Carbon nanotube thin film transistor platforms for small molecule aptasensors

Abstract

Network films of carbon nanotubes have shown a great deal of promise as the active channel in field effect transistors (CNT FETs) for utilisation as aptasensors. The detection of small biological molecules or ions using biosensors remains a challenge, partly due to issues of selectivity. This thesis presented CNT FET aptasensors for selectively and quantitatively detecting 17 β-estradiol (estrogen, E2) and potassium ions (K⁺) in real time. By using this device architecture, the conformational change of the aptamer upon the exposure to analytes induced electrostatic gating of electronic conduction in CNT FET aptasensors clearly demonstrate their ability as selective biosensors.

Initially device fabrication and electrical characterisation of CNT FETs are presented in this thesis. CNT network films have been reproducibly fabricated on both rigid SiO₂/Si and flexible polyimide substrates via a surfactant free solution deposition route. Standard microelectronic processing is used to fabricate the devices in controlled locations. This thesis presents the dependency of electrical performances on CNT tube densities which are controlled by the CNT suspension concentrations. It is found that the tube density must reach the percolation threshold for an optimised device. The CNT FETs display reliable p-type semiconducting properties with low operating voltages (< 1 V) in liquid environments. The flexible CNT FETs on polyimide show good electrical performance with 5000 on/off current ratios and 4 cm²/V·s hole mobilities, making the platform a viable candidate for aptasensors.

To fabricate an aptasensor, noncovalent functionalisation of the CNT surface with 1-pyrenebutanoic acid, succinimidyl ester (PBASE) is carried out before linking amine functionalised aptamers. It is shown that the selective tethering is required to fabricate a reliable sensor, and that non selective aptamer adsorption on the CNT surfaces blocks any analyte detection. The real time current response for the 35-mer E2 aptamer functionalised CNT FET shows a clear increase in current over the range of 50 nM to 1.6 μM of E2, using 35-mer random sequenced DNA as a control. The E2 response using the 75-mer aptamer functionalised CNT FETs where the aptamer/E2 binding occurs beyond the Debye length, shows no obvious evidence of sensing, confirming that the sensing mechanism is due to conformational changes in the aptamer structure bringing charges closer to the CNT surface. The successful realisation of CNT FET aptasensors on flexible substrates (polyimide) for detection of the K⁺ ion is also presented. Electrostatic gating is established via the specificity and the sign of the current response, and by observing its suppression when higher ionic strength decreases the Debye length at the CNT/water interface. A detection limit of 10 pM has been achieved in resistive sensing mode, and 100 pM in real time FET current response mode.

Overall the research shows successful application on a CNT FET device platform for detecting E2 and K⁺ based on aptamer functionalised CNT FETs. The reliable electrical performance of flexible CNT FETs fabricated by a facile solution deposition route is an advantage of this sensing platform. The effective device architecture presented, along with the identification of clear real time response signatures, which inform the development of flexible electronic biosensors using aptamer as receptors. At the end of thesis, the plans for future work are presented.