| Summary: | Magnetite nanoparticles (MNPs) are a preferable material for different bioassays because of their quite low toxicity both for cells and for mammals, and they have a big variety of surface functionalization approaches. We have synthesized MNPs via a simple and convenient co-precipitation method with preliminary filtration of FeCl<sub>2</sub> and FeCl<sub>3</sub> solution, under argon atmosphere and non-magnetic stirring. MNPs were citrate-stabilized and then modified stage by stage with tetraethoxysilane (TEOS), (3-Aminopropyl)triethoxysilane (APTES) and acylated with succinic anhydride, resulting in carboxylated MNPs. Carboxylated MNPs were covalently bounded with folic acid antibody (FA-1) via 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). MNP-EDC-FA-1 were passed through a test-stripe with the line consisting of folic acid–gelatin conjugate. The conjugation of MNP-EDC-FA-1 with folic acid was observed visually, and the magnetic signal distribution was scanned through the test-stripe with the magnetic particle quantification technique (MPQ) developed earlier. Visually, the line with folic acid–gelatin conjugate on the test-stripe turned dark, with color intensity strongly depending on the MNP-EDC-FA-1 concentration. MPQ has shown that the great majority of MNP-EDC-FA-1 was bound with the acid–gelatin conjugate. The MPQ technique allowed quantification down to 5 ng of MNP-EDC-FA-1 in this experiment with MNPs synthesized, with a strong peak at the acid–conjugate line.
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