Poly(ferrocenylsilane) (PFS) is a redox-active polymer that can be utilized in a variety of applications, such as sensing, nanoparticle foundries or dual-responsive hydrogels, due to the fact that it can be partially or completely oxidized, reversibly, by (electro)chemical means. In this research, PFS is tethered to a cysteamine-modified gold-coated microchannel electrode via amine alkylation, following a recent published procedure for conventional macro-sized electrodes. The PFS-coated microfluidic device is successfully employed as an electrochemical sensor for sodium l-ascorbate, or vitamin C, with a detection limit of 0.5 mM in aqueous solution. The fabrication of the device, that comprises gold-coated electrodes of micrometers in width, is discussed, as well as the static and in-line detection of sodium l-ascorbate. The analyte concentration is sensed through an electrocatalytic process in both static and in-line cases, by cyclic voltammetry and amperometry, respectively. The findings presented in this work open novel opportunities for local surface modification of chip-integrated gold electrodes by redox-responsive polymers used, e.g., as electrochemical sensors.