Immobilization of tetra-amine substituted metallophthalocyanines at gold surfaces modified with mercaptopropionic acid or DTSP-SAMs

This paper shows that amine substituted cobalt phthalocyanine (CoTAPc) can be deposited on gold surfaces by using an interconnecting layer of a self-assembled monolayer (SAM) of mercaptopropionic acid or Lomant's reagent (dithiobis(N-succinimidyl propionate) (DTSP)). In both cases the new bond formed is obtained by the creation of an amide. The layers were characterized by electrochemistry and showed high coverage fractions (near 100%). Reductive and oxidative desorption of the SAMs limit the useful potential window from −0.6 to +0.5 V versus Ag|AgCl. The SAM-CoTAPc layers show electrocatalytic activities towards oxygen reduction through the Co(I) central metal ion. The amount of CoTAPc molecules deposited (obtained from the Co central metal ion activity in nitrogen purged solutions) revealed that the CoTAPc molecules are bonded in a perpendicular manner at the surface. Taking into account a surface of 200 Å² for a flatly bonded MPc, this should result in a four times less amount of deposited CoTAPc compared to the experimental value obtained. Both methods showed good results and promising long-term stability and will be interesting tools for further research in surface modification and sensor development.     

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.     

Investigations on the monolayer structure of thiol SAMs and the influence of conjugated π-bonds on the electronic molecular conductivity

Thiols have been in the focus of recent research because of their capability to form self assembled monolayers (SAM) on noble and semi-noble metals opening a new field of fundamental research and its application in various branches, as e.g. in nano technology. In this paper, the investigation of SAMs of six specially tailored thiols with an aromatic head group on a preferentially (1 1 1) orientated Au-surface are described with special interest in their structure and the electronic conductivity in dependence on the number of conjugated π-bonds in the chain group. Potentiodynamic polarization curves in 0.5 M KOH with and without [Fe(CN)₆]^3−/4− were used to obtain information on the quality and conductivity of the SAMs while scanning tunneling microscopy (STM) and XPS were used for investigations on the monolayer structure. Additionally molecular dynamic calculations were performed to check the possibility and reliability of these calculations to predict the arrangement of the thiol molecules within the SAMs. The electron conductivity of these SAMs rises significantly with the amount of conjugated π-bonds. A naphthalene head group causes the formation of SAMs with a typical herringbone structure whereas anthraquinone leads to a mixture of parallel and herringbone arrangements.     

In situ stress measurements during the electrochemical adsorption/desorption of self-assembled monolayers

This paper examines the surface stress associated with the electrochemical desorption of 4-mercaptobenzoic acid (4-MBA) from (1 1 1) textured Au in aqueous 0.1 mol L⁻¹ KOH. Self-assembled monolayers of varying coverage were adsorbed onto the Au electrode surface from a 0.1 mol L⁻¹ aqueous KOH solution containing 1 mmol L⁻¹ 4-MBA. Adsorption follows Langmuir kinetics and fully formed monolayers, corresponding to 0.29 coverage with respect to the Au surface, are formed in about 120 min. XP spectra confirm the formation of the Au-S bond while FTIR spectra indicate that the 4-MBA is orientated with the carboxylate pointed away from the surface. The one-electron reductive desorption of 4-MBA occurs at a potential of −0.9 to −1.0 V vs. SSE, depending on coverage, and causes a surface stress change in the tensile direction, indicating that 4-MBA adsorption induces a compressive surface stress to the Au. At short immersion times and low monolayer coverage, the surface stress increases with coverage as the stress response is primarily governed by the Au-S bond density. SAM desorption following longer immersion times produces large stress changes with little corresponding change in SAM coverage. We attribute the additional compressive stress to stabilization of the Au-S bonding regions and the coulombic repulsion between neighboring molecules, both associated with ordering of the 4-MBA on the Au surface.     

Characterization of manganese tetraarylthiosubstituted phthalocyanines self assembled monolayers

Manganese tetraarylthiosubstituted phthalocyanines (complexes 1-5) have been deposited on Au electrode surfaces through the self assembled monolayer (SAM) technique. SAM characteristics reported in this work are: ion barrier factor (∼1); interfacial capacitance (303-539 μF cm⁻²) and surface coverage (1.06 × 10⁻¹⁰-2.80 × 10⁻¹⁰ mol cm⁻²). Atomic force microscopy was employed in characterizing a SAM. SAMs of complexes 1-5 were employed to detect l-cysteine (with limit of detection ranging from 2.83 × 10⁻⁷ to 3.14 × 10⁻⁷ M at potentials of 0.68-0.75 V vs. Ag|AgCl) and nitrite (limit of detection ranging from 1.78 × 10⁻⁷ to 3.02 × 10⁻⁷ M at potentials of 0.69-0.76 V vs. Ag|AgCl).     

Unobstructed electron transfer on porous polyelectrolyte nanostructures and its characterization by electrochemical surface plasmon resonance

Thin organic films with desirable redox properties have long been sought in biosensor research. We report here the development of a polymer thin film interface with well-defined hierarchical nanostructure and electrochemical behavior, and its characterization by electrochemical surface plasmon resonance (ESPR) spectroscopy. The nano-architecture build-up is monitored in real time with SPR, while the redox response is characterized by cyclic voltammetry in the same flow cell. The multilayer assembly is built on a self-assembled monolayer (SAM) of 1:1 (molar ratio) 11-ferrocenyl-1-undecanethiolate (FUT) and mercaptoundecanoic acid (MUA), and constructed using a layer-by-layer deposition of cationic poly(allylamine hydrochloride) (PAH) and anionic poly(sodium 4-styrenesulfonate) (PSS). Electron transfer (ET) on the mixed surface and the effect of the layer structures on ET are systematically studied. Under careful control, multiple layers can be deposited onto the 1:1 FUT/MUA SAM that presents unobstructed redox chemistry, indicating a highly ordered, extensively porous structure obtained under this condition. The use of SPR to trace the minute change during the electrochemical process offers neat characterization of local environment at the interface, in particular double layer region, allowing for better control over the redox functionality of the multilayers. The 1:1 SAM has a surface coverage of 4.1 ± 0.3 × 10⁻¹⁰ mol cm⁻² for ferrocene molecules and demonstrates unperturbed electrochemistry activity even in the presence of a 13 nm polymer film adhered to the electrode surface. This thin layer possesses some desirable properties similar to those on a SAM while presenting ∼15 nm exceedingly porous structure for high loading capacity. The high porosity allows perchlorate to freely partition into the film, leading to high current density that is useful for sensitive electrochemical measurements.     

Electrochemical and surface analytical studies of the self-assembled monolayer of 5-methoxy-2-(octadecylthio)benzimidazole in corrosion protection of copper

5-Methoxy-2-(octadecylthio)benzimidazole (MOTBI) monolayer was self-assembled on fresh copper surface obtained after etching with nitric acid at ambient temperature. The optimum conditions for formation of self-assembled monolayer (SAM) were established using impedance studies. The optimum conditions are methanol as solvent, 10 mM concentration of the organic molecule and an immersion period of 24 h. The MOTBI SAM on copper surface was characterized by contact angle measurements, X-ray photoelectron spectroscopy and reflection absorption FTIR spectroscopy and it is inferred that chemisorption of MOTBI on copper surface is through nitrogen. Corrosion protection ability of MOTBI SAM was evaluated in aqueous NaCl solution using impedance, electrochemical quartz crystal nanobalance, potentiodynamic polarization and weight-loss studies. While bare copper showed a charge-transfer resistance (R_ct) value of 1.89 kΩ cm² in 0.20 M NaCl aqueous environment, the R_ct value for SAM covered copper surface is 123.4 kΩ cm². The MOTBI SAM on copper afforded corrosion inhibition efficiency of 98-99% in NaCl solution in the concentration range and in the temperature range studied. The SAM functions as a cathodic inhibitor. Quantum chemical calculations showed that MOTBI has relatively small ΔE between HOMO and LUMO and large negative charge in its benzimidazole ring, which facilitate formation of a polymeric [Cu⁺-MOTBI] complex on copper surface.     

Electrochemistry of conductive polymers: 41. Effects of self-assembled monolayers of aminothiophenols on polyaniline films

Effects of self-assembled monolayers (SAMs) of a few different aminothiophenols (ATPs) on growth, morphology, and electrical properties of polyaniline have been studied at gold electrodes employing current sensing atomic force microscopy. The 4-aminothiophenol (4-ATP) SAM showed the best capability of shuttling electrons from the gold electrode to the solution species, and the 4-ATP SAM covered electrode not only showed the best kinetics for the film growth but also produced the film of the highest electrical conductivity. The vertical conductivity of the film ranged from 58 S cm⁻¹ for a film prepared at a bare gold electrode to as high as 148 S cm⁻¹ for a film obtained at the 4-ATP SAM covered electrode. The conductivity was shown to depend on the thickness of the films as well as how they were prepared.     

Artificially phase-separated binary self-assembled monolayers composed of 11-amino-1-undecanethiolate and 10-carboxy-1-decanethiolate on Au(1 1 1): A comparative study of two preparing methods

Two methods have been compared for preparing artificially phase-separated two-component SAMs on Au(1 1 1) composed of 11-amino-1-undecanethiolates (AUTe) and 10-carboxyl-1-decanethiolates (CDTe), which would form, thermodynamically, a homogeneously mixed binary SAMs. The first method starts with the formation of a phase-separated binary SAM of AUTe and 2-hydroxy-1-ethanethiolate (HETe) as a template of the artificially phase-separated SAM, followed by the selective desorption of HETe domains and succeeding filling of the vacancy with CDTe. The second method utilizes fluoren-9-ylmethyl N-(11-mercaptoundecyl) carbamate (FMUCe) instead of 11-amino-1-undecanethiol in preparing the template. After the filling with CDTe, the 9-fluorenylmethyloxycarbonyl (Fmoc) group is removed to obtain AUTe domains. Both methods yield artificially phase-separated binary SAMs having AUTe domains of tens nanometer across. The molecularly flat SAM surface with nanometer-scale domains of different acid-base and electrostatic properties are thus created. For preparing binary SAMs with a higher degree of phase separation, the second method is a better choice; a more clear-cut phase separation is achieved.     

Electron-transfer characteristics of ferrocene attached to single-walled carbon nanotubes (SWCNT) arrays directly anchored to silicon(1 0 0)

High-density, surface-mounted ferrocene has been prepared using covalent immobilisation of an alcohol substituted ferrocene derivative to a pre-assembled single-walled carbon nanotubes directly anchored to silicon(1 0 0) surface (SWCNTs-Si). The formation of these ferrocene-modified electrodes (Fc-SWCNTs-Si) has been followed using X-ray photoelectron spectroscopy and atomic force microscopy. Electrochemical results show the surface concentration of ferrocenemethanol molecules is 9.26 × 10⁻⁸ mol cm⁻², which is about 500-1000 times greater than the experimentally measured coverage of ferrocene directly attached to flat Si(1 0 0) surfaces. The reversible one-electron wave of the ferrocene/ferrocenium couple was observed at 490 mV versus Ag⁺/Ag and the apparent rate constant of electron transfer, k_app, was 21 s⁻¹. These results suggest these ferrocene-modified electrodes are excellent candidates for molecular memory devices.     

A new cleaning methodology for efficient Au-SAM removal

The desorption of a self-assembled monolayer (SAM) of 11-amino-1-undecanethiol (AUT) formed on an Au polycrystalline electrode was investigated with the purpose of establishing the most efficient method for SAM removal prior to electrode re-use. The cleanliness of the surface was evaluated by assessing the characteristics (and their reproducibility) of a newly prepared SAM (on a freshly cleaned electrode) in the presence of Ru(NH₃)₆^3+/2+ probe species. The simple flame annealing of the modified electrodes showed poor reproducibility. Later, cleaning treatments based on previous reports about the electrochemical desorption of SAMs, were investigated in NaOH, HClO₄, and KCl solutions. The anodic removal in alkaline and chloride solutions was not efficient enough due to the effect of the Au surface oxidation and dissolution. The commonly used reductive desorption in alkaline solution did also not offer a high efficiency (in contrast with the behavior described in those reports) probably due to the low stability of the thiolate molecules at high-pH values. In addition, such procedures did not provide a way to visualize the evolution of the process. The oxidative desorption in perchloric acid showed a higher efficiency at each single desorption cycle, and allowed to monitor the extension of the removal by comparing the cyclic voltammograms obtained in that medium with the expected fingerprint for the bare Au. However the voltammetric characterization of new AUT films, prepared on such electrochemically treated surfaces by re-incubation in the AUT solution, showed that the conditions of substrate cleanliness and smoothness necessaries to reproduce a close-packed and compact AUT monolayer were not reached. A new cleaning strategy based on the coupled use of oxidative removal in HClO₄ 0.1 M and flame annealing showed to be efficient and reproducible, providing the proper substrate precursors for the formation of highly ordered AUT films. The results also showed that the methodology works well in the removal of other SAMs like the one formed by the neutral 1-dodecanethiol (1-DT).     

Self-assembled monolayers (SAMs) of cobalt tetracarboxylic acidchloride phthalocyanine covalently attached onto a preformed mercaptoethanol SAM: A novel method

A feasible method of fabricating phthalocyanine sensor was developed by covalent attachment of cobalt tetracarboxylic acidchloride phthalocyanine (CoTCACIPc) onto a preformed 2-mercaptoethanol (2-ME) self-assembled monolayer (SAM) modified gold electrode (designated as CoTCACIPc-2-ME-SAM). The surface concentration of the CoTCACIPc was found to be 4.58 × 10⁻¹⁰ mol/cm². The sensor gave a linear response to l-cysteine over the concentration range 0.28-20 μM with a detection limit of 5 × 10⁻⁷ M and best response time of 2 s.     

Adsorption dynamics and interfacial properties of thiol-based cobalt terpyridine monolayers

Spontaneously adsorbed monolayers of [Co(ttp-CH₂-SH)₂](PF₆)₂ have been formed on platinum microelectrodes by exposure to micromolar solutions of the complex in 0.1 M TBABF₄ in acetonitrile, ttp-CH₂-SH is 4′-(p-(thiolmethyl)-phenyl)-2,2′:6′,2″-terpyridine. Resonance Raman spectroscopy on roughened polycrystalline platinum macro electrodes show that the molecule undergoes adsorption through the sulphur atom onto the platinum surface. The monolayers show reversible and well defined cyclic voltammetry when switched between Co²⁺ and Co³⁺ forms, with a peak to peak splitting of 0.040 ± 0.005 V up to 200 V s⁻¹ and an FWHM of 0.138 ± 0.010 V. Adsorption is irreversible leading to the maximum surface coverage, 6.3 ± 0.3 × 10⁻¹¹ mol cm⁻² for 2.5 ≤ [Co(ttp-CH₂-SH)₂] ≤ 10 μM. The rate of monolayer formation appears to be controlled not by mass transport or interfacial binding but by surface diffusion of the complex. The surface diffusion coefficient is 5.5 ± 1.1 × 10⁻⁷ cm² s⁻¹ indicating that prior to formation of an equilibrated monolayer, the adsorbates have significant mobility on the surface. The electron transfer process across the monolayer-electrode interface has been probed by high speed chronoamperometry and the standard heterogeneous electron transfer rate constant, k°, is approximately 3.06 ± 0.03 × 10⁴ s⁻¹. The reorganization energy is at least 18.5 kJ mol⁻¹.     

Cyclic voltammetry and scanning electrochemical microscopy studies of methylene blue immobilized on the self-assembled monolayer of n-dodecanethiol

Electron transfer (ET) kinetics through n-dodecanethiol (C₁₂SH) self-assembled monolayer on gold electrode was studied using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). An SECM model for compensating pinhole contribution, was used to measure the ET kinetics of solution-phase probes of ferrocyanide/ferricyanide (Fe(CN)₆^4−/3−) and ferrocenemethanol/ferrociniummethanol (FMC^0/+) through the C₁₂SH monolayer yielding standard tunneling rate constant () of (4 ± 1) × 10⁻¹¹ and (3 ± 1) × 10⁻¹⁰ cm s⁻¹ for Fe(CN)₆^4−/3− and FMC^0/+ respectively. Decay tunneling constants (β) of 0.97 and 0.96 Å⁻¹ for saturated alkane thiol chains were obtained using Fe(CN)₆⁴⁻ and FMC respectively. Also, it was found that methylene blue (MB) molecules are effectively immobilized on the C₁₂SH monolayer and can mediate the ET between the solution-phase probes and underlying gold substrate. SECM-mediated model was used to simultaneously measure the bimolecular ET between the solution-phase probes and the monolayer-immobilized MB molecules, as well as tunneling ET between the monolayer-immobilized MB molecules and the underlying gold electrode, allowing the measurement of k_BI = (5 ± 1) × 10⁶ and (4 ± 2) × 10⁷ cm³ mol⁻¹ s⁻¹ for the bimolecular ET and and (7 ± 3) × 10⁻² s⁻¹ for the standard tunneling rate constant of ET using Fe(CN)₆^4−/3− and FMC^0/+ probes respectively.     

Rapid formation of high-quality self-assembled monolayers of dodecanethiol on polycrystalline gold under ultrasonic irradiation

Self-assembled monolayers of dodecanethiol (C₁₂SH-SAMs) on polycrystalline gold were prepared under ultrasonic irradiation at 100 W (the actual ultrasonic power intensity is about 0.1 W cm⁻² including the heat loss) for different time and investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that the differential capacitance C_d values of the C₁₂SH-SAM prepared under ultrasonic irradiation at 100 W (0.1 W cm⁻²) for 15 min are independent of the scan rate, the thickness d value of this monolayer is 17.5 Å, the tilt angle φ value of the molecules in this monolayer from the gold surface normal was calculated to be 30° and the difference value of the current density at −0.2 and 0.5 V (Δi_p) is only 0.69 μA cm⁻². From the EIS experiments, we find that the phase angle value at 1 Hz Φ_1 Hz of the C₁₂SH-SAM prepared under ultrasonic irradiation at 100 W (0.1 W cm⁻²) for 15 min is 89°, the charge transfer resistance R_ct value of this monolayer is 1.40 × 10⁶ Ω cm² and the surface coverage θ value of this monolayer was calculated to be 99.997% from R_ct. These results indicate that the C₁₂SH-SAM of almost defect-free structure and very low ionic permeability can be formed under ultrasonic irradiation at 100 W (0.1 W cm⁻²) in a short time (15 min).     

Self-assembled monolayers-based immunosensor for detection of Escherichia coli using electrochemical impedance spectroscopy

An electrochemical impedance immunosensor for the detection of Escherichia coli was developed by immobilizing anti-E. coli antibodies at an Au electrode. The immobilization of antibodies at the Au electrode was carried out through a stable acyl amino ester intermediate generated by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydrosuccinimide (NHS), which could condense antibodies reproducibly and densely on the self-assembled monolayer (SAM). The surface characteristics of the immunosensor before and after the binding reaction of antibodies with E. coli were characterized by atomic force microscopy (AFM). The immobilization of antibodies and the binding of E. coli cells to the electrode could increase the electro-transfer resistance, which was directly detected by electrochemical impedance spectroscopy (EIS) in the presence of Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ as a redox probe. A linear relationship between the electron-transfer resistance and the logarithmic value of E. coli concentration was found in the range of E. coli cells from 3.0 × 10³ to 3.0 × 10⁷ cfu mL⁻¹ with the detection limit of 1.0 × 10³ cfu mL⁻¹. With preconcentration and pre-enrichment steps, it was possible to detect E. coli concentration as low as 50 cfu/mL in river water samples.     

Functional gold nanorod particles on conducting polymer poly(3-octylthiophene) as non-enzymatic glucose sensor

The immobilization of surface-functionalized self-assembled monolayer (SAM) gold nanoparticles onto poly(3-octylthiophene) (POT) was achieved by the cooperation of hydrophobic forces. SAMs were prepared by 11-mercaptoundecanoicacid (MUA), 4-mercaptophenyl boronic acid (MPB), and 1-decanethiol (DT) hydrophobic substrates. Nanoparticles-SAM-POT system was characterized by cyclic voltammetry, SEM, EDAX and contact angle measurements. SAMs (MUA) is closely packed providing effective blocking of the underlying platinum electrode and preventing a ferrocyanide molecule from penetrating. However, potential scanning was applied at SAMs (MUA) modified electrode on which electron penetrating holes or defects were occured. Since SAMs (MPB) is poorly packed according to SAMs (MUA), ferrocyanide molecules could penetrate to SAMs (MPB) modified electrode surface. POT-Au-SAM (MPB) electrode was used for glucose determination as potentiometric non-enzymatic glucose sensor. The analytical performance was evaluated and linear calibration graphs were obtained in the concentration range of 5-30 mM glucose including the level of human blood glucose.     

SAM-modified microdisc electrode arrays (MDEAs) with functionalized carbon nanotubes

The construction of a novel voltammetric transducer employing mixed acid-chopped, oxidatively modified, single-walled carbon nanotubes (HOOC-SWNT-COOH) covalently coupled to alkane thiol self-assembled monolayers (SAMs) that were deposited onto gold microdisc electrode arrays (MDEA-Au) (1296 microdiscs, 100 μm diameter each) has been demonstrated. The chemisorption of cysteamine (CA) and 11-amino-1-undecanethiol (11-AUT) SAMs onto the MDEAs resulted in MDEA-Au|CA and MDEA-Au|11-AUT, which when conjugated with the HOOC-SWNT-COOH produced MDEA-Au|CA|SWNT and MDEA-Au|11-AUT|SWNT electrodes. Electrodes were characterized by multiple scan rate cyclic voltammetry in the presence of ferrocene monocarboxylic acid and by AFM and compared. As expected, CA modification of MDEA-Au produced a disordered film that did not alter the effective surface area of the MDEA while 11-AUT modification resulted in passivation and a 50% reduction in effective surface area. The conjugation of the SWNT to the CA-modified and the 11-AUT-modified MDEA-Au surface returned well-resolved voltammograms with highest peak currents (I_pa = 5.54 × 10⁻⁴ A, I_pc = 2.34 × 10⁻⁴ A at 100 mV/s). The effective area of MDEA-Au|CA|SWNT was increased by 200% while that of MDEA-Au|11-AUT|SWNT was increased by 100%. AFM showed SWNTs to be vertically oriented on the CA-modified surface but to be horizontally arranged and bundled on the 11-AUT surface.     

Metal deposition onto a thiol-covered gold surface: A new approach

In this study, we report on details of a new technique for depositing metal on top of a self-assembled monolayer (SAM). The experiments were performed with 4,4′-dithiodipyridine (4-PySSPy) SAMs on Au(1 1 1) electrodes, which were immersed without potential control into a Pd(II)-containing solution, where Pd(II) adsorbs on the surface by forming a complex with the pyridine nitrogen. The complexed Pd(II) ions were reduced electrochemically to Pd(0) after transferring the electrode to a Pd ion-free solution. Upon reduction, monoatomic high Pd islands were observed in STM. From cyclic voltammetry, in situ STM and ex situ angle-resolved XPS we conclude that these islands reside on top of the SAM, not underneath. It is shown that palladium complexation and deposition on self-assembled monolayers of 2-mercaptopyridine is not possible.     

Feasible attachment of dinuclear ruthenium complex to gold electrode surface via new ligand substitution reaction

A general method has been developed for accumulation of a dinuclear ruthenium complex [Ru₂(dhpta)(μ-O₂CCH₃)₂]⁻ (H₅dhpta = 1,3-diamino-2-hydroxypropane-N,N,N′,N′-tetraacetic acid) on a gold surface. The accumulation using a ligand substitution reaction of bridging acetate in the complex by terminal benzoic acid in a self-assembled monolayer (SAM) with ω-mercaptoalkoxy benzoic acid (HOOC-C₆H₄-O-(CH₂)_n-SH) (n = 4, 6, 12) is undergone. The methyl benzoate-containing alkyl disulfides capable to form SAMs on gold electrode have been synthesized utilizing reductive dimerization of the corresponding alkyl thiocyanates with tetraphenylphosphonium tetrathiomolybdate. The methyl benzoate group in the SAM was converted into benzoic acid group by base hydrolysis, which was confirmed by surface-enhanced Raman scattering measurements for silver electrode. After the ligand substitution reactions to accumulate the complex on the gold electrode surface, in the case of n = 6 and n = 12, voltammetric waves for surface confined redox process, which corresponds to Ru^IIIRu^III/Ru^IIIRu^II redox couple are observed, respectively, and these surfaces of gold electrodes are covered with the complex completely. The present ligand substitution reaction should be widely applicable for the accumulation of other complexes and useful for designing of functional electrodes.