Detection of Aβ oligomers based on magnetic-field-assisted separation of aptamer-functionalized Fe3O4 magnetic nanoparticles and BaYF5:Yb,Er nanoparticles as upconversion fluorescence labels

We developed a sensitive and stable bioassay for detection of Aβ oligomers (Aβo), a potentially promising candidate biomarker for Alzheimer's disease (AD) diagnosis. We first prepared the amino-functionalized Fe3O4 magnetic nanoparticles (MNPs) and BaYF5:Yb,Er upconversion nanoparticles (UCNPs), which were conjugated with avidin by a classical glutaraldehyde method. Then the biotinylated Aβo aptamer DNA1 was immobilized on avidin-conjugated MNPs, and its complementary DNA2 was linked to avidin-conjugated UCNPs, all through the biotin-avidin affinity reaction, in order to prepare the DNA1-functionalized MNPs and DNA2-functionalized UCNPs. Then the aptamer DNA1 hybridized with its part complementary DNA2 to form the duplex structure on the surface of the MNPs/UCNPs nanoprobe, therefore were assembled onto the surface of MNPs giving a background upconversion fluorescent signal. In the presence of Aβo, the aptamer DNA1 preferentially bound with Aβo and caused the dissociation of some complementary DNA2, liberating some UCNP-labeled complementary DNA2 and leading to a decreased upconversion fluorescent signal on the surface of MNPs. Then after magnetic separation with an external magnet, highly specific and sensitive detection of Aβo was achieved in connection with measurement of the upconversion fluorescent signal of the unreleased UCNPs on the surface of MNPs. The decreased fluorescence intensity of UCNPs was related to the concentration of Aβo in the range of 0.2-15 nM with a detection limit of 36 pM. The developed method then was successfully applied to measure Aβo in artificial cerebrospinal fluid. Benefiting from the magnetic separation and concentration effect of MNPs, the high sensitivity of UCNPs, as well as the selectivity and stability of the aptamer, the present strategy offered valuable information related to early diagnosis of AD process.232