Multireflection Polarimetry in Microfluidics

Abstract

The field of microfluidics promises new portable, low-cost sensing systems, as well as the capabilities to measure the physical or chemical properties of precious samples, for which only small volumes are available. However, when using microfluidic channels with millimeter to micron scale dimmensions, together with optical sensing methods, these configurations result in short path lengths over which the signal can be acquired. Whilst polarimetry would greatly benefit from using small volumes, providing important information on the structure of chiral biomarkers in life sciences, the small interrogation volumes associated with the use of minute samples decreases the numbers of molecules in the light path that cause an optical rotation, and so reduces the sensitivity of the technique. Here we show that when an optical beam, passing through a chiral sample, undergoes multiple reflections from suitably aligned and configured external micromirrors, the usual cancelling out of the optical rotation that occurs when the rotated polarized beam is passed back through a solution following reflection at a single mirror can be negated. This enables the chirality of molecular species present in a microfluidic sample to be measured with increased sensitivity. This approach was validated experimentally using solutions of D-(+)-glucose as a model system, by investigating the effect of multiple reflections of a linearly polarized He-Ne laser beam and a 403 nm diode laser beam across the microfluidic channel. It was found that there was a 30 fold enhancement in the limit of detection with as few as 11 reflections through the sample.

Review of Non-Invasive Glucose Sensing Techniques: Optical, Electrical and Breath Acetone

“…On the other hand, the photodetector will not detect light when the polarization axis of the analyzer is perpendicular to the angle of rotation of the electric field. The angle of rotation of the electric field is dependent on the concentration of glucose in the optically active aqueous solution and is modeled by Equation (5) [83]:…”