In Situ Selective Determination of Cysteine in Crops Employing a Novel Colorimetric and NIR-Emitting Ratiometric Fluorescent Probe along with a Smartphone-Assisted Portable Detection Device

Cysteine (Cys) is the first organic sulfur nutrient produced by crops. There is an urgent need to construct a reliable analytical method to quantitatively detect Cys in crops. Herein, a colorimetric and NIR-emitting ratiometric fluorescent probe for in situ quantitative detection of Cys in crops has been developed. The probe presented highly specific response to Cys over other biothiols including Hcy and GSH. The fluorescence ratios (I₅₄₅/I₆₅₅) exhibited a linearity with Cys concentration in the range of 0.113–300 μM, and the detection limit was measured to be 0.034 μM (S/N = 3). Importantly, the specific sensing reaction between the probe and Cys is achieved through a unique two-step recognition process. The probe was employed to detect Cys in living cells through fluorescence imaging. Additionally, alterations in Cys levels within Gynura cusimbua leaves, triggered by atmospheric H₂S, have been monitored. Furthermore, the probe has been utilized to trace changes in the Cys concentration in G. cusimbua roots under external Cd stress. Notably, to facilitate in situ quantitative detection of Cys in crops, a smartphone-assisted portable detection device was made up. The probe and portable detection device were successfully employed for in situ quantitative detection of Cys in cabbage and apple.