Carbon Nanotube and Graphene Biosensing with Liquid Potential Control
Bioelectronics is a rapidly expanding technology that has seen much success across the field of biosensing. This dissertation explores the biosensing application of graphene field-effect transistors (gFETs) and single-walled carbon nanotube field-effect transistors (SWCNT-FETs). Liquid potentials are methodically controlled throughout the experiments performed herein to demonstrate the importance of electrostatics in gFET sensor responses and biomolecular processes such as receptor-ligand binding and enzyme catalysis.
After a brief introduction to the field of biosensing and current methods, the dissertation is organized into two halves. Chapters 2 and 3 describe testing of a commercial gFET biosensor in a collaboration with Cardea, a startup company in San Diego, CA. Chapters 4 and 5 describe single-molecule measurements of Taq polymerase using SWCNT-FETs.
Two main experiments were completed with the gFET biosensors. First, Chapter 2 details measurements of interleukin-6 (IL-6) antibody-antigen binding. Measurements using externally applied liquid potentials ranging from -0.1 V to 0.4 V revealed that changes in source-drain current (Isd) previously attributed to specific antibody-antigen binding were instead a result of electrochemical charging effects. After proper characterization and subtraction of electrochemical charging signals, changes in Isd due to antigen adsorption on the graphene surface were found to decrease monotonically from 0 % to -5 % with increasing applied potentials, indicating that applied liquid potentials play a critical role in sensor output signals and proper interpretation of those signals.
Chapter 3 describes the use of Cardea gFETs in the sensing of single-nucleotide polymorphisms (SNPs). This experiment involved immobilizing Cas9 molecules on gFET biosensors then incubating them with target-specific guide RNA (gRNA) related to sickle cell disease (SCD). When incubated with target genomic DNA, the gFET biosensor produced signals twice as large on average as when incubated with genomic DNA varying by a SNP, demonstrating successful discrimination between the two genomic DNA samples. Follow-up experiments instead using gRNA related to familial amyotrophic lateral sclerosis (fALS) achieved similar discrimination between genomic DNA varying by a SNP, as did experiments instead using a Cas9 orthologue from Mycoplasma gallisepticum (MgaCas9). These results establish Cardea’s gFET biosensor as a viable platform for rapid and highly programmable SNP detection without the need for DNA amplification.
In the second half of this dissertation, SWCNT-FETs were used to study individual Taq polymerase molecules. Chapter 4 introduces the use of SWCNT-FETs as biosensors with experimental procedures, sample signals, and subsequent signal analysis methods specific to the study of Taq polymerase all contained therein. Various analyses confirmed the Poisson-like behavior of free-running Taq polymerase, with catalysis occurring at a rate of approximately 4 s-1 at T = 27 °C.
Chapter 5 expands on the study of Taq with additional external potentials. Taq only generated electrical signals in a narrow 200-mV range of liquid potentials, Vlg. Outside that potential range, the devices became quiet. Follow-up experiments showed that Taq polymerase catalysis deviated from its Poisson-like behavior when subjected to time-varying potentials with one boundary lying within the active range of Vlg and the other outside. More specifically, when subjected to time-varying potentials, Taq generated signals that lasted longer, occurred more frequently, and less randomly than when subjected to time-independent potentials. These results indicate the importance of external potential control in single-molecule SWCNT-FET studies and provide evidence that external potentials can even be tuned to influence the timing of enzyme motions.