The condensation reaction between methyldopa sesquihydrate and ethylenediamine is reported and the assembled nanoplatform possesses green luminescence at 520 nm with excitation‐independent feature. The employment of sodium borohydride during synthesis has stabilized the nanoparticle (NP) size and suppressed uncontrolled autoxidation processes. It is appealing to observe that the fluorescence as well as colorimetric titration demonstrates trace level monitoring of silver ions, and the detection limits are determined to be 68 × 10−9 m (fluorometric) and 71 × 10−9 m (colorimetric), respectively. The optical response to Ag+ ions is assigned to a static quenching process and the corresponding redox reaction including particle aggregation may cause the “on‐off” change. During in vitro studies, two living cell lines (HeLa cells and T‐cells) are used and the negligible cytotoxicity of such NP is verified via flow cytometry and 3‐(4,5‐dimethythiazol‐. 2‐yl)‐2,5‐diphenyl tetrazolium bromide assays. More importantly, the visualization of silver recognition in two cells is realized and the availability of polydopamine‐type NPs for the purpose of bio‐medical evaluation can generate considerable interests in future studies. Methyldopa sesquihydrate and ethylenediamine are used to fabricate the quantum dots, and the role of stabilizing agent (sodium borohydride) is evaluated. Both fluorometric and colorimetric titrations can realize the recognition of silver ions with high sensitivity. Cellular entry experiments are performed in HeLa cells and T‐cells, enabling the identification of silver ions in cellular environment with high accuracy.