Z. Li, T. Pang. J. Shen, P. J. Fletcher, K. Mathwig, F. Marken, Micro and Nano Engineering 16 (2022) 100157. [link, pdf]

Microscale ionic rectifier effects are commonly observed in devices based on semipermeable ionomer coated on an array of microholes with potential applications in alternating current (AC) driven desalination and/or electroosmotic pumping. The efficiency of devices is dependent on ionic diode switching speed, the rectification ratio, and the design of materials and the ionic circuit. Here, a new circuit is proposed based on coupling in parallel (i) a cationic diode based on the cation conductor Nafion and (ii) an anionic diode based on the anion conducting Sustainion. With an alternating driving voltage, a net desalination effect is observed without any moving parts and without significant side reactions. Experimentally, a 4-electrode configuration and a 2-electrode configuration are compared. The ionic diode desalination system is shown to work with only two carbon mat driver electrodes, but the performance in particular at higher ionic strengths (>10 mM) still needs to be improved. Based on the experimental prototype, the current/power efficiency are investigated and challenges for future improvements are discussed. 

J. Mestres, F. Leonardi, K. Mathwig, Micromachines 13 (2022) 362. [link, pdf]

Electrochemical sensors are powerful tools for the detection and real-time monitoring of a wide variety of analytes. However, the long-term operation of Faradaic sensors in complex media is challenging due to fouling. The protection of the electrode surface during in vivo operation is a key element for improving the monitoring of analytes. Here, we study different EUDRAGIT^® controlled release acrylate copolymers for protecting electrode surfaces. The dissolution of these polymers—namely EUDRAGIT^® L 30 D-55 and EUDRAGIT^® FS 30 D—is triggered by a change in pH of the environment, and it is electrochemically monitored by detecting electrode access by means of a redox probe. The full dissolution of the polymer is achieved within 30 min and the electrode response indicates a complete recovery of the original electrochemical performance. We demonstrate that amperometric sensing is a practical and straightforward technique for real-time and in situ sensing of EUDRAGIT^® dissolution profiles. It will find future applications in determining the protection of polymer electrode coating in real matrices and in vivo applications. 

M. Fontana, Š. Ivanovaitė, S. Lindhoud, E. van der Wijk, K. Mathwig, W. van den Berg, D. Weijers, J. Hohlbein, Advanced Biology 6 (2022) 2100953. [link, pdf]

Single-molecule fluorescence detection offers powerful ways to study biomolecules and their complex interactions. Here, nanofluidic devices and camera-based, single-molecule Förster resonance energy transfer (smFRET) detection are combined to study the interactions between plant transcription factors of the auxin response factor (ARF) family and DNA oligonucleotides that contain target DNA response elements. In particular, it is shown that the binding of the unlabeled ARF DNA binding domain (ARF-DBD) to donor and acceptor labeled DNA oligonucleotides can be detected by changes in the FRET efficiency and changes in the diffusion coefficient of the DNA. In addition, this data on fluorescently labeled ARF-DBDs suggest that, at nanomolar concentrations, ARF-DBDs are exclusively present as monomers. In general, the fluidic framework of freely diffusing molecules minimizes potential surface-induced artifacts, enables high-throughput measurements, and proved to be instrumental in shedding more light on the interactions between ARF-DBDs monomers and between ARF-DBDs and their DNA response element

Z. Li, R. Malpass-Evans, N. B. McKeown, M. Carta, K. Mathwig, J. P. Lowe, F. Marken, Electrochemistry Communications 103 (2021) 107110. [link, pdf]

Tertiary-amine-based Polymers of Intrinsic Microporosity (PIMs) provide a class of highly porous molecularly rigid materials for the electrochemical transport of both ionic and neutral species. Here, the transport of water molecules together with chloride anions (i.e. the electroosmotic drag coefficient) is studied for the intrinsically microporous polyamine PIM-EA-TB immersed in aqueous 0.01 M NaCl (i) when protonated for pH < 4 or (ii) when not protonated for pH > 4. Preliminary data suggest that in both cases a high electroosmotic drag coefficient is observed based on direct H₂O transport into a D₂O-filled compartment (quantified by ¹H-NMR). For PIM-EA-TB there is a strong pH dependence with a higher electroosmotic drag coefficient in less acidic solutions (going from approx. 400 H₂O per anion at pH 3 to approx. 4000 H₂O per anion at pH 7), although the underlying absolute rate of water transport at a fixed voltage of −1 V appears to be essentially pH independent. Water transport through the PIM-EA-TB microchannels is rationalised based on the relative populations of chloride anions and of water in the micropores (essentially a ‘piston’ mechanism).

A. K. Thompson, K. Mathwig, P. J. Fletcher, R. Castaing and F. Marken, J. Solid State Electrochem. 25 (2021) 2881. [link, pdf]

Commercial resin microbeads are widely applied in ion exchange and extraction. Here, a single anion-selective and phosphate binding resin microbead (FerrIX™) is mounted into an epoxy membrane and investigated by 4-electrode membrane voltammetry and membrane impedance spectroscopy. Anion transport properties are observed to dominate associated with three distinct potential domains: (I) a low bias ohmic potential domain (dominant at high electrolyte concentration), (II) a concentration polarisation potential domain, and (III) an over-limiting potential domain. Voltammetric responses show transient diffusion-migration features at higher scan rates and quasi-steady state features at lower scan rates. Inherent microbead conductivity is shown to be linked to two resistive elements, electrolyte concentration dependent and independent, in series. The effects of phosphate binding are revealed as transient pattern in impedance spectroscopy data. Preliminary data suggest phosphate concentration-dependent peak features in the imaginary impedance versus frequency plot due to phosphate binding into the microbead.

Z. Li, L. Wang, R. Malpass-Evans, M. Carta, N. B. McKeown, K. Mathwig, P. J. Fletcher and F. Marken, ChemElectroChem 8 (2021) 2044. Invited contribution to the Marcin Opałło Festschrift. [link, pdf]

The polymer of intrinsic microporosity PIM‐EA‐TB provides a molecularly rigid micropore structure containing tertiary amine sites and is shown here to interact with hydrogen bonding guest molecules such as caffeic acid. Voltammetric data with a PIM‐EA‐TB film on glassy carbon electrodes show that in both acidic solution (pH 2; PIM‐EA‐TB is protonated) and in neutral solution (pH 6; PIM‐EA‐TB is not protonated) caffeic acid is slowly accumulated into the microporous host. Binding constants are estimated and suggested to be linked to hydrogen bonding causing accumulation of caffeic acid. When employing PIM‐EA‐TB as an asymmetric membrane coated onto a 5 mm thick Teflon support film with 10 mm diameter microholes (using either a single microhole or a 10 × 10 array of microholes), binding of caffeic acid is shown to cause a modulation of the ionic current without affecting the pH‐dependent ionic diode behaviour. Two complementary types of effects of caffeic acid guests are discussed based on blocking anion diffusion pathways and based on removal of positive charges. The caffeic acid transport mechanism/efficiency is investigated in view of selective molecular pumping.

B. R. Putra, L. Tshwenya, M. A. Buckingham, J. Chen, K. J. Aoki, K. Mathwig, O. A. Arotiba, A. K. Thompson and F. Marken, Electroanalysis 33 (2021) 1398. [link, pdf]

Ionic rectifier membranes or devices generate uni‐directional ion transport to convert an alternating current (AC) ion current input into stored energy or direct current (DC) in the form of ion/salt gradients. Electrochemical experiments 80 years ago were conducted on biological membrane rectifier systems, but today a plethora of artificial ionic rectifier types has been developed and electroanalytical tools are employed to explore mechanisms and performance. This overview focuses on microscale ionic rectifiers with a comparison to nano‐ and macroscale ionic rectifiers. The potential is surveyed for applications in electrochemical analysis, desalination, energy harvesting, electrochemical synthesis, and in selective ion extraction.

S. Voci, H. Al-Kutubi, L. Rassaei, K. Mathwig and N. Sojic Anal. Bioanal. Chem. 41, (2020) 4067. [link, pdf]

Nanofluidic electrochemical devices confine the volume of chemical reactions to femtoliters. When employed for light generation by electrochemiluminescence (ECL), nanofluidic confinement yields enhanced intensity and robust luminescence. Here, we investigate different ECL pathways, namely coreactant and annihilation ECL in a single nanochannel and compare light emission profiles. By high-resolution imaging of electrode areas, we show that different reaction schemes produce very different emission profiles in the unique confined geometry of a nanochannel. The confrontation of experimental results with finite element simulation gives further insight into the exact reaction ECL pathways. We find that emission strongly depends on depletion, geometric exclusion, and recycling of reactants in the nanofluidic device.

L. Tshwenya, F. Marken, K. Mathwig and O. A. Arotiba, ACS Applied Materials and Interfaces 12 (2020) 3214. [link, pdf]

Membrane materials with semi-permeability for anions or for cations are of interest in electrochemical and nanofluidic separation and purification technologies. In this study, partially hydrolyzed poly-acrylonitrile (phPAN) is investigated as a pH-switchable anion/cation conductor. When switching from anionic to cationic semi-permeability, also the ionic current rectification effect switches for phPAN materials deposited asymmetrically onto a 5, 10, 20, or 40 µm diameter microhole in a 6 µm thick polyethylene-terephthalate (PET) film substrate. Therefore, ionic rectifier behavior can be tuned and used to monitor and characterize semi-permeability. Effects of electrolyte type and concentration, and pH (relative to the zeta potential at approximately 3.1) are investigated by voltammetry, chronoamperometry, and impedance spectroscopy. A computational model provides good qualitative agreement with observed electrolyte concentration data. High rectification effects are observed for both cations (pH > 3.1) and anions (pH < 3.1), but only at relatively low ionic strengths.

K. Mathwig and N. Sojic, Journal of Analysis and Testing 3 (2019) 160. [link, pdf]

In ion-annihilation electrochemiluminescence (ECL), luminophore ions are generated by oxidation as well as reduction at electrodes surfaces, and subsequently recombine into an electronically excited state, which emits light. The intensity of the emitted light is often limited by the kinetic rate of recombination of the luminophore ion species. Recombination or annihilation rates are high ranging up to approximately 10¹⁰ M⁻¹ s⁻¹ and can be difficult to determine using scanning electrochemical microscopy or high-frequency oscillations of an electrode potential. Here, we propose determining annihilation kinetics by measuring the relative change of the emitted light intensity as a function of luminophore concentration. Using finite element simulations of annihilation ECL in a geometry of two closely spaced electrodes biased at constant potentials, we show that, with increasing concentrations, luminescence intensity crosses over from a quadratic dependence on concentration to a linear regime—depending on the rate of annihilation. Our numerical results are applicable to scanning electrochemical microscopy as well as nanofluidic electrochemical devices to determine fast ion-annihilation kinetics.