Lanthanide Metal Organic Framework as A New Unlabeled Fluorescence Anisotropy Probe for Detection of Phosphate Ions

Fluorescence anisotropy (FA) analysis has many advantages such as no requirement of separation, high throughput and real-time detection, and thus has been widely used in many fields, including biochemical analysis, food safety detection, environmental monitoring, etc. However, due to the small volume or mass of the target, its combination with the fluorescence probe cannot produce significant signal change. To solve this issue, researchers often use nanomaterials to enhance the mass or volume of fluorophore to improve the sensitivity. Nevertheless, this FA amplification strategy also has some disadvantages. Firstly, nanomaterials are easy to quench fluorescence. As a result, the FA value is easily influenced by light scattering, which reduces the detection accuracy. Secondly, fluorescent probes in most methods require complex modification steps. Therefore, it is necessary to develop new FA probes that do not require the amplification of volume and mass or modification. As a new kind of nanomaterials, luminescent metal-organic framework (MOF) has a large volume (or mass) and strong fluorescence emission. It does not require additional signal amplification materials. As a consequence, it can be used as a potential FA probe. This study successfully synthesized a lanthanide metal organic framework (Ce-TCPP MOF) using cerium ion (Ce3+) as the central ion and 5,10,15,20-tetra (4-carboxylphenyl) porphyrin (H2TCPP) as the ligand through microwave assisted method, and used it as a novel unmodified FA probe to detect phosphate ions (Pi). In the absence of Pi, Ce-TCPP MOF had a significant FA value (r). After addition of Pi, Pi reacted with Ce3+ in MOF and destroyed the structure of MOF into the small pieces, resulting in a decrease in r. The experimental results indicated that with the increase of Pi concentration, the change of the r of Ce-TCPP MOF (Delta r) gradually increased. The Delta r and Pi concentration showed a good linear relationship within the range of 0.5-3.5 mu mol/L (0.016-0.108 mg/L). The limit of detection (LOD, 3 sigma/k) was 0.41 mu mol/L. The concentration of Pi in the Jialing River water detected by this method was about 0.078 mg/L, and the Pi value detected by ammonium molybdate spectrophotometry was about 0.080 mg/L. The two detection results were consistent with each other, and the detection results also meet the Class II water quality standard, proving that this method could be used for the detection of Pi in complex water bodies