OPTIMIZATION OF PLASMONIC GOLD-TITANIA NANOSTRUCTURES SYNTHESIZED VIA PULSED LASER ABLATION IN LIQUID FOR OPTICAL FIBER-BASED PETROLEUM SENSING

Abstract

The growing demand for advanced nanophotonic and sensing technologies has intensified interest in noble metal-based nanostructures (NSs) and nanocomposites, particularly those exhibiting synergistic plasmonic effects. Conventional synthesis methods for plasmonic NSs often involve toxic chemicals, complex surfactants, or high-temperature processes, which limit scalability and environmental safety. Moreover, the integration of plasmonic gold nanoparticles (AuNPs) with titania nanoparticles (TiO2NPs) remains challenging in achieving uniform core–shell structures with tunable optical features and stable dispersion suitable for sensing applications. This thesis focuses on the synthesis, characterization, optimization, and sensing evaluation of AuNPs, TiO2NPs, and their core– shell nanostructures (Au–TiO2CSNSs), fabricated through a green, chemical-free pulsed laser ablation in liquid (PLAL) technique. Pure Au and Ti targets were ablated in deionized water (DIW) and ethanol (ET) as growth medium under various laser fluences (LFs) ranging from 4.5 to 13.6 Jcm-2 using a Q-switched Nd:YAG laser (1064 nm). The influence of LFs and growth medium on the morphological, structural, optical, and sensing properties of the produced NSs was determined using comprehensive analytical tools, including transmission electron microscopy (TEM), selective area electron diffraction (SAED), energy-dispersive X￾ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, ultraviolet-visible spectroscopy (UV–Vis), and photoluminescence spectroscopy. TEM analysis confirmed the formation of well-dispersed Au–TiO2CSNSs with average core diameters of 8.11 ± 4.28 nm in DIW and 11.59 ± 10.02 nm in ET. SAED patterns exhibited bright concentric rings and halo spots, indicative of a nanocrystalline structure. EDX maps validated the homogeneous elemental distribution across all NSs. XRD confirmed the face-centred cubic and tetragonal phases for Au and TiO2, respectively. FTIR and Raman spectra revealed both anatase and rutile phases of TiO2, high suspension purity, and the presence of Ti–O and Au–O bonds. UV–Vis spectra showed localized surface plasmon resonance (LSPR) bands between 260 and 533 nm, exhibiting solvent-dependent red and blue shifts. The optical bandgap energies of Au–TiO2CSNSs were in the ranged of 3.61 to 4.00 eV in DIW and 3.62 to 3.85 eV in ET, indicating significant plasmonic enhancement. Photoluminescence analysis displayed sharp emission peaks at 331 (DIW) and 398 nm (ET), with fluorescence lifetimes of 2.9 to 3.1 µs, demonstrating excellent colloidal ability. The enhanced zeta potentials further reflected high colloidal stability, critical for long- term sensor performance. The synthesized Au–TiO2CSNSs were used as active sensing layers in fiber-optic sensors (FOSs) and their sensing responses were evaluated at varied petroleum (N95 grade) concentrations. Au–TiO2CSNSs-coated FOSs fabricated at 9.1 Jcm-2 demonstrated superior sensitivity, selectivity, and robustness toward petroleum vapor detection compared to individual AuNPs and TiO2NPs. The synergistic integration of Au plasmonic properties with TiO2 chemical affinity enabled rapid, sensitive, and selective detection of hydrocarbons and sulfur-containing compounds. Overall, the PLAL-based synthesis offers a green, scalable, and surfactant-free route for tailoring NSs with tunable optical properties, while the engineered Au–TiO2CSNSs provide a sustainable, cost-effective, and high-performance platform for real-time petroleum sensing, with strong implications for environmental monitoring, industrial safety, and next-generation FOS technologies.