Metal clad leaky waveguides for chemical and biosensing applications

Abstract

Novel metal clad leaky waveguide (MCLW) sensor devices have been developed for sensing applications. These chips are designed to confine the light in a low refractive index waveguide that encompasses the chemically-selective layer, maximising the overlap between the optical mode and the chemistry, thus improving the sensitivity. In this work, a thin metal layer was inserted between the substrate and the thick waveguide layer, increasing the reflectivity of the waveguide/metal interface and decreasing the light lost at each of reflection in the leaky mode, which in turn increases the propagation distance. The device has been used for a range of biosensing applications, including the detection of organophosphoros pesticides. The limit of detection for paraoxon, based on absorbance detection, was calculated to be 6 nM. Refractive index detection was demonstrated by monitoring the change in the out-coupled angle resulting from the binding of protein A to anti-protein A immobilized on agarose. The sensor was also used for detecting the quenching of the fluorescence of an acid–base sensitive ruthenium complex immobilized within the sol–gel and with glucose oxidase enzyme. The limit of detection for glucose was 3 μM. The advantage of using the metal layer in the MCLW was that an electrical potential could be applied to accelerate the diffusion of the analyte to the immobilised antibody, which resulted in a shortened analysis time and a reduction in non-specific binding.