| Summary: | A ratiometric fluorescent sensor with hydrogen sulfide (H<sub>2</sub>S) and methanthiol (CH<sub>3</sub>SH) sensitivity was developed to real-time monitor beef freshness. A silicon quantum dots (SiQD) and silver nanoclusters (AgNC) complex, namely SiQD-AgNC, was used as the dual emission fluorescence materials. Due to the fluorescence resonance energy transfer (FRET) effect between SiQD and AgNC, when the fluorescence of AgNC (610 nm) was quenched by H<sub>2</sub>S or CH<sub>3</sub>SH, the fluorescence of SiQD (468 nm) recovered, resulting in an increase of the fluorescent intensity ratio (<i>I</i><sub>468</sub>/<i>I</i><sub>610</sub>). <i>I</i><sub>468</sub>/<i>I</i><sub>610</sub> showed a linear relationship with the H<sub>2</sub>S concentration within the concentration range of 1.125–17 μM, with a limit of detection (LOD) value of 53.6 nM. Meanwhile, <i>I</i><sub>468</sub>/<i>I</i><sub>610</sub> presented two linear relationships with the CH<sub>3</sub>SH concentration within the concentration range of 1.125–17 μM and 23.375–38.25 μM, respectively, with a LOD value of 56.5 nM. The SiQD-AgNC complex was coated on a polyvinylidene fluoride (PVDF) film to form a portable SiQD-AgNC/PVDF film sensor. This film showed purplish red-to-cyan color changes in response to H<sub>2</sub>S and CH<sub>3</sub>SH, with LOD values of 224 nM and 233 nM to H<sub>2</sub>S and CH<sub>3</sub>SH, respectively. When the film was used to monitor beef freshness at 4 °C, its fluorescent color gradually changed from purplish red to cyan. Hence, this study presented a new ratiometric fluorescent sensor for intelligent food packaging.
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