Ultra-sensitive boric acid detection using a plasmonic fiber-optic spectral comb and MEA modified with gold nanoparticles.

Even trace amounts of boric acid (BA) could cause serious harm to human beings and the environment. Therefore, trace BA detection is very important. Here, we propose a tilted fiber Bragg grating (TFBG)-based plasmonic fiber-optic spectral comb for ultra-low limit BA detection. The TFBG is coated with a gold film to excite the surface plasmon resonance (SPR) effect, and mercaptoethylamine (MEA) molecules are self-assembled and fixed on the gold film to form a BA probe. MEA molecules can form a stable coordination structure with BA molecules to realize TFBG-SPR BA detection. To enhance the sensor's sensitivity, using MEA modified with gold nanoparticles (AuNPs) to combine with BA molecules, the combination-induced aggregation of AuNPs induces a significant spectral response. The proposed sensor shows highly sensitive detection of BA under the detection range of 1 fM-1 nM, with a detection limit of 1.01 × 10^-15 mol/L (1.01 fM), which is more than five orders of magnitude lower than those of the previously reported methods. The proposed sensor not only shows practical application of BA detection but also provides a new, to the best of our knowledge, way for the TFBG-SPR biochemical sensor to achieve high RI sensitivity.