MIP-MEPS based sensing strategy for the selective assay of dimethoate. Application to wheat flour samples

The aim of this work was to demonstrate the potentialities of the use of a molecularly imprinted (MIP) sensor coupled to a microextraction by packed sorbent (MEPS) strategy for the selective and sensitive detection of dimethoate in real samples. A dimethoate-polypyrrole MIP film was realised by cyclic voltammetry (CV) on the surface of a glassy carbon electrode (GCE). Being dimethoate electro-inactive, K₃[Fe(CN)₆] was used as probe for the indirect quantification of the analyte via the decrease of redox peaks observed upon binding of the target analyte. Detection of dimethoate at low nanomolar range was achieved with linearity in the 0.1-1nM range. Relative standard deviation calculated for different electrodes at 0.5nM of dimethoate was < 3% and selectivity was very satisfactory being the response for omethoate only 23% of dimethoate. A MEPS strategy for the extraction of dimethoate from a challenging matrix as wheat flour was then used in conjunction with the MIP electrochemical sensor. The procedure applied to flour samples spiked with dimethoate at 0.5 MRL, MRL, and 1.5 MRL gave very favourable comparison with a validated UHPLC-MS/MS method with deviations in the -21% /+17% range, demonstrating the feasibility of the approach as screening assay. This work clearly shows that the sequential use of a microextraction based procedure and electrochemical sensing system is low cost, easy to realise and use and can open new perspectives for the development of selective sensing system to be used in field or decentralised lab testing for the selective screening of target analytes.

Gold nanoparticles-peptide based gas sensor arrays for the detection of food aromas

A gas sensor array based on peptide modified gold nanoparticles deposited onto 20MHz quartz crystal microbalances has been realized. Glutathione and its constituting aminoacids and dipeptides have been used as ligands. A great increase in sensitivity (2 orders of magnitude) was achieved using gold nanoparticles versus monolayer modified QCMs. The sensors have been characterised in terms of sensitivity for hexane, water, trimethylammine and ethanol. Highest sensitivity was found for water. The ability to discriminate typical food aromas as cis-3-hexenol, isopentylacetate, ethylacetate, and terpinen-4-ol dissolved in different solvents was studied using a gas sensor array constituted by gold nanoparticles modified with the glutathione peptides, thioglycolic acid and an heptapeptide. The array was found able to discriminate the food aromas, the response being dependent on the polarity of the solvent used. Tests on real olive oil samples gave a satisfactory separation among samples having defects versus non defected samples demonstrating that this approach has high potential for the development of gas sensor arrays to be used in real samples.

An electrochemical bioassay for dichlorvos analysis in durum wheat samples

The use of an acetylcholinesterase inhibition assay for the detection of dichlorvos in durum wheat samples by a simplified extraction procedure is reported. After an incubation step, the residual activity was determined with an amperometric biosensor using a portable potentiostat. The use of electric eel and recombinant acetylcholinesterase was compared. The effect of the matrix extract was evaluated by using various sample:solvent ratios, 1:2.5, 1:5, 1:10, and 1:20. The optimal extraction ratio, considering the electrochemical interferences and the effect on enzyme activity and bioavailability of the pesticide, was 1:10. Calibrations were performed in buffer and durum wheat extract. The calculated detection limits in buffer solution were 10 ng/ ml and 0.045 ng/ml for electric eel and recombinant acetylcholinesterase, respectively, whereas operating in the matrix extract they increased up to 45 ng/ml and 0.07 ng/ml, corresponding to 0.45 mg/kg (extraction ratio 1:10) and 0.07 mg/kg in samples. These characteristics allowed the detection of contaminated samples at the maximum residue limit, which is 2 mg/kg and well below. Fortified samples of durum wheat were obtained with both dichlorvos and the commercial product Didivane, which contains dichlorvos as an active molecule. At all the tested levels, the occurrence of contaminant was detected with an average recovery of 75%. The total assay time, including the extraction step, was 30 min. Because several extractions as well as most of the assay steps can be run simultaneously, the throughput for one operator is 12 determinations per hour.

Evaluation of the antioxidant capacity of individual phenolic compounds in virgin olive oil

Virgin olive oil has a high resistance to oxidative deterioration due to its tryacylglycerol composition low in polyunsaturated fatty acids and due to the presence of a group of phenolic antioxidants composed mainly of polyphenols and tocopherols. We isolated several phenolic compounds of extra virgin olive oil (phenyl-ethyl alcohols, lignans, and secoiridoids) by semipreparative high-performance liquid chromatography (HPLC) and identified them using ultraviolet, atmospheric pressure chemical ionization, and electrospray ionization MS detection. The purity of these extracts was confirmed by analytical HPLC using two different gradients. Finally, the antioxidant capacity of the isolated compounds was evaluated by measuring the radical scavenging effect on 1,1-diphenyl-2-picrylhydrazyl radical, by accelerated oxidation in a lipid model system (OSI, oxidative stability instrument), and by an electrochemical method.

Piezoelectric sensors for dioxins: a biomimetic approach

The aim of this work was to design a fast, cheap and easy to use analytical system for dioxins. Piezoelectric sensors coupled with the pentapeptides as biomimetic traps (the receptors), selective for the dioxins, were used for the realisation of this analytical system. A methodology to select specific receptors among all possible pentapeptides randomly generated was represented by the use of molecular modelling software. Three peptides called later on A, B and C (A:[N]Asn-Phe-Gln-Gly-Ile[C]; B:[N]Asn-Phe-Gln-Gly-Gln[C]; C:[N]Asn-Phe-Gln-Gly-Phe[C]), were selected and evaluated for their potential usage as artificial receptors in solid-gas analysis by using a quartz crystal microbalance (QCM) sensors array. The peptide sequences were functionalised by two terminal cysteine residues in order to achieve a covalent interaction with the QCM gold surface. A manganese-porphyrin complex and two other pentapeptides, a pentaglutamine (pentapeptide D) and a pentalysine (pentapeptide E), were used as negative control sensors. The QCM sensors (A, B and C) gave a good linearity against different sample concentrations of the 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD) and a mixture of dioxins. In particular, the selectivity against 2,3,7,8-TCDD was nicely correlated to the estimated binding energy of the receptors calculated by computational modelling. The cross-reactivity of the system was quantified using commercial polychlorinated biphenyls (PCBs) mixtures (dioxin-like compounds).

Contribution of the phenolic fraction to the antioxidant activity and oxidative stability of olive oil

Antioxidant activity of the phenolic fraction of extra virgin olive oil (EVOO) was measured by means of a chemical and an electrochemical method. Both methods were tested in predicting the oxidative spoilage and stability to oxidation of 22 EVOO samples and resulted correlated with peroxide values and oxidative stability measured by Rancimat. The main phenolic compounds of EVOOs were detected by HRGC. To study the contribution of single polyphenols (PPs) to antioxidant activity of phenolic fraction and oxidative stability of EVOOs, multivariate statistical analyses were applied on HRGC data. An isomer of oleuropein aglycon was shown to affect significantly antioxidant activity of phenolic fraction but not oil stability to oxidation. No individual compounds was identified as the main cause of the overall antioxidant activity, and the total polyphenol determination by the Folin reagent was better correlated to antioxidant activity and oxidative stability than each tested PP or PPs groups such as o-diphenols.

Comparison among differential pulse voltammetry, amperometric biosensor, and HPLC/DAD analysis for polyphenol determination

Polyphenols are widespread in vegetables and fruits. They can play an important role in human diet and health, and they influence the sensorial properties of many foods, and act as natural antioxidants. This study was conducted using HPLC/DAD, tyrosinase biosensor, and differential pulse voltammetry (DPV) analyses to detect polyphenolic compounds in natural complex matrices. The analyses were applied to a series of both standards and natural extracts derived from grape, olives, and green tea. The pure compounds include phenolic acids, flavones, flavonols, catechins, tannins, and oleuropein. HPLC/DAD, DPV, and the biosensor approach were used as independent analytical techniques. Bare graphite screen-printed electrodes were employed in DPV and in the biosensor analysis. The most accurate data were obtained by HPLC/DAD analysis, while the DPV approach using screen-printed electrodes could represent a quick screening method for the determination of polyphenols in natural extracts. Use of the biosensor for the analysis of complex matrices needs further study in order to improve its performance.