Proteins at interfaces play important roles in cell biology, immunology, bioengineering, and biomimetic material design. Many biological processes are based on interfacial protein action, ranging ...from cellular communication to immune responses and the protein-driven mineralization of bone. Despite the importance of interfacial proteins, comparatively little is known about their structure. The standard methods for studying crystalline or solution-phase proteins (X-ray diffraction and NMR spectroscopy) are not well-suited for studying proteins at interfaces, and for these proteins we still lack a corresponding technique that can provide the same level of structural resolution. This is not surprising in view of the challenges involved in probing the structure of proteins within monomolecular films assembled at a very thin interface in situ. Vibrational sum-frequency generation (SFG) spectroscopy has the potential to overcome this challenge and investigate the structure and dynamics of proteins at interfaces at the molecular level with subpicosecond time resolution. While SFG studies were initially limited to simple model peptides, the past decade has seen a dramatic advancement of experimental techniques and data analysis methods that has made it possible to also study interfacial proteins and their folding, binding, orientation, hydration, and dynamics. In this review, we first explain the principles of SFG spectroscopy and the experimental and theoretical methods to measure and analyze protein SFG spectra. Then we give an extensive overview of the interfacial proteins studied to date with SFG. We highlight representative examples to demonstrate recent advances in probing the structure of proteins at the interfaces of liquids, membranes, minerals, and synthetic materials.
Carbon dots (CDs) are an astonishing class of fluorescent materials with many applications in bioimaging, drug delivery, photovoltaics and photocatalysis due to their outstanding luminescence ...properties and low toxicity. However, the internal CD structure of bottom-up synthesized CDs is still the subject of considerable debate. Unambiguous analysis of the internal CD composition is hampered by the fact that reaction products usually contain mixtures of several CD fractions as well as molecular intermediate and side products. Therefore, purification and careful separation of the various CD fractions is vital for structural analysis and isolation of pure CDs possessing optimized optical properties. In this study, CD solutions were synthesized from citric acid and cysteine
via
a one-pot hydrothermal synthesis. A simple column chromatography unit was used to systematically study the influence of the molar precursor ratios and synthesis conditions (temperature, reaction time) on the CD solution composition. By investigating the structural and optical properties of the chromatographically separated fractions, three different fluorescent species could be identified. Freely floating molecular fluorophores left the column first, followed by highly fluorescent CDs with fluorophores bound to the carbon core, finally followed by low-fluorescent carbon particles without fluorophores. We demonstrate that the CD solution composition and the internal structure of the individual fluorescent components can be clarified
via
chromatographic separation. This information can be further applied to isolate pure CDs with optimized optical properties.
Purification and separation of CD solution into three fluorescent fractions.
Applications of gold nanoparticles often demand that the particle’s ligand shell is modified after particle formation. Obviously, there is a great need for a molecular understanding of this process ...which is often not accessible in situ. Here, we have applied second-harmonic light scattering (SHS) to investigate the ligand exchange at the surface of colloidal gold nanoparticles in situ and in real time. We demonstrate that the ligand exchange at the surface of citrate-covered Au nanoparticles with 3-mercapto-1-propanesulfonate (MPS) must be described by a fast (<100 s) and a slow reaction process (<23 min), which can be attributed to MPS adsorption on low- and high-coordinated Au surface sites. Using a modified Langmuir isotherm, the average Gibbs free energy of adsorption ΔG (−46 kJ/mol) and the surface coverage Γ (∼3.5 μmol/m2) for MPS on Au nanoparticles were determined. The latter was found to be much smaller compared to planar gold surfaces which points to coadsorption of MPS with citrate on high-coordinated sites, i.e., Au terraces. On more reactive low-coordinated Au sites, i.e., edge sites, citrate is easily replaced by MPS. In fact, we find that a substantial portion (49%) of the surface-adsorbed MPS is present on these low-coordinated sites.
The presence of electrochemical reactions occurring in an electrospray processes at the point where the current enters the liquid is discussed since the early 1990's. This current transfer to the ...liquid results in oxidation or reduction of either electrolyte species in the liquid sprayed or of the electrode material in contact with the liquid. As a result, new chemical species are generated. These products of the electrochemical reaction might be detected as altered species in mass spectra; they might be volatile and not recognized at all or accumulate on the electrode surface and cause cross contamination later on. In other cases, it might happen that the products of the electrochemical reactions are the only detectable species formed from an otherwise nondetectable analyte. An electrospray setup in which electrochemical reactions do not interfere with the analyte under investigation excludes the electrochemical reaction as source of sample contamination and sample altering and may serve as reference setup for experiments focused on the electrochemical reaction itself. We present a simple and inexpensive current coupling approach and specify operation conditions for which any impact of the electrochemical reaction on the sample under investigation is inherently excluded. On the basis of a practical example, we show the impact of the electrochemical reaction on sample composition and demonstrate the benefit of using the proposed current coupling method. Because of the obvious benefit of this method and its simple realization, it has the potential to be employed as standard feeding approach, especially for electrosprays operated at small flow rates.
In this work we performed a detailed investigation of the photostability of bottom-up produced carbon nanodots (CDs) prepared from citric acid and urea by solvothermal synthesis. Analytical ...ultracentrifugation (AUC) reveals that the CDs have a hydrodynamic diameter of <1 nm and a very narrow size distribution. In the community it is widely assumed that CDs are photo-stable. In contrast, we found that CDs exposed to UV-irradiation exhibit noteworthy fluorescence degeneration compared to freshly prepared CDs or CDs stored in the dark, indicating that fluorescence bleaching is caused by a photochemical process. We found that fluorescence intensity decay due to exposure to UV-irradiation is accelerated in the presence of oxygen and identified the surface status of CDs as the decisive factor of fluorescence bleaching of CDs. Based on a discussion on the underlying mechanisms we show how to avoid photobleaching of CDs.
The topological metal-organic framework isomers MIL-53 and MIL-68 form from similar educts but differ in their pore geometries. They have been known for several years, but their synthesis is always ...reported separately. In consequence, the underlying mechanism and decisive synthesis parameters leading to the formation of either MIL-53 or MIL-68 are not understood. The present study shows how to induce the formation of MIL-68(Al) rather than MIL-53(Al) at low synthesis temperatures in
,
-dimethylformamide (DMF) using a modulated synthesis approach. MIL-68(Al) is identified as the intermediate product of formic acid modulated synthesis, which converts into the thermodynamically stable MIL-53(Al) product at longer synthesis times. The interactions of formic acid with the synthesis precursors responsible for inducing MIL-68(Al) formation are investigated with
Raman and FTIR spectroscopy. In contrast to the commonly assumed modulation mechanism of competitive coordination of linker and modulator with the metal node, formic acid is shown to form hydrogen bonds
the carboxylic group of the terephthalic acid (H
BDC) linker, slowing prenucleation building unit and subsequent crystal growth. MIL-68(Al) formation is favored by the combination of a deficiency of terephthalic acid in solution and a slow MOF growth rate. Dissolved H
BDC in solution is proposed to hinder MIL-68(Al) formation by serving as a molecular template for the rhombic MIL-53(Al) pore channels.
After spilling coffee, a tell-tale stain is left by the drying droplet. This universal phenomenon, known as the coffee ring effect, is observed independent of the dispersed material. However, for ...many technological processes such as coating techniques and ink-jet printing a uniform particle deposition is required and the coffee ring effect is a major drawback. Here, we present a simple and versatile strategy to achieve homogeneous drying patterns using surface-modified particle dispersions. High-molecular weight surface-active polymers that physisorb onto the particle surfaces provide enhanced steric stabilization and prevent accumulation and pinning at the droplet edge. In addition, in the absence of free polymer in the dispersion, the surface modification strongly enhances the particle adsorption to the air/liquid interface, where they experience a thermal Marangoni backflow towards the apex of the drop, leading to uniform particle deposition after drying. The method is independent of particle shape and applicable to a variety of commercial pigment particles and different dispersion media, demonstrating the practicality of this work for everyday processes.
The metal–organic framework MIL-53(Al) serves as a model system in this study. Its formation mechanism in N,N-dimethylformamide (DMF) is elucidated through simultaneous FTIR and Raman spectroscopy ...and turbidity measurements collected under in-situ synthesis conditions in a custom-designed solvothermal reactor coupled with reaction sampling. Different synthesis steps are followed over synthesis time including a prenucleation building unit (PNBU) consisting of one linker molecule and one aluminum atom, the assembly of the PNBUs to MIL-53 nuclei in solution, the decomposition of DMF to formic acid and dimethylammonium, and finally the precipitation of the crystalline MOF phase. The rearrangement of the PNBU to form MIL-53 is identified as the rate-limiting reaction step responsible for the long induction time at low temperatures (<80 °C). MOF nucleation and particle growth is followed directly in-situ through a novel methodcomparison of the temporal evolution of the respective FTIR and Raman bands. Analysis of the particles isolated after various synthesis times indicates that MIL-53(H2BDC) of low crystalline long-range order is formed initially, which quickly rearranges to form more ordered, crystalline particles with DMF inside of the pores (MIL-53(DMF)). It is to be expected that the synthesis steps elucidated herein can be generalized to the solvothermal synthesis of other metal–organic framework structures from metal salts and organic acid linkers.
Dry powder coating is used in many industries to tailor the bulk solid characteristics of cohesive powders. Within this paper, the state of the art of dry coating of feedstock materials for powder ...based additive manufacturing (AM) processes will be reviewed. The focus is on feedstock materials for powder bed fusion AM processes, such as powder bed fusion of polymers with a laser beam and powder bed fusion of metals with lasers or an electron beam. Powders of several microns to several ten microns in size are used and the feedstock’s bulk solid properties, especially the flowability and packing density are of immanent importance in different process steps in particular for powder dosing and spreading of powder layers onto the building area. All these properties can be tuned by dry particle coating. Moreover, possibilities to improve AM processability and to manipulate the resulting microstructure (c.f. grain refinement, dispersion strengthening) by adhering nanoparticles on the powders will be discussed. The effect of dry coating on the obtained powder properties along the whole AM process chain and the resulting part properties is assessed. Moreover, appropriate characterization methods for bulk solid properties of dry-coated AM powders are critically discussed.
We present a model to calculate particle size distributions (PSDs) of colloidal ZnO nanoparticles from their absorbance spectra. Using literature values for the optical properties of bulk ZnO and ...correlating the measurement wavelengths in the UV−visible regime with distinct particle sizes by a tight binding model (TBM), an algorithm deconvolutes the absorbance spectra into contributions from size fractions. We find an excellent agreement between size distributions determined from TEM images and the calculated PSDs. For further validation, bimodal PSDs have been investigated and an approach to determine not only particle size but also solid concentration is introduced. We will show the applicability of our model by the determination of temperature-dependent ripening rates, which enables the calculation of solubilities, surface tensions, and the activation enthalpy of ripening. In principle, our methodology is applicable to different semiconductor nanoparticles in various solvents as long as their bulk properties are known and scattering is negligible.
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