A ratiometric fluorescent aptasensor based on energy transfer between [Ru(bpy)] and silica quantum dots (silica QDs) for assaying OTA was fabricated. The aptamer for OTA was used as the gate to shield the fluorescent reagent [Ru(bpy)] into mesoporous silica nanoparticle (MSN). In the presence of OTA, the constrained [Ru(bpy)] was released from MSN due to a target-induced aptamer conformational change. The released [Ru(bpy)] adsorbed onto the negatively charged silica QDs through electrostatic interaction. This creates appearance of fluorescence from [Ru(bpy)] at 625 nm and decrease of the fluorescence from silica QDs at 442 nm owing to the energy transfer. The value of FL/FL was in proportion to the concentration of OTA in the range 0.5~100 ng mL with a LOD of 0.08 ng mL. Practical applicability of this method was validated by the determination of OTA in flour samples. Graphical abstract The sensing principle of this sensor.