Maab Al-hafidh
University of Glasgow/Biomedical Engineering
Maab has been a PhD student in the division of Biomedical Engineering at Glasgow University/UK since 2014. She was awarded her BSc. and MSc. in Laser and Optoelectronic Engineering at Alnahrain University/Iraq in 2008, and since 2009 she has been a lecturer in Medical Engineering at Alnahrain University. She is a member of Iraqi Engineering Union since 2006. Her research interests include free electron laser oscillators, nonlinear optics, biosensors, MEMS machining and microfluidics.
Introduction
Polarimetry can provide important insights into the concentration and the molecular structure of chemical compounds in solution. The technique has been used in a variety of biochemical applications such as drug screening [1]. Here we demonstrate a new method that significantly reduces the instrumentation required, without compromising performance, opening up the use of polarimetry in low volume, hand-held and multiplexed systems [2].
We overcome the short optical path lengths inherent to microfluidic systems, which decrease optical rotation sensitivity, by using metal micromirrors to create multiple reflections across a capillary channel. Arranging the mirrors to obtain two reflections before passing through the sample volume (Figure 1) nullifies the 180 degrees flipping of the polarisation at each mirror before interrogating the sample through the subsequent pass, thus adding up rotation through multiple passes.
Methods
Our set-up (Figures 1 and 2) was modelled using simulations of the optical components in a Matlab environment, confirming functionality. It is controlled via computer programs (LabVIEW-Matlab), enabling to measure the polarisation of the He-Ne laser after three passes through solutions of D-glucose, using two reflections off silver mirrors at each step (Figures 1 and 2). The process was then repeated along the length of a capillary channel.
Results and Discussions
We analysed the increase in performance of the system but studying the rotation produced by different concentrations of glucose. The detection limit was enhanced 20 times when using only three passes with the mirrors (10 mM D-Glucose with (blue line) and 200 mM without reflections (red line) – Figure 3), while achieving an excellent linear correlation (Linear correlation R2>0.99). The system achieved an angle resolution of 0.001 degrees, demonstrating the potential impact of our approach to increase sensitivity and thus open up microfluidic applications. We are now manufacturing micromirrors by taking advantage of the 45 degrees wet etching of silicon (Figure 4), to realise microfluidics-based polarimetry.
References
[1] Sofikitis, D., et al., “Evanescent-wave and ambient chiral sensing by signal-reversing cavity ringdown polarimetry.” Nature, 514(7520): p. 76-79 (2014). [2] Tan, C., et al., Frontiers of optofluidics in synthetic biology. Lab on a Chip, 12(19): p. 3654-3665 (2012).
