It has recently been demonstrated that dopamine solutions put in contact with a variety of solid substrates allow the production of thin coatings probably made of melanin (Science 2007, 318, 426). In this article, we show that the thickness of these coatings can be controlled to allow a growth regime that is proportional to the reaction time if fresh dopamine is regularly proVided. We propose that the growth is initiated by the adsorption of a radical compound. When dopamine polymerization or aggregation has reached a steady state in solution, the produced species do not adhere anymore to the substrate, emphasizing the role played by unoxidized dopamine. X-ray photoelectron spectroscopy showed that the thickness of the deposit increases linearly with the number of immersion steps, but the thickness measured in ultravacuum is about 4 times smaller than the thickness measured by ellipsometry in conditions of ambient humidity. This suggests that the drying of the deposit has a considerable influence on its properties. The Si2p signal characteristic of the silicon substrate decreases progressively when the number of deposition steps increases but does vanish even after 32 deposition steps. This observation will be discussed with respect to the formation of a continuous film. Cyclic voltammetry experiments showed that a deposit impermeable to ferrocyanide is obtained after the immersion in nine freshly prepared dopamine solutions, demonstrating the formation of a film. The atomic composition of the film determined by X-ray photoelectron spectroscopy is compatible with that of melanin. Finally, we show that the deposit can be quantitatively removed from the substrate when put in a strongly alkaline solution.

Morphology influences the functionality of covalent organic networks and determines potential applications. Here, we report for the first time the use of Zincke reaction to fabricate, under either solvothermal or microwave conditions, a viologen-linked covalent organic network in the form of hollow particles or nanosheets. The synthesized materials are stable in acidic, neutral, and basic aqueous solutions. Under basic conditions, the neutral network assumes radical cationic character without decomposing or changing structure. Solvent polarity and heating method determine product morphology. Depending upon solvent polarity, the resulting polymeric network forms either uniform self-templated hollow spheres (HS) or hollow tubes (HT). The spheres develop via an inside-out Ostwald ripening mechanism. Interestingly, microwave conditions and certain solvent polarities result in the formation of a robust covalent organic gel framework (COGF) that is organized in nanosheets stacked several layers thick. In the gel phase, the nanosheets are crystalline and form honeycomb lattices. The use of the Zincke reaction has previously been limited to the synthesis of small viologen molecules and conjugated viologen oligomers. Its application here expands the repertoire of tools for the fabrication of covalent organic networks (which are usually prepared by dynamic covalent chemistry) and for the synthesis of viologen-based materials. All three materials-HT, HS, and COGF-serve as efficient adsorbents of iodine due to the presence of the cationic viologen linker and, in the cases of HT and HS, permanent porosity.