Scientists synthesize more sensitive sensor

David Shenkenberg

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 materials.

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.