Loading carboxyfluorescein into porous SiO2 spheres and UCNPs-cored porous SiO2 spheres: emission turn-on sensing with a warning signal for tumor-biomarker 5-HIAA urine test

Abstract In this paper, a sensing probe (denoted as CFE) for tumor-biomarker 5-hydroxyindoleacetic acid (5-HIAA) was firstly synthesized. Its sensing mechanism was confirmed as the recovery of fluorescein structure via an ester-exchange reaction, accompanied with emission turn on effect towards 5-HIAA. To improve photostability and minimize self-quenching/aggregation of CFE molecules, they were loaded into silica based MCM-41 spheres and UCNPs-cored (up-conversion nanoparticles) MCM-41 spheres. Corresponding composite samples, denoted as CFE@MCM-41 and CFE@MCM-41@UCNPs, were carefully discussed to confirm their structure, including XRD, N2 adsorption and desorption, SEM/TEM and TGA. The energy transfer from secondary excitation source UCNPs to probe CFE was confirmed by Forester radius calculation and emission lifetime comparison. Sensing performance of CFE@MCM-41 and CFE@MCM-41@UCNPs were finally discussed in detail. They showed emission turn on effect towards 5-HIAA with good selectivity. CFE@MCM-41 showed linear response, while CFE@MCM-41@UCNPs showed exponential response, resulting in a warning signal when analyte concentration was higher than a critical level. The reason for this exponential response was discussed and explained. Their practical sensing result for human urine test was compared to HLPC method and literature methods, which confirmed the promising result of CFE@MCM-41 and CFE@MCM-41@UCNPs.