Symmetry-Breaking Dendrimer Synthons in Colloidal Crystal Engineering with DNA.

Breaking symmetry in colloidal crystals is challenging due to the inherent chemical and structural isotropy of many nanoscale building blocks. If a non-particle component could be used to anisotropically encode such building blocks with orthogonal recognition properties, one could expand the scope of structural and compositional possibilities of colloidal crystals beyond what is thus far possible with purely particle-based systems. Herein, we report the synthesis and characterization of novel DNA dendrimers that function as symmetry-breaking synthons, capable of programming anisotropic and orthogonal interactions within colloidal crystals. When the DNA dendrimers have identical sticky ends, they hybridize with DNA-functionalized nanoparticles to yield three distinct colloidal crystals, dictated by dendrimer size, including a structure not previously reported in the field of colloidal crystal engineering, Si2Sr. When used as symmetry-breaking synthons (when the sticky ends deliberately consist of orthogonal sequences), the synthesis of binary and ternary colloidal alloys with structures that can only be realized through directional interactions is possible. Furthermore, by modulating the extent of shape anisotropy within the DNA dendrimers, the local distribution of the nanoparticles within the crystals can be directed.

Programming "Atomic Substitution" in Alloy Colloidal Crystals Using DNA.

Although examples of colloidal crystal analogues to metal alloys have been reported, general routes for preparing 3D analogues to random substitutional alloys do not exist. Here, we use the programmability of DNA (length and sequence) to match nanoparticle component sizes, define parent lattice symmetry and substitutional order, and achieve faceted crystal habits. We synthesized substitutional alloy colloidal crystals with either ordered or random arrangements of two components (Au and Fe3O4 nanoparticles) within an otherwise identical parent lattice and crystal habit, confirmed via scanning electron microscopy and small-angle X-ray scattering. Energy dispersive X-ray spectroscopy reveals information regarding composition and local order, while the magnetic properties of Fe3O4 nanoparticles can direct different structural outcomes for different alloys in an applied magnetic field. This work constitutes a platform for independently defining substitution within multicomponent colloidal crystals, a capability that will expand the scope of functional materials that can be realized through programmable assembly.

Understanding Nanoparticle Toxicity Mechanisms To Inform Redesign Strategies To Reduce Environmental Impact.

There has been a surge of consumer products that incorporate nanoparticles, which are used to improve or impart new functionalities to the products based on their unique physicochemical properties. With such an increase in products containing nanomaterials, there is a need to understand their potential impacts on the environment. This is often done using various biological models that are abundant in the different environmental compartments where the nanomaterials may end up after use. Beyond studying whether nanomaterials simply kill an organism, the molecular mechanisms by which nanoparticles exhibit toxicity have been extensively studied. Some of the main mechanisms include (1) direct nanoparticle association with an organism's cell surface, where the membrane can be damaged or initiate internal signaling pathways that damage the cell, (2) dissolution of the material, releasing toxic ions that impact the organism, generally through impairing important enzyme functions or through direct interaction with a cell's DNA, and (3) the generation of reactive oxygen species and subsequent oxidative stress on an organism, which can also damage important enzymes or an organism's genetic material. This Account reviews these toxicity mechanisms, presenting examples for each with different types of nanomaterials. Understanding the mechanism of nanoparticle toxicity will inform efforts to redesign nanoparticles with reduced environmental impact. The redesign strategies will need to be chosen based on the major mode of toxicity, but also considering what changes can be made to the nanomaterial without impacting its ability to perform in its intended application. To reduce interactions with the cell surface, nanomaterials can be designed to have a negative surface charge, use ligands such as polyethylene glycol that reduce protein binding, or have a morphology that discourages binding with a cell surface. To reduce the nanoparticle dissolution to toxic ions, the toxic species can be replaced with less toxic elements that have similar properties, the nanoparticle can be capped with a shell material, the morphology of the nanoparticle can be chosen to minimize surface area and thus minimize dissolution, or a chelating agent can be co-introduced or functionalized onto the nanomaterial's surface. To reduce the production of reactive oxygen species, the band gap of the material can be tuned either by using different elements or by doping, a shell layer can be added to inhibit direct contact with the core, or antioxidant molecules can be tethered to the nanoparticle surface. When redesigning nanoparticles, it will be important to test that the redesign strategy actually reduces toxicity to organisms from relevant environmental compartments. It is also necessary to confirm that the nanomaterial still demonstrates the critical physicochemical properties that inspired its inclusion in a product or device.

Programming Nucleation and Growth in Colloidal Crystals Using DNA.

Colloidal crystal engineering with DNA has advanced beyond controlling the lattice symmetry and parameters of ordered crystals to now tuning crystal habit and size. However, the predominately used slow-cooling procedure that enables faceted crystal habits also limits control over crystal size and uniformity because nucleation and growth cannot be separated. Here, we explore how DNA sequence design can be used to deliberately separate nucleation and growth in a given crystallization process. Specifically, two batches of complementary particles are created with one batch exhibiting perfectly complementary base pairs while the other has a strategically introduced mismatch. This design enables the weaker binding "growth" particles to participate in heterogeneous growth on the nucleates formed from the stronger binding "seed" particles, effectively eliminating secondary nucleation pathways. By eliminating secondary nucleation events, this approach improves crystal uniformity, as measured by polydispersity (from PDI = 0.201 to 0.091). By using this approach with two different particle cores (gold and silver), we show how core-shell colloidal crystals can be synthesized in a one-pot fashion. This work shows how tuning DNA interaction strength can profoundly impact crystal size, uniformity, and structure, parameters central to using such materials as device components.

Using an environmentally-relevant panel of Gram-negative bacteria to assess the toxicity of polyallylamine hydrochloride-wrapped gold nanoparticles.

We aim to establish the effect of environmental diversity in evaluating nanotoxicity to bacteria. We assessed the toxicity of 4 nm polyallylamine hydrochloride-wrapped gold nanoparticles to a panel of bacteria from diverse environmental niches. The bacteria experienced a range of toxicities as evidenced by the different minimum bactericidal concentrations determined; the sensitivities of the bacteria was A. vinelandii = P. aeruginosa > S. oneidensis MR-4 > A. baylyi > S. oneidensis MR-1. Interactions between gold nanoparticles and molecular components of the cell wall were investigated by TEM, flow cytometry, and computational modeling. Binding results showed a general trend that bacteria with smooth LPS bind more PAH AuNPs than bacteria with rough LPS. Computational models reveal that PAH migrates to phosphate groups in the core of the LPS structure. Overall, our results demonstrate that simple interactions between nanoparticles and the bacterial cell wall cannot fully account for observed trends in toxicity, which points to the importance of establishing more comprehensive approaches for modeling environmental nanotoxicity.