A fluorescence assay mediated peroxidase-like activity of magnetic nanoparticles for dopamine detection in human blood serum with a smartphone-based portable device

Dopamine (DA) concentration in the blood is a critical diagnostic indicator for human illnesses. Thus, the platform-based fluorescence method for DA detection with a smartphone was proven. The detection assay used the complimentary properties of 3-aminopropyl triethoxysilane-functionalized magnetic nanoparticles (MNPs@APTES). In the absence of DA, MNPs@APTES is capable of catalyzing the oxidation of o-phenylenediamine (OPD) by H2O2 to produce the fluorescent species (2,3-diaminophenazine; DAP). The smartphone-based portable fluorescence device can measure the fluorescence intensity of the DAP, which can then be translated into the RGB signal corresponding to the DAP concentration via the smartphone application. However, the RGB intensity was diminished in the presence of DA as a result of the oxidation of DA in an alkaline medium to produce polydopamine (PDA) passivated onto the surface of the MNPs@APTES, which inhibited catalytic activity and produced less DAP. Thus, there is a correlation between the concentration of DA and the degree of fluorescence decrease. This platform demonstrated a high degree of selectivity for DA relative to other analogous substances and potential interferences. In addition, it successfully detected DA levels in human blood serum samples over a linear working range of 1–50 μM with a detection limit of 0.48 μM.