Electrochemical reduction of aryl diazonium salts: a versatile way for carbon nanotubes functionalisation † Matei Raicopol, Luiza Necula, Mariana Ionita and Luisa Pilan* Single-walled carbon nanotubes (SWCNTs) possess a wealth of exceptional structural, mechanical and electronic properties. An interesting challenge is their functionalisation without altering their electronic properties by a route that ensures a stable and a strong grafting of the desired molecules. Aryl diazonium salts are promising candidates for introducing molecular tools by surface modifications. We report here the functionalisation of SWCNTs through the electrochemical reduction of aryl dia- zonium salts. Experiments were carried out first on a glassy carbon electrode, taking into account its low porosity and struc- tural similarity with SWCNTs. The surface modification of both substrates is checked by electrochemical signals related to the different immobilised moieties obtained by voltammetric methods and the blocking action of the modified electrodes for Fe (CN) 6 3À/4À redox probe in cyclic voltammetry measurements. This procedure allowed the use of a variety of diazonium salts, including those that provide moieties favourable to further elaboration after attachment to the SWCNTs. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: diazonium salt; carbon nanotubes; functionalization; cyclic voltammetry; redox probe Introduction Scientific and technological interest in one-dimensional nanoma- terials, in particular single-walled carbon nanotubes (SWCNTs), is a result of their fascinating structural, mechanical and electronic properties. SWCNTs are ideal systems for investigating funda- mental properties in one-dimensional electronic systems. Their chemical modification can pave the way to many applications, including the preparation of composite materials or the immobi- lisation of biological molecules as enzymes for electrochemical biosensors. [1,2] According to Hirsch, [3] the different approaches to functionalise SWCNTs are classified into five categories: (i) defect-group functionalisation, [4] (ii) covalent sidewall functio- nalisation, [5] (iii) noncovalent exohedral functionalisation with amphiphilic molecules [6] or surfactant, [7] (iv) noncovalent exohedral functionalisation with polymers [8] and (v) noncovalent endohedral functionalisation with, for example, C 60 . [9] The grafting procedure considered in our study falls in ‘covalent sidewall functionalisation’ category. We report here the functionalisation of SWCNTs through the electrochemical reduction of aryl diazonium salts, in a manner similar to that explored by Pinson. [10] The electrochemical functionalisation based on the electrogeneration of reactive radicals near the nanotube surface shows several advantages, compared with some other grafting methods such as clean and nondestructive chemical functionalisation or selective electrochemical modifica- tion of individual objects. However, very few electrochemical methods of functionalisation and characterisation of SWCNTs have been described so far in the literature. [2,11–13] The reason may be due to the difficulties in maintaining the nanotubes on the electrode surfaces. Bucky paper electrodes, formed by filtration of a suspension of carbon nanotubes, were usually used for the electrochemical characterisation of SWCNTs, although their fabrication led to random samples that offered high resistivities. [1,11] Therefore, SWCNTs thin film electrodes prepared by electrophoretic deposition (EPD) were used in our study, this technique having the advantages of short formation time, simple apparatus and suitability for electrochemistry measurements. In this article, four types of diazonium salts bearing different substituents on the aryl groups were used. The p-nitrophenyl diazonium (p-NPD) salt is the most frequently used of all the aryl diazonium salts in electroanalytical chemistry. [14–17] One reason consists in the ability to convert the nitro moiety to amine func- tionality that can further allow the coupling of biomolecules to electrode surfaces and hence biosensors preparation. [15] Thus, choosing this compound is relevant because understanding the grafting of this salt on SWCNTs electrode surfaces provides a guide to electroanalytical chemistry researchers on how to design these surfaces for biomolecule immobilisation. The grafting behaviour of p-NPD and another three synthesised p-substituted phenyl compounds (p-N-phenylaniline diazonium [p-PAD], p-tolyl diazonium [p-TD] and p-flourophenyl diazonium [p-FPD]) has been compared. This allowed us both to understand the general trends of the grafting behaviour and to establish the functionalisation conditions enforceable to all kinds of diazonium * Correspondence to: Luisa Pilan, Applied Physical Chemistry and Electrochem- istry Department, University Politehnica of Bucharest, 1 Polizu, 010737, Bucharest, Romania. E-mail: luisa_pilan@yahoo.com † Paper published as part of the ECASIA 2011 special issue. Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1 Polizu, 010737, Bucharest, Romania Surf. Interface Anal. (2012) Copyright © 2012 John Wiley & Sons, Ltd. ECASIA special issue paper Received: 23 August 2011 Revised: 16 November 2011 Accepted: 6 December 2011 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/sia.4830