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How to build a nanothermometer

(Nanowerk Spotlight) One of the problems nanoscientists encounter in their forays into the nanoworld is the issue of accurate temperature measurement. Ever since Galileo Galilei invented a rudimentary water thermometer in 1593, accurate temperature measurement has been a challenging research topic and thermosensing technologies have become a field in their own right. Now, that technology has reached the nanoscale, temperature gradients are becoming essential in areas such as thermoelectricity, nanofluidics, design of computer chips, or hyperthermal treatment of cancer. Currently there is no established method how to measure the temperature gradients at the nanoscale. Most of today's probes are traditional bulk probes, the kind that gets inserted into a sample and measures the temperature. Liquid crystal films which change colors depending on temperature also have at least microscale thickness and lateral dimensions. A recent review addresses these issues and gives an overview of current and future developments for nanoscale temperature measurement technologies.
"The development of a nanoscale thermometer is not only a matter of size, but also requires materials with novel physical properties, because all physicochemical and thermodynamic properties are drastically altered at this tiny scale" Dr. Nicholas Kotov explains to Nanowerk. "The measurement of local temperature in a single cell and in volumes less than 10-18 liters are becoming the technological frontier. The development of thermometers able to operate within the spatial constraints of the operating environment should be addressed using precise, nanoscale sensing modules, which potentially can be employed in biological applications as a result of their biocompatibility."
Kotov, an Associate Professor in Chemical Engineering at the University of Michigan, together with Dr. Jaebeom Lee, a previous research fellow in his group, published a review article ("Thermometer design at the nanoscale") in which they cover current developments in various nanoscale thermometers and describe their advantages and applications. In particular, progress in thermal sensors using molecular and biological moieties, as well as nanoscale superstructures, is stressed as a novel approach to thermometry development.
"The accurate temperature regime is critical for medical applications" says Kotov. "All biological systems are temperature sensitive. We are designed to operate at 37 degrees celsius. If the temperature gets higher or lower that makes a tremendous change in our feeling of health. Similarly, large differences occur with biomedical measurements based on protein reactions. For example a microfluidics diagnostic device that is becoming more common now may show you that you have cancer when you don't, just because the temperature of the process is slightly off."
Carbon nanotubes (CNTs) have proven very useful in fabricating nanothermometer. In one example, a 7 nm thin film of CNTs forms a fairly simple thermometer with a measurable range of 373-600°K under vacuum. The ambient temperature of the nanotube directly corresponds to the turn-on field and emission current at a given applied electronic field. In another example of CNT use for thermometers, in what is analogous in shape to that of a conventional mercury thermometer but a billion times smaller, the gallium in a gallium-filled carbon/MnO nanotube serves as a temperature indicator by expanding and contracting inside the nanotube in the range of 303-2478°K (see our Nanowerk Spotlight: "Carbon nanotube nanothermometers").
According to Kotov, nanoparticles and nanocolloids are the natural choice for nanoscale temperature probes because of the size and optical activity that can be used to monitor temperature. The nanothermometers can be made from dynamic nanoscale assemblies. "The dynamic aspect of these assemblies is quite essential because it allows the thermometer to operate reversibly, i.e. it is not a one-shot measurement" says Kotov. "Polymers are excellent choices for making dynamic nanoscale assemblies, and superstructures from particles and polymers are a logical choice for such structures."
Now the question is how to make such a nanothermometer – how can scientists learn from nature and use bioconjugation reactions to assemble them?
Kotov and his group came up with a structure that looks like a planet surrounded by many satellites (corona assemblies). The "planet" in the center is a fairly "big" gold nanoparticle (approx 20 nm). The "satellites" are smaller cadmium telluride (CdTe) semiconductor particles (3-7 nm) tethered to the gold particle with a flexible polymeric PEG chain. The polymer "spring" acts like a coiled garden hose that contracts and tightens in the cold and relaxes in the heat. As the polymer responds to heat or cold, the particles attached to the ends move closer together or farther apart. As the distance is altered the particles produce a concomitant change in the luminescence output. ("Nanoparticle Assemblies with Molecular Springs: A Nanoscale Thermometer")
Blown bubble film with nanoparticles
Schematic of the nanoparticle superstructure of the nanothermometer. (Image: Dr. Kotov)
"Temperature changes make the PEG chains expand or contract and this movement affects the emission of the entire corona assembly" explains Kotov. "The emission becomes weaker when temperature rises and the polymer swells and stronger when the temperature drops and the PEG contracts."
He notes that the optical behavior of the corona assemlies and their responce to temperature was explained by using electromagnetic theory of particle interactions developed by Professor Alexander Govorov from Ohio University.
Kotov points out that these thermometers can be particularly useful in measuring the temperature profiles in fluid media, for instance in lab-on-a-chip devices and in hyperthermal eradication of cancer. In this technique, gold nanoparticles are illuminated with pulsed laser irradiation, which raises the temperature of the attached targeted cancer cells and they die. "The temperature change in the tissue is fairly small, but even the difference by 2-3 degrees is significant, whether the cancer is eliminated completely or not" he says. "Without appropriate temperature profiling in live tissue, when movement of fluids (e.g. blood) is present, this is very difficult to predict or calculate. Nanothermometers, in principle, allow to measure it with spatial resolution smaller than the typical cell."
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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