Gold coated magnetic beads for electrochemical detection

This post summarizes an interesting article, recently accepted for publication in Analytical Chemistry, which presents an innovative use of magnetic beads. Researchers from the Osaka Prefecture University at Japan have proposed the synthesis of magnetic microbeads covered with a smooth gold layer that are used for the electrochemical detection of streptavidin (1).

The synthesis, which is deeply described in the manuscript, consists of three well defined steps. First of all, the magnetic beads are introduced in a colloidal gold dispersion containing Au-nanoparticles (Au-NPs). Thanks to the opposite zeta potential of both materials (microbeads are negatively charged while Au-NPs present a positive charge), their assembly takes place by electrostatic interactions. However, the Au-NPs are not connected to each other and therefore the electrical resistance of the microbeads is too high. In a second step, the modified microbeads are dispersed in an aqueous medium containing HAuCl₄ as gold precursor, which is reduced with the appropriate reagent to cover the microbeads surface completely and to connect the Au-NPs of their surface. Finally, thiolated biotin is immobilized on the surface thanks to the thiol-Au interaction (see Figure 1). As it is well know, biotin-streptavidin complex is very stable and therefore the synthesized microbeads present a high capability to isolate streptavidin from an aqueous environment.

Figure 1. Magnetic microbeads coated with gold and thiolated biotin for the detection of streptavidin (ST) by using electrochemically labeled (X) biotin in a sandwich format

The electrochemical detection of streptavidin is performed by means of a sandwich complex which requires the presence of electrochemically labeled biotin in the solution. In this sense, the derivatized microbeads are dispersed in an aqueous sample containing the target which is retained thanks to its interaction with biotin. Then, labeled biotin is added to the medium and it is adsorbed due to its interaction with retained streptavidin. In general words the higher the concentration of streptavidin, the higher the amount of labeled biotin retained. Finally, the microbeads are recovered on the Au/Fe electrode by the application of an external magnet. The electrochemical signal depends on the amount of labeled biotin which is directly related to the streptavidin concentration.

We strongly recommend the reading of the manuscript where the optimization of the process as well as the comparison with commercial beads is described.

Reference

(1) Efficient Collection and Sensitive Detection Using Conducting Magnetic Microbeads. Link to the article