Modulating the ion-transfer electrochemistry of perfluorooctanoate with serum albumin and ß-cyclodextrin.

Per- and polyfluoroalkyl substances (PFAS) are durable synthetic pollutants that persist in the environment and resist biodegradation. Ion-transfer electrochemistry at aqueous-organic interfaces is a simple strategy for the detection of ionised PFAS. Herein, we investigate the modulation of the ion transfer voltammetry of perfluorooctanoate (PFOA) at liquid-liquid micro-interface arrays by aqueous phase bovine serum albumin (BSA) or ß-cyclodextrin (ß-CD) and examine the determination of association constants for these binding interactions. By tracking the ion transfer current due to ionised, uncomplexed PFOA as a function of BSA or ß-CD concentration, titration curves are produced. Fitting of a binding isotherm to these data provides the association constants. The association constant of PFOA with the BSA determined in this way was ca. 105 M-1 assuming a 1 : 1 binding. Likewise, the association constant for PFOA with ß-CD was ca. 104 M-1 for a 1 : 1 ß-CD-PFOA complex. Finally, the simultaneous effect of both BSA and ß-CD on the ion transfer voltammetry of PFOA was studied, showing clearly that PFOA bound to BSA is released (de-complexed) upon addition of ß-CD. The results presented here show ion transfer voltammetry as a simple strategy for the study of molecular and biomolecular binding of ionised PFAS and is potentially useful in understanding the affinity of different PFAS with aqueous phase binding agents such as proteins and carbohydrates.

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.

Detection of perfluorooctane sulfonate by ion-transfer stripping voltammetry at an array of microinterfaces between two immiscible electrolyte solutions.

Per- and polyfluoroalkyl substances (PFAS) are a category of persistent environmental contaminants that have been linked to health issues in humans. In this work, we investigate the detection of perfluorooctanesulfonate (PFOS-), one such PFAS, by ion-transfer voltammetry at an array of microinterfaces between two immiscible electrolyte solutions (µITIES). Cyclic voltammetry, differential pulse voltammetry and differential pulse stripping voltammetry (DPSV) indicated the ion-transfer behaviour and detection of PFOS-, with the latter enabling detection at picomolar concentrations. Using a 5 min preconcentration time, during which PFOS- was preconcentrated into the organic phase of the µITIES array, a limit of detection (LOD) of 0.03 nM (0.015 µg L-1) in aqueous electrolyte was achieved. This performance is attributed to the enhanced mass transport (radial diffusion) to the µITIES that occurs during preconcentration. To investigate the potentiality for applications of this analytical approach to environmental samples, measurements in a range of water matrices were investigated. Drinking water, laboratory tap water and seawater matrices were assessed by spiking with PFOS- over the 0.1-1 nM range. A matrix effect was observed, with changes in sensitivity and LOD relative to those in pure aqueous electrolyte solutions. Such matrix effects need to be considered in designing applications of these PFOS- measurements to environmental samples. The results presented here indicate that DPSV at a µITIES array can form the basis for a fast and sensitive screening method for PFOS- contamination that is suited to portable and on-site applications.

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 ß-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.

Electrochemical Behavior and Detection of Sulfated Sucrose at a Liquid|Organogel Microinterface Array.

The electrochemical behavior and detection of sulfated carbohydrates were investigated at an array of microinterfaces between two immiscible electrolyte solutions where the organic phase was gelled. It was found that the electrochemical signal was dependent on the organic phase electrolyte cation. Cyclic voltammetry (CV) of sucrose octasulfate (SOS) with bis(triphenylphosphoranylidene)ammonium BTPPA+ as the organic phase cation did not provide a response to a 10 µM SOS concentration. However, when the organic phase cation was tetradodecylammonium TDDA+, a distinct peak was present in the CV at ca. -0.47 V, indicative of a desorption process following adsorption during the preceding scan. This detection peak shifted to ca. -0.28 V when tridodecylmethylammonium TDMA+ was the organic phase cation, indicating an increased binding strength between this alkylammonium cation and SOS. By combining electroadsorption with TDMA+ as the organic phase electrolyte cation, detection limits of 0.064 µM SOS in 10 mM LiCl and 0.16 µM in a synthetic urine aqueous phase were achieved. The detection limit was improved to 0.036 µM SOS (10 mM LiCl) when the electroadsorption time was increased to 180 s, indicating the analytical capability for the detection of SOS and related sugars by ion-transfer adsorptive stripping voltammetry.

Electroactivity of Aptamer at Soft Microinterface Arrays.

The electrochemical behavior of a synthetic oligonucleotide, thrombin-binding aptamer (TBA, 15-mer), was explored at a liquid-organogel microinterface array. TBA did not display any response when only background electrolytes were present in both phases. On the basis of literature reports that surfactants can influence nucleic acid detection, the response in the presence of cetyltrimethylammonium (CTA+) was examined. With both TBA and CTA+ in the aqueous phase, the transfer current for CTA+ was diminished, signifying the interaction of CTA+ with TBA. Experiments with CTA+ spiked into the organic phase revealed a sharp current peak, consistent with the interfacial formation of a CTA+-TBA complex. However, use of CTA+ as the organic phase electrolyte cation, as the salt with tetrakis(4-chlorophenyl)borate, greatly improved the response to TBA. In this case, a distinctive peak response (at ca. -0.25 V) was attributed to the transfer of CTA+ across the soft interface to complex with aqueous phase TBA. Employing this process as a detection step enabled a detection limit of 0.11 µM TBA (by cyclic voltammetry). Furthermore, the presence of magnesium cations at physiological concentration resulted in the disappearance of the TBA response because of Mg2+-induced folding of TBA. Also, the current response of TBA was decreased by the addition of thrombin, indicating TBA interacted with this binding partner. Finally, the interfacial surfactant-aptamer interaction was explored in a synthetic urine matrix that afforded a detection limit of 0.29 µM TBA. These results suggest that aptamer-binding interactions can be monitored by electrochemistry at aqueous-organic interfaces and open up a new possibility for detection in aptamer-binding assays.

Paper-Based System for Ion Transfer Across the Liquid-Liquid Interface.

While ion transfer studies were shown to be a promising method for fundamental electrochemistry, pharmacokinetic studies, and sensing, they suffer from inherent instability of the interface formed between the organic and aqueous phases. This limits to some extent the range of solvents which can be used and confines these studies to the laboratory. We propose here the use of paper, which has revolutionized the way we think of miniaturized analytical devices during the past decade, as a perfect substrate for ion transfer studies across the liquid-liquid interface. We describe the design of a simple three-phase electrode paper-based setup for redox-driven transfer of anions from an aqueous to an organic phase. Electrochemical measurements of seven different anions and concentration dependence studies are in good agreement with the results obtained with traditional setups. Additionally, we show the applicability and limitations of such setups to the analysis of anion mixtures.

Investigation of modified nanopore arrays using FIB/SEM tomography.

The investigation of electrochemical processes at the interface of two immiscible electrolyte solutions (ITIES) is of great interest for sensing applications, and serves as a surrogate to the study of biological transport phenomena, e.g. ion channels. Alongside e-beam lithography, focused ion beam (FIB) milling is an attractive method to prototype and fabricate nanopore arrays that support nanoITIES. Within this contribution, we explore the capability of FIB/scanning electron microscopy (SEM) tomography to visualize the actual pore structure and interfaces at silica-modified nanoporous membranes. The nanopores were also characterized by atomic force microscopy (AFM) using ultra-sharp AFM probes to determine the pore diameter, and using scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) spectroscopy, providing additional information on the elemental composition of deposits within the pores. Si-rich particles could be identified within the pores as well as at the orifice that had faced the organic electrolyte solution during electrochemical deposition. The prospects of the used techniques for investigating the interface at or within FIB-milled nanopores will be discussed.

Electrochemical behaviour at a liquid-organogel microinterface array of fucoidan extracted from algae.

Fucoidans are sulfated polysaccharides mostly derived from algae and used in a number of applications (e.g. nutrition, cosmetics, pharmaceuticals and biomaterials). In this study, the electrochemical behaviour of fucoidans extracted from two algal species (Undaria pinnatifida and Fucus vesiculosus) was assessed using voltammetry at an array of micro-interfaces formed between two immiscible electrolyte solutions (µITIES) in which the organic electrolyte phase was gelled. Cyclic voltammetry revealed an adsorption process when scanning to negative potentials, followed by a desorption peak at ca. -0.50 V on the reverse scan, indicating the electroactivity of both fucoidans. U. pinnatifida fucoidan showed a more intense voltammetric signal compared to F. vesiculosus fucoidan. In addition, use of tridodecylmethylammonium (TDMA+) or tetradodecylammonium (TDDA+) as the organic phase electrolyte cation provided improved detection of both fucoidans relative to the use of bis(triphenylphosphoranylidene)ammonium (BTPPA+) cation. Application of adsorptive stripping voltammetry provided a linear response of current with fucoidan concentration in the range 2-20 µg mL-1 for U. pinnatifida fucoidan (with TDMA+) and 10-100 µg mL-1 for F. vesiculosus fucoidan (with TDDA+). The combination of TDMA+ in the organic phase and adsorptive pre-concentration for 180 s afforded a detection limit of 1.8 µg mL-1 fucoidan (U. pinnatifida) in aqueous phase of 10 mM NaOH and 2.3 µg mL-1 in synthetic urine (pH adjusted). These investigations demonstrate the electroactivity of fucoidans at the µITIES array and provide scope for their detection at low µg mL-1 concentrations using this approach.

Reactive Conjugated Polymers: Synthesis, Modification, and Electrochemical Properties of Polypentafluorophenylacetylene (Co)Polymers.

(Co)Polymers containing pentafluorophenylacetylene (F5 PA) have been prepared for the first time mediated by [Rh(nbd)Cl]2 /NEt3 to give materials with properties typical of poly(phenylacetylene)s prepared with this catalyst/co-catalyst combination. It is demonstrated that the F5 PA repeat units in these new (co)polymers serve as convenient reactive species for post-polymerization modification with thiols via para-fluoro aromatic nucleophilic substitution reactions to give an entirely new family of novel thioether-functional polyene materials accompanied by absorption maxima shifts of up to 130 nm. Finally, the electrochemical properties of these new fluorinated polyene materials are briefly examined and the distinct difference in behavior of the F5 PA homopolymer versus polyphenylacetylene, copolymers, and functional derivatives is highlighted.

Detection of Prostate Specific Membrane Antigen at Picomolar Levels Using Biocatalysis Coupled to Assisted Ion Transfer Voltammetry at a Liquid-Organogel Microinterface Array.

A label-free electrochemical strategy for the detection of a cancer biomarker, prostate specific membrane antigen (PSMA), at picomolar concentrations without the use of antibodies, was investigated. The approach is based on the assisted ion transfer of protons, generated by a series of enzymatic reactions, at an array of microinterfaces between two immiscible electrolyte solutions (µ-ITIES). This nonredox electrochemical approach based on biocatalysis-coupled proton transfer at the µ-ITIES array opens a new way to detect the prostate cancer biomarker, with detection capability achieved at concentrations below those indicative of disease presence. The strategy is expected to contribute to cancer diagnostics, recurrence monitoring, and therapeutic treatment efficacy.

Achievement of Prolonged Oxygen Detection in Room-Temperature Ionic Liquids on Mechanically Polished Platinum Screen-Printed Electrodes.

The demonstration of prolonged amperometric detection of oxygen in room-temperature ionic liquids (RTILs) was achieved by the use of mechanical polishing to activate platinum screen-printed electrodes (Pt-SPEs). The RTILs studied were 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) and N-butyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyrr][NTf2]). It was found that voltammetry on polished Pt-SPEs exhibited less deterioration (in terms of voltammogram shapes, stability of peak currents, and appearance of contaminant peaks) from long-term consecutive cycling under 100% vol oxygen flow in both RTILs. The detection capability of these RTIL/Pt-SPE systems, initially subjected to long-term consecutive voltammetric cycling, was also investigated by cyclic voltammetry (CV) and long-term chronoamperometry (LTCA). Current versus concentration plots were linear on both unpolished and polished electrodes for 10-100% vol O2 (using CV) and 0.1-5% vol O2 (using LTCA). However, sensitivities and limits of detection (LODs) from CV were found to improve significantly on polished electrodes compared to unpolished electrodes, particularly in [C2mim][NTf2], but also moderately in [C4mpyrr][NTf2]. The lowest LODs (of ca. 0.1% vol O2) were found on polished SPEs using LTCA, with the most stable responses observed in [C4mpyrr][NTf2]. Calibration graphs could not be obtained on unpolished electrodes in both RTILs using LTCA. The results show that polishing markedly improves the analytical performances of Pt-SPEs for oxygen sensing in RTILs. The reusability of such disposable Pt-SPEs, after the surfaces had been experimentally fouled, was also demonstrated through the use of polishing. Mechanical polishing of Pt-SPE devices offers a viable approach to performance improvement for amperometric gas sensing.

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.

Visualization of Diffusion within Nanoarrays.

The direct experimental characterization of diffusion processes at nanoscale remains a challenge that could help elucidate processes in biology, medicine and technology. In this report, two experimental approaches were employed to visualize ion diffusion profiles at the orifices of nanopores (radius (ra) of 86 ± 6 nm) in array format: (1) electrochemically assisted formation of silica deposits based on surfactant ion transfer across nanointerfaces between two immiscible electrolyte solutions (nanoITIES); (2) combined atomic force - scanning electrochemical microscopy (AFM-SECM) imaging of topography and redox species diffusion through the nanopores. The nature of the diffusion zones formed around the pores is directly related to the interpore distance within the array. Nanopore arrays with different ratios of pore center-to-center separation (rc) to pore radius (ra) were fabricated by focused ion beam (FIB) milling of silicon nitride (SiN) membranes, with 100 pores in a hexagonal arrangement. The ion diffusion profiles determined by the two visualization methods indicated the formation of overlapped or independent diffusion profiles at nanopore arrays with rc/ra ratios of 21 ± 2 and 91 ± 7, respectively. In particular, the silica deposition method resulted in formation of a single deposit encompassing the complete array with closer nanopore arrangement, whereas individual silica deposits were formed around each nanopore within the more widely spaced array. The methods reveal direct experimental evidence of diffusion zones at nanopore arrays and provide practical illustration that the pore-pore separation within such arrays has a significant impact on diffusional transport as the pore size is reduced to the nanoscale. These approaches to nanoscale diffusion zone visualization open up possibilities for better understanding of molecular transport processes within miniaturized systems.

Ion-transfer voltammetric behavior of propranolol at nanoscale liquid-liquid interface arrays.

In this work, the ion-transfer voltammetric detection of the protonated ß-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.

Achievement of Diffusional Independence at Nanoscale Liquid-Liquid Interfaces within Arrays.

In this work, independent radial diffusion at arrayed nanointerfaces between two immiscible electrolyte solutions (nanoITIES) was achieved. The arrays were formed at nanopores fabricated by focused ion beam milling of silicon nitride (SiN) membranes, enabling the reproducible and systematic design of five arrays with different ratios of pore center-to-center distance (rc) to pore radius (ra). Voltammetry across water-1,6-dichlorohexane nanoITIES formed at these arrays was examined by the interfacial transfer of tetrapropylammonium ions. The diffusion-limited ion-transfer current increased with the ratio rc/ra, reaching a plateau for rc/ra ≥ 56, which was equivalent to the theoretical current for radial diffusion to an array of independent nanoITIES. As a result, mass transport to the nanoITIES arrays was greatly enhanced due to the decreased overlap of diffusion zones at adjacent nanoITIES, allowing each interface in the array to behave independently. When the rc/ra ratio increased from 13 to 56, the analytical performance parameters of sensitivity and limit of detection were improved from 0.50 (±0.02) A M(-1) to 0.76 (±0.02) A M(-1) and from 0.101 (±0.003) µM to 0.072 (±0.002) µM, respectively. These results provide an experimental basis for the design of arrayed nanointerfaces for electrochemical sensing.

Void-Assisted Ion-Paired Proton Transfer at Water-Ionic Liquid Interfaces.

At the water-trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) ionic liquid interface, the unusual electrochemical transfer behavior of protons (H(+)) and deuterium ions (D(+)) was identified. Alkali metal cations (such as Li(+), Na(+), K(+)) did not undergo this transfer. H(+)/D(+) transfers were assisted by the hydrophobic counter anion of the ionic liquid, [FAP](-), resulting in the formation of a mixed capacitive layer from the filling of the latent voids within the anisotropic ionic liquid structure. This phenomenon could impact areas such as proton-coupled electron transfers, fuel cells, and hydrogen storage where ionic liquids are used as aprotic solvents.

Impact of a surfactant on the electroactivity of proteins at an aqueous-organogel microinterface array.

The impact of surfactant addition to the organic phase on the electroactivity of proteins at the aqueous-organogel interface was examined by voltammetry. The presence of bis(2-ethylhexyl)sulfosuccinate (AOT) in the organogel phase, as the sodium salt, caused marked changes in the peak currents for myoglobin detection. The protein desorption voltammetric peak exhibited a 6-fold increase in the current compared to the corresponding experiment without surfactant. Interfacial coverage showed a 17-fold increase in the adsorbed protein at the interface, from 50 pmol cm(-2), in the absence of surfactant, to 850 pmol cm(-2), in the presence of 10 mM surfactant. Additionally, the presence of the surfactant resulted in a second pair of adsorption/desorption peaks at lower potentials and in a change in the capacitance of the system. The formation of surfactant-protein and surfactant-protein-organic anion deposits is proposed on the basis of these features, leading to increased voltammetric signals for myoglobin, hemoglobin, and cytochrome c. The mechanism of protein-surfactant interaction was probed by using the surfactant as the anion in the organic phase electrolyte salt. Repetitive cyclic voltammetry of cytochrome c showed that in the presence of surfactant there was an enhancement of the signal, caused by a buildup of the protein-surfactant-electrolyte anion assembly at the interface. These findings provide the basis for surfactant-modified interfaces to enhance the electroanalytical performance for protein detection.