Environmental Applications of Supramolecular Materials

Supra: there's a prefix you have probably never heard before. It defines a fascinating class of materials with a broad range of applications, including in environmental protection. In general, supramolecular materials are composed of molecules that self-assemble into large complex structures that are held together by noncovalent interactions, namely by hydrogen bonding. And these assemblies really do get very big and very complex, as the image below shows. Supramolecules come in many different forms, including hydrogels, which is the form that opens the door to supramolecule applications in tackling environmental challenges. 

https://phys.org/news/2017-09-covalent-post-assembly-modification-cascade-self-assembled.html

How do supramolecular materials self-assemble?

According to an article in the Proceedings of the National Academy of Sciences (PNAS), there are five
main characteristics that define molecular self-assembly: components, interactions, reversibility, environment, and mass transport. In short, it means that groups of the same molecule or different molecules interact and transition from a disorganized structure into an organized structure, held together by noncovalent interactions. Motion by heat brings the molecules together to form these interactions. Now here's the really interesting piece, for an organized structure to be formed the association between molecules must be reversible, meaning that the molecules continually self-assemble, and subsequently disassemble. If the association is not reversible then we are left with a glass structure, which is not organized because it shatters very easily into different sized and shaped pieces. And of course the environment in which the self-assembly occurs influences the interactions.

Image: Whitesides and Boncheva

How can we apply supramolecular molecular materials to combat environmental challenges?

Massive production of goods has led to high volumes of waste products like heavy metals, dyes, pharmaceuticals, pesticides and fertilizer entering water bodies resulting in severe health problems for people and decreased biodiversity in water bodies. An article from the Royal Society of Chemistry discusses the application of supramolecular materials to remove pollutants from water bodies. The methods of removing pollutants that are currently used have severe limitations, which underscores the need for new innovative approaches. That promising new approach is using gels. 

Gels are already used in various products like cosmetics, pharmaceuticals, and food sealants. Because gels can easily enter that reversible reaction cycle discussed above, while maintaining a solid state, they can adsorb and hold on to a lot of a given liquid. There are different types of gels, depending on what liquid the supramolecular structure self-assembles into a gel. They are called organogels if they self-assemble into gels in oils, and hydrogels if they assemble into gels in water.

Oil spills contribute to aquatic pollution, and are economically wasteful. I found the project using a "bio-refinery" approach that was discussed to be very interesting, as it was based on the principles of green chemistry. These principles connect to the idea that in order to reduce pollution and other negative impacts on the environment while conserving resources, chemically engineered processes should not rely on substances that damage the environment. With these principles in mind researchers designed a renewable gel derived from sugar. Upon its addition to a variety of oil compounds like diesel, mineral oil, silicone oil, and crude oil, this gel was attracted to the oil and soon began to self-assemble.

Image: James Edward Bates/Biloxi Sun Herald/TNS

Another group of researchers used gels to prevent oil spills in the first place. An inactive gel (in this case one that would not self-assemble in the presence of oil) was applied in an oil pipe. The moment the pipe had a leak, the gel was activated to self-assemble triggered by the water at the site of the leak, essentially putting a band-aid on the leak to prevent oil from escaping into the environment. This mechanism is similar to how fibrinogen (a protein) is activated to clot, when there is damage to blood vessels.  

Another harmful pollutant that may be removed from water using gels are dyes, which are used in textiles, paints, plastics, printing, and cosmetics, among other areas. Of course these unnatural, even toxic compounds in water are detrimental to the health of those near the water source, and well as the organisms inhabiting it. Removing dyes from water is more complicated than removing oil because oil does not mix with water, whereas dyes do. Yet, most dyes are not biodegradable, so they will remain in the water. So, the dyes have to be separated from water. Researchers have used sugar-derived gels for dye removal as well, specifically, a xerogel. The xerogel is very efficient and can be reused, which along with other aspects of the project also aligns with green chemistry principles. 

Image: Sustain by Kat

The gels' capacity to adsorb these harmful substances can be used to detect these substances in water or other liquids. Researchers also developed a gel that can detect lead by predicting the form that the gel would self-assemble into upon exposure to lead. Therefore if they were to detect this form of gel, they would know that there must be lead present. This approach was discussed as a way to detect lead in paint, but as it is capable of detecting lead in water and removing it, it can safeguard people from this contaminant while reducing its presence in the environment. The idea of activating self-assembly of gels upon exposure to distinct compounds could be used in many applications, which is very exciting.

Gel properties could also be exploited for reusing materials that had been thrown away, particularly electronic waste which is making up an ever-larger portion of our waste.  Electronic devices contain precious metals that end up in the landfill along with the rest of the device once it is discarded. This is becoming an issue because as the demand for electronics grows, more precious metals get mined from the Earth, so soon there will not be anything new to mine. While the precious metals in the thrown-away devices can be reused, the main challenge is that it is very difficult to separate the metals from the device. So, researchers used a gel to selectively remove precious metals from devices, for reuse. This approach was particularly successful with gold. The gel enhanced the properties of gold that make it a great material for electronic devices to the point that the gold could immediately be reused, without any further remediation. This just goes to show how science and engineering are connected with the circular economy.

Image: Geneva Environment Network

What’s next?

This post discussed supramolecular applications that are in-equilibrium, which in simple terms means that the gel is able to activate with compounds in the surrounding environment. Since everything is balanced, energy is not input or released in the process. Supramolecular interactions are also present in living things, as alluded to with the blood clotting example. However these interactions are not in-equilibrium, meaning there is energy involved. So, a new area of research is synthetically creating materials that behave like those in our skin and bones. I am very excited that I will be involved in this emerging field of research as an intern at a biomolecular and chemical engineering lab at the University of Maryland College Park, so I will have the opportunity to experience the research process first-hand! Supra-excited!

Works Cited

Amabilino, David B., et al. "Supramolecular materials." Chemical Society Reviews, vol. 46, no. 9, 7 May 2017, pp. 2404-20, https://doi.org/10.1039/C7CS00163K. 

John, George, et al. "Biorefinery: A Design Tool for Molecular Gelators." Langmuir, vol. 26, no. 23, May 2010, pp. 17843-51, https://doi.org/10.1021/la100785r. 

McNeil, Anne J., et al. "Developing a Gel-Based Sensor Using Crystal Morphology Prediction." Journal of the American Chemical Society, vol. 138, no. 37, Sept. 2016, pp. 12228-33, https://doi.org/10.1021/jacs.6b06269. 

Okesola, Babatunde O., and David K. Smith. "Applying low-molecular weight supramolecular gelators in an environmental setting – self-assembled gels as smart materials for pollutant removal." Chemical Society Review, vol. 45, no. 15, 7 Aug. 2016, pp. 4226-51, https://doi.org/10.1039/ C6CS00124F. 

Raghavan, Srinivasa R., et al. "Gelation of Oil upon Contact with Water: A Bioinspired Scheme for the Self-Repair of Oil Leaks from Underwater Tubes." Langmuir, vol. 31, no. 19, May 2015, pp. 5259-64, https://doi.org/10.1021/acs.langmuir.5b00676. 

Rieß, Benedikt, et al. "The Design of Dissipative Molecular Assemblies Driven by Chemical Reaction Cycles." Chem, vol. 6, no. 3, 12 Mar. 2020, pp. 552-78, https://doi.org/10.1016/ j.chempr.2019.11.008. 

Smith, David K., et al. "Selective Extraction and In Situ Reduction of Precious Metal Salts from Model Waste To Generate Hybrid Gels with Embedded Electrocatalytic Nanoparticles." Angewandte Chemie, vol. 55, no. 1, 4 Jan. 2016, pp. 183-87, https://doi.org/10.1002/anie.201507684. 

Whitesides, George M., and Mila Boncheva. "Beyond molecules: Self-assembly of mesoscopic and macroscopic components." Proceedings of the National Academy of Sciences, vol. 99, no. 8, Apr. 2002, pp. 4769-74, https://doi.org/10.1073/pnas.082065899. 

Yu, Haitao, et al. "Phase-selective gelators based on closed-chain glucose derivatives: their applications in the removal of dissolved aniline/nitrobenzene, and toxic dyes from contaminated water." Journal of Materials Chemistry A, vol. 3, no. 37, Oct. 2015, pp. 18953-62, https://doi.org/10.1039/C5TA01232E.