Autocatalytic Growth Research Articles

A Four-Step Mechanism for the Formation of Supported-Nanoparticle Heterogenous Catalysts in Contact with Solution: The Conversion of Ir(1,5-COD)Cl/γ-Al2O3to Ir(0)∼170/γ-Al2O3

Product stoichiometry, particle-size defocusing, and kinetic evidence are reported consistent with and supportive of a four-step mechanism of supported transition-metal nanoparticle formation in contact with solution: slow continuous nucleation, A → B (rate constant k1), autocatalytic surface growth, A + B → 2B (rate constant k2), bimolecular agglomeration, B + B → C (rate constant k3), and secondary autocatalytic surface growth, A + C → 1.5C (rate constant k4), where A is nominally the Ir(1,5-COD)Cl/γ-Al2O3 precursor, B the growing Ir(0) particles, and C the larger, catalytically active nanoparticles. The significance of this work is at least 4-fold: first, this is the first documentation of a four-step mechanism for supported-nanoparticle formation in contact with solution. Second, the proposed four-step mechanism, which was obtained following the disproof of 18 alternative mechanisms, is a new four-step mechanism in which the new fourth step is A + C → 1.5C in the presence of the solid, γ-Al2O3 support. Third, the four-step mechanism provides rare, precise chemical and kinetic precedent for metal particle nucleation, growth, and now agglomeration (B + B → C) and secondary surface autocatalytic growth (A + C → 1.5C) involved in supported-nanoparticle heterogeneous catalyst formation in contact with solution. Fourth, one now has firm, disproof-based chemical-mechanism precedent for two specific, balanced pseudoelementary kinetic steps and their precise chemical descriptors of bimolecular particle agglomeration, B + B → C, and autocatalytic agglomeration, B + C → 1.5C, involved in, for example, nanoparticle catalyst sintering.

Modeling of scale-dependent bacterial growth by chemical kinetics approach.

We applied the so-called chemical kinetics approach to complex bacterial growth patterns that were dependent on the liquid-surface-area-to-volume ratio (SA/V) of the bacterial cultures. The kinetic modeling was based on current experimental knowledge in terms of autocatalytic bacterial growth, its inhibition by the metabolite CO2, and the relief of inhibition through the physical escape of the inhibitor. The model quantitatively reproduces kinetic data of SA/V-dependent bacterial growth and can discriminate between differences in the growth dynamics of enteropathogenic E. coli, E. coli JM83, and Salmonella typhimurium on one hand and Vibrio cholerae on the other hand. Furthermore, the data fitting procedures allowed predictions about the velocities of the involved key processes and the potential behavior in an open-flow bacterial chemostat, revealing an oscillatory approach to the stationary states.

Autocatalytic growth of ZnO nanorods from flat Au(111)-supported ZnO films.

Physical vapour deposition of ZnO on an Au(111) support has been investigated as a function of the oxygen chemical potential by means of scanning tunnelling microscopy and luminescence spectroscopy. Whereas a layer-by-layer growth of ZnO is revealed in oxygen excess, formation of oxide nanorods with large height-to-diameter ratio prevails at lower oxygen chemical potentials. We ascribe the formation of 3D nanostructures in the latter case to traces of Au atoms on the surface that promote trapping and dissociation of the incoming oxygen molecules. The Au residuals, acting as catalyst for the oxide formation, are indeed found on top of the ZnO nanorods.

A biofabrication approach for controlled synthesis of silver nanoparticles with high catalytic and antibacterial activities

We report simple, facile and size-controllable synthesis of uniform Ag nanoparticles with tobacco mosaic virus (TMV) as a biomediator in the absence of external reducing agents. UV–vis and TEM analysis show that Ag nanoparticles with average diameter of 2, 4 and 9nm were obtained by simply tuning the ratio of TMV/Ag(NH3)2+. The Ag formation in the presence of TMV showed autocatalytic growth followed by coalescence. The as-prepared TMV-mediated Ag nanoparticles show substantially higher catalytic and antibacterial activities than previous results. For the 4-nitrophenol hydrogenation reaction, the rate constants per surface area for 2 and 9nm Ag nanoparticles were determined to be 0.64 and 1.2Lm−2s−1 respectively. Both Kirby–Bauer disk diffusion test and tube culture results demonstrate high antibacterial activity of TMV-mediated Ag particles against Escherichia coli, with minimal inhibition concentration (MIC) of 2.3 and 2.5ppm for 2 and 9nm Ag nanoparticles respectively. We expect that our biomediated Ag synthesis approach can be readily extended to other biomaterials and metal nanoparticle systems.

Carbon dots mediated room-temperature synthesis of gold nanoparticles in poly(ethylene glycol)

We report an efficient room-temperature synthesis of Au nanoparticles (NPs) using carbon dots (C-dots) as mediator in poly(ethylene glycol). The synthesis does not require any irradiation or heating for the reduction of the metal precursor and it yields smaller sized particles of ~15 nm, mostly octahedron in shape. The effect of varying concentrations of the Au precursor and C-dots on the synthesis was studied, which demonstrated the variation in particle size and shape with change in either the precursor or C-dots concentration. Time-resolved absorbance study for the synthesis of Au NPs showed the sigmoidal behavior for the autocatalytic growth having the lagging phase of induction period. The optimum concentration of the precursor and the C-dots were determined for the synthesis of well-dispersed Au NPs. The stability of the prepared Au NPs was also determined, showing that at optimum concentration of the precursor and C-dots, the particles were stable and did not precipitate for several days.

Kinetic Factors in the Synthesis of Silver Nanoparticles by Reduction of Ag+ with Hydrazine in Reverse Micelles of Triton N-42

The growth kinetics of silver nanoparticles (AgNPs) during the reduction of AgNO3 by hydrazine in the droplets of dispersed aqueous phase encapsulated in the reverse micelles of oxyethylated surfactant Triton N-42 with decane as disperse medium was studied in situ by UV–vis spectroscopy. The mechanism of the process includes two steps that are slow, continuous nucleation and fast, autocatalytic surface growth. Both steps are under kinetic control and are limited by the rate of Ag+ reduction. The rate of nucleation is limited by reaction in the droplets of the aqueous phase forming the cores of reverse micelles, and the rate of the growth is limited by the reaction on the surface of AgNPs growing inside the micelles. The reduction of Ag+ is a second-order process with respect to N2H4. It includes the formation of the intermediate complex Ag(N2H4)+ and its reaction with another N2H4 molecule. The concentration effects of N2H4 (c′N2H4) and NH3 (c′NH3) as competing ligand, medium effects of ionic strength (I) and of the background salt in dispersed aqueous phase, the effect of solubilization capacity of the micellar solution (Vs/Vo), and the effect of temperature (T) on the observed rate constants for the steps were studied. An increase in c′N2H4, I, Vs/Vo and T can be used to accelerate the rates of both steps, whereas an increase in c′NH3 inhibits them. The background salts have a positive effect on the rate of nucleation, whereas their effect on the growth rate is small and has probably a negative trend. The size and composition of AgNPs were characterized by means of DLS, TEM, EDXA, XRD, UV–vis, and IR spectroscopy.

Effect of Surfactant Concentration and Aggregation on the Growth Kinetics of Nickel Nanoparticles

The effect of trioctylphosphine (TOP) concentration on the growth of nickel nanoparticles is studied using in situ synchrotron small-angle X-ray scattering. The growth kinetics are fitted using a two-step nucleation and autocatalytic growth model. TOP acts as a nucleating agent and then acts as an inhibitor against rapid particle growth. Increasing the TOP concentration results in smaller nanoparticles. Once there is a critical concentration of nickel particles above a certain size, they start to aggregate. This results in a broadening of the particle size distribution at later times due to particles on the outside of the aggregates continuing to grow, while those on the inside cease to grow as the nickel precursor is locally depleted.

Hydroxyapatite supported ruthenium(0) nanoparticles catalyst in hydrolytic dehydrogenation of ammonia borane: Insight to the nanoparticles formation and hydrogen evolution kinetics

When a solution of ammonia borane is added to the suspension of ruthenium(III) ions supported on hydroxyapatite, both reduction of ruthenium(III) to ruthenium(0) nanoparticles and hydrogen release from the hydrolysis of ammonia borane occur concomitantly at room temperature. Using the hydrogen evolution from the hydrolysis of ammonia borane as reporter reaction provides valuable insights to the formation kinetics of ruthenium(0) nanoparticles. Thus, the rate constants for the slow nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles could be obtained. Furthermore, the evaluation of rate constants at various temperatures provides the estimation of activation energies for both reactions; Ea=166±7kJ/mol for the nucleation and Ea=59±2kJ/mol for the autocatalytic surface growth of ruthenium(0) nanoparticles. The ruthenium(0) nanoparticles, in situ formed during the hydrolysis of ammonia borane and supported on hydroxyapatite, could be isolated from the reaction solution and characterized by a combination of advanced analytical techniques. The results show that (i) highly dispersed ruthenium(0) nanoparticles of 4.7±0.7nm size were formed on the surface of hydroxyapatite, (ii) they are highly active catalyst in the hydrolytic dehydrogenation of ammonia borane with a turnover frequency value of 137min−1 at 25.0±0.1°C, and (iii) they are long lived and reusable catalyst providing 87,000 turnovers for hydrogen generation from the hydrolysis of ammonia borane and preserving 92% of their initial catalytic activity even after the fifth run of hydrolysis of ammonia borane at 25.0±0.1°C. The results of kinetic study on the hydrogen generation from the hydrolysis of ammonia borane were also reported including the activation energy of 58±2kJ/mol for the hydrolytic dehydrogenation of ammonia borane.

Unifying autocatalytic and zeroth-order branching models for growing actin networks

The directed polymerization of actin networks is an essential element of many biological processes, including cell migration. Different theoretical models considering the interplay between the underlying processes of polymerization, capping, and branching have resulted in conflicting predictions. One of the main reasons for this discrepancy is the assumption of a branching reaction that is either first order (autocatalytic) or zeroth order in the number of existing filaments. Here we introduce a unifying framework from which the two established scenarios emerge as limiting cases for low and high filament numbers. A smooth transition between the two cases is found at intermediate conditions. We also derive a threshold for the capping rate above which autocatalytic growth is predicted at sufficiently low filament number. Below the threshold, zeroth-order characteristics are predicted to dominate the dynamics of the network for all accessible filament numbers. Together, these mechanisms allow cells to grow stable actin networks over a large range of different conditions.

The Bainite Controversy

There has long existed a controversy regarding the formation of the bainite phase in steels. Nonetheless, following the clear identification of bainite in 1930, several characteristic features associated with the bainite transformation became well established. These features include: the classification into upper and lower bainite; the existence of a bainite start temperature; and the incomplete reaction phenomenon. Accordingly, two competing theories have been developed to explain the bainite transformation. However, no overriding consensus has been reached as to which is correct. The first theory invokes a diffusion controlled transformation, describing the growth of bainite via the propagation of ledges – a series of steps on the transformation interface. The second interpretation favours a displacive, diffusionless transformation. Bainite growth occurs via the autocatalytic nucleation and growth of successive subunits or platelets. Over time, a wealth of techniques has been implemented in order to increase the understanding of the mechanism by which bainite forms. From the early approaches that involved thermodynamic and kinetic considerations; through detailed work on the crystallography of the transformation; to studies involving advanced characterisation techniques that focused on the distribution of atoms, etc. Some of the theories have been progressively adapted to new evidence and new concepts as they emerge. For example, the concepts of the T0 curve and of paraequilibrium transformation. This work presents a chronological summary of the two theories of the bainite transformation, charting their progression and noting the variety of evidence both in support of and in contradiction to each viewpoint. The bainite controversy is still debated by steel metallurgists today, although it has mostly been reduced to the question of whether or not bainitic ferrite initially forms with a supersaturation of carbon. This review gathers and evaluates some of the accumulated work in the hope that this question may finally be answered.

Mechanistic Investigation of Seeded Growth in Triblock Copolymer Stabilized Gold Nanoparticles

We report the seeded synthesis of gold nanoparticles (GNPs) via the reduction of HAuCl4 by (L31 and F68) triblock copolymer (TBP) mixtures. In the present study, we focused on [TBP]/[Au(III)] ratios of 1-5 (≈1 mM HAuCl4) and seed sizes ~20 nm. Under these conditions, the GNP growth rate is dominated by both the TBP and seed concentrations. With seeding, the final GNP size distributions are bimodal. Increasing the seed concentration (up to ~0.1 nM) decreases the mean particle sizes 10-fold, from ~1000 to 100 nm. The particles in the bimodal distribution are formed by the competitive direct growth in solution and the aggregative growth on the seeds. By monitoring kinetics of GNP growth, we propose that (1) the surface of the GNP seeds embedded in the TBP cavities form catalytic centers for GNP growth and (2) large GNPs are formed by the aggregation of GNP seeds in an autocatalytic growth process.

The Transition Towards Immortality: Non-linear Autocatalytic Growth of Citations to Scientific Papers

We discuss microscopic mechanisms of complex network growth, with the special emphasis of how these mechanisms can be evaluated from the measurements on real networks. As an example we consider the network of citations to scientific papers. Contrary to common belief that its growth is determined by the linear preferential attachment, our microscopic measurements show that it is driven by the nonlinear autocatalytic growth. This invalidates the scale-free hypothesis for the citation network. The nonlinearity is responsible for a dramatic dynamical phase transition: while the citation lifetime of majority of papers is 6–10 years, the highly-cited papers have practically infinite lifetime.

Exact solutions of kinetic equations in an autocatalytic growth model

Kinetic equations are introduced for the transition-metal nanocluster nucleation and growth mechanism, as proposed by Watzky and Finke [J. Am. Chem. Soc. 119, 10382 (1997)]. Equations of this type take the form of Smoluchowski coagulation equations supplemented with the terms responsible for the chemical reactions. In the absence of coagulation, we find complete analytical solutions of the model equations for the autocatalytic rate constant both proportional to the cluster mass, and the mass-independent one. In the former case, ξ(k)=s(k)(ξ(1))[proportionality]ξ(1)(k)/k was obtained, while in the latter, the functional form of s(k)(ξ(1)) is more complicated. In both cases, ξ(1)(t)=h(μ)(M(μ)(t)) is a function of the moments of the mass distribution. Both functions, s(k)(ξ(1)) and h(μ)(M(μ)), depend on the assumed mechanism of autocatalytic growth and monomer production, and not on other chemical reactions present in a system.

Growth of textured thin Au coatings on iron oxide nanoparticles with near infrared absorbance

A homologous series of Au coated iron oxide nanoparticles with hydrodynamic diameters smaller than 60 nm was synthesized with very low Au-to-iron mass ratios, as low as 0.15. The hydrodynamic diameter was determined by dynamic light scattering and the composition by atomic absorption spectroscopy and energy dispersive x-ray spectroscopy. Unusually low Au precursor supersaturation levels were utilized to nucleate and grow Au coatings on iron oxide relative to the formation of pure Au nanoparticles. This approach produced unusually thin coatings by lowering autocatalytic growth of Au on Au, as shown by transmission electron microscopy. Nearly all of the nanoparticles were attracted by a magnet, indicating a minimal number of pure Au particles. The coatings were sufficiently thin to shift the surface plasmon resonance to the near infrared with large extinction coefficients, despite the small particle hydrodynamic diameters observed from dynamic light scattering to be less than 60 nm.

Inhibitory Mechanism of Pancreatic Amyloid Fibril Formation: Formation of the Complex between Tea Catechins and the Fragment of Residues 22–27

Islet amyloid polypeptide (IAPP) is a major component of pancreatic amyloid deposits associated with type 2 diabetes. Polyphenols contained in plant foods have been found to inhibit amyloid fibril formation of proteins and/or peptides. However, the inhibition mechanism is not clear for a variety of systems. Here the inhibition mechanism of green tea polyphenols, catechins, on amyloid fibril formation of the IAPP fragment (IAPP22-27), which is of sufficient length for formation of β-sheet-containing amyloid fibrils, was investigated by means of kinetic analysis. A quartz crystal microbalance (QCM) determined that the association constants of gallate-type catechins [epicatechin 3-gallate (ECg) and epigallocatechin 3-gallate] for binding to IAPP22-27 immobilized on the gold plate in QCM were 1 order of magnitude larger than those of the free IAPP22-27 peptide, and also those of epicatechin and epigallocatechin. Kinetic analysis using a two-step autocatalytic reaction mechanism revealed that ECg significantly reduced the rate constants of the first nucleation step of amyloid fibril formation, while the rate of autocatalytic growth was less retarded. (1)H nuclear magnetic resonance studies clarified that a IAPP22-27/ECg complex clearly forms as viewed from the (1)H chemical shift changes and line broadening. Our study suggests that tea catechins specifically inhibit the early stages of amyloid fibril formation to form amyloid nuclei by interacting with the unstructured peptide and that this inhibition mechanism is of great therapeutic value because stabilization of the native state could delay the pathogenesis of amyloid diseases and also the toxicity of the small oligomer (protofibril) is reported to be greater than that of the mature fibril.

Size homeostasis can be intrinsic to growing cell populations and explained without size sensing or signalling

The cell division cycle orchestrates cellular growth and division. The machinery underpinning the cell division cycle is well characterized, but the actual cue(s) driving the cell division cycle remains unknown. In rapidly growing and dividing yeast cells, this cue has been proposed to be cell size. Presumably, a mechanism communicating cell size acts as gatekeeper for the cell division cycle via the G(1) network, which triggers G(1) exit only when a critical size has been reached. Here, we evaluate this hypothesis with a minimal core model linking metabolism, growth and the cell division cycle. Using this model, we (a) present support for coordinated regulation of G(1)/S and G(2)/M transition in Saccharomyces cerevisiae in response to altered growth conditions, (b) illustrate the intrinsic antagonism between G(1) progression and cell size and (c) provide evidence that the coupling of growth and division is sufficient to allow for size homeostasis without directly communicating or measuring cell size. We show that even with a rudimentary version of the G(1) network consisting of a single unregulated cyclin, size homeostasis is maintained in populations during autocatalytic growth when the geometric constraint on nutrient supply is considered. Taken together, our results support the notion that cell size is a consequence rather than a regulator of growth and division.

Autocatalytic growth of Co on pure Co surfaces using Co2(CO)8 precursor

The autocatalytic growth of Co on different surfaces using the Co2(CO)8 precursor is investigated. It is observed that Co2(CO)8 molecules dissociate spontaneously on pure Co surfaces grown by sputtering, forming a pure Co film. The microstructure of this film consists of Co nanocrystals with size below 100nm. However, when the same type of experiment is done on a Co surface grown by focused-electron-beam induced deposition there is no autocatalytic growth of Co. On other surfaces such as Si substrates and Al films grown by sputtering, the spontaneous dissociation of the Co2(CO)8 molecules does not occur. The origin and implications of these results are discussed.

Spontaneous dissociation of Co(2)(CO)(8) and autocatalytic growth of Co on SiO(2): A combined experimental and theoretical investigation.

We present experimental results and theoretical simulations of the adsorption behavior of the metal–organic precursor Co2(CO)8 on SiO2 surfaces after application of two different pretreatment steps, namely by air plasma cleaning or a focused electron beam pre-irradiation. We observe a spontaneous dissociation of the precursor molecules as well as autodeposition of cobalt on the pretreated SiO2 surfaces. We also find that the differences in metal content and relative stability of these deposits depend on the pretreatment conditions of the substrate. Transport measurements of these deposits are also presented. We are led to assume that the degree of passivation of the SiO2 surface by hydroxyl groups is an important controlling factor in the dissociation process. Our calculations of various slab settings, using dispersion-corrected density functional theory, support this assumption. We observe physisorption of the precursor molecule on a fully hydroxylated SiO2 surface (untreated surface) and chemisorption on a partially hydroxylated SiO2 surface (pretreated surface) with a spontaneous dissociation of the precursor molecule. In view of these calculations, we discuss the origin of this dissociation and the subsequent autocatalysis.

Thin membranes versus bulk substrates: investigation of proximity effects in focused electron beam-induced processing

The resolution of focused electron beam induced processing techniques is limited by electron scattering processes. General wisdom holds that using a membrane, this can be effectively improved due to a cutoff of the electron interaction volume and thus diminished proximity effects. Recently, we demonstrated that in contrast to the expectation, proximity effects can be indeed larger on a 200 nm silicon nitride membrane than on the respective bulk substrate, due to charging-induced surface activation. Herein, we expand these investigations on proximity effects in electron beam-induced surface activation to other substrates and to electron beam-induced deposition followed by autocatalytic growth.

Effect of the Volume-to-Surface Ratio of Cultures on Escherichia coli Growth: An Experimental and Theoretical Analysis

The growth dynamics of bacterial populations are usually represented by the classical S-shaped profiles composed of lag, exponential and stationary growth phases. Although exceptions to this classical behavior occur, they are normally produced under non-standard conditions such as supply of two carbohydrates as sole carbon source. However, we here report variations in the classic S-shaped growth profiles of Escherichia coli under standard culturing conditions; explicitly, we found growth during transition to the stationary phase wherein the bacterial growth rate inversely depended on the volume-to-surface ratio of cultures (V/S); the reasons for this behavior were experimentally explored. To complement our experimental analysis, a theoretical model that rationalizes the bacterial response was developed; simulations based on the developed model essentially reproduced experimental growth curves. We consequently conclude that the effect of V/S on E. coli growth reflects an interplay between auto-catalytic bacterial growth, bacterial growth auto-inhibition, and, the relief of that inhibition.