We introduce a novel solid-phase colorimetric sensor facilely fabricated by loading unmodified gold nanoparticles into poly(oligo (ethylene glycol) methacrylate) (POEGMA) brushes grown on glass. Our work reports the first synergistic combination of metallic nanoparticles acting as a colorimetric sensing module with a non-fouling polymer matrix acting both as a non-rigid scaffold and a screen to reduce interference from non-target molecules. In addition, as the nanocomposite is formed on a transparent substrate, solid-phase detection can be performed in the same manner as in the solution-phase. We demonstrate the use of this unique platform for label-free lead detection based on the release of gold nanoparticles from the polymer brush upon exposure to lead ions. An ultralow limit-of-detection of 25 pM (S/N=3) and a dynamic range of 100 pM to 100 nM (R2=0.987) are achieved. Furthermore, the detection is up to 1000 fold more selective to lead over other common heavy metal ions.