The accurate and rapid assessment of okadaic acid (OA) levels in shellfish is of paramount importance for ensuring seafood safety. In this study, a competitive electrochemiluminescence (ECL) immunosensor was proposed for the precise quantitation of OA, utilizing a novel self-enhanced solid-state ECL marker. Graphene quantum dots doped with nitrogen and sulfur (N,S-GQDs) were synthesized through the electrolysis of graphite in 3-(N-morpholine) propane sulfonic acid (MOPS) solution. Intriguingly, these N,S-GQDs exhibited exceptional co-reactant properties, significantly enhancing the anodic ECL performance of Ru(bpy)32+ in a phosphate-buffered saline solution. Following the functionalization of Ru(bpy)32+-doped silica nanoparticles (RuSiNPs) with poly(diallyldimethylammonium) chloride (PDDA), we achieved a well-dispersed assembly of N,S-GQDs on the exterior of the RuSiNPs through electrostatic interactions. Importantly, the core-shell structure of RuSiNPs@N,S-GQDs efficiently encapsulated both the luminophore and co-reactant, facilitating improved electron transfer rates, shorter interaction distances, and reduced energy loss during light emission. Consequently, the RuSiNPs@N,S-GQDs displayed enhanced ECL properties compared to bare RuSiNPs. Leveraging this "bright" ECL beacon, our ECL immunosensor demonstrated remarkable analytical performance, yielding a low half maximal inhibitory concentration (IC50) of 0.14 ng mL− 1, an extensive linear range spanning 0.003-40 ng mL− 1, and impressively low limit of detection of 1 fg mL− 1 for OA determination.