Carbon nanomaterials as platforms for biosensors and atomic switch network devices
Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene, have attracted growing attention as sensing platforms for field effect transistor (FET) aptasensors due to their large surface area and real-time detection capability. Moreover, carbon nanotubes offer high levels of sensitivity for various analytes detection and use as network material to increase the stability in atomic switch network (ASN) devices. This thesis investigated the detection of small molecules using different carbon nanomaterial platforms, as well as the use of CNT networks for atomic switch networks.
First, CNT FET aptasensors have been investigated for the detection of adenosine using two different aptamer sequences, a 35-mer and a 27-mer. Limits of detection were found for adenosine of 100 pM and 320 nM for the 35-mer and 27-mer aptamers, with dissociation constants of 1.2 nM and 160 nM, respectively. Upon analyte recognition, the 35-mer adenosine aptamer adopts a compact G-quadruplex structure, while the 27-mer adenosine aptamer changes to a folded duplex. Using the CNT FET aptasensor platform, adenosine could be detected with high sensitivity over the range of 100 pM to 10 µM, highlighting the suitability of the CNT FET aptasensor platform for high-performance adenosine detection. The aptamer restructuring format is critical for high sensitivity with the G-quadruplex aptasensor having a 130-fold smaller dissociation constant than the duplex forming aptasensor.
Second, a comparison between CNT and graphene FET aptasensors has been conducted. CNT and graphene both showed high sensitivity over the detection range of 1 nM to 10 mM, with dissociation constants of 7.7 nM for CNT and 5.7 nM for graphene. Compared to graphene FET aptasensors, CNT FET showed eight times higher sensitivity. CNT FETs have also performed for the study of dose-dependent sensing to evaluate the feasibility of realizing accurate biosensors. Regardless of the initial ligand concentration, the real-time current responses showed a similar signal on the first added concentration. These sensing signals could be caused by CNT platforms with sensing hotspots between CNT junctions, or by over-detecting sensing sites, rather than accurately responding to analyte levels. Overall, the sensing performance of CNT FET aptasensors is influenced by the initial concentrations, with the sensitivity increasing for lower initial concentrations near the detection limit.
Finally, distribution of silver nanowires (AgNWs) in the ASN device results in Joule heating at nanowire contact points, resulting in failure of AgNWs in the network. To solve this issue, a combination of CNTs has been used to improve the electrical conductivity of ASN devices. A simple fabrication and measurement method for ASN-based CNT devices has been developed. This technique involves drop-casting silver nanowires onto a patterned carbon nanotube network and then exposing it to sulfur gas to activate the nanowires. Because of the combination of AgNWs and carbon nanotubes, the ASN system improved contact resistance and switching events. This work is the first to introduce an Ag/Ag2S/Ag carbon nanotube that can be potentially applied to neural computing applications. Plans for future work are presented at the end of this thesis.