Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec Full length article Cascaded optical polymer waveguide for enhanced fuorescence evanescent wave spectroscopy Jalal Abdul-Hadi a , Marc A. Gauthier a , Muthukumaran Packirisamy b, ⁎ a Institut National de la Recherche Scientifique, EMT Research Center, 1650 Lionel-Boulet, Varennes, Quebec J3X 1S2, Canada b Optical Bio-Microsystems Laboratory, Dept. of Mechanical and Industrial Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada HIGHLIGHTS • Increased sensitivity of optical biosensing with cascaded waveguide. • Detection limit as low as 9 pg/mL of Biotin-Alexa 647. • Optical design optimization of the cascaded waveguide with beam propagation method. ABSTRACT A cascaded waveguide optical biosensor was developed for enhancing the sensitivity of evanescent wave fuorescence spectroscopy by increasing the area of analyte immobilization. Analysis of the coupled excitation and fuorescence light guided through the sensor was performed by numerical simulations with the beam propagation method and experimental characterization. A fuorescence evanescent wave spectroscopy experiment was done with Alexa 647-labeled biotin as the model analyte, and showed a sensitivity increase of 1.39 fold for the cascaded waveguide design compared to a single waveguide. Also, concentrations of biotin-Alexa 647 fuor/PBS solutions as low as 14.62 pg/mL and 8.88 pg/mL were detected for single and cascaded waveguides respectively. 1. Introduction The detection and monitoring of biological analytes are essential for the for the felds of biology, biochemistry, and the biomedical industry. Fortunately, biosensors were developed to respond to this need. Biosensors are analytical devices that turn a biological response to an electric signal. They are mainly composed of a biological or chemical receptor that recognize the bio target and produces a signal, a trans- duction component that convert the produced signal in an electrical signal which is then analyzed by a reading device. A multitude of biosensors were developed with diferent bio transducing technology as electrochemical, gravimetric, mechanical, electronic, acoustic, optical, etc. Interestingly, optical biosensors compared to others ofer high sensitivity, fast detection, immunity to electromagnetic interference, suitable for miniaturization, multiplexing, labeled and label free de- tection capabilities [1–5]. The sensing mechanism of most optical bio- sensors arise from a property of light when confned in an optical wa- veguide called evanescent wave. Indeed, frst described by Hirschfeld [6], the evanescent wave appears after total refection of an incident light at the boundary between two dielectric media. Where the incident light propagates from a high index dielectric medium and totally refected at the boundary with a lower index dielectric medium. The evanescent wave is a feld which is exponentially decaying inside the lower index medium and is used to probe changes in the vicinity at the surface of the platform. Optical fbers were the frst and still widely used sensing platforms for optical bio sensing [7]. Indeed, their ability to perform remote sensing with good geometrical fexibility is attractive for many biomedical applications [8]. However, the main drawback about optical fbers is their fragility. Especially when their cladding is removed or subjected to extreme tapering to allow the core to be ex- posed to a sample for example for absorption [9,10] or fuorescence detection [11,12]. Planar waveguides arose as alternative to optical fbers since they provide a more robust platform. Most importantly, they allow for miniaturization and patterning of biological materials as antibodies or antigens for real time multi-analyte assays. Diferent op- tical methods were employed with the planar waveguide platform for bio detection such as surface plasmon resonance (SPR) [13], localized surface plasmon resonance (LSPR) [14], surface plasmon-coupled emission (SPCE) [15], reverse symmetry [16], fuorescence evanescent wave spectroscopy [17], etc. Among others, fuorescence evanescent wave spectroscopy combines the surface restricted probing of the eva- nescent feld and the high sensitivity and specifcity provided by the https://doi.org/10.1016/j.optlastec.2019.105904 Received 26 February 2019; Received in revised form 10 September 2019; Accepted 12 October 2019 ⁎ Corresponding author. E-mail address: pmuthu@alcor.concordia.ca (M. Packirisamy). Optics and Laser Technology 123 (2020) 105904 0030-3992/ © 2019 Elsevier Ltd. All rights reserved. T