Electrochemical Control Research Articles

The modified liquid | liquid interface: An electrochemical route for the electrode-less synthesis of MoS2 metal composite thin films

An interfacial route is described to form composites of molybdenum disulfide with metals. Metal deposition occurs at the liquid| liquid interface, on to the pre-adsorbed MoS2 film. The method is exemplified via the interfacial deposition of Cu and Pd at the MoS2-modified water/1,2,-dichlorobenzene interface, under electrochemical control (interface between two immiscible electrolyte solutions). Differences in morphology between the two metals are related to their differing modes of growth. The voltammetric measurements were analysed to elucidate the effect of the interfacial MoS2 film on the reduction and growth of the metals. Overall, the MoS2 film was found to impact the metallic deposition reaction, and the effect depends on the nature of the metallic precursor.

Nanogravimetric Monitoring of Electrochemically Driven Fluoride Ion Extraction from Water By Aniline-Based Copolymer Films

Sustainable access to safe drinking water is a global societal goal [1]. Accordingly, much attention has been given to the development of technologies for remediation of the unintended effects of agricultural practices and industrial processes. The case of fluoride in water is an interesting case in two respects: its desirability up to a point and its natural origins. The beneficial effects of fluoride on dental health are recognized: in some countries drinking water is routinely fluoridated. However, high levels of fluoride can have detrimental effects on teeth, bones and internal organs; the recommended upper limit is 1.5 mg/L. The fluoride concentration in natural waters - the source of most drinking water - is dictated by the presence of calcium ions, through the solubility product of calcium fluoride. Dependent on the local geology, the calcium ion concentration may be relatively low or high, such that either the beneficial effects of fluoride may not be realized or its injurious effects may dominate. Determination of fluoride concentration using ion selective electrodes is well-established. The question then is how to remedy high fluoride levels. Existing methods for removal of excess fluoride from drinking water and wastewater include precipitation (as Ca or Al salts), membrane technologies and adsorption. Each of these has limitations, e.g. high costs, toxic by-products and slow or complex processes.Here we explore electrochemically controlled fluoride ion extraction by electroactive polymer films. This approach has been studied for water softening (using polypyrrole [2]) and perchlorate removal (using polypyrrole composites [3] and polyaniline-based copolymers [4]). Here we explore the feasibility of using a range of polyaniline-based materials in an electrochemically switched ion exchange system for fluoride removal from water [5]. The concept is based on F- ion uptake as counter ions in the oxidation (p-doping) of the conducting polymer film. Upon oxidation, the film will “capture” F- ions from solution. After separation of the purified water, the F- ions would then be ejected into a concentrated waste stream by reduction (un-doping) of the oxidized polymer.In this presentation we compare the characteristics and performance of polyaniline, poly(o-aminophenol) and poly(o-toluidine) homopolymer films with each other and with those of their copolymers of various composition. Film deposition is controlled electrochemically and monitored nanogravimetrically using the EQCM. Acoustic impedance enables distinction between gravimetric and viscoelastic interpretation of the response [8]. The extent of redox-driven fluoride uptake is then determined upon exposure to solutions of varying fluoride (and in some cases chloride) concentration. Correlation of EQCM-derived film mass and charge responses is used to assay fluoride and solvent uptake during film oxidation and reduction. Comparison with the total redox site population (from the response in fluoride-free media) yields the efficiency. Observations for these aniline-based homopolymers and copolymers reveal behavior that is quite different to that seen for typical small anionic dopants, such as chloride, nitrate and perchlorate. Further, there are surprisingly diverse responses to fluoride for these relatively similar polymeric materials, notably the extent of film solvation change during fluoride uptake. Use of different electrochemical control functions and timescales reveals differences in fluoride ion uptake and release rates. The relevance of these data to defluoridation will be discussed.

Atomic-Scale Friction Study: Underpotential Deposition (UPD) of Silver on I-Modified Au(111) in Aprotic Electrolyte

Friction in general is one of the main sources of energy loss, and therefore finding ways for better controlling friction is of uttermost importance. Atomic-scale friction is measured using electrochemical lateral force microscopy (EC-LFM) where the AFM tip slides on the surface of electrode under electrochemical control. It helps us to understand interfacial and tribological behavior on nanoscale. In ionic liquids (ILs) including solvate ILs, the friction as function of normal load has been studied on HOPG and Au(111), which shows that the structure of ions is a decisive factor for the friction behavior [1-4]. Our interest was focused on the adsorption of foreign metals, ranging from (sub)monolayer (underpotential deposition (UPD)) to multilayer (bulk) deposition and the adsorption of organic adsorbates, and its influence on friction as function of potential on single crystal electrodes in aqueous electrolyte[5-7].The underpotential deposition (UPD) of silver had often been studied on gold in aqueous electrolyte since the lattice constants of silver and gold are fairly identical (Å, Å) [8-12]. Previous results clearly show that the Ag UPD process is very sensitive to the presence of other adsorbates (e.g. SO4 2-, ClO4 -). Especially, the Ag UPD on iodine covered surface of Au(111) shows completely different atomic structures after the 1st, 2nd, and 3rd Ag UPD compared to the results on Au(111) [9, 13].In this study, we investigate the interfacial properties on I-modified Au(111) during the Ag UPD in aprotic electrolyte. We observed two sets of Ag UPD peaks in cyclic voltammetry (CV). The charge density for these peaks is about 216 µC/cm² close to the theoretical charge density for the monolayer (222 µC/cm²). We observed the iodine structure at low normal load (FN < 20nN), which shows that the iodine adlayer on Au(111) and Ag monolayer forms a (˅3×˅3)R30°. It indicates that the iodine adlayer is stable during the Ag UPD. At high normal load, the AFM tip penetrates into iodine layer and reveals the structure of Au(111) or Ag monolayer depending on the potential. It is remarkable that there is no irreversible wear during the penetration of AFM tip meaning that the iodine structure appears again with decreasing normal loads. At high normal load where the tip penetrates into iodine layer and interacts with substrates (Au and Ag), the friction on Ag monolayer is higher than on Au(111). Interestingly, even though the cyclic voltammetry (CV) shows that the amount of water in the electrolyte has minor influence, the friction increases with an increase in water content on the I/Ag/Au(111).[1] H. Li, M.W. Rutland, R. Atkin, Physical Chemistry Chemical Physics 15 (2013) 14616-14623.[2] H. Li, M.W. Rutland, M. Watanabe, R. Atkin, Faraday discussions 199 (2017) 311-322.[3] O.Y. Fajardo, F. Bresme, A.A. Kornyshev, M. Urbakh, Scientific Reports 5 (2015) 6.[4] A. Elbourne, J. Sweeney, G.B. Webber, E.J. Wanless, G.G. Warr, M.W. Rutland, R. Atkin, Chem. Commun. 49 (2013) 6797-6799.[5] S. Iqbal, S. Wezisla, N. Podgaynyy, H. Baltruschat, Electrochimica Acta 186 (2015) 427-435.[6] M. Nielinger, H. Baltruschat, Physical Chemistry Chemical Physics 9 (2007) 3965-3969.[7] N. Podgaynyy, S. Iqbal, H. Baltruschat, Surface Science 631 (2015) 67-72.[8] T. Hachiya, K. Itaya, Ultramicroscopy 42 (1992) 445-452.[9] K. Ogaki, K. Itaya, Electrochimica acta 40 (1995) 1249-1257.[10] M. Espladiu, M. Schneeweiss, D. Kolb, Physical Chemistry Chemical Physics 1 (1999) 4847-4854.[11] C.H. Chen, S.M. Vesecky, A.A. Gewirth, Journal of the American Chemical Society 114 (1992) 451-458.[12] P. Mrozek, Y.-e. Sung, M. Han, M. Gamboa-Aldeco, A. Wieckowski, C.-h. Chen, A.A. Gewirth, Electrochimica acta 40 (1995) 17-28.[13] S. Sugita, T. Abe, K. Itaya, The Journal of Physical Chemistry 97 (1993) 8780-8785.

(Digital Presentation) Synthesis of Conjugated Polymer Alloy Prepared By Electrochemical Polymerization in Chiral Liquid Crystal

In this research, we synthesized polymer alloy by electrochemical polymerization in chiral liquid crystal. Homopolymers and the copolymer can be synthesized when electrochemical polymerization was conducted in the presence of several types of monomers. The polymer alloy thus prepared contains both blend of homopolymers and copolymers. The polymer alloy shows optical activity. The chirality derived from transcription of liquid crystal during the polymerization reaction results to show optical activity. The polymer alloy can show optical activity with no asymmetric centers. The optical activity is due to existence of charge carriers (polarons) in the main chain as a form of conjugated polymers. This is called chiral charge carrier “chiralions”. This research found that the chiral polymer alloy has chiralions.Polymer alloys have been prepared by blending several kinds of polymers in the reaction. Polymer alloys also can be prepared by melt method. Unsubstituted conjugated polymers have poor solubility and fusibility because of the rigid π-bonds in the skeleton, which is drawback for preparation of π-conjugated polymer-based alloys. We have studied on preparation of chiral conjugated polymer alloys with combination of achiral polymers and chiral polymers.In the present study, conjugated polymer alloys were synthesized with the method described in Figure 1a. Cholesteryl acetate (chiral inducer), 2,7-di(2-thienyl)fluorene (mono1), N-methyl-3,6-di(2-thienyl)carbazole (mono2) and TBAP (supporting salt) were dissolved in 4-cyano-4'-pentylbiphenyl (5CB) as a host liquid crystal to prepare a chiral liquid crystal electrolyte. The electrolyte exhibited a fingerprint structure under observation of polarizing optical microscopy. For the electrochemical polymerization reaction, the electrolyte solution is charged in the cell consisted of two indium-tin-oxide (ITO) glass plates as electrodes and a poly(tetrafluoroethylene) spacer (200 μm thickness). Next, dc voltage of 3.0 V was applied to the ITO glass cell for 30 min for electrochemical polymerization. The polymer was deposited on the anode electrode as a thin film form. After polymerization, the remaining electrolyte solution on the ITO glass electrode was carefully washed with a large amount of hexane, yielding dark blue film. This film is abbreviated as Alloy1. The molecular structure was confirmed by infrared spectroscopy with the KBr method. Alloy1 shows a fingerprint structure under POM observations (Figure 1b). The macroscopic structure of cholesteric liquid crystal was transcribed from the liquid crystal electrolyte solution to Alloy1. Synchrotron XRD measurement was carried out. The peak at 10.3 Å is derived from monomer repeat units. The XRD signal at 5.1 Å and 3.4 Å can be derived from π-stacking of Alloy1. Alloy1 shows UV-vis absorption due to the three signals derived from π-π* transition of the main chain, polarons (radical cations) and bipolarons (radical dications). Electrochemical properties of Alloy1 were evaluated by the cyclic voltammetry (CV). Alloy1 film deposited on the ITO glass was used as a working electrode. An Ag/Ag+ electrode and a platinum wire served as the reference and counter electrodes, respectively. A propylene carbonate solution with 0.1 M of TBAP was employed as an electrolyte solution for the CV measurements. Cyclic voltammograms of Alloy1 shows the oxidation peak and reduction trough (Figure 1c). Plots of the oxidation peak current (I pa) and the reduction peak current (I pc) as a function of the square root of the scan rate show a linear shape of the anodic and cathodic peak current values when scan rates from 10 to 100 mV/s with intervals of 10 mV/s. This result indicated that the redox reaction of Alloy1 was reversible and controlled by electron transfer processes. In-situ circular dichromism (CD) and in-situ optical rotatory dispersion (ORD) spectra of Alloy1 with application of voltages were examined in a propylene carbonate solution with 0.1 M of TBAP. The application voltages were between 0-1.2 V (vs. Ag/Ag+) with intervals of 0.1 V. A negative Cotton effect was observed from the in-situ CD, indicating Alloy1 has left-handed helical structure. The ORD shows negative signals, and changes with applied voltage. The ellipticity signal in the CD and optical rotation in the ORD could control in the electrochemical method, accompanied by electrochemical redox process.We synthesized the polymer alloy by electrochemical polymerization in chiral liquid crystal. The conjugated polymer alloy showed chirality with no chiral centers in the main chain. The CD and ORD bands at NIR range are assigned to doping band as a form of polarons (radical cations). This research evaluated electrochemical control of “chiralions”. Figure 1

Investigation of the Mechanism of Cocrystal Formation at the Polarized Liquid-Liquid Interface

The cocrystals of two reagents of mismatch solubility, hydrophilic caffeine and lipophilic 1-hydroxy-2-naphthoic acid, were obtained at a water-oil interface. 1 By analysing the product collected at the phase interface by X-ray diffraction and Raman spectroscopy, we confirmed that we obtained the molecular cocrystals of caffeine and 1-hydroxy-2-naphthoic acid, free of starting reagents and solvents. Polarisation of the interface allowed the electrochemical control of the cocrystallisation process: for non-polarised interfaces, a mixture of phase I and II cocrystals were obtained, whereas cocrystals of phase I were formed when a high interfacial potential was applied. The difference between the polymorphs obtained in the absence or in the presence of interfacial polarisation is attributed to the different cocrystallisation mechanisms. In the absence of interfacial polarisation, caffeine and 1-hydroxy-2-naphthoic acid are cocrystallising through an interfacial process. When the interface is positively polarised, cationic form of caffeine is transferred to the organic phase, the complex with naphthoic acid is formed and the proton is released. The pH increased after cocrystallisation, suggesting that the liberated H+ remained in the organic phase. On the contrary, in the absence of polarisation or in the case of low interfacial potential the protons are remaining in the aqueous phase what resulted in a decrease in pH after cocrystallisation process. We demonstrated an indirect evidence of the transfer of caffeine upon chemical polarization of the interface. We observed almost no deposits in the organic phase by analysing it with a scanning electron microscope.We investigated here the mechanism behind the formation of cocrystals by using the polarized interface between two immiscible electrolyte solutions (ITIES).2 For the purpose of this study, the two solutions have been placed in contact3 : one being an aqueous solution of caffeine, as a model drug substance, and the second being organic solution of 1-hydroxy-2-naphthoic acid (cocrystal component) and the salt inducing the suitable imposed potential difference. The formation of the caffeine: 1-hydroxy-2-naphthoic acid complex was studied by changing the salts present in the aqueous phase, the chemical compounds imposing potential in organic phase, varying the organic solvents and the temperature at the liquid-liquid interface. The cocrystals collected at the interface were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and low frequency Raman spectroscopy. We studied the proportions of polymorphic phases as a function of changing conditions. To understand the mechanism behind the caffeine transfer and the formation of different forms of polymorphic crystals at the water-oil interface, Electrochemical Noise Experiments4 have been carried out. A four-electrode system was used to apply zero volts between the working electrode and the counter electrode and then measuring the current and the potential versus the reference electrode. Such a setup allows the measurement of potential and current as the function of time simultaneously. We could thus follow the phenomena that are taking place at the interface. The measurements were carried out by varying the cations (tetraalkylammoniums, Li+, K+) in aqueous and organic phase as well as the anions (derivatives of tetrakis(phenyl-borate) in organic phase). The experimental results were supported by theoretical equations and applied to the system containing caffeine.This novel strategy of cocrystallization, was followed by the investigation of other coformers (3-hydroxy-2-naphthoic acid) and drugs (cocrystals of ketoprofen with amino acids and carboxylic acids) in order to validate the method. Such an electrocrystallization process at the ITIES will open new opportunities for the formation of drugs cocrystals. M. Kaliszczak et al., CrystEngComm (2022) doi: 10.1039/D1CE01281A.Z. Samec, Pure Appl. Chem., 76, 2147–2180 (2004).S. J. Diez et al., Cryst. Growth Des., 18, 3263–3268 (2018).I. B. Obot, I. B. Onyeachu, A. Zeino, and S. A. Umoren, J. Adhes. Sci. Technol., 33, 1453–1496 (2019).

Electrochemical Control of Biofilm Formation and Approaches to Biofilm Removal

This review deals with microbial adhesion to metal-based surfaces and the subsequent biofilm formation, showing that both processes are a serious problem in the food industry, where pathogenic microorganisms released from the biofilm structure may pollute food and related material during their production. Biofilm exhibits an increased resistance toward sanitizers and disinfectants, which complicates the removal or inactivation of microorganisms in these products. In the existing traditional techniques and modern approaches for clean-in-place, electrochemical biofilm control offers promising technology, where surface properties or the reactions taking place on the surface are controlled to delay or prevent cell attachment or to remove microbial cells from the surface. In this overview, biofilm characterization, the classification of bacteria-forming biofilms, the influence of environmental conditions for bacterial attachment to material surfaces, and the evaluation of the role of biofilm morphology are described in detail. Health aspects, biofilm control methods in the food industry, and conventional approaches to biofilm removal are included as well, in order to consider the possibilities and limitations of various electrochemical approaches to biofilm control with respect to potential applications in the food industry.

Reversible vs Standard Hydrogen Electrode Scale in Interfacial Electrochemistry from a Theoretician’s Atomistic Point of View

It is a general notion in interfacial electrochemistry that the stability of adsorbate phases that only contain hydrogen atoms should be independent of the pH value of the electrolyte on the scale of the reversible hydrogen electrode, whereas the stability of adsorbate phases that do not contain any hydrogen should be independent of the pH value on the scale of the standard hydrogen electrode. In this Perspective, it will be argued on the basis of a grand-canonical approach that such a Nernstian behavior can only be reproduced if the free energy of the adsorbate phase is independent of the electrochemical control parameters. In general, this should not be true, so that the Nernstian behavior should be the exception rather than the rule. Still, structural and chemical factors will be discussed that might lead to a Nernstian behavior. This requires an analysis of the electrochemical electrolyte/electrode interface on the atomistic level. At the same time, this analysis also provides a guideline for the validity of grand-canonical simulations using the concept of the computational hydrogen electrode in which the dependence of the energy of adsorbate phases on pH and electrode potential is neglected.

Bioelectrocatalytic sulfide oxidation by a haloalkaliphilic electroactive microbial community dominated by Desulfobulbaceae

Sulfide-oxidizing microorganisms (SOMs) play a vital role in biogeochemical sulfur cycling and are useful for the bioremediation of sulfide-rich wastewaters and environments. The electroactive SOMs remain poorly explored from extreme environments. This study presents the autotrophic electroactive microbial biofilm capable of sulfide oxidation under haloalkaline conditions (pH – 9.5; salinity – 20 g/L). It was obtained through electrochemical microbial enrichment from sub-surface sediments of the haloalkaline Lonar lake at an applied anode potential of -330 mV vs. Ag/AgCl in microbial electrolysis cells (MECs). The sulfide-oxidizing electroactive biofilm achieved a maximum average current density of 315±23 µA/cm2 with 87±10% coulombic efficiency and 34±8% sulfide oxidation to sulfate. The bioelectrocatalytic sulfide oxidation at the anode facilitated 0.16±0.02 mM/day hydrogen production at the cathode of MECs. Low current densities and neither sulfate nor hydrogen production were observed in abiotic electrochemical controls suggesting the bioelectrocatalytic role of anodic microbial biofilm in the sulfide to sulfate oxidation process. The sulfide oxidation process was faster in bioelectrochemical (0.94±0.2 mM/day) than the abiotic reactors (0.49±0.01 mM/day), confirming the microbial role in the enhanced sulfide oxidation bioelectrocatalysis. Sulfur was detected at the anode surface in both abiotic and biotic electrochemical experiments, suggesting the occurrence of electrochemical sulfide oxidation to sulfur process under here-tested conditions. The bio-anode analysis through scanning electron microscopic imaging confirmed the microbial growth at the anode surface, and 16S-rRNA based amplicon sequencing revealed the dominance of unclassified Desulfobulbaceace at a relative sequence abundance of 61.8 ± 9.9% in anodic biofilm. This work on electroactive SOMs from a natural haloalkaline habitat contributes to the limited understanding of extreme electroactive microorganisms and their role in sulfur cycling in such environments.

Redox-Switchable Chalcogen Bonding for Anion Recognition and Sensing.

Inspired by the success of its related sigma-hole congener halogen bonding (XB), chalcogen bonding (ChB) is emerging as a powerful noncovalent interaction with a plethora of applications in supramolecular chemistry and beyond. Despite its increasing importance, the judicious modulation of ChB donor strength remains a formidable challenge. Herein, we present, for the first time, the reversible and large-scale modulation of ChB potency by electrochemical redox control. This is exemplified by both the switching-ON of anion recognition via ChB oxidative activation of a novel bis(ferrocenyltellurotriazole) anion host and switching-OFF reductive ChB deactivation of anion binding potency with a telluroviologen receptor. The direct linking of the redox-active center and ChB receptor donor sites enables strong coupling, which is reflected by up to a remarkable 3 orders of magnitude modulation of anion binding strength. This is demonstrated through large voltammetric perturbations of the respective receptor ferrocene and viologen redox couples, enabling, for the first time, ChB-mediated electrochemical anion sensing. The sensors not only display significant anion-binding-induced electrochemical responses in competitive aqueous-organic solvent systems but can compete with, or even outperform similar, highly potent XB and HB sensors. These observations serve to highlight a unique (redox) tunability of ChB and pave the way for further exploration of the reversible (redox) modulation of ChB in a wide range of applications, including anion sensors as well as molecular switches and machines.

Synthetic biology and bioelectrochemical tools for electrogenetic system engineering

Synthetic biology research and its industrial applications rely on deterministic spatiotemporal control of gene expression. Recently, electrochemical control of gene expression has been demonstrated in electrogenetic systems (redox-responsive promoters used alongside redox inducers and electrodes), allowing for the direct integration of electronics with biological processes. However, the use of electrogenetic systems is limited by poor activity, tunability, and standardization. In this work, we developed a strong, unidirectional, redox-responsive promoter before deriving a mutant promoter library with a spectrum of strengths. We constructed genetic circuits with these parts and demonstrated their activation by multiple classes of redox molecules. Last, we demonstrated electrochemical activation of gene expression under aerobic conditions using a novel, modular bioelectrochemical device. These genetic and electrochemical tools facilitate the design and improve the performance of electrogenetic systems. Furthermore, the genetic design strategies used can be applied to other redox-responsive promoters to further expand the available tools for electrogenetics.

Electrosynthesis of Biocompatible Free-Standing PEDOT Thin Films at a Polarized Liquid|Liquid Interface.

Conducting polymers (CPs) find applications in energy conversion and storage, sensors, and biomedical technologies once processed into thin films. Hydrophobic CPs, like poly(3,4-ethylenedioxythiophene) (PEDOT), typically require surfactant additives, such as poly(styrenesulfonate) (PSS), to aid their aqueous processability as thin films. However, excess PSS diminishes CP electrochemical performance, biocompatibility, and device stability. Here, we report the electrosynthesis of PEDOT thin films at a polarized liquid|liquid interface, a method nonreliant on conductive solid substrates that produces free-standing, additive-free, biocompatible, easily transferrable, and scalable 2D PEDOT thin films of any shape or size in a single step at ambient conditions. Electrochemical control of thin film nucleation and growth at the polarized liquid|liquid interface allows control over the morphology, transitioning from 2D (flat on both sides with a thickness of <50 nm) to “Janus” 3D (with flat and rough sides, each showing distinct physical properties, and a thickness of >850 nm) films. The PEDOT thin films were p-doped (approaching the theoretical limit), showed high π–π conjugation, were processed directly as thin films without insulating PSS and were thus highly conductive without post-processing. This work demonstrates that interfacial electrosynthesis directly produces PEDOT thin films with distinctive molecular architectures inaccessible in bulk solution or at solid electrode–electrolyte interfaces and emergent properties that facilitate technological advances. In this regard, we demonstrate the PEDOT thin film’s superior biocompatibility as scaffolds for cellular growth, opening immediate applications in organic electrochemical transistor (OECT) devices for monitoring cell behavior over extended time periods, bioscaffolds, and medical devices, without needing physiologically unstable and poorly biocompatible PSS.

Electrochemical control and protonation of the strontium iron oxide SrFeOy by using proton-conducting electrolyte

To electrochemically control structural and transport properties of oxygen-deficient perovskite SrFeOy (2.5 ≦ y ≦ 3) (SFO) epitaxial films, we employed electric-field-effect transistor structures in which the proton-conducting solid electrolyte Nafion is used as a gate insulator. When a positive gate voltage (VGS) is applied and protons are injected toward the film channel layer, the SFO films are electrochemically reduced, leading to increases in the channel resistance. On the other hand, when a negative VGS is applied and protons are removed, the SFO films are oxidized, and as a result, the channel resistances decrease. In addition, we found that the electrochemically reduced SFO films accommodate protons, forming the proton-containing oxide HxSrFeO2.5 whose proton concentration is determined by elastic recoil detection analysis to be x ∼ 0.11. Our results indicate the usefulness of the proton-conducting solid electrolyte for electrochemically controlling transition metal oxides and for exploring proton-containing oxides.

Coupling Respirometric HER and ORR Monitoring with Electrochemical Measurements

An experimental setup for respirometric real-time monitoring of the kinetics of the H2 evolution reaction and the O2 reduction reaction coupled with electrochemical control is presented. Cathodic polarization of different metal electrodes was carried out to verify the accuracy, and sensitivity of this approach. It was found that the respirometric response was in excellent agreement with the applied electrical signal. Dissolution of H2 in the liquid phase can lead to a delayed detection of the evolved H2 in the gas phase. However, reducing the volume of electrolyte in the setup can diminish this lag in response time. The influence of H2 oxidation on reactive Pt and the generation of H2O2 on Au on the respirometric monitoring of cathodic reactions was investigated. Finally, the combined respirometric and electrochemical approach was applied to quantify dissolution of Al during cathodic polarization by relating the total cathodic charge with the electrical signal.

Unique Electronic Excitations at Highly Localized Plasmonic Field.

ConspectusUnder visible light illuminations, noble metal nanostructures can condense photon energy into the nanoscale region. By precisely tuning the metal nanostructures, the ultimate confinement of photoenergy at the molecular scale can be obtained. At such a confined photon energy field, various unique photoresponses of molecules, such as efficient visible light energy conversion processes or efficient multielectron transfer reactions, can be observed. Light-matter interactions also increase with the condensation of photons with nanoscale regions, leading to efficient light energy utilizations. Moreover, the strong field confinement can often modulate electronic excitations beyond normal selection rules. Such unique electronic excitations could realize innovative photoenergy conversion systems. On the other hand, such interactions lead to changes in the optical absorption property of the system via the formation of hybridized electronic energy states. This hybridized state is expected to have the potential to modulate the chemical reaction pathways. Taking these facts into consideration, a probe for the molecular absorption process with high sensitivity allows us to find novel ways for further precise tuning of light-matter interactions. In this Account, we review phenomena of unique electronic excitations from the perspective of our previous investigations using surface-enhanced Raman scattering (SERS) spectroscopy at electrified interfaces. Because the enhancement mechanism of Raman scattering at interfaces is deeply correlated with the photon absorption process accompanied by the electronic excitations between molecules and electrode surfaces, the detailed SERS investigations of the well-defined system can provide information on the electronic excitation processes. Through SERS observations of single-molecule junctions at electrodes or well-defined low-dimensional carbon materials, we have observed the characteristic Raman bands containing additional polarization tensors, indicating the occurrence of electronic polarization induced by electronic excitations based on a distinct selection rule. The origins for the observed facts were attributed to the highly condensed electric field producing the huge intensity gradient at the nano scale. The electrochemical potential control of the system would be valuable for the control of the excitation process. Additionally, from Raman spectra of dye molecules coupled to the plasmonic field, the changes in the Raman scattering intensity depending on the strength of interactions suggested the modulation of the absorption characteristics of the system. In addition, we have proved that the electrochemical potential control method can be a powerful tool for the active tuning of the light-matter interaction, leading to the change in the light absorption property. The molecular behaviors of dyes in the strong-coupling regime were reversibly tuned to show intense SERS. The current descriptions provide novel insights for these unique electronic excitations, realized by the plasmon excitation, that lead to advanced photoenergy conversions beyond the limits of present systems.

Synthesis and covalent immobilization of redox-active metallopolymers for organic phase electrochemistry

Redox-active metallopolymers are promising stimuli-responsive platforms for a range of applications including sensing, energy storage, and selective separations. In particular, heterogeneously-functionalized metallopolymers can modulate the capture and release of target molecules, driven by redox electron-transfer. However, prior metallopolymer-functionalized electrodes have been fabricated by non-covalent methods, and tailored for aqueous phase applications. As such, despite the existing potential for heterogeneous applications in organic phase, there are significant constraints to the stability of metallopolymers in organic solvents, including high solubility in solvents such as chloroform or tetrahydrofuran. We propose the immobilization of thiol-functionalized redox-active metallopolymers on metallic surfaces as a facile way to enhance stability and cyclability in organic media, and thus broaden the applicability of redox-metallopolymers for organic phase applications. We explore the anionic polymerization of metal-containing monomers vinylferrocene (VFc) and ferrocenyldimethylsilane (FS), and their thiol end-functionalization by living anionic polymerization strategies. PFS and PVFc with molar masses ranging from 1800 to 49900 g mol−1 and 2900 to 6300 g mol−1 respectively were prepared with a segment of poly(ethylene sulfide), as characterized by size-exclusion chromatography, NMR spectroscopy, MALDI/ToF, thermogravimetry, and elemental analysis. Both metallopolymers were immobilized on gold substrates by a grafting-to protocol, with demonstrated redox-responsiveness by electrochemical control. In the case of immobilized PVFc, operando electrochemical testing demonstrated the stable and reversible electrochemical cycling capabilities (>74% maximum current retained after 100 oxidation/reduction cycles) in several organic solvents including chloroform, tetrahydrofuran, ethanol, methanol, acetonitrile, and acetone. Immobilized PFS was stable in chloroform, with a 83% maximum current retained after 100 oxidation/reduction cycles. We envision future applications of these covalently immobilized metallopolymers for a broad range of fields from selective separations to sensing and energy storage.

Room-Temperature Molecular Manipulation via Plasmonic Trapping at Electrified Interfaces.

For the motion control of individual molecules at room temperature, optical tweezers could be one of the best approaches to realize desirable selectivity with high resolution in time and space. Because of physical limitations due to the thermal fluctuation, optical manipulation of small molecules at room temperature is still a challenging subject. The difficulty of the manipulation also emerged from the variation of molecular polarizability depending on the choice of molecules as well as the molecular orientation to the optical field. In this article, we have demonstrated plasmonic optical trapping of small size molecules with less than 1 nm at the gap of a single metal nanodimer immersed in an electrolyte solution. In situ electrochemical surface-enhanced Raman scattering measurements prove that a plasmonic structure under electrochemical potential control realizes not only the selective molecular condensation but also the formation of unique mixed molecular phases which is distinct from those under a thermodynamic equilibrium. Through detailed analyses of optical trapping behavior, we established the methodology of plasmonic optical trapping to create the novel adsorption isotherm under applying an optical force at electrified interfaces.

Covalent Biosensing Polymer Chain Reaction Enabling Periphery Blood Testing to Predict Tumor Invasiveness with a Platelet Procancerous Protein.

Periphery blood testing is an attractive and relatively less invasive way of early cancer screening. In this work, based on the latest understanding of the pivotal role of platelets in promoting cancer invasion, a method for detecting a procancerous protein overexpressed both on platelets and in cancer cells is developed. As a kinase, the enzymatic activity, abundance, and self-phosphorylation of this protein are all important factors influencing its procancerous activity. To simultaneously determine these three important biochemical parameters, electrochemical control is called upon to connect or disconnect a polymer chain reaction (PCR) primer with a small-molecule synthetic probe, and with the target protein, in a target-specific manner. The resulting PCR signal amplification greatly improves the sensitivity of the design and also enables direct detection of the protein and its catalytic activity as well as its self-phosphorylation in clinical periphery blood samples from hepatocellular carcinoma (HCC) patients. This may point to future application of the proposed method in the early screening of HCC to assist its diagnosis and treatment.

Oxygen deficiency in Sr2FeO4-x: electrochemical control and impact on magnetic properties.

The oxygen-deficient system Sr2FeO4-x was explored by heating the stoichiometric Fe4+ oxide Sr2FeO4 in well-defined oxygen partial pressures which were controlled electrochemically by solid-state electrolyte coulometry. Samples with x up to about 0.2 were obtained by this route. X-ray diffraction analysis reveals that the K2NiF4-type crystal structure (space group I4/mmm) of the parent compound is retained. The lattice parameter a slightly decreases while the c-parameter increases with increasing x, which is in contrast to the Ruddlesden-Popper system Sr3Fe2O7-x and suggests removal of oxygen atoms from FeO2 lattice planes. The magnetic properties were studied by magnetization, 57Fe Mössbauer, and powder neutron diffraction experiments. The results suggest that extraction of oxygen atoms from the lattice progressively changes the elliptical spiral spin ordering of the parent compound to an inhomogeneous magnetic state with coexistence of long-range ordered regions adopting a circular spin spiral and smaller magnetic clusters.

Electrochemical gating for single-molecule electronics with hybrid Au|graphene contacts.

The single-molecular conductance of a redox active viologen molecular bridge between Au|graphene electrodes has been studied in an electrochemical gating configuration in an ionic liquid medium. A clear "off-on-off" conductance switching behaviour has been achieved through gating of the redox state when the electrochemical potential is swept. The Au|viologen|graphene junctions show single-molecule conductance maxima centred close to the equilibrium redox potentials for both reduction steps. The peak conductance of Au|viologen|graphene junctions during the first reduction is significantly higher than that of previously measured Au|viologen|Au junctions. This shows that even though the central viologen moiety is not directly linked to the enclosing electrodes, substituting one gold contact for a graphene one nevertheless has a significant impact on junction conductance values. The experimental data was compared against two theoretical models, namely a phase coherent tunnelling and an incoherent "hopping" model. The former is a simple gating monoelectronic model within density functional theory (DFT) which discloses the charge state evolution of the molecule with electrode potential. The latter model is the collective Kuznetsov Ulstrup model for 2-step sequential charge transport through the redox centre in the adiabatic limit. The comparison of both models to the experimental data is discussed for the first time. This work opens perspectives for graphene-based molecular transistors with more effective gating and fundamental understanding of electrochemical electron transfer at the single molecular level.

Development of defective molybdenum oxides for photocatalysis, thermal catalysis, and photothermal catalysis.

The localized surface plasmon resonance (LSPR) of noble metals has been investigated for decades for applications in various catalysis reactions and optical research studies, but its development has been hampered by inefficient light absorption and high costs. In comparison, the creation of less expensive semiconductors (metal oxides) with strong plasmonic absorption is an appealing option, particularly defective molybdenum oxide (HxMoO3-y) has received considerable attention and investigation as a promising plasmonic material for a variety of catalytic reactions (photocatalysis, thermocatalysis, photothermal catalysis, etc.).The LSPR effect of HxMoO3-y can be tuned throughout a broad spectrum range from visible to near-infrared (NIR) by altering the doping amount by electrochemical control, chemical reduction, or photochemical control. Notably, defects (oxygen vacancies) in HxMoO3-y arise in conjunction with the LSPR effect, resulting in the formation of unique and useful active sites in a range of catalytic processes. In this review, we explore the formation mechanism of HxMoO3-y with plasmonic features and discuss its applications in photocatalysis, thermocatalysis, and photothermal catalysis.