Large-scale synthesis of dual sensing nitrogen-doped graphene quantum dots for real-time detection of pH and antibiotic

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-up approach 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 μM. 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 between RGB values and pH values is obtained (R2 = 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.