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Cationic naked nanocrystals (NCs) are useful building units for assembling hierarchical mesostructured materials. Until now, their preparation required strongly electrophilic reagents that irreversibly sever bonds between native organic ligands and the NC surface. Colloidal instabilities can occur during ligand stripping if exposed metal cations desorb from the surface. We hypothesized that cation desorption could be avoided were we able to stabilize the surface during ligand stripping via ion pairing. We were successful in this regard by carrying out ligand stripping under equilibrium control with Lewis acid-base adducts of BF3. To better understand the microscopic processes involved, we studied the reaction pathway in detail using in situ NMR experiments and electrospray ionization mass spectrometry. As predicted, we found that cationic NC surfaces are transiently stabilized post-stripping by physisorbed anionic species that arise from the reaction of BF3 with native ligands. This stabilization allows polar dispersants to reach the NC surface before cation desorption can occur. The mechanistic insights gained in this work provide a much-needed framework for understanding the interplay between NC surface chemistry and colloidal stability. These insights enabled the preparation of stable naked NC inks of desorption-susceptible NC compositions such as PbSe, which were easily assembled into new mesostructured films and polymer-nanocrystal composites with wide-ranging technological applications.

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.

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.

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.

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.