Amperometric Glucose Biosensor Based on Rhodium Dioxide‐Modified Carbon Ink

An amperometric method for the determination of glucose using a screen printed carbon electrode is reported. The electrode material was bulk modified with rhodium dioxide and the enzyme glucose oxidase immobilized in a Nafion‐film on the electrode surface and investigated for its ability to serve as a detector of glucose in flow injection analysis. The sensor exhibited a linear increase of the amperometric signal with the concentration of glucose in the range of 1–250 mg L^?1 with a detection limit (evaluated as 3σ) of 0.2 mg L^?1 under optimized flow rate of 0.4 mL min^?1 in 0.1 M phosphate buffer (pH 7.5) carrier. At the potential applied (?0.2 V vs. Ag/AgCl), interferences from redox species present in the sample matrix were negligible. The biosensor reported here retained its activity for more than 40 injections or 4 months of storage at 6 °C. The RSD was determined as 1.8% for a glucose concentration of 25 mg L^?1 (n=5) with a typical response time of about 28 s.     

Amperometric sensor for L-ascorbic acid determination based on MnO2 bulk modified screen printed electrode

A simple biosensor constructed by bulk-modification of carbon ink with manganese dioxide as a mediator was investigated for its ability to serve as amperometric detector for L-ascorbic acid in hydrodynamic mode. The sensor could be operated at pH 5.0 (0.05 M phosphate buffer) and exhibited excellent reproducibility and stability. Optimization of measurement parameters such as applied working potential and pH value were studied in detail. The screen printed electrode exhibited a linear amperometric increase with the concentration of ¶L-ascorbic acid from 50 mg L^–1 to 250 mg L^–1 and gave a (LOD = 3σ) detection limit of 0.2 mg L^–1 (1.172 μmol L^–1). The manganese dioxide modified screen printed electrode shows long term stability.     

Simultaneous determination of oxalate, thiosulfate, and thiocyanate in urine by ion-exclusion chromatography with electrochemical detection

Summary A sensitive ion-exclusion chromatographic method has been developed for determination of oxalate, thiosulfate, and thiocyanate. The method is based on separation of these anions on a polymethacrylate-based, weakly acidic cation-exchange resin (TSKgel OApak-A) and detection by means of a glassy carbon (GC) electrode electrochemically modified with polyvinylpyridine (PVP), palladium, and iridium oxide (PVP/Pd/IrO₂). The electrochemical behavior of oxalate, thiosulfate, and thiocyanate at this chemically modified electrode (CME) have been investigated by cyclic voltammetry. The results indicated that electrocatalytic oxidation of these anions by the electrode was efficient and that the sensitivity, stability, and lifetime of the electrode were relatively high. Combined with ion-exclusion chromatography the PVP/Pd/IrO₂ electrode was used as the working electrode for amperometric detection of these anions. All linear ranges were over two orders of magnitude and detection limits, defined asS/N=3, were 9.0×10⁻⁷ mol L⁻¹ for oxalate, 6.7×10⁻⁷ mol L⁻¹ for thiosulfate, and 5.6×10⁻⁷ mol L⁻¹ for thiocyanate. Correlation coefficients were all>0.998. Coupled with microdialysis sampling the method has been successfully applied to the determination of oxalate, thiosulfate, and thiocyanate in urine.     

Electrogenerated Chemiluminescence of Tris(2,2′‐bipyridyl)ruthenium(II) Immobilized in Humic Acid‐Silica‐Poly(vinyl alcohol) Composite Films

In this paper, we report the first attempt to use humic acid (HA) as modifiers to prepare the organic‐inorganic hybrid modified glassy carbon electrodes based on HA‐silica‐PVA (poly(vinyl alcohol)) sol‐gel composite. Electroactive species of tris(2,2′‐bipyridyl)ruthenium(II) (Ru(bpy) ) can easily incorporate into the HA‐silica‐PVA films to form Ru(bpy) modified electrodes. The amount of Ru(bpy) incorporated in the composite films strongly depends on the amount of HA in the hybrid sol. Electrochemical and electrogenerated chemiluminescence (ECL) of Ru(bpy) immobilized in HA‐silica composite films coated on a glassy carbon electrode have been studied with tripropylamine (TPA) as the coreactant. The analytical performance of this modified electrode was evaluated in a flow injection analysis (FIA) system with a homemade flow cell. The as‐prepared electrode showed good stability and high sensitivity. The detection limits (S/N=3) were 0.050 μmol L^?1 for TPA and 0.20 μmol L^?1 for oxalate, and the linear ranges were from 0.10 μmol L^?1 to 1.0 mmol L^?1 for TPA and from 1.0 μmol L^?1 to 1.0 mmol L^?1 for oxalate, respectively. The resulting electrodes were stable over two months.     

FIA Determination of Paracetamol in Pharmaceutical Drugs by Using Gold Electrodes Modified with a 3‐Mercaptopropionic Acid Monolayer

A flow injection analysis (FIA) method for the determination of paracetamol in pharmaceutical drugs using a gold electrode modified with a self‐assembled monolayer (SAM) of 3‐mercaptopropionic acid is described. At optimized experimental conditions the dynamic concentration range was 0.15 to 15.0 mg L^?1 with a detection limit of 0.2 μg mL^?1 (S/N=3). The repeatability of current responses for injections of 10 μmol L^?1 paracetamol was evaluated to be 3.2% (n=30) and the analytical frequency was 180 h^?1. The lifetime of the modified electrode was found to be 15 days. The results obtained by using the proposed amperometric method for paracetamol determination in four different drug samples compared well with those found by spectrophotometry.     

Flow Injection Amperometric System Coupled with a Well-Plate for Fast Screening of Total Antioxidant Capacity

A flow injection (FI) system with an amperometric detection in combination with a 96-well-plate has been developed for high throughput screening of total antioxidant capacity. It is based on the reaction between dichromate and antioxidants Chromium reducing antioxidant capacity (CHROMAC) assay. Antioxidant reduces dichromate leading to a decrease of dichromate concentration, in which reacts further with triiodide reagent. The slow reaction between antioxidant and dichromate takes place in parallel in a 96-well microplate, with the reaction time sufficiently long to reach steady state conditions. Microliter volumes of the remaining dichromate solution were manually injected into the FI amperometric system. Reaction with iodide in acidic medium produces triiodide which undergoes electrochemical reduction on a glassy carbon working electrode at 200?mV versus a Ag/AgCl reference electrode. The decrease of electrical current was directly proportional to the antioxidant capacity. The optimum conditions have been achieved as follow: 160?mg?L^?1 of potassium dichromate, pH of dichromate solution of 2.82, 0.005?mol?L^?1 of potassium iodide, 0.20?mol?L^?1 of hydrochloric acid, and 3.0?mL?min^?1 total flow rate. The developed method was applied for the determination of total antioxidant capacity of tea and herbal infusions and fruit juice samples. The results obtained indicated that the developed method had good correlation with the batch CHROMAC spectrophotometric method and 1,1-diphenyl-2-picrylhydrazyl scavenging assay. The proposed method provides fast and high throughput analysis, convenient operation, and low consumption of chemical reagents and samples.     

Amperometric Detection of Histamine with a Pyrroloquinoline‐Quinone Modified Electrode

A modified glassy carbon (GC) electrode was developed for the amperometric detection of biogenic amines, particularly histamine. The electrode was modified with the co‐enzyme pyrroloquinoline quinone (PQQ) by entrapment during electropolymerziation of pyrrole to form polypyrrole (PPy). This method formed a thin film on the electrode surface possessing very good stability with a shelf‐life exceeding one month without loss of signal. Optimal conditions for the PQQ/PPy electrode were determined and a linear response was found for histamine in phosphate buffer (pH 6) at +550 mV from 40 to 170 mg L^?1 with a limit of detection (S/N≥3) of 38 mg L^?1. The practical linear range offered by this method suggests ideal use for spoilage detection in fermented foods.     

Simultaneous Flow Injection Analysis of Paracetamol and Ascorbic Acid with Multiple Pulse Amperometric Detection

The present work reports a simple and quick strategy for simultaneous determination of paracetamol (PC) and ascorbic acid (AA) in pharmaceutical formulations using flow injection method with multiple pulse amperometric detection. The method allows the resolution of the mixture without chemical pretreatment of the sample or electrode modification or the use of chemometric techniques for data analysis. The compounds are detected by applying four sequential pulses (waveform) in function of time to a three‐electrode amperometric system that uses a wall‐jet cell with gold as working electrode. AA is direct detected at +0.40 V and PC is indirectly detected at 0.0 V by the reduction (desorption) of the oxidation product (N‐acetyl‐p‐benzoquinoneimine) electrochemically generated at +0.65 V. The fourth potential pulse (?0.05 V) is applied for the complete regeneration (cleaning) of the gold electrode surface. The linear response range was optimized between 5 and 24 mg L^?1 for AA and 50 and 240 mg L^?1 for PC. The difference between the two responses ranges (10‐fold) present correlation with the concentration of these compounds in two different pharmaceutical formulations available in the Brazilian market. The analytical frequency was calculated in 60 injections per hour. The use of the proposed methodology for PC quantification in the presence of higher AA concentrations was also carried out. Using the standard addition method, it was possible to detect PC in trace levels (LD=0.2 mg L^?1) in the presence of 880‐fold more of AA (176 mg L^?1).     

Study on Electrochemiluminesce of Ru(bpy3)2+ Immobilized in a Titania Sol‐Gel Membrane

The immobilization of tris(2,2′‐bipyridyl)ruthenium(II), Ru(bpy)₃²⁺, at a glassy carbon electrode was achieved by entrapping the Ru(bpy)₃²⁺ in a vapor deposited titania sol‐gel membrane. The electrogenerated chemiluminescence (ECL) of the immobilized Ru(bpy)₃²⁺ was studied. The Ru(bpy)₃²⁺ modified electrode showed a fast ECL response to both oxalate and proline. The ECL intensity was linearly related to concentrations of oxalate and proline over the ranges from 20 to 700 μmol L^?1 and 20 to 600 μmol L^?1, respectively. The detection limits for oxalate and proline at 3σ were 5.0 μmol L^?1 and 4.0 μmol L^?1, respectively. This electrode possessed good precision and stability for oxalate and proline determinations. The electrogenerated chemiluminescence mechanism of proline system was discussed. This work provided a new way for the immobilization of Ru(bpy)₃²⁺ and the application of titania sol‐gel membrane in electrogenerated chemiluminescence.     

Development of an Amperometric Biosensor for β‐N‐Oxalyl‐L‐α,β‐Diaminopropionic Acid (β‐ODAP)

An amperometric method for the determination of the neurotoxic amino acid β‐N‐oxalyl‐L ‐α,β‐diaminopropionic acid (β‐ODAP) using a screen printed carbon electrode (SPCE) is reported. The electrode material was bulk‐modified with manganese dioxide and used as a detector in flow injection analysis (FIA). The enzyme glutamate oxidase (GlOx) was immobilized in a Nafion‐film on the electrode surface. The performance of the biosensor was optimized using glutamate as an analyte. Optimum parameters were found as: operational potential 440 mV (vs. Ag/AgCl), flow rate 0.2 mL min^?1, and carrier composition 0.1 mol L^?1 phosphate buffer (pH 7.75). The same conditions were used for the determination of β‐ODAP. The signal was linear within the concentration range 53–855 μmol L^?1 glutamate and 195–1950 μmol L^?1 β‐ODAP. Detection limits (as 3σ value) for both analytes were 9.12 and 111.0 μmol L^?1, respectively, with corresponding relative standard deviations of 3.3 and 4.5%. The biosensor retained more than 73% of its activity after 40 days of on‐line use.     

Optimized multilayer oxalate biosensor

The optimization of a biosensor prepared by the immobilization of oxalate oxidase (OOX) with a cross-linking agent onto a multilayer inorganic/organic modified electrode, is presented. A very thin Prussian Blue (PB) film covered by a self-doped polyaniline (SPAN) layer acts as very sensitive amperometric sensor for the H₂O₂ formed by the enzymatic reaction. The electrode allows the very reliable and sensitive oxalate detection in the 0.08 to 0.45 mmol l⁻¹ concentration range. The observed sensitivity was 131.3 μA mmol⁻¹ cm⁻² at the operation potential of 0.05 V versus Ag/AgCl in a succinate buffer solution (pH=3.8). The bilayer Prussian blue/SPAN leads to a very stable, sensitive and selective system that not only minimizes the interference caused by ascorbic and uric acids but also forms a very adherent sensing film that allows repetitive successive determinations.     

Cathodic Electrochemical Detection of Nitrophenols at a Bismuth Film Electrode for Use in Flow Analysis

The bismuth film electrode (BiFE) is presented for use in both batch voltammetric and flow injection (FI) amperometric detection of some nitrophenols (2‐nitrophenol, 2‐NP; 4‐nitrophenol, 4‐NP; 2,4‐dinitrophenol, 2,4‐DNP). The bismuth film was deposited ex situ (batch measurements) and in‐line (FI) onto a glassy carbon substrate electrode. Batch analysis of the nitrophenols was carried out in 0.04 M Britton Robinson (BR) buffer pH 4, while for FI measurements, a carrier/electrolyte solution composed of 0.1 M BR buffer pH 4 mixed with methanol (20+80, v/v%) was employed to resemble media used in preconcentration/clean‐up and flow separation sample pretreatment procedures. Under batch conditions, the voltammetric behavior of the nitrophenols was examined for dependence on medium pH in the range of 2 to 10. Employing the square‐wave voltammetry mode, the limits of detection were 0.4 μg L^?1, 1.4 μg L^?1, and 3.3 μg L^?1 for 2‐NP, 4‐NP, and 2,4‐DNP, respectively. Under flow conditions, a simple in‐line electrochemical bismuth film renewal procedure was tested and shown to provide very good inter‐ and intra‐electrode reproducibility of the current signals at low μg L^?1 analyte concentrations. The limits of detection for 2‐NP, 4‐NP and 2,4‐DNP obtained using FI and amperometric detection at ?1.0 V (vs. Ag/AgCl) were 0.3 μg L^?1, 0.6 μg L^?1 and 0.7 μg L^?1, respectively, with linear ranges extending up to 20 μg L^?1. The attractive performance of the BiFE under flow analysis conditions offers great promise with respect to its detection capability and to its use for a prolonged period of time with no need for inconvenient removal of the electrode from the system for mechanical surface treatment.     

Flow‐Injection Pulsed‐Amperometric Determination of Free Glycerol in Biodiesel at a Gold Electrode

This work reports the highly‐sensitive amperometric determination of free glycerol in biodiesel at a gold electrode adapted in a flow‐injection analysis (FIA) cell. The amperometric method involved the continuous application of three sequential pulses to the working electrode (+250 mV, +700 mV, and ?200 mV, for 100 ms each). This sequence of potential pulses eliminated electrode passivation and dramatically increased the analytical signal. The proposed FIA‐amperometric method presented low relative standard deviation between injections (1.5 %, n=15), high analytical frequency (85 h^?1), satisfactory recovery values (93–118 %) for spiked samples, wide linear range (from 1 to 300 µmol L^?1), and low detection limit (0.5 µmol L^?1).     

An amperometric sensor for uric acid based on ordered mesoporous carbon-modified pyrolytic graphite electrode

A novel amperometric sensor for uric acid based on ordered mesoporous carbon modified pyrolytic graphite electrode was developed. Uric acid oxidation was easily catalyzed by this electrode in a phosphate buffer solution at pH 7.0, with an anodic potential decrease about 140 mV compared to bare pyrolytic graphite electrode. The uric acid level was determined by the amperometric method, at a constant potential of 0.31 mV, the catalytic current of uric acid vs. its concentration showed a good linearity in the range of 1.0 × 10⁻⁶−1.0 × 10⁻⁴ mol L⁻¹, with a correlation coefficient of 0.999. The detection limit was 4.0 × 10⁻⁷ mol L⁻¹. The proposed method could be effectively used for uric acid amperometric sensing in human urine.     

Flow injection amperometric determination of acetaminophen at a gold nanoparticle modified carbon paste electrode

An amperometric detector with a gold nanoparticle modified carbon paste electrode (GNMCPE) was applied applied to flow injection analysis for the determination of acetaminophen. An obvious shift of the peak potential and increase of the current peak were observed for the GNMCPE in comparison to that of the bare carbon paste electrode. The experimental conditions, such as species of buffer, pH, flow rate, detection volume, injection volume, and injection time were investigated. Under the optimized conditions, the calibration curve was obtained over the concentration range of 0.1–80 mg L^?1 of acetaminophen with a linear correlation coefficient of 0.9994. The detection limit (3σ) was estimated to be 0.05 mg L^?1 (n?=?7). The recoveries of acetaminophen were between 98.40% and 104.1%, and the relative standard deviation varied between 1.66% and 2.74% for the different samples. This method was applied to analyze six types of tablets obtained from a local drugstore. The contents of acetaminophen were found to be 0.498, 0.323, 0.249, 0.324, 0.319 and 0.323 g of each tablet, respectively. These results are consistent with the values obtained by high performance liquid chromatography.     

Amperometric Biosensor Based on Enzymatic Reactor for Choline Determination in Flow Systems

A simple, selective and stable biosensor with the enzymatic reactor based on choline oxidase (ChOx) was developed and applied for the determination of choline (Ch) in flow injection analysis with amperometric detection. The enzyme ChOx was covalently immobilized with glutaraldehyde to mesoporous silica powder (SBA‐15) previously covered by NH₂‐groups. This powder was found as an optimal filling of the reactor. The detection of Ch is based on amperometric monitoring of consumed oxygen during the enzymatic reaction, which is directly proportional to Ch concentration. Two arrangements of an electrolytic cell in FIA, namely wall‐jet cell with working silver solid amalgam electrode covered by mercury film and flow‐through cell with tubular detector of polished silver solid amalgam were compared. The experimental parameters affecting the sensitivity and stability of the biosensor (i. e. pH of the carrier solution, volume of reactor, amount of the immobilized enzyme, the detection potential, flow rate, etc.) were optimized. Under the optimized conditions, the limit of detection was found to be 9.0×10^?6 mol L^?1. The Michaelis‐Menten constant for covalently immobilized ChOx on SBA‐15 was calculated. The proposed amperometric biosensor with the developed ChOx‐based reactor exhibits good repeatability, reproducibility, long‐term stability, and reusability. Its efficiency has been confirmed by the successful application for the determination of Ch in two commercial pharmaceuticals.     

An Amperometric Biological Toxic Hydrazine Sensor Based on Multiwalled Carbon Nanotubes and Iron Tetrasulfonated Phthalocyanine Composite Modified Electrode

Hydrazines are well‐known for their diverse biological properties but especially for their toxicity. An amperometric hydrazine sensor was developed at multi‐walled carbon nanotubes (MWCNT) and iron tetrasulfonated phthalocyanine (FeTsPc) composite modified electrode for the first time. The TEM and UV‐Vis spectroscopy results revealed the successful formation of MWCNT/FeTsPc composite. Compared with the response of MWCNT and FeTsPc modified electrodes, the MWCNT/FeTsPc composite showed enhanced oxidation current response with lower overpotential for hydrazine. Under optimum conditions, the amperometric i–t response of hydrazine was linear in the concentration range from 100 nM L^?1 to 3 μM L^?1 with the detection limit of 7.6 nM L^?1. The response time of hydrazine was found as 6 s with a high sensitivity of 7.615 μA/μM L^?1 cm^?2.     

Electroanalytical performance of self-assembled monolayer gold electrode for chloramphenicol determination

Monolayers of 2-mercapto-5-methylbenzimidazole (MMB) were prepared on a polycrystalline gold electrode via a self-assembly process to produce a self-assembled monolayer. The resulting electrode was investigated by cyclic voltammetry and electrochemical impedance spectroscopy, and applied to the determination of chloramphenicol (CAP) in a pharmaceutical formulation using flow injection analysis along with amperometric detection. The amperometric cell was operated at ?0.75 V (vs Ag/AgCl) at a flow rate of 3 mL min^?1. The method was applied to the determination of CAP in ophthalmic solutions, and its performance was compared to a previously validated HPLC method. The response to CAP is linear in the range from 0.050 to 1.000 µmol L^?1 (r?=?0.9990), and the limit of detection is 44 µmol L^?1.     

Direct Determination of Phosphite in Fertilizers by Flow‐Injection Amperometry

A sensor based on graphite electrode modified with palladium‐platinum‐palladium film is proposed for phosphite determination by flow‐injection amperometry. The modified electrode was prepared by a sequential cathodic deposition of Pd, Pt and Pd on a graphite electrode from 0.5% m/v PdCl₂+28% m/v NH₄OH and 2% m/v H₂PtCl₆+10% v/v H₂SO₄ solutions. After suitable conditioning, the electrode showed catalytic activity for phosphite oxidation when 0.15 V was applied. The proposed system handles approximately 50 samples per hour (0.01–0.05 mol L^?1 Na₂HPO₃; R²=0.9997), consuming ca. 70 μL of sample per determination. The limit of detection and amperometric sensibility were 5×10^?4 mol L^?1 and 1.5 mA L mol^?1, respectively. The proposed method was applied to analysis of fertilizer samples without pre‐treatment. Results are in agreement with those obtained by spectrophotometry and titrimetry at 95% confidence level and good recoveries (96–109%) of spiked samples were found. Relative standard deviation (n= 12) of a 0.01 mol L^?1 Na₂HPO₃ sample was 2%. The useful lifetime of modified electrode was around 220 determinations. For routine purposes it means that this electrode can be continuously used for 5 hours.     

Trace Level Determination of Hydrogen Peroxide at a Carbon Ceramic Electrode Modified with Copper Oxide Nanostructures

An electrochemical sensor was developed for determination of hydrogen peroxide based on nanocopper oxides modified carbon sol‐gel or carbon ceramic electrode (CCE). The modified electrode was prepared by electrodeposition of metallic copper on the CCE surface and derivatized in situ to copper oxides nanostructures and characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD) techniques. The modified electrode responded linearly to the hydrogen peroxide (H₂O₂) concentration over the range 0.78–193.98 µmol L^?1 with a detection limit of 71 nmol L^?1 (S/N=3) and the sensitivity of 0.697 A mol^?1 L cm^?2. This electrode was used as selective amperometric sensor for determination of H₂O₂ contents in hair coloring creams.