Flow-injection amperometry at microfabricated silicon-based μ-liquid–liquid interface arrays

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

Electrochemically Modulated Liquid-Liquid Extraction of Ionized Drugs under Physiological Conditions

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 samp matrix

Electrochemical detection of dopamine using arrays of liquid–liquid micro-interfaces created withinmicromachined silicon membranes

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 (ITIES) is presented. Microfabricated porous silicon membranes (consisting of eight pores, 26.6min radius and 500mpore–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 ascorbatewas achieved by the use of dibenzo-18-crown-6 (DB18C6) for the facilitated ion transfer of dopamine across the ITIES 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. 0.5M. 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 membrane

Microfluidic chip for electrochemically-modulated liquid[vertical bar]liquid extraction of ions

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

Detection of food additives by voltammetry at the liquid-liquid interface

Electrochemistry at the liquid-liquid interface enables the detection of nonredoxactive species withelectroanalytical techniques. In this work, the electrochemical behavior of two food additives, aspartameand 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 µM with a detection limit of 30 µM. 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

Potentiometric characterisation of a dual-stream electrochemical microfluidic device

A microfluidic device is presented with offchipelectrodes residing in a reservoir and connected viamicro-capillaries to the Y-shaped microfluidic channel.The device is tested by potentiometric measurementsinvolving dual-stream laminar flow of two aqueoussolutions carrying different electrolytes at various concentrations.Open circuit potentials are measured for aseries of solutions of alkali metal chlorides and tetraalkylammoniumchlorides as well as for dilute hydrochloricacid. The open circuit potential for the microfluidic chipwas calculated by taking into account the diffusionpotential at finite ionic strength as well as the potentialdifference introduced by the reference electrode system.The liquid junction potential developed at the boundary ofthe co-flowing aqueous solutions may be manipulated tohave greater or lesser relative contributions to the measuredopen circuit potential based on use of electrolyte saltshaving cation and anion pairs of similar or dissimilarmobilities in solution. A reasonable agreement betweentheoretical and experimental values of the open circuitpotential is observed for these situations. The results showthat simple microfluidic structures possess a rich environmentfor exploration and application of the solutionchemistry of ions

Voltammetry of chromium(VI) at the liquid|liquid interface

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[15]-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)