In this work the ion-selective response of an electrolyte-gated carbon-nanotube field-effect transistor (CNT-FET) towards K(+), Ca(2+) and Cl(-) in the biologically relevant concentration range from 10(-1) M to 10(-6) M is demonstrated. The ion-selective response is achieved by modifying the gate-electrode of an electrolyte-gated CNT-FET with ion-selective membranes, which are selective towards the respective target analyte ions. The selectivity, assured by the ion-selective poly(vinyl chloride) based membrane, allows the successful application of the herein proposed K(+)-selective CNT-FET to detect changes in the K(+) activity in the μM range even in solutions containing different ionic backgrounds. The sensing mechanism relies on a superposition of both an ion-sensitive response of the CNT-network as well as a change of the effective gate potential present at the semiconducting channel due to a selective and ion activity-dependent response of the membrane towards different types of ions. Moreover, the combination of a CNT-FET as a transducing element gated with an ion-selective coated-wire electrode offers the possibility to miniaturize the already well-established conventional ion-selective electrode setup. This approach represents a valuable strategy for the realization of portable, multi-purpose and low-cost biosensing devices.

In this work we fabricate and characterize field-effect transistors based on the solution-processable semiconducting polymer poly(3-hexylthiophene) (P3HT). Applying two independent gate potentials to the electrolyte-gated organic field-effect transistor (EGOFET), by using a conventional SiO(2) layer as the back-gate dielectric and the electrolyte-gate as the top-gate, allows the measurement of the electrical double layer (EDL) capacitance at the semiconductor-electrolyte interface. We record the transfer curves of the transistor in salt solutions of different concentration by sweeping the bottom gate potential for various constant electrolyte-gate potentials. A change of the electrolyte-gate potential towards more negative voltages shifts the threshold voltage of the bottom-gate channel towards more positive back-gate potentials, which is directly proportional to the capacitive coupling factor. By operating the EGOFET in the dual-gate mode, we can prove the dependency of the EDL capacitance on the molarity of the electrolyte according to the Debye-Hückel theory, and additionally show the difference between a polarizable and non-polarizable electrolyte-gate electrode. With the experimentally obtained values for the EDL capacitance at the semiconductor-electrolyte interface we can model the electrolyte-gate transfer characteristics of the P3HT OTFT.

We demonstrate the ion-selective response of an electrolyte-gated carbon nanotube network based field-effect transistor fabricated on a flexible polyimide substrate. Selective response towards the two prominent second messengers for cellcell communication, namely K + and Ca 2+ is demonstrated by modifying the carbon nanotube network with different polymeric ion-selective membranes. The sensing mechanism relies on the transduction of the ionic signal in an electrical one due to an ionactivity dependent change of the membrane potential at the membrane/electrolyte interface, which leads to a change in the effective gate-potential affecting the charge transport in the semiconducting channel. These sensors can be successfully used to selectively detect concentrations of primary ions down to a concentration in the μM range even in solutions with a highly concentrated background of interfering ions. Our approach allows the realization of low-cost, flexible, portable and multipurpose biosensing devices.Index Terms-carbon nanotubes, electrolyte-gated field-effect transistor, flexible, ion-selective membrane, ion-sensitive fieldeffect transistor

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We report on the reversible detection of CaptAvidin, a tyrosine modified avidin, with single-walled carbon nanotube (SWNT) field-effect transistors (FETs) noncovalently functionalized with biotin moieties using 1-pyrenebutyric acid as a linker. Binding affinities at different pH values were quantified, and the sensor’s response at various ionic strengths was analyzed. Furthermore, protein “fingerprints” of NeutrAvidin and streptavidin were obtained by monitoring their adsorption at several pH values. Moreover, gold nanoparticle decorated SWNT FETs were functionalized with biotin using 1-pyrenebutyric acid as a linker for the CNT surface and (±)-α-lipoic acid linkers for the gold surface, and reversible CaptAvidin binding is shown, paving the way for potential dual mode measurements with the addition of surface enhanced Raman spectroscopy (SERS).