CdS/TiO2–fluorescein isothiocyanate nanoparticles as fluorescence resonance energy transfer probe for the determination of trace alkaline phosphatase based on affinity adsorption assay

The CdS/TiO2–fluorescein isothiocyanate (FITC) luminescent nanoparticles (CdS/TiO2–FITC) with the particle size of 20 nm have been synthesized by sol–gel method. CdS/TiO2–FITC could emit the fluorescence of both FITC and CdS/TiO2. The fluorescence resonance energy transfer (FRET) occurred between the donor CdS/TiO2 and the acceptor FITC in the CdS/TiO2–FITC. Taking advantages of the excellent characteristics of FRET, a new CdS/TiO2–FITC FRET labeling reagent and a CdS/TiO2–FITC–wheat germ agglutinin (CdS/TiO2–FITC–WGA) fluorescent probe have been developed. The FRET occurring between the donor CdS/TiO2 and the acceptor FITC in the labelled product CdS/TiO2–FITC–WGA–AP, formed in the affinity adsorption reaction between the WGA in this CdS/TiO2–FITC–WGA fluorescent probe and alkaline phosphatase (AP), sharply enhanced the fluorescence signal of FITC and quench the fluorescence signal of CdS/TiO2. Moreover, the ΔF (the change of the fluorescence signal) of FITC and CdS/TiO2 were proportional to the content of AP, respectively. Thus, a new method that CdS/TiO2–fluorescein isothiocyanate nanoparticles for the determination of trace AP based on FRET-affinity adsorption assay has been established. The limit of quantification (LOQ) of the method was 1.3×10−17 g AP mL−1 for CdS/TiO2 and 1.1×10−17 g AP mL−1 for FITC, respectively. This sensitive, rapid, high selective and precise method has been applied to the determination of AP in human serum and the prediction of human disease with the results agreed well with enzyme-linked immunosorbent assay (ELISA) in Zhangzhou Municipal Hospital of Fujian Province. Simultaneously, the reaction mechanism for the determination of AP was also discussed.