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.