Scanning force microscopy(SFM) has become a powerful tool for probing and manipulating nanometer scale objects on surfaces since the invention of the atomic force microscope. SFM probes can interact with surfaces through a wide spectrum of forces including Van der Waals(VDW), electrostatic and magnetic forces. To understand and exploit SFM, it is crucial to develop probe technologies that (i) deconvolute different interaction forces (ii) localize the tip-sample interaction at the nanometer or subnanometer length scales and (iii) enable active control of the cantilever. Here we report recent progress in addressing these critical issues. First, chemical force microscopy is shown to be sensitive to one single base-pair mismatch in DNA oligomers. Second, controlled growth of carbon nanotubes on AFM probes is demonstrated to give unprecedented high resolution probes. Last, magnetic force feedback spectroscopy(MFFS) has been used for accurate control of the cantilever position. These advances are producing a better understanding of molecular level chemistry and biology, and offer novel approaches to the fabrication of molecular scale structures and devices.
Synthetic oligonucleotides were used to functionalize CFM tips. The resulting probe is capable of recognizing complementary strands on surfaces. Single base-pair mismatches were introduced at the center of DNA duplexes. The presence of a single base pair mismatch leads to a ~30% drop in the unbinding force for a 14-base oligonucleotide. Elastic deformation of duplexes prior to separation into single strands was also studied. Combining the unbinding force and elastic response, CFM was able to distinguish the T-T "open" mismatch from "wobble" mismatches such as G-A and G-T, and fully complementary duplexes.
In addition to the enormous interests in using carbon nanotubes to build molecular devices, carbon nanotubes make potentially ideal tips for SFM because of the intrinsic small diameters, high aspect ratios and reversible buckling. We have recently developed a technique for growing individual carbon nanotube probe tips directly by chemical vapor deposition from the ends of silicon tips. The single nanotube tips have been used to image gold nanoparticles and biological macromolecules (Fig.1). Routine imaging using these tips shows excellent resolution as shown in the figure. It is also possible to functionalize the nanotube tips thus combining the high spatial resolution with chemical and biological sensitivity. Modified nanotube probes offer the possibility of subnanometer functional imaging of chemical and biological supramolecular complexes, and the creation of structures at the molecular scale.
Magnetic force feedback spectroscopy has been developed to overcome the mechanical instability intrinsic to AFM force curve measurements. The technique has far-reaching implications in both chemical physics and biophysics because a stable probe tip allows the complete free energy profile along a specific direction to be probed. Since MFFS provides control over the tip position independently from sample position, this technique should have significant impact on nanomanipulation and nanofabrication as well.
Figure 1. Imaging IgM macromolecules with a CVD nanotube tip at high resolution. Inset, high resolution image of IgM.
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