Trace Chromium Ion Detection Based on 4-Mercaptobenzoic Acid Functionalized Plasmonic Fiber-Optic Spectral Combs

High sensitivity chromium ions (Cr<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup>) sensor was developed based on the combination of gold nanomaterials-coated tilted fiber Bragg grating (TFBG) and 4-mercaptobenzoic acid (4-MBA). The TFBG was first deposited with a 50 nm gold film to excite sensitive surface plasmonic resonance (SPR) spectral combs and then functionalized with a 4-MBA layer that is available for specifically capturing Cr<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions. To further enhance the SPR-Cr<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> interaction, additional gold nanoparticles (AuNPs)-4-MBA composite was prepared and bonded with the Cr<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> captured by the TFBG. Thus, the SPR response to the capture of Cr<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> and the sensing performance were greatly enhanced. Experimental results show that the sensor exhibits a high sensitivity of 1.23 dB/log(nM) for Cr<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> with a detection limit of 0.04 nM, which is lower than that of the current similar optical Cr<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> detection sensor by one order of magnitude.