Youngeun Kim

Bio: Youngeun Kim is an academic researcher from Harvard University. The author has contributed to research in topics: Colloidal gold & DNA origami. The author has an hindex of 11, co-authored 13 publications receiving 445 citations. Previous affiliations of Youngeun Kim include Northwestern University & Wyss Institute for Biologically Inspired Engineering.

Transmutable nanoparticles with reconfigurable surface ligands

Abstract: Unlike conventional inorganic materials, biological systems are exquisitely adapted to respond to their surroundings. Proteins and other biological molecules can process a complex set of chemical binding events as informational inputs and respond accordingly via a change in structure and function. We applied this principle to the design and synthesis of inorganic materials by preparing nanoparticles with reconfigurable surface ligands, where interparticle bonding can be programmed in response to specific chemical cues in a dynamic manner. As a result, a nascent set of “transmutable nanoparticles” can be driven to crystallize along multiple thermodynamic trajectories, resulting in rational control over the phase and time evolution of nanoparticle-based matter.

pH-Responsive Nanoparticle Superlattices with Tunable DNA Bonds.

Abstract: Stimuli-responsive nanomaterials with reconfigurable structures and properties have garnered significant interest in the fields of optics, electronics, magnetics, and therapeutics. DNA is a powerful and versatile building material that provides programmable structural and dynamic properties, and indeed, sequence-dependent changes in DNA have already been exploited in creating switchable DNA-based architectures. However, rather than designing a new DNA input sequence for each intended dynamic change, it would be useful to have one simple, generalized stimulus design that could provide multiple different structural outputs. In pursuit of this goal, we have designed, synthesized, and characterized pH-dependent, switchable nanoparticle superlattices by utilizing i-motif DNA structures as pH-sensitive DNA bonds. When the pH of the solution containing such superlattices is changed, the superlattices reversibly undergo: (i) a lattice expansion or contraction, a consequence of the pH-induced change in DNA length,...

Dynamically Interchangeable Nanoparticle Superlattices Through the Use of Nucleic Acid-Based Allosteric Effectors

Abstract: DNA is a powerful tool for programmably assembling colloidal crystals, and has been used to generate nanoparticle superlattices with synthetically adjustable lattice parameters and crystal symmetries. However, the majority of these superlattice structures remain static once constructed, and factors such as interparticle distance cannot be controlled in a facile and rapid manner. Incorporation of these materials into functional devices would be greatly benefitted by the ability to change various aspects of the crystal assembly after the lattice has been synthesized. Herein, we present a reversible, rapid, and stoichiometric on-the-fly manipulation of nanoparticle superlattices with allosteric effectors based upon DNA. This approach is applicable to multiple different crystal symmetries, including FCC, BCC, CsCl, and AlB2.

Epitaxial Growth of DNA-Assembled Nanoparticle Superlattices on Patterned Substrates

Abstract: DNA-functionalized nanoparticles, including plasmonic nanoparticles, can be assembled into a wide range of crystalline arrays via synthetically programmable DNA hybridization interactions. Here we demonstrate that such assemblies can be grown epitaxially on lithographically patterned templates, eliminating grain boundaries and enabling fine control over orientation and size of assemblies up to thousands of square micrometers. We also demonstrate that this epitaxial growth allows for orientational control, systematic introduction of strain, and designed defects, which extend the range of structures that can be made using superlattice assembly. Ultimately, this will open the door to integrating self-assembled plasmonic nanoparticle materials into on-chip optical or optoelectronic platforms.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Abstract: Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micro...

Programmable materials and the nature of the DNA bond

Abstract: For over half a century, the biological roles of nucleic acids as catalytic enzymes, intracellular regulatory molecules, and the carriers of genetic information have been studied extensively. More recently, the sequence-specific binding properties of DNA have been exploited to direct the assembly of materials at the nanoscale. Integral to any methodology focused on assembling matter from smaller pieces is the idea that final structures have well-defined spacings, orientations, and stereo-relationships. This requirement can be met by using DNA-based constructs that present oriented nanoscale bonding elements from rigid core units. Here, we draw analogy between such building blocks and the familiar chemical concepts of "bonds" and "valency" and review two distinct but related strategies that have used this design principle in constructing new configurations of matter.

Small Angle X-ray Scattering for Nanoparticle Research

Abstract: X-ray scattering is a structural characterization tool that has impacted diverse fields of study. It is unique in its ability to examine materials in real time and under realistic sample environments, enabling researchers to understand morphology at nanometer and angstrom length scales using complementary small and wide angle X-ray scattering (SAXS, WAXS), respectively. Herein, we focus on the use of SAXS to examine nanoscale particulate systems. We provide a theoretical foundation for X-ray scattering, considering both form factor and structure factor, as well as the use of correlation functions, which may be used to determine a particle’s size, size distribution, shape, and organization into hierarchical structures. The theory is expanded upon with contemporary use cases. Both transmission and reflection (grazing incidence) geometries are addressed, as well as the combination of SAXS with other X-ray and non-X-ray characterization tools. We conclude with an examination of several key areas of research w...

Physics of active jamming during collective cellular motion in a monolayer

Abstract: Although collective cell motion plays an important role, for example during wound healing, embryogenesis, or cancer progression, the fundamental rules governing this motion are still not well understood, in particular at high cell density. We study here the motion of human bronchial epithelial cells within a monolayer, over long times. We observe that, as the monolayer ages, the cells slow down monotonously, while the velocity correlation length first increases as the cells slow down but eventually decreases at the slowest motions. By comparing experiments, analytic model, and detailed particle-based simulations, we shed light on this biological amorphous solidification process, demonstrating that the observed dynamics can be explained as a consequence of the combined maturation and strengthening of cell−cell and cell−substrate adhesions. Surprisingly, the increase of cell surface density due to proliferation is only secondary in this process. This analysis is confirmed with two other cell types. The very general relations between the mean cell velocity and velocity correlation lengths, which apply for aggregates of self-propelled particles, as well as motile cells, can possibly be used to discriminate between various parameter changes in vivo, from noninvasive microscopy data.

Three-dimensional cell migration does not follow a random walk

Abstract: Cell migration through 3D extracellular matrices is critical to the normal development of tissues and organs and in disease processes, yet adequate analytical tools to characterize 3D migration are lacking Here, we quantified the migration patterns of individual fibrosarcoma cells on 2D substrates and in 3D collagen matrices and found that 3D migration does not follow a random walk Both 2D and 3D migration features a non-Gaussian, exponential mean cell velocity distribution, which we show is primarily a result of cell-to-cell variations Unlike in the 2D case, 3D cell migration is anisotropic: velocity profiles display different speed and self-correlation processes in different directions, rendering the classical persistent random walk (PRW) model of cell migration inadequate By incorporating cell heterogeneity and local anisotropy to the PRW model, we predict 3D cell motility over a wide range of matrix densities, which identifies density-independent emerging migratory properties This analysis also reveals the unexpected robust relation between cell speed and persistence of migration over a wide range of matrix densities