Researchers focus on the interference pattern created by the coupling of light with electrons.

The healthcare industry has a huge need for fast-acting, ultrasensitive, compact biosensors that allow biological processes to be monitored in real time. The ability to detect the different types of proteins being secreted in cell lines would open doors for researchers working on tissue regeneration.

The most promising devices capable of achieving this are based on a surface plasmon polariton (SPP), a type of electromagnetic wave generated when an incident beam of light couples with an oscillating wave of electrons in the surface of a metal.

Writing in ACS Nano, a research team led by Filbert Bartoli, professor of electrical and computer engineering and member of Lehigh’s bioengineering program, has reported a new type of microfluidic chip-based plasmonic biosensor that outperforms current nanoplasmonic devices by an order of magnitude.

“Most commercial plasmonicbased sensing devices employ a prism to couple incident light with the surface plasmon,” says Bartoli. “While this approach has an advantage in terms of sensitivity, the intrinsic size and alignment requirements are a significant limitation.

“To overcome these problems, scientists have designed nanoplasmonic sensors based on nanoparticles, nanoslit arrays and nanohole arrays. However, a significant improvement in sensitivity for detecting proteins has to be made before they can compete with prism-based systems.”

Bartoli’s simple device contains two parallel nanoslits etched a few microns apart into a thin film of silver deposited on a glass slide. When an incident light beam is focused onto one of these slits, the electrons at the outermost surface of the metal film oscillate, causing an SPP to propagate along the surface of the metal.

“Two SPPs are generated,” says graduate student Yongkang Gao. “One travels along the metal-air interface on the film’s top surface and the other along the metal-glass interface on its bottom surface.”

On reaching the second slit, these two waves interact to form an interference pattern whose fringes are highly dependent on the difference in the effective refractive index of the interfaces along which they have traveled. The light emanating from the second slit is collected by a modified optical microscope that ensures only SPPmediated radiation is collected. It is then passed through an optical fiber-based compact spectrometer to obtain information on the interference pattern.

“As the optical field of an SPP is strongly confined to a very thin region along the metal surface,” says Bartoli, “it is extremely sensitive to changes in the local refractive index, such as those induced by proteins and other biomolecules binding to the metal surface.”

Working with Xuanhong Cheng, assistant professor of materials science and engineering, Bartoli’s group tested the device on the affinity that biotin has for the protein streptavidin. They coated the top metal surface with a monolayer of biotinylated bovine serum albumen. A buffer solution was then introduced to define a base line.

“We then focused on the interference peak around 690nm and recorded its spectral position,” says Gao. “The injection of a very dilute solution of streptavidin caused a peak shift of 15.7nm, which was much larger than that previously reported for devices based on nanoparticles or nanohole arrays.”

A control experiment was carried out using a monolayer coating of bovine serum albumen without the biotin linkage. A peak shift of only 0.7nm was observed. “This proved that the large shift of 15nm was due solely to the specific binding of the streptavidin to biotin,” says Gao.

“The next step is to modify the device to detect molecules secreted by stem cells during neuronal differentiation,” says Sabrina Jedlicka, assistant professor of materials science and engineering. “This will allow us to monitor real-time neuronal differentiation and ultimately neuronal response to certain stimuli, including novel drug compounds.”

The team is also designing a multiplex device comprising a 2-D array of sensors to allow simultaneous detection of a mixture of proteins or spatial distribution of a protein.

The project is funded by NSF and is part of the engineering college’s Healthcare Research Cluster.