Aqueous sol-gel encapsulation of genetically engineered Moraxella spp. cells for the detection of organophosphates

An all-aqueous sol-gel method for encapsulation of bacterial cells in porous silicate matrices towards the development of a biosensor is described. The sol-gel encapsulation of cells is achieved at room temperature and neutral pH. Furthermore, use of sodium silicate as precursor avoids generation of alcohol that can be detrimental to cells in contrast to the traditional alkoxide sol-gel encapsulation process. Moraxella spp. cells engineered to express recombinant organophosphorus hydrolase (OPH) on the cell surface were encapsulated and OPH enzymatic activity was measured for paraoxon hydrolysis. Kinetic parameters (Km and Vmax) as well as pH behavior of surface-expressed OPH were determined to evaluate the effect of encapsulation. Cells encapsulated by the sodium silicate method displayed higher activity retention compared to those by the traditional alkoxide process. Time-course studies over a 2-month period indicate that immobilization through the sodium silicate process led to a reduction in activity of ∼5% as compared to ∼30% activity reduction in case of free cells in buffer indicating that immobilization leads to stabilization, a key parameter in biosensor development.