Effective diffusion coefficient, Df

Dependent upon the nature of the immobilized cells and the lipophilicity of the mediator molecule, the reduced mediator may be formed at redox active sites on the outside of the plasma membrane or at sites within the 

Consequently, the ratio of the diffusion coefficient in bulk aqueous solution and the effective diffusion coefficient in the biological layer is likely to be large (i.e., D Df 1). The actual magnitude will depend on the size, shape, membrane structures, coverage, and packing arrangement of the immobilized cells. 

Redox mediated whole cell biosensors for toxicity assessment 

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