Finite-element simulations of the influence of pore wall adsorption on cyclic voltammetry of ion transfer across a liquid–liquid interface formed at a micropore

Adsorption onto the walls of micropores was explored by computational simulations involving cyclic voltammetry of ion transfer across an interface between aqueous and organic phases located at the micropore. Micro-interfaces between two immiscible electrolyte solutions (micro-ITIES) have been of particular research interest in recent years and show promise for biosensor and biomedical applications. The simulation model combines diffusion to and within the micropore, Butler–Volmer kinetics for ion transfer at the liquid–liquid interface, and Langmuir-style adsorption on the pore wall. Effects due to pore radius, adsorption and desorption rates, surface adsorption site density, and scan rates were examined. It was found that the magnitude of the reverse peak current decreased due to adsorption of the transferring ion on the pore wall; this decrease was more marked as the scan rate was increased. There was also a shift in the half-wave potential to lower values following adsorption, consistent with a wall adsorption process which provides a further driving force to transfer ions across the ITIES. Of particular interest was the disappearance of the reverse peak from the cyclic voltammogram at higher scan rates, compared to the increase in the reverse peak size in the absence of wall adsorption. This occurred for scan rates of 50 mV/s and above and may be useful in biosensor applications using micropore-based ITIES

The performance of differential pulse stripping voltammetry at micro-liquid-liquid interface arrays

Microporous silicon membranes were recently introduced to create hexagonally-patterned arrays ofmicro-scale interfaces between two immiscible electrolyte solutions (lITIES). In this report we presenta simulation study of the application of differential pulse stripping voltammetry (DPSV) using theselITIES arrays for ion sensing. Simulations showed that the stripping current for ion detection wasenhanced by use of relatively deep pores (i.e. a low pore aspect ratio) and a viscous organic phase. These factors decrease the speed of escape of the pre-concentrated ion from the organic side of the ITIES. The stripping current initially increased steeply with pre-concentration time but eventually reached a plateau. Experiments performed using a lITIES array with micropores of radius 26 lm, depth of 100 lm and with a gelified organic phase demonstrated the saturation of the stripping peak with increasing pre-concentration time for the DPSV detection of tetraethylammonium ion. The reasons for the saturations are discussed in terms of diffusion coefficients and depth of the micropores

Voltammetric behaviour of biological macromolecules at arrays of aqueous|organogel micro-interfaces

The behaviour of two biological macromolecules, bovine pancreatic insulin and hen-egg-whitelysozyme (HEWL), at aqueous-organogel interfaces confined within an array of solid-state membrane micropores was investigated via cyclic voltammetry (CV). The behaviour observed is discussed in terms of possible charge transferring species and mass transport in the interfacial reaction. Comparison of CV results for HEWL, insulin, and the well-characterised model ion tetraethylammonium cation (TEA+) revealed that the biomacromolecules undergo an interfacial reaction comprising biomacromolecular adsorption and facilitated transfer of electrolyte anions from the organic phase to a protein layer on the aqueous side of the interface, whereas TEA+ undergoes a simple ion transfer process. Evidence for biomacromolecular adsorption on the aqueous side of the micro-interfaces is provided by comparison of the CVs for TEA+ ion transfer in the presence and absence of the biomacromolecules. Similar experiments in the presence of the low generation polypropylenimine tetraamine dendrimer, (DAB-AM-4), a smaller synthetic molecule, revealed it to be non-adsorbing. The behaviour of biological macromolecules at miniaturised aqueous-organogel interfaces involves adsorption on the aqueous side of the interface and transfer of organic phase electrolyte anions across the interface to associate with the adsorbed biomacro molecule. The data presented support the previously suggested mechanism forbiomacromolecular voltammetry at liquid-liquid interfaces, involving adsorption and facilitated ion-transfer of organic electrolyte anions

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

Chronoamperometric response at nanoscale liquid–liquid interface arrays

In this work, potential step chronoamperometry (PSCA) was used to study the behaviour of arrays of nanoscale interfaces between two immiscible electrolyte solutions (nanoITIES). The nanoITIES arrays were formed at nanoporous silicon nitride membranes containing 400 nanopores in a hexagonal close-packed arrangement. Three membrane designs, with nanopore radii of 75, 50 and 17 nm, were studied by ion-transfer of tetrapropylammonium cations across the nanopore array-supported water|1,6-dichlorohexane interface. The cell time constants and charging times were determined prior to experimental PSCA. The three membrane designs studied exhibited charging times in the range of 0.08–0.46 s, with the smallest pore configuration (17 nm radius) exhibiting the longest charging time. The experimental steady-state currents were 30–50% lower than of the calculated inlaid disc model currents, due to diffusion zone overlap at adjacent interfaces. The three nano-interface arrays studied also showed response times of 6 ± 1 s, being the time required to reach 95% of the steady-state current

Nanofabrication of Robust Nanoelectrodes for Electrochemical Applications

Individual gold nanowire devices for use as nanoelectrodes in electrochemical studies were fabricated using a low-cost robust fabrication approach. Nanowires were characterized by a combination of direct electrical current-voltage measurements and cyclic voltammetry using the model analyte ferrocene monocarboxylic acid. We observed low electrical resistances to contacted nanowires and steady-state sigmoidal electrochemical voltammograms that may be described by classical Butler-Volmer kinetics. The potential of nanowires for use in future biosensing applications was explored by demonstrating mediated bioelectrocatalytic oxidation of glucose

An Electrochemical Sensing Platform Based on Liquid-Liquid Microinterface Arrays Formed in Laser-Ablated Glass Membranes

Arrays of microscale interfaces between two immiscible electrolyte solutions (µITIES) were formed using glass membranes perforated with microscale pores by laser ablation. Square arrays of 100 micropores in 130 µm thick borosilicate glass coverslips were functionalized with trichloro(1H,1H,2H,2H-perfluorooctyl)silane on one side, to render the surface hydrophobic and support the formation of aqueous-organic liquid-liquid microinterfaces. The pores show a conical shape, with larger radii at the laser entry side (26.5 µm) than at the laser exit side (11.5 µm). The modified surfaces were characterized by contact angle measurements and X-ray photoelectron spectroscopy. The organic phase was placed on the hydrophobic side of the membrane, enabling the array of µITIES to be located at either the wider or narrower pore mouth. The electrochemical behavior of the µITIES arrays were investigated by tetrapropylammonium ion transfer across water-1,6-dichlorohexane interfaces together with finite element computational simulations. The data suggest that the smallest microinterfaces (formed on the laser exit side) were located at the mouth of the pore in hemispherical geometry, while the larger microinterfaces (formed on the laser entry side) were flatter in shape but exhibited more instability due to the significant roughness of the glass around the pore mouths. The glass membrane-supported µITIES arrays presented here provide a new platform for chemical and biochemical sensing systems. © 2016 American Chemical Society

Optimisation of the conditions for stripping voltammetric analysis at liquid-liquid interfaces supported at micropore arrays: a computational simulation

Micropore membranes have been used to form arrays of micro interfaces between immiscible electroly tesolutions (μITIES) as a basis for the sensing of non-redoxactiveions. Implementation of stripping voltammetry as asensing method at these arrays of μITIES was applied recently to detect drugs and biomolecules at low concentrations. The present study uses computational simulation to investigate the optimum conditions for stripping voltammetricsensing at the μITIES array. In this scenario, thediffusion of ions in both the aqueous and the organic phasescontributes to the sensing response. The influence of the preconcentration time, the micropore aspect ratio, the location of the micro interface within the pore, the ratio of the diffusion coefficients of the analyte ion in the organic and aqueous phases, and the pore wall angle were investigated. The simulations reveal that the accessibility of the microinterfaces during the preconcentration period should not be hampered by a recessed interface and that diffusional transport in the phase where the analyte ions are preconcentrated should be minimized. This will ensure that the ions are accumulated within the micropores close to the interface and thus be readily available for back transferduring the stripping process. On the basis of the results, an optimal combination of the examined parameters is proposed,which together improve the stripping voltammetric signal and provide an improvement in the detection limit

Ion-Transfer Voltammetric Behavior of Propranolol at Nanoscale Liquid-Liquid Interface Arrays

In this work, the ion-transfer voltammetric detection of the protonated β-blocker propranolol was explored at arrays of nanoscale interfaces between two immiscible electrolyte solutions (ITIES). Silicon nitride nanoporous membranes with 400 pores in a hexagonal arrangement, with either 50 or 17 nm radius pores, were used to form regular arrays of nanoITIES. It was found that the aqueous-to-organic ion-transfer current continuously increased steadily rather than reaching a limiting current plateau after the ion-transfer wave; the slope of this limiting current region was concentration dependent and associated with the high ion flux at the nanointerfaces. Electrochemical data were examined in terms of an independent nanointerface approach and an equivalent microdisc approach, supported by finite element simulation. In comparison to the larger interface configuration (50 nm radius), the array of 17 nm radius nanoITIES exhibited a 6.5-times higher current density for propranolol detection due to the enhanced ion flux arising from the convergent diffusion to smaller electrochemical interfaces. Both nanoITIES arrays achieved the equivalent limits of detection, 0.8 μM, using cyclic voltammetry. Additionally, the effect of scan rate on the charging and faradaic currents at these nanoITIES arrays, as well as their stability over time, was investigated. The results demonstrate that arrays of nanoscale liquid–liquid interfaces can be applied to study electrochemical drug transfer, and provide the basis for the development of miniaturized and integrated detection platforms for drug analysis

On the electronic structures of metaphosphate PX(3)((-)) and metaphosphite PX(2)((-)) anions, X=CH2,SIH2,NH,PH,O,S

Schoeller W, Strutwolf J. On the electronic structures of metaphosphate PX(3)((-)) and metaphosphite PX(2)((-)) anions, X=CH2,SIH2,NH,PH,O,S. PHOSPHORUS SULFUR AND SILICON AND THE RELATED ELEMENTS. 1996;111(1-4):673-673