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

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Abstract

Microporous silicon membranes were recently introduced to create hexagonally-patterned arrays of micro-scale interfaces between two immiscible electrolyte solutions (μITIES). In this report we present a simulation study of the application of differential pulse stripping voltammetry (DPSV) using these μITIES arrays for ion sensing. Simulations showed that the stripping current for ion detection was enhanced 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 μITIES array with micropores of radius 26 μm, depth of 100 μm 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.

Introduction

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) [1], [2], [3], [4] is of special interest in analytical chemistry because it enables the measurement of electroanalytical signals of charged analyte species in solution without the need for their oxidation or reduction [1], [5], [6], [7], [8]. Ionised substances can therefore be detected in a label-free manner even if their oxidation/reduction is not easily achieved or is masked by the presence of an interfering substance at solid electrodes, e.g. [8], [9].

In general, electroanalytical performance is improved by miniaturisation of the electroactive surface. The introduction of micron-sized solid electrodes [10] led to major progress in electroanalysis. The analogous introduction of micron-scaled ITIES (μITIES) also brings several advantages [11], [12]. The smaller size of the interface results in smaller currents and therefore a reduced Ohmic potential drop especially in the resistive organic phase. The μITIES is subject to increased diffusional transport of the analyte compared to their macroscopic counterparts, resulting in an increased current density and in an enhancement of sensitivity and a lowering of detection limits. The strength of the electroanalytical signal can be amplified if several μITIES are operated in parallel, in the form of an array of μITIES [13].

Recently, arrays of μITIES were developed using micropores fabricated within silicon solid-state membranes [14]. Different array designs were fabricated, possessing between 3 and 120 micropores arranged in an hexagonal pattern with between-pore centre-to-centre distances of 100–1000 μm and pore radii between 5 and 25 μm. The silicon membrane thickness was 100 μm. These were used to form arrays of μITIES to investigate simple and facilitated ion transfer [14], [15] and for the detection of dopamine [6], oligopeptides [7], and propranolol [5]. The performance of the μITIES array in a hydrodynamic arrangement was also investigated [16].

Cyclic voltammetry (CV) of ion transfer across the μITIES confined within such solid-state micropore arrays was studied in detail by simulations [15]. The simulation model took into account the geometric properties of the membrane as well as possible interactions of the diffusion zones around neighbouring micropores. By comparing simulation with experiments, it was shown that the ITIES was practically co-planar with the membrane surface on the aqueous side, i.e. the pores were filled with the organic phase. In the experiments, the organic phase was gelified to increase the mechanical stability of the interface and to facilitate the experimental set-up. By comparing experimental and simulated voltammograms, the ratio of the diffusion coefficients of the transferring ion in the organic and aqueous phases was determined. The diffusion coefficient in the organo-gel was reduced by a factor of 8.7 compared to that in the aqueous phase.

Applications of stripping voltammetric techniques at ITIES have been investigated by Senda and co-workers [17], [18], [19], [20], [21] and by Lee at et al. [22]. Ohkouchi et al. [17] used a micropipette filled with nitrobenzene in contact with an aqueous phase. Acetylcholine was pre-concentrated into the nitrobenzene and stripped back into the aqueous phase. Theoretical considerations predicted a dependence of the stripping peak height on the square root of the pre-concentration time and this was in agreement with experiments. Katano and Senda developed a thin film organo-gel/water interface electrode which was applied to stripping analysis of Hg+ and Pb2+ ions [18] and polyoxyethylene non-ionic surfactants [19]. The fact that the sensitivity of the stripping step can be enhanced if the diffusion of the analyte in the organic phase is diminished by gelification of the organic phase was also exploited by Lee et al., who used a micromachined organo-gel membrane for stripping analysis of choline ions [22].

In our recent work, improved sensitivity was achieved by combination of stripping and pulse voltammetric techniques (e.g. differential pulse stripping voltammetry (DPSV)) at the gelified μITIES array formed in solid-state membranes. It was found that DPSV improved the detection limits obtained at the μITIES array for oligopeptides [7] and for the beta-blocker drug propranolol [5] in comparison to linear sweep voltammetry, differential pulse voltammetry (DPV) or linear sweep stripping voltammetry.

In this report, we present a detailed examination of the performance characteristics of DPSV at a μITIES array formed at a microporous membrane. The purpose of the work was to develop an understanding of the application of advanced voltammetric methods at arrays of μITIES for the eventual sensitive detection of biological molecules. The simulation results presented clearly indicate the necessary control parameters for the design of new micropore arrays. The influence on the stripping current of the pore depth, of the ratio of the diffusion coefficients of the analyte in the organic and aqueous phases and of the pre-concentration time, were examined by computer simulation. The results show that the combination of DPSV with a μITIES array formed at the orifice of relatively deep micropores (100 μm depth) in a solid-state membrane serves as an excellent platform for the electroanalytical detection of charged species. The results of the simulated DPSV curves were compared to experiments with tetraethylammonium ions as the analyte.

Section snippets

Reagents

All reagents were purchased from Sigma–Aldrich Ireland Ltd. (unless indicated otherwise) and used without further purification, with the exception of 1,6-dichlorohexane (1,6-DCH) which was purified according to the published procedure [23]. The aqueous phase electrolyte solution of 10 mM LiCl was prepared in ultrapure water (resistivity: 18  cm), obtained from an Elgastat Maxima – HPLC purifier (Elga, UK). The model analyte species studied was the tetraethylammonium cation (TEA+) prepared as the

Theory

The simulation of ion transfer across the ITIES confined in micropore arrays was described in detail recently [15]. A summary is given here. Transport is assumed to occur by diffusion only and is described by a diffusion equation in cylindrical coordinates. A sketch of the computational domain including the boundary conditions is presented in Fig. 1. The pore is filled with the organic phase and the ITIES is co-planar with the membrane surface on the aqueous side. Initially (at time t = 0 s), the

Results and discussions

In order to justify and verify some of the model assumptions used, such as γ, Δowφi0 and geometric features of the micropore array, the comparison of an experimental background-subtracted CV for the transfer of TEA+ ions across the aqueous/gelified organic phase boundary to a simulated voltammogram is first presented. The solid line in Fig. 3 shows the experimental CV recorded at a scan rate of 10 mV s−1. The experimental current was normalised to its limiting current of 2.68 nA. The open circles

Conclusions

The combination of μITIES based on regular arrays of micropores in solid-state membranes and stripping voltammetric techniques provides an effective platform for ion sensing at liquid–liquid interfaces. This combination has been investigated by simulations and compared to experimental data. It was found that an enhancement of the stripping signal was achieved by increasing the length of the pores within which the organic phase is located and by slowing down the diffusion speed of the analyte in

Acknowledgements

The support of Science Foundation Ireland (Grant number 07/IN.1/B967), the Irish Research Council for Science Engineering and Technology (PhD scholarship to MDS, Grant number RS/2005/122) and the European Commission’s Framework Programme 6 (Marie Curie Transfer of Knowledge fellowship to JS, Grant number MTKD-CT-2005-029568)) are gratefully acknowledged. The fabrication facilities employed in this work were supported by the Irish High Education Authority’s Programme for Research in Third Level

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