Linamarin sensors: interference-based sensing of linamarin using linamarase and peroxidase

An interference-based linamarin sensor is developed. Horseradish peroxidase (HRP) is adsorbed on a pyrolytic graphite (PG) electrode, and then linamarase from cassava is cross-linked with glutaraldehyde on the electrode surface. The prepared bienzyme electrode is poised at -300 mV vs Ag/AgCl for 40 s to reduce dissolved O(2) to H(2)O(2) at the PG surface. The potential is then stepped to 0 mV, at which point the accumulated H(2)O(2) is reduced, though the O(2) reduction does not proceed. Since the H(2)O(2) reduction is catalyzed by HRP, the transient cathodic current is inhibited by cyanide, which is liberated from linamarin by linamarase. Therefore, the transient current is a function of the linamarin concentration. This sensor responds to 1 × 10(-5)-5 × 10(-3) M linamarin and can estimate a linamarin concentration of a cassava extract.     

Prussian blue based nanoelectrode arrays for H(2)O(2) detection

We propose to form nanoelectrode arrays by deposition of the electrocatalyst through lyotropic liquid crystalline templates onto inert electrode support. Whereas Prussian Blue is known to be a superior electrocatalyst in hydrogen peroxide reduction, carbon materials used as electrode support demonstrate only a minor activity. We report on the possibility for nanostructuring of Prussian Blue by its electrochemical deposition through lyotropic liquid crystalline templates, which is noticed from atomic force microscopy images of the resulting surfaces. The resulting Prussian Blue based nanoelectrode arrays in flow injection analysis mode demonstrate a sub-part-per-billion detection limit (1 x 10(-)(8) M) and a linear calibration range starting exactly from the detection limit and extending over 6 orders of magnitude of H(2)O(2) concentrations (1 x 10(-)(8) to 1 x 10(-)(2) M), which are the most advantageous analytical performances in hydrogen peroxide electroanalysis.     

Cathodic detection of H2O2 using iodide-modified gold electrode in alkaline media

Oxidative chemisorption and cathodic stripping reductive desorption of iodide have been studied at a smooth polycrystalline gold (Au (poly)) electrode. Potential-dependent surface coverage of iodide has been controlled on the basis of its reductive desoprtion in 0.1 M KOH alkaline media and its quantitative oxidation to aqueous iodates in acidic media. The Au (poly) electrode surface catalyzes the decomposition of H2O2 to O2. Specific adsorption of iodide on the Au electrode inhibits fully the catalytic decomposition and electrochemical oxidation of H2O2 as well as the adsorption of unknown impurities and the oxidative degradation of the electrode surface by H2O2. A quantitative characterization/detection of H2O2 at the iodide-modified Au (poly) electrode in the alkaline media has, thus, been achieved. Performance of the electrode toward the detection of H2O2 with respect to response time and sensitivity as well as operational stability has been evaluated. It has a sensitivity of 0.272 mA cm(-2) mM(-1) in amperometric measurements with a detection limit of 1.0 x 10(-5) M H2O2, and the response time to achieve 95% of the steady-state current is <20 s. The effect of O2 in the air-saturated solution can be minimized by subtracting the additional current for the O2 reduction. Experimental measurements were based upon cyclic voltametric and amperometric techniques.     

Anodic deposition of colloidal iridium oxide thin films from hexahydroxyiridate(IV) solutions

A facile, in-situ deposition route to stable iridium oxide (IrO(x)·nH(2)O) nanoparticle thin films from [Ir(OH)(6)](2-) solutions is reported. The [Ir(OH)(6)](2-) solution, made by alkaline hydrolysis of [IrCl(6)](2-), is colorless and stable near neutral pH, and forms blue IrO(x)·nH(2)O nanoparticle suspensions once it is adjusted to acidic or basic conditions. IrO(x)·nH(2)O nanoparticle thin films are grown anodically on glassy carbon, fluorine-doped tin oxide, and gold electrodes by electrolyzing [Ir(OH)(6)](2-) solutions at +1.0-1.3 V versus Ag/AgCl. The thickness of the IrO(x)·nH(2)O films can be controlled by varying the concentration of [Ir(OH)(6)](2-) , the deposition potential, and/or the deposition time. These thin films are stable between pH 1 and 13 and have the lowest overpotential (η) for the oxygen evolution reaction (OER) of any yet reported. Near neutral pH, the Tafel slope for the OER at a IrO(x)·nH(2)O film/Au rotating disk electrode was 37-39 mV per decade. The exchange current density for the OER was 4-8 × 10(-10) A cm(-2) at a 4 mC cm(-2) coverage of electroactive Ir.     

Electrochemical synthesis and characterization of TiO(2) nanoparticles and their use as a platform for flavin adenine dinucleotide immobilization and efficient electrocatalysis

Here, we report the electrochemical synthesis of TiO(2) nanoparticles (NPs) using the potentiostat method. Synthesized particles have been characterized by using x-ray diffraction (XRD) studies, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results revealed that the TiO(2) film produced was mainly composed of rutile and that the particles are of a size in the range of 100 ± 50?nm. TiO(2) NPs were used for the modification of a screen printed carbon electrode (SPE). The resulting TiO(2) film coated SPE was used to immobilize flavin adenine dinucleotide (FAD). The flavin enzyme firmly attached onto the metal oxide surface and this modified electrode showed promising electrocatalytic activities towards the reduction of hydrogen peroxide (H(2)O(2)) in physiological conditions. The electrochemistry of FAD confined in the oxide film was investigated. The immobilized FAD displayed a pair of redox peaks with a formal potential of -0.42?V in pH?7.0 oxygen-free phosphate buffers at a scan rate of 50?mV?s(-1). The FAD in the nanostructured TiO(2) film retained its bioactivity and exhibited excellent electrocatalytic response to the reduction of H(2)O(2), based on which a mediated biosensor for H(2)O(2) was achieved. The linear range for the determination of H(2)O(2) was from 0.15 × 10(-6) to 3.0 × 10(-3)?M with the detection limit of 0.1 × 10(-6)?M at a signal-to-noise ratio of 3. The stability and repeatability of the biosensor is also discussed.     

Amperometric sensors for simultaneous superoxide and hydrogen peroxide detection

A two-channel sensor capable of almost instantaneous simultaneous detection of superoxide radical and hydrogen peroxide in the concentration range 10(-)(7)-10(-)(4) M is very important for understanding of a number of rapid kinetics processes. A glassy carbon working microelectrode covered by an electrodeposited polypyrrole/horseradish peroxidase (PPy/HRP) membrane was employed as a H(2)O(2) sensor. Another glassy carbon microelectrode covered by a composite membrane of an inside layer of PPy/HRP and an outside layer of superoxide dismutase was employed as a working electrode for superoxide detection. These two working electrodes with Pt counter and tungsten oxide (WO(3)) reference electrodes were contained in one 6 mm diameter Teflon cylinder. Simultaneous measurements were performed at a potential of -60 mV (vs WO(3) reference, pH 5.1). Additional sensor characterization was performed for pH 5.1-9.0. Superoxide sensor behavior as a function of membrane deposition conditions and coating time is reported. Sensors' mutual influence, selectivity, response times, linearity, stability, and sensitivity for hydrogen peroxide and superoxide are presented and discussed. A mathematical model of sensors' responses is proposed, with model calculation corresponding to experiment within 10%.     

Detection of approximately 10(3) copies of DNA by an electrochemical enzyme-amplified sandwich assay with ambient O(2) as the substrate

The electrochemical sandwich-type, enzyme-amplified assay of Zhang, Kim, and Heller (Anal. Chem. 2003, 75, 3267-3269) was simplified by replacing the amplifying horseradish peroxidase with bilirubin oxidase (BOD). BOD catalyzes the reduction of ambient O(2) to water and obviates the need for adding H(2)O(2). Femtomolar (10(-)(15) M) concentrations of DNA were detected at a 10-microm-diameter tip of a carbon fiber electrode. Correspondingly, a few thousand copies of DNA were detected in approximately 5-microL samples. The sandwich is formed in an electron-conducting redox hydrogel, to the polymer of which a DNA capture sequence is bound. Capture of the analyte DNA and its hybridization with a BOD-labeled complementary DNA sequence, electrically connects the BOD label to the electron-conducting redox polymer, which is in electrical contact with the electrode. Placing the BOD in contact with the redox polymer thus converts the noncatalytic base layer into a catalyst for the electroreduction of O(2) to water at +0.12 V (vs Ag/AgCl) (Figure 1). In an exemplary assay, approximately 3000 copies of the iron transporting sequence of the sit gene of Shigella flexneri were detected without PCR amplification.     

Equilibrium and kinetic behavior of Fe(CN)6(3-/4-) and cytochrome c in direct electrochemistry using a film electrode thin-layer transmission cell

We report on the design and performance of a thin-layer electrochemical cell optimized for use with optically transparent film electrodes in combination with UV/vis and IR transmission spectroscopic measurements. The cell allows for measurements under both aerobic and anaerobic conditions. The direct, unmediated electron transfer, as assessed by the current transient, and the corresponding optical response observed for the Fe(CN)(6)(3-/4-) couple were in good agreement with theoretical predictions for voltammetry and optical absorption by an analyte confined in a thin layer. Chronoamperometric and spectroscopic measurements of Fe(CN)(6)(3-/4-) on gold mesh electrode revealed fast kinetics strongly influenced by the electrolyte concentration. Maximal apparent rates exceeding 2 s(-1) in 1 M KCl were observed optically. The direct kinetic and thermodynamic behavior of cytochrome c was compared with several electrode materials using the cell. The results showed heme ligand-dependent changes in the protein-electrode interactions. Mid-UV/visible spectral changes upon redox transitions in native cytochrome c and its cyanide derivative, as well as dissociation of the ferrous cytochrome c-CN complex, are reported.     

Perchlorate-selective polymeric membrane electrode based on bis(dibenzoylmethanato)cobalt(II) complex as a neutral carrier

A synthesized bis(dibenzoylmethanato)Co(II) complex (Co(DBM)(2)), has been used as a ionophore for the preparation of a new perchlorate ion-selective electrode. The electrode exhibits a Nernstian response over the perchlorate concentration range of 8.0x10(-7)-1.0x10(-1)M with a slope of 60.3+/-0.5 mV per decade of concentration. The limit of detection as determined from the intersection of the extrapolated linear segments of the calibration plot is 5.6x10(-7)M. The electrode shows good selectivity towards perchlorate with respect to many common anions. The response time of the sensor is very fast (< or = 5s), and can be used for at least 2 months in the pH range of 2.0-9.0. The electrode was used to determine perchlorate in water and human urine. The interaction of the ionophore with perchlorate ions was demonstrated by UV-vis spectroscopy.     

A hydrogen peroxide biosensor based on peroxidase activity of hemoglobin in polymeric film

A Hydrogen peroxide (H2O2) biosensor, based on hemoglobin (Hb) and ortho-phenylenediamine (o-PD) gold electrode, was fabricated. Hb was immobilized onto the electrode surface by electrochemical polymerize method with o-PD. The designed biosensor showed a well defined redox peak which was attributed to the direct electrochemical response of Hb. The immobilized Hb exhibited an excellent electrocatalytical response to the reduction of hydrogen peroxide, enabling the sensitivity determination of H2O2. Factors and performances such as pH, potential, influencing the designed biosensor, were studied carefully. The amperometric detection of H2O2 was carried out at -300 mV in phosphate buffer solution (PBS) (0.1 M) with pH 6.0. This biosensor showed a fast amperometric response (less then 5 s) to H2O2. The levels of the (Relative standard deviation) RSDs (< 3.5%) for the entire analyses reflected a highly reproducible sensor performance. Using the optimized conditions, the detection limit of the biosensor was 1 x 10(-7) M and linear range was from 5 x 10(-6) to 1.25 x 10(-4) M. In addition, this sensor showed long-term stability and good sensitivity.     

Potential-pH diagram for "Leucobacter sp. Ch-1-Cr-H(2)O" system

Laboratory experiments were used to investigate the effects of initial pH and applied potential on the growth of Leucobacter sp. Ch-1 and Cr (VI) reduction and establish the pH-Eh diagram for Leucobacter sp. Ch-1-Cr-H(2)O system. The results showed that the preferred initial pH for Leucobacter sp. Ch-1 growth was from 7.0 to 11.0. At this pH range, 56.2-99.5% of Cr (VI) was reduced by Ch-1 strain. The applied potentials from -700 to 0 mV, from -800 to +300 mV and from -800 to +400 mV at 7.0, 9.0 and 11.0 of initial pH values were favorable for the bacterial growth. The corresponding ranges of applied potentials for bio-reduction of Cr (VI) were from -200 to 0 mV, from -800 to +200 mV, and from -700 to +100 mV at above initial pH values. In the potential-pH diagram, the region of initial pH and Eh for Cr (VI) bio-reduction was included in the region for Leucobacter sp. Ch-1 growth and the stable region of Cr(OH)(3), which implied that Cr (VI) could be reduced to trivalent chromium existing in the forms of Cr(OH)(3) precipitate under the presence of Leucobacter sp. Ch-1 in alkaline condition. The results suggest that Leucobacter sp. Ch-1 has potential application for remediation of Cr (VI) contamination sites.     

PVC-Based Mn(III) Porphyrin Membrane-Coated Graphite Electrode for Determination of Histidine

The construction, performance characteristics, and application of a polymeric membrane coated on a graphite electrode with unique selectivity toward histidine are reported. The electrode was prepared by incorporating chloro(5,10,15,20-tetraphenylporphyrinato) manganese(III) [Mn(TPP)Cl] into a plasticized poly(vinyl chloride) membrane. The influences of membrane composition, pH, and foreign ions were investigated. Calibration plots with near Nernstian slopes for histidine were observed, -55.4 mV/decade, over a linear range of four decades of concentration (1 × 10(-)(5) to 1 × 10(-)(1) M). The electrode has a detection limit of 5 × 10(-)(6) M histidine and shows a fast response time of about 1 min. The electrode shows high selectivities toward histidine over several amino acids and common inorganic anions.     

Sensing of H2O2 at low surface density assemblies of Pt nanoparticles in polyelectrolyte

Amperometric detection of H2O2 was studied at random arrays of 2.5 nm polyacrylate-capped Pt nanoparticles (NP) assembled in poly(diallydimethylammonium chloride), PDDA, as a function of NP surface coverage. The arrays were assembled by varying the adsorption time of PDDA-modified electrodes in the nanoparticles solution. Pt NP-on-PDDA assemblies exhibited significant sensitivity and stability facing constant anodic polarization and a low limit of detection at small Pt mass in submonolayer coverage. The current output was measured at approximately 0.5 A M(-1) cm(-2)(geom) over a linear range from 42 nM to 0.16 mM H2O2 at a loading of 0.87 microg(Pt)/cm(2) or an estimated coverage of 0.4 of an assumed monolayer, or higher, and decreased with decreasing NP surface density to 0.2 A M(-1) cm(-2)(geom) at a Pt loading of 190 ng/cm. On the other hand, the intrinsic sensitivity measured relative to the real Pt surface area increased with decreasing coverage and reached a significant limiting value of 0.9 A M(-1) cm(-2) real at approximately 190-380 ng/cm(2). The behavior shows a significant effective turnover rate per Pt site and mass (1 A M(-1)/microg of Pt) in loosely packed assemblies, while overlap of individual diffusion fields (of particles or islands) and inaccessibility of some active sites lowers the sensitivity per nanoparticle in densely packed arrays. The reported trend agrees with the behavior of ultramicroelectrode arrays.     

Voltammetric reduction and determination of hydrogen peroxide at an electrode modified with a film containing palladium and iridium

Cyclic voltammetry of a mixture containing 0.2 mM Na2IrCl6, 0.1 mM PdCl2, 0.2 M K2SO4, and 0.1 M HCl between 1.2 and -0.3 V vs Ag/AgCl for five cycles at 50 mV s-1 yields a stable film on a glassy carbon electrode. The reduction of hydrogen peroxide in 0.1 M KCl is diffusion controlled at that modified electrode. Calibration curves obtained at a 100 mV s-1 scan rate are linear in the range 0.2-1.8 mM H2O2. The slope, 28 microA L mmol-1, is independent of film thickness. Since dissolved oxygen is reduced at about the same potential as H2O2, -0.3 V, at the modified electrode, it will act as an interferent in solutions that are not deaerated; however, the currents are additive. A second limitation of the described procedure is that with the KCl electrolyte the immobilized film must be reoxidized prior to each measurement. Preliminary data are described which suggest that this problem is alleviated by switching to a basic supporting electrolyte.     

Poly(vinyl) chloride membrane copper-selective electrode based on 1-phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione

1-Phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione (H(2)L) was used as an effective ionophore for copper-selective poly(vinyl) chloride (PVC) membrane electrodes. Optimization of the composition of the membrane and of the conditions of the analysis was performed, and under the optimized conditions the electrode has a detection limit of 6.30×10(-7) M Cu(II) at pH 4.0 with response time 10s and displays a linear EMF versus log[Cu(2+)] response over the concentration range 2.0×10(-6) to 5.0×10(-3) M Cu(II) with a Nernstian slope of 28.80±0.11 mV/decade over the pH range of 3.0-8.0. The sensor is stable for 9 weeks and exhibits good selectivity with respect to alkali, alkali earth and transition metal ions (e.g. Na(+), K(+), Ba(2+), Ca(2+), Zn(2+), Cd(2+), Co(2+), Mn(2+), Ni(2+), Fe(2+), Al(3+)) in the 3.0-8.0 pH range. It was successfully applied for the direct determination of copper(II) in zinc, aluminum and nickel based alloys, in soils polluted by oil, and as an indicator electrode for potentiometric titration of copper ions with EDTA.     

Triiodide PVC Membrane Electrode Based on a Charge-Transfer Complex of Iodine with 2,4,6,8- Tetraphenyl-2,4,6,8-tetraazabicyclo[3.3.0]octane

A novel triiodide ion-selective electrode based on a charge-transfer complex of iodine with 2,4,6,8-tetraphenyl-2,4,6,8-tetraazabicyclo[3.3.0]octane as membrane carrier was prepared. The electrode has a linear dynamic range between 5.0 × 10(-)(2) and 3.5 × 10(-)(6) M, with a near-Nernstian slope of 54.7 ± 0.8 mV decade(-)(1) and a detection limit of 2.0 × 10(-)(6) M. The potentiometric response is independent of the pH of the solution in the pH range 4.0-10.5. The electrode possesses the advantages of low resistance, short conditioning time, fast response, and, especially, very good selectivities over a wide variety of other anions. The electrode can be used for at least 10 months without any considerable divergence in potentials. It was used as an indicator electrode in potentiometric titration of triiodide ions.     

Preconcentration and Voltammetric Determination of Mercury(II) at a Chemically Modified Glassy Carbon Electrode

In this work, the organic compound 2-mercaptobenzimidazole was covalently bound on the surface of a glassy carbon rod, via silanization, yielding a material capable of selectively complexing Hg(2+) ions. This material was applied as an electrode for voltammetric determination of mercury(II) following its nonelectrolytic preconcentration. After exchanging the medium, the voltammetric measurements were carried out by anodic stripping in the differential pulse mode (pulse amplitude, 50 mV; scan rate, 1.25 mV s(-)(1)) using 10(-)(2) mol L(-)(1) NaSCN solution as supporting electrolyte. An anodic stripping peak was obtained at 0.06 V (vs SCE) by scanning the potential from -0.3 to +0.3 V. After a 5 min preconcentration period in a pH 4.0 Hg(2+) solution, this electrode shows increasing voltammetric response in the range 0.1-2.2 μg mL(-)(1), with a relative standard deviation of 5% and a practical detection limit of 0.1 μg mL(-)(1) (5.0 × 10(-)(7) mol dm(-)(3)). Compared with the conventional stripping approach, this chemically modified glassy carbon electrode procedure presented good discrimination against interference from Cu(II) in up to 10-fold molar excess.     

Carbon nanotube composite coated platinum electrode for detection of Ga(III)

This study demonstrates the application of composite multi-walled carbon nanotube (MWCNT) polyvinylchloride (MWNT-PVC) based on 7-(2-hydroxy-5-methoxybenzyl)-5,6,7,8,9,10-hexahydro-2H benzo [b][1,4,7,10,13] dioxa triaza cyclopentadecine-3,11(4H,12H)-dione ionophore for gallium sensor. The sensor shows a good Nernstian slope of 19.68 ± 0.40 mV/decade in a wide linear range concentration of 7.9 × 10(-7) to 3.2 × 10(-2)M of Ga(NO(3))(3). The detection limit of this electrode is 5.2 × 10(-7)M of Ga(NO(3))(3). This proposed sensor is applicable in a pH range of 2.7-5.0. It has a short response time of about 10s and has a good selectivity over nineteen various metal ions. The practical analytical utility of this electrode is demonstrated by measurement of Ga(III) in river water.     

Kinetics of decolorization of an azo dye in UV alone and UV/H(2)O(2) processes

The decolorization of C.I. Acid Red 27 (AR27), a monoazo anionic dye, was studied in the ultraviolet radiation (UV) alone and UV plus hydrogen peroxide (UV/H(2)O(2)) processes. The experimental results indicated that the kinetics of both oxidation processes fit well by pseudo-first order kinetics. The reaction rate was sensitive to the operational parameters and increased with increasing H(2)O(2) concentration and light intensity. The reaction orders of H(2)O(2) concentration and light intensity in both processes were obtained with linear regression method. A regression model was developed for pseudo-first order rate constant (k(ap,UV/H(2)O(2))) as a function of the Cconcentration and UV light intensity. (k(ap,UV/H(2)O(2)))=(2 x 10(-4)I(0.75)(0) + k(3)I(1.38)(0)[H(2)O(2)](n)(0))phi(AR27). As a result of two opposing effects of H(2)O(2) concentration at low and high concentrations, n has a value of 0.49 and -0.39 and k(3) has a value of 3 x 10(-4) and 0.1 for the regions of 0 mg l(-1) < [H(2)O(2)](0) < 650 mg l(-1) and 650 mg l(-1) < [H(2)O(2)](0) < 1500 mg l(-1)1, respectively. PhiAR27 is the initial dye concentration correlation index for developing of model for different initial concentrations of AR27. This rate expression can be used for predicting k(ap,UV/H(2)O(2) at different conditions in UV alone and UV/H2O2 processes. The results show that UV alone cannot be an efficient method for decolorization of AR27 in comparison with UV/H(2)O(2) process, therefore the first term of the model can be neglected.     

Dumbbell-like PtPd-Fe(3)O(4) Nanoparticles for Enhanced Electrochemical Detection of H(2)O(2)

Dumbbell-like Pt(x)Pd(100-x)-Fe(3)O(4) nanoparticles (NPs) were synthesized and studied for electrocatalytic reduction and sensing of H(2)O(2). In 0.1 M phosphate buffered saline (PBS) solution, the 4-10 nm Pt(x)Pd(100-x)-Fe(3)O(4) NPs showed the Pt/Pd composition-dependent catalysis with Pt(48)Pd(52)-Fe(3)O(4) NPs having the best activity. The Pt(48)Pd(52)-Fe(3)O(4) NPs were tested for H(2)O(2) detection, and their H(2)O(2) detection limit reached 5 nM, which was suitable for monitoring H(2)O(2) generated from Raw 264.7 cells. These dumbbell-like PtPd-Fe(3)O(4) NPs are the most sensitive probe ever reported and can be used to achieve real-time quantitative detection of H(2)O(2) in biological environment for biological and biomedical applications.