Oxidative Self-Polymerization of Dopamine in an Acidic Environment

Subscriber access provided by UNIV OF LETHBRIDGE

Article

Oxidative Self-polymerization of Dopamine in Acidic Environment Weichao Zheng, Hailong Fan, Le Wang, and Zhaoxia Jin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02757 • Publication Date (Web): 06 Oct 2015 Downloaded from http://pubs.acs.org on October 10, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Oxidative Self-polymerization of Dopamine in Acidic Environment Weichao Zheng, Hailong Fan, Le Wang and Zhaoxia Jin* Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China.

ABSTRACT: Weak alkaline condition (pH > 8) is a general requirement for oxidative self-polymerization of dopamine. Here we first demonstrated the generation of polydopamine in acidic environment via hydrothermal method. The pH scope of self-polymerization of dopamine is extended to pH~1 in hydrothermal process. Polydopamine generated via hydrothermal method shows similar chemical features and radical scavenging activity with that generated in basic environment.

ACS Paragon Plus Environment

1

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

INTRODUCTION In nature, extensive kinds of organic carbon are generated through hydrothermal process.1-9 Oligopeptides are proposed to be formed in submarine hydrothermal systems. Concentration, time and temperature strongly influence the peptide synthesis in submarine hydrothermal systems.2 Lipid is also synthesized under hydrothermal conditions by Fischer-Tropsch-Type reactions.3 It is heavily debated in scientific communities if hydrothermal vents at oceanic ridge crests may act as the place for life origin.4-9 However, as a fabrication process, hydrothermal path is usually utilized in synthesis of inorganic nanomaterials.10-13 Few studies report the synthesis of organic materials via hydrothermal route, which maybe because organic materials will easily be decomposed or carbonized under high temperature and high pressure.14-19 Thus optimizing processing parameters to avoid hydrothermal carbonization is critical for exploring the synthesis of organic materials via hydrothermal route. Recently the study of polydopamine (PDA), a kind of natural melanin-like materials, attracts extensive attention because of its unique photophysical properties,20 versatile adhesiveness and multifunctional coating abilities for a broad range of applications in biomedical,21-25 electrochemical,26 nanotechnology,27,28 membrane application,29 bio-optoelectric materials30 and solar energy31. Their photophysical properties are related to their heterogenerous molecular structures and complex aggregated-structures.32 The mussel-inspired polydopamine coating has been found great application in biomaterials.

33,34,35,36

The exploration in synthesis methods and

formation mechanism of PDA will benefit the controllable fabrication of PDA with manipulated functional properties in future.37 In general, dopamine is stable towards oxidation in acidic media in the complete absence of metal ions.38 In most reported synthesis of PDA, weak alkaline condition (pH > 8) is a fundamental requirement, except in some synthesis routes using specific


2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

oxidants such as enzymes, CuSO4, (NH4)2S2O8, NaIO4 and KClO3.39,40 However, the pH scope in these experiments is still weak acidic (pH~4.5) or neutral (pH~7).40 Herein, for the first time, we demonstrated that the oxidative self-polymerization of dopamine happens in strong acidic condition (pH~1) via hydrothermal process. This study is not only important for understanding the formation mechanism of polydopamine, but also critical for extending wider application for polydopamine. Particularly, because hydrothermal process is a useful method to generate inorganic nanostructures, one-pot synthesis of inorganic/polydopamine hybrid nanostructure is highly

desirable.

The

existence

of

polydopamine-coating

layer

will

benefit

the

post-functionalization of obtained nanostructures. EXPERIMENTAL SECTION Materials. Dopamine hydrochloride (purity 98%) was purchased from Sigma-Aldrich. Hydrochloric acid is the product of Sinopharm Chemical Reagent Co. Ltd. Chloroauric acid (HAuCl4) was purchased from J&K Scientific Ltd. All solutions were prepared by using Millipore water. Fabrication of PDA via hydrothermal method In a typical synthesis, 5.0 mg or 45 mg dopamine hydrochloride was dissolved in 10 mL water (pH of dopamine solution is about 5) or hydrochloric acid solution (pH was adjusted to 3 or 1). Abovementioned solution was kept in Teflon-lined autoclave at 120 °C or 160 °C for 16 hrs. After reaction, the autoclave was cooled down to room temperature. The produced dark suspension was washed by deionized water several times and collected by ultracentrifuge at


3

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

15000 rpm for 10 minutes, and finally dried in freeze-drying. The dried powder was used to conduct morphological and chemical characterizations. It is named as H-PDA. Fabrication of gold@H-PDA nanoparticles We fabricated gold@H-PDA to check the reactivity of H-PDA. To prepare gold@H-PDA nanoparticles, 20 µL HAuCl4 solution (1 mM) was added in abovementioned aqueous suspension of H-PDA nanoparticles (2 mL), then the mixture was stirred for 1 min and incubated for 20 hrs at 1 °C. After reaction, the mixture was centrifuged at 15000 rpm for 5 min, the sediment was washed by deionized water several times. Clean gold@H-PDA nanoparticles were characterized by SEM. Characterizations of H-PDA nanoparticles and the supernatant of product suspensions after centrifugation Fourier Transform Infrared Spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) characterizations were conducted to identify their chemical features. FTIR spectra were recorded in KBr pellets in which dried H-PDA nanoparticles are dispersed (Bruker tensor 27). X-ray Photoelectron Spectroscopy was performed on a Imaging Photoelectron Spectrometer (Axis Ultra, Kratos Analytical Ltd.) with monochromatic Al Kα (hv = 1486.7 eV) radiation as the excitation and X-ray power of 150 W. All spectra were calibrated using the hydrocarbon C1s peak (284.6 eV). High-resolution scans were acquired for C 1s, N 1s regions. The peak-fitting procedure is consistent with the best fit with consideration of peak position, full width at a half-maximum and intensity. UV-Vis absorption of nanoparticles suspension was measured using a Shimadzu UV-3600 spectrophotometer. Fluorescence spectra were measured using a HITACHI F-4600 fluorescence spectrophotometer. Zeta potential was measured using a


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

MALVERN zeta sizer Nanozs 90. Measurements were performed three times for each sample at 25 °C. Scanning electron microscope (SEM, JEOL 7401) was performed at an accelerating voltage of 5 kV. The sample was coated with a thin layer of gold before SEM characterization. High-resolution transmission electron microscopy (HRTEM) was conducted by using a JEM-2100 (JEOL) at 200 kV. MALDI-TOF mass spectrometry characterizations of H-PDA nanoparticles α-Cyano-4-hydroxycinnamic acid (CHCA) 98% purity was purchased from Sigma Aldrich Inc. (St. Louis, MO). HPLC-grade acetonitrile and trifluoroacetic acid (TFA) were purchased from Fisher Scientific (USA). Triple deionized water was prepared from a Millipore water purification system (Millipore, Billerica, MA, USA) in our laboratory whenever necessary. The solution of matrix CHCA (10 mg/mL) was prepared in CH3CN/water (1 : 1, v / v) containing 0.1% TFA. 1 mL of the analyte was premixed with 1 mL of the matrix in a centrifuge tube, and then 1 mL of the resulting mixture was pipetted on the MALDI target plate and air-dried for MALDI-TOF mass spectrometry analysis. MALDI-TOF mass spectrometry analysis was performed on an Ultraflextreme MALDI-TOF/TOF mass spectrometer (Bruker Daltonics, Billerica, MA) equipped with a Nd:YAG /355 nm Smart Beam TM laser. The laser was operated at 2000 Hz in the positive reflectron mode. The mass spectrometer parameters were set as recommended by the manufacturer and adjusted for optimal acquisition performance. The laser spot size was set at medium focus (~50 mm laser spot diameter). The mass spectra data were acquired over a mass range of m/z 0–3000 Da and each mass spectrum was collected from the accumulation of 200 laser shots. Mass calibration was carried out with external standards prior to data acquisition.


5

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

qNano Analyzer (Izon Sciences, Christchurch, New Zealand) was used for microspheres size determination with NP1000 nanopore and 1000 nm calibration particles. First, the nanopore and cells were cleaned with KCl aqueous solution (0.05 M) and a baseline current (70–140 nA) was developed. Diluted sample or calibration particles (40 µL) were loaded in the upper fluid cell and the lower fluid cell was filled with 80 µL of KCl aqueous solution. All samples were run under the same applied voltage (0.18 V) and stretch (46.5 mm). Each recorded measurement was based on at least 500 particles. Data was analyzed using Izon control suite 3.0 software. DPPH assay for antioxidant activities of H-PDA nanoparticles. The antioxidant activities of H-PDA nanoparticles were measured by DPPH assays as described in literature.41 Fresh DPPH /ethanol (0.1 mM) solution was used in measurements. 180 µL of different concentrations of H-PDA suspensions was added to 4 mL of DPPH. The amount of H-PDA in test was changed from 5 to 240 µg. The scavenging activity was evaluated by measuring the absorbance change at 520 nm after tested mixture was kept in the dark for 20 min.

RESULTS AND DISCUSSION In a typical synthesis, 5.0 mg dopamine hydrochloride was dissolved in 10 mL water and kept in Teflon-lined autoclave at 120 °C or 160 °C for 16 hrs. The pH of original dopamine solution is about 5. After reaction, the autoclave was cooled down to room temperature. The produced dark suspension was washed by deionized water several times and collected by ultracentrifuge at 15000 rpm for 20 minutes, and finally dried in freeze-drying. This synthesis was also conducted in acidic environment, in which HCl was added in dopamine solution to adjust pH to 1 or 3.


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1 show morphological features of H-PDA fabricated at pH 1 and pH 5. All experimental results in various experimental conditions (different pH values, concentrations of dopamine and temperatures) are presented in Table 1 and Figure S1. In a strong acidic condition (pH = 1), there is no product in low or high concentration of dopamine at 120 °C for 16 hrs. However, the increase of temperature significantly accelerates the oxidative polymerization of dopamine, H-PDA can be produced at 160 °C even at a low dopamine concentration (Figure 1a). In pH = 3 condition, H-PDA microspheres are obtained at either low (0.5 mg/mL) or high (4.5 mg/mL) dopamine concentrations at 120 °C (Figure S1). H-PDA products show three different morphologies: aggregate of nanoparticles (Figure 1a and 1b), microspheres (Figure 1c) and aggregate of microspheres and nanoparticles (Figure 1d). The size of microspheres (Figure 1c) is about 700 nm in diameter (Figure S2). The extension of reaction time has showed influence to morphology of H-PDA, from aggregates of nanoparticles to aggregates of microspheres and nanoparticles (Figure S3). The morphologies of H-PDA are similar with the products of PDA fabricated in basic condition.42 Based on these studies, we observed that although high acidic environment (pH = 1) goes against the formation of PDA in hydrothermal process, higher temperature (160 °C) and extension of reaction time (from 16 hrs to 36 hrs) will benefit the generation of solid H-PDA product (Figure S4). On the other hand, we noticed that the typical morphology of H-PDA in high concentration conditions is aggregates of nanoparticles. Because the H-PDA obtained at pH = 1 is the aggregate of nanoparticles, such a morphology may suggest that it is the result of a hindered reaction. Further characterizations by UV-Vis, FTIR and XPS spectroscopy revealed the chemical nature of H-PDA products (Figure 2, Figure S5-S8). UV-Vis absorption spectrum of H-PDA presents the featureless broadband absorption (Figure 2a, Figure S5) that agrees with other melanin-like


7

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

PDA. In the FTIR spectrum of H-PDA (Figure 2b, Figure S6), the broad peak at 3433 cm-1 is due to the stretching of (N-H) and (O-H). Strong peak at 1635 cm-1 and shoulder peak at 1554 cm-1 can be assigned to aromatic C=C bonds of indole and ring(C=N).43,44 FTIR spectrum of H-PDA confirms that its chemical structure is similar with that generated in basic environment.45 XPS characterization further helps us to identify the functional groups of H-PDA (Figure S7, S8). In these H-PDA products, the N/C ratio is 0.09, which falls into the N/C range reported in literatures (0.08 to 0.17).45-47 Detailed analysis of XPS spectra of high resolution C1s and N1s of H-PDA (pH = 1) and H-PDA (pH = 5) are presented in Figure S7, S8 and Table 2. The C1s region is fit with four species, CH/C-NH2 (67.16% of pH 1 and 62.5% of pH 5), C-O/C-N (26.22% of pH 1 and 21.91% of pH 5), C=O/C=N (6.07% of pH 1 and 11.77% of pH 5), π→π* (0.55% of pH 1 and 3.82% of pH 5). The significant amount of C=O can be assigned to the tautomers of catechole units.45 The π→π* species is related to the intramolecular and intermolecular response of the electronic system upon the excitation process.48 This species shows clear difference for samples generated in different pH values, indicating the intramolecular and intermolecular response of H-PDA (pH 1) in excitation process is limited due to their generation in high acidic environment. The N1s peak is fit with three components assigned to primary (R-NH2, 19.28% of pH 1 and 18.43% of pH 5), secondary (R-NH-R, 67.99% of pH 1 and 68.26% of pH 5) and tertiary/aromatic (=N-R, 12.73% of pH 1 and 13.31% of pH 5) amine functionalities (Table 2). The secondary amine belongs to intermediate species and polydopamine,44 that confirms the chemical nature of H-PDA. The tertiary/aromatic amine, assigned to the intermediate species 5,6-dihyroxyindole and 5,6-indolequinone, are 12.73% and 13.31% in H-PDA fabricated at pH 1 and pH 5, respectively. They are in agreement with that reported in literature (13.6%).49 Dopamine may also aggregate with itself or oligomers of


8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

polydopamine via noncovalent interactions,50 giving a significant amount of primary amine in XPS result. Thermogravimetric analysis (TGA) and mass spectroscopy (MALDI-TOF) of H-PDA presented in Figure S9 and S10 also supported that H-PDA are basically similar with that generated in basic condition.51,52 Qu et al. have reported the carbon dots prepared by hydrothermal treatment of dopamine.53 Because the temperature they used is higher (180 °C) than we used, their main product is carbon dots (3.8 nm in diameter). In summary, dopamine hydrochloride can be converted to H-PDA in Teflon-lined autoclave at a temperature range of 120-160 °C for 16 hrs without significant carbonization, the pH scope is extended to 1, which means the basic environment is no longer necessary in the generation of PDA. The surface charge of dopamine-melanin films and nanoparticles has been demonstrated by zeta potential values.51,54-56 Bernsmann et al. have measured the zeta potential of dopamine-melanin films, which is -39 ± 3 mV (pH 8.5) after 12 immersion steps.54 The negative surface charge originates from dissociation of quinone imine (pKa = 6.3) and catechol groups (their dimers have pKa values between 9 and 13).55 The isoelectric point of dopamine-melanin films is close to 4.56 In our previous study, the isoelectric point of folic acid/PDA nanoparticles is also close to 4.51 However, H-PDA fabricated via hydrothermal route shows different isoelectric point that is between 2~3 (Figure 2c). It indicates that functional groups on H-PDA surface may be different compared with that generated in alkaline environment. d’Ischia et al. have summarized the molecular architectures generated from the 5,6-dihydroxyindole system and they found the architectures are varied at acidic environment.57 The generation of different functional groups on H-PDA in strong or weak acidic hydrothermal processes is possible. Melanin-like polydopamine shows good free radical scavenging effect.41,51,58,59 To further confirm the H-PDA has similar chemical properties with PDA, we characterized its free radical


9

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

scavenging activities using DPPH assay. The scavenging activity (Figure 2d) was compared with ascorbic acid (AA). The efficient concentration (EC50) value, at which the initial DPPH concentration is dropped to 50%, is 33 µg for H-PDA, which is comparable with PDA generated in basic environment.51 The reactivity of H-PDA was also checked through the generation of gold nanoparticles on H-PDA nanostructures (Figure S11). Without using any reducing agent, H-PDA itself can reduce gold ions adsorbed on H-PDA surface to gold nanoparticles, showing H-PDA surface functional groups have similar reductive capacity as normal PDA layer has.47,60 In addition, we characterized the detailed structure of carbonized H-PDA particles by using HRTEM. H-PDA particles were heated in N2 environment from room temperature to 750 °C with a heating rate of 5 °C/min, and then kept at 750 °C for 1 hr. Figure 3 presents HRTEM images of carbonized H-PDA nanoparticles. Well-graphitized stacking layers show a layer distance of 0.35 nm that agrees with our previous report.52 We noticed that graphitized parts are not so obvious in those carbonized H-PDA microspheres (Figure S12). It may be due to the stacking of amorphous and graphitized layers in large microspheres. The supernatant of H-PDA suspension after centrifugation at 15000 rpm still shows yellow or brown colour depending on the concentration of dopamine used in hydrothermal process (Figure 4a). It may contain unreacted dopamine and some oligomers in hydrothermal process which are not large enough to aggregate to solid particles. Previous studies observed that the fluorescence of eumelanin and pheomelanin is related to their chemically distinct oligomeric units, their sizes of oligomers and stacking conditions of oligomers.20,32,61-64 Therefore, we investigated the fluorescence properties of these supernatant and hope to learn something about small oligomers. Figure 4a presents the absorbance spectrum of supernatant (after hydrothermal process, pH = 5, concentration of dopamine is 4.5 mg/mL, 160 °C for 16 hrs). Its fluorescence spectra at different


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

excitation wavelengths are shown in Figure 4b. The oligomers and polymers of melanins have weak fluorescence (from 450 to 600 nm) based on literature. The supernatant of hydrothermal product shows an asymmetric emission peak at ~500 nm, which is similar with synthetic eumelanin and Sepia ink eumelanin.65,66 However, carbon dots generated from hydrothermal of dopamine show emission wavelength from 380 to 530 nm.53 The characteristic emission of H-PDA supernatant does not show “mirror image roles”, which is due to the mixed chromophores in eumelanin-like solution (Figure S13).67 The emission wavelength of melanins is varied from 500 nm to 570 nm depending on the different sources and processing. Because molecular structures in H-PDA are different from that of eumelanin, excitation wavelengths in our case (290 nm to 370 nm) are unlike theirs.65 MALDI-TOF mass spectrum of this supernatant also confirms the existence of mixed oligomers like trimers, tetramers and pentamers (Figure S14). Since these small oligomers are soluble in water, the solid sediment after centrifugation may contain larger oligomers although the MALDI-TOF mass spectrum of H-PDA solid product presented only small oligomers (Figure S10). A further investigation by using mass spectrum will reveal deeper structural information of H-PDA. Above experimental investigation demonstrates that H-PDA is similar with that fabricated in basic environment. Why can dopamine be polymerized to H-PDA via hydrothermal process in acidic conditions? We thought the following reasons contribute to the successful produce of H-PDA. First of all, higher temperature accelerates the generation of polydopamine. Secondly, water at elevated temperatures and high pressures is a special medium for chemical reaction. The autoionization of water shows different equilibrium constant at high temperatures and high pressures. Self-ionization of water increases with increasing temperature and pressure thus the concentrations of hydronium and hydroxyl ion are higher in hydrothermal conditions. In the


11

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

proposed formation mechanism of PDA, the oxidative polymerization of dopamine starts with the transformation of catechol moieties into the corresponding quinone moieties via oxidation reaction. In these cascade reactions, the first step from dopamine to dopamine quinone via 2-electron oxidation process (removing two H+ and two electrons) requires a basic environment. More hydroxyl groups will assist the leaving of hydrogen from dopamine molecules. Thus the generation of oxidative polymerization of dopamine in hydrothermal environment is somehow reasonable. On the other hand, the radical pathways may also involve in the oxidation and cyclization processes.68 Herlinger and coworkers have proposed that the reaction from dopaminonchrome to polymeric melanin may require a catalytic amount of OH• free radicals.38 Hydrothermal environment may benefit the generation of OH• free radicals, thus further promoting the polymerization of dopamine. Although we still have not fully understood the formation mechanism of polydopamine in strong acidic environment (pH~1) yet, the knowledge of different formation processes of PDA will help us understand the construction of multilevel structures of PDA. In particular, it will also contribute to recognizing reactivity of organic molecules in hydrothermal environment, which is somehow meaningful to know the origin of life. CONCLUSIONS In conclusion, we demonstrated a synthesis path of polydopamine via hydrothermal process. For the first time, the pH value for self-polymerization of dopamine is extended to strong acidic environment (pH~1) without using special oxidants. The chemical property of polydopamine generated via hydrothermal method is similar with that fabricated in basic environment, which provides a good opportunity to synthesize inorganic/polydopamine hybrid nanostructure via one-pot hydrothermal process.


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1. SEM images of H-PDA products in various experimental conditions. (a) pH = 1, 160 °C, 0.5 mg/mL, (b) pH = 1, 160 °C, 4.5 mg/mL, (c) pH = 5, 120 °C, 0.5 mg/mL, and (d) pH = 5, 160 °C, 0.5 mg/mL.


13

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

Figure 2. Characterizations of H-PDA products. (a) UV-vis absorbance spectrum, (b) FT-IR spectrum, (c) Zeta-potential, (d) Free-radical scavenging effect. The sample was produced at 160 °C for 16 hrs, pH = 5, the concentration of dopamine was 4.5 mg/mL.


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 3. (a, b) HRTEM images of carbonized H-PDA particles, the layer distance is 0.35 nm. The sample was produced at 160°C for 16 hrs, pH=5, the concentration of dopamine was 4.5 mg/mL.


15

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Figure 4. Characterizations of the supernatant of product suspensions after centrifugation. (a) UV-vis absorbance spectrum of supernatant, the picture inset is the supernatant solution. (b) Fluorescence spectra of supernatant at various excitation wavelengths. Table 1. Summary of polydopamine synthesized via hydrothermal method in various experimental conditions (The reaction times were kept for 16 hrs in listed experiments). 120 °C

160 °C

0.5 mg/mL

No product

0.5 mg/mL

Aggregation of nanoparticles

4.5 mg/mL

No product

4.5 mg/mL

Aggregation of nanoparticles

0.5 mg/mL

microspheres

0.5 mg/mL

Aggregation of microspheres

4.5 mg/mL

microspheres

4.5 mg/mL


0.5 mg/mL

microspheres

0.5 mg/mL

Aggregation of microspheres with nanoparticles

4.5 mg/mL


4.5 mg/mL

Aggregation of microspheres with nanoparticles

pH = 1

pH = 3

pH = 5


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table 2. XPS functional groups percentages of H-PDA obtained in different pH values *. C 1s C-C C=C C-NH2

N 1s C=O

C-O C-N

π-π*

RNH2

R1-NH-RH2

C=NR

C=N

284.6 eV

286 eV

287.7 eV

290.0 eV

401.8 eV

399.9 eV

398.6 eV

pH=1

67.16%

26.22%

6.07%

0.55%

19.28%

67.99%

12.73%

pH=5

62.5%

21.91%

11.77%

3.82%

18.43%

68.26%

13.31%

* The samples for XPS were produced at 160 °C for 16 hrs, pH = 1 or pH = 5, the concentration of dopamine was 4.5 mg/mL.

ASSOCIATED CONTENT Supporting Information. Supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail [email protected] (Z.J.). Notes


17

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 21374132, 51173201) for financial support. We thank Dr. Y. Liu (Peking University) for HRTEM characterization. REFERENCES 1. Comita, P. B.; Gagosian, R. B.; Williams, P. M. Suspended Particulate Organic Material from Hydrothermal Vent Waters at 21° N. Nature 1984, 307, 450-453. 2. Cleaves, H. J.; Aubrey, A. D.; Bada, J. L. An Evaluation of the Critical Parameters for Abiotic Peptide Synthesis in Submarine Hydrothermal Systems. Origins Life Evol. Biospheres 2009, 39, 109-126. 3. McCollom, T.; Ritter, G.; Simoneit, B. T. Lipid Synthesis Under Hydrothermal Conditions by Fischer- Tropsch-Type Reactions. Origins Life Evol. Biospheres 1999, 29, 153-166. 4. Rabenau, A. The Role of Hydrothermal Synthesis in Preparative Chemistry. Angew. Chem., Int. Ed. Engl. 1985, 24, 1026-1040. 5. McCollom, T. M.; Seewald, J. S. Abiotic Synthesis of Organic Compounds in Deep-Sea Hydrothermal Environments. Chem. Rev. 2007, 107, 382-401. 6. Maher, K. A.; Stevenson, D. J. Impact Frustration of the Origin of Life. Nature 1988, 331, 612-614.


18

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

7. Miller, S. L.; Bada, J. L. Submarine Hot Springs and the Origin of Life. Nature 1988, 334, 609-611. 8. Hennet, R. J. C.; Holm, N. G.; Engel, M. H. Abiotic Synthesis of Amino Acids under Hydrothermal

Conditions

and

the

Origin

of

Life:

A

Perpetual

Phenomenon?

Naturwissenschaften 1992, 79, 361-365. 9. Amend, J. P.; Shock, E. L. Energetics of Amino Acid Synthesis in Hydrothermal Ecosystems. Science 1998, 281, 1659-1662. 10. Feng, S.; Xu, R. New Materials in Hydrothermal Synthesis. Acc. Chem. Res. 2001, 34, 239-247. 11. Wang, C.-C.; Ying, J. Y. Sol−Gel Synthesis and Hydrothermal Processing of Anatase and Rutile Titania Nanocrystals. Chem. Mater. 1999, 11, 3113-3120. 12. Liu, B.; Zeng, H. C. Hydrothermal Synthesis of ZnO Nanorods in the Diameter Regime of 50 nm. J. Am. Chem. Soc. 2003, 125, 4430-4431. 13. Cundy, C. S.; Cox, P. A. The Hydrothermal Synthesis of Zeolites: History and Development from the Earliest Days to the Present Time. Chem. Rev. 2003, 103, 663-702. 14. Pan, L. J.; Pu, L.; Shi, Y.; Sun, T.; Zhang, R.; Zheng, Y. O. Hydrothermal Synthesis of Polyaniline Mesostructures. Adv. Funct. Mater. 2006, 16, 1279-1288. 15. Hu, B.; Wang, K.; Wu, L.; Yu, S.-H.; Antonietti, M.; Titirici, M.-M. Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass. Adv. Mater. 2010, 22, 813-828.


19

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

16. Titirici, M.-M.; Antonietti, M. Chemistry and Materials Options of Sustainable Carbon Materials Made by Hydrothermal Carbonization. Chem. Soc. Rev. 2010, 39, 103-116. 17. Zhao, H.; Qu, Z.-R.; Ye, H.-Y.; Xiong, R.-G. In Situ Hydrothermal Synthesis of Tetrazole Coordination Polymers with Interesting Physical Properties. Chem. Soc. Rev. 2008, 37, 84-100. 18. Titirici, M. M.; Thomas, A.; Yu, S.-H.; Müller, J.-O.; Antonietti, M. A Direct Synthesis of Mesoporous Carbons with Bicontinuous Pore Morphology from Crude Plant Material by Hydrothermal Carbonization. Chem. Mater. 2007, 19, 4205-4212. 19. Ranganathan, A.; Pedireddi, V. R.; Rao, C. N. R. Hydrothermal Synthesis of Organic Channel Structures: 1:1 Hydrogen-Bonded Adducts of Melamine with Cyanuric and Trithiocyanuric Acids. J. Am. Chem. Soc. 1999, 121, 1752-1753. 20. Meredith, P.; Sarna, T. The Physical and Chemical Properties of Eumelanin. Pigm. Cell Res. 2006, 19, 572-594. 21. Ball, V.; Apaydin, K.; Laachachi, A.; Toniazzo, V.; Ruch, D. Changes in Permeability and in Mechanical

Properties

of

Layer-by-Layer

Films

Made

from

Poly(allylamine)

and

Montmorillonite Postmodified upon Reaction with Dopamine. Biointerphases 2012, 7, 1-9. 22. della Vecchia, N. F.; Cerruti, P.; Gentile, G.; Errico, M. E.; Ambrogi, V.; D’Errico, G.; Longobardi, S.; Napolitano, A.; Paduano, L.; Carfagna, C.; d’Ischia, M. Artificial Biomelanin: Highly Light-Absorbing Nano-Sized Eumelanin by Biomimetic Synthesis in Chicken Egg White. Biomacromolecules 2014, 15, 3811-3816.


20

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

23. Yu, B.; Wang, D. A.; Ye, Q.; Zhou, F.; Liu, W. Robust Polydopamine Nano/Microcapsules and Their Loading and Release Behavior. Chem. Commun. 2009, 6789-6791. 24. Zhang, Y.; Panneerselvam, K.; Ogaki, R.; Hosta-Rigau, L.; van der Westen, R.; Jensen, B. E. B.; Teo, B. M.; Zhu, M.; Städler, B. Assembly of Poly(dopamine)/Poly(N-isopropylacrylamide) Mixed Films and Their Temperature-Dependent Interaction with Proteins, Liposomes, and Cells. Langmuir 2013, 29, 10213-10222. 25. Lynge, M. E.; van der Westen, R.; Postma, A.; Stadler, B. Polydopamine-a Nature-Inspired Polymer Coating for Biomedical Science. Nanoscale 2011, 3, 4916-4928. 26. Fang, C.; Deng, Y.; Xie, Y.; Su, J.; Chen, G. Improving the Electrochemical Performance of Si Nanoparticle Anode Material by Synergistic Strategies of Polydopamine and Graphene Oxide Coatings. J. Phys. Chem. C 2015, 119, 1720-1728. 27. Ogaki, R.; Bennetsen, D. T.; Bald, I.; Foss, M. Dopamine-Assisted Rapid Fabrication of Nanoscale Protein Arrays by Colloidal Lithography. Langmuir 2012, 28, 8594-8599. 28. Sun, K.; Song, L.; Xie, Y.; Liu, D.; Wang, D.; Wang, Z.; Ma, W.; Zhu, J.; Jiang, X. Using Self-Polymerized Dopamine to Modify the Antifouling Property of Oligo(ethylene glycol) Self-Assembled Monolayers and Its Application in Cell Patterning. Langmuir 2011, 27, 5709-5712. 29. Saito, Y.; Kawano, T.; Shimomura, M.; Yabu, H. Fabrication of Mussel-Inspired Highly Adhesive Honeycomb Films Containing Catechol Groups and Their Applications for Substrate-Independent Porous Templates. Macromol. Rapid Commun. 2013, 34, 630-634.


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

30. d'Ischia, M.; Napolitano, A.; Pezzella, A.; Meredith, P.; Sarna, T. Chemical and Structural Diversity in Eumelanins: Unexplored Bio-Optoelectronic Materials. Angew. Chem., Int. Ed. 2009, 48, 3914-3921. 31. 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. 32. Meredith, P.; Powell, B. J.; Riesz, J.; Nighswander-Rempel, S. P.; Pederson, M. R.; Moore, E. G. Towards Structure-Property-Function Relationships for Eumelanin. Soft Matter 2006, 2, 37-44. 33. Jeong, K. J.; Wang, L.; Stefanescu, C. F.; Lawlor, M. W.; Polat, J.; Dohlman, C. H.; Langer, R. S.; Kohane, D. S. Polydopamine Coatings Enhance Biointegration of a Model Polymeric Implant. Soft Matter 2011, 7, 8305-8312. 34. 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. 35. Kang, S. M.; Hwang, N. S.; Yeom, J.; Park, S. Y.; Messersmith, P. B.; Choi, I. S.; Langer, R.; Anderson, D. G.; Lee, H. One-Step Multipurpose Surface Functionalization by Adhesive Catecholamine. Adv. Funct. Mater. 2012, 22, 2949-2955. 36. 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. 37. d’Ischia, M.; Napolitano, A.; Ball, V.; Chen, C.-T.; Buehler, M. J. Polydopamine and Eumelanin: From Structure–Property Relationships to a Unified Tailoring Strategy. Acc. Chem. Res. 2014, 47, 3541-3550.


22

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

38. Herlinger, E.; Jameson, R. F.; Linert, W. Spontaneous Autoxidation of Dopamine. J. Chem. Soc., Perkin Trans.2 1995, 259-263. 39. Bernsmann, F.; Ball, V.; Addiego, F.; Ponche, A.; Michel, M.; Gracio, J. J. d. A.; Toniazzo, V.; Ruch, D. Dopamine−Melanin Film Deposition Depends on the Used Oxidant and Buffer Solution. Langmuir 2011, 27, 2819-2825. 40. Wei, Q.; Zhang, F.; Li, J.; Li, B.; Zhao, C. Oxidant-Induced Dopamine Polymerization for Multifunctional Coatings. Polym. Chem. 2010, 1, 1430-1433. 41. 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. 42. Li, N.; Wang, H.-B.; Thia, L.; Wang, J.-Y.; Wang, X. Enzymatic-Reaction Induced Production of Polydopamine Nanoparticles for Sensitive and Visual Sensing of Urea. Analyst 2015, 140, 449-455. 43. Kong, J.; Yee, W. A.; Yang, L.; Wei, Y.; Phua, S. L.; Ong, H. G.; Ang, J. M.; Li, X.; Lu, X. Highly Electrically Conductive Layered Carbon Derived from Polydopamine and Its Functions in SnO2-Based Lithium Ion Battery Anodes. Chem. Commun. 2012, 48, 10316-10318. 44. Zangmeister, R. A.; Morris, T. A.; Tarlov, M. J. Characterization of Polydopamine Thin Films Deposited at Short Times by Autoxidation of Dopamine. Langmuir 2013, 29, 8619-8628.


23

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

45. Liebscher, J.; Mrówczyński, R.; Scheidt, H. A.; Filip, C.; Hădade, N. D.; Turcu, R.; Bende, A.; Beck, S. Structure of Polydopamine: A Never-Ending Story? Langmuir 2013, 29, 10539-10548. 46. Yu, X.; Fan, H.; Wang, L.; Jin, Z. Formation of Polydopamine Nanofibers with the Aid of Folic Acid. Angew. Chem., Int. Ed. 2014, 53, 12600-12604. 47. Lee, H.; Dellatore, S. M.; Miller, W. M. Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426-430. 48. Schöll, A.; Zou, Y.; Jung, M.; Schmidt, T.; Fink, R.; Umbach, E. Line Shapes and Satellites in High-Resolution X-Ray Photoelectron Spectra of Large π-Conjugated Organic Molecules. J. Chem. Phys. 2004, 121, 10260-10267. 49. Bernsmann, F.; Ponche, A.; Ringwald, C.; Hemmerlé, J.; Raya, J.; Bechinger, B.; Voegel, J.-C.; Schaaf, P.; Ball, V. Characterization of Dopamine−Melanin Growth on Silicon Oxide. The J. Phys. Chem. C 2009, 113, 8234-8242. 50. Hong, S.; Na, Y. S.; Choi, S.; Song, I. T.; Kim, W. Y.; Lee, H. Non-Covalent Self-Assembly and Covalent Polymerization Co-Contribute to Polydopamine Formation. Adv. Funct. Mater. 2012, 22, 4711-4717. 51. Fan, H.; Yu, X.; Liu, Y.; Shi, Z.; Liu, H.; Nie, Z.; Wu, D.; Jin, Z. Folic Acid-Polydopamine Nanofibers Show Enhanced Ordered-Stacking via π-π Interactions. Soft Matter 2015, 11, 4621-4629.


24

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

52. Yu, X.; Fan, H.; Liu, Y.; Shi, Z.; Jin, Z. Characterization of Carbonized Polydopamine Nanoparticles Suggests Ordered Supramolecular Structure of Polydopamine. Langmuir 2014, 30, 5497-5505. 53. Qu, K.; Wang, J.; Ren, J.; Qu, X. Carbon Dots Prepared by Hydrothermal Treatment of Dopamine as an Effective Fluorescent Sensing Platform for the Label-Free Detection of Iron(III) Ions and Dopamine. Chem. - Eur. J. 2013, 19, 7243-7249. 54. Bernsmann, F.; Frisch, B.; Ringwald, C.; Ball, V. Protein Adsorption on Dopamine–Melanin Films: Role of Electrostatic Interactions Inferred from ζ-Potential Measurements Versus Chemisorption. J. Colloid Interface Sci. 2010, 344, 54-60. 55. Szpoganicz, B.; Gidanian, S.; Kong, P.; Farmer, P. Metal Binding by Melanins: Studies of Colloidal Dihydroxyindole-Melanin, and its Complexation by Cu(II) and Zn(II) Ions. J. Inorg. Biochem. 2002, 89, 45-53. 56. Ball, V. Impedance Spectroscopy and Zeta Potential Titration of Dopa-Melanin Films Produced by Oxidation of Dopamine. Colloids Surf., A 2010, 363, 92-97. 57. d'Ischia, M.; Napolitano, A.; Pezzella, A. 5,6-Dihydroxyindole Chemistry: Unexplored Opportunities Beyond Eumelanin. Eur. J. Org. Chem. 2011, 2011, 5501-5516. 58. Tan, D.-X.; Reiter, R. J.; Manchester, L. C.; Yan, M.-t.; El-Sawi, M.; Sainz, R. M.; Juan C. Mayo; Kohen, R.; Allegra, M. C.; Hardelan, R. Chemical and Physical Properties and Potential Mechanisms: Melatonin as a Broad Spectrum Antioxidant and Free Radical Scavenger. Curr. Top. Med. Chem. 2002, 2, 181-197.


25

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

59. Bustamante, J.; Bredeston, L.; Malanga, G.; Mordoh, J. Role of Melanin as a Scavenger of Active Oxygen Species. Pigm. Cell Res. 1993, 6, 348-353. 60. Zhou, J.; Wang, P.; Wang, C.; Goh, Y. T.; Fang, Z.; Messersmith, P. B.; Duan, H. Versatile Core–Shell Nanoparticle@Metal–Organic Framework Nanohybrids: Exploiting Mussel-Inspired Polydopamine for Tailored Structural Integration. ACS Nano 2015, 9, 6951-6960. 61. Perna, G.; Frassanito, M. C.; Palazzo, G.; Gallone, A.; Mallardi, A.; Biagi, P. F.; Capozzi, V. Fluorescence Spectroscopy of Synthetic Melanin in Solution. J. Lumin. 2009, 129, 44-49. 62. Nighswander-Rempel, S. P.; Riesz, J.; Gilmore, J.; Bothma, J. P.; Meredith, P. Quantitative Fluorescence Excitation Spectra of Synthetic Eumelanin. J. Phys. Chem. B 2005, 109, 20629-20635. 63. Huijser, A.; Pezzella, A.; Sundstrom, V. Functionality of Epidermal Melanin Pigments: Current Knowledge on UV-Dissipative Mechanisms and Research Perspectives. Phys. Chem. Chem. Phys. 2011, 13, 9119-9127. 64. Nighswander-Rempel, S. P. Quantitative Fluorescence Spectra and Quantum Yield Map of Synthetic Pheomelanin. Biopolymers 2006, 82, 631-637. 65. Teuchner, K.; Ehlert, J.; Freyer, W.; Leupold, D.; Altmeyer, P.; Stücker, M.; Hoffmann, K. Fluorescence Studies of Melanin by Stepwise Two-Photon Femtosecond Laser Excitation. J. Fluoresc. 2000, 10, 275-275. 66. Brian Nofsinger, J.; Simon, J. D. Radiative Relaxation of Sepia Eumelanin is Affected by Aggregation. Photochem. Photobiol. 2001, 74, 31-37.


26

Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

67. Lackowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006. 68. 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.


27

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

Table of Contents


28

Oxidative Self-Polymerization of Dopamine in an Acidic Environment

Recommend Documents