Multicolor fluorescent probe for visual point-of-care detection of water via a smartphone

The application of multicolor fluorescent probes in visual detection of targets has attracted considerable interest. Herein, a multicolor fluorescent probe (G/R-CDs) was fabricated by combining green carbon dots (G-CDs) with red carbon dots (R-CDs). As water content increased, the fluorescence of R-CDs at 683 nm was quenched significantly, resulting in the decrease of red fluorescence. To improve the visualization effect, G-CDs were introduced. The fluorescence peak of G-CDs was red shifted from 500 nm to 535 nm, and the fluorescence intensity was basically unchanged with addition of water. Therefore, G-CDs and R-CDs served as reference signal and response signal, respectively. The fluorescence intensity ratio (F-535/F-683Y) showed a good linear relationship with water content in the ranges of 3%-30% and 40%-60% with a low detection limit of 1.2%. More importantly, G-CDs also acted as a toner to enrich the fluorescence color of G/R-CDs. Under the toning of G-CDs, G/R-CDs presented a variety of fluorescent colors such as pink, orange, gray and green towards different water contents. Based on the rich color changes, the visual analysis was achieved by the naked eye. In addition, a smartphone was used to read out the RGB values for establishing the RGB method. The method was successfully applied for the detection of water in medical alcohol and commercial liquor, showing good practicability in visual point-of-care detection of water.