A polymer substrate, such as a polymer coating or a polymer hydrogel network, includes carboxyl moieties that can be used as conjugation sites to which receptor or analyte molecules can be attached. In an example, the polymer substrate includes a polyacrylamide polymer network having alkanoic acid moieties or derivatives thereof, which can react with carboxyl activating compounds to provide an activated alkanoate moieties on the polyacrylamide network. Amine-terminated nucleic acids can react with the activated alkanoate moieties to capture the nucleic acid to the polymer network through an alkylamide moiety.