Stable non-covalent functionalisation of multi-walled carbon nanotubes by pyrene–polyethylene glycol through p–p stackingw Jie Liu,* a Olivier Bibari, a Pascal Mailley, b Jean Dijon, c Emmanuelle Rouvie`re, c Fabien Sauter-Starace, a Patrice Caillat, a Franc¸oise Vinet a and Gilles Marchand a Received (in Montpellier, France) 31st July 2008, Accepted 20th November 2008 First published as an Advance Article on the web 15th December 2008 DOI: 10.1039/b813085j Carbon nanotubes (CNTs) are good candidates to construct nanostructured, implantable micro- electrodes since they are conductive materials and may increase the overall electrode surface area, and thus the signal/noise ratio. However, the adsorption of biomolecules on CNTs is well-known to lead to a surface passivation. In this context, a surface modification appears essential to overcome these limitations. In this paper, the non-covalent functionalisation of multi-walled carbon nanotubes (MWNTs) by pyrene–PEG molecules through p–p stacking is presented. We describe in the first part the functionalisation of MWNT powders in aqueous solution for which we obtained a stable dispersion of functionalised MWNTs. The stability of the non-covalently functionalised MWNT dispersion during 7 cycles of dialysis in H 2 O was studied by UV spectroscopy. The density of pyrene–PEG on the MWNT surface stabilised at about 4  10 11 molecules mm 2 after 4 cycles of dialysis (all the free pyrene–PEG molecules were removed by dialysis). Next, this non-covalent functionalisation of MWNT arrays on a substrate was examined. After the functionalisation, the increase of the MWNT wettability led to a 50-fold increase of the capacitance of the MWNT nanostructured electrode. Finally, we chose streptavidin, a well-known adhesive protein, as an example to test the efficiency of functionalised MWNTs towards preventing non-specific adsorption. The result shows that the presence of pyrene–PEG on the MWNT surface is indeed efficient. Introduction In recent years, researchers have developed systems to inter- face living cells, such as neurons, with electronics to create a brain–computer interface (BCI) system using micro-electrodes and so to record action potentials. 1,2 The use of electrodes with a nanostructured surface can be a way to: (i) make small diameter electrodes with large active surface areas, (ii) increase the signal intensity, (iii) increase the signal to noise ratio and consequently improve the electrode sensitivity. In this way, carbon nanotube (CNT) arrays can be good candidates to make nanostructured, electrically conductive and implantable micro-electrodes. Indeed, CNTs allow an increase in the over- all surface area leading to a higher signal and tend to reduce the electrode output impedance and increase its capacitance. Consequently, the signal to noise ratio is increased by limiting the thermal noise proportional to the interfacial impedance. 3 However, the design of implants with stable electrical proper- ties in long-term applications is still a challenge since the non-specific adsorption of some proteins can generate the formation of fibroblasts around the electrodes, which degrade their electrical performance over time. A functionalisation of the CNTs may be an efficient way to prevent the adsorption of these bio-molecules. Polyethylene glycol (PEG) is a biocompatible polymer which has been widely used in the field of biology to create hydrophilic surfaces and to prevent the non-specific adsorp- tion of bio-molecules on the surface. The functionalisation of CNT-covered electrodes by PEG chains may be an efficient way to prevent the non-specific adsorption of some bio- molecules on the surface. Many works have been reported on the functionalisation of single-walled carbon nanotubes (SWNTs) by PEG chains. For example, after oxidation of SWNTs, the generated carboxylic groups have been modified in acyl chloride groups which can react with hydroxyl- terminated PEG to give PEG-chain covalently-functionalised SWNT bundles. 4,5 PEGylated individual SWNTs have also been obtained through doping by lithium to undo the bundles before reacting with PEG chains. 6 However, the covalent functionalisation can introduce defects on the sidewall of CNTs, which will reduce their electrical conductivity. 7 To overcome this problem, several works have been described on the non-covalent functionalisation of CNTs by PEG chains: (i) adsorption of PEG chains onto SWNTs assisted by Triton X-100 or Triton X-405; 8 (ii) adsorption of PEG–phospholipids on the SWNTs through hydro- phobic interaction; 9 (iii) functionalisation of SWNTs by a CEA-LETI-DTBS Minatec, 17 rue des Martyres, 38054 Grenoble, France. E-mail: jie.liu@cea.fr; Tel: +33 (0)4 38 78 21 46 b CEA-DSM-DRFMC, 17 rue des Martyres, 38054 Grenoble, France c CEA-LITEN-LCH, 17 rue des Martyres, 38054 Grenoble, France w Electronic supplementary information (ESI) available: NMR and MS spectrum of pyrene–PEG; calibration curve of MWNTs; TGA curve of MWNT-1 after 7 cycles of dialysis; XPS C 1s results of MWNTs/Si functionalised with only MeO-PEG-NH 2 ; XPS results of MWNTs/Si and f-MWNTs/Si before and after the adsorption of streptavidin. Angle contact measurements. See DOI: 10.1039/b813085j This journal is  c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1017–1024 | 1017 PAPER www.rsc.org/njc | New Journal of Chemistry