The residue of tetracycline from food and environment poses a potential threat to public health. Developing a more convenient portable optical detection system has become more demanding than fluorescence detection. This study describes a sensor of sulfur quantum dots (SQDs) with a positively charged passivator for tetracycline in the concentrations from 0.20 to 100 μM and a limit of detection of 0.15 μM (3σ/k). A new optical sensing device was also designed with a smartphone as the detector for sensing the color change from blue to yellow-green upon the addition of tetracycline. In the presence of Eu3+, the detectable color change is extended to red. The verification of the sensing device was applied to the real detection of lake water and milk, with a standard recovery from 93.1 to 109%. This emissive excitation-independent SQD first ensures a sensitive and accurate analysis at very low concentrations and also demonstrates evident color changes upon interaction with tetracycline. The application of this method has provided great convenience for practical detection.

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