Researchers Develop Robust Strategy to Produce Rich Variety of Nanorods

A new approach for crafting one-dimensional nanorods from a variety of precursor materials has been developed by materials scientists. The system, based on a cellulose backbone, depends on the growth of block copolymer “arms” that help produce a compartment to serve as a nanometer-scale chemical reactor. Aggregation of the nanorods is prevented by the outer blocks of the arms.

The produced structures resemble small bottlebrushes comprising polymer “hairs” on the nanorod surface. The nanorods are available in varied sizes ranging from a few hundred nanometers to a few micrometers in length, and a few tens of nanometers in diameter. This new technique firmly controls the length, diameter and surface properties of the nanorods, whose magnetic, electrical, optical and catalytic properties rely on the dimensions of the nanorods and the precursor materials used.

The nanorods are ideal for applications in areas like cancer treatment, drug delivery, energy conversion and storage, sensory devices, and electronics. With this method, the researchers have until now fabricated uniform metallic, thermoelectric, semiconducting, upconversion and ferroelectric nanocrystals, including combinations thereof. Air Force Office of Scientific Research supported this study, which was published in the recent issue of the journal 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. 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.

Zhiqun Lin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology.

Nanorod structures are not new, but the method used by Lin’s lab develops nanorods of even sizes – such as iron oxide and barium titanate, which are yet to be illustrated via wet-chemistry approaches in the literature– and highly-uniform core-shell nanorods produced by integrating two dissimilar materials. Lin and former postdoctoral research associate Xinchang Pang state that precursor materials ideal for the technique are almost limitless.

There are many precursors of different materials available that can be used with this robust system. 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.

Lin

Fabrication of the nanorods commences with the functionalization of separate lengths of cellulose, a cost-effective long-chain biopolymer harvested from trees. A single cellulose unit comprises three hydroxyl groups, which are chemically altered with a bromine atom. The brominated cellulose is then used as macroinitiator for the growth of the block copolymer arms with well-monitored lengths using the atom transfer radical polymerization (ATRP) process, with, poly(acrylic acid)-block-polystyrene (PAA-b-PS) yielding cellulose that is grafted densely with PAA-b-PS (i.e., cellulose-g-[PAA-b-PS]) that provide the bottlebrush appearance.

The next step deals with the preferential partitioning of precursors in the inner PAA compartment that is used as a nanoreactor for instigating the nucleation and growth of nanorods. The block copolymer arms that are grafted densely and the firm cellulose backbone offer researchers the potential to avoid aggregation of the resulting nanorods and also to prevent 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.”

Lin and coworkers, examined the difference in the chemistry and the number of blocks in the arms of the bottlebrush-like block copolymers, in order to develop an array of hollow nanorods – nanotubes, core-shell nanorods, and oil-soluble and water-soluble plain nanorods – of varied compositions and dimensions.

For instance, by using bottlebrush-like triblock copolymers comprising densely grafted amphiphilic triblock copolymer arms, the core-shell nanorods can be developed from two varied materials. In a number of cases, a big lattice mismatch existing between shell and core materials would avoid the development of superior quality core-shell structures, but the method overcomes that limitation.

By using this approach, we can grow the core and shell materials independently in their respective nanoreactors. 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.

Lin

Lin believes in the existence of several significant applications for the nanorods.

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. Potential applications include optics, electronics, photonics, magnetic technologies, sensory materials and devices, lightweight structural materials, catalysis, drug delivery, and bio-nanotechnology.

Lin

For instance, plain gold nanorods of varied lengths may permit efficient plasmonic absorption in the near-infrared range for use in converting solar energy with enhanced harvesting of solar spectrum. The upconversion nanorods can relatively harvest IR solar photons, followed by the absorption of high-energy photons that are emitted, thus producing extra photocurrent in solar cells. They can also find suitable applications in biological labeling due to their chemical stability, low toxicity, and strong luminescence when excited by near-IR radiation, which can go through tissues in a much improved manner than higher energy radiation like ultraviolet, as is mostly needed with quantum dot labels.

The gold-iron oxide core-shell nanorods could also be used in cancer therapy, with MRI imaging carried out by the iron oxide shell, and local heating developed by the photothermal effect on gold nanorod core killing cancer cells.

Besides the already mentioned researchers, co-authors include graduate research assistant Yanjie He and postdoctoral researcher Jaehan Jung in Georgia Tech’s School of Materials Science and Engineering.

The Air Force Office of Scientific Research under grant FA9550-16-1-0187 supported this study.