Hygroscopic Micro/Nanolenses along Carbon Nanotube Ion Channels.

Nanolenses of alkali metal halides can be a unique optical element due to their hygroscopicity, optical transparency, and high mobility of constituent ions. It has been challenging, however, to form and place such lenses in a controlled manner. Here, we report micro/nanolenses of various alkali metal halides arranged as a one-dimensional (1D) array, using the exterior of single-walled carbon nanotubes (SWNTs) as a template for forming the lenses. Applying an electrical bias to an aqueous solution of alkali metal halides placed at the end of an SWNT array causes ionic transport along the exterior of SWNTs and the subsequent formation of salt micro/nanocrystals. The crystals serve as micro/nanolenses that optically visualize individual SWNTs and amplify their Raman scattering by orders of magnitude. Molecules dissolved in the ionic solution can be electrokinetically transported along the nanotubes, captured within the lenses, and analyzed by Raman spectroscopy, which we demonstrate by detecting ∼12 attomoles of glucose and 2 femtomoles of urea. The hygroscopic salt nanolenses are robust under various ambient conditions indefinitely, by transitioning to liquid droplets above their deliquescence relative humidity, yet can be removed nondestructively by water. Our approach could have broad implications in the optical visualization of 1D nanostructures, molecular transport or chemical reactions in 1D space, and molecular spectroscopy in salty environments.

Compartmentalized Arrays of Matrix Droplets for Quantitative Mass Spectrometry Imaging of Adsorbed Peptides.

Mass spectrometry imaging (MSI) based on matrix-assisted laser desorption/ionization (MALDI) provides information on the identification and spatial distribution of biomolecules. Quantitative analysis, however, has been challenging largely due to heterogeneity in both the size of the matrix crystals and the extraction area. In this work, we present a compartmentalized elastomeric stamp for quantitative MALDI-MSI of adsorbed peptides. Filling the compartments with matrix solution and stamping onto a planar substrate extract and concentrate analytes adsorbed in each compartment into a single analyte-matrix cocrystal over the entire stamped area. Walls between compartments help preserve spatial information on the adsorbates. The mass intensity of the cocrystals directly correlates with the surface coverage of analytes, which enables not only quantitative analysis but estimation of an equilibrium constant for the adsorption. We demonstrate via MALDI-MSI relative quantitation of peptides adsorbed along a microchannel with varying surface coverages.

Three-Dimensional, High-Resolution Printing of Carbon Nanotube/Liquid Metal Composites with Mechanical and Electrical Reinforcement.

The formation of three-dimensional (3D) interconnections is essential in integrated circuit packaging technology. However, conventional interconnection methods, including the wire-bonding process, were developed for rigid structures of electronic devices, and they are not applicable to the integration of soft and stretchable electronic devices. Hence, there is a strong demand for 3D interconnection technology that is applicable to soft, stretchable electronic devices. Herein, we introduce the material and the processing required for stretchable 3D interconnections on the soft forms of devices and substrates with high resolutions. Liquid-metal-based composites for use as stretchable interconnection materials were developed by uniformly dispersing Pt-decorated carbon nanotubes in a liquid metal matrix. The inclusion of carbon nanotubes in the liquid metal improves the mechanical strength of the composite, thereby overcoming the limitation of the liquid metal that has a low mechanical strength. The composites can be 3D printed with various dimensions: the minimum diameters are about 5 µm and have a breakdown current density comparable to that of metal wires.

N-Carbon-Doped Binary Nanophase of Metal Oxide/Metal-Organic Framework for Extremely Sensitive and Selective Gas Response.

Metal-organic frameworks (MOFs), which are highly ordered structures exhibiting sub-nanometer porosity, possess significant potential for diverse gas applications. However, their inherent insulative properties limit their utility in electrochemical gas sensing. This investigation successfully modifies the electrical conductivity of zeolitic imidazolte framework-8 (ZIF-8) employing a straightforward surface oxidation methodology. A ZIF-8 polycrystalline layer is applied on a wafer-scale oxide substrate and subjects to thermal annealing at 300 °C under ambient air conditions, resulting in nanoscale oxide layers while preserving the fundamental properties of the ZIF-8. Subsequent exposure to NO2 instigates the evolution of an electrically interconnected structure with the formation of electron-rich dopants derived from the decomposition of nitrogen-rich organic linkers. The N-carbon-hybridized ZnO/ZIF-8 device demonstrates remarkable sensitivity (≈130 ppm-1 ) and extreme selectivity in NO2 gas detection with a lower detection limit of 0.63 ppb under 150 °C operating temperature, surpassing the performance of existing sensing materials. The exceptional performances result from the Debye length scale dimensionality of ZnO and the high affinity of ZIF-8 to NO2 . The methodology for manipulating MOF conductivity through surface oxidation holds the potential to accelerate the development of MOF-hybridized conductive channels for a variety of electrical applications.

Alkalide-Assisted Direct Electron Injection for the Noninvasive n-Type Doping of Graphene.

Although the doping of graphene grown by chemical vapor deposition is crucial in graphene-based electronics, noninvasive methods of n-type doping have not been widely investigated in comparison with p-type doping methods. We developed a convenient and robust method for the noninvasive n-type doping of graphene, wherein electrons are directly injected from sodium anions into the graphene. This method involves immersing the graphene in solutions of [K(15-crown-5)2]Na prepared by dissolving a sodium-potassium (NaK) alloy in a 15-crown-5 solution. The n-type doping of the graphene was confirmed by downshifted G and 2D bands in Raman spectra and by the Dirac point shifting to a negative voltage. The electron-injected graphene showed no sign of structural damage, exhibited higher carrier mobilities than that of pristine graphene, and remained n-doped for over a month of storage in air. In addition, we demonstrated that electron injection enhances noncovalent interactions between graphene and metallomacrocycle molecules without requiring a linker, as used in previous studies, suggesting several potential applications of the method in modifying graphene with various functionalities.

Packaging vertically aligned carbon nanotubes into a heat-shrink tubing for efficient removal of phenolic pollutants.

Owing to their extremely high surface-to-volume ratio, carbon nanotubes (CNTs) are excellent adsorbents for the removal of organic pollutants. However, retrieval or collection of the CNTs after adsorption in existing approaches, which utilize CNTs dispersed in a solution of pollutants, is often more challenging than the removal of pollutants. In this study, we address this challenge by packaging vertically aligned CNTs into a PTFE heat-shrink tubing. Insertion of CNTs into the tubing and subsequent thermal shrinkage densified the CNTs radially by 35% and also reduced wrinkles in the nanotubes. The CNT-based adsorption tube with a circular cross-section enabled both easy functionalization of CNTs and facile connection to a source of polluted water, which we demonstrated for the removal of phenolic compounds. We purified and carboxylated CNTs, by flowing a solution of nitric acid through the tubing, and obtained adsorption capacities of 115, 124, and 81.2 mg g-1 for 0.5 g L-1 of phenol, m-cresol, 2-chlorophenol, respectively. We attribute the high adsorption capacity of our platform to efficient adsorbate-CNT interaction within the narrow interstitial channels between the aligned nanotubes. The CNT-based adsorption tubes are highly promising for the simple and efficient removal of phenolic and other types of organic pollutants.

Probing sub-diffraction optical confinement via the polarized Raman spectroscopy of a single-walled carbon nanotube.

Polarized Raman spectroscopy of a single-walled carbon nanotube (SWNT) was shown to serve as a simple alternative to sophisticated imaging tools for probing sub-diffraction optical phenomena. As a model system, we used TiO2 nanoparticles (n ∼ 2.67), which confine plane-polarized incident light (λ = 532 nm) into two bands less than 150 nm apart. After depositing the nanoparticles onto SWNTs and measuring the nanoparticle-SWNT distance, Raman spectra of individual SWNTs were obtained with the excitation laser polarized either parallel (θ = 0°) or perpendicular (θ = 90°) to the nanotubes. The spectral intensity increased by the nanoparticles only at θ = 90°, with the degree of enhancement being greater when the nanotube was located farther from the particle-substrate contact. Finite-difference time-domain simulations explained that such an enhancement at θ = 90° was a sub-diffraction phenomenon, which occurred when the nanotubes were located within one of the two confined bands formed by the TiO2 nanoparticles. On repeating the measurements on a two-dimensional graphene sheet, only diminished Raman scattering of the graphene with no polarization dependence was observed, confirming the advantage of the one-dimensional nanostructure for studying sub-diffraction optics.