Probe Emits Light in Living, Unharmed Cells

A new class of light-emitting probes small enough to be injected into individual cells without harming them could provide new ways of exploring the dynamics of living cells.

The Stanford University engineers are the first to demonstrate that sophisticated engineered light resonators can be inserted inside cells without damaging them. Their nanobeam — only a few microns in length and just a few hundred nanometers in width and thickness — looks similar to a piece from an old erector set with holes through the beams that act like a nanoscale hall of mirrors, focusing and amplifying light at the center of the beam in what are known as photonic cavities. These are the building blocks for nanoscale lasers and LEDs.

“Devices like the photonic cavities we have built are quite possibly the most diverse and customizable ingredients in photonics,” said senior author Jelena Vuckovic, an electrical engineering professor. “Applications span from fundamental physics to nanolasers and biosensors that could have profound impact on biological research.”

At the cellular level, nanobeams act like a needle that can penetrate cell walls without injury. Upon insertion, the beam emits light, yielding a remarkable array of research applications and implications. While 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.

“We think this is quite a dramatic shift from existing applications and will enable expanded opportunities for understanding and influencing cellular biology,” said doctoral candidate Gary Shambat, who works at the Nanoscale and Quantum Photonics Lab directed by Vuckovic.

A clever design
Structurally, the device is a sandwich of extremely thin gallium arsenide layers alternated with similarly thin layers of light-emitting crystal — quantum dots. The structure is carved out of chips or wafers, and once sculpted, the devices remain tethered to the thick substrate.

Similar optical devices for use in ultrafast, ultra-efficient computer applications are being developed by Shambat and colleagues.

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

For biological applications, however, the thick, heavy substrate presents a serious hurdle for interfacing with single cells. The underlying nanocavities are locked in position on the rigid material and are unable to penetrate cell walls.

Shambat’s breakthrough came when he peeled away the photonic nanobeams, leaving the bulky wafers behind. The ultrathin photonic device was then glued to a fiber optic cable with which he steers the needlelike probe toward and into the cell.

Anticipating that gallium arsenide could be toxic to cells, Shambat devised a clever way to encapsulate the devices in a thin, electrically insulating coating of alumina and zirconia. The coating serves two purposes: It both protects the cell from the potentially toxic gallium arsenide, and protects the probe from degrading in the cell environment.

Magnetic appeal
In this case, the studied cells came from a prostate tumor, indicating possible application for the probe in cancer research. The primary and most immediate use for the nanobeams would be in the real-time sensing of specific proteins within the cells, but the probe could also be adapted for sensing any important biomolecules such as DNA or RNA.

To detect these molecules, researchers coat the probe with certain organic molecules or antibodies that are known to attract the target proteins, just like iron to a magnet. If the desired proteins are present within the cell, they accumulate on the probe and cause a slight-but-detectable shift in the wavelength of the light being emitted. This shift is a positive indication that the protein is present and in what quantity.

“Let’s say you have a study that is interested in whether a certain drug produces or inhibits a specific protein. Our biosensor would tell definitively if the drug was working and how well based on the color of the light from the probe. It would be quite a powerful tool,” said Dr. Sanjiv Sam Gambhir, chair of the department of radiology at the Stanford School of Medicine and director of Stanford’s Canary Center for Early Cancer Detection.

Embeddable nanoscale optical sensors would represent a key development in the quest for patient-specific cancer therapies, or personalized medicine, in which drugs are targeted to the patient based on efficacy.

A photonic nanobeam inserted in a cell. Clearly visible are 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.

"Stunning” results
Once inserted inside the cell, the probe emits light, which can be observed from the outside. For engineers, this means that almost any current application or use of these photonic devices can be translated into the previously off-limits environment of the cell interior.

In an experiment the authors described as stunning, they loaded the nanobeams into cells and watched as the cells grew, migrated around the research environment, and reproduced. Each time a cell divided, one of the daughter cells inherited the nanobeam from the parent, and the beam continued to function as expected.

This enables researchers to study living cells over long periods of time, a research advantage not possible with existing detection techniques, which require that cells be either dead or fixed in place.

“Our nanoscale probes can reside in cells for long periods of time, potentially providing sensor feedback or giving control signals to the cells down the road,” Shambat said. “We tracked one cell for eight days. That’s a long time for a single-cell study.”

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

For more information, visit: www.stanford.edu