The short version.
We want to make dissipative synthetic systems that are out of thermodynamic equilibrium, to obtain adaptive / dynamic materials that are “alive”. We believe this is the way to get materials with a level of sophistication approaching that of living beings. To get a better understanding of how to obtain such non-equilibrium systems, we try to learn from the microtubule/centrosome system found in most living cells, by performing quantitative measurements on their structure and dynamics in vitro. In addition, we are developing artificial supramolecular polymers that are driven by chemical fuels (analogous to microtubules that use guanosine triphosphate as fuel to drive their assembly/disassembly cycles).
The way most synthetic self-assembled structures are currently made is by adding the various dissolved building blocks (for example molecules or proteins) together in closed container – and then the magic happens!1 Well, it’s not magic really, the building blocks interact and form ordered structures. This approach works well if the kinetics of formation are relatively fast (“they are formed fast”) and the desired structure is the thermodynamically favorable one. The result is a wealth of intricate molecular assemblies, which have had an important impact in e.g. materials science, catalysis, medicine.2–5Living systems found in Nature, on the other hand, operate far away from thermodynamic equilibrium and are able to do so for extended periods of time (until they die and then reach thermodynamic equilibrium). They seem to defy the second law of thermodynamics, which states that in a closed system, entropy should be maximized and therefore disorder should be the preferred outcome.6 As we can see around us, living systems are far from being disordered and, more impressively, they are able to achieve highly organized states from disordered starting materials (e.g. sun light, minerals and gasses). Prigogine, a Belgian physical chemist, showed that this tour de force is done at the cost of increasing entropy of the larger system in which the non-equilibrium system is embedded.7,8 In order for this to happen the organized system has to be able to exchange energy (and/or material) across its boundaries (Fig. 1b). The theoretical thermodynamic description of such non-equilibrium systems is described for states close to the thermodynamic, but fails for strong perturbations.7,9-11
Let us consider the example of Bénard convection cells (click here for a movie), which are formed when a liquid is heated from the bottom and cooled from the top.12
A small temperature gradient between the two parallel plates, i.e the system is pushed out of equilibrium only slightly, is countered by heat transfer through molecule-to-molecule interactions (left side, “conduction”). For a larger temperature difference the system reacts coherently by creating highly ordered hexagonal Bénard convection cells (see middle, “conduction + convection”), which greatly increase the rate at which the thermal gradient is dissipated. When pushed too far out of equilibrium the hexagonal structure breaks down to give rise to chaos (right side, “chaos”).
It is such emergent (click for more info) molecular behavior – like that observed in the Bénard cells – that we apply to make new classes of supramolecular materials / systems. Great progress has been made in macroscopic non-equilibrium systems,13–16 but on the molecular scale there is still a world to explore. So far, we have shown that kinetically stable supramolecular complexes can be obtained using a step-wise non-covalent synthetic approach,17 leading to emergent properties such as “dilution-induced self-assembly”18 and new material properties19–21. Currently, our efforts focus on new methods of pushing supramolecular assemblies out of equilibrium and in this way create highly organized non-equilibrium structures. Various gradients (see above) – and specifically the tendency of molecular systems to counter them by creating order – are very promising to push systems out of their thermodynamic equilibrium. We’re also in the process of extracting new design rules from these non-equilibrium assembly studies that allow for the design of new materials capable of performing complex functions like those found in living systems.
Join the lab!
Motivated master students, PhD candidates and postdocs are encouraged to apply!
References and further reading:
1. Lehn, J.-M. Supramolecular chemistry : concepts and perspectives. (VCH: Weinheim, 1995).
2. Cragg, P. Supramolecular chemistry : from biological inspiration to biomedical applications. (Springer: Dordrecht; New York, 2010).
3. Vögtle, F. Supramolecular chemistry : an introduction. (Wiley: Chichester England, 1993).
4. Steed, J.W. & Atwood, J.L. Supramolecular chemistry. (Wiley Online Library: 2000).
5. Reinhoudt, D. & Crego-Calama, M. Synthesis beyond the molecule. Science 295, 2403-2407 (2002).
6. Hatsopoulos, G.N. & Keenan, J.H. Principles of general thermodynamics. (Krieger: 1981).
7. Nicolis, G. & Prigogine, I. Self-organization in nonequilibrium systems. (Wiley: 1977).
8. Nicolis, G., Prigogine, I. & Nocolis, G. Exploring Complexity. (W.H. Freeman & Company: 1989).
9. Addy, P. The Driving Force for Life’s Emergence: Kinetic and Thermodynamic Considerations. Journal of Theoretical Biology 220, 393-406 (2003).
10. Kestin, J. Course in thermodynamics. (Taylor & Francis: New York, 1979).
11. Schneider, E.D. & Kay, J.J. Life as a manifestation of the second law of thermodynamics. Mathematical and Computer Modelling 19, 25-48 (1994).
12. Chandrasekhar, S. Hydrodynamic and hydromagnetic stability. International Series of Monographs on Physics, Oxford: Clarendon, 1961 1, (1961).
13. Grzybowski, B.A. & Campbell, C.J. Complexity and dynamic self-assembly. Chemical engineering science 59, 1667–1676 (2004).
14. Grzybowski, B.A. & Whitesides, G.M. Macroscopic synthesis of self-assembled dissipative structures. The J. Phys. Chem. B 105, 8770–8775 (2001).
15. Grzybowski, B.A. & Whitesides, G.M. Dynamic aggregation of chiral spinners. Science 296, 718 (2002).
16. Fialkowski, M. et al. Principles and implementations of dissipative (dynamic) self-assembly. J. Phys. Chem. B 110, 2482–2496 (2006).
17. Hermans, T.M. et al. Stepwise noncovalent synthesis leading to dendrimer-based assemblies in water. J. Am. Chem. Soc. 129, 15631–15638 (2007).
18. Hermans, T.M. et al. Self-assembly of soft nanoparticles with tunable patchiness. Nature Nanotechnology 4, 721-726 (2009).
19. Hermans, T.M. et al. Application of Solvent‐Directed Assembly of Block Copolymers to the Synthesis of Nanostructured Materials with Low Dielectric Constants. Angew. Chem. Int. Edit. 45, 6648-6652 (2006).
20. Galeazzi, S. et al. Multivalent supramolecular dendrimer-based drugs. Biomacromolecules 11, 182–186 (2009).
21. Choi, J. et al. Monolayered Organosilicate Toroids and Related Structures: A Phase Diagram for Templating from Block Copolymers. Nano Lett. 6, 1761-1764 (2006).