In the grounds around manufacturing you could spot leaks in waste or as a cheap way of detecting contaminants in drinking water. Specifically I'm thinking of the fracking fears and the groundwater contamination.
If they can be trained with more sophisticated particles you could get a cheap Cryptosporidium detector, which has been elusive as fuck.
If it is fast acting, and good sensitivity it could potentially be an early warning system.
"Quick kids the spinach sentry is fluorescing, get to the car, don't worry about grandma, she's already dead."
The only thing I would be worried about is introducing carbon nanotubes into the food cycle, slugs are bad enough without them excreting bullet proof armour.