Carbon Nanotube Tips: High-Resolution Probes for Imaging Biological Systems

Abstract

Herein, we report the first atomic force microscopy (AFM) imaging studies with carbon nanotube tips that address their potential to improve lateral resolution and to probe biological systems. Multiwall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) have been attached to the ends of single-crystal silicon cantilever tip assemblies and used to image amyloid â(1-40) derived protofibrils and fibrils by tapping mode AFM. Image analysis shows that the average resolution obtained with the nanotube tips is significantly better than that obtained with our best silicon tips and provides new insight into the structure and assembly mechanism of amyloid fibrils. The potential for imaging with molecular resolution using these tips is discussed. The feature resolution obtained by AFM is determined in large part by the size and shape of the probe tip used for imaging.1,2 Commercially available probes consist of microfabricated pyramids of Si or Si3N4 that have end radii of curvature as small as 10 nm but are often much larger.2,3 These tips place significant constraints on potential lateral resolution, and furthermore, the pyramidal shape restricts the ability of these tips to access narrow and deep features. Small cone angle carbon protrusions with radii of 10-30 nm deposited on the ends of conventional tips significantly improve depth but not lateral resolution.4 Recently, a potential breakthrough in probe technology was achieved by attaching MWNTs to the ends of Si tips.5 In this work, the cylindrical geometry of the MWNT tips was exploited to image a deep grating with excellent fidelity, although potential improvements in lateral resolution were not investigated. A typical scanning electron microscopy (SEM) image of a MWNT tip attached to a conventional single-crystal silicon cantilever tip assembly is shown in Figure 1.5,6 This MWNT tip extends ca. 1.8 μm from the end of the pyramidal Si tip to which it is attached. The diameter of the major portion of the nanotube tip, 100 nm, is much larger than a single MWNT and corresponds to a bundle of tightly packed tubes.7 The MWNT bundles attached to the Si probes have diameters of 75 ( 27 nm; attached SWNT bundles8 typically are smaller with diameters of 45 ( 8 nm. The high aspect ratio nanotube tips have obvious advantages for probing deep crevices and steep features.5 Additionally, it is known that nanotubes elastically buckle above a critical force.5,9 Buckling limits the maximum force applied to a sample,10 which can prevent damage to delicate organic and biological samples, and at the same time makes the tips very robust.9 To explore the resolution of these tips and their applicability to high-resolution imaging of biological samples, we have investigated amyloid-â 1-40 (Aâ40) fibrils produced in vitro. Aâ fibrils are the primary constituent of the insoluble core of amyloid plaques that are characteristic of Alzheimer’s disease.11 Previous AFM studies have shown that Aâ initially forms protofibril structures12,13 and subsequently larger fibrils;12-14 branching of fibrils has also been observed.13,14 A typical image15 of an Aâ fibril obtained with a MWNT tip (Figure 2) shows higher resolution than observed in previous AFM studies of the fibrils. Key features include (1) a fibril Y-branch (left arrow) with the