464320 High-Transconductance Ionic Transistors for Ionic Conductance Biosensing Platforms
The gating action is based on non-equilibrium ion transport through an ion-selective nanoporous membrane or nanochannel. Because of its charge selectivity, asymmetric concentration polarization occurs across such a membrane upon the application of a normal electric field. The cross current is controlled by the length of the low-conductance depletion front, which must have a strong correlation to the gating voltage to achieve high transconductance. Earlier efforts with nanoslot-microchannel designs have produced a gating-voltage independent depletion front length, either a very short one with a length-scale comparable to the nanoslot dimension [Yossifon et al, Phys Rev E, 81, 046301 (2010)] or a long one that extends all the way to the electrode [Chang et al, Annu Rev Fluid Mech, 44, 401-426 (2012)]. The former occurs when the nanoslot width is small compared to the external microfluidic channel width and the latter when the opposite is true.
To achieve a gating voltage-sensitive depletion front length, we configure the voltages of the draining current such that the coion flux balance for this draining current, necessary to maintain electro-neutrality, is only possible for a particular depletion length. The depletion front produces an unfavorable concentration gradient for this coion flux at the gating membrane, corresponding to a funnel-shaped energy surface. The size of the funnel and its depth, which control the local coion draining current, is sensitive to the gating voltage. The depletion front length must hence correspond to a specific value to ensure coion flux balance throughout the cross channel. We are hence able to produce a gating-voltage sensitive depletion front length which is insensitive to the external geometries.
This positive feedback mechanism due to a two-dimensional depletion front evolution controlled by two competing electrodes is confirmed by fluorescence imaging and numerical simulation. The membrane ionic transistor chip we have designed based on this principle is able to achieve a transconductance of more than 12 µS, which is the larger than previous reported for ionic transistors by a factor of 40. It corresponds to a voltage amplifier with an amplification factor of 10. The ion current amplification design will be shown to improve a particular nucleic acid ionic conductance membrane sensor by orders of magnitude with fM sensitivity and a tunable 3 orders dynamic range that can be shifted with the adjustable limit of detection. More details of the bipolar membrane sensor design will be presented in another talk in Session T3003.