Oxidative Electrochemistry Research Articles

Cleavage of [Pd2(PP)2(μ-Cl)2][BArF24]2 (PP = Bis(phosphino)ferrocene, BArF24 = Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) with Monodentate Phosphines.

The addition of Na[BArF24] (BArF24 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) to [Pd(PP)Cl2] (PP = 1,1'-bis(phosphino)ferrocene ligands) compounds results in the loss of a chloride ligand and the formation of the dimeric species [Pd2(PP)2(μ-Cl)2][BArF24]2. In most cases, the addition of a monodentate phosphine, PR3, to these dimeric species leads to cleaving of the dimer and formation of [Pd(PP)(PR3)Cl][BArF24]. While these reactions are readily observed via a significant color change, the 31P{1H} NMR spectra offer more significant support, as the singlet for the dimer is replaced with three doublets of doublets. The reaction seems to take place for a wide range of PR3 ligands, although there do appear to be steric limitations to the reaction. The compounds were thoroughly characterized by NMR, and X-ray crystal structures of several of the compounds were obtained. In addition, the ferrocenyl backbone of the 1,1'-bis(phosphino)ferrocene ligands provides an opportunity to examine the oxidative electrochemistry of these compounds. In general, the potential at which oxidations of these compounds occurs shows a dependence on the phosphine substituents.

Electroreductively Induced Radicals for Organic Synthesis.

Organic electrochemistry has attracted tremendous interest within the novel sustainable methodologies that have not only reduced the undesired byproducts, but also utilized cleaner and renewable energy sources. Particularly, oxidative electrochemistry has gained major attention. On the contrary, reductive electrolysis remains an underexplored research direction. In this context, we discuss advances in transition-metal-free cathodically generated radicals for selective organic transformations since 2016. We highlight the electroreductive reaction of alkyl radicals, aryl radicals, acyl radicals, silyl radicals, fluorosulfonyl radicals and trifluoromethoxyl radicals.

Ionic Liquids Effect on the Stability of 17-Electron Cation Product of the Electrochemical Oxidation of Cymantrene.

The oxidative electrochemistry of cymantrene, CpMn(CO)3 (1; Cp = [η5-C5H5]-), was examined in ionic liquids (ILs) composed of anions of varying Lewis base properties. It was observed that the cyclic voltammetric responses strongly depended on the nucleophilic properties of the IL anion. Still, all observations are consistent with the initial formation of 1+ followed by an attack from the IL anion. In bis(trifluoromethylsulfonyl)amide [NTf2]-based ILs, the process shows close to ideal electrochemical reversibility as the reaction between 1+ and [NTf2] anion is very slow. On the other hand, in tetrafluoroborate and trifluoromethanesulfonate-based IL, the oxidation of 1 shows different levels of electrochemical reversibility with a marked sign of anion attack to 1+. In contrast, 1 exhibits an irreversible oxidation process in hexafluorophosphate-based IL. The reaction rate constants for the interaction of 1+ with the different IL anions were estimated by fitting the experimental data to digital simulations of the proposed mechanism. Besides, the use of [NTf2]-based ILs as a supporting electrolyte in CH2Cl2 was also examined. The oxidation process of 1 shows a close to ideal electrochemical reversibility but low to non-chemical reversibility. This study illustrates the wide range of electrochemical environments available with ILs and demonstrates their limited utility for investigating the redox properties of metal carbonyl compounds. It also intends to warn the reader on how the IL media may influence an electrochemical study if care is not exercised.

Cleavage of the dimeric heterometallic complexes [Pd(dppf)(μ-Cl)]2[BArF24]2 (dppf = 1,1′-bis(diphenylphosphino)ferrocene, BArF24 = tetrakis(bis-3,5-trifluoromethylphenyl)borate) via addition of monodentate phosphine ligands

The cleavage of the heterometallic, chloro-bridged dimer [Pd2(dppf)2(µ-Cl)2][BArF24]2 (dppf = 1,1′-bis(diphenylphosphino)ferrocene; BArF24 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) was of interest within this study. The addition of the monodentate phosphines, PPh3, PMe3 and diphenylferrocenylphosphine (PPh2Fc), resulted in the formation of three new [Pd(dppf)(PR3)Cl][BArF24] complexes. These new compounds were characterized by NMR and the X-ray crystal structures of all three compounds were obtained. Structural parameters for these compounds were compared to other square-planar Pd(II) compounds with a dppf ligand. In addition, the oxidative electrochemistry of these three compounds was examined using cyclic voltammetry and the electronic communication between the two different iron centers in [Pd(dppf)(PPh2Fc)Cl][BArF24] was explored.

Synthesis, characterization and electrochemistry of [Pd(PP)MeCl] compounds with 1,1′-bis(phosphino)ferrocene ligands

Three new [Pd(κ2-PP)MeCl] (PP = 1,1′-bis(dicyclohexylphosphino)ferrocene (dcpf), 1-diphenylphosphino-1′-ditert-butylphosphinoferrocene (dppdtbpf) or 1,1ʹ-bis(5-methyl-2-furanylphosphino)ferrocene (dfurpf)) compounds were prepared and characterized. The X-ray crystal structure of [Pd(dppdtbpf)MeCl] was obtained which allowed for unambiguous determination of the cis/trans- relationship between the methyl and chloride ligands with respect to the different phosphine groups. The oxidative electrochemistry of the three new compounds as well as that of the reported [Pd(κ2-PP)MeCl] (PP = 1,1′-bis(diphenylphosphino)ferrocene (dppf), or 1,1′-bis(diiso-propylphosphino)ferrocene (dippf)) and [Pd(κ3-dtbpf)Me]Cl (dtbpf = 1,1′- bis(ditert-butylphosphino)ferrocene) was examined. The compounds with the κ2-bis(phosphino)ferrocene ligands react with sodium tetrakis(3,5-bis(trifluoromethyl)phenylborate) to lose a chloride ligand and the catalytic activity of these compounds was investigated for the polymerization of phenylacetylene.

Versatile Synthesis of Siloxy Silicon Tetrabenzotriazacorroles and Insight into the Mode of Macrocycle Formation.

Tetrabenzotriazacorroles (Tbcs) are a family of molecules related to phthalocyanines but have the unique ability to intensely absorb both blue and red light. Here, we report the synthesis of four novel silicon tetrabenzotriazacorrole derivatives (SiTbcs) with varying sized axial ligands. SiTbcs are formed starting from bis(hydroxy) silicon phthalocyanine ((OH)2-SiPc) via a simple in situ axial functionalization and reductive chemical process using magnesium metal and the respective chlorosilane in pyridine. Systematic probing of the reaction conditions revealed that the reaction is acid-promoted and that the formation of the Tbc macrocycle occurs at temperatures as low as 40 °C. Results imply this chemistry can be extended to SiTbcs with any axial ligands using pyridine hydrochloride as an acid source. Single crystals of all compounds were grown and showed significant π-π interactions between the macrocycles in the solid state. Optical, electrochemical, and thermal characterization of these materials is also described. The SiTbcs exhibit interesting highly oxidative electrochemistry as well as high thermal stability and tunable phase transition behavior.

Capacitance-Assisted Sustainable Electrochemical Carbon Dioxide Mineralisation.

An electrochemical cell comprising a novel dual‐component graphite and Earth‐crust abundant metal anode, a hydrogen producing cathode and an aqueous sodium chloride electrolyte was constructed and used for carbon dioxide mineralisation. Under an atmosphere of 5 % carbon dioxide in nitrogen, the cell exhibited both capacitive and oxidative electrochemistry at the anode. The graphite acted as a supercapacitive reagent concentrator, pumping carbon dioxide into aqueous solution as hydrogen carbonate. Simultaneous oxidation of the anodic metal generated cations, which reacted with the hydrogen carbonate to give mineralised carbon dioxide. Whilst conventional electrochemical carbon dioxide reduction requires hydrogen, this cell generates hydrogen at the cathode. Carbon capture can be achieved in a highly sustainable manner using scrap metal within the anode, seawater as the electrolyte, an industrially relevant gas stream and a solar panel as an effective zero‐carbon energy source.

The Dual Roles of Phenylenediamines: Using their Voltammetric Behavior to Measure the Hydrogen Donor and Acceptor Abilities of Alcohols in Acetonitrile.

Cyclic voltammetry experiments on 2,3,5,6-tetramethyl-1,4-phenylenediamine (P) in acetonitrile in the presence of varying concentrations of alcohols indicate that the oxidized forms of the compound (P.+ and P2+ ) interact with the alcohols through a hydrogen-bonding mechanism where P.+ and P2+ act as the hydrogen donor and the alcohols act as acceptors. However, the neutral form (P) largely acts as a hydrogen acceptor but only for strong hydrogen donors that do not undergo proton-transfer reactions with the phenylenediamine. These results were ascertained based on measuring the difference in potential of the two one-electron transfer reactions (ΔEPox(1, 2) =|EPox(1) -EPox(2) |) in the oxidative electrochemistry of P, which thereby allows a simple measure of relative hydrogen bonding strengths.

Late Transition Metal Compounds with 1,1′‐Bis(phosphino)ferrocene Ligands

The syntheses of several new zinc compounds with 1,1′‐bis(phosphino)ferrocene ligands and their phosphine chalcogenide derivatives are reported. The X‐ray crystal structures of [Zn(dppdtbpf)Cl2] [dppdtbpf = 1‐diphenylphosphino‐1′‐(di‐tert‐butylphosphino)ferrocene], [Zn(dippfO2)Cl2] [dippf = 1,1′‐bis(diisopropylphosphino)ferrocene], [Zn(dtbpfO2)Cl2] [dtbpf = 1,1′‐bis(di‐tert‐butylphosphino)ferrocene], and [Zn(4S)Cl2] (4 = dppdtbpf) were determined. The oxidative electrochemistry of the zinc compounds was examined and typically produced irreversible oxidations. The reactions of [(AuCl)2(PP)] [PP = 1,1′‐bis(diphenylphosphino)ferrocene (dppf), dtbpf, or dppdtbpf] with [N(p‐C6H4Br)3][B(C6F5)4] result in the loss of a chlorido ligand to form [Au2(µ‐Cl)(PP)][B(C6F5)4]. The structures of [(AuCl)2(dtbpf)] and [Au2(µ‐Cl)(dtbpf)][B(C6F5)4] were determined, and [Au2(µ‐Cl)(dtbpf)]+ displays a weak Au–Au interaction that is not present in [(AuCl)2(dtbpf)]. The oxidative electrochemistry of [Au2(µ‐Cl)(PP)]+ displays two reversible waves, the first of which occurs at a potential similar to those of the corresponding [(AuCl)2(PP)] species.

Electrochemical studies of n'-ferrocenylmethyl-n'-phenylbenzohydrazide at glassy carbon electrode in different medium

The oxidative electrochemistry of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide FcX was studied in acetonitrile with tetrabutylammonium hexafluorophosphate as the supporting electrolyte and aqueous ethanol using the electrochemical technique. This study using cyclic (CV) and rotating disk electrode (RDE) voltammetry showed that the FcX/FcX + redox couple is reversible in this electrolytes. The effects of changing the scan rate on the electrochemical behavior of ferrocene have been examined. A comparison of the electrochemical behavior of ferrocene and N'-Ferrocenylmethyl-N'-Phenylbenzohydrazid have been examined. Keywords: Cyclic voltammetry, ferrocene, N'-Ferrocenylmethyl-N'-Phenylbenzo-hydrazid, half-wave potential

X-ray structures and oxidative electrochemistry of phosphine sulfides and phosphine selenides

A variety of mono- and bisphosphine sulfides and selenides were synthesized and structurally characterized by NMR spectroscopy. In addition, the X-ray crystal structures were determined for eleven of these compounds. The percent buried volume was determined from the X-ray structures and the use of this measurement in estimating the steric bulk in phosphine sulfides and selenides was further developed. The oxidation of the mono- and bisphosphine chalcogenides was examined using cyclic voltammetry in methylene chloride and acetonitrile. The solvent had little effect on the observed trends in the potentials at which oxidation occurred. Both the mono- and bisphosphine chalcogenides display an irreversible oxidative wave. Many of the phosphine selenides also display a follow-up, irreversible reductive wave indicative of the formation of a Se–Se bond upon oxidation. However, the products for the oxidation of the phosphine selenides could not be isolated by chemical oxidation. The potential at which oxidation of the phosphine sulfides occurs is more positive than the analogous phosphine selenides.

Synthesis and spectroelectrochemistry of transition metal carbonyls with 1,1′-bis(phosphino)metallocene ligands

The oxidative electrochemistry of group VI and group VII metal carbonyl compounds containing 1,1′-bis(phosphino)ferrocene ligands with the general formula [M(CO)4(P∩P)] (M = Cr, Mo, or W, P∩P = 1,1′-bis(diisopropylphosphino)ferrocene (dippf) or 1,1′-bis(dicyclohexylphosphino)ferrocene (dcpf)) and [M(CO)3Br(P∩P)] (M = Mn, P∩P = 1,1′-bis(diphenylphosphino)ferrocene (dppf) or dippf; M = Re, P∩P = dppf, dippf or dcpf) was investigated. Two reversible waves were observed in the electrochemistry of the Cr compounds, while the other compounds exhibited one wave with varying reversibility. The compounds were examined using spectroelectrochemistry, which was particularly useful in assigning the two waves in the Cr species. In addition, the X-ray structures of [Cr(CO)4(dippf)], [Cr(CO)4(dcpf)] and [Mn(CO)3Br(dippf)] were determined. The %Vbur values for dippf, dcpf and other bidentate phosphines have been calculated for these pseudo-octahedral Cr and Mn compounds and these structural studies have been supplemented by DFT optimized geometries for the Cr complexes.

A nanomaterial composed of cobalt nanoparticles, poly(3,4-ethylenedioxythiophene) and graphene with high electrocatalytic activity for nitrite oxidation

We have investigated the oxidative electrochemistry of nitrite on glassy carbon electrodes modified with cobalt nanoparticles, poly(3,4-ethylenedioxythiophene) (PEDOT), and graphene. The modified electrode was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and scanning electron microscopy. The results suggest that this new type of electrode combines the advantages of PEDOT-graphene films and cobalt nanoparticles and exhibits excellent electrocatalytic activity towards the oxidation of nitrite. There is a linear relationship between the peak current and the nitrite concentration in the range from 0.5 μM to 240 μM, and the detection limit is 0.15 μM. The modified electrodes also enable the determination of nitrite at low potentials where the noise level and interferences by other electro-oxidizable compounds are weak.

Electrogenerated chemiluminescence properties of bisalicylideneethylenediamino (salen) metal complexes

The spectroscopy, electrochemistry and electrogenerated chemiluminescence (ECL) of eight bisalicylideneethylenediamino (salen) metal complexes are reported. Two of the complexes contain an unsubstituted salen ligand and either cobalt(II) or nickel(II). The others have 1,2-cyclohexanediamonio- N, N′-bis(3,5-di- t-butylsalicylidene) as the ligand, and chromium(III), aluminum(III), cobalt(II), cobalt(III) or manganese(II) as the metal center. The complexes have lowest energy absorption maxima between 350 and 430 nm. When excited at these wavelengths, the complexes emit between 417 and 594 nm in acetonitrile. Photoluminescence efficiencies ( ϕ em) were between 0.0310 and 23.8 compared to Ru(bpy) 3 2+ (bpy = 2,2′-bipyridine; ϕ em = 1), with the aluminum complexes displaying the most intense photoluminescence. Both reversible and irreversible oxidative electrochemistry is displayed by the metal–salen complexes with oxidation potentials ranging between +0.152 and +1.661 V versus Ag/AgCl. The ECL intensity peaks at a potential corresponding to oxidation of both TPrA and the salen systems, indicating that both are involved in the ECL reaction sequence. ECL efficiencies ( ϕ ecl) were between 0.0018 and 0.0086 when compared to Ru(bpy) 3 2+ ( ϕ ecl = 1) in acetonitrile (0.05 M tri- n-propylamine (TPrA) as an oxidative–reductive ECL coreactant). Also, qualitative studies using transmission filters suggest that the complexes emit ECL in approximately the same region as their photoluminescence, indicating that the same excited state is formed in both experiments.

Photophysical and Electrochemical Properties of Phenanthroline‐Based Bis‐cyclometallated Iridium Complexes in Aqueous and Organic Media

AbstractThe electrochemical and photophysical properties of biscyclometallated iridium(III) complexes containing phenanthroline‐based ligands have been investigated and compared in organic and aqueous media. Complexes having general formula [Ir(ppy)2(N∩N)]+/– were synthesized, where ppy is the cyclometallating ligand, 2‐phenylpyridine, and N∩N represents one of the following phenanthroline based ligands: 1,10‐phenanthroline (phen), 4,7‐diphenyl‐1,10‐phenanthroline (dpp) and 4,7‐diphenyl‐1,10‐phenanthrolinedisulfonate (BPS). In the case of [Ir(ppy)2(BPS)]–, the SO3– group located on the phenyl ring of BPS dramatically increases the aqueous solubility of the complex compared to the dpp complex, which is only soluble in organic media. The chloride salt of the underivatized phen complex, though not as soluble as the BPS chelated complex, is sufficiently soluble (> 1 mM) for sensing and other applications in aqueous media. The electrochemical response depends markedly on the medium; all the complexes showed reversible behaviour in organic media, whereas the oxidative electrochemistry of [Ir(ppy)2BPS]– and [Ir(ppy)2phen]+ was irreversible. Results from spectroelectrochemical experiments indicate that electrolysis produces significant changes to the UV region of the spectrum associated with the ppy moieties. Significantly, this demonstrates that the highest occupied molecular orbital (HOMO) is not entirely metal based but is also associated with the cyclometallating ligand. The variation in emission maxima between the iridium complexes was small in organic and aqueous media, but the colour of the emission depends on solvent polarity and strongly on temperature. Most significantly, the quantum yields for these species are also strongly influenced by the solvent: high quantum yields, ranging from 14.0 to 28.6 %, are observed in CH2Cl2. However, in aqueous media the water‐soluble complexes, [Ir(ppy)2BPS]– and [Ir(ppy)2phen]+, exhibited significantly diminished quantum yields of 2.5 and 2.6 %, respectively. Theoretical calculations confirm the spectroscopic assignments and that the HOMO is significantly delocalized over the metal and cyclometallating ligands. The relationship between the electrochemical, spectroscopic and theoretical results is discussed.

A Novel Composite Anode: The Electrooxidation of Organic Molecules via Formation of Highly Energetic Holes

This paper describes, for the first time, the electrochemical characteristics of a novel, composite electrode, comprising a thin TiO2 layer sandwiched between a silicon wafer and a metal grid. Holes thermally and/or photochemically generated near the Si/TiO2 interface in the Si are able to reach the surface of the TiO2 and oxidize water and/or species in solution. Hole transport across the TiO2 is facilitated by the application of a bias voltage across the silicon and metal grid. At low bias voltages, oxidation of water takes place at the metal grid; at higher voltages, the oxidation takes place directly at the TiO2 as surface states are accessed. The generation of holes is enhanced significantly if the TiO2 surface is irradiated with visible light. A theoretical model is presented to explain the observed data. The anodes represent a completely new area of oxidative electrochemistry with potential application across a wide range of technology, from fuel cells-on-a-chip to electroorganic chemistry.

Electrochemistry of P(CH 2Fc) 3 and derivatives

The oxidative electrochemistry of P(CH 2Fc) 3 and three of its derivatives was examined. The electrochemistry of these compounds is sensitive to the functionality added to the phosphorus lone pair and the supporting electrolyte used.

A novel poly(3,4-ethylenedioxythiophene)/iron phthalocyanine/multi-wall carbon nanotubes nanocomposite with high electrocatalytic activity for nitrite oxidation

In the present work, the oxidative electrochemistry of nitrite on the poly(3,4-ethylenedioxythiophene)/iron phthalocyanine/multi-wall carbon nanotubes-(PEDOT/FePc/MWCNT) modified screen-printed carbon electrodes (SPCE) has been investigated. The parameters, such as overpotential, current density and rate constant at PEDOT/FePc/MWCNT-modified SPCE, were compared with an un-modified, FePc-, and FePc/MWCNT-modified SPCE for electro-oxidation of nitrite. As compared with the un-modified SPCE, an increase in the anodic peak current density ( J pa) (∼100%) along with a decrease in the anodic peak potential ( E pa) of ∼150 mV for electro-oxidation of nitrite at the FePc-modified SPCE was observed. When an under-layer of MWCNT was introduced onto FePc-modified SPCE, denoted as FePc/MWCNT-modified SPCE, and the number of FePc/MWCNT bilayer was optimized, the heterogeneous electron transfer rate constant ( k) at FePc/MWCNT-modified SPCE was enhanced about 7.8 times as compared with that at FePc-modified SPCE. Moreover, as a layer of PEDOT film was electrodeposited onto the FePc/MWCNT-modified SPCE, denoted as PEDOT/FePc/MWCNT-modified SPCE, a significant increase in current response along with a remarkable decrease in E pa were noticed. This can be attributed to the pre-concentration effect induced by the electrostatic interaction between the negatively charged nitrite and oxidized PEDOT film. On the whole, the PEDOT/FePc/MWCNT-modified SPCE greatly reduces the overpotential of ∼330 mV along with 3.5 times enhanced the peak current density for the electro-oxidation of nitrite as compared with un-modified SPCE. The sensitivity and limit of detection (S/N = 3) for the PEDOT/FePc/MWCNT-modified SPCE were found to be as 638 mA cm −2 M −1 and 71 nM, respectively. Notably, PEDOT/FePc/MWCNT-modified SPCE has a lower sensing potential than compared to several other modified electrodes. The developed sensor was also applied for the determination of nitrite in tap water sample.

Electrooxidation of [(η5-C5H5)Fe(CO)2]2 As a Probe of the Nucleophilic Properties of Ionic Liquid Anions

The oxidative electrochemistry of [CpFe(CO)(2)](2), 1 (Cp = [eta(5)-C(5)H(5)](-)), was examined in detail in ionic liquids (ILs) composed of ions of widely varying Lewis acid-base properties. Cyclic voltammetric responses were strongly dependent on the nucleophilic properties of the IL anion, but all observations are consistent with the initial formation of 1(+) followed by attack from the IL anion. In [NTf(2)](-)-based ILs ([NTf(2)](-) = bis(trifluoromethylsulfonyl)amide), the process shows nearly ideal chemical reversibility as the reaction between 1(+) and [NTf(2)](-) is very slow. This is highly significant, as 1(+) is known to be highly susceptible to nucleophilic attack and its stability indicates a remarkable lack of coordinating ability of these ILs. In 1-methyl-3-butylimidazolium hexafluorophosphate, [bmim][PF(6)], the oxidation of 1 is still largely reversible, but there is more pronounced evidence of [PF(6)](-) coordination. In contrast, 1 exhibits an irreversible two-electron oxidation process in a dicyanamide-based IL. This overall oxidation process is thought to proceed via an ECE mechanism, details of which are presented. Rate constants were estimated by fitting the experimental data to digital simulations of the proposed mechanism. The use of [NTf(2)](-)-based ILs as a supporting electrolyte in CH(2)Cl(2) was examined by using this solvent/electrolyte as a medium in which to perform bulk electrolyses of 1 and 1*, the permethylated analogue [Cp*Fe(CO)(2)](2) (Cp* = [eta(5)-C(5)(CH(3))(5)](-)). These cleanly yielded the corresponding binuclear radical-cation species, 1(+) and 1*(+), which were subsequently characterized by electron paramagnetic resonance (EPR) spectroscopy. In addition to the above oxidation studies, the reduction of 1 was studied in each of the ILs; differences in cathodic peak potentials are attributed, in part, to ion-pairing effects. This study illustrates the wide range of electrochemical environments available with ILs and demonstrates their utility for the investigation of the redox properties of metal carbonyls and other organometallic compounds.

Effects of thin-layer diffusion in the electrochemical detection of nicotine on basal plane pyrolytic graphite (BPPG) electrodes modified with layers of multi-walled carbon nanotubes (MWCNT-BPPG)

The oxidative electrochemistry of nicotine is measured in aqueous solution at a multi-walled carbon nanotube modified basal plane pyrolytic graphite (MWCNT-BPPG) electrode. Quantitative detection of nicotine is obtained with a limit of detection of 1.5 μM (based on 3 σ) and a linear range of at least up to 1 mM. Evidence is found for a mass transport regime that includes thin-layer (within the MWCNT layers) as well as semi-infinite (from bulk solution) diffusional signatures, adding to the growing picture that the fundamental source of the ‘electrocatalytic’ properties claimed of many porous and multi-walled carbon nanotube-based modified electrodes may, at least in some cases, be due to mass transport effects rather than electronic or structural peculiarities of the modifying layers.