Tag Archives: self-assembly

Carbon nanotubes self-assembling into transistors on a gold substrate

I’m not sure this work is ready for commercialization (I think not) but it’s certainly intriguing. From an April 5, 2017 news item on ScienceDaily,

Carbon nanotubes can be used to make very small electronic devices, but they are difficult to handle. University of Groningen scientists, together with colleagues from the University of Wuppertal and IBM Zurich, have developed a method to select semiconducting nanotubes from a solution and make them self-assemble on a circuit of gold electrodes. …

An April 5, 2017 University of Groningen (Netherlands) press release on EurekAlert, which originated the news item, explains the work in more detail,

The results look deceptively simple: a self-assembled transistor with nearly 100 percent purity and very high electron mobility. But it took ten years to get there. University of Groningen Professor of Photophysics and Optoelectronics Maria Antonietta Loi designed polymers which wrap themselves around specific carbon nanotubes in a solution of mixed tubes. Thiol side chains on the polymer bind the tubes to the gold electrodes, creating the resultant transistor.

Patent

‘In our previous work, we learned a lot about how polymers attach to specific carbon nanotubes’, Loi explains. These nanotubes can be depicted as a rolled sheet of graphene, the two-dimensional form of carbon. ‘Depending on the way the sheets are rolled up, they have properties ranging from semiconductor to semi-metallic to metallic.’ Only the semiconductor tubes can be used to fabricate transistors, but the production process always results in a mixture.

‘We had the idea of using polymers with thiol side chains some time ago’, says Loi. The idea was that as sulphur binds to metals, it will direct polymer-wrapped nanotubes towards gold electrodes. While Loi was working on the problem, IBM even patented the concept. ‘But there was a big problem in the IBM work: the polymers with thiols also attached to metallic nanotubes and included them in the transistors, which ruined them.’

Solution

Loi’s solution was to reduce the thiol content of the polymers, with the assistance of polymer chemists from the University of Wuppertal. ‘What we have now shown is that this concept of bottom-up assembly works: by using polymers with a low concentration of thiols, we can selectively bring semiconducting nanotubes from a solution onto a circuit.’ The sulphur-gold bond is strong, so the nanotubes are firmly fixed: enough even to stay there after sonication of the transistor in organic solvents.

The production process is simple: metallic patterns are deposited on a carrier , which is then dipped into a solution of carbon nanotubes. The electrodes are spaced to achieve proper alignment: ‘The tubes are some 500 nanometres long, and we placed the electrodes for the transistors at intervals of 300 nanometres. The next transistor is over 500 nanometres away.’ The spacing limits the density of the transistors, but Loi is confident that this could be increased with clever engineering.

‘Over the last years, we have created a library of polymers that select semiconducting nanotubes and developed a better understanding of how the structure and composition of the polymers influences which carbon nanotubes they select’, says Loi. The result is a cheap and scalable production method for nanotube electronics. So what is the future for this technology? Loi: ‘It is difficult to predict whether the industry will develop this idea, but we are working on improvements, and this will eventually bring the idea closer to the market.’

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

On-Chip Chemical Self-Assembly of Semiconducting Single-Walled Carbon Nanotubes (SWNTs): Toward Robust and Scale Invariant SWNTs Transistors by Vladimir Derenskyi, Widianta Gomulya, Wytse Talsma, Jorge Mario Salazar-Rios, Martin Fritsch, Peter Nirmalraj, Heike Riel, Sybille Allard, Ullrich Scherf, and Maria A. Loi. Advanced Materials DOI: 10.1002/adma.201606757 Version of Record online: 5 APR 2017

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

Synthesized nanoparticles with the complexity of protein molecules

Caption: The structure of the largest gold nanoparticle to-date, Au246(SR)80, was resolved using x-ray crystallography. Credit: Carnegie Mellon University

Carnegie Mellon University (CMU) researchers synthesized a self-assembled nanoparticle of gold as they built on their 2015 work described in my April 14, 2015 posting (Nature’s patterns reflected in gold nanoparticles). Here’s the latest from the team in a Jan. 23, 2017 news item on phys.org,

Chemists at Carnegie Mellon University have demonstrated that synthetic nanoparticles can achieve the same level of structural complexity, hierarchy and accuracy as their natural counterparts – biomolecules. The study, published in Science, also reveals the atomic-level mechanisms behind nanoparticle self-assembly.

The findings from the lab of Chemistry Professor Rongchao Jin provide researchers with an important window into how nanoparticles form, and will help guide the construction of nanoparticles, including those that can be used in the fabrication of computer chips, creation of new materials, and development of new drugs and drug delivery devices.

Caption: By resolving the structure of Au246, Carnegie Mellon researchers were able to visualize its hierarchical assembly into artificial solid. Credit: Carnegie Mellon University

A Jan.  23, 2017 CMU news release on EurekAlert, which originated the news item, expands on the theme,

“Most people think that nanoparticles are simple things, because they are so small. But when we look at nanoparticles at the atomic level, we found that they are full of wonders,” said Jin.

Nanoparticles are typically between 1 and 100 nanometers in size. Particles on the larger end of the nanoscale are harder to create precisely. Jin has been at the forefront of creating precise gold nanoparticles for a decade, first establishing the structure of an ultra-small Au25 nanocluster and then working on larger and larger ones. In 2015, his lab used X-ray crystallography to establish the structure of an Au133 nanoparticle and found that it contained complex, self-organized patterns that mirrored patterns found in nature.

In the current study, they sought to find out the mechanisms that caused these patterns to form. The researchers, led by graduate student Chenjie Zeng, established the structure of Au246, one of the largest and most complex nanoparticles created by scientists to-date and the largest gold nanoparticle to have its structure determined by X-ray crystallography. Au246 turned out to be an ideal candidate for deciphering the complex rules of self- assembly because it contains an ideal number of atoms and surface ligands and is about the same size and weight as a protein molecule.

Analysis of Au246’s structure revealed that the particles had much more in common with biomolecules than size. They found that the ligands in the nanoparticles self-assembled into rotational and parallel patterns that are strikingly similar to the patterns found in proteins’ secondary structure. This could indicate that nanoparticles of this size could easily interact with biological systems, providing new avenues for drug discovery.

The researchers also found that Au246 particles form by following two rules. First, they maximize the interactions between atoms, a mechanism that had been theorized but not yet seen. Second the nanoparticles match symmetric surface patterns, a mechanism that had not been considered previously. The matching, which is similar to puzzle pieces coming together, shows that the components of the particle can recognize each other by their patterns and spontaneously assemble into the highly ordered structure of a nanoparticle.

“Self-assembly is an important way of construction in the nanoworld. Understanding the rules of self-assembly is critical to designing and building up complex nanoparticles with a wide-range of functionalities,” said Zeng, the study’s lead author.

In future studies, Jin hopes to push the crystallization limits of nanoparticles even farther to larger and larger particles. He also plans to explore the particles’ electronic and catalytic power.

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

Emergence of hierarchical structural complexities in nanoparticles and their assembly by Chenjie Zeng, Yuxiang Chen, Kristin Kirschbaum, Kelly J. Lambright, Rongchao Jin. Science  23 Dec 2016: Vol. 354, Issue 6319, pp. 1580-1584 DOI: 10.1126/science.aak9750

This paper is behind a paywall.

New design strategy for synthesizing metal-organic frameworks (MOFs)

A Jan. 24, 2017 news item on Nanowerk announces new research from South Korea,

The accurate interpretation of particle sizes and shapes in nanoporus materials is essential to understanding and optimizing the performance of porous materials used in many important existing and potentially new applications. However, only a few experimental techniques have been developed for this purpose.

A team of researchers, led by Professor Wonyoung Choe of Natural Science and Professor Ja Hun Kwak of Energy and Chemical Engineering [ at Ulsan National Institute of Science and Technology {UNIST}] has recently developed a novel design strategy for synthesizing various forms of functional materials, especially for metal-organic materials (MOMs).

The research team expects that this synthetic approach might open up a new direction for the development of diverse forms in MOMs, with highly advanced areas such as sequential drug delivery/release and heterogeneous cascade catalysis targeted in the foreseeable future.

A Jan. 6, 2017 UNIST press release, which originated the news item, provides more detail,

In the last decades, much research has been developed to the synthesis and design of functional materials, but only a few of them could control the walls of the interior of the particles within the nanoporous materials.

In the study, Professor Choe and his team denomstrated sequential self-assembly strategy for synthesizing various forms of MOM crystals, including double-shell hollow MOMs, based on single-crystal to single-crystal transformation from MOP to MOF.

Schematic representation of various forms of micro-/nanostructures. From left are Solid, core-shell, hollow, matryoshka, yolk-shell and multi-shell hollow structures.

Porous materials are highly utilized as catalysts or gas capture materials because they supply abundant surface active sites for chemical reaction. Although materials, like Zeolites, which can be obtained from nature, have the ability to act as catalysts for chemical reactions, they suffer from the difficulty of controlling pore sizes and shapes.

As one solution, scientists have developed self-assembled porous materials using organic molecules and metals. Metal-Organic Frameworks (MOFs) and Metal-Organic Polyhedral (MOPs) are notable examples and they both have holes all over their surfaces. MOPs dissolve easily in chemical solvent, while MOFs are practically insoluble.

“MOFs take the form of three-dimensional (3D) structure, linking metals with organic molecules, while MOPs agglomerate together to form larger clusters,” says Jiyoung Lee, the first contributor of the study and a graduate student in the combined master-doctoral program from Chemistry department.

Schematic illustration of form evolution.

Schematic illustration of form evolution.

According to the research team, this synthetic strategy also yields other forms, such as solid, core-shell, double and triple matryoshka, and single-shell hollow MOMs, thereby exhibiting form evolution in MOMs.

“The best feature of this technique is that it allows two very different substances to coexist within a single crystal,” says Professor Choe. “This technique also permits greater control over size and shape of the pore, which can be then used to regulate the entrance and exit of molecules.”

This particular synthetic approach also has the potential to generate new type of porous materials containing micropores with diameters less than 2nm, macropores with diameters between 20 to 50nm, as well as pores of larger than 50 nm. Such hierarchical pore structure plays a critical role during catalysis, adsorption, and separation processes.

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

Evolution of form in metal–organic frameworks by Jiyoung Lee, Ja Hun Kwak & Wonyoung Choe. Nature Communications 8, Article number: 14070 (2017) doi:10.1038/ncomms14070 Published online: 04 January 2017

This is an open access paper.

Novel self-assembly at 102 atoms

A Jan. 13, 2017 news item on ScienceDaily announces a discovery about self-assembly of 102-atom gold nanoclusters,

Self-assembly of matter is one of the fundamental principles of nature, directing the growth of larger ordered and functional systems from smaller building blocks. Self-assembly can be observed in all length scales from molecules to galaxies. Now, researchers at the Nanoscience Centre of the University of Jyväskylä and the HYBER Centre of Excellence of Aalto University in Finland report a novel discovery of self-assembling two- and three-dimensional materials that are formed by tiny gold nanoclusters of just a couple of nanometres in size, each having 102 gold atoms and a surface layer of 44 thiol molecules. The study, conducted with funding from the Academy of Finland and the European Research Council, has been published in Angewandte Chemie.

A Jan. 13, 2017 Academy of Finland press release, which originated the news item, provides more technical information about the work,

The atomic structure of the 102-atom gold nanocluster was first resolved by the group of Roger D Kornberg at Stanford University in 2007 (2). Since then, several further studies of its properties have been conducted in the Jyväskylä Nanoscience Centre, where it has also been used for electron microscopy imaging of virus structures (3). The thiol surface of the nanocluster has a large number of acidic groups that can form directed hydrogen bonds to neighbouring nanoclusters and initiate directed self-assembly.

The self-assembly of gold nanoclusters took place in a water-methanol mixture and produced two distinctly different superstructures that were imaged in a high-resolution electron microscope at Aalto University. In one of the structures, two-dimensional hexagonally ordered layers of gold nanoclusters were stacked together, each layer being just one nanocluster thick. Modifying the synthesis conditions, also three-dimensional spherical, hollow capsid structures were observed, where the thickness of the capsid wall corresponds again to just one nanocluster size (see figure).

While the details of the formation mechanisms of these superstructures warrant further systemic investigations, the initial observations open several new views into synthetically made self-assembling nanomaterials.

“Today, we know of several tens of different types of atomistically precise gold nanoclusters, and I believe they can exhibit a wide variety of self-assembling growth patterns that could produce a range of new meta-materials,” said Academy Professor Hannu Häkkinen, who coordinated the research at the Nanoscience Centre. “In biology, typical examples of self-assembling functional systems are viruses and vesicles. Biological self-assembled structures can also be de-assembled by gentle changes in the surrounding biochemical conditions. It’ll be of great interest to see whether these gold-based materials can be de-assembled and then re-assembled to different structures by changing something in the chemistry of the surrounding solvent.”

“The free-standing two-dimensional nanosheets will bring opportunities towards new-generation functional materials, and the hollow capsids will pave the way for highly lightweight colloidal framework materials,” Postdoctoral Researcher Nonappa (Aalto University) said.

Professor Olli Ikkala of Aalto University said: “In a broader framework, it has remained as a grand challenge to master the self-assemblies through all length scales to tune the functional properties of materials in a rational way. So far, it has been commonly considered sufficient to achieve sufficiently narrow size distributions of the constituent nanoscale structural units to achieve well-defined structures. The present findings suggest a paradigm change to pursue strictly defined nanoscale units for self-assemblies.”

References:

(1)    Nonappa, T. Lahtinen, J.S. Haataja, T.-R. Tero, H. Häkkinen and O. Ikkala, “Template-Free Supracolloidal Self-Assembly of Atomically Precise Gold Nanoclusters: From 2D Colloidal Crystals to Spherical Capsids”, Angewandte Chemie International Edition, published online 23 November 2016, DOI: 10.1002/anie.201609036

(2)    P. Jadzinsky et al., “Structure of a thiol-monolayer protected gold nanoparticle at 1.1Å resolution”, Science 318, 430 (2007)

(3)    V. Marjomäki et al., “Site-specific targeting of enterovirus capsid by functionalized monodispersed gold nanoclusters”, PNAS 111, 1277 (2014)

Here’s the figure mentioned in the news release,

Figure: 2D hexagonal sheet-like and 3D capsid structures based on atomically precise gold nanoclusters as guided by hydrogen bonding between the ligands. The inset in the top left corner shows the atomic structure of one gold nanocluster.

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

Template-Free Supracolloidal Self-Assembly of Atomically Precise Gold Nanoclusters: From 2D Colloidal Crystals to Spherical Capsids by Dr. Nonappa, Dr. Tanja Lahtinen, M. Sc. Johannes. S. Haataja, Dr. Tiia-Riikka Tero, Prof. Hannu Häkkinen, and Prof. Olli Ikkala. Angewandte Chemie International Edition Volume 55, Issue 52, pages 16035–16038, December 23, 2016 Version of Record online: 23 NOV 2016 DOI: 10.1002/anie.201609036

© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

Ocean-inspired coatings for organic electronics

An Oct. 19, 2016 news item on phys.org describes the advantages a new coating offers and the specific source of inspiration,

In a development beneficial for both industry and environment, UC Santa Barbara [University of California at Santa Barbara] researchers have created a high-quality coating for organic electronics that promises to decrease processing time as well as energy requirements.

“It’s faster, and it’s nontoxic,” said Kollbe Ahn, a research faculty member at UCSB’s Marine Science Institute and corresponding author of a paper published in Nano Letters.

In the manufacture of polymer (also known as “organic”) electronics—the technology behind flexible displays and solar cells—the material used to direct and move current is of supreme importance. Since defects reduce efficiency and functionality, special attention must be paid to quality, even down to the molecular level.

Often that can mean long processing times, or relatively inefficient processes. It can also mean the use of toxic substances. Alternatively, manufacturers can choose to speed up the process, which could cost energy or quality.

Fortunately, as it turns out, efficiency, performance and sustainability don’t always have to be traded against each other in the manufacture of these electronics. Looking no further than the campus beach, the UCSB researchers have found inspiration in the mollusks that live there. Mussels, which have perfected the art of clinging to virtually any surface in the intertidal zone, serve as the model for a molecularly smooth, self-assembled monolayer for high-mobility polymer field-effect transistors—in essence, a surface coating that can be used in the manufacture and processing of the conductive polymer that maintains its efficiency.

An Oct. 18, 2016 UCSB news release by Sonia Fernandez, which originated the news item, provides greater technical detail,

More specifically, according to Ahn, it was the mussel’s adhesion mechanism that stirred the researchers’ interest. “We’re inspired by the proteins at the interface between the plaque and substrate,” he said.

Before mussels attach themselves to the surfaces of rocks, pilings or other structures found in the inhospitable intertidal zone, they secrete proteins through the ventral grove of their feet, in an incremental fashion. In a step that enhances bonding performance, a thin priming layer of protein molecules is first generated as a bridge between the substrate and other adhesive proteins in the plaques that tip the byssus threads of their feet to overcome the barrier of water and other impurities.

That type of zwitterionic molecule — with both positive and negative charges — inspired by the mussel’s native proteins (polyampholytes), can self-assemble and form a sub-nano thin layer in water at ambient temperature in a few seconds. The defect-free monolayer provides a platform for conductive polymers in the appropriate direction on various dielectric surfaces.

Current methods to treat silicon surfaces (the most common dielectric surface), for the production of organic field-effect transistors, requires a batch processing method that is relatively impractical, said Ahn. Although heat can hasten this step, it involves the use of energy and increases the risk of defects.

With this bio-inspired coating mechanism, a continuous roll-to-roll dip coating method of producing organic electronic devices is possible, according to the researchers. It also avoids the use of toxic chemicals and their disposal, by replacing them with water.

“The environmental significance of this work is that these new bio-inspired primers allow for nanofabrication on silicone dioxide surfaces in the absence of organic solvents, high reaction temperatures and toxic reagents,” said co-author Roscoe Lindstadt, a graduate student researcher in UCSB chemistry professor Bruce Lipshutz’s lab. “In order for practitioners to switch to newer, more environmentally benign protocols, they need to be competitive with existing ones, and thankfully device performance is improved by using this ‘greener’ method.”

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

Molecularly Smooth Self-Assembled Monolayer for High-Mobility Organic Field-Effect Transistors by Saurabh Das, Byoung Hoon Lee, Roscoe T. H. Linstadt, Keila Cunha, Youli Li, Yair Kaufman, Zachary A. Levine, Bruce H. Lipshutz, Roberto D. Lins, Joan-Emma Shea, Alan J. Heeger, and B. Kollbe Ahn. Nano Lett., 2016, 16 (10), pp 6709–6715
DOI: 10.1021/acs.nanolett.6b03860 Publication Date (Web): September 27, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall but the scientists have made an illustration available,

An artist's concept of a zwitterionic molecule of the type secreted by mussels to prime surfaces for adhesion Photo Credit: Peter Allen

An artist’s concept of a zwitterionic molecule of the type secreted by mussels to prime surfaces for adhesion Photo Credit: Peter Allen

Lawrence Berkeley National Laboratory (US) and five of its nanoscience projects

An Aug 3, 2016 Lawrence Berkeley National Laboratory news release (also on Azonano as an Aug. 5, 2016 news item) features a selection of their nanoscience projects (Note: Links, embedded images, and embedded videos have been removed),

1. A DIY paint-on coating for energy efficient windows

This “cool” DIY retrofit tech could improve the energy efficiency of windows and save money. Researchers are developing a polymer-based heat-reflective coating that makes use of the unusual molecular architecture of a polymer.

It has the potential to be painted on windows at one-tenth the cost of current retrofit approaches. Window films on the market today reflect infrared solar energy back to the sky while allowing visible light to pass through, but a professional contractor is needed to install them. A low-cost option could significantly expand adoption and result in potential annual energy savings equivalent to taking 5 million cars off the road.

2. Nanowires that move data at light speed

Researchers have found a new way to produce nanoscale wires that can serve as tiny, tunable lasers. The excellent performance of these tiny lasers is promising for the field of optoelectronics, which is focused on combining electronics and light to transmit data, among other applications. Miniaturizing lasers to the nanoscale could further revolutionize computing, bringing light-speed data transmission to desktop, and ultimately, handheld computing devices.

3. Nano sponges that fight climate change

Scientists are developing nano sponges that could capture carbon from power plants before it enters the atmosphere. Initial tests show the hybrid membrane, composed of nano-sized cages (called metal-organic frameworks) and a polymer, is eight times more carbon dioxide permeable than membranes composed only of the polymer.

Boosting carbon dioxide permeability is a big goal in efforts to develop carbon capture materials that are energy efficient and cost competitive. Watch this video for more on this technology.

4. Custom-made chemical factories

Scientists have recently reengineered a building block of a nanocompartment that occurs naturally in bacteria, greatly expanding the potential of nanocompartments to serve as custom-made chemical factories. Researchers hope to tailor this new use to produce high-value chemical products, such as medicines, on demand

The sturdy nanocompartments are formed by hundreds of copies of just three different types of proteins. Their natural counterparts, known as bacterial microcompartments, encase a wide variety of enzymes that carry out highly specialized chemistry in bacteria.

5. Nanotubes that assemble themselves

Researchers have discovered a family of nature-inspired polymers that, when placed in water, spontaneously assemble into hollow crystalline nanotubes. What’s more, the nanotubes can be tuned to all have the same diameter of between five and ten nanometers.

Controlling the diameter of nanotubes, and the chemical groups exposed in their interior, enables scientists to control what goes through. Nanotubes have the potential to be incredibly useful, from delivering cancer-fighting drugs inside cells to desalinating seawater.

It’s nice to see projects grouped together like that as it gives you a bigger picture of what’s taking place at the lab than you’re likely to get reading news releases about individual projects and breakthroughs.

Berkeley Lab has also got an introductory video which does one of the best jobs I’ve seen of conveying the concept of the nanoscale,

H/t to Aug. 10, 2016 news item on Nanowerk for the Berkeley Lab’s ‘nano penny’ video.

Directing self-assembly of multiple molecular patterns within a single material

Self-assembly in this context references the notion of ‘bottom-up engineering’, that is, following nature’s engineering process where elements assemble themselves into a plant, animal, or something else. Humans have for centuries used an approach known as ‘top-down engineering’ where we take materials and reform them, e.g., trees into paper or houses.

Theoretically, bottom-up engineering (self-assembly) is more efficient than top-down engineering but we have yet to become as skilled as Nature at the process.

Scientists at the US Brookhaven National Laboratory believe they have taken a step in the right direction with regard to self-assembly. From an Aug. 8, 2016 Brookhaven National Laboratory news release (also on EurekAlert) by Justin Eure describes the research (Note: A link has been removed),

To continue advancing, next-generation electronic devices must fully exploit the nanoscale, where materials span just billionths of a meter. But balancing complexity, precision, and manufacturing scalability on such fantastically small scales is inevitably difficult. Fortunately, some nanomaterials can be coaxed into snapping themselves into desired formations-a process called self-assembly.

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have just developed a way to direct the self-assembly of multiple molecular patterns within a single material, producing new nanoscale architectures. The results were published in the journal Nature Communications.

“This is a significant conceptual leap in self-assembly,” said Brookhaven Lab physicist Aaron Stein, lead author on the study. “In the past, we were limited to a single emergent pattern, but this technique breaks that barrier with relative ease. This is significant for basic research, certainly, but it could also change the way we design and manufacture electronics.”

Microchips, for example, use meticulously patterned templates to produce the nanoscale structures that process and store information. Through self-assembly, however, these structures can spontaneously form without that exhaustive preliminary patterning. And now, self-assembly can generate multiple distinct patterns-greatly increasing the complexity of nanostructures that can be formed in a single step.

“This technique fits quite easily into existing microchip fabrication workflows,” said study coauthor Kevin Yager, also a Brookhaven physicist. “It’s exciting to make a fundamental discovery that could one day find its way into our computers.”

The experimental work was conducted entirely at Brookhaven Lab’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility, leveraging in-house expertise and instrumentation.

Cooking up organized complexity

The collaboration used block copolymers-chains of two distinct molecules linked together-because of their intrinsic ability to self-assemble.

“As powerful as self-assembly is, we suspected that guiding the process would enhance it to create truly ‘responsive’ self-assembly,” said study coauthor Greg Doerk of Brookhaven. “That’s exactly where we pushed it.”

To guide self-assembly, scientists create precise but simple substrate templates. Using a method called electron beam lithography-Stein’s specialty-they etch patterns thousands of times thinner than a human hair on the template surface. They then add a solution containing a set of block copolymers onto the template, spin the substrate to create a thin coating, and “bake” it all in an oven to kick the molecules into formation. Thermal energy drives interaction between the block copolymers and the template, setting the final configuration-in this instance, parallel lines or dots in a grid.

“In conventional self-assembly, the final nanostructures follow the template’s guiding lines, but are of a single pattern type,” Stein said. “But that all just changed.”

Lines and dots, living together

The collaboration had previously discovered that mixing together different block copolymers allowed multiple, co-existing line and dot nanostructures to form.

“We had discovered an exciting phenomenon, but couldn’t select which morphology would emerge,” Yager said. But then the team found that tweaking the substrate changed the structures that emerged. By simply adjusting the spacing and thickness of the lithographic line patterns-easy to fabricate using modern tools-the self-assembling blocks can be locally converted into ultra-thin lines, or high-density arrays of nano-dots.

“We realized that combining our self-assembling materials with nanofabricated guides gave us that elusive control. And, of course, these new geometries are achieved on an incredibly small scale,” said Yager.

“In essence,” said Stein, “we’ve created ‘smart’ templates for nanomaterial self-assembly. How far we can push the technique remains to be seen, but it opens some very promising pathways.”

Gwen Wright, another CFN coauthor, added, “Many nano-fabrication labs should be able to do this tomorrow with their in-house tools-the trick was discovering it was even possible.”

The scientists plan to increase the sophistication of the process, using more complex materials in order to move toward more device-like architectures.

“The ongoing and open collaboration within the CFN made this possible,” said Charles Black, director of the CFN. “We had experts in self-assembly, electron beam lithography, and even electron microscopy to characterize the materials, all under one roof, all pushing the limits of nanoscience.”

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

Selective directed self-assembly of coexisting morphologies using block copolymer blends by A. Stein, G. Wright, K. G. Yager, G. S. Doerk, & C. T. Black. Nature Communications 7, Article number: 12366  doi:10.1038/ncomms12366 Published 02 August 2016

This paper is open access.

Watching artificial nanofibres self-sort in real-time

A May 31, 2016 news item on phys.org describes research on self-assembling fibres at Kyoto University (Japan) by referencing the ancient Greek mythological figure, Psyche,

The Greek goddess Psyche borrowed help from ants to sort a room full of different grains. Cells, on the other hand, do something similar without Olympian assistance, as they organize molecules into robust, functional fibers. Now scientists are able to see self-sorting phenomena happen in real time with artificial molecules.

The achievement, reported in Nature Chemistry, elucidates how two different types of nanofibers sort themselves into organized structures under artificial conditions.

“Basic cellular structures, such as actin filaments, come into being through the autonomous self-sorting of individual molecules, even though a tremendous variety of proteins and small molecules are present inside the cell,” says lead author Hajime Shigemitsu, a researcher in Itaru Hamachi’s lab at Kyoto University.

A May 30, 2016 Kyoto University news release (also on EurekAlert), which originated the news item, expands on the theme,

“Imagine a box filled with an assortment of building blocks — it’s as if the same type of blocks started sorting themselves into neat bundles all on their own. In living cells, such phenomena always happen, enabling accurate self-assembling of proteins, which is essential for cell functions.”

“If we are able to control self-sorting with artificial molecules, we can work toward developing intelligent, next-generation biomimics that possess the flexibility and diversity of functions that exist in a living cell.”

Study co-author Ryou Kubota explains that previous studies have already made artificial molecules build themselves into fibers — but only when there was one type of molecule around. Having a jumble of types, on the other hand, made the molecules confused.

“The difficulty in inducing self-assembly with artificial molecules is that they don’t recognize the same type of molecule, unlike molecules in the natural world. Different types of artificial molecules interact with each other and make an unsorted cluster.”

From a database of structural analyses, Hamachi and colleagues discovered a combination of nanofibers — namely a peptide-based and lipid-based hydrogelator — that would make sorted fibers without mixing with the other. They then tethered the fibers with fluorescent probes; with a type of microscope typically used in cell imaging, the team was able to observe directly and in real-time how the artificial molecules sorted themselves.

“Ultimately, this finding could help develop new materials that respond dynamically to different environments and stimuli,” elaborates Hamachi. “This insight is not only useful for materials science, but may also provide useful clues for understanding self-organization in cells.”

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

In situ real-time imaging of self-sorted supramolecular nanofibres by Shoji Onogi, Hajime Shigemitsu, Tatsuyuki Yoshii, Tatsuya Tanida, Masato Ikeda, Ryou Kubota, & Itaru Hamachi. Nature Chemistry (2016) doi:10.1038/nchem.2526 Published online 30 May 2016

This paper is behind a paywall bu the researchers have made a video of the self-sorting proteins freely available,

Self-assembling molecular rings from McMaster University (Canada)

An April 21, 2016 news item on Nanotechnology Now highlights some research from Canada’s McMaster University,

Imagine throwing Lego pieces into the air and seeing them fall to the ground assembled into the shape of a house or plane.

Nature effortlessly does the equivalent all the time, using molecules as building blocks.

The right combination of ingredients and conditions spontaneously assembles structures as complex as viruses or cellular membranes. Chemists marvel at this very efficient approach to creating large molecular structures and keep searching for new ways to emulate the process using their own components.

Now, in a McMaster University laboratory, chemistry researchers have managed to coax molecules known as tellurazole oxides into assembling themselves into cyclic structures – a major advance in their field that creates a new and promising set of materials.

An April 20, 2016 McMaster University news release by Wade Hemsworth, which originated the news item, provides more detail,

“This is a seed we have found – one we have never seen. It has sprouted, now we need to see how tall the tree will grow and what kind of fruit it will bear,” says Ignacio Vargas Baca, an associate professor in McMaster’s Department of Chemistry and Chemical Biology. “Once we understand the properties of these new materials, we can look at their potential applications.”

Barca’s group works in the realm of supramolecular chemistry, where the key is to exploit the forces that keep molecules together. Hydrogen atoms, for example, can form strong bridges between water molecules or pairs of DNA strands.

Earlier, the realization that atoms of iodine and bromine can act in a similar way had sparked great excitement in chemistry circles, giving rise to the hot field of “halogen bonding,” where other researchers have had success with enormous assemblies, but have had difficulties controlling the association of just a few molecules.

Meanwhile, Vargas’ group moved over one column on the periodic table of elements to work with chalcogens instead.

They discovered that certain molecules that contain the element tellurium assemble automatically into rings in solution, a success that has no rival in halogen bonding and constitutes a significant advance in supramolecular chemistry.

For now, he and his team envision uses in areas as diverse as communication technologies, gas storage and catalysis.

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

Supramolecular macrocycles reversibly assembled by Te…O chalcogen bonding by Peter C. Ho, Patrick Szydlowski, Jocelyn Sinclair, Philip J. W. Elder, Joachim Kübel, Chris Gendy, Lucia Myongwon Lee, Hilary Jenkins, James F. Britten, Derek R. Morim, & Ignacio Vargas-Baca.    Nature Communications 7, Article number: 11299  doi:10.1038/ncomms11299 Published 19 April 2016

This is an open access paper.

4D printing: a hydrogel orchid

In 2013, the 4th dimension for printing was self-assembly according to a March 1, 2013 article by Tuan Nguyen for ZDNET. A Jan. 25, 2016 Wyss Institute for Biologically Inspired Engineering at Harvard University news release (also on EurekAlert) points to time as the fourth dimension in a description of the Wyss Institute’s latest 4D printed object,

A team of scientists at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences has evolved their microscale 3D printing technology to the fourth dimension, time. Inspired by natural structures like plants, which respond and change their form over time according to environmental stimuli, the team has unveiled 4D-printed hydrogel composite structures that change shape upon immersion in water.

“This work represents an elegant advance in programmable materials assembly, made possible by a multidisciplinary approach,” said Jennifer Lewis, Sc.D., senior author on the new study. “We have now gone beyond integrating form and function to create transformable architectures.”

In nature, flowers and plants have tissue composition and microstructures that result in dynamic morphologies that change according to their environments. Mimicking the variety of shape changes undergone by plant organs such as tendrils, leaves, and flowers in response to environmental stimuli like humidity and/or temperature, the 4D-printed hydrogel composites developed by Lewis and her team are programmed to contain precise, localized swelling behaviors. Importantly, the hydrogel composites contain cellulose fibrils that are derived from wood and are similar to the microstructures that enable shape changes in plants.

By aligning cellulose fibrils (also known as, cellulose nanofibrils or nanofibrillated cellulose) during printing, the hydrogel composite ink is encoded with anisotropic swelling and stiffness, which can be patterned to produce intricate shape changes. The anisotropic nature of the cellulose fibrils gives rise to varied directional properties that can be predicted and controlled. Just like wood, which can be split easier along the grain rather than across it. Likewise, when immersed in water, the hydrogel-cellulose fibril ink undergoes differential swelling behavior along and orthogonal to the printing path. Combined with a proprietary mathematical model developed by the team that predicts how a 4D object must be printed to achieve prescribed transformable shapes, the new method opens up many new and exciting potential applications for 4D printing technology including smart textiles, soft electronics, biomedical devices, and tissue engineering.

“Using one composite ink printed in a single step, we can achieve shape-changing hydrogel geometries containing more complexity than any other technique, and we can do so simply by modifying the print path,” said Gladman [A. Sydney Gladman, Wyss Institute a graduate research assistant]. “What’s more, we can interchange different materials to tune for properties such as conductivity or biocompatibility.”

The composite ink that the team uses flows like liquid through the printhead, yet rapidly solidifies once printed. A variety of hydrogel materials can be used interchangeably resulting in different stimuli-responsive behavior, while the cellulose fibrils can be replaced with other anisotropic fillers of choice, including conductive fillers.

“Our mathematical model prescribes the printing pathways required to achieve the desired shape-transforming response,” said Matsumoto [Elisabetta Matsumoto, Ph.D., a postdoctoral fellow at the Wyss]. “We can control the curvature both discretely and continuously using our entirely tunable and programmable method.”

Specifically, the mathematical modeling solves the “inverse problem”, which is the challenge of being able to predict what the printing toolpath must be in order to encode swelling behaviors toward achieving a specific desired target shape.

“It is wonderful to be able to design and realize, in an engineered structure, some of nature’s solutions,” said Mahadevan [L. Mahadevan, Ph.D., a Wyss Core Faculty member] , who has studied phenomena such as how botanical tendrils coil, how flowers bloom, and how pine cones open and close. “By solving the inverse problem, we are now able to reverse-engineer the problem and determine how to vary local inhomogeneity, i.e. the spacing between the printed ink filaments, and the anisotropy, i.e. the direction of these filaments, to control the spatiotemporal response of these shapeshifting sheets. ”

“What’s remarkable about this 4D printing advance made by Jennifer and her team is that it enables the design of almost any arbitrary, transformable shape from a wide range of available materials with different properties and potential applications, truly establishing a new platform for printing self-assembling, dynamic microscale structures that could be applied to a broad range of industrial and medical applications,” said Wyss Institute Founding Director Donald Ingber, M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital and Professor of Bioengineering at Harvard SEAS [School of Engineering and Applied Science’.

Here’s an animation from the Wyss Institute illustrating the process,

And, here’s a link to and a citation for the paper,

Biomimetic 4D printing by A. Sydney Gladman, Elisabetta A. Matsumoto, Ralph G. Nuzzo, L. Mahadevan, & Jennifer A. Lewis. Nature Materials (2016) doi:10.1038/nmat4544 Published online 25 January 2016

This paper is behind a paywall.