Tag Archives: Vinayak P. Dravid

Cloaking devices made from DNA and gold nanoparticles using top-down lithography

This new technique seems promising but there’ve been a lot of ‘cloaking’ devices announced in the years I’ve been blogging and, in all likelihood, I was late to the party so I’m exercising a little caution before getting too excited. For the latest development in cloaking devices, there’s a January 18, 2018 news item on Nanowerk,

Northwestern University researchers have developed a first-of-its-kind technique for creating entirely new classes of optical materials and devices that could lead to light bending and cloaking devices — news to make the ears of Star Trek’s Spock perk up.

Using DNA [deoxyribonucleic acid] as a key tool, the interdisciplinary team took gold nanoparticles of different sizes and shapes and arranged them in two and three dimensions to form optically active superlattices. Structures with specific configurations could be programmed through choice of particle type and both DNA-pattern and sequence to exhibit almost any color across the visible spectrum, the scientists report.

A January 18, 2018 Northwestern University news release (also on EurekAlert) by Megan Fellman, which originated the news item, delves into more detail (Note: Links have been removed),

“Architecture is everything when designing new materials, and we now have a new way to precisely control particle architectures over large areas,” said Chad A. Mirkin, the George B. Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences at Northwestern. “Chemists and physicists will be able to build an almost infinite number of new structures with all sorts of interesting properties. These structures cannot be made by any known technique.”

The technique combines an old fabrication method — top-down lithography, the same method used to make computer chips — with a new one — programmable self-assembly driven by DNA. The Northwestern team is the first to combine the two to achieve individual particle control in three dimensions.

The study was published online by the journal Science today (Jan. 18). Mirkin and Vinayak P. Dravid and Koray Aydin, both professors in Northwestern’s McCormick School of Engineering, are co-corresponding authors.

Scientists will be able to use the powerful and flexible technique to build metamaterials — materials not found in nature — for a range of applications including sensors for medical and environmental uses.

The researchers used a combination of numerical simulations and optical spectroscopy techniques to identify particular nanoparticle superlattices that absorb specific wavelengths of visible light. The DNA-modified nanoparticles — gold in this case — are positioned on a pre-patterned template made of complementary DNA. Stacks of structures can be made by introducing a second and then a third DNA-modified particle with DNA that is complementary to the subsequent layers.

In addition to being unusual architectures, these materials are stimuli-responsive: the DNA strands that hold them together change in length when exposed to new environments, such as solutions of ethanol that vary in concentration. The change in DNA length, the researchers found, resulted in a change of color from black to red to green, providing extreme tunability of optical properties.

“Tuning the optical properties of metamaterials is a significant challenge, and our study achieves one of the highest tunability ranges achieved to date in optical metamaterials,” said Aydin, assistant professor of electrical engineering and computer science at McCormick.

“Our novel metamaterial platform — enabled by precise and extreme control of gold nanoparticle shape, size and spacing — holds significant promise for next-generation optical metamaterials and metasurfaces,” Aydin said.

The study describes a new way to organize nanoparticles in two and three dimensions. The researchers used lithography methods to drill tiny holes — only one nanoparticle wide — in a polymer resist, creating “landing pads” for nanoparticle components modified with strands of DNA. The landing pads are essential, Mirkin said, since they keep the structures that are grown vertical.

The nanoscopic landing pads are modified with one sequence of DNA, and the gold nanoparticles are modified with complementary DNA. By alternating nanoparticles with complementary DNA, the researchers built nanoparticle stacks with tremendous positional control and over a large area. The particles can be different sizes and shapes (spheres, cubes and disks, for example).

“This approach can be used to build periodic lattices from optically active particles, such as gold, silver and any other material that can be modified with DNA, with extraordinary nanoscale precision,” said Mirkin, director of Northwestern’s International Institute for Nanotechnology.

Mirkin also is a professor of medicine at Northwestern University Feinberg School of Medicine and professor of chemical and biological engineering, biomedical engineering and materials science and engineering in the McCormick School.

The success of the reported DNA programmable assembly required expertise with hybrid (soft-hard) materials and exquisite nanopatterning and lithographic capabilities to achieve the requisite spatial resolution, definition and fidelity across large substrate areas. The project team turned to Dravid, a longtime collaborator of Mirkin’s who specializes in nanopatterning, advanced microscopy and characterization of soft, hard and hybrid nanostructures.

Dravid contributed his expertise and assisted in designing the nanopatterning and lithography strategy and the associated characterization of the new exotic structures. He is the Abraham Harris Professor of Materials Science and Engineering in McCormick and the founding director of the NUANCE center, which houses the advanced patterning, lithography and characterization used in the DNA-programmed structures.

Here’s a link to and a citation for the paper,

Building superlattices from individual nanoparticles via template-confined DNA-mediated assembly by Qing-Yuan Lin, Jarad A. Mason, Zhongyang, Wenjie Zhou, Matthew N. O’Brien, Keith A. Brown, Matthew R. Jones, Serkan Butun, Byeongdu Lee, Vinayak P. Dravid, Koray Aydin, Chad A. Mirkin. Science 18 Jan 2018: eaaq0591 DOI: 10.1126/science.aaq0591

This paper is behind a paywall.

As noted earlier, it could be a while before cloaking devices are made available. In the meantime, you may find this image inspiring,

Caption: Northwestern University researchers have developed a new method to precisely arrange nanoparticles of different sizes and shapes in two and three dimensions, resulting in optically active superlattices. Credit: Northwestern University

The reason the findings in a popular thermoelectricity paper can’t be replicated

It seems to me that over the last few years there’s been a lot more discussion about errors in science. There have always been scandals but this public interest in reproducibility of scientific results seems relatively new. In any event, a Nov. 17, 2016 news item on Nanowerk highlights research that explains why scientists have been unable to reproduce results of an influential 2014 paper (Note: A link has been removed),

A team of physicists in Clemson University’s College of Science and Academia Sinica in Taiwan has determined why other scientists have been unable to replicate a highly influential thermoelectricity study published in a prestigious, peer-reviewed journal.

In the April 2014 issue of the journal Nature (“Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals”), a group of scientists described an emerging crystalline material made of tin selenide that provided the highest efficiency ever recorded for thermoelectricity, the process of capturing wasted energy which is released as heat and making it available again as electricity. The paper has been viewed 45,000 times and its findings have been referenced in 600 subsequent studies, according to Google Scholar.

A thermoelectricity (TE) module captures waste energy, released as heat, converts it to electricity and returns it to a device. Image Credit: Thomas Masservy, Clemson University

There appears to have been a mistake in the original research. A Nov. 17, 2016 Clemson University news release, which originated the news item, expands on the theme (Note: A link has been removed),

“If it were true, basically, they would have found a crown jewel,” said Apparao Rao, the Robert A. Bowen professor of Physics and the director of the Clemson Nanomaterials Institute.

On Nov. 3, 2016, Nature ran a brief communication by the Clemson-Sinica team explaining why the 2014 data could not be replicated.

Thermoelectricity could provide enormous monetary and environmental savings because it is sustainable; instead of requiring fuel it continually captures wasted heat energy and puts it to use. And there’s a lot of wasted energy; about 70 percent in most machines, including cars.

“When your laptop gets hot, energy is released as waste heat because it doesn’t use all the supplied electricity. Machines have limited efficiency,” according to Ramakrishna Podila, assistant professor of physics and astronomy at Clemson who co-authored the paper solving the mystery.

But, so far, the perfect material for capturing and creating thermoelectricity has proven elusive.

Heat and electrical current can flow through any material when heat is applied to one side. But to efficiently harness thermoelectricity, the material has to trap heat on one side while letting the current flow. The difference in temperature, from one side to the other, generates energy.

Imagine cookware. Expensive pots and pans are copper or they have copper cores. Copper is a great heat-conducting material: it quickly and evenly spreads heat so food cooks evenly. Copper makes for good cookware, but poor thermoelectric material.

In an ideal thermoelectric material, current-carrying electrons should flow unimpeded from the hot side to the cold side, but heat-carrying phonons, which are atomic vibrations, must be blocked, either by large atoms or defects where the material is of lower density.

Rao; Podila; Sriparna Bhattacharya, a research assistant professor in astronomy and physics; and Jian He, an associate professor in physics and astronomy at Clemson and a thermoelectrics expert, performed their own study on tin selenide in collaboration with Academia Sinica’s Institute of Physics in Taipei.

Right away, Bhattacharya noticed a problem. “The most puzzling thing was that when we measured our own tin-selenide material, we observed the same electrical flow as reported in the 2014 article, but the heat carried by the phonons was relatively higher,” Bhattacharya said.

The original research group “made tin-selenide crystal that was not fully dense,” Bhattacharya said. Ideally, a crystalline material matches its “theoretical density,” meaning it’s as dense as it can be expected to get.

“Instead of reaching 100 percent theoretical density, it reached 89 percent. A 10 percent difference might not seem like much,” she said, but it can have a huge implication on the electron and phonon flow.

The Clemson-Taiwan collaborators are now focusing on their own assessment of thermoelectricity in tin-selenide. They expect to publish soon.

Here’s a link to and a citation to the 2014 thermoelectricity paper and a link to and a citation for the 2016 paper critiquing it,

Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals by Li-Dong Zhao, Shih-Han Lo, Yongsheng Zhang, Hui Sun, Gangjian Tan, Ctirad Uher, C. Wolverton, Vinayak P. Dravid, & Mercouri G. Kanatzidis. Nature 508, 373–377 (17 April 2014) doi:10.1038/nature13184 Published online 16 April 2014

The intrinsic thermal conductivity of SnSe by Pai-Chun Wei, S. Bhattacharya, J. He, S. Neeleshwar, R. Podila, Y. Y. Chen, & A. M. Rao. Nature 539, E1–E2 (03 November 2016) doi:10.1038/nature19832 Published online 02 November 2016

Both papers are behind a paywall.

One final observation, scientists may mistakes as do we all. The point after all is to contribute and the mistakes can be as useful as the successes.

Ultimate discovery tool?

For anyone familiar with the US nanomedicine scene, Chad Mirkin’s appearance in this announcement from Northwestern University isn’t much of a surprise.  From a June 23, 2016 news item on ScienceDaily,

The discovery power of the gene chip is coming to nanotechnology. A Northwestern University research team is developing a tool to rapidly test millions and perhaps even billions or more different nanoparticles at one time to zero in on the best particle for a specific use.

When materials are miniaturized, their properties—optical, structural, electrical, mechanical and chemical—change, offering new possibilities. But determining what nanoparticle size and composition are best for a given application, such as catalysts, biodiagnostic labels, pharmaceuticals and electronic devices, is a daunting task.

“As scientists, we’ve only just begun to investigate what materials can be made on the nanoscale,” said Northwestern’s Chad A. Mirkin, a world leader in nanotechnology research and its application, who led the study. “Screening a million potentially useful nanoparticles, for example, could take several lifetimes. Once optimized, our tool will enable researchers to pick the winner much faster than conventional methods. We have the ultimate discovery tool.”

A June 23, 2016 Northwestern University news release (also on EurekAlert), which originated the news item, describes the work in more detail,

Using a Northwestern technique that deposits materials on a surface, Mirkin and his team figured out how to make combinatorial libraries of nanoparticles in a very controlled way. (A combinatorial library is a collection of systematically varied structures encoded at specific sites on a surface.) Their study will be published June 24 by the journal Science.

The nanoparticle libraries are much like a gene chip, Mirkin says, where thousands of different spots of DNA are used to identify the presence of a disease or toxin. Thousands of reactions can be done simultaneously, providing results in just a few hours. Similarly, Mirkin and his team’s libraries will enable scientists to rapidly make and screen millions to billions of nanoparticles of different compositions and sizes for desirable physical and chemical properties.

“The ability to make libraries of nanoparticles will open a new field of nanocombinatorics, where size — on a scale that matters — and composition become tunable parameters,” Mirkin said. “This is a powerful approach to discovery science.”

“I liken our combinatorial nanopatterning approach to providing a broad palette of bold colors to an artist who previously had been working with a handful of dull and pale black, white and grey pastels,” said co-author Vinayak P. Dravid, the Abraham Harris Professor of Materials Science and Engineering in the McCormick School of Engineering.

Using five metallic elements — gold, silver, cobalt, copper and nickel — Mirkin and his team developed an array of unique structures by varying every elemental combination. In previous work, the researchers had shown that particle diameter also can be varied deliberately on the 1- to 100-nanometer length scale.

Some of the compositions can be found in nature, but more than half of them have never existed before on Earth. And when pictured using high-powered imaging techniques, the nanoparticles appear like an array of colorful Easter eggs, each compositional element contributing to the palette.

To build the combinatorial libraries, Mirkin and his team used Dip-Pen Nanolithography, a technique developed at Northwestern in 1999, to deposit onto a surface individual polymer “dots,” each loaded with different metal salts of interest. The researchers then heated the polymer dots, reducing the salts to metal atoms and forming a single nanoparticle. The size of the polymer dot can be varied to change the size of the final nanoparticle.

This control of both size and composition of nanoparticles is very important, Mirkin stressed. Having demonstrated control, the researchers used the tool to systematically generate a library of 31 nanostructures using the five different metals.

To help analyze the complex elemental compositions and size/shape of the nanoparticles down to the sub-nanometer scale, the team turned to Dravid, Mirkin’s longtime friend and collaborator. Dravid, founding director of Northwestern’s NUANCE Center, contributed his expertise and the advanced electron microscopes of NUANCE to spatially map the compositional trajectories of the combinatorial nanoparticles.

Now, scientists can begin to study these nanoparticles as well as build other useful combinatorial libraries consisting of billions of structures that subtly differ in size and composition. These structures may become the next materials that power fuel cells, efficiently harvest solar energy and convert it into useful fuels, and catalyze reactions that take low-value feedstocks from the petroleum industry and turn them into high-value products useful in the chemical and pharmaceutical industries.

Here’s a diagram illustrating the work,

 Caption: A combinatorial library of polyelemental nanoparticles was developed using Dip-Pen Nanolithography. This novel nanoparticle library opens up a new field of nanocombinatorics for rapid screening of nanomaterials for a multitude of properties. Credit: Peng-Cheng Chen/James Hedrick

Caption: A combinatorial library of polyelemental nanoparticles was developed using Dip-Pen Nanolithography. This novel nanoparticle library opens up a new field of nanocombinatorics for rapid screening of nanomaterials for a multitude of properties. Credit: Peng-Cheng Chen/James Hedrick

Here’s a link to and a citation for the paper,

Polyelemental nanoparticle libraries by Peng-Cheng Chen, Xiaolong Liu, James L. Hedrick, Zhuang Xie, Shunzhi Wang, Qing-Yuan Lin, Mark C. Hersam, Vinayak P. Dravid, Chad A. Mirkin. Science  24 Jun 2016: Vol. 352, Issue 6293, pp. 1565-1569 DOI: 10.1126/science.aaf8402

This paper is behind a paywall.