The biomimetic structures of carbon nanotube porins and lipid membranes provide a membrane coating to isolate biosensor surface from potential foulants present in biological fouling solution. The lipid membranes mimic cellular membrane while the ultrashort carbon nanotube porins mimic the structure and functionalities of membrane protein channels. This versatile biosensor platform enables the ion sensing at nano-bio interfaces and opens up the potential for intracellular and multimodal sensing.

In this dissertation, I will review the properties and advantages of carbon nanotubes in the nanofluidics field. I will report the optimization of high-yield synthesis of carbon nanotube porins, the biomimetic nanochannels in membranes. I will also discuss the fully synthetic membrane with incorporation of carbon nanotube porins into a block-copolymer matrix. In addition, I will present a biomimetic approach for creating fouling-resistant pH sensors by integrating silicon nanoribbon transistor sensors with an antifouling lipid bilayer coating that contains proton-permeable carbon nanotube porin channels and demonstrate robust pH detection in a variety of complex biological fluids. And lastly I will describe potential applications of the carbon nanotube porin based biosensor platform.

Nanofluidic channels confine water and ions down to length scales that are comparable to the sizes of individual molecules. This strong confinement creates unique transport phenomena, such as enhanced water flow, unusual selectivity patterns, and strong electroosmotic coupling. To probe transport behavior and the underlying physics under extreme confinement, our group developed a model system - carbon nanotube porins (CNTPs), which are synthetic analogs of aquaporins made of carbon nanotubes. CNTPs have well-defined, nanometer-sized diameters and ultrashort lengths of ca. 10 nm. In addition, CNTPs have the ability to self-insert into lipid bilayers, forming artificial membrane channels. Using ionic transport measurement setups adapted from protein channels, I was able to obtain ionic conductance scaling and selectivity values through CNTPs with average diameters of 0.8 nm and 1.5 nm. While the 0.8 nm-diameter CNTPs showed exclusive-cation selectivity, the 1.5 nm-diameter CNTPs demonstrated strong ion-water coupling during transport. Furthermore, I demonstrated that built-in charges (end-groups), environmental changes (such as altering pH), or external forces (such as applying a gate voltage) could alter the ionic distribution and selectivity of the CNTP channels. Our CNTPs represent a versatile nanofluidic model system that we can utilize to advance our understanding and control of ion and water behavior at the nanoscale, which could further benefit desalination membrane design and lab-on-a-chip sensing.

Carbon nanotube porins (CNTPs), short segments of carbon nanotubes stabilized by a lipid coating, are a promising example of artificial membrane channels that mimic a number of key behaviors of biological ion channels and promising model system to investigate the role of the electronic properties of the channel wall on ion and proton transport.

While the lipid-assisted synthesis of CNTPs may facilitate their subsequent incorporation into lipid bilayers, it limits the applicability of these pores in other self-assembled membrane materials and precludes the use of large-scale purified carbon nanotube (CNT) feedstocks. Here I demonstrate that CNTPs can be synthesized by sonochemical cutting of long CNT feedstocks in the presence of different surfactants, producing CNTs with transport properties identical to those obtained by the lipid-assisted procedure.

CNTs of different chiralities can be synthesized which are either metallic or semiconducting. Here, I studied short chirality-separated pure (6,5), (7,4), and (7,5)/(8,4) CNTPs, which vary in electronic properties, yet are almost identical in diameter. Single channel conductance from potassium ion transport through these CNTPs indicates that the electronic properties have a weak effect on ion transport.

In addition to intrinsic electronic properties, I also investigated the role of the functional group and defects on the CNT wall. I measured potassium ion transport through fluorescent ultrashort carbon nanotube porins (FUNPs)– functionalized channels with single defined p-nitroaryl defect incorporated into its structure. These channels exhibited significantly lower ion conductance compared to the pure (6, 5) CNTPs regardless of the surfactant used to solubilize the CNTPs.

Overall, my study highlights the role of the electronic properties of the CNTs, including defects, and functional groups, on the nanofluidic transport of ions through single channel CNTs.