Covalently-Functionalized Single-Walled Carbon Nanotube Probe Tips for Chemical Force Microscopy

Abstract

Herein, we report the first covalent modification of singlewalled carbon nanotubes 1 (SWNTs) to create high-resolution, chemically sensitive probe microscopy tips. Carboxylic acid groups at the open ends of SWNTs were coupled to amines to create additional probes with basic or hydrophobic functionality. Force titrations recorded between the ends of the SWNT tips and hydroxy-terminated self-assembled monolayers (SAMs) confirmed the chemical sensitivity and robustness of these SWNT tips. In addition, images recorded on patterned SAM and partial bilayer surfaces have demonstrated chemically sensitive imaging with nanometer-scale resolution. These studies show that well-defined covalent chemistry can be exploited to create functionalized SWNT probes that have the potential for true molecular-resolution, chemically sensitive imaging. Recent atomic force microscopy (AFM) studies of amyloid fibrils showed 2 that multiwalled carbon nanotube (MWNT)3 tips and especially SWNT tips provide significantly better lateral resolution compared with commercial Si and Si3N4 AFM tips. Indeed, the ca. 0.5-nm radii of individual SWNTs indicate that such tips could enable true molecular-resolution imaging. 2 Sharp tips offer advantages for structural imaging but do not necessarily provide information about the functionality central to binding and reactivity in chemical and biological systems. Such chemical information can be obtained by using functionalized probes in chemical force microscopy (CFM). In the past, we 4-6 and others 7,8 have used SAMs to functionalize conventional tips and have used CFM to measure intermolecular forces and map chemical functionality. We have also recently shown that the ends of MWNT tips can be covalently modified to present chemically and biologically active functionality. 6 However, covalent functionalization of the ends of SWNTs, which offer the highest spatial resolution possible in force microscopy, has not been demonstrated