Biosensor Applicationof CMOS Inverter and Ring Oscillatorwith Organic Field-Effect Transistors

Organic field-effect transistors (FETs) were fabricated for use in a biosensor that detects urea hydrolysis under urease. Here, a 60 nm thick 2,6-diphenylanthracene (DPA) thin film was used to prepare a p-channel FET that effectively detected this enzymatic reaction. The absolute drain current, |<i>I</i>_D|, of the transfer curve rapidly increased at any gate voltage, <i>V</i>_G, when a droplet of 0.1 M phosphate buffer solution (pH = 6.7 at 25 °C) containing 25 mM urea and 49 nM urease was placed on a different gate electrode (i.e., sensor electrode) than the <i>V</i>_G electrode. Thus, urea hydrolysis directly affected the value of <i>V</i>_G, enhancing |<i>I</i>_D|. The usage of high-<i>k</i> gate dielectric ZrO₂ in the FET device caused the absolute value of <i>V</i>_G, |<i>V</i>_G|, to decrease by ≈1.0 V in the transfer curve. The value of |<i>I</i>_D| at any absolute drain voltage |<i>V</i>_D| and |<i>V</i>_G| was plotted as a function of time and urea concentration [S], showing a rapid increase and reaching saturation within ≈1.0 min or up to [S] = 250 mM. Furthermore, we prepared the complementary metal oxide semiconductor (CMOS) inverter with thin films of DPA as a p-channel and <i>N</i>,<i>N</i>′-di-<i>n</i>-octyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C8) as an n-channel to detect urea hydrolysis. The logic threshold voltage <i>V</i>_TIC in the CMOS inverter increased by placing a droplet containing urea and urease on a different gate electrode than that delivering the input voltage <i>V</i>_in. Finally, a ring oscillator consisting of five CMOS inverters was fabricated to detect the variation in oscillation frequency throughout the reaction, revealing an unambiguous decrease in the frequency. The successful detection of urea hydrolysis is the first step toward the realization of a practical biosensor incorporating a CMOS inverter and ring oscillator.