Supramolecular Hydrogel of Bile Salts Triggered by Single‐Walled Carbon Nanotubes

Abstract

IO N Supramolecular self-assembly is a branch of smart chemistry that focuses on chemical systems made up of a discrete number of assembled molecular subunits or components. [ 1 ] It has been applied to carbon nanotubes (CNTs) for the development of new materials and molecular systems. [ 2 ] Many organic surfactants are reported to form supramolecular self-assemblies on the surface of CNTs when the concentration is higher than the critical micelle concentration (CMC). [ 3 ] This phenomenon is generally used to disperse CNTs in water and to change the surface physical/chemical behavior of CNTs. Here, we report the synthesis of a supramolecular hydrogel based on singlewalled carbon nanotubes (SWCNTs) triggered by a bile salt biosurfactant, sodium deoxycholate (NaDC). We found that this supramolecular hydrogel had excellent viscoelastic properties. The shearing modulus was found to be ≈ 2 × 10 5 Pa over the shear stress range from 1 to 1000 Pa, and the dynamic elastic modulus ( G ′) and viscous modulus ( G ′′) were 10 5 Pa and 10 4 Pa, respectively, at 20 ° C. We prepared nanowires and nanopatterns by a direct print method that using the hydrogel as a “solid” ink. The electrical conductivity ( σ ) of such nanowire was ≈ 55 S cm − 1 . It suggests that the hydrogel is a suitable material to construct SWCNT-based devices by the top-down process. The supramolecular hydrogel has the potential for use in rheological materials [ 4 ] and fl exible, stretchable electronics applications. [ 5 ] NaDC is a bile salts that is an anionic biological surfactant widely used in biomedicine. NaDC is a surfactant often used to dissolve and stabilize individual SWCNTs in water. [ 6 ] The stabilization mechanism can be explained by the adsorption of NaDC molecules onto the surface of SWCNTs driven by the hydrophobic interaction. NaDC has a large, rigid, and planar hydrophobic moiety consisting of a steroid nucleus with two hydroxyl groups. This unique structure helps NaDC molecules adsorb stably onto the surfaces of SWCNTs through hydrophobic interactions, resulting in the dissolution and dispersion of SWCNTs in water. However, NaDC exhibits a rich and complex phase behavior in aqueous solution because of its unique molecular structure. It forms primary micelles with a small size of 1 nm at a CMC of 0.05% and forms secondary micelles (large rod-like self-assemblies) at a CMC of 0.1%. [ 7 ] The secondary self-assembly can be phase transformed into hydrated nanotubes and hexagonal liquid crystals