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

Abstract

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.

Fabrication of Fully Positively Charged Layer-by-Layer Polyelectrolyte Nanofilms with pH-Dependent Swelling Properties

“…Azide-modified PLL was synthesized following our previously described method . Briefly, PLL (60 mg, 0.47 mmol) was dissolved in 10 mL of a bicarbonate buffer solution (pH 8.5, 0.1 M).…”

“…The film assembly was driven by the CuAAC reaction. Because of the high charge density, the FNC films were sensitive to changes in pH and showed a pH-induced thickness and a surface morphology transition . In this study, we expanded the film fabrication strategy to prepare a fully positively charged (FPC) LbL nanofilm.…”

In situ fabrication of fully negatively-charged layer-by-layer nanofilms on a living cell surface

“…2,3 Recently, our group also fabricated fully single-charged LbL nanofilms using negatively-charged poly(acrylic acid) (PAA) as the backbone and CuAAC as the driving force to overcome electrostatic repulsion. 4 Compared to polyelectrolyte (PE) LbL nanofilms, the fully negatively-charged (FNC) nanofilms showed extraordinary pH-responsive properties and high capacity of loading positively-charged molecules due to the high negative charge density.…”

“…We suggest that covalent binding of azide and DBCO can overcome the electrostatic repulsion between deprotonated carboxyl groups of PAA. Compared to the traditional PE films, the FNC nanofilms with high charge density have been proved to show higher loading capacity to positive-charged molecules, 4 that can be used for the drug and growth factor loading in tissue engineering. The FNC nanofilms avoid the use of polycations, which are cytotoxic by disrupting cell membrane.…”

“…The 5-bilayer nanofilm showed a total Δ F of 1074 Hz. Unlike PE LbL nanofilms that dissociate when the pH breaks the electrostatic interactions between layers, 4 the Cu-free FNC nanofilms were able to maintain the interactions even in a harsh pH environment due to the covalent bonding between the azide and DBCO (Fig. 2c), showing good stability.…”