Gradient sensitive microscopic probes prepared by gold evaporation and chemisorption on latex spheres

We describe a simple yet general method for preparing microscopic probes that are gradient sensitive. Gold was first evaporated on one side of latex spheres, which were then chemically modified by a thiol. As one example, by using thiols with ionizable groups, we prepared microscopic spheres with pH dependent electric dipole moments. Under an electric field such spheres reoriented themselves depending on the pH value of the environment. This method can be naturally extended to other thiol-modified molecules such as dyes and antibodies to prepare various probes that are sensitive to the gradient of the physicochemical microenvironment or that are directional in binding.