Background: Recently, some new nanobiosensors using different nanoparticles or microarray systems for detection ofmycotoxins have been designed. However, rapid, sensitive and early detection of aflatoxicosis would be very helpful todistinguish high-risk persons.Objectives: We report a highly sensitive competitive immunoassay using magnetic/silica core shell as a signal intensifierfor the determination of aflatoxin B1 using fluorescence resonance energy transfer (FRET) from Cd/Te quantum dots (antiaflatoxinB1 antibody immobilized on the surface of Cd/Te quantum dots) to Rhodamine 123 (Rho 123-labeled aflatoxinB1 bound to albumin). The specific immune-reaction between the anti-aflatoxin B1 antibody on the QDs and the labeledaflatoxinB1 brings the Rho 123 fluorophore (acting as the acceptor) and the QDs (acting as the donor) in close spatialproximity and causes FRET to occur upon photo-excitation of the QDs. Using magnetic/silica core shell to intensify theobtained signal is the novelty of this study.Materials and Methods: Cd/Te QDs were synthesized by the simultaneous reduction of cadmium chloride and telluriumin the presence of sodium borohydride under nitrogen atmosphere. Magnetic nanoparticles were synthesized using FeSO4 and FeCl3 (1: 2 molar ratio) and ammonia as an oxidizing agent under nitrogen atmosphere. The prepared magneticnanoparticles shelled by silica using tetraethoxysilane in the presence of ammonia. Nanoparticles synthesis and monodispersity confirmed by TEM. Immobilization of Cd/Te QDs to antibodies and labeling of aflatoxin B1-albumin by Rho 123 were performed by EDC/NHS reaction in reaction mixture buffer, pH 6, at room temperature.Results: By using the magnetic/silica core shell sensitivity of the system changed from 2×10-11 in our previous study to2×10-12 in this work. The feasibility of the method established by the detection of aflatoxin B1 in spiked human serum.There is a linear relationship between the decreased fluorescence intensity of Rho 123 with increasing concentration ofaflatoxin B1 in spiked samples, over the range of 0.01-0.06 μmol.mL-1.Conclusions: This homogeneous competitive detection scheme is simple, rapid and efficient, and does not require multipleseparation steps and excessive washing.

Introduction: Histone modifying enzymes include several classes of enzymes that are responsible for various post-translational modifications of histones such as methylation and acetylation. They are important epigenetic factors, which may involve several diseases and so their assay, as well as screening of their inhibitors, are of great importance. Herein, a bioassay based on terbiumto-quantum dot (Tb-to-QD) time-resolved Fö rster resonance energy transfer (TR-FRET) was developed for monitoring the activity of G9a, the euchromatic histone-lysine N-methyltransferase 2. Overexpression of G9a has been reported in some cancers such as ovarian carcinoma, lung cancer, multiple myeloma and brain cancer. Thus, inhibition of this enzyme is important for therapeutic purposes. Methods: In this assay, a biotinylated peptide was used as a G9a substrate in conjugation with streptavidin-coated ZnS/CdSe QD as FRET acceptor, and an anti-mark antibody labeled with Tb as a donor. Time-resolved fluorescence was used for measuring FRET ratios. Results: We examined three QDs, with emission wavelengths of 605, 655 and 705 nm, as FRET acceptors and investigated FRET efficiency between the Tb complex and each of them. Since the maximum FRET efficiency was obtained for Tb to QD705 (more than 50%), this pair was exploited for designing the enzyme assay. We showed that the method has excellent sensitivity and selectivity for the determination of G9a at concentrations as low as 20 pM. Furthermore, the designed assay was applied for screening of an enzyme inhibitor, S-(5’-Adenosyl)-L-homocysteine (SAH). Conclusion: It was shown that Tb-to-QD FRET is an outstanding platform for developing a homogenous assay for the G9a enzyme and its inhibitors. The obtained results confirmed that this assay was quite sensitive and could be used in the field of inhibitor screening.