Fluorescent carbon dots (CQDs) have drawn increasing attention in recent years owing to exceptional advantages such as high sensitivity, chemical stability and low toxicity. However, it remains a challenge to accurately control lateral dimensions and the resulting surface chemistry, especially the quantum yields. A simple synthesis approach to accurate control the characteristics of fluorescent CQDs has been developed by a one-step thermal decomposition in this work. With citric acid and ethylenediamine as raw materials, polymer-like CQDs in diameter of 3.2 nm are obtained. The as-prepared CQDs have been characterized by TEM, FT-IR, Raman spectrometry, UV spectrometry and Fluorescence spectrometry. The CODs exhibit excellent fluorescent property and high stability, which can be used as an effective fluorescent probe for the detection of metal ions with good selectivity and sensitivity in an aqueous solution. This is a pioneer work for chemical sensing of aqueous metal ions.