Microcystin-LR, produced by cyanobacteria, or blue-green algae is suspected to cause liver damage and possibly liver cancer. It's been suspected to have caused mass poisonings going back to early human history.

The paper infused with carbon nanotubes is small (about the size of a home pregnancy test kit), inexpensive enough for frequent use in developing countries (not to mention water treatment plants everywhere), can be adapted to detect other toxins. What's not to like?

http://www.ns.umich.edu/htdocs/releases/story.php?id=7481