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Posted: Aug 27, 2008

Nanotechnology tunes - listening to the music of molecules

(Nanowerk Spotlight) Detecting the presence of a given substance at the molecular level, down to a single molecule, remains a considerable challenge for many nanotechnology sensor applications that range from nanobiotechnology research to environmental monitoring and antiterror or military applications.
Currently, chemical functionalization techniques are used to specify what a nanoscale detector will sense. For biological molecules, this might mean developing an antibody/antigen (i.e., lock-key) pair, or an alternative synthetically generated ligand. For chemical gases, it is much more challenging to develop the right 'glue' that sticks a given gas (and only that gas) to a substrate. Thus, for many gas-sensing applications, appropriate functionalization may not even possible.
The advantage of spectroscopic techniques – measuring and interpreting electromagnetic spectra arising from either emission or absorption of radiant energy by various substances – such as Raman, infrared, and nuclear magnetic resonance spectroscopy is that they are label-free, i.e. they require no preconditioning in order to identify a given analyte. They are also highly selective, capable of distinguishing species that are chemically or functionally very similar. On the downside, spectroscopic methods face enormous challenges in measuring dilute concentrations of an analyte and generally involve the use of large, expensive equipment.
"We have been working on ways to overcome the functionalization bottleneck in sensing and, instead of trying to see a molecule by using photons or electrons – as in optical spectroscopy or electron microscopy – we have been using vibrational energy exchange to in effect 'listen' to the vibrations of the molecule," Jeffrey Grossman tells Nanowerk. "The concept is much like bringing a set of nano tuning forks up to a molecule and seeing which ones become excited. Those would form a chord of 'notes' that are unique to that particular molecule. Thus, the molecule can be identified."
In his recent work, funded by the National Science Foundation, Grossman, who leads the Computational Nanoscience Group at UC Berkeley and is Executive Director of Berkeley's Center of Integrated Nanomechanical Systems, has been taking advantage of the unique manner in which vibrational energy transfers between nanoscale objects, with applications spanning chemical, biological, radiation, and even acoustic sensing.
"The scientific core of our work is aimed at utilizing the unique way in which mechanical energy – in other words, heat – is exchanged at the nanoscale" Grossman explains. "Specifically, we have shown that if one nanoscale object vibrates at the same frequency as another – 'in resonance' – then it is possible for these two objects to exchange heat extremely efficiently. At the same time, if they are not vibrating at the same frequency then the flow of heat is blocked and little or no energy is exchanged."
In essence, what Grossman and his group have done is to demonstrate that one can take advantage of this nanomechanical exchange of energy for detection or characterization of an unknown molecule type. They have termed their novel chemical detection technique nanomechanical resonance spectroscopy (NRS). NRS basically employs an array of nanomechanical resonators that are used to directly interrogate a heated ('exited thermally') analyte's vibrational frequencies.
They laid some of the groundwork for the theory behind nanomechanical energy exchange, which allowed them to demonstrate this concept now, in a paper last year in Physical Review Letters ("Nanomechanical Energy Transfer and Resonance Effects in Single-Walled Carbon Nanotubes").
The new proposed NRS sensor can 'listen' to the 'music' a molecule makes without needing to adhere the molecule to a surface, thereby allowing for continuous measurements with little or no cleaning, resetting, or degradation of the sensor. The result would be a nanodetection system that can detect many different species without functionalization chemistry steps. The NRS is only possible because of nanotechnology's ability to synthesize nanoscale objects that resonate at the same frequencies of the natural vibrations of molecules.
listening to the music of molecules
Left: Illustration of nanotube or nanowire when its fundamental mode of oscillation is unexcited (bottom) or excited (top). Right: analogy with a piano – the frequency of oscillation of the nanotube or nanowire can be thought of as a note on a piano. (Image: Dr. Grossman, UC Berkeley)
First-authored by P. Alex Greaney, a post-doc in Grossman's group, the scientists published their findings in Nano Letters ("Nanomechanical Resonance Spectroscopy: A Novel Route to Ultrasensitive Label-Free Detection").
This work is part of an emerging field of study of frequency dependent thermal phenomena in nanoscale systems – that is, recognizing and exploiting the wave nature of heat. In these small systems the macroscopic concept of temperature (the time averaged thermal energy) is insufficient to describe how a system will behave.
Grossman points out that one must also be concerned about how the thermal energy is distributed in frequency and this distribution can influence the way in which heat is transported (or blocked) as well as how heat effects other important properties such as electron transport.
"There are other researchers who are studying the importance of the frequency dependence of heat in nanostructures, for example in thermoelectric materials, or in thermally rectifying materials" he says. "However, our work is unique by proposing an application that depends solely on frequency dependent phenomena and also by utilizing these phenomena for label-free detection."
each molecule has its own unique set of vibrational excitations
Illustration of how each molecule has its own unique set of vibrational excitations which, taken together, form a unique “chord” in the NRS approach. (Image: Dr. Grossman, UC Berkeley)
Grossman notes that in their recent paper, he and Greaney present the NRS concept and use atomistic computational modeling to demonstrate a proof-of-principle of the idea. An actual NRS device has yet to be built, although there are many ways in which the scientists believe this could be done.
"One could envision several NRS device setups" says Grossman. "For example, a series of carbon nanotubes of differing length (or radius) suspended over a trench, similar to the strings in a harp. Alternatively one could pass analytes through holes in graphene membranes or over substrates coated in fullerenes of different radius. The design of a practical NRS is more limited by capabilities for detecting the excitation of a vibrational mode than the ability to fabricate nanoscale devices."
The number of applications for a sensitive, label-free detection system is quite extensive. It would be extremely useful in areas such as medicine, homeland security, environmental monitoring, and clean energy and water.
Research in this area of thermal phenomena and heat flow at the nanoscale is just beginning. "With the development of theory for describing heat transport in nanoscale systems and advances in nanoscale fabrication and characterization techniques we are now well equipped to study these phenomena" Grossman says. "As a result we should look forward to the development of more applications that exploit the wave nature of heat."
By . Copyright Nanowerk LLC

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