“Peak tracking chip” for label-free optical detection of bio-molecular interaction and bulk sensing

Abstract

A novel imaging method for bulk refractive index sensing or label-free bio-molecular interaction sensing is presented. This method is based on specially designed “Peak tracking chip” (PTC) involving “tracks” of adjacent resonant waveguide gratings (RWG) “micropads” with slowly evolving resonance position. Using a simple camera the spatial information robustly retrieves the diffraction efficiency, which in turn transduces either the refractive index of the liquids on the tracks or the effective thickness of an immobilized biological layer. Our intrinsically multiplex chip combines tunability and versatility advantages of dielectric guided wave biochips without the need of costly hyperspectral instrumentation. The current success of surface plasmon imaging techniques suggests that our chip proposal could leverage an untapped potential to routinely extend such techniques in a convenient and sturdy optical configuration toward, for instance for large analytes detection. PTC design and fabrication are discussed with challenging process to control micropads properties by varying their period (step of 2 nm) or their duty cycle through the groove width (steps of 4 nm). Through monochromatic imaging of our PTC, we present experimental demonstration of bulk index sensing on the range [1.33–1.47] and of surface biomolecule detection of molecular weight 30 kDa in aqueous solution using different surface densities. A sensitivity of the order of 10⁻⁵ RIU for bulk detection and a sensitivity of the order of ∼10 pg mm⁻² for label-free surface detection are expected, therefore opening a large range of application of our chip based imaging technique. Exploiting and chip design, we expect as well our chip to open new direction for multispectral studies through imaging.