Fluorescent core–shell SiO₂@COF composite for ultra-sensitive detection of cysteine and homocysteine

To improve the fluorescence properties of covalent organic framework (COF) materials, core–shell SiO₂@COF composite was prepared, in which SiO₂ was used as inner substrate, and BMTH (2,5-bis (2-methoxyethoxy) p-phenylhydrazide) and HB (2-hydroxy-1,3,5-phenyltrialdehyde) acted as precursors to construct COF_BMTH-HB layer on the surface of SiO₂. Compared with COF_BMTH-HB, SiO₂@COF_BMTH-HB composite showed better dispersion property and improved fluorescence performance under the same experimental conditions. Using SiO₂@COF_BMTH-HB as a fluorescence probe, the fluorescence intensity was gradually decreased with the addition of Cu²⁺, while it was restored by introducing Cys or Hcy, realizing “ON–OFF-ON” fluorescence sensing detection. Other amino acids with five times the concentration of Cys showed little effect on the determination of Cys or Hcy. The SiO₂@COF_BMTH-HB-Cu²⁺ probe exhibited ultrahigh sensitivity and good selectivity for the detection of Cys and Hcy, with a detection range of 0.23 to 250.0 µM for Cys and 0.31 to 170.0 µM for Hcy. The detection limits (LOD = 3 σ/s) were 75 and 104 nM for Cys and Hcy, respectively. The system also showed a good recovery and low relative standard deviations in actual sample tests for Cys or Hcy, demonstrating its potential for practical applications.