Jörg Strutwolf

Serum-protein effects on the detection of the β-blocker propranolol by ion-transfer voltammetry at a micro-ITIES array

In this work, the effect of the serum protein, bovine serum albumin (BSA), on the detection of propranolol in artificial serum by ion-transfer voltammetry at an array of micro-interfaces between two immiscible electrolyte solutions (microITIES) is presented. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), and differential pulse stripping voltammetry (DPSV) were examined for the detection of low concentrations of propranolol. Both CV and DPV had an interference effect from BSA, manifested as lower currents in the presence of the protein. DPSV proved to be the most effective technique, enabling the detection of 0.05 microM propranolol in the presence of BSA. The DPSV method employed a preconditioning step as well as a preconcentration step followed by the analytical signal generation step. The latter was based on the back-transfer of the drug across the microITIES. The preconcentration step was crucial to prevention of the adverse effects of BSA on the voltammetric detection. These results demonstrate that serum-protein effects on drug detection at low concentrations can be eliminated by use of DPSV at arrays of microITIES. CVs of propranolol with increasing concentrations of BSA revealed the influence of the drug-protein binding interaction, with decreases in current but no change in transfer potential. Therapeutic concentrations of propranolol were detected, demonstrating the viability of this approach for bioanalytical investigations.

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

The behaviour of two biological macromolecules, bovine pancreatic insulin and hen-egg-white lysozyme (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 biomacromolecule. The data presented support the previously suggested mechanism for biomacromolecular voltammetry at liquid-liquid interfaces, involving adsorption and facilitated ion-transfer of organic electrolyte anions.

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.

Microelectrochemical Systems on Silicon Chips for the Detection of Pollutants in Seawater

The ever-growing demand for simple, fast and reliable techniques for the detection of pollutants and contaminants in the environment has sparked the development of remote detection and monitoring systems which include application specific sensors, instrumentation and signal processing. We report here the design, fabrication and characterisation of four designs of microelectrochemical systems on silicon chip for the detection of pollutants in artificial seawater. These systems were fabricated by photolithography and incorporate a Pt working microelectrode array (squares or bands), a Pt counter electrode and a Ag|AgCl reference electrode. They have been characterised by cyclic voltammetry of ferricyanide and behaved in good agreement with the theory. These systems were evaluated over 72 hours and showed good stability. Underpotential Deposition – Stripping Voltammetry experiments of Cu2+ in artificial seawater have been carried out at an array of 35 microsquares of 20u2005µmu2009×u200920u2005µm. The sensitivity achieved was (2.93±0.14)u2005µA cm−2 µM−1, with 1u2005µM being the lowest Cu2+ concentration measured. These devices provide the basis for the development into sensor systems for remote analysis applications.

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

AbstractMicropore membranes have been used to form arrays of microinterfaces between immiscible electrolyte solutions (µITIES) as a basis for the sensing of non-redox-active ions. Implementation of stripping voltammetry as a sensing 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 voltammetric sensing at the µITIES array. In this scenario, the diffusion of ions in both the aqueous and the organic phases contributes to the sensing response. The influence of the preconcentration time, the micropore aspect ratio, the location of the microinterface 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 transfer during 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.n FigureSimulation was used to study stripping voltammetry at micro-liquid-liquid interface arrays. Optimum conditions include the minimisation of diffusion within the gelled organic phase.

Electrochemical Fabrication of Nanostructured Surfaces for Enhanced Response

The objective of this work is to explore approaches to enhance electrochemical signals through sequential deposition and capping of gold particles. Gold nanoparticles are electrodeposited from KAuCl(4) solution under potentiostatic conditions on glassy carbon substrates. The number density of the nanoparticles is increased by multiple deposition steps. To prevent secondary nucleation processes, the nanoparticles are isolated after each potentiostatic deposition step by self-assembled monolayers (SAMs) of decanethiol or mercaptoethanol. The increasing number of particles during five deposition/protection rounds is monitored by assembling electroactive SAMs using a ferrocene-labeled alkanethiol. A precise estimation of the surface area of the gold nanoparticles by formation of an oxide layer on gold is difficult due to oxidation of the glassy carbon surface. As an alternative approach, the charge flow of the electroactive SAM is used for surface measurement of the gold surface area. A sixfold increase in the redox signal in comparison to a bulk gold surface is observed, and this increase in redox signal is particularly notable given that the surface area of the deposited nanoparticles is only a fraction of the bulk gold surface. After five rounds of deposition there is a gold loading of 1.94 mug cm(-2) of the deposited nanoparticles as compared to 23.68 mug cm(-2) for the bulk gold surface. Remarkably, however, the surface coverage of the ferrocene alkanethiol on the bulk material is only 10 % of that achieved on the deposited nanoparticles. This enhancement in signal of the nanoparticle-modified surface in comparison to bulk gold is thus demonstrated not to be attributable to an increase in surface area, but rather to the inherent properties of the surface atoms of the nanoparticles, which are more reactive than the surface atoms of the bulk material.

Digital simulation of thermal reactions

Abstract Simulations of the equation for thermal expansion of a reacting gas have been carried out, exploring both the (possible) steady states and time-marching solutions. The critical Frank-Kamenetskii parameter δ cr has been evaluated to seven decimal places for the slab, cylinder and spherical geometries and the role of the critical activation parameter ϵ was explored. It was found that there exist one or more mathematical steady states for any δ if ϵ > 0 , the curves for steady temperature at the center of the geometry plotted against δ tending to a straight line at large δ . Critical values of ϵ , the values above which this plot has a single solution for a given δ , have been computed to eight decimals. Time marching simulations showed that the Crank–Nicolson method, applied consistently, produces very accurate results, compared with the implementation in which the nonlinear term is rendered explicit. Where for a given δ there are several mathematical steady states, a time march usually settles on the lowest such state (if it settles at all), regardless of where the simulation is started, within the possible limits. The mathematical multiple steady states are not attained by time marching simulations, and are also physically unlikely.

A Capacitive Humidity Sensor Suitable for CMOS Integration

This paper describes the design, fabrication, and performance of a thin film humidity sensor fabricated in standard CMOS process, hence it may be combined with an integrated circuit. The sensor is based on a capacitance between interdigitated electrodes in the top metal layer and water adsorption in the polyimide layer. The design is optimized by analytical and then finite element models which show that, within the constraint of the CMOS structure, the sensitivity can be no greater than one third of the sensitivity of the polyimide alone. Experimental sensors were fabricated in-house before an improved design was fabricated in a commercial foundry. The different behavior of these sensors, despite their similar designs, leads to an investigation into the effects of fabrication process on the sensor linearity. Characterizing the polyimide film by contact angle, AFM and FTIR revealed that the difference in linearity of the response between the two sensors resulted from different etching techniques employed to pattern the film.

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(-1) and above and may be useful in biosensor applications using micropore-based ITIES.

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.