Effect of Water on a Hydrophobic Deep Eutectic Solvent.

Deep eutectic solvents (DESs) formed by hydrogen bond donors and acceptors are a promising new class of solvents. Both hydrophilic and hydrophobic binary DESs readily absorb water, making them ternary mixtures, and a small water content is always inevitable under ambient conditions. We present a thorough study of a typical hydrophobic DES formed by a 1:2 mole ratio of tetrabutyl ammonium chloride and decanoic acid, focusing on the effects of a low water content caused by absorbed water vapor, using multinuclear NMR techniques, molecular modeling, and several other physicochemical techniques. Already very low water contents cause dynamic nanoscale phase segregation, reduce solvent viscosity and fragility, increase self-diffusion coefficients and conductivity, and enhance local dynamics. Water interferes with the hydrogen-bonding network between the chloride ions and carboxylic acid groups by solvating them, which enhances carboxylic acid self-correlation and ion pair formation between tetrabutyl ammonium and chloride. Simulations show that the component molar ratio can be varied, with an effect on the internal structure. The water-induced changes in the physical properties are beneficial for most prospective applications but water creates an acidic aqueous nanophase with a high halide ion concentration, which may have chemically adverse effects.

Electroanalytical cells pencil drawn on PVC supports and their use for the detection in flexible microfluidic devices.

A simple, effective and low-cost technique is here presented for assembling flexible and robust electrochemical devices on transparent PVC supports, using ordinary tools, all installed on a commercial desktop digitally controlled plotter/cutter. Small diamond burs were first set up to rough precise and well defined patterns on the surface of smooth and flexible PVC transparent films. Subsequently, reference, counter and working carbon electrodes were drawn onto abraded patterns by using micropencils (4B graphite leads, 0.5 mm in diameter), in their turn installed on the plotter/cutter. The effective active working surface of electrochemical cells was then defined by a thin adhesive strip or by covering the patterned support with a suitably cut adhesive layer, depending upon whether they were intended for use in batch or drop mode. After optimization of fabrication parameters, such as pressure and speed adopted during bur abrasion and pencil drawing, the electrochemical characterization of these cells was performed by using potassium hexacyanoferrate(II) as redox probe. Voltammetric responses displayed a good inter-device reproducibility (5.6%), thus confirming the effectiveness of this easy and fast assembling strategy. These PVC-based pencil-drawn electrochemical cells were then integrated as thin-layer detectors in adhesive-tape based microfluidic channels, cut and prepared in their turn using the digitally controlled plotter/cutter. These detectors offer the advantage given by the impermeability of PVC supports, thus avoiding absorption of the flowing carrier and consequent analyte broadening, instead occurring when electrochemical cells are pencil drawn on hydrophilic materials as paper. After optimization of the complete fabrication process, the effectiveness of these devices was tested by a proof-of-concept direct quantification of ascorbic acid in commonly used drugs.

A Flexible Platform of Electrochemically Functionalized Carbon Nanotubes for NADH Sensors.

A flexible electrode system entirely constituted by single-walled carbon nanotubes (SWCNTs) has been proposed as the sensor platform for ß-nicotinamide adenine dinucleotide (NADH) detection. The performance of the device, in terms of potential at which the electrochemical process takes place, significantly improves by electrochemical functionalization of the carbon-based material with a molecule possessing an o-hydroquinone residue, namely caffeic acid. Both the processes of SWCNT functionalization and NADH detection have been studied by combining electrochemical and spectroelectrochemical experiments, in order to achieve direct evidence of the electrode modification by the organic residues and to study the electrocatalytic activity of the resulting material in respect to functional groups present at the electrode/solution interface. Electrochemical measurements performed at the fixed potential of +0.30 V let us envision the possible use of the device as an amperometric sensor for NADH detection. Spectroelectrochemistry also demonstrates the effectiveness of the device in acting as a voltabsorptometric sensor for the detection of this same analyte by exploiting this different transduction mechanism, potentially less prone to the possible presence of interfering species.

A cotton thread fluidic device with a wall-jet pencil-drawn paper based dual electrode detector.

A simple and sensitive device is presented based on the use of pencil-drawn paper based electrochemical detector placed at the end of a cotton thread fluidic channel in wall-jet configuration. This innovative and fast responding electroanalytical system can be adopted for both single and dual electrode electrochemical detection, this last achieved by applying two different potentials at two independent working electrodes drawn on the opposite faces of the paper based detector. Its performance was preliminarily optimized by adopting hexacyanoferrate(II) as probe species undergoing reversible electrochemical processes. These devices were then used for the single electrode detection of ascorbic acid in aqueous samples and the dual electrode detection of orthodiphenols in extra virgin olive oils (EVOOs). In fact, these devices enable hydrophilic orthodiphenols, typically present in EVOOs (extracted by a 80:20% v/v acetonitrile/water mixture), to be discriminated from hydrophilic monophenols instead present in almost all vegetable oils. Flow-injections runs were conducted by using a 0.01 M H2SO4 + 0.5 KCl running electrolyte allowing the rapid and selective detection of hydrophilic orthodiphenols with satisfactory sensitivity and a low enough detection limit (2 µM). Different real samples of EVOOs and sunflower oils were analyzed. Abundant enough contents of orthidiphenols were found in EVOO samples, while no trace of these antioxidants was found in sunflower oils.

Digitally Controlled Procedure for Assembling Fully Drawn Paper-Based Electroanalytical Platforms.

A simple, reliable, and low-cost fabrication method is proposed here for assembling paper-based electrochemical devices (PEDs) using a commercial desktop digitally controlled plotter/cutter, together with ordinary writing tools. Permanent markers (tips of 1 mm) were used to create effective hydrophobic barriers on paper, while micromechanical pencils (mounting 4B graphite leads, 0.5 mm in diameter) were adopted for automatically drawn precise reference, counter, and working carbon electrodes. Fabrication parameters, such as writing pressure and speed, were first optimized, and the electrochemical performance of these devices was then evaluated by using potassium hexacyanoferrate(II) as redox probe. The good interdevice reproducibility (4.8%) displayed by the relevant voltammetric responses confirmed that this strategy can be profitably adopted to easily assemble paper-based electrochemical devices in a highly flexible manner. The simplicity of the instrumentation used and the low cost of each single device (about $0.04), together with the speed of fabrication (about 2 min), are other important features of the proposed strategy. Finally, to confirm the effectiveness of this prototyping method for the analysis of real samples and rapid controls, PEDs assembled by this simple approach were successfully exploited for the analysis of vitamin B6 in food supplements.

A paper-based platform with a pencil-drawn dual amperometric detector for the rapid quantification of ortho-diphenols in extravirgin olive oil.

A simple, sensitive and fast responding device is described for the discrimination of hydrophilic ortho-diphenols, whose presence in abundant enough amounts is typical for extra virgin olive oils (EVOOs), from hydrophilic mono-phenols instead present in almost all vegetable oils. It consists of a dual electrode detector pencil-drawn at the end of a paper microfluidic channel, defined by hydrophobic barriers, where samples of these antioxidants, extracted from vegetable oils by a 80:20% v/v acetonitrile/water mixture, were applied. Thin-layer chromatographic runs conducted by using a 0.01 M H2SO4 + 1 M KCl running buffer allowed the selective detection of hydrophilic ortho-diphenols by profiting from the fact that they undergo reversible oxidation at less positive potentials than those required by monophenols for displaying their irreversible anodic process. On this basis, a potential for the oxidation of hydrophilic ortho-diphenols was applied to the upstream pencil-drawn electrode (W1) (at which a minor fraction of mono-phenols was also oxidized), while a potential for the reverse process involving the sole product (ortho-quinones) of the reversible oxidation of ortho-diphenols was imposed at the downstream pencil-drawn working electrode (W2). Thus, cathodic peak currents linearly dependent on analyte concentrations could be recorded at W2 which led to a satisfactory detection limit (8 µM, equivalent to 1.23 mg/L) even when working electrodes W1 and W2 with same dimensions were employed. Improved sensitivities and lower detection limits were achieved by increasing the dimensions of W2 with respect to W1, thanks to the improvement of the collection efficiency. Throughout this investigation, hydroxytyrosol (HTy) and tyrosol (Ty) were adopted as models of ortho-diphenols and mono-phenols, respectively, in view of their abundant presence in EVOOs. Real samples of EVOO from different production companies, of a simple olive oil and of a sunflower oil were analyzed. Different hydrophilic ortho-diphenol contents were found in EVOO samples (up to 40.8 mg/kg), while only a negligible amount turned out to be present in simple olive oil. No trace of these antioxidants were instead found in sunflower oil, as expected. All concentrations found were in good agreement with those detected by a more frequently employed spectrophotometric method used for the sake of comparison.

Determination of polyphenol content and colour index in wines through PEDOT-modified electrodes.

Poly(3,4-ethylenedioxythiophene)-modified electrodes have been used for the estimation of the polyphenolic content and of the colour index of different samples of wines. Synthetic wine solutions, prepared with different amount of oenocyanins, have been analysed spectrophotometrically and electrochemically in order to find a correlation between the total polyphenolic content or colour index and the current peak. The regression curves obtained have been used as external calibration lines for the analysis of several commercial wines, ranging from white to dark red wines. In this way, a rapid estimation of the total polyphenolic content and of the colour index may be accomplished from a single voltammetric measurement. Furthermore, principal component analysis has also been used to evaluate the effect of total polyphenolic content and colour index on the whole voltammetric signals within a selected potential range, both for the synthetic solutions and for the commercial products. Graphical abstract Electrochemical sensors for the rapid determination of colour index and polyphenol content in wines.

Simple pencil-drawn paper-based devices for one-spot electrochemical detection of electroactive species in oil samples.

We propose here simple electrochemical cells assembled with electrodes pencil drawn on paper for conducting one-spot tests enabling olive oil to be easily distinguished from other vegetable oils. They consist of small circular pads of hydrophilic paper defined by hydrophobic barriers, these last printed by using custom-designed rubber stamps, where working, reference, and counterelectrodes are drawn by pencil leads. These cells were first wetted with a small volume of aqueous electrolyte, avoiding coating of the upper surface of electrodes. A controlled volume of edible oil samples was then applied on top of the moist cell. The results found proved that these devices can be adopted as effective platforms suitable for the detection of electroactive compounds present in edible oils. In fact, they allow voltammetric profiles to be recorded not only for the oxidation of water soluble species (ortho-diphenols, as well as some monophenols and polyphenols) present in olive oils, but also for electroactive hydrophobic components (e.g., α-tocopherol) present in sunflower oils, which were chosen as model of seed oils. The whole of collected findings pointed out that simple one-spot tests performed by these devices enable olive oils to be easily distinguished from other edible oils on the basis of their clearly different voltammetric profiles. A satisfactory interdevice reproducibility (±13%) was estimated by applying strictly similar extra virgin olive oil samples onto seven different cells carefully prepared by the same procedure. An operating mechanism able to account for the detection of also electroactive hydrophobic compounds present in oils is proposed.

Ti metal electrode as an unconventional amperometric sensor for determination of Au(III) species.

The control of the noble metal concentration is crucial in order to increase the efficiency of hydrometallurgic processes in mining and in the recovery of precious materials from electronic waste. The present study is devoted to the development of an effective procedure for the quantification of Au(III) species dissolved in aqueous solutions, similar real complex matrices included. In particular, a novel electrode system based on Ti has been studied. This electrode material is still poorly investigated in the framework of electroanalysis, despite its lack of sensitivity to common interfering species, such as oxygen; hence, the determination of metal species can be carried out without performing deaeration of the solution. In addition, the interfering effect due to the presence of other heavy metal ions, such as Ag, Fe and Pb, has been minimised by a proper choice of the conditions adopted for the amperometric measurements. Ti electrodes exhibit reproducible electrochemical responses, even in the presence of high concentration of organic fouling species typical of biosorption processes.

Development of a sensor system for the determination of sanitary quality of grapes.

n instrument for the automatic quantification of glycerol in grapes has been developed. We verify here that this analyte can be used as a benchmark of a serious disease affecting the grapevines, namely Botrytis cinerea. The core of the instrument is an amperometric biosensor consisting of a disposable screen printed electrode, generating the analytical signal thanks to a bi-enzymatic process involving glycerol dehydrogenase and diaphorase. The full automation of the analysis is realised by three micropumps and a microprocessor under control of a personal computer. The pumps allow the correct and constant dilution of the grape juice with a buffer solution also containing [Fe(CN)6]3- redox mediator and the injection of NAD+ cofactor when the baseline signal reaches a steady state; the instrument leads to automated reading of the analytical signal and the consequent data treatment. Although the analytical method is based on an amperometric technique that, owing to heavy matrix effects, usually requires an internal calibration, the analyses indicate that a unique external calibration is suitable for giving accurate responses for any grapes, both white and black ones.

Graphene-modified electrode. Determination of hydrogen peroxide at high concentrations.

A gold electrode partially coated by graphene multilayer is developed and tested with respect to high concentrations of hydrogen peroxide. The effective use of conventional electrode materials for the determination of such an analyte by anodic oxidation or cathodic reduction is prevented by the occurrence of adsorptions fouling the electrode surface. This prevents reliable and repeatable voltammetric curves for being recorded and serious problems arise in quantitative analysis via amperometry. The gold-graphene electrode is shown to be effective in quantitative evaluation, by cathodic reduction, of hydrogen peroxide at concentration levels that are of interest in an industrial. Acid, neutral, and basic pH values have been tested through correct adjustment of a Britton Robinson buffer. The experiments have been performed both by cyclic voltammetry and with amperometry at constant potential in unstirred solution. The latter technique has been employed in drawing a calibration linear plot. In particular, the performances of the developed electrode system have been compared with those of both pure gold and pure graphene electrode materials. The bi-component electrode was more sensitive; co-catalytic action by the combination of the two components is hypothesised. The system is stable over many potential cycles, as checked by surface-enhanced Raman spectra recorded over time.

Studies of the interface of conducting polymers with inorganic surfaces.

Many of the properties of multi-material systems and relevant devices depend on the interfaces between the different components. This review focuses on characterization of the interfaces between intrinsically conducting polymers and inorganic materials consisting of metals and metal oxides. These materials are chosen because of their importance in several analytical applications. Although use of conducting polymers and metals or metal oxides in analytical systems, specifically in sensing, is well established, the number of novel materials used for analytical purposes is continuously increasing. This further increases the possible number of effective combinations of different materials within multicomponent systems. As a consequence, innovative characterization techniques have become as important as more conventional techniques. On the other hand, sophisticated characterisation techniques are increasingly widespread and, consequently, also readily accessible. This critical review is not an exhaustive discussion of all possible analytical techniques suitable for characterization of interfaces. It is, instead, limited to an overview of the most effective, relatively widespread techniques, emphasising their most significant recent advances. Critical analysis of the individual techniques is complemented by a few selected examples.

Peptide nucleic acids tagged with four lysine residues for amperometric genosensors.

A homothymine PNA decamer bearing four lysine residues has been synthesized as a probe for the development of amperometric sensors. On one hand, the four amino groups introduced make this derivative nine times more soluble than the corresponding homothymine PNA decamer and, on the other hand, allow the stable anchoring of this molecule on Au nanostructured surface through the terminal -NH 2 moieties. In particular, XPS and electrochemical investigations performed with hexylamine, as a model molecule, indicate that the stable deposition of primary amine derivatives on such a nanostructured surface is possible and involves the free electron doublet on the nitrogen atom. This finding indicates that this PNA derivative is suitable to act as the probe molecule for the development of amperometric sensors. Thanks to the molecular probe chosen and to the use of a nanostructured surface as the substrate for the sensor assembly, the device proposed makes possible the selective recognition of the target oligonucleotide sequence with very high sensitivity.

UPS, XPS, and NEXAFS study of self-assembly of standing 1,4-benzenedimethanethiol SAMs on gold.

We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 °C for 30 min and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free -SH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24° with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units.

Water-soluble full-length single-wall carbon nanotube polyelectrolytes: preparation and characterization.

HiPco single-wall carbon nanotubes (SWNTs) have been noncovalently modified with ionic pyrene and naphthalene derivatives to prepare water-soluble SWNT polyelectrolytes (SWNT-PEs), which are analogous to polyanions and polycations. The modified nanotubes have been characterized with UV-vis-NIR, fluorescence, Raman and X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The nanotube-adsorbate interactions consist of pi-pi stacking interactions between the aromatic core of the adsorbate and the nanotube surface and specific contributions because of the substituents. The interaction between nanotubes and adsorbates also involves charge transfer from adsorbates to SWNTs, and with naphthalene sulfonates the role of a free amino group was important. The ionic surface charge density of the modified SWNTs is constant and probably controlled by electrostatic repulsion between like charges. The linear ionic charge density of the modified SWNTs is similar to that of common highly charged polyelectrolytes.

3-methylthiophene self-assembled monolayers on planar and nanoparticle Au surfaces.

In this article the adsorption of 3-methylthiophene on planar and nanoparticle Au surfaces is investigated. The resulting systems are compared with a benchmark system based on 1-decanethiol. The characterization data collected evidence the formation of a packed 3-methylthiophene SAM on the planar surface. In particular, spectroscopic investigations suggest that 3-methylthiophene aromatic system is not adsorbed on the surface through the pi-electron system but rather through the S atom alone. On the other hand, the behavior of 3-methylthiophene on nanoparticle surfaces is notably different from that of the alkanethiol. Only a limited fraction of the surface of Au nanoparticles results to be actually coated after purification; this notwithstanding, the nanoparticle growth seems to be strongly influenced by the presence of such a labile encapsulating agent.