A poly(ethylene glycol) (PEG)-based hydrogel was synthesized by the copolymerization of PEG diacrylate (PEGDA) and PEG methacrylate (PEGMA) linked with N,N′-di(tert-butoxycarbonyl)-dityrosine (DBDY). Owing to the presence of the dityrosine derivative DBDY, the resulting hydrogel exhibited pH-dependent fluorescence and contained multi-functional groups (1 carboxylic acid group and 2 amine groups protected by tert-butoxycarbonyl groups). A high molecular mass chemical can be bound to DBDY in the hydrogel through one of these functional groups (i.e. the formation of amide bonding). Then the porosity of the resulting hydrogel is increased by the action of an enzyme such as proteinase. Also, if a drug is bound directly to DBDY, the enzyme can control its detachment. Hence, the PEGMA–DBDY hydrogel could be useful for drug release near the site of the disease where the expression level of a certain enzyme is especially high (e.g. matrix metalloproteinases in cancers). Binding of a UV-absorbing compound to DBDY was monitored by measuring the change in fluorescence intensity, which is known as the phenomenon of fluorescence resonance energy transfer (FRET).