bobstar 2008-09-20 11:02
Nanoholography goes beneath the surface
Conventional imaging techniques only scan the surface of cells, but a related method called scanning near-field ultrasonic holography can be used to image nanoparticles inside them. The technique could be useful for nanotoxicology studies.
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Imaging nanoparticles
"We have shown that at ultrasonic frequencies, intracellular nanomaterial causes sufficient wave scattering that a probe outside the cell can detect it," said Ali Passian of Oak Ridge National Laboratory in Tennessee. "This provides a non-invasive way of looking inside a cell, so eliminating the need to cut up the cell or inject artificial light-emitting molecules into it to find out whether or not a certain type of nanoparticle is present."
Nanoparticles show great promise for medical applications, such as drug delivery, but their toxic effects need to be investigated. Scientists would therefore like to visualize nanoparticles inside cell structures to see how the particles enter and interact with cells.
The team, which includes researchers from the University of Tennessee and Northwestern University in Illinois, exposed mice to single-walled carbon nanohorn particles. The researchers killed the mice a few days later and isolated macrophages from the animals' lungs and red blood cells. They placed the cells on a substrate that they vibrated at ultrasonic frequencies of around 4 MHz.
Travelling vibrations
As the vibrations travel through the cells, different delays or phase shifts are created depending on the cell's composition. By measuring these phase shifts, Passian and colleagues were able to build up a map of the cell interior (see figure). Analysis of the maps revealed nanoparticles between about 70 and 110 nm in size inside lung macrophages and blood cells.
"Our method provides an alternative way to study a cell under ambient conditions without the need to place it in a vacuum, coat it with metal, bombard it with electrons or insert other molecules into it," explained Passian. "This is not the case with other techniques, such as electron microscopy or fluorescence tagging."
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Potential applications for the new imaging technique include nanotoxicology studies and drug delivery.
The team now plans to improve its technique so that a larger number of samples can be investigated at the same time. "Another challenge is to use our approach to visualize cell interiors in their natural environment – cell fluid," added Passian.
The researchers reported their work in Nature Nanotechnology.