Molecularly Imprinted Polymer-Coated Optical Waveguide for Attogram Sensing

Ultrahigh sensitivity and selectivity are the ultimate goals of sensor development. For such purposes, we propose a sensing platform in which an optical fiber–waveguide–fiber (OFWF) structure is integrated with a molecularly imprinted polymer (MIP). The OFWF works as a highly efficient probe light launcher and signal light collector, and the MIP layer acts as a highly selective and sensitive sensing interface. In the MIP design, a high-molecular refractive index monomer (2-phenylphenoxyethyl acrylate) was copolymerized with a MIP functional monomer (acrylic acid). The resulting high-refractive index MIP layers could effectively extract the probe light from the waveguide and send it to the MIP sensing interface. Moreover, a highly elastic cross-linker (poly(ethylene glycol) 600 diacrylate) was employed to increase the MIP mesh size, which could effectively increase the penetrability of the analyte. Rhodamine B (Rh B) is widely used in the textile industry, and its contamination may lead to serious public health problems. As a proof of concept, the Rh B chromophore was used as a molecular template, and the thin MIP layer was cured on the waveguide surface by utilizing the evanescent wave of the 405 nm propagating light in the waveguide. The MIP-OFWF sensing platform afforded highly selective monitoring of the absorption spectra of the components in a mixture solution of Rh B and methyl blue. It also afforded an extremely low detection limit of approximately 6.5 × 10–17 g/mL, with an absolute mass of 20–30 ag.