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On the Ytterbium Valence and the Physical Properties in Selected Intermetallic Phases. ACS ORGANIC & INORGANIC AU 2024; 4:188-222. [PMID: 38585514 PMCID: PMC10996054 DOI: 10.1021/acsorginorgau.3c00054] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 10/13/2023] [Revised: 11/16/2023] [Accepted: 11/17/2023] [Indexed: 04/09/2024]
Abstract
The present review summarizes important aspects of the crystal chemistry of ytterbium-based intermetallic compounds along with a selection of their outstanding physical properties. These originate in many cases from the ytterbium valence. Different valence states are possible here, divalent (4f14), intermediate-valent, or trivalent (4f13) ytterbium, resulting in simple diamagnetic, Pauli or Curie-Weiss paramagnetic, or valence fluctuating behavior. Especially, some of the Yb3+ intermetallics have gained deep interest due to their Kondo or heavy Fermion ground states. We have summarized their property investigations using magnetic and transport measurements, specific heat data, NMR, ESR, and Mössbauer spectroscopy, elastic and inelastic neutron scattering, and XAS data as well as detailed thermoelectric measurements.
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Spectroscopic Insights into the Influence of Filling Carbon Nanotubes with Atomic Nanowires for Photophysical and Photochemical Applications. ACS APPLIED NANO MATERIALS 2023; 6:2883-2893. [PMID: 36875181 PMCID: PMC9972344 DOI: 10.1021/acsanm.2c05266] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/07/2022] [Accepted: 01/30/2023] [Indexed: 06/18/2023]
Abstract
Studying the optical performance of carbon nanotubes (CNTs) filled with guest materials can reveal the fundamental photochemical nature of ultrathin one-dimensional (1D) nanosystems, which are attractive for applications including photocatalysis. Here, we report comprehensive spectroscopic studies of how infiltrated HgTe nanowires (NWs) alter the optical properties of small-diameter (d t < 1 nm) single-walled carbon nanotubes (SWCNTs) in different environments: isolated in solution, suspended in a gelatin matrix, and heavily bundled in network-like thin films. Temperature-dependent Raman and photoluminescence measurements revealed that the HgTe NW filling can alter the stiffness of SWCNTs and therefore modify their vibrational and optical modes. Results from optical absorption and X-ray photoelectron spectroscopy demonstrated that the semiconducting HgTe NWs did not provide substantial charge transfer to or from the SWCNTs. Transient absorption spectroscopy further highlighted that the filling-induced nanotube distortion can alter the temporal evolution of excitons and their transient spectra. In contrast to previous studies on functionalized CNTs, where electronic or chemical doping often drove changes to the optical spectra, we highlight structural distortion as playing an important role.
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Atomic Layer Grown Zinc-Tin Oxide as an Alternative Buffer Layer for Cu 2ZnSnS 4-Based Thin Film Solar Cells: Influence of Absorber Surface Treatment on Buffer Layer Growth. ACS APPLIED ENERGY MATERIALS 2022; 5:13971-13980. [PMID: 36465259 PMCID: PMC9710524 DOI: 10.1021/acsaem.2c02579] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/11/2022] [Accepted: 10/03/2022] [Indexed: 06/17/2023]
Abstract
Zn1-x Sn x O y (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnO x and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na-CO x species from the CZTS surface, may be useful for the ZTO/CZTS device performance.
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Submolecular Resolution Imaging of P3HT:PCBM Nanostructured Films by Atomic Force Microscopy: Implications for Organic Solar Cells. ACS APPLIED NANO MATERIALS 2022; 5:13794-13804. [PMID: 36338328 PMCID: PMC9623582 DOI: 10.1021/acsanm.2c01399] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
The efficiency of organic bulk-heterojunction (BHJ) solar cells depends greatly on both the bulk and surface structure of the nanostructured bicontinuous interpenetrating network of materials, known as the active layer. The morphology of the top layer of a coated film is often resolved at the scale of a few nanometers, but fine details of the domains and the order within them are more difficult to identify. Here, we report a high-resolution atomic force microscopy (AFM) investigation of various stoichiometries of the well-studied poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) active layer mixture. Images of the surface were obtained using AC-mode AFM exciting higher-order resonance frequencies of a standard silicon probe, a promising technique for acquiring real-space images of organic-based thin films with nanoscale and even submolecular resolution. We provide firm evidence of the nanoscale organization of the P3HT polymer and of the P3HT:PCBM stoichiometric mixtures at the surface-air interface of the BHJ architecture. Our study shows the characteristic periodicity of the regioregular P3HT identified in the nanoscale domain areas with submolecular resolution. Such areas are then distorted in place when adding different quantities of PCBM forming stoichiometric mixtures. When the samples were exposed to ambient light, the morphologies were very different, and submolecular resolution was not achieved. This approach is shown to provide a precise view of the active layer's nanostructure and will be useful for studies of other materials as a function of various parameters, with particular attention to the role of the acceptor in tuning morphology for understanding optimum performance in organic photovoltaic devices.
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On the Contact Optimization of ALD-Based MoS 2 FETs: Correlation of Processing Conditions and Interface Chemistry with Device Electrical Performance. ACS APPLIED ELECTRONIC MATERIALS 2021; 3:3185-3199. [PMID: 34337417 PMCID: PMC8320240 DOI: 10.1021/acsaelm.1c00379] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/23/2021] [Accepted: 06/11/2021] [Indexed: 06/13/2023]
Abstract
Despite the extensive ongoing research on MoS2 field effect transistors (FETs), the key role of device processing conditions in the chemistry involved at the metal-to-MoS2 interface and their influence on the electrical performance are often overlooked. In addition, the majority of reports on MoS2 contacts are based on exfoliated MoS2, whereas synthetic films are even more susceptible to the changes made in device processing conditions. In this paper, working FETs with atomic layer deposition (ALD)-based MoS2 films and Ti/Au contacts are demonstrated, using current-voltage (I-V) characterization. In pursuit of optimizing the contacts, high-vacuum thermal annealing as well as O2/Ar plasma cleaning treatments are introduced, and their influence on the electrical performance is studied. The electrical findings are linked to the interface chemistry through X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) analyses. XPS evaluation reveals that the concentration of organic residues on the MoS2 surface, as a result of resist usage during the device processing, is significant. Removal of these contaminations with O2/Ar plasma changes the MoS2 chemical state and enhances the MoS2 electrical properties. Based on the STEM analysis, the observed progress in the device electrical characteristics could also be associated with the formation of a continuous TiS x layer at the Ti-to-MoS2 interface. Scaling down the Ti interlayer thickness and replacing it with Cr is found to be beneficial as well, leading to further device performance advancements. Our findings are of value for attaining optimal contacts to synthetic MoS2 films.
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True Nature of the Transition-Metal Carbide/Liquid Interface Determines Its Reactivity. ACS Catal 2021; 11:4920-4928. [PMID: 33898080 PMCID: PMC8057231 DOI: 10.1021/acscatal.1c00415] [Citation(s) in RCA: 16] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2021] [Revised: 03/17/2021] [Indexed: 01/01/2023]
Abstract
Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO2RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo2C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO2 activation. The oxides are stable down to potentials as low as -1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo2C indeed shows CO2RR activity.
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Characterization of GaAs Solar Cells under Supercontinuum Long-Time Illumination. MATERIALS 2021; 14:ma14020461. [PMID: 33477904 PMCID: PMC7833441 DOI: 10.3390/ma14020461] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/14/2020] [Revised: 01/13/2021] [Accepted: 01/15/2021] [Indexed: 02/07/2023]
Abstract
This work is dedicated to the description of the degradation of GaAs solar cells under continuous laser irradiation. Constant and strong exposure of the solar cell was performed over two months. Time-dependent electrical characteristics are presented. The structure of the solar cells was studied at the first and last stages of degradation test. The data from Raman spectroscopy, reflectometry, and secondary ion mass spectrometry confirm displacement of titanium and aluminum atoms. X-ray photoelectron spectroscopy showed a slight redistribution of oxygen bonds in the anti-corrosion coating.
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Approaches for the quantitative analysis of oxidation state in cerium oxide nanomaterials. NANOTECHNOLOGY 2019; 30:085703. [PMID: 30240366 PMCID: PMC6351072 DOI: 10.1088/1361-6528/aae364] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/21/2023]
Abstract
Cerium oxide nanomaterials (nanoceria, CNMs) are receiving increased attention from the research community due to their unique chemical properties, most prominent of which is their ability to alternate between the Ce3+ and Ce4+ oxidation states. While many analytical techniques and methods have been employed to characterize the amounts of Ce3+ and Ce4+ present (Ce3+/Ce4+ ratio) within nanoceria materials, to-date no studies have used multiple complementary analytical tools (orthogonal analysis) with technique-independent oxidation state controls for quantitative determinations of the Ce3+/Ce4+ ratio. Here, we describe the development of analytical methods measuring the oxidation states of nanoceria analytes using technique-independent Ce3+ (CeAlO3:Ge) and Ce4+ (CeO2) control materials, with a particular focus on x-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) approaches. The developed methods were demonstrated in characterizing a suite of commercial nanoceria products, where the two techniques (XPS and EELS) were found to be in good agreement with respect to Ce3+/Ce4+ ratio. Potential sources of artifacts and discrepancies in the measurement results were also identified and discussed, alongside suggestions for interpreting oxidation state results using the different analytical techniques. The results should be applicable towards producing more consistent and reproducible oxidation state analyses of nanoceria materials.
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Enhancing Graphene Protective Coatings by Hydrogen-Induced Chemical Bond Formation. ACS APPLIED NANO MATERIALS 2018; 1:4509-4515. [PMID: 32596648 PMCID: PMC7311049 DOI: 10.1021/acsanm.8b00610] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/13/2018] [Accepted: 08/23/2018] [Indexed: 06/11/2023]
Abstract
Increased interactions at the graphene-metal interface are here demonstrated to yield an effective prevention of intercalation of foreign species below the graphene cover. Hereby, an engineering pathway for increasing the usability of graphene as a metal coating is demonstrated. Graphene on Ir(111) (Gr/Ir(111)) is used as a model system, as it has previously been well-established that an increased interaction and formation of chemical bonds at the graphene-Ir interface can be induced by hydrogen functionalization of the graphene from its top side. With X-ray photoelectron spectroscopy, it is shown that hydrogen-induced increased interactions at the Gr/Ir(111) interface effectively prevents intercalation of CO in the millibar range. The scheme leads to protection against at least 10 times higher pressure and 70 times higher fluences of CO, compared to the protection offered by pristine Gr/Ir(111).
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Cell wall-bound silicon optimizes ammonium uptake and metabolism in rice cells. ANNALS OF BOTANY 2018; 122:303-313. [PMID: 29788158 PMCID: PMC6070024 DOI: 10.1093/aob/mcy068] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/07/2018] [Accepted: 04/18/2018] [Indexed: 05/09/2023]
Abstract
BACKGROUND AND AIMS Turgor-driven plant cell growth depends on cell wall structure and mechanics. Strengthening of cell walls on the basis of an association and interaction with silicon (Si) could lead to improved nutrient uptake and optimized growth and metabolism in rice (Oryza sativa). However, the structural basis and physiological mechanisms of nutrient uptake and metabolism optimization under Si assistance remain obscure. METHODS Single-cell level biophysical measurements, including in situ non-invasive micro-testing (NMT) of NH4+ ion fluxes, atomic force microscopy (AFM) of cell walls, and electrolyte leakage and membrane potential, as well as whole-cell proteomics using isobaric tags for relative and absolute quantification (iTRAQ), were performed. KEY RESULTS The altered cell wall structure increases the uptake rate of the main nutrient NH4+ in Si-accumulating cells, whereas the rate is only half in Si-deprived counterparts. CONCLUSIONS Rigid cell walls enhanced by a wall-bound form of Si as the structural basis stabilize cell membranes. This, in turn, optimizes nutrient uptake of the cells in the same growth phase without any requirement for up-regulation of transmembrane ammonium transporters. Optimization of cellular nutrient acquisition strategies can substantially improve performance in terms of growth, metabolism and stress resistance.
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Quantifying the Impact of Nanoparticle Coatings and Nonuniformities on XPS Analysis: Gold/Silver Core-Shell Nanoparticles. Anal Chem 2016; 88:3917-25. [PMID: 26950247 PMCID: PMC4821750 DOI: 10.1021/acs.analchem.6b00100] [Citation(s) in RCA: 47] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Spectral modeling of photoelectrons can serve as a valuable tool when combined with X-ray photoelectron spectroscopy (XPS) analysis. Herein, a new version of the NIST Simulation of Electron Spectra for Surface Analysis (SESSA 2.0) software, capable of directly simulating spherical multilayer NPs, was applied to model citrate stabilized Au/Ag-core/shell nanoparticles (NPs). The NPs were characterized using XPS and scanning transmission electron microscopy (STEM) to determine the composition and morphology of the NPs. The Au/Ag-core/shell NPs were observed to be polydispersed in size, nonspherical, and contain off-centered Au-cores. Using the average NP dimensions determined from STEM analysis, SESSA spectral modeling indicated that washed Au/Ag-core-shell NPs were stabilized with a 0.8 nm layer of sodium citrate and a 0.05 nm (one wash) or 0.025 nm (two wash) layer of adventitious hydrocarbon, but did not fully account for the observed XPS signal from the Au-core. This was addressed by a series of simulations and normalizations to account for contributions of NP nonsphericity and off-centered Au-cores. Both of these nonuniformities reduce the effective Ag-shell thickness, which effect the Au-core photoelectron intensity. The off-centered cores had the greatest impact for the particles in this study. When the contributions from the geometrical nonuniformities are included in the simulations, the SESSA generated elemental compositions that matched the XPS elemental compositions. This work demonstrates how spectral modeling software such as SESSA, when combined with experimental XPS and STEM measurements, advances the ability to quantitatively assess overlayer thicknesses for multilayer core-shell NPs and deal with complex, nonideal geometrical properties.
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Sorption and desorption of Cr(VI) ions from water by biochars in different environmental conditions. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH INTERNATIONAL 2015; 22:5985-94. [PMID: 25378029 PMCID: PMC4381096 DOI: 10.1007/s11356-014-3752-4] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/20/2014] [Accepted: 10/20/2014] [Indexed: 05/11/2023]
Abstract
In the present research, the potential of two biochars produced by the thermal decomposition of wheat straw (BCS) and wicker (BCW) for Cr(VI) ions removing from wastewater was investigated. The pH and the presence of chlorides and nitrates were also investigated. The Freundlich and Langmuir models were applied for the characterization of adsorption isotherms. The Langmuir model has better fitting of adsorption isotherms than the Freundlich model. The sorption process can be described by the pseudo second-order equation. The optimal adsorption capacities were obtained at pH 2 and were 24.6 and 23.6 mg/g for BCS and BCW, respectively. X-ray photoelectron spectroscopy (XPS) studies confirmed that Cr(III) ions were the most abundant chromium species on the biochars' surface. The results indicated that the sorption mechanism of Cr(VI) on biochar involves anionic and cationic adsorption combined with Cr(VI) species reduction.
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Surface softening in metal-ceramic sliding contacts: an experimental and numerical investigation. ACS NANO 2015; 9:1478-1491. [PMID: 25530212 DOI: 10.1021/nn505968m] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten-carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation.
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Surface chemistry of SnO2 nanowires on Ag-catalyst-covered Si substrate studied using XPS and TDS methods. NANOSCALE RESEARCH LETTERS 2014; 9:43. [PMID: 24461127 PMCID: PMC3913376 DOI: 10.1186/1556-276x-9-43] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/04/2013] [Accepted: 12/22/2013] [Indexed: 05/27/2023]
Abstract
In this paper we investigate the surface chemistry, including surface contaminations, of SnO2 nanowires deposited on Ag-covered Si substrate by vapor phase deposition (VPD), thanks to x-ray photoelectron spectroscopy (XPS) in combination with thermal desorption spectroscopy (TDS). Air-exposed SnO2 nanowires are slightly non-stoichiometric, and a huge amount of C contaminations is observed at their surface. After the thermal physical desorption (TPD) process, SnO2 nanowires become almost stoichiometric without any surface C contaminations. This is probably related to the fact that C contaminations, as well as residual gases from air, are weakly bounded to the crystalline SnO2 nanowires and can be easily removed from their surface. The obtained results gave us insight on the interpretation of the aging effect of SnO2 nanowires that is of great importance for their potential application in the development of novel chemical nanosensor devices.
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Assessment of cytotoxicity of (N-isopropyl acrylamide) and poly(N-isopropyl acrylamide)-coated surfaces. Biointerphases 2013; 8:19. [PMID: 24706136 PMCID: PMC3979476 DOI: 10.1186/1559-4106-8-19] [Citation(s) in RCA: 127] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2013] [Accepted: 07/15/2013] [Indexed: 12/30/2022] Open
Abstract
Poly(N-isopropyl acrylamide) (pNIPAM) is one of the most popular stimulus-responsive polymers for research. It is especially of great interest in the field of tissue engineering. While it is known that the NIPAM monomer is toxic, there is little conclusive research on the cytotoxicity of the polymer. In this work, the relative biocompatibility of the NIPAM monomer, pNIPAM, and pNIPAM-coated substrates prepared using different polymerization (free radical and plasma polymerization) and deposition (spin coating and plasma polymerization) techniques was evaluated using appropriate cytotoxicity tests (MTS, Live/Dead, plating efficiency). Four different mammalian cell types (endothelial, epithelial, smooth muscle, and fibroblasts) were used for the cytotoxicity testing. The pNIPAM-coated surfaces were evaluated for their thermoresponse and surface chemistry using X-ray photoelectron spectroscopy and goniometry. We found that while cell viability on pNIPAM surfaces decreases when compared to controls, the viability also seems to be deposition type dependent, with sol-gel based pNIPAM surfaces being the least biocompatible. Long term experiments proved that all pNIPAM-coated surfaces were not cytotoxic to the four cell types evaluated in a direct contact test. Plating efficiency experiments did not show cytotoxicity. Cellular sensitivity to pNIPAM and to the NIPAM monomer varied depending on cell type. Endothelial cells consistently showed decreased viability after 48 hours of exposure to pNIPAM extracts and were more sensitive than the other cell lines to impurities in the polymer.
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Introducing carbon diffusion barriers for uniform, high-quality graphene growth from solid sources. NANO LETTERS 2013; 13:4624-31. [PMID: 24024736 PMCID: PMC3813970 DOI: 10.1021/nl401601x] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
Carbon diffusion barriers are introduced as a general and simple method to prevent premature carbon dissolution and thereby to significantly improve graphene formation from the catalytic transformation of solid carbon sources. A thin Al2O3 barrier inserted into an amorphous-C/Ni bilayer stack is demonstrated to enable growth of uniform monolayer graphene at 600 °C with domain sizes exceeding 50 μm, and an average Raman D/G ratio of <0.07. A detailed growth rationale is established via in situ measurements, relevant to solid-state growth of a wide range of layered materials, as well as layer-by-layer control in these systems.
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Role of surface composition in morphological evolution of GaAs nano-dots with low-energy ion irradiation. NANOSCALE RESEARCH LETTERS 2012; 7:552. [PMID: 23035733 PMCID: PMC3489695 DOI: 10.1186/1556-276x-7-552] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 06/12/2012] [Accepted: 08/01/2012] [Indexed: 06/01/2023]
Abstract
The surface chemistry of GaAs (100) with 50-keV Ar+ ion beam irradiation at off-normal incidence has been investigated in order to elucidate the surface nano-structuring mechanism(s). Core level and valence band studies of the surface composition were carried out as a function of fluences, which varied from 1 × 1017 to 7 × 1017 ions/cm2. Core-level spectra of samples analyzed by X-ray photoelectron spectroscopy confirmed the Ga enrichment of the surface resulting in bigger sized nano-dots. Formation of such nano-dots is attributed to be due to the interplay between preferential sputtering and surface diffusion processes. Valence band measurement shows that the shift in the Fermi edge is higher for Ga- rich, bigger sized nano-dots due to the partial oxide formation of Ga. 'One-dimensional power spectral density' extracted from atomic force micrographs also confirms the significant role of surface diffusion in observed nano-structuring.
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Nanoscale clustering of carbohydrate thiols in mixed self-assembled monolayers on gold. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2012; 28:6950-9. [PMID: 22435511 PMCID: PMC3350752 DOI: 10.1021/la300444h] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
Self-assembled monolayers (SAMs) bearing pendant carbohydrate functionality are frequently employed to tailor glycan-specific bioactivity onto gold substrates. The resulting glycoSAMs are valuable for interrogating glycan-mediated biological interactions via surface analytical techniques, microarrays, and label-free biosensors. GlycoSAM composition can be readily modified during assembly by using mixed solutions containing thiolated species, including carbohydrates, oligo(ethylene glycol) (OEG), and other inert moieties. This intrinsic tunability of the self-assembled system is frequently used to optimize bioavailability and antibiofouling properties of the resulting SAM. However, until now, our nanoscale understanding of the behavior of these mixed glycoSAMs has lacked detail. In this study, we examined the time-dependent clustering of mixed sugar + OEG glycoSAMs on ultraflat gold substrates. Composition and surface morphologic changes in the monolayers were analyzed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. We provide evidence that the observed clustering is consistent with a phase separation process in which surface-bound glycans self-associate to form dense glycoclusters within the monolayer. These observations have significant implications for the construction of mixed glycoSAMs for use in biosensing and glycomics applications.
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"Soft and rigid" dithiols and Au nanoparticles grafting on plasma-treated polyethyleneterephthalate. NANOSCALE RESEARCH LETTERS 2011; 6:607. [PMID: 22117780 PMCID: PMC3240945 DOI: 10.1186/1556-276x-6-607] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/04/2011] [Accepted: 11/25/2011] [Indexed: 05/11/2023]
Abstract
Surface of polyethyleneterephthalate (PET) was modified by plasma discharge and subsequently grafted with dithiols (1, 2-ethanedithiol (ED) or 4, 4'-biphenyldithiol) to create the thiol (-SH) groups on polymer surface. This "short" dithiols are expected to be fixed via one of -SH groups to radicals created by the plasma treatment on the PET surface. "Free" -SH groups are allowed to interact with Au nanoparticles. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and electrokinetic analysis (EA, zeta potential) were used for the characterization of surface chemistry of the modified PET. Surface morphology and roughness of the modified PET were studied by atomic force microscopy (AFM). The results from XPS, FTIR, EA and AFM show that the Au nanoparticles are grafted on the modified surface only in the case of biphenyldithiol pretreatment. The possible explanation is that the "flexible" molecule of ethanedithiol is bounded to the activated PET surface with both -SH groups. On the contrary, the "rigid" molecule of biphenyldithiol is bounded via only one -SH group to the modified PET surface and the second one remains "free" for the consecutive chemical reaction with Au nanoparticle. The gold nanoparticles are distributed relatively homogenously over the polymer surface.
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Structure and order of phosphonic acid-based self-assembled monolayers on Si(100). LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2010; 26:14747-54. [PMID: 20735054 PMCID: PMC2939309 DOI: 10.1021/la1021438] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
Organophosphonic acid self-assembled monolayers (SAMs) on oxide surfaces have recently seen increased use in electrical and biological sensor applications. The reliability and reproducibility of these sensors require good molecular organization in these SAMs. In this regard, packing, order, and alignment in the SAMs is important, as it influences the electron transport measurements. In this study, we examine the order of hydroxyl- and methyl-terminated phosphonate films deposited onto silicon oxide surfaces by the tethering by aggregation and growth method using complementary, state-of-art surface characterization tools. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and in situ sum frequency generation (SFG) spectroscopy are used to study the order of the phosphonate SAMs in vacuum and under aqueous conditions, respectively. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry results show that these samples form chemically intact monolayer phosphonate films. NEXAFS and SFG spectroscopy showed that molecular order exists in the octadecylphosphonic acid and 11-hydroxyundecylphosphonic acid SAMs. The chain tilt angles in these SAMs were approximately 37° and 45°, respectively.
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XPS and SPR analysis of glycoarray surface density. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2009; 25:2181-7. [PMID: 19199748 PMCID: PMC2719674 DOI: 10.1021/la8031122] [Citation(s) in RCA: 60] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/07/2023]
Abstract
Despite the fact that the carbohydrate microarray has seen increasing use within the field of glycobiology, the surface chemistry of the glycoarray remains largely unexplored. Motivated by the need to develop surface analytical techniques to characterize carbohydrate-modified surfaces, we developed a quantitative X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance imaging (SPR imaging) method to study glycan biosensors. We performed a comparative analysis on the relative coverage of mixed self-assembled monolayers (SAMs) on gold, consisting of a thiol-functionalized trimannoside (Manalpha1,2Manalpha1,2Manalpha-OEG-SH) at varying concentrations (0-100%) mixed separately with two thiol-containing polyethylene glycol oligomers. XPS C1s core level analysis was used to identify the O-C-O functionality unique to the carbohydrate acetal moiety and to separate and quantify the relative coverage of sugar in carbohydrate/OEG mixed SAMs. XPS spectra of the mixed SAMs demonstrated a proportional increase in the acetal signature of the glycan with increasing sugar concentration. To relate surface glycan density with biological function, we carried out a kinetic analysis of Concanavalin A (ConA) binding to SAMs of varying densities of carbohydrate using SPR imaging. We observed protein binding that was highly dependent on both glycan density and the nature of the OEG-thiol used in the mixed self-assembly. These results illustrate the utility of surface analytical techniques such as XPS and SPR in carbohydrate biosensor characterization and optimization.
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The effects of cell culture parameters on cell release kinetics from thermoresponsive surfaces. JOURNAL OF APPLIED BIOMATERIALS & BIOMECHANICS : JABB 2008; 6:81-88. [PMID: 20740450 PMCID: PMC3842098] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
The thermoresponsive properties of poly(N-isopropyl acrylamide) (pNIPAM) have led to its wide use in bioengineering applications, including the reversible adhesion of mammalian cells. The groups performing this research have used different solutions to initiate cell release and have varied the temperature of the solution during detachment. To our knowledge, there has been no direct correlation between the solution identity or temperature on the efficiency of cell release from pNIPAM films. In this work, we present a study of the effect of the solution type and temperature used to initiate detachment on the time required to achieve 100% detachment of bovine aortic endothelial cells (BAECs) from pNIPAM. The pNIPAM films used in this work were obtained using a novel technique using a spin-coated solution containing pNIPAM (spNIPAM). We found that the fastest, most reliable release of cells occurred below the LCST of the polymer at 4 degrees C in serum free media (SFM). As it is sometimes desirable to stop cell metabolism at the time of detachment (e.g., to "freeze" protein expression prior to subsequent analysis), the use of extremely cold SFM would be ideal in such cases.
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Abstract
Performance improvements in DNA-modified surfaces required for microarray and biosensor applications rely on improved capabilities to accurately characterize the chemistry and structure of immobilized DNA molecules on micropatterned surfaces. Recent innovations in imaging X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) now permit more detailed studies of micropatterned surfaces. We have exploited the complementary information provided by imaging XPS and imaging TOF-SIMS to detail the chemical composition, spatial distribution, and hybridization efficiency of amine-terminated single-stranded DNA (ssDNA) bound to commercial polyacrylamide-based, amine-reactive microarray slides, immobilized in both macrospot and microarray diagnostic formats. Combinations of XPS imaging and small spot analysis were used to identify micropatterned DNA spots within printed DNA arrays on slide surfaces and quantify DNA elements within individual microarray spots for determination of probe immobilization and hybridization efficiencies. This represents the first report of imaging XPS of DNA immobilization and hybridization efficiencies for arrays fabricated on commercial microarray slides. Imaging TOF-SIMS provided distinct analytical data on the lateral distribution of DNA within single array microspots before and after target hybridization. Principal component analysis (PCA) applied to TOF-SIMS imaging datasets demonstrated that the combination of these two techniques provides information not readily observable in TOF-SIMS images alone, particularly in identifying species associated with array spot nonuniformities (e.g., "halo" or "donut" effects often observed in fluorescence images). Chemically specific spot images were compared to conventional fluorescence scanned images in microarrays to provide new information on spot-to-spot DNA variations that affect current diagnostic reliability, assay variance, and sensitivity.
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Structure and DNA hybridization properties of mixed nucleic acid/maleimide-ethylene glycol monolayers. Anal Chem 2007; 79:4390-400. [PMID: 17492838 PMCID: PMC2518630 DOI: 10.1021/ac0703395] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The surface structure and DNA hybridization performance of thiolated single-strand DNA (HS-ssDNA) covalently attached to a maleimide-ethylene glycol disulfide (MEG) monolayer on gold have been investigated. Monolayer immobilization chemistry and surface coverage of reactive ssDNA probes were studied by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Orientation of the ssDNA probes was determined by near-edge X-ray absorption fine structure (NEXAFS). Target DNA hybridization on the DNA-MEG probe surfaces was measured by surface plasmon resonance (SPR) to demonstrate the utility of these probe surfaces for detection of DNA targets from both purified target DNA samples and complex biological mixtures such as blood serum. Data from complementary techniques showed that immobilized ssDNA density is strongly dependent on the spotted bulk DNA concentration and buffer ionic strength. Variation of the immobilized ssDNA density had a profound influence on the DNA probe orientation at the surface and subsequent target hybridization efficiency. With increasing surface probe density, NEXAFS polarization dependence results (followed by monitoring the N 1s --> pi* transition) indicate that the immobilized ssDNA molecules reorient toward a more upright position on the MEG monolayer. SPR assays of DNA targets from buffer and serum showed that DNA hybridization efficiency increased with decreasing surface probe density. However, target detection in serum was better on the "high-density" probe surface than on the "high-efficiency" probe surface. The amounts of target detected for both ssDNA surfaces were several orders of magnitude poorer in serum than in purified DNA samples due to nonspecific serum protein adsorption onto the sensing surface.
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Influence of PEG architecture on protein adsorption and conformation. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2005; 21:12327-32. [PMID: 16343010 PMCID: PMC2515350 DOI: 10.1021/la051726h] [Citation(s) in RCA: 236] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers with various grafting ratios were adsorbed to niobium pentoxide-coated silicon wafers and characterized before and after protein adsorption using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Three proteins of different sizes, myoglobin (16 kD), albumin (67 kD), and fibrinogen (340 kD), were studied. XPS was used to quantify the amount of protein adsorbed to the bare and PEGylated surfaces. ToF-SIMS and principal component analysis (PCA) were used to study protein conformational changes on these surfaces. The smallest protein, myoglobin, generally adsorbed in higher numbers than the much larger fibrinogen. Protein adsorption was lowest on the surfaces with the highest PEG chain surface density and increased as the PEG layer density decreased. The highest adsorption was found on lysine-coated and bare niobium surfaces. ToF-SIMS and PCA data evaluation provided further information on the degree of protein denaturation, which, for a particular protein, were found to decrease with increasing PEG surface density and increase with decreasing protein size.
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