Direct Characterization of Buried Interfaces in 2D/3D Heterostructures Enabled by GeO2 Release Layer.

Inconsistent interface control in devices based on two-dimensional materials (2DMs) has limited technological maturation. Astounding variability of 2D/three-dimensional (2D/3D) interface properties has been reported, which has been exacerbated by the lack of direct investigations of buried interfaces commonly found in devices. Herein, we demonstrate a new process that enables the assembly and isolation of device-relevant heterostructures for buried interface characterization. This is achieved by implementing a water-soluble substrate (GeO2), which enables deposition of many materials onto the 2DM and subsequent heterostructure release by dissolving the GeO2 substrate. Here, we utilize this novel approach to compare how the chemistry, doping, and strain in monolayer MoS2 heterostructures fabricated by direct deposition vary from those fabricated by transfer techniques to show how interface properties differ with the heterostructure fabrication method. Direct deposition of thick Ni and Ti films is found to react with the monolayer MoS2. These interface reactions convert 50% of MoS2 into intermetallic species, which greatly exceeds the 10% conversion reported previously and 0% observed in transfer-fabricated heterostructures. We also measure notable differences in MoS2 carrier concentration depending on the heterostructure fabrication method. Direct deposition of thick Au, Ni, and Al2O3 films onto MoS2 increases the hole concentration by >1012 cm-2 compared to heterostructures fabricated by transferring MoS2 onto these materials. Thus, we demonstrate a universal method to fabricate 2D/3D heterostructures and expose buried interfaces for direct characterization.

A High Entropy Oxide Designed to Catalyze CO Oxidation Without Precious Metals.

The chemical complexity of single-phase multicationic oxides, commonly termed high entropy oxides (HEOs), enables the integration of conventionally incompatible metal cations into a single-crystalline phase. However, few studies have effectively leveraged the multicationic nature of HEOs for optimization of disparate physical and chemical properties. Here, we apply the HEO concept to design robust oxidation catalysts in which multicationic oxide composition is tailored to simultaneously achieve catalytic activity, oxygen storage capacity, and thermal stability. Unlike conventional catalysts, HEOs maintain single-phase structure, even at high temperature, and do not rely on the addition of expensive platinum group metals (PGM) to be active. The HEOs are synthesized through a facile, relatively low temperature (500 °C) sol-gel method, which avoids excessive sintering and catalyst deactivation. Nanostructured high entropy oxides with surface areas as high as 138 m2/g are produced, marking a significant structural improvement over previously reported HEOs. Each HEO contained Ce in varying concentrations, as well as four other metals among Al, Fe, La, Mn, Nd, Pr, Sm, Y, and Zr. All samples adopted a fluorite structure. First row transition metal cations were most effective at improving CO oxidation activity, but their incorporation reduced thermal stability. Rare earth cations were necessary to prevent thermal deactivation while maintaining activity. In sum, our work demonstrates the utility of entropy in complex oxide design and a low-energy synthetic route to produce nanostructured HEOs with cations selected for a cooperative effect toward robust performance in chemically and physically demanding applications.

Ultra-thin enzymatic liquid membrane for CO2 separation and capture.

The limited flux and selectivities of current carbon dioxide membranes and the high costs associated with conventional absorption-based CO2 sequestration call for alternative CO2 separation approaches. Here we describe an enzymatically active, ultra-thin, biomimetic membrane enabling CO2 capture and separation under ambient pressure and temperature conditions. The membrane comprises a ~18-nm-thick close-packed array of 8 nm diameter hydrophilic pores that stabilize water by capillary condensation and precisely accommodate the metalloenzyme carbonic anhydrase (CA). CA catalyzes the rapid interconversion of CO2 and water into carbonic acid. By minimizing diffusional constraints, stabilizing and concentrating CA within the nanopore array to a concentration 10× greater than achievable in solution, our enzymatic liquid membrane separates CO2 at room temperature and atmospheric pressure at a rate of 2600 GPU with CO2/N2 and CO2/H2 selectivities as high as 788 and 1500, respectively, the highest combined flux and selectivity yet reported for ambient condition operation.

Structurally Deformed MoS2 for Electrochemically Stable, Thermally Resistant, and Highly Efficient Hydrogen Evolution Reaction.

The emerging molybdenum disulfide (MoS2 ) offers intriguing possibilities for realizing a transformative new catalyst for driving the hydrogen evolution reaction (HER). However, the trade-off between catalytic activity and long-term stability represents a formidable challenge and has not been extensively addressed. This study reports that metastable and temperature-sensitive chemically exfoliated MoS2 (ce-MoS2 ) can be made into electrochemically stable (5000 cycles), and thermally robust (300 °C) while maintaining synthetic scalability and excellent catalytic activity through physical-transformation into 3D structurally deformed nanostructures. The dimensional transition enabled by a high throughput electrohydrodynamic process provides highly accessible, and electrochemically active surface area and facilitates efficient transport across various interfaces. Meanwhile, the hierarchically strained morphology is found to improve electronic coupling between active sites and current collecting substrates without the need for selective engineering the electronically heterogeneous interfaces. Specifically, the synergistic combination of high strain load stemmed from capillarity-induced-self-crumpling and sulfur (S) vacancies intrinsic to chemical exfoliation enables simultaneous modulation of active site density and intrinsic HER activity regardless of continuous operation or elevated temperature. These results provide new insights into how catalytic activity, electrochemical-, and thermal stability can be concurrently enhanced through the physical transformation that is reminiscent of nature, in which properties of biological materials emerge from evolved dimensional transitions.

Laser Direct Write Synthesis of Lead Halide Perovskites.

Lead halide perovskites are increasingly considered for applications beyond photovoltaics, for example, light emission and detection, where an ability to pattern and prototype microscale geometries can facilitate the incorporation of this class of materials into devices. Here we demonstrate laser direct write of lead halide perovskites, a remarkably simple procedure that takes advantage of the inverse dependence between perovskite solubility and temperature by using a laser to induce localized heating of an absorbing substrate. We demonstrate arbitrary pattern formation of crystalline CH3NH3PbBr3 on a range of substrates and fabricate and characterize a microscale photodetector using this approach. This direct write methodology provides a path forward for the prototyping and production of perovskite-based devices.

Mesoporous Silica Nanoparticle-Supported Lipid Bilayers (Protocells) for Active Targeting and Delivery to Individual Leukemia Cells.

Many nanocarrier cancer therapeutics currently under development, as well as those used in the clinical setting, rely upon the enhanced permeability and retention (EPR) effect to passively accumulate in the tumor microenvironment and kill cancer cells. In leukemia, where leukemogenic stem cells and their progeny circulate within the peripheral blood or bone marrow, the EPR effect may not be operative. Thus, for leukemia therapeutics, it is essential to target and bind individual circulating cells. Here, we investigate mesoporous silica nanoparticle (MSN)-supported lipid bilayers (protocells), an emerging class of nanocarriers, and establish the synthesis conditions and lipid bilayer composition needed to achieve highly monodisperse protocells that remain stable in complex media as assessed in vitro by dynamic light scattering and cryo-electron microscopy and ex ovo by direct imaging within a chick chorioallantoic membrane (CAM) model. We show that for vesicle fusion conditions where the lipid surface area exceeds the external surface area of the MSN and the ionic strength exceeds 20 mM, we form monosized protocells (polydispersity index <0.1) on MSN cores with varying size, shape, and pore size, whose conformal zwitterionic supported lipid bilayer confers excellent stability as judged by circulation in the CAM and minimal opsonization in vivo in a mouse model. Having established protocell formulations that are stable colloids, we further modified them with anti-EGFR antibodies as targeting agents and reverified their monodispersity and stability. Then, using intravital imaging in the CAM, we directly observed in real time the progression of selective targeting of individual leukemia cells (using the established REH leukemia cell line transduced with EGFR) and delivery of a model cargo. Overall, we have established the effectiveness of the protocell platform for individual cell targeting and delivery needed for leukemia and other disseminated disease.

Engineered Theranostic Magnetic Nanostructures: Role of Composition and Surface Coating on Magnetic Resonance Imaging Contrast and Thermal Activation.

Magnetic nanostructures (MNS) have emerged as promising functional probes for simultaneous diagnostics and therapeutics (theranostic) applications due to their ability to enhance localized contrast in magnetic resonance imaging (MRI) and heat under external radio frequency (RF) field, respectively. We show that the "theranostic" potential of the MNS can be significantly enhanced by tuning their core composition and architecture of surface coating. Metal ferrite (e.g., MFe2O4) nanoparticles of ∼8 nm size and nitrodopamine conjugated polyethylene glycol (NDOPA-PEG) were used as the core and surface coating of the MNS, respectively. The composition was controlled by tuning the stoichiometry of MFe2O4 nanoparticles (M = Fe, Mn, Zn, ZnxMn1-x) while the architecture of surface coating was tuned by changing the molecular weight of PEG, such that larger weight is expected to result in longer length extended away from the MNS surface. Our results suggest that both core as well as surface coating are important factors to take into consideration during the design of MNS as theranostic agents which is illustrated by relaxivity and thermal activation plots of MNS with different core composition and surface coating thickness. After optimization of these parameters, the r2 relaxivity and specific absorption rate (SAR) up to 552 mM(-1) s(-1) and 385 W/g were obtained, respectively, which are among the highest values reported for MNS with core magnetic nanoparticles of size below 10 nm. In addition, NDOPA-PEG coated MFe2O4 nanostructures showed enhanced biocompatibility (up to [Fe] = 200 µg/mL) and reduced nonspecific uptake in macrophage cells in comparison to other well established FDA approved Fe based MR contrast agents.

Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide.

Establishing processing-structure-property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T' (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔG(H)), and, with respect to catalysis, the 1T' transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T' molybdenum disulfide and a lowering of ΔG(H) from +1.6 eV for 2H to +0.18 eV for 1T', comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.

Controlling the metal to semiconductor transition of MoS2 and WS2 in solution.

Lithiation-exfoliation produces single to few-layered MoS2 and WS2 sheets dispersible in water. However, the process transforms them from the pristine semiconducting 2H phase to a distorted metallic phase. Recovery of the semiconducting properties typically involves heating of the chemically exfoliated sheets at elevated temperatures. Therefore, it has been largely limited to sheets deposited on solid substrates. Here, we report the dispersion of chemically exfoliated MoS2 sheets in high boiling point organic solvents enabled by surface functionalization and the controllable recovery of their semiconducting properties directly in solution. This process connects the scalability of chemical exfoliation with the simplicity of solution processing, ultimately enabling a facile method for tuning the metal to semiconductor transitions of MoS2 and WS2 within a liquid medium.

Synthetic fossilization of soft biological tissues and their shape-preserving transformation into silica or electron-conductive replicas.

Structural preservation of complex biological systems from the subcellular to whole organism level in robust forms, enabling dissection and imaging while preserving 3D context, represents an enduring grand challenge in biology. Here we report a simple immersion method for structurally preserving intact organisms via conformal stabilization within silica. This self-limiting process, which we refer to as silica bioreplication, occurs by condensation of water-soluble silicic acid proximally to biomolecular interfaces throughout the organism. Conformal nanoscopic silicification of all biomolecular features imparts structural rigidity enabling the preservation of shape and nano-to-macroscale dimensional features upon drying to form a biocomposite and further high temperature oxidative calcination to form silica replicas or reductive pyrolysis to form electrically conductive carbon replicas of complete organisms. The simplicity and generalizability of this approach should facilitate efforts in biological preservation and analysis and could enable the development of new classes of biomimetic composite materials.

Thermoresponsive magnetic hydrogels as theranostic nanoconstructs.

We report the development of thermoresponsive magnetic hydrogels based on poly(N-isopropylacrylamide) encapsulation of Fe3O4 magnetic nanostructures (MNS). In particular, we examined the effects of hydrogels encapsulated with poly-ethylene glycol (PEG) and polyhedral oligomeric silsesquioxane (POSS) surface modified Fe3O4 MNS on magnetic resonance (MR) T2 (transverse spin relaxation) contrast enhancement and associated delivery efficacy of absorbed therapeutic cargo. The microstructural characterization reveal the regular spherical shape and size (∼200 nm) of the hydrogels with elevated hydrophilic to hydrophobic transition temperature (∼40 °C) characterized by LCST (lower critical solution temperature) due to the presence of encapsulated MNS. The hydrogel-MNS (HGMNS) system encapsulated with PEG functionalized Fe3O4 of 12 nm size (HGMNS-PEG-12) exhibited relaxivity rate (r2) of 173 mM(-1) s(-1) compared to 129 mM(-1) s(-1) obtained for hydrogel-MNS system encapsulated with POSS functionalized Fe3O4 (HGMNS-POSS-12) of the same size. Further studies with HGMNS-PEG-12 with absorbed drug doxorubicin (DOX) reveals approximately two-fold enhance in release during 1 h RF (radio-frequency) field exposure followed by 24 h incubation at 37 °C. Quantitatively, it is 2.1 µg mg(-1) (DOX/HGMNS) DOX release with RF exposure while only 0.9 µg mg(-1) release without RF exposure for the same period of incubation. Such enhanced release of therapeutic cargo is attributed to micro-environmental heating in the surroundings of MNS as well as magneto-mechanical vibrations under high frequency RF inside hydrogels. Similarly, RF-induced in vitro localized drug delivery studies with HeLa cell lines for HGMNS-PEG-12 resulted in more than 80% cell death with RF field exposures for 1 h. We therefore believe that magnetic hydrogel system has in vivo theranostic potential given high MR contrast enhancement from encapsulated MNS and RF-induced localized therapeutic delivery in one nanoconstruct.

Negatively charged metal oxide nanoparticles interact with the 20S proteasome and differentially modulate its biologic functional effects.

The multicatalytic ubiquitin-proteasome system (UPS) carries out proteolysis in a highly orchestrated way and regulates a large number of cellular processes. Deregulation of the UPS in many disorders has been documented. In some cases, such as carcinogenesis, elevated proteasome activity has been implicated in disease development, while the etiology of other diseases, such as neurodegeneration, includes decreased UPS activity. Therefore, agents that alter proteasome activity could suppress as well as enhance a multitude of diseases. Metal oxide nanoparticles, often developed as diagnostic tools, have not previously been tested as modulators of proteasome activity. Here, several types of metal oxide nanoparticles were found to adsorb to the proteasome and show variable preferential binding for particular proteasome subunits with several peptide binding "hotspots" possible. These interactions depend on the size, charge, and concentration of the nanoparticles and affect proteasome activity in a time-dependent manner. Should metal oxide nanoparticles increase proteasome activity in cells, as they do in vitro, unintended effects related to changes in proteasome function can be expected.

Ligand conjugation of chemically exfoliated MoS2.

MoS2 is a two-dimensional material that is gaining prominence due to its unique electronic and chemical properties. Here, we demonstrate ligand conjugation of chemically exfoliated MoS2 using thiol chemistry. With this method, we modulate the ζ-potential and colloidal stability of MoS2 sheets through ligand designs, thus enabling its usage as a selective artificial protein receptor for ß-galactosidase. The facile thiol functionalization route opens the door for surface modifications of solution processable MoS2 sheets.

Enhanced field-emission behavior of layered MoS2 sheets.

Field emission studies are reported for the first time on layered MoS2 sheets at the base pressure of ∼1 × 10⁻8 mbar. The turn-on field required to draw a field emission current density of 10 µA/cm² is found to be 3.5 V/µm for MoS2 sheets. The turn-on values are found to be significantly lower than the reported MoS2 nanoflowers, graphene, and carbon nanotube-based field emitters due to the high field enhancement factor (∼1138) associated with nanometric sharp edges of MoS2 sheet emitter surface. The emission current-time plots show good stability over a period of 3 h. Owing to the low turn-on field and planar (sheetlike) structure, the MoS2 could be utilized for future vacuum microelectronics/nanoelectronic and flat panel display applications.

Nanoscale graphene oxide (nGO) as artificial receptors: implications for biomolecular interactions and sensing.

The role of conventional graphene-oxide in biosensing has been limited to that of a quenching substrate or signal transducer due to size inconsistencies and poor supramolecular response. We overcame these issues by using nanoscale GOs (nGO) as artificial receptors. Unlike conventional GO, nGOs are sheets with near uniform lateral dimension of 20 nm. Due to its nanoscale architecture, its supramolecular response was enhanced, with demonstrated improvements in biomacromolecular affinities. This rendered their surface capable of detecting unknown proteins with cognizance not seen with conventional GOs. Different proteins at 100 and 10 nM concentrations revealed consistent patterns that are quantitatively differentiable by linear discriminant analysis. Identification of 48 unknowns in both concentrations demonstrated a >95% success rate. The 10 nM detection represents a 10-fold improvement over analogous arrays. This demonstrates for the first time that the supramolecular chemistry of GO is highly size dependent and opens the possibility of improvement upon existing GO hybrid materials.

Graphene oxide as an enzyme inhibitor: modulation of activity of α-chymotrypsin.

We have investigated the efficacy of graphene oxide (GO) in modulating enzymatic activity. Specifically, we have shown that GO can act as an artificial receptor and inhibit the activity of α-chymotrypsin (ChT), a serine protease. Most significantly, our data demonstrate that GO exhibits the highest inhibition dose response (by weight) for ChT inhibition compared with all other reported artificial inhibitors. Through fluorescence spectroscopy and circular dichroism studies, we have shown that this protein-receptor interaction is highly biocompatible and conserves the protein's secondary structure over extended periods (>24 h). We have also explored GO-enzyme interactions by controlling the ionic strength of the medium, which attenuates the host-guest electrostatic interactions. These findings suggest a new generation of enzymatic inhibitors that can be applied to other complex proteins by systematic modification of the GO functionality.

Hybrid magnetic nanostructures (MNS) for magnetic resonance imaging applications.

The development of MRI contrast agents has experienced its version of the gilded age over the past decade, thanks largely to the rapid advances in nanotechnology. In addition to progress in single mode contrast agents, which ushered in unprecedented R(1) or R(2) sensitivities, there has also been a boon in the development of agents covering more than one mode of detection. These include T(1)-PET, T(2)-PET T(1)-optical, T(2)-optical, T(1)-T(2) agents and many others. In this review, we describe four areas which we feel have experienced particular growth due to nanotechnology, specifically T(2) magnetic nanostructure development, T(1)/T(2)-optical dual mode agents, and most recently the T(1)-T(2) hybrid imaging systems. In each of these systems, we describe applications including in vitro, in vivo usage and assay development. In all, while the benefits and drawbacks of most MRI contrast agents depend on the application at hand, the recent development in multimodal nanohybrids may curtail the shortcomings of single mode agents in diagnostic and clinical settings by synergistically incorporating functionality. It is hoped that as nanotechnology advances over the next decade, it will produce agents with increased diagnostics and assay relevant capabilities in streamlined packages that can meaningfully improve patient care and prognostics. In this review article, we focus on T(2) materials, its surface functionalization and coupling with optical and/or T(1) agents.