Positions for grad students (Diplomarbeit) and doctorate students (Dissertation)...

... are available (Diplomarbeiten und Dissertationen) and can be funded by one of our research projects. If you are interested, write to Clemens (dot) Heitzinger (at) univie or just drop by (C710).

Research interests

My research interest is applied mathematics for bio-nano-technology. We are currently working on the theory of field-effect biosensors.

What are field-effect biosensors?

A BioFET (biologically sensitive field-effect transistor) is simply a MOSFET (metal-oxide-semiconductor field-effect transistor) whose conventional gate structure is replaced by a biologically sensitive structure, usually a layer of immobilized molecules. The main idea is that the binding of target molecules to the immobilized probe molecules at the surface modulates the conductance of the semiconductor transducer so that the resulting conductance variation can be measured and thus enables detection. A DNAFET is a special kind of BioFET. The probe molecules are ssDNA (single-stranded DNA), and the target molecules are complementary ssDNA. The advantages of BioFETs are their direct, label-free, continuous, and real-time operation. It is expected that they will eventually be used in point-of-care applications.

Although the concept of BioFETs was proposed more than two decades ago, its realization has only started in the last decade. In recent years DNAFETs based on conventional MOSFET device structures as well as based on silicon-nanowires were fabricated and characterized.

A fundamental problem: the Debye length in an electrolyte solution

The fundamental problem of using semiconducting transducers in aqueous solutions was realized early: the Debye length of the partial charges of the analyte is a function of the ionic concentration of the solution, and it decreases rapidly as the ionic concentration increases. At the salt concentration of blood or serum the Debye length is less than 1nm. The figure shows the Debye length as a function of Na+Cl- concentration. Physiologically relevant concentrations correspond to a 150mM or 160mM solution. This implies that the probe molecules must be attached as close as possible to the oxide surface.

Modeling the sensors must take into account the electrostatics of the solution and the probe and target molecules, the binding efficiency of the probes and targets, and conductance of the solid-state transducer. Due to the small dimensions of the silicon-nanowires used in experiments (with diameters of about 20nm down to 5nm) quantum-mechanical effects are important for this kind of devices.

Simulation method

We have developed multi-scale models for the simulation of nano-plate and nanowire BioFETs in order to elucidate the functioning and to investigate the limits of the biosensors. The conductance variations that we have observed in our simulations so far are in qualitative and good quantitative agreement with published experiments. The simulations indicate that the conductance change observed is indeed a field-effect (and not, e.g., a leakage current).

References

See here or here.