Morphogen-driven self-construction of covalent films built from polyelectrolytes and homobifunctional spacers: buildup and pH response

Self-construction of films using polyelectrolytes bearing complementary chemical groups is a recent emerging concept in the coating field. Herein, we describe the one-pot morphogen film self-construction through formation of covalent bonds between polyelectrolytes and homobifunctional spacers by Huisgen click-chemistry. One-pot solution of poly(acrylic acid) bearing alkyne groups (PAA-Alk), bisazides ethylene glycol (Az-EGn-Az) spacers and CuSO4 was brought into contact with a gold electrode. Cu(I), the morphogen, was generated electrochemically from Cu(II) at the electrode/one-pot solution interface. The PAA-Alk/Az-EGn-Az film buildup was investigated by varying the length of the EGn spacer (n = 2, 13 and 50) leading to different growth rates. By increasing the pH of the solution, a reversible swelling of the films (up to 400% for n = 13 or 50) is obtained due to electrostatic repulsion between PAA chains. Further investigations on film swelling were performed by using PAA-Alk and an original bisazide spacer based on two pyrene units connected by a triamine spacer (Az-bispyrene-Az). This spacer has the property to adopt an extended conformation under acidic conditions and a “stacked” conformation under basic conditions, which both can be monitored by fluorimetry. Despite the mechanical tension due to film swelling at basic pH, bispyrene spacers adopt a stacked conformation. This allowed us to monitor the pH within PAA-Alk/Az-bispyrene-Az films. The pH remains fairly stable and below the pKa of PAA when the pH of the contacting solution varies from pH 5 to pH 7 and increases above pH 8, which explains why film swelling takes place essentially above pH 8.