© (2006) AIP

Microfluidic sensors promise detection of very low concentrations of analytes in volumes on the nanolitre scale. Now, a team of French researchers has shown that incorporating lasers into such devices significantly increases their sensitivity1. Microfluidics entails passing liquids along channels that are only tens or hundreds of micrometres across, allowing analysis of chemicals in a small area, the so called lab-on-a-chip. The dynamics of fluid flow at this scale differs greatly from that at macroscopic channel widths, allowing chemicals to be controlled in totally new ways.

Lasers offer a particularly sensitive means of chemical and biological sensing. The output properties of a laser are altered by very small changes within the optical cavity, such as fluctuations in the refractive index or light absorption. Now, J. C. Galas and colleagues have integrated a laser onto a microfluidic chip and investigated the device as a chemical detector.

In their structure, a 600-µm long laser cavity was bounded by the cleaved ends of two optical fibres, each coated in gold to increase their reflectivity. Two microchannels run between these mirrors. The first carries the laser gain material: in this case, a fluorescent dye, rhodamine 6G, dissolved in ethanol. The dye flows through the optical cavity at a speed of 5 mm s-1 and, when pumped by an external laser, lases at a wavelength of 570 nm. The team monitored the power output of the microfluidic laser as different densities of a second dye, methylene blue, flow through the second channel. In this way, concentrations down to 10-6 mol l-1 can be detected. Its small size, low cost and simplicity give this type of device key advantages over alternative techniques.