Rosanna Toniolo

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.

Amperometric monitoring of sulphur dioxide in liquid and air samples of low conductivity by electrodes supported on ion-exchange membranes

An amperometric sensor is described for the determination of sulphur dioxide in both gaseous atmospheres and solutions of low conductivity. It consists of a porous Pt electrode (facing the sample) supported on one face of an ion-exchange membrane (Nafion 417) which serves as a solid polymer electrolyte. The other side of the membrane faces an internal electrolyte solution (1 mol dm–3 aqueous perchloric acid) containing the counter and reference electrodes. This sensor is inserted into a flow cell in which gaseous or electrolyte-free aqueous samples are fed by a peristaltic pump placed in a closed-loop path and SO2 is oxidized at an applied potential of 0.65 V versus Ag–AgCl. The device is found to be characterized by a high current sensitivity and a short response time, 24 A cm–2 mol–1 dm3 and 1 s respectively for gaseous samples; (0.4 A cm–2 mol–1 dm–3 and 4 s, respectively, for water solutions), and by good stability and low background noise. The dynamic range extends up to 2 × 10–4 mol dm–3(gaseous samples) and 1 × 10–3 mol dm–3(water samples) with good linearity, and detection limits of 8 × 10–9 mol dm–3(gaseous samples) and 4 × 10–7 mol dm–3(water samples) are predicted for a signal-to-noise ratio of 3. The advantages offered by this type of sensor over conventional gas-permeation membrane electrodes are discussed.

An electroanalytical investigation on the redox properties of lacidipine supporting its anti-oxidant effect.

The redox properties of lacidipine (PyH2), one of the most pharmacologically active N-unsubstituted 1,4-dihydropyridines, have been studied by cyclic voltammetry and controlled potential electrolysis in acetonitrile, an aprotic solvent that is, at best, a mimic of the lipofilic layer of biological membranes. PyH2 undergoes a two-electron oxidation process involving two consecutive one-electron releases, the latter requiring potentials much less positive than the former. The overall process occurs through a primary one-electron step accompanied by a fast proton release, with the formation of a neutral radical (PyH*), which undergoes a further and quite easier one-electron step, thus providing the main ultimate product (PyH+) consisting in the protonated form of the parent pyridine derivative. This appears relevant for the anti-oxidant effect since the radical intermediate is much more prone to be oxidized than to be reduced, thus preventing the propagation of the oxidative chain reaction. The mentioned release of protons in the primary electrode step causes the overall process to be complicated by a parassite side reaction involving the coupling between one of the electrode products (H+) and the starting species. The protonation of PyH2 subtracts part of the original species from the electrode process because the parent cationic species (PyH3+) is no longer electroactive. This parassite reaction occurs rather slowly in the timescale of electroanalytical measurements (the relevant kinetic constant has been estimated to be 6.4 l mol(-1) s(-1)), thus markedly affecting the process only in the presence of relatively high PyH2 concentrations and progressively decreasing with the starting PyH2 concentration. All the products formed in the oxidation process (PyH+, H+ and PyH3+) have been identified by voltammetric evidences based on deep investigations on their cathodic behaviour. The advantageous anti-oxidant properties displayed by PyH2 with respect to those exhibited by phenolic anti-oxidants such as vitamin E are also discussed.

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.

Effect of the sample ionic strength on the preconcentration attained in ion exchange voltammetry

A solid state cell proposed elsewhere, enabling the extension of ion exchange voltammetry to trace determination of small ionic species with low ion exchange selectivity coefficients, dissolved in electrolyte-free media, has been adopted to verify whether the degrees of preconcentration achieved with such a voltammetric technique actually increase with the decrease of the sample ionic strength—in agreement with thermodynamic expectations—or whether they are otherwise conditioned by kinetic complications. With this purpose, a series of experiments has been conducted by accumulating either monovalent or divalent cationic analytes (Tl+ and Pb2+) at glassy carbon electrodes modified with Nafion® films loaded with either monovalent or divalent cations (Na+ and Ca2+) from aqueous solutions containing different concentrations of both the analyte and the loading ion introduced as the cation of the supporting electrolyte. These preconcentrated ions were then assayed by inserting the modified electrodes in the solid state cell mentioned. The results obtained indicate that the expected inverse proportional relation between the degree of preconcentration achieved and the ionic strength of the sample analyzed is found only at fairly high concentrations of supporting electrolyte, the values of which depend on both the charge and concentration of the analyte as well as on the charge of the counterion from the supporting electrolyte. In contrast, at lower ionic strengths, the degree of preconcentration is kinetically controlled. This finding is discussed in terms of interdiffusion effects as well as of the decrease in both diffusion coefficients in the polymer and equilibrium constants, owing to cross-linking changes involved in the ion exchange processes. Also the outcome of such kinetic control on analytical determinations of trace ionic species in poorly conducting samples is considered.

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.

Improved microwave digestion procedure for inductively coupled plasma mass spectrometric determinations of inorganic bromide residues in foodstuffs fumigated with methyl bromide

Abstract An improved microwave digestion procedure is described for the inductively coupled plasma mass spectrometric (ICP-MS) determination of inorganic bromide residues in foodstuffs fumigated with methyl bromide. It is based on the addition to the usual acid oxidizing mixture of small amounts of silver ions which cause bromides from the mineralized matrix to precipitate as sparingly soluble AgBr, thus, avoiding losses due to their conversion into volatile products. The silver bromide precipitate separated from the digested sample is dissolved with ammonia and the cationic complex [Ag(NH3)2]+ thus formed is removed on a cation-exchange column. The resulting solution is finally acidified and added with indium internal standard, to make it suitable for inductively coupled plasma mass spectrometric (ICP-MS) analysis. This modified procedure has been tested for the analysis of synthetic KBr samples by obtaining totally satisfactory results, in that bromide recoveries ranging from 98.6 to 100.2% were found, with an overall standard deviation of 5.17%. Its application to the bromide determination in some mushroom samples is reported and the results found are compared with those obtained on the same samples mineralized by conventional alkali fusion. The proposed procedure can be extended to the determination in biological samples of other anions which may be lost during a conventional acid oxidizing microwave mineralization, but are able to form insoluble silver salts.