The exceptional nature of WO3-x dots has inspired widespread interests (Science, 2017, 358, 1192; Adv. Mater., 2016, 28, 10518), but it is still a major challenge to synthesize high-quality WO3-x dots without using unstable reactants, expensive equipment, and complex synthetic processes. Chemical tailoring of nanosheets is an essential way for the synthesis of nanodots, however it is NOT applicable to exfoliate bulk WO3 due to its covalently-bound layers. As such, most of the synthesis rely on a bottom-up method by using WCl6 as a typical precursor but water-free conditions are usually required due to the highly hydrolytic property of WCl6 (J. Am. Chem. Soc. 2005, 127, 15595). In addition, to diminish the anisotropic growth during the synthesis, the surface of the WO3-x dots is usually anchored with aliphatic amines or oleic acid as surfactant/template, which leads to SLOW Faradic electrochemistry (Adv. Mater. 2014, 26, 4260). Along these lines, it is of both fundamental and technical importance to overcome these deficiencies in the synthesis of high-quality WO3-x dots.
In this work, we report the synthesis of WO3-x dots by a facile exfoliation of bulk WS2 instead of bulk WO3 followed by a mild chemical conversion. The WO3-x dots were not only ligand-FREE and highly water-dispersible but also had tunable oxygen-vacancies, ready for a varity of high-demanding applications. As an example, the WO3-x dots were emerged as a new generation of coreactants for the electrochemiluminescence (ECL) of Ru(bpy)32+ with a tremendous enhancement factor up to 500-fold, owing to the unique electrochemical and catalytic properties. More importantly, compared to the commonly used tripropylamine (TPA) coreactant in the clinics, the WO3-x dots displayed a factor of ca. 300 less ANIMAL toxicity. Along these lines, the enormous potential of WO3-x dots as ECL coreactants in replacing TPA for clinic diagnosis was further exemplified by cytosensing circulating tumor cells with a uncompromised performance. This work would not only open a new way to synthesize WO3-x dots with superior properties but also stimulate an emerging application in clinic ECL diagnosis as coreactants with uncompromised high performance and unprecedented low toxicity.