On-the-flow absorption and refractive index measurements in a microfluidic device

Abstract

Measuring the refractive index and optical absorption of an analyte (e.g., fluid, particle, and cells) in a microfluidic device with high accuracy and sensitivity is highly desirable for many applications. In conventional absorption spectroscopy the detection of small absorption changes requires a long interaction length, typically on the order of the absorption length. In addition, the required sample volumes conflict with the lab-on-a-chip approach. To integrate absorption measurements on such devices the interaction length must be prolonged. We enhance the sensitivity by increasing the interaction length using reflective layers inside the microfluidic channel, thereby creating a Fabry-Prot etalon. While the changes of the refractive index result in a shift of the Fabry-Prot peak position, absorption within the optical cavity is causing a change of the amplitude and the width of the modes. The method is insensitive to intensity variations outside the fluidic channel since the transmitted light also provides the referencing signal. We have measured absorption coefficients on the order of 1 cm-1 in sub-nanoliter sample volumes inside a 25 m high microfluidic channel, and reflective index changes as small as 10-6 using a 400 m cavity. This work is partially funded under ONR contract N00014-05-C-0430 and by the Alexander-von-Humboldt-Foundation.