Amperometric flow-through biosensor for the determination of cholinesterase inhibitors

Organophosphorous Pesticide Detection in Olive Oil by Using a Miniaturized, Easy-to-Use, and Cost-Effective Biosensor Combined with QuEChERS for Sample Clean-Up

Herein, we report a portable electrochemical biosensor based on butyrylcholinesterase (BChE) immobilized on carbon black (CB)-modified screen-printed electrodes (SPEs) for the detection of organophosphorous pesticides in olive oil. The BChE/CB-SPE biosensor was developed to detect paraoxon in standard solutions as well as in olive oil samples previously treated with the QuEChERS method to extract pesticides from the whole fatty matrix. The biosensor shows a linear concentration range of between 20 and 100 ppb for paraoxon both in standard solutions (phosphate buffer 0.05 M) and in olive oil extracts, with a detection limit of 6 ppb in olive oil extract, corresponding to 10% of inhibition. The accuracy of this biosensor in olive oil samples was assessed with olive oil spiked with paraoxon, obtaining satisfactory recovery values.

Electrochemical enzymatic biosensor with long-term stability using hybrid mesoporous membrane

An acetylcholinesterase-immobilized sensor unit was successfully prepared by encapsulating the enzyme within hybrid mesoporous silica membranes (F127-MST).

Self-assembly of acetylcholinesterase on gold nanoparticles electrodeposited on graphite

The immobilisation of AChE enzyme through chemisorption on Au-modified graphite was examined with view of its prospective application in the design of membraneless electrochemical biosensors for the assay of enzyme inhibitors. The developed immobilisation protocol has been based on a two-stage procedure, comprising i) electrodeposition of gold nanostructures on spectroscopic graphite; followed by ii) chemisorption of the enzyme onto gold nanoparticles. Both the coverage of the electrode surface with Au nanostructures and the conditions for enzyme immobilisation were optimised. The proposed electrode architecture together with the specific type of enzyme immobilisation allow for a long-term retaining of the enzyme catalytic activity. The extent of inhibition of the immobilised acetylcholinesterase enzyme by the organophosphorous compound monocrotophos has been found to depend linearly on its concentration over the range from 50 to 400 nmol mL−1 with sensitivity 77.2% inhibition per 1 µmol mL−1 of monocrotophos.

Critical evaluation of acetylthiocholine iodide and acetylthiocholine chloride as substrates for amperometric biosensors based on acetylcholinesterase

Numerous amperometric biosensors have been developed for the fast analysis of neurotoxic insecticides based on inhibition of cholinesterase (AChE). The analytical signal is quantified by the oxidation of the thiocholine that is produced enzymatically by the hydrolysis of the acetylthiocholine pseudosubstrate. The pseudosubstrate is a cation and it is associated with chloride or iodide as corresponding anion to form a salt. The iodide salt is cheaper, but it is electrochemically active and consequently more difficult to use in electrochemical analytical devices. We investigate the possibility of using acetylthiocholine iodide as pseudosubstrate for amperometric detection. Our investigation demonstrates that operational conditions for any amperometric biosensor that use acetylthiocholine iodide must be thoroughly optimized to avoid false analytical signals or a reduced sensitivity. The working overpotential determined for different screen-printed electrodes was: carbon-nanotubes (360 mV), platinum (560 mV), gold (370 mV, based on a catalytic effect of iodide) or cobalt phthalocyanine (110 mV, but with a significant reduced sensitivity in the presence of iodide anions).

Biosensors based on enzyme inhibition

The present chapter describes the use of biosensors based on enzyme inhibition as analytical tools. The parameters that affect biosensor sensitivity, such as the amount of immobilized enzyme, incubation time, and immobilization type, were critically evaluated, highlighting how the knowledge of enzymatic kinetics can help researchers optimize the biosensor in an easy and fast manner. The applications of these biosensors demonstrating their wide application have been reported. The objective of this survey is to give a critical description of biosensors based on enzyme inhibition, of their assembly, and their application in the environmental, food, and pharmaceutical fields.

Binding of acetylcholinesterase to multiwall carbon nanotube-cross-linked chitosan composite for flow-injection amperometric detection of an organophosphorous insecticide

A novel method for immobilization of acetylcholinesterase (AChE) by binding covalently to a cross-linked chitosan-multiwall carbon nanotube (MWNT) composite is described. In addition a sensitive, fast, cheap and automatizable flow injection detection of an organophosphorous insecticide was developed. The MWNTs were homogeneously distributed in the chitosan membrane which showed a homogeneous porous structure. The immobilized AChE could catalyze the hydrolysis of acetylthiocholine with a K(M)app value of 177 microM to form thiocholine, which was then oxidized to produce detectable signal in a linear range of 1.0-500 microM and fast response. MWNTs could catalyze the electrooxidation of thiocholine, thus increasing detection sensitivity. Based on the inhibition of an organophosphorous insecticide on the enzymatic activity of AChE, using Sulfotep as a model compound, the conditions for the flow-injection detection of the insecticide were optimized. Both biocompatibility of chitosan and inherent conductive properties of MWNTs favored the detection of the insecticide from 1.5 to 80 microM along with good stability and reproducibility. 95 % reactivation from inhibited AChE could be regenerated by using 2-pyridinealdoxime methiodide within 15 min for 15 times. The detection of Sulfotep samples exhibited satisfactory results. The proposed flow-injection analysis device can be applied to automated determination and characterization of enzyme inhibitors.

Detection of carbamic and organophosphorous pesticides in water samples using a cholinesterase biosensor based on Prussian Blue-modified screen-printed electrode

In the present paper, a comparative study using Co-phthalocyanine and Prussian Blue-modified screen-printed electrodes, has been performed. Both the electrodes have demonstrated an easiness of preparation together with high sensitivity towards thicoholine (LOD=5 x 10(-7) and 5 x 10(-6) M for Co-phthalocyanine and Prussian Blue, respectively) with high potentialities for pesticide measurement. Prussian Blue-modified screen-printed electrodes were then selected for successive enzyme immobilization due to their higher operative stability demonstrated in previous works. AChE and BChE enzymes were used and inhibition effect of different pesticides was studied with both the enzymes. AChE-based biosensors have demonstrated a higher sensitivity towards aldicarb (50% inhibition with 50 ppb) and carbaryl (50% inhibition with 85 ppb) while BChE biosensors have shown a higher affinity towards paraoxon (50% inhibition with 4 ppb) and chlorpyrifos-methyl oxon (50% inhibition with 1 ppb). Real samples were also tested in order to evaluate the matrix effect and recovery values comprised between 79 and 123% were obtained.

Subnanomolar cyanide detection at polyphenol oxidase/clay biosensors

A novel, inexpensive, and simple amperometric biosensor based on immobilization of polyphenol oxidase (PPO) into Zn-Al layered double hydroxides, also called anionic clays, is applied for determination of cyanide. The detection of cyanide was performed via its inhibiting action on the PPO electrode. Measurement was carried out with 3,4-dihydroxyphenylacetic acid as enzyme substrate, the enzymatically generated quinoid products being electroreduced at -0.2 V. An extremely sensitive detection limit (0.1 nM) was obtained for cyanide. Enzyme immobilization into an anionic exchanger clay seems to cause an increase in cyanide inhibition effects because of anion accumulation in the clay matrix.

New principle of direct real-time monitoring of the interaction of cholinesterase and its inhibitors by piezolectric biosensor

This paper describes a new method for the sensitive detection of cholinesterase inhibitors based on real-time monitoring using a piezoelectric biosensor. The cholinesterase inhibitor paraoxon was immobilized on the sensing surface via a chelate complex as the recognition element. At first, the conjugate of N-mercaptoundecanoic acid (MUA) with Nalpha,Nalpha-bis (carboxymethyl)-L-lysine (NTA-Lys) was chemisorbed to form a self-assembled monolayer on the surface of the gold electrode of the piezosensor. In the next step, paraoxon-spacer-hexahistidine conjugate was linked to the MUA-Lys-NTA layer via the chelate complex with Ni2+. The paraoxon-modified surface thus obtained was applied for the binding of human butyrylcholinesterase (BChE). Regeneration of the sensing surface was achieved by splitting the chelate complex with EDTA and depositing a fresh layer of Ni2+ followed by addition of the paraoxon-spacer-hexahistidine. In the presence of free inhibitors like diisopropylfluorophosphate (DFP), binding of BChE to the surface-bound paraoxon was decreased. In this way, a competitive affinity assay for organophosphorus compounds was developed. The limit of detection for DFP as a model compound was 10 nmol/l (ca. 2 microg/l). This new concept seems suitable for constructing biosensors for the group-specific detection of cholinesterase-inhibiting substances like insecticides in the field.

Design of acetylcholinesterases for biosensor applications

In recent years, the use of acetylcholinesterases (AChEs) in biosensor technology has gained enormous attention, in particular with respect to insecticide detection. The principle of biosensors using AChE as a biological recognition element is based on the inhibition of the enzyme's natural catalytic activity by the agent that is to be detected. The advanced understanding of the structure-function-relationship of AChEs serves as the basis for developing enzyme variants, which, compared to the wild type, show an increased inhibition efficiency at low insecticide concentrations and thus a higher sensitivity. This review describes different expression systems that have been used for the production of recombinant AChE. In addition, approaches to purify recombinant AChEs to a degree that is suitable for analytical applications will be elucidated as well as the various attempts that have been undertaken to increase the sensitivity of AChE to specified organophosphates and carbamates using side-directed mutagenesis and employing the enzyme in different assay formats.

Enzyme inhibition-based biosensors and biosensing systems: questionable analytical devices

An overview of methods based on enzyme inhibition is presented. Both biosensors and biosensing systems (implemented in continuous and discontinuous approaches) are considered. The aim of the overview is to alert users on the use of these devices which, despite they can involve selective biocatalysts, the inhibition effect is not selective. Tables and examples illustrate this assertion.

Biocapteurs ampérométriques à cholinestérases pour la détermination des pesticides organophosphorés

The purpose of this study is a comparative presentation of the different types of the amperometric biosensors based on cholinesterases for the determination of organophosphorous pesticides using the bibliographical information of the last 20 years. The study contains the presentation of the structure and properties of the cholinesterases, the main reactions implied in the functioning of the amperometric biosensors, their applications and factors influencing the detection or (and) the inhibition process. The detection limit of the mono- or bi-enzymatic amperometric biosensors are relatively higher than those corresponding with the immunobiosensors or with gas and liquid chromatography, which are still considered as the reference methods. As shown, for many other amperometric biosensors, the Michaelis–Menten's kinetic treatment used for reactions catalyzed by free enzymes can be extended to describe the response of amperometric biosensors based on immobilized cholinesterases. The positive compromise between advantages and drawbacks, as well as the "soft" experimental conditions, point to the amperometric monoenzymatic bioelectrode, as an attractive analytical tool for the detection of organophosphorous pesticides.Key words: amperometric biosensor, acetylcholinesterase, organophosphorous pesticides, kinetic, inhibition.

Development of a quantitative relationship between inhibition percentage and both incubation time and inhibitor concentration for inhibition biosensors--theoretical and practical considerations

Theoretical and practical insights into the design and development of immobilised enzyme inhibition biosensors are reported. A general mathematical expression relating the percent of enzyme inhibition (i.e. the analytical signal) to both the inhibitor concentration and the incubation time is presented. The relevant physical, chemical and biochemical parameters required by the model are developed and discussed in terms of the inhibition of acetylcholinesterase by the organophosphorous pesticide, paraoxon. A second enzyme, choline oxidase and an amperometric transducer are used to facilitate the determination acetylcholinesterase inhibitor.

Flow-injection spectrophotometric determination of paraoxon by its inhibitory effect on the enzyme acetylcholinesterase

A spectrophotometric enzymatic flow injection (FI) system for the determination of diethyl-p-nitrophenylphosphate (paraoxon) is proposed. The method was based on the determination of the acetic acid formed by the enzymatic reaction of the acetylcholinesterase, immobilized on glass beads, with the substrate acetylcholine. The acetic acid formed permeates through a PTFE membrane and is received by a solution (pH 7.0) containing the acid-base indicator Bromocresol Purple (B.C.P.), leading to a pH change and therefore to a color change. The variation of the absorbance of the solution is detected spectrophotometrically at 400 nm. The determination of paraoxon is related to its inhibitory action on the enzyme. Therefore the analytical signal is the difference between the signal that corresponds to the free and the one that corresponds to the inhibited enzyme, considering a fixed acetylcholine concentration. The correlation between the peak height and paraoxon concentration at a given acetylcholine concentration is linear in the range from 5.0 x 10(-7) mol L-1 to 5.0 x 10(-5) mol L-1 (r = 0.998) of paraoxon, with a relative estimated standard deviation (R.S.D.) of +/- 1.7% (n = 10) considering a solution containing 5.0 x 10(-6) mol L-1 of paraoxon and a solution containing 5.0 x 10(-3) mol L-1 of acetylcholine. Therefore, the quantitative limit detection is about 2.5 x 10(-7) of paraoxon (3 sigma). A 1,1'-trimethylene-bis(4-formylpyridinium bromide)dioxime (TMB-4) solution was used to reactivate the enzyme.

Environmental toxicity monitoring using electrochemical biosensing systems

Environmental monitoring faces the challenge of measuring an increasing number of analytes at ever decreasing concentrations. Since not all species of a given analyte have the same detrimental impact on the environment, new analytical devices and techniques are required to distinguish between the different species of a pollutant or different groups of pollutants. This paper describes analytical techniques based on biomaterials that are toxically sensitive to pollutants. This approach permits the biomonitoring of certain compounds by looking at their toxic properties. Although these techniques are based on a sound analytical strategy, their applications are limited because most of the interactions between the biological material and the analyte are irreversible. Additionally, the immobilised biological material has a limited stability. Several biomonitoring strategies based on electrochemical biosensing are discussed here and how to recover the bioactivity of biosensing system, both in discrete and automated procedures, is also reviewed.