The mechanism of electrode surface modification via

The electrode surface modification method via the electrochemical reduction of aryldiazonium salts was first introduced by Pinson and co-workers in 1992 [41].

In general, this process follows a two-step pathway and the proposed reaction mechanism of that is depicted in Scheme 1.

Scheme 1. Proposed reaction mechanism for electrografting by diazonium reduction [5].

In the first step (Scheme 1), the formation of aryl radical and evolution of nitrogen occurs during the one-electron reduction of aryldiazonium compound [1]. Thereafter, the formed aryl radical reacts with the electrode surface, giving a strong bond between the electrode surface and modifier [1]. In case of carbon materials it has been proposed that during the electrografting, the surface carbons bearing the aryl group most likely pass from a sp² to a sp³ hybridisation [58].

The surface grafting with aryldiazonium salts can be performed spontaneously or electrochemically. The spontaneous grafting is achieved by

dipping the electrode into a solution of the diazonium salt for a certain time period [59] and this method has proven to be very useful for modification for example graphene sheets [60]. In contrast, the electrochemical grafting involves the potential cycling or electrolysis at certain potential by applying the potential with a potentiostat [4]. Although spontaneous modification is easier than electrochemical grafting, the first method usually lacks control during the modification, is time-consuming and is not very reproducible [5, 7]. Therefore, the latter method (electrografting) is more preferable due to the better control of the formation of an aryl film on the electrode surface. Usually, the electro-grafting is performed in a narrow potential range using potential cycling. On the first cycle, single and broad, one-electron wave is observed at potentials close to 0 V (vs. SCE) [1]. It has been proposed that this reduction wave corresponds to the formation of an aryl radical which reacts with the electrode surface. On the subsequent cycles, this reduction wave disappears, which is indicative of the blocking of the electrode surface by the organic groups [5].

The electrochemical reduction of aryldiazonium salts can be carried out both in aprotic (e.g. acetonitrile) or in aqueous acidic solutions. It has been proposed that the grafted aryl-layers on Au surface in aqueous solution are less compact than those grafted in acetonitrile [32]. Based on the Brooksby and Downard study [40], the aryl-film formation on pyrolysed photoresist films in aqueous acid medium yielded lower surface coverage as well as thinner films compared to the aryl films formed in acetonitrile.

The diazonium reduction method has been applied to different electrode materials [7]. One of the most important issues concerns the nature of the bond between the aryl group and electrode surface. Generally, it has been suggested that one of the advantages of the diazonium reduction method is the long-term stability of the aryl layer on electrode surface which indicates that this method allows a strong (covalent) bond between the surface modifier and substrate. It is well established that aryl groups are covalently bonded to the GC surface [61].

This finding has been supported by Raman spectroscopy [62, 63] or time-of-flight secondary ion mass spectroscopy [64]. Also, the calculated bonding energy is very high between the aryl group and carbon electrode surface (105 kcal mol^–1) indicating the formation of covalent bond [65]. However, the nature of the bond between an aryl group and HOPG (especially basal plane) is still not fully clear. Liu and McCreery have reported about the chemisorbed (covalently attached) aryl groups on both basal and edge plane HOPG by the electrochemical reduction of the corresponding diazonium salts [62, 66]. In addition, Saveant and co-workers have claimed that the aryl radicals are able to attach to both edge and basal plane graphite [41, 58]. Furthermore, Ray and McCreery concluded that the chemisorption of aryl radicals formed during the diazonium reduction may form both basal and edge regions but more rapidly at edge sites [67]. Very recently, Kirkman et al. [68] proposed that aryl groups are covalently attached both basal plane and step edge sites of HOPG. In contrast, Ma et al. [69] presented an interesting finding on the attachment of aryl groups

onto HOPG via diazonium reduction and these authors concluded that there is no clear evidence for covalent attachment of aryl groups to basal plane HOPG.

There is also a disagreement about the nature of the bonding (covalent or noncovalent bond) between the aryl group and graphene surface. One point of view is that grafting the graphene (including the basal plane graphene) by diazonium reduction yielded a covalent carbon-carbon bond between the aryl group and graphene surface [60, 70, 71]. In contrast, Jiang et al. concluded in their paper that the isolated phenyl groups might be weakly bonded on the basal plane graphene via diazonium reduction although a carbon-carbon bond is formed between the aryl group and graphene converting a sp²-carbon in the graphene sheet to sp³ [72]. Furthermore, it has been proposed that the diazonium reduction appears more rapidly at edges and the reactivity of the sp² sites is much higher for single layer than bilayer graphene and decreases further as the number of graphene layers increases and it is still not clear how aryl radicals bind to multilayer graphene [69, 73].

It has been proposed that the modification by the reduction of diazonium salts on metal surfaces (including Au) is more complex and difficult than that of carbon electrodes [5]. The first attempt of modifying metal electrodes (including Au) by diazonium salts was reported by Ahlberg et al. already in 1980 [74]. However a systematic characterisation of diazonium-functionalised Au was first reported by Laforgue et al. [75]. It has been considered, that for Au substrate, the nature of the bond on Au surface is also controversial and there is still lack of information about the electrochemical grafting process and the physico-chemical properties of the deposited layer [32].In general, it has been shown that the reduction of aryldiazonium salts on Au surface leads to the formation of Au–C bonds [76]. Furthermore, the Au–C bond formed by diazonium-based radical during electrografting is more stable and stronger compared to the Au–S (sulphur) bond produced by self-assembled monolayers (SAMs) of thiols on gold [44, 77–79]. Laurentius et al. [76] have claimed that the interaction between the organic layer derived from diazonium chemistry and Au substrate is Au–C covalent bond, which was confirmed by surface-enhanced Raman scattering studies. It has been claimed that the calculated bonding energy between the organic molecule and Au surface is 24 kcal mol^–1 [80], which is much higher than that reported for thiol SAMs on Au (5 kcal mol^–1) [5], but at the same time it is much lower than that calculated in case of carbon-based materials (105 kcal mol^–1). Besides the strong bond between aryl layer and substrate, and the presence of azo bonds inside the multilayer films, the formation of Au–N=N–C bonds is also evident [81].

In general, the attachment of the aryl films on electrode surface is strong and persistent. In order to remove the organic films from the surface, a mechanical abrasion is required. It has been shown that the aryl films on GC and Au surface are even able to withstand ultrasonication in different solvents and long time exposure to ambient conditions [5]. Lately, Lee et al. [82] studied the stability of aminophenyl films electrografted on GC and Au surfaces and the results

revealed that the aryl films grafted on Au surface were more stable than those grafted to GC.

An interesting study has been carried out by Scholz et al. [83] where the authors showed that SAMs of thiols on gold and mercury electrodes were degraded by hydroxyl radicals (OH^●). It is known that OH^● radical is a highly reactive species degrading almost all kinds of organic compounds. Usually, the OH^● radicals are used in advanced oxidation processes for removal or degradation of pollutants, e.g., azo compounds (dyes) in different wastewaters.

OH^● radicals can be produced by Fenton reactions or UV photolysis of water and hydrogen peroxide. In the Fenton process, OH^● radicals are generated in acid media from H₂O₂ in the presence of Fe²⁺ ions [84]. As shown by Scholz workgroup, OH^● radicals were able to degrade SAMs of thiols on Au [83] and in addition to remove surface asperities from mechanically polished Au electrodes [85] whereby active centres were knocked out [86]. Therefore, it would be highly interesting to study the degradation of aryl layers on GC and Au substrate by OH^● radicals in order to get some insight into the stability of the aryl layers formed on either GC and Au substrate and it was performed recently [IV].