Ratiometric fluorescence sensor for Escherichia coli detection using fluorescein isothiocyanate–labeled metal–organic frameworks

A ratiometric fluorescence sensor for detecting Escherichia coli (E. coli) was fabricated based on the fluorescein isothiocyanate (FITC)–labeled zirconium (Zr)–tetraphenylporphyrin tetrasulfonic acid (TPPS) hydrate metal–organic frameworks (ZTMs@FITC). The ZTMs have strong red fluorescence emission at 683 nm, which can be quenched by Cu²⁺. E. coli can capture and convert external Cu²⁺ into Cu⁺ through its distinctive metabolic activities. To minimize environmental and instrumental influences and enhance detection precision, green FITC with an emission peak at 515 nm was utilized as the fluorescence labeling agent to fabricate the ratiometric fluorescence probe (ZTMs@FITC). The prepared ZTMs@FITC probe showed excellent performance in the detection of E. coli. As the concentration of E. coli increased, the fluorescence intensity at 683 nm (ZTMs, F₆₈₃) increased considerably, while the fluorescence intensity at 515 nm (FITC, F₅₁₅) decreased. By monitoring the increase in the ratio of F₆₈₃ to F₅₁₅, this sensor achieved rapid and sensitive detection of E. coli within the concentration range from 1.0 × 10¹ to 5.0 × 10⁵ CFU/mL. The limit of detection was 6 CFU/mL. When observed under a 365 nm ultraviolet lamp, the fluorescence color of the solution changed from yellow to red. Additionally, the dual-signal ratiometric fluorescence method exhibited high selectivity for E. coli and was successfully utilized to detect E. coli in juice samples, demonstrating its practical application potential in food analysis.

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