Detection of haemoglobin using an adsorption approach at a liquid–liquid microinterface array

Electroanalytical applications of ITIES - A review

Over the last decades, the interface between two immiscible electrolyte solutions (ITIES) attracted considerable attention of the scientific community due to their vast applications, such as extraction, catalysis, partition studies and sensing. The aim of this Review is to highlight the potential of electrochemistry at the ITIES for analytical purposes, focusing on ITIES-based sensors for detection and quantification of chemically and biologically relevant (bio)molecules. We start by addressing the evolution of ITIES in terms of number of publications over the years along with an overview of their main applications (Chapter 1). Then, we provide a general historical perspective about pioneer voltammetric studies at water/oil systems (Chapter 2). After that, we discuss the most impacting improvements on ITIES sensing systems from both perspectives, set-up design (interface stabilization and miniaturization, selection of the organic solvent, etc.) and optimization of experimental conditions to improve selectivity and sensitivity (Chapter 3). In Chapter 4, we discuss the analytical applications of ITIES for electrochemical sensing of several types of analytes, including drugs, pesticides, proteins, among others. Finally, we highlight the present achievements of ITIES as analytical tool and provide future challenges and perspectives for this technology (Chapter 5).

Phenylethylamine sensing at the electrified liquid-liquid interface. Can electrochemistry be used to follow the UHT milk spoilage process?

This study involved an investigation into the electrochemical characteristic of a few biogenic amines (BAs) occurring at the polarized interface between two immiscible electrolyte solutions (ITIES) with ion transfer voltammetry (ITV). The main focus of this research was the comprehensive electroanalytical and physicochemical analysis of phenylethylamine (PEA), allowing the determined of the formal Galvani potential of the ion transfer reaction (ΔorgaqΦ'), diffusion coefficients (D), formal free Gibbs energy of the ion transfer reaction (ΔG'aq→org) and water-1,2-dichloroethane partition coefficient (logPwater/DCEPEA). Furthermore, the collected data were employed to calculate analytical parameters, including voltametric detection sensitivity, limits of detection and the target analyte quantification. Moreover, the influence of the presence of 7 other BAs (histamine, spermine, spermidine, putrescine, cadaverine, tyramine and tryptamine) on the recorded signals originating from the PEA ion transfer was checked. The feasibility of the developed method was corroborated through experimentation with milk samples. Additionally, utilizing the devised methodology, an electrochemical assessment of the spoilage progression in milk samples was undertaken.

Interfacial behaviour and quantitative analysis of hexadecyl phosphocholine drug at a polarized liquid/liquid interface

The interfacial behaviour of the amphiphilic drug hexadecyl phosphocholine (HePC, also called miltefosine) was analysed by cyclic voltammetry applied at the water/1,2-dichloroethane interface. HePC is the only oral drug currently approved for the treatment of visceral, mucosal  and cutaneous leishmaniasis. Because of its amphiphilic character, it can interact with biological membranes, solubilizing their compounds and leading to cell disruption. These interactions are responsible for its side effects and toxicity; therefore, HePC quantification in biological fluids and pharmaceutical preparations is extremely important. However, the lack of a chromophore in its structure prevents its spectroscopic determination. For this reason, the main challenge of this work was to propose an electroanalytical method for the quantification of this drug, which constitutes a simpler alternative than liquid chromatography-tandem mass spectrometry already reported. With this aim, in the first part of this work, the mechanism of the electrochemical process occurring after polarizing the interface was studied. By varying the experimental conditions, it was possible to determine that in a first step, at open circuit or at low potential values, HePC spontaneously adsorbed to the interface. Later, as the potential increased, the transfer of the anions present in the organic phase towards the aqueous side of the interface, where the HePC polar head groups were present, occurred thus forming adsorbed "ion pairs" and producing an increase in positive current. Subsequently, in the negative sweep, the "ion pairs" dissociated and desorbed giving rise to a negative peak. In this way, both negative and positive currents were considered useful for quantitative purposes. In the second part of this work, an appropriate experimental procedure was designed and proposed as a quantitative methodology for the HePC determination, which consisted of cleaning the interface and controlling the time at open circuit, followed by the voltammetric analysis. A linear response of both, positive or negative, peak currents with drug concentration was obtained within an acceptable range, providing a simple solution for the HePC quantification problem. Future studies will be carried out to evaluate the quantification and selectivity in real matrices containing polymer micelles working as HePC nanocarriers with the aim of avoiding the adverse effects of HePC when it is orally or intravenously administered.

Structural Changes in Insulin at a Soft Electrochemical Interface

Understanding the interaction of proteins at interfaces, which occurs at or within cell membranes and lipoprotein vesicles, is central to our understanding of protein function. Therefore, new experimental approaches to understand how protein structure is influenced by protein-interface interactions are important. Herein we build on our previous work exploring electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) to investigate changes in protein secondary structure that are modulated by protein-interface interactions. The ITIES provides an experimental framework to drive protein adsorption at an interface, allowing subsequent spectroscopic analysis (e.g., Fourier transform infrared spectroscopy) to monitor changes in protein structure. Here, we reveal that the interaction between insulin and the interface destabilizes native insulin secondary structure, promoting formation of α helix secondary structures. These structural alterations result from protein-interface rather than protein-protein interactions at the interface. Although this is an emerging approach, our results provide a foundation highlighting the value of the ITIES as a tool to study protein structure and interactions at interfaces. Such knowledge may be useful to elucidate protein function within biological systems or to aid sensor development.

Ion Transfer Voltammetry with an Electrochemical Pen

We present a new electrochemical system that combines paper-based sensing and ion-transfer voltammetry, bringing the latter a step closer toward point-of-care applications. Studies at the interface between two immiscible electrolyte solutions (ITIES) are often performed to detect redox-inactive species; unfortunately, due to the inherent instability of the interface, it is rather poorly explored outside specialized laboratories. Here, we address this limitation by combining a pen-like device containing the gelled organic phase with a paper-supported aqueous phase. This combination makes the system more user-friendly, potentially low-cost, and easy to assemble. We show the applicability of the new cell to analyze both simple and ionophore-facilitated transfer of ions and proteins, preconcentration of species, and analysis of mixtures through combination with paper chromatography. The native ion content of the paper also enabled measurements without added electrolytes. Those studies could broaden the scope for the application of the label-free electrochemical detection of nonredox-active species at points-of-need.

Electrochemistry of catalase at a liquid|liquid micro-interface array

The electrochemistry of catalase (CAT) was investigated at the interface between two immiscible electrolyte solutions (ITIES) as a step towards its detection. Electrochemistry at the ITIES offers advantages such as the non-redox detection of biomolecules. The electrochemical behaviour of CAT at the ITIES, in a micro-interface array format, displayed a distinct cyclic voltammogram when the aqueous phase pH was lower than the isoelectric point (pI) of CAT. No voltammetric response was observed when the aqueous phase pH > pI of CAT, indicating that neutral or negatively charged CAT has no capability to facilitate anion transfer from the organic phase. Adsorptive stripping voltammetry (AdSV) was assessed for detection of low concentrations at the µITIES array. Application of a positive preconcentration potential for a fixed time enabled interfacial accumulation of CAT as a complex; subsequently, a voltammetric scan to lower potentials desorbed the complex, providing the electroanalytical signal. Assessment of sample matrix effects by examining the electrochemistry of CAT in artificial serum indicated that detection in pH-adjusted samples is feasible. Together, these results demonstrate that CAT is electroactive at the liquid-liquid interface and this may be useful as a strategy to detect and characterize the enzyme in a label-free manner.

Secondary Structural Changes in Proteins as a Result of Electroadsorption at Aqueous-Organogel Interfaces

The electroadsorption of proteins at aqueous-organic interfaces offers the possibility to examine protein structural rearrangements upon interaction with lipophilic phases, without modifying the bulk protein or relying on a solid support. The aqueous-organic interface has already provided a simple means of electrochemical protein detection, often involving adsorption and ion complexation; however, little is yet known about the protein structure at these electrified interfaces. This work focuses on the interaction between proteins and an electrified aqueous-organic interface via controlled protein electroadsorption. Four proteins known to be electroactive at such interfaces were studied: lysozyme, myoglobin, cytochrome c, and hemoglobin. Following controlled protein electroadsorption onto the interface, ex situ structural characterization of the proteins by FTIR spectroscopy was undertaken, focusing on secondary structural traits within the amide I band. The structural variations observed included unfolding to form aggregated antiparallel β-sheets, where the rearrangement was specifically dependent on the interaction with the organic phase. This was supported by MALDI ToF MS measurements, which showed the formation of protein-anion complexes for three of these proteins, and molecular dynamic simulations, which modeled the structure of lysozyme at an aqueous-organic interface. On the basis of these findings, the modulation of protein secondary structure by interfacial electrochemistry opens up unique prospects to selectively modify proteins.

Amperometric Ion Sensing Approaches at Liquid/Liquid Interfaces for Inorganic, Organic and Biological Ions

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) has become an invaluable tool for the selective and sensitive detection of cationic and anionic species, including charged drug molecules and proteins. In addition, neutral molecules can also be detected at the ITIES via enzymatic reactions. This chapter highlights recent developments towards creating a wide spectrum of sensing platforms involving ion transfer across the ITIES. As well as outlining the basic principles needed for performing these sensing applications, the development of ITIES-based detection strategies for inorganic, organic, and biological ions is discussed.

Recent developments in electrochemistry at the interface between two immiscible electrolyte solutions for ion sensing

The most recent developments on electrochemical sensing of ions at the liquid–liquid interface are reviewed here.