In this study, we report that both CN and I can enhance the fluorescent intensity of Hg-graphene quantum dots (Hg-GQDs). However, the selectivity of the sensor was poor. Accordingly, simple specific masking agents can be directly used to solve this problem. Here, for the first time, we report the use of persulfate ion (SO ) as a turn-on fluorescent probe of Hg-GQDs for selective CN detection, while hydrogen peroxide (HO) was selected for its sensing ability towards I ion detection. Interestingly, the signal was immediately measured after addition of the masking agent to Hg-GQDs and the sample because its interaction was very fast and efficient. The method had a linear response in the concentration ranges of 0.5-8 μM ( = 0.9994) and 1-12 μM ( = 0.9998) with detection limits of 0.17 and 0.20 μM for CN and I, respectively. The sensor was successfully used for the dual detection of both CN and I in real water samples with satisfactory results. In conclusion, the specific masking agents in a Hg-GQDs system appeared to be good candidates for fluorometric "turn-on" sensors for CN and I with excellent selectivity over other ions.