Evaluation of an amperometric glucose biosensor based on a ruthenium complex mediator of low redox potential

An amperometric glucose ring-disk biosensor based on a ruthenium complex mediator of low redox potential was fabricated and evaluated. This thin-layer radial flow microsensor (10 μl) with ring-disk working electrode displayed remarkable amperometric sensitivity. For Ru₃(μ₃-O)(AcO)₆(Py)₃(ClO₄) (Ru-Py), a trinuclear oxo-acetate bridged cluster, a reversible redox curve of low redox potential and narrow potential window (redox potentials were −0.190 and −0.106 V versus Ag/AgCl wire, respectively) was observed, which is comparable to many reported mediators such as ferrocene derivatives and other ruthenium complexes. The glucose and hydrogen peroxide assays were carried out with this complex-modified electrode Ru-Py-HRP-GOx/Nafion. The sensitivity was obtained 24 nA (15.4 mA M⁻¹ cm⁻²) for 10 μM glucose and 126 nA (160 mA M⁻¹ cm⁻²) for 5 μM H₂O₂, respectively with a working potential at 0 V versus Ag/AgCl. Ascorbic acid was studied as interference to the glucose assay. The application of 0 V potential versus Ag/AgCl did not avoid the occurrence of the oxidation of ascorbic acid, however, the pre-coating of ascorbate oxidase on the disk part of the ring-disk working electrode efficiently pre-oxidized the ascorbic acid and hence eliminated its interference on the glucose response. The practical reliability was also evaluated by assaying the dialysate from the prefrontal cortex of Wistar rats.     

Enthalpies of combustion and formation of 3-formylchromones

The enthalpies of combustion of 3-formylchromone (3F), 3-formyl-6-methylchromone (3F6M) and 3-formyl-6-isopropylchromone (3F6I) were determined by combustion calorimetry. The molar combustion energies () of the 3F, 3F6M and 3F6I are: −(4452.4 ± 1.8), −(5115.6 ± 2.7) and −(6411.4 ± 2.5) kJ mol⁻¹, respectively. The formation enthalpies in the crystalline state () are: −(340.2 ± 2.2), −(355.1 ± 3.1) and −(415.5 ± 3.0) kJ mol⁻¹, respectively.s     

Design of a composite amperometric enzyme electrode for the control of the benzoic acid content in food

A graphite–Teflon–tyrosinase composite biosensor for the determination of benzoic acid in foodstuffs is reported. The biosensor functioning is based on the inhibition effect of benzoic acid on the biocatalytic activity of the enzyme in a reversed micelle working medium formed with ethyl acetate as the continuous phase, a 0.05 mol l⁻¹ phosphate buffer solution of pH 7.4 (5%) as the aqueous dispersed phase, and 0.10 mol l⁻¹ dioctyl sulfosuccinate (AOT) as the emulsifying agent. A potential value of −0.10 V, and a constant enzyme-substrate (phenol) concentration of 2.0×10⁻⁴ mol l⁻¹ were selected to carry out the amperometric inhibition measurements. The tyrosinase inhibition process by benzoic acid is reversible and of the competitive type, with an apparent inhibition constant of 0.016 mmol l⁻¹. The composite bioelectrodes allow the regeneration of the electrode surface by polishing and exhibit long-term operation and stability. A limit of detection of 9.0×10⁻⁷ mol l⁻¹ benzoic acid was obtained. An interference study from other substances which can be found in foodstuffs together with benzoic acid was performed. Taking advantage of the capabilities of reversed micelles as universal solubilization media, the composite tyrosinase electrode was used for the determination of benzoic acid in two different kind of samples: mayonnaise sauce, which is a highly hydrophobic matrix, and Cola soft drinks, a hydrophilic matrix for which practically no sample treatment is necessary.     

Enzymatic biosensor for the electrochemical detection of 2,4-dinitrotoluene biodegradation derivatives

In this work, we demonstrate for the first time that 4-methyl-5-nitrocatechol (4M5NC) and 2,4,5-trihydroxytoluene (2,4,5-THT), two compounds obtained from the 2,4-DNT biodegradation are recognized by polyphenol oxidase as substrates. An amperometric biosensor is described for detecting these compounds and for evaluating the efficiency of the 2,4-DNT conversion into 4M5NC in the presence of bacteria able to produce the 2,4-DNT-biotransformation. The biosensor format involves the immobilization of polyphenol oxidase into a composite matrix made of glassy carbon microspheres and mineral oil. The biosensor demonstrated to be highly sensitive for the quantification of 4M5NC and 2,4,5-THT. The analytical parameters for 4M5NC are the following: sensitivity of (7.5 ± 0.1) × 10⁵ nAM⁻¹, linear range between 1.0 × 10⁻⁵ and 8.4 × 10⁻⁵ M, and detection limit of 4.7 × 10⁻⁶ M. The sensitivity for the determination of 2,4,5-THT is (6.2 ± 0.6) × 10⁶ nAM⁻¹, with a linear range between 1.0 × 10⁻⁶ and 5.8 × 10⁻⁶ M, and a detection limit of 2.0 × 10⁻⁷. Under the experimental conditions, it was possible to selectively quantify 4M5NC even in the presence of a large excess of 2,4-DNT. The suitability of the biosensor for detecting the efficiency of 2,4-DNT biotransformation into 4M5NC is demonstrated and compared with HPLC-spectrophotometric detection, with very good correlation. This biosensor holds great promise for decentralized environmental testing of 2,4-DNT.     

An alcohol oxidase biosensor using PNR redox mediator at carbon film electrodes

A new amperometric biosensor for ethanol monitoring has been developed and optimised. The biosensor uses poly(neutral red) (PNR), as redox mediator, which is electropolymerised on carbon film electrodes and alcohol oxidase (AlcOx) from Hansenula polymorpha as recognition element, immobilised by cross-linking with glutaraldehyde (GA) in the presence of bovine serum albumin (BSA) as carrier protein. Optimisation of variables affecting the system was performed and, for chronoamperometric measurements, a potential of −0.300 V versus saturated calomel electrode was chosen in 0.1 M sodium phosphate buffer saline at pH 7.5. The optimised biosensor showed a good sensitivity of 171.8 ± 14.8 nA mM⁻¹ and the corresponding detection limit (signal-to-noise-ratio = 3) of 29.7 ± 1.5 μM. Stability studies showed a good preservation of the bioanalytical properties of the sensor, 57.6% of its initial sensitivity remaining after 3 weeks (the sensor was used two to three times per week). No significant interferences were found from compounds usually present in wine. The biosensor was used for the determination of ethanol in Portuguese red and white wines.     

Penicillamine determination using a tyrosinase micro-rotating biosensor

Tyrosinase [EC 1.14.18.1], immobilized on a rotating disk, catalyzed the oxidation of catechols to o-benzoquinone, whose back electrochemical reduction was detected on glassy carbon electrode surface at −150 mV versus Ag/AgCl/NaCl 3 M. Thus, when penicillamine (PA) was added to the solution, this thiol-containing compound participate in Michael type addition reactions with o-benzoquinone to form the corresponding thioquinone derivatives, decreasing the reduction current obtained proportionally to the increase of its concentration. This method could be used for sensitive determination of PA in drug and human synthetic serum samples. A linear range of 0.02–80 μM (r = 0.999) was obtained for amperometric determination of PA in buffered pH 7.0 solutions (0.1 M phosphate buffer). The biosensor has a reasonable reproducibility (R.S.D. < 4.0%) and a very stable amperometric response toward this compound (more than 1 month).     

A sensitive method for the detection of catecholamine based on fluorescence quenching of CdSe nanocrystals

A sensitive method for the detection of catecholamine based on the fluorescence quenching of CdSe nanocrystals was developed. The sodium citrate-protected CdSe nanocrystals were synthesized in water solution. The fluorescence quenching of CdSe nanocrystals by dopamine, uric acid, ascorbic acid and catechol was studied; the results showed that all of these four kinds of compounds could quench the fluorescence of nanocrystals, and the quenching constant was 6.3 × 10⁴, 2.57 × 10³, 2.14 × 10³ and 1.168 × 10³, respectively. The order of sensitivity for the biosensor was: dopamine > lactic acid > ascorbic acid > catechol. This method shows good selectivity for dopamine, the detection limit reaches 5.8 × 10⁻⁸ M.     

A peroxidase-tetrathiafulvalene biosensor based on self-assembled monolayer modified Au electrodes for the flow-injection determination of hydrogen peroxide

The construction and performance under flow-injection conditions of an integrated amperometric biosensor for hydrogen peroxide is reported. The design of the bioelectrode is based on a mercaptopropionic acid (MPA) self-assembled monolayer (SAM) modified gold disk electrode on which horseradish peroxidase (HRP, 24.3 U) was immobilized by cross-linking with glutaraldehyde together with the mediator tetrathiafulvalene (TTF, 1 μmol), which was entrapped in the three-dimensional aggregate formed.

The amperometric biosensor allows the obtention of reproducible flow injection amperometric responses at an applied potential of 0.00 V in 0.05 mol L⁻¹ phosphate buffer, pH 7.0 (flow rate: 1.40 mL min⁻¹, injection volume: 150 μL), with a range of linearity for hydrogen peroxide within the 2.0 × 10⁻⁷–1.0 × 10⁻⁴ mol L⁻¹ concentration range (slope: (2.33 ± 0.02) × 10⁻² A mol⁻¹ L, r = 0.999). A detection limit of 6.9 × 10⁻⁸ mol L⁻¹ was obtained together with a R.S.D. (n = 50) of 2.7% for a hydrogen peroxide concentration level of 5.0 × 10⁻⁵ mol L⁻¹. The immobilization method showed a good reproducibility with a R.S.D. of 5.3% for five different electrodes. Moreover, the useful lifetime of one single biosensor was estimated in 13 days.

The SAM-based biosensor was applied for the determination of hydrogen peroxide in rainwater and in a hair dye. The results obtained were validated by comparison with those obtained with a spectrophotometric reference method. In addition, the recovery of hydrogen peroxide in sterilised milk was tested.     

Theoretical study of activation Fe–O bond of FeO by CO in the gas phase

The entire reaction mechanism for the gas phase CO–CO₂ conversion by FeO⁺ is discussed by means of the density functional theory and the intrinsic reaction coordinate approach. The calculated results have strongly indicated that the reaction of is a spin-forbidden reaction between the quartet and the sextet potential energy surfaces (PESs). There is a crossing point between the quartet and the sextet potential energy surfaces which may play a significant role in this reaction, by which the activation energy can be decreased from −15.1 to −56.4 kJ mol⁻¹ at the reaction system.     

Phase equilibria and crystal structure of the solid solution LaFe1−xNixO3−δ (0 ≤ x ≤ 1)

Phase equilibria in the LaFeO₃–“LaNiO₃” were studied at 1100 °C in air. The samples were synthesized by standard ceramic and/or solution route via nitrate or citrate precursors. According to the results of XRD it was found that the homogeneity ranges of LaFe_1−xNi_xO_3−δ solid solution lay within 0.0 ≤ x ≤ 0.4 (sp.gr. Pbnm) and 0.6 ≤ x ≤ 0.8 (sp.gr. ). The structural parameters (bond lengths, atom coordinates) for the single-phase samples were refined using Rietveld analysis. The unit cell parameters versus LaFe_1−xNi_xO_3−δ composition are presented.     

Highly selective dopamine quantification using a glassy carbon electrode modified with a melanin-type polymer

A highly selective dopamine quantification at a new polymer-modified electrode in the presence of large excess of ascorbic acid and 3,4-dihydroxyphenyl acetic acid (dopac) is described. The electrochemical detection was performed at a glassy carbon electrode modified with a melanin-type polymer obtained by polymerization of 3.0×10⁻³ M -dopa in 0.050 M phosphate buffer solution pH 7.40 by applying 1.00 V for 60 min. The polymeric film exhibits attractive permselectivity excluding anionic species such as potassium ferricyanide, ascorbic acid, dopac and uric acid. Cationic species such as epinephrine, norepinephrine and dopamine and neutral ones such as catechol and hydrogen peroxide can be oxidized at the polymer-modified electrode. The use of ascorbic acid in the measurement solution allows the amplification of dopamine oxidation signal due to the reduction of the electrochemically generated dopaminequinone. By using 1.0×10⁻³ M ascorbic acid, the detection limit for dopamine is 5.0 nM. The interference for the maximum physiological concentrations of ascorbic acid and dopac in nervous centers, i.e. 500 μM ascorbic acid and 50 μM dopac is 8.1 and 1.4%, respectively.     

Determination of organophosphate pesticides at a carbon nanotube/organophosphorus hydrolase electrochemical biosensor

An amperometric biosensor for oganophosphorus (OP) pesticides based on a carbon nanotube (CNT)-modified transducer and an organophosphorus hydrolase (OPH) biocatalyst is described. A bilayer approach with the OPH layer atop of the CNT film was used for preparing the CNT/OPH biosensor. The CNT layer leads to a greatly improved anodic detection of the enzymatically generated p-nitrophenol product, including higher sensitivity and stability. The sensor performance was optimized with respect to the surface modification and operating conditions. Under the optimal conditions the biosensor was used to measure as low as 0.15 μM paraoxon and 0.8 μM methyl parathion with sensitivities of 25 and 6 nA/μM, respectively.     

In situ electrodeposited nanoparticles for facilitating electron transfer across self-assembled monolayers in biosensor design

Gold nanoparticles (AuNPs) were synthesized in situ and electrodeposited onto Au substrate. The AuNPs modified interface facilitates electron transfer across self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA). After activation of surface carboxyl groups with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide, the interface displayed good stability for immobilization of biomolecules. These modification processes were characterized by contact angle measurement, cyclic voltammetry and electrochemical impedance spectra. The immobilized acetylcholinesterase (AChE), as a model, showed excellent activity to its substrate, leading to a stable AChE biosensor. Under the optimal experimental conditions, the inhibition of malathion on AChE biosensor was proportional to its concentration in two ranges, from 0.001 to 0.1 μg mL⁻¹ and from 0.1 to 25 μg mL⁻¹, with detection limit of 0.001 μg mL⁻¹. The simple method showed good reproducibility and acceptable stability, which had potential application in biosensor design.     

Detection of zeptomolar concentrations of alkaline phosphatase based on a tyrosinase and horse-radish peroxidase bienzyme biosensor

A bienzyme biosensor based on tyrosinase and horse-radish peroxidase is described in a flow injection analysis and cyclic voltammetry for measurement of phenol. Tyrosinase and horse-radish peroxidase were immobilized on the surface of a glassy carbon electrode by bovine serum albumin and glutaric dialdehyde. Phenol was oxidized by tyrosinase and horse-radish peroxidase via catechol to o-quinone in the presence of oxygen and hydrogen peroxide. The o-quinone was reduced to produce catechol (the substrate recycling) on the electrode surface. The enhanced sensitivity of the bienzyme electrode to phenol was observed in the flow injection system comparing with tyrosinase and horse-radish peroxidase monoenzyme electrodes. The mechanisms for enhanced amperometric response to phenol of bienzyme electrode were discussed. The biosensor was used to detect alkaline phosphatase (ALP). A detection limit of 1.4×10⁻¹⁵ M ALP (140 zmol/100 μl) was obtained after 1 h incubation with phenyl phosphate.     

Structure and redox properties of CexPr1−xO2−δ mixed oxides and their catalytic activities for CO, CH3OH and CH4 combustion

A series of Ce_xPr_1−xO_2−δ mixed oxides were synthesized by a sol–gel method and characterized by Raman, XRD and TPR techniques. The oxidation activity for CO, CH₃OH and CH₄ on these mixed oxides was investigated. When the value x was changed from 1.0 to 0.8, only a cubic phase CeO₂ was observed. The samples were greatly crystallized in the range of the value x from 0.99 to 0.80, which is due to the formation of solid solutions caused by the complete insertion of Pr into the CeO₂ crystal lattices. Raman bands at 465 and 1150 cm⁻¹ in Ce_xPr_1−xO_2−δ samples are attributed to the Raman active F_2g mode of CeO₂. The broad band at around 570 cm⁻¹ in the region of 0.3 ≤ x ≤ 0.99 can be linked to oxygen vacancies. The new band at 195 cm⁻¹ may be ascribed to the asymmetric vibration caused by the formation of oxygen vacancies. The TPR profile of Pr₆O₁₁ shows two reduction peaks and the reduction process is followed: . The reduction temperature of Ce_xPr_1−xO_2−δ mixed oxides is lower than those of Pr₆O₁₁ or CeO₂. TPR results indicate that Ce_xPr_1−xO_2−δ mixed oxides have higher redox properties because of the formation of Ce_xPr_1−xO_2−δ solid solutions. The presence of the oxygen vacancies favors CO and CH₃OH oxidation, while the activity of CH₄ oxidation is mostly related to reduction temperatures and redox properties.     

A new oxamato-bridged heterotrinuclear NiCuNi complex with irregular spin state structure: Synthesis, spectroscopy, crystal structure and magnetism

A new oxamato-bridged Ni^IICu^IINi^II species, [Ni(tacn)(H₂O)]₂[μ-Cu(pba)](ClO₄)₂·6H₂O 1, (tacn=1,4,7-triazacyclononane; pba=1,3-propylenebis(oxamato)) has been synthesized and structurally as well as magnetically characterized. Complex 1 has a discrete trinuclear Ni^IICu^IINi^II structure: Two nickel(II) ions are bridged by [Cu(pba)]²⁻ anion, macrocyclic ligand tacn works as terminal ligand of the nickel(II) center. The magnetic data of compound 1 was analyzed by means of

leading to g_Cu=2.19, g_Ni=2.18, J=−112.8 cm⁻¹, D=±4.31 cm⁻¹. The parameters are compared with the similar complexes and the irregular spin state structure of complex 1 is also described here.     

Compact amperometric algal biosensors for the evaluation of water toxicity

Unicellular microalga Chlorella vulgaris was entrapped in an alginate gel or a polyion complex membrane immobilized directly on the surface of a transparent indium tin oxide electrode. Photosynthetically generated oxygen of the immobilized algae was monitored amperometically. Responses of the algal biosensor to four toxic compounds, 6-chloro-N-ethyl-N-isopropyl-1,3,5-triazine-2,4-diamine (atrazine), 3-(3,4-dichlorophenyl)-1,1-diethylurea (DCMU), toluene and benzene, were evaluated as inhibition ratios of the reduction current. The concentrations that give 50% inhibition of the oxygen reduction current (IC^′₅₀) for atrazine, DCMU, toluene and benzene were 2.0, 0.05, 1550 and 3000 μmol dm⁻³, respectively. There was a good correlation between these data and those of the conventional standard growth test. In comparison with the conventional algal biosensors based on the Clark-type oxygen electrode, the present sensor is much smaller and less expensive, and its assay time is much shorter (≤200 s).     

Excess molar enthalpies of binary mixtures containing ethylene glycols or poly(ethylene glycols) + ethyl alcohol at 308.15 K and atmospheric pressure

Excess molar enthalpies, , of binary mixtures containing ethylene glycols and poly(glycols) + ethyl alcohol were measured by a flow microcalorimeter at 308.15 K and at atmospheric pressure over the whole composition range. Binary mixtures contain ethyl alcohol + ethylene glycol, + di(ethylene glycol), + tri(ethylene glycol), + tetra(ethylene glycol), + poly(ethylene glycol)-200, + poly(ethylene glycol)-300, + poly(ethylene glycol)-400, + poly(ethylene glycol)-600. Effects of the molecular weight distribution (MWD), of the polymer were investigated too, by preparing three additional samples of poly(ethylene glycol) with the same number average molecular weight (M_n ≈ 300), but different MWD. For all mixtures, results were fitted to the Redlich–Kister polynomial. curves are asymmetrical, showing positive values which vary from 280 J mol⁻¹ (diethylene glycol + ethyl alcohol) to 1034 J mol⁻¹ (mixture containing PEGs (200 + 400) + ethyl alcohol). Effects of changes in the glycols chain length and in MWD on the molecular interactions among the mixture components are discussed.     

Preparation of MoVTe(Sb)Nb mixed oxide catalysts using a slurry method for selective oxidative dehydrogenation of ethane

The preparation of bulk MoVTe(Sb)Nb mixed oxide catalysts using a traditional slurry method, results in highly active catalysts for oxidative dehydrogenation of ethane to ethene. Several major phases including orthorhombic M1, hexagonal M2 or Mo_xM_1−xO_2.8 (M = V or Nb) have been detected in the catalysts from characterization results such as X-ray diffraction (XRD), SEM and EDX analyses. Ethane conversion and yield to ethene increase with increasing content of the M1 phase in the catalysts. The maximum yield of ethene (ca. 87% selectivity and ca. 90% conversion, STY_C2H4 of 176 g  h⁻¹) has been obtained with a MoV_0.31Te_0.2Nb_0.14 mixed oxide catalyst, calcined at 873 K under nitrogen, containing almost pure orthorhombic M1 phase and small amounts of unidentified impurity phases, operating at a relatively low reaction temperature of 673 K. The orthorhombic M1 phase has been shown to be the most active in ethane activation and the most selective for ethene formation. The hexagonal M2 phase is relatively inactive in ethane activation and less selective for ethene formation. The Te-free phases such as Sb₄Mo₁₀O₃₁ and Mo_xM_1−xO_2.8 (M = V or Nb) show the lowest selectivity to ethene.     

Biosensor for phenol based on the direct electron transfer blocking of peroxidase immobilising on silica–titanium

Horseradish peroxidase (HRP) was immobilised on silica gel modified with titanium oxide. This material was employed to prepare modified carbon paste electrode. The direct electron transfer of the hydrogen peroxide reduction by HRP was blocked when immobilised on silica–titanium. This biosensor presented a very sensitive response for phenol (1 μmol l⁻¹) at an applied potential of 0 mV vs SCE. The best condition was achieved in phosphate buffer pH 6.8, ratio of hydrogen peroxide/phenol higher than 0.35. The biosensor showed a linear response range between 10 and 50 μmol l⁻¹ of phenol, adjusted by the equation j=−32.8+16.3 [phenol], for n=5 with a correlation coefficient of 0.9995. The response time of the biosensor was about 3 s.