The exceptional nature of WO3−x dots has inspired widespread interest, but it is still a significant challenge to synthesize high‐quality WO3−x dots without using unstable reactants, expensive equipment, and complex synthetic processes. Herein, the synthesis of ligand‐free WO3−x dots is reported that are highly dispersible and rich in oxygen vacancies by a simple but straightforward exfoliation of bulk WS2 and a mild follow‐up chemical conversion. Surprisingly, the WO3−x dots emerged as co‐reactants for the electrochemiluminescence (ECL) of Ru(bpy)32+ with a comparable ECL efficiency to the well‐known Ru(bpy)32+/tripropylamine (TPrA) system. Moreover, compared to TPrA, whose toxicity remains a critical issue of concern, the WO3−x dots were ca. 300‐fold less toxic. The potency of WO3−x dots was further explored in the detection of circulating tumor cells (CTCs) with the most competitive limit of detection so far.

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The development of electrochemiluminescent (ECL) emitters with both intense ECL and excellent film-forming properties is highly desirable for biosensing applications. Herein, a facile one-pot preparation strategy was proposed for the synthesis of a self-enhanced ECL emitter by co-doping Ru­(bpy)3 2+ and (diethylaminomethyl)­triethoxysilane (DEAMTES) into an in situ-produced silica nanohybrid (DEAMTES@RuSiO2). DEAMTES@RuSiO2 not only possessed improved ECL properties but also exhibited outstanding film-forming ability, which are both critical for the construction of ECL biosensors. By coupling branched catalytic hairpin assembly with efficient signal amplification peculiarity, a label-free ECL biosensor was further constructed for the convenient and highly sensitive detection of miRNA-21. The as-fabricated ECL biosensor displayed a detection limit of 8.19 fM, much lower than those in previous reports for miRNA-21 and showed superior reliability for detecting miRNA-21-spiked human serum sample, demonstrating its potential for applications in miRNA-associated fundamental research and clinical diagnosis.

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