Scientists synthesize more sensitive sensor
Devices for biological, chemical and photonic sensing have uses in medical diagnostics,
environmental monitoring, high-throughput drug screening and biological research.
Label-free sensors such as those based on photonic crystals can offer less experimental
uncertainty, complexity and cost than labeled sensors made of biological or chemical
Brian T. Cunningham’s Nano Sensors Group
at the University of Illinois at Urbana-Champaign recently created a photonic crystal
biosensor that incorporates a low-refractive-index nanoporous dielectric with a
250-nm period to produce an optical resonator that functions at near-UV wavelengths.
The team tested its sensitivity for detecting biological compounds.
As detailed in the July 10 issue of
Applied Physics Letters, the researchers created the sensor by depositing
titanium oxide onto a periodic surface structure formed with low-refractive-index
spin-on glass from Honeywell. They used rigorous coupled-wave analysis to predict
that the low-index dielectric would confine the electric field to the sensor surface,
resulting in an increased bulk-to-surface sensitivity ratio. Then, they calculated
the bulk sensitivity by measuring the change in the peak-wavelength value as the
refractive index varied for water, isopropyl alcohol and air. Finally, they characterized
the sensitivity of the sensor to adsorbed surface material by using it to detect
a single layer of poly(lysine phenylalanine).
They compared their sensor with a previously
developed near-IR sensor. The near-UV sensor had a bulk shift of 49.1 nm per refractive
index units and a peak wavelength value of 3.19 nm, versus 302 and 4.07 nm, respectively,
for the near-IR sensor. The surface-to-bulk sensitivity for the near-UV sensor was
4.5 times greater than that of the near-IR detector.
The researchers ultimately desire to
resolve single molecules, and the increased sensitivity of the detector will bring
them closer to that goal.
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