Space-tiled colloidal crystals from DNA-forced shape-complementary polyhedra pairing.

Generating space-filling arrangements of most discrete polyhedra nanostructures of the same shape is not possible. However, if the appropriate individual building blocks are selected (e.g., cubes), or multiple shapes of the appropriate dimensions are matched (e.g., octahedra and tetrahedra) and their pairing interactions are subsequently forced, space-filled architectures may be possible. With flexible molecular ligands (polyethylene glycol-modified DNA), the shape of a polyhedral nanoparticle can be deliberately altered and used to realize geometries that favor space tessellation. In this work, 10 new colloidal crystals were synthesized from DNA-modified nanocrystal building blocks that differed in shapes and sizes, designed to form space-filling architectures with micron-scale dimensions. The insights and capabilities provided by this new strategy substantially expand the scope of colloidal crystals possible and provide an expanded tool kit for researchers interested in designing metamaterials.

Arrays of Colloidal Single Crystals Engineered with DNA in Lithographically Defined Microwells.

Lithographically defined microwell templates are used to study DNA-guided colloidal crystal assembly parameters, including superlattice position, habit orientation, and size, in an effort to increase our understanding of the crystallization process. In addition to enabling the synthesis of arrays of individual superlattices in arbitrary predefined patterns, the technique allows one to study the growth pathways of the crystals via ex situ scanning electron microscopy. Importantly, a Volmer-Weber (VM) (island formation)-like growth mode is identified, which has been reproduced via simulations. Notably, both experiment and simulation reveal that the crystallites merge and reorient within the microwells that defined the crystal growth to form single-crystalline structures, an observation not common for VM pathways. The control afforded by this platform will facilitate efforts in constructing metamaterials from colloidal crystals as well as their integration into optical devices and applications.

Entropic formation of a thermodynamically stable colloidal quasicrystal with negligible phason strain.

Quasicrystals have been discovered in a variety of materials ranging from metals to polymers. Yet, why and how they form is incompletely understood. In situ transmission electron microscopy of alloy quasicrystal formation in metals suggests an error-and-repair mechanism, whereby quasiperiodic crystals grow imperfectly with phason strain present, and only perfect themselves later into a high-quality quasicrystal with negligible phason strain. The growth mechanism has not been investigated for other types of quasicrystals, such as dendrimeric, polymeric, or colloidal quasicrystals. Soft-matter quasicrystals typically result from entropic, rather than energetic, interactions, and are not usually grown (either in laboratories or in silico) into large-volume quasicrystals. Consequently, it is unknown whether soft-matter quasicrystals form with the high degree of structural quality found in metal alloy quasicrystals. Here, we investigate the entropically driven growth of colloidal dodecagonal quasicrystals (DQCs) via computer simulation of systems of hard tetrahedra, which are simple models for anisotropic colloidal particles that form a quasicrystal. Using a pattern recognition algorithm applied to particle trajectories during DQC growth, we analyze phason strain to follow the evolution of quasiperiodic order. As in alloys, we observe high structural quality; DQCs with low phason strain crystallize directly from the melt and only require minimal further reduction of phason strain. We also observe transformation from a denser approximant to the DQC via continuous phason strain relaxation. Our results demonstrate that soft-matter quasicrystals dominated by entropy can be thermodynamically stable and grown with high structural quality--just like their alloy quasicrystal counterparts.

Photonic Capsule Sensors with Built-In Colloidal Crystallites.

Technologies to monitor microenvironmental conditions and its spatial distribution are in high demand, yet remain unmet need. Herein, photonic microsensors are designed in a capsule format that can be injected, suspended, and implanted in any target volume. Colorimetric sensors are loaded in the core of microcapsules by assembling core-shell colloids into crystallites through the depletion attraction. The shells of the colloids are made of a temperature-responsive hydrogel, which enables the crystallites to rapidly and widely tune the structural color in response to a change in temperature while maintaining close-packed arrays. The spherical symmetry of the microcapsules renders them optically isotropic, i.e., displaying orientation-independent color. In addition, as a solid membrane is used to protect the delicate crystallites from external stresses, their high stability is assured. More importantly, each microcapsule reports the temperature of its microenvironment so that a suspension of capsules can provide information on the spatial distribution of the temperature.

Inertial-ordering-assisted droplet microfluidics for high-throughput single-cell RNA-sequencing.

Single-cell RNA-seq reveals the cellular heterogeneity inherent in the population of cells, which is very important in many clinical and research applications. Recent advances in droplet microfluidics have achieved the automatic isolation, lysis, and labeling of single cells in droplet compartments without complex instrumentation. However, barcoding errors occurring in the cell encapsulation process because of the multiple-beads-in-droplet and insufficient throughput because of the low concentration of beads for avoiding multiple-beads-in-a-droplet remain important challenges for precise and efficient expression profiling of single cells. In this study, we developed a new droplet-based microfluidic platform that significantly improved the throughput while reducing barcoding errors through deterministic encapsulation of inertially ordered beads. Highly concentrated beads containing oligonucleotide barcodes were spontaneously ordered in a spiral channel by an inertial effect, which were in turn encapsulated in droplets one-by-one, while cells were simultaneously encapsulated in the droplets. The deterministic encapsulation of beads resulted in a high fraction of single-bead-in-a-droplet and rare multiple-beads-in-a-droplet although the bead concentration increased to 1000 µl-1, which diminished barcoding errors and enabled accurate high-throughput barcoding. We successfully validated our device with single-cell RNA-seq. In addition, we found that multiple-beads-in-a-droplet, generated using a normal Drop-Seq device with a high concentration of beads, underestimated transcript numbers and overestimated cell numbers. This accurate high-throughput platform can expand the capability and practicality of Drop-Seq in single-cell analysis.

Reaction-Diffusion-Mediated Photolithography for Designing Pseudo-3D Microstructures.

Microstructures with 3D features provide advanced functionalities in many applications. Reaction-diffusion process has been employed in photolithography to produce pseudo-3D microstructures in a reproducible manner. In this work, the influences of various parameters on growth behavior of polymeric structures are investigated and the use of the reaction-diffusion-mediated photolithography (RDP) is expanded to a wide range of structural dimensions. In addition, how a lens effect alters the growth behavior of microstructures in conjunction with reaction-diffusion process is studied. For small separation between reaction sites in the array, ultraviolet (UV) exposure time is optimized along with the separation to avoid film or plateau formation. It is further proved that the RDP process is highly reproducible and applicable to various photocurable resins. In a demonstrative purpose, the use of microdomes created by the RDP process as microlens arrays is shown. The RDP process enables the production of pseudo-3D microstructures even with collimated UV light in the absence of complex optical setups, thereby potentially serving as a useful means to create micropatterns and particles with unique structural features.