Ultralow detection of 3,4-methylenedioxymethylamphetamine using an immunofluorescence nanoconjugate of heavy metal-free alloyed quantum dots and NiCeFe₃O₄ magnetic nanoparticles

The recreational use of 3,4-methyl​enedioxy​methylamphetamine (MDMA) remains consistently high on a global level. This reinforces the importance of early, rapid and accurate detection of MDMA. This work reports on the novel development of a rapid, ultrasensitive and selective immunofluorescence nanobiosensor probe for MDMA. To develop the nanoprobe, heavy metal-free AuZnFeSeS alloyed quantum dots (QDs) were synthesized and coated with carboxy (COOH)-silica and subsequently conjugated to an anti-MDMA antibody (Ab). Secondly, spinel NiCeFe3O4 magnetic nanoparticles (MNPs) were synthesized, surface functionalized with L-cysteine and conjugated to the same anti-MDMA Ab. An immunocomplex was established where both AuZnFeSeS QDs-Ab and NiCeFe3O4-Ab each captured the target MDMA drug. The bound QDs fluorescently reported the surface biomolecular interaction between the nanomaterials and Ab while the bound NiCeFe3O4 MNPs functioned as an adsorbent and as a signal amplifier. The Ab binding on AuZnFeSeS QDs surface switched off the fluorescence of the QDs but upon interaction of AuZnFeSeS QDs-Ab with NiCeFe3O4-Ab, the fluorescence of the bound QDs was switched on. Experimental analysis revealed the inefficiency of AuZnFeSeS QDs-Ab (without NiCeFe3O4-Ab) to detect MDMA ultrasensitively and selectively; hence, the use of NiCeFe3O4-Ab was justified in the probe system. Under optimum reaction conditions, MDMA was quantitatively detected using the AuZnFeSeS QDs-Ab-NiCeFe3O4-Ab nanocomplex in the concentration range of 0.05–50,000 nM and a detection limit of 0.02 nM (0.0046 ng/mL) was obtained. The developed AuZnFeSeS QDs-Ab-NiCeFe3O4-Ab nanoprobe was successfully used to detect MDMA in urine with satisfactory recoveries.