A method for causing a membrane to become permeable to a first chemical
species, in which the membrane is contacted with a solution comprising a
second chemical species. The membrane comprises a polyelectrolyte complex
film comprising a positively-charged polyelectrolyte and a
negatively-charged polyelectrolyte. The second chemical species
selectively interacts with the membrane, and selective interaction of the
second chemical species with the membrane causes the membrane to become
permeable to the first chemical species, thereby causing the first
chemical species to permeate the membrane.