Ratiometric fluorescence system for pH sensing and urea detection based on MoS₂ quantum dots and 2,...

Herein, we developed a novel and sensitive ratiometric fluorescence strategy for pH sensing and urea detection based on MoS₂ quantum dots (MQDs) and 2, 3-diaminophenazine (DAP). The DAP was pH-sensitive and can be used as pH indicator while MQDs was pH-insensitive and acted as a reference fluorescent signal. The fluorescence intensity of DAP at 568 nm exhibited increases gradually while the fluorescence intensity of MQDs at 420 nm kept unchanged as the pH changed from 3.0 to 9.0. So the ratio of the two fluorescence intensities at 568 nm and 420 nm can be used to monitor the change of pH. The pH-sensing system showed a linear and reversible respond to pH in the range of 3.8-6.0 with an interval of 0.2 pH unit. Furthermore, the enzymatic reaction of urea in the presence of urease can lead to the pH increase of the solution, which can be used to detect urea by using the above-mentioned pH-sensing system. There exhibited a good linear relationship between the ratiometric fluorescence intensity of I₅₆₈/I₄₂₀ and urea concentrations ranging from 5 to 700 μM. And a low detection limit of 1.8 μM was obtained. The proposed strategy exhibited excellent selectivity and sensitivity. Additionally, the developed ratiometric fluorescence strategy was successfully applied to detect urea in water samples.