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(a) Illustration of the electrostatic micromechanical actuator device structure utilized for the biosensor. The suspended beam can be electrostatically pulled down (or pulled in) by the applied gate (G) bias. At the pull-in voltage (VPI ), the dimple of the beam contacts the drain (D), thereby establishing a direct current path through between the drain and source (S). (b) A schematic illustration of the sensor where the oligonucleotides are immobilized on the gate electrodes. The intrinsic negative charges from oligonucleotides on the gate surface alter the amount of electrostatic force acting on the cantilever in accordance with a pre-charging mechanism, thereby changing VPI . (c)-(e) SEM images of the fabricated sensor: (c) a bird’s-eye-view, (d) a close-up side view at the gap between the suspended beam and the electrodes, (e) a magnified side view at the gap between the dimple of the beam and the drain electrode.
Simulation results for a conceptual demonstration of the sensor. (a) 3-dimensional schematic model designed for the computational simulation. (b) The displacement of the suspended beam towards the drain is plotted as a function of the applied gate voltage. VPI is the voltage when the dimple of the beam contacts the drain (d = 0.3 μm). (c) Simplified capacitance model for a comprehensive analysis of the detection mechanism.
(a) The experimental setup for electrical measurement of the sensor. (b) Typical measured transfer characteristics of the sensor for each step of the oligonucleotide binding process. The plot shifts to the right hand side (VPI is increased) under negative and positive voltage domains due to the intrinsic negative charges of the oligonucleotides.
(a) The modulation of the shift in pull-in voltage (ΔVPI ) for each status: the mechanical biosensor after being dipped in deionized water; after immobilization of the probe oligonucleotides; and after hybridization of the complementary target oligonucleotides. The specificity of the DNA hybridization is also investigated with non-complementary oligonucleotides. (b) Comparative results of ΔVPI of the sensor for various concentrations of complementary target oligonucleotides.
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