As a first step in the development of an optical biosensor for the detection of organophosphorus compounds, we describe the immobilization of conjugates of enzyme (AChE) and pH-sensitive fluorophore (SNAFL-1) within the PEG hydrogel microparticles via a dispersion photopolymerization. The fluorescent response of the PEG microparticles containing AChE-SNAFL conjugates showed that AChE-SNAFL conjugates were successfully encapsulated within the microparticles and that AChE was still active after the encapsulation procedure. Using a quantitative analysis of enzyme activity, we found that 70% of AChE activity was maintained after conjugation and encapsulation. A leaching test showed that there was no significant AChE leaching into the surrounding media. These results indicate that AChE-SNAFL conjugates could be encapsulated within PEG hydrogel microparticles without a significant loss of the enzyme activity and that these microparticles could potentially be used as optical biosensors for the detection of organophosphorus compounds. (C) 2009 Elsevier B.V. All rights reserved.