Understanding the Aggregation of Lanthanum(III) Nitrate Clusters in Pure Methanol: A Molecular Dynamics Investigation

A detailed analysis of the structure and speciation of La3+ clusters in the 0.1 mol L-1 La(NO3)3 salt methanol (MeOH) solution has been performed by means of molecular dynamics (MD) simulations. The time distribution and NO3-/MeOH ligand composition of these clusters have been computed using graph theory techniques. These analyses revealed the formation of branched-like polynuclear clusters in the solution, the predominant clusters being the 3, 7, and 8 La3+ clusters. In these clusters, the La3+ cations are bound by "monodentate" nitrate bridges. Moreover, the mechanism of aggregation of the La3+ clusters has been examined with the development of a 3-step model. Finally, the origin of the aggregation process has been identified by estimating the binding constant for the ion pair La3+-NO3- using the Bjerrum theory of dilute solutions, with pK° = 5.32 at 25 °C. The low value of the dielectric constant of methanol promotes the binding of the ion pair La3+-NO3- and the nitrato-bridging polymerization, resulting in the formation of clusters.

Autocatalytic Nucleation and Self-Assembly of Inorganic Nanoparticles into Complex Biosimilar Networks

Self-replication of bioorganic molecules and oil microdroplets have been explored as models in prebiotic chemistry. An analogous process for inorganic nanomaterials would involve the autocatalytic nucleation of metal, semiconductor, or ceramic nanoparticles-an area that remains largely uncharted. Demonstrating such systems would be both fundamentally intriguing and practically relevant, especially if the resulting particles self-assemble into complex structures beyond the capabilities of molecules or droplets. Here, we show that autocatalytic nucleation occurs with silver nanoparticles, which subsequently self-assemble into chains through spatially restricted attachment. In dispersions containing "hedgehog" particles, these reactions produce complex colloids with hierarchical spike organization. On solid surfaces, autocatalytic nucleation of nanoparticles yields conformal networks with hierarchical organization, including nanoparticle "colonies." We analyzed the complexity of both types of solid-stabilized particle assemblies via graph theory (GT). The complexity index of idealized spiky colloids is comparable to that of complex algal skeletons. The GT analysis of the percolating nanoparticle networks revealed their similarities to the bacterial, but not fungal, biofilms. We conclude that coupling autocatalytic nucleation with self-assembly enables the generation of complex, biosimilar particles and films. This work establishes mathematical and structural parallels between biotic and abiotic matter, integrating self-organization, autocatalytic nucleation, and theoretical description of complex systems. Utilization of quantitative descriptors of connectivity patterns opens possibility to GT-based biomimetic engineering of conductive coatings and other complex nanostructures.

Exploration of anion effects in solvothermal synthesis using in situ X-ray diffraction

Solvothermal synthesis presents a facile and highly flexible approach to chemical processing and it is widely used for preparation of micro- and nanosized inorganic materials. The large number of synthesis parameters in combination with the richness of inorganic chemistry means that it is difficult to predict or design synthesis outcomes, and it is demanding to uncover the effect of different parameters due to the sealed and complex nature of solvothermal reactors along with the time demands related to reactor cleaning, sample purification, and characterization. This study explores the effect on formation of crystalline products of six common anions in solvothermal treatment of aqueous and ethanolic precursors. Three different cations are included in the study (Mn2+, Co2+, Cu2+) representing chemical affinities towards different regions of the periodic table with respect to the hard soft acid base (HSAB) classification and the Goldschmidt classification. They additionally belong to the commonly used 3d transition metals and display a suitable variety in solvothermal chemistry to highlight anion effects. The results of the solvothermal in situ experiments demonstrate a clear effect of the precursor anions, with respect to whether crystallization occurs or not and the characteristics of the formed phases. Additionally, some of the anions are shown to be redox active and to influence the formation temperature of certain phases which in turn relates to the observed average crystallite sizes.

Atomic Mechanisms of Nanocrystallization via Cluster-Clouds in Solution Studied by Liquid-Phase Scanning Transmission Electron Microscopy

The formation of nanocrystals is at the heart of various scientific disciplines, but the atomic mechanisms underlying the early stages of crystallization from supersaturated solutions are still rather unclear. Here, we used in situ liquid-phase scanning transmission electron microscopy to study at the atomic level the very early stages of gold nanocrystal growth, and the evolution of its crystallinity. We found that the nucleation is initiated by the formation of poorly crystalline nanoparticles. These are transformed into monocrystals via nanocrystallization governed by a complex process of multiple out-and-in exchanges of matter between a crystalline-core and a disordered-shell, referred to as the cluster-cloud. Our observations at the crystal/cluster-cloud interface during growth demonstrate that the initially formed nanocrystals expel the poorly crystallized phases as nanoclusters into the cluster-cloud, then readsorb it by two distinct pathways, namely, by (i) monomer attachments and (ii) nanocluster coalescence. This growth process eventually leads to the formation of monocrystalline nanoparticles.

Molecular dynamics simulations of the formation of Ag nanoparticles assisted by PVP

Understanding the formation mechanisms of nanoparticles is essential for the synthesis of nanomaterials with controlled properties. In solution synthesis, capping agents are used to mediate this process and control the final size and shape of the particles. In this work, the synthesis of silver nanoparticles, with polyvinylpyrrolidone (PVP) as the capping agent, is studied through molecular dynamics simulations. Nucleation of clusters of atoms and subsequent growth to form nanoparticles are analyzed, with focus on the role of PVP. No finite critical nucleus is detected, and amorphous particles seem to form by spinodal growth. In this timescale, PVP seems to have no effect on particle growth, which is ascribed to the competition between the protective effect and "bridging" (where a molecule of PVP is adsorbed to two different clusters, bringing them together). As the process evolves, a sequence of ordered structures appears within the particles: icosahedral, BCC, and FCC, the last one being the equilibrium configuration of bulk silver. In addition, for a low PVP content an apparent acceleration is observed in particle growth after these ordered phases appear, indicating that the growth of ordered particles from the solution is faster than the growth of amorphous particles. For a high PVP content, this acceleration is not observed, indicating that the protective effect prevails on particle growth in this regime. In addition, due to the bridging effect, the final overall configuration is strongly dependent on the PVP content. In the absence of PVP, large but dispersed particles are observed. When the PVP content is low, due to strong bridging, particles form agglomerates (with no strong coalescence in the timescale of simulations). When the PVP content is large enough, particles are smaller in size and do not show a strong tendency to agglomerate.

Recent experimental explorations of non-classical nucleation

This work provides a clearer picture for non-classical nucleation by revealing the presence of various intermediates using advanced characterization techniques.

High-Resolution Analysis of Small Silver Clusters by Analytical Ultracentrifugation

Although silver particles are used in various applications and a countless amount of synthesis routes exists, their formation mechanism is still poorly understood. Especially the first species formed directly after nucleation challenge analysis methods with their small size and transient nature. Analytical ultracentrifugation (AUC) has already proven to provide high size resolution and therefore enables the characterization of early nucleation species. Herein, we present an experiment of multiwavelength (MWL)-AUC of silver clusters, which revealed seven different cluster species. They consist of less than 10 atoms and therefore represent the first species formed after nucleation. Using MWL-AUC, UV/vis spectra could be allocated to each of them, which is shown for the first time. These findings establish MWL-AUC as a high-resolution tool to investigate a nucleation mechanism for silver and other metal nanoparticles.

Revealing the Cluster-Cloud and Its Role in Nanocrystallization

Elucidating the early stages of crystallization from supersaturated solutions is of critical importance, but remains a great challenge. An in situ liquid cell transmission electron microscopy study reveals an intermediate state of condensed atomic clusters during Pd and Au crystallizations, which is named a "cluster-cloud." It is found that nucleation is initiated by the collapse of a cluster-cloud, first forming a nanoparticle. The subsequent particle maturation proceeds via multiple out-and-in relaxations of the cluster-cloud to improve crystallinity: from a poorly crystallized phase, the particle evolves into a well-defined single-crystal phase. Both experimental investigations and atomistic simulations suggest that the cluster-cloud-mediated nanocrystallization involves an order-disorder phase separation and reconstruction, which is energetically favored compared to local rearrangements within the particle. This finding grants new insights into nanocrystallization mechanisms, and provides useful information for the improvement of synthesis pathways of nanocrystals.

Band Sedimentation Experiment in Analytical Ultracentrifugation Revisited

The band sedimentation experiment in analytical ultracentrifugation (AUC) allows for the performance of a chemical reaction inside the AUC and also offers separation of individual pure components in a sedimentation velocity experiment. Although this experiment offers exciting possibilities for application, it is barely used. This is related to the bad definition of the initial conditions. Both the duration and the time of the solution overlay during rotor acceleration are not known. In this study, we investigate these conditions under the variation of the overlay volume using recording of interference patterns in a continuous mode during the acceleration of the rotor. It was found that the overlay occurs at rotor speeds between 770 and 2000 rpm, which is very low compared to typical experimental rotor speeds from 3 000 to 60 000 rpm and therefore elucidates that the generated reaction products, respectively, overlaid species are subject to the centrifugal force almost from the beginning. Also, the duration of the overlay is less than 1.2 s, which is very fast compared to hours of centrifugation time for an experiment and we demonstrated that the overlay compartment is completely emptied during overlay allowing for the precise calculation of the meniscus using the known sample sector geometry. Our results show that the initial conditions of the experiment are defined and should make an adapted analysis possible if the interdiffusion of the two solvents is taken into account, which lead to a dynamic density gradient.

From Solute, Fluidic and Particulate Precursors to Complex Organizations of Matter

The organization of matter from its constitutive units recruits intermediate states with distinctive degrees of self-association and molecular order. Existing as clusters, droplets, gels as well as amorphous and crystalline nanoparticles, these precursor forms have fundamental contributions towards the composition and structure of inorganic and organic architectures. In this personal account, we show that the transitions from atoms, molecules or ionic species to superstructures of higher order are intertwined with the interfaces and interactions of precursor and intermediate states. Structural organizations distributed across different length scales are explained by the multistep nature of nucleation and crystallization, which can be guided towards functional hybrid materials by the strategic application of additives, templates and reaction environments. Thus, the non-classical pathways for material formation and growth offer conceptual frameworks for elucidating, inducing and directing fascinating material organizations of biogenic and synthetic origins.

Analytical band centrifugation revisited

Analytical band centrifugation (ABC) is a powerful tool for the analysis of macromolecules and nanoparticles. Although it offers several advantages over the sedimentation velocity (SV) experiment like a physical separation of the individual components and the possibility to perform chemical reactions, its analysis is still very restricted. Therefore, we investigated the integration of ABC data as an alternative approach, as this results in data similar to SV, which can then be evaluated by many established evaluation programs. We investigated this method using two different test systems, myoglobin as a biopolymer with significant diffusion and 100 nm polystyrene latex as a large particle with negligible diffusion, and found some limiting issues. These are namely, broadening of the initial boundary by diffusion of the sample, which can be taken into account and the dynamic density gradient between the solvent in the sector and the overlaid solution, which deforms the initial band upon movement through the gradient and is currently not taken into account. We show the influence these two factors have on the evaluation and show that it is possible to calculate the time-dependent change in solvent density and viscosity in the AUC cell using the integrated form of Fick's second law. We conclude that taking the dynamic density gradient into account will open ABC for the sophisticated methods based on the analysis of the whole sedimentation boundary and not just the determination of an average sedimentation coefficient.

Solubility curves and nucleation rates from molecular dynamics for polymorph prediction – moving beyond lattice energy minimization

Seeded nucleation simulations allow for the screening of low energy structures to predict final structure present in solution.

Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution

We report an unsurpassed solution characterization technique based on analytical ultracentrifugation, which demonstrates exceptional potential for resolving particle sizes in solution with sub-nm resolution. We achieve this improvement in resolution by simultaneously measuring UV/Vis spectra while hydrodynamically separating individual components in the mixture. By equipping an analytical ultracentrifuge with a novel multi-wavelength detector, we are adding a new spectral discovery dimension to traditional hydrodynamic characterization, and amplify the information obtained by orders of magnitude. We demonstrate the power of this technique by characterizing unpurified CdTe nanoparticle samples, avoiding tedious and often impossible purification and fractionation of nanoparticles into apparently monodisperse fractions. With this approach, we have for the first time identified the pure spectral properties and band-gap positions of discrete species present in the CdTe mixture.

Shape Analysis of DNA-Au Hybrid Particles by Analytical Ultracentrifugation

Current developments in nanotechnology have increased the demand for nanocrystal assemblies with well-defined shapes and tunable sizes. DNA is a particularly well-suited building block in nanoscale assemblies because of its scalable sizes, conformational variability, and convenient self-assembly capabilities via base pairing. In hybrid materials, gold nanoparticles (AuNPs) can be assembled into nanoparticle structures with programmable interparticle distances by applying appropriate DNA sequences. However, the development of stoichiometrically defined DNA/NP structures is still challenging since product mixtures are frequently obtained and their purification and characterization is the rate-limiting step in the development of DNA-NP hybrid assemblies. Improvements in nanostructure fractionation and characterization techniques offer great potential for nanotechnology applications in general. This study reports the application of analytical ultracentrifugation (AUC) for the characterization of anisotropic DNA-linked metal-crystal assemblies. On the basis of transmission electron microscopy data and the DNA primary sequence, hydrodynamic bead models are set up for the interpretation of the measured frictional ratios and sedimentation coefficients. We demonstrate that the presence of single DNA strands on particle surfaces as well as the shape factors of multiparticle structures in mixtures can be quantitatively described by AUC. This study will significantly broaden the possibilities to analyze mixtures of shape-anisotropic nanoparticle assemblies. By establishing insights into the analysis of nanostructure mixtures based on fundamental principles of sedimentation, a wide range of potential applications in basic research and industry become accessible.

Detection and characterisation of sub-critical nuclei during reactive Pd metal nucleation by X-ray absorption spectroscopy

The interfacial reduction of aqueous [PdCl₄]²⁻ at the interface with an organic solution of ferrocene has been characterised by X-ray absorption fine structure (XAFS) spectroscopy.

Aluminal speciation in the crystal nucleus: a mass spectral interpretation

Classical constructions are revisited in the light of aperiodic tiling. Recent experimental observations are re-interpreted.

Next-Generation AUC Adds a Spectral Dimension: Development of Multiwavelength Detectors for the Analytical Ultracentrifuge

We describe important advances in analytical ultracentrifugation (AUC) hardware, which add new information to the hydrodynamic information observed in traditional AUC instruments. In contrast to the Beckman-Coulter XLA UV/visible detector, multiwavelength (MWL) detection is able to collect sedimentation data not just for one wavelength, but for a large wavelength range in a single experiment. The additional dimension increases the data density by orders of magnitude, significantly improving the statistics of the measurement and adding important information to the experiment since an additional dimension of spectral characterization is now available to complement the hydrodynamic information. The new detector avoids tedious repeats of experiments at different wavelengths and opens up new avenues for the solution-based investigation of complex mixtures. In this chapter, we describe the capabilities, characteristics, and applications of the new detector design with biopolymers as the focus of study. We show data from two different MWL detectors and discuss strengths and weaknesses of differences in the hardware and different data acquisition modes. Also, difficulties with fiber optic applications in the UV are discussed. Data quality is compared across platforms.