Abstract: | 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. |