Mario Morin

Formation and reactivity of 3-diazopyridinium cations and influence on their reductive electrografting on glassy carbon.

The in situ generation of 3-diazonium cations from 3-aminopyridine and their subsequent stability under experimental conditions used for electrografting of pyridine groups were investigated by spectroscopy and electrochemistry. UV spectroscopy revealed the rapid kinetics for the reaction of 3-aminopyridine with sodium nitrite in HCl to form the 3-diazopyridinium cation with a second-order rate constant of 550 ± 20 L mol(-1) s(-1) at 22 °C. UV spectroscopy showed that the 3-diazopyridinium ion was relatively unstable and its transformation into 3-hydroxypyridine was proven by (1)H NMR. Its hydrolytic decomposition was investigated by NMR and followed first-order kinetics with a rate constant of (53 ± 5) × 10(-3) s(-1) at 22 °C. These results enable us to establish the appropriate conditions for the electrografting of pyridine from the corresponding diazonium cations generated in situ. The electrochemical modification of glassy carbon electrodes with pyridine was characterized by cyclic voltammetry and the resulting grafted layer by electrochemical impedance spectroscopy in the presence of Fe(CN)(6)(3-/4-) as redox probes. The effect of diazotization time before electrochemical reduction on the blocking effect of the grafted layer was investigated and showed that an increase of the diazotization time led to less efficient grafting. The presence of immobilized pyridine on the electrode surface was demonstrated by X-ray photoelectron spectroscopy measurements, and a surface coverage of 8.8 × 10(-10) mol cm(-2) was estimated for the grafted pyridine groups. The significance of these results for researchers using the in situ generation approach for electrochemical and chemical grafting is discussed.

Isomeric effect on the oxidative formation of bilayers of benzenedimethanethiol on Au(111)

We report on the oxidative electrodeposition of the three isomers of benzenedimethanethiol (BDMT) on Au(111) from an alkaline aqueous solution. All isomers are found to adsorb onto gold via one sulfur and to form a complete monolayer. The IRAS spectra of the monolayers indicate that the isomers adopt an orientation that maximizes the exposure of the unreacted thiolate to the electrolyte solution. The monolayers can be made to react with dithiolates in solution by applying a sufficiently positive potential to the gold substrate. The extent of the monolayer that can react to form bilayer, through the oxidative formation of a disulfide, is related to the availability of the second sulfur endgroup of the adsorbed dithiolate. The orientation of the BDMT isomers in the bilayer also seems to favour the exposure of the unreacted thiolate to the electrolyte solution.

Glycodendrimer coated gold nanoparticles for proteins detection based on surface energy transfer process

We report here a novel and simple approach on which nanometer-size, water soluble glycodendrimer (mannose-functionalized poly(amidoamine) PAMAM generation G0 cystamine core dendrimer) coated gold nanoparticles (Au–man) allows the detection of protein–carbohydrate interactions based on surface energy transfer process (SET). This sensitive SET sensing approach enables quantitative analysis of the binding constant of a mannose-binding protein, lectin Concanavalin A (Con A) labelled with a fluorophore to a sensing Au–man in aqueous samples. Competitive binding and inhibition assays were done to investigate the SET efficiency. The binding constant of Con A to Au–man interactions of (5.6 ± 0.1) × 106 M−1 is 100-times higher than the binding constant values obtained in the interactions with the glycodendrimer G2 alone. The simple modification of the gold nanoparticles with glycodendrimers provides a convenient method to obtain biocompatible and selective carbohydrate-based SET probes for protein biorecognition.

Electrodeposition of bilayers of dithiols

We report an electrochemical method to form a bilayer of dithiol. The cyclic voltammogram of the oxidative deposition of an aromatic dithiol on gold from an alkaline aqueous solution reveals two current peaks separated by more than 400 mV. The integrated charge of the oxidative current peak (B) at the most positive potential is twice that of the other oxidative current peak (A). These two oxidative current peaks were characterized by differential capacitance and electrochemical quartz crystal microbalance (EQCM) measurements. A decrease of the capacity by a factor of two, and an increase of the EQCM frequency change by a factor of two were observed when the potential was scanned from a value where only the first oxidative peak (A) is obtained, to a potential where both oxidative current peaks (A and B) are obtained. Infrared spectra show that the aromatic dithiols adsorb vertically at potentials corresponding to the current peak A and they become tilted for potentials corresponding to the current peak B. The simple relationships between the properties of the two oxidative current peaks are found to be compatible with a step-wise oxidative deposition of a bilayer of dithiol.

Modification of p-type Silicon for the Photoelectrochemical Reduction of CO2

Silicon surface is modified using electrochemical deposition methods to improve its reactivity for photoelectrochemical reduction of CO2. Starting from a solution of CuSO4, copper is deposited by potentiostatic and a pulsed deposition technique providing metal particles. These catalytic sites lower the overpotential for the CO2 reduction. The electrodes were characterized by scanning electron microscopy and X-rays photoelectron spectroscopy to determine the distribution of the size of the particles and their dispersion on the silicon surface as well as the interaction between the copper particles and the silicon substrate. Finally, electrochemical and photoelectrochemical reductions of CO2 were done to assess the reaction efficiency and the catalytic effect of copper particles.

Surface band structure of aryl-diazonium modified p-Si electrodes determined by X-ray photoelectron spectroscopy and electrochemical measurements

The influence of grafted organic layers, obtained by aryldiazonium cations reduction, on the surface properties of p-type silicon was investigated. These layers were characterized by X-ray photoelectron spectroscopy (XPS), infrared spectroscopy and electrochemical methods. The results indicate a passivation of the silicon surface, a decrease of the oxidation of silicon. The results from electrochemical and XPS measurements can be used together to quantify the shift of the surface energy bands of the semiconductor caused by the presence of these layers at the surface. These measurements are important to understand the interfacial properties of the semiconductor and this grafting method can thus be used to adjust the surface energy levels positions toward those of different chemical reactions.

Stability of Created Nano-Holes from Electrodeposited Organic SAMs (Thiols) on Gold Polycrystalline Films

Self-assembled monolayers CH3(CH2)n-1SH with different chain lengths have been elaborated by electrodeposition on gold films. Enhanced blocking properties were obtained by sweeping the potential between oxidative and reductive regions. Once complete and almost defects free layers were obtained, partial reduction was found to lead to the creation of almost circular nanosized holes in the SAMs; which can be used as nanoelectrodes. As verified by the EIS analysis of the charge and the mass transfer through the electrodeposited layers, the stability of the system permeability was found deeply depending on the chain length.

Synthesis of Novel Alkyl- and Aryl Sulfides and Thiols as Precursors for Self-Assembled Monolayers on Gold

A series of 4-alkyl-1-bromosulfanylbenzenes having S-methyl 2, S-t-butyl 3, S-trityl 4, S-benzyl 5, and S-silylethoxymethyl 6 substituents were prepared and evaluated for their ability to form a monolayer consisting of anS-aryl adsorbate on Au (111) surface. The monolayer was formed via the selective hydrolytic removal of an alkyl group protecting the sulfur functionality upon adsorbtion onto the gold surface. Thiols having variable carbon chain length and a peripheral thiophene moiety 9, 10, 17, 18 and 23-26 were also prepared.