Article pubs.acs.org/Langmuir
Characterization of Carbonized Polydopamine Nanoparticles Suggests Ordered Supramolecular Structure of Polydopamine Xiang Yu,† Hailong Fan,† Yang Liu,‡ Zujin Shi,‡ and Zhaoxia Jin*,† †
Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
‡
ABSTRACT: Polydopamine is not only a multifunctional biopolymer with promising optoelectronic properties but it is also a versatile coating platform for different surfaces. The structure and formation of polydopamine is an active area of research. Some studies have supposed that polydopamine is composed of covalently bonded dihydroxyindole, indoledione, and dopamine units, but others proposed that noncovalent self-assembly contributes to polydopamine formation as well. However, it is difficult to directly find the details of supramolecular structure of polydopamine via self-assembly. In this study, we first report the graphite-like nanostructure observed in the carbonized polydopamine nanoparticles in nitrogen (or argon) environment at 800 °C. Raman characterization, which presents the typical D band and G band, confirmed the existence of graphite-like nanostructures. Our observation provides clear evidence for a layered-stacking supramolecular structure of polydopamine. Particularly, the size of graphite-like domains is similar to that of disk-shaped aggregates hypothesized in previous study about the polymerization of 5,6-dihydroxyindole [Biomacromolecules 2012, 13, 2379]. Analysis of the hierarchical structure of polydopamine helps us understand its formation.
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INTRODUCTION Melanins are widely found in human skin, hair, eyes, inner ears, and brain. They are also present in other living organisms, such as the ink sacs of Sepia Of f icinalis.1 They not only provide a photoprotective effect for human but also have antioxidation and radical-scavenging effect as well.2 Melanins have also been observed in Parkinson’s disease, age-related macular degeneration (Alzheimer’s diseases), and other diseases.3,4 At the same time, researchers have intense scientific interest in their biooptoelectronic properties. It is observed that melanins are potential biocompatible and multifunctional materials for a broad range of technological interests, ranging from organoelectronics, photothermal therapeutic agents to thermooxidative stabilizer of polymers.5−9 All these studies of their special physical properties and functions in human require an elucidated structure of melanins. For example, the interpretation of photoprotective functionality of melanins needs knowledge of their structures at the molecular and supramolecular level. Meredith and Sarna have pointed that the ultimate goal of melanin research is clarifying the relationship between molecular and cellular scale structure of melanins and macroscopically observable properties and functions.10 This fundamental knowledge will help researchers manipulate properties of melanins. The debate on the nature of melanins has lasted for several decades, especially about the structure at the nanoscale which is related to the self-assembly of oligomers generated in oxidative polymerization. Choi first studied structure of melanins by using wide-angle X-ray diffraction.11 Chen et al. characterized natural sepia melanin and synthetic © 2014 American Chemical Society
melanin powder by using synchrotron radiation X-ray diffraction.12,13 They reported that the size of fundamental melanin particles is about 15 Å, consisting of 4−5 stacking layers (stacking spacing is ∼3.45 Å) which are organized inplane by protomolecules composed of 4−8 monomers of 5,6dihydroxylindole (DHI). Morphology characterization of Sepia melanin revealed that it is an aggregated structure consisting of subunits that have a lateral dimension of 150 nm.14,15 Atomic force microscopy observation suggests that it is a hierarchical structure with small units, the diameter of which is 15−25 nm, assembling into 100 nm structures, which then aggregated to form large spherical particles.16 Recently, Kaxiras proposed that porphyrin-like tetramers are protomolecules which are stacked in planar, graphite-like arrangements.17 Sutter et al. studied eumelanin formation in situ by using fluorescence of thioflavin T. They proposed that the formation of eumelanins is based on the generation and assembly of protomolecules, rather than a randomized heteropolymer formed by monomer addition.18 Arzillo et al. have focused on the dynamics of the aggregation process before precipitation, in which the size of aggregates composed of polymerized products of 5,6-dihydroxyindole is at the nanoscale level.19 On the basis of dynamic light scattering and small-angle neutron scattering investigation, they found that the aggregate is a 2-D disklike structure ∼55 nm thick before the formation and precipitation of larger particles. Received: January 18, 2014 Revised: April 9, 2014 Published: April 28, 2014 5497
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Figure 1. (a) SEM image of obtained PDA nanoparticles. (b) TEM image of PDA nanoparticles. (c) UV−vis spectrum of PDA nanoparticles suspension. (d) FTIR characterization of PDA nanoparticles.
that both noncovalent self-assembly and covalent polymerization contribute to polydopamine formation.27 To solve the puzzles of polydopamine structure, we need investigate the supramolecular structure of polydopamine from different points of view. In this study, we first report here the graphite-like nanostructure existing in carbonized PDA nanoparticles by using high-resolution TEM and Raman spectroscopy. The size of graphite-like nanostructures is around 15 nm consisting of several tens of stacking carbon layers. The dimension and shape of assembled sheets in our observation are in keeping with what was reported by Arzillo et al. Detailed investigation of polydopamine supramolecular structure not only helps us understand the origin of its chemical and physical properties but also sheds light on changing its properties through manipulating the supramolecular structure of PDA.
However, Watt et al. have observed the onionlike stacking sheets of protomolecules by using low-voltage high-resolution transmission electron microscopy.20 The intersheet spacing they observed is between 3.7 and 4.0 Å, which is consistent with that of noncovalent π stacking. Recently, the oxidation and self-polymerization of dopamine attracted much attention because of its numerous applications in surface modification of various materials.21,22 Polydopamine (PDA), the polymerized product of dopamine, a kind of spherical granule, is also recognized as synthetic eumelanin.23−25 Liebscher et al. have characterized the chemical structure of polydopamine by using cross-polarization polarization-inversion magic angle spinning NMR, electrospray ionization high-resolution mass spectrometry, X-ray photoelectron spectroscopy, and FTIR spectroscopy.26 They proposed that PDA is a mixture of indole units with different degrees of saturation and open-chain dopamine units. The intermolecular interaction is charge transfer between o-quinoid and catechol. Dreyer et al. proposed a new structure for polydopamine, in which the supramolecular aggregate of monomers is dominated.25 The study from Hong et al. shows
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EXPERIMENTAL SECTION
Dopamine hydrocholoride (purity 98%) was purchased from SigmaAldrich. Tris(hydroxymethyl)aminomethane (Tris)-HCl buffer (pH = 8.8) was obtained from Shanghai Double-Helix Biotech. Co. Ltd. All these reagents are used as received. Polydopamine nanoparticles were 5498
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Figure 2. HRTEM image of PDA nanoparticles. The stacking of several layers (2−4 layers) could be observed in parts b and c, highlighted by white circles. synthesized through oxidative polymerization of dopamine. Typically, dopamine (0.3 mg/mL) was dissolved in Tris-HCl buffer solution (10 mM, pH = 8.8). The solution was stirred for 24 h, and oxidation of dopamine was achieved by saturated O2 in solution. Then the obtained dark suspension was centrifuged at 10 000 rpm for 30 min to collect the sediment. The sediment was washed several times with fresh water to remove unreacted dopamine, and it was then dried by freeze-drying (−60 °C, 5−6 Pa). The obtained dried sample is the PDA nanoparticles we used in this study. PDA nanoparticles were coated with a thin layer of gold before the characterization using scanning electron microscopy (SEM, JEOL 7401). A drop of PDA nanoparticle suspension in water was placed onto copper grid for TEM characterization (Hitachi TEM, H-7650B). The optical absorption of pale-brown aqueous suspension of PDA nanoparticles was measured by using a UV−vis spectrophotometer (Varian Cary 50). Dried powder of PDA nanoparticles was further characterized by using a Fourier transform infrared spectrometer (Shimadzu Cor. IRPrestige21). The carbonization of PDA nanoparticles was conducted in N2 or Ar environment. PDA nanoparticles were heated from room temperature to 800 °C at a heating rate of 10 or 5 °C/min and then annealed at 800 °C for different periods (0.5 or 1 h). The residue of carbonized PDA nanoparticles was redispersed in water via sonication and conducted morphological and structural characterizations. High-resolution transmission electron microscopy (HRTEM) was conducted by using a JEM-2100 (JEOL) at 200 kV. Raman spectra of the nanoparticles were measured with a microzone confocal Raman spectroscope (HORIBA Jobin Yvon, LabRam HR 800) equipped with a color charge coupled device. The excitation wavelength was 632.8
nm. Carbonized PDA nanoparticles were directly deposited on Si/ SiO2 wafer for measurement.
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RESULTS AND DISCUSSION
Figure 1 shows morphological and chemical characterizations of PDA nanoparticles. The size distribution of obtained PDA nanoparticles is around 100 nm (Figure 1, parts a and b). Figure 1c shows a monotonic, broad-band UV−vis absorption spectrum for PDA aqueous suspension. We suppose it is caused by the heterogeneity or chemical disorder of polydopamine. The broad-band UV−vis absorption of melanin relates to its photoprotective function in vivo, and there are many scientific discussions about it lasting for several decades.28−31 Three different models are proposed to interpret this property: scattering effect,28 semiconductor model,29 and chemical disorder model.30,31 Recent study indicates that the optical scattering coefficient of dilute eumelanin solution is less than 6%.32 Because of the complexity of oxidative self-polymerization of dopamine, we prefer the chemical disorder model. FTIR characterization gives some information on chemical structure of polydopamine (Figure 1d). The broad band from 3700−3300 cm−1 is assigned to ν(N−H) and ν(O−H) stretching modes, and peaks at 2936−2881 cm−1 are due to the C−H stretching mode. The peak at 1622 cm−1 is assigned to stretching of aromatic C−C bonds of indole, and the peak at 1506 cm−1 is attributed to C−N bending in indolequinone, 5499
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Figure 3. (a) TGA curve of PDA nanoparticles. (b) SEM and (c, d) TEM images of carbonized PDA nanoparticles; scale bar = 100 nm. The heating rate was 10 °C/min.
Figure 4. TEM images of carbonized PDA nanoparticles. The heating rate was 5 °C/min. The heavy rupture of carbonized nanoparticles was effectively prevented.
while the peak at 1040 cm−1 is attributed to C−H in-plane deformation. 33,34 This is in agreement with literature reports.27,33−35 It indicates that PDA nanoparticles are composed of protomolecules with indolequinone units as natural melanins are.36 Figure 2 presents HRTEM images of
PDA nanoparticles. We found only small domains which contain few stacking layers, typically 2−4 layers (parts b and c in Figure 2). Thermal gravimetric analysis (TGA) shows that the weight loss of PDA nanoparticles in carbonized process is about 50− 5500
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Figure 5. HRTEM images of carbonized PDA nanoparticles. (a) TEM image of carbonized PDA nanoparticles. There are some dark domains. (b) Magnified image of dark domains in (a), which shows well-organized graphite-like nanostructure. The size of one typical domain is about 14 × 20 nm2. (c, d) Distance between stacking layers can be obtained from enlarged images. (e) HRTEM image shows that there are some parts containing only two or three stacking layers, except large domains with 40−50 stacking layers.
55% (Figure 3a). Parts b and c of Figure 3 show SEM/TEM images of carbonized PDA nanoparticles which were heated at a heating rate of 10 °C/min. It is clear that after heating at 800 °C PDA nanoparticles still keep their round shape except for some broken parts. The breakage of carbonized PDA nanoparticles is possibly induced by the fast heating rate which leads to burst evaporation of moisture inside PDA nanoparticles. To avoid it, we have further conducted carbonization of PDA nanoparticles using reduced heating rate (5 °C/min). It is clear that structural rupture has been effectively prevented (Figure 4). Liu et al. first reported that dopamine can work as carbon source.37 They also observed that pure polydopamine produces nearly 60% carbon yield in N2 at 800 °C. Such a high carbon yield indicates that these
mesoporous carbon nanospheres could be applied in catalyst loading and act as a conductive filler for batteries.38−40 Figure 5 shows HRTEM images of carbonized PDA nanoparticles which were heated in a heating rate of 10 °C/ min. It is interesting that there are many dark domains distributed in carbonized PDA nanoparticles (Figure 5a). The magnified image (Figure 5b) shows that these dark domains consist of graphite-like nanostructure which contains several tens (∼40) of stacking layers with whole thickness of ∼15 nm. We have carefully measured the layer distance of these nanostructures and found that it is around 0.34 nm, which is the typical distance between graphite layers (parts c and d in Figure 5).41 The carbon nanostructure between these dark graphite-like domains is similar to amorphous carbon, with 5501
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Figure 6. HRTEM images of carbonized PDA nanoparticles. The heating rate was 5 °C/min. Part c shows the enlarged graphite-like domain in carbonized PDA nanoparticles, which contains over 50 stacking layers. Part d presents the magnified part in (b), which includes two areas with shortrange and ordered stacking layers, highlighted by white circles.
modified carbon nitride42 but lacks a clear stage edge to distinguish their band gap.43 Comparing FTIR spectra of original PDA nanoparticles and carbonized PDA nanoparticles, we found that PDA experiences clear structural changes after pyrolysis: the strong band at 3300−3400 cm−1 which is assigned to the N−H and O−H stretching mode nearly disappears. Peaks at 3155 and 2921 cm−1 are recognized as C− H stretching mode, and the strong peak at 1507 cm−1 is assigned to C−N bending, while the peak at 1401 cm−1 is due to the heterocyclic stretching.34 The peak at 1157 cm−1 is due to the heterocyclic N−H in-plane deformation breathing. Much information comes from the nitrogen heteroaromatic skeleton. These characterizations indicated that after carbonization PDA nanoparticles change to nitrogen-doped carbon nanoparticles as previous studies expected.38 On the other hand, Raman spectroscopy is a noninvasive technique for the characterization of structural and electronic properties of carbon-based materials.44,45 Figure 7c presents the Raman spectrum of carbonized PDA nanoparticles. The sharp peak at 510.4 cm−1 corresponds to Si, which is used as a mark. Two dominating peaks appear at 1331 and 1578 cm−1, corresponding to D band and G band, respectively. The D band is due to A1g breathing mode of sp3 carbon, and the G peak corresponds to the inplane bond-stretching of all pairs of C sp2 atoms in both rings and chains.45 This Raman spectrum shows typical feature of the mixture of amorphous carbon and nanocrystalline graphite, which is in agreement with our HRTEM observation.
some short-range stacking layers (2−3 layers) (Figure 5e, highlighted by dotted-line squares). Figure 6 presents HRTEM images of carbonized PDA nanoparticles that were carbonized at a heating rate of 5 °C/min. The change of heating rate shows no significant influence on the graphitization of the two samples. Both the size of graphite-like domains and the distance between stacking layers consist with those in Figure 5. Besides, we also found several domains containing a few ordered stacking layers in shell parts (Figure 6d). Watt et al. demonstrated that melanin has onionlike nanostructure composed of stacking sheets of protomolecules, with an intersheet spacing range of 3.7−4.0 Å.20 Kong et al. have reported that highly electrically conductive layered carbon can be derived from polydopamine coating by carbonization.39 The interlayer spacing of their PDA coating on SnO2 is 0.37 nm. It is clear that the stacking sheets in carbonized PDA nanoparticles are more tightly aggregated than that in carbonized PDA coating on SnO2 as well as in natural melanins. Figure 7 presents UV−vis absorption (Figure 7a), FTIR (Figure 7b), and Raman (Figure 7c) spectra of carbonized PDA nanoparticles. Carbonized PDA nanoparticles also show a broad band absorption (black line in Figure 7a), but with different features compared with pure PDA nanoparticles (blue line in Figure 7a). It has broad stagelike absorption. We cannot totally avoid the scattering effect in this measurement because carbonized PDA nanoparticles are heavily aggregated, and their suspension in water is unstable. The optical absorption of carbonized PDA nanoparticles is somehow similar to that of 5502
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Figure 7. (a) UV−vis absorption of carbonized PDA nanoparticles. (b) FTIR spectrum of carbonized PDA nanoparticles. The spectra of PDA nanoparticles before carbonization are shown for a comparison. (c) Raman spectrum of carbonized PDA nanoparticles.
The formation of nanocrystalline graphite from PDA nanoparticles through carbonization at 800 °C is noteworthy. It is known that the graphite-like structure in carbonized polymers requires an ordered in-plane orientation of polymers.46 For example, the in-plane orientation of polyimide correlates very well with in-plane orientation of graphitized films.47 On the other hand, generally graphitization of polymer happens at a relatively higher temperature, for example, 2000− 3000 °C.48 Recently, Byun et al. have demonstrated the formation of a few layers of graphene by heating thin polymer films covered with a metal capping layer at 1000 °C.49 Tiwari et al. have reported a low-temperature treatment at 750 °C for the transformation of polymer to graphene films under metal capping layer.50 The metal capping layer is required to catalyze the formation of graphene at lower carbonizing temperatures. An additional effect of a metal capping layer is preventing vaporization of dissociated molecules. In our case, the observation of well-organized graphite-like domains in carbonized PDA nanoparticles at 800 °C, without any metal catalyst, suggests the existence of a supramolecular structure of PDA within the nanoparticles, which is hardly identified by using HRTEM at an accelerate voltage of 200 kV (Figure 2). First, PDA molecules are composed of some aggregates with orderly in-plane orientation and the size of aggregates is quite large. The nanocrystallized graphite in carbonized PDA is composed of several tens of stacking layers (Figures 5 and 6), indicating that these layers may be orderly stacked in PDA nanoparticles as previously hypothesized “nanoaggregates”.19
Arzillo et al. have reported the fundamental aggregates on the basis of combined characterizations of oxidative polymerization of 5,6-dihydroxyindole by using dynamic light scattering and small-angle neutron scattering.19 They showed that these aggregates have a disklike shape with an intermediate value of disk thickness at 33 ± 1 nm. In our carbonized PDA nanoparticles, we have not observed graphite-like domains with near 100 stacking layers (that may correspond to the thickness of 33 nm); the number of stacking layers we observed is around 40−50, and the thickness (15−20 nm) of nanocrystallized graphite domains is slightly smaller than the hypothesized disk thickness. However, considering the difference in the characterization methods, their data are obtained by using DLS, which observes the polymerized product of 5,6hydroxyindole in solution, while our data are measured directly in HRTEM image of carbonized PDA nanoparticles. We think that the reduced thickness in our case is reasonable because the stacking of protomolecular sheets will become tighter in carbonization, inducing aggregates with decreased thickness. Also of note, the shape of domains we observed is similar to a disk shape and is quite different from the onionlike stacking nanostructures of natural eumelanin reported in another study.20 Second, PDA nanoparticles are unevenly aggregated. Carbonized PDA nanoparticles showed mixed structural features of amorphous carbon and nanosized graphite, indicating that the formation of PDA is through a “heterogeneous nucleation” process. Arzillo et al. also reported 5503
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(3) Zucca, F. A.; Giaveri, G.; Gallorini, M.; Albertini, A.; Toscani, M.; Pezzoli, G.; Lucius, R.; Wilms, H.; Sulzer, D.; Ito, S.; Wakamatsu, K.; Zecca, L. The Neuromelanin of Human Substantia Nigra: Physiological and Pathogenic Aspects. Pigm. Cell Res. 2004, 17, 610−617. (4) Pezzella, A.; d’Ischia, M.; Napolitano, A.; Misuraca, G.; Prota, G. Iron-Mediated Generation of the Neurotoxin 6-Hydroxydopamine Quinone by Reaction of Fatty Acid Hydroperoxides with Dopamine: A Possible Contributory Mechanism for Neuronal Degeneration in Parkinson’s Disease. J. Med. Chem. 1997, 40, 2211−2216. (5) d’Ischia, M.; Napolitano, A.; Pezzella, A.; Meredith, P.; Sarna, T. Chemical and Structural Diversity in Eumelanins: Unexplored BioOptoelectronic Materials. Angew. Chem., Int. Ed. 2009, 48, 3914−3921. (6) Bothma, J. P.; de Boor, J.; Divakar, U.; Schwenn, P. E.; Meredith, P. Device-Quality Electrically Conducting Melanin Thin Films. Adv. Mater. 2008, 20, 3539−3542. (7) Cho, J. H.; Shanmuganathan, K.; Ellison, C. J. Bioinspired Catecholic Copolymers for Antifouling Surface Coatings. ACS Appl. Mater. Interfaces 2013, 5, 3794−3802. (8) Yang, L. P.; Phua, S. L.; Teo, J. K. H.; Toh, C. L.; Lau, S. K.; Ma, J.; Lu, X. A Biomimetic Approach to Enhancing Interfacial Interactions: Polydopamine-Coated Clay as Reinforcement for Epoxy Resin. ACS Appl. Mater. Interfaces 2011, 3, 3026−3032. (9) Liu, Y. L.; Ai, K. L.; Liu, J. H.; Deng, M.; He, Y. Y.; Lu, L. H. Dopamine-Melanin Colloidal Nanospheres: An Efficient Near-Infrared Photothermal Therapeutic Agent for In Vivo Cancer Therapy. Adv. Mater. 2013, 25, 1353−1359. (10) Meredith, P.; Sarna, T. The Physical and Chemical Properties of Eumelanin. Pigm. Cell Res. 2006, 19, 572−594. (11) Choi, S. X-ray Diffraction and ESR Studies on Amorphous Melanin. Ph.D Thesis, University of Houston, Houston, TX, 1977. (12) Cheng, J.; Moss, S. C.; Eisner, M.; Zschack, P. X-Ray Characterization of Melanins. I. Pigm. Cell Res. 1994, 7, 255−262. (13) Cheng, J.; Moss, S. C.; Eisner, M. X-Ray Characterization of Melanins. II. Pigm. Cell Res. 1994, 7, 263−273. (14) Ogaki, R.; Bennetsen, D. T.; Bald, I.; Foss, M. DopamineAssisted Rapid Fabrication of Nanoscale Protein Arrays by Colloidal Lithography. Langmuir 2012, 28, 8594−8599. (15) Zeise, L.; Murr, B. L.; Chedekel, M. R. Melanin Standard Method: Particle Description. Pigm. Cell Res. 1992, 5, 132−142. (16) Clancy, C. M. R.; Nofsinger, J. B.; Hanks, R. K.; Simon, J. D. A Hierarchical Self-Assembly of Eumelanin. J. Phys. Chem. B 2000, 104, 7871−7873. (17) Kaxiras, E.; Tsolakidis, A.; Zonios, G.; Meng, S. Structural Model of Eumelanin. Phys. Rev. Lett. 2006, 97, 218102. (18) Sutter, J. U.; Bidlakova, T.; Karolin, J.; Birch, D. J. S. Eumelanin Kinetics and Sheet Structure. Appl. Phys. Lett. 2012, 100, 113701. (19) Arzillo, M.; Mangiapia, G.; Pezzella, A.; Heenan, R. K.; Radulescu, A.; Paduano, L.; d’Ischia, M. Eumelanin Buildup on the Nanoscale: Aggregate Growth/Assembly and Visible Absorption Development in Biomimetic 5,6-Dihydroxyindole Polymerization. Biomacromolecules 2012, 13, 2379−2390. (20) Watt, A. A. R.; Bothma, J. P.; Meredith, P. The Supramolecular Structure of Melanin. Soft Matter 2009, 5, 3754−3760. (21) Lee, B. P.; Messersmith, P. B.; Israelachvili, J. N.; Waite, J. H. Mussel-Inspired Adhesives and Coatings. Annu. Rev. Mater. Res. 2011, 41, 99−132. (22) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426−430. (23) Ju, K. Y.; Lee, Y.; Lee, S.; Park, S. B.; Lee, J. K. Bioinspired Polymerization of Dopamine to Generate Melanin-Like Nanoparticles Having an Excellent Free-Radical-Scavenging Property. Biomacromolecules 2011, 12, 625−632. (24) Zhang, X. Y.; Wang, S. Q.; Xu, L. X.; Feng, L.; Ji, Y.; Tao, L.; Li, S. X.; Wei, Y. Biocompatible Polydopamine Fluorescent Organic Nanoparticles: Facile Preparation and Cell Imaging. Nanoscale 2012, 4, 5581−5584.
that DHI monomer can be irreversibly depleted from aqueous solution by addition of eumelanin suspensions.19 Hong et al. have shown that 14.2% (w/w) of unpolymerized dopamine was entrapped in the polydopamine.27 We suppose that the wellaggregated protoparticles of PDA may work as a nucleation site, inducing assembly of oligomers with a lower degree of polymerization surrounding these well-organized “nuclei”. Third, PDA may somehow cross-link to prevent vaporization of dissociated molecules in thermal annealing in Ar or N2. Kong et al. have proposed that PDA molecules may cross-link via C− O−C bonds during carbonization on the basis of FTIR characterization.39 We noticed that PDA nanoparticles became nanocages after carbonization in some cases (Figure 3d). Although the carbonization at low heating rate (5 °C/min) may prevent the structural breakage of carbonized PDA nanoparticles, the structural feature in their shell part is distinct from the inner part (Figure 6d); more stacking layers in the shell than that in the inside part are observed. This may be due to different degrees of aggregation or degrees of polymerization in the shell and core parts of the PDA nanoparticles. Because PDA has similar structure to phenolic resins, which will cross-link via elimination of water in pyrolysis, the hydroxyl groups in neighboring units of PDAs have an opportunity to react and cross-link. In future studies, we will investigate the relation of experimental parameters of PDA fabrication and the shape and size of graphite-like domains in carbonized PDA nanoparticles. More quantitative experimental results are needed to understand their connection. The enlargement of graphite-like domains shows improved order of aggregated sheets of protomolecules in PDA. It may be particularly useful to manipulate the optoelectric properties of PDA nanostructures.
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CONCLUSION In summary, we are the first to investigate the nanostructures of carbonized polydopamine nanoparticles by using high-resolution TEM and Raman spectroscopy. We observed that in carbonized PDA nanoparticles there are many graphite-like nanoaggregates which are composed of 40−50 stacking layers, with thicknesses of about 15 nm. The quite large size and graphitization of these domains indicate an in-plane orientation of stacking sheets composed of protomolecules of polydopamine.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (Z.J.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 21374132 and 51173201) for financial support.
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REFERENCES
(1) Liu, Y.; Simon, J. D. The Effect of Preparation Procedures on the Morphology of Melanin from the Ink Sac of Sepia officinalis. Pigm. Cell Res. 2003, 16, 72−80. (2) Panzella, L.; Gentile, G.; D’Errico, G.; Della Vecchia, N. F.; Errico, M. E.; Napolitano, A.; Carfagna, C.; d’Ischia, M. Atypical Structural and π-Electron Features of a Melanin Polymer That Lead to Superior Free-Radical-Scavenging Properties. Angew. Chem., Int. Ed. 2013, 52, 12684−12687. 5504
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dx.doi.org/10.1021/la500225v | Langmuir 2014, 30, 5497−5505
