In this study, we report a switchable fluorescence probe based on nitrogen-doped graphene quantum dots (N-GQDs) to detect the presence of tetracycline and L-Cystence. Strong blue emissive N-GQDs were successfullyprepared by the hydrothermal treatment of urea and benzophenone, with a quantum yield of 34.9%. Both transmission electron microscopy and atomic force microscope images reveal a nearly monodispersed N-GQDs with spherical morphologies, and provide evidences of 1–3 layered structures of N-GQDs. Meanwhile, the obtained N-GQDs can be utilized for fluorescent labelling due to their superior optical stability and favourable biological compatibility. A strong fluorescence quenching was observed for the presence of tetracycline. The following addition of L-Cysteine can motivate the regeneration of fluorescence. Therefore, a simple strategy for the detection of tetracycline and L-Cystence in natural water was developed due to the relevant sensitivity of N-GQDs nanosensor.

Recently, graphene quantum dots (GQDs) with bright fluorescence have emerged as a novel carbon nanomaterial because of their distinctive optical properties and robust chemical inertness. Herein, a simple bottom-upapproach is used to prepare nitrogen-doped GQDs (N-GQDs) by using citric acid as the carbon source and tris (hydroxymethyl) aminomethane (Tris) as the nitrogen source. The prepared N-GQDs have a high yield of 61.50% and the quantum yield is 14.42%. Meanwhile, the N-GQDs exhibit clearly fluorescence quenching with the increase of pH value from 3.0 to 12.0. In addition, the N-GQDs possess excellent fluorescence quenching response to tetracycline (TC) due to the inner filter effect. And the fluorescence intensity of N-GQDs exhibits a good linear relationship with the addition of TC in range of 1–50 ${\mu}$M. The detection limit is determined to be 94 nM. Furthermore, the smartphone-based handheld device is developed to track the fluorescence colour changes caused by the variation of pH and TC. Through analysing the RGB values from the fluorescence images, good linearity betweenRGB values and pH values is obtained ($R^2$ = 0.996), while the detection limit for TC detection is 97 nM. This method has been shown to be effective and reliable along with great promise for real-time visual monitoring of pH and TC values in real samples.