Development of plasmonic fiber-optic probes for highly sensitive and selective detection of dopamine <i>via</i> citrate–Fe ³⁺ –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) 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 (1.61 × 10^-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 <i>in situ</i> detection of trace analytes, offering significant potential for applications in healthcare and biomedical fields, particularly in diagnosing and monitoring dopaminergic dysfunction-related diseases.