Researchers at JILA have demonstrated the use of infrared laser light to quickly and precisely heat the water in "nano bathtubs"-tiny sample containers-for microscopy studies of the biochemistry of single molecules and nanoparticles. Described in a new paper published in J. Phys. Chem. Lett., the JILA technique is faster, more controllable, and less prone to damaging expensive optics or accidentally altering chemistry than conventional methods using electric currents for bulk heating of microscope stages, optics and samples. The demonstration extends a technique used to study single living cells to the field of single-molecule microscopy. Fast, noncontact heating of very small samples is expected to enable new types of experiments with single molecules. For example, sudden, controlled jumps in temperature could be used to activate molecular processes and observe them in real time.
JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder (CU).
The JILA "bathtubs" consist of about 35 picoliters of water on a glass slide. Gently focused infrared laser light is used to heat a nanoscale column of water. By moving the laser beam, this column can be made to warm single RNA molecules attached to the slide. The samples are mounted above an inverted fluorescence microscope, used to study folding of tagged RNA molecules.
The heating laser is directed at the samples from above, with the beam focused to a spot size of about 20 micrometers. The near-infrared light is just the right wavelength to excite vibrations in chemical bonds in the water molecules; the vibrations quickly turn into heat. The laser offers a much larger dynamic temperature range (20 to 90 degrees Celsius, or 68 to 194 degrees Fahrenheit) than bulk heating methods, according to the paper. In early trials, the technique controlled bathtub heating to an accuracy of half a degree Celsius in less than 20 milliseconds across a micrometer-scale sample area.
Holmstrom E.D. and Nesbitt D.J.: Real-Time Infrared Overtone Laser Control of Temperature in Picoliter H2O Samples: "Nanobathtubs" for Single Molecule Microscopy.
Journal of Physical Chemistry Letters 1, page 2264-2268. Published online: July 7, 2010.
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