Carbon nanotube and graphene field-effect transistors for aptamer and odorant receptor-based biosensors
Authors:   , Murugathas Thanihaichelvan 
Published: May 2023

<p>Carbon nanotubes (CNTs) and graphene field-effect transistors (FETs) are investigated here for their utility as sensors incorporating aptamers and insect odorant receptors (ORs) as the sensing element. This thesis investigated the junction dominated sensing mechanism of CNT network FET, as well as the use of CNT network and graphene FETs for insect OR-based biosensors. Firstly, I focused on the unique case of CNT network FETs that were formed with a channel composed of a network of CNTs. The CNTs were purchased as a Bucky paper with 99% semiconducting tubes. However, the fabrication process resulted in bundles of CNTs forming the network. These bundles were predominantly semiconducting CNT with a maximum metallic component of 24%. Using a K+ aptamer, which is known to act as a molecular scale electrostatic gate, the significance of the metallic-semiconducting junctions within the network was demonstrated. CNT networks were fabricated by a simple solution process method, and the CNT density was tailored by controlling the deposition time. By tuning the density of the CNTs within the FET channel, it was found that the devices where the film is closest to the percolation network show the greatest sensitivity as aptasensors. This result confirms that a small number of junctions can dominate the sensing response in these devices. These findings highlight the critical role of metallic tubes in CNT FET biosensor devices and demonstrate how the CNT network composition is an important variable to boost the performance of electronic biosensors.</p>
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<p>The CNT network FETs were then functionalised with insect ORs in nanodiscs and tested against several volatile organic compounds. Four fruit fly (Drosophila melanogaster) ORs (DmelOR10a, DmelOR22a, DmelOR35a, and DmelOR71a) were expressed in Sf9 cells, purified and reconstituted into lipid nanodiscs. Each of these OR nanodisc immobilised sensors produces a selective and highly sensitive electrical response to their respective ligands, methyl salicylate, methyl hexanoate, trans-2-hexen-1-al and 4-ethylguaiacol, with limits of detection in the low fM range. No detection was observed for each OR against control ligands, and empty nanodiscs showed no specific sensor signal for any of the odorant molecules. The sensing results reported in this thesis are the first evidence that insect ORs integrated into lipid nanodiscs can be used as primary sensing elements with CNT network FETs for biosensor technologies.</p>
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<p>The OR nanodisc, as well as OR expressed liposome, were also tested for biosensing with graphene FETs. The sensors were fabricated by functionalising the graphene channel with OR nanodisc as well as OR liposomes with and without the obligate in vivo co-receptor (Orco). DmelOR10a and DmelOR22a were used for this sensing study. The OR nanodisc immobilised sensors showed highly sensitive electrical responses to their respective positive ligands with limits of detection in fM range, and no signal was observed for the negative ligands. The liposome sensors also showed selective response to the positive ligand but at higher concentrations. Similar to the OR nanodiscs, the OR liposome sensors also showed no significant response to the negative ligands and empty liposomes showed no specific sensor signal for any of the ligands used. Interestingly, an enhancement in the sensitivity was found with the Orco co-expressed DmelOR10a and DmelOR22a liposomes when compared to OR only liposomes. The results reported in this thesis are the first to demonstrate the enhanced sensitivity of Orco co-expressed DmelOR10a and DmelOR22a liposomes with graphene FETs. Further studies are now required to determine the sensing mechanism of OR nanodisc and liposome-based sensor and to evaluate the role of Orco in in-vitro sensors.</p>