Transmembrane Transport in Biomimetic Assemblies of One-Dimensional Nanomaterials
- Author(s): Kim, Kyunghoon
- Advisor(s): Grigoropoulos, Costas P.
- et al.
The creation of biomimetic structures based on one-dimensional nanomaterials and lipid membranes will provide a unique platform for achieving functionalities of biological machines and mimicking nature at the nanoscale. Silicon nanowires (SiNW) and carbon nanotubes (CNT) are of significant interest due to the novel properties not present in bulk materials as well as characteristic dimensions comparable to the size of biological molecules. My thesis describes the creation of fabricated nanomaterials integrated with biomaterials such lipid membranes and their constitutive proteins to create biomimetic assemblies.
In the first part of my dissertation, I report transmembrane carbon nanotube pores as a biological ion channel analogs. Biological ion channels in nature transport ions across cellular membranes showing two functions of gating and ion selectivity. CNT pores give structural and functional mimic of an ion channel, in part because smooth, narrow and hydrophobic inner pores of the CNT are remarkably similar to natural biological pores. First, CNTs served as a materials platform that can replicate the features of biological channels. I successfully created ultrashort CNTs (ca. ~10nm) using lipid-assisted sonication-cutting method. Lipid molecules self-assemble on the long CNTs to form a template for sonication cutting. These short CNT pores with their length comparable to the lipid membrane thickness provide a much closer match to protein-channel dimensions. Short CNT pores were incorporated into lipid vesicles to mimic membrane ion channels and study transport properties through CNT pores. These short CNTs in a lipid membrane can transport water, protons, and small ions and reject large uncharged species. Ion rejection in CNT channels is determined by charge repulsion at the CNT rim. Electrophoretic ion transport measurements for individual CNT pores revealed an ion conductance value of 0.63ns which is comparable to those of biological channels. CNT pores inserted in the membrane exhibited stochastic gating behavior common for biological ion channels. These fluctuations result from a spontaneous reversible ionic penetration-exclusion transition previously reported in nanofluidic transport of sub-2-nm pores. Electrophoretically-driven translocation of individual single-stranded DNA molecules through CNT pores produced well-defined ion current blockades. Overall, short CNT mimics transport properties of a biological protein channel. Since the structure and functionality of short CNT pores self-inserted in a lipid membrane resemble the β-barrel structure of a porin, they are termed as "carbon nanotube porins".
In the second part, I describe synthesis of SiNWs grown via vapor-liquid-solid (VLS) mechanism. Silicon nanowires were grown on silicon substrates via chemical vapor deposition (CVD) using silane as a precursor gas and diborane for p-type doing of wires. These nanowires were utilized for a bioelectronics platform for integration of membrane protein functionality based on one-dimensional lipid bilayer. This lipid bilayer provides shielding the nanowires from the solution species and environment for proteins preserving their functionality, integrity, and even vectorality. Here, I report a hybrid lipid bilayer- silicon nanowire bioelectronic device with output controlled via light-induced proton pump protein, bacteriorhodopsin (bR). SiNW field effect transistors (FET) were fabricated via conventional micro/nanofabrication process. bR proteins were incorporated into SiNW transistors covered with a lipid bilayer shell and different ionophore molecules, valinomycin and nigericin were co-assembled to create biologically-tunable bioelectronics devices. In this way, the devices convert photoactivated proton transport by bR protein into an electronic signal. The addition of ionophores tuned the device output by altering membrane ion permeability and the two ionophores were able to modulate different system parameters.