Chip-based refractive index detection using a single point evanescent waveprobe

This paper presents a novel approach for performing spectroscopic refractiv e index detection within microfluidic channel environments. Based on the pr inciple of total internal reflection (TIR), changes in the refractive index of an analyte stream passing through a microfabricated channel are detecte d through interaction with an optical evanescent field formed at the channe l wall. Refractive index variations within the microchannel environment mod ify the critical angle at the liquid-solid inter-face, thereby altering the characteristics of evanescent field formation in solution. These variation s are evidenced through measurement of fluorescence intensities. Initially, the design and testing of the method are described, Subsequently, refracti ve index values for bulk sucrose solutions (0-35% w/v sucrose in water) are measured using the single point evanescent wave probe and compared with va lues obtained through conventional refractometry and the literature. Close agreement between all three approaches is demonstrated. The method is then applied to the detection of sucrose plugs (10-500 mM) hydrodynamically flow ing through microfabricated channels on a planar glass chip. The evanescent wave probe is also used to selectively monitor specific analytes within a multicomponent system, by precise angular control in the vicinity of the cr itical angle. Although detection limits using the prototype system are non- ideal (similar to 5 muM carbohydrate), they compare favourably with existin g methods for on-chip refractive index detection.