1. A method of biosensor-based detection of toxins, comprising the steps of: providing for an effective fluorescent signal level by concentrating a plurality of photosynthetic organisms in a fluid to be analyzed into a concentrated region using biased AC electro-osmosis; obtaining a measured photosynthetic activity of said photosynthetic organisms in said concentrated region, wherein chemical, biological or radiological agents reduce a nominal photosynthetic activity of said photosynthetic organisms, and determining a presence of at least one of said chemical, biological or radiological agents, or precursors thereof, in said fluid based on said measured photosynthetic activity of said concentrated plurality of photosynthetic organisms.