Credit: ACS
Credit: ACS

A new design of absorber could light the way to more sensitive optical biosensors, claim US researchers.

Optical biosensors can detect a range of analytes such as chemicals and toxins in air and water, and antibodies and enzymes in the body that could be indicative of diseases. When a target molecule is detected, biosensors send an optical signal that is then absorbed by an absorber. The energy of this absorbed light is converted to heat − that can be both measured and quantified.   

The narrower the wavelength of light that is absorbed, the more sensitive the biosensor will be. “Currently, plasmonic absorbers used in biosensors have a resonant bandwidth of 50nm,” explains lead researcher Koray Aydin from Northwestern University in the US. It is challenging to design plasmonic absorbers with narrower bandwidths due to their inherent optical properties, he adds.

Aydin and his team have now designed, fabricated and tested a new absorber that absorbs a very narrow range of light: it has a bandwidth of just 12nm. This work is published in ACS Nano [Li Z., Butun S. & Aydin K., ACS Nano (2014), DOI: 10.1021/nn502617t].

“There have been several plasmonic-based absorbers that have larger resonance bandwidths,” Aydin tells Materials Today. “Most of these utilize a three layer film, where an insulating film is sandwiched between a metallic mirror at the bottom and nanostructured metal film at the top. In our design, we got rid of the middle insulator layer and realized an all-metallic absorber which resulted in significant reduction in the absorption bandwidth.” Gold was the metal used by the team, and this absorber was fabricated used electron-beam lithography.

When light shines on the team’s absorber, photons interact with the electrons in the gold and force electrons to oscillate along the surface of the absorber. “Since electrons move back and forth, the electric field at the surface is significantly enhanced which in turns results in absorption of light at that specific resonance wavelength,” says Aydin. The amount of light absorbed was also shown to be high, exceeding 90 percent at visible frequencies. 

Aydin predicts that this design of absorbers could find a range of future uses aside from biosensors such as catalysis, thermophotovoltaics and solar steam generation. “I believe that this material will find use in many other applications in the fields of physics, chemistry and biology that I cannot foresee at this point,” he says.