Chong Mou Wang

Characterizations of Iron‐Containing Clay Modified Electrodes and Their Applications for Glucose Sensing

An amperometric biosensor was constructed on the basis of ruthenium purple-containing clay and glucose oxidase for the direct assay of glucose. These clay-modified electrodes were characterized by cyclic voltammetry and flow injection analysis techniques. The electrodes were very sensitive to the presence of glucose. Linear calibration curves between 10 μM and 15 mM with an electrode sensitivity of 3.88 μA/M and a detection limit of 10 μM were registered in aerated solutions (pH 5.1). Uric acid, ascorbic acid, and oxygen interfere very little with the detection of glucose. Long-term tests showed these clay electrodes were quite durable, e.g., the electrode sensitivity only decreases by ca. 25% over a period of six weeks.

Peroxidase mimicking: Fe(Salen)Cl modified electrodes, fundamental properties and applications for biosensing

Abstract Iron(III) N , N ′-bis(salicylidene)ethylenediamine (denoted as Fe(Salen) + ) was prepared and characterized for its application in chemical analysis. From the stability constant of Fe(III)(Salen) + (7.1×10 25 M −1 ) and the formal potentials of Fe 3+/2+ and Fe(Salen) +/0 , the stability constant of Fe(II)(Salen) was calculated to be 3×10 17 M −1 . This relatively weaker stability constant, compared with that of Fe(III)(Salen) + , led to the occurrence of the electron transfer reactions between Fe(Salen) and electron acceptors, like oxygen and H 2 O 2 . Experimental results supported this hypothesis, showing that the pseudo-first order rate constants for the reactions of Fe(II)(Salen) with O 2 (DMSO–H 2 O, v/v 4:1) and H 2 O 2 (pH 7) are 330 and 4400 M −1 s −1 , respectively. Because of this catalytic effect, a sensing electrode for glucose or uric acid was constructed on the basis of Fe(Salen) + and glucose oxidase (GOx) or uricase (UOx). According to the flow injection analysis (FIA), the detection limits were 1 μM for glucose at pH 7 and 0.1 μM for uric acid at pH 8.5, respectively. The linear response to each substrate covered a region of 1 μM–10 mM for glucose and 5–40 μM for uric acid. Fe(Salen) + might form a 1:1 adduct with β-cyclodextrin (β-CDx); the equilibrium constant was determined to be about 6 M −1 . Although this chemical equilibrium, in terms of the numerical value, was not significant, the formation of {Fe(Salen)} 2 O was effectively limited as β-CDx was incorporated.

Electrochemical characterization of the clay-enhanced luminol ecl reaction

Abstract Clay modified electrodes were prepared with iron-containing clay particles (montmorillonite K10) and characterized for their ability to promote electrogenerated chemiluminescence (ecl) in a solution containing luminol and hydrogen peroxide. Electrochemical impedance spectroscopic (EIS) measurements showed that the charge-transfer resistance measured for the clay electrode decreased systematically with the addition of luminol or H 2 O 2 , leading to a linear relationship between the exchange current and the concentration of H 2 O 2 or luminol in the region where [luminol]/[H 2 O 2 ] or [H 2 O 2 ]/[luminol]

Characterization of Iron‐Containing Clay Modified Electrodes and Their Applications for the Detection of Hydrogen Peroxide and Ascorbic Acid

Clays containing ruthenium purple (denoted clay/RP) were prepared and characterized by diffuse-reflectance ultraviolet-visible absorption spectroscopic and electrochemical methods. The characteristics of the clay-modified electrodes with or without RP were obtained and compared. The clay/RP electrodes displayed a remarkable ability for detection of hydrogen peroxide and ascorbate. When H 2 O 2 or ascorbic acid was present, the current response was dramatically enhanced and tended to a limiting value, leading to a linear relationship with the bulk activity of the substrate. The rate constants (pseudo-first-order) were determined from Δi/i o vs. v -1 plots and calculated to be 530 M -1 s -1 (pH 3.3) for H 2 O 2 , and 150 M -1 s -1 (pH 4) for ascorbic acid. Flow injection analysis showed that the detection limits for both substances were about 1 ppm at pH 5. Oxygen reduction causes little interference with the detection of H 2 O 2 , and this was tentatively ascribed to unfavorable electron transfer between oxygen and clay/RP particles

Characterizations of iron-rich clay modified electrodes and their applications in optical recognition

Abstract Optical recognition was studied with modified electrodes based on iron-rich clays. According to energy-dispersive X-ray spectroscopy (EDX), several clay minerals including montmorillonite K10 (mont. K10) contain a lot of iron. Cyclic voltammetry suggested that electrochemically active iron species exist in most iron-rich clays and are likely to reside at different sites. The associated electrochemical activity is strongly pH-dependent and photosensitive. Under UV irradiation (λ ≤ 420 nm), these iron species were activated, and a pronounced photocurrent resulted. When these electrodes were flow-inJEACted with 2-pyridylcarboxylic acids (λab,max = 260 nm), the originally monotonic photocurrent could be modulated into a more recognizable a.c. pattern and the 260 nm optical signal became distinguishable. The photoresponse was highly reproducible, and the response time (t90) was less than 10s.

Evidence and applications for electron transfer between vitamin K3 and oxygen

Abstract In this paper we describe the electrochemical properties of vitamin K 3 (denoted VK 3 ) in aerated solutions, and the application potential of VK 3 in the degradation of 4-chlorophenol, 3-chloropropanl and sodium 2,3-dichloropropionate. Cyclic voltammetry revealed that the cathodic current of VK 3 decreased systematically with the addition of 3-chloropropanol and sodium 2,3-dichloropropionate. Concomitantly, a pair of new waves grew at less negative potentials. This electrode behavior suggested that VK 3 might form 1:1 complexes with 3-chloropropanol and sodium 2,3-dichloropropionate. The equilibrium constants were estimated to be about 1000 and 100 M −l , respectively, for the reactions of VK 3 with 3-chloropropanol and sodium 2,3-dichloropropionate. Emission spectroscopy supported our postulate, although the determined equilibrium constants (10 4 M −l ) had deviations from those obtained based on the electrochemical method. Apart from these discrepancies, VK 3 was found to be an effective catalyst for the reduction of oxygen as it was reduced. By using superoxide dismutase (SOD) and amplex red as probes, we confirmed that superoxide anion radical could be produced in aerated VK 3 solutions. In the light of these properties, attempts were made using the reduced VK 3 to degrade 4-chlorophenol, 3-chloropropanol and sodium 2,3-dichloropropionate in aerated solutions. According to the in-situ monitoring of proton levels in aerated solutions biased at −0.80 V versus SCE, 4-chlorophenol, 3-chloropropanol and sodium 2,3-dichloropropionate could be oxygenated under the catalysis of VK 3 , Fe 2+ and SOD.

Is the iron center important? Comparison of the electrochemistry between poly-phen-NH2 and poly[Fe(phen-NH2)32+] modified electrodes

Abstract 5-Amino-1,10-phenanthroline (denoted phen-NH 2 ) and its iron(II) complex, Fe(phen-NH 2 ) 3 2+ , were prepared as polymer modified electrodes for electrochemical studies. According to the cyclic voltammograms and in situ electrochemical quartz-crystal-microbalance spectra (EQCM) recorded for Fe(phen-NH 2 ) 3 2+ , phen-NH 2 and 1,10-phenanthroline (denoted phen), Fe(phen-NH 2 ) 3 2+ can be immobilized on the Pt-sputtered crystal electrode via an anodic polymerization. This polymerization is likely to be initiated by the oxidation of the amino group in the ligand. Although phen-NH 2 can also be polymerized on the electrode surface, the resulting polymer film is less stable compared with its iron derivative. The poly[Fe(phen-NH 2 ) 3 2+ ] electrode showed a significant sensitivity to hydrogen peroxide, leading to a linear calibration curve up to 10 mM at pH 5. The detection limits reached a level of 10 μM. The rate constant (pseudo-first-order) of the reaction between the reduced poly[Fe(phen-NH 2 ) 3 2+ ] and H 2 O 2 was determined to be 470 M −1 s −1 at this pH. Due to this catalytic property, a glucose sensor was developed. Although experiments suggested that some catalytic sites in the polymer film might be buried by the bulky enzyme (GOx), this modified electrode showed a significant ability in the detection of glucose. The linear sensitivity covered a range of 0.1–60 mM at pH 5. The detection limits reached a level of 0.1 mM.

Poly[Fe(phen-NH2)3]2+ modified electrodes: proton-gated charge transfer reactions and applications in current rectification

Abstract Poly[Fe(phen-NH 2 ) 3 ] 2+ electrodes were prepared on Au-sputtered quartz crystals (9 MHz) by oxidizing iron(II) tris(5-amino-1,10-phenanthroline) (denoted Fe(phen-NH 2 ) 3 2+ ) complex in acetonitrile. According to the study with the electrochemical quartz crystal microbalance (EQCM) technique, the decisive step in this electrode preparation involved an anodic polymerization. The resulting polymer (denoted poly[Fe(phen-NH 2 ) 3 ] 2+ ) as prepared on the electrodes showed two redox waves in aqueous solutions. One was ascribed to the electron removal from the metal center (Fe II ), poly[Fe(phen-NH 2 ) 3 ] 3+/2+ , and the other one to an electron addition to the resulting polymer, poly[Fe(phen-NH 2 ) 3 ] 2+/+ . For the latter waves, the peak potential shifted to more negative values in a linear manner with an increased pH. From the slope (ca. 72 mV pH −1 ) and the half-height-peak width ( W 1/2 ≈120 mV), a one-electron-one-proton transfer reaction was ascribed. Because of this property, the poly[Fe(phen-NH 2 ) 3 ] 2+ electrode behaved as a pH sensor. The sensitivity covered a pH range from 2 to 10. Energetic probing with Fe(CN) 6 3−/4− , Ru(NH 3 ) 6 3+/2+ and methylviologen (MV 2+/+/0 ), in addition, revealed that this pH dependence could be employed to regulate the electron transfer taking place through the polyFe(phen-NH 2 ) 3 2+ film. As a result, current rectifications for Fe(CN) 6 3−/4− and Ru(NH 3 ) 6 3+/2+ were achieved. In this study a short-range interaction between MV 2+ and the polymeric Fe(phen-NH 2 ) 3 2+ film was also found. The associated Gibbs energy change was estimated to be −33 kJ. Long-term experiments, in addition, suggested that although a decrease in pH might deactivate the electrochemical activity of the adsorbate, the adsorbed MV 2+ did not diffuse away from the electrode. In consequence, a vivid ‘on-off’ pattern in terms of current versus time was found during the variation in pH.

Fabrication of Photomagnetic Carbon Surfaces via Redox Assembly

3-Aminophenylboronic acid (APBA) and the complex Ru(bpy)2(phendione)2+ (bpy = 2,2′-bipyridine, phendione = 1,10-phenanthroline-5,6-dione) were found to be useful building blocks for preparing photomagnetic carbon surfaces. Scanning tunneling microscopy (STM) showed that when APBA was diazotized in acidic sodium nitrite solutions and cathodically reduced with highly ordered pyrolytic graphite (HOPG) electrodes, nanoscale films formed on the electrodes. The resulting HOPG had strong affinities for phendione and Ru(bpy)2(phendione)2+ as the electrodes were biased in the presence of them, respectively, with voltages more negative than the cathodic peak potentials for phendione/phendiol and Ru(bpy)2(phendione)2+/Ru(bpy)2(phendiol)2+ (phendiol = 1,10-phenanthroline-5,6-diol). However, if APBA was excluded, the affinities did not exist. Boronate ester formation featured prominently in these intermolecular interactions. The average increments in the HOPG surface roughness contributed by APBA and Ru(bpy)2(phendione)2+ were roughly 1 : 2, suggesting that the reaction stoichiometry between APBA and Ru(bpy)2(phendione)2+ be 1 : 1. Ru(bpy)2(phendione)2+ could also be grafted to carbon nanotubes (CNTs) under conditions similar to those for the HOPG using ascorbate as sacrificial donor. The resulting CNTs and HOPG exhibited photomagnetism when exposed to the 473 nm light. The ruthenium complex was shown to be a room-temperature photomagnetism precursor, and APBA was shown to be an effective molecular bridge for the complex and carbon substrates.