Magnetic particles as labels in bioassays : Interactions between a biotinylated gold substrate and streptavidin magnetic particles

Magnetic particles (MPs) have been attracting much interest as a labeling material for advanced biological and medical applications, such as biomagnetic separation, drug delivery, magnetic resonance imaging, and hyperthermia. In most of these applications, the MPs have been designed to specifically interact with a target, such as cells or proteins, moving freely in a solution. However, for surface-based applications, such as magnetic biosensing, these MPs must bind specifically with a target that is immobilized onto a planar substrate. Consequently, new interaction phenomena, which influence the binding of the MPs to the substrate, have to be taken into account. To achieve adequate binding characteristics and to optimize the MPs toward substrate labeling, these physicochemical interactions should be properly identified. In this paper, the interactions between 16 commercially available streptavidin MPs and a biotinylated gold substrate were monitored in real time by surface plasmon resonance technology and the particle surface coverage was calculated by optical microscopy. On the basis of the type of interactions, the MPs studied in this paper could be classified into three different cases: (I) MPs that bind to the biotinylated substrate via the specific streptavidin-biotin interactions, without showing any nonspecific interactions; (II) MPs that do not bind to the substrate; and (III) MPs that bind to the biotinylated substrate via nonspecific interactions rather than via specific streptavidin-biotin interactions. The three cases were understood by determining the surface charges of both the particle and the substrate in ζ potential measurements. It was found that binding of MPs to the substrate was strongly dependent on the amount and the sign of the charges on both surfaces. The strong influence of electrostatic interactions was validated by simulating the total interaction force between a streptavidin MP and a biotinylated substrate by use of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, while the gravitational force and the streptavidin-biotin force were accounted for. Finally, we conclude that apart from a well-controlled streptavidin coating, the surface charge of the particle and the substrate plays a pivotal role in the construction of MP assays on surfaces.