A Versatile and Simple Approach to Electrochemical Colloidal Probes for Direct Force Measurements

The colloidal probe technique, which is based on micrometer-sized colloidal particles that are attached to the end of a cantilever, revolutionized direct force measurements by atomic force microscopy (AFM). Its major advantages are a defined interaction geometry and a high force sensitivity. Here, we present a versatile and simple approach for preparing spherical electrodes in the micrometer range on an otherwise insulated AFM cantilever. Thereby, it becomes possible to combine direct force measurements and potentiostatic control of the probe for various types of electrode materials. Two examples for the use of such electrochemical colloidal probes (eCP) are presented: First, on soft, conductive films of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) the adhesion behavior was studied. The current through the contact area between the probe and film remained constant until the jump-out of contact, indicating a constant geometrical contact area. Second, the long-range forces due to diffuse layer overlap between an eCP and a glass surface have been determined as a function of the externally applied potential. The resulting interaction force profiles are in good agreement with those calculated based on charge regulation and solutions of the full Poisson-Boltzmann equation.