A novel colorimetric immunoassay strategy using iron(iii) oxide magnetic nanoparticles as a label for signal...

Although bioenzymes or artificial nanoenzymes-catalyzed reactions can efficiently amplify the detection signal in immunosorbent assays, the cascade reactions are sensitive to various conditions, including pH, temperature, time, and so on, resulting in poor reproducibility and dose-dependent response. In this work, a novel colorimetric immunoassay strategy was developed using an iron oxide magnetic nanoparticle (MNP) as a label for signal generation and amplification without the need for subsequent cascade reactions. The signal intensity is directly proportional to the level of targeting analytes, providing a more accurate quantification and a better reproducibility. The measurable color change results from the formation of a soluble red complex between bathophenanthrolinedisulfonic acid disodium salt (BPT) and Fe²⁺ released from dissolution of iron oxide MNP. The MNP, acting as an iron pool, provides a large amount of Fe²⁺ after its dissolution, thus promoting an efficient signal amplification. A qualitative evaluation is directly observed with the naked eye, while quantitative detection is achieved by measuring the absorbance of the colored solution using a UV-Vis spectrophotometer. Under optimal conditions, a limit of detection (LOD) at 3.6 pg mL^-1 was achieved with a cancer biomarker carcinoembryonic antigen (CEA) as a model analyte, demonstrating its great potential for early cancer diagnosis. Moreover, the developed colorimetric assay method can be easily adapted to detection of other biomolecules (nucleic acids, saccharides, small molecules and so on) by simply changing the recognition pairs.