Anton Alexandru Ciucu

Prussian Blue and enzyme bulk-modified screen-printed electrodes for hydrogen peroxide and glucose determination with improved storage and operational stability

The addition to the carbon ink, a major component of a screen-printed electrode (SPE), of an aliquot (10%) of Prussian Blue (PB)-modified glassy carbon (PB-GC) particles, resulted in an interference free “ready to use” amperometric H 2O2 sensor (Vapp =− 50 mV) with a LOD of 3 × 10 −7 mol/l and a sensitivity of 135 mA (mol l −1 cm 2 ). A storage stability of up to 8 months and an operational stability of 3 days has been achieved making these sensors suitable for mass-production. Glucose and lysine biosensors have been assembled immobilizing glucose oxidase (Gox) and lysine oxidase (LyOx) with glutaraldehyde and Nafion ® onto the PB bulk-modified electrodes. A LOD of 4 × 10 −6 mol/l for glucose and 5 × 10 −6 mol/l for lysine with a linear range up to 0.5 and 0.7 mmol/l, respectively, have been observed. A ready to use glucose biosensor was then developed mixing PB-modified glassy carbon (7.5%), carbon ink (87.5%) and glucose oxidase (GOx, 5%) and using the mixture for the printing step of a SPE working electrode. A LOD of 3× 10 −5 mol/l and a linearity range up to 6 × 10 −3 mol/l of glucose have been achieved, together with a storage stability up to 20 weeks (at RT) and an operational stability of 1 day with 11 calibration curves performed.

Detection of pesticides using an amperometric biosensor based on ferophthalocyanine chemically modified carbon paste electrode and immobilized bienzymatic system

A new highly sensitive amperometric method for the detection of organophosphorus compounds has been developed. The method is based on a ferophthalocyanine chemically modified carbon paste electrode coupled with acetylcholinesterase and choline oxidase co-immobilized onto the surface of a dialysis membrane. The activity of cholinesterase is non-competitively inhibited in the presence of pesticides. The highest sensitivity to inhibitors was found for a membrane containing low enzyme loading and this was subsequently used for the construction of an amperometric biosensor for pesticides. Analyses were done using acetylcholine as substrate; choline produced by hydrolysis in the enzymatic layer was oxidized by choline-oxidase and subsequently H(2)O(2) produced was electrochemically detected at +0.35 V vs. Ag/AgCl. The decrease of substrate steady-state current caused by the addition of pesticide was used for evaluation. With this approach, up to 10(-10) M of paraoxon and carbofuran can be detected.

Development of a cellobiose dehydrogenase modified electrode for amperometric detection of diphenols

A new amperometric biosensor based on cellobiose dehydrogenase (CDH) was created for the detection of ortho- and para-diphenolic compounds. The developed electrode efficiently discriminates between diphenolic and monophenolic compounds. The analyte, a diphenolic compound, is oxidised on the surface of a graphite electrode at an applied potential of +300 mV vs. Ag/AgCl. The diphenol is then regenerated by the adsorbed CDH in the presence of cellobiose, thus allowing an amplified response signal. Different parameters of the CDH–electrode system were optimised, e.g., applied potential, immobilisation time, flow rate, substrate concentration and storage conditions. Using the optimised parameters the sensitivity and detection limits for various diphenolic compounds were evaluated, resulting in detection limits below 5 nM for most of the compounds tested. The highest sensitivity recorded was obtained for dopamine, 3.6 A l mol–1 cm–2. The operational stability of the electrodes was high: during 2 h of continuous operation only a 1–2% decrease in response signal was observed.

Flow injection analysis of mercury(II) based on enzyme inhibition and thermometric detection

An enzymatic procedure for the determination of mercury(II) is described, based on inhibition of invertase using glucose oxidase and catalase co-immobilised on controlled-pore glass (CPG) coupled to a thermometric continuous-flow sensor system to follow the invertase activity. A small amount of invertase (0.66 U ml−1) was incubated for a short time in sucrose solution and 20 μl of the mixture were injected into the enzyme thermistor system to give a temperature change corresponding to 100% enzyme activity. Addition of a mercury(II) sample to the mixture caused a decrease in the invertase activity, that allowed the determination of mercury(II) concentrations in the 5–80 ppb range with RSD ⩽ 0.74%. The analysis time was 2–6 min including incubation. The main advantages of this thermometric biosensor assay are as follows: simplicity, with no need for regeneration due to the use of a cheap, soluble sensing enzyme; robustness, with excellent reproducibility and repeatability; and long operational and storage stability of the enzymes involved in the detection system.

Rapid determination of total polyphenolic content in tea samples based on caffeic acid voltammetric behaviour on a disposable graphite electrode

The present paper describes the voltammetric behaviour and the quantitative determination of caffeic acid (CA) on a disposable pencil graphite electrode (PGE). The anodic peak current of CA recorded by differential pulse voltammetry (DPV) varies linearly with CA concentration in the range 1×10(-7)-3×10(-3) M. The detection and quantification limits were 8.83×10(-8) M and 2.94×10(-7) M caffeic acid, respectively. The mean recoveries of CA from Turkish green, white and black teas were 98.30%, 99.57% and 91.46%. For these three tea types the corresponding total polyphenolic contents (TPCs) evaluated by DPV on PGE were 35.81, 34.59 and 31.21 mg caffeic acid equivalent/g tea, respectively. These TPC values were in good accordance with those obtained by the Folin-Ciocalteu method. The developed DPV on PGE method constitutes a simple and inexpensive tool for the rapid assessment of TPC of tea samples.

Voltammetric determination of B1 and B6 vitamins using a pencil graphite electrode

Due to the importance of B1 and B6 vitamins for human health it is useful to develop new cheap and rapid methods for their determination. Voltammetric behavior of these vitamins on a pencil graphite electrode was investigated using cyclic voltammetry in different media. Direct quantitative determination of the two vitamins, one in the presence of the other, was done by differential pulse voltammetry. Vitamin B1 was electroactive only in a NaOH solution generating two irreversible oxidation peaks; the first peak obtained at 250 mV is well-defined and was used in quantitative determinations. In case of vitamin B6, a well-defined oxidation peak was observed in all investigated supporting electrolytes except for HCl. The linear concentration ranges were 10−5–10−3 M for vitamin B1 in a NaOH solution and 5 × 10−6–10−3 M for vitamin B6 in an acetate buffer solution. The obtained detection limits were 5.34 × 10−6 M and 2.81 × 10−6 M for vitamin B1 and vitamin B6, respectively. The developed method is simple and rapid and it was successfully applied in the determination of the two vitamins in pharmaceuticals.

SELECTIVE DPV METHOD OF DOPAMINE DETERMINATION IN BIOLOGICAL SAMPLES CONTAINING ASCORBIC ACID

A modified carbon paste electrode that incorporates the anionic surfactant sodium dodecyl sulfate (SDS) in the paste (SDS-CP) is reported. It is shown that the SDS-CP electrode discriminates effectively between the cationic form of dopamine and the anionic electroactive species existing in biological fluids at the physiological pH. Therefore, a differential pulse voltammetry method has been developed. It has the detection limit comprised in the submicromolar range and the capability to remove the interference of the ascorbic acid and to diminish the interference of the uric acid. The method has been tested with good results on real samples of deproteinized serum.

Electropolymerized molecular imprinting on glassy carbon electrode for voltammetric detection of dopamine in biological samples.

A simple and reliable method for preparing a selective dopamine (DA) sensor based on a molecularly imprinted polymer of ethacridine was proposed. The molecularly imprinted polymer electrode was prepared through electrodepositing polyethacridine-dopamine film on the glassy carbon electrode and then removing DA from the film via chemical induced elution. The molecular imprinted sensor was tested by cyclic voltammetry as well as by differential pulse voltammetry (DPV) to verify the changes in oxidative currents of DA. In optimized DPV conditions the oxidation peak current was well-proportional to the concentration of DA in the range from 2.0×10(-8)M up to 1×10(-6)M. The limit of detection (3σ) of DA was found to be as low as 4.4nM, by the proposed sensor that could be considered a sensitive marker of DA depletion in Parkinsons disease. Good reproducibility with relative standard deviation of 1.4% and long term stability within two weeks were also observed. The modified sensor was validated for the analysis of DA in deproteinized human serum samples using differential pulse voltammetric technique.

DNA-wrapped multi-walled carbon nanotube modified electrochemical biosensor for the detection of Escherichia coli from real samples

This paper introduces DNA-wrapped multi-walled carbon nanotube (MWCNT)-modified genosensor for the detection of Escherichia coli (E. coli) from polymerase chain reaction (PCR)-amplified real samples while Staphylococcus aureus (S. aureus) was used to investigate the selectivity of the biosensor. The capture probe specifically recognizing E. coli DNA and it was firstly interacted with MWCNTs for wrapping of single-stranded DNA (ssDNA) onto the nanomaterial. DNA-wrapped MWCNTs were then immobilised on the surface of disposable pencil graphite electrode (PGE) for the detection of DNA hybridization. Electrochemical behaviors of the modified PGEs were investigated using Raman spectroscopy and differential pulse voltammetry (DPV). The sequence selective DNA hybridization was determined and evaluated by changes in the intrinsic guanine oxidation signal at about 1.0V by DPV. Numerous factors affecting the hybridization were optimized such as target concentration, hybridization time, etc. The designed DNA sensor can well detect E. coli DNA in 20min detection time with 0.5pmole of detection limit in 30µL of sample volume.

Rapid voltammetric method for quinine determination in soft drinks

A novel voltammetric assay for quinine (QN) determination using an electrochemically pretreated pencil graphite electrode is described. The detection limit of QN was 2 × 10-7 M. The method possesses some obvious advantages including extreme simplicity, rapid response, and low cost.