Progress Toward Carbon-Nanotube Arrays for Probing Cells

A process for deposition of presynthesized SWNTs holds great promise.

February 1, 2008

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Progress has been reported in research oriented toward the goal of fabricating arrays of interconnected single-walled carbon nanotubes (SWNTs) that could serve as probes for measuring localized events within living cells. As used here, "interconnected" signifies mechanically and electrically connected to patterned metal films (interconnections) that lead to contact pads, which, in turn, enable electrical connection to external electronic instrumentation. This research poses a high economic risk because it entails pushing several technological disciplines beyond their present limits. There is no previously reported combination of fabrication techniques and processes for producing carbon nanotubes that have the required properties at the required positions and orientations on patterned metal films.

In this Conceptual Process, an array of interconnected SWNTs would be deposited through a hole in an SiNx onto a patterned metal film. Only one SWNT is shown here for simplicity, but a typical device as envisioned would contain multiple SWNTs.

This research follows three parallel tracks: (1) photolithography and other processing to pattern interconnection metals on quartz substrates and seed metals (catalysts for growth of carbon nanotubes) on the interconnections, (2) fabrication of SWNTs and integration of the SWNT-fabrication process with the interconnection-fabrication process, and (3) for SWNTs that are in place in an array, developing strategies for chemically functionalizing the nanotubes for specific applications.

A process for fabricating the interconnections (see figure) was developed under the original assumption that vertically aligned SWNTs would be grown by chemical vapor deposition (CVD) in nanoscale holes (vias) formed at desired locations (by use of either electron beam lithography and reactive ion etching or focussed-ionbeam drilling) through a layer of silicon nitride (SiNx) covering the metal layers. It was found that the SiNx layer could not withstand the thermal cycles involved in CVD, making it necessary to abandon CVD.

The most significant accomplishment of the research effort thus far, prompted by the abandonment of CVD, was development of a process for depositing presynthesized SWNTs in the vias. In an experiment to demonstrate the feasibility of this process, examination by scanning electron microscopy revealed that all of the vias were only partially filled with SWNTs, and each of a significant number of vias contained only one SWNT (or a small bundle of SWNTs). The presence of SWNTs was verified by use of micro- Raman spectroscopy: the Raman spectra from selected areas containing vias matched those of pristine SWNTs used for the deposition. Finite-element modeling of the deposition process was performed to show that it should be possible to control both the number and placement of the SWNTs; this finding has broad implications for numerous potential applications in which SWNTs could be used.

A potential further benefit of the deposition of presynthesized SWNTs is that the SWNTs could be sorted according to their electrical properties, prior to deposition, so that only SWNTs having desired properties would be deposited. For example, metallic SWNTs could be separated from semiconducting SWNTs, and semiconducting SWNTs could be further sorted into narrow bandgap ranges suitable for specific applications.

Notwithstanding the promise of the deposition-of-presynthesized-SWNTs approach, there is still significant risk associated with fabrication and with measurement of cell signaling events. Problems of passivating deposited SWNTs, functionalization of SWNTs for specific applications, defining measurement requirements, and understanding interactions of SWNTs with cells during measurements have yet to be solved. The risk of this research may be justified by the potential for contributing to understanding of cellular dynamics and thereby contributing to medical advances.

This work was done by Reginald, C. Farrow of New Jersey Institute of Technology for the Air Force Research Laboratory.