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1.
J Phys Chem B ; 2019 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-31513402

RESUMO

Heterogeneous ozone chemistry occurring on aerosols is driven by interfacial chem- istry and thus affected by the surface state of aerosol particles. Therefore, the effect of electrolytes on the structure of interfacial water has been under intensive investi- gation. However, consequences for energy dissipation rates and mechanisms at the interface are largely unknown. Here, we use time-resolved sum frequency generation spectroscopy to reveal that the relaxation pathway is the same for neat water-air as for aqueous solutions of Na2SO4 and Na2CO3 , although the interfacial aqueous structure is substantially modified. We further show that similar lifetimes are extracted from all investigated systems and that these lifetimes show an excitation frequency dependent relaxation time from 0.2 ps up to 1 ps. Hence, despite static SFG on the same systems revealing that the interfacial aqueous structure changes upon adding electrolytes, the dynamics are indistinguishable for both pure water and different electrolyte solutions.

2.
ACS Nano ; 13(8): 9431-9441, 2019 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-31386338

RESUMO

Graphene has been the subject of widespread research during the past decade because of its outstanding physical properties which make it an ideal nanoscale material to investigate fundamental properties. Such characteristics promote graphene as a functional material for the emergence of disruptive technologies. However, to impact daily life products and devices, high-quality graphene needs to be produced in large quantities using an environmentally friendly protocol. In this context, the production of graphene which preserves its outstanding electronic properties using a green chemistry approach remains a key challenge. Herein, we report the efficient production of electrode material for micro-supercapacitors obtained by functionalization of water-dispersed high-quality graphene nanosheets with polydopamine. High-frequency (terahertz) conductivity measurements of the graphene nanosheets reveal high charge carrier mobility up to 1000 cm-2 V-1 s-1. The fine water dispersibility enables versatile functionalization of graphene, as demonstrated by the pseudocapacitive polydopamine coating of graphene nanosheets. The polydopamine functionalization causes a modest, i.e., 20%, reduction of charge carrier mobility. Thin film electrodes based on such hybrid materials for micro-supercapacitors exhibit excellent electrochemical performance, namely a volumetric capacitance of 340 F cm-3 and a power density of 1000 W cm-3, thus outperforming most of the reported graphene-based micro-supercapacitors. These results highlight the potential for water-dispersed, high-quality graphene nanosheets as a platform material for energy-storage applications.

3.
J Phys Chem Lett ; : 4914-4919, 2019 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-31393136

RESUMO

Density functional theory-based molecular dynamics simulations are increasingly being used for simulating aqueous interfaces. Nonetheless, the choice of the appropriate density functional, critically affecting the outcome of the simulation, has remained arbitrary. Here, we assess the performance of various exchange-correlation (XC) functionals, based on the metrics relevant to sum-frequency generation spectroscopy. The structure and dynamics of water at the water-air interface are governed by heterogeneous intermolecular interactions, thereby providing a critical benchmark for XC functionals. We find that the XC functionals constrained by exact functional conditions (revPBE and revPBE0) with the dispersion correction show excellent performance. The poor performance of the empirically optimized density functional (M06-L) indicates the importance of satisfying the exact functional condition. Understanding the performance of different XC functionals can aid in resolving the controversial interpretation of the interfacial water structure and direct the design of novel, improved XC functionals better suited to describing the heterogeneous interactions in condensed phases.

4.
J Am Chem Soc ; 141(28): 10972-10977, 2019 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-31268310

RESUMO

In this Communication, we report the first synthesis of structurally well-defined graphene nanoribbons (GNRs) functionalized with dendritic polymers. The resultant GNRs possess grafting ratios of 0.59-0.68 for the dendrons of different generations. Remarkably, the precise 3D branched conformation of the grafted dendrons affords the GNRs unprecedented 1D supramolecular self-assembly behavior in tetrahydrofuran (THF), yielding nanowires, helices and nanofibers depending on the dimension of the dendrons. The GNR superstructures in THF exhibit near-infrared absorption with maxima between 650 and 700 nm, yielding an optical bandgap of 1.2-1.3 eV. Ultrafast photoconductivity analyses unveil that the helical structures exhibit the longest free carrier (3.5 ps) and exciton lifetime (several hundred ps) among the three superstructure systems. This study opens pathways for tunable construction of ordered GNR superstructures with promising optoelectronic applications.

5.
Nat Commun ; 10(1): 3260, 2019 Jul 22.
Artigo em Inglês | MEDLINE | ID: mdl-31332187

RESUMO

The realization of ferromagnetism in semiconductors is an attractive avenue for the development of spintronic applications. Here, we report a semiconducting layered metal-organic framework (MOF), namely K3Fe2[(2,3,9,10,16,17,23,24-octahydroxy phthalocyaninato)Fe] (K3Fe2[PcFe-O8]) with spontaneous magnetization. This layered MOF features in-plane full π-d conjugation and exhibits semiconducting behavior with a room temperature carrier mobility of 15 ± 2 cm2 V-1 s-1 as determined by time-resolved Terahertz spectroscopy. Magnetization experiments and 57Fe Mössbauer spectroscopy demonstrate the presence of long-range magnetic correlations in K3Fe2[PcFe-O8] arising from the magnetic coupling between iron centers via delocalized π electrons. The sample exhibits superparamagnetic features due to a distribution of crystal size and possesses magnetic hysteresis up to 350 K. Our work sets the stage for the development of spintronic materials exploiting magnetic MOF semiconductors.

6.
Nat Commun ; 10(1): 2893, 2019 06 28.
Artigo em Inglês | MEDLINE | ID: mdl-31253797

RESUMO

The solubilities of polyethers are surprisingly counter-intuitive. The best-known example is the difference between polyethylene glycol ([-CH2-CH2-O-]n) which is infinitely soluble, and polyoxymethylene ([-CH2-O-]n) which is completely insoluble in water, exactly the opposite of what one expects from the C/O ratios of these molecules. Similar anomalies exist for oligomeric and cyclic polyethers. To solve this apparent mystery, we use femtosecond vibrational and GHz dielectric spectroscopy with complementary ab initio calculations and molecular dynamics simulations. We find that the dynamics of water molecules solvating polyethers is fundamentally different depending on their C/O composition. The ab initio calculations and simulations show that this is not because of steric effects (as is commonly believed), but because the partial charge on the O atoms depends on the number of C atoms by which they are separated. Our results thus show that inductive effects can have a major impact on aqueous solubilities.

7.
Biophys J ; 116(12): 2346-2355, 2019 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-31153590

RESUMO

Lipid droplets (LDs), present in many cell types, are highly dynamic organelles that store neutral lipids, primarily triacylglycerols (TAGs). With the discovery of new LD functions (e.g., in immune response, protein clearage, and occurrence with disease), new methods to study LD chemical composition in situ are necessary. We present an approach for in situ, quantitative TAG analysis using label-free, coherent Raman microscopy that allows deciphering LD TAG composition in different biochemically complex samples with submicrometer spatial resolution. Employing a set of standard TAGs, we generate a spectral training matrix capturing the variation caused in Raman-like spectra by TAG backbone, chain length, and number of double bonds per chain, as well as the presence of proteins or other diluting molecules. Comparing our fitting approach to gas chromatography measurements for mixtures of standard TAGs and food oils, we find the root mean-square error for the prediction of TAG chemistry to be 0.69 CH2 and 0.15 #C=C. When progressing to more complex samples such as oil emulsions and LDs in various eukaryotic cells, we find good agreement with bulk gas chromatography measurements. For differentiated adipocytes, we find a significant increase in the number of double bonds in small LDs (below 2 µm in diameter) compared to large LDs (above 2 µm in diameter). Coupled with a relatively limited sample preparation requirement, this approach should enable rapid and accurate TAG LD analysis for a variety of cell biology and technological applications.

8.
J Phys Chem Lett ; 10(9): 2170-2174, 2019 May 02.
Artigo em Inglês | MEDLINE | ID: mdl-30978286

RESUMO

The fabrication of two-dimensional (2D) biomineral nanosheets is of high interest owing to their promise for applications in electronics, filtration, catalysis, and chemical sensing. Using a facile approach inspired by biomineralization in nature, we fabricate laterally macroscopic calcium oxalate nanosheets using ß-folded peptides. The template peptides are composed of repetitive glutamic acid and leucine amino acids, self-organized at the air-water interface. Surface-specific sum frequency generation spectroscopy and molecular dynamics simulations reveal that the formation of oxalate nanosheets relies on the peptide-Ca2+ ion interaction at the interface, which not only restructures the peptides but also templates Ca2+ ions into a calcium oxalate dihydrate lattice. Combined, this enables the formation of a critical structural intermediate in the assembly pathway toward the oxalate sheet formation. These insights into peptide-ion interfacial interaction are important for designing novel inorganic 2D materials.

9.
Proc Natl Acad Sci U S A ; 116(18): 8715-8720, 2019 Apr 30.
Artigo em Inglês | MEDLINE | ID: mdl-30988207

RESUMO

Water must be effectively transported and is also essential for maximizing proton conductivity within fuel-cell proton-exchange membranes (PEMs). Therefore, identifying relationships between PEM properties, water transport, and proton conductivity is essential for designing optimal PEMs. Here, we use coherent Raman spectroscopy to quantify real-time, in situ diffusivities of water subspecies, bulk-like and nonbulk-like (interfacial) water, in five different perfluorosulfonic acid (PFSA) PEMs. Although the PEMs were chemically diverse, water transport within them followed the same rule: Total water diffusivity could be represented by a linear combination of the bulk-like and interfacial water diffusivities. Moreover, the diffusivity of interfacial water was consistently larger than that of bulk-like water. These measurements of microscopic transport were combined with through-plane proton conductivity measurements to reveal the correlation between interfacial water transport and proton conductivity. Our results demonstrate the importance of maximizing the diffusivity and fractional contribution of interfacial water to maximize the proton conductivity in PFSA PEMs.

10.
Phys Chem Chem Phys ; 21(17): 8956-8964, 2019 Apr 24.
Artigo em Inglês | MEDLINE | ID: mdl-30989163

RESUMO

Photocatalytic splitting of water into hydrogen and oxygen by utilizing sunlight and a photocatalyst is a promising way of generating clean energy. Here, we report a molecular-level study on heavy water (D2O) interacting with TiO2 as a model photocatalyst. We employed the surface specific technique Sum-Frequency-Generation (SFG) spectroscopy to determine the nature of the hydrogen bonding environment and the orientation of interfacial water molecules using their OD-stretch vibrations as reporters. By examining solutions with various pD-values, we observe an intensity-minimum at around pD 5, corresponding to the balance of protonation and deprotonation of TiO2 (point of zero charge). The majority of water molecules' deuterium atoms point away from the interface when the pD is below 5, and point towards the surface when the pD is higher than 5, with strong hydrogen bonds towards the surface.

11.
J Phys Chem Lett ; 10(7): 1431-1436, 2019 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-30848919

RESUMO

Graphene quantum dots (GQDs) are emerging as environmentally friendly, low-cost, and highly tunable building blocks in solar energy conversion architectures, such as solar (fuel) cells. Specifically, GQDs constitute a promising alternative for organometallic dyes in sensitized oxide systems. Current sensitized solar cells employing atomically precise GQDs are based on physisorbed sensitizers, with typically limited efficiencies. Chemisorption has been pointed out as a solution to boost photoconversion efficiencies, by allowing improved control over sensitizer surface coverage and sensitizer-oxide coupling strength. Here, employing time-resolved THz spectroscopy, we demonstrate that chemisorption of atomically precise C42-GQDs (hexa- peri-hexabenzocoronene derivatives consisting of 42 sp2 carbon atoms) onto mesoporous metal oxides, enabled by their functionalization with a carboxylate group, enhances electron transfer (ET) rates by almost 2 orders of magnitude when compared with physisorbed sensitizers. Density functional theory (DFT) calculations, absorption spectroscopy and valence band X-ray photoelectron spectroscopy reveal that the enhanced ET rates can be traced to stronger donor-acceptor coupling strength enabled by chemisorption.

12.
Acc Chem Res ; 52(4): 1006-1015, 2019 Apr 16.
Artigo em Inglês | MEDLINE | ID: mdl-30925035

RESUMO

The ice premelt, often called the quasi-liquid layer (QLL), is key for the lubrication of ice, gas uptake by ice, and growth of aerosols. Despite its apparent importance, in-depth understanding of the ice premelt from the microscopic to the macroscopic scale has not been gained. By reviewing data obtained using molecular dynamics (MD) simulations, sum-frequency generation (SFG) spectroscopy, and laser confocal differential interference contrast microscopy (LCM-DIM), we provide a unified view of the experimentally observed variation in quasi-liquid (QL) states. In particular, we disentangle three distinct types of QL states of disordered layers, QL-droplet, and QL-film and discuss their nature. The topmost ice layer is energetically unstable, as the topmost interfacial H2O molecules lose a hydrogen bonding partner, generating a disordered layer at the ice-air interface. This disordered layer is homogeneously distributed over the ice surface. The nature of the disordered layer changes over a wide temperature range from -90 °C to the bulk melting point. Combined MD simulations and SFG measurements reveal that the topmost ice surface starts to be disordered around -90 °C through a process that the topmost water molecules with three hydrogen bonds convert to a doubly hydrogen-bonded species. When the temperature is further increased, the second layer starts to become disordered at around -16 °C. This disordering occurs not in a gradual manner, but in a bilayer-by-bilayer manner. When the temperature reaches -2 °C, more complicated structures, QL-droplet and QL-film, emerge on the top of the ice surface. These QL-droplets and QL-films are inhomogeneously distributed, in contrast to the disordered layer. We show that these QL-droplet and QL-film emerge only under supersaturated/undersaturated vapor pressure conditions, as partial and pseudopartial wetting states, respectively. Experiments with precisely controlled pressure show that, near the water vapor pressure at the vapor-ice equilibrium condition, no QL-droplet and QL-film can be observed, implying that the QL-droplet and QL-film emerge exclusively under nonequilibrium conditions, as opposed to the disordered layers formed under equilibrium conditions. These findings are connected with many phenomena related to the ice surface. For example, we explain how the disordering of the topmost ice surface governs the slipperiness of the ice surface, allowing for ice skating. Further focus is on the gas uptake mechanism on the ice surface. Finally, we note the unresolved questions and future challenges regarding the ice premelt.

13.
J Chem Phys ; 150(4): 044706, 2019 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-30709297

RESUMO

Specific ion effects at interfaces are important for a variety of thermodynamic properties of electrolyte solutions, like surface tension and the phase behavior of surfactants. We report the relative surface affinity of Na+ and D3O+ at both the D2O-air and the sodium dodecyl sulfate (surfactant)-covered D2O surface by studying the alignment of interfacial D2O, using vibrational sum frequency generation spectroscopy. The surface propensity of ions is found to be a function of both the nature of the ion and the nature of the surface. Specifically, for the charged, surfactant-covered interface, Na+ has a higher affinity than D3O+. In contrast, D3O+ has a higher affinity than Na+ at the air-D2O interface. The relative surface affinity of cations thus depends on both details of the cation and the type of interface.

14.
J Phys Chem B ; 123(8): 1831-1839, 2019 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-30717596

RESUMO

The dynamics of probe molecules is commonly used to investigate the structural dynamics of room-temperature ionic liquids; however, the extent to which this dynamics reflects the dynamics of the ionic liquids or is probe specific has remained debated. Here, we explore to what extent the vibrational and rotational dynamics of the dicyanamide anion, a common ionic liquid anion, correlates with the structural relaxation of ionic liquids. We use polarization-resolved, ultrafast infrared spectroscopy to probe the temperature- and probe-concentration-dependent dynamics of samples with small amounts of 1-ethyl-3-methylimidazolium ([emim]+) dicyanamide ([DCA]-) dissolved in four [emim]+-based ionic liquids with tetrafluoroborate ([BF4]-), bis(trifluoromethylsulfonyl)imide ([NTf2]-), ethylsufate ([EtSO4]-), and triflate ([OTf]-) as anions. The transient spectra after broad-band excitation at 2000-2300 cm-1, resonant with the symmetric and antisymmetric C≡N stretching vibrations, initially contain oscillatory signatures due to the vibrational coherence between both modes. Vibrational population relaxation occurs on two distinct time scales, ∼6-7 and ∼15-20 ps. The vibrational dynamics is rather insensitive to the details of the ionic liquid anion and temperature, except for the slow vibrational relaxation component. The decay of the excitation anisotropy, a measure of the rotational dynamics of [DCA]-, markedly depends on temperature, and the obtained decay time exhibits an activation energy of ∼15-21 kJ/mol. Remarkably, neither the rotation time nor the activation energy can be simply explained by the variation of the macroscopic viscosity. Hence, our results suggest that the dynamics of dicyanamide is only in part representative of the ionic liquid structural dynamics. Rather, the dynamics of the probe anion seems to be determined by the specific interaction of [DCA]- with the ionic liquid's ions for the class of [emim]+-based ionic liquids studied here.

15.
BMC Bioinformatics ; 20(1): 39, 2019 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-30658582

RESUMO

BACKGROUND: Image segmentation and quantification are essential steps in quantitative cellular analysis. In this work, we present a fast, customizable, and unsupervised cell segmentation method that is based solely on Fiji (is just ImageJ)®, one of the most commonly used open-source software packages for microscopy analysis. In our method, the "leaky" fluorescence from the DNA stain DRAQ5 is used for automated nucleus detection and 2D cell segmentation. RESULTS: Based on an evaluation with HeLa cells compared to human counting, our algorithm reached accuracy levels above 92% and sensitivity levels of 94%. 86% of the evaluated cells were segmented correctly, and the average intersection over union score of detected segmentation frames to manually segmented cells was above 0.83. Using this approach, we quantified changes in the projected cell area, circularity, and aspect ratio of THP-1 cells differentiating from monocytes to macrophages, observing significant cell growth and a transition from circular to elongated form. In a second application, we quantified changes in the projected cell area of CHO cells upon lowering the incubation temperature, a common stimulus to increase protein production in biotechnology applications, and found a stark decrease in cell area. CONCLUSIONS: Our method is straightforward and easily applicable using our staining protocol. We believe this method will help other non-image processing specialists use microscopy for quantitative image analysis.


Assuntos
Antraquinonas/metabolismo , Separação Celular/métodos , Processamento de Imagem Assistida por Computador/métodos , Algoritmos , Humanos
16.
Proc Natl Acad Sci U S A ; 116(5): 1520-1525, 2019 01 29.
Artigo em Inglês | MEDLINE | ID: mdl-30655339

RESUMO

Interfaces between water and silicates are ubiquitous and relevant for, among others, geochemistry, atmospheric chemistry, and chromatography. The molecular-level details of water organization at silica surfaces are important for a fundamental understanding of this interface. While silica is hydrophilic, weakly hydrogen-bonded OH groups have been identified at the surface of silica, characterized by a high O-H stretch vibrational frequency. Here, through a combination of experimental and theoretical surface-selective vibrational spectroscopy, we demonstrate that these OH groups originate from very weakly hydrogen-bonded water molecules at the nominally hydrophilic silica interface. The properties of these OH groups are very similar to those typically observed at hydrophobic surfaces. Molecular dynamics simulations illustrate that these weakly hydrogen-bonded water OH groups are pointing with their hydrogen atom toward local hydrophobic sites consisting of oxygen bridges of the silica. An increased density of these molecular hydrophobic sites, evident from an increase in weakly hydrogen-bonded water OH groups, correlates with an increased macroscopic contact angle.

17.
Angew Chem Int Ed Engl ; 58(11): 3620-3624, 2019 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-30601600

RESUMO

Small organic molecules on ice and water surfaces are ubiquitous in nature and play a crucial role in many environmentally relevant processes. Herein, we combine surface-specific vibrational spectroscopy and a controllable flow cell apparatus to investigate the molecular adsorption of acetone onto the basal plane of single-crystalline hexagonal ice with a large surface area. By comparing the adsorption of acetone on the ice/air and the water/air interface, we observed two different types of acetone adsorption, as apparent from the different responses of both the free O-H and the hydrogen-bonded network vibrations for ice and liquid water. Adsorption on ice occurs preferentially through interactions with the free OH group, while the interaction of acetone with the surface of liquid water appears less specific.

18.
Artigo em Inglês | MEDLINE | ID: mdl-30516968

RESUMO

Synthetic polymers are commonly used as protein repelling materials for a variety of biomedical applications. Despite their widespread use, the fundamental mechanism underlying protein repellence is often elusive. Such insights are essential for improving existing, and developing new materials. Here, we investigate how subtle differences in the chemistry of hydrophilic polyphosphoesters influence the adsorption of the human blood proteins serum albumin and fibrinogen. Using thermodynamic measurements, surface-specific vibrational spectroscopy and Brewster angle microscopy, we investigate protein adsorption, hydration as well as the steric repulsion properties of the polyphosphoester polymers. Whereas both surface hydration and polymer conformation of the polymers vary substantially as a consequence of the chemical differences in the polymer structure, the protein repellency ability of these hydrophilic materials appears to be dominated by steric repulsion.

19.
Artigo em Inglês | MEDLINE | ID: mdl-30403434

RESUMO

Ion-protein interactions are important for protein function, yet challenging to rationalize owing to the multitude of possible ion-protein interactions. To explore specific ion effects on protein binding sites, we investigate the interaction of different salts with the zwitterionic peptide triglycine in solution. Dielectric spectroscopy shows that salts affect the peptide's reorientational dynamics, with a more pronounced effect for denaturing cations (Li+ , guanidinium (Gdm+ )) and anions (I- , SCN- ) than for weakly denaturing ones (K+ , Cl- ). The effects of Gdm+ and Li+ were found to be comparable. Molecular dynamics simulations confirm the enhanced binding of Gdm+ and Li+ to triglycine, yet with a different binding geometry: While Li+ predominantly binds to the C-terminal carboxylate group, bidentate binding to the terminus and the nearest amide is particularly important for Gdm+ . This bidentate binding markedly affects peptide conformation, and may help to explain the high denaturation activity of Gdm+ salts.

20.
J Phys Chem C Nanomater Interfaces ; 122(43): 24760-24764, 2018 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-30450149

RESUMO

Heterogeneous ice nucleation at the water-sapphire interface is studied using sum-frequency generation spectroscopy. We follow the response of the O-H stretch mode of interfacial water during ice nucleation as a function of time and temperature. The ice and liquid states each exhibit very distinct, largely temperature-independent responses. However, at the moment of freezing, a transient response with a significantly different intensity is observed, with a lifetime between several seconds and several minutes. The presence of this transient signal has previously been attributed to a transient phase of ice. Here, we demonstrate that the transient signal can be explained without invoking a transient ice phase, as the transient signal can simply be accounted for by a linear combination of time-dependent liquid and ice responses.

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