Outer-sphere Probes Research Articles

Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study

Salts of hexacyanoferrate II/III anions have been widely used as redox couple probe molecules to determine the characteristics of electrode surfaces. Examples include the assessment of electrocatalysts for energy applications and electrocatalysts for the detection of biological or chemical species, as well as the determination of electrochemically active surface areas. An examination of the electrochemical literature, based largely on cyclic voltammetric investigations, reveals a wide range of peak separation and/or heterogeneous electron transfer rate constants, classified sometimes as inner or outer sphere electron transfer processes. Originally developed for the mechanistic interpretation of inorganic transition metal compounds in solution, this terminology has since been extended to account for heterogeneous electron transfer occurring at electrodes. In the case of the hexacyanoferrate II/III anions, there can be a number of reasons why it sometimes behaves as an outer sphere probe and at other times displays inner sphere electron transfer characteristics. After examining some of the structural and chemical properties of the hexacyanoferrate II/III species, the methods used to determine such classifications are described. The most common method involves measuring peak-to-peak separation in a cyclic voltammogram to ascertain a heterogeneous rate constant, but it has inherent flaws. This paper reviews the reasons for the classification disparity, including the effects of various oxygen surface species, the influence of organic surface films, the nature of the cation counter-ion, surface adsorption and surface hydrophilicity/hydrophobicity. Other surface interactions may also take place, such as those occurring with Au corrosion or pH effects. These can impact the electrical double layer and thus may affect the electron transfer process. Consequently, it is recommended that hexacyanoferrate II/III should be considered a multi-sphere or alternatively a surface-sensitive electron transfer species.

All-in-One Single-Print Additively Manufactured Electroanalytical Sensing Platforms.

This manuscript provides the first report of a fully additively manufactured (AM) electrochemical cell printed all-in-one, where all the electrodes and cell are printed as one, requiring no post-assembly or external electrodes. The three-electrode cell is printed using a standard non-conductive poly(lactic acid) (PLA)-based filament for the body and commercially available conductive carbon black/PLA (CB/PLA, ProtoPasta) for the three electrodes (working, counter, and reference; WE, CE, and RE, respectively). The electrochemical performance of the cell is evaluated first against the well-known near-ideal outer-sphere redox probe hexaamineruthenium(III) chloride (RuHex), showing that the cell performs well using an AM electrode as the pseudo-RE. Electrochemical activation of the WE via chronoamperometry and NaOH provides enhanced electrochemical performances toward outer-sphere probes and for electroanalytical performance. It is shown that this activation can be completed using either an external commercial Ag|AgCl RE or through simply using the internal AM CB/PLA pseudo-RE and CE. This all-in-one electrochemical cell (AIOEC) was applied toward the well-known detection of ascorbic acid (AA) and acetaminophen (ACOP), achieving linear trends with limits of detection (LODs) of 13.6 ± 1.9 and 4.5 ± 0.9 μM, respectively. The determination of AA and ACOP in real samples from over-the-counter effervescent tablets was explored, and when analyzed individually, recoveries of 102.9 and 100.6% were achieved against UV-vis standards, respectively. Simultaneous detection of both targets was also achieved through detection in the same sample exhibiting 149.75 and 81.35% recoveries for AA and ACOP, respectively. These values differing from the originals are likely due to electrode fouling due to the AA oxidation being a surface-controlled process. The cell design produced herein is easily tunable toward different sample volumes or container shapes for various applications among aqueous electroanalytical sensing; however, it is a simple example of the capabilities of this manufacturing method. This work illustrates the next step in research synergising AM and electrochemistry, producing operational electrochemical sensing platforms in a single print, with no assembly and no requirements for exterior or commercial electrodes. Due to the flexibility, low-waste, and rapid prototyping of AM, there is scope for this work to be able to span and impact a plethora of research areas.

Rapid industrial scale synthesis of robust carbon nanotube network electrodes for electroanalysis

Carbon nanotubes (CNT) have been extensively investigated for various electroanalytical applications. As the properties of CNTs heavily depend on the fabrication conditions, it is expected that the electrochemical performance will also vary between CNTs from different processes. However, it is still not well known how the different synthesis conditions affect the electrochemical properties of CNTs. Thus, here we investigate the effect of synthesis rate on the physicochemical properties of CNT networks. Through extensive structural and chemical analysis, we show that the widely different synthesis rates, fast and slow, produced CNT networks with surprisingly similar properties. The only distinct differences were seen in the TEM tomography 3D reconstructions, where the faster synthesis produced a less dense network with larger bundle size. Moreover, minor changes were seen in the composition of Fe catalyst particles where the faster rate network mainly exhibited metallic Fe, whereas carbide and oxidized Fe phases were observed in the slower rate network. Although no changes were seen in the electron transfer kinetics with outer-sphere probes, it was clear that even these small changes in physicochemical properties affected the surface sensitive inner-sphere analytes. With slower synthesis rate i) sensitivity towards all analgesics, especially oxycodone, was enhanced and ii) oxidation potential of all analytes shifted to cathodic direction in comparison to higher synthesis rate. In the wider context, we propose that good quality CNTs can be fabricated rapidly in industrial scale for biosensing purposes. However, in electroanalytical applications properties of CNTs should be optimized for the analyte of interest.

Edge vs. basal plane electrochemistry of layered pnictogens (As, Sb, Bi): Does edge always offer faster electron transfer?

Materials with layered structure are at the forefront of material research. Graphite, MoS2 and black phosphorus, as layered materials, exhibit, in general, fast heterogeneous transfer at the edge planes and slow at the basal plane for many electroactive molecules. There is a fundamental question whether this is general behaviour of layered materials. Here, we examined fundamental electrochemistry of layered pnictogens (As, Sb and Bi) using single crystal edge- and basal-plane electrodes. Significant differences in behaviour of edge and basal plane surface for the electrochemical properties were found. The surface morphology of electrodes was examinated in detail. The electrochemical activity was investigated by both, inner and outer sphere probes. Several trends related to the pnictogen position in periodic table were observed. Importantly, an increasing activity of basal plane with atomic number was observed and attributed to the decreasing degree of anisotropy. However, catalytic activity towards hydrogen evolution reaction (HER) was observed only for the edge-planes of the electrodes.