A novel gold nanoparticle-based protocol for detection of DNA hybridization based on a magnetically trigged direct electrochemical detection of gold quantum dot tracers is described. It relies on binding target DNA (here called DNA1) with Au(67) quantum dot in a ratio 1:1, followed by a genomagnetic hybridization assay between Au(67)-DNA1 and complementary probe DNA (here called DNA2) marked paramagnetic beads. Differential pulse voltammetry is used for a direct voltammetric detection of resulting Au(67) quantum dot-DNA1/DNA2-paramagnetic bead conjugate on magnetic graphite-epoxy composite electrode. The characterization, optimization, and advantages of the direct electrochemical detection assay for target DNA are demonstrated. The two main highlights of presented assay are (1) the direct voltammetric detection of metal quantum dots obviates their chemical dissolution and (2) the Au(67) quantum dot-DNA1/DNA2-paramagnetic bead conjugate does not create the interconnected three-dimensional network of Au-DNA duplex-paramagnetic beads as previously developed nanoparticle DNA assays, pushing down the achievable detection limits.
A novel electrochemical immunosensing strategy for the detection of atrazine based on magnetic beads is presented. Different coupling strategies for the modification of the magnetic beads with the specific anti-atrazine antibody have been developed. The immunological reaction for the detection of atrazine performed on the magnetic bead is based on a direct competitive assay using a peroxidase (HRP) tracer as the enzymatic label. After the immunochemical reactions, the modified magnetic beads can be easily captured by a magnetosensor made of graphite-epoxy composite, which is also used as the transducer for the electrochemical immunosensing. The electrochemical detection is thus achieved through a suitable substrate and mediator for the enzyme HRP. The electrochemical approach is also compared with a novel magneto-ELISA based on optical detection. The performance of the electrochemical immunosensing strategy based on magnetic beads was successfully evaluated using spiked real orange juice samples. The detection limit for atrazine using the competitive electrochemical magnetoimmunosensing strategy with anti-atrazine-specific antibody covalent coupled with tosyl-activated magnetic beads was found to be 6 x 10(-3) microg L(-1) (0.027 nmol L(-1)). This strategy offers great promise for rapid, simple, cost-effective, and on-site analysis of biological, food, and environmental samples.
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