Micro-manipulation using combined optical tweezers and atomic force microscope

In this paper, we report a novel approach to functionalize the tip of a micro-cantilever by selectively positioning a functionalized polystyrene bead using optical tweezers as a lever arm. We present a design that consists of an Atomic Force Microscope (AFM) combined with an optical tweezers setup to study specific interactions between complementary protein molecules. A BSA protein coated polystyrene bead, held stationary in an optical trap, is chemically grafted to AFM cantliever tip functionalized by the complementary protein- anti BSA. This arrangement also provides a flexible means of reversibly and irreversibly fusing polystyrene beads thermally at desired specified locations on the micro-cantilever, by heating the silicon tip with the focused laser beam of the optical tweezers. We use optical tweezers as a micro-manipulation tool for grafting pre-specified number of beads to cantilever in a controlled fashion as against the other widely used methods where an aggregate of molecules are chemically attached. Further, we study the changes in cantilever's resonant frequency and find it in good agreement with the expected change due to the additional bead mass. This study opens up opportunities in the area of biosensors by providing a method to standardize the calibration of chemically modified cantilevers.