Voltammetry of proteins at liquid-liquid interfaces

The voltammetric behaviour of proteins at interfaces between two immiscible electrolyte solutions (ITIES) is reviewed. This behaviour is of interest for a number of reasons, including the basis of label-free detection methods and the understanding of the stability of biopharmaceutical and food formulations. The review discusses electrochemical strategies for protein and polypeptide detection, and the mechanisms involved in protein detection at the ITIES. Results obtained by DC and AC voltammetry are included together with data from other complementary techniques

Electrochemical behaviour of myoglobin at an array of microscopic liquid–liquid interfaces

Electrochemistry at liquid–liquid interfaces, or at interfaces between two immiscible electrolyte solutions ITIES, provides a basis for the non-redox detection of biological molecules, based on ion-transferor adsorption processes. The electroactivity of myoglobin at an array of micron-sized liquid–organogel interfaces was investigated. The μITIES array was patterned with a silicon membrane consisting of an array of eight pores with radii of ~12.8 μm and a pore to pore separation of ~400 μm. Using cyclic voltammetry at the ITIES, the protein was shown to adsorb at the interface and facilitate the transfer of the organic phase electrolyte anions to the aqueous side of the interface. The electrochemical current response was linear with concentration in the range of 1–6 μM, with corresponding surface coverage of 10–50 pmol cm-2. The reverse peak currents was found to be proportional to the voltammetric scan rate, indicating a desorption process. The detection of the protein was only possibly when the pH of the aqueous phase solution was below the pI of the protein. The steady-state simple ion transfer behaviour of tetraethylammonium cation was decreased on the forward sweep, providing a qualitative indication of the presence of adsorbed protein at the interface. Increasing the ionic strength of the aqueous phase resulted in enhanced peak currents, possibly due to aggregation of protein precipitates in the aqueous solution. UV/Vis absorbance spectroscopy was used to investigate the effects of various aqueous electrolyte solutions on the structure of the protein, and it was shown that at low pH the protein is at least partially denatured. These results provide the basis for label-free detection of myoglobin at the ITIES

Electrochemical detection of ractopamine at arrays of micro-liquid | liquid interfaces

The behaviour of protonated ractopamine (RacH+) at an array of micro-interfaces between two immiscible electrolyte solutions (micro-ITIES) was investigated via cyclic voltammetry (CV) and linear sweep stripping voltammetry (LSSV). The micro-ITIES array was formed at silicon membranes containing 30 pores of radius 11.09±0.12 µm and pore centre-to-centre separation of 18.4±2.1 times the pore radius. CV shows that RacH+ transferred across the water |1,6-dichlorohexane µITIES array at a very positive applied potential, close to the upper limit of the potential window. Nevertheless, CV was used in the estimation of some of the drug’s thermodynamic parameters, such as the formal transfer potential and the Gibbs transfer energy. LSSV was implemented by pre-concentration of the drug, into the organic phase, followed by voltammetric detection, based on the back-transfer of RacH+ from the organic to aqueous phase. Under optimised pre-concentration and detection conditions, a limit of detection of 0.1 µM was achieved. In addition, the impact of substances such as sugar, ascorbic acid, metal ions, amino acid and urea on RacH+ detection was assessed. The detection of RacH+ in artificial serum indicated that the presence of serum protein interferes in the detection signal, so that sample deproteinisation is required for feasible bioanalytical applications

Electrochemical signature of hen egg white lysozyme at the glycerol-modified liquid-liquid interface

Electrochemical characterization of hen egg white lysozyme (HEWL) at a glycerol-modified interface between two immiscible electrolyte solutions (ITIES) was conducted using a microporous silicon membrane-supported gelled-1,6-dichlorohexane|water-glycerol interface. The electrochemical response of HEWL under these conditions is of interest for the system's potential application to the formation of isorefractive emulsified-ITIES, which offer practical opportunities for spectrophotometric analysis of interfacial processes. Importantly, the voltammetric signature for HEWL seen under glycerol-rich conditions was similar but with some differences from that for glycerol-free conditions. Specifically, the potential at which facilitated transfer of the organic phase electrolyte anion tetrakis-(4-chlorophenyl) borate (TPBCl-) occurred was shifted to lower potential with increasing glycerol concentration. However, features in the voltammetry associated with adsorption/desorption processes were observed to remain constant. The simple ion transfer response of tetraethylammonium cations (TEA+) at the same glycerol-modified ITIES provides insight into the nature of changes that determine the atypical HEWL signature. Lower ion transfer current with respect to increasing glycerol concentration and a shift in transfer potential were the key findings here. The results indicate that the electrochemistry which determines the HEWL signature is similar in environments that are rich in glycerol or purely aqueous

Array of water|room temperature ionic liquid micro-interfaces

Cyclic Voltammetry and AC Voltammetry were used to characterise the micro-interface array between water and a commercially available room temperature ionic liquid (RTIL) trihexyltetradecylphosphonium tris (pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) for the first time. The interface array was formed within the micropores of a silicon chip membrane (30 pores and 23 m diameter). The polarisable potential window and capacitance curves were recorded, and the transfers of three cations (tetraalkylammoniums) and three anions (tetraphenylborate, hexafluorophosphate and tetrafluoroborate) across the interface were studied. The shapes of the voltammograms revealed that the RTIL filled the pores and that the interface was located at/near the pore mouths. This is the first report of voltammetry at an array of water|RTIL microinterfaces, rather than at a single interface or porous polymer supported-interface. This work is particularly relevant to the sensing/extraction of redox-inactive ions

Liquid / Liquid Interface-Based Electrochemical Sensing of Ractopamine and Salbutamol

AbstractAn electrochemical sensor based on ion transfer across micro- and nano-liquid / liquid interfaces has been developed for the detection of protonated drugs, ractopamine (RacH+) and salbutamol (SalH+) via cyclic voltammetry (CV) and linear sweep stripping voltammetry (LSSV). These voltammetric methods enabled the detection and characterisation of the ionised drugs despite that they transferred at high applied potentials. The drugs’ thermodynamic and analytical parameters were determined. The limit of detection in the sub-μM range is suitable for applications to detection in real samples. Electrochemistry at the liquid / liquid interfaces was shown to be a viable technique for drug sensing

Investigation into the voltammetric behaviour and detection of selenium(IV) at metal electrodes in diverse electrolyte media

The voltammetric behaviour of selenium(IV) was studied at platinum and gold electrodes in sulphuric acid, perchloric acid and potassium chloride media as a basis for its voltammetric detection. The best voltammetric behaviour was recorded at gold electrodes with perchloric acid as the supporting electrolyte. The concomitant presence of metals, such as copper or lead, and of model biomolecules, such as bovine serum albumin, in the solution resulted in a deterioration of the electrochemical response for selenium(IV). Quantitative detection of selenium(IV) by square wave anodic stripping voltammetry at both a millimetre-sized gold disc electrode and a microband electrode array revealed linear responses to selenium concentration in the ranges of 5-15 μM and 0.1-10 μM, respectively, with 60 s preconcentration. The sensitivities were 6.4 μA μM-1 cm-2 and 100 μA μM-1 cm-2 at the disc and the microband array, respectively. The detection limit at the microband electrode array was 25 nM, illustrating the potentiality of such microelectrodes for the development of mercury-free analytical methods for the trace detection of selenium(IV)

Ubiquinone electrochemistry in analysis and sensing

Ubiquinone (UQ) is a lipophilic compound present in most living organisms,where UQ’s interesting but complex electrochemistry serves an important rolein the transfer of electrons and protons within and across the mitochondrialmembrane. We briefly review the electrochemical characteristics of UQ and itsreduced state, ubiquinol, in solution and immobilized on electrodes, togetherwith its application in electrochemical sensing and detection systems, for exam-ple, measuring redox status with reference to reactive oxidative species. Theimportance of the local environment, solvent, electrolyte, organic membrane,and pH, on the electrochemical behavior of UQ, is also discussed. We discusstechniques used for the direct detection of UQ such as liquid chromatography-electrochemistry. Mediated electrochemistry of UQ allows for quantitative mea-surements of ions, small molecules, and other analytes such as glucose via chem-ical sensors and biosensors

Macromolecular sensing at the liquid-liquid interface

We report here the electrochemical sensing of macromolecules, such as polyLysine dendrimers, at the polarised liquid | liquid interface. Electrochemistry at the liquid | liquid interface is a powerful analytical technique which allows the detection of non-redox active molecules via ion transfer reactions at a polarised water – oil interface. We demonstrate here that different parameters of the polyLysine dendrimers (charge number, molecular weight) have a strong influence on the sensitivity and limit of detection of these macromolecules. This work will help to the development of sensors based on charge transfer at the liquid | liquid interface

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