Hollow, Porous Nanocubes Show Promise for Catalysis, Biomedical, and Optical Sensing Applications

Researchers at the Center for Functional Nanomaterials (CFN)—a U.S. Department of Energy Office of Science User Facility at Brookhaven National Laboratory—have swathed a different kind of box this holiday season.

With the help of a one-step chemical synthesis technique, the researchers created hollow metallic nanosized boxes whose corners contain cube-shaped pores. The team showed how these “nanowrappers” could be used for transporting and releasing nanoparticles coated with DNA in a controlled manner. The study has been reported in a paper published in ACS Central Science, a journal of the American Chemical Society (ACS), on December 12^th, 2018.

Imagine you have a box but you can only use the outside and not the inside. This is how we’ve been dealing with nanoparticles. Most nanoparticle assembly or synthesis methods produce solid nanostructures. We need methods to engineer the internal space of these structures.

Oleg Gang, Study Co-Author and Lead, Soft and Bio Nanomaterials Group, CFN

“Compared to their solid counterparts, hollow nanostructures have different optical and chemical properties that we would like to use for biomedical, sensing, and catalytic applications,” added Fang Lu, corresponding author and a scientist in Gang’s group. “In addition, we can introduce surface openings in the hollow structures where materials such as drugs, biological molecules, and even nanoparticles can enter and exit, depending on the surrounding environment.”

Although artificial approaches have been created for producing hollow nanostructures with surface pores, the location, shape, and size of these pores cannot be typically controlled in a suitable way. Arbitrarily distributed across the surface, the pores result in a Swiss-cheese-like structure. It is essential to have a high level of control over the surface openings so that nanostructures can be used in practical applications—for instance, for loading and releasing nanocargo.

In this analysis, the researchers revealed a novel pathway through which gold-silver alloy nanowrappers with cube-shaped corner holes can be chemically sculptured from solid nanocube particles. A chemical reaction called nanoscale galvanic replacement was used by the team. At the time of this reaction, gold ions in an aqueous solution replace the atoms present in a silver nanocube at room temperature. The researchers added a molecule—surface-capping agent or surfactant—to the solution to guide the leaching of silver as well as the deposition of gold on certain crystalline facets.

“The atoms on the faces of the cube are arranged differently from those in the corners, and thus different atomic planes are exposed, so the galvanic reaction may not proceed the same way in both areas,” Lu explained. “The surfactant we chose binds to the silver surface just enough—not too strongly or weakly—so that gold and silver can interact. Additionally, the absorption of surfactant is relatively weak on the silver cube’s corners, so the reaction is most active here. The silver gets ‘eaten’ away from its edges, resulting in the formation of corner holes, while gold gets deposited on the rest of the surface to create a gold and silver shell.”

At the CFN, the researchers used electron microscopes to capture the chemical and structural composition changes of the entire structure at the atomic level in 2D and at the nanoscale in 3D as the reaction continued for more than three hours. It was confirmed by the 2D electron microscope images with energy-dispersive X-ray spectroscopy (EDX) elemental mapping that the cubes are not only hollow but also contain a gold-silver alloy. The 3D images, thus achieved through electron tomography, showed that these hollow cubes include huge cube-shaped holes at the corners.

“In electron tomography, 2D images collected at different angles are combined to reconstruct an image of an object in 3D,” Gang stated. “The technique is similar to a CT [computerized tomography] scan used to image internal body structures, but it is carried out on a much smaller size scale and uses electrons instead of X-rays.”

The conversion of nanocubes to nanowrappers was also confirmed via spectroscopy experiments that captured optical changes. The spectra demonstrated that the nanowrappers’ optical absorption can be adjusted based on the reaction time. Infrared light is absorbed by the nanowrappers at their final state.

“The absorption spectrum showed a peak at 1250 nanometers, one of the longest wavelengths reported for nanoscale gold or silver,” Gang stated. “Typically, gold and silver nanostructures absorb visible light. However, for various applications, we would like those particles to absorb infrared light—for example, in biomedical applications such as phototherapy.”

With the help of the synthesized nanowrappers, the investigators subsequently showed how the DNA-coated, spherical gold nanoparticles of a suitable size can possibly be loaded into and discharged from the corner openings by altering the salt concentration in the solution. In response to decreasing or increasing concentrations of a positively charged ion like salt, the negatively charged DNA alters its configuration. DNA is negatively charged due to the presence of oxygen atoms in its phosphate backbone. DNA chains, in high salt concentrations, contract because the salt ions tend to reduce their repulsion. DNA chains, in low salt concentrations, can stretch because their repulsive forces separate them apart.

Contraction of the DNA strands causes the nanoparticles to become so small that they can fit into the openings and penetrate the hollow cavity. Subsequently, the nanoparticles can be locked inside the nanowrapper by reducing the concentration of salt. The DNA strands stretch at this lower concentration and thus make the nanoparticles extremely large to pass through the pores. The nanoparticles can exit the structure via a reverse process of decreasing and increasing the concentration of salt.

“Our electron microscopy and optical spectroscopy studies confirmed that the nanowrappers can be used to load and release nanoscale components,” Lu stated. “In principle, they could be used to release optically or chemically active nanoparticles in particular environments, potentially by changing other parameters such as pH or temperature.”

In the future, the researchers are interested in organizing the nanowrappers into larger scale architectures, so that they can apply their technique to other bimetallic systems as well, and also compare the nanowrappers’ external and internal catalytic activity.

We did not expect to see such regular, well-defined holes. Usually, this level of control is quite difficult to achieve for nanoscale objects. Thus, our discovery of this new pathway of nanoscale structure formation is very exciting. The ability to engineer nano-objects with a high level of control is important not only to understanding why certain processes are happening but also to constructing targeted nanostructures for various applications, from nanomedicine and optics to smart materials and catalysis. Our new synthesis method opens up unique opportunities in these areas.

Oleg Gang, Study Co-Author and Lead, Soft and Bio Nanomaterials Group, Center for Functional Nanomaterials

This work was made possible by the world-class expertise in nanomaterial synthesis and capabilities that exist at the CFN. In particular, the CFN has a leading program in the synthesis of new materials by assembly of nanoscale components, and state-of-the-art electron microscopy and optical spectroscopy capabilities for studying the 3D structure of these materials and their interaction with light. All of these characterization capabilities are available to the nanoscience research community through the CFN user program. We look forward to seeing the advances in nano-assembly that emerge as scientists across academia, industry, and government make use of the capabilities in their research.

Charles Black, Director, CFN