Polymer structures serve as 'nanoreactors' for nanocrystals with uniform sizes, shapes

Jun 11, 2013
Georgia Tech professor Zhiqun Lin examines a gold nanoparticle toluene solution. The work is part of research on using star-shaped block co-polymers to create nanocrystals of uniform size and shape. Credit: Gary Meek

Using star-shaped block co-polymer structures as tiny reaction vessels, researchers have developed an improved technique for producing nanocrystals with consistent sizes, compositions and architectures – including metallic, ferroelectric, magnetic, semiconductor and luminescent nanocrystals. The technique relies on the length of polymer molecules and the ratio of two solvents to control the size and uniformity of colloidal nanocrystals.

The technique could facilitate the use of nanoparticles for optical, electrical, optoelectronic, magnetic, catalysis and other applications in which tight control over size and structure is essential to obtaining desirable properties. The technique produces plain, core-shell and hollow nanoparticles that can be made soluble either in water or in .

"We have developed a general strategy for making a large variety of nanoparticles in different size ranges, compositions and architectures," said Zhiqun Lin, an associate professor in the School of at the Georgia Institute of Technology. "This very robust technique allows us to craft a wide range of nanoparticles that cannot be easily produced with any other approaches."

The technique was described in the June issue of the journal Nature Nanotechnology. The research was supported by the Air Force Office of Scientific Research.

The star-shaped block co-polymer structures consist of a central beta-cyclodextrin core to which multiple "arms" – as many as 21 linear block co-polymers – are covalently bonded. The star-shaped block co-polymers form the unimolecular micelles that serve as a reaction vessel and template for the formation of the nanocrystals.

The inner blocks of unimolecular micelles are poly(acrylic) acid (PAA), which is hydrophilic, which allows to enter them. Once inside the tiny reaction vessels made of PAA, the ions react with the PAA to form nanocrystals, which range in size from a few up to a few tens of nanometers. The size of the nanoparticles is determined by the length of the PAA chain.

Georgia Tech professor Zhiqun Lin (standing) watches research scientist Xinchang Pang tuning the experimental condition in the nanocrystal synthesis. Credit: Gary Meek

The block co-polymer structures can be made with hydrophilic inner blocks and hydrophobic outer blocks – amphiphilic block co-polymers, with which the resulting nanoparticles can be dissolved in organic solvents. However, if both inner and outer blocks are hydrophilic – all hydrophilic block co-polymers – the resulting nanoparticles will be water-soluble, making them suitable for biomedical applications.

Lin and collaborators Xinchang Pang, Lei Zhao, Wei Han and Xukai Xin found that they could control the uniformity of the nanoparticles by varying the volume ratio of two solvents – dimethlformamide and benzyl alcohol – in which the nanoparticles are formed. For ferroelectric lead titanate (PbTiO3) nanoparticles, for instance, a 9-to-1 solvent ratio produces the most uniform nanoparticles.

The researchers have also made iron oxide, zinc oxide, titanium oxide, cuprous oxide, cadmium selenide, barium titanate, gold, platinum and silver nanocrystals. The technique could be applicable to nearly all transition or main-group metal ions and organometallic ions, Lin said.

"The crystallinity of the nanoparticles we are able to create is the key to a lot of applications," he added. "We need to make them with good crystalline structures so they will exhibit good physical properties."

Earlier techniques for producing polymeric micelles with linear block co-polymers have been limited by the stability of the structures and by the consistency of the nanocrystals they produce, Lin said. Current fabrication techniques include organic solution-phase synthesis, thermolysis of organometallic precursors, sol-gel processes, hydrothermal reactions and biomimetic or dendrimer templating. These existing techniques often require stringent conditions, are difficult to generalize, include a complex series of steps, and can't withstand changes in the environment around them.

Georgia Tech professor Zhiqun Lin (standing) and research scientist Xinchang Pang compare two cadmium selenide (CdSe) nanocrystals made by Pang. The researchers are examining the absorption spectra of the nanocrystals in front of the computer. Credit: Gary Meek

By contrast, nanoparticle production technique developed by the Georgia Tech researchers is general and robust. The nanoparticles remain stable and homogeneous for long periods of time – as much as two years so far – with no precipitation. Such flexibility and stability could allow a range of practical applications, Lin said.

"Our star-like block co-polymers can overcome the thermodynamic instabilities of conventional linear block co-polymers," he said. "The chain length of the inner PAA blocks dictates the size of the nanoparticles, and the uniformity of the nanoparticles is influenced by the solvents used in the system."

The researchers have used a variety of star-like di-block and tri-block co-polymers as nanoreactors. Among them are poly()-block-polystyrene (PAA-b-PS) and poly(acrylic acid)-blockpoly(ethylene oxide) (PAA-b-PEO) diblock co-polymers, and poly(4-vinylpyridine)-block-poly(tert-butyl acrylate)-block-polystyrene (P4VP-b-PtBA-b-PS), poly(4-vinylpyridine)-block-poly (tert-butyl acrylate)-block-poly(ethylene oxide) (P4VP-b-PtBA-b-PEO), polystyrene-block-poly(acrylic acid)-block-polystyrene (PS-b-PAA-b-PS) and polystyrene-block-poly(acrylic acid)-block-poly(ethylene oxide) (PS-b-PAA-b-PEO) tri-block co-polymers.

For the future, Lin envisions more complex with multifunctional shells and additional shapes, including nanorods and so-called "Janus" that are composed of biphasic geometry of two dissimilar materials.

Explore further: Coming round to stable self-assembly

More information: Xinchang Pang, Lei Zhao, Wei Han, Xukai Xin and Zhiqun Lin, "A general and robust strategy for the synthesis of nearly monodisperse colloidal nanocrystals," Nature Nanotechnology, 8, 426, 2013 dx.doi.org/10.1038/nnano.2013.85

Related Stories

Coming round to stable self-assembly

May 28, 2013

Nanostructures that assemble themselves from polymer molecules could prove to be useful tools in chemistry and industry. However, it is difficult to develop structurally robust self-assembling materials because ...

Building electronics from the ground up

Jan 14, 2013

(Phys.org)—There's hardly a moment in modern life that doesn't involve electronic devices, whether they're guiding you to a destination by GPS or deciding which incoming messages merit a beep, ring or vibration. ...

Titanium dioxide nanoreactor

Feb 21, 2013

Tiny particles of titanium dioxide are found as key ingredients in wall paints, sunscreens, and toothpaste; they act as reflectors of light or as abrasives. However with decreasing particle size and a corresponding ...

Accidental discovery may lead to improved polymers

Apr 05, 2013

Chemical Engineering Professor Tim Bender and Post-Doctoral Fellow Benoit Lessard's discovery of an unexpected side product of polymer synthesis could have implications for the manufacture of commercial polymers used in sealants, ...

Recommended for you

Thinnest feasible nano-membrane produced

Apr 17, 2014

A new nano-membrane made out of the 'super material' graphene is extremely light and breathable. Not only can this open the door to a new generation of functional waterproof clothing, but also to ultra-rapid filtration. The ...

Wiring up carbon-based electronics

Apr 17, 2014

Carbon-based nanostructures such as nanotubes, graphene sheets, and nanoribbons are unique building blocks showing versatile nanomechanical and nanoelectronic properties. These materials which are ordered ...

Making 'bucky-balls' in spin-out's sights

Apr 16, 2014

(Phys.org) —A new Oxford spin-out firm is targeting the difficult challenge of manufacturing fullerenes, known as 'bucky-balls' because of their spherical shape, a type of carbon nanomaterial which, like ...

User comments : 0

More news stories

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...