Article pubs.acs.org/Langmuir
Control of Heterogeneous Nucleation and Growth Kinetics of Dopamine-Melanin by Altering Substrate Chemistry Luke Klosterman,† John K. Riley,‡ and Christopher John Bettinger*,†,§ †
Department of Materials Science and Engineering, ‡Department of Chemical Engineering, and §Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
ABSTRACT: Dopamine-melanin (DM or “polydopamine”) can be deposited on virtually any substrate from solution through autoxidation of dopamine. The versatility of this process has allowed surface-mediated assembly of DM for a wide variety of functional coatings. Here we report the impact of well-defined surface chemistries on the nucleation and growth of such films. DM was deposited on silicon dioxide (SiO2) and SiO2 substrates modified with self-assembled monolayers (SAMs) bearing octadecyl (C18), phenethyl, and aminopropyl functional groups. Atomic force microscopy revealed three-dimensional islands whose areal density and surface coverage are lowest on bare SiO2 substrates and highest on the neutral aromatic and aliphatic substrates. Increasing the pH of the solution from 8.2 to 10 dissociates catechol moieties in DM and inhibits adsorption on negatively charged SiO2 substrates. The growth rate of DM films on SAM-modified SiO2 is maximized at pH 9.5 and almost completely abolished at pH 10 because of increased DM solubility. The initial rates of DM adsorption were measured using quartz crystal microbalance with dissipation measurements. The initial adsorption rate is proportional to the nucleation density, which increases as the hydrophobicity of the substrate increases. Taken together, these data provide insight into the rates of heterogeneous nucleation and growth of DM on substrates with well-defined chemistries.
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INTRODUCTION The aqueous deposition of dopamine-melanin (DM, often termed “polydopamine”) has emerged in recent years as a versatile coating technique. Conformal DM films can be formed on a wide range of materials, which is an important processing advantage.1 DM films exhibit diverse practical chemical properties,2,3 including promiscuous affinity for many materials, including cells,4−8 redox activity, and chelation of multivalent ions.9−15 DM films can also serve a functional coatings for tuning the surface chemistry of hydrophobic interfaces.16,17 DM is a heterogeneous mixture of interacting aromatic oligomers with multiple levels of structural disorder.18 Aqueous oxidation of dopamine produces 5,6-dihydroxyindole (DHI) and indole-5,6-quinone (IQ). These precursors then covalently bond into insoluble oligomers that aggregate because of π−π stacking, charge transfer, and hydrogen bonding.19−23 DM thin films form from precursor solutions at the interface of many materials, including noble metals, oxides, semiconductors, ceramics, and polymers.1,9 Atomic force microscopy (AFM) contact force measurements suggest that adsorption is governed primarily by catechol groups. The force of adhesion of catechols to TiO2 interfaces is 4 times larger than that of oxidized o-quinone counterparts and 8 times larger than that of tyrosine.24 o-Quinones are also capable of forming covalent bonds with primary amines. Guardingo et al. reported that the average adhesion of a catechol monolayer to a silicon AFM tip © XXXX American Chemical Society
was slightly higher than the maximal adhesive force of an amorphous DM film. These results suggest that catechol groups are primarily responsible for interfacial adhesion of DM films.25 Ball and co-workers found that the film thickness of DM formed on SiO2 surfaces plateaus when using oxygenated tris(hydroxymethyl)aminomethane (Tris) buffer but grows continuously when it is formed using phosphate buffer or Cu2+ as an oxidant.26,27 The former observation may be attributed to the incorporation of Tris into the dopaminemelanin.28 Increasing the pH or dopamine concentration increases the rate of deposition and maximal thickness, which in turn is correlated with greater root-mean-square (RMS) roughness.29 Additionally, the deposition rate decays exponentially, which may arise because of the evolution of the size distribution of DM aggregates. Nanometer scale granule structures form rapidly on the surface during deposition from solution.30 Kim et al. observed that the rate of the dopamine reaction greatly increases with an increasing PO2 of the solution, along with a significant decrease in roughness compared to that of stirring with ambient oxygenation.31 The chemical evolution of DM and its subsequent properties are comparable to those of the biological pigment eumelanin,18 Received: January 10, 2015 Revised: March 3, 2015
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DOI: 10.1021/acs.langmuir.5b00105 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Scheme 1. Proposed Synthesis of Dopamine-Melanin (DM) on Surface Chemistries Prepared in This Study19−23,a
DM film formation proceeds via oxidation and subsequent cyclization to form oligomers based on dihydroxyindole/indolequinone that nucleate and grow on the surface. The substrate chemistries are (a) silicon dioxide, and self-assembled monolayers of (b) 3-aminopropyltrimethoxysilane, (c) phenethyltrichlorosilane, and (d) octadecyltrichlorosilane.
a
the nomenclature. Surface modification via self-assembled monolayers (SAMs) was verified by water-in-air contact angles using the sessile drop method. Measurements were repeated in triplicate at different locations on the surface. Dopamine-Melanin Deposition. Dopamine-melanin (DM) films were prepared by dissolving 2 mg/mL dopamine hydrochloride in 12 mL of 50 mM carbonate/bicarbonate buffer contained in a sealed 20 mL vial. Substrates were oriented at approximately 30° relative to horizontal. DM deposition proceeded for 24 h after which samples were rinsed with H2O and dried under a N2 stream. The pH values of sample solutions were measured using a Ag/AgCl electrode (model 5014T, Hach, Loveland, CO). Measurements of Deposition Rate. The volume of carbonate buffer for QCM-D (Q-Sense E4, QSoft401) measurements was 40 mL at 50 mM, prepared at pH 9.09. The QCM-D flow cell and tubing were cleaned prior to each experiment using 2% (w/w) sodium dodecyl sulfate and extensive deionized water rinsing. Functionalized QCM-D sensors were installed; the cell was filled with carbonate buffer, and the temperature was equilibrated at 25 °C. Another buffer of the same volume, concentration, and pH was prepared, and upon addition of 2 mg/mL dopamine, the pH decreases to pH 8.5. Immediately after the dopamine had been dissolved, this solution was passed through the system at 50 μL/min for approximately 4 h, and the frequency and dissipation shifts were monitored. The sensors were then rinsed with H2O and dried under a N2 stream. New sensors were used for each measurement. Morphological Characterization of Dopamine-Melanin Films. Film thicknesses and areal island densities were measured using atomic force microscopy (NTegra AFM, NT-MDT, Tempe, AZ) in tapping mode. Thickness scans were 10 μm × 10 μm at 0.8 Hz using tips with a reported radius of
