The preparation of pH-sensitive yellow-fluorescent carbon dots and their application in visual detection of urease and Proteus bacteria

Fluorescent colorimetric dual-mode pH-sensitive probes based on functional carbon dots (CDs) have significant potential in biological enzyme sensing and pathogen detection. Herein, we report the synthesis of pH-sensitive functional carbon dots (Cu-CDs) via a one-step hydrothermal method using 2,5-diaminobenzenesulfonic acid, 2,4-dihydroxybenzoic acid, and CuCl₂ as precursors. Under acidic conditions, the Cu-CDs exhibit optimal excitation and emission wavelengths at 400 nm and 540 nm, respectively. In alkaline environments, these wavelengths shift to 510 nm and 589 nm. Within the pH range of 5–8, the fluorescence color transitions from yellow to red, with the fluorescence intensity ratio (F₅₈₉/F₅₄₀) displaying a strong linear correlation. At higher concentrations, the solution undergoes a visible color change from yellow to red, accompanied by a red shift in UV–Vis absorption from 360 nm to 490 nm. Leveraging urease's ability to hydrolyze urea into CO₂ and NH₃—thereby increasing the solution's pH—a simple, rapid, and visual method for urease detection was developed using Cu-CDs as a fluorescent colorimetric probe. The method demonstrated a detection range of 1–80 U/L with a limit of detection (LOD) of 0.27 U/L. Furthermore, as Proteus bacteria naturally produce urease, Cu-CDs were successfully applied for their specific detection, achieving a detection range of 5.6 × 10 2 –1.6 × 10 5  CFU/mL and an LOD of 5.6 × 10 2  CFU/mL. This work presents a novel strategy for the rapid, direct, and visual detection of urease and Proteus bacteria.