogen peroxide biosensor based on immobilization of horseradish peroxidase on an electrode modified with magnetic dextran microspheres

A new kind of magnetic dextran microsphere (MDMS) with uniform shape and narrow diameter distribution has been prepared from magnetic iron nanoparticles and dextran. Horseradish peroxidase (HRP) was successfully immobilized on the surface of an MDMS-modified glassy-carbon electrode (GCE), and the immobilized HRP displayed excellent electrocatalytic activity in the reduction of H₂O₂ in the presence of the mediator hydroquinone (HQ). The effects of experimental variables such as the concentration of HQ, solution pH, and the working potential were investigated for optimum analytical performance. This biosensor had a fast response to H₂O₂ of less than 10 s and an excellent linear relationship was obtained in the concentration range 0.20 μmol L⁻¹–0.68 mmol L⁻¹, with a detection limit of 0.078 μmol L⁻¹ (S/N = 3) under the optimum conditions. The response showed Michaelis–Menten behavior at larger H₂O₂ concentrations, and the apparent Michaelis–Menten constant was estimated to be 1.38 mmol L⁻¹. Moreover, the selectivity, stability, and reproducibility of the biosensor were evaluated, with satisfactory results. Figure Amperometric response of the biosensor to successive additions of H₂O₂ and the plot of amperometric response vs. H₂O₂ concentration