Weak polyelectrolytes, such as poly(carboxylic acid) [97], poly(phosphoric acid) [98] and
polyamine [99], contain ionizable groups and are able to accept or donate protons at a specific pH.
These pH-sensitive polymers exhibit a conformational change upon variation of the pH which translates
into swelling/shrinking behavior when present as gels. Such hydrogels are the most common candidates
for pH-responsive systems. In addition, dendrimers such as polyethyleneimine (PEI) modified with
methacryl groups was can also reversibly be opened and closed in response to pH [100]. Therefore,
MIPs based on pH-responsive polyelectrolytes should demonstrate pH-dependent recognition and such
systems have therefore been considered as promising materials for controlled drug delivery [59,60,101].