A ratio-metric fluorescent off-on method for quantitative analysis of methylene blue in water and shrimp meat

Accurate detection of methylene blue (MB), a widely utilized agent in fish transportation, is of paramount significance for the stringent regulation of aquatic product quality and the safeguarding of consumer health. In this work, we establish a ratio-metric fluorescence method for the quantitative determination of MB. The sensing mechanism is based on an "OFF-ON" fluorescence switching process of carbon dots (VCDs): initially, the fluorescence of VCDs is quenched (OFF) by Fe(OH)₃,based on the redox reaction between VCDs and Fe³⁺and Fe³⁺ was reduced to Fe²⁺ Upon the introduction of MB, Fe²⁺ was reduced to Fe³⁺, which leads to the gradual recovery of VCD fluorescence (ON), thereby facilitating the quantitative detection of MB. Under optimized experimental conditions, the method exhibits a linear response range for MB from 1.337 to 13.37 nM, with a low limit of detection (LOD) of 0.31 nM. Furthermore, the system can also be applied for the detection of Fe(OH)₃, showing a good linear relationship between the fluorescence signal variation of VCDs and Fe(OH)₃ concentrations within the range of 0.468 - 5.616 mM, with a corresponding LOD of 0.21 mM. The excellent anti-interference capability of the sensor, coupled with its good uniformity RSD confirm the superiority of the developed ratio-metric fluorescence strategy. For practical applications, the method was successfully applied for MB detection in real samples including shrimp meat and Songhua River water, yielding satisfactory recovery rates ranging from 81.53% to 101.2%. Collectively, these results demonstrate that the proposed "OFF-ON" ratio-metric fluorescence sensing system holds considerable potential as a reliable and efficient tool for MB monitoring in complex matrices.