Ultra-sharp metal and nanotube-based probes for applications in scanning microscopy and neural recording,
by I. Borzenets, I. Yoon, M. Prior, B. R. Donald, R. Mooney, and G. Finkelstein.
Journal of Applied Physics 2012; 111(7):74703-747036.
(doi: 10.1063/1.3702802.)

Image: Top-left: Schematic of the setup used for pulling a nanotube fibril from solution. A seed metal wire that is sharpened to ~100 nm tip radius is attached to a motor-controlled stage and is dipped into a suspension of dispersed nanotubes. A ring counter-electrode is positioned right below the surface level of the solution. A 1-20 MHz, 1-10 V voltage signal is applied between the seed wire and counter-electrode. The wire is slowly pulled up at a rate of 40 microns/sec. The fibril is truncated with a sharp tip by ramping the voltage to zero. Bottom: SEM micrograph of a nanotube fibril (thick diagonal line) attached to a metal tip (visible at the left). The fibril length can be controlled to range between 50 microns and 5 mm. Top-right: The very tip of this fibril has a radius of 20 nm, but probes with a single nanotube at the tip are not uncommon and have radii down to a few nm.

Summary of paper: The techniques of Scanning Probe Microscopy (such as Scanning Tunneling Microscopy and Atomic Force Microscopy) require extremely sharp probes that must be robust so that they are not easily damaged when touching the surface. In many cases, the probes must also be conductive in order to make electrical contact to a sample being measured. Fortunately, the scanned surfaces are usually smooth (nearly atomic smoothness for STM and typically less than 100nm roughness in AFM applications), meaning that the shape of the probe several hundred of nanometers beyond the tip apex is not crucial.

In the field of cell biology and electrophysiology, ultra-sharp probes are needed to perform intra-cellular recordings of neural signals. Unlike scanning microscopy, a probe of about one hundred nanometers in diameter at the tip is sufficient to penetrate the cell membrane. However, the probes in neural applications must be conductive at the apex and isolated everywhere else. They must also be compatible with the biological environment, otherwise they will get fouled or possibly poison the cell.

This paper is a collection of methods for making probes that have a tip diameter as low as several nanometers, borrowed from the scanning probe microscopy for the potential use in neurophysiology. Metallic probes are made from wires of two different materials, each having their advantages and disadvantages: Tungsten and Platinum/Iridium. Further, carbon nanotube fibrils may be attached at the end of the metal probes, thus improving the probe robustness and bio-compatibility. We demonstrate how a carbon nanotube (CNT) fibril can be grown at the end of the metal probes. These fibrils probes are straightforward to manufacture, very robust, conductive and bio-compatible. Finally, two methods for electrically isolating the probes are shown. The resulting probes are suitable to perform in vivo measurements, and are light enough to be successfully mounted on MEMS actuators as a first step to MEMS-based neural probe arrays. While the probes fabricated in this paper are geared towards either scanning probe microscopy or neural probing, they may be easily adapted for other uses.