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.