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  • Living cells probed without harm

BioPhotonics
Apr 2013

STANFORD, Calif. – Exploring the dynamics of living cells without harming them – the holy grail of biological research – just got easier with a new class of light-emitting probes small enough to be injected into individual cells.

Stanford University engineers in the Nanoscale and Quantum Photonics Lab directed by electrical engineering professor Jelena Vuckovic are the first to demonstrate that sophisticated light resonators can be inserted inside cells without damaging them, transforming how biologists study and influence living cells. The nanobeam – only 20 µm x 200 nm x 500 nm – is made of thin gallium arsenide and features a periodic array of holes through the beam to form the nanocavity, electrical engineering doctoral candidate Gary Shambat told BioPhotonics. The device looks similar to a piece from an old erector set.

At the cellular level, nanobeams act like a needle that can penetrate cell walls and emit light without injury. Although other investigators have shown that it is possible to insert simple nanotubes and electrical nanowires into cells, no scientists have yet realized such complicated optical components inside biological cells.


This scanning electron microscope image shows Stanford University’s nanobeam probe, including a large part of the handle tip, inserted into a typical cell. Images courtesy of Gary Shambat, Stanford University School of Engineering.


Unlike a nanowire, “ours is an optical structure, which is the basis behind many photonic devices like lasers, LEDs and optical sensors,” Shambat said.

Structurally, the device is a sandwich of alternating layers of thin gallium arsenide and light-emitting crystal – quantum dots. The structure is carved out of chips or wafers and remains tethered to the thick substrate, which poses a serious hurdle for biological applications because the underlying nanocavities are locked in position on the rigid material and cannot penetrate cell walls.

By peeling away the photonic nanobeams, Shambat rid the device of bulky wafers. He then glued the ultrathin photonic device to a fiber optic cable, with which he could steer the needlelike probe into the cell.


A photonic nanobeam inserted in a cell shows the etched holes through the beam as well as the sandwichlike layer structure of the beam itself. The beam structure alternates between layers of gallium arsenide and photonic crystal containing the photon-producing quantum dots.


The instrument was encapsulated with a thin, electrically insulating coating of alumina and zirconia to protect the cell from potentially toxic gallium arsenide and the probe from degrading.

Cells from a prostate tumor were used in the Stanford study, indicating possible application for the probe in cancer research.

“The most immediate application of this result would be to measure biomolecules such as proteins and DNA/RNA inside cells in a real-time, single-cell method over a long period of time,” Shambat said. “This ability currently does not exist with current biological techniques and, if available, could shed light on complex matters like drug response, disease progression and fundamental cell biology.”

Next, the investigators will try to “combine the techniques of cellular insertion and protein detection to detect specific biomarkers inside a cell line,” Shambat said. “For example, we might want to detect PSA (prostate specific antigen) inside these prostate cancer cells in real time as it is being produced and, therefore, measure protein production dynamics.”

The probe was described in Nano Letters (doi: 10.1021/nl304602d).


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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