Effects of shape and size of cobalt ferrite nanostructures on their MRI contrast and thermal activation

Cobalt ferrite magnetic nanostructures were synthesized via a high temperature solution phase method. Spherical nanostructures of various sizes were synthesized with the help of seed mediated growth of the nanostructures in organic phase, while faceted irregular (FI) cobalt ferrite nanostructures were synthesized via the same method but in the presence of a magnetic field. Magnetic properties were characterized by SQUID magnetometry, relaxivity measurements and thermal activation under RF field, as a function of size and shape. The results show that the saturation magnetization of the nanostructures increases with an increase in size, and the FI nanostructures exhibit lower saturation magnetization than their spherical counterparts. The relaxivity coefficient of cobalt ferrite nanostructures increases with increase in size; while FI nanostructures show a higher relaxivity coefficient than spherical nanostructures with respect to their saturation magnetization. In the case of RF thermal activation, the specific absorption rate (SAR) of nanostructures increases with increase in the size. The contribution sheds light on the role of size and shape on important magnetic properties of the nanostructures in relation to their biomedical applications.     

碳纳米结构构建:十二烷基二甲基苄基氯化铵@氯化钠体系

碳纳米材料用途十分广泛,但其形貌的控制和制备方法的完善仍然是个研究热点和亟需攻克的难点。在此,基于阳离子表面活性剂十二烷基二甲基苄基氯化铵(DDBAC)@氯化钠(NaCl)体系成功制备了碳纳米结构（碳纳米管、纳米碳球、棒状和层状碳纳米结构）。以DDBAC为碳源。碳化过程中使用NaCl晶体分隔DDBAC的有序聚集体。结果表明,通过控制DDBAC浓度制备出不同形貌的碳纳米结构,在1倍临界胶束浓度（CMC）~5CMC下制备了球形、棒形碳纳米结构和碳纳米管,在10CMC下制备了矩形层状碳纳米结构。利用TEM、PL、Raman等手段对制备的碳纳米结构进行表征。通过TEM分析DDBAC @ NaCl体系结构的转变,建立了胶束结构的演变过程。表面活性剂浓度的增加最终使得碳纳米形态从球形碳纳米结构到层状碳纳米结构的变化。本实验结论表明了表面活性剂@盐体系是一种潜在的制备碳纳米结构的方法。     

Kinetics-driven growth of orthogonally branched single-crystalline magnesium oxide nanostructures

Orthogonally branched single-crystalline magnesium oxide nanostructures were synthesized through a simple chemical vapor transport and condensation process in a flowing Ar/O(2) atmosphere. Other morphologies, such as cubes and nanowires, can also be obtained under different controlled conditions. The formation of different types of nanostructures can be tuned by modifying oxygen partial pressure during the synthesis. All the nanostructures are cubic single-crystalline enclosed by low-index {100} facets. Growth mechanisms for the nanostructures are discussed in detail: different supersaturation ratios, relatively high substrate temperatures, and surface defects in certain crystallographic planes cooperatively take important effects on determining the product morphologies. Structural defect-related blue light emission of the three types of MgO nanostructures was investigated. The MgO nanostructures with abundant morphologies may find applications in various nanodevices, and the kinetics-driven methodology might be exploited to synthesize similar nanostructures of other functional oxide materials.     

Photoactive Nanostructures of Polypyrrole

The creation of nanostructures of the photoactive polymer polypyrrole (PPy) on glass substrates with the spin‐coating method is described. No additional post‐production treatment is necessary to obtain uniformly distributed photoactive nanostructures on macroscopically scaled substrates. Based on X‐ray reflectivity measurements, the critical solution concentration of PPy below which these nanostructures develop is determined. The PPy nanostructures are displayed with atomic force microscopy (AFM) measurements, which prove that the nanostructures form directly on the substrate. With UV/Vis spectroscopy the absorption behavior of the nanostructures is probed in comparison to PPy films and PPy solutions. A linear dependence of the absorption of the nanostructure on the surface coverage measured with AFM is detected. The influence of confinement on the conjugation length results in a modified absorption behavior of the nanostructures.     

Collective buckling of a two-dimensional array of nanoscale columns

Periodic soft nanostructures are building blocks for small devices. However, mechanical failure in the form of structure buckling or distortion from their original shape is often reported when the dimension of these soft structures were reduced to below submicron scale. Such a phenomenon seriously limits a reliable impact of these nanostructures to greater applications. Current understandings of buckling of soft 2-D nanostructures are limited. The substrate for these soft nanostructures is usually very compliant. Neighboring nanostructures could interact through the deformation of the substrate. We analyze the collective buckling of a two-dimensional array of nanoscale columns with their lower ends built into an elastic substrate. Buckling of these nanostructures is mathematically described by an eigenvalue problem. Numerical analyses show patterned collapse for these 2-D nanostructures, qualitatively matching reported experimental findings. Our efforts are useful toward the understanding and manufacturing of many two-dimensional nanoscale features.     

金属表面分子纳米结构的构筑及性质研究

结合近期研究工作, 简要介绍了在溶液环境下, 利用有机分子在金属表面构筑纳米结构, 利用光化学反应方法调控所得的纳米结构, 利用电化学扫描隧道显微镜对这些结构进行观察, 及利用毛细管隧道结方法测量纳米结构电学性质的相关结果. 并展望了表面纳米结构的构筑、控制和性质研究领域的发展趋势.     

Growth and reactions of SiOx/Si nanostructures on surface-templated molecule corrals

Surface-templated nanostructures on the highly oriented pyrolytic graphite (HOPG) basal plane were created by controlled Cs+- or Ga+)ion bombardment, followed by subsequent oxidation at high temperature, forming molecule corrals. The corrals were then used for template growth of SiOx/Si nanostructures. We demonstrate here that, for SiOx/Si nanostructures formed in controlled molecule corrals, the amount of silicon deposited on the surface is directly correlated with the corral density, making it possible to generate patterned SiOx/Si nanostructures on HOPG. Since the size, depth, position, and surface density of the nanostructures can be controlled on the HOPG, it is possible to produce surfaces with patterned or gradient functionalities for applications in fields such as biosensors, microelectronics, and biomaterials (e.g., neuron pathfinding). If desired, the SiOx structures can be reduced in size by etching in dilute HF, and further oxidation of the nanostructures is slow enough to provide plenty of time to functionalize them using ambient and solution reactions and to perform surface analysis. Organosilane monolayers on surface-templated SiOx/Si nanostructures were examined by X-ray photoelectron spectroscopy, time-of-flight secondary ion mas spectrometry, and atomic force microscopy. Silanes with long alkyl chains such as n-octadecyltrichlorosilane (C18) were found to both react on SiOx/Si nanostructures and to condense on the HOPG basal plane. Shorter-chain silanes, such as 11-bromoundicyltrimethoxysilane (C11) and 3-mercaptopropyltrimethoxysilane (C3) were found to react preferentially with SiOx/Si nanostructures, not HOPG. The SiOx/Si nanostructures were also found to be stable toward multiple chemical reactions. Selective modification of SiOx/Si nanostructures on the HOPG basal plane is thus achievable.     

Controlled growth of two-dimensional and one-dimensional ZnO nanostructures on indium tin oxide coated glass by direct electrodeposition

A simple electrochemical deposition technique is used to deposit ZnO nanostructures with diverse morphology directly on ITO-coated glass substrates at 70 degrees C. The concentration of the Zn(NO 3) 2.6H 2O electrolyte is important to controlling the dimensionality of the nanostructures, with formation of one-dimensional (1D) nanospikes and nanopillars (with 50-500 nm diameter) below 0.01 M and of two-dimensional (2D) nanowalls and nanodisks (with 50-100 nm wall/disk thickness) above 0.05 M. Glancing-incidence X-ray diffraction study shows their wurtzite structure and confirms the change in the preferred crystal plane orientation with the dimensionality of ZnO nanostructures. UV-vis spectroscopy reveals a higher transmittance from 2D nanostructures than from 1D nanostructures and their optical direct band gaps estimated to be 3.12-3.27 eV. Depth-profiling X-ray photoemission studies show the presence of Zn(OH) 2 outer layers on the ZnO nanostructures, with a higher Zn(OH) 2 moiety for 2D nanostructures relative to 1D nanostructures. Furthermore, a substantial quantity of Cl (provided by the KCl supporting electrolyte) is detected throughout the 2D nanostructures only. The photoemission data therefore affirm our proposed growth mechanism that involves capping of the preferred [0001] growth direction by Cl (-) ions under fast hydroxylation kinetics condition as observed at a higher Zn(NO 3) 2.6H 2O electrolyte concentration.     

One-step controlled synthesis of anisotropic gold nanostructures with aniline as the reductant in aqueous solution

We report a general and versatile method for controlled synthesis of anisotropic gold nanostructures through the reduction of HAuCl4 by aniline in aqueous solution, without the need for an additional stabilizer or capping agent. In this approach, the reduction kinetics of AuCl-4 can be altered by simply adjusting the initial pH and temperature, inducing the formation of a wide variety of anisotropic nanostructures such as dispersed or multilayered plates, wires with networked or paramecium-like structures, and ginger-shaped particles. AFM, TEM, XRD, EDX, FTIR, and UV-vis-NIR measurements were used to characterize the resulting gold nanostructures. Investigation reveals that in situ formed polyaniline serves effectively as a capping agent to direct the shape of gold nanostructures during the slow growth process. These as-synthesized gold nanostructures exhibit strongly shape-dependent optical properties. This facile approach may be extended to the synthesis of some other anisotropic metal nanostructures such as platinum or palladium.     

Green synthesis of nanowire-like Pt nanostructures and their catalytic properties

We reported a simple and effective green chemistry route for facile synthesis of nanowire-like Pt nanostructures at one step. In the reaction, dextran acted as a reductive agent as well as a protective agent for the synthesis of Pt nanostructures. Simple mixing of precursor aqueous solutions of dextran and K₂PtCl₄ at 80 °C could result in spontaneous formation of the Pt nanostructures. Optimization of the experiment condition could yield nanowire-like Pt nanostructures at 23:1 molar ratio of the dextran repeat unit to K₂PtCl₄. Transmission electron microscopy results revealed that as-prepared nanowire-like Pt nanostructures consisted of individual Pt nanoparticles with the size range from 1.7 to 2.5 nm. Dynamic light scattering analysis indicated that as-prepared nanowire-like nanostructures have already formed in solution. The as-prepared nanowire-like Pt nanostructures were further characterized by UV-vis spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. In addition, the ratio dependence and temperature dependence of this reaction have also been investigated. The as-prepared nanowire-like Pt nanostructures can be immobilized on glassy carbon electrodes using an electrochemical coupling strategy, and the resulting nanowire-like Pt nanostructures modified film exhibited an excellent electrocatalytic activity for the reduction of oxygen and the oxidation of NADH.     

The effect of solvent on the morphology of ZnO nanostructure assembly by dielectrophoresis and its device applications

Different zinc oxide nanostructured morphologies were grown on photolithographically patterned silicon/silicon dioxide substrates by dielectrophoresis technique using different solvents, such as water and ethanol, obtaining rod-like and net-like nanostructures, respectively. The formation of continuous nanostructures was confirmed by scanning electron microscopic, atomic force microscopic images, and electrical characterizations. The rod-like zinc oxide nanostructures were observed in the 10 μm gap between the fingers in the pattern, whereas net-like nanostructures were formed independently of microgap. A qualitative study about the mechanism for the assembly of zinc oxide continuous nanostructures was presented. Devices were electrically characterized, at room temperature, in controlled environment to measure the conductance behavior in ultraviolet and humidity environment. Devices based on zinc oxide nanostructures grown in ethanol medium show better responses under both ultraviolet and humidity, because of the net-like structure with high surface-to-volume ratio.     

Synthesis and bioanalytical applications of specific-shaped metallic nanostructures: A review

Many successful synthesis routes for producing different shapes of metallic nanostructures, including sphere, rod, cube, and hollow shapes, have been developed in the past few decades. Many applications using these nanostructures have been studied because the outstanding properties of the nanostructures are not exhibited by their bulk-state counterparts. This review paper reports some recent developments in clinical and biosensor applications. The first part focused on the synthesis methods of metallic nanostructures having various shapes along with their optical properties. The second and third part is an introduction of the gold nanoparticle assemblies and arrays, explaining the conjugation methods of metallic nanostructures with biological entities. The final part reviews on the recent bioanalytical applications using various shapes of metallic nanostructures.     

Recent Development on Controlled Synthesis of Metal Sulfides Hollow Nanostructures via Hard Template Engaged Strategy: A Mini‐Review

In this mini‐review, we highlighted the recent progresses in the controlled synthesis of metal sulfides hollow nanostructures via hard template technique. After a brief introduction about the formation mechanism of the inorganic hollow nanostructures via hard template technique, the discussions primarily focused on the emerging development of metal sulfides hollow nanostructures. Various synthetic strategies were summarized concerning the use of the hard template engaged strategies to fabricate various metal sulfides hollow nanostructures, such as hydrothermal method, solvothermal method, ion‐exchange, sulfidation or calcination etc. Finally, the perspectives and summaries have been presented to demonstrate that a facile synthetic technique would be widely used to fabricate metal sulfides hollow nanostructures with multi‐shells and components.     

Biomolecule-assisted synthesis and electrochemical hydrogen storage of porous spongelike Ni3S2 nanostructures grown directly on nickel foils

Nanothread-based porous spongelike Ni3S2 nanostructures were synthesized directly on Ni foil by using a simple biomolecule-assisted method. By varying the experimental parameters, other novel Ni3S2 nanostructures could also be fabricated on the nickel substrate. The electrochemical hydrogen-storage behavior of these novel porous Ni3S2 nanostructures was investigated as an example of the potential properties of such porous materials. The thread-based porous spongelike Ni3S2 could electrochemically charge and discharge with the high capacity of 380 mAh g(-1) (corresponding to 1.4 wt % hydrogen in single-walled nanotubes (SWNT)). A novel two-charging-plateaux phenomenon was observed in the synthesized porous spongelike Ni3S2 nanostructures, suggesting two independent steps in the charging process. We have demonstrated that the morphology of the synthesized Ni3S2 nanostructures had a noticeable influence on their electrochemical hydrogen-storage capacity. This is probably due to the size and density of the pores as well as the microcosmic morphology of different nickel sulfide nanostructures. These novel porous Ni3S2 nanostructures should find wide applications in hydrogen storage, high-energy batteries, luminescence, and catalytic fields. This facile, environmentally benign, and solution-phase biomolecule-assisted method can be potentially extended to the preparation of other metal sulfide nanostructures on metal substrates, such as Cu, Fe, Sn, and Pb foils.     

Self-assembly of ionic liquids-stabilized Pt nanoparticles into two-dimensional patterned nanostructures at the air-water interface

In this paper, we demonstrate the self-assembly of ionic liquids (ILs)-stabilized Pt nanoparticles into two-dimensional (2D) patterned nanostructures at the air-water interface under ambient conditions. Here, ILs are not used as solvents but as mediators by virtue of their pronounced self-organization ability in synthesis of self-assembled, highly organized hybrid Pt nanostructures. It is also found that the morphologies of the 2D patterned nanostructures are directly connected with the quantities of ILs. Due to the special structures of ILs-stabilized Pt nanoparticles, 2D patterned Pt nanostructures can be formed through the pi-pi stack interactions and hydrogen bonds. The resulting 2D patterned Pt nanostructures exhibit good electrocatalytic activity toward oxygen reduction.     

Stimulated emission in ZnO nanostructures: A time-resolved study

Stimulated emission was studied using time-integrated and time-resolved photoluminescence in ZnO comb, tetrapod, and rod nanostructures. All the measurements were performed on ensembles of the nanostructures. The nanostructures were fabricated by vapor deposition (combs, tetrapods) and hydrothermal methods (rods). While stimulated emission was detected in all of the nanostructures, significant differences in the behavior of the stimulated emission, as well as the lasing threshold power, were found for different morphologies. The differences in the time evolution of the lasing spectra were particularly pronounced. The observed differences in the stimulated emission spectra of the three types of nanostructures in both exciton-exciton scattering and electron-hole plasma regimes are discussed.     

Hydrothermal synthesis of momordica-like CuO nanostructures using egg white and their characterisation

In this paper, we report on the preparation of momordica-like CuO nanostructures by the hydrothermal method using freshly extracted egg white protein (ovalbumin) in an aqueous medium. These momordica-like CuO nanostructures were characterised by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). XRD patterns showed that these nanostructures had a polycrystalline nature with a monoclinic structure. FE-SEM images indicated that the momordica-like CuO nanostructures obtained at 180°C for 15 h were composed of CuO nanorods with a length of less than 100 nm and a width ranging from 30 nm to 50 nm. Finally, a possible growth mechanism for the momordica-like CuO nanostructures is proposed and discussed.     

Morphological control of synthesis and anomalous magnetic properties of 3-D branched Pt nanoparticles

Morphology-controllable platinum nanostructures could be obtained by modulating the growth kinetics in oleylamine. The nanostructures evolve from spherical particles to branched networks with decreasing reaction temperature, and the complexity of the branched-network nanostructures increases with the extended reaction period. Size-dependent magnetic properties and enhanced ferromagnetism in dodecanethiol-capped Pt branched nanostructures indicate that the permanent magnetic moments are probably introduced by broken symmetry and charge transfer because charge transfers more effectively from dodecanethiol than from oleylamine.     

Effects of quenching on the morphology and crystal structure of ZnO nanostructures

Zinc oxide nanostructures were prepared by a simple wet chemical procedure using zinc acetate and sodium hydroxide as precursors. The process was subjected to quenching treatment and the effect of the treatment on the formation of the nanostructures was studied using atomic force and scanning electron microscopies. The change in crystal structure of the nanostructures due to quenching was studied using an X-ray diffractometry that established that physical and structural properties of the nanostructures were largely influenced by the quenching treatment.     

Au-Pd alloy and core-shell nanostructures: one-pot coreduction preparation, formation mechanism, and electrochemical properties

It is a known fact that Pd-based bimetallic nanostructures possess unique properties and excellent catalytic performance. In this work, the Au-Pd alloy and core-shell nanostructures have been prepared by a simple one-pot hydrothermal coreduction route, and their formation process and mechanism are discussed in detail. A reducing capacity-induced controlled reducing mechanism is proposed for the formation process of Au-Pd bimetallic nanostructures. CTAB plays a key role in the formation of alloy Au-Pd nanostructures. When CTAB is absent, the products are typical core-shell nanostructures. Moreover, the as-prepared nanostructures exhibit excellent electrocatalytic ORR performance in alkaline media, especially for Au-Pd alloy nanostructures. The overpotential of oxygen reduction gets reduced significantly, and the peak potential is positive-shifted by 44 and 34 mV in comparison with the core-shell ones and Pd/C catalyst, respectively. Thus, the controllable preparation and excellent electrocatalytic properties will make them become a potentially cheaper Pd-based cathodic electrocatalyst for DAFCs in alkaline media.