Samuel Woojoo Jun

Multifunctional wearable devices for diagnosis and therapy of movement disorders

Wearable systems that monitor muscle activity, store data and deliver feedback therapy are the next frontier in personalized medicine and healthcare. However, technical challenges, such as the fabrication of high-performance, energy-efficient sensors and memory modules that are in intimate mechanical contact with soft tissues, in conjunction with controlled delivery of therapeutic agents, limit the wide-scale adoption of such systems. Here, we describe materials, mechanics and designs for multifunctional, wearable-on-the-skin systems that address these challenges via monolithic integration of nanomembranes fabricated with a top-down approach, nanoparticles assembled by bottom-up methods, and stretchable electronics on a tissue-like polymeric substrate. Representative examples of such systems include physiological sensors, non-volatile memory and drug-release actuators. Quantitative analyses of the electronics, mechanics, heat-transfer and drug-diffusion characteristics validate the operation of individual components, thereby enabling system-level multifunctionalities.

Large-Scale Synthesis of Bioinert Tantalum Oxide Nanoparticles for X-ray Computed Tomography Imaging and Bimodal Image-Guided Sentinel Lymph Node Mapping

Ever since Au nanoparticles were developed as X-ray contrast agents, researchers have actively sought alternative nanoparticle-based imaging probes that are not only inexpensive but also safe for clinical use. Herein, we demonstrate that bioinert tantalum oxide nanoparticles are suitable nanoprobes for high-performance X-ray computed tomography (CT) imaging while simultaneously being cost-effective and meeting the criteria as a biomedical platform. Uniformly sized tantalum oxide nanoparticles were prepared using a microemulsion method, and their surfaces were readily modified using various silane derivatives through simple in situ sol-gel reaction. The silane-modified surface enabled facile immobilization of functional moieties such as polyethylene glycol (PEG) and fluorescent dye. PEG was introduced to endow the nanoparticles with biocompatibility and antifouling activity, whereas immobilized fluorescent dye molecules enabled simultaneous fluorescence imaging as well as X-ray CT imaging. The resulting nanoparticles exhibited remarkable performances in the in vivo X-ray CT angiography and bimodal image-guided lymph node mapping. We also performed an extensive study on in vivo toxicity of tantalum oxide nanoparticles, revealing that the nanoparticles did not affect normal functioning of organs.

High-resolution three-photon biomedical imaging using doped ZnS nanocrystals

Three-photon excitation is a process that occurs when three photons are simultaneously absorbed within a luminophore for photo-excitation through virtual states. Although the imaging application of this process was proposed decades ago, three-photon biomedical imaging has not been realized yet owing to its intrinsic low quantum efficiency. We herein report on high-resolution in vitro and in vivo imaging by combining three-photon excitation of ZnS nanocrystals and visible emission from Mn(2+) dopants. The large three-photon cross-section of the nanocrystals enabled targeted cellular imaging under high spatial resolution, approaching the theoretical limit of three-photon excitation. Owing to the enhanced Stokes shift achieved through nanocrystal doping, the three-photon process was successfully applied to high-resolution in vivo tumour-targeted imaging. Furthermore, the biocompatibility of ZnS nanocrystals offers great potential for clinical applications of three-photon imaging.

Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst

A highly active and stable non-Pt electrocatalyst for hydrogen production has been pursued for a long time as an inexpensive alternative to Pt-based catalysts. Herein, we report a simple and effective approach to prepare high-performance iron phosphide (FeP) nanoparticle electrocatalysts using iron oxide nanoparticles as a precursor. A single-step heating procedure of polydopamine-coated iron oxide nanoparticles leads to both carbonization of polydopamine coating to the carbon shell and phosphidation of iron oxide to FeP, simultaneously. Carbon-shell-coated FeP nanoparticles show a low overpotential of 71 mV at 10 mA cm-2, which is comparable to that of a commercial Pt catalyst, and remarkable long-term durability under acidic conditions for up to 10 000 cycles with negligible activity loss. The effect of carbon shell protection was investigated both theoretically and experimentally. A density functional theory reveals that deterioration of catalytic activity of FeP is caused by surface oxidation. Extended X-ray absorption fine structure analysis combined with electrochemical test shows that carbon shell coating prevents FeP nanoparticles from oxidation, making them highly stable under hydrogen evolution reaction operation conditions. Furthermore, we demonstrate that our synthetic method is suitable for mass production, which is highly desirable for large-scale hydrogen production.

Sizing by Weighing: Characterizing Sizes of Ultrasmall-Sized Iron Oxide Nanocrystals Using MALDI-TOF Mass Spectrometry

We present a rapid and reliable method for determining the sizes and size distributions of <5 nm-sized iron oxide nanocrystals (NCs) using matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS). MS data were readily converted to size information using a simple equation. The size distribution obtained from the mass spectrum is well-matched with the data from transmission electron microscopy, which requires long and tedious analysis work. The size distribution obtained from the mass spectrum is highly resolved and can detect size differences of only a few angstroms. We used this MS-based technique to investigate the formation of iron oxide NCs, which is not easy to monitor with other methods. From ex situ measurements, we observed the transition from molecular precursors to clusters and then finally to NCs.

Magnetically recyclable core–shell nanocatalysts for efficient heterogeneous oxidation of alcohols

We describe the designed fabrication of magnetically recyclable core–shell Pd nanocatalysts for the efficient oxidation of alcohols under base-free reaction conditions in water. The Pd NPs that are half-partitioned in the polymer matrix can provide not only high catalytic activity but also stabilization of the nanocatalysts under harsh reaction conditions. Furthermore the magnetic separation provides a convenient method for removing and recycling the active Pd nanocatalysts from the reaction mixture. The designed nanocatalysts can be readily synthesized in a large scale and were able to be reused for five consecutive cycles of the oxidation of cycloheptanol. The nanocatalysts present high catalytic activity in other types of catalytic reactions involving Pd NPs such as Suzuki cross-coupling and reduction of nitroarenes.

Large-Scale Synthesis of Ultra-Small-Sized Silver Nanoparticles

Metallic silver nanoparticles have been intensely investigated for their applications in various areas due to their unique properties including excellent electrical conductivity, catalysis, plasmon light scattering, and antibacterial activity. These properties critically depend on the mean size and the size distribution of the nanoparticles as well as their shape. Accordingly, great efforts have been made for the last decades to develop synthetic methods of improved controllability on the size and shape of silver nanoparticles. Especially, small sized Ag nanoparticles below 5 nm that have a high surface-to-volume ratio showed unique catalytic activities and have been applied in low-cost printed electronics. Among various methods for the chemical synthesis of Ag nanoparticles published so far, there are only few of those that are capable of producing uniform Ag nanoparticles with mean sizes smaller than 5 nm. Moreover, synthetic methods for such small Ag nanoparticles with good productivity are even rarer. In general, for the synthesis of very small-sized Ag nanoparticles, low concentrations of silver precursor have been preferred in order to suppress the growth of the nanoparticles, but this is undesirable for the large-scale synthesis. Herein, we report a large-scale synthetic method to produce uniform and ultra-small-sized Ag nanoparticles with good productivity. This method is simple and efficient in that it can produce Ag nanoparticles within 20 min by heating a reaction mixture containing only three chemicals. In the synthesis of ultra-small Ag nanoparticles, oleylamine was used both as the reducing agent and surfactant, and oleic acid as the co-surfactant and co-solvent. Although there are several previous reports on the synthesis of Ag nanoparticles using a silver-alkylamine complex, the sizes achieved are generally larger than 3 nm. In the current synthesis, we were able to synthesize Ag nanoparticles as small as 1.7 nm by controlling the growth process. The size was controlled by changing the heating rate. 1.7 nm Ag nanoparticles were obtained with the heating rate of 10 8C min 1 while a heating rate of 1 8C min 1 yielded 3.5 nm-sized nanoparticles. In Figure 1, transmission electron microscopy (TEM) images of 1.7 nm and 3.5 nm Ag nanoparticles are shown. The relative standard deviations estimated from the TEM images were 26 % and 12 % for the 1.7 nmand 3.5 nm-sized nanoparticles, respectively. According to fast Fourier transform (FFT) analysis on hexagonal close-packed arrays of Ag nanoparticles (Figures 1 b and d), the interparticle distance was ~3.7 nm for 1.7 nm nanoparticles and ~4.8 nm for 3.5 nm ones. Considering that the chain length of the oleyl group is ~15 , these values indicate that the surfactant layers of the neighboring nanoparticles are largely overlapping. This can be explained by the high surface curvature of the ultra-small-sized nanoparticles, which leads to the loose packing of hydrocarbon chains of surfactants on the surface of the nanoparticles. The Ag nanoparticles were highly dispersible in many organic solvents including toluene, chloroform, hexane, and tetrahydrofuran. Because of the high initial concentration of the silver precursor and the simple synthetic method, this synthetic method can be easily scaled-up. As a demonstration, we carried out the synthesis using 10 mmol of silver nitrate as precursor for a single-batch reaction which yielded 2.06 g of 1.7 nm Ag nanoparticles (Figure S1 in the Supporting Information). To monitor the formation of ultra-small Ag nanoparticles in the solution during synthesis, we carried out ex situ measureFigure 1. TEM images of Ag nanoparticles with sizes of 1.7 nm (a, b) and 3.5 nm (c, d). In panels (b) and (d), the nanoparticles are in hexagonal closepacked arrays. The insets show FFT images of the corresponding nanoparticle arrays.

One-pot synthesis of magnetically recyclable mesoporous silica supported acid–base catalysts for tandem reactions

We report one-pot synthesis of magnetically recyclable mesoporous silica catalysts for tandem acid-base reactions. The catalysts could be easily recovered from the reaction mixture using a magnet, and the pore size of the catalysts could be controlled by introducing a swelling agent, resulting in the significant enhancement of the reaction rate.