The final step in the supramolecular buildup of eumelanin particles is investigated regarding the involved species and mechanism. Time-resolved in situ light scattering and scanning electron microscopy reveal an aggregation of particles with a narrow size distribution around 40 nm, previously only observed as substructures. These form larger particles with again very uniform size and diameters around 200 nm. Aggregation of each single particle takes only a few minutes to complete, whereas the entire process goes on for at least 3 h, partly due to the kinetics of the precursors. The individual particles also undergo an additional consolidation step toward their final form, which takes up to 24 h. Atomic force microscopy shows that the size before consolidation is around twice the size of the final particles, due to free space between the substructures. Light scattering also reveals that the aggregation is random with respect to where the particles attach, as the shape of aggregates changes from sphere to coil, before it returns to a spherical shape at the end. Application of enzyme mediated autodeposition finally shows the potential to stop the supramolecular buildup at each level, and therefore enables isolation of the respective eumelanin particles at will. This may enable the full potential for melanin materials in nanotechnology deriving from its unique (for biological polymers) properties like paramagnetism, electrical conductivity, and many more.

Research on the supramolecular buildup of eumelanin has gained high momentum in the last years. Several new aspects regarding the involved structures and mechanisms have been established, which has led to a better understanding of the entire process. This review intends to provide a clearly laid-out summary of previous and new findings regarding structures, mechanisms, and controllability. With respect to materials applications, the aspect of controllability is of supreme importance. A focus of this review is therefore set on a novel method with high potential for specific synthesis of various, isolated particle morphologies. Finally, open questions and possibilities for their elucidation are discussed.

A method for in situ formation and controlled deposition of eumelanin nanoparticles is presented. The particles are built up by enzymatic reaction of l-DOPA with tyrosinase. The enzyme is tethered onto the support surface to get site-specific deposition of eumelanin. Due to the immediate deposition, the particles are monodisperse, with diameters of about 30-60 nm. Up to now, eumelanin particles have only been observed with sizes of about 200 nm. Deposition of those particles is site-specific on the areas where enzyme is present and results in different kinds of patterns on the support surface, including versatile monolayer structures.

Fibrinogen not only forms fibrin networks if assisted by thrombin but also exhibits self-assembly in dilute aqueous solutions in the absence of thrombin. It could be shown that self-assembly can be triggered in a controlled way by diluting the ionic strength set to a value of 0.14 M NaCl in the starting solutions. The present work unravels the mechanism of this self-assembly process by means of a combination of time-resolved multiangle static and dynamic light scattering and atomic force microscopy. Analysis was carried out as a function of the ionic strength adjusted by the drop in ionic strength and at variable salt compositions at a given final ionic strength. Composition was varied by changing the ratio of NaCl and phosphate buffer. The self-assembly induced by the drop of the ionic strength depends on the final value. The lower the final ionic strength gets, the faster is the self-assembly process. The variation of the salt composition at a given ionic strength has only a marginal effect, which depends on the ionic strength. The self-assembly obeys a step-growth process, where any intermediate cluster can coalesce with any other cluster. Interpretation of the data with a kinetic model based on the approach of von Smoluchowski follows a diffusion-limited cluster aggregation at ionic strength values lower than 30 mM. At an ionic strength of 30 mM, the model has to take into account a size dependence of the rate constant, and at 60 mM a transition is observed to a reaction-limited cluster aggregation.

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The previously introduced process for enzyme‐mediated in situ synthesis and deposition of eumelanin is investigated with covalent immobilization of the tyrosinase. It results in a monolayer structure of non‐coalesced melanin particles, with a film thickness of 5–8 nm. The reaction is self‐terminating due to overlay of the enzymes with particles. The melanin particles are rodlike with lengths down to 6 nm. Isolated melanin structures of such small size have not been observed before and might be a kind of protoparticle in the supramolecular buildup of melanin oligomers. Utilization of melanin particles with such small size can enable nanotechnological applications in the areas of bioelectronics and biosensors.

Formation of nanoscaled monolayer protein structures via enzyme mediated autodeposition is investigated on the example of casein as protein and chymosin as enzyme. The key of this method is tethering of enzyme to the support. This ensures that destabilization and subsequent deposition of casein particles occurs only in direct proximity to the support surface. In this work, covalent enzyme coupling (with and without polymeric spacer) is applied to obtain high site‐specificity and self‐terminating properties of the autodeposition process. Direct covalent coupling results in defined deposition of monolayer films or single particles. Use of polymeric spacers increases the amount and radius of deposition by a factor of 4, due to higher mobility of enzyme and delayed self‐termination. Deposited casein structures show DMT‐moduli of 1.2–1.4 GPa, indicating higher flexibility compared to conventional casein coatings. Applications might arise in the fields of implantology and biosensor technology as well as renewable coatings.

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