(237b) Mechanistic Study of Biosensing Using Ion Selective Membranes/Gels

We report a mechanistic study of biosensors using an ion selective environment (ISE) as a sensing element. ISE contains a fixed charge (either positive or negative) which enhances/suppresses the flux of counter-ions/co-ions. This imbalance of flux of ions gives rise to formation of a depletion and enrichment region in the electrolyte adjacent to the ISE under DC voltage. Under a sufficiently high electric field, hydrodynamic vortices develop in the depletion region resulting in the mixing with bulk of the electrolyte. This increases the ions concentration in the depletion region (limiting region) that results in the sudden increase of current (over-limiting region). Ion selective membranes, ion selective hydrogels or fluidic nanochannels are examples of these ion selective environments.

When the surface of such an ISE is covered with a layer of biomolecules having an opposite charge, the flux of all ions through this ion selective environment is drastically changed, quite analogous to the nonlinear effects of PN junctions on doped semiconductor IV characteristics. This change can easily be picked up as an electrical signal in measuring current-voltage characteristics, chronoamperometric characteristics or impedance spectrometry. This phenomenon is referred to as charge inversion. We will show that the charge inversion on the ISE results in: (i) suppression of the hydrodynamic vortices due to extended space charge and (ii) water-splitting reaction producing new ions responsible for the observed over-limiting current. The transition from “vortices-controlled” to “water-spitting controlled” over-limiting region is dependent on the degree of coverage of ISE with biomolecules. The detailed description of this new way of sensing technique allows the development of sensitive and specific DNA, RNA and protein biosensors.