(276f) Designing Ion Selective Membranes for Biosensing

A number of interesting phenomena occur at the interface of an ion selective environment (nanochannel or ion exchange membrane) and the surrounding electrolyte under DC or AC electric fields. These phenomena include formation of depletion and concentration regions around the nanoporous membrane or nanochannel, development of a space charge region with the length exceeding the length of Debye layer, development of electrically driven vortices, Warburg-like Electrical Impedance Spectroscopy (EIS) etc. Our efforts are aimed at using such ion selective environments to develop a highly specific and sensitive DNA/RNA biosensing platform. The biosensing is based on a charge inversion phenomenon that occurs when negatively charged DNA/RNA molecules hybridize with specific probes immobilized on an ion exchange membrane bearing a positive fixed charge. Expensive and bulky optical biosensing technologies can thus be replaced by miniature electronic circuits connected to on-chip nanoporous membrane sensors. Such nanoprobes are not grown or fabricated by expensive nanofabrication techniques, but are rather loaded on-site by patented dielectrophoretic assembly technologies or by standard sol-gel or UV curable chemistry. Ion depletion action by the nanoporous membranes controls the ionic strength around the sensor to ensure robustness to sample variability. The ion-depleted region also dielectrophoretically trap and concentrate the target molecules from the flowing sample to hybridize/dock with molecular probes functionalized onto the nanosensors. Instead of the classical electrochemical sensing technique, we utilize a much more sensitive conductance and impedance signature based on sensitivity of ion current across the nanoporous membrane to surface charge density change effected by hybridized molecules. The sensitivity involves a nonlinear correction to the conductance and is hence much more sensitive than EIS or other linear electrochemical sensing techniques.