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Aspergillus is known as one of the most frequent toxigenic fungi in Europe. It produces aflatoxin B1 transformed into aflatoxin M1 (AFM1) present in milk. The International Agency for Research on Cancer (IARC) has included AFM1 in the group I human carcinogens. Acceptable maximum level of AFM1 in milk according to EU regulation is 50 ppt equivalent to 15.2×10-2 nM (the molecular weight of AFM1 is 328.27 g/mol). Up to now, the most used techniques for AFM1 detection in milk are time consuming Enzyme-Linked Immuno-Sorbent Assay (ELISA) and high-performance liquid chromatography associated to fluorescence (HPLC). Silicon photonics based biosensors such as Mach-Zehnder Interferometers and Microring Resonators thanks to their ability to be miniaturize and integrated with electronics and microfluidics in lab-on-a-chip devices, are new candidates to become faster, cheaper and more accurate tools for AFM1 detection in milk. Here, we validate photonic AFM1 biosensors based on an array of four Si3N4 asymmetric Mach-Zehnder Interferometers (aMZI) functionalized by F(ab’)2 fragments and passivated with casein.
Analyses of AFM1 binding by Fab’ in MES buffer and in real milk samples, using different concentrations of AFM1, are performed. The dependence of the phase shifts to the AFM1 concentration allows to calculate the association and dissociation rate constants. The proposed biosensor is capable to detect AFM1 concentration down to 50 ppt. For the average dissociation constant (KD) of AFM1-Fab’ interaction, values of (1.8÷5)×10-8 M in MES buffer and 0.8×10-9 M in milk samples are measured. These are in the same order of magnitude as published results. The difference of one order of magnitude between KD in MES buffer and in milk might be caused by the fact that during the preparation of milk samples, an additional concentration of salt is added to the solution which yields a stronger ionic interaction to occur. Finally, the specificity of the interaction is confirmed by using blank solutions that are free from AFM1.
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