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Acid-base equilibria in ethaline. An approach providing a strategy for the pH modulation in deep eutectic solvents
Get PDFThe possibility of changing the pH in deep eutectic solvents when they are used as solvent media is fundamental in chemistry, since acid-base equilibria are of critical importance for almost all areas of chemistry, such as synthetic reactions, isolation of specific analytes, chromatographic separations, protein stability and enzyme catalysis. Such a possibility of modulating the pH of DESs is particularly interesting for enhancing the electrocatalytic properties of some electrode surfaces or promoting the stability and in some cases improving the activity of biorecognition systems such as antibodies, enzymes and aptamers that are particularly useful for biosensor applications. The aim of this investigation was the evaluation of the possibility of easily modulating the acid-base properties of the DES ethaline, adopted in our tests as the prototype of DESs because it displays a moderate viscosity at room temperature, by simply adding typical pH buffers such as weak acids or bases or salts consisting of weak acids and strong bases. This approach results to be particularly advantageous compared to that often used of modifying the proton activity of DESs by changing one or both of its precursors (HBA or HBD), since it allows their chemical-physical properties, such as density, viscosity, conductivity and surface tension, to be kept unchanged. With this purpose, acid-base equilibria in the DES ethaline were studied using cyclic voltammetry, pH measurements at glass electrodes, acid-base titrations and spectrophotometric measurements by exploiting suitable electrochemical probes, such as hydroquinone and alizarin: this last being an acid-base indicator. The results obtained with these measurements are all consistent with each other and provide an effective strategy to achieve pH modulation in DES, since the approach taken here in ethaline can be easily transferred to any other DES, provided it is capable of solubilizing the desired buffer species
A simple method for patterning poly(dimethylsiloxane) barriers in paper using contact-printing with low-cost rubber stamps
No full textThis paper presents a simple and low-cost method for patterning poly(dimethylsiloxane) (PDMS) barriers in porous support such as paper for the construction of flexible microfluidic paper-based analytical devices (μPADs). The fabrication method consisted of contact-printing a solution of PDMS and hexane (10:1.5 w/w) onto chromatographic paper using custom-designed rubber stamps containing the patterns of μPADs. After penetrating the paper (∼30 s), the PDMS is cured to form hydrophobic barriers. Under optimized conditions, hydrophobic barriers and hydrophilic channels with dimensions down to 949±88 μm and 771±90 μm (n=5), respectively, were obtained. This resolution is well-suitable for most applications in analytical chemistry. Chemical compatibility studies revealed that the PDMS barriers were able to contain some organic solvents, including acetonitrile and methanol, and aqueous solutions of some surfactants. This find is particularly interesting given that acetonitrile and methanol are the most used solvents in chromatographic separations, non-aqueous capillary electrophoresis and electroanalysis, as well as aqueous solutions of surfactants are suitable mediums for cell lyses assays. The utility of the technique was evaluated in the fabrication of paper-based electrochemical devices (PEDs) with pencil-drawn electrodes for experiments in static cyclic voltammetry and flow injection analysis (FIA) with amperometric detection, in both aqueous and organic mediums
Electropolymerisation and electrochemical behavior of polypyrrole in deep eutectic solvents. An EQCM study
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A sensor based on electrodes supported on ion-exchange membranes for the flow-injection monitoring of suplhur dioxide in wines and grape juices
No full textA 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
Amperometric sniffer for volatile amines based on paper-supported room-temperature ionic liquids enabling rapid assessment of fish spoilage
No full textSimultaneous determination of derivatized light aldehydes by microchip electrophoresis with electrochemical detection
No full textA 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 15 mM borate buffer (pH 9.2) added with 25 mM 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 350 s 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, 6.8,13.1 mu M were obtained for acetaldehyde-DNPH, propionaldehyde-DNPH, butyraldehyde-DNPH and hexylaldehyde-DNPH derivatives, respectively
Application of microchip electrophoresis with electrochemical detection to environmental aldehyde monitoring
No full textA 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 2500V 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 mu g/mL). Detection limits, estimated for a SIN of 3, equal to 9.5, 7.2 and 9.2 mu M 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 detection of peracetic acid and hydrogen peroxide by amperometry at Pt and Au electrodes
No full textBased on preliminary voltammetric investigations at both Pt and Au electrodes in aqueous solutions buffered at different pH values in the range 0-10, two possible profitable triple-pulse amperometric approaches were developed for determining simultaneously peroxyacetic acid (PAA) and hydrogen peroxide present in the same samples. At both surfaces a pulsed waveform applied at rotating-disc electrodes was adopted to take advantage on one hand of the optimized signal reproducibility achieved by this potential multi-step anti-fouling approach and on the other hand of the constant thickness of the diffusion layer, which is necessary when the recording of time-independent currents is desired. At a rotating-disc Pt electrode an anodic selective signal was indeed recorded for H2O2 alone, while PAA contents could be inferred only from the difference of convenient signals, since at all pHs explored its sole cathodic reaction could be observed at potentials coincident with those proper for the reduction of H2O2 too. The same pulse approach at Au electrodes instead provided totally independent signals for the two analytes considered, thus proving to be suitable for their independent detection. In fact, H2O2 alone undergoes anodic oxidation also at this surface, while the reduction of PAA occurs at potentials less cathodic than those required for H2O2. At both electrodes, the best results turned out to be achieved at pH = 0 in terms of both precision (± 2 - 4 %) and detection limits (0.2 - 0.3 mM), as well as of linear range which extended for about three orders of magnitude. The kinetics of the equilibrium involving the generation of H2O2 from the reaction of PAA with water was also evaluated, since it was suspected of making unreliable the proposed amperometric approaches
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