2019-10-15, Schneider, Cornelia M., Cölfen, Helmut

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.

2019-02-13, Xu, Xufeng, Franke, Tina, Schilling, Kristian, Sommerdijk, Nico A. J. M., Cölfen, Helmut

Binary colloidal nanoparticles have been found to form different types of crystalline phases at varied radial positions in a centrifugal field by Chen et al (ACS nano 2015, 9, 6944-50). The variety of binary phase behaviors resulted from the two different nanoparticle concentration gradients but to date the gradients can only be empirically controlled. For the first time, we are able to measure, fit and simulate binary hard sphere colloidal nanoparticle concentration gradients at high particle concentration up to 30 vol%, which enables tailor-made gradients in a centrifugal field. By this means, a continuous range of binary particle concentration ratios can be accessed in one single experiment to obtain an extended phase diagram. By dispersing two differently sized silica nanoparticles labeled with two different fluorescence dyes in a refractive index matching solvent, we can use a Multi-Wavelength Analytical Ultracentrifuge (MWL-AUC) to measure the individual concentration gradient for each particle size in sedimentation-diffusion equilibrium. The influence of the remaining slight turbidity at high concentration can be corrected using the MWL spectra from the AUC data. We also show that the experimental concentration gradients can be fitted using a non-interacting non-ideal sedimentation model. By using these fitted parameters, we are able to simulate nanoparticle concentration gradients, which agreed with the subsequent experiments at a high concentration of 10 vol% and thus allowed for the simulation of binary concentration gradients of hard sphere nanoparticles in preparative ultracentrifuges (PUC). Finally we demonstrated that by simulating the concentration gradients in PUC, a continuous and extended binary nanoparticle phase diagram can be obtained by simply studying the structure evolution along the centrifugal field for one single sample instead of a large number of experiments with discrete compositions in conventional studies.

2018-11-01, Pearson, Joseph, Nguyen, Tich Lam, Cölfen, Helmut, Mulvaney, Paul

Virtually all dynamic methods for determining particle size on the nanoscale use the Stokes–Einstein–Sutherland (SES) equation to convert the diffusion coefficient into a hydrodynamic radius. The validity of this equation on the nanoscale has not been rigorously validated by experiment. Here we measure the sedimentation rates and diffusion coefficients of C₆₀ and C₇₀ in toluene using analytical ultracentrifugation and compare the results to the SES equation. We find that the SES equation for the drag force (nonslip boundary condition) works down to 1 nm length scales.

2018-09-04, Schneider, Cornelia M., Haffke, Dirk, Cölfen, Helmut

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.

2019-03-12, Chen, Song, Krumova, Marina, Cölfen, Helmut, Sturm, Elena V.

Monetite (CaHPO₄, DCPA) and hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) are promising biomaterials with wide application in the biomedical field. In this study, we propose a novel method to prepare fiber-like DCPA without organic additives for the first time and show that nanostructured HAp can be synthesized without changing the macroscopic morphology of DCPA. Our results show that temperature, pH, calcium to phosphate ratio (Ca/P) and concentration of NH₄⁺ are essential factors in the formation of fiber-like DCPA. The synthesized materials were characterized by PXRD, SEM and TEM, and the DCPA→HAp phase transformation process was examined. The growth of HAp on the DCPA crystal can be regulated by the composition of the hydrolyzing solution, forming HAp with its c-axis perpendicular (HAp (001)// DCPA (100)) or parallel (HAp (001)// DCPA (010)) to the elongation direction of DCPA crystal. At a high concentration of Ca²⁺ in NH₄OH solution, octacalcium phosphate (OCP) was observed as an intermediate phase with its c-axis parallel to elongation of initial DCPA crystal. The HAp crystals formed after complete transformation of the precursor phases have a preferred crystallographic orientation with c-axis co-axial to the elongation direction of DCPA. In the concentrated NH₄OH solution, the direct transformation of DCPA to HAp was observed. At initial stages of the hydrolysis, the c-axis of newly formed HAp is perpendicular to the elongation direction of DCPA. This study provides a new insight in regulating the crystal growth of DCPA without organic additives and a facile route to form HAp with hierarchical structures.

2018-11-27, Gebauer, Denis, Raiteri, Paolo, Gale, Julian D., Cölfen, Helmut

Classical nucleation theory (CNT) is based on the notion of critical nuclei serving as transition states between supersaturated solutions and growing particles. Their excess standard free energy depends on supersaturation, and determines the height of the barrier for phase separation. However, predictions of CNT nucleation rates can deviate from experimental observations by many orders of magnitude. We argue that this is due to oversimplifications within CNT, rendering the critical nucleus essentially a conceptual notion, rather than a truly existing physical entity. Still, given adequate parametrization, CNT is useful for predicting and explaining nucleation phenomena, since it is currently the only quantitative framework at hand. In the recent years, we have been introducing an alternative theory, the so-called pre-nucleation cluster (PNC) pathway. The truly “non-classical” aspect of the PNC pathway is the realization that critical nuclei, as defined within CNT, are not the key to nucleation, but that the transition state relevant for phase separation is based on a change in dynamics of PNCs rather than their size. We provide a summary of CNT and the PNC pathway, thereby highlighting this major difference. The discussion of recent works claiming to provide scientific evidence against the existence of PNCs reveals that such claims are indeed void. Moreover, we illustrate that an erroneous interpretation of the concentration dependence of the free energy has led to a postulated rationalization of the standard free energy of ion pairs and stable ion associates within CNT, which is not sustainable. In fact, stable ion associates are stuck in a free energy trap from the viewpoint of CNT and cannot be considered in a straightforward manner. On the other hand, the notions of the PNC pathway, by dismissing the idea of the CNT-type critical nucleus as a required transition state, overcome this issue. While a quantitative theory of the PNC pathway is eagerly anticipated, the rationalization of experimental observations that are inconsistent with CNT proves its qualitative explanatory power, underpinning great promise towards a better understanding of, for instance, polymorph selection and crystallization control by additives.

2018-09-18, Wang, Bingjun, Mao, Li-Bo, Li, Ming, Chen, Yupeng, Liu, Ming-Feng, Xiao, Chuanlian, Jiang, Yuan, Wang, Shutao, Yu, Shu-Hong, Liu, Xiang Yang, Cölfen, Helmut

In biomineralization and bioinspired mineralization, substrates and additives function synergistically in providing structural control of the mineralized layers including their orientation, polymorph, morphology, hierarchical architecture, etc. Herein, a novel type of granular aragonitic CaCO₃-poly(acrylic acid) substrate guides the mineralization of prismatic CaCO₃ thin films of distinct morphology and polymorph in the presence of different additives including organic compounds and polymers. For instance, weakly charged amino acids lead to columnar aragonite overlayers, while their charged counterparts and organic acids/bases inhibit the overgrowth. Employment of several specific soluble polymer additives in overgrowth instead results in calcitic overlayers with distinct hierarchical architecture, good hardness/Young's modulus, and under-water superoleophobicity. Interestingly, self-organized patterns in the CaCO₃-poly(l-glutamic acid) overlayer are obtained under proper mineralization conditions. We demonstrate that the granular seed comprised of mineralized and polymeric constituents is a versatile platform for obtaining prismatic CaCO₃ thin films, where structural control can be realized by the employment of different types of additives in overgrowth. We expect the methodology to be applied to a broad spectrum of bioinspired, prismatic-type crystalline products, aiming for the development of high-performance hybrids.

2019-03, Zhu, Guomin, Reiner, Holger, Cölfen, Helmut, De Yoreo, Jim

In situ liquid phase scanning/ transmission electron microscopy (LP-S/TEM), enables direct observation of particle formation and evolution in the hydrated liquid state. Though powerful, this technique has significant technical limitations, which need to be carefully addressed in order to obtain reliable quantitative data. In this paper, we highlight several common artifacts seen in the LP-TEM including electron-beam induced particle dissolution and motion, particle-membrane adhesion, contamination, and triggering reactions. We describe our efforts to overcome these problems by modifying solution and interface chemistry, maintaining solution flow, performing systematic post in situ analyses on the samples and applying controlled heating.

2018-11-13, Schmid, Marius, Häusele, Benedikt, Junk, Michael, Brookes, Emre, Frank, Jürgen, Cölfen, Helmut

Asymmetrical flow field-flow fractionation (AF4) is a chromatographic separation technique that can be used for a broad range of particles or macromolecules. As an orthogonal method to size exclusion chromatography (SEC) with a much broader separation size range (1–800 nm) AF4 is gaining importance. However, the data evaluation capacities are far behind in comparison to other techniques like analytical ultracentrifugation (AUC). A program for evaluation of data from AF4 with a coupled multiangle laser light scattering (MALLS) detector was developed that allows the determination of the distributions of diffusion coefficients (D), hydrodynamic radii (R_h), molecular weights (M_w), and relative concentrations (RC) of the obtained species. In addition, two algorithms to remove broadening effects via deconvolution were implemented and tested for their validity. The first is an extension of the known diffusion broadening correction applying the entire diffusion coefficient distribution instead of a single diffusion coefficient. The second applies the Richardson–Lucy algorithm for the deconvolution of overlapping signals from stars in astronomy. This program allows a reproducible strong enhancement of the fractogram resolution allowing for entire baseline separations of proteins. The comparison of the values for M_w determined by a partial Zimm plot from each data point of the original fractogram and the deconvolved results shows that especially the Richardson–Lucy algorithm maintains a high degree of data robustness.

2018-09-10, Rao, Ashit, Drechsler, Markus, Schiller, Stefan, Scheffner, Martin, Gebauer, Denis, Cölfen, Helmut

Biogenic crystallization reactions produce hybrid nanostructured materials under physiological conditions. In article 1802063, Helmut Cölfen and co‐workers identify the disorder to order transitions of biomacromolecules as a regulatory feature of additive‐controlled mineralization. This molecular conditioning generates conformational sub‐ensembles and supramolecular assemblies adept at controlling the pathways of nucleation and crystallization.