COVALENTLY FUNCTIONALIZED NANOTUBES AS NANOMETRE-SIZED PROBES IN CHEMISTRY AND BIOLOGY

Carbon nanotubes combine a range of properties that make them well sui ted for use as probe tips in applications such as atomic force microsc opy (AFM)(1-3), Their high aspect ratio, for example, opens up the pos sibility of probing the deep crevices(4) that occur in microelectronic circuits, and the small effective radius of nanotube tips significant ly improves the lateral resolution beyond what can be achieved using c ommercial silicon tips(5). Another characteristic feature of nanotubes is their ability to buckle elastically(4,6), which makes them very ro bust while limiting the maximum force that is applied to delicate orga nic and biological samples, Earlier investigations into the performanc e of nanotubes as scanning probe microscopy tips have focused on topog raphical imaging, but a potentially more significant issue is the ques tion of whether nanotubes can be modified to create probes that can se nse and manipulate matter at the molecular level(7), Here we demonstra te that nanotube tips with the capability of chemical and biological d iscrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the ca rboxyl groups that are present at the open tip ends. We have used thes e modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to mea sure the binding force between single protein-ligand pairs. As carboxy l groups are readily derivatized by a variety of reactions(8), the pre paration of a wide range of functionalized nanotube tips should be pos sible, thus creating molecular probes with potential applications in m any areas of chemistry and biology.