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Dissolution of solid matter into aqueous solution is one of the most challenging physicochemical aspects related to drug development. While influenced by several parameters, the effect of pH remains the most important one to be fully understood. The dissolution process is essentially controlled by activity at the surface of the molecular crystals, which is difficult to characterize experimentally. To address this, a combination of

Protein/peptide self-assembly into amyloid structures associates with major neurodegenerative disorders such as Alzheimer's disease (AD). Soluble assemblies (oligomers) of the A beta peptide and their aggregates are perceived as neurotoxic species in AD. While screening for synthetic cleavage agents that could break down such aberrant assemblies through hydrolysis, we observed that the assemblies of A beta oligopeptides, containing the nucleation sequence A beta(14-24) (H(14)QKLVFFAEDV(24)), could act as cleavage agents by themselves. Autohydrolysis showed a common fragment fingerprint among various mutated A beta(14-24) oligopeptides, A beta(12-25)-Gly and A beta(1-28), and full-length A beta(1-40/42), under physiologically relevant conditions. Primary endoproteolytic autocleavage at the Gln(15)-Lys(16), Lys(16)-Leu(17) and Phe(19)-Phe(20) positions was followed by subsequent exopeptidase self-processing of the fragments. Control experiments with homologous d-amino acid enantiomers A beta(12-25)-Gly and A beta(16-25)-Gly showed the same autocleavage pattern under similar reaction conditions. The autohydrolytic cascade reaction (ACR) was resilient to a broad range of conditions (20-37 degrees C, 10-150 mu M peptide concentration at pH 7.0-7.8). Evidently, assemblies of the primary autocleavage fragments acted as structural/compositional templates (autocatalysts) for self-propagating autohydrolytic processing at the A beta(16-21) nucleation site, showing the potential for cross-catalytic seeding of the ACR in larger A beta isoforms (A beta(1-28) and A beta(1-40/42)). This result may shed new light on A beta behaviour in solution and might be useful in the development of intervention strategies to decompose or inhibit neurotoxic A beta assemblies in AD.

Solvents can dramatically affect molecular crystals. Obtaining favorable properties for these crystals requires rational design based on molecular level understanding of the solid–solution interface. Here we show how atomic force microscopy combined with molecular dynamics simulations can be utilized for understanding critical surface properties, namely crystallinity and hydrophobicity, as crystals are exposed to water–ethanol mixtures. We report the formation of dynamic heterogeneous disordered surface (DHDS) layers at the solid–solution interface. The observed DHDS layer was affected by the solvent composition and a variation in the water–ethanol ratio caused significant changes in surface properties.

Protoplanetary disks are dust- and gas-rich structures surrounding protostars. Depending on the distance from the protostar, this dust is thermally processed to different degrees and accreted to form bodies of varying chemical compositions. The primordial accretion processes occurring in the early protoplanetary disk such as chondrule formation and metal segregation are not well understood. One way to constrain them is to study the morphology and composition of forsteritic grains from the matrix of carbonaceous chondrites. Here, we present high-resolution ptychographic X-ray nanotomography and multimodal chemical micro-tomography (X-ray diffraction and X-ray fluorescence) to reveal the early history of forsteritic grains extracted from the matrix of the Murchison CM2.5 chondrite. The 3D electron density maps revealed, at unprecedented resolution (64~nm), spherical inclusions containing Fe-Ni, very little silica-rich glass and void caps (i.e., volumes where the electron density is consistent with conditions close to vacuum) trapped in forsterite. The presence of the voids along with the overall composition, petrological textures, and shrinkage calculations is consistent with the grains experiencing one or more heating events with peak temperatures close to the melting point of forsterite ($\sim$2100~K) and subsequently cooled and contracted, in agreement with chondrule-forming conditions., 15 pages, 11 figures, accepted for publication in ApJ

Interactions between organic molecules in aqueous environments, whether in the fluid phase or adsorbed on solids, are often affected by the cations present in the solution. We investigated, at nanometre scale, how surface carboxylate interactions are influenced by dissolved divalent cations: Mg2+, Ca2+, Sr2+ and Ba2+. Self assembled monolayer (SAM) surfaces with exposed terminations of alkyl, -CH3, carboxylate, -COO- or dicarboxylate, -DiCOO-, were deposited on gold coated tips and substrates. We used atomic force microscopy (AFM), in chemical force mapping (CFM) mode, to measure adhesion forces between various combinations of SAMs on the tip and substrate, in solutions of 0.5 M NaCl, that contained 0.012 M of one of the divalent cations. The type of cation, the number of carboxyl groups that interact and their structure on the SAM influenced adhesion between the surfaces. The effect of the reference solution, which only contains Na+ cations, on adhesion force was mainly attributed to van der Waals and hydrophobic forces, explaining lower force in systems that are more hydrophilic, i.e., -COO--COO-, and higher force for more hydrophobic systems. For charged surfaces, i.e., -COO- and -DiCOO-, in divalent cation solutions, results were consistent with ion bridging. The inclusion of a hydrophobic surface, i.e., the -CH3-COO- or -CH3-DiCOO- system, decreased the possibility for strong cation bridging with the charged surface, resulting in lower adhesion. For systems including -COO-, the adhesion force series followed the inverse cation hydrated radius trend (Na+~ Mg2+2+2+2+) whereas -DiCOO- was responsible for lower adhesion force and modified trends, depending on the corresponding surface in the system. Differences in force magnitude between the monolayers were correlated with lower charge availability on the -DiCOO- surface as a result of fewer active sites, probably because of the tendency of exposed malonate surface groups to interact between them, as well as high rigidity, resulting from the molecule structure. The characteristic response of the -DiCOO- surface in solutions of Sr2+ and Ca2+ was correlated with possible malonate complexation modes. Comparison with previous studies suggested that the strong response of a -DiCOO- surface to Sr2+ resulted from bidentate chelation, whereas Ca2+ response was attributed to alpha mode association to malonate.

Interactions between graphene oxide (GO) and organic molecules play a role in processes such as environmental remediation and water treatment. However, little is known about underlying molecular level processes with the presence of ions. In this study, we utilized atomic force microscopy (AFM) in chemical force mapping (CFM) mode to directly probe their adhesion interactions. AFM tips were functionalised to serve as models for nonpolar and polar organic molecules, i.e. with alkyl, –CH3, and carboxyl, –COO(H). For experiments with –COO(H) tips, adhesion between GO and tips decreased in the order: Ba2+ > Ca2+ > Mg2+ > Na+, whereas for the –CH3 tips, ion dependent adhesion was relatively low but followed the same: Ba2+ > Ca2+ > Mg2+ ≈ Na+. Calculations with Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and the Schulze–Hardy rule could not account for the observations. We propose that ion bridging plays a definitive role in adhesion between –COO(H) tips and the GO surface. This is consistent with proposed models with density functional theory (DFT) calculations. Adhesion of –CH3 tips is a response to the hydrophilic interactions and the ion dependent part is suggested to arise from ion bridging between slightly negative charged –CH3 tips and the GO surface. High pH had a notable influence on the adhesion of the –COO(H) tip but a negligible effect on the –CH3 tip. These results offer important insights into interactions between solutions and mineral surfaces with adsorbed organic molecules.

Simultaneous retention of SMM behaviour and of optical activity is demonstrated upon surface deposition for a chiral SMM., The synthesis and characterization of a chiral, enneanuclear Mn(iii)-based, Single-Molecule Magnet, [Mn9O4(Me-sao)6(L)3(MeO)3(MeOH)3]Cl (1; Me-saoH2 = methylsalicylaldoxime, HL = lipoic acid) is reported. Compound 1 crystallizes in the orthorhombic P212121 space group and consists of a metallic skeleton describing a defect super-tetrahedron missing one vertex. The chirality of the [MnIII9] core originates from the directional bridging of the Me-sao2– ligands via the –N–O– oximate moieties, which define a clockwise (1ΔΔ) or counter-clockwise (1ΛΛ) rotation in both the upper [MnIII3] and lower [MnIII6] subunits. Structural integrity and retention of chirality upon dissolution and upon deposition on (a) gold nanoparticles, 1@AuNPs, (b) transparent Au(111) surfaces, 1ΛΛ@t-Au(111); 1ΔΔ@t-Au(111), and (c) epitaxial Au(111) on mica surfaces, 1@e-Au(111), was confirmed by CD and IR spectroscopies, mass spectrometry, TEM, XPS, XAS, and AFM. Magnetic susceptibility and magnetization measurements demonstrate the simultaneous retention of SMM behaviour and optical activity, from the solid state, via dissolution, to the surface deposited species.

Experiments in quartz ampoules that simulate the saltation of quartz grains in a methane atmosphere show that the solid phase acquires a reddish color, reminiscent of the color observed on Triton, Pluto, and Charon. Reflection spectroscopy of the coated grains show a wide, continuous absorption spectrum peaking at near-UV wavelengths, in line with the reddish color. X-ray photoelectron spectroscopy indicates the grains are coated with a substance containing Csbnd C and Csbnd O bonds, and estimates the average thickness of the surface layer to 2.3 Å. Solid state Raman measurements of the coating shows a transition at 1540 cm-1. A model is proposed to describe these measurements. The model is a polydiene, anchored to the quartz surface. Electron structure calculations show that a polydiene with around 8 CH units reproduces the measurements. AFM-IR experiments support this result. Our findings suggest a pathway for synthesis of complex molecules with Cdbnd C bonds on planets and moons with a solid surface and a methane-containing atmosphere.

Metal ion-induced self-assembly (SA) of proteins into higher-order structures can provide new, dynamic nano-assemblies. Here, the synthesis and characterization of a human insulin (HI) analog modified at Lys

By introducing hydrophilic polymers into silicone medical devices, highly beneficial biomedical properties can be realized. An established solution to introduce hydrophilic polymers is to form an interpenetrating polymer network (IPN) by performing the hydrogel synthesis in the presence of silicone swollen in supercritical carbon dioxide. The precise distribution of the two polymers is not known, and determining this is the goal of this study. Neutron scattering and microscopy were used to determine the distribution of the hydrophilic guest polymer. Atomic force microscopy revealed that the important length scale on the surface of these materials is 10–100 nm, and spin-echo small-angle neutron scattering (SESANS) on IPNs submerged in D2O revealed structures of the same scale within the interior and enabled quantification of their size. SESANS with hydration by D2O proved to be the only scattering technique that could determine the structure of the bulk of these types of materials, and it should be used as an important tool for characterizing polymer medical devices.

Metal ion-induced self-assembly (SA) of proteins into higher-order structures can provide new, dynamic nano-assemblies. Here, the synthesis and characterization of a human insulin (HI) analog modified at LysB29 with the tridentate chelator 2,2′:6′,2′′-terpyridine (Tpy) is described. SA of this new insulin analog (LysB29Tpy-HI) in the presence of the metal ions Fe2+ and Eu3+ at different concentrations was studied in solution by fluorescence luminescence and CD spectroscopy, dynamic light scattering, and small-angle X-ray scattering, while surface assembly was probed by AFM. Unique oligomerization was observed in solution, as Fe2+ yielded small magenta-colored discrete non-native assemblies, while Eu3+ caused the formation of large fractal assemblies. Binding of both metal ions to Tpy was demonstrated spectroscopically, and emission lifetime experiments revealed a distinct Eu3+ coordination geometry that included two water molecules. SAXS suggested that LysB29Tpy-HI with Fe2+ oligomerized to a discrete, roughly octameric species, while LysB29Tpy-HI with Eu3+ gave very large assemblies that could be modelled as fractals. The fractal dimensionality increased with the Eu3+ concentration. We propose that this is a consequence of Eu3+ binding to both Tpy and to free carboxylic acid groups on the insulin surface. LysB29Tpy-HI maintained insulin receptor affinity, and showed extended blood glucose lowering and plasma concentration after subcutaneous injection in rats. The combination of metal ion directed SA and native SA provides control of nano-scale fractal dimensionality and points towards use in therapeutics.

Phosphorus induced eutrophication of freshwaters is one of the great global challenges. As the critical threshold concentration for eutrophication is very low, there is a strong need for development of phosphate sorbents with high affinity and selectivity. Single sheet iron oxide (SSI), a nanomaterial prepared from oxidation and exfoliation of layered iron(II)-iron(III) hydroxide (green rust), is a fast reacting and promising sorbent. Phosphate sorption affinity and selectivity may be improved by incorporation of lanthanum (La) in the structure. Lanthanum was added during SSI synthesis resulting in La-SSIs with 0 to 22.5 wt% of La content. XPS and EXAFS showed all La was present as La(OH)3 nanoclusters between iron hydroxide layers and on the surface of SSI. The sorbent showed fast phosphate sorption with 90% completion within 30 min and high stability with minimal La leaching. All La-SSI nanomaterials showed better sorption affinity than non-doped SSI, and the La-SSI with the highest La content showed an extreme affinity with Kd of 105 L/kg at solution concentration of 0.1 mg P/L. The sorption affinity was not seriously affected by pH. The La-SSIs showed high selectivity for phosphate with

Metasedimentary rocks from Isua, West Greenland (> 3,700 million years old) contain carbonaceous compounds, compatible with a biogenic origin (Hassenkam, Andersson, Dalby, Mackenzie, & Rosing, 2017; Ohtomo, Kakegawa, Ishida, Nagase, & Rosing, 2014; Rosing, 1999). The metamorphic mineral assemblage with garnet and quartz intergrowths contains layers of carbonaceous inclusions contiguous with carbon-rich sedimentary beds in the host rock. Previous studies (Hassenkam et al., 2017; Ohtomo et al., 2014; Rosing, 1999) on Isua rocks focused on testing the biogenic origin of the carbonaceous material, but here we searched for evidence which could provide new insights into the nature of the life that generated this carbonaceous material. We studied material trapped in inclusions armoured within quartz grains inside garnet porphyroblasts by non-destructive ptychographic X-ray nanotomography (PXCT). The 3D electron density maps generated by PXCT were correlated with maps from X-ray fluorescence tomography and micro-Raman spectroscopy. We found that the material trapped inside inclusions in the quartz grains consist of disordered carbon material encasing domains of iron-rich carbonaceous material. These results corroborate earlier claims (Hassenkam et al., 2017; Ohtomo et al., 2014; Rosing, 1999) for biogenic origins and are compatible with relics of metamorphosed biological material originally containing high iron/carbon ratios, comparable to ratios found in most extant organisms. These iron-rich domains represent the oldest evidence for organic iron complexes in the geologic record and are consistent with Fe-isotopic evidence for metabolic iron fractionation in > 3,700 Ma Isua banded iron formation (Czaja et al., 2013; Whitehouse & Fedo, 2007).

We have developed a method for predicting the solvation contribution to solid-liquid interfacial tension (IFT) based on density functional theory and the implicit solvent model COSMO-RS. Our method can be used to predict wetting behavior for a solid surface in contact with two liquids. We benchmarked our method against measurements of contact angle from water-inoil on silica wafers and a range of self-assembled monolayers (SAMs) with different compositions, ranging from oil-wet to water-wet. We also compared our predictions to literature data for wetting of a polydimethylsilane surface. By explicitly including deprotonation for silica surfaces and carboxylic acid SAMs, very good agreement was obtained with experimental data for nearly all surfaces. Poor agreement was found for amine-terminated SAMs, which could be the result of both method and model insufficiencies and impurities known to be present for such surfaces. Solid-liquid IFT cannot be measured directly, making predictions such as from our method all the more important.

Structural characterization of aggregates and fibrils of the Aβ protein is pivotal to the molecular-level elucidation of Alzheimer's disease (AD). AFM-IR spectroscopy provides nanoscale resolution, and thus allows the interrogation of individual aggregates and fibrils. During aggregation of Aβ, we observed mainly disordered Aβ at t=15 min, but substantial structural diversity including the co-existence of parallel and antiparallel β-sheets within a large amorphous aggregate at t=2 hours, while fibrils exhibited the expected signature of parallel β-sheets at t=1 week. The resonance observed for parallel β-sheets at t=2 hours coincides with that observed for fibrils (at 1634 cm

It is possible that early life relied on RNA polymers that served as ribozyme-like catalysts and store genetic information1,2. The source of such polymers is uncertain, but previous investigations reported that wet-dry cycles simulating prebiotic hot springs provide sufficient energy to drive condensation reactions of mononucleotides to form oligomers3. The aim of the study reported here was to visualize the products by atomic force microscopy. In addition to globular oligomers, ring-like structures ranging from 10-200 nm in diameter with an average around 30-40 nm were abundant, particularly when nucleotides capable of base pairing were present. The thickness of the rings was consistent with single stranded products, but some had thicknesses indicating base pair stacking. Others had more complex structures in the form of short polymer attachments and pairing of rings. These observations suggest the possibility that template-directed synthesis may occur during wet-dry cycling followed by solvation of the rings. We conclude that RNA-like rings and structures could have been synthesized non-enzymatically on the prebiotic Earth with sizes sufficient to fold into ribozymes and genetic molecules required for life to begin.

The properties and behavior of the interface between mineral surfaces, adsorbed organic compounds, and water are important for oil recovery. Low-salinity (LS) water flooding releases more oil from sandstone reservoirs than conventional flooding with seawater or formation water. However, the role of strongly adsorbed organic material, as an anchor for oil molecules, is not yet completely understood. Here, we mimic reservoir pore surfaces using graphene oxide sheets deposited on flat silicon wafers. The LS response was quantified using atomic force microscopy (AFM) in chemical force mapping mode to directly measure the adhesion force. AFM tips were functionalized to serve as models for hydrophobic and polar oil molecules, i.e., with alkyl, −CH3, and carboxyl, −COO(H). Adhesion force, measured with −CH3 tips, was 18% lower in LS (∼1500 ppm) than high-salinity (HS, ∼35 600 ppm) solutions, while for −COO(H) tips, adhesion force was 13% lower in LS than HS solutions. The Dejarguin–Landau–Verwey–Overbeek theory ...

Transglutaminase (TG) catalyzes formation of covalent bonds between lysine and glutamine side chains and has applications in manipulation of food structure. Physical properties of a whey protein mixture (SPC) denatured either at elevated pH or by heat-treatment and followed by TG catalyzed crosslinking, have been characterised using dynamic light scattering, size exclusion chromatography, flourescence spectroscopy and atomic force microscopy. The degree of enzymatic crosslinking appeared higher for pH- than for heat-denatured SPC. The hydrophobic surface properties depended on the treatment, thus heating caused the largest exposure of the hydrophobic core of SPC proteins, which was decreased by crosslinking. The particle size of the treated SPC samples increased upon crosslinking by TG. Moreover, the particle morphology depended on the type of denaturing treatment, thus heat-treated SPC contained fibrillar structures, while pH-denatured SPC remained globular as documented by using atomic force microscopy. Finally, the in vitro digestability of the different SPC samples was assessed under simulated gastric and intestinal conditions. Notably heat-treatment was found to lower the gastric digestion rate and enzymatic crosslinking reduced both the gastric and the intestinal rate of digestion. These characteristics of the various SPC samples provide a useful basis for design of isoenergic model foods applicable in animal and human studies on how food structure affects satiety.

Enhanced oil recovery using low-salinity solutions to sweep sandstone reservoirs is a widely-practiced strategy. The mechanisms governing this remain unresolved. Here, we elucidate the role of Ca2+ by combining chemical force microscopy (CFM) and molecular dynamics (MD) simulations. We probe the influence of electrolyte composition and concentration on the adsorption of a representative molecule, positively-charged alkylammonium, at the aqueous electrolyte/silica interface, for four electrolytes: NaCl, KCl, MgCl2, and CaCl2. CFM reveals stronger adhesion on silica in CaCl2 compared with the other electrolytes, and shows a concentration-dependent adhesion not observed for the other electrolytes. Using MD simulations, we model the electrolytes at a negatively-charged amorphous silica substrate and predict the adsorption of methylammonium. Our simulations reveal four classes of surface adsorption site, where the prevalence of these sites depends only on CaCl2 concentration. The sites relevant to strong adhesion feature the O− silica site and Ca2+ in the presence of associated Cl−, which gain prevalence at higher CaCl2 concentration. Our simulations also predict the adhesion force profile to be distinct for CaCl2 compared with the other electrolytes. Together, these analyses explain our experimental data. Our findings indicate in general how silica wettability may be manipulated by electrolyte concentration.

In situ infrared spectroscopy maps the occurrences of chemical bonds within tiny inclusions in 3,700-million-year-old metasedimentary rocks from West Greenland, finding greater evidence for organic life at this early date. From stromatolites in the sunlit surface waters of oceans to hydrothermal vents in the ocean depths, the evidence for organic life having existed 3.7 billion years ago is growing. Here, the researchers describe very tiny inclusions of lightly metamorphosed organic material inside garnets from 3.7-billion-year-old rocks in Greenland. They use a relatively new technique that combines atomic force microscopy and infrared spectroscopy to map the occurrences of various chemical bonds within these inclusions including carbonyl, anhydride, phosphate and nitrile. This corroborates earlier work on carbon isotopes in the same rocks but goes further by identifying these specific chemical bonds and finding much greater evidence for organic life at this early date. Metasedimentary rocks from Isua, West Greenland (over 3,700 million years old) contain 13C-depleted carbonaceous compounds, with isotopic ratios that are compatible with a biogenic origin1,2,3. Metamorphic garnet crystals in these rocks contain trails of carbonaceous inclusions that are contiguous with carbon-rich sedimentary beds in the host rock, where carbon is fully graphitized. Previous studies4,5 have not been able to document other elements of life (mainly hydrogen, oxygen, nitrogen and phosphorus) structurally bound to this carbonaceous material. Here we study carbonaceous inclusions armoured within garnet porphyroblasts, by in situ infrared absorption on approximately 10−21 m3 domains within these inclusions. We show that the absorption spectra are consistent with carbon bonded to nitrogen and oxygen, and probably also to phosphate. The levels of C–H or O–H bonds were found to be low. These results are consistent with biogenic organic material isolated for billions of years and thermally matured at temperatures of around 500 °C. They therefore provide spatial characterization for potentially the oldest biogenic carbon relics in Earth’s geological record. The preservation of Eoarchean organic residues within sedimentary material corroborates earlier claims2,6 for the biogenic origins of carbon in Isua metasediments.

Flooding sandstone oil reservoirs with low salinity water can lead to a significant increase in oil recovery, a phenomenon called “the low salinity effect”. Although there are many factors that contribute to this response, the surface tension on the pore walls is an important one. Sandstone is composed predominantly of quartz with some clay, but feldspar grains are often also present. While the wettability of quartz and clay surfaces has been thoroughly investigated, little is known about the adhesion properties of feldspar. We explored the interaction of model oil compounds, molecules that terminate with carboxyl, −COO(H), and alkyl, −CH3, with freshly cleaved, museum quality perthitic microcline, KxNa(1–x)AlSi3O8, a K-feldspar. Microcline is a member of the orthoclase family, a type of feldspar that weathers more slowly than the plagioclase series, and thus is more likely to be preserved in well sorted sandstone. Adhesion forces, measured with the chemical force mapping (CFM) mode of atomic force micros...

We measured the binding energy and bonding parameters between model nucleotide functional groups and model clay mineral surfaces in solutions of acidic pH. We demonstrate that basal surfaces of clay minerals interact most strongly with nucleobases and show that the adsorption of the phosphate group to clay edges could facilitate polymerisation. Our results suggest that Al- and Fe-rich edge sites behave similarly in nucleotide polymerisation through change of the phosphodiester bond strength. We present an internally consistent set of thermodynamic parameters that represent the nucleotide-clay mineral system.

Laboratory core flood and field scale tests have demonstrated that 5–40% more oil can be released from sandstone reservoirs by injecting low salinity water, rather than high salinity fluids, such as seawater or formation water. The effect has been explained by a change in wettability of the minerals that form the pore surfaces, as a result of the decrease in divalent cation concentration. We have previously demonstrated that, even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material and this affects wettability and response to changed salinity. Here we quantified the response of sandstone core plug material in its preserved state (i.e., after storage in kerosene) and after the same core plug material was treated with ethanol and ozone to remove adsorbed organic compounds. We used the chemical force microscopy (CFM) mode of atomic force microscopy (AFM) to directly measure the adhesion force for two types of molecules on pore surfaces of individual ...

The wettability of mineral surfaces controls a range of phenomena in natural and industrial processes. In reservoirs, rock wettability determines the effectiveness of oil production; thus, modification of mineral surface properties can lead to enhanced oil recovery. Recent work reports that potential determining ions in seawater, Mg2+, Ca2+, and SO42–, are responsible for altering the wettability of calcite surfaces. In favorable conditions, e.g., elevated temperature, calcium at the calcite surface can be replaced by magnesium, making organic molecules bind more weakly and water molecules bind more strongly, rendering the surface more hydrophilic. We used atomic force microscopy in chemical force mapping mode to probe the adhesion forces between a hydrophobic CH3-terminated AFM tip and a freshly cleaved calcite {10.4} surface to investigate wettability change in the presence of Mg2+ and SO42– at 75 and 80 °C. We made submicrometer scale maps of adhesion force and contact angle and demonstrated that the a...

The unambiguous determination of the chemical functionality over graphene oxide (GO) is important to unleash its potential applications. However, the mapping of oxygen functionalities distribution remains to be unequivocally determined because of highly inhomogeneous non-stoichiometric structures and ultra-thin layers of GO. In this study, we report an experimental observation of the spatial distribution of oxygen functional groups on monolayer and multilayer GO using AFM-IR, atomic force microscopy coupled with infrared spectroscopy. Overcoming conventional IR diffraction limit for several micrometers, the novel AFM-IR reaches high spatial resolution ∼20 nm and could detect IR absorption on ∼1 nm thickness of monolayer GO. With nanoscale chemical mapping, the distribution of different oxygen functional groups is distinguished with AFM-IR over the GO surface. It allows us to observe that these oxygen functional groups prefer to sit on the fold areas, in discrete domains and on the edges of GO, which gave more insights into its chemical nature. The determination of the position of functional groups through precise imaging contributes to our understanding of GO structure-properties relations and paves the way for targeted tethering of polymers, biomaterials, and other nanostructures.

All known eukaryotic topoisomerases are only able to relieve torsional stress in DNA. Nevertheless, it has been proposed that the introduction of positive DNA supercoiling is required for efficient sister-chromatid disjunction by Topoisomerase 2a during mitosis. Here we identify a eukaryotic enzymatic activity that introduces torsional stress into DNA. We show that the human Plk1-interacting checkpoint helicase (PICH) and Topoisomerase 3a proteins combine to create an extraordinarily high density of positive DNA supercoiling. This activity, which is analogous to that of a reverse-gyrase, is apparently driven by the ability of PICH to progressively extrude hypernegatively supercoiled DNA loops that are relaxed by Topoisomerase 3a. We propose that this positive supercoiling provides an optimal substrate for the rapid disjunction of sister centromeres by Topoisomerase 2a at the onset of anaphase in eukaryotic cells.

Core plug and field tests have shown that significantly more oil can be produced from sandstone reservoirs when the water that is injected to maintain pressure in the reservoir has lower salinity than regular seawater. We investigated the adhesion between functional groups known to dominate in crude oil and grains from reservoir sandstone as a function of pH and salinity. We used atomic force microscopy (AFM) in chemical force mapping mode, and we functionalized the AFM tips both with −CH3, to model adhesion of hydrophobic molecules, and with −COO(H), to model polar compounds. As the pH increased, the adhesion force decreased. The behavior was similar for the two types of tips, but adhesion was higher for the polar tip. The average adhesion at pH 4.5 for the −COO(H) tip in artificial seawater (ASW) was close to 300 pN, and 200 pN in ASW diluted by a factor of 20. For the −CH3 tip, adhesion was ∼200 pN in high salinity and 140 pN in low salinity. Regardless of the salinity, adhesion decreased as pH increas...

Summary Core tests demonstrated that decreasing the salinity of injection water can increase oil recovery. Optimizing injection-water salinity, however, would offer a clear economic advantage for several reasons. Too-low salinity risks swelling of the clays that would lead to permanent reservoir damage, but evidence of effectiveness with moderate-salinity solutions would make it less difficult to dispose of produced water. The goal is to define boundary conditions so injection-water salinity is high enough to prevent reservoir damage and low enough to induce the low-salinity (LS) effect, while keeping costs and operational requirements at a minimum. Traditional core-plug testing for optimizing conditions has some limitations. Each test requires a fresh sample; core-testing requires sophisticated and expensive equipment; and reliable core-test data require several months because cores must be cleaned, restored, and aged before the tests can begin. It is also difficult to compare data from one core with results from another because no two cores are identical, making it difficult to distinguish between effects resulting from different conditions and effects resulting from different cores. Gathering statistics is limited by the time required for each test and the fact that core material is in short supply. Thus, our aim was to explore the possibility of a less-expensive, faster alternative by probing the fundamental chemical mechanisms behind the LS effect. We developed a method that uses atomic-force microscopy (AFM) to investigate the relationship between the wettability of pore surfaces and water salinity. We functionalize AFM tips with organic molecules and use them to represent tiny oil droplets of nonpolar molecules, and we use sand grains removed from core plugs to represent the pore walls in sandstone. We bring our “oil”-wet tip close to the sand-grain surface and measure the work of adhesion between the tip and the surface. Repeated probing of the surface with the tip produces data that one can convert to maps of adhesion, and we can estimate contact angle. Adhesion work is proportional to wettability and is directly correlated with the salinity of the fluid in contact with the tip and the particle surface. From our measurements, the threshold values for the onset of the LS response are 5,000 to 8,000 ppm, which benchmark remarkably well with observations from core-plug tests. From a mechanistic perspective, the correlation between salinity and adhesion provides evidence for the role of electrical-double-layer (EDL) expansion in the LS response; expansion of the double layer decreases oil wettability. Because AFM experiments can be performed relatively quickly on very little material, they give the possibility of testing salinity response on many samples throughout a reservoir and for gathering statistics. Our approach provides a range of data that one can use to screen conditions to maximize the value of the core-plug testing and to provide extra data that would be too time consuming or too expensive to gather with traditional methods alone. Thus, AFM force mapping is an excellent complement to traditional core-plug testing.

It is possible that early life relied on RNA polymers that served as ribozyme-like catalysts and for storing genetic information. The source of such polymers is uncertain, but previous investigations reported that wet&ndash, dry cycles simulating prebiotic hot springs provide sufficient energy to drive condensation reactions of mononucleotides to form oligomers. The aim of the study reported here was to visualize the products by atomic force microscopy. In addition to globular oligomers, ring-like structures ranging from 10&ndash, 200 nm in diameter, with an average around 30&ndash, 40 nm, were abundant, particularly when nucleotides capable of base pairing were present. The thickness of the rings was consistent with single stranded products, but some had thicknesses indicating base pair stacking. Others had more complex structures in the form of short polymer attachments and pairing of rings. These observations suggest the possibility that base-pairing may promote polymerization during wet&ndash, dry cycling followed by solvation of the rings. We conclude that RNA-like rings and structures could have been synthesized non-enzymatically on the prebiotic Earth, with sizes sufficient to fold into ribozymes and genetic molecules required for life to begin.

Surface charge density can be derived from atomic force microscopy (AFM) by using Derjaguin, Landau, Vervey and Overbeek (DLVO) theory. The sub-micrometer data allows observation of local differences in charge density and changes with time or solution composition, which has interesting applications in crystal growth and inhibition, bone formation and colloid behavior. To calibrate this type of AFM data and verify DLVO assumptions, it has to be correlated with an established technique. We successfully matched AFM derived surface charge densities with zeta potential measurements on a mica surface within one order of magnitude. A reproducible difference between surface charge of the mica substrate exposed to solutions cations with monovalent and divalent charge was also observed. The results provide confidence that the AFM method is valid for obtaining local surface charge information.

Graphene oxide (GO) is an interesting material that has the potential for a wide range of applications. Critical for these applications are the type of oxygen bond and its spatial distribution on the individual GO sheets. This distribution is not yet well understood. Few techniques offer a resolution high enough to unambiguously identify oxygen configuration. We used a new, label free spectroscopic technique to map oxygen bonding on GO, with spatial resolution of nanometres and high chemical specificity. AFM-IR, atomic force microscopy coupled with infrared spectroscopy, overcomes conventional IR diffraction limits, producing IR spectra from specific points as well as chemical maps that are coupled to topography. We have directly observed oxygen bonding preferentially on areas where graphene is folded, in discrete domains and on edges of GO. From these observations, we propose an updated structural model for GO, with C O on its edge and plane, which confirms parts of earlier proposed models. The results have interesting implications. Determining atomic position and configuration from precise imaging offers the possibility to link nanoscale structure and composition with material function, paving the way for targeted tethering of ions, polymers and biomaterials.

The long-term mechanical strength of calcite-bearing rocks is highly dependent on the presence and nature of pore fluids, and it has been suggested that the observed effects are due to changes in nanometer-scale surface forces near fracture tips and grain contacts. In this letter, we present measurements of forces between two calcite surfaces in air and water-glycol mixtures using the atomic force microscope. We show a time- and load-dependent adhesion at low water concentrations and a strong repulsion in the presence of water, which is most likely due to hydration of the strongly hydrophilic calcite surfaces. We argue that this hydration repulsion can explain the commonly observed water-induced decrease in strength in calcitic rocks and single calcite crystals. Furthermore, this relatively simple experimental setup may serve as a useful tool for analyzing surface forces in other mineral-fluid combinations.

The interactions between mineral surfaces and organic molecules in water control many processes in nature and in the production of modern materials. To improve the understanding of fluid-surface interactions, we investigated the interface behavior of quartz and muscovite, a model for clay minerals, in aqueous solutions where the pH and composition were controlled. We used atomic force microscopy (AFM) in chemical force mapping (CFM) mode to measure adhesion using tips functionalized with alkyl, -CH3. By combining adhesion forces measured as a function of pH, with data from streaming potential experiments and DLVO calculations, we were able to determine the surface charge density. We observed increased adhesion between the mineral surface and the hydrophobic tips as the contact time increased from 7 ms to ∼2 s. The diffusion of dissolved ions takes time, and density functional theory (DFT) calculations did not indicate a strong hydration of the mineral surfaces. Therefore, we interpret that the loss of ions from the confined space between the tip and sample is a likely explanation of the correlation between the dwell time and adhesion. The maximum adhesion increase with dwell time for muscovite, i.e., 400 ± 77 pN, was considerably larger than for quartz, 84 ± 15 pN, which fits with the different surface structure and composition of the two minerals. We propose two mechanisms to explain these results: (1) cations that are structured in the solution and on the surface remain associated at the tip-sample interface initially but diffuse away during extended contact time and (2) adventitious carbon, the organic material that comes spontaneously from air and solution, can diffuse to the tip-sample interface during contact. This material decreases the surface energy by aggregating near the alkyl tip and increases adhesion between the tip and sample.

Interactions between cations and organic molecules are found throughout nature, from the functionality and structure of proteins in humans and animals to the exchange of ions in minerals in soil and oil reservoirs with the fluid phases. We have explored the behavior of the s-block elements that are most common in the natural world, namely, Na(+), K(+), Mg(2+), and Ca(2+). Specifically, we investigated how these ions affect the interactions between surfaces covered by self-assembled monolayers (SAMs) terminated with benzene molecules. We used a flat oxidized silicon substrate and an atomic force microscopy (AFM) tip that were both functionalized with 11-phenoxyundecane-1-thiol and measured the adhesion force between them in solutions of each of the four chloride salts. We observed that the adhesion increased in the order of the Hofmeister series: K(+)Na(+) ≈ Mg(2+)Ca(2+). Supplementary evidence from X-ray photoelectron spectroscopy (XPS) allowed us to conclude that K(+) binds in the benzene layers, creating a positive surface charge on the benzene-covered surfaces, thus leading to lower adhesion in KCl solutions than in pure water. Evidence suggested that Ca(2+) does not bind to the surfaces but forms bridges between the layers, leading to higher adhesion than in pure water. In Na(+) and Mg(2+) solutions, adhesion is quite similar to that in pure water, indicating a lack of interaction between these two ions and the surfaces, or at least that the interaction is too weak to be detected by our measurements. The results of our studies clearly show that even a nonpolar, hydrophobic molecule, such as benzene, has a role to play in the behavior of aqueous solutions and that it interacts differently depending on which ions are present. Even ions from the same column in the periodic table behave differently.

We investigated the adhesion of two functional groups to α-alumina as a model for the adsorption of organic molecules on clay minerals. Interactions between organic compounds and clay minerals play an important role in processes such as drinking water treatment, remediation of contaminated soil, oil recovery, and fabricating complicated nanomaterials, and there have been claims that organic compound-clay mineral interaction created the ordering that is necessary for the genesis of life. In many organisms, interaction between organic molecules and biominerals makes it possible to control the growth of bones, teeth, and shells. Adhesion of carboxylic acid, -COO(H), and pyridine, -C5H5N(H(+)), on the {0001} plane of α-alumina wafers has been investigated with atomic force microscopy (AFM) in chemical force mapping (CFM) mode. Both functional groups adhered to α-alumina in deionized water at pH5, and adhesion decreased as NaCl or CaCl2 concentration increased. X-ray photoelectron spectroscopy (XPS) showed that Na(+) and Ca(2+) adsorbed to the α-alumina surface at pH5, decreasing surface interaction with the carboxylic acid and pyridine groups. We interpret the results as evidence that the tips adhere to alumina through hydrogen bonding when only water is present. In solutions containing NaCl and CaCl2, cations are adsorbed but Cl(-) is not. When NaCl solutions are replaced by CaCl2, Ca(2+) replaces Na(+), but rinsing with ultrapure deionized water (pH 5.6) could not restore the original protonated surface. The results demonstrate that the alumina surface at pH 3 has a higher affinity for inorganic cations than for -COO(H) or -C5H5N(H(+)), in spite of the known positive surface charge of α-alumina {0001} wafers. These results demonstrate that solution salinity plays an important role in surface properties, controlling surface tension (i.e., contact angle) and adsorption affinity on α-alumina and, by analogy, on clay minerals.

Laboratory core flood and field scale tests have demonstrated that about 5 to 40% more oil can be released from sandstone reservoirs by injecting low salinity water, rather than high salinity fluids such as seawater or formation water. The effect has been explained by a change in wettability of the minerals that form the pore wall, as a result of the decrease in divalent cation concentration. Using X-ray photoelectron spectroscopy, we have demonstrated that even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material. Thus, pore wall wettability is more likely dominated by tightly adsorbed organic material than by the character of the underlying minerals. To test this hypothesis, we used the chemical force mapping (CFM) mode of atomic force microscopy (AFM) to directly measure adhesion forces on individual quartz grains that were plucked from core plugs. We functionalized AFM tips with model oil compounds so they would represent tiny oil droplets, and...

The demand for oil is increasing, but many reservoirs are reaching the end of their productive lifetime. A clearer understanding of the fundamental chemical and physical controls on the wetting behavior of reservoir pore surfaces would provide clues for developing methods to improve, or enhance, recovery of the currently inaccessible oil (improved/enhanced oil recovery, IOR/EOR). In this work, the surfaces of chalk were investigated to understand hydrophobicity at nanometer scale spatial resolution. Chalk samples from the gas and water zones of the Danish sector in the North Sea Basin were used. With inverse gas chromatography (IGC), the surface characteristics were compared. Chalk from the gas zone has a lower surface energy, dispersive as well as specific, than chalk from the water zone, clearly indicating that the gas zone pore surfaces are more hydrophobic. X-ray photoelectron spectroscopy shows that the concentration of hydrocarbons is higher in gas zone chalk than in water zone chalk, which is consi...

Highly fluorescent, discotic trioxatriangulenium dyes were aligned by simple spin-casting on substrates with friction transferred PTFE layers. The fluorescent crystalline thin films show near perfect macroscopic alignment on centimeter large areas directly from spin-casting. Gracing Incidence X-ray Diffraction (GIXD) unambiguously allowed the determination of a long-range order unit cell as well as its orientation with respect to the PTFE fibers. Further analysis of the X-ray data, in conjunction with polarized absorption spectroscopy, suggest a lamellar packing model with alternating layers of alkyl chains and ionic dyes oriented parallel to the substrate. This structure results in a highly anisotropic electrostatic potential around the cationic chromophore, causing significant shifts in energy and orientation of the optical transitions. Thus, the optical properties of the material are, to a large extent, controlled by the position of the otherwise inert PF6(-) counterions. The bright fluorescence from the films is also polarized parallel to the PTFE alignment layer. Doping of the thin films with fluorescent energy acceptor traps shows that efficient exciton migration takes place in the thin films. The excellent exciton transfer capabilities, in conjunction with the perfect alignment, might be of interest in future applications in solar energy harvesting or as thin film sensors.

Chalk is a sedimentary rock consisting mostly of micrometer sized particles of biogenic calcite (CaCO 3 ). It forms the reservoirs for oil in the Danish part of the North Sea and important drinking water aquifers in northern Europe. The interaction between organic molecules and the particle surfaces has a strong influence on the migration of oil in the reservoirs. Adhesion of oil components to chalk particle surfaces is thought to significantly reduce production and efforts are being made to determine the mechanisms behind enhanced/improved oil recovery (EOR/IOR) during water flooding. Aquifers beneath industrial areas can be heavily polluted with organic compounds and similar adsorption mechanisms could influence the geographical extent of the contaminant plume. Chalk, as a sedimentary rock, has been studied extensively for many years but the properties of the particle surfaces, where adsorption takes place, are largely unknown. In this study, we have used atomic force microscopy (AFM) to show that the grain surfaces in offshore and onshore chalk are more heterogeneous than previously assumed. The particles are not simply calcite surfaces but are partially covered by clay that is only 1–4 nm thick. With chemical force mapping (CFM), we have probed the surface using a tip coated with organic molecules to represent a tiny, mono-functionalized oil droplet. The functional groups of the self assembled monolayer on the tip were either –CH 3 or –COO − . From maps of adhesion, it is evident that in calcite saturated water, both the polar and the nonpolar functional groups adhere to the nano sized clay particles but not to calcite. This is fundamentally important information for the development of conceptual and chemical models to explain wettability alterations in chalk reservoirs.

Enhanced oil recovery has been documented in sandstone reservoirs where solutions of decreased salinity have been used for water flooding. Injected water with salinities (total dissolved solids) of 8000 ppm or less has increased oil production by incremental amounts ranging from 5 to 38% over conventional water flooding in some reservoirs. Explanations of the mechanism for the low salinity effect have proposed the presence of clays, polar compounds in the crude oil and the activity of divalent ions such as Ca2+ but there is no clear and consistent molecular level explanation for the effect. We used atomic force microscopy to investigate the fundamental properties of surfaces during the low salinity response and especially the role of illite clay. We designed model systems to reflect the minerals present in sandstone, namely illite attached to a flat SiO2 surface, in the form of an oxidized silicon wafer, freshly cleaved mica and an ozone cleaned SiO2 surface. We used tips that had been functionalized with hydrocarbons terminated either with carboxylate ( COOH) or methyl ( CH3), to simulate the polar and nonpolar components of crude oil. We used artificial sea water (ASW), 36,500 ppm total dissolved solids (TDS) for the high salinity solutions and diluted it to 1400 ppm TDS for the low salinity solutions. The adhesion maps derived from the interaction of the functionalized tips with the model surfaces show that reduced salinities have little or no effect on illite adhesion; there is only a 0–2% change. Adhesion on the fresh, clean muscovite surface varies considerably and the CH3 tip records a 10–20% decrease in response to low salinity solutions. In contrast, the carboxylate tip records a distinct response to salinity decrease, with a 20–30% drop. On a clean SiO2 surface, the low salinity response is evident, reproducible and repeatable through two cycles of high to low salinity changes. The range in decreased adhesion for these model mineral surfaces corresponds well with what is observed for core plug and field tests.

We describe herein the synthesis of a triptycene-based surfactant designed with the ability to solubilise single-walled carbon nanotubes (SWNTs) and C(60) in water through non-covalent interactions. Furthermore, an amphiphilic naphthalene-based surfactant with the same ability to solubilise SWNTs and C(60) has also been prepared. The compounds synthesised were designed with either two ionic or non-ionic tails to ensure a large number of supramolecular interactions with the solvent, thereby promoting strong solubilisation. The surfactants produced stable suspensions in which the SWNTs are dispersed and the surfactant/SWNT complexes formed are stable for more than one year. UV/Vis/NIR absorption spectroscopy, TEM and AFM were employed to probe the solubilisation properties of the dispersion of surfactants and SWNTs in water.

Enhanced oil recovery has been observed in sandstone reservoirs when the salinity of the injected water is decreased during water flooding. In core plug tests, enhancement ranges from 5 to 38% over traditional water flooding using sea water or formation water. Previous work suggests that clays must be present in the reservoir, the oil must have polar components and divalent cations must be present in the formation water, but there is no clear explanation of exactly what controls the molecular level processes at the interface between the fluid and the sand grain surfaces. We have used a new method, atomic force mapping, where functionalised atomic force microscopy (AFM) tips feel a surface, to simulate the interaction between mineral, water and crude oil, under a variety of salinity conditions. We used contact mode AFM to image the internal pore surfaces from fresh outcrop sandstone and from reservoir sandstone that had been cleaned with strong solvents, the traditional treatment method for samples used in core plug testing. We see features that suggest clay nanoparticles attached to the sand grain surfaces. The tips were functionalised with carboxylic acid to model adhesion of polar components in crude oil to pore surfaces. Adhesion forces were extracted from force maps generated while the samples were exposed to solutions of high salinity (artificial seawater, ASW, 36,500 ppm) and low salinity (diluted ASW, 1400 ppm). The maps displayed heterogenous adhesion, which in some cases could be linked to features on the surface. The low salinity effect was observed on most sand grains from the sandstones; there was no significant difference in behaviour for the outcrop and solvent cleaned reservoir samples. Adhesion decrease ranged from 0 to 57% with an average of about 30%. For most cases, the effect was reversible when the salinity was changed, even through several salinity change cycles, and the effect was consistent on several individual sand grains.