Gold plating and biofunctionalization of ferromagnetic magnetic tweezers: Application for local studies of soft surface-grafted polymer films

We present a method for the gold plating and biofunctionalization of microm eter-sized, ferromagnetic cobalt and iron particles for applications as mag netic tweezers to characterize the elasticity of surface-grafted polymer fi lms and to measure viscoelastic moduli at, cell surfaces locally on microme ter length scales. Gold plating of ferromagnetic particles is achieved by r eduction of tetrachlorogold(III) on the metallic surface in acidic solution -a spontaneous reaction-without any electrical current. Agglomeration of th e ferromagnetic particles during the plating reaction is avoided by dispers ing the beads in an aqueous dextrane solution. The resulting gold layer is a versatile platform for further biofunctionalization using a wide variety of standard coupling protocols based on thiol chemistry. Several methods, s uch as electron microscopy, elemental analysis, X-ray powder diffraction, a nd X-ray photoelectron spectroscopy, have been used for quantitative and qu alitative characterization of the coatings. The magnetic tweezers are used for local quantitative characterization of the elasticity of soft surface-g rafted films of hyaluronic acid. A method for the calibration of the magnet ization of each bead chosen for the measurement is introduced which is base d on the simultaneous analysis of the Brownian motion.