Dongchoul Kim

Facile Preparation of Ultrasmall Void Metallic Nanogap from Self-Assembled Gold-Silica Core-Shell Nanoparticles Monolayer via Kinetic Control.

A facile preparation of ultrasmall 1-2 nm void metallic nanogaps on various solid substrates is proposed by utilizing the self-assembly of a uniform gold-silica core-shell nanoparticle monolayer at interfaces and chemical etching. The ultrasmall void metallic nanogap shows key advantages such as a strong near-field enhancement and free diffusion of analytes to the gap, which are useful in molecular sensing and monitoring.

Multifunctional hydrogel nano-probes for atomic force microscopy

Since the invention of the atomic force microscope (AFM) three decades ago, there have been numerous advances in its measurement capabilities. Curiously, throughout these developments, the fundamental nature of the force-sensing probe—the key actuating element—has remained largely unchanged. It is produced by long-established microfabrication etching strategies and typically composed of silicon-based materials. Here, we report a new class of photopolymerizable hydrogel nano-probes that are produced by bottom-up fabrication with compressible replica moulding. The hydrogel probes demonstrate excellent capabilities for AFM imaging and force measurement applications while enabling programmable, multifunctional capabilities based on compositionally adjustable mechanical properties and facile encapsulation of various nanomaterials. Taken together, the simple, fast and affordable manufacturing route and multifunctional capabilities of hydrogel AFM nano-probes highlight the potential of soft matter mechanical transducers in nanotechnology applications. The fabrication scheme can also be readily utilized to prepare hydrogel cantilevers, including in parallel arrays, for nanomechanical sensor devices.

Controlled Unusual Stiffness of Mechanical Metamaterials

Mechanical metamaterials that are engineered with sub-unit structures present unusual mechanical properties depending on the loading direction. Although they show promise, their practical utility has so far been somewhat limited because, to the best of our knowledge, no study about the potential of mechanical metamaterials made from sophisticatedly tailored sub-unit structures has been made. Here, we present a mechanical metamaterial whose mechanical properties can be systematically designed without changing its chemical composition or weight. We study the mechanical properties of triply periodic bicontinuous structures whose detailed sub-unit structure can be precisely fabricated using various sub-micron fabrication methods. Simulation results show that the effective wave velocity of the structures along with different directions can be designed to introduce the anisotropy of stiffness by changing a volume fraction and aspect ratio. The ratio of Young’s modulus to shear modulus can be increased by up to at least 100, which is a 3500% increase over that of isotropic material (2.8, acrylonitrile butadiene styrene). Furthermore, Poisson’s ratio of the constituent material changes the ratio while Young’s modulus does not influence it. This study presents the promising potential of mechanical metamaterials for versatile industrial and biomedical applications.

Hollow Microtube Resonators via Silicon Self-Assembly toward Subattogram Mass Sensing Applications

Fluidic resonators with integrated microchannels (hollow resonators) are attractive for mass, density, and volume measurements of single micro/nanoparticles and cells, yet their widespread use is limited by the complexity of their fabrication. Here we report a simple and cost-effective approach for fabricating hollow microtube resonators. A prestructured silicon wafer is annealed at high temperature under a controlled atmosphere to form self-assembled buried cavities. The interiors of these cavities are oxidized to produce thin oxide tubes, following which the surrounding silicon material is selectively etched away to suspend the oxide tubes. This simple three-step process easily produces hollow microtube resonators. We report another innovation in the capping glass wafer where we integrate fluidic access channels and getter materials along with residual gas suction channels. Combined together, only five photolithographic steps and one bonding step are required to fabricate vacuum-packaged hollow microtube resonators that exhibit quality factors as high as ∼ 13,000. We take one step further to explore additionally attractive features including the ability to tune the device responsivity, changing the resonator material, and scaling down the resonator size. The resonator wall thickness of ∼ 120 nm and the channel hydraulic diameter of ∼ 60 nm are demonstrated solely by conventional microfabrication approaches. The unique characteristics of this new fabrication process facilitate the widespread use of hollow microtube resonators, their translation between diverse research fields, and the production of commercially viable devices.

Label-free density difference amplification-based cell sorting

The selective cell separation is a critical step in fundamental life sciences, translational medicine, biotechnology, and energy harvesting. Conventional cell separation methods are fluorescent activated cell sorting and magnetic-activated cell sorting based on fluorescent probes and magnetic particles on cell surfaces. Label-free cell separation methods such as Raman-activated cell sorting, electro-physiologically activated cell sorting, dielectric-activated cell sorting, or inertial microfluidic cell sorting are, however, limited when separating cells of the same kind or cells with similar sizes and dielectric properties, as well as similar electrophysiological phenotypes. Here we report a label-free density difference amplification-based cell sorting (dDACS) without using any external optical, magnetic, electrical forces, or fluidic activations. The conceptual microfluidic design consists of an inlet, hydraulic jump cavity, and multiple outlets. Incoming particles experience gravity, buoyancy, and drag forces in the separation chamber. The height and distance that each particle can reach in the chamber are different and depend on its density, thus allowing for the separation of particles into multiple outlets. The separation behavior of the particles, based on the ratio of the channel heights of the inlet and chamber and Reynolds number has been systematically studied. Numerical simulation reveals that the difference between the heights of only lighter particles with densities close to that of water increases with increasing the ratio of the channel heights, while decreasing Reynolds number can amplify the difference in the heights between the particles considered irrespective of their densities.

General and programmable synthesis of hybrid liposome/metal nanoparticles

Programmable liposomes are designed to selectively produce various liposome-nanoparticle hybrids. Hybrid liposome/metal nanoparticles are promising candidate materials for biomedical applications. However, the poor selectivity and low yield of the desired hybrid during synthesis pose a challenge. We designed a programmable liposome by selective encoding of a reducing agent, which allows self-crystallization of metal nanoparticles within the liposome to produce stable liposome/metal nanoparticles alone. We synthesized seven types of liposome/monometallic and more complex liposome/bimetallic hybrids. The resulting nanoparticles are tunable in size and metal composition, and their surface plasmon resonance bands are controllable in visible and near infrared. Owing to outer lipid bilayer, our liposome/Au nanoparticle shows better colloidal stability in biologically relevant solutions as well as higher endocytosis efficiency than gold nanoparticles without the liposome. We used this hybrid in intracellular imaging of living cells via surface-enhanced Raman spectroscopy, taking advantage of its improved physicochemical properties. We believe that our method greatly increases the utility of metal nanoparticles in in vivo applications.

Immiscible oil-water interface: dual function of electrokinetic concentration of charged molecules and optical detection with interfacially trapped gold nanorods.

In this paper, we report that an immiscible oil-water interface can achieve the dual function of electrokinetic molecular concentration without external electric fields and sensitive optical detection without a microscope. As a proof-of-concept, we have shown that the concentration of positively charged molecules at the oleic acid-water interface can be increased significantly simply by controlling the pH. Three-dimensional phase field simulation suggests that the concentration of positively charged rhodamine 6G can be increased by about 10-fold at the interface. Surface-enhanced Raman spectroscopy (SERS) is utilized for label-free detection by taking advantage of this molecular accumulation occurring at the interface, since gold nanorods can be spontaneously trapped at the interface via electrostatic interaction. SERS measurements suggest that the immiscible oleic acid-water interface allows the limit of detection to be improved by 1-3 orders of magnitude.

A study on the interfacial effect on cancer-cell invasion

Cancer-cell invasion is a complex biological process involving cell migration through the extracellular matrix, which is driven by haptotaxis, and the interactions between cancer cells and the surrounding matrix. In this paper, a three-dimensional haptotaxis model that simulates the migration of a cancer cell population, including cell–cell adhesion and cell–matrix adhesion, is proposed. We employ a diffuse interface model that incorporates the mechanism of haptotaxis and the interface energy of cancer cells as well as that between cancer cells and the matrix. The semi-implicit Fourier spectral scheme is applied for high efficiency and numerical stability. The simulations systematically reveal the dynamics of cancer-cell migration and the effect of interface energy on the invasion of cancer cells.

Spontaneous Self-Formation of 3D Plasmonic Optical Structures.

Self-formation of colloidal oil droplets in water or water droplets in oil not only has been regarded as fascinating fundamental science but also has been utilized in an enormous number of applications in everyday life. However, the creation of three-dimensional (3D) architectures by a liquid droplet and an immiscible liquid interface has been less investigated than other applications. Here, we report interfacial energy-driven spontaneous self-formation of a 3D plasmonic optical structure at room temperature without an external force. Based on the densities and interfacial energies of two liquids, we simulated the spontaneous formation of a plasmonic optical structure when a water droplet containing metal ions meets an immiscible liquid polydimethylsiloxane (PDMS) interface. At the interface, the metal ions in the droplet are automatically reduced to form an interfacial plasmonic layer as the liquid PDMS cures. The self-formation of both an optical cavity and integrated plasmonic nanostructure significantly enhances the fluorescence by a magnitude of 1000. Our findings will have a huge impact on the development of various photonic and plasmonic materials as well as metamaterials and devices.

Computational kinetic study of chemotactic cell migration

The interaction between the cell and the substrate is the most prominent feature of the crawling cell. Here, a three-dimensional dynamic chemotaxis model for a crawling cell is proposed based on the diffuse interface description. From the computational analysis, the interfacial effect on the chemotactic migration is systematically analyzed with respect to an energetic and kinetic view. Quantitative information about the interfacial effect on the chemotactic migration is provided with a suggested correlation coefficient that defines the relation between the surface tension and the adhesion strength. Moreover, the analyzed kinetic effect elucidates the chemotactic migration of cells on morphologically patterned substrates. The developed approach provides considerably reliable information for the effective experimental control of crawling cells with the condition of a substrate.