Selective ion-sensing with membrane-functionalized electrolyte-gated carbon nanotube field-effect transistors.

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.

Random CNT network and regioregular poly(3-hexylthiophen) FETs for pH sensing applications: a comparison.

BACKGROUND: Nowadays, there is a tremendous need for cheap disposable sensing devices for medical applications. Materials such as Carbon Nanotubes (CNTs) and regioregular P3HT are proven to offer a huge potential as cost-effective and solution processable semiconductors for (bio)sensing applications. METHODS: CNT-based field-effect transistors (CNT-FETs) as well as regioregular P3HT-based ones (P3HT-FETs) are fabricated and operated in the so-called electrolyte-gated configuration. The active layer of the P3HT-FETs consists of a spin-coated regioregular P3HT layer, which serves on one hand as the active sensing element and on the other hand as passivation layer for the transistor's metal contacts. The active layer of the nanotube transistors consists of a randomly distributed single walled CNT-network (>90% semiconducting tubes) deposited from a CNT-ink solution by spin-coating. RESULTS: We compare both devices concerning their stability in aqueous environment and their response when exposed to buffers with different pH. We found that even if P3HT shows lower stability its pH sensitivity is reproducible even after long-term measurements. CONCLUSION: CNT-FETs and P3HT-FETs offer different advantages and drawbacks concerning their stability in solution and the ease of fabrication. A discussion of their different sensing mechanisms as well as sensitivity is given here. GENERAL SIGNIFICANCE: This work reports on fast and cost-effective production of solution processable thin-film transistors based on carbon nanotubes and regioregular P3HT and demonstrates their suitability as reliable pH sensors. This article is part of a Special Issue entitled Organic Bioelectronics - Novel Applications in Biomedicine.

Back-gated spray-deposited carbon nanotube thin film transistors operated in electrolytic solutions: an assessment towards future biosensing applications.

We report on back-gated carbon nanotube (CNT) thin-film transistors (CNTFETs) and their performance in electrolytic solutions to assess their suitability for future application as biosensors. Spray-deposited CNT networks were used as the sensitive active layer which offers the opportunity for integration on flexible sensing platforms at low-cost. We characterized the transistors' behavior in electrolytes by analyzing the response to different KCl solutions and buffers over a wide pH range. We observed a linear response of the drain current upon changing the pH in low molarity buffers and obtained an exponential dependence on the salt concentration of the electrolyte. These responses can be attributed to electrostatic gating effects that go along with shifts in the threshold voltage. Even though a lot of effort has been put into understanding the biosensing mechanism a detailed theory is still missing. Back-gated CNTFETs operated in electrolytic solutions can be a further tool to investigate and clarify the existing unsolved phenomena.

Fabrication and electrical characterization of a pore-cavity-pore device.

We present a solid state nanopore device structure comprising two nanopores which are stacked above each other and connected via a pyramidal cavity of 10 fl volume. The process of fabrication of the pore-cavity-pore device (PCP) relies on the formation of one pore in a Si(3)N(4) membrane by electron beam lithography, while the other pore is chemically etched into the Si carrier by a feedback controlled process. The dimensions of the two nanopores as well as the cavity can be adjusted independently, which is confirmed by transmission electron microscopy. The PCP device is characterized with respect to its electrical properties, including noise analysis and impedance spectroscopy. An equivalent circuit model is identified and resistance, capacitance, and dielectric loss factors are obtained. Potential and electric field distributions inside the electrically biased device are simulated by finite element methods. The low noise characteristics of the PCP device (comparable to a single solid state nanopore) make it suitable for the stochastic sensing of single molecules; moreover, the pore-cavity-pore architecture allows for novel kinds of experiments including the trapping of single nano-objects and single molecule time-of-flight measurements.