Fabry–Perot Cavity by a Bio-Inspired Hollow Tetragonal Waveguide: Toward Femtomolar Detection

We have developed a box-like closed-end hollow tetragonal waveguide (Fabry–Perot cavity) out of a peptide mimetic composed of fenamic acid and N,N′-dicyclohexylurea. From X-ray crystallography, the peptide mimetic adopts a kink-like conformation stabilized by a hydrogen bond and assembled in sheet-like structures through hydrophobic interactions. The hollow tetragonal structure was characterized by FE-SEM and fluorescence microscopy. We have excited longitudinal modes, along both the length and the width of the rectangle employing broadband light. The longitudinal modes arise due to the closed ends of the hollow waveguides, and the quality factor (Q) of the modes along the longer side of the waveguides is 2 orders of magnitude higher than that along the shorter side. The broadband coupling enables the sensing of dye at emission wavelengths as well as due to a refractive index change, both of which lead to broadening of the longitudinal modes. This may lead to the development of highly sensitive yet flexible waveguide-based femtomolar biosensors.