Jean Pinson

Attachment of organic layers to conductive or semiconductive surfaces by reduction of diazonium salts

Surface chemistry is the topic of this tutorial review. It describes the electrochemical reduction of aryl diazonium salts on carbon, silicon or metals which leads to the formation of an aromatic organic layer covalently bonded to the surface. The method which permits such a modification is set forth. The proof for the existence of the organic layer is brought forward. The grafting mechanism and the covalent bonding between the surface and the aryl group are discussed. The formation of mono or multilayers depending on the experimental conditions is rationalized. Finally some examples of the possible uses of this reaction are given.

Immobilization of glucose oxidase on a carbon surface derivatized by electrochemical reduction of diazonium salts

Glassy carbon disk electrodes grafted with superficial 4-phenylacetic groups through electrochemical reduction of the corresponding diazonium salt were used for glucose oxidase covalent immobilization. Protein attachment was confirmed by means of X-ray photoelectron spectroscopy and the activity of the enzyme modified electrode was ascertained by performing enzymatic electrocatalysis. The present method does not cause any roughening of the electrode surface and concomitant increase of a background current thus allowing precise measurements using transient electrochemical techniques such as cyclic voltammetry. As diazonium salts (which upon reaction provide the anchoring site) can be synthesized with various substituents, it is conceivable that customized attachment of enzymes and biochemical reagents could be designed.

Sterically hindered diazonium salts for the grafting of a monolayer on metals.

An organic monolayer is obtained on Cu, Au, and SiH by electrografting 3,5-bis-tert-butyl benzenediazonium tetrafluoroborate, as evidenced by cyclic voltammetry, IR-ATR, and ellipsometry. This results from the bulky groups at the 3,5-positions that sterically hinder the growth of the layer.

Organic monolayers on Si(111) by electrochemical method

Abstract This work details the formation of organic monolayers on Si(111) by electrochemical methods. We show that grafting of phenyl groups is possible by reduction of + N 2 –Ar–X cations where the substituent X may be Br, NO 2 , COOH, CN, C n H 2 n +1 ( n =1, 4, 12). Characterizations show that the electrochemical process is self stopped after completion of the first monolayer whose structure is (2×1) close packed in the case X=Br and CH 3 as observed by STM. The stability and passivating properties of films are also investigated.

Fast sweep cyclic voltammetry at ultra-microelectrodes: Evaluation of the method for fast electron-transfer kinetic measurements

With ultra-microelectrodes (in the 10 μ m diameter range), it is possible to decrease the ohmic drop to values that allow the use of very high scan rates in cyclic voltammetry (10–1000 kV s−1. Although reduced as compared with standard size microelectrodes, the ohmic drop is, however, not negligible and thus must be appropriately corrected for. On the other hand, at these high sweep rates the double-layer charging current is a substantial portion of the total current. The correct treatment of the cyclic voltammetric data thus necessitates an ohmic drop correction that takes into account the mutual dependence of the faradaic and double-layer charging currents. Such a treatment is applied to the reduction of anthracene in dimethylformamide, a test example of very fast electron-transfer kinetics (kSap = 3.5 cm s−1), at sweep rates ranging from 20 to 250 kV s−1. The system is analysed both by direct simulation of the cyclic voltammograms and by means of convolution of the voltammograms with the diffusion function (πt)−12. The effect of band-pass limitations of the potentiostat and current measurer on the cyclic voltammetric responses is also discussed.

Oxidation of caffeic acid and related hydroxycinnamic acids

Oxidation of caffeic acid (3,4-dihydroxycinnamic acid) 1H a has been studied by electrochemical methods and by pulse radiolysis in aqueous and organic solvents. The results have been compared with the behaviour of 4-coumaric acid 2H 2 and ferulic acid 3H 2. The first oxidative intermediates have been characterised by their UV spectra and oxidation potentials. In the case of 2H 2 and 3H 2, the initial radicals decay by a second order process indicating a radical-radical coupling mechanism. On the contrary, for caffeic acid the oxidation leads to the formation of the corresponding o-quinone through disproportionation of the initial semiquinone radical.

Degradation of chlorophenoxyacid herbicides in aqueous media, using a novel electrochemical method†

The degradation of five chlorophenoxyacid herbicides has been studied using an electrochemical method based on the Fenton reaction (simultaneous reduction of dioxygen and ferric ions). The method consists of electrosynthesizing OH˙ radicals, which react rapidly with chlorophenoxyacids in aqueous media. HPLC and GC-MS analysis show the formation of polyhydroxyphenols and quinones in a first step, and the complete destruction of the aromatic nucleus upon exhaustive electrolysis. This offers a possible way for the depollution of natural waters containing chlorophenoxyacid pesticide residues.

Steric Effects in the Reaction of Aryl Radicals on Surfaces

Steric effects are investigated in the reaction of aryl radicals with surfaces. The electrochemical reduction of 2-, 3-, 4-methyl, 2-methoxy, 2-ethyl, 2,6-, 2,4-, and 3,5-dimethyl, 4-tert-butyl, 3,5-bis-tert-butyl benzenediazonium, 3,5-bis(trifluoromethyl), and pentafluoro benzenediazonium tetrafluoroborates is examined in acetonitrile solutions. It leads to the formation of grafted layers only if the steric hindrance at the 2- or 2,6-position(s) is small. When the 3,5-positions are crowded with tert-butyl groups, the growth of the organic layer is limited by steric effects and a monolayer is formed. The efficiency of the grafting process is assessed by cyclic voltammetry, X-ray photoelectron spectroscopy, infrared, and ellipsometry. These experiments, together with density functional computations of bonding energies of substituted phenyl groups on a copper surface, are discussed in terms of the reactivity of aryl radicals in the electrografting reaction and in the growth of the polyaryl layer.

Electrochemical functionalization of nanotube films: growth of aryl chains on single-walled carbon nanotubes

Covalent exohedral derivatizations of HiPco nanotubes, through electrochemical reduction of aryldiazonium salts, is described. Four different aryldiazonium salts have been used under the same experimental conditions: 4-bromophenyl, 4-chloromethylphenyl, 4-sulfophenyl and 4-carboxyphenyl. Derivatized samples were characterized through X-ray photoelectron spectroscopy and micro-Raman diffusion. The evolution of the spectra (area of the D-band), as a function of the number of grafted groups, led us to the conclusion that the electrochemical reduction of aryldiazonium salts into radicals gives rise to the growth of aryl chains on the sidewalls of the nanotubes.

Trace crossings in cyclic voltammetry and electrochemic electrochemical inducement of chemical reactions: Aromatic nucleophilic substitution

Abstract Crossing of anodic and cathodic traces is frequently observed on cyclic voltammograms featuring the electrochemical induction of a chemical reaction in the case where the product standard potential is positive to the reactant reduction potential. The theory of this phenomenon has been established in the contaxt of aromatic nucleophilic substitution. The reaction of potassium diethyl phosphite on 4-chlorobenzonitrile in liquid ammonia was investigated as an example illustrating this type of phenomenon and its interpretation. The simulation of the experimental voltammograms demonstrates the proposed mechanistic and kinetic model and allows the rate constants of the various steps to be determined. Much higher rate constants can thus been attained than by the standard application of electrochemical techniques (the gain may reach five or six orders of magnitude). A procedure is derived from these observations and then a rationalization for inducing chemical reactions with a very low electricity consumption as opposed to that which occurs when the electrode potential is settled at the level of the reactant wave.