Development of Plasmonic Fiber-Optic Probes for Highly Sensitive and Selective Detection of Dopamine via Citrate–Fe3+–Dopamine Chelate Complexation

Dopamine (DA) is a key biomarker for neurodegenerative disorders. One-dimensional (1D) optical fiber (OF) sensing offers remote, real-time detection but faces a limited signal reflection of 5–10% compared to two- or three-dimensional (2D/3D) planar configurations, restricting detection efficiency to a nanomolar threshold. To overcome these limitations, we developed a novel fiber-optic sensing system (FOSS) by integrating gold-silver core–shell nanostars (Au@AgNSs), functionalized with a citrate-ferric (Fe3+) complex, onto the flattened facets of OFs, creating highly effective SERS-active substrates for enhancing the sensitivity and selectivity of DA. This customized design exploits the synergistic interplay of electromagnetic and chemical enhancements, primarily through Fe3+–DA chelation, significantly amplifying the SERS signals. Optimizing plasmonic nanostructures, ferric ion concentration, and stepwise Fe3+ addition improved sensitivity, achieving a sub-nanomolar (1.61ⅹ10⁻10 M) detection limit, surpassing previous OF-based sensors. Moreover, its strong affinity for DA conferred exceptional selectivity, allowing it to effectively distinguish DA from other coexisting biomolecules. Our work advances bio-optrode sensing technologies for in-situ sensitive detection of trace analytes, offering significant potential for applications in healthcare and biomedical fields, particularly in diagnosing and monitoring dopaminergic dysfunction-related diseases.