A dual-channel ratiometric fluorometric assay is described for the determination of the activity of the enzyme tyrosinase (TYR). It is making use of blue-emitting nitrogen-doped graphene quantum dots (bQDs) and of red-emitting dopamine-modified CdTe quantum dots (DA-rQDs). A mixture of the two kinds of quantum dots was prepared, with the ratiometric fluorescence intensity of red to blue adjusted to 5:1. The dopamine on the rQDs is catalytically oxidized by TYR to form dopamine quinone, and this leads to a reduction of the intensity of red fluorescence (peaking at 644 nm). The emission of the bQDs (peaking at 440 nm) represents a stable reference signal. After adding different activities of TYR, the color of the fluorescence of the system continuously changes from red to blue. This can also be visually observed. The ratio of luminescence intensities (at 644/440 nm) can be used to quantify the activity of TYR, and the detection limit is 4.5 mU mL. In addition, a test strip is described based on the above method. Graphical abstract Schematic representation of the ratiometric fluorometric method for determination of the activity of tyrosinase (TYR). (The full name of the abbreviation in the Scheme: 1-ethyl-3-[3-(dimethylamino)-propyl] carbodiimide hydrochloride (EDC), dopamine (DA), N-hydroxysuccinimide (NHS), nitrogen-doped graphene quantum dots (bQDs), CdTe quantum dots (rQDs)).