Nano Focus: Photonic-crystal nanolasers shown to be highly sensitive biosensors
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M
ove over, ELISA. While the Enzyme-Linked ImmunoSorbent Assay has long been one of the most popular ways of detecting and quantifying the presence of antibodies or antigens in solution, nanolasers may be poised to share the spotlight on the biosensor stage. Using a photonic-crystal nanolaser developed by their team, engineering professor Toshihiko Baba and colleagues at Yokohama National University’s Department of Electrical and Computer Engineering in Japan have demonstrated the utility of their device as a disposable sensor for several biomedical applications. Their experiments, presented in MRS Communications (DOI: 10.1557/ mrc.2015.73) show promise for detecting targets including proteins, endotoxins, and DNA sequences. Enzyme immunoassays determine whether body fluids contain proteins related to certain diseases. Contemporary immunoassays often require expensive fluorescent labels and complicated procedures. Furthermore, their detection limit can be insufficient for many important proteins. Now, Baba aims to provide “an immunosensor beyond the current standard technology,” exploiting the nanolaser’s sensitivity to changes in refractive index and surface charge. “Our interest is using the nanolaser for immunosensing. But it has the potential to detect toxins, cells, and chemicals. However, photonic sensors are not [yet] available for medical applications. ELISA has been superior to those reported so far,” says Baba. The Yokohama group fabricated a nanolaser from an indium gallium arsenide phosphide wafer with a lattice of holes comprising the photonic crystal separated by an air layer from an indium phosphide substrate. Shifting several holes resulted in a nanolaser cavity that lases at ~1550 nm when pumped with light at 980 nm. As the refractive index of the fluid in the cavity changes upon immersion in solution containing various concentrations of proteins, so does the nanolaser wavelength.
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MRS BULLETIN
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VOLUME 41 • JANUARY 2016
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Schematic of nanolaser biosensor and laser mode profile. (Top right): Modal profile without nanoslot; mode is most intense at the nanoslot position. (Bottom): GaInAsP slab of quantum wells on InP substrate. Periodic holes, which form a photonic crystal, are modified to form the central slot (top left), which is the main part of the sensor. Credit: Toshihiko Baba/Yokohama National University.
Functionalizing the nanolaser with a molecule known to bind with the target molecule provides biological specificity. To detect a protein, this means fixing an antibody to the nanolaser surface. “We can detect a smaller amount of protein than the limit of ELISA. This means we can use a lower concentration of protein as a biomarker for severe diseases, which offers a higher diagnostic probability,” adds Baba. Proteins in body fluids help to identify particular diseases. However, existing tests are limited by detection sensitivity. If lower concentrations could be detected, diagnoses could be improved. For prostate cancer screening, current tests for detecting the prostate-specific ant
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