Wednesday, January 14, 2015

ALD ZrO2 protects Photonic crystal nanolaser biosensor for DNA detection

As reported by : A simple method to sense DNA, as well as potential biomarker proteins of cancer or other diseases such as Alzheimer's, may soon be within reach thanks to the work of a team of Yokohama National Univ. researchers in Japan.

As the team reports in Applied Physics Letters, they created a photonic crystal nanolaser biosensor capable of detecting the adsorption of biomolecules based on the laser's wavelength shift.

Equally impressive, the nanolaser biosensor enables detection of the surface charge from its laser emission intensity, which in turn can also be used to sense the adsorption of biomolecules. Using laser intensity to detect biomolecules is potentially less expensive than the fluorescent tagging or spectroscopy techniques typically used in biosensors because it is a simpler procedure.

When the team first set out to explore photonic crystal nanolaser sensors, they weren't focusing on the intensity of the laser emission because it's sensitive to the quality of the fabricated laser and, frankly, they didn't expect it to show sensing signals.

"In the beginning we focused on wavelength behavior, but quickly noticed that [the laser emission] intensity is influenced by both pH and polymers," noted Toshihiko Baba, a professor in Yokohama National Univ.'s Dept. of Electrical and Computer Engineering. "Our results were very reproducible and, interestingly, we found that the behaviors of the wavelength and intensity are independent." 
This image shows a top view of the group's nanolaser, in which the center narrow slot (horizontal line) is the main part of the sensor. The periodic holes form a photonic crystal, and although the size of the holes appears to fluctuate they've been intentionally modified so the laser's emission is effectively extracted to the top. Image: Toshihiko Baba/Yokohama National Univ.

The team was surprised by these results, which they discovered when they deposited a protective film of thin zirconium dioxide (ZrO2) over the device using atomic layer deposition, and then tried sensing in liquids of high or low pH and liquids containing charged polymers. The coating was necessary to protect the nanolaser from damage and unwanted wavelength drift.

The nanolaser device can sense surface charge because the surface charge changes the occupancy rate of electrons at the surface states in the semiconductor of the nanolaser, Baba explained. "This modifies the semiconductor's emission efficiency." 

Keisuke Watanabe, Yoji Kishi, Shoji Hachuda, Takumi Watanabe, Mai Sakemoto, Yoshiaki Nishijima and Toshihiko Baba
Appl. Phys. Lett. 106, 021106 (2015)

The emission intensity of a GaInAsP photonic crystal nanolaser is affected by the pH of the solution, in which the nanolaser is immersed. This phenomenon can be explained by the change in the redox potential, which modifies the filling of electrons at surface states of the semiconductor and hence the nonradiative surface recombination. This phenomenon allows the nanolaser to simultaneously and independently detect the refractive index and electric charges near the surface on the basis of the variation in emission wavelength and intensity, respectively. This paper demonstrates this function through alternate deposition of charged polyelectrolytes and hybridization of deoxyribonucleic acids.

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