SERS in a Capillary Could Aid Biochemical Sensing on the Lab-on-a-Chip Scale

Lynn M. Savage

Raman spectroscopy is useful for applications where high-specificity biological or chemical sensing is required. Unfortunately, sensitive detection using Raman-based sensors has proved more difficult, especially at the scale of lab-on-a-chip devices, because analytes passing through microchannels provide Raman signals that are very small. However, exploiting the effects of surface-enhanced Raman scattering (SERS), in which nanometer-scale metal particles interact with photons from a laser beam, can help boost sensitivity in microfluidic systems.

A schematic illustrates the design of a liquid-core optical ring resonator (LCORR) through which passes an analyte infused with silver particles (a). A photograph shows the optical fiber probe bringing light into the capillary wall (b). Radiative scattering sends some of the light a short distance along the longitudinal axis of the capillary, but most of the light resonates around the circumference of the tube via total internal reflection. Reprinted with permission of Optics Express.

Researchers have developed methods to integrate SERS techniques with microfluidics, achieving detection limits of about 10³ to 10⁶ pM. But now a group from the University of Missouri-Columbia, led by assistant professor Xudong Fan, has created a technique that achieves a detection limit of 400 pM.

The investigators used a glass capillary ∼125 μm in diameter to form a liquid-core optical ring resonator, bringing a tapered 1- to 2-μm-diameter optical fiber into contact with the capillary. Through the fiber, they shone the light from a 785-nm distributed feedback laser made by Toptica Photonics AG of Gräfelfing, Germany. After the beam enters the outer wall of the capillary, it travels around the capillary several hundred times, resonating between the inner and outer surfaces via total internal reflection.

Through the capillary, the investigators passed the dye rhodamine 6G, into which they had added silver nanoparticles. As the silver-infused analyte passed the cross section of the capillary that was undergoing resonance, the particles closest to the inner wall entered an intense evanescent field emanating from the light traversing the capillary walls. A second fiber collected the resulting Raman signal and sent it to a spectrometer made by Jobin Yvon (now Horiba Jobin Yvon of Edison, N.J.). The average Raman enhancement achieved was 10⁷.

According to Fan, using silver made optimizing the system easy, and not much of the metal is required. Other methods of introducing silver for signal enhancement, such as layering silver nanostructures onto the inner wall, would be more difficult and, ultimately, self-defeating. “Too much silver can completely destroy the ability of the ring resonator to resonate,” he said.

The light in the ring resonator had about 25 times more power than within the input fiber, enabling the evanescent field to penetrate about 100 nm into the sample. The investigators subsequently etched away the inner wall of the capillary, repeating the experiment with wall thicknesses of 3.3, 2.85 and 2.15 μm. As the thickness shrank, the depth of penetration by the evanescent field increased, further enhancing the Raman signal.

Fan said that etching might introduce surface roughness, which in turn could deteriorate the system’s effectiveness. “The best way is to fabricate the thin-walled capillary with direct pulling from a draw tower, like the way optical fiber is made,” he said.

The group is working to enhance the system’s detection limit by increasing the light intensity, optimizing the wavelength used, improving the connection between the input fiber optic and the capillary, and developing a technique to better attract the silver particles to the inner wall.

Optics Express, Dec. 10, 2007, pp. 17433-17442.