Applications of Ion Tracks: Filtration, Sensing and Nanostructures
The uses of ion tracks all rest on one simple property: in a polymer film, a track forms wherever a charged particle has passed through. With the right treatment it can be turned into a pore of controlled shape. Several separate fields have grown out of this, from laboratory filtration to the dating of rocks. This page gives an overview and shows what filters, sensors and dating methods have in common.
Filtration Membranes and Particle Separation
A filtration membrane made from ion tracks acts like a sieve with a precisely defined pore size. Particles larger than the pore stay on the surface while the liquid flows through. That sets these membranes apart from fiber materials, where a dense, irregular web decides the path of the liquid. In the lab they are used for microfiltration, which means separating particles or cells from a liquid. A sample is split into a retained fraction and a filtrate. What counts here is that the pore size is set in advance.
Sensing with Nanopores
Sensors built around a single nanopore are another field. When a molecule travels through the pore, the ion current changes. The change is recorded as a short electrical current pulse.
Each pulse registers one event, so molecules can be watched one at a time. Geometry is the deciding factor: the pore shape, cylindrical or conical, influences the ion flow and is chosen to suit the purpose. Depending on how the track is treated, the same material can show different sensor properties. For that reason the pore shape sits at the center of any description of such sensors.
Track Detectors and Dating in Geology
Ion tracks are also used in nuclear physics. Polymer films can serve as nuclear track detectors, because the tracks of alpha particles or fission products become visible under a microscope after etching. Such a detector passively records what has passed through the material.
Geology relies on a similar mechanism. In minerals such as apatite and zircon, fission tracks are used to date rocks. The tracks form naturally, and researchers read them under a microscope. The reading principle is the same as with detectors made from polymer film.
Templates for Nanostructures
Making materials is another field of use. In the template method a metal is deposited inside the pores. After the polymer is dissolved, nanowires remain whose diameter is set by the pore. The template works like a casting mold on the nanoscale and passes its features on to the finished product.
All of these uses depend on the same step: a track becomes a pore of known size and shape. The pore can then be used to filter, measure, record or mold. For us, this pore is the common core of the whole field.