Using a technique called DNA origami, researchers in the US and China have succeeded in designing biomimetic silica nanocomposites with pre-defined sizes and shapes at sub-5-nm resolution. The technique is a new way of exploiting the structure of DNA to guide silica mineralization and it could be extended to other inorganic materials as well as metal oxides. The structures produced could find applications in nano-electronics, photonics and robotics.

DNA origami relies on exploiting the base pairing of DNA’s four nucleotides, A, T, C and G, to produce an infinite variety of self-assembled engineered shapes. The resulting nanostructures can be used as scaffolding or as miniature circuit boards for precisely building components such as carbon nanotubes and nanowires.

Powerful though it is, one of the main limitations of the technique is that the DNA shapes need to be formed in a saltwater solution, which produces surface charging that can actually prevent material assembly. It can also damage surfaces such as silicon wafers.

Modified Stöber method

Researchers led by Chunhai Fan of the Chinese Academy of Sciences and Shanghai Jiaotong University and Hao Yan of Arizona State University have now used the so-called Stöber method, which is widely employed to produce silica nanostructures in industry, to overcome these problems.

“The Stöber method is a chemical process that makes use of tetraethyl orthosilicate (TEOS) to prepare SiO₂ particles of controllable and uniform size for applications in materials science,” explains Fan. “It was first pioneered by Werner Stöber in 1968. We modified this method by pre-mixing TEOS with N-trimethyloxysilylpropyl-N,N,N-trimethylammonium chloride (TMAPS) to form prehydrolyzing clusters of silica that have enough positive charge to electrostatically bind to the surface of the negatively-charged DNA templates.

“The process, which we have dubbed DNA Origami Silification (DOS), induces the growth of amorphous silica on the DNA.”

2D and 3D nanostructures

The researchers made different-shaped nanostructures, including 2D squares, triangles, crosses and structures that mimic diatoms (marine unicellular organisms). They also made 3D objects, including cubes, tetrahedrons, hemispheres, toroids and ellipsoids. The structures could be made to be anywhere between 10 nm and 1 µm in size and their thickness (and thus rigidity) could be controlled by varying growth time.

Importantly, the structures have high specific strength (a measure of how resistant a material is to breaking, relative to its density), say Fan and study lead author Xiaoguo Liu. “We measured a compressive E-modulus of the DNA-silica hybrids that was 10 times higher than pure DNA nanostructures, and the 3D frameworks were found to be rigid and flexible – like a spring.”

In this respect, the structures resemble diatoms, whose silica exoskeletons boast the highest specific strength of any known biomaterial.

“In our work, we have proved that DNA nanostructures can be used as templates for synthesizing inorganic materials,” Fan tells Physics World. “The technique we have presented allows us to transfer geometric information from ‘designer’ DNA structures (the nucleic acids framework, for example) to the inorganics. Provided suitable synthetic conditions can be found, it should, in principle, be possible to create composites in which other inorganic materials coat the DNA origami shapes. Multi-component composites, through the stepwise deposition of more than one inorganic material on the DNA templates, should also be achievable.”

The technique is detailed in Nature 10.1038/s41586-018-0332-7.