Martin Dulle

Protein-Assisted Assembly of Modular 3D Plasmonic Raspberry-like Core/Satellite Nanoclusters: Correlation of Structure and Optical Properties

We present a bottom-up assembly route for a large-scale organization of plasmonic nanoparticles (NPs) into three-dimensional (3D) modular assemblies with core/satellite structure. The protein-assisted assembly of small spherical gold or silver NPs with a hydrophilic protein shell (as satellites) onto larger metal NPs (as cores) offers high modularity in sizes and composition at high satellite coverage (close to the jamming limit). The resulting dispersions of metal/metal nanoclusters exhibit high colloidal stability and therefore allow for high concentrations and a precise characterization of the nanocluster architecture in dispersion by small-angle X-ray scattering (SAXS). Strong near-field coupling between the building blocks results in distinct regimes of dominant satellite-to-satellite and core-to-satellite coupling. High robustness against satellite disorder was proved by UV/vis diffuse reflectance (integrating sphere) measurements. Generalized multiparticle Mie theory (GMMT) simulations were employed to describe the electromagnetic coupling within the nanoclusters. The close correlation of structure and optical property allows for the rational design of core/satellite nanoclusters with tailored plasmonics and well-defined near-field enhancement, with perspectives for applications such as surface-enhanced spectroscopies.

Structure of Polyglycerol Oleic Acid Ester Nonionic Surfactant Reverse Micelles in Decane: Growth Control by Headgroup Size

The structure of polyglycerol oleic acid ester nonionic surfactant micelles in n-decane has been investigated at room temperature by small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and densiometry techniques. The scattering data were evaluated by indirect Fourier transformation (IFT) or generalized indirect Fourier transformation (GIFT) methods depending on the volume fractions of the surfactants and also by model fit. A simple route to the growth control of reverse micelles by headgroup size of the surfactant was investigated. Additionally, the dependence of reverse micellar structure (shape and size) on temperature, composition, and added water was also investigated. The indirect Fourier transformation gives the real space pair-distance distribution function, p(r): a facile way for the quantitative estimation of structure parameters of the aggregates. It was found that the size of the reverse micelles increases with increasing the headgroup size of the surfactant. Globular type of micelles with maximum diameter ca. 6 nm observed in the monoglycerol oleic acid ester/decane system at 25 degrees C transferred into elongated prolate type micelles with maximum diameter ca. 19.5 nm in the hexaglycerol oleic acid ester/decane system. In a particular surfactant and oil system, increasing temperature decreased the micellar size. The size of the micelle was decreased by approximately 25% upon increasing temperature from 25 to 75 degrees C in the 5 wt % diglycerol oleic acid ester/decane system. Concentration could not modulate the structure of micelles despite a wide variation in the surfactant concentration (5-25 wt %). Nevertheless, increasing surfactant concentration reduces the intermicellar distance, and a strong repulsive interaction peak was observed in the scattering curves at higher surfactant concentrations. Besides, the results obtained from the dynamic light scattering have shown the signature of diffusion hindrance relative to hard sphere with the surfactant concentration. Interestingly, the reverse micelles of the 10 wt % diglycerol oleic acid ester/decane system could incorporate approximately 1.2% water in the micellar core and cause a dramatic growth to the micelles size. The size of the water swollen micelles was approximately 40% bigger than the empty micelle.

Influence of phosphocholine alkyl chain length on peptide-micelle interactions and micellar size and shape.

The interaction with biological membranes is of functional importance for many peptides and proteins. Structural studies on such membrane-bound biomacromolecules are often carried out in solutions containing small membrane-mimetic assemblies of detergent molecules. To investigate the influence of the hydrophobic chain length on the structure, diffusional and dynamical behavior of a peptide bound to micelles, we studied the binding of three peptides to n-phosphocholines with n ranging from 8 to 16. The peptides studied are the 15 residue antimicrobial peptide CM15, the 25-residue transmembrane helix 7 of yeast V-ATPase (TM7), and the 35-residue bacterial toxin LdrD. To keep the dimension of the peptide-membrane-mimetic assembly small, micelles are typically used when studying membrane-bound peptides and proteins, for example, by solution NMR spectroscopy. Since they are readily available in deuterated form most often sodium-dodecylsulfate (SDS) and dodecylphosphocholine (DPC) are used as the micelle-forming detergent. Using NMR, CD, and SAXS, we found that all phosphocholines studied form spherical micelles in the presence and absence of small bound peptides and the diameters of the micelles are basically unchanged upon peptide binding. The size of the peptide relative to the micelle determines to what extent the secondary structure can form. For small peptides (up to approximately 25 residues) the use of shorter chain phosphocholines is recommended for solution NMR studies due to the favorable spectral quality and since they are as well-structured as in DPC. In contrast, larger peptides are better structured in micelles formed by detergents with chain lengths longer than DPC.

Simultaneous SAXS/WAXS/UV–Vis Study of the Nucleation and Growth of Nanoparticles: A Test of Classical Nucleation Theory

Despite the increasing interest in the applications of functional nanoparticles, a comprehensive understanding of the formation mechanism starting from the precursor reaction with subsequent nucleation and growth is still a challenge. We for the first time investigated the kinetics of gold nanoparticle formation systematically by means of a lab-based in situ small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering (WAXS)/UV-vis absorption spectroscopy experiment using a stopped-flow apparatus. We thus could systematically investigate the influence of all major factors such as precursor concentration, temperature, the presence of stabilizing ligands and cosolvents on the temporal evolution of particle size, size distribution, and optical properties from the early prenucleation state to the late growth phase. We for first time formulated and numerically solved a closed nucleation and growth model including the precursor reaction. We observe that the results can be well described within the framework of classical nucleation and growth theory, including also results of previous studies by other research groups. From the analysis, we can quantitatively derive values for the rate constants of precursor reaction and growth together with their activation free enthalpies. We find the growth process to be surface-reaction limited with negligible influence of Ostwald ripening yielding narrow disperse gold nanoparticles.

SERF Protein Is a Direct Modifier of Amyloid Fiber Assembly

Summary The inherent cytotoxicity of aberrantly folded protein aggregates contributes substantially to the pathogenesis of amyloid diseases. It was recently shown that a class of evolutionary conserved proteins, called MOAG-4/SERF, profoundly alter amyloid toxicity via an autonomous but yet unexplained mode. We show that the biological function of human SERF1a originates from its atypical ability to specifically distinguish between amyloid and nonamyloid aggregation. This inherently unstructured protein directly affected the aggregation kinetics of a broad range of amyloidogenic proteins in vitro, while being inactive against nonamyloid aggregation. A representative biophysical analysis of the SERF1a:α-synuclein (aSyn) complex revealed that the amyloid-promoting activity resulted from an early and transient interaction, which was sufficient to provoke a massive increase of soluble aSyn amyloid nucleation templates. Therefore, the autonomous amyloid-modifying activity of SERF1a observed in living organisms relies on a direct and dedicated manipulation of the early stages in the amyloid aggregation pathway.

Internally Self-Assembled Submicrometer Emulsions Stabilized with a Charged Polymer or with Silica Particles

Internally self-assembled submicrometer emulsions were stabilized by F127, by the charged diblock copolymer K151, by L300 particles, and by sodium dodecyl sulfate (SDS). The stabilization of all investigated internal phases and the impact of the stabilizer on them are discussed. The use of charged stabilizers results in a highly negative zeta potential of the emulsion droplets, which can be exploited as a means to control their adsorption onto charged surfaces. Small-angle X-ray scattering and dynamic light scattering were used to determine the internal structure and size of the emulsion droplets, respectively.

Fast diffusion-limited lyotropic phase transitions studied in situ using continuous flow microfluidics/microfocus-SAXS.

Fast concentration-induced diffusion-limited lyotropic phase transitions can be studied in situ with millisecond time resolution using continuous flow microfluidics in combination with microfocus small-angle X-ray scattering. The method was applied to follow a classical self-assembly sequence where amphiphiles assemble into micelles, which subsequently assemble into an ordered lattice via a disorder/order transition. As a model system we selected the self-assembly of an amphiphilic block copolymer induced by the addition of a nonsolvent. Using microchannel hydrodynamic flow-focusing, large concentration gradients can be generated, leading to a deep quench from the miscible to the microphase-separated state. Within milliseconds the block copolymers assembly via a spinodal microphase separation into micelles, followed by a disorder/order transition into an FCC liquid-crystalline phase with late-stage domain growth and shear-induced domain orientation into a mesocrystal. A comparison with a slow macroscopic near-equilibrium kinetic experiment shows that the fast structural transitions follow a direct pathway to the equilibrium structure without the trapping of metastable states.

SAXS Analysis of Shell Formation During Nanocapsule Synthesis via Inverse Miniemulsion Periphery RAFT Polymerization

Currently available methods for synthesis of polymeric nanocapsules only offer limited control over the shell thickness, even though it is an important parameter for various applications. Furthermore, suitable methods to critically measure this parameter in a facile way are still nonexistent. Here, lab-scale small-angle X-ray scattering (SAXS) is utilized to in situ measure the evolution of shell thickness during nanocapsule synthesis via inverse miniemulsion periphery reversible addition-fragmentation chain transfer (RAFT) polymerization (IMEPP). The measured shell thickness is consistent with estimates from the commonly used transmission electron microscopy (TEM) technique. Moreover, the individual thicknesses of two concentric shells comprising different polymeric materials (the outer shell formed via IMEPP chain extension of the inner shell) can be determined, thus further demonstrating the versatility of this approach.

Monodisperse hollow silica spheres: An in-depth scattering analysis

Herein, we fabricate hollow silica nanoparticles with exceptionally narrow size distributions that inherently possess two distinct length scales—tens of nanometers with regards to the shell thickness, and hundreds of nanometers in regards to the total diameter. We characterize these structures using dynamic and static light scattering (DLS and SLS), small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM), and we demonstrate quantitative agreement among all methods. The ratio between the radius of gyration (SLS) and hydrodynamic radius (DLS) in these particles equals almost unity, corresponding to ideal capsule behavior. We are able to resolve up to 20 diffraction orders of the hollow sphere form factor in SAXS, indicating a narrow size distribution. Data from light and X-ray scattering can be combined to a master curve covering a q-range of four orders of magnitude assessing all hierarchical length scales of the form factor. The measured SLS intensity profiles noticeably change when the scattering contrast between the interior and exterior is altered, whereas the SAXS intensity profiles do not show any significant change. Tight control of the aforementioned length scales in one simple and robust colloidal building block renders these particles suitable as future calibration standards.

Bottom-Up Meets Top-Down: Patchy Hybrid Nonwovens as an Efficient Catalysis Platform

Heterogeneous catalysis with supported nanoparticles (NPs) is a highly active field of research. However, the efficient stabilization of NPs without deteriorating their catalytic activity is challenging. By combining top-down (coaxial electrospinning) and bottom-up (crystallization-driven self-assembly) approaches, we prepared patchy nonwovens with functional, nanometer-sized patches on the surface. These patches can selectively bind and efficiently stabilize gold nanoparticles (AuNPs). The use of these AuNP-loaded patchy nonwovens in the alcoholysis of dimethylphenylsilane led to full conversion under comparably mild conditions and in short reaction times. The absence of gold leaching or a slowing down of the reaction even after ten subsequent cycles manifests the excellent reusability of this catalyst system. The flexibility of the presented approach allows for easy transfer to other nonwoven supports and catalytically active NPs, which promises broad applicability.