Nanopores are ubiquitous in nature and technology, yet relatively little is known about how surface-grafted polymers can affect the interaction forces at the pore. By fabricating Au nanorings on Si substrates, we have constructed a unique experimental platform that allows for direct atomic force microscope (AFM) measurements to be made on polyethylene glycol (PEG) chains locally anchored onto a geometric pore surface. Force measurements show that the PEG gives rise to a steric repulsive barrier that envelops the entire nanoring, signifying polymer brush formation. This is confirmed by a direct imaging of the PEG brush, which reversibly collapses by switching between poor and good solvent conditions to "open" and "close" the pore, respectively. From the view of interaction forces, these results highlight possible functionalities in which polymer brushes may play a role in minimizing fouling/clogging effects in synthetic nanopores and biological nuclear pore complexes (NPCs). By the mechanism of a reversible collapse, this work illustrates how polymer brush-gated nanopores may be used as nonfouling sieves for small molecules and/or solvent-controlled chemical valves that regulate solute traffic.