POM@PMO plastic electrode for phosphate electrochemical detection: a further improvement of the detection limit.

The development of a highly sensitive electrochemical sensor (E-sensor) is described based on stand-alone plastic electrodes (PE) for phosphate detection, being an essential nutrient in the marine environment. The detection mechanism is based on the chemical affinity between polyoxomolybdate anions (POM) and orthophosphate to form an electroactive phosphomolybdate complex. The custom-made E-sensor was formulated with an organic octamolybdate derivative (TBA4Mo8O26) incorporated with periodic mesoporous organosilica (PMO) to obtain a significant improvement in the analytical performances of phosphate determination. This POM@PMO combination was found to be advantageous in the determination of low concentrations of phosphate in standard solutions ranging from 1 to 500 nM, using square wave voltammetry as the detection technique. This sensitivity enhancement can be attributed to the effect of hydrophobic PMO in loading more POM moieties, owing to its highly porous structure and charged shell. Consequently, the POM@PMO-PE sensor achieved a competitive sensitivity of 4.43 ± 0.14 µA.nM-1.cm-2 and a limit of detection of 0.16 nM with good selectivity against silicates. Finally, seawater and treated wastewater samples have been tested to validate the sensor response in comparison to the official method of phosphate determination.

When microplastics meet electroanalysis: future analytical trends for an emerging threat.

Microplastics are a major modern challenge that must be addressed to protect the environment, particularly the marine environment. Microplastics, defined as particles ≤5 mm, are ubiquitous in the environment. Their small size for a relatively large surface area, high persistence and easy distribution in water, soil and air require the development of new analytical methods to monitor their presence. At present, the availability of analytical techniques that are easy to use, automated, inexpensive and based on new approaches to improve detection remains an open challenge. This review aims to outline the evolution and novelties of classical and advanced methods, in particular the recently reported electroanalytical detectors, methods and devices. Among all the studies reviewed here, we highlight the great advantages of electroanalytical tools over spectroscopic and thermal analysis, especially for the rapid and accurate detection of microplastics in the sub-micron range. Finally, the challenges faced in the development of automated analytical methods are discussed, highlighting recent trends in artificial intelligence (AI) in microplastics analysis.

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.

Plastic electrode decorated with polyhedral anion tetrabutylammonium octamolybdate [N(C4H9)4]4 Mo8O26 for nM phosphate electrochemical detection.

Inorganic phosphorous (as phosphate (PO43-), is one of the essential nutrients for all living forms, either terrestrial or marine. In oligotrophic seawaters, this macronutrient is limited (10-9 M) and its ratio with other elements (nitrogen or carbon) is denoting the health state of the marine environment; a small variation of its concentration can produce eutrophication. The gold standard method used for PO43- detection is based on colorimetric detection of phosphomolybdate. The colored complex is obtained by mixing water-soluble molybdenum salts (Mo(VI)) and reducing agents in acid media, along with the sample containing PO43-. Moreover, the kinetic of complex formation is slow, about 1 h is generally required for color to develop, exposing the assay to the drawbacks of interferences as those from silica. The detection is preferably performed in a controlled environment (i.e. in a laboratory) because several chemicals and steps of preparations are required as well as the optical instrumentation is not intended for in-field use. Electrochemical sensors offer portability and simplicity making them a practical option for on-site detection applications. To gain an analytical alternative in measuring low quantities of PO43- (10-9 M), and overcome some of the drawbacks of the classical approaches, we optimised a new easy way to produce a plastic electrode decorated with an alkyl Mo-polyoxometalate (Mo8O264-), that is soluble in organic solvents. This tetra-butyl-ammonium octamolybdate powder, [N (C4H9)4]4 Mo8O26, purposely synthetized was identified with FTIR, Raman, MS methods, and the electroactivity and reactivity with PO43- was confirmed in solution with cyclic voltammetry (CV). When the Mo-decorated electrode was in contact with PO43-, an electroactive phosphomolybdate aggregate formed at the electrode surface that was electrochemically detectable with square wave voltammetry (SWV). A remarkably low detection limit of 6.1 nM, to PO43-, as well as good stability and selectivity were obtained also in real samples. In fact, PO43- was measured in saline simulated and real seawater samples at nM concentrations in less than 5 min. The present investigation provides a new alternative to the current standard colorimetric methods to detect low phosphate concentrations, showing the potential to be used for monitoring nutrients in oligotrophic seawater.

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 microL/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.

Imprinting Pentaphyrin on Conductive Electropolymerized Dipyrromethane Films: A New Strategy towards the Synthesis of Photokilling Materials.

We report herein the synthesis and photoinduced bactericidal activity of two new polymeric materials, obtained by imprinting the photosensitizer 20-(4-carboxyphenyl)-2,13-dimethyl-3,12-diethyl-[22]pentaphyrin (PCox, 1) into suitable electropolymerized dipyrromethane films. 5-Phenyl-dipyrromethane (5-ph-DP) and 5-(4-pyridyl)dipyrromethane (5-py-DP) have been selected as the monomers for the synthesis of the materials in order to assess the correlation between the substituent in C5 and the capability in Pcox uptake. Both films have been tested in their photokilling ability toward Staphylococcus aureus by using a multi-LED blue lamp at a fluence rate of 40 W/m2 . Poly-5-py-DP/PCox, with a PCox load of 10-8  mol/cm2 , achieved a 4-log reduction in microbial viability after 60 min of irradiation. The polymeric films proved to be stable over time and under oxidation conditions; in addition, no loss of photosensitizer was observed during the experiments, thus demonstrating that the bactericidal action was effectively brought by the ROS generated by PCox immobilized in the material. After use, the films were recharged with PCox, with almost complete recovery of their photodynamic efficiency.

CNN pincer ruthenium complexes for efficient transfer hydrogenation of biomass-derived carbonyl compounds.

The ligand HCNNOMe (6-(4-methoxyphenyl)-2-aminomethylpyridine) is easily prepared from the commercially available 6-(4-methoxyphenyl)pyridine-2-carbaldehyde by the reaction of hydroxylamine and hydrogenation (H2, 1 atm) with Pd/C. The pincer complexes cis-[RuCl(CNNOMe)(PPh3)2] (1) and [RuCl(CNNOMe)(PP)] (PP = dppb, 2; and dppf, 3) are synthesized from [RuCl2(PPh3)3], HCNNOMe and PP (for 2 and 3) in 2-propanol with NEt3 at reflux and are isolated in 85-93% yield. Carbonylation of 1 (CO, 1 atm) gives [RuCl(CNNOMe)(CO)(PPh3)] (4) (79% yield) which cleanly reacts with Na[BArf4] and PCy3, affording the cationic trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5) (92% yield). These robust pincer complexes display remarkably high catalytic activity in the transfer hydrogenation (TH) of lignocellulosic biomass carbonyl compounds, using 2-propanol at reflux in a basic medium (NaOiPr or K2CO3). Thus, furfural, 5-(hydroxymethyl)furfural and Cyrene are reduced to the corresponding alcohols with 2 and 3, at S/C in the range of 10 000-100 000, within minutes or hours (TOF up to 1 500 000 h-1). The monocarbonyl complex 5 was found to be extremely active in the TH of cinnamaldehyde, vanillin derivatives and ethyl levulinate at S/C in the range of 10 000-50 000. Vanillyl alcohol is also obtained by the TH of vanillin with 5 (S/C = 500) in 2-propanol in the presence of K2CO3.

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 microg/mL). Detection limits, estimated for a S/N of 3, equal to 9.5, 7.2 and 9.2 microM 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.

Preparation of monocarbonyl ruthenium complexes bearing bidentate nitrogen and phosphine ligands and their catalytic activity in carbonyl compound reduction.

Monocarbonyl complexes [RuCl2(CO)(PR3)(NN)] (R = Cy, NN = en 1, ampy 2; R = iPr; NN = en 3) have been prepared in a one pot reaction from [RuCl2(CO)(dmf)(PPh3)2], PR3 and the NN ligand in CH2Cl2. Treatment of [Ru(OAc)2(CO)(PPh3)2] with NN ligands in methanol gives the cationic derivatives [Ru(OAc)(CO)(PPh3)(NN)]OAc (NN = en 4, ampy 5) in which one acetate acts as a bidentate ligand, whereas the other is not coordinated. Diphosphine complexes [RuCl2(CO)(PP)(PPh3)] (PP = dppb 6, dppf 7, (R)-BINAP 8, (R,Sp)-Josiphos 9 and (R,R)-Skewphos 10) have been obtained starting from [RuCl2(CO)(dmf)(PPh3)2] and the PP ligand in CHCl3 or toluene at reflux. The reaction of [Ru(OAc)2(CO)(PPh3)2] with PP in CH2Cl2 or toluene affords the fluxional acetate derivatives [Ru(OAc)2(CO)(PP)] (PP = dppb 11, dppf 12, (R)-BINAP 13, and (R,R)-Skewphos 14). The cationic diphosphine complexes [RuCl(CO)(PP)(en)]Cl (PP = dppb 15, dppf 16) are prepared from [RuCl2(CO)(dmf)(PPh3)2], PP and en in CH2Cl2 or, alternatively, from [RuCl2(CO)2]n or the 6, 7 derivatives. Similarly, [Ru(OAc)(CO)(PP)(NN)]OAc (PP = dppb, NN = en 17, ampy 18; PP = dppf, NN = en 19, ampy 20) are isolated starting from [Ru(OAc)2(CO)(PPh3)2], PP and NN ligands or from 11, 12. The derivatives [Ru(OAc)2(CO)(PP)] show a fluxional behavior in solution as the result of the flexible coordination of acetate ligands. These complexes are found to be active in the transfer hydrogenation and hydrogenation of ketones and aldehydes, including furfural derivatives, at an S/C up to 10 000 and a TOF up to 18 000 h-1.

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.

Quenching of superoxide ions by curcumin. A mechanistic study in acetonitrile.

The quenching of superoxide ions, O2.-, by curcumin has been studied by electrogenerating this anion radical from oxygen dissolved in acetonitrile solvent (that is, at best, a mimic of the lipofilic layer of biological membranes), containing known amounts of curcumin. Voltammetric tests, combined with coulometric and spectrophotometric measurements, pointed out that each mol of curcumin is able to react with six mols of such anion radical, through a process initiated by an acid-base step, which provides the perhydroxyl radical, HO2.; that disproportionates rapidly to the anionic form of hydrogen peroxide, HO2-, and oxygen, which is thus partially regenerated. At the same time, curcumin is converted to the corresponding three-charged anion. The strict resemblance existing between the mechanism of the rapid superoxide radical decay caused by curcumin and that involved in the presence of the superoxodismutase enzyme (SOD) is also underlined.

ICP-MS determination of toxic-metal release from pumping systems for food processing.

The release of heavy metals from uncovered and nickel-covered brass pumps has been evaluated by ICP-MS analysis in both simple ultrapure water and 3% acetic acid solution (mimic of neutral and acid edible liquids, respectively), following a procedure similar to that recommended by the National Sanitation Foundation (NSF) International, Test Procedure P203. The results found highlight that the main release regards zinc, copper and lead, i.e. the three major metals present in brass alloys. The first contact of brass surfaces with the extraction solvent leads to an extensive Pb release which is comparable with that observed for Cu and Zn. Subsequent washings reduce markedly the Pb release, thus rising in evidence a progressive surface passivation. In particular, the Pb release found after four repeated washings turns out to approach the limit set by both Italian and USA governments for liquids used for food purposes when determined in neutral media, while it remains quite higher when evaluated in acid media. Release analyses conducted on nickel-covered brass pumps point out that the Niploy nickel coating process is very effective for brass surface protection, in that the Pb release is reduced of about three orders of magnitude, but a Ni release exceeding the relevant permitted level is in this case observed.

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 comparison among different instrumental approaches for bromide analysis in foodstuffs digested by a suitably modified microwave procedure.

Cathodic stripping voltammetry (CSV), ion-chromatography (IC) and spectrophotometry (SP) have been tested as instrumental approaches alternative to inductively coupled plasma mass spectrometry (ICP-MS) for the determination of inorganic bromide residues in foodstuffs fumigated with brominated pesticides and digested by a suitably improved microwave procedure proposed previously. They were chosen in view of the fact that the relevant instrumentation is less expensive than that required for ICP-MS and more frequently available in analytical laboratories designed for routine food control. These approaches were compared with one another, as well as with the ICP-MS method previously adopted, not only with regard to their performance, but also in terms of the interferences caused by the composition of final samples coming from the microwave digestion procedure. The results found pointed out unambiguously that IC turns out to be well suited for replacing ICP-MS, thanks to its consistency with the composition of digested samples and its good sensitivity which allows a quite low detection limit for bromides (0.2 mg kg(-1)) to be achieved. Conversely, CSV and SP performance appears to be significantly affected by interferences caused by the presence in digested samples of chlorides and ammonium ions, respectively.