Abstract: Field-effect nano-biosensors (or BioFETs, biologically sensitive field-effect transistors) have recently been demonstrated experimentally and have thus gained interest as a technology for direct, label-free, real-time, and highly sensitive detection of biomolecules. The experiments have not been accompanied by a quantitative understanding of the underlying detection mechanism.
In recent work we have been developing models and simulation tools for these field-effect sensors to explain their functioning, to investigate their limits, and to improve their design.
The modeling of field-effect biosensors poses a multi-scale problem due to the different length scales in the sensors: the charge distribution and the electric potential of the biofunctionalized surface layer changes on the Angstrom scale, whereas the exposed sensor area is measured in micrometers squared. We developed multi-scale models for the electrostatics of nanowire sensors, of nanoplate sensors, and of sensors of arbitrary geometry by homogenization of the biofunctionalized boundary layer. The resulting interface conditions depend on the surface charge density and dipole moment density of the boundary layer. The multi-scale model can be coupled to any charge transport model and hence makes the self-consistent quantitative investigation of the physics of field-effect sensors possible.
We present results from the simulation of charge transport in a nanowire biosensor to elucidate the influence of the surface charge density and the dipole moment density on the conductance of the semiconductor transducer. The numerical evidence shows the conductance varies exponentially as a function of both charge and dipole moment. Therefore the dipole moment of the surface layer must be included in biosensor models. The conductance variations observed in experiments can be explained by the field effect and they could be caused by a change in dipole moment alone.
We have also performed Monte-Carlo simulations and Poisson-Boltzmann calculations of the charge distribution around biomolecules at charged surfaces to investigate the microscopic length scale. These simulations are the first quantitative explanation of the functioning of BioFETs by a model that does not contain any fitting parameters.
Finally we present a simulation study where it is shown that by introducing an electro-diffusion current flow in an electrolyte, the electrostatic screening of biomolecules can be significantly suppressed. An improvement of the sensed signal strength by a factor of more than 10× is observed. Therefore the screening-induced performance limits of silicon-nanowire biosensors can be overcome based on such an operation principle.
The results have been done in collaboration with Norbert Mauser, Christian Ringhofer, Robert Dutton and Yang Liu.