Time-resolved spectroscopic and SERS characterization of plasmonic optical fiber sensor using glutathione-capped gold nanourchins for detection of lead in water

Fabrication and optical characterization of evanescent plasmonic fiber sensor for detection of Lead (Pb²⁺) ions in water are described. Glutathione-capped gold nanourchins (GNU-GSH) were immobilized onto fluorinated silicon dioxide (SiO₂) clad using (3-Aminopropyl) triethoxysilane (APTES). At 488 and 638 nm Raman wavelength, a penetration depth of 98 and 128 nm was obtained where the sensing region acts as nanoantenna at far-field for detection of analyte. The absorbance of LSPR decreased by time indicating the binding of Pb²⁺ to GSH ligand. Two types of fiber sensors were prepared using 10 and 30 min of GNU incubation time (i.e., F10 and F30) each irradiated in horizontal and vertical configurations. Dynamic SERS study of GNU-GSH-Pb²⁺ showed that F10 and F30 exhibit a similar non-linear response, where the former reached the maximum intensity faster than the latter one. Fluorescence spectroscopy of GNU-GSH in horizontal configuration indicated the maximum emission wavelength of 633 and 605 nm for F10 and F30, respectively. In the vertical configuration, the wavelengths of 607 and 727 nm were observed, respectively, for F10 and F30. When Pb²⁺ was added, a hyperchromic shift of fluorescence occurred from 652 to 655 nm in 2 min for F10, and the intensity decreased linearly by ≈ 72% in 13 min indicating the gradual binding of Pb⁺² ions to GSH sites. In the case of F30, the emission gradually red-shifted from 551 to 569 nm within 15 min, and the emission reached the maximum at 561 nm after 10 min of interaction.