Colloidal superparticles from nanoparticle assembly.

Colloidal superparticles are size- and shape-controlled nanoparticle assemblies in the form of colloidal particles. Because these superparticles can exhibit physical and chemical properties different from both individual nanoparticles and their bulk assemblies, the development of superparticle synthesis has attracted significant research attention and is emerging as a new frontier in the field of nanotechnology. In this review, we discuss theoretical considerations on the nucleation and growth of colloidal superparticles. We then present recent progress in the synthesis and characterization of monodispersed colloidal superparticles, which are important for applications such as biomedical diagnosis, biological separation, and light emitting devices.

Formation of heterodimer nanocrystals: UO2/In2O3 and FePt/In2O3.

This Article reports a mechanistic study on the formation of colloidal UO(2)/In(2)O(3) and FePt/In(2)O(3) heterodimer nanocrystals. These dimer nanocrystals were synthesized via the growth of In(2)O(3) as the epitaxial material onto the seed nanocrystals of UO(2) or FePt. The resulting dimer nanocrystals were characterized using X-ray powder diffraction (XRD), energy dispersion spectroscopy, transmission electron microscopy (TEM), scanning transmission electron microscopy, and high-resolution TEM (HRTEM). The results from XRD and HRTEM clearly show that lattice strains exist in both of these dimer nanocrystals. Interestingly, the lattice of In(2)O(3) expands in UO(2)/In(2)O(3) dimers, whereas FePt/In(2)O(3) dimers exhibit compressed In(2)O(3) lattices. Using HRTEM and nanocrystal structure simulations, we have identified the crystallographic orientation of the attachment of the two segments in these two types of dimers. An unconventional Miller index was introduced to describe the crystallographic orientation of these heterodimer nanocrystals. On the basis of the results herein as well as those from other researchers, we propose an empirical law for the determination of the crystallographic attachment orientation in heterodimers: instead of growth on the facet of the seed nanocrystals where lattice mismatch is minimized, the growth of an epitaxial material often chooses the crystal facets where the first atomic monolayer of this material has the strongest affinity for the seed nanocrystals.

Gas-bubble effects on the formation of colloidal iron oxide nanocrystals.

This paper reports that gas bubbles can be used to tailor the kinetics of the nucleation and growth of inorganic-nanocrystals in a colloidal synthesis. We conducted a mechanistic study of the synthesis of colloidal iron oxide nanocrystals using gas bubbles generated by boiling solvents or artificial Ar bubbling. We identified that bubbling effects take place through absorbing local latent heat released from the exothermic reactions involved in the nucleation and growth of iron oxide nanocrystals. Our results show that gas bubbles display a stronger effect on the nucleation of iron oxide nanocrystals than on their growth. These results indicate that the nucleation and growth of iron oxide nanocrystals may rely on different types of chemical reactions between the iron-oleate decomposition products: the nucleation relies on the strongly exothermic, multiple-bond formation reactions, whereas the growth of iron oxide nanocrystals may primarily depend upon single-bond formation reactions. The identification of exothermic reactions is further consistent with our results in the synthesis of iron oxide nanocrystals with boiling solvents at reaction temperatures ranging from 290 to 365 °C, by which we determined the reaction enthalpy in the nucleation of iron oxide nanocrystals to be -142 ± 12 kJ/mol. Moreover, our results suggest that a prerequisite for effectively suppressing secondary nucleation in a colloidal synthesis is that the primary nucleation must produce a critical amount of nuclei, and this finding is important for a priori design of colloidal synthesis of monodispersed nanocrystals in general.

Carboxyl-functionalized superparamagnetic Fe3O4/poly(St-co-MPS)/SiO2 composite particles for rapid and sensitive immunoassay.

Superparamagnetic Fe3O4/poly(St-co-MPS)/SiO2 composite particles with the average size of 140 nm were functionalized with carboxyl group by emulsion polymerization. Functionalized particles with carboxyl contents of 13.6 and 136 micromol/mL were prepared by changing the amount of acrylic acid monomer used in the polymerization. After conjugation with human chorionic gonadotrophin (hCG) antibody, the particles were used to construct lateral flow immunoassays (LFIA) for the detection of hCG in solution. The quantitative analysis could be finished in 20 min by using the magnetic particles as labels. The detection limit of LFIA was determined to be 1 and 5 IU/L respectively for the magnetic particles with carboxyl contents of 13.6 and 136 micromol/mL.

Synthesis of water-soluble 2,2'-diphenyl-1-picrylhydrazyl nanoparticles: a new standard for electron paramagnetic resonance spectroscopy.

This communication reports a size-controlled synthesis of water-soluble 2,2'-diphenyl-1-picrylhydrazyl (DPPH) nanoparticles (NPs). These nanoparticles exhibit size-dependent absorption spectra and fast spin exchange-narrowed single-line EPR spectra. The linewidths of the EPR spectra of these water-soluble nanoparticles are approximately 1.5-1.8 G, which are equal or close to the narrowest line width (1.5 G) of the common DPPH standard in the form of water-insoluble microcrystals. In addition, these NPs are stable over a wide pH range of 3.0 to 10.0. These properties make these water-soluble DPPH NPs suitable for use as a new type of EPR standard, which is important for fundamental research and practical applications in fields such as the food industry and the life sciences. Furthermore, the DPPH NPs can potentially be used as a spin probe in biomedical studies.

Cylindrical superparticles from semiconductor nanorods.

In this communication, we report a synthesis of anisotropic colloidal superparticles (SPs) from CdSe/CdS semiconductor nanorods. These anisotropic SPs are cylindrical disks or stacked-disk arrays. We attribute the major driving forces controlling the SP shape to interparticle interactions between nanorods and solvophobic interactions between a superparticle and its surrounding solvent. According to their sizes (or volumes), the SPs adopt either single- or multilayered structures. In addition, these SPs exhibit linearly polarized emissions, demonstrating their potential role as useful components in devices such as polarized light-emitting diodes and electrooptical modulators.

Size-selective separation of DNA fragments by using lysine-functionalized silica particles.

In this work, a facile and efficient approach has been demonstrated for size-selective separation of DNA fragments by using lysine-functionalized silica particles. At a given pH, the environmental ionic strength can be utilized to alter the electrostatic interactions of lysine-functionalized silica particles with DNA fragments and in turn the DNA fragments on the silica particle surfaces, which exhibits a clear dependence on the DNA fragment sizes. By carefully adjusting the environmental pH and salt concentration, therefore, the use of the lysine-functionalized silica particles allows effective separation of binary and ternary DNA mixtures, for example, two different DNA fragments with sizes of 101 and 1073 bp, 101 and 745 bp, 101 and 408 bp, respectively, and three different DNA fragments with sizes of 101, 408 and 1073 bp.

Facile deposition of continuous gold shells on Tween-20 modified Fe3O4 superparticles.

A facile method to deposit continuous Au shells on Fe3O4 colloidal superparticles was developed by introducing Tween-20 as a surface modification agent to maintain the colloidal stability of the Fe3O4 superparticles and provide nucleation and growth sites for the Au shells. The Fe3O4-Au core-shell particles showed excellent chemical stability, superparamagnetic properties and efficient photothermal conversion performance under laser irradiation at 808 nm, and were expected to be a useful material for biodetection and cancer therapy.

Self-assembled colloidal superparticles from nanorods.

Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide-cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.