BIOSENSING MECHANISM OF FLOATING GATE TRANSISTORS
UTILIZING ELECTROLYTE DIELECTRICS
Scott P. White, Kevin D. Dorfman, C. Daniel Frisbie,
Department of Chemical Engineering and Materials Science
University of Minnesota, Minneapolis, MN 55455, USA
In a recent publication,1we reported a new label-free biosensing strategy based on floating gate transistors (FGTs) that uses only solution processed, electrolyte dielectrics rather than vacuum deposited solid dielectrics. The device uses the benchmark organic polymer poly(3-hexylthiophene) or P3HT, in conjunction with an ion-gel that permits low-voltage operation. Our previous experiments demonstrated high sensitivity for DNA hybridization, which we hypothesized was due to changes in the surface potential upon binding of the analyte. In this presentation, we show that our device is sensitive to changes in both the interfacial capacitance and the surface potential of the active sensing area. This new insight allows the electrolyte dielectric FGT strategy to be readily applied to non-charged analytes (in contrast to previous FGT biosensors) while also simplifying the design of capture molecules.
To demonstrate the ability to sense interfacial capacitance, we designed a set of experiments where we systematically changed the interfacial area between the gold electrode (the sensing area) and the ion-gel. The total interfacial capacitance can thus be tuned and correlated to: (1) the current through the semiconductor and (2) the voltage when the semiconductor spikes in conductivity, VT. When the interfacial area adjacent to the semiconductor is altered, we show that the device response can be predicted by a simple model of two capacitors in series. However, when the interfacial area of the sensing surface (decoupled from the semiconductor) is altered it results in large changes in output due to charge storage in the floating gate. These experiments and the corresponding scaling laws that we have developed highlight differences between operating with electrolyte over solid dielectrics and outline the capacitive component of the sensing mechanism.
To demonstrate the different sensing responses for capacitance and surface potential, we adjusted the work function of the floating gate electrode by selectively immobilizing self-assembled monolayers (SAMs) onto a single electrode/ion-gel interface through microfluidic channels. The SAMs were formed through thiol/gold chemistry but with a varied functional group chemistry that changes the work function from -250 to +320 meV when measured in an inert environment. The effective VTof the semiconductor changes significantly when SAMs are formed with (1) a direction highly dependent on the orientation of the SAM but (2) largely independent on the functional group chemistry. These results are explained using a simple thermodynamic model with the high ionic strength of the ion-gel (~3.8 M) screening the electric field of the SAM functional group dipole.
- White, S.P.; Dorfman, K.D.; Frisbie, C.D. Label-Free DNA Sensing Platform with Low-Voltage Electrolyte-Gated Transistors. Anal. Chem. 2015, 87 (3), 1861-1866