Highly-ordered covalent anchoring of carbon nanotubes on electrode surfaces by diazonium salt reactions.

Since their discovery in 1991, carbon nanotubes (CNTs) have attracted growing interest in view of their unique mechanical and physicochemical properties, and have successfully been applied in microelectronics such as memory devices, switches, supercapacitors, and in the biomedical area as drug-delivery devices and biosensors. The oriented assembly of CNTs on an electrode surface offers further possibilities with respect to both electrochemical biosensors because of the high electron-transfer rate along the tube, and the attachment of redoxactive molecules (or redox enzymes) to the ends of the CNTs. Electrodes modified with vertically aligned CNTs have been accomplished through direct growth or self-assembly of CNTs. Although different procedures can be followed to achieve a covalent attachment of molecules, the most applied method to anchor CNTs is perhaps that based on the formation of amide bonds from the reaction between the amines located on the modified electrode and the carboxylic groups at the ends and side-wall defects of the nanotubes. Since 1992, when an aryl diazonium salt was used for the covalent functionalization of a carbon electrode, this approach has been applied to a variety of surfaces such as carbon-based materials, metals, and semiconductors. The method has also been used for the derivatization of CNTs, that is, multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs). To the best of our knowledge, no report of CNTanchoring on a surface by the use of diazonium salt reactions has been published to date. The aim of the present work is to develop a new approach based on diazonium salt reactions that provides a simple, stable, and well-organized assembly of CNTs for a wide range of substrates. To chemically modify an electrode surface, a diazonium salt is reduced to the corresponding aryl radical, which binds to the surface rapidly. The process is promoted by a variety of experimental conditions, and can even be used in combination with click chemistry. Herein, the procedures adopted for the derivatization of a glassy carbon electrode (GCE) are shown in Figure 1. For the immobilization of unfunctionalized SWCNTs on the chemically modified GCE surface, the p-nitro diazonium ions, which are formed in situ from p-nitroaniline (see Experimental Section for details), bind to the CGE surface (Figure 1a). The amine groups obtained by electrochemical reduction of nitro groups are converted into the corresponding diazonium functionalities, which covalently bind SWCNTs. The incorporation of phenyl groups to form a mixed monolayer, which is anticipated to prevent the diazo coupling, a reaction that occurs in a densely packed p-aniline monolayer. Indeed, the electrophilic attack of the diazonium functionality at the ortho position of a vicinal p-aniline (diazo coupling) or to the vicinal amine group, avoids CNTs immobilization. Alternatively, SWCNTs are first functionalized with p-nitrobenzene groups, then the amines formed by reduction of the nitro groups are converted into diazonium ions, which covalently bind SWCNTs on a clean GCE surface (Figure 1b). The orientation of SWCNTs attached to a GCE surface was investigated by scanning electron microscopy (SEM). A typical SEM image, taken from a 458 tilted view, is shown in Figure 1c; the image clearly shows that SWCNTs are arranged vertically on the surface. The obtained arrangement is independent of the reaction route. TEM images clearly show that SWCNTs are perpendicularly oriented with only one end anchored on the CGE surface (Figure 1d,e). The SWCNTs attached to the GCE were 80–120 nm in length, with a diameter of 2–8 nm. Cyclic voltammograms (CVs) were recorded in a solution of K3[Fe(CN)6] in water (2 10 3 molL ) using bare GCEs or GCEs modified with SWCNTs according to the procedure described above (Figure 2). The assembly of CNTs on the electrode leads to a 1.5-fold increase of the current intensity, which results from the increased active electrode surface; the double-layer capacitance increases accordingly. Sonication of the modified electrode slightly decreases the current intensity, thus suggesting that some SWCNTs may be adsorbed on the surface. Also, the electrode modification slightly improves the reversibility, as indicated by the peak separation values. The electron-transfer rate constants (kET) determined for both bare and SWCNT-modified GCEs, were calculated from the peak separation assuming the [Fe(CN)6] 3 /4 diffusion coef[*] Prof. T. Ferri, V. Paolini Department of Chemistry, “Sapienza” University of Rome Piazzale Aldo Moro 5, 00185 Rome (Italy) E-mail: [email protected]