Modeling of suspended vs. immobilized whole-cell amperometric biosensors

Here we present a simple 1D modeling of an electrochemical cell integrated with viable bacterial cells expressing intracellular enzyme which responds to added substrate and the generated product is secreted and oxidized on electrode's surface. The effect of bacteria distribution was analysed comparing two cases: 1. Suspended cells, assuming equal distribution and homogenous response and 2. Cells immobilized on beads attached to electrode's surface, forming an inhomogeneous product generation in narrow region near the electrode. The model shows that when the total number of sensing cells is kept constant, it is preferred to locate them in close proximity to the working electrodes. Although the model is an approximation, since the beads form a 3D structure where the transport is via the gaps between the beads, the 1D model explains well the main experimental results as was shown by fitting to COMSOL Mulyiphysics™ simulations. Assuming that the diffusion of the products towards the electrode is the dominant factor and bacteria's response is independent of its immobilization and density, we showed that as the total population gets close to the electrode the response time becomes faster, while the long-time electrochemical current reaches the same value, which depends on the total number of bacterial cells.