Living organisms, the most sophisticated collections of organic molecules, are built on molecular self-assembly. For example, cell membranes are formed by the spontaneous aggregation of molecular components, dubbed amphiphiles, which – within the same molecule – have domains of orthogonal interactions with their aqueous environment. These differing interactions trigger the organization of amphiphiles into nanostructures with tunable dynamics and optimized geometries.
Inspired by this natural bottom-up processing strategy, scientists have reported innumerable designs of synthetic amphiphiles that self-assemble into supramolecular (i.e., non-covalent) nanostructures that harness high surface areas and directed environmental interactions. Nevertheless, incorporating a sufficiently hydrophilic domain into the molecular design has remained compulsory to maintaining the assembly of amphiphiles in water. Consequently, the surface chemistries of supramolecular nanostructures are limited to hydrophilic functional groups, even though a wide range of chemical groups with limited to no solubility in water are desirable for critical applications. Therefore, enabling the assembly of “insufficiently amphiphilic” small molecules might allow us to greatly broaden the application space for supramolecular nanostructures.
Our group has identified an unusual pathway to overcome this challenge. Recently, we published a molecular design that incorporates aromatic amides (aramids), a feature mimetic of bulletproof Kevlar materials, to suppress the dynamic behavior of small molecule assemblies and enhance their mechanical stability. These aramid amphiphiles (AAs) self-assemble in water into high-aspect-ratio nanoribbons with ~5 nm cross-sectional dimensions and ~20 µm lengths. The strong cohesion from the aramid domain prevents the disassembly of nanoribbons after being removed from water, and results in GPa-scale nanoribbon strengths and stiffnesses. Because Kevlar is also widely considered a heat-resistant polymer, we were curious to see how AAs responded to extreme temperature environments. The results from our first thermogravimetric analysis (TGA) trial, which tracks sample mass under variable temperature conditions, surprised us tremendously: we observed a distinct plateau in mass loss upon heating (Fig. 1a). We hypothesized the features of this plot resulted from a multi-step thermal decomposition, where the hydrophilic surfaces of the nanoribbons burned off at lower temperatures while the hydrophobic aramid core persisted.
Figure 1 | (A) Thermogravimetric analysis of aramid amphiphiles (AAs) shows mass loss upon heating that first plateaus at approx. 250°C. A triaramid control containing an analogous molecular structure as AAs but without a hydrophilic head group exhibits no mass loss in this range. (B) Representative AFM of AA nanoribbons before (left) and after (right) annealing at 250 °C for 1 h in air demonstrates that the nanoribbon morphology is maintained after annealing.
Through careful analysis of the chemical composition of TGA exhaust gasses, annealed AAs, and control compounds, we confirmed that AAs exhibit the proposed domain-selective thermal decomposition at 250°C. These results encouraged us to investigate how, or if, the thermal decomposition process impacted the morphologies of the assembled nanostructures. Through atomic force microscopy and transmission electron microscopy, we discovered the nanoribbon architectures remain intact after annealing at 250°C (Fig. 1b). X-ray scattering and contact angle measurements further confirmed that the hydrophilic surface groups are selectively removed with maintaining the internal order of the nanoribbons.
We believe this concept of domain-selective decomposition could have significant implications in generating supramolecular nanostructures composed of less soluble and insoluble surface chemistries that don’t naturally self-assemble (Fig. 2). Constructing a supramolecular nanostructure by molecular self-assembly from aramid molecules without hydrophilic surface groups is conventionally impossible. By combining selectively removable and hydrophilic surface groups with the hydrophobic aramid motif, self-assembly could be used as an intermediate step to create otherwise unattainable supramolecular nanostructures via post-assembly surface cleavage. Integrating the benefits of supramolecular nanostructures with a broader range of surface chemistries unhindered by traditional solvophilic and geometric constraints may enable countless new targets for applying molecular assemblies.
Figure 2 | Site-selective thermal cleavage provides a novel pathway to engineering small-molecule nanostructures without extensive amphiphilic character. Generally, constructing a nanostructure without orthogonal hydrophilic and hydrophobic driving forces has to overcome significant entropic costs and is unlikely to occur. Our reported pathway generates entirely hydrophobic nanostructures through hydrophilic modification, self-assembly, and thermal cleavage.
This work is featured in recent Editors’ Highlights webpage of “Materials science and chemistry”, and you can find the whole article in Nature Communication.
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