Carbon dots rooted PVDF membrane for fluorescence detection of heavy metal ions

PVDF-g-PAA-CDs composite membrane with stable fluorescence was prepared by grafting carbon dots (CDs) onto the surface of PVDF-g-PAA membrane through amidation reaction for the first time. The results of Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and fluorescence properties clearly showed the formation of CDs with a strong blue fluorescence under UV light, which was ascribed to the influence of surface functional groups or edge defects. The effect of CDs on the surface composition, morphology, hydrophilicity and pure water flux of PVDF-g-PAA-CDs composite membranes was investigated. Consistent with CDs, PVDF-g-PAA-CD composite membrane also exhibits photoluminescence behavior that is dependent on excitation. The addition of CDs gives the composite membrane a significant fluorescence quenching effect on Cu²⁺, Hg²⁺ and Fe³⁺ without being affected by K⁺, Na⁺, Cl⁻. This may be because the complexation of Cu²⁺, Hg²⁺ and Fe³⁺ with coordination bonds such as -NH₂ and -COOH on CDs surface can accelerate the extinction of electron/hole non-radiative recombination through effective electron or energy transfer, leading to fluorescence quenching of CDs on the surface of PVDF-g-PAA-CD composite membrane. Furthermore, the optical detection limit of the respective metal ion is found to be 1 nmol/L for Cu²⁺ and Hg²⁺ and 1 μmol/L for Fe³⁺ by studying the changes in fluorescence spectra of PVDF-g-PAA-CD composite membrane. In summary, PVDF-g-PAA-CDs fluorescent composite membrane has the advantages of fast, stable, environmentally friendly, simple, and inexpensive as a solid platform for heavy metal ion detection, and has great potential in the field of water purification.