Concentration-dependent multicolor fluorescent carbon dots for colorimetric and fluorescent bimodal detections of Fe3+ and l-ascorbic acid

We present a green and facile strategy for fabrication of concentration-dependent multicolor fluorescent carbon dots (CDs) by using coffee, salvia, and urea as the starting material via a one-step hydrothermal method. The obtained CDs emit three different colors of fluorescence (i. e. orange, yellow, and blue) with their concentration reduction. Moreover, yellow fluorescent CDs (Y-CDs) are utilized as a platform for colorimetric and fluorescent dual-mode detections of Fe3+ and L-ascorbic acid (L-AA) with high selectivity and sensitivity. The fluorescence of Y-CDs can be effectively quenched by Fe3+ based on static quenching and subsequently recovered upon the addition of L-AA due to the competitive complexation of L-AA with Fe3+. Therefore, the as-prepared Y-CDs could serve as a highly efficient fluorescent probe for "on-off-on" sequential determination of Fe3+ and L-AA. It is more attractive that Fe3+ and L-AA can also be visualized by this nanosensor via evident color changes under daylight. Furthermore, the designed nanosensor can be extended to detect intracellular Fe3+ and L-AA with outstanding biocompatibility and cellular imaging capability, which holds great promise in biomedical applications.