Tetracarboxylic acid cobalt phthalocyanine SAM on gold: Potential applications as amperometric sensor for H2O2 and fabrication of glucose biosensor

This report describes the applications of cobalt tetracarboxylic acid phthalocyanine (CoTCAPc) self-assembled monolayer (SAM) immobilized onto a preformed 2-mercaptoethanol (Au-ME) SAM on gold surface (Au-ME-CoTCAPc SAM) as a potential amperometric sensor for the detection of hydrogen peroxide (H₂O₂) at neutral pH conditions. The Au-ME-CoTCAPc SAM sensor showed a very fast amperometric response time of approximately 1 s, good linearity at the studied concentration range of up to 5 μM with a coefficient R² = 0.993 and a detection limit of 0.4 μM oxidatively. Also reductively, the sensor exhibited a very fast amperometric response time (∼1 s), linearity up to 5 μM with a coefficient R² = 0.986 and a detection limit of 0.2 μM. The cobalt tetracarboxylic acid phthalocyanine self-assembled monolayer was then evaluated as a mediator for glucose oxidase (GOx)-based biosensor. The GOx (enzyme) was immobilized covalently onto Au-ME-CoTCAPc SAM using coupling agents: N-ethyl-N(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide (NHS), and the results demonstrated a good catalytic behavior. Kinetic parameters associated with the enzymatic and mediator reactions were estimated using electrochemical versions of Lineweaver-Burk and Hanes equation, and the stability of the sensor was tested. The biosensor (Au-ME-CoTCAPc-GOx SAM) electrode showed good sensitivity (7.5 nA/mM) with a good detection limit of 8.4 μM at 3σ, smaller Michaelis-Menten constant (4.8 mM from Hanes plot) and very fast response time of approximately 5 s.     

Evaluation of various strategies to formation of pH responsive hydroquinone-terminated films on carbon electrodes

The hydroquinone/quinone (H₂Q/Q) redox system was tethered to glassy carbon surfaces using first an electrochemical pre-oxidation treatment to afford carboxylic acid functionalities followed by immobilizing the H₂Q precursor, n-(2,5-dimethoxyphenyl)alkan-1-amine (general structure: H₂N-(CH₂)_n-C₆H₃(OCH₃)₂, n = 1, 2, 4, 8, and 12), by carbodiimide chemistry and a final demethylation reaction. The resultant surfaces exhibited the expected chemical reversibility in aqueous solution with a pH-sensitive position of the formal potential (∼55 mV/pH unit), and an increase in the peak potential separation going from 0.02 V for n = 1 to 0.21 V for n = 12. The films were very robust and could withstand prolonged sonication and relatively large potential excursions. While the films followed the expected kinetic distance dependence for up to 4 methylene units the electrode kinetics was faster than expected for longer alkyl spacers. We suggest that film disorder, electrode-mediating effects, and a roughened electrode material could account for these apparent inconsistencies. To further understand such effects, two complementary electrode modification strategies leading to better film ordering on carbon were adapted; immobilizing a thin layer of benzoic acid by oxidative deposition of 4-aminobenzoic acid or employing a plasma deposition process to tether an acid analogue. Analysis of the various electrodes was accomplished by electrochemical methods, atomic force microscopy, and X-ray photoelectron spectroscopy.     

A study of adsorption kinetics and thermodynamics of ω-mercaptoalkylhydroquinone self-assembled monolayer on a gold electrode

The adsorption kinetics and thermodynamics for the formation of redox active self-assembled monolayer (SAM) of 2-(n-mercaptoalkyl)hydroquinone (abbreviated as H₂Q(CH₂)_nSH, where n = 4, 6, 8, 10, and 12) on gold electrode has been investigated by cyclic voltammetry to study the effects of concentration and alkyl chain length. The time dependence of surface coverage, differential capacitance, and formal potential of electroactive hydroquinone(H₂Q) moiety supports that the adsorption of H₂Q(CH₂)_nSH molecules typically processes with a two-step adsorption consisted of a fast initial adsorption and a slowly following reorganization. The adsorption processes can be satisfactorily described by simple Langmuir adsorption kinetics, irrespective of concentration and alkyl chain length of adsorbate molecule. Based on Langmuir kinetics, the adsorption rate constant was determined at the initial step for the formation of all H₂Q(CH₂)_nSH-SAMs studied in this work. The rate constant value was found to be decreased with increasing alkyl chain length and decreasing bulk solution concentration (≤10 μM). The dependence of a surface coverage (Γ_e) at adsorption equilibrium on the bulk concentration is accurately described by the Langmuir isotherm at several concentrations ranging from 8 × 10⁻⁶ to 1 × 10⁻⁵ M for all H₂Q(CH₂)_nSH molecules. Parameters characterizing the adsorption thermodynamics, such as Γ_s, adsorption coefficient (β), and adsorption free energy (ΔG_ads) were determined from this isotherm.     

Effects of imidazolium salts as cocatalysts on the copolymerization of CO2 with epoxides catalyzed by (salen)CrCl complex

Imidazolium salts, most of which are room temperature ionic liquids (ILs), have been introduced as effective and tunable cocatalysts in the copolymerization of CO₂ with epoxides catalyzed by (salen)Cr^IIICl complex for the first time. Effects of imidazolium salts with different alkyl chains as well as with different anions on the copolymerization were investigated. The results showed that the copolymerization was influenced obviously by the property of anion. In addition, the cation of imidazolium salts with longer alkyl chain length such as n-dodecyl (TOF, 242.5 h⁻¹, carbonate linkages > 99%) displays better activities and selectivity in the copolymerization as compared with N-MeIm (TOF, 72.5 h⁻¹, carbonate linkages 94%). These results are instructive for further design of task-specific ILs as effective cocatalysts to improve the copolymerization of CO₂ with epoxides.     

Ion-exchange voltammetry of tris(2,2′-bipyridine) nickel(II), cobalt(II), and Co(salen) at polyestersulfonated ionomer coated electrodes in acetonitrile: Reactivity of the electrogenerated low-valent complexes

The electrochemical behaviour of [Ni(bpy)₃(BF₄)₂], [Co(bpy)₃(BF₄)₂], and Co(salen) (where bpy = 2,2′-bipyridine, and salen = N,N′-bis(salicylidene)ethylenediamine) is studied at a glassy carbon electrode modified with the poly(estersulfonate) ionomer Eastman AQ 55 in acetonitrile (MeCN). It is shown that the nickel complex is strongly incorporated into the polymer. The reduction of the divalent nickel compound features a two-electron process leading to a nickel(0) species which is released from the coating because of the lack of electrostatic attraction with the ionomer. Yet, the neutral zerovalent nickel-bipyridine complex is reactive towards ethyl 4-iodobenzoate and di-bromocyclohexane despite the presence of the polymer. The activation of the aryl halide occurs through an oxidative addition, whereas, an electron transfer is involved in the presence of the alkyl halide making the catalyst regeneration much faster in the latter case. The electrochemical study of [Co(bpy)₃(BF₄)₂] shows that incorporation of the cobalt complex into the polymer is efficient, provided excess bpy is used. This excess bpy does not interfere with the electrocatalytic activity of the cobalt complex incorporated in the AQ coating and efficient electrocatalysis is observed towards di-bromocyclohexane and benzyl-bromide as substrates. Finally, replacement of the bpy ligand with the macrocycle N,N′-bis(salicylidene)ethylenediamine, salen, leads to the incorporation of the non-charged Co^II(salen) complex into the AQ 55 polymer showing the relevancy of hydrophobic interactions. The reaction between the electrogenerated [Co^I(salen)]⁻ with 1,2-dibromocyclohexane exhibits a fast inner sphere electron transfer.     

Swelling behavior of amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate

The amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate with different 1-bromoalkanes (1-C_nH_2n+1Br, n = 2, 4, 6, 8, 12) were synthesized by radiation-induced polymerization and crosslinking. The length of alkyl side chains had significant influence on the swelling behavior of the resulting gels. The swelling degree of the gels decreased with the increase of side chain length, and the gel hardly swelled when n = 12. The effect of temperature and ionic strength on the swelling behavior of the resulting gels revealed that (1) the gels with longer side chains (n ≥ 8) had upper critical solution temperature, while other gels were not thermo-sensitive. (2) Antipolyelectrolyte effect was observed when immersing the gels (n ≥ 8) in NaCl solutions in certain concentration range. The dramatic difference in swelling behavior was attributed to the different gel structures. The gels with short side chains (n ≤ 6) had cellular structure of normal polyelectrolyte gels. The gels (n ≥ 8) had an aggregation gel structure caused by the hydrophobic interaction among alkyl groups and the formation of ion-cluster between tetra-alkyl ammonium cation and Br⁻, which had been analyzed with the aid of SEM, Br⁻-selective electrode and fluorescence molecular probe.     

Surface chemistry and electrocatalytic behaviour of tetra-carboxy substituted iron, cobalt and manganese phthalocyanine monolayers on gold electrode

Surface chemistry and electrocatalytic properties of self-assembled monolayers of metal tetra-carboxylic acid phthalocyanine complexes with cobalt (Co), iron (Fe) and manganese (Mn) as central metal ions have been studied. These phthalocyanine molecules are immobilized on gold electrode via the coupling reaction between the ring substituents and pre-formed mercaptoethanol self-assembled monolayer (Au-ME SAM). X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy confirmed chemisorption of mercaptoethanol via sulfur group on gold electrode and also coupling reaction between phthalocyanines and Au-ME SAM. Electrochemical parameters of the immobilized molecules show that these molecules are densely packed with a perpendicular orientation. The potential applications of the gold modified electrodes were investigated towards l-cysteine detection and the analysis at phthalocyanine SAMs. Cobalt and iron tetra-carboxylic acid phthalocyanine monolayers showed good oxidation peak for l-cysteine at potentials where metal oxidation (M^III/M^II) takes place and this metal oxidation mediates the catalytic oxidation of l-cysteine. Manganese tetra-carboxylic acid phthalocyanine monolayer also exhibited a good catalytic oxidation peak towards l-cysteine at potentials where Mn^IV/Mn^III redox peak occurs and this redox peak mediates l-cysteine oxidation. The analysis of cysteine at phthalocyanine monolayers displayed good analytical parameters with good detection limits of the orders of 10⁻⁷ mol L⁻¹ and good linearity for a studied concentration range up to 60 μmol L⁻¹.     

Effect of the alkyl group on the synthesis and the electrochemical properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids

The effect of the alkyl side group on the synthesis and the electrochemical properties of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR_1ATFSI) ionic liquids (ILs) is reported. The investigation was focused on the PYR_1ATFSI ionic liquid family because of the interesting electrochemical properties of the members with propyl and butyl side chains. Side alkyl groups (A = C_nH_2n+1 with n ranging from 1 to 10) of different length and structure were used for the synthesis of PYR_1ATFSI materials. NMR and DSC have shown that the ionic liquids were correctly synthesized with the exception of the compounds with tertiary side chains. Most of the materials exhibited a conductivity higher than 10⁻³ S cm⁻¹ already at 12 °C. In the molten state a moderate conductivity decrease was observed with increasing the length and the branching of the side chain (C₂H_2n+1) group according with the change of viscosity of the ionic liquids. Most of the PYR_1ATFSI samples exhibited an electrochemical stability window exceeding 5 V.     

Kinetics and PEMFC performance of RuxMoySez nanoparticles as a cathode catalyst

Kinetics of Ru_xMo_ySe_z nanoparticles dispersed on carbon powder was studied in 0.5 M H₂SO₄ electrolyte towards the oxygen reduction reaction (ORR) and as cathode catalysts for a proton exchange membrane fuel cell (PEMFC). Ru_xMo_ySe_z catalyst was synthesized by decarbonylation of transition-metal carbonyl compounds for 3 h in organic solvent. The powder was characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. Catalyst is composed of uniform agglomerates of nanocrystalline particles with an estimated composition of Ru₆Mo₁Se₃, embedded in an amorphous phase. The electrochemical activity was studied by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. Tafel slopes for the ORR remain invariant with temperature at −0.116 V dec⁻¹ with an increase of the charge transfer coefficient in dα/dT = 1.6 × 10⁻³, attributed to an entropy turnover contribution to the electrocatalytic reaction. The effect of temperature on the ORR kinetics was analyzed resulting in an apparent activation energy of 45.6 ± 0.5 kJ mol⁻¹. The catalyst generates less than 2.5% hydrogen peroxide during oxygen reduction. The Ru_xMo_ySe_z nanoparticles dispersed on a carbon powder were tested as cathode electrocatalyst in a single fuel cell. The membrane-electrode assembly (MEA), included Nafion^® 112 as polymer electrolyte membrane and commercial carbon supported Pt (10 wt%Pt/C-Etek) as anode catalyst. It was found that the maximum performance achieved for the electro-reduction of oxygen was with a loading of 1.0 mg cm⁻² Ru_xMo_ySe_z 20 wt%/C, arriving to a power density of 240 mW cm⁻² at 0.3 V and 80 °C.     

Investigation of photoelectrocatalytic activity of Cu2O nanoparticles for p-nitrophenol using rotating ring-disk electrode and application for electrocatalytic determination

A cuprous oxide (Cu₂O) nanoparticles modified Pt rotating ring-disk electrode (RRDE) was successfully fabricated, and the electrocatalytic determination of p-nitrophenol (PNP) using this electrode was developed. Cu₂O nanoparticles were obtained by reducing the copper-citrate complex with hydrazine hydrate (N₂H₄·H₂O) in a template-free process. The hydrodynamic differential pulse voltammetry (HDPV) technique was applied for in situ monitor the photoelectrochemical behavior of PNP under visible light using nano-Cu₂O modified Pt RRDE as working electrode. PNP undergoes photoelectrocatalytic degradation on nano-Cu₂O modified disk to give electroactive p-hydroxylamino phenol species which is compulsive transported and can only be detected at ring electrode at around 0.05 V with oxidation signal. The effects of illumination time, applied bias potential, rotation rates and pH of the reaction medium have been discussed. Under optimized conditions for electrocatalytic determination, the anodic current is linear with PNP concentration in the range of 1.0 × 10⁻⁵ to 1.0 × 10⁻³ M, with a detection limit of 1.0 × 10⁻⁷ M and good precision (RSD = 2.8%, n = 10). The detection limit could be improved to 1.0 × 10⁻⁸ M by given illumination time. The proposed nano-Cu₂O modified RRDE can be potentially applied for electrochemical detection of p-nitrophenol. And it also indicated that modified RRDE technique is a promising way for photoelecrocatalytic degradation and mechanism analysis of organic pollutants.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.     

A carboxyalkanethiol monolayer modified electrode for blocking of ascorbic acid oxidation and its application to a biosensor

Upon the application of amperometric biosensor to the biological fluid, ascorbic acid interferes the amperometric determination of analytes, because the oxidative potential of ascorbic acid is lower than that of electro active substances such as H₂O₂ produced by the enzymatic reaction. In this study we propose a method to block ascorbic acid based on the electrostatic interaction with self-assembled monolayer (SAM) and its application of the surface modified electrode to biosensor. In order to form SAM on the gold electrode with carboxyl group, 7-carboxy-heptanethiol (7-CHT) was used. The 7-CHT modified electrode did not show anodic response to ascorbic acid, but oxidized phenanthroline cobalt complex [Co(phen)₃²⁺], which can be used as a mediator of biosensor. Thus, the 7CHT-modified electrode was applied to biosensor mediated with Co(phen)₃²⁺. Fructose dehydrogenase (FDH) was immobilized to the 7-CHT modified electrode. Fructose was determined selectively with the FDH/7-CHT modified electrode at the range of 0.2-2 mM.     

Simultaneous determination of paracetamol and ascorbic acid using tetraoctylammonium bromide capped gold nanoparticles immobilized on 1,6-hexanedithiol modified Au electrode

Tetraoctylammonium bromide stabilized gold nanoparticles (TOAB-AuNPs) attached to 1,6-hexanedithiol (HDT) modified Au electrode was used for the simultaneous determination of paracetamol (PA) and ascorbic acid (AA) at physiological pH. The attachment of TOAB-AuNPs on HDT modified Au surface was confirmed by attenuated total reflectance (ATR)-FT-IR spectroscopy and atomic force microscope (AFM). The ATR-FT-IR spectrum of TOAB-AuNPs attached to the HDT monolayer showed a characteristic stretching modes corresponding to -CH₂ and -CH₃ of TOAB, confirming the immobilization of AuNPs with surface-protecting TOAB ions on the surface of the AuNPs after being attached to HDT modified Au electrode. AFM image showed that the immobilized AuNPs were spherical in shape and densely packed to a film of ca. 7 nm thickness. Interestingly, TOAB-AuNPs modified electrode shifted the oxidation potential of PA towards less positive potential by 70 mV and enhanced its oxidation current twice when compared to bare Au electrode. In addition, the AuNPs modified electrode separated the oxidation potentials of AA and PA by 210 mV, whereas bare Au electrode failed to resolve them. The amperometry current of PA was increased linearly from 1.50 × 10⁻⁷ to 1.34 × 10⁻⁵ M with a correlation coefficient of 0.9981 and the lowest detection limit was found to be 2.6 nM (S/N = 3). The present method was successfully used to determine the concentration of PA in human blood plasma and commercial drugs.     

Redox polymer covalently modified multiwalled carbon nanotube based sensors for sensitive acetaminophen and ascorbic acid detection

A sensitive electrochemical detection method was developed involving multiwalled carbon nanotubes (MWCNTs) covalently modified with osmium-based redox polymer. The polycationic redox polymer, poly[4-vinylpyridine Os(bipyridine)₂Cl]-co-ethylamine (POs-EA), was first synthesized and covalently attached to MWCNTs. The redox polymer modified MWCNTs were then trapped in a hydrogel formed from polyethyleneglycol diacrylate (PEG-DA) using 1-phenyl-2-hydroxy-2-methyl-1-propanone as a photoinitiator. Upon exposure to aqueous media, the gel swelled to allow movement of analytes in and out of the gel without having any effect on the redox polymer modified nanotube signal. Cyclic voltammetry showed reversible pairs of oxidation-reduction peaks at 0.35 V (vs Ag/AgCl) corresponding to the Os^II/Os^III. This assembly was able to catalytically oxidize both acetaminophen and ascorbic acid (AA). Amperometric data showed a linearity between 0 and 100 μM (R² of 0.999, n = 10) 0.5 mV vs Ag/AgCl (sensitivity 0.003 μA/μM) for ascorbic acid, while for acetaminophen the linearity was between 0 and 1.5 μM (R² of 0.9999, n = 8) with a sensitivity of 65 μA/μM. This sensing system was found to exhibit remarkable stability over several weeks with excellent reproducibility.     

Electrocatalytic oxidation of ascorbic acid by [Fe(CN)6] redox couple electrostatically trapped in cationic N,N-dimethylaniline polymer film electropolymerized on diamond electrode

Multinegatively charged metal complex, hexacyanoferrate ([Fe(CN)₆]⁴⁻), was electrostatically trapped in the cationic polymer film of N,N-dimethylaniline (PDMA) which was electrochemically deposited on the boron-doped diamond (BDD) electrode by controlled-potential electro-oxidation of the monomer. This ferrocyanide-trapped PDMA film was used to catalyze the oxidation of ascorbic acid (AA). Increase in the oxidation current response with a negative shift of the anodic peak potential was observed at the cationic PDMA film-coated BDD (PDMA|BDD) electrode, compared with that at the bare BDD electrode. A more drastic enhancement in the oxidation peak current as well as more negative shift of oxidation potential was found at the ferrocyanide-trapped PDMA film-coated BDD ([Fe(CN)₆]^3−/4−|PDMA|BDD) electrode. This [Fe(CN)₆]^3−/4−|PDMA|BDD electrode can be used as an amperometric sensor of AA. Ferrocyanide, electrostatically trapped in the polymer film shows more electrocatalytic activity than that coordinatively attached to the polymer film or dissolved in the solution phase. The electrocatalytic current depends on the surface coverage of ferricyanide, Γ_Fe, within the polymer film. Diffusion coefficient (D) of AA in the solution was estimated by rotating disk electrode voltammetry: D = (5.8 ± 0.3) × 10⁻⁶ cm² s⁻¹. The second-order rate constant for the catalytic oxidation of AA by ferricyanide was also estimated to be 9.0 × 10⁴ M⁻¹ s⁻¹. In the hydrodynamic amperometry using the [Fe(CN)₆]^3−/4−|PDMA|BDD electrode, a successive addition of 1 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.233 μA cm⁻² μM⁻¹.     

Extraction kinetics of Fe(III) by di-(2-ethylhexyl) phosphoric acid using a Y–Y shaped microfluidic device

The extraction kinetics of Fe^III by di-(2-ethylhexyl) phosphoric acid (D2EHPA) were investigated using a Y–Y shaped microfluidic device. Finite-volume simulations were used to examine the accuracy of a single-step dimeric reaction mechanism in fitting the experimental data. Results demonstrate the validity of the proposed mechanism and show that Fe^III extraction occurred at a slow rate (second-order forward rate constant of k₁ = (3.0 ± 0.1) × 10⁻⁶ m⁴/mol s) under a mixed reaction–diffusion resistance regime. The present study provides insight for the control of Fe^III extraction rates in hydrometallurgical processes.     

Chlorophyll assembled electrode for photovoltaic conversion device

Chlorophyll-a (Chl-a) assembled in hydrophobic domain by fatty acid with long alkyl hydrocarbon chain such as myristic acid (Myr), stearic acid (Ste) and cholic acid (Cho) modified onto nanocrystalline TiO₂ electrode is prepared and the photovoltaic properties of the nanocrystalline TiO₂ film by Chl-a are studied. Incident photon to current efficiency (IPCE) value at 660 nm in photocurrent action spectrum of Chl-a/Ste-TiO₂, Chl-a/Myr-TiO₂ and Chl-a/Cho-TiO₂ electrodes are 5.0%, 4.1% and 4.1%, respectively. Thus, the IPCE is maximum using Chl-a/Ste-TiO₂ electrode. From the results of photocurrent responses with light intensity of 100 mW cm⁻² irradiation or monochromatic light with 660 nm, generated photocurrent increases using Chl-a/Ste-TiO₂ electrode compared with the other Chl-a assembled TiO₂ electrodes. These results show that the hydrophobic domain formed by stearic acid with long alkyl hydrocarbon chain is suitable for fixation of Chl-a onto TiO₂ film electrodes and photovoltaic performance is improved using Chl-a onto Ste-TiO₂ film electrode.     

Electrocatalytic activities of gold-5-amino-2-mercaptobenzimidazole-M self-assembled monolayer complexes (M: Ag, Cu) for hydroquinone oxidation investigated by CV and EIS

Electrocatalytic activity of a new catalyst toward the oxidation reaction of hydroquinone as a model compound is described. The catalyst was formed by immobilizing metal cations on the topside of a gold-5-amino-2-mercaptobenzimidazole, self-assembled monolayer (Au-5A2MBI-Mⁿ⁺ SAM, Mⁿ⁺: Cu²⁺, Ag⁺) electrode. Preparation steps and the electrocatalytic activity of the catalyst were studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The EIS data were approximated by appropriate electronic equivalent circuit models from which kinetic parameters, such as charge transfer resistance, double layer capacitance, and apparent rate constant (k_app), were estimated. Excellent activity was observed for Au-5A2MBI-Ag⁺ SAM with the following order: Au-5A2MBI-Ag⁺ > Au-5A2MBI-Cu²⁺ > Au-5A2MBI, after testing many modified electrodes. The increased activity originates from a modification of the Au-5A2MBI structure by mediating the effect of Ag⁺. This behavior was understood from significant increases in the k_app without significant changes in the double layer capacitance.     

RAFT polymerization and self-assembly of thermoresponsive poly(N-decylacrylamide-b-N,N-diethylacrylamide) block copolymers bearing a phenanthrene fluorescent α-end group

Phenanthrene α-end-labeled poly(N-decylacrylamide-b-N,N-diethylacrylamide) (PDcA_n-b-PDEA_m) block copolymers consisting in a highly hydrophobic block (n = 11) and a thermoresponsive block with variable length (79 ≤ m ≤ 468) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. A new phenanthrene-labeled chain transfer agent (CTA) was synthesized and used to control the RAFT polymerization of a hydrophobic acrylamide derivative, N-decylacrylamide (DcA). This first block was further used as macroCTA to polymerize N,N-diethylacrylamide (DEA) in order to prepare diblock copolymers with the same hydrophobic block of PDcA (number average molecular weight: M_n = 2720 g mol⁻¹, polydispersity index: M_w/M_n = 1.13) and various PDEA blocks of several lengths (M_n = 10,000-60,000 g mol⁻¹) with a very high blocking efficiency. The resulting copolymers self-assemble in water forming thermoresponsive micelles. The critical micelle concentration (CMC) was determined using Förster resonance energy transfer (FRET) between phenanthrene linked at the end of the PDcA block and anthracene added to the solution at a low concentration (10⁻⁵ M), based on the fact that energy transfer only occurs when phenanthrene and anthracene are located in the core of the micelle. The CMC (∼2 μM) was obtained at the polymer concentration where the anthracene fluorescence intensity starts to increase. The size of the polymer micelles decreases with temperature increase around the lower critical solution temperature of PDEA in water (LCST ∼ 32 °C) owing to the thermoresponsiveness of the PDEA shell.     

Mechanistic comparison between oxidative and reductive charge transport by [(bpy)2(H2O)RuORu(H2O)(bpy)2] confined in a Nafion membrane as studied by potential-step chronocoulospectrometry

Charge transport (CT) in a Nafion membrane containing μ-oxobis[aquabis(2,2′-bipyridine)ruthenium(III)] complex, [(bpy)₂(H₂O)RuORu(H₂O)(bpy)₂]⁴⁺ (bpy=2,2′-bipyridine, abbreviated to Ru^IIIORu^III) was investigated by potential-step chronocoulospectrometry (PSCCS). Electrochemical reduction of Ru^IIIORu^III in the membrane occurred irreversibly to form [Ru(bpy)₂(OH₂)₂]²⁺ monomer. The CT by reduction of Ru^IIIORu^III in the membrane was suggested to take place by physical displacement of the complex, which is quite different from the mechanism in the CT by oxidation of Ru^IIIORu^III in the same membrane in which charge is transported by charge hopping based on reversible redox reaction between Ru^IIIORu^III and Ru^IIIORu^IV. The fractions of the electrochemically reacted complex in the membrane for the oxidative CT was dependent on the complex concentration, and the yield was low (maximum fraction=0.42 at 0.87 M) relative to the reductive CT. By contrast, the fraction for the reductive CT was independent of the concentration over 0.12 M and close to unity. The different concentration dependence of the fraction was discussed related to the difference in the CT mechanism.