Breakthrough in sensing at the nanoscale

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The super-sensitive nanocrystals enable the micro-structured optical fibre to detect and track the movement of a single nanocrystal remotely. Credit: Dr Mathieu Juan

Researchers have made a breakthrough discovery in identifying the world's most sensitive nanoparticle and measuring it from a distance using light. These super-bright, photostable and background-free nanocrystals enable a new approach to highly advanced sensing technologies using optical fibres.

This discovery, by a team of researchers from Macquarie University, the University of Adelaide, and Peking University, opens the way for rapid localisation and measurement of cells within a living environment at the nanoscale, such as the changes to a single living cell in the human body in response to chemical signals.

Published in Nature Nanotechnology today, the research outlines a new approach to advanced sensing that has been facilitated by bringing together a specific form of nanocrystal, or "SuperDot" with a special kind of optical fibre that enables light to interact with tiny (nanoscale) volumes of liquid.

"Up until now, measuring a single nanoparticle would have required placing it inside a very bulky and expensive microscope," says Professor Tanya Monro, Director of the University of Adelaide's Institute for Photonics and Advanced Sensing (IPAS) and ARC Australian Laureate Fellow. "For the first time, we've been able to detect a single nanoparticle at one end of an optical fibre from the other end. That opens up all sorts of possibilities in sensing."

"Using optical fibres we can get to many places such as inside the living human brain, next to a developing embryo, or within an artery ? locations that are inaccessible to conventional measurement tools.

"This advance ultimately paves the way to breakthroughs in medical treatment. For example, measuring a cell's reaction in real time to a cancer drug means doctors could tell at the time treatment is being delivered whether or not a person is responding to the therapy."

The micro-structured optical fibre has been employed as a nanoliter-volume spectroscope to analyse the optical properties of nanocrystals. Credit: Matthew Henderson

The performance of sensing at single molecular level had previously been limited by both insufficient signal strength and interference from background noise. The special optical fibre engineered at IPAS also proved useful in understanding the properties of nanoparticles. "Material scientists have faced a huge challenge in increasing the brightness of nanocrystals," says Dr. Jin, ARC Fellow at Macquarie University's Advanced Cytometry Laboratories. "Using these optical fibres, however, we have been given unprecedented insight into the light emissions. Now, thousands of emitters can be incorporated into a single SuperDot – creating a far brighter, and more easily detectable nanocrystal."

Under infrared illumination, these SuperDots selectively produce bright blue, red and infrared light, with a staggering thousand times more sensitivity than existing materials. "Neither the glass of the optical fibre nor other background biological molecules respond to infrared, so that removed the background signal issue. By exciting these SuperDots we were able to lower the detection limit to the ultimate level – a single nanoparticle," says Jin.

"The trans-disciplinary research from multiple institutions has paved the way for this innovative discovery," says Jin, "with the interface of experts in nanomaterials, photonics engineering, and biomolecular frontiers."

"These joint efforts will ultimately benefit patients around the world - for example, our industry partners Minomic International Ltd and Patrys Ltd are developing uses for SuperDots™ in cancer diagnostic kits, detecting incredibly low numbers of biomarkers within conditions like prostate and multiple myeloma cancer." Macquarie is now actively seeking other industrial partners with the capacity to jointly develop solutions outside of these fields.

Read more at: http://phys.org/

Mysteries of Thunderstorms

Mysteries of Thunderstorms
Atmospheric Scientists Link Lightning to Ice Particles In Clouds

October 1, 2006 — Satellite imaging is now helping atmospheric scientists link the amount of charged ice in clouds to lightning activity. Ice particles in thunderstorms can help increase precipitation, the scientists found. Different-sized ice particles within a cloud also carry a positive or negative charge, and as the particles collide, that charge builds up, leading to lightning.

HUNTSVILLE, Ala. -- There's no mistaking the billowing clouds, the noise, the rain, and the lightning of a thunderstorm. But why do some dark and ominous clouds form into huge masses of rain and lightning while others just pass us by?

We'll likely see a big storm roll-in on warm days, but you might be surprised to learn thunderstorms are also filled with ice!

"Ice plays a big role in the amount of rain that you see," says Walter Petersen, an atmospheric scientist at University of Alabama, Huntsville.

He says ice in clouds is the key to really big electrical storms. Ice creates lightning and often heavy rain.

"A fair amount of rain that you see over continents actually is the result of melting ice that's created high up in the, high up in the development of thunderstorms," Petersen says.

Ice is vital to the development of lightning. Different-sized ice particles within a cloud carry a positive or negative charge. As the particles collide, that charge builds up. When the charge is released -- we see lightning.

Satellites watch lightning flashes from space, helping scientists to learn more about them.

"We know how much ice is associated with a given number of lightning flashes," Dr. Petersen says. "Then we can say something about the amount of rain that falls out of those clouds." Knowing the rain that falls was once ice above you -- a heads up about what's really inside a thunderstorm.

BACKGROUND: Most people know that thunderstorms tend to form on warm days, but new satellite observations indicate that in order for lightning to form, thunderstorm clouds need to have a high content of ice.

THE RESEARCH: Walter Petersen, a meteorologist at the University of Alabama, Huntsville, analyzed observations of lightning and precipitation from 1998 to 2000 taken from the Tropical Rainfall Measuring Mission (TRIMM), launched in 1997. The relationship between the number of lightning strikes and how much ice crystals are present in clouds is the same regardless of different atmospheric environments over oceans, coasts, and continents. The relationship between rain and lightning, in comparison, does not show this same level of consistency. The results support previous assumptions about the basic physics of lightning and ice. As a result, the density of lightning in a storm could be used in the future to predict the amount of ice that is present.

HOW STORMS DEVELOP: Storm clouds form as moisture evaporates from the earth into the atmosphere, where the droplets jostle against each other. The air cools off rapidly with as it reaches higher altitude. Sometimes a cold front – the boundary between where the cold air from one thunderstorm meets the air outside the storm for example – will force the moist air upward into the colder air. This moist air cools off and the water vapor "condenses" into liquid drops, forming clouds. The process continues: more and more water vapor turns into liquid, and the moist air warms up even more and rises higher and higher. A thunderstorm results.

WHAT CAUSES LIGHTNING? As more and more water droplets collide inside a cloud, their atoms bounce off each other more forcefully. This knocks off electrons. The ousted electrons gather at the lower portion of the cloud, giving it a negative charge, while the upper part of the cloud becomes positively charged. Eventually the growing negative charge becomes so intense that electrons on the Earth's surface are repelled and burrow deeper into the Earth. The Earth's surface becomes positively charged, and hence very attractive to the negative charge accumulating in the bottom of the cloud. All that is needed is a conductive path between cloud and Earth, in the form of ionized air.

The American Meteorological Society contributed to the information contained in the TV portion of this report.

Magnetic Charge Crystals Imaged in Artificial Spin Ice

Aug. 28, 2013 — A team of scientists, led by University of Illinois physicist Peter Schiffer, has reported direct visualization of magnetic charge crystallization in an artificial spin ice material, a first in the study of a relatively new class of frustrated artificial magnetic materials-by-design known as "Artificial Spin Ice." These charges are analogs to electrical charges with possible applications in magnetic memories and devices.

3-D depiction of the honeycomb artificial spin ice topography after the annealing and cooling protocols. The light and dark colors represent the north and south magnetic poles of the islands. (Credit: Ian Gilbert, Department of Physics & Frederick Seitz Materials Research Laboratory)

he research team's findings appear in the August 29 issue of the journalNature.

The unique properties of spin ice materials have fascinated scientists since they were first discovered in the late 1990s in naturally occurring rare earth titanites. The material is aptly named: the highly complex ordering of nanoscale magnets in spin ice obey the same rules that determine the positional ordering of hydrogen and oxygen atoms in frozen water ice. Both have "spin" -- degrees of freedom -- with frustrated interactions that prevent complete freezing, even at absolute zero.

In 2006, an interdisciplinary team of physicists and materials scientists designed the first artificial spin ice, a two-dimensional array of magnetic nanoislands that are fabricated to interact in complex ways, depending on the chosen design of the array. The islands were lithographically printed onto a substrate, arranged in a square-lattice pattern, with the north and south poles of each nanomagnet meeting and interacting at their four-pronged vertices.

Now the same research team has developed a new annealing protocol that allows the artificial material's full potential for highly complex magnetic interactions to be realized. The new protocol was applied to two artificial spin ice materials, one configured in a square-lattice pattern, the other in a hexagonal-honeycomb pattern with three-pronged vertices.

In the honeycomb pattern, where three magnetic poles intersect, a net charge of north or south is forced at each vertex. The magnetic "monopole charge" at each vertex influences the magnetic "charge" of the surrounding vertices. The team was able to image the crystalline structure of the magnetic charges using magnetic force microscopy.

"Nanomagnets are so small that their behavior becomes relatively simple. We can arrange the magnets in a particular lattice pattern -- square or honeycomb -- and they interact in a way that we can predict and control," Schiffer expained. "The challenge -- you have to get the nanomagnets to flip their north and south poles to show how they interact. It's hard to force them to show the effects of interaction, since they get stuck in one particular arrangement."

The research team's new annealing protocol -- heating the material to a high temperature where their magnetic polarity is suppressed (here, about 550 degrees Celsius) -- allows the nanomagnets to flip their polarity and freely interact. As the material cools, the nanomagnets are ordered according to the interactions of their poles at the vertices.

The collective thermal behavior of the arrays is studied through statistical mechanics, a branch of fundamental physics. As theorized, the monopole charge of each vertex was found to contribute to the order of the entire system in a manner analogous to the interactions of electric charges at the atomic scale during water ice crystal growth.

Los Alamos National Laboratory staff scientist Cristiano Nisoli explained, "The emergence of magnetic monopoles in spin ice systems is a particular case of what physicists call fractionalization, or deconfinement of quasi-particles that together are seen as comprising the fundamental unit of the system, in this case the north and south poles of a nanomagnet. We have seen how arranging magnets in a honeycomb configuration allows for these charges to be sort of 'stripped' from the magnetic islands to which they belong and become relevant degrees of freedom."

The ability to use the magnetic charges as degrees of freedom has implications for future technological applications.

"Magnetic technology generally concerns itself with manipulation of localized dipolar degrees of freedom," Nisoli said. "The ability of building materials containing delocalized monopolar charges is very exciting with possible technological implications in data storage and computation."

An advantage of artificial spin ice is that it can be designed in different topologies, and examined subsequently to see the effects of those topologies. That allows physicists to explore a wide range of possible behaviors that are not accessible in natural crystals.

"This work demonstrates a direction in condensed matter physics that is quite opposite to what has been done in the last sixty decades or so," said Nisoli. "Instead of imagining an emergent theoretical description to model the behavior of a nature-given material and validating it indirectly, we engineer materials of desired emergent properties that can be visualized directly."

The theoretical work for this research was performed at Los Alamos National Laboratory under Cristiano Nisoli and Gia-Wei Chern, and at Penn State University under Vincent Crespi and Paul Lammert. Synthesis of the magnetic materials and the high temperature treatment was performed at the University of Minnesota's Department of Chemical Engineering and Materials Science under Chris Leighton. Magnetic measurements and lithography were performed at Penn State University and the University of Illinois' Frederick Seitz Materials Research Laboratory by graduate students Sheng Zhang and Ian Gilbert, under the direction of Peter Schiffer.

This research was supported by the U.S. Department of Energy and the National Science Foundation.