Surface functionalization of optical fibers with PDDA-stabilized AuNPs for enhanced interfacial stability in sensing applications

• PDDA-stabilized AuNP layer developed for optical fiber functionalization. • Systematic optimization of PDDA concentration, weight, and gold deposition time. • Improved AuNP/PDDA layer with enhanced interfacial stability for sensing. • FTIR and microscopic analyses confirmed NH₃ adsorption on functionalized layers. • Demonstrates a stable surface-functionalized material for optical sensing applications. Ammonia sensing is important for environmental monitoring, industrial safety, and food spoilage detection, but achieving a stable and reliable sensing response at low concentrations remains challenging. This work reports the development and optimization of a gold nanoparticle (AuNP) layer stabilized with poly(diallyldimethylammonium chloride) (PDDA) for application in optical fiber ammonia sensing. To optimize the coating process, PDDA concentrations (0.5, 0.10, and 1 wt%), molecular weights (low and medium), and AuNP deposition times (5–20 min) were systematically varied. The optimum condition, identified as 1 wt% medium-molecular-weight PDDA with a 20 min AuNP deposition, produced uniform nanoparticle coverage, enhanced interfacial stability, and a strong plasmonic response. The homogeneous distribution and surface coverage of AuNPs, as well as the adsorption of NH₃ at the AuNP/PDDA interface, were verified by FE-SEM, AFM, and FTIR. The optimized layer was applied onto an optical fiber to demonstrate its structural stability and practical sensing capability, exhibiting concentration-dependent spectral shifts upon NH₃ exposure at 0.5, 1, 5, 10, 15, 20, and 25 ppm under room temperature conditions. A sensitivity of 0.023 nm/ppm with excellent linearity (R² = 0.97) was achieved, and nitrogen-assisted desorption enabled repeatable operation. This study demonstrates that systematic material optimization provides a stable, uniform, and robust interfacial layer, supporting the potential of the AuNP/PDDA-coated optical fiber for real-world ammonia detection in environmental, industrial, and healthcare applications.