Fabrication of Fully Negatively Charged Layer-by-Layer Nanofilms with pH-Induced Remarkable Swelling–Shrinking Properties

Layer-by-layer (LbL) assembly by polyelectrolytes (PE) with opposite charges has been widely used for nanofilm fabrications with diverse functionalities. However, in a typical PE LbL assembly, the charges distributed along the polymer chains are used, resulting in the retention of charges only on the outermost layer. If the charges residing within the inner layers could be harnessed, then we could obtain nanofilms full of charge. Herein, we present a novel approach by employing click moiety-functionalized poly(acrylic acid) (PAA) to achieve LbL nanofilms with preserved charges. The film assembly was driven by copper-catalyzed azide–alkyne cycloaddition (CuAAC), thereby yielding fully negatively charged (FNC) nanofilms possessing unique properties compared to conventional PE nanofilms, including high stability over a wide pH range (pH 2–12) and reversible pH-induced remarkable swelling–shrinking properties in response to cyclic transitions between alkaline and acidic conditions. The swelling induced by the increase in pH reached 201% with a thickness change from 138 (pH 2) to 278 (pH 12) nm. AFM measurements showed distinct morphological changes for nanofilms post-treated by solutions with different pH values, and structure colors depending on pH were observed. This kind of FNC nanofilm holds high potential for application as a diagnostic nanomaterial such as positively charged ions and molecules in biomedical and environmental fields.