Polydopamine and Eumelanin: From Structure–Property Relationships to a Unified Tailoring Strategy

Abstract

CONSPECTUS: Polydopamine (PDA), a black insoluble biopolymer produced by autoxidation of the catecholamine neurotransmitter dopamine (DA), and synthetic eumelanin polymers modeled to the black functional pigments of human skin, hair, and eyes have burst into the scene of materials science as versatile bioinspired functional systems for a very broad range of applications. PDA is characterized by extraordinary adhesion properties providing efficient and universal surface coating for diverse settings that include drug delivery, microfluidic systems, and water-treatment devices. Synthetic eumelanins from dopa or 5,6-dihydroxyindoles are the focus of increasing interest as UV-absorbing agents, antioxidants, free radical scavengers, and water-dependent hybrid electronic-ionic semiconductors. Because of their peculiar physicochemical properties, eumelanins and PDA hold considerable promise in nanomedicine and bioelectronics, as they are biocompatible, biodegradable, and exhibit suitable mechanical properties for integration with biological tissues. Despite considerable similarities, very few attempts have so far been made to provide an integrated unifying perspective of these two fields of technology-oriented chemical research, and progress toward application has been based more on empirical approaches than on a solid conceptual framework of structure-property relationships. The present Account is an attempt to fill this gap. Following a vis-à-vis of PDA and eumelanin chemistries, it provides an overall view of the various levels of chemical disorder in both systems and draws simple correlations with physicochemical properties based on experimental and computational approaches. The potential of large-scale simulations to capture the macroproperties of eumelanin-like materials and their hierarchical structures, to predict the physicochemical properties of new melanin-inspired materials, to understand the structure-property-function relationships of these materials from the bottom up, and to design and optimize materials to achieve desired properties is illustrated. The impact of synthetic conditions on melanin structure and physicochemical properties is systematically discussed for the first time. Rational tailoring strategies directed to critical control points of the synthetic pathways, such as dopaquinone, DAquinone, and dopachrome, are then proposed, with a view to translating basic chemical knowledge into practical guidelines for material manipulation and tailoring. This key concept is exemplified by the recent demonstration that varying DA concentration, or using Tris instead of phosphate as the buffer, results in PDA materials with quite different structural properties. Realizing that PDA and synthetic eumelanins belong to the same family of functional materials may foster unprecedented synergisms between research fields that have so far been apart in the pursuit of tailorable and marketable materials for energy, biomedical, and environmental applications.

Highly controlled coating of biomimetic polydopamine in TiO2 nanotubes

“…In the second example, we took advantage of the strong adhesive properties of PDA [13,14] for decorating it with PtNPs. In this case, the bulk formation of PtNPs, done by reducting H 2 PtCl 6 with NaBH 4 was performed in the presence of the PDA-coated TiO 2 NTs and the arrays were kept in the colloidal solution for 30 min, followed by rinsing and drying.…”

“…Polydopamine (PDA) is a synthetic eumelanin polymer mimicking the biopolymer secreted by mussels to attach to surfaces with a high binding strength [12][13][14]. It exhibits unique adhesive properties and has recently attracted considerable interest as a multifunctional thin film coating.…”

“…It exhibits unique adhesive properties and has recently attracted considerable interest as a multifunctional thin film coating. PDA coatings are most of the time performed using the procedure developed by Lee et al [12] in which, dopamine spontaneously polymerizes at a slightly basic pH, which leads to the coating of the immersed surfaces with PDA [13,14]. Among others, it has been applied for sensing, [15][16][17][18] for the passive dispensing of aqueous solutions, [19] for making graphitic transparent stretchable electrodes, [20] and it can moreover be used as a platform for the decoration of surfaces with NPs [21][22][23][24][25][26].…”

Novel Intrapolymerization Doped Manganese‐Eumelanin Coordination Nanocomposites with Ultrahigh Relaxivity and Their Application in Tumor Theranostics

“…It likely involves the oxidation of catechol to benzoquinone, intramolecular cyclization through Michael reaction to generate indole, polymerization to eumelanin, and Mn chelation 19. During polymerization, low valence states of Mn ions reduced from KMnO 4 are continuously incorporated into MnEMNPs accompanied by nanostructure formation.…”

Polydopamine as a Catalyst for Thiol Coupling

“…[26][27][28][29][30][31][32][33] Although there are still challenges to clarify the chemical structure of synthetic PDA, it is generally agreedthat dopamine must be oxidized into dopamine quinone and that there are abundant quinone motifs in the corresponding polymer. [34][35][36] These mechanismsh aveb een summarized by d'Ischia et al [37] Therefore, it is reasonable for us to develop abiomimeticr oute for the synthesis of disulfides by using the quinone motifs in PDA.…”