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news digest ♦ Novel Devices


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


In one finding that the authors describe as “stunning”, they loaded their 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.


Scanning electron microscope (SEM) image shows a nanobeam probe, including a large part of the handle tip, inserted in a typical cell.


“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 colour of the light from the probe. It would be quite a powerful tool,” explains Sanjiv Sam Gambhir, MD, co-author of the paper and chair of the Department of Radiology at the Stanford School of Medicine as well as director of Stanford’s Canary Centre for Early Cancer Detection.


As such, embeddable nanoscale optical sensors would represent a key development in the quest for patient-specific cancer therapies - often referred to as personalised medicine - in which drugs are targeted to the patient based on efficacy.


Structurally, the new device is a sandwich of extremely thin layers of GaAs alternated with similarly thin layers of light- emitting crystal, a sort of photonic fuel known as quantum dots. The structure is carved out of chips or wafers, much like sculptures are chiselled out of rock. Once sculpted, the devices remain tethered to the thick substrate.


Shambat and his fellow engineers have been working on similar optical devices for use in ultrafast, ultra-efficient computer applications where having devices immobilised on chips and wafers does not matter so much since they will ultimately be integrated with microelectronics.


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


Shambat’s breakthrough came when he was able to peel away the photonic nanobeams, leaving the bulky wafer behind. He then glued the ultrathin photonic device to a fibre optic cable with which he steers the needle-like probe toward and into the cell.


Similarly, anticipating that GaAs could be toxic to cells, Shambat also devised a clever way to encapsulate his 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 GaAs and protects the probe from degrading in the cell environment.


140 www.compoundsemiconductor.net March 2013


This inheritability frees researchers to study living cells over long periods of time, a research advantage not possible with existing detection techniques, which require 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,” explains Shambat. “We tracked one cell for eight days. That’s a long time for a single- cell study.”


Further details of this work have been published in the paper, “ Single-Cell Photonic Nanocavity Probes,” by Shambat et al in Nano Letters. DOI: 10.1021/nl304602d


Funding for this study was provided by The Beckman Centre for Molecular and Genetic Medicine at Stanford, the Canary Foundation and the Centre for Cancer and Nanotechnology Excellence.


New production method improves quantum-dot performance


Altering the creation of CdSe-CdS QDs could enable everything from more efficient computer displays to enhanced biomedical testing


Quantum dots, tiny particles that emit light in a dazzling array of glowing colours have the potential for many applications, but have faced a series of hurdles to improving performance.


Now an MIT team says that it has succeeded in overcoming all these obstacles at once, while earlier efforts have only been able to tackle them one or a few at a time.


Quantum dots, in this case, a specific type called colloidal quantum dots, are tiny particles of semiconductor material that are so small that their properties differ from those of the bulk material.


They are governed in part by the laws of quantum mechanics that describe how atoms and subatomic particles behave. When illuminated with ultraviolet light, the dots fluoresce brightly in a range of colours, determined by the sizes of the particles.


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