Chip-based refractive index detection using a single point evanescent wave probe

Abstract

This paper presents a novel approach for performing spectroscopic refractive index detection within microfluidic channel environments. Based on the principle of total internal reflection (TIR), changes in the refractive index of an analyte stream passing through a microfabricated channel are detected through interaction with an optical evanescent field formed at the channel wall. Refractive index variations within the microchannel environment modify the critical angle at the liquid–solid interface, thereby altering the characteristics of evanescent field formation in solution. These variations are evidenced through measurement of fluorescence intensities. Initially, the design and testing of the method are described. Subsequently, refractive 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 values 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 flowing 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 critical angle. Although detection limits using the prototype system are non-ideal (∼5 µM carbohydrate), they compare favourably with existing methods for on-chip refractive index detection.