Hollow nanostructures have the potential to load, carry, and release molecules and nanoparticles, so long as they can be synthesized with the appropriate surface features and functionalities. Such technology could form the basis of smart materials with stimulus-dependent properties or reconfigurable structures. With this goal in mind, a team of researchers from the United States and China has developed a single-step process for synthesizing hollow nanocubes with well-defined surface openings, as reported in the ACS Central Science.

“Synthesis strategies have been developed to produce hollow nanostructures with surface pores, but typically the size, shape, and location of these pores cannot be well-controlled,” says Brookhaven National Laboratory’s (BNL) Fang Lu, who led the research. Most current strategies for producing hollow nanostructures with surface openings are multi-step processes that yield small, irregular pores unsuitable for nanoparticle entry.

Drawing from previous work with the surfactant cetylpyridinium chloride (CPC), Lu and colleagues at BNL, the Chinese Academy of Sciences, and Columbia University devised a single-step chemical process for creating what they call nanowrappers, hollow nanocubes with large, well-defined cubic openings at the corners. Each nanowrapper consists of a single-crystalline AuAg alloy. The pores are about 31 nm in edge length, large enough for some nanoparticles to pass through.

The nanowrappers were created from solid silver nanocubes about 100 nm on each side, with sharp corners. First, the researchers dispersed the nanocubes in a solution of CPC, which acted as a surface capping agent that covered the surface of the nanocubes and kept them from agglomerating. While the solution was at room temperature, the researchers mixed in chloroauric acid (HAuCl₄). Over the next three hours, a galvanic replacement reaction took place in which one gold(0) atom was generated at the expense of every three silver(0) atoms.

A time-dependent, three-dimensional tomography analysis by Huolin Xin, who is now at the University of California, Irvine, showed that as the nanocubes oxidized, their sharp corners were truncated and gave way to small pores that grew over time. Simultaneously, the inside of each cube was hollowed out through oxidation. The end result was a set of hollow nanowrappers with high uniformity, allowing for self-assembly into a large-area well-ordered superlattice.

The key to this single-step process was the use of CPC as a surface capping agent. CPC absorbs weakly on the sharp corners of a nanocube. This subjects the corners to greater oxidative etching and designates them at preferred pore sites. “We selected CPC as the surfactant to allow both the corner truncation and hollowing-out reactions to happen simultaneously, due to the CPC’s unique surface-absorption preference,” explains Lu.

This work suggests that the surface pores on hollow nanostructures can be tuned by capping agents with different surface absorption preferences. In addition, the researchers say, the work provides insight into the more general process of redox in the presence of surface capping agents—insight that “can be extended to a wide range of materials and reactions,” according to Lu. 

Through a collaboration with Oleg Gang from Columbia University, the researchers demonstrated that the nanowrappers could hold and release cargo. They suspended the nanowrappers and spherical, DNA-coated gold nanoparticles in a phosphate buffer saline (PBS) solution with low ionic strength. They then increased the ionic strength by adding sodium chloride. This change reduced the electrostatic interactions within the DNA shells, causing them to adopt tighter arrangements and decreasing the effective diameter of the DNA-coated particles from approximately 34.7 nm to 25.6 nm. As a result, the particles—initially too large to fit through the nanowrapper pores (31 nm)—could be loaded into the nanowrappers. 

After allowing the particles to diffuse into the nanowrappers for several hours, the researchers decreased the ionic strength of the solution, washed away the unloaded nanoparticles, and redispersed the nanowrappers in a buffer with low ionic strength. Suspended in the low-ionic-strength solution, the effective size of the loaded particles increased beyond the pore size, which kept the particles locked inside the nanowrappers. To release the particles, the researchers increased the ionic strength of the solution again and shook the mixture for several hours.

Moving forward, the research team plans to pursue other optical and chemical mechanisms for cargo loading and release by changing parameters such as pH and temperature. “We are also interested in assembling the nanowrappers into larger scale architectures, extending the method to other bimetallic systems, and comparing the internal and external catalytic activity of the nanowrappers,” Lu says.

“To the best of my knowledge, this is a first demonstration of precise control of the porosity at the level of individual nanoparticles. Moreover, the size and positions of the pores can be controlled by simple adjustment of the reaction conditions,” says Elena Shevchenko, an expert in nanostructures and their assembly at Argonne National Laboratory who was not associated with this research. “The synthesis of AuAg hollow nanowrappers is a one-step process that simplifies the scalability of the synthetic protocol,” she says. As a result, she expects this work to motivate researchers to study how different surface coatings influence nanoparticles with other shapes and compositions during similar reactions.

Read the article in ACS Central Science.