Nanoseparation and concomitant purification of nanoparticles by ultracentrifugation in a nonhydroxylic organic density gradient has been demonstrated by separating several typical colloidal nanoparticles, including Au, Ag, and CdSe. Successful separation of Au nanowires from their spherical counterparts showed that colloidal particles can be separated not only by size but also morphology. In addition to extending the range of colloidal systems which can be separated and providing monodisperse samples that cannot be obtained by synthesis optimization alone, this method simplifies the postsynthesis treatment process and facilitates subsequent bulk assembly of the monodisperse colloids. Dissolution of organic polymers in the gradient medium both enhances the separation efficiency and also allows the direct fabrication of functional composite films with discrete monodisperse nanoparticles embedded inside.
Identifying the phase purity of CdS nanorods (NRs) is complicated by the serious overlap between the X-ray diffraction peaks of zinc blende and wurtzite phases as well as anisotropic growth, which might hide a mixed phase. Here we show that the density gradient ultracentrifugation rate separation method can be used to sort CdS NRs synthesized under nitrogen according to differences in particle size and morphology. Furthermore, it was found that the different sized NRs formed in a single batch synthesis had different phases: the thinner ones (<3.5 nm in diameter) were predominantly wurtzite phase, while the thicker ones (>5 nm in diameter) were mainly zinc blende phase. Dark-field transmission electron microscopy (TEM) and high-resolution TEM images indicated the presence of numerous stacking faults in the thick zinc blende rods, while the wurtzite thin rods were exclusively single crystals. As a result of the differences in phase and stacking faults, the NRs showed different photoluminescent properties. The development of an effective way of separating such NRs thus leads to further insight into the differences in phase, structure, and optical properties between individual colloidal particles synthesized in a single batch. A preliminary mathematical model of the separation process has been proposed.
Parameters such as solution concentrations and composition of the ambient atmosphere are known to be important in phase and morphology control in the solvothermal synthesis of CdS semiconductor nanorods (NRs), but a clear understanding of the underlying mechanisms involved is lacking. In this work, a series of experiments were performed to demonstrate that the key factor affecting the phase and morphology of CdS NRs is the amount of O(2) in the space above the reaction solution in the sealed vessel relative to the amount of precursors in solution: O(2)-depleted conditions resulted in more cubic phase CdS and thick polycrystalline NRs with an aspect ratio usually less than 3, which have small blue shifts in band-edge emission and little surface trap emission, while O(2)-rich conditions resulted in more hexagonal-phase CdS and slim single-crystal NRs, which have significantly blue shifted band-edge emission and relatively strong surface trap emission. Thus, increasing the amount of solution in the vessel, changing the ambient atmosphere from air to N(2), and increasing the reagent concentration all lower the molar ratio of O(2) to reagents and lead to more cubic phase and thicker NRs. The results indicate that the composition of the "empty" section of the reaction vessel plays as important a role as the composition of the liquid in determining the phase and morphology, something that has been overlooked in earlier work. A mechanism to explain the effect of oxygen on the nucleation and growth stages has been proposed on the basis of those results and further supported by shaking experiments and ZnS NR synthesis manipulation. The CdS NRs synthesized under different conditions showed obvious differences in photocatalytic activity, which indicated that controlling the synthetic process can lead to materials with tailored photocatalytic activity.
CdS nanorods have been sorted by length using a density gradient ultracentrifuge rate separation method. The fractions containing longer rods showed relatively stronger oxygen-related surface trap emission, while the shorter ones had dominant band-edge emission. These results suggest that the final length distribution of CdS nanorods is not a result of random nucleation and growth, but is related to the local synthesis conditions. Inspired by these findings, different synthesis environments (N 2 , air, and O 2 ) have been employed in order to tailor the length distribution. In addition to the rod length, the photoluminescence properties of CdS nanorods can also be manipulated. Increasing the oxygen partial pressure significantly changed the growth behavior of CdS nanorods by improving the anisotropic growth.
A novel highly sensitive electrochemiluminescence (ECL) detection protocol for mercury(II) ions was developed. Based on the strong and stable thyminethymine mismatches complexes coordination chemistry, mercury(II) ions can specifically bind to a designed DNA strand, leading to the release of the complimentary DNA strand. The released DNA strand was then captured by magnetic beads modified with specific DNA, and then through the formation of DNA‐linked luminol‐Au nanoparticles (NPs) superstructure, a specific ECL system for mercury(II) ions was developed. Using 3‐aminopropyl‐triethoxysilane as an effective enhancer, the ECL system can detect Hg2+ ion within a linear range from 2.0×10−10 mol L−1 to 2.0×10−8 M, with a detection limit as low as 1.05×10−10 M (3σ). Moreover, this ECL system is highly specific for Hg2+, without interference from other commonly coexisted metal ions, and it can be used for the analysis of real samples.
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