Carbonaceous nanodots (CDs) have drawn increasing attention owing to their inspiring properties. CDs based composites have been attracting extensive research interest because of the properties improvement of materials and introduction of new physical or chemical properties. In this work, we designed a quick, handy and extra-reductant needless method to synthesize Ag@CDs composites according to the direct reduction of Ag(NH₃)₂OH. TEM, XPS, XRD and optical properties were investigated, and a hypothetical synthesis mechanism was provided. Interestingly, glucose lowered the reaction difficulty and shortened the reaction time, which was well known as silver mirror reaction. Furthermore, a dual signal sensor for glucose was established through optical variations during the synthetic process. The simultaneous synthesis and detection avoided complicated pre-modification of the materials and thus simplified the procedure. The synthesis by a simple elementary reaction and “detection during the synthesis” would attract much interest.