Room-temperature surface-assisted reactivity of a melanin precursor: silver metal–organic coordination versus covalent dimerization on gold

Abstract

The ability of catecholamines to undergo oxidative self-polymerization provides an attractive route for preparation of coatings for biotechnology and biomedicine applications. However, efforts toward developing a complete understanding of the mechanism that underpins polymerization have been hindered by the multiple catechol crosslinking reaction pathways that occur during the reaction. Scanning tunneling microscopy allows the investigation of small molecules in a reduced-complexity environment, providing important insight into how the intermolecular forces drive the formation of supramolecular assemblies in a controlled setting. Capitalizing on this approach, we studied the self-assembly of 5,6-dihydroxy-indole (DHI) on Au(111) and Ag(111) to investigate the interactions that affect the two-dimensional growth mechanism and to elucidate the behavior of the catechol group on these two surfaces. X-ray photoelectron spectroscopy, together with density functional theory and Monte Carlo modeling, helps unravel the differences between the two systems. The molecules form large ordered domains, yet with completely different architectures. Our data reveal that some of the DHI molecules deposited on Ag are in a modified redox state, with their catechol group oxidized into quinone. On Ag(111), the molecules are deposited in long-range lamellar patterns stabilized by metal-organic coordination, while covalent dimer pairs are observed on Au(111). We also show that the oxidation susceptibility is affected by the substrate, with the DHI/Au remaining inert even after being exposed to O2 gas.

Melanins as Sustainable Resources for Advanced Biotechnological Applications

“…[ 399,400 ] Studies of reduction/oxidation processes and electron transfer using cyclic voltammetry are particularly useful for melanins, with fundamental studies on melanins formed chemically from single monomers (e.g., l ‐DOPA, [ 401 ] DHI, [ 402 ] 3,4‐dihydroxyphenylacetic acid, [ 403 ] HGA, [ 404,405 ] DHN [ 231 ] ), combinations of DHI and DHICA, [ 406–408 ] and natural melanins from bacteria (e.g., Shewanella oneidensis MR‐1, [ 157 ] Pseudomonas aeruginosa [ 409 ] ), plants (including fungi: basidial fungi, [ 410 ] Cryptococcus neoformans [ 411 ] and Nigella sativa [ 412 ] ), cuttlefish ( Sepia officinalis [ 407 ] ), and human hair‐derived pheomelanins. [ 413 ] Electrochemical impedance spectroscopy and dielectric spectroscopy enabled the rational investigation of the protonic and electronic contributions, suggesting melanins are protonic conductors, [ 414–416 ] which is important because the electrical properties of melanins [ 417–422 ] underpin their potential technical and medical applications, [ 423–425 ] and it is noteworthy that the potential for melanins in electronics has seen an explosion of interest (see Figure [ 426 ] ).…”

“…Scanning tunneling microscopy (STM) is a form of scanning probe microscopy [ 506 ] capable of generating high resolution images (down to the Å scale) based on quantum tunneling of electrons between the surface and the STM tip. STM has been employed to analyze synthetic melanins (e.g., based on tyrosine, [ 507–509 ] DHI, [ 418,510,511 ] DHICA, [ 417 ] DHI and DHICA [ 419 ] ), and cuttlefish melanins ( Sepia officinalis [ 201,512 ] ), often in combination with computational studies.…”

Two‐Sidedness of Surface Reaction Mediation

“…Both observations indicate a stronger interaction of the polymeric intermediate with Cu(111) than with Au(111). Additionally, the catechol group in the precursor molecule, 5,6‐dihydroxy‐indole, can be stepwise oxidized into quinone on Ag(111) upon exposure to oxygen, but the precursor can only form covalent dimers on Au(111) instead of oxidation reactions even in oxygen atmosphere . The difference in the reaction performances on Ag(111) and Au(111) stems from distinct strength of the molecule–substrate interaction as well as the oxidation susceptibility affected by the substrate.…”

Oxygen‐Induced 1D to 2D Transformation of On‐Surface Organometallic Structures

“…[ 1–16 ] The surface can act both as a template to confine reactants and as a catalyst, driving reactions to proceed along a different pathway than in traditional solution chemistry. [ 17–20 ] On‐surface Ullmann‐type coupling, involving dehalogenation and subsequent CC coupling of aromatic halides, is a widely investigated reaction due to its potential to synthesize π‐conjugated polymers with control over dimensionality. [ 21–31 ] This is exemplified by quasi 1D graphene nanoribbons, where the edge structure and width, and therefore the electronic properties, can be rationally controlled by the choice of the halogenated precursors.…”

“…[ 39,40 ] In addition, H 2 S was observed to suppress Ullmann‐type coupling on Au(111), while the reaction could be reactivated by subsequent exposure to H 2 . [ 41 ] Oxygen plays a critical role in many surface‐chemistry processes, [ 19,42–45 ] such as CH activation, [ 46–48 ] yet its effect on the Ullmann‐type reaction has not been investigated. Moreover, the oxidation of the substrate to a semiconducting metal oxide could be a simple method to remove the influence of the metal surface, [ 49,50 ] facilitating the characterization of the intrinsic polymer properties.…”