Andrea Pizzariello

Pencil-drawn paper supported electrodes as simple electrochemical detectors for paper-based fluidic devices.

A simple procedure for preparing inexpensive paper‐based three‐electrode electrochemical cells is described here. They consist of small circular pads of hydrophilic paper defined by hydrophobic barriers printed on paper with wax‐based ink. The back face of these pads is insulated by thermally laminating a polyethylene layer and working, reference and counter electrodes are drawn on paper by using commercial pencil leads. At last, a controlled volume of sample containing a supporting electrolyte was laid to soak in paper channels. Their performance was evaluated by assaying these devices as both simple cells suitable for recording voltammograms on static samples and low‐cost detectors for flowing systems. Voltammetric tests, conducted by using potassium hexacyanoferrate(II) as model prototype, were also exploited for identifying the brand and softness of graphite sticks enabling paper to be marked with lines displaying the best conductivity. By taking advantage of the satisfactory information thus gained, pencil drawn electrodes were tested as amperometric detectors for the separation of ascorbic acid and sunset yellow, which were chosen as prototype electroactive analytes because they are frequently present concomitantly in several food matrices, such as soft drinks and fruit juices. This separation was performed by planar thin layer chromatography conducted on microfluidic paper‐based devices prepared by patterning on filter paper two longitudinal hydrophobic barriers, once again printed with wax‐based ink. Factors affecting both separation and electrochemical detection were examined and optimised, with best performance achieved by using a 20 mM acetate running buffer (pH 4.5) and by applying a detection potential of 0.9 V. Under these optimum conditions, the target analytes could be separated and detected within 6 min. The recorded peaks were well separated and characterized by good repeatability and fairly good sensitivity, thus proving that this approach is indeed suitable for rapidly assembling inexpensive and reliable electrochemical detectors for flow analysis systems.

Effect of TiO2 photocatalytic activity in a HDPE-based food packaging on the structural and microbiological stability of a short-ripened cheese.

A high density polyethylene (HDPE)/calcium carbonate (CaCO(3)) film containing TiO(2) was prepared via blown film extrusion process. The photocatalytic properties of this film were evaluated by voltammetric, UV-Vis spectrophotometric and gas chromatographic measurements following the decomposition rate of suitably selected molecular probes, such as 4-hydroxybenzoic acid and methylene blue. The film containing 1% w/w of TiO(2) displayed a profitable and reproducible photoinduced degradation activity towards target organic compounds. The effect of packaging photocatalytic activity on the structural and microbiological stability of a short-ripened cheese was studied. Cheese structure was assessed by dynamic, small deformation rheological tests. A container consisting of a multilayer material, where the layer brought in contact with the food, made from the HDPE+CaCO(3)+TiO(2) composite matrix, was able to provide a greater maintenance of the original cheese structure than a rigid container currently used, mainly due to the inhibition of lactic acid bacteria and coliforms.

Application of microchip electrophoresis with electrochemical detection to environmental aldehyde monitoring

A method based on microchip electrophoresis with electrochemical detection has been developed for the simultaneous determination at trace levels of the main small‐chain aldehydes (formaldehyde, acetaldehyde and 2‐propenal) present in the atmosphere. Sampling was performed by forcing atmospheres through silica‐gel cartridges coated with 2,4‐dinitrophenylhydrazine (DNPH), where aldehydes were derivatized to form the corresponding hydrazones, which were then injected and eluted into the electrophoresis system. Factors affecting both separation and detection processes were optimized, with best performance achieved by applying a voltage of 2500 V both in the separation and in the electrokinetic injection (5 s) and using a 15 mM borate buffer (pH 9.2) added with 25 mM of SDS and 20% v/v ACN plus 10% v/v 1‐propanol. Under these optimal conditions, well satisfactory resolution could be achieved, so that the analytes could be separated and detected within about 400 s, by applying a detection potential of – 1.0 V versus Ag/Ag/Cl to the glassy carbon‐working electrode. The recorded peaks were characterized by both a good repeatibility (RSD <3%) and a linear dependence over a wide concentration range (2–100 μg/mL). Detection limits, estimated for a S/N of 3, equal to 9.5, 7.2 and 9.2 μM were inferred for the DNPH derivatives of formaldehyde, acetaldehyde, 2‐propenal, respectively. The application of the method to aldehyde analysis in real air samples is also presented.

Simultaneous determination of derivatized light aldehydes by microchip electrophoresis with electrochemical detection

A method, based on microchip electrophoresis with electrochemical detection, has been developed for the simultaneous determination of light aliphatic aldehydes (acetaldehyde, propionaldehyde, butyraldehyde and hexylaldehyde) derivatized with 2,4-dinitrophenylhydrazine (DNPH). Optimal conditions for the derivatization reaction, providing recoveries of 70+/-1.8% for all analytes, were identified by application to real samples, consisting of vegetable oils enriched with known amounts of the aldehydes considered. DNPH hydrazones thus obtained in acetonitrile solution were added to the electrophoresis running medium consisting of a 15mM borate buffer (pH 9.2) added with 25mM of sodium dodecyl sulfate and 35% (v/v) of acetonitrile. Factors affecting both separation and electrochemical detection were examined and optimised, with best performance achieved by using the running medium above and applying a voltage of 2250V in both separation and electrokinetic injection. Under these optimal conditions, the target analytes could be separated and detected within 350s by applying a detection potential of -1.0V (vs. Ag/AgCl) to the glassy carbon working electrode. The recorded peaks were well separated and characterized by good repeatability (RSD=1.6-3.8%), high sensitivity and a wide linear range. Detection limits of 4.5, 6.6, 6.8, 13.1microM were obtained for acetaldehyde-DNPH, propionaldehyde-DNPH, butyraldehyde-DNPH and hexylaldehyde-DNPH derivatives, respectively.

A modified electrode for the electrochemical detection of biogenic amines and their amino acid precursors separated by microchip capillary electrophoresis.

The use of a mixed‐valent ruthenium oxide/hexacyanoruthenate polymeric film electrochemically deposited onto glassy carbon electrodes is proposed here for the detection of biogenic amines and their amino acid precursors, following their separation by microchip capillary electrophoresis. The ability of this ruthenium coating to electrocatalyze the oxidation of aliphatic and heterocyclic amines, as well as their amino acid precursors, was checked by using ethanolamine, tryptamine and tryptophane as prototype compounds and adopting a 25 mM sulphuric acid as the electrolyte in the detection cell, where a constant potential of 1.05 V versus Ag/AgCl, 3 M KCl was applied to the modified working electrode. Optimization of parameters affecting both detection and separation steps led to satisfactory separations when performed by using a 20 mM phosphate running buffer (pH 2.5) and applying a high voltage of 2.5 kV both in the separation and in the electrokinetic injection (duration 4 s). The recorded peaks were characterized by good repeatability (RSD≤3.6%), high sensitivity and a wide linear range. Detection limits of 23 μM (1.4 mg/L), 27 μM (4.3 mg/L) and 34 μM (6.8 mg/L) were inferred for ethanolamine, tryptamine and tryptophane, respectively. The approach proposed here was also applied for the analysis of some double malt dark beers spiked with a controlled amount of the analytes considered.

Composite Biosensor for Sulfite Assay: Use of Water‐Insoluble Hexacyanoferrate(III) Salts as Electron‐Transfer Mediators

Water-insoluble salts of hexacyanoferrate(III) and cationic surface active agents were synthetized and used as electron-mediators for sulfite oxidase. The biosensor was prepared from a composite consisting of modified graphite (50 % w/w) and n-eicosane (50 % w/w). Graphite was modified with mediators or with both mediator and sulfite oxidase for surface- and bulk-modified electrode, respectively. The main advantage of biosensors with insoluble mediators is the possibility to operate at a potential of 0 mV (vs. SCE), thus less interferences are expected, in comparison to soluble hexacyanoferrate(III) where a potential of +300 mV must be used. The maximum sensitivity 7.8 × 10−4 μA/μM was obtained for bulk-modified biosensor, prepared from graphite modified with 5 % w/w of hexadecyltrimethylammonium hexacyanoferrate(III) and 1.25 units/mg (of graphite) of sulfite oxidase. The sensitivity of the biosensor decreased to 24 % of the initial sensitivity after one month storage in dry state at ambient temperature. The use of trehalose as an enzyme stabilization agent has led to the improved stability: 40 % of the initial stability was retained after one month.

A biosensing method for detection of caffeine in coffee

A specific inhibition of 3′,5′-cyclic phosphodiesterase (CPDE) from bovine heart by methylxanthines was used in combination with a pH electrode to develop a new biosensing method for the detection of caffeine in coffee. The potential response changes of the sensor were proportional to the concentration of caffeine in the range 0–4 mg ml−1. The response time was about 2–4 min. The standard deviation of five measurements of a 0.2 mg ml−1 caffeine solution was ±7.1 µg ml−1. The electrode gave a detection limit of 0.6 mg l−1 caffeine. The concentration of caffeine in espresso coffee was analysed. This model gave excellent correlation between observed and predicted caffeine values. This electrode exhibits advantages such as fast response, short conditioning time and low cost of the instrumentation used. We also expected to be able to perform the detection of caffeine in food and clinical analysis. © 1999 Society of Chemical Industry

A sensor based on electrodes supported on ion-exchange membranes for the flow-injection monitoring of sulphur dioxide in wines and grape juices.

A sensitive and fast responding electrochemical sensor is described for the determination of free and total sulphur dioxide in wines and grape juices which prevents interferences coming from ethanol and other natural components. It consists of a cell provided with a porous gold working electrode supported on one face of an ion-exchange membrane, acting as a solid polymer electrolyte (SPE), which allows gaseous electroactive analytes to be detected. This sensor was used as an amperometric detector for a flow injection system in which controlled volumes of headspace equilibrated with samples were injected. This approach was adopted to make also possible the determination of total SO(2), avoiding drawbacks caused by the high relative humidity generated by the sample heating resulting from the neutralization reaction of excess NaOH, whose addition was required to release sulphur dioxide from its combined forms. Factors affecting the detection process were examined and optimised. Under the identified optimal conditions, SO(2) detection resulted in sharp peaks which allowed to infer detection limits for a signal-to-noise ratio of 3, referred to liquid samples, of 0.04 and 0.02 mg L(-1) for free and total SO(2) which were determined at 20 and 35 degrees C, respectively. Moreover, the responses were found to be characterized by good repeatability (+/-2% and +/-4%, respectively) and linear dependence on the SO(2) concentration over a wide range (0.2-500 mg L(-1) for both free and total SO(2)). Finally, the long-term stability of the sensor turned out to be totally satisfactory in that responses changed of +/-9% alone even after long periods of continuous use. The application to some commercial wines and grape juices is also presented.

A simple approach to the hydrodynamic injection in microchip electrophoresis with electrochemical detection

A simple hydrodynamic injection method is proposed here for microchip CE coupled to electrochemical detection. It is based on the use of a precise syringe pump to push the sample into the microfluidic circuit, accompanied by the application of a secondary electric field to the injection channel, soon after the end of the injection step. In such a way, any counter pressure effect taking place when the sample plug enters the micrometric channel is prevented. Suitable optimization of this secondary electric field enables pushing of sample excess to be avoided and a narrow sample plug during the separation step to be maintained. Best conditions for hydrodynamic injection were achieved injecting catechol as model analyte by pressure with a syringe pump set at a flow rate of 8 μL/min for 6 s and applying to the injection channel a secondary high voltage of 700 V soon after the injection was completed. The reliability of this injection procedure has been proved by comparing electropherograms found for samples containing either catechol alone or catechol and dopamine together with those recorded under the same conditions by electrokinetic injection. Repeatability, expressed as RSD and estimated for seven replicate injections, turned out to be 2.1% for peak height of catechol used as single analyte and 0.9 and 1.1% for catechol and dopamine respectively, simultaneously injected.

Single-step microwave digestion of food and biological samples for the quantitative conversion of Se into the +4 oxidation state.

An improved single step microwave digestion procedure is described for providing the fast and easy exhaustive mineralisation of biological samples concomitantly with the quantitative conversion of any type of selenium compounds into Se(IV). In such a way, digested samples are directly suitable for the subsequent Se analysis at trace and ultratrace levels by both spectrometric methods such as HG-ICP-MS or HG-ICP-OES and differential pulse cathodic stripping voltammetry (DPCSV). It is based on the use, under suitably optimised microwave irradiation conditions, of a digestion mixture with a carefully tailored composition such that its redox potential is made lower than that allowing Se(IV) to be oxidized to Se(VI), but high enough to permit total destruction of biological or, in general, organic matrices. It consists of a nitric acid (65%, w/w) and hydrogen peroxide (30%, w/w) mixture in a volume ratio 5:1, frequently adopted for the mineralisation of organic and biological samples, but added simply with 0.25 g mL(-1) of NaCl. Successful application of the procedure, in terms of both repeatability and accuracy, to the quantification of selenium by the instrumental methods above in standard compounds and in a certified biological sample proved its good performance. The application to the Se determination in human blood plasma and in a wide variety of foods is also reported.