Polymer brushes grafted by surface-initiated atom transfer radical polymerization (SI-ATRP) from the surface of Ir-based microelectrode arrays are explored as a platform for the fabrication of sensory coatings for the voltammetric detection of Hg2+. The polymer brush coatings are post-modified with a metallothionein derived peptide to enable the selective detection of Hg2+. The performance of the polymer brush modified microelectrode arrays was evaluated using both cyclic voltammetry (CV) as well as square-wave anodic stripping voltammetry (SWASV) experiments. These studies revealed that the polymer brush based coatings allowed the selective detection of Hg2+ with detection limits in the subnanomolar range.