Pronounced aromatic carboxylic acid detection using a layer-by-layer mesoporous coating on optical fibre long period grating

In this work a new optical fibre sensor for the detection of aromatic carboxylic acids (ACAs) is proposed. The operation of the sensor is based on the measurement of the change in refractive index of a functional coating deposited onto the cladding of the optical fibre. Mesoporous thin films were fabricated using the layer-by-layer self-assembly of poly(allylamine hydrochloride) (PAH) and silica nanospheres (SiO2) with a diameter in the range of 40-50 nm. The film was successfully deposited onto three different transducers; an optical fibre long-period grating (LPG), a quartz crystal microbalance (QCM) and a silicon wafer used in reflectometric interference spectroscopy (RIfS). The amino (NH2) functional group of PAH was used as the binding site for the detection of ACAs. High sensitivity, with binding constants of 1.36 ± 0.01 × 106 and 5.6 ± 0.01 × 108 M−1 for benzoic acid and mellitic acid, respectively, was achieved. The lowest measured concentration of 1 nM of mellitic acid was achieved selectively over structurally (phenol) and functionally (acetic acid) related compounds. Concomitant refractive index (RI) and QCM mass changes were measured to validate the LPG response.