RAFAL KLAJN GROUP
NANOSCALE REACTIVITY AND SELF-ASSEMBLY
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Chemistry in confined spaces

Nature inspires chemists with abilities to develop strategies to stabilize ephemeral chemical species, perform chemical reactions with unprecedented rates and selectivities, and synthesize complex molecules and exquisite inorganic nanostructures. What natural systems consistently exploit—which is yet fundamentally different from how chemists perform reactions—is the aspect of nanoscale confinement. Our research focuses on studying the behavior of chemical species under a variety of types of nanoconfinement including cavities of coordination cages, surfaces of colloidal nanoparticles, and nanopores of porous materials (such as porous aromatic frameworks). We also develop novel families of synthetic materials featuring confined spaces; examples include reversibly self-assembling colloidal crystals (“dynamic nanoflasks”), bowl-shaped metallic nanoparticles, and non–close-packed nanoparticle superlattices.

While these objectives are predominantly of a fundamental nature, they can easily evolve into a vast array of applications. Our ultimate goals are as diverse as preparing a new family of inverse opals, studying protein folding inside “artificial chaperones”, and controlling polymerization reactions by the size and shape of the “nanoflask”. We believe that fundamental studies of chemistry under nanoconfinement have the potential to provide novel ways to perform chemical reactions, paving the way to discovery of new phenomena and unique structures.
 
Representative publications

– A. B. Grommet, M. Feller, R. Klajn, Chemical reactivity under nanoconfinement, Nat. Nanotech. 2020, 15, 256-271.
– H. Zhao, S. Sen, T. Udayabhaskararao, M. Sawczyk, K. Kucanda, D. Manna, P. K. Kundu, J.-W. Lee, P. Král, R. Klajn, Reversible trapping and reaction acceleration within dynamically self-assembling nanoflasks, Nat. Nanotech. 2016, 11, 82–88.
– Z. Chu, Y. Han, T. Bian, S. De, P. Král, R. Klajn, Supramolecular control of azobenzene switching on nanoparticles, J. Am. Chem. Soc. 2019, 141, 1949–1960.
– D. Samanta, J. Gemen, Z. Chu, Y. Diskin-Posner, L. J. W. Shimon, R. Klajn, Reversible photoswitching of encapsulated azobenzenes in water, Proc. Natl. Acad. Sci. USA 2018, 115, 9379–9384.


 
Self-assembly at the nanoscale
Inorganic nanoparticles (i.e., particles in the size range 1–100 nm) exhibit a wide range of fascinating physicochemical properties, including light upconversion, superparamagnetism, and localized plasmon resonance (which gives rise to the wine-red color of colloidal suspensions of gold nanoparticles). Development of such functional materials requires a precise control over the self-assembly of individual nanoparticles into higher-ordered arrays. We are therefore interested in pursuing fundamental studies of how nanoparticles interact with each other, and ultimately how we can use this information as a tool to generate complex nanomaterials in the most efficient and precise manner. For example, by combining short- and long-range forces of different symmetries, we have developed an efficient way to assemble simple cubic nanoparticles into complex double-helical superstructures. In parallel, we are also interested in controlled chemical transformations of nanoparticle assemblies as a conceptually new route to functional nanomaterials.

 
Representative publications

– G. Singh, H. Chan, A. Baskin, E. Gelman, N. Repnin, P. Král, R. Klajn, Self-assembly of magnetite nanocubes into helical superstructures, Science 2014, 345, 1149–1153.
– T. Udayabhaskararao, T. Altantzis, L. Houben, M. Coronado-Puchau, J. Langer, R. Popowitz-Biro, L. M. Liz-Marzán, L. Vukovic, P. Král, S. Bals, R. Klajn, Tunable porous nanoallotropes prepared by post-assembly etching of binary nanoparticle superlattices, Science 2017, 358, 514–518.
– M. Sawczyk, R. Klajn, Out-of-equilibrium aggregates and coatings during seeded growth of metallic nanoparticles, J. Am. Chem. Soc. 2017, 139, 17973–17978.


 
Stimuli-responsive materials

Living organisms are sophisticated self-assembled structures that exist and operate far from thermodynamic equilibrium. They remain stable at highly organized (low-entropy) states owing to the continuous consumption of energy stored in “chemical fuels”, which they eventually convert into low-energy waste. This so-called dissipative self-assembly is ubiquitous in nature, where it gives rise to complex structures and properties such as self-healing, homeostasis, and camouflage. In sharp contrast, nearly all man-made materials are static: they are designed to serve a given purpose rather than to exhibit different properties dependent on external conditions.

In our research, we are developing principles for designing systems capable of reversible, dynamic, and dissipative self-assembly. We employ novel, unconventional approaches based on integrating organic and nanoparticulated building blocks into hybrid structures, which can be programmed to self-assemble into larger structures and, ultimately, materials. Materials assembled from nanoparticles often exhibit distinctive properties compared to those of individual building blocks – for example, optical, magnetic, electronic, and catalytic properties have been all manipulated by adjusting the interparticle distance. Therefore, achieving programmable assembly and disassembly of individual nanoparticles in a reversible fashion could lead to dynamically tunable materials. We are particularly interested in designing nanoparticles that assemble in response to external stimuli such as light, magnetic fields, and chemical fuels. Our efforts could lead to new classes of “driven” materials with features such as tunable lifetimes, time-dependent catalysis, and dynamic exchange of building blocks. We hope that these efforts will lay the foundations for developing new synthetic dissipative materials that could rival in complexity and functionality those found in nature.
 
Representative publications

– T. Bian, Z. Chu, R. Klajn, The many ways to assemble nanoparticles using light, Adv. Mater. 2020, 32, 1905866.
– P. K. Kundu, D. Samanta, R. Leizrowice, B. Margulis, H. Zhao, M. Börner, T. Udayabhaskararao, D. Manna, R. Klajn, Light-controlled self-assembly of non-photoresponsive nanoparticles, Nat. Chem. 2015, 7, 646–652.
– D. Manna, T. Udayabhaskararao, H. Zhao, R. Klajn, Orthogonal self-assembly of nanoparticles using differently substituted azobenzenes, Angew. Chem. Int. Ed. 2015, 54, 12394–12397.
– P. K. Kundu, G. L. Olsen, V. Kiss, R. Klajn, Nanoporous frameworks exhibiting multiple stimuli responsiveness, Nat. Commun. 2014, 5, 3588.