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.

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.

The ratio of Cd to Se (Cd/Se) within colloidal CdSe quantum dots (QDs) synthesized with 90% trioctylphosphine oxide (TOPO) as the coordinating solvent increases from 1.2:1 for QDs with radius R ≥ 3.3 nm to 6.5:1 for R = 1.9 nm, as measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The highest value of Cd/Se reported previously for CdSe QDs was 1.8:1. The dependence of Cd/Se on R fits a geometric model that describes the QDs as CdSe cores with Cd/Se = 1:1 encapsulated by a shell of Cd−organic complexes. Use of 99% TOPO as the coordinating solvent produces QDs with Cd/Se ≈ 1:1 for all values of R, and use of 99% TOPO “doped” with n-octylphosphonic acid (OPA), an impurity in 90% TOPO, produces QDs with values of Cd/Se up to 1.5:1. These results imply that Cd enrichment of the QDs is driven by tight-binding Cd2+−alkylphosphonate complexes that stabilize the interface between the polar CdSe core and the organic medium.

We report on responsive photoluminescent hybrid materials with quantum dots immobilized in organized manner fabricated by spin-assisted layer-by-layer assembly (SA LbL). The strongly interacting polyelectrolytes such as poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) serve for confining CdTe nanoparticles stabilized by thioglycolic acid, while a poly(methacrylic acid) (PMAA) hydrogel matrix presents an elastomeric network with pH-responsive properties. Quantum dot layers encapsulated in PSS-PAH bilayers are confined inside this hybrid hydrogel matrix. The system undergoes reversible changes in photoluminescent intensity in response to pH variations. Photoluminescent intensity of the hybrid matrix is suppressed in excess negative charge at high pH, but excess positive charge at low pH results in significant photoluminescence increase. Such hybrid quantum dot-containing hydrogel-LbL assemblies provide a way for a novel design of materials with precisely controlled structure and pH-triggered optical properties which might be developed into or pH-or chemical sensors.

The spontaneous self-assembly of II−IV stabilizer depleted nanoparticles (NPs) into nanowires (NWs) is a complex process that is only partially understood. This paper examines the mechanism governing changes in the growth pattern of CdTe NWs that are induced by the addition of dimethyl sulfoxide (DMSO) to the NW growth solution. We propose that, after the initial step of formation of NP pearl necklace assemblies, the assemblies recrystallize and subsequently grow into long NWs by Ostwald ripening. The addition of DMSO allows for improved control over the NW length and diameter. As the DMSO concentration in the NW growth solution is increased, the resulting NW length and diameter increase. When DMSO concentrations are raised above 70%, there is no NW formation, which is attributed to inhibition of the formation of pearl necklace assemblies. DMSO influence on NW morphology is attributed to its effect on the electrostatic interactions between the nanoparticles and mass exchange between the growing nuclei.