Controlled Assembly of Carbon Nanotube Nanohybrids for Single-Molecule Investigations
Authors: Palma Matteo 
Published: February 2020

We present a universal approach for the generation of multifunctional nanomaterials that employ molecular building blocks assembled on carbon nanotube (CNTs) electrodes. We will demonstrate single-molecule control in the formation of nanohybrids via the in-solution assembly of classes of molecular materials (organic, inorganic, and biological with promising attributes) to DNA wrapped CNTs. Notably, both static and dynamic heterostructures were assembled; this allowed us to study and tailor various systems with single-molecule resolution.
We will first discuss the in-solution linking of metallic single-walled CNTs (SWCNTs) with different conjugated molecular wires. CNT-based molecular junctions were interfaced to macroscopic electrodes, and conductive atomic force microscopy measurements were performed at different locations along the junctions (see Figure 1a). This allowed us to measure the molecular conductance of a series of oligophenyls, highlighting the potential of an all-carbon based approach for solution-processable molecular electronics.[1]
Furthermore, we produced organic-inorganic heterostructures consisting of single Quantum Dots (QDs) univocally linked at the terminal ends of individual SWCNTs. Monofunctionalized SWCNT-QD heterostructures were obtained and photophysical investigations at the single nanohybrid level showed evidence of electronic coupling (see Figure 1b).[2] Moreover, DNA linkers of differing lengths were used as molecular rulers to control the distance, and hence tune the energy/charge transfer between the two nanostructures in these organic-inorganic nanohybrids. Additionally, a dynamic SWCNT-QD hybrid was designed and assembled using a G-quadruplex DNA aptamer linker, so that the distance between the SWCNT and QD could then be dynamically modulated by the introduction and removal of potassium ions (K+);[3] the system was further found to be sensitive to K+ concentrations between 1pM and 25mM. By and large, these approaches open the possibility of assembling tailored optoelectronic and light harvesting systems with single-particle control.
Finally, we will demonstrate site-specific assembly of single proteins on individual SWCNTs. As a proof of concept, we investigated different CNT-protein configurations and obtained evidence, with single-molecule resolution, of site-specific coupling between SWCNTs and specific proteins of interest (see Figure 1c).[4] Notably, only the right bioengineered system exhibited the expected direct protein-nanotube communication, paving the way to selective electrical addressability of proteins via the use of carbon nanoelectrodes.
In summary, we demonstrate a novel approach for the assembly of individual molecules on carbon nanotube electrodes with single-molecule control. The assembled heterostructures are of interest for the fabrication of solution-processable nanoelectronic devices with applications that range from molecular (bio)electronics to light harvesting systems.
[1] Journal of the American Chemical Society, 2016, 138, 2905-2908
[2] Small, 2017, 13, 1603042
[3] Submitted, 2017
[4] Journal of the American Chemical Society
, 2017, in press

Figure 1