The two brothers, of Egyptian origin, "had tutorials effectively indicating how to make the poison based on ricin," Collomb told broadcaster BFMTV.
Authorities tracking extremist activity on the internet had come across their plot when they used the Telegram messaging app, Collomb added.
The news came six days after a man killed one person and injured five in a series of knife attacks in central Paris, claimed by Daesh.
More than 230 people were killed in attacks, mostly claimed by Daesh, in France in 2015 and 2016. Eight more have died in smaller attacks in the last two years.
Authorities say that more than 50 attacks have been foiled since early 2015.