Ratiometric fluorescence immunoassay based on MnO2-o-phenylenediamine-fluorescent carbon nanodots for detection of α-fetoprotein via fluorescence resonance energy transfer

Ratiometric fluorescence immunoassay is known as a spectrofluorimetric variant for sensitive and quantitative analyses of biomarkers. However, there are few comparative studies between it and the traditional, single-wavelength-based immunoassay. In this work, a ratiometric fluorescence immunoassay was designed via the fluorescence resonance energy transfer (FRET) in a system comprising MnO2 nanosheets, o-phenylenediamine (OPD), and fluorescent carbon nanodots (FCNs). The MnO2 nanosheet oxidizes OPD, producing a fluorescence peak at 580 nm, while FCNs exhibit a fluorescence peak at 490 nm. Under the effect of FRET, the fluorescence at 580 nm increases significantly while the fluorescence at 490 nm is reduced. The FRET effect was confirmed by our fluorescence lifetime measurements. We optimized the experimental conditions that include solution pH, OPD concentration, and reaction time. By electrostatically coating the MnO2 nanosheets with antibody molecules and using the resultant materials as a detection probe, the ratiometric fluorescence sensing was extended to the detection of alpha fetoprotein (AFP) in human serum samples on a 96-well titer plate. Our ratiometric fluorescence immunoassay possesses a good dynamic range (from 0.14 to 130.43 pM). The detection limit was estimated to be 0.13 pM (0.009 ng/mL), which is 7.6-fold lower than that of the single-wavelength-based immunoassay. The measurement results of serum samples from healthy and patient donors are highly comparable to those measured by chemiluminescence for the same donors, demonstrating the viability for serological assays of important cancer markers.