Fluorescent Paper Based on CQDs/Rhodamine B: A Ratio and Sensitive Detection Platform for On-Site Fe³⁺ Sensing

Fluorescent sensors with single reading are generally subject to unpredictable disturbs from environmental and artificial factors. In order to overcome this barrier of detection reliability, a paper-based optical sensor with proportional fluorescence was established and further combined with a smartphone for visual, on-site and quantitative detection of Fe³⁺, which affects the color, smell and taste of water, and endangers the health of plants and animals. The ratio fluorescent probe was fabricated by rhodamine B and carbon quantum dots derived from xylan. The red fluorescence of rhodamine B was inert to Fe³⁺, which was referred to as background. And blue emitting carbon quantum dots functioned as signal report units, which would be quenched by Fe³⁺ and make the fluorescence of the ratio probe change from purple to red. The quantitative detection of Fe³⁺ was conducted by investigating the RGB value of fluorescent images with a smartphone. With the increase of Fe³⁺ concentration, the R/B (red/blue) value of the fluorescent paper gradually increased. The linear detection range was 10-180 μM, and the limit of detection was 198.2 nM. The application of ratio fluorescent paper with a smartphone provides a facile method for the rapid detection of ions.