Oh my... lol... Biodegredation is organic matter decaying into other materials with the aide of bacterias (I think that's right, someone correct me).
So if you are able to dectect the type of chemical bonding and atoms present in a substance with a PIC16F84, then you would be mighty rich and mighty popular.
If you want other ideas, how about some access control system, Biometric, RFID, Smart Card (It's already been done but you could improve?) Don't know really... Weather Station? A controller of some description?