Traffic signals are one of the few lights we can't afford to turn off to save energy. Advances in materials science have made light-emitting diodes (LEDs) more cost effective―they have longer lifespans and draw less electricity―and many municipalities have replaced their incandescent traffic lights with LEDs. However, materials scientists continue pushing the envelope to make improvements to existing optical and electronic technologies. One avenue for continued improvement uses nanoparticles as building blocks for alloy colloidal crystals. Recent research by a team conducting experiments at the U.S. Department of Energy’s Advanced Photon Source (APS) demonstrates a technique based on colloidal crystal engineering with DNA that synthesizes three-dimensional finished crystals composed of DNA-modified nanoparticles in both ordered and random arrangements. The researchers' results were published in the journal Nanoletters.
Nanoparticles―particles whose diameter is between 1 and 100 nanometers―can be used as the building blocks for colloidal crystals. These crystals have a repeating lattice structure that incorporates one or more types of nanoparticles. A single-component colloidal crystal includes only one type of nanoparticle, while a multi-component includes multiple types of nanoparticles. In this research effort, the team used gold and iron (II,III) oxide (Fe3O4) nanoparticles in both single-component and multi-component colloidal crystals; some crystals contained only gold nanoparticles, some only iron, and some both types of nanoparticles. The arrangement of the nanoparticles within the multi-component colloidal crystal gives it its useful optical properties.
The ideal process for synthesizing multi-component colloidal crystals would produce three-dimensional crystals from components whose arrangement was programmable. However, creating a process with these traits is challenging. First, most existing processes generate two-dimensional crystals. These lack the optical properties of three-dimensional crystals (such as specific light propagation effects), which are necessary for innovative applications. Second, the success of a programmable technique depends on two characteristics: nanoparticles of similar size, and well-understood chemical interactions between the molecules, which link the nanoparticles together in the crystal.
To overcome these challenges, the team chose DNA to program the synthesis of the crystals. This choice results in a three-dimensional crystal, and by utilizing DNA, offers methods for adjusting the nanoparticles' sizes using well-understood chemical interactions between the DNA nucleotides. In the team's DNA programming technique, they first coat the nanoparticles with a DNA shell. They can then equalize the sizes of the nanoparticles by using different lengths of DNA when they assemble the nanoparticles into a lattice. The ability to adjust nanoparticle size is one of the main advantages of engineering with DNA.
The team began by coating the nanoparticles with two types of DNA shells, denoted A and B, and linking the shells with lengths of DNA. To assess their synthesis process, the team first created single-component crystals: gold nanoparticles in both the A-type and B-type DNA shells and iron (II,III) oxide nanoparticles in both the A-type and B-type DNA shells. Their observations of these synthesized crystals using scanning electron microscopy and small-angle x-ray scattering (SAXS) at the DuPont-Northwestern-Dow Collective Access Team insertion device beamline 5-ID at Sector 5 of the APS showed both resulting crystals had a body-centered cubic lattice in a rhombic dodecahedral habit. The APS is an Office of Science user facility at Argonne National Laboratory.
The team then synthesized multi-component crystals with both ordered and random arrangements of the nanoparticles. In the ordered cases, the gold nanoparticles were coated with A-type DNA shells and the iron (II,III) oxide with B-type DNA shells. In the random cases, half of each type of nanoparticle were coated with A-type DNA shells and half with B-type DNA shells. The team examined the resulting crystals using SAXS as well as energy dispersive x-ray spectroscopy. They observed that both crystals were also a body-centered cubic lattice in a rhombic dodecahedral habit and that the nanoparticles were distributed throughout the entire crystal rather than segregated by type of nanoparticle.
To test that the ratio of types of nanoparticles within the synthesized crystal could be dictated by the ratio of nanoparticles in the crystallizing mixtures, the team synthesized random crystals with varied nanoparticle ratios. Observations of the results showed that the ratio of nanoparticles within the finished crystals were identical to those within the initial mixture. Fig. 1 shows both schematic and electron microscopy imagery of the resulting crystals.
Additionally, the team investigated the effects of a magnetic field on the resulting crystals. They synthesized both the random and ordered crystals within a 3800-Gauss magnetic field. The starting materials used to make the ordered alloys produced a crystal of segmented rods, while the starting materials used to make the random alloys produced the rhombic dodecahedron habit seen previously. Energy dispersive x-ray spectroscopy of the random crystals also showed a core-shell structure, which previous researchers had only achieved using more lengthy synthesis techniques.
The team's use of DNA to successfully synthesize high-quality, three-dimensional crystals with ordered or random arrangements of its nanoparticles components is an important demonstration not only of the applicability of colloidal crystal engineering with DNA to create substitutional alloy colloidal crystals, but also of the continued enhancement of materials for use in optical and electronic technologies. – Mary Alexandra Agner
See: Kaitlin M. Landy, Kyle J. Gibson, Zachary J. Urbach, Sarah S. Park, Eric W. Roth, Steven Weigand, and Chad A. Mirkin*, “Programming 'Atomic Substitution' in Alloy Colloidal Crystals Using DNA,” Nano Lett. 22, 280 (2022). DOI: 10.1021/acs.nanolett.1c03742
Author affiliation: Northwestern University
Correspondence: * firstname.lastname@example.org
This material is based upon work supported by the Air Force Office of Scientific Research Award FA9550-17-1-0348 (DNA synthesis and nanoparticle functionalization, fundamental colloidal crystal assembly) and the Center for Bio-Inspired Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE) Office of Science-Basic Energy Sciences, Award DE-SC0000989 (colloidal crystal assembly under magnetic field). Z.J.U. gratefully acknowledges support from the National Defense Science and Engineering Graduate Fellowship. The DuPont- Northwestern-Dow Collaborative Access Team located at sector 5 of the Advanced Photon Source (DOE DE-AC02- 06CH11357). This work made use of the Electron Probe Instrumentation Center and BioCryo facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental Resource (NSF ECCS-2025633), the International Institute for Nanotechnology, and the Northwestern University Materials Research Science and Engineering Center (NSF DMR-1720139). Metal analysis was performed at the Northwestern University Quantitative Bio-element Imaging Center. The DuPont-Northwestern-Dow Collaborative Access Team is supported by Northwestern University, The Dow Chemical Company, and DuPont de Nemours, Inc. This research used resources from the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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