Roles of nucleation, denucleation, coarsening, and aggregation kinetics in nanoparticle preparations and neurological disease

Kinetic models for nucleation, denucleation, Ostwald ripening (OR), and nanoparticle (NP) aggregation are presented and discussed from a physicochemical standpoint, in terms of their role in current NP preparations. Each of the four solid-state mechanisms discussed predict a distinct time dependence for the evolution of the mean particle radius over time. Additionally, they each predict visually different particle size distributions (PSDs) under limiting steady-state (time-independent) conditions. While nucleation and denucleation represent phase transformation mechanisms, OR and NP aggregation do not. Thus, when modeling solid-state kinetics relevant to NP processing, either the time evolution of the mean particle radius or the fractional conversion data should be fit using appropriate models (discussed herein), without confusing/combining the two classes of models. Experimental data taken from the recent literature are used to demonstrate the usefulness of the models in real-world applications. Specifically, the following examples are discussed: the preparation of bismuth NPs, the synthesis of copper indium sulfide nanocrystals, and the aggregation of neurological proteins. Because the last process is found to obey reaction-limited colloid aggregation (RLCA) kinetics, potential connections between protein aggregation rates, the onset of neurological disease, and population lifespan dynamics are suggested by drawing a parallel between RLCA kinetics and Gompertz kinetics. The physical chemistry underpinning NP aggregation is investigated, and a detailed definition of the rate constant of aggregation, k(a), is put forth that provides insight into the origin of the activation energy barrier of aggregation. For the two nanocrystal preparations investigated, the initial kinetics are found to be well-described by the author's dispersive kinetic model for nucleation-and-growth, while the late-stage NP size evolution is dominated by OR. At intermediate times, it is thought that the two mechanisms both contribute to the NP growth, resulting in PSD focusing as discussed in a previous work [Skrdla, P. J. J. Phys. Chem. C2012, 116, 214-225]. On the basis of these two mechanisms, a synthetic procedure for obtaining monodisperse NP PSDs, of small and/or systematically targeted mean sizes, is proposed.     

General shape control of colloidal CdS, CdSe, CdTe quantum rods and quantum rod heterostructures

We report a general synthetic method for the formation of shape-controlled CdS, CdSe and CdTe nanocrystals and mixed-semiconductor heterostructures. The crystal growth kinetics can be manipulated by changing the injection rate of the chalcogen precursor, allowing the particle shape-spherical or rodlike-to be tuned without changing the underlying chemistry. A single injection of precursor leads to isotropic spherical growth, whereas multiple injections promote epitaxial growth along the length of the c-axis. This method was extended to produce linear type I and type II semiconductor nanocrystal heterostructures.     

Precursor conversion kinetics and the nucleation of cadmium selenide nanocrystals

The kinetics of cadmium selenide (CdSe) nanocrystal formation was studied using UV-visible absorption spectroscopy integrated with an automated, high-throughput synthesis platform. Reaction of anhydrous cadmium octadecylphosphonate (Cd-ODPA) with alkylphosphine selenides (1, tri-n-octylphosphine selenide; 2, di-n-butylphenylphosphine selenide; 3, n-butyldiphenylphosphine selenide) in recrystallized tri-n-octylphosphine oxide was monitored by following the absorbance of CdSe at λ = 350 nm, where the extinction coefficient is independent of size, and the disappearance of the selenium precursor using {(1)H}(31)P NMR spectroscopy. Our results indicate that precursor conversion limits the rate of nanocrystal nucleation and growth. The initial precursor conversion rate (Q(o)) depends linearly on [1] (Q(o)(1) = 3.0-36 μM/s) and decreases as the number of aryl groups bound to phosphorus increases (1 > 2 > 3). Changes to Q(o) influence the final number of nanocrystals and thus control particle size. Using similar methods, we show that changing [ODPA] has a negligible influence on precursor reactivity while increasing the growth rate of nuclei, thereby decreasing the final number of nanocrystals. These results are interpreted in light of a mechanism where the precursors react in an irreversible step that supplies the reaction medium with a solute form of the semiconductor.     

Colloidal CdSe nanocrystals synthesized in noncoordinating solvents with the addition of a secondary ligand: exceptional growth kinetics

The addition of a secondary ligand, trioctylphosphine oxide, in the synthesis of cadmium selenide nanocrystals performed in a system with oleic acid as the primary ligand and octadecene as the noncoordinating solvent gives rise to the improvement of nanocrystal size distribution. This phenomenon, which is more significant in the nucleation process than in the growth process, demonstrates that the existence of trioctylphosphine oxide allows for superior nucleation control and permits the facile and reproducible production of extremely small CdSe nanocrystals with narrow size distribution. A systematic study of the nanocrystal formation processes shows that the well-established colloidal nanocrystal growth mechanism, in which nucleation is followed by focusing of size distribution and ended with defocusing of size distribution, cannot be applied to our reactions. Instead, we observed an exceptional type of growth mechanism in which, after nucleation, clear defocusing instead of focusing follows; then slight focusing occurs.     

Study of the Growth of Capped ZnO Nanocrystals: A Route to Rational Synthesis

We report the study of complex and unexpected dependencies of nanocrystal size as well as nanocrystal‐size distribution on various reaction parameters in the synthesis of ZnO nanocrystals using poly(vinyl pyrollidone) (PVP) as a capping agent. This method establishes a qualitatively different growth mechanism to the anticipated Ostwald ripening behavior. The study of size‐distribution kinetics and an understanding of the observed non‐monotonic behaviors provides a route to rational synthesis. We used a simple, but accurate, approach to estimate the size‐distribution function of nanocrystals from the UV‐absorption spectrum. Our results demonstrate the accuracy and generality of this approach, and we also illustrate its application to various semiconducting nanocrystals, such as ZnO, ZnS, and CdSe, over a wide size range (1.8–5.3 nm).     

Application of a microfluidic reaction system for CdSe nanocrystal preparation: their growth kinetics and photoluminescence analysis

We used a microreactor for CdSe nanocrystal preparation and explored the effects of experimental conditions on the properties of the products. The particle growth kinetics and photoluminescence properties of the nanocrystals showed identical trends to previous reports, indicating the efficiency of the current method for analysis of rapid nanocrystal synthesis as well as industrial production of CdSe nanocrystals.     

Multigram scale synthesis and characterization of monodisperse tetragonal zirconia nanocrystals

A new and simple method has been developed to synthesize large quantities of highly monodisperse tetragonal zirconia nanocrystals. In this synthesis, a nonhydrolytic sol-gel reaction between zirconium(IV) isopropoxide and zirconium(IV) chloride at 340 degrees C generated 4 nm sized zirconia nanoparticles. A high-resolution transmission electron microscopic (HRTEM) image showed that the particles have a uniform particle size distribution and that they are highly crystalline. These monodisperse nanoparticles were synthesized without any size selection process. X-ray diffraction studies combined with Rietveld refinement revealed that the ZrO(2) nanocrystals are the high-temperature tetragonal phase, and very close to a cubic phase. When zirconium(IV) bromide is used as a precursor instead of zirconium chloride, zirconia nanoparticles with an average size of 2.9 nm were obtained. The UV-visible absorption spectrum of 4 nm sized zirconia nanoparticles exhibited a strong absorption starting at around 270 nm. A fluorescence spectrum with excitation at 300 nm showed a broad fluorescence band centered around 370 nm. FTIR spectra showed indication of TOPO binding on the ZrO(2) nanoparticle surface. These optical studies also suggest that the nanoparticles are of high quality in terms of narrow particle size distribution and relatively low density of surface trap states.     

Synthesis and characterization of Ln,Yb:BaGdF5 (Ln = Er,Ho) nanocrystals by hydrothermal method

The Er,Yb: BaGdF₅ and Ho,Yb: BaGdF₅ nanocrystal materials with the narrow particle size distribution was synthesized by a hydrothermal method at 180°C for 24 h. The phase structure and fluorescence properties were investigated by X-ray diffraction, scanning electron microscopy and up-conversion spectroscopy, respectively. The phase composition does not change with increasing the pH value, leading to the formation of a pure phase BaGdF₅, while the solution was turbid state. The products show a good crystallinity, dispersion and uniform particle size distribution. Under the LD excitation at 980 nm, Er,Yb: BaGdF₅ and Ho,Yb: BaGdF₅ nanocrystal materials were researched in the visible range of the fluorescence spectrum. Er,Yb: BaGdF₅ nanocrystals were achieved the launch of green, blue, and red. Ho,Yb: BaGdF₅ nanocrystal with green and red light output were realized. The light-emitting belong to the two-photon transition process. And the possible mechanism for the corresponding up-conversion luminescence was also discussed. The Er,Yb: BaGdF₅ and Ho,Yb: BaYbF₅ nanocrystals with the narrow particle size distribution have potential applications in biological field as luminescence probes.

     

单源前体合成水溶性的CdS和ZnS纳米晶

0引言量子点(QuantumDots)一般指半径小于或接近玻尔激子半径的半导体纳米晶颗粒。和有机染料分子相比,无机半导体纳米晶的带隙宽度可通过简单     

Seed‐Mediated and Iodide‐Assisted Synthesis of Gold Nanocrystals with Systematic Shape Evolution from Rhombic Dodecahedral to Octahedral Structures

We report the development of a seed‐mediated and iodide‐assisted method for the synthesis of monodisperse gold nanocrystals with systematic shape evolution from rhombic dodecahedral to octahedral structures. Particle growth is complete in 15 min at room temperature, so the process is fast and energy‐efficient. By progressively increasing the volume of KI used in a growth solution while keeping the amount of ascorbic acid added constant, nanocrystals with morphologies that vary from rhombic dodecahedral to rhombicuboctahedral, edge‐ and corner‐truncated octahedral, corner‐truncated octahedral, and octahedral structures were synthesized. The nanocrystals are monodisperse in size and readily form self‐assembled structures on substrates. By simply adjusting the volume of gold seed solution added to a growth solution, particle sizes of the octahedral gold nanocrystals can be tuned with average opposite corner‐to‐corner distances of 42, 48, 54, 60, 68, 93, 107, and 125 nm. In the presence of HAuCl₄, iodide may act as a reducing agent. Variation of its volume in the solution may slightly modulate the reduction rate and affect the final crystal morphology. Intermediate structures collected during crystal growth reveal the presence of many twisted structures that surround a developing nanocrystal core. This nanocrystal growth mechanism and the less important role of surfactant in directing the polyhedral nanocrystal morphology is discussed.     

Highly luminescent silicon nanocrystals with discrete optical transitions.

A new synthetic method was developed to produce robust, highly crystalline, organic-monolayer passivated silicon (Si) nanocrystals in a supercritical fluid. By thermally degrading the Si precursor, diphenylsilane, in the presence of octanol at 500 degrees C and 345 bar, relatively size-monodisperse sterically stabilized Si nanocrystals ranging from 15 to 40 A in diameter could be obtained in significant quantities. Octanol binds to the Si nanocrystal surface through an alkoxide linkage and provides steric stabilization through the hydrocarbon chain. The absorbance and photoluminescence excitation (PLE) spectra of the nanocrystals exhibit a significant blue shift in optical properties from the bulk band gap energy of 1.2 eV due to quantum confinement effects. The stable Si clusters show efficient blue (15 A) or green (25-40 A) band-edge photoemission with luminescence quantum yields up to 23% at room temperature, and electronic structure characteristic of a predominantly indirect transition, despite the extremely small particle size. The smallest nanocrystals, 15 A in diameter, exhibit discrete optical transitions, characteristic of quantum confinement effects for crystalline nanocrystals with a narrow size distribution.     

Gelification: an effective measure for achieving differently sized biocompatible Fe3O4 nanocrystals through a single preparation recipe

Biocompatible Fe(3)O(4) nanocrystals were synthesized through the pyrolysis of ferric acetylacetonate (Fe(acac)(3)) in diphenyl oxide, in the presence of α,ω-dicarboxyl-terminated polyethylene glycol (HOOC-PEG-COOH) and oleylamine. Unusual gelification phenomena were observed from the aliquots extracted at different reaction stages after they were cooled to room temperature. By reaction time, the average size of the Fe(3)O(4) nanocrystals was tuned from 5.8 to 11.7 nm with an equilibrium size around 11.3 nm. By increasing the gelification degree of the stock solution, the equilibrium size of the Fe(3)O(4) nanocrystals was further increased from 11.3 to 18.9 nm. The underlying gel formation mechanism was investigated by using ultraviolet-visible absorption spectroscopy and Fourier transform infrared spectroscopy. The results suggest that the complexation between HOOC-PEG-COOH and Fe(acac)(3), with the help of oleylamine, results in large molecular networks, which are responsible for the gelification of the stock solution, while the interaction between the fragment of the molecular network and Fe(3)O(4) nanocrystal is responsible for the second gelification process observed during the early stage of reflux. To further investigate the particle growth behavior, small molecules released during the preparation were collected and analyzed by using photoelectron spectroscopy/photoionization mass spectroscopy (PES/PIMS). It was demonstrated that the pyrolysis of the Fe precursor is strongly correlated with the particle growth process. Further numerical simulations reveal that the first gelification process induced by the complexation between HOOC-PEG-COOH and Fe(acac)(3) largely alters the pyrolysis behavior of the Fe precursor; consequently, the equilibrium size of the resultant Fe(3)O(4) nanocrystals can effectively be tuned by the gelification degree of the stock solution.     

Photoassisted tuning of silicon nanocrystal photoluminescence

Silicon is a rather inefficient light emitter due to the indirect band gap electronic structure, requiring a phonon to balance the electron momentum during the interband transition. Fortunately, momentum requirements are relaxed in the 1-5 nm diameter Si crystals as a result of quantum confinement effects, and bright photoluminescence (PL) in the UV-vis range is achieved. Photoluminescent Si nanocrystals along with the C- and SiC-based nanoparticles are considered bioinert and may lead to the development of biocompatible and smaller probes than the well-known metal chalcogenide-based quantum dots. Published Si nanocrystal production procedures typically do not allow for the fine control of the particle size. An accepted way to make the H-terminated Si nanocrystals consists of anodic Si wafer etching with the subsequent breakup of the porous film in an ultrasound bath. Resulting H-termination provides a useful platform for further chemical derivatization and conjugation to biomolecules. However, a rather polydisperse mixture is produced following the ultrasonic treatment, leading to the distributed band gap energies and the extent of surface passivation. From the technological point of view, a homogeneous nanoparticle size mixture is highly desirable. In this study, we offer an efficient way to reduce the H-terminated Si nanocrystal diameter and narrow size distribution through photocatalyzed dissolution in a HF/HNO3 acid mixture. Si particles were produced using the lateral etching of a Si wafer in a HF/EtOH/H2O bath followed by sonication in deaerated methanol. Initial suspensions exhibited broad photoluminescence in the red spectral region. Photoassisted etching was carried out by adding the HF/HNO3 acid mixture to the suspension and exposing it to a 340 nm light. Photoluminescence and absorbance spectra, measured during dissolution, show the gradual particle size decrease as confirmed by the photoluminescence blue shift. The simultaneous narrowing of the photoluminescence spectral bandwidth suggests that the dissolution rate varies with the particle size. We show that the Si nanoparticle dissolution rate depends on the amount of light adsorbed by the particle and accounts for the etching rate variation with the particle size. Significant improvement in the PL quantum yield is observed during the acid treatment, suggesting improvement in the dangling bond passivation.     

Phosphine-free synthesis of CdSe nanocrystals

High quality CdSe nanocrystals have been prepared using elemental selenium as the chalcogenide precursor dispersed in 1-octadecene (ODE). The conditions used to prepare the Se precursor were found to be critical for successful nanocrystal synthesis. Systematic titration of the Se precursor solution with tri-n-octylphosphine (TOP) allowed the Se reactivity to be tuned and the final particle size to be controlled. Band-edge and surface related emission were observed for samples prepared in the presence and absence of added TOP. In the absence of a selenium passivant, the crystal structure of CdSe nanocrystals could be altered from zinc blende to wurtzite by the addition of bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA).     

First synthesis by liquid-liquid phase transfer of magnetic CoxPt100-x nanoalloys

Controlled synthesis of magnetic nanoparticles with a well-known size and composition is always a challenge. A soft chemical synthesis was developed to obtain magnetic alloy nanocrystals with a high ability to control composition, size, and polydispersity. Cobalt-platinum alloy nanocrystals were synthesized using a colloidal approach by the liquid-liquid phase transfer method. Structural characterization using HRTEM and XRD was carried out on nanocrystals in the range of 25-75% cobalt composition, which indicated the formation of nanoalloy bimetallic CoxPt100-x. Adjusting the alkylamine capping agent and the kinetics of the reduction process allowed tuning of the size in the range of 1.8-4 nm while keeping an equiatomic composition. The narrow size distribution led to the possibility of inducing nanoparticle self-organization over a long range. The magnetic properties of the Co50Pt50 nanoalloy in the disordered face-centered cubic phase A1 were studied for different nanoparticle sizes.     

One-step synthesis of highly water-soluble magnetite colloidal nanocrystals

A high-temperature solution-phase hydrolysis approach has been developed for the synthesis of colloidal magnetite nanocrystals with well-controlled size and size distribution, high crystallinity, and high water solubility. The synthesis was accomplished by the hydrolysis and reduction of iron(III) cations in diethylene glycol with a rapidly injected solution of sodium hydroxide at an elevated temperature. The high reaction temperature allows for control over size and size distribution and yields highly crystalline products. The superior water solubility is achieved by using a polyelectrolyte, that is, poly(acrylic acid) as the capping agent, the carboxylate groups of which partially bind to the nanocrystal surface and partially extend into the surrounding water. The direct synthesis of water-soluble nanocrystals eliminates the need for additional surface modification steps which are usually required for treating hydrophobic nanocrystals produced in nonpolar solvents through the widely recognized pyrolysis route. The abundant carboxylate groups on the nanocrystal surface allow further modifications, such as bioconjugation, as demonstrated by linking cysteamine to the particle surface. The monodisperse, highly water-soluble, superparamagnetic, and biocompatible magnetite nanocrystals should find immediate important biomedical applications.     

How Chemical Bonding Impacts Halide Perovskite Nanocrystals Growth to Bulk Films: Implication of Pb−X Bond on Growth Kinetics

Lead halide perovskites nanocrystals have emerged as a leading candidate in perovskite solar cells and light-emitting diodes. Given their favorable, tunable optoelectronic properties through modifying the size of nanocrystals, it is imperative to understand and control the growth of lead halide perovskite nanocrystals. However, during the nanocrystal growth into bulk films, the effect of halide bonding on growth kinetics remains elusive. To understand how a chemical bonding of Pb−X (covalency and ionicity) impact on growth of nanocrystals, we have examined two different halide perovskite nanocrystals of CsPbCl₃ (more ionic) and CsPbI₃ (more covalent) derived from the same parent CsPbBr₃ nanocrystals. Tracking the growth of nanocrystals by monitoring the spectral features of bulk peaks (at 445 nm for Cl and at 650 nm for I) enables us to determine the growth activation energy to be 92 kJ/mol (for CsPbCl₃) versus 71 kJ/mol (for CsPbI₃). The electronegativity of halides in Pb−X bonds governs the bond strength (150–240 kJ/mol), characteristics of bonding (ionic versus covalent), and growth kinetics and resulting activation energies. A fundamental understanding of Pb−X bonding provides a significant insight into controlling the size of the perovskite nanocrystals with more desired optoelectronic properties.     

Spatially-resolved analysis of nanoparticle nucleation and growth in a microfluidic reactor

Microfluidic systems provide a unique platform for investigation of fundamental reaction processes, which is critical to understanding how to control nanostructure synthesis on a production scale. We have examined the synthesis of cysteine-capped CdS quantum dot nanocrystals (CdS-Cys) between two interdiffusing reagent streams in a continuous-flow microfluidic reactor. Using spatially resolved photoluminescence imaging and spectroscopy of the microreactor, we have acquired kinetic and mechanistic data on the CdS-Cys nanoparticle nucleation and growth, and observed a binary shift in the particle emission spectrum from a higher (2.9 eV) to lower (2.5 eV) energy emission peak within the first second of residence time. Several reactor models have been tested against the spatially and spectrally resolved signals, which suggest that homogeneous reaction and particle nucleation are diffusion-limited and occur only at the boundary between the two laminar streams, while a slower activation process occurs on a longer (seconds) time scale. The results provide direct insight into the rapid processes that occur during crystallization in microfluidic mixing channels, and demonstrate the potential of using controlled microfluidic environments with spatially resolved monitoring to conduct fundamental studies of nanocrystal nucleation and growth.     

Study of the growth of capped ZnO nanocrystals: a route to rational synthesis

We report the study of complex and unexpected dependencies of nanocrystal size as well as nanocrystal-size distribution on various reaction parameters in the synthesis of ZnO nanocrystals using poly(vinyl pyrollidone) (PVP) as a capping agent. This method establishes a qualitatively different growth mechanism to the anticipated Ostwald ripening behavior. The study of size-distribution kinetics and an understanding of the observed non-monotonic behaviors provides a route to rational synthesis. We used a simple, but accurate, approach to estimate the size-distribution function of nanocrystals from the UV-absorption spectrum. Our results demonstrate the accuracy and generality of this approach, and we also illustrate its application to various semiconducting nanocrystals, such as ZnO, ZnS, and CdSe, over a wide size range (1.8-5.3 nm).