Optical tweezers-based immunosensor

Summary form only given. The authors have developed an extremely sensitive immunoassay that uses optical tweezers to detect antigen-antibody bonds. An optical tweezers is used to manipulate a microscopic object with respect to a surface. The adjustable force applied by the tweezers is used to sense adhesion between the objects, which can either naturally or artificially present binding partners. This measurement can be used either to directly detect small amounts of ligand on a surface or to detect low concentrations of free ligand in solution via competitive-binding inhibition of the adhesion. To develop this immunosensor, the authors have studied a cell-free system consisting of bovine serum albumin (BSA), covalently coupled to 4.5-/spl mu/m-diameter spheres, interacting with a surface. In the first experiments, the sensor was used to compare the adhesion force between a sphere and surfaces with and without an anti-BSA coating. The sensor is able to detect bio-specific adhesion due to only a few antigen-antibody binding pairs, even in the presence of nonspecific binding forces. To demonstrate the detection of free ligands in solution, the binding force between a surface with anti-BSA and a BSA-coated sphere was measured as a function of the concentration of free BSA in solution. The free BSA competes with the sphere-bound BSA for binding sites, thereby lowering the adhesion force between spheres and the surface. For concentrations above a picomol/L, the binding force was indistinguishable from that to a surface without anti-BSA, indicating complete displacement of sphere-bound BSA by free BSA. Between a picomol/L and a femtomol/L, the method was sensitive to changes in the binding forces, with the force at a femtomol/L still distinguishable from the force with no BSA in solution.