Self-Assembly of Colloidal Crystals from DNA Coated Colloids
Creators: name: Hensley, Alexander
Publisher: name: Brandeis University
Year: 2022
Subjects: colloids
micro-fluidics
nucleation
self-assembly
DNA
Photonics

description: DNA-coated colloids are in a constantly advancing field of programmable self-assembly that promises to allow researchers to assemble structures out of components that are neither on the macroscopic scale, nor on the atomic scale: a region that is surprisingly difficult to work in. Colloidal crystals have been one of the first structures people researching these particles have tried to assemble. This is no surprise as these particles have been described as ``programmable atom equivalents" and crystallization is one of the defining phenomena of the atomic world. However, colloidal crystals have an important distinction in that they can be made up of particles of a similar size to light which allows researchers to envision self-assembling materials that have arbitrary photonic properties by exploiting the incredible flexibility a DNA-based system provides. In this work we studied the self-assembly of colloidal crystals made from these particles and arrived at conclusions about how they nucleate and grow and what has been missing in prior attempts to assemble large crystals.
description: We perform a variety studies of the self-assembly of DNA-coated particles into colloidal crystals and find that a modified classical nucleation theory that includes the rate at which loosely bound particles on a pre-critical nuclei surface roll into a crystalline binding spot quantitatively and accurately describes the nucleation rate of these crystals in a range of conditions. We make an emulsion of monodisperse nanoliter scale droplets filled with DNA-coated particles and precisely measure the nucleation rate of crystals within the droplets as a function of temperature and particle concentration. We also measure the rate of growth and the equilibrium concentration of the crystal phase by measuring the gas density of each droplet over the course of the experiment. We then fit the equilibrium concentration with respect to system temperature to an exponential which we use to redefine the droplet temperature and concentration as a degree of supersaturation. We find the nucleation rate increases with the supersaturation in accordance with classical nucleation theory, and with a prefactor that scales linearly with particle concentration which is an unexpected deviation from current theory and is a unique feature of DNA-coated colloids which mediate their interaction through several transient DNA bonds. This leads to a timescale in which particles that impinge on a crystal surface must diffuse along that surface via the constant severing and reforming of DNA bonds until the particle is in a crystalline position. After this initial stage of nucleation and growth we find that the crystal grows in a deterministic and diffusion limited way.
description: We then show that particles in nanoliter droplets are excellent incubators for single crystals with sizes that can be precisely defined by changing the number of particles within the droplets, allowing for near-digital control of the final crystal size. We find that once a crystal begins growing in a droplet that the nucleation rate within the gas phase of the droplet decreases in accordance to our nucleation and growth model. We define a parameter which represents the timescale it takes for a growing crystal to sufficiently deplete the gas phase enough to suppress further nucleation which leads to a way to compare the rates of crystal nucleation and crystal growth in order to engineer a system that preferentially self-assembles single crystals. With this advanced understanding of all phases of the nucleation and growth of DNA-coated particles, we develop a protocol to more easily self-assemble single crystals which may find use in fields such as optical photonics and medicine. Namely, we find that by using a slow staircase temperature ramp protocol as opposed to a single isothermal step, we can much more easily self-assemble droplets with single crystals without having to worry about the extreme precision necessary to grow single crystals normally and we demonstrate that this protocol is actually near optimal given the constraints of our system.
description: Despite the promise of droplets in making monodisperse crystals, we find that this system is fundamentally unable to grow a high yield single crystals beyond a certain size as the region of temperature where the growth rate and the nucleation rate are comparable is too narrow when the droplets are large, and practical solutions like decreasing the temperature ramp rate or increasing the droplet particle fraction are not feasible to the degree they would be required. We get around these issues by developing a novel two-step protocol that involves in the first step using small droplets to make tiny crystals to act as seeds for further growth. A small number of these seeds are then removed from the emulsion and added to a bulk system of new particles that has a melting temperature a couple degrees below that of the seeds, allowing the seed crystals to be stable while the bulk is in its gas phase. We slowly cool this system down until the seeds begin to grow. This occurs at a supersaturation where further nucleation from the bulk is incredibly unlikely, thus preserving the number of crystals in the system from start to finish, removing the usual difficulties of assembling monodisperse single crystals in the bulk.
description: We find that the growth of these crystals are diffusion limited and fully predictable as long as the seeds are not within a few crystal diameters of each other as they will begin to compete for the particles in the gas phase. This second growth stage is theoretically unbounded, with our method being able to produce the largest DNA-coated crystal to date at 0.3 mm in length containing 30,000,000 particles and being visible to the naked eye.
description: We finally demonstrate that these crystals have photonic properties by imaging them in reflection and cross polarized transmission. We find that in reflection, crystals made from different sized particles tend to shine different colors. And in transmission we find a variety of vibrant colors that depend on the orientation of the crystal with respect to the polarizer, the presented face of the crystal, and the size of the crystal. We show that the vibrancy of the transmitted color of the DNA-coated crystals increases with crystal thickness, noting that due to our understanding of how the crystals grow in this second step we would be able to determine a precise protocol to make large crystals of a given thickness for its desired photonic properties.
description: This research promises to make the self-assembly of colloidal crystals much more achievable and allow for the assembly of structures of larger size than previously possible, opening a pathway towards making functional photonic devices with programmed photonic properties which is the ultimate goal of DNA-coated particle colloidal crystallization.