(Self)-assembling tomorrow: Designing materials that build themselves
As a scientist, working with researchers in the same field as oneself is often the path of least resistance; however, connecting with researchers in complementary fields empowers all parties to achieve more than they could alone. Energy Frontier Research Centers (EFRCs) embody this gap-bridging philosophy by bringing together scientists from different fields and institutions to tackle grand challenges.
Just as “like attracts like” among people, similar molecules tend to stick together rather than interact with different molecules. Molecules that include two connected segments with different properties present exciting implications for researchers, such as understanding, predicting, and controlling the lipid bilayers that are essential to human life. Phospholipids contain both hydrophilic (water-loving) and hydrophobic (oil-loving) components and, therefore, arrange themselves into a two-layered sheet with the hydrophilic portions pointing outward. Such arrangements of discrete molecules into an organized structure due to interactions between (or within) the molecules rather than external influences is called self-assembly. This process is illustrated in Figure 1. Molecules can self-assemble into more than just lipid bilayers, however. For example, proteins consist of polypeptide chains that fold upon themselves into exquisitely complex structures, each with unique biological functions. Following self-assembly principles, individual proteins are then able to come together into organized superstructures that can carry out even more complex functions. The geometry of self-assembled architectures can be controlled by adjusting the identity and sizes of each component, much like the optimal team make-up for one EFRC might be two chemists and four mechanical engineers, while another EFRC needs one of each plus two physicists to achieve its goals.
Several EFRCs are both using self-assembled materials to address energy challenges and building on our fundamental understanding of this design space. The examples described here span self-assembly in synthetic block copolymers, biopolymers, and small molecules.
Materials for water purification
Scientists in the Center for Materials for Water and Energy Systems (M-WET) leverage self-assembly to design next generation water filtration membranes. Improved water filtration is essential for providing billions of people worldwide with adequate sanitation and drinking water, as well as enabling energy and food production. Membranes remove impurities such as heavy metals, hydrocarbons, and bacteria from water by passing the water through a porous material that serves as a barrier to the larger molecules dissolved in the fluid. This technology has been transformational to water security and human health in many capacities, from reducing arsenic content in well-water to purifying blood by hemodialysis. Traditional membrane technology, however, is limited because increasing pore sizes to increase the water flowrate through the membrane also results in larger, undesired molecules or particles passing through. One source of membrane inefficiency is that pore sizes are non-uniform, but self-assembled block copolymers can yield uniform pore sizes and spacings. Block copolymers are polymer chains that contain multiple monomer types organized into “blocks” of like monomers. Altering the identity and length of these “blocks” tailors the pore geometry to improve water flux while ensuring target molecule removal, as illustrated in Figure 2. M-WET researchers have used simulations to demonstrate that membrane morphology is strongly connected to performance and are following this guidance to inform experimental trials.1,2
Bio-inspired energy materials
Growing global energy demands and continued reliance upon fossil fuels necessitate development of creative energy generation strategies. Some powerful biological systems rely on multiple classes of compounds to manage energy, such as the light harvesting apparatus of a bacterial cell. Taking inspiration from nature, the Center for the Science of Synthesis Across Scales (CSSAS) focuses on building a fundamental understanding of how high information content building blocks self-assemble into hierarchical structures to build hybrid materials that integrate polymers, proteins, and inorganic compounds. Recently, researchers from CSSAS synthesized a tunable, multilayered material with polypeptoid, protein, and nanoparticle layers. Such a material could be modified by adjusting the architecture and chemistry of the components to enable a broad range of biological functionalities, such as photocatalysis.3 The ability to mimic natural energy systems presents a path towards sustainable fuel and chemical production, a necessary part of reducing fossil fuel consumption.
The Center for Bio-Inspired Energy Science (CBES) also draws inspiration from natural energy systems to address increasing global energy demands. CBES structures small molecules with light-harvesting and catalytic capabilities in soft materials. These small molecules reversibly self-assemble based on their interactions with each other and their environment. For example, researchers in CBES added light absorbing molecules called chromophores to a swollen polymer matrix then triggered crystallization of the chromophores by changing the solvent. Crystallizing these light absorbing molecules immobilized them within the matrix. This system shows promise in harvesting light which can be a critical component of sustainable energy efforts.4
As EFRC research illustrates, well designed molecules can arrange themselves into materials with the potential to address many of the energy challenges faced by humanity. By utilizing and further studying self-assembly, several EFRC’s are developing teams of molecules that can build the materials of tomorrow.
(1) Aryal, D.; Howard, M. P.; Samanta, R.; Antoine, S.; Segalman, R.; Truskett, T. M.; Ganesan, V. Influence of Pore Morphology on the Diffusion of Water in Triblock Copolymer Membranes. J. Chem. Phys. 2020, 152 (1), 014904. https://doi.org/10.1063/1.5128119.
(2) Howard, M. P.; Lequieu, J.; Delaney, K. T.; Ganesan, V.; Fredrickson, G. H.; Truskett, T. M. Connecting Solute Diffusion to Morphology in Triblock Copolymer Membranes. Macromolecules 2020, 53 (7), 2336–2343. https://doi.org/10.1021/acs.macromol.0c00104.
(3) Ma, J.; Cai, B.; Zhang, S.; Jian, T.; De Yoreo, J. J.; Chen, C.-L.; Baneyx, F. Nanoparticle-Mediated Assembly of Peptoid Nanosheets Functionalized with Solid-Binding Proteins: Designing Heterostructures for Hierarchy. Nano Lett. 2021, acs.nanolett.0c04285. https://doi.org/10.1021/acs.nanolett.0c04285.
(4) Sai, H.; Erbas, A.; Dannenhoffer, A.; Huang, D.; Weingarten, A.; Siismets, E.; Jang, K.; Qu, K.; Palmer, L. C.; Olvera de la Cruz, M.; Stupp, S. I. Chromophore Amphiphile–Polyelectrolyte Hybrid Hydrogels for Photocatalytic Hydrogen Production. J. Mater. Chem. A 2020, 8 (1), 158–168. https://doi.org/10.1039/C9TA08974H.
- We thank Professor Benny Freeman and other members of M-WET for many useful discussions. We also thank Ms. Pam Cook for suggestions and revisions which improved the stylistic aspects of the preprint. This work was supported by the Center for Materials for Water and Energy Systems, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019272. Acknowledgment is also made to the Robert A. Welch Foundation (Grant No. F-1599 to VG and Grant No. F-1696 to TMT). We acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources.
- We thank Wesley Reinhart for helpful discussions on the random-forest regression. This work was supported as part of the Center for Materials for Water and Energy Systems (MWET), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0019272. V.G. and T.M.T. acknowledge financial support from the Welch Foundation (grant nos. F-1599 and F-1696). Use was made of computational facilities purchased with funds from the National Science Foundation (no. CNS-1725797) and administered by the Center for Scientific Computing (CSC). The CSC is supported by the California NanoSystems Institute and the Materials Research Science and Engineering Center (MRSEC; no. NSF DMR-1720256) at UC Santa Barbara.
- We are grateful to Alexander Thomas for helping with the initial experiments. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, as part of the Energy Frontier Research Centers program: CSSAS, The Center for the Science of Synthesis Across Scales under award number DE-SC0019288. XRD work was conducted at the Advanced Light Source with support from the Molecular Foundry, at Lawrence Berkeley National Laboratory, both of which are supported by the Office of Science, under contrast no. DE-AC02-05CH11231. AFM and SEM imaging was conducted at the University of Washington Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure (NNCI) site which is supported in part by the National Science Foundation, the University of Washington, the Molecular Engineering and Sciences Institute, and the Clean Energy Institute. PNNL is a multiprogram national laboratory operated for the Department of Energy by Battelle under contract no. DE-AC05-76RL01830.
- This work was primarily supported by the Center for BioInspired Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award # DESC0000989. Molecular synthesis was supported by the National Science Foundation under NSF Award Number DMR1121262. The authors thank the Sherman Fairchild Foundation for computational support. E. S. acknowledges the Materials Research Science and Engineering Center REU program, supported by the National Science Foundation under NSF Award Number DMR-1121262. The authors also thank Prof. Michael Wasielewski (Northwestern University) for access to the gas chromatography instrument, Zaida Alvarez-Pinto (Northwestern University) for assistance with optical imaging of the hydrogels, and Garrett Lau (Northwestern University) for assistance in acquiring thermogravimetric analysis data. This work made use of the following facilities at Northwestern University: the J. B. Cohen X-Ray Diffraction Facility, the EPIC facility (cryoSEM), the Keck Biophysics facility (UV-Vis), the Biological Imaging Facility (CLSM), the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS) (X-ray scattering), and the IMSERC facility (TGA). The J. B. Cohen X-Ray Diffraction Facility is supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University. The EPIC facility of the NUANCE Center has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. The Keck Biophysics facility has received support from a Cancer Center Support Grant (NCI CA060553). DND-CAT is supported by Northwestern University, E. I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. IMSERC facility has received support from the NSF (CHE1048773); Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the State of Illinois and International Institute for Nanotechnology (IIN).