Elsevier

Electrochimica Acta

Volume 56, Issue 5, 1 February 2011, Pages 2213-2220
Electrochimica Acta

Diazonium salt derivatives of osmium bipyridine complexes: Electrochemical grafting and characterisation of modified surfaces

https://doi.org/10.1016/j.electacta.2010.11.070Get rights and content

Abstract

Osmium bipyridine complexes were directly grafted to carbon electrodes through electroreduction of the diazonium salts [Os(bpy-ph-N2+)3](PF6)5 (1) and [Os(bpy-ph-N2+)2Cl2](PF6)2 (2). Growth of the films was not self-limiting as is usually found for grafting from diazonium salts. It appears that electron hopping through the bipyridine ligands of the immobilised complexes enables film growth to continue at a constant rate during grafting by potential cycling. The surface concentrations of deposited films were measured electrochemically and the film thicknesses were measured by depth-profiling using the atomic force microscope. Films up to 42 nm thick were prepared with no evidence for slowing of film growth. The grafted films exhibited high stability when repetitively cycled through the Os2+/3+ couple and electron transfer rate constants of 11.4 s−1 and 35.4 s−1 were measured in ACN and PBS, respectively, for the Os2+/3+ couple of the film grafted from 1.

Introduction

Surface modification aims to give new properties to a surface while maintaining the bulk properties of the material. Biocompatibility, wettability, adhesion, resistance to corrosion and wear-and-tear, and chemical functionality are examples of important surface properties that can be modulated by surface coatings. Methods for covalent attachment of thin organic layers to conducting substrates via formation of radicals which attack the surface have received much recent attention [1], [2], [3], [4], [5], [6]. The stability of the covalent attachment [7], [8], [9], the simplicity of the procedures and the wide substrate compatibility are important aspects of these methods [10], [11], [12], [13], [14], [15].

Reduction of aryldiazonium cations is the best-known example of this general modification strategy [1], [3]. Following one-electron reduction, dinitrogen is eliminated leaving an aryl radical which binds to the surface [7], [16]. Many types of organic films have been electrografted to carbon [7], [12], [16], metal [11], [13], [15], [17] and semiconducting [15], [18], [19] substrates from diazonium salt solutions. However, there have been relatively few reports of immobilisation of metal complexes or organometallic compounds via diazonium salts. Three general approaches towards modification with well-defined metal-centred compounds have emerged. In the first strategy, the ligand is immobilised on the surface via reduction of the corresponding diazonium salt derivative [20], [21], [22] or by coupling the ligand to a tether layer grafted by diazonium salt reduction [23]. The immobilised ligand layer is subsequently reacted with the metal to form a complex. The second approach involves coupling of a suitable derivative of the metal complex to a surface-immobilised tether layer which has been grafted from diazonium salts [24], [25], [26], [27]. The third method is direct grafting of metal-centred compounds from the corresponding diazonium derivative [28], [29], [30], [31]. The latter strategy offers advantages in terms of minimising the number of surface reactions required, giving greater control and hence better reproducibility of the surface concentration of the metal-centred compound.

Geiger and co-workers demonstrated the first example of direct grafting of a metal-centred compound via a diazonium salt by preparing cobaltacenium-modified glassy carbon electrodes [31]. More recently, Bidan's and Mailley's groups reported the preparation and characterisation of surfaces modified with ruthenium bipyridine and terpyridine complexes via reduction of the corresponding diazonium salt derivatives [28], [29]. An interesting aspect of that work was the formation of layers with very high surface concentrations, up to 35 × 10−9 mol cm−2 [29]. During electrografting of Ru(bpy)2(bpy-ph-N2+)·3PF6 and the analogous terpyridine complex, the electrodes were not passivated by the growing film as is usually found during electrografting from diazonium salts. Instead the growing film acted as a redox polymer, allowing electron hopping through the film and continuous reduction of the diazonium complex at the film-solution interface. A patent by the same authors describes a range of diazonium salts of metal complexes based on ruthenium, osmium and iridium but no details of synthetic methods or electrochemistry of specific complexes are given [32].

Osmium complexes of bipyridine-type ligands are gaining popularity as redox mediators in a number of biological applications. Osmium bipyridine complexes have several attractive properties as redox mediators: the Os2+/3+ couple has well-defined redox chemistry and both oxidation states have suitable stability for mediation purposes. Further, by altering the ligand shell of the complex, the potential of the Os2+/3+ couple can be tuned across a wide potential window [33], [34]. There are examples of surface-confined osmium complexes successfully mediating electron transfer in both microbial [35], [36] and enzymatic systems [37], [38], [39], [40] for biosensing and biofuel cell applications.

This paper describes the preparation and electrochemistry of diazonium salts of three osmium bipyridine complexes (Fig. 1). Precursor complexes with one, two or three aminophenyl-substituted bipyridine ligands were diazotised and directly electrochemically grafted to glassy carbon (GC) and pyrolysed photoresist (PPF) electrodes. Modified surfaces were characterised using electrochemistry and atomic force microscopy (AFM).

Section snippets

Synthesis of diazonium tetrafluoroborate salts of osmium complexes 1a, 2a, and 3a

The synthesis of amine complexes 1a, 2a, and 3a (Fig. 1) and the complexes Os(bpy)3.2PF6 and Os(bpy)2Cl2 is described in the supporting information. Positive ion ESI-MS spectra of complexes 1a, 2a, and 3a are shown in Fig. S1 (supporting information). To convert the amine derivatives to the corresponding diazonium complexes (1, 2 and 3), 10 mg of the osmium complex for diazotisation was added to 1.5 mL of 25% HBF4 (diluted from 50% HBF4 (Acros Organics)), briefly sonicated to aid suspension and

Results and discussion

Fig. 1 shows the osmium bipyridine complex diazonium salts used in this work. Complexes 13 were prepared from the precursor amine complexes (1a3a) by treatment with sodium nitrite in aqueous fluoroboric acid. Preliminary electrochemical investigations of 13 were made in the diazotisation solutions giving clear evidence for grafting of films of the complexes (Fig. S3). However the electrochemical response was complex and hence in situ grafting was not pursued. For the work described here, 13

Conclusions

Diazonium derivatives of osmium biyridine complexes can be directly electrografted to carbon surfaces to give films with well-behaved Os2+/3+ couples. It appears that electron transport to diazonium cations in solution through the growing film is mediated by bipyridine reduction processes and hence grafting does not lead to passivating layers. As a consequence, thick films with high surface concentrations can be prepared, similar to those previously reported for a diazonium derivative of a

Acknowledgements

This work was supported by the RSNZ Marsden Fund (contract UOC 0605), the MacDiarmid Institute for Advanced Materials and Nanotechnology and the University of Canterbury. D.J.G. thanks the Tertiary Education Commission for a doctoral scholarship and Wayne K. Pilgrim, National University of Ireland, Galway, for obtaining mass spectrometry data.

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