Reflective leaky waveguide gratings (LWGs) with internal referencing for sensing

Current transmission grating sensors are unsuitable for use in typical samples that absorb and scatter light. To overcome this limitation, we report leaky waveguide gratings (LWGs), which operate in reflective mode. The LWGs comprising alternating strips with and without recognition elements (herein, streptavidin) in (chitosan) hydrogel films were deposited on glass substrates. The strips were fabricated by laser interference lithography using a photocleavable biotin linker. The strips of LWGs were parallel to the plane of incidence, which contrasts with widely known waveguide grating couplers and surface plasmon resonance gratings. Hence, two-dimensional (2D) diffraction patterns were observed in the orthogonal spaces of angles of diffraction and incidence. Plots of intensity profiles of first diffracted orders of LWGs versus angles of incidence showed peaks close to resonance angles of LW strips without and with streptavidin. Differential measurements between these peaks allowed sensing of exemplar analytes, biotin–protein A and immunoglobulin G, and suppressed changes in sample composition by between 94.1 and 99.9%, providing a high degree of internal referencing.