Tris Buffer Modulates Polydopamine Growth, Aggregation, and

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Tris Buffer Modulates Polydopamine Growth, Aggregation, and Paramagnetic Properties Nicola Fyodor Della Vecchia,† Alessandra Luchini,† Alessandra Napolitano,† Gerardino D’Errico,†,‡ Giuseppe Vitiello,‡,§ Noemi Szekely,∥ Marco d’Ischia,*,† and Luigi Paduano*,†,‡ †

Dipartimento di Scienze Chimiche, Università degli Studi di Napoli “Federico II”, Via Cintia, 80126 Napoli, Italy CSGI − Consorzio interuniversitario per lo sviluppo dei Sistemi a Grande Interfase, via della Lastruccia 3, 50019, Sesto Florentino, Italy § Dipartimento dei Materiali e della Produzione Industriale, Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli “Federico II”, Piazzale V. Tecchio 80, 80125 Napoli, Italy ∥ Jülich Centre for Neutron Science, Lichtenbergstrasse 1, 85747 Garching, Germany

Langmuir 2014.30:9811-9818. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/29/18. For personal use only.

‡

ABSTRACT: Despite the growing technological interest of polydopamine (dopamine melanin)-based coatings for a broad variety of applications, the factors governing particle size, shape, and electronic properties of this bioinspired multifunctional material have remained little understood. Herein, we report a detailed characterization of polydopamine growth, particle morphology, and paramagnetic properties as a function of dopamine concentration and nature of the buffer (pH 8.5). Dynamic Light Scattering data revealed an increase in the hydrodynamic radii (Rh) of melanin particles with increasing dopamine concentration in all buffers examined, especially in phosphate buffer. Conversely, a marked inhibition of particle growth was apparent in Tris buffer, with Rh remaining as low as 4 may be characteristic for diffuse interfaces.

(2)

where k is the Boltzmann constant, T is the absolute temperature, and n0 is the solvent viscosity. For non spherical particles, RH represents the radius of equivalent spherical aggregates.30 Due to the high dilution it is possible to make the approximation D ≅ D∞. In this hypothesis, eq 2 can 9812

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Figure 1. Hydrodynamic radii of polydopamine growing from 0.5 mM (panel a), 1.0 mM (panel b), and 2.0 mM (panel c) dopamine in three buffer solutions, K2PO4 (●), NaHCO3 (□), and Tris (▲).

Figure 2. Concentration dependence of the diffusion coefficients of polydopamine growing in (●) K2PO4, (□) NaHCO3, and (▲) Tris buffer, from 0.5 mM (panel a), 1.0 mM (panel b), and 2.0 mM (panel c) dopamine. The solid curves represent the fitting of the diffusion coefficients according to eq 5

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RESULTS AND DISCUSSION Dynamic Light Scattering. In a first series of experiments the autoxidation of dopamine at three different concentrations, i.e. 0.5, 1.0, and 2.0 mM, was examined in 50 mM phosphate, bicarbonate, and Tris buffers, all at pH 8.5. Under these conditions, a progressive darkening of the solution was observed in all cases which was eventually associated with precipitation of a black eumelanin-like polymer. Figure 1 shows the time evolution of the hydrodynamic radius distributions associated with polydopamine particles obtained from dopamine autoxidation at three different starting concentrations and in three different buffers, as detailed in the figure caption. In all cases the dependence of aggregation kinetics on initial dopamine concentration, as well as, even more interesting, on buffer chemical composition, was observed. Notably, the growth process proved to be almost negligible from 0.5 mM dopamine in Tris buffer (panel a) with a maximum Rh value at less than 200 nm from 2 mM dopamine (panel c). In contrast, at the same concentration in phosphate buffer Rh attains a value of about 600 nm. Worthy of note is also the role of bicarbonate, which exerts an apparent inhibitory effect on polydopamine particle growth as compared to phosphate buffer, leading to Rh values only slightly higher than those measured in Tris buffer. Altogether, the DLS results reported above are in agreement with a kinetic model of dopamine oxidation similar to that recently proposed for DHI polymerization.40 This model is based on the sequential interactions between the monomer, or small oligomers not greater than of 2−5 monomeric units, and the growing aggregates, according to the following equation.

considered at each growing step. It should be noted that even in the early stages of the polymerization a single distribution population of aggregates was observed. Such a population gradually shifted toward larger values of the average radii with time. If small oligomers and growing polymeric chains coexisted during polymerization, a bimodal distribution would be detected, with the small oligomer population remaining almost invariant in size during the whole process. This situation was never observed during our experiments, suggesting that dopamine polymers grow from small oligomers as seeds. The proposed kinetic model does not take into account whether particle growth occurs by covalent bonding or by simple adsorption and aggregation phenomena, since no data are available to assess the relative importance of the two mechanisms. During the reaction, assuming that each step is characterized by the same kinetic constant, k, it is possible to derive an equation correlating the mean diffusion coefficient, D, to the kinetic characteristics of the process (eq 5). The equation can be obtained considering momenta of the polymer size distribution, according to an analysis reported elsewhere.41,42 ⎛ D = ⎜⎜Ddopamine(mNa − c)exp( −kNat ) + Dpolydopamine ⎝ ⎧1 ⎨ [(mNa − c)exp( −kNat ) + c]2 + (mNa − c) ⎩ Na ⎫⎞ ⎛ (exp( −kNat ) − 1)⎬⎟⎟ /⎜⎜(mNa − c)exp( −kNat ) ⎭⎠ ⎝ ⎧1 + ⎨ [(mNa − c)exp( −kNat ) + c]2 + (xNa − c) ⎩ Na

polymer n + dopaminem = polymer n + m with 1 < m ≤ 5

⎫⎞ (exp( −kNat ) − 1)⎬⎟⎟ ⎭⎠

Because of the limitations set by the instrumental resolution, in the present model small oligomers not exceeding 5 monomeric units, roughly corresponding to a size of 1.0−1.5 nm, were 9813

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where c is the stoichiometric monomer concentration, m is the mean number of monomeric units in oligomers in the early stage of reaction, Ddopamine is the diffusion coefficients of dopamine unimer, and Dpolydopamineand Na represent the diffusion coefficient and the molar concentration of polydopamine at the end of reaction, or according to Figure 2, at the plateau, respectively. Figure 2 shows the diffusion coefficients collected from the DLS measurements and the relative fitting curve from which the parameters presented in Table 1 have been obtained. For each Table 1. Parameters Obtained from Fitting the Experimental DLS Data to Eq 5 0.5 mM dopamine m Na 104 (mol dm−3) k 10−5 (h−1 M−1) Dpolydopamine 108 (cm2 s−1)

KH2PO4 5±1 1.0 ± 0.2 1.6 ± 0.1 0.52 ± 0.1 1.0 mM Dopamine

m Na 104 (mol dm−3) k 10−5 (h−1 M−1) Dpolydopamine 108 (cm2 s−1)

NaHCO3 KH2PO4 5±1 5±1 1.87 ± 0.02 1.9 ± 0.02 1.41 ± 0.06· 1.45 ± 0.08 0.44 ± 0.01 1.36 ± 0.07 2.0 mM Dopamine

TRIS 5±1 1.8 ± 0.2 1.75 ± 0.06 2.30 ± 0.08

KH2PO4 5±1 3.80 ± 0.06 0.54 ± 0.01 (0.426 ± 0.003

TRIS 5±1 3.95 ± 0.01 1.10 ± 0.07 1.39 ± 0.06

m Na 104 (mol dm−3) k 10−5 (h−1 M−1) Dpolydopamine 108 (cm2 s−1)

NaHCO3 5±1 1.0 ± 0.4 1.7 ± 0.1 1.99 ± 0.03

NaHCO3 5±1 3.95 ± 0.08 0.51 ± 0.03 9.48 ± 0.08

TRIS 5±1 1.0 ± 0.2 2.3 ± 0.2 3.5 ± 0.1

Figure 3. Neutron scattering intensity profiles of polydopamine growing in 0.05 M Tris (■), NaHCO3 (□), and K2HPO4 (●) buffers after 3 h. The fitting curve is reported as a solid line (see text eq 3).

K2HPO4 the exponent is −2. These data clearly indicate that whereas in bicarbonate or phosphate the scattering behavior reflects mainly bidimensional objects (paillettes), as previously observed for DHI melanin,40 Tris markedly affects polydopamine particles morphology to induce three-dimensional mass fractal aggregates. The scattering profiles of polydopamine in K2HPO4 and NaHCO3 were modeled according to eq 3 using as fitting parameters the transversal length d of the discs and its radius, r. Based on fitting, such parameters were estimated to be about 17 ± 1 nm and 105 ± 7 nm, respectively, in both cases. Electron Paramagnetic Resonance. To gain an insight into the effects of buffer-dependent structural modifications and aggregation mechanisms on the paramagnetic centers as probes of the π-electron system, in a final set of experiments dry polydopamine samples obtained in the three buffers were characterized by EPR.33 An EPR spectrum of polydopamine was reported recently.44 As shown in Figure 4, left panel, all spectra exhibit similar line shapes, i.e. a single, roughly symmetric signal at a g value of ∼2.004, (see Table 2), typical of carbon-centered radicals. However, on closer inspection, the spectrum of the polydopamine sample obtained in Tris buffer revealed a significantly broader signal. The difference in line shapes was determined by

curve the fitting parameters were Dpolydopamine, Na, and m, while the diffusion coefficients of dopamine, Ddopamine, was imposed to be 0.60 × 10−5 cm2 s−1.43 Small oligomers (with m ∼ 5), formed in the early stages of the dopamine oxidation process, are the “seeds” from which polymer growth originates. Polydopamine concentration, Na, in the three buffers is substantially constant and its value increases linearly as dopamine concentration is raised. The value of diffusion coefficients of the polymer increases in the order phosphate < bicarbonate < Tris, which corresponds to a decrease in the hydrodynamic volume of the polydopamine, as discussed above. Since the amount of polydopamine at the end of the reaction, at any given concentration, is substantially similar, whereas its particle size increases from Tris to phosphate, it is expected that the unpolymerized dopamine in equilibrium with the polymeric form is larger in Tris and bicarbonate with respect to phosphate. In other words, in Tris the polymerization reaction is less favorite. Interestingly, the kinetic constant for the polymerization process, k, increases following the same sequence observed for the diffusion coefficients, being significantly faster in Tris buffer. Small-Angle Neutron Scattering. The morphology of the polydopamine particles was then assessed by SANS. Figure 3 reports the intensity scattering profile, I(q), of solutions prepared in the three different buffers at 0.05 M concentration using dopamine at 1 mM concentration after several hours after the beginning of reaction. The data revealed a power law decay for I(q) in Tris buffer markedly different from those in bicarbonate and phosphate. On the basis of Figure 3, the scattering profile for the particles in Tris decays with a power law of −3.6 whereas in NaHCO3 and

Figure 4. EPR spectra of polydopamine samples registered at 0.6 mW microwave power (left panel) and at 60 mW microwave power, i.e. under power saturation conditions (right panel) obtained in Tris buffer (A), bicarbonate buffer (B), and phosphate buffer (C). 9814

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elongation and aggregation steps. Each single step would be dependent on the aerobic oxidation of dopamine leading to an oquinone serving as the electrophilic acceptor for the attacking oligomer. Deprotonation of the amine side chain would then be critical for intramolecular cyclization steps. Although the detailed mechanisms by which Tris buffer affects polydopamine are difficult to elucidate, useful insights have been provided in a recent study demonstrating the covalent incorporation of the amine-containing buffer into growing dopamine oligomers.20 Mass spectral analysis provided a reasonable basis for postulating well-defined dimers and higher oligomer species, as well as oxidative breakdown products thereof, bearing a Tris functionality. Chemical arguments based on the known reactivity of amines with o-quinones would then allow proposing a reliable sequence of reaction pathways for Tris incorporation into polydopamine, which are schematically illustrated in Figure 6. The key step in these pathways is the nucleophilic addition of Tris to the relatively long-lived dopamine quinone20 which may occur in principle at various levels of the oxidation process, including oxidized dimers and possibly higher oligomers. The incorporation of Tris into polydopamine structure was previously found to be higher at the highest Tris-to-dopamine ratio.20 This observation would be consistent with the partitioning of dopamine quinone among several competitive pathways at the branching point, namely dimerization (i.e,. trapping by dopamine), intramolecular cyclization, and covalent trapping by Tris, DHI, or other nucleophiles in the medium. Incorporation of Tris would be prevalent only at high buffer concentrations or low dopamine concentrations, as demonstrated by solid-state NMR.20 Addition of Tris to other quinone species putatively formed in the pathway, e.g. the highly unstable 5,6-indolequinone from DHI, can be ruled out since the latter can only be trapped by highly reactive nucleophiles, such as thiols and thioureylene groups,47 and no evidence has been provided so far about the conjugation of 5,6-indolequinone with amines. Despite the lack of direct evidence, several chemical arguments suggest that Tris residues on dopamine quinone moieties may slow down or even inhibit chain elongation reactions due to (a) the occupation of a critical reactive site (the 6-position of the quinone ring), (b) steric hindrance effects on adjacent sites, and (c) the electron-donating properties of the amine group decreasing the electrophilic character of the chain elongating dopamine o-quinone and its susceptibility to nucleophilic attack. Furthermore, formation of Tris adducts as in Figure 6 would entail a decreased ability of these structures to give rise to stacking interactions due to steric hindrance opposed by the Tris residues. In consequence of these effects, aggregate growth would be blocked causing the failure of DLS to detect significant particles despite progressive dopamine polymerization and consumption, as observed in the reaction from 0.5 mM dopamine. On the basis of this scheme, it appears that the effects of Tris on dopamine polymerization are irreversible in character and would not be comparable with those of other modulators of polymerization processes, such as chain transfer agents, which impart control to the reaction by reversible deactivation of a propagating center. Unlike Tris, phosphate and bicarbonate buffers are unlikely to give rise to covalent adducts with dopamine quinone. The origin of their different effects on polydopamine formation is not obvious, and may be due to a complex combination of effects. The larger size of polydopamine particles observed in phosphate can be ascribed to the ability of the buffer to favor the aggregation

Table 2. EPR Spectral Parameters for Pigments Obtained from Dopamine 10 mM in Different Buffers (50 mM) g-factor ΔB(G)

Tris buffer

bicarbonate buffer

phosphate buffer

2.0039 ± 0.0004 5.4 ± 0.2

2.0042 ± 0.0004 5.0 ± 0.2

2.0041 ± 0.0004 4.9 ± 0.2

quantitative measurement of the signal amplitude (ΔB), showing that the ΔB value for the sample obtained in Tris buffer was larger than those from the other buffers. Notably, when the spectra of the same samples were recorded at a higher microwave power (Figure 4, right panel) the superposition of two main signals became evident for all polymers, and especially for the sample obtained in Tris. A similar behavior was recently reported for Dopa melanins obtained under basic conditions, and was interpreted as being due to the simultaneous presence of carboncentered radicals and semiquinone radicals.45 On this basis, we conclude that in our samples, carbon-centered radicals are largely predominant, but the semiquinone signal is detectable in all cases, and gives a particularly relevant contribution to the spectrum of the sample obtained in Tris. This could explain the signal broadening observed at low microwave power. Inspection of the normalized power saturation profiles in Figure 5 revealed a steeper decrease at high microwave power for

Figure 5. Plot of amplitude vs. power intensities of polydopamine samples obtained in Tris buffer (●), bicarbonate buffer (■), and phosphate buffer (⧫).

the polydopamine sample obtained in Tris, suggesting a lower degree of molecular disorder. This may be a consequence of the lower tendency to aggregation and stacking evidenced by DLS analysis, leading to a less extensive delocalization of the paramagnetic centers across the molecular scaffolds. It is plausible that the different EPR properties of the sample from Tris buffer depend on the previously reported covalent incorporation of Tris into polydopamine structure.20 Binding of the amine functionality to the catechol ring of dopamine may profoundly modify the π-electron properties of polydopamine by blocking reactive positions on the aromatic ring. This may both limit the degree of polymerization/aggregation and influence the equilibria of catechol−semiquinone−quinone systems45,46 affecting the overall free radical population of the samples. Whether this effect has any influence on the electrical, antioxidant, and free radical scavenging properties33 of polydopamine is the focus of ongoing studies. Mechanistic Issues and Structure−Property Relationships. With all buffer systems used, DLS data suggest that polydopamine grows from small oligomers via sequential chain 9815

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Figure 6. Scheme illustrating the possible effect of Tris as covalent modifier of dopamine and polydopamine structural components. Putative Triscontaining structures are based on specific peaks at m/z 393, 409, 419, and 689 reported in previous mass spectral data.20

indicate that nucleophilic buffers, such as ammonia or primary amines, behave like Tris and lead to polydopamine polymers with properties different from those of materials prepared in nonnucleophilic buffers. This aspect will be addressed in separate studies currently underway. Besides particle size, the consequences of Tris incorporation include a profound modification of polydopamine morphology and paramagnetic properties, as evidenced by the SANS and EPR data. Whereas phosphate and bicarbonate do not affect the usual mode of aggregation of eumelanin-type materials to form compact 2D objects, bulky Tris moieties covalently bound to the polydopamine scaffolds apparently hinder this process as revealed by the formation of irregular fractal particles. Thus, both SANS and DLS data concur to support a role of Tris in modifying aggregate growth and shape. In addition, covalent coupling of the amino group of Tris to the catechol ring of dopamine profoundly modifies the π-electron and spin properties of polydopamine. Besides controlling polymerization/aggregation, the electrondonating group would increase the oxidation potential of the catechol, stabilizing o-quinone moieties in the polymer and affecting the semiquinone free radical population as evidenced by EPR.44,45 Although data measured for SANS and EPR experiments refer to samples under different conditions, namely aqueous suspensions and dry solids, respectively, they can nonetheless be safely compared since desiccation has been shown to not alter the morphology and EPR properties of eumelanin polymers.33

of polydopamine structures. With pKa1 and pKa2 values of 8.86 and 10.5 for dopamine,48 it can be calculated that at pH 8.5 polydopamine would retain a significant positive charge on the uncyclized side chains, which would favor coordination by the prevalent dianion form of phosphate. Initial aggregating effects promoted by electrostatic interactions between the negatively charged buffer and the polycationic oligomers might thus serve as the primer to direct more profound stacking and charge-transfer interactions between the aromatic moieties. Bicarbonate buffer, on the other hand, at pH 8.5 exists mainly in the monoanion form, which would not be so effective as phosphate in inducing aggregation. Another relevant observation is that the ionic strength is higher in phosphate buffer than in Tris or bicarbonate, a condition that may promote physical aggregation, typically when oligomers become hydrophobic. Thus, both charge-driven aggregation and salting out effects may account for the efficient aggregation promoting effects of phosphate on polydopamine growth and aggregation. Phosphate-driven aggregation also provides an explanation as to why polydopamine films are thicker and less adhesive than in Tris.23 Retention of phosphate within the polymer scaffold may promote uncontrolled aggregation with consequent film thickening and detachment from the surface due to interference of the negatively charged buffer with the adhesion mechanisms. Bicarbonate, on the other hand, would exert a weaker aggregating influence on polydopamine synthesis than phosphate due to the smaller charge and lower ionic strength imparted to the medium. Very preliminary data (not shown) 9816

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(3) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Musselinspired surface chemistry for multifunctional coatings. Science 2007, 318, 426−430. (4) Waite, J. H. Surface chemistry - Mussel power. Nat. Mater. 2008, 7, 8−9. (5) Nam, H. J.; Kim, B.; Ko, M. J.; Jin, M.; Kim, J. M.; Jung, D. Y. A new mussel-inspired polydopamine sensitizer for dye-sensitized solar cells: Controlled synthesis and charge transfer. Chem.Eur. J. 2012, 18, 14000−14007. (6) Liu, Q.; Wang, N. Y.; Caro, J.; Huang, A. S. Bio-inspired polydopamine: A versatile and powerful platform for covalent synthesis of molecular sieve membranes. J. Am. Chem. Soc. 2013, 135, 17679− 17682. (7) Pop-Georgievski, O.; Popelka, S.; Houska, M.; Chvostova, D.; Proks, V.; Rypacek, F. Poly(ethylene oxide) layers grafted to dopaminemelanin anchoring layer: Stability and resistance to protein adsorption. Biomacromolecules 2011, 12, 3232−3242. (8) Liu, X. S.; Cao, J. M.; Li, H.; Li, J. Y.; Jin, Q.; Ren, K. F.; Ji, J. Musselinspired polydopamine: A biocompatible and ultrastable coating for nanoparticles in vivo. ACS Nano 2013, 7, 9384−9395. (9) Ambrico, M.; Ambrico, P. F.; Cardone, A.; Della Vecchia, N. F.; Ligonzo, T.; Cicco, S. R.; Talamo, M. M.; Napolitano, A.; Augelli, V.; Farinola, G. M.; d’Ischia, M. Engineering polydopamine films with tailored behaviour for next-generation eumelanin-related hybrid devices. J. Mater. Chem. C 2013, 1, 1018−1028. (10) Kang, K.; Choi, I. S.; Nam, Y. A biofunctionalization scheme for neural interfaces using polydopamine polymer. Biomaterials 2011, 32, 6374−6380. (11) Cui, J. W.; Wang, Y. J.; Postma, A.; Hao, J. C.; Hosta-Rigau, L.; Caruso, F. Monodisperse polymer capsules: Tailoring size, shell thickness, and hydrophobic cargo loading via emulsion templating. Adv. Funct. Mater. 2010, 20, 1625−1631. (12) Ho, C. C.; Ding, S. J. The pH-controlled nanoparticles size of polydopamine for anti-cancer drug delivery. J. Mater. Sci.: Mater. Med. 2013, 24, 2381−2390. (13) Ball, V.; Del Frari, D.; Toniazzo, V.; Ruch, D. Kinetics of polydopamine film deposition as a function of pH and dopamine concentration: Insights in the polydopamine deposition mechanism. J. Colloid Interface Sci. 2012, 386, 366−372. (14) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Perspectives on poly(dopamine). Chem. Sci. 2013, 4, 3796−3802. (15) Sun, T. L.; Qing, G. Y.; Su, B. L.; Jiang, L. Functional biointerface materials inspired from nature. Chem. Soc. Rev. 2011, 40, 2909−2921. (16) Lee, H.; Rho, J.; Messersmith, P. B. Facile conjugation of biomolecules onto surfaces via mussel adhesive protein inspired coatings. Adv. Mater. 2009, 21, 431−434. (17) Liu, Y.; Ai, K.; Lu, L. Polydopamine and its derivative materials: Synthesis and promising applications in energy, environmental, and biomedical fields. Chem. Rev. 2014, 114, 5057−5115. (18) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Elucidating the structure of poly(dopamine). Langmuir 2012, 28, 6428−6435. (19) Hong, S.; Na, Y. S.; Choi, S.; Song, I. T.; Kim, W. Y.; Lee, H. Noncovalent self-assembly and covalent polymerization co-contribute to polydopamine formation. Adv. Funct. Mater. 2012, 22, 4711−4717. (20) Della Vecchia, N. F.; Avolio, R.; Alfe, M.; Errico, M. E.; Napolitano, A.; d’Ischia, M. Building-block diversity in polydopamine underpins a multifunctional eumelanin-type platform tunable through a quinone control point. Adv. Funct. Mater. 2013, 23, 1331−1340. (21) Liebscher, J.; Mrowczynski, R.; Scheidt, H. A.; Filip, C.; Hadade, N. D.; Turcu, R.; Bende, A.; Beck, S. Structure of polydopamine: A never-ending story? Langmuir 2013, 29, 10539−10548. (22) Bernsmann, F.; Ponche, A.; Ringwald, C.; Hemmerle, J.; Raya, J.; Bechinger, B.; Voegel, J. C.; Schaaf, P.; Ball, V. Characterization of dopamine-melanin growth on silicon oxide. J. Phys. Chem. C 2009, 113, 8234−8242. (23) Bernsmann, F.; Frisch, B.; Ringwald, C.; Ball, V. Protein adsorption on dopamine-melanin films: Role of electrostatic inter-

CONCLUSIONS We presented herein a detailed investigation of the role of dopamine concentration and nature of the buffer on the process of polydopamine buildup and particle aggregation at pH 8.5 using an integrated approach. DLS analysis showed that (a) polydopamine particles evolve via addition of the monomer or small oligomers to the growing chain with a very low size dispersion throughout the entire process, while aggregation of larger oligomers does not play a significant role, (b) aggregation depends on dopamine concentration, and (c) the average size of growing polydopamine particles varies significantly with the chemical nature of the buffer, being larger in phosphate and remarkably small in Tris buffer. The modeling of the SANS data suggested two-dimensional structures in phosphate and bicarbonate, in line with previous data on DHI melanins,40 but a profound alteration in Tris buffer consistent with a threedimensional fractal morphology. Comparison of the EPR spectra indicated amplitude values and power saturation profiles that were compatible with a greater homogeneity of the spin systems in Tris buffer compared to phosphate and bicarbonate. These results confirm the notion that polydopamine particles are built upon a monomer−polymer, rather than polymer− polymer, growth regime that can be efficiently controlled by experimental factors. In this model, small oligomer chains rapidly aggregate to form distinct entities that gradually grow in a relatively homogeneous fashion to form larger structures. As highlighted recently,20 dopamine quinone is the critical control point in the pathway, which governs covalent and noncovalent interactions and may be targeted by Tris or other nucleophiles in the medium. Although the scheme proposed in this study is clearly an oversimplification of the actual complexity of the polydopamine assembly process, it supports the concept of polydopamine as a sui-generis case of living polymer, as in the case of DHI-derived eumelanins,40 in which monomer and low oligomers are rapidly adsorbed and oxidized on the growing chain particles. Judicious selection of the buffer system, in addition to metal cations,49 control of pH,12 oxygen diffusion,50 proteins,51 and soft templates,11 may thus provide a new means of controlling or tuning polydopamine particle size, film thickness, and properties.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.P. and A.L. wish to thank the JCNS for the SANS beam time. This work was carried out in the frame of the PRIN 2010-2011 (PROxi) project (to M.d’I.) within the aims of the EuMelaNet special interest group (www.espcr.org/eumelanet/).

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REFERENCES

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dx.doi.org/10.1021/la501560z | Langmuir 2014, 30, 9811−9818