Evaluation of Molecularly Imprinted Polyurethane as an Optical Waveguide for PAH Sensing Yin-Chu Chen a , Jennifer J. Barzier b , Mingdi Yan b , Scott A. Prahl a a Department of Biomedical Engineering, OHSU, Portland, OR b Department of Chemistry, PSU, Portland, OR ABSTRACT We developed a numerical model for the fluorescence output efficiency of a molecularly imprinted polymer (MIP) waveguide sensing system. A polyurethane waveguide imprinted with a polycyclic aromatic hydrocarbon (PAH) molecule was fabricated using micromolding in capillaries. The coupling of light into a 5mm long MIP segment was verified by comparing the output transmission signals of a deuterium lamp from the MIP waveguide collected by an optical fiber with the background lamp signals collected by the same optical fiber. It was found that polyurethane MIP was an effective waveguide but absorbed much shorter wavelengths, especially in the UV region, thereby the transmission of light appeared orange/red in color. The high background absorption of polyurethane in the spectrometric regions of interest was found to be a critical problem for sensor sensitivity. Our numerical model shows that the fluorescence output is only 2×10 −6 of the input excitation for 25 mM anthracene for a 5 mm polyurethane waveguide. A 10 fold decrease of background absorption will increase the fluorescence output 250 times. Keywords: polycyclic aromatic hydrocarbon, molecularly imprinted polymers, optical fiber sensor, light cou- pling efficiency 1. INTRODUCTION Evanescent-wave fluorescence-based fiber-optic biosensors detect the binding of an antigen to an antibody immo- bilized in the distal end of an optical fiber. 1–4 Detected refractive index changes caused by binding of an antigen and an antibody are limited to the evanescent sensing region (typically less than 1 μm thick). For immunoassay recognition elements, this is an advantage because fluorophores outside the evanescent field don’t contribute to the emission signal. Molecularly imprinted polymer techniques allow much greater detection volumes that may capture more analytes. For instance, a 600 μm fiber coated for 5 cm with MIP with an active sensing depth of 1 μm will have a detecting volume of ∼10 −2 mm 3 . On the other hand, if the fiber itself is a MIP, which acts as both a detecting element and a waveguide, a 100 μm×100 μm×1cm long MIP waveguide will have 10 times more detecting volume than an evanescent-wave sensor. Another advantage is that the light intensity inside a MIP waveguide that directly excites the analytes is stronger than that in the evanescent field (which decays exponentially). Yet another advantage is that a greater proportion of the fluorescence signal, generated inside the MIP, will be guided directly to the output. A potential problem of a MIP waveguide, however, is the attenuation of the signals due to the background absorption of polymers, and an increase in the equilibrium time of the analytes and MIP. The concept of using the biochemical sensing layer itself as an optical waveguide was presented by Hisamoto et al.. 5 They used an “active polymer-waveguide platform” where the sensing layer also acted as the guiding layer. The poly(vinylchloride) membrane was used both as a sensing layer and as the evanescent-wave waveguide core layer. The absorbance signal was measured, and the sensitivity of such a system was shown to be greater than that in the evanescent-wave sensing mode. Although optical sensors based on molecularly imprinted polymers have been constructed, 6–10 few publications have used MIPs directly as an optical waveguide. 11 For biochemical sensing use, the attenuation of light may not be as critical as an optical fiber for optical communications. Further author information: (Send correspondence to S. A. P.) Y.-C. C.: E-mail: yinchu@bme.ogi.edu, Tel: (503) 216-6830 S. A. P.: E-mail: prahl@bme.ogi.edu, Tel: (503) 216-2197