[Show abstract][Hide abstract] ABSTRACT: A microfluidic device is presented with off-chip electrodes residing in a reservoir and connected via micro-capillaries to
the Y-shaped microfluidic channel. The device is tested by potentiometric measurements involving dual-stream laminar flow
of two aqueous solutions carrying different electrolytes at various concentrations. Open circuit potentials are measured for
a series of solutions of alkali metal chlorides and tetraalkylammonium chlorides as well as for dilute hydrochloric acid.
The open circuit potential for the microfluidic chip was calculated by taking into account the diffusion potential at finite
ionic strength as well as the potential difference introduced by the reference electrode system. The liquid junction potential
developed at the boundary of the co-flowing aqueous solutions may be manipulated to have greater or lesser relative contributions
to the measured open circuit potential based on use of electrolyte salts having cation and anion pairs of similar or dissimilar
mobilities in solution. A reasonable agreement between theoretical and experimental values of the open circuit potential is
observed for these situations. The results show that simple microfluidic structures possess a rich environment for exploration
and application of the solution chemistry of ions.
Microfluidics and Nanofluidics 04/2009; 6(2):231-240. DOI:10.1007/s10404-008-0319-z · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Geometrically regular silicon membrane-based micropore arrays were employed for defined arrays of micrometer-sized interfaces between two immiscible electrolyte solutions (μITIES). These were incorporated into a poly(tetrafluoroethylene) (PTFE) hydrodynamic cell. Electrochemistry at the μITIES array was undertaken following gellification of the organic phase using polyvinyl chloride (PVC) and flowing an aqueous phase over the array surface. Cyclic voltammetric characterization of asymmetric diffusion profiles on either side of the μITIES was accomplished under flowing conditions using positively and negatively charged (TEA+ and 4-OBSA−, respectively) model analyte species. Incorporation of an ionophore (dibenzo-18-crown-6 ether) into the organogel allowed the ion-transfer detection of two oligopeptides (phenylalanine dipeptide and lysine dipeptide) within the available potential window under stationary and flowing conditions. Flow rate studies with TEA+ indicated that the amperometric peak currents do not obey the Levich equation, due to diffusion dominating the mass transport, as opposed to convection. The influence of the applied potential () on the amperometric response of the oligopeptides was studied and hydrodynamic voltammograms (HDVs) for the individual oligopeptides were subsequently constructed. The data presented provide a basis for the use of silicon membrane-based μITIES arrays in flow analytical methods.
[Show abstract][Hide abstract] ABSTRACT: Electrochemically modulated liquid-liquid extraction (EMLLE) enables the selective extraction and separation of ions from mixtures by choice of an applied interfacial potential difference. The extraction of ionized drugs from artificial urine is reported in this paper. The artificial urine matrix was characterized by cyclic voltammetry at the interface between two immiscible electrolyte solutions (ITIES), showing that components of that aqueous phase truncate the available potential window at the ITIES. The transfer of three cationic drugs from aqueous artificial urine to the 1,2-dichloroethane organic electrolyte phase was examined. Both propranolol and timolol were found to transfer across the artificial urine-organic interface. However, sotalol transfer was not possible within the available potential window. Extraction of propranolol and timolol from artificial urine into an organogel phase, by electrochemically modulated liquid-liquid extraction, was examined. The application of potentials positive of the drugs' formal transfer potentials enabled the selective extraction of both propranolol and timolol, with a higher potential being required for timolol. This work demonstrates the practical utility of EMLLE for the selective extraction of target compounds from a complex sample matrix.
[Show abstract][Hide abstract] ABSTRACT: Electrochemistry at the liquid-liquid interface enables the detection of nonredoxactive species with electroanalytical techniques. In this work, the electrochemical behavior of two food additives, aspartame and acesulfame K, was investigated. Both ions were found to undergo ion-transfer voltammetry at the liquid-liquid interface. Differential pulse voltammetry was used for the preparation of calibration curves over the concentration range of 30-350 microM with a detection limit of 30 microM. The standard addition method was applied to the determination of their concentrations in food and beverage samples such as sweeteners and sugar-free beverages. Selective electrochemically modulated liquid-liquid extraction of these species in both laboratory solutions and in beverage samples was also demonstrated. These results indicate the suitability of liquid-liquid electrochemistry as an analytical approach in food analysis.
Journal of Agricultural and Food Chemistry 07/2008; 56(12):4304-10. DOI:10.1021/jf7035966 · 2.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The detection of protonated dopamine by differential pulse voltammetry (DPV) and square wave voltammetry (SWV) at arrays of micro-interfaces between two immiscible electrolyte solutions (microITIES) is presented. Microfabricated porous silicon membranes (consisting of eight pores, 26.6 microm in radius and 500 microm pore-pore separation, in a hexagonal layout) were prepared by photolithographic and etching procedures. The membrane pores were fabricated with hydrophobic internal walls so that the organic phase filled the pores and created the liquid interface at the aqueous side of the membrane. These were used for harnessing the benefits of three-dimensional diffusion to the interface and for interface stabilisation. The liquid-liquid interface provides a simple method to overcome the major problem in the voltammetric detection of dopamine at solid electrodes due to the co-existence of ascorbate at higher concentrations. Selectivity for dopamine over ascorbate was achieved by the use of dibenzo-18-crown-6 (DB18C6) for the facilitated ion transfer of dopamine across the microITIES array. Under these conditions, the presence of ascorbate in excess did not interfere in the detection of dopamine and the lowest concentration detectable was ca. 0.5 microM. In addition, the drawback of current signal saturation (non-linear increase of the peak current with the concentration of dopamine) observed at conventional (millimetre-sized) liquid-liquid interfaces was overcome using the microfabricated porous membranes.
[Show abstract][Hide abstract] ABSTRACT: A microfluidic device with integrated electrodes for the electrochemically-modulated extraction of ions across immiscible aqueous–organic liquid–liquid interfaces is presented. Using a Y-shaped microfluidic channel with in situ electrodes and co-flowing aqueous and organic immiscible electrolyte solutions, the manipulation of the applied interfacial potential enabled the extraction of ions from the aqueous phase into the organic phase. Data for the extraction of tetraethylammonium cations from aqueous electrolyte into 1,2-dichloroethane electrolyte are presented. The device demonstrates the benefits of combination of microfluidics and liquid–liquid electrochemistry.
[Show abstract][Hide abstract] ABSTRACT: The electrochemistry of a series of dendrimers was examined at the interface between two immiscible electrolyte solutions (ITIES), enabling study of non-redox-active dendrimers. Different generations of poly(propylenimine) (DAB-AM-n) and poly(amidoamine) (PAMAM) dendrimers were studied. In their protonated states, the dendrimers were transferred across the ITIES, with the electrochemical behavior observed depending on the dendrimer family, the generation number, and the experimental pH. The electrochemistry of the lower generations studied was characterized by well-defined peaks for both dendrimer families and with small peak-peak separations in the case of the PAMAM family. The voltammetry of the higher generations was more complex, showing distorted voltammograms and instability of the interface. The charges of the transferring dendrimers were calculated by convolution of the voltammetric data and were similar to the theoretical charges for DAB-AM-n. For PAMAM, only the lowest generation exhibited reversible behavior, with higher generations having irreversible behavior. Using cyclic voltammetry, low micromolar concentrations of the dendrimers were detected. The results show that electrochemistry at the ITIES can be a useful method for characterization of ionizable dendrimers and that voltammetry can be a simple method for detection of low concentrations of these multicharged species.
[Show abstract][Hide abstract] ABSTRACT: This paper describes the electrochemical characterisation of a range of gold and platinum microelectrode arrays (MEAs) fabricated by standard photolithographic methods. The inter-electrode spacing, geometry, numbers and dimensions of the electrodes in the arrays were found to influence the voltammetric behaviours obtained. Excellent correlation was found between experimental data and theoretical predictions employing published models of microelectrode behaviour. Gold MEAs were evaluated for their applicability to copper determination in a soil extract sample, where agreement was found between the standard analytical method and a method based on underpotential deposition-anodic stripping voltammetry (UPD-ASV) at the MEAs, offering a mercury-free alternative for copper sensing.
[Show abstract][Hide abstract] ABSTRACT: Electrochemistry at the interface between two immiscible electrolyte solutions has been presented as a method of electrochemically modulated liquid-liquid extraction, where ions in a mixture can be selectively partitioned as a function of the applied interfacial potential difference. In this study, a mixture comprising 4-octylbenzenesulfonate (4-OBSA-) and tetraethylammonium (TEA+) ions was evaluated. The application of negative potential differences enabled the selective extraction of 4-OBSA- into the organic phase, and more positive potential differences enabled the selective extraction of TEA+. However, intermediate potentials lead to the coextraction of both ions into the organic phase, with apparent selectivity for TEA+ over 4-OBSA-. An increased concentration of either ion in the mixture inhibited the extraction response of the other ion, but the order of the extraction at these intermediate potentials was always TEA+ followed by 4-OBSA-. The reasons for the selectivity for the cation over the anion are discussed.
[Show abstract][Hide abstract] ABSTRACT: The development of ion extraction methods under electrochemical control via electrochemistry at the interface between two immiscible electrolyte solutions is discussed. A hydrodynamic flow injection system was used for the potentiostatic extraction of non-redox-active species from a flowing aqueous phase into a stationary organogel phase. The ions tetraethylammonium, 4-octylbenzenesulfonate (4-OBSA-), and p-toluenesulfonate (p-TSA-) were studied as model analytes. The extraction study comprised examination of the influence of extraction potentials, aqueous-phase flow rate, and target species concentration. The extraction process can be monitored in situ by means of the ion-transfer current, which has opposing signs for anions and cations. Hydrodynamic voltammograms were obtained from these experiments. The selective extraction of 4-OBSA-, from its mixture with p-TSA-, as well as coextraction of both anions is shown. The results demonstrate the utility of electrochemical modulation for the controlled extraction of ions from an aqueous phase into an organogel electrolyte phase. This offers potential benefits for various analytical processes including sample preparation and cleanup.
[Show abstract][Hide abstract] ABSTRACT: The voltammetry of hexavalent chromium (ammonium dichromate) at the interface between two immiscible electrolyte solutions is reported. Detection of Cr(VI) by ion transfer voltammetry is possible by use of an organic phase ionophore, which facilitates the transfer of Cr(VI) from the aqueous into the organic phase. The ionophore was the penta protonated form of polyamine macrocycle 2,5,8,11,14-pentaaza-16,29-phenanthrolinophane (NeoTT). Cyclic voltammetry showed an increase of the peak current on increasing the concentration of Cr(VI). Square wave voltammetry with background subtraction was employed for low level concentration detection. The lowest concentration detected was 0.25 parts per million of Cr(VI).
[Show abstract][Hide abstract] ABSTRACT: Platinum and gold microelectrode arrays (MEAs), fabricated on silicon substrates with different geometric characteristics, were surface-modified by the potentiostatic electropolymerization of the pyrrole-ferrocene derivative Py(CH2)3NHCOFc, in the case of the platinum MEAs, and chemisorption of the thiol-functionalized ferrocene HS(CH2)6N+(CH3)2Fc, in the case of the gold MEAs. Cyclic voltammetry of these MEAs was typical of thin film behavior. The modified MEAs were investigated for the detection of the dihydrogen phosphate mono-anion in non-aqueous media via differential pulse voltammetry. This was based on electrostatic interaction and/or hydrogen-bonding between the target anion and the amide-ferrocene or ammonium-ferrocene functionalized electrode surfaces. A decrease in the ferrocene (Fc) oxidation peak current with a concomitant increase in the peak current of a new peak at lower potentials was observed when the concentration of the dihydrogen phosphate was increased.