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1.
Nat Commun ; 9(1): 2998, 2018 07 31.
Artigo em Inglês | MEDLINE | ID: mdl-30065278

RESUMO

The growing interest in two-dimensional imine-based covalent organic frameworks (COFs) is inspired by their crystalline porous structures and the potential for extensive π-electron delocalization. The intrinsic reversibility and strong polarization of imine linkages, however, leads to insufficient chemical stability and optoelectronic properties. Developing COFs with improved robustness and π-delocalization is highly desirable but remains an unsettled challenge. Here we report a facile strategy that transforms imine-linked COFs into ultrastable porous aromatic frameworks by kinetically fixing the reversible imine linkage via an aza-Diels-Alder cycloaddition reaction. The as-formed, quinoline-linked COFs not only retain crystallinity and porosity, but also display dramatically enhanced chemical stability over their imine-based COF precursors, rendering them among the most robust COFs up-to-date that can withstand strong acidic, basic and redox environment. Owing to the chemical diversity of the cycloaddition reaction and structural tunability of COFs, the pores of COFs can be readily engineered to realize pre-designed surface functionality.

2.
Arch Oral Biol ; 63: 82-92, 2016 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26741830

RESUMO

INTRODUCTION: Covalently bound functional GAGs orchestrate tissue mechanics through time-dependent characteristics. OBJECTIVE: The role of specific glycosaminoglycans (GAGs) at the ligament-cementum and cementum-dentin interfaces within a human periodontal complex were examined. Matrix swelling and resistance to compression under health and modeled diseased states was investigated. MATERIALS AND METHODS: The presence of keratin sulfate (KS) and chondroitin sulfate (CS) GAGs at the ligament-cementum and cementum-dentin interfaces in human molars (N=5) was illustrated by using enzymes, atomic force microscopy (AFM), and AFM-based nanoindentation. The change in physical characteristics of modeled diseased states through sequential digestion of keratin sulfate (KS) and chondroitin sulfate (CS) GAGs was investigated. One-way ANOVA tests with P<0.05 were performed to determine significant differences between groups. Additionally, the presence of mineral within the seemingly hygroscopic interfaces was investigated using transmission electron microscopy. RESULTS: Immunohistochemistry (N=3) indicated presence of biglycan and fibromodulin small leucine rich proteoglycans at the interfaces. Digestion of matrices with enzymes confirmed the presence of KS and CS GAGs at the interfaces by illustrating a change in tissue architecture and mechanics. A significant increase in height (nm), decrease in elastic modulus (GPa), and tissue deformation rate (nm/s) of the PDL-C attachment site (215±63-424±94nm; 1.5±0.7-0.4±0.2GPa; 21±7-48±22nm/s), and cementum-dentin interface (122±69-360±159nm; 2.9±1.3-0.7±0.3GPa; 18±4-30±6nm/s) was observed. CONCLUSIONS: The sequential removal of GAGs indicated loss in intricate structural hierarchy of hygroscopic interfaces. From a mechanics perspective, GAGs provide tissue recovery/resilience. The results of this study provide insights into the role of GAGs toward conserved tooth movement in the socket in response to mechanical loads, and modulation of potentially deleterious strain at tissue interfaces.


Assuntos
Glicosaminoglicanos/fisiologia , Proteoglicanas/fisiologia , Raiz Dentária/ultraestrutura , Adolescente , Adulto , Cemento Dentário/fisiologia , Dentina/fisiologia , Humanos , Imuno-Histoquímica , Técnicas In Vitro , Microscopia de Força Atômica , Microscopia Eletrônica de Varredura , Dente Molar , Ligamento Periodontal/fisiologia , Desmineralização do Dente
3.
Chem Sci ; 6(11): 6295-6304, 2015 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-30090247

RESUMO

Our understanding of structure and bonding in nanoscale materials is incomplete without knowledge of their surface structure. Needed are better surveying capabilities responsive not only to different atoms at the surface, but also their respective coordination environments. We report here that d-block organometallics, when placed at nanocrystal surfaces through heterometallic bonds, serve as molecular beacons broadcasting local surface structure in atomic detail. This unique ability stems from their elemental specificity and the sensitivity of their d-orbital level alignment to local coordination environment, which can be assessed spectroscopically. Re-surfacing cadmium and lead chalcogenide nanocrystals with iron- or ruthenium-based molecular beacons is readily accomplished with trimethylsilylated cyclopentadienyl metal carbonyls. For PbSe nanocrystals with iron-based beacons, we show how core-level X-ray spectroscopies and DFT calculations enrich our understanding of both charge and atomic reorganization at the surface when beacons are bound.

4.
Bone ; 57(2): 455-67, 2013 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-24063947

RESUMO

This study investigates bony protrusions within a narrowed periodontal ligament space (PDL-space) of a human bone-PDL-tooth fibrous joint by mapping structural, biochemical, and mechanical heterogeneity. Higher resolution structural characterization was achieved via complementary atomic force microscopy (AFM), nano-transmission X-ray microscopy (nano-TXM), and microtomography (MicroXCT™). Structural heterogeneity was correlated to biochemical and elemental composition, illustrated via histochemistry and microprobe X-ray fluorescence analysis (µ-XRF), and mechanical heterogeneity evaluated by AFM-based nanoindentation. Results demonstrated that the narrowed PDL-space was due to invasion of bundle bone (BB) into PDL-space. Protruded BB had a wider range with higher elastic modulus values (2-8GPa) compared to lamellar bone (0.8-6GPa), and increased quantities of Ca, P and Zn as revealed by µ-XRF. Interestingly, the hygroscopic 10-30µm interface between protruded BB and lamellar bone exhibited higher X-ray attenuation similar to cement lines and lamellae within bone. Localization of the small leucine rich proteoglycan biglycan (BGN) responsible for mineralization was observed at the PDL-bone interface and around the osteocyte lacunae. Based on these results, it can be argued that the LB-BB interface was the original site of PDL attachment, and that the genesis of protruded BB identified as protrusions occurred as a result of shift in strain. We emphasize the importance of bony protrusions within the context of organ function and that additional study is warranted.


Assuntos
Osso e Ossos/fisiologia , Articulações/fisiologia , Ligamento Periodontal/fisiologia , Dente/fisiologia , Biglicano/metabolismo , Fenômenos Biomecânicos , Osso e Ossos/diagnóstico por imagem , Osso e Ossos/ultraestrutura , Cálcio/metabolismo , Cemento Dentário/diagnóstico por imagem , Cemento Dentário/fisiologia , Módulo de Elasticidade , Proteínas da Matriz Extracelular/metabolismo , Fibromodulina , Fluorescência , Humanos , Imuno-Histoquímica , Articulações/ultraestrutura , Microscopia de Força Atômica , Modelos Biológicos , Ligamento Periodontal/citologia , Ligamento Periodontal/diagnóstico por imagem , Ligamento Periodontal/ultraestrutura , Fósforo/metabolismo , Proteoglicanas/metabolismo , Propriedades de Superfície , Dente/diagnóstico por imagem , Dente/ultraestrutura , Microtomografia por Raio-X , Zinco/metabolismo
5.
Nano Lett ; 12(3): 1295-9, 2012 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-22339758

RESUMO

Transmission electron microscopy was used to determine the structure of molecular films of self-assembled monolayers of pentathiophene derivatives supported on various electron transparent substrates. Despite the extreme beam sensitivity of the monolayers, structural crystallographic maps were obtained that revealed the nanoscale structure of the film. The image resolution is determined by the minimum beam diameter that the radiation hardness of the monolayer can support, which in our case is about 90 nm for a beam current of 5 × 10(6) e(-)/s. Electron diffraction patterns were collected while scanning a parallel electron beam over the film. These maps contain uncompromised information of the size, symmetry and orientation of the unit cell, orientation and structure of the domains, degree of crystallinity, and their variation on the micrometer scale, which are crucial to understand the electrical transport properties of the organic films. This information allowed us to track small changes in the unit cell size driven by the chemical modification of the support film.


Assuntos
Membranas Artificiais , Microscopia Eletrônica/métodos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Compostos Orgânicos/química , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Propriedades de Superfície
6.
Nano Lett ; 11(10): 4107-12, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21848283

RESUMO

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.

7.
Chem Commun (Camb) ; 46(45): 8579-81, 2010 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-20972497

RESUMO

Symmetrically substituted hexakis(alkoxy)triphenylene (HAT) derivatives were assembled into single molecular thick 2D nanosheets, which stacked further to give multilayered nanofibers through a convenient solution process. Detailed information on molecular arrangement was unraveled by various imaging techniques and diffraction studies.

8.
ACS Nano ; 4(8): 4762-8, 2010 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-20604526

RESUMO

We demonstrate a process for batch production of large-area (100-3000 microm(2)) patterned free-standing graphene membranes on Cu scaffolds using chemical vapor deposition (CVD)-grown graphene. This technique avoids the use of silicon and transfers of graphene. As one application of this technique, we fabricate transmission electron microscopy (TEM) sample supports. TEM characterization of the graphene membranes reveals relatively clean, highly TEM-transparent, single-layer graphene regions ( approximately 50% by area) and, despite the polycrystalline nature of CVD graphene, membrane yields as high as 75-100%. This high yield verifies that the intrinsic strength and integrity of CVD-grown graphene films is sufficient for sub-100 microm width membrane applications. Elemental analysis (electron energy loss spectroscopy (EELS) and X-ray energy-dispersive spectroscopy (EDS)) of the graphene membranes reveals some nanoscaled contamination left over from the etching process, and we suggest several ways to reduce this contamination and improve the quality of the graphene for electronic device applications. This large-scale production of suspended graphene membranes facilitates access to the two-dimensional physics of graphene that are suppressed by substrate interactions and enables the widespread use of graphene-based sample supports for electron and optical microscopy.


Assuntos
Carbono/química , Membranas Artificiais , Nanotecnologia/métodos , Cobre/química , Microscopia Eletrônica de Transmissão , Nanotecnologia/economia , Fenômenos Ópticos , Propriedades de Superfície , Suspensões , Volatilização
9.
J Am Chem Soc ; 132(25): 8697-703, 2010 Jun 30.
Artigo em Inglês | MEDLINE | ID: mdl-20521788

RESUMO

Three series of bimetallic nanoparticle catalysts (Rh(x)Pd(1-x), Rh(x)Pt(1-x), and Pd(x)Pt(1-x), x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized Rh(x)Pd(1-x) and Pd(x)Pt(1-x) nanoparticles have a core-shell structure whereas the Rh(x)Pt(1-x) alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The Rh(x)Pd(1-x) and Rh(x)Pt(1-x) nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O(2) at 300 degrees C) to reducing (H(2) or CO at 300 degrees C) or catalytic (mixture of NO and CO at 300 degrees C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the Pd(x)Pt(1-x) nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.

10.
Proc Natl Acad Sci U S A ; 106(27): 10917-21, 2009 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-19541601

RESUMO

The development of probes for single-molecule imaging has dramatically facilitated the study of individual molecules in cells and other complex environments. Single-molecule probes ideally exhibit good brightness, uninterrupted emission, resistance to photobleaching, and minimal spectral overlap with cellular autofluorescence. However, most single-molecule probes are imperfect in several of these aspects, and none have been shown to possess all of these characteristics. Here we show that individual lanthanide-doped upconverting nanoparticles (UCNPs)--specifically, hexagonal phase NaYF(4) (beta-NaYF(4)) nanocrystals with multiple Yb(3+) and Er(3+) dopants--emit bright anti-Stokes visible upconverted luminescence with exceptional photostability when excited by a 980-nm continuous wave laser. Individual UCNPs exhibit no on/off emission behavior, or "blinking," down to the millisecond timescale, and no loss of intensity following an hour of continuous excitation. Amphiphilic polymer coatings permit the transfer of hydrophobic UCNPs into water, resulting in individual water-soluble nanoparticles with undiminished photophysical characteristics. These UCNPs are endocytosed by cells and show strong upconverted luminescence, with no measurable anti-Stokes background autofluorescence, suggesting that UCNPs are ideally suited for single-molecule imaging experiments.


Assuntos
Elementos da Série dos Lantanídeos/química , Luminescência , Nanopartículas/química , Animais , Sobrevivência Celular , Fibroblastos/citologia , Membranas Artificiais , Camundongos , Células NIH 3T3 , Nanopartículas/ultraestrutura , Compostos de Silício/química , Solubilidade , Água
11.
Nano Lett ; 8(7): 1936-9, 2008 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-18505301

RESUMO

We report the results of charge transport studies on single CdTe nanocrystals contacted via evaporated Pd electrodes. Device charging energy, E c, monitored as a function of electrode separation drops suddenly at separations below approximately 55 nm. This drop can be explained by chemical changes induced by the metal electrodes. This explanation is corroborated by ensemble X-ray photoelectron spectroscopy studies of CdTe films as well as single particle measurements by transmission electron microscopy and energy dispersive X-rays. Similar to robust optical behavior obtained when nanocrystals are coated with a protective shell, we find that a protective SiO 2 layer deposited between the nanocrystal and the electrode prevents interface reactions and an associated drop in E c,max. This observation of interface reactivity and its effect on electrical properties has important implications for the integration of nanocrystals into conventional fabrication techniques and may enable novel nanomaterials.

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