"Cloud" assemblies: quantum dots form electrostatically bound dynamic nebulae around large gold nanoparticles.

Dynamic self-assembled structures of nanoparticles can be produced using predominantly electrostatic interactions. Such assemblies were made from large, positively charged Au metal nanoparticles surrounded by an electrostatically bound cloud of smaller, negatively charged CdSe/ZnS or CdTe quantum dots. At low concentrations they are topologically similar to double electric layers of ions and corona-like assemblies linked by polymer chains. They can also be compared to the topological arrangement of some planetary systems in space. The great advantages of the cloud assemblies are (1) their highly dynamic nature compared to more rigid covalently bound assemblies, (2) simplicity of preparation, and (3) exceptional versatility in components and resulting optical properties. Photoluminescence intensity enhancement originating from quantum resonance between excitons and plasmons was observed for CdSe/ZnS quantum dots, although CdTe dots displayed emission quenching. To evaluate more attentively their dynamic behavior, emission data were collected for the cloud-assemblies with different ratios of the components and ionic strengths of the media. The emission of the system passes through a maximum for 80 QDs ∶ 1 Au NP as determined by the structure of the assemblies and light absorption conditions. Ionic strength dependence of luminescence intensity contradicts the predictions based on the Gouy-Chapman theory and osmotic pressure at high ionic strengths due to formation of larger chaotic colloidally stable assemblies. "Cloud" assemblies made from different nanoscale components can be used both for elucidation of most fundamental aspects of nanoparticle interactions, as well as for practical purposes in sensing and biology.

Light-controlled self-assembly of semiconductor nanoparticles into twisted ribbons.

The collective properties of nanoparticles manifest in their ability to self-organize into complex microscale structures. Slow oxidation of tellurium ions in cadmium telluride (CdTe) nanoparticles results in the assembly of 1- to 4-micrometer-long flat ribbons made of several layers of individual cadmium sulfide (CdS)/CdTe nanocrystals. Twisting of the ribbons with an equal distribution of left and right helices was induced by illumination with visible light. The pitch lengths (250 to 1500 nanometers) varied with illumination dose, and the twisting was associated with the relief of mechanical shear stress in assembled ribbons caused by photooxidation of CdS. Unusual shapes of multiparticle assemblies, such as ellipsoidal clouds, dog-bone agglomerates, and ribbon bunches, were observed as intermediate stages. Computer simulations revealed that the balance between attraction and electrostatic repulsion determines the resulting geometry and dimensionality of the nanoparticle assemblies.

In vitro toxicity testing of nanoparticles in 3D cell culture.

Common 2D cell cultures do not adequately represent the functions of 3D tissues that have extensive cell-cell and cell-matrix interactions, as well as markedly different diffusion/transport conditions. Hence, testing cytotoxicity in 2D cultures may not accurately reflect the actual toxicity of nanoparticles (NPs) and other nanostructures in the body. To obtain more adequate and detailed information about NP-tissue interactions, we here introduce a 3D-spheroid-culture-based NP toxicology testing system. Hydrogel inverted colloidal crystal (ICC) scaffolds are used to create a physiologically relevant and standardized 3D liver tissue spheroid model for in vitro assay application. Toxicity of CdTe and Au NPs are tested in both 2D and 3D spheroid cultures. The results reveal that NP toxic effects are significantly reduced in the spheroid culture when compared to the 2D culture data. Tissue-like morphology and phenotypic change are identified to be the major factors in diminishing toxicity. Acting as an intermediate stage bridging in vitro 2D and in vivo, our in vitro 3D cell-culture model would extend current cellular level cytotoxicity to the tissue level, thereby improving the predictive power of in vitro NP toxicology.