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Ion transfer across interfaces between immiscible liquids provides a means for the nonredox electrochemical detection of ions. Miniaturization of such interfaces brings the benefits of enhanced mass transport. Here, the electrochemical behavior of geometrically regular arrays of nanoscale interfaces between two immiscible electrolyte solutions (nanoITIES arrays) is presented. These were prepared by supporting the two electrolyte phases within silicon nitride membranes containing engineered arrays of nanopores. The nanoITIES arrays were characterized by cyclic voltammetry of the interracial transfer of tetraethylammonium cation (TEAr) between the aqueous phase and the gelled organic phase. Effects of pore radius, pore center-to-center separation, and number of pores in the array were examined. The ion transfer produced apparent steady-state voltammetry on the forward and reverse sweeps at all experimentally accessible scan rates and at all nanopore array designs. However, background-subtraction of the voltammograms revealed the evolution of a peak-shaped response on the reverse sweep with increasing scan rate, indicative of pores filled with the organic phase to a certain extent. The steady-state voltammetric behavior at the nanoITIES arrays on the forward sweep for arrays with significant diffusion zone overlap between adjacent nanoITIES is indicative of the dominance of radial diffusion to interfaces at the edge of the arrays over linear diffusion to interfaces within the arrays. This implies that nanoITIES arrays, which occupy an overall area of micrometer dimensions, behave like a single [mu]ITIES of corresponding area to the nanoITIES array. 10.1021/ac1008282

The development of new methods for the detection of proteins and peptides is of widespread importance. In this work, the electrochemical behavior of peptide mixtures resulting from proteolytic digestion of proteins was investigated at the polarized liquid/liquid interface (or the interface between two immiscible electrolyte solutions, ITIES). The influence of pepsin digestion on three proteins (hemoglobin, lysozyme, and cytochrome c) was studied, and it was revealed that resulting cyclic voltammograms of the three protein digests were different due to the unique peptide mixtures for a given protein. Differential pulse stripping voltammetry of protein digests enabled the detection of digested proteins at concentrations ranging between 0.55 and 4.22 [micro]M. A limit of detection of 0.55 [micro]M of the initial concentration of protein was achieved, demonstrating the analytical possibilities of such an electrochemical method. These results show that ion-transfer voltammetry offers the opportunity to study and develop label-free detection of peptides resulting from enzymatic digestions of proteins and may thus have a role in development of new proteomic technologies. 10.1021/ac901909j

The occurrence of reactions at boundaries between adjacent miscible but unmixed aqueous streams coflowing in a microfluidic channel structure has been studied by simulation of the diffusion potentials that develop between the two coflowing aqueous electrolyte streams and by measurement of the effects of aqueous ion complexation on diffusion potentials. The microfluidic structure consisted of a Y-shaped microchannel with off-chip electrodes immersed in electrolyte reservoirs connected by capillaries to the Y-microchannel. The time-dependent, one-dimensional Nernst--Planck equation employing the electroneutrality condition was solved numerically to calculate the diffusion potentials established at the boundary between the two coflowing aqueous streams. Under the experimental conditions (channel length and width, flow rate) employed, it was shown that the use of the Henderson equation was appropriate. It was also shown that the cross-channel diffusion potential remained constant from the entrance of the channel to the exit. The influence of cation complexation by a neutral ionophore was investigated by experimentally measured diffusion potentials. It was found that potassium complexation by the cyclic polyether 18-crown-6 altered the experimental diffusion potental, whereas the interaction of sodium or lithium cations with the ionophore did not perturb the diffusion potential. The results are consistent with the literature data for aqueous-phase complexation of these cations by this ionophore. The results of these investigations demonstrate that relatively simple diffusion potential measurements between coflowing streams in microchannels may be used as a basis for study of ion complexation reactions occurring at boundaries between miscible fluids. 10.1021/ac901061r

In this work, the ion-transfer voltammetric detection of the protonated [beta]-blocker propranolol in artificial saliva is presented. Cyclic voltammetry, differential pulse voltammetry, and differential pulse stripping voltammetry (DPSV) were employed in the detection of the cationic drug based on ion-transfer voltammetry across arrays of microinterfaces between artificial saliva and an organogel phase. It was found that the artificial saliva matrix decreased the available potential window for ion-transfer voltammetry at this liquid/liquid interface but transfer of protonated propranolol was still achieved. The DPSV method employed a preconditioning step as well as a preconcentration step followed by analytical signal generation based on the back-transfer of the drug across the array of microinterfaces. The DPSV peak current response was linear with drug concentration in the artificial saliva matrix over the concentration range of 0.05-1 [micro]M ([i.sub.p] = -8.13 (nA [micro][M.sup.-1])(concentration) + 0.07 (nA), R = 0.9929, n = 7), and the calculated detection limit (3[S.sub.b]) was 0.02 [micro]M. These results demonstrate that DPSV at arrays of liquid|liquid microinterfaces is a viable analytical approach for pharmaceutical determinations in biomimetic matrixes.

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.

The detection of peptides is an important bioanalytical challenge, as they are a generic class of potent molecules of biomedical and biopharmaceutical significance. In this work, the electrochemistry of seven oligopeptides at microscaled interfaces between two immiscible electrolyte solutions ([micro]ITIES) was investigated. Their transfer across the polarized interface was assisted by dibenzo-18-crown-6 (DB18C6). The ion transfer potentials of these oligopeptides were dependent on their hydrophobicities and their interaction with DB18C6. Micropore arrays, which were fabricated in silicon by a combination of wet and dry etch techniques, were used to enhance mass transfer and thus analytical sensitivities. The use of a gellified organic phase allowed the implementation of voltammetric stripping techniques at the liquid--organogel interface. The combination of interface miniaturization and stripping voltammetry provided limits of detection at submicromolar concentration levels. The sensitivities (calibration graph slopes) were -3205 nA [micro][M.sup-1] [cm.sup-2] for Phe-Phe, -1791 nA[micro]m i [cm.sup-2] for Leu-Leu, -6014 nA p[M.sup-1] [cm.sup-2] for Lys-Lys, and -9611 nA p[M.sup-1] [cm.sup-2] for Lys-Lys-Lys. Mixtures of peptides were also investigated with this technique, illustrating the possibility to detect certain mixture combinations.

Single nanopore electrodes and nanopore electrode arrays have been fabricated using a focused ion beam (FIB) method. High aspect ratio pores (~150-400-nm diameter and 500-nm depth) were fabricated using direct-write local ion milling of a silicon nitride layer over a buried platinum electrode. This local milling results in formation of a recessed platinum electrode at the base of each nanopore. The electrochemical properties of these nanopore metal electrodes have been characterized by voltammetry. Steady-state voltammograms were obtained for a range of array sizes as well as for single nanopore electrodes. High-resolution scanning electron microscopy imaging of the arrays showed that the pores had truncated cone, rather than cylindrical, conformations. A mathematical model describing diffusion to an electrode located at the base of a truncated conical pore was developed and applied to the analysis of the electrode geometries. The results imply that diffusion to the pore mouth is the dominant mass transport process rather than diffusion to the electrode surface at the base of the truncated cone. FIB milling thus represents a simple and convenient method for fabrication of prototype nanopore electrode arrays, with scope for applications in sensing and fundamental electrochemical studies.

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 interracial potential difference. In this study, a mixture comprising 4-octylbenzene-sulfonate (4-[OBSA.sup.-]) and tetraethylammonium ([TEA.sup.+]) ions was evaluated. The application of negative potential differences enabled the selective extraction of 4-[OBSA.sup.-] into the organic phase, and more positive potential differences enabled the selective extraction of [TEA.sup.+] However, intermediate potentials lead to the coextraction of both ions into the organic phase, with apparent selectivity for [TEA.sup.+] over 4-[OBSA.sup.-]. 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.sup.+] followed by 4-[OBSA.sup.-]. The reasons for the selectivity for the cation over the anion are discussed.

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.sup.-]), andp-toluenesulfonate ([p-TSA.sup.-])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.sup.-], 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.

Development of an approach to prevention of electrode surface fouling by surfactants in samples is demonstrated. Spontaneously adsorbed monolayer systems employing short alkyl chains and bulky end groups are used to form porous disorganized monolayers on gold electrodes. Detection of copper by stripping of underpotential deposits formed at electrodes modified with disorganized films of mercaptoethanesuffonate (MES), mercaptopropanesulfonate, mercaptoacetic acid, and mercaptopropanoic acid was possible, and to a much lesser extent at amino-ethanethiol and L-cysteine films. Use of short deposition times in conjunction with linear sweep anodic stripping voltammetry allowed detection of [Cu.sup.2+] ions down to 1 x [10.sup.-6] M in sulfuric acid solution, using underpotential deposition as the deposition step of the procedure. Calibration graphs were linear in the concentration range (1-80) x [10.sup.-6] M [Cu.sup.2+] using 15-s deposition at 0.00 V versus Ag|AgCl. The surfactants Tween 20, Tween 80, and Triton X-100 were found to have no affect on detection of [Cu.sup.2+] ions in the calibration curve concentration range using MES-modified gold electrodes, whereas at unmodified gold electrodes very severe attenuation of the detection capability was manifested. The average slope for all calibration curves at the MES-modified electrode in the absence and presence of the surfactants at two different concentration levels was 0.0710 [+ or -] 0.0024 [micro]A [micro][M.sup.-1]; in contrast, the slope of the calibration line at uncoated gold electrodes in the presence of surfactant was 0.0268 [micro]A [micro][M.sup.-1]. These results indicate the excellent ability of a disorganized, porous monolayer for prevention of fouling of the electrode surface by the surfactants.