Georgia Tech researchers (left to right) Yanjie He, Zhiqun Lin and Jaehan Jung demonstrate how magnetic nanorods fabricated with their new technique are attracted to a magnet. Image: Rob Felt, Georgia Tech.
Georgia Tech researchers (left to right) Yanjie He, Zhiqun Lin and Jaehan Jung demonstrate how magnetic nanorods fabricated with their new technique are attracted to a magnet. Image: Rob Felt, Georgia Tech.

Materials scientists have developed a new strategy for crafting one-dimensional nanorods from a wide range of precursor materials. Based on a cellulose backbone, the strategy relies on the growth of block copolymer ‘arms’ that help to create a compartment that serves as a nanometer-scale chemical reactor. The outer blocks of the arms prevent aggregation of the nanorods.

The produced structures resemble tiny bottlebrushes with polymer ‘hairs’ on the nanorod surface. The nanorods range in length from a few hundred nanometers to a few micrometers, and are a few tens of nanometers in diameter. This new technique provides tight control over the diameter, length and surface properties of the nanorods, whose optical, electrical, magnetic and catalytic properties depend on both the precursor materials used and the dimensions of the nanorods.

These nanorods could have applications in such areas as electronics, sensory devices, energy conversion and storage, drug delivery, and cancer treatment. Using their technique, the researchers have so far fabricated uniform metallic, ferroelectric, upconversion, semiconducting and thermoelectric nanocrystals, as well as combinations thereof. The research, supported by the US Air Force Office of Scientific Research, is reported in a paper in Science.

“We have developed a very general and robust strategy to craft a rich variety of nanorods with precisely-controlled dimensions, compositions, architectures and surface chemistries,” said Zhiqun Lin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “To create these structures, we used nonlinear bottlebrush-like block copolymers as tiny reactors to template the growth of an exciting variety of inorganic nanorods.”

The technique developed by Lin’s lab can produce various novel nanorods of uniform sizes – such as nanorods made of barium titanate and iron oxide, which have not yet been demonstrated via wet-chemistry approaches in the literature. It can also produce highly-uniform core-shell nanorods by combining two dissimilar materials. According to Lin and former postdoctoral research associate Xinchang Pang, the technique can work with a virtually limitless range of precursor materials.

“There are many precursors of different materials available that can be used with this robust system,” Lin said. “By choosing a different outer block in the bottlebrush-like block copolymers, our nanorods can be dissolved and uniformly dispersed in organic solvents such as toluene or chloroform, or in water.”

Fabrication of the nanorods begins with the functionalization of individual lengths of cellulose, an inexpensive long-chain biopolymer harvested from trees. Each unit of cellulose has three hydroxyl groups, which are chemically modified with a bromine atom. The brominated cellulose then serves as a macroinitiator for the growth of block copolymer arms with well-controlled lengths using the atom transfer radical polymerization (ATRP) process. Thus, for example, poly(acrylic acid)-block-polystyrene (PAA-b-PS) yields cellulose densely grafted with PAA-b-PS arms (i.e., cellulose-g-[PAA-b-PS]) that confer the bottlebrush appearance.

The next step involves the preferential partitioning of precursors in the inner PAA compartment that serves as a nanoreactor, initiating the nucleation and growth of the nanorods. The densely grafted block copolymer arms, together with the rigid cellulose backbone, not only prevent the resulting nanorods from aggregating together but also keep them from bending.

“The polymers are like long spaghetti and they want to coil up,” Lin explained. “But they cannot do this in the complex macromolecules we make because with so many block copolymer arms formed, there is no space. This leads to the stretching of the arms, forming a very rigid structure.”

By varying the chemistry and the number of blocks in the arms of the bottlebrush-like block copolymers, Lin and his co-workers produced an array of oil-soluble and water-soluble plain nanorods, core-shell nanorods and hollow nanorods, or nanotubes, of different dimensions and compositions.

For example, by using bottlebrush-like triblock copolymers containing densely-grafted amphiphilic triblock copolymer arms, they were able to produce core-shell nanorods from two different materials. In most cases, a large lattice mismatch between core and shell materials would prevent the formation of such high-quality core-shell structures, but the technique overcomes that limitation.

“By using this approach, we can grow the core and shell materials independently in their respective nanoreactors,” Lin said. “This allows us to bypass the requirement for matching the crystal lattices and permits fabrication of a large variety of core-shell structures with different combinations that would otherwise be very challenging to obtain.”

“With a broad range of physical properties – optical, electrical, optoelectronic, catalytic, magnetic and sensing – that are dependent sensitively on their size and shape as well as their assemblies, the produced nanorods are of both fundamental and practical interest,” he added. “Potential applications include optics, electronics, photonics, magnetic technologies, sensory materials and devices, lightweight structural materials, catalysis, drug delivery, and bio-nanotechnology.”

For example, plain gold nanorods of different lengths may allow effective plasmonic absorption in the near-infrared range for use in solar energy conversion with improved harvesting of the solar spectrum. These upconversion nanorods can preferentially harvest the infrared solar photons, followed by the absorption of emitted high-energy photons, to generate extra photocurrent in solar cells.

The nanorods can also be used for biological labeling because of their low toxicity, chemical stability and intense luminescence when excited by near-infrared radiation, which can penetrate tissue much better than higher energy radiation such as ultraviolet. In addition, gold-iron oxide core-shell nanorods may be useful for cancer therapy, with the iron oxide shell allowing MRI imaging of the tumor before local heating created by the photothermal effect on the gold nanorod core kills the cancer cells.

This story is adapted from material from the Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.