Single-wall carbon nanotubes (SWNTs) are attractive for sensor applications especially due to their large surface-to-volume ratio [1] . Pristine semiconducting SWNTs have been successfully used to detect gas molecules such as NH 3 , NO 2 , oxygen [2,3] as well as humidity [4] , via changes in electrical conductivity. Furthermore, the possibility to use water with or without an electrolyte as a gating medium [5,6] has sparked interest in the realization of SWNT-field effect transistors (FETs) for detecting molecules such as ammonia in solution. [7] Towards the aim of sensing specific molecules in solution, SWNTs have been chemically modified, in most cases through non-covalent functionalization.[8] Most of the devices demonstrated until now, rely in one way or another on the analyte-dependent gating mechanism of semiconducting SWNTs. While this imparts good sensitivity to the devices, they have the disadvantage that parasitic charges on the substrate have a strong influence on sensor stability and selectivity. Here, we present a novel route to chemical sensors which uses metallic SWNT as the starting basis. Our strategy comprises the covalent attachment of analyte-sensitive functional groups in controlled density to individual contacted metallic SWNTs. The attached moieties function as scattering sites, whose effect on charge transport through the tubes is modulated by the analyte concentration.In this study, we demonstrate a pH sensor using this principle, where the sensitivity to pH is attained by covalent attachment of amino-substituted phenyl radicals to individual metallic SWNTs via gentle electrochemical modification (ECM). The low functionalization degree allows for minimal destruction of the p-conjugated framework, ensuring that the tubes show sufficient conductivity after modification. Electrochemistry represents an excellent tool for nanotube functionalization, which enables for instance exclusive modification of the metallic tube fraction of a mixture of metallic and semiconducting SWNTs. [9] Moreover, preferential decoration of defect sites along the tubes with metal clusters has been achieved by application of a low overpotential within metal salt solutions.[10] Here, we demonstrate that the density of moieties covalently attached to a SWNT can be effectively controlled by a series of electrochemical couplings.Metallic SWNTs were identified by their weak (< 2) back-gate dependence ratio (bGDR), defined as the ratio of the maximum resistance at a back-gate voltage of V bG = + 5 V to the minimum resistance at V bG = À5 V. Figure 1 displays the back gate-
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