Single Wall Carbon Nanotube Tips
| A remaining development that would go far in realizing the full potential of AFM in the Life Sciences is to make single wall carbon nanotube (SWNT) tips, 1-2 nm in diameter, available in quantity. |
lifeScan is highly compatible with nanotube tips and the two in combination would make 1 nm resolution available routinely to users.
Also important, carbon nanotubes interact relatively weakly with biomolecules and therefore contaminate less rapidly.
Partnership
In 2003, LifeAFM partnered with Molecular Nanosystems, Inc., a developer of improved technology for growing SWNTs by chemical vapor deposition on commercial AFM tips.
Molecular Nanosystems fabricated SWNTs for LifeAFM on stiff and soft cantilevers. Their length as grown however is several thousand nm and must be shortened to 50 nm or less to avoid degradation of lateral resolution by transverse thermal motion of the nanotube tip.
Electrical discharge shortening works reasonably well for stiff cantilevers but shortening remains a difficult problem for soft cantilevers. A new approach to the shortening problem is under development at LifeAFM.
SW Carbon Nanotube Grown In Situ on a Soft Silicon Cantilever
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Image was acquired in a Leo™ field emission SEM. The scale bar indicates 1000 nm. |
It is interesting to note that the free end of this ~2600 nm long SWNT appears blurred and thick due to vibration. At its base end at the silicon tip, the SWNT is 1-2 nm in diameter. By comparison, the silicon tip is 7-10 nm in diameter.
Results
lifeScan has imaged DNA in buffer with shortened SWNT tips on stiff and soft silicon cantilevers.
It is notable that the nanotube tips remain attached, although held in place only by van der Waals forces, because compressive force in lifeScan is minimal.
DNA imaged with a stiff cantilever, k=3 N/m, in a 31 nm field shows 3.3 nm spacing of the double helical backbone strands, but the DNA width is 25 nm due to the stiff cantilever’s high applied force.
Soft cantilever tips, k=~0.1 N/m, resolve the 3.3 nm helical repeat in 3 nm wide DNA, close to its 2 nm native width, when imaged in 135 nm fields. The addition of 1 nm to DNA’s native 2 nm width indicates the diameter of the SWCNT tip is 1 nm.
Shortening SWNTs Project
LifeAFM has a program underway to develop an alternate method for shortening SWNTs grown on AFM tips.
A successful method must be able to shorten some 20%, and preferably more, of the long nanotubes grown by CVD in order to keep SWNT tips affordable.
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