Surface-induced Nucleation Research Articles

Mechanistic Insights Behind the Self-Assembly of Human Insulin under the Influence of Surface-Engineered Gold Nanoparticles.

Elucidating the underlying principles of amyloid protein self-assembly at nanobio interfaces is extremely challenging due to the diversity in physicochemical properties of nanomaterials and their physical interactions with biological systems. It is, therefore, important to develop nanoscale materials with dynamic features and heterogeneities. In this work, through engineering of hierarchical polyethylene glycol (PEG) structures on gold nanoparticle (GNP) surfaces, tailored nanomaterials with different surface properties and conformations (GNPs-PEG) are created for modulating the self-assembly of a widely studied protein, insulin, under amyloidogenic conditions. Important biophysical studies including thioflavin T (ThT) binding, circular dichroism (CD), surface plasmon resonance (SPR), and atomic force microscopy (AFM) showed that higher-molecular weight GNPs-PEG triggered the formation of amyloid fibrils by promoting adsorption of proteins at nanoparticle surfaces and favoring primary nucleation rate. Moreover, the modulation of fibrillation kinetics reduces the overall toxicity of insulin oligomers and fibrils. In addition, the interaction between the PEG polymer and amyloidogenic insulin examined using MD simulations revealed major changes in the secondary structural elements of the B chain of insulin. The experimental findings provide molecular-level descriptions of how the PEGylated nanoparticle surface modulates protein adsorption and drives the self-assembly of insulin. This facile approach provides a new avenue for systematically altering the binding affinities on nanoscale surfaces by tailoring their topologies for examining adsorption-induced fibrillogenesis phenomena of amyloid proteins. Together, this study suggests the role of nanobio interfaces during surface-induced heterogeneous nucleation as a primary target for designing therapeutic interventions for amyloid-related neurodegenerative disorders.

Modeling and Designing Particle-Regulated Amyloid-like Assembly of Synthetic Polypeptides in Aqueous Solution.

In cells, actin and tubulin polymerization is regulated by nucleation factors, which promote the nucleation and subsequent growth of protein filaments in a controlled manner. Mimicking this natural mechanism to control the supramolecular polymerization of macromolecular monomers by artificially created nucleation factors remains a largely unmet challenge. Biological nucleation factors act as molecular scaffolds to boost the local concentrations of protein monomers and facilitate the required conformational changes to accelerate the nucleation and subsequent polymerization. An accelerated assembly of synthetic poly(l-glutamic acid) into amyloid fibrils catalyzed by cationic silica nanoparticle clusters (NPCs) as artificial nucleation factors is demonstrated here and modeled as supramolecular polymerization with a surface-induced heterogeneous nucleation pathway. Kinetic studies of fibril growth coupled with mechanistic analysis demonstrate that the artificial nucleators predictably accelerate the supramolecular polymerization process by orders of magnitude (e.g., shortening the assembly time by more than 10 times) when compared to the uncatalyzed reaction, under otherwise identical conditions. Amyloid-like fibrillation was supported by a variety of standard characterization methods. Nucleation followed a Michaelis-Menten-like scheme for the cationic silica NPCs, while the corresponding anionic or neutral nanoparticles had no effect on fibrillation. This approach shows the effectiveness of charge-charge interactions and surface functionalities in facilitating the conformational change of macromolecular monomers and controlling the rates of nucleation for fibril growth. Molecular design approaches like these inspire the development of novel materials via biomimetic supramolecular polymerizations.

Surface Nucleation of Dispersed Polyethylene Droplets in Immiscible Blends Revealed by Polypropylene Matrix Self-Nucleation

Usually, the nature of surface-induced nucleation in polymer blends is not easily disclosed. A novel approach for studying surface-induced crystallization in blends of semicrystalline polymers is proposed here. It consists of detecting variations in the crystallization kinetics of the dispersed phase with changing the crystalline state of the matrix through self-nucleation. It can be used only when the dispersed phase has a lower melting temperature than the matrix phase. As a case study, the crystallization behavior of dispersed polyethylene droplets in a polypropylene matrix was investigated. An enhancement of crystallization kinetics of polyethylene was achieved when the lamellar thickness of polypropylene increased, and it was proved by the formation of a transcrystalline layer of polyethylene at the interface, as observed by scanning electron microscopy. Compared to the self-nucleated neat polyethylene, the efficiency of the nucleating effect of polypropylene toward polyethylene was estimated around 140%. This result together with a very low value for the interfacial free energy difference as obtained from isothermal crystallization measurements is evidence that such surface-induced nucleation occurs through epitaxial growth. Moreover, a mechanism of polyethylene nuclei formation through epitaxy, which was proposed in the literature, was proved to be valid for blends of the two polymers through small- and wide-angle X-ray scattering structural analysis. While epitaxy between polyethylene and polypropylene was previously shown only for ideal systems such as thin-layered films, it is hereby reported for common melt mixed blends of the two polyolefins.

Predominant Effect of Material Surface Hydrophobicity on Gypsum Scale Formation.

Scale formation is an important challenge in water and wastewater treatment systems. However, due to the complex nature of membrane surfaces, the effects of specific membrane surface characteristics on scale formation are poorly understood. In this study, the independent effect of surface hydrophobicity on gypsum (CaSO4·2H2O) scale formation via surface-induced nucleation and bulk homogeneous nucleation was investigated using quartz crystal microbalance with dissipation (QCM-D) on self-assembled monolayers (SAMs) terminated with -OH, -CH3, and -CF3 functional groups. Results show that higher surface hydrophobicity enhances both surface-induced nucleation of gypsum and attachment of gypsum crystals formed from homogeneous nucleation in the bulk solution. The enhanced surface-induced nucleation is attributed to the lower nucleation energy barrier on a hydrophobic surface, while the increased gypsum crystal attachment results from the favorable hydrophobic interactions between gypsum and more hydrophobic surfaces. Contrary to previous findings, the role of Ca2+ adsorption in surface-induced nucleation was found to be relatively small and similar on the different SAMs. Therefore, increasing material hydrophilicity is a potential approach to reduce gypsum scaling.

Nucleation modalities in poly(lactide), poly(butylene succinate), and poly(ε‐caprolactone) ternary blends with partial wetting morphology

Ternary biodegradable polymer blends of poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and poly(ε-caprolactone) (PCL) with the composition 45/10/45 wt% and exhibiting partial-wetting morphology were prepared. In this morphology, the minor phase is present as self-assembled droplets at the co-continuous interface of the other two major phases. The crystallization of the components in the various blends was thoroughly investigated. Differential scanning calorimetry highlighted minor differences in the overall kinetics of a given component in the ternary blend, with respect to the neat polymer. On the other hand, several unusual nucleation mechanisms could be studied by polarized optical microscopy (PLOM). With reference to the major phases, PLA spherulites displayed surface-induced nucleation from the interface with molten PBS or PCL droplets. On lowering the crystallization temperature, the PBS phase effectively nucleated at the interface with previously crystallized PLA domains, forming a transcrystalline morphology. Concerning the minor phase, weak partial-wetting PBS droplets displayed a droplet-to-droplet percolation of the nucleation events. Strongly partial-wetting PCL droplets were confined between previously crystallized PLA and PBS co-continuous phases and, instead, solidified as isolated domains randomly in space. This work provides further insights in the relationship between morphology and crystallization in immiscible ternary blends.

Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of a structure. Here, we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene. We observe the nucleation and growth of the surface and bulk spherulites, which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite-size effects in percolation phenomena and via surface-induced nucleation. The modulus-crystallinity relationship can be described through a general effective medium theory. It indicates that for thicker gaps, the viscoelastic liquid to solid transition can be described via percolation, whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enable the estimation of when gap-dependent superstructure effects can be anticipated.

How Confinement Affects the Nucleation, Crystallization, and Dielectric Relaxation of Poly(butylene succinate) and Poly(butylene adipate) Infiltrated within Nanoporous Alumina Templates.

This work describes the successful melt infiltration of poly(butylene succinate) (PBS) and poly(butylene adipate) (PBA) within 70 nm diameter anodic aluminum oxide (AAO) templates. The infiltrated samples were characterized by SEM, Raman, and FTIR spectroscopy. The crystallization behaviors and crystalline structures of both polymers, bulk and confined, were analyzed by differential scanning calorimetry (DSC) and grazing incidence wide angle X-ray scattering (GIWAXS). DSC revealed that a change in the nucleation process occurred from heterogeneous nucleation for bulk samples to homogeneous nucleation for infiltrated PBA and to surface-induced nucleation for infiltrated PBS. GIWAXS results indicate that PBS nanofibers crystallize in the α-phase, as well as their bulk samples. However, PBA nanofibers crystallize just in the β-phase, whereas PBA bulk samples crystallize in a mixture of α- and β-phases. The crystal orientation within the pores was determined, and differences between PBS and PBA were also found. Finally, broadband dielectric spectroscopy was applied to study the segmental dynamics for bulk and infiltrated samples. The glass temperature was found to significantly decrease in the PBS case upon infiltration, while that of PBA remained unchanged. These differences were correlated with the higher affinity of PBS to the AAO walls than PBA, in accordance with their nucleation behavior (surface-induced versus homogeneous nucleation, respectively).

Structure and High Performance of Lead-Free (K0.5Na0.5)NbO3 Piezoelectric Nanofibers with Surface-Induced Crystallization at Lowered Temperature.

Lead-free potassium and sodium niobate (KNN) nanofiber webs with random and aligned configurations were prepared by the electrospinning process from polymer-modified chemical solution. The crystallization process, structure, composition, dielectric, ferroelectric, and piezoelectric properties of the nanofibers and nanofiber webs were investigated. Theoretical analysis and experimental results showed that the surface-induced heterogeneous nucleation resulted in the remarkable lower crystallization temperature for the KNN nanofibers with the {100} orientation of the perovskite phase in contrast to the bulk KNN gel and thus well-controlled chemical stoichiometry. Low dielectric loss, large electric polarization, and high piezoelectric performance were obtained in the nanofiber webs. In particular, the aligned nanofiber web exhibited further improved piezoelectric strain and voltage coefficients and higher FOM than their thin film counterparts and is promising for high-performance electromechanical sensor and transducer applications.

Self-Alignment of Cylinder-Forming Silicon-Containing Block Copolymer Films

Well-ordered perpendicular cylinders of star-block copolymers (BCPs) composed of polystyrene and poly(dimethylsiloxane) blocks with high aspect ratio can be achieved by using a combination of architecture effect (entropy effect) and surface air plasma treatment (enthalpy effect). An interesting morphological evolution from disordered texture to hexagonally packed cylinders with perpendicular orientation has been observed, in which perpendicular cylinders are initiated at the top surface and bottom substrate of the thin film, followed by a self-alignment process, resulting in span-thru cylinders through the thin film. The self-alignment process is systematically examined by a combination of real space imaging using transmission electron microscopy, reciprocal space imaging using grazing incidence small-angle X-ray scattering, and three-dimensional tomography. The experimental results reveal a unique mechanism for the formation of highly ordered BCP thin films via surface-induced nucleation of perpendicular...

Mechanistic investigations in sonochemical degradation of trihalomethanes in presence of non–porous and mesoporous silica nanospheres

This study reports mechanistic investigations in sonochemical degradation of four Trihalomethanes (THMs), viz. CHCl3, CHCl2Br, CHClBr2 and CHBr3, in presence of non–porous and mesoporous silica nanospheres. Degradation reactions of THMs were carried out under identical conditions with addition of silica nanospheres. Concurrently, simulations of cavitation bubble dynamics were carried out. Mesoporous silica nanospheres with the largest surface area and the highest mean pore diameter yielded the highest enhancement in degradation of the THMs. These results were also corroborated by H2O2 dosimetry. The concentration profiles of the THMs were fitted to pseudo 1st order kinetic model. Although mean pore sizes of mesoporous silica nanospheres were smaller than minimum threshold radius for transient cavitation, these nanospheres provided surface-induced and pore-induced nucleation to augment cavitation bubble population in medium. The highest nanosphere–induced enhancement in degradation was obtained for CHBr3, while the highest total degradation was obtained for CHCl3. With concurrent analysis of experimental and simulations results, an attempt is made to deduce the predominant physical mechanism of degradation of the THMs.

Mechanistic Origin of the Combined Effect of Surfaces and Mechanical Agitation on Amyloid Formation.

Interactions between proteins and surfaces in combination with hydrodynamic flow and mechanical agitation can often trigger the conversion of soluble peptides and proteins into aggregates, including amyloid fibrils. Despite the extensive literature on the empirical effects of surfaces and mechanical forces on the formation of amyloids, the molecular details of the mechanisms underlying this behavior are still elusive. This limitation is, in part, due to the complex reaction network underlying the formation of amyloids, where several microscopic reactions of nucleation and growth can occur both at the interfaces and in bulk. In this work, we design a high-throughput assay based on nanoparticles and we apply a chemical kinetic platform to analyze the mechanisms underlying the effect of surfaces and mechanical forces on the formation of amyloid fibrils from human insulin under physiological conditions. By considering a variety of polymeric nanoparticles with different surface properties we explore a broad range of repulsive and attractive interactions between insulin and surfaces. Our analysis shows that hydrophobic interfaces induce the formation of amyloid fibrils by specifically promoting the primary heterogeneous nucleation rate. In contrast, mechanical forces accelerate the formation of amyloid fibrils by favoring mass transport and further amplify the number of fibrils by promoting fragmentation events. Thus, surfaces and agitation have a combined effect on the kinetics of protein aggregation observed at the macroscopic level but, individually, they each affect distinct microscopic reaction steps: the presence of interfaces generates primary nucleation events of fibril formation, which is then amplified by mechanical forces. These results suggest that the inhibition of surface-induced heterogeneous nucleation should be considered a primary target to suppress aggregation and explain why in many systems the simultaneous presence of surfaces and hydrodynamic flow enhances protein aggregation.

In situ synthesis of spherical CdS1-xSex red pigment used for ceramic ink-jet printing

A CdS1-xSex red pigment with spherical particles is urgently needed for red ceramic ink preparation to enrich the available ink types and enhance the decoration effect of ceramic tiles. In this study, spherical CdS0.75Se0.25 red pigment was synthesized through a facile two-step in situ reaction and the sphere formation mechanism was elucidated in detail. The CIE L*a*b* parameters of synthesized CdS0.75Se0.25 were as follows: L* = 47.76, a* = 43.91, b* = 30.05, implying a bright red color. As the first step, CdS spheres were prepared through a process of gradual random attachment and surface-induced nucleation; as the second step, CdS1-xSex spheres were synthesized through an in situ reaction based on the previously prepared CdS spheres. The spherical “core”, CTAB template and controlled reaction speed are crucial to the spherical morphology of the CdS1-xSex formations. Spherical CdS1-xSex exhibited a considerable red-shift compared with spherical CdS as a result of the partial substitution of Se2− ions for S2− ions. The synthesized spherical CdS0.75Se0.25 red pigment had a mean particle size of about 448 nm, which is consistent with the requirements for ceramic ink preparation. Hence, it is a promising candidate for further study regarding their potential use as ceramic ink in ink-jet printing.

Selective crystallisation of carbamazepine polymorphs on surfaces with differing properties

Surface-induced nucleation of carbamazepine (CBZ) in ethanol was investigated with different surface materials: glass, polytetrafluoroethylene (PTFE) and tin.

One-Step Hydrothermal Synthesis of TiO2@MoO3 Core–Shell Nanomaterial: Microstructure, Growth Mechanism, and Improved Photochromic Property

Photochromic TiO2@MoO3 core–shell (TM) nanopowder was synthesized by a one-step hydrothermal method and characterized with XRD, TEM, Raman, and a spectrophotometer. The nanopowder has a very small particle size of 5–10 nm in diameter, and a well-defined core–shell structure with the tight interface; that is, an anatase TiO2 core is tightly surrounded with amorphous MoO3, to form an ideal heterojunction structure. The growth mechanism was proposed with a surface-induced nucleation of amorphous MoO3 from a well-mixed precursor, where the mutual inhibition between core and shell led to the confined core size. The photochromic performance of TiO2@MoO3 was evaluated in solution as well as casted thin film on glass using a spectrophotometer, with a comparison to that of single MoO3. A significant enhancement in the photochromic properties was demonstrated for TM; that is, the change of the absorbance in water at 600 nm is about 30 times larger than that of α-MoO3, and a 20% transmittance regulation at 500–800 n...

Fatigue strength scatter characteristics of JIS SUS630 stainless steel with duplex S–N curve

The scatter behavior of fatigue strength for 17-4 PH stainless steel (JIS SUS630, equivalent to ASTM-A563 Grade 630), which exhibited a duplex S–N curve, was investigated. It was found that the fatigue strength scatter of the SUS630 at 106 and 107 cycles, which were corresponding to the fatigue strengths for the surface-induced and the subsurface-inclusion-induced crack nucleation modes, respectively, were significantly small and comparable to each other. The scatter of inclusion size in the present material was small and resulted in the small scatter of fatigue strength comparable to that for the surface-induced crack nucleation mode. The variation in size of non-metallic inclusions is known as one of the main factors for the fatigue strength scatter (Murakami, 2002) [1]. It was also found that the scatter of fatigue strength was in the same order of tensile strength.

Electrical percolation and crystallization kinetics of semi-crystalline polystyrene composites filled with graphene nanosheets

Syndiotactic polystyrene (sPS) is a semi-crystalline polymer with high melting temperature and good mechanical strength. Composites of sPS filled with different contents of graphene nanosheets (GNS) are prepared by coagulation method. Two types of GNS with different thicknesses (denoted as G1 and G10) are studied to unveil the effect of aspect ratio on electrical conductivity and crystallization kinetics of the composite. Atomic force microscopy and transmission electron microscopy (TEM) show that G1 is a wrinkled sheet with an average thickness of ∼2 nm and that G10 is a smooth flake with a thickness of ∼50 nm; both possess a similar basal dimension of ∼5 μm. The percolation thresholds for electrical conductivity (φc) of the G1-filled and G10-filled composites are 0.46 and 3.84 vol%, respectively. At a given GNS content, the electrical conductivity of the G1-filled composites is higher than that of the G10-filled composites. Both findings are attributed to the larger GNS aspect ratio of G1 compared with G10. The deduced φc of the G1-filled composites is significantly larger than that of GNS-filled amorphous atactic PS composites, indicating that the crystallizability of the matrix has an important influence on formation of GNS networks. Both G1 and G10 nanofillers are found to be good nucleating agents for the heterogeneous nucleation of sPS. Because of its wrinkled surface, G1 is less effective than G10 in inducing sPS crystallization. Compared with 2D sheet-like GNS, 1D CNTs are more effective in enhancing sPS crystallization through surface-induced nucleation as well as the chain-tube wrapping behavior in the sPS/CNT composites.

Enhanced Crystallization from the Glassy State of Poly(l-lactic acid) Confined in Anodic Alumina Oxide Nanopores

The crystallization behavior of poly(l-lactic acid) (PLLA) infiltrated in anodic alumina oxide templates (AAO) was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). During heating from the glassy state, the crystallization of infiltrated PLLA was unexpectedly enhanced as compared with bulk PLLA. The cold crystallization temperature of infiltrated PLLA from the glassy state was much lower than that of bulk PLLA. The half-crystallization time (t1/2) of infiltrated PLLA at 75 °C decreased with the diameter of AAO nanopores. The glass transition temperature of PLLA was not influenced by the geometrical confinement. The enhanced crystallization from the glassy state was explained by surface-induced nucleation of AAO walls on PLLA. Our results provide the first observation of enhanced cold crystallization of polymers in confined geometry.

Isothermal crystallization of syndiotactic polystyrene induced by graphene nanosheets and carbon nanotubes: a comparative study

Composites of sPS filled with different contents of graphene nanosheets (GNS) are prepared by coagulation method. Two types of GNS with different thicknesses (denoted as G1 and G10) are studied to unveil the effect of aspect ratio on the crystallization kinetics of the composite. Atomic force microscopy and transmission electron microscopy (TEM) show that G1 is a wrinkled sheet with an average thickness of ~2 nm and that G10 is a smooth flake with a thickness of ~50 nm; both possess a basal dimension of ~5 μm. TEM studies on the melt-quenched composites reveal that G1 has a more uniform dispersion in the sPS matrix than G10. Short lamellae of sPS are observed in the G10-filled composites with GNS content higher than 0.5 wt.%. The results of wide-angle X-ray diffraction show that the produced sPS crystallites are in β form under severe cooling conditions in liquid N2. Regardless of the GNS content and type, the glass transition and equilibrium melting temperature of the sPS matrix are unchanged at ~96 and ~290 °C, respectively. Both G1 and G10 nanofillers are good nucleating agents for the heterogeneous nucleation of sPS. With increasing GNS loading, the isothermal crystallization rate of sPS increases. G10 is more effective than G1 in inducing sPS crystallization despite the higher concentration required to form the GNS network. Compared with 1D CNT nanofiller, 2D GNS is less effective in enhancing sPS crystallization through surface-induced nucleation because of the geometrical difference between the two materials.

Internal stress-induced melting below melting temperature at high-rate laser heating

In this Letter, continuum thermodynamic and phase field approaches (PFAs) predicted internal stress-induced reduction in melting temperature for laser-irradiated heating of a nanolayer. Internal stresses appear due to thermal strain under constrained conditions and completely relax during melting, producing an additional thermodynamic driving force for melting. Thermodynamic melting temperature for Al reduces from 933.67 K for a stress-free condition down to 898.1 K for uniaxial strain and to 920.8 K for plane strain. Our PFA simulations demonstrated barrierless surface-induced melt nucleation below these temperatures and propagation of two solid-melt interfaces toward each other at the temperatures very close to the corresponding predicted thermodynamic equilibrium temperatures for the heating rate Q≤1.51×1010K/s. At higher heating rates, kinetic superheating competes with a reduction in melting temperature and melting under uniaxial strain occurs at 902.1 K for Q = 1.51 × 1011 K/s and 936.9 K for Q = 1.46 × 1012 K/s.

Surface induced nucleation of a Lennard-Jones system on an implicit surface at sub-freezing temperatures: A comparison with the classical nucleation theory

The aggregation-volume-bias Monte Carlo method was employed to study surface-induced nucleation of Lennard-Jonesium on an implicit surface below the melting point. It was found that surfaces catalyze not only the formation of the droplets (where the nucleation free energy barriers were shown to decrease with increasing surface interaction strength), but also the transition of these droplets into crystal structures due to the surface-induced layering effects. However, this only occurs under suitable interaction strength. When surface attraction is too strong, crystallization is actually inhibited due to the spread of the particles across the surface and corresponding formation of two-dimensional clusters. The simulation results were also used to examine the bulk-droplet based classical nucleation theory for surface-induced nucleation, particularly the additional contact angle term used to describe both the nucleation free energy barrier heights and the critical cluster sizes compared to its homogeneous nucleation formalism. Similar to what has been found previously for homogeneous nucleation, the theory does poorly toward the high-supersaturation region when the critical clusters are small and fractal, but the theoretical predictions on both barrier heights and critical cluster sizes improve rapidly with the decrease of the supersaturation.