Massachusetts Institute of Technology, Cambridge, USA

A chemist predicts a bright future for sensors based on carbon nanotubes.

I am struck by the parallels between the development of polymer-based chemical sensors and those made from carbon nanotubes.

About ten years ago, I started to develop sensors from conjugated organic polymers, which took advantage of the materials' optical properties, rather than the electrical properties that had been exploited in devices until that time. This work led to fluorescent sensors, which are now being used in Iraq to detect explosives.

As with polymers, early work on nanotube sensors focused on detecting changes in a tube's electrical conduction when it binds to a molecule of interest. But electrical responses are sensitive to stray electric fields, which create interference in the signal.

Now, researchers working with nanotubes are also moving towards optical methods. A demonstration of a biosensor for glucose (P. W. Barone and M. S. Strano Angew. Chem. Int. Edn 45, 8138–8141; 2006) sets the stage.

To make the sensor, the team first attached glucose groups to nanotubes. They then mixed these nanotubes with a large molecule, known as concanavalin A, which can bind to four glucose molecules at once. The glucose-decked nanotubes end up caught in clumps around the concanavalin A, which attenuates their emission. This system is sensitive to glucose because any glucose in solution loosens the nanotube clusters, and so boosts fluorescence.

A significant advantage of nanotubes is that they emit near infrared light, a longer wavelength than that accessible with polymers. And it just happens that human tissue is almost transparent in this spectral region. As a result, sensors based on these materials might be used for in vivo clinical diagnostics.

Posted by Jenny Hogan at 06:00 PM | Permalink | Comments (0) | TrackBacks (0)

University of Wisconsin-Madison, USA

A geophysicist wonders how and why faults behave in so many different ways.

I'm involved, with colleagues, in a project of the Integrated Ocean Drilling Program (IODP) to drill deep into the Nankai Trough subduction zone off southwestern Japan — a site of numerous great earthquakes and tsunamis.

Major unknowns in the generation of tsunamis include how far earthquake fault slip can propagate up towards the sea bed and what factors control how that slip stops in accretionary wedges — the submarine mountain ranges created as sediment and rock are scraped off the sinking plate.

My research focus is on faults in such wedges, which are generally thought of as aseismic, or incapable of earthquakes. By drilling into the wedge faults at Nankai Trough, we hope to learn how aseismic faults give way with depth to the seismic faulting associated with tsunamis.

Recently, a new kind of slow-motion earthquake was observed in this wedge (Y. Ito & K. Obara Geophys. Res. Lett. 33, L02311; 2006). Suddenly, wedges don't seem so aseismic after all.

These 'very-low-frequency' earthquakes, some as large as magnitude 4.4, have previously gone unrecognized because their seismic waves don't show up in the frequency range in which earthquakes are normally detected.

By chance, the quakes were detected exactly where my IODP team plans to start drilling later this year. We hope to install sensors deep in the subsurface to record the earthquakes up close and to measure pore fluid pressure and strain in the rock. We'll also collect samples for laboratory studies of the frictional properties of the rock.

Taken together, the in situ and sample data should yield insight into the processes responsible for these slow-motion quakes. This might help us to understand the aseismic–seismic transition.

Posted by Jenny Hogan at 06:00 PM | Permalink | Comments (0) | TrackBacks (0)

University of Arizona, Tucson, USA

An expert on instabilities jumps from optically bound plastic beads to the brain.

It's not often that reading scientific papers turns my mind to the melancholic work of great Russian writers, but a recent one did.

The paper reports observations of 'bistability' in a simple optical system. Bistable systems have two stable output states for the same input. In this case, the researchers had studied the behaviour of two plastic spheres, trapped side-by-side in a pair of counter propagating laser beams (N. K. Metzger et al. Phys. Rev. Lett. 96, 068102; 2006). They found that the beads could adopt two stable arrangements, differing in the beads' separation.

Bistability arises in optical systems that show nonlinear responses to changes in light intensity and include some kind of feedback process. Here, one bead feels the position of the other because each affects the light field around it, creating the necessary feedback.

The researchers modelled how the two stable states come about, combining equations that describe the propagation of the light with others that predict the forces on the beads. I was impressed by how many physical effects are taken into account in the model.

And this is what turned my thoughts away from the physics of my research to the literature of my homeland. It is believed that some Russian authors, including Leo Tolstoy, may have suffered from what is now known as a bipolar disorder, characterized by states of euphoria and depression.

I have wondered before whether bistability in optical systems might serve as a simple model to help understand the mechanisms that underlie bistability in the human brain. Papers such as this one put that challenge in perspective — modelling a system that involves just two beads is already nontrivial.

Posted by Jenny Hogan at 06:00 PM | Permalink | Comments (0) | TrackBacks (0)

Max-Planck Institute for Biogeochemistry, Jena, Germany

A biogeochemist finds inspiration for life on the ocean floor.

My research interests lie in understanding the interplay between the physical and chemical conditions that constrain life, and the feedback processes by which life shapes the Earth's environment.

I want to understand these interactions in terms of a thermodynamic hypothesis that states that systems dissipate as much energy as possible. Can life be seen as an emergent outcome of this tendency for the whole Earth system? To test this, one would need to show that it is possible to predict the emergence of life from the hypothesis, as well as its impact on Earth's early environment.

Two articles (M. J. Russell & A. J. Hall GSA Memoir 198, 1–32; 2006, and M. J. Russell Am. Sci. 94, 32–39; 2006) could provide a starting point. The authors give a detailed picture of the thermodynamics of life emerging at hydrothermal mounds on the ocean floor.

One of the earliest metabolic reactions would have involved the conversion of hydrogen, carbon dioxide and sulphur compounds into organic carbon, acetate and water. This would have happened in the hot, mineral-rich spring water seeping into the hollow mound.

But its influence would have been felt more widely. Removing sulphur from the environment would have changed atmospheric composition and cloud cover, affecting the amount of sunlight reaching the ground. And acetate may have served as fuel for methanogens, methane-producing organisms known to live in vents. Increased methane production would have raised its levels in the atmosphere, resulting in higher surface temperatures on Earth.

Quantifying these interactions should help us to understand whether the evolution of our planet emerged from general thermodynamic trends.

Posted by Jenny Hogan at 06:00 PM | Permalink | Comments (1) | TrackBacks (0)