(Invited) Carbon Nanotube Field Effect Transistor for Biological Detection in Liquids
Authors: Plank Natalie Olivia Victoria , Zheng Han Yue , Alsager Omar A , Hodgkiss Justin Mark 
Published: February 2020

Carbon nanotube (CNT) network films are an effective semiconducting platform for field effect transistor (FET) device applications. Their desirable properties include simple reproducible fabrication procedures1 with performances that are maintained during measurements in aqueous environments.2It is also well known that CNTFETs often show strong electronic responses to changes in their surrounding environment. Due to this one of the key challenges with electronic sensing is the reliable detection of a specific target. The emergence of synthetic receptors such as DNA aptamers which bind to targets with high selectivity3 has stimulated new opportunities for CNTFET biosensors.4 These short strands of DNA can be generated via a synthetic evolution process to bind with specificity for different types of targets including cells, proteins, viruses and small molecules.5 Once the correct sequence for a given target is discovered, DNA aptamers can be synthesized and readily functionalised for integration in biosensor devices. Figure 1 shows a schematic of a CNTFET biosensor, where the CNTs are directly functionalized with aptamers, and the sensor is operated in a liquid buffer gated through an Ag/AgCl electrode. The source and drain electrodes are encapsulated with an insulating material to ensure no leakage paths directly form the gate to the source, and the ensure all sensing is taking place on the CNTFET channel region.
Although these CNTFET apatasensors have shown immense promise, some challenges remain. Is it possible to detect small molecules? What are the limiting factors of the CNTFET aptasensor platform? Each analyte and aptamer system presents different challenges with implications in the design of aptamers for operational sensors. For example, we have demonstrated a small molecule, 17 β-estradiol (E2),6 sensor based on CNTFET aptasensors using both 35-mer, 75-mer E2 aptamer sequences and a random 35-mer aptamer sequence.2 The 35-mer E2 aptamer functionalized CNTFETs show a clear increase in current whereas the 75-mer E2 aptamer functionalised CNTFET where the aptamer/E2 binding occurs beyond the Debye length, shows no obvious evidence of sensing. The performance of this and other CNTFET aptasensors will be discussed with emphasis on the role of electrostatic gating effects and the implications for future aptamer design.
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Figure 1