Ligand bonding and dynamics on colloidal nanocrystals at room temperature: the case of alkylamines on CdSe nanocrystals

With CdSe nanocrystals stabilized with very weak ligands (pyridine) as the starting materials, NMR techniques were applied to distinguish the bonded and free alkylamine ligands in an equilibrated adsorption/desorption system for the CdSe-amine nanocrystal-ligand pair. NMR and photoluminescence (PL) measurements were further correlated to identify the linear relationship between PL intensity and the surface ligand coverage of the amine-coated CdSe nanocrystals. For 3.5 nm CdSe nanocrystals and octylamine ligands, the chemical equilibrium constant (K) of the CdSe-amine nanocrystal-ligand adsorption/desorption process was found to be around 50-100, and the corresponding Delta(r)G(o) was calculated as 9.8-11.5 kJ/mol. With a proposed mathematic method, the corresponding chemical kinetic constants for the desorption (kd) and adsorption (ka) processes were measured to be 0.01 s(-1) and 0.5 L mol(-1) s(-1), respectively. K, kd, and ka obtained here are generally 2-4 magnitudes different from those estimated in literature. Analysis indicates that these constants are well consistent with the existing experimental observations.     

Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols

The photochemical instability of CdSe nanocrystals coated by hydrophilic thiols was studied nondestructively and systematically in water. The results revealed that the photochemical instability of the nanocrystals actually included three distinguishable processes, namely the photocatalytic oxidation of the thiol ligands on the surface of nanocrystals, the photooxidation of the nanocrystals, and the precipitation of the nanocrystals. At first, the thiol ligands on the surface of a nanocrystal were gradually photocatalytically oxidized using the CdSe nanocrystal core as the photocatalyst. This photocatalytic oxidation process was observed as a zero-order reaction in terms of the concentration of the free thiols in the solution. The photogenerated holes in a nanocrystal were trapped onto the thiol ligands bound on the surface of the nanocrystal, which initiated the photooxidation of the ligands and protected the nanocrystal from any photooxidation. After nearly all of the thiol ligands on the surface of the nanocrystals were converted into disulfides, the system underwent several different pathways. If the disulfides were soluble in water, then all of the disulfides fell into the solution at the end of this initial process, and the nanocrystals precipitated out of the solution without much variation over their size and size distribution. When the disulfides were insoluble in water, they likely formed a micelle-like structure around the nanocrystal core and kept it soluble in the solution. In this case, the nanocrystals only precipitated after severe oxidation, which took a long period of time. If the system contained excess free thiol ligands, they replaced the photochemically generated disulfides and maintained the stability and solubility of the nanocrystals. The initiation stage of the photooxidation of CdSe nanocrystals themselves increased as the thickness and packing density of the ligand shell increased. This was explained by considering the ligand shell on the surface of a nanocrystal as the diffusion barrier of the oxygen species from the bulk solution into the interface between the nanocrystal and the surface ligands. Experimental results clearly indicated that the initiation stage of the photooxidation was not caused by the chemical oxidation of the system kept in air under dark conditions or the hydrolysis of the cadmium-thiol bonds on the surface of the nanocrystals, both of which were magnitudes slower than the photocatalytic oxidation of the surface ligands if they occurred at all. The results described in this contribution have already been applied for designing new types of thiol ligands which dramatically improved the photochemical stability of CdSe nanocrystals with a ligand shell that is as thin as approximately 1 nm.     

Control of the optical properties of CdTe nanocrystals by selective exchange of Te with thiolate: effect of organic ligands on the formation of core-shell structures

Changes in the optical properties of CdTe nanocrystals through selective surface exchange reaction with thiolate molecules in the organic phase are studied with an aim to investigate the mechanism and the role of organic ligands. The reaction was mediated by dissociation of Te anions via oxidation in air from CdTe nanocrystals, followed by attachment of thiolate molecules in a 1:1 stoichiometric manner. This results in a gradual shell formation and a corresponding decrease in the size of the fluorescent CdTe cores, which induces a blue shift of both the absorption edge and emission wavelength in the visible region. A systematic study including the addition of ligands at different concentrations revealed that Te dissociation is the rate-determining step for the process and the degree of blue shift is significantly dependent on the amount of organic ligands present. The process could also be kinetically controlled through the addition of an excess amount of thiolate ligands, allowing systematic tuning of the emission properties of nanocrystals under ambient conditions.     

Nearly monodisperse and shape-controlled CdSe nanocrystals via alternative routes: nucleation and growth

The nucleation and growth of colloidal CdSe nanocrystals with a variety of elongated shapes were explored in detail. The critical size nuclei for the system were magic sized nanoclusters, which possessed a sharp and dominated absorption peak at 349 nm. The formation of the unique magic sized nuclei in a broad monomer concentration range was not expected by the classic nucleation theory. We propose that this was a result of the extremely high chemical potential environment, that is, very high monomer concentrations in the solution, required for the growth of those elongated nanocrystals. The shape, size, and size/shape distributions of the resulting nanocrystals were all determined by two related factors, the magic sized nuclei and the concentration of the remaining monomers after the initial nucleation stage. Without any size sorting, nearly monodisperse CdSe quantum structures with different shapes were reproducibly synthesized by using the alternative cadmium precursors, cadmium-phosphonic acid complexes. A reasonably large excess of the cadmium precursor, which is less reactive than the Se precursor, was found beneficial for the system to reach the desired balance between nucleation and growth. The shape evolution and growth kinetics of these elongated nanocrystals were consistent with the diffusion-controlled model proposed previously. The branched nanocrystals had to grow at very high monomer concentrations because the multiple growth centers at the end of each branch must be fed with a very high diffusion flux to keep all branches in the 1D-growth mode. The rice-shaped nanocrystals were found as special products of the 3D-growth stage. The growth of the nanocrystals in the 1D-growth stage was proven to be not unidirectional after the length of the nanocrystals reached a certain threshold. Experimental results indicate that coordinating solvents and two ligands with distinguishable coordinating abilities are both not intrinsic requirements for the growth of elongated CdSe nanocrystals.     

Shape evolution of single-crystalline iron oxide nanocrystals

Shape- and size-controlled synthesis of single-crystalline maghemite (gamma-Fe2O3) nanocrystals are performed by utilizing a solution-based one-step thermolysis method. Modulating the growth parameters, such as the type and amount of capping ligands as well as the growth time, is shown to have a significant effect on the overall shape and size of the obtained nanocrystals and on the ripening process itself. The resulting shapes of the novel structures are diverse, including slightly faceted spheres, diamonds, prisms, and hexagons, all of which are in fact truncated dodecahedron structures with different degrees of truncation along the {111}, {110}, or {100} faces. Spherical nanocrystals are easily assembled into the three-dimensional superlattices, demonstrating the uniformity of these nanocrystals. The size-dependent magnetic properties are examined, and large hexagon-shaped gamma-Fe2O3 nanocrystals are shown to be ferrimagnetic at room temperature.     

Colloidal synthesis and self-assembly of CoPt(3) nanocrystals

Reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid were employed in different coordinating mixtures to produce monodisperse, highly crystalline CoPt(3) nanoparticles. The mean particle size can be varied from 1.5 to 7.2 nm by controlling the reaction conditions and the type of coordinating mixture. As-synthesized CoPt(3) particles represent single crystal domains and have chemically disordered face-centered cubic (fcc) structure. Nearly spherical CoPt(3) nanocrystals were found to assemble into two- (2D) and three-dimensional (3D) structures. An AB(5) type superlattice is observed by TEM after mixing two nanoparticle samples with different mean sizes. Slow precipitation led to the formation of facetted colloidal crystals with sizes up to 20 microm.     

Luminescent CdSe/CdS core/shell nanocrystals in dendron boxes: superior chemical,photochemical and thermal stability

The surface ligands, generation-3 (G3) dendrons, on each semiconductor nanocrystal were globally cross-linked through ring-closing metathesis (RCM). The global cross-linking of the dendron ligands sealed each nanocrystal in a dendron box, which yielded box-nanocrystals. Although the dendron ligands coated CdSe nanocrystals (CdSe dendron-nanocrystals) were already quite stable, the stability of CdSe box-nanocrystals against chemical, photochemical, and thermal treatments were dramatically improved in comparison to that of the original dendron-nanocrystals. Furthermore, the box structure of the ligands monolayer coupled with the stable inorganic CdSe/CdS core/shell nanocrystals resulted in a class of extremely stable nanocrystal/ligands complexes. The band edge photoluminescence of the core/shell dendron-nanocrystals and box-nanocrystals were partially remained, and could be further brightened through controlled chemical oxidation or photooxidation. Practically, the stability of the box-nanocrystals is sufficient for most fundamental studies and technical applications. The box-nanocrystals may represent a general solution for the commonly encountered instability for many types of colloidal nanocrystals. The size distribution of the empty dendron boxes formed by the dissolution of the inorganic nanocrystals in concentrated HCl was very narrow. The empty boxes as new types of polymer capsules are soluble in solution, mesoporous, and with a very thin but stable peripheral. Those nanometer-sized cavities should be of interest for many purposes in the field of solution host-guest chemistry.     

Multidentate-protected colloidal gold nanocrystals: pH control of cooperative precipitation and surface layer shedding

Colloidal gold nanocrystals (AuNCs) with broad size tunability and unusual pH-sensitive properties have been synthesized using multidentate polymer ligands. Because they contain both carboxylic functional groups and sterically hindered aliphatic chains, the multidentate ligands can not only reduce gold precursors but also stabilize gold nanoclusters during nucleation and growth. The "as-synthesized" AuNCs are protected by an inner coordinating layer and an outer polymer layer and are soluble in water and polar solvents. When the solution pH is lowered by just 0.6 units (from 4.85 to 4.25), the particles undergo a dramatic cooperative transition from being soluble to insoluble, allowing rapid isolation, purification, and redispersion of the multidentate-protected AuNCs. A surprising finding is that when a portion of the surface carboxylate groups are neutralized by protonation, the particles irreversibly shed their outer polymer layer and become soluble in nonpolar organic solvents. Furthermore, the multidentate polymer coatings are permeable to small organic molecules, in contrast to the tightly packed self-assembled monolayers of alkanethiols on gold. These insights are important in regard to the design of "smart" imaging and therapeutic nanoparticles that are activated by small pH changes in the tumor interstitial space or endocytic organelles.     

PVP封端剂对Pd纳米晶电催化氧化甲醇和乙醇性能的影响

Direct alcohol fuel cells (DAFCs) have attracted considerable research interest because of their potential application as alternative power sources for automotive systems and portable electronics. Pd-based catalysts represent one of the most popular catalysts for DAFCs due to their excellent electrocatalytic activities in alkaline electrolytes. Thus, it is of great importance to understand the structure-activity relationship of Pd electrocatalysts for alcohol electrocatalysis. Recently, size- and shape- controlled Pd nanocrystals have been successfully synthesized and subsequently used to study the size and shape effects of Pd electrocatalysts on alcohol electrocatalysis, in which the Pd (100) facet exhibited higher electrocatalytic oxidation activity for small alcohol molecules than the Pd (111) and (110) facets. Although it is well known that capping ligands, which are widely used in wet chemistry for the size- and shape-controlled synthesis of metal nanocrystals, likely chemisorb onto the surfaces of the resulting metal nanocrystals and influence their surface structure and surface-mediated properties, such as catalysis, this issue was not considered in previous studies of Pd nanocrystal electrocatalysts for electrocatalytic oxidation of small alcohol molecules. In this study, we prepared polyvinylpyrrolidone (PVP)-capped Pd nanocrystals with different morphologies and sizes and comparatively studied their electrocatalytic activities for methanol and ethanol oxidation in alkaline solutions. The chemisorbed PVP molecules transferred charge to the Pd nanocrystals, and the finer Pd nanocrystals had a higher coverage of chemisorbed PVP, and thus exposed fewer accessible surface sites, experienced more extensive PVP-to-Pd charge transfer, and were more negatively charged. The intrinsic electrocatalytic activity, represented by the electrochemical surface area (ECSA)-normalized electrocatalytic activity, of Pd nanocubes with exposed (100) facets increases with the particle size, indicating that the more negatively-charged Pd surface is less electrocatalytically active. The Pd nanocubes with average sizes between 12 and 19 nm are intrinsically more electrocatalytically active than commercial Pd black electrocatalysts, while the activity of Pd nanocubes with an averages size of 8 nm is less. This suggests that the enhancement effect of the exposed (100) facets surpasses the deteriorative effect of the negatively charged Pd surface for the Pd nanocubes with average sizes between 12 and 19 nm, whereas the deteriorative effect of the negatively charged Pd surface surpasses the enhancement effect of the exposed (100) facets for the Pd nanocubes with average sizes of 8 nm due to the extensive PVP-to-Pd charge transfer. Moreover, the Pd nanocubes with average sizes of 8 nm exhibit similar intrinsic electrocatalytic activity to the Pd nanooctahedra with (111) facets exposed and average sizes of 7 nm, indicating that the electronic structure of Pd electrocatalysts plays a more important role in influencing the electrocatalytic activity than the exposed facet. Since the chemisorbed PVP molecules block the surface sites on Pd nanocrystals that are accessible to the reactants, all Pd nanocrystals exhibit lower mass-normalized electrocatalytic activity than the Pd black electrocatalysts, and the mass-normalized electrocatalytic activity increases with the ECSA. These results clearly demonstrate that the size- and shape-dependent electrocatalytic activity of Pd nanocrystals capped with PVP for methanol and ethanol oxidation should be attributed to both the exposed facets of the Pd nanocrystals and the size-dependent electronic structures of the Pd nanocrystals resulting from the size-dependent PVP coverage and PVP-to-Pd charge transfer. Therefore, capping ligands on capped metal nanocrystals inevitably influence their surface structures and surface-mediated properties, which must be considered for a comprehensive understanding of the structure-activity relationship of capped metal nanocrystals.     

高温热解法制备硒化镉纳米晶

以脂肪酸和三辛基氧化膦为表面活性剂,采用高温热解硒与镉的前聚体,制备出分散性好的CdSe纳米晶.吸收光谱和荧光光谱研究表明,控制反应时间可以改变CdSe纳米晶的荧光强度.     

Effects of size of noncovalent complexes on their stability during collision-induced dissociation

The size-dependent stability of noncovalent complexes under collision-induced dissociation (CID) conditions was studied on a quadrupole ion trap mass spectrometer. Complexes of heme, tetraphenylporphyrin iron(III) (TPP-Fe), and tetraphenylporphyrin manganese(III) (TPP-Mn) with several histidine-containing peptides and model compounds were formed by electrospray ionization (ESI) and their stability was probed by variable-energy CID. It was found that the stability of complexes with the same (or nearly the same) binding energy has a linear dependence on the complex size (or total number of degrees of freedom). This approach will allow comparisons of variable-energy CID data for noncovalent complexes with different binding energies and could be used to help in structural elucidation of some complexes formed by multidentate ligands. The linearity of size effects on the stability of the complexes was also tested in three crown ether/protonated primary amine systems.     

Surface-functionalization-dependent optical properties of II-VI semiconductor nanocrystals

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3/2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylamine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleate-capped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.     

铒镱共掺磷酸镧纳晶的合成和近红外荧光性质

以磷酸三丁酯作为配体,使用高温溶液法合成了掺铒磷酸镧纳米晶体,高分辨电镜和XRD结果表明产物尺寸均一,直径4nm左右,具有高度的结晶性,在甲苯等有机溶剂中分散性良好.近红外发光光谱表明,产物在1535 nm处有发光峰,半高全宽为50 nm,高于目前掺铒光纤放大器(EDFA)的增益带宽.通过IR、元素分析研究发现,使用真空干燥等方法不能有效去除纳晶表面的羟基.为了进一步消除表面缺陷和猝灭基团对纳晶发光性质的影响,我们尝试在其外围生长一层LaPO₄壳做成核壳粒子以消除表面缺陷和隔离猝灭基团,结果表明,新的纳晶的发光强度得到了显著提高.     

Isothermal titration microcalorimetric method for studying the combined ligand binding with application to the binding of ethylurea and (N,N)dimethylurea on urease

A simple, novel method was introduced for determining equilibrium constants and enthalpies of binding of two different competitive ligands on a macromolecule by isothermal titration microcalorimetry technique. This method was applied to the simultaneous binding of ethylurea (I) and (N,N)dimethylurea (X), on jack-bean urease at pH 7.0 (tris-base; 30 mM) at 27°C. The dissociation equilibrium constants measured by this method were markedly consistent with inhibition constants obtained from assay of enzyme activities in the presence of I and X.     

Synthesis and properties of nanosilicon stabilized by butyl and perfluorobutyl ligands

Nanocrystalline silicon stabilized by butyl and perfluorobutyl ligands that form improper surface states of silicon nanocrystals were synthesized. The presence of perfluorobutyl ligands on the surface of silicon nanocrystals was proved by IR spectroscopy. Nanocrystals with perfluorobutyl ligands form aggregates, which decreases the efficiency of photoluminescence. The nanocrystals with butyl ligands have smaller size but their photoluminescence can be clearly recorded.     

Study of nucleation and growth in the organometallic synthesis of magnetic alloy nanocrystals: the role of nucleation rate in size control of CoPt3 nanocrystals

High quality CoPt(3) nanocrystals were synthesized via simultaneous reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid and hexadecylamine as stabilizing agents. The high flexibility and reproducibility of the synthesis allows us to consider CoPt(3) nanocrystals as a model system for the hot organometallic synthesis of metal nanoparticles. Different experimental conditions (reaction temperature, concentration of stabilizing agents, ratio between cobalt and platinum precursors, etc.) have been investigated to reveal the processes governing the formation of the metal alloy nanocrystals. It was found that CoPt(3) nanocrystals nucleate and grow up to their final size at an early stage of the synthesis with no Ostwald ripening observed upon further heating. In this case, the nanocrystal size can be controlled only via proper balance between the rates for nucleation and for growth from the molecular precursors. Thus, the size of CoPt(3) nanocrystals can be precisely tuned from approximately 3 nm up to approximately 18 nm in a predictable and reproducible way. The mechanism of homogeneous nucleation, evolution of the nanocrystal ensemble in the absence of Ostwald ripening, nanocrystal faceting, and size-dependent magnetic properties are investigated and discussed on the example of CoPt(3) magnetic alloy nanocrystals. The developed approach was found to be applicable to other systems, e.g., FePt and CoPd(2) magnetic alloy nanocrystals.     

Facile synthesis of size-tunable ZIF-8 nanocrystals using reverse micelles as nanoreactors

This paper describes a robust method for the synthesis of high-quality ZIF-8 nanocrystals using reverse micelles as discrete nanoscale reactors.The precise size control of ZIF-8 nanocrystals is conveniently achieved by tuning the concentration of precursors,reaction temperatures,the amount of water,and the structure of surfactants.The as-synthesized ZIF-8 nanocrystals are of narrow distribution and tunable size.A size-dependent catalytic activity for Knoevenagel condensation reaction is further demonstrated by using ZIF-8 nanocrystals with different sizes as the catalysts.This facile method opens up a new opportunity in the synthesis of various ZIFs nanocrystals.     

Electrophoretic properties of BSA-coated quantum dots

Low toxic InP/ZnS quantum dots (QDs), ZnS:Mn²⁺/ZnS nanocrystals and CdSe/ZnS nanoparticles were rendered water-dispersible by different ligand-exchange methods. Eventually, they were coated with bovine serum albumin (BSA) as a model protein. All particles were characterised by isotachophoresis (ITP), laser Doppler velocimetry (LDV) and agarose gel electrophoresis. It was found that the electrophoretic mobility and colloidal stability of ZnS:Mn²⁺/ZnS and CdSe/ZnS nanoparticles, which bore short-chain surface ligands, was primarily governed by charges on the nanoparticles, whereas InP/ZnS nanocrystals were not charged per se. BSA-coated nanoparticles showed lower electrophoretic mobility, which was attributed to their larger size and smaller overall charge. However, these particles were colloidally stable. This stability was probably caused by steric stabilisation of the BSA coating.     

pH- and Dopant-Dependent CdMoO(4):Mn Nanocrystals: Luminescence and Magnetic Properties

CdMoO(4):Mn nanocrystals with a tetragonal crystal structure were prepared by aqueous coprecipitation method at a low temperature of 2 °C under different pH values. The size of the CdMoO(4):Mn nanocrystals of spherical morphology increases with the Mn dopant concentration from 35 to 55 nm for pH = 4. The morphology could be tuned from nanocrystals to microstructures consisting of smaller nanoparticles by the Mn concentration when the pH value of the precursor was increased to 8. The thermal stability of the luminescence and magnetic properties of the Mn-doped samples also depend on the pH and the doping level. The effects of the pH and dopant on the luminescence and magnetic properties, including magnetic susceptibility and electron paramagnetic resonance, were investigated. This approach contributes to better understanding of aqueous chemistry methods to control the growth of nanocrystals.     

Minimizing the hydrodynamic size of quantum dots with multifunctional multidentate polymer ligands

We report a new strategy to minimize the hydrodynamic size of quantum dots (QDs) and to overcome their colloidal stability and photobleaching problems based on the use of multifunctional and multidentate polymer ligands. A novel finding is that a balanced composition of thiol (-SH) and amine (-NH 2) coordinating groups grafted to a linear polymer chain leads to highly compact nanocrystals with exceptional colloidal stability, a strong resistance to photobleaching, and high fluorescence quantum yields. In contrast to the standing brushlike conformation of PEGylated dihydrolipoic acid molecules, mutlidentate polymer ligands can wrap around the QDs in a closed "loops-and-trains" conformation. This structure is highly stable thermodynamically and is responsible for the excellent colloidal and optical properties. We have optimized this process for the preparation of ultrastable CdTe nanocrystals and have found the strategy to be broadly applicable to a wide range of nanocrystalline materials and heterostructures. This work has led to a new generation of bright and stable QDs with small hydrodynamic diameters between 5.6 and 9.7 nm with tunable fluorescence emission from the visible (515 nm) to the near-infrared (720 nm). These QDs are well suited for molecular and cellular imaging applications in which the nanoparticle hydrodynamic size must be minimized.