Article pubs.acs.org/Langmuir
Oxidative Self-Polymerization of Dopamine in an Acidic Environment Weichao Zheng, Hailong Fan, Le Wang, and Zhaoxia Jin* Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China
Langmuir 2015.31:11671-11677. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/05/18. For personal use only.
S Supporting Information *
ABSTRACT: A weak alkaline condition (pH > 8) is a general requirement for oxidative self-polymerization of dopamine. Here, we first demonstrated the generation of polydopamine in an acidic environment via a hydrothermal method. The pH scope of self-polymerization of dopamine is extended to pH ∼ 1 in a hydrothermal process. Polydopamine generated via a hydrothermal method shows similar chemical features and radical scavenging activity with that generated in a basic environment.
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7).40 Herein, for the first time, we demonstrated that the oxidative self-polymerization of dopamine happens in a strong acidic condition (pH ∼ 1) via the hydrothermal process. This study is not only important for understanding the formation mechanism of PDA but also critical for extending wider application for PDA. Particularly, because the hydrothermal process is a useful method to generate inorganic nanostructures, a one-pot synthesis of the inorganic/PDA hybrid nanostructure is highly desirable. The existence of a PDAcoating layer will benefit the post-functionalization of obtained nanostructures.
INTRODUCTION In nature, extensive kinds of organic carbon are generated through the 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 origination.4−9 However, as a fabrication process, the hydrothermal path is usually used in synthesis of inorganic nanomaterials.10−13 Few studies report the synthesis of organic materials via the hydrothermal route, which may be 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 the hydrothermal route. Recently, the study of polydopamine (PDA), a kind of natural melanin-like material, attracts extensive attention because of its unique photophysical properties,20 versatile adhesiveness, and multifunctional coating abilities for a broad range of applications in biomedical materials,21−25 electrochemistry,26 nanotechnology,27,28 membrane application,29 biooptoelectric materials,30 and solar energy.31 Their photophysical properties are related to their heterogenerous molecular structures and complex aggregated structures.32 The mussel-inspired PDA coating has found great application in biomaterials.33−36 The exploration in synthesis methods and formation mechanism of PDA will benefit the controllable fabrication of PDA with manipulated functional properties in the future.37 In general, dopamine is stable toward oxidation in acidic media in the complete absence of metal ions.38 In most reported synthesis of PDA, a weak alkaline condition (pH > 8) is a fundamental requirement, except in some synthesis routes using specific oxidants, such as enzymes, CuSO4, (NH4)2S2O8, NaIO4, and KClO3.39,40 However, the pH scope in these experiments is still weakly acidic (pH ∼ 4.5) or neutral (pH ∼ © 2015 American Chemical Society
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EXPERIMENTAL SECTION
Materials. Dopamine hydrochloride (98% purity) 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 using Millipore water. Fabrication of PDA via the Hydrothermal Method. In a typical synthesis, 5.0 or 45 mg dopamine hydrochloride was dissolved in 10 mL of water (pH of dopamine solution is about 5) or hydrochloric acid solution (pH was adjusted to 3 or 1). The above-mentioned solution was kept in a Teflon-lined autoclave at 120 or 160 °C for 16 h. After reaction, the autoclave was cooled to room temperature. The produced dark suspension was washed by deionized water several times, collected by an ultracentrifuge at 15 000 rpm for 10 min, and finally dried in a freeze dryer. 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@HPDA nanoparticles, 20 μL of HAuCl4 solution (1 mM) was added in the above-mentioned aqueous suspension of H-PDA nanoparticles (2 mL) and then the mixture was stirred for 1 min and incubated for 20 h at 1 °C. After reaction, the mixture was centrifuged at 15 000 rpm for 5 min and the sediment was washed by deionized water several times. Clean gold@H-PDA nanoparticles were characterized by scanning electron microscopy (SEM). Received: July 26, 2015 Revised: September 30, 2015 Published: October 6, 2015 11671
DOI: 10.1021/acs.langmuir.5b02757 Langmuir 2015, 31, 11671−11677
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Langmuir
Figure 1. SEM images of H-PDA products in various experimental conditions: (a) pH 1, 160 °C, and 0.5 mg/mL, (b) pH 1, 160 °C, and 4.5 mg/ mL, (c) pH 5, 120 °C, and 0.5 mg/mL, and (d) pH 5, 160 °C, and 0.5 mg/mL. 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). XPS was performed on an imaging photoelectron spectrometer (Axis Ultra, Kratos Analytical, Ltd.) with monochromatic Al Kα (hν = 1486.7 eV) radiation as the excitation and X-ray power of 150 W. All spectra were calibrated using the hydrocarbon C 1s peak (284.6 eV). Highresolution scans were acquired for C 1s and N 1s regions. The peakfitting procedure is consistent with the best fit with consideration of the peak position, full width at half maximum, and intensity. Ultraviolet−visible (UV−vis) absorption of nanoparticle suspension was measured using a Shimadzu UV-3600 spectrophotometer. Fluorescence spectra were measured using a HITACHI F-4600 fluorescence spectrophotometer. ζ potential was measured using a MALVERN Zetasizer Nano ZS90. Measurements were performed 3 times for each sample at 25 °C. 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 using a JEM2100 (JEOL) at 200 kV. Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (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). High-performance liquid chromatography (HPLC)-grade acetonitrile and trifluoroacetic acid (TFA) were purchased from Fisher Scientific (Waltham, MA). Triple deionized water was prepared from a Millipore water purification system (Millipore, Billerica, MA) 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% trifluoroacetic
acid (TFA). A total of 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 MALDITOF 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 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. qNano analyzer (Izon Sciences, Christchurch, New Zealand) was used for microsphere 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 were analyzed using Izon Control Suite 3.0 software. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) Assay for Antioxidant Activities of H-PDA Nanoparticles. The antioxidant activities of HPDA nanoparticles were measured by DPPH assays as described in the literature.41 Fresh DPPH/ethanol (0.1 mM) solution was used in measurements. A total of 180 μL of different concentrations of H-PDA suspensions was added to 4 mL of DPPH. The amount of H-PDA in the test was changed from 5 to 240 μg. The scavenging activity was 11672
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Langmuir evaluated by measuring the absorbance change at 520 nm after the tested mixture was kept in the dark for 20 min.
nanoparticles, such a morphology may suggest that it is the result of a hindered reaction. Further characterizations by UV−vis, FTIR, and XPS revealed the chemical nature of H-PDA products (Figure 2
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RESULTS AND DISCUSSION In a typical synthesis, 5.0 mg of dopamine hydrochloride was dissolved in 10 mL of water and kept in a Teflon-lined autoclave at 120 or 160 °C for 16 h. The pH of original dopamine solution is about 5. After reaction, the autoclave was cooled to room temperature. The produced dark suspension was washed by deionized water several times, collected by ultracentrifuge at 15 000 rpm for 20 min, and finally dried in a freeze dryer. This synthesis was also conducted in an acidic environment, in which HCl was added in dopamine solution to adjust pH to 1 or 3. Figure 1 show morphological features of HPDA fabricated at pH 1 and 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 Table 1. Summary of PDA Synthesized via the Hydrothermal Method in Various Experimental Conditionsa 120 °C pH 1
pH 3
pH 5
a
160 °C
0.5 mg/mL
no product
0.5 mg/mL
4.5 mg/mL
no product
4.5 mg/mL
0.5 mg/mL
microspheres
0.5 mg/mL
4.5 mg/mL
microspheres
4.5 mg/mL
0.5 mg/mL
microspheres
0.5 mg/mL
4.5 mg/mL
aggregation of microspheres
4.5 mg/mL
aggregation of nanoparticles aggregation of nanoparticles aggregation of microspheres aggregation of microspheres aggregation of microspheres with nanoparticles aggregation of microspheres with nanoparticles
Figure 2. Characterizations of H-PDA products: (a) UV−vis absorbance spectrum, (b) FTIR spectrum, (c) ζ potential, and (d) free radical scavenging effect. The sample was produced at 160 °C for 16 h, pH 5, and the concentration of dopamine of 4.5 mg/mL.
and Figures S5−S8). UV−vis absorption spectrum of H-PDA presents the featureless broadband absorption (Figure 2a and Figure S5) that agrees with other melanin-like PDA. In the FTIR spectrum of H-PDA (Figure 2b and Figure S6), the broad peak at 3433 cm−1 is due to the stretching of N−H and O−H. A strong peak at 1635 cm−1 and shoulder peak at 1554 cm−1 can be assigned to aromatic CC bonds of indole and ring(CN).43,44 The FTIR spectrum of H-PDA confirms that its chemical structure is similar to that generated in a basic environment.45 XPS characterization further helps us to identify the functional groups of H-PDA (Figures S7 and S8). In these H-PDA products, the N/C ratio is 0.09, which falls into the N/ C range reported in the literature (0.08−0.17).45−47 Detailed analyses of XPS spectra of high-resolution C 1s and N 1s of HPDA (pH 1) and H-PDA (pH 5) are presented in Figures S7 and S8 and Table 2. The C 1s 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), CO/CN (6.07% of pH 1 and 11.77% of pH 5), and π → π* (0.55% of pH 1 and 3.82% of pH 5). A significant amount of CO can be assigned to the tautomers of catechole units.45 The π → π* species is related to the intra- and intermolecular responses of the electronic system upon the excitation process.48 This species shows a clear difference for samples generated in different pH values, indicating that the intra- and intermolecular responses of H-PDA (pH 1) in the excitation process are limited as a result of their generation in a high acidic environment. The N 1s 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 PDA,44 which confirms the chemical nature of H-PDA. The tertiary/
The reaction times were kept for 16 h in listed experiments.
product in low or high concentration of dopamine at 120 °C for 16 h. However, the increase of the temperature significantly accelerates the oxidative polymerization of dopamine, and HPDA 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 (panels a and b of Figure 1), 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 shown an 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 to the products of PDA fabricated in a basic condition.42 On the basis of these studies, we observed that, although a high acidic environment (pH 1) goes against the formation of PDA in a hydrothermal process, a higher temperature (160 °C) and an extension of the reaction time (from 16 to 36 h) will benefit the generation of a 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 H-PDA obtained at pH 1 is the aggregate of 11673
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Langmuir Table 2. XPS Functional Group Percentages of H-PDA Obtained in Different pH Valuesa C 1s
pH 1 pH 5 a
N 1s
C−C/CC/C−NH2
C−O/C−N
CO/CN
π → π*
R−NH2
R1−NH−RH2
CN−R
284.6 eV (%)
286 eV (%)
287.7 eV (%)
290.0 eV (%)
401.8 eV (%)
399.9 eV (%)
398.6 eV (%)
67.16 62.5
26.22 21.91
6.07 11.77
0.55 3.82
19.28 18.43
67.99 68.26
12.73 13.31
The samples for XPS were produced at 160 °C for 16 h, pH 1 or 5, and the concentration of dopamine of 4.5 mg/mL.
functional groups have a reductive capacity similar to that of a normal PDA layer.47,60 In addition, we characterized the detailed structure of carbonized H-PDA particles using HRTEM. H-PDA particles were heated in a N2 environment from room temperature to 750 °C with a heating rate of 5 °C/min and then kept at 750 °C for 1 h. Figure 3 presents HRTEM images of carbonized H-
aromatic amines, assigned to the intermediate species 5,6dihyroxyindole and 5,6-indolequinone, are 12.73 and 13.31% in H-PDA fabricated at pH 1 and 5, respectively. They are in agreement with that reported in the literature (13.6%).49 Dopamine may also aggregate with itself or oligomers of PDA via non-covalent interactions,50 giving a significant amount of primary amine in the XPS result. Thermogravimetric analysis (TGA) and MALDI-TOF mass spectroscopy of H-PDA presented in Figures S9 and S10 also supported that H-PDA is basically similar to that generated in a basic condition.51,52 Qu et al. have reported the carbon dots prepared by hydrothermal treatment of dopamine.53 Because the temperature that they used is higher (180 °C) than what we used, their main product is carbon dots (3.8 nm in diameter). In summary, dopamine hydrochloride can be converted to H-PDA in a Teflon-lined autoclave at a temperature range of 120−160 °C for 16 h without significant carbonization and the pH scope is extended to 1, which means that 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 ζ potential values.51,54−56 Bernsmann et al. have measured the ζ 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 (their dimers have pKa values between 9 and 13) groups.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 the hydrothermal route shows a different isoelectric point that is between 2 and 3 (Figure 2c). It indicates that functional groups on the H-PDA surface may be different compared to that generated in an 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 an acidic environment.57 The generation of different functional groups on H-PDA in strong or weak acidic hydrothermal processes is possible. Melanin-like PDA shows a good free radical scavenging effect.41,51,58,59 To further confirm that H-PDA has chemical properties similar to PDA, we characterized its free radical scavenging activities using the DPPH assay. The scavenging activity (Figure 2d) was compared to that of ascorbic acid (AA). The efficient concentration (EC50) value, at which the initial DPPH concentration is dropped to 50%, is 33 μg for HPDA, which is comparable to PDA generated in a 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 a H-PDA surface to gold nanoparticles, showing that H-PDA surface
Figure 3. (a and b) HRTEM images of carbonized H-PDA particles. The layer distance is 0.35 nm. The sample was produced at 160 °C for 16 h, pH 5, and the concentration of dopamine of 4.5 mg/mL.
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 15 000 rpm still shows a yellow or brown color depending upon the concentration of dopamine used in the hydrothermal process (Figure 4a). It may contain unreacted dopamine and some oligomers in the hydrothermal process, which are not large enough to aggregate to solid particles. Previous studies
Figure 4. Characterizations of the supernatant of product suspensions after centrifugation. (a) UV−vis absorbance spectrum of the supernatant. The picture inset is the supernatant solution. (b) Fluorescence spectra of the supernatant at various excitation wavelengths. 11674
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Langmuir 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 supernatants and hope to learn something about small oligomers. Figure 4a presents the absorbance spectrum of the supernatant (after the hydrothermal process, pH 5, concentration of dopamine of 4.5 mg/mL, and 160 °C for 16 h). Its fluorescence spectra at different excitation wavelengths are shown in Figure 4b. The oligomers and polymers of melanins have weak fluorescence (from 450 to 600 nm) based on the literature. The supernatant of the hydrothermal product shows an asymmetric emission peak at ∼500 nm, which is similar to synthetic eumelanin and Sepia ink eumelanin.65,66 However, carbon dots generated from the hydrothermal process of dopamine show an emission wavelength from 380 to 530 nm.53 The characteristic emission of the 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 to 570 nm depending upon the different sources and processes. Because molecular structures in H-PDA are different from that of eumelanin, excitation wavelengths in our case (290−370 nm) are unlike theirs.65 MALDI-TOF mass spectrum of this supernatant also confirms the existence of mixed oligomers, such as trimers, tetramers, and pentamers (Figure S14). Because these small oligomers are soluble in water, the solid sediment after centrifugation may contain larger oligomers, although the MALDI-TOF mass spectrum of the H-PDA solid product presented only small oligomers (Figure S10). A further investigation using the mass spectrum will reveal deeper structural information on H-PDA. The above experimental investigation demonstrates that HPDA is similar to that fabricated in a basic environment. Why can dopamine be polymerized to H-PDA via a hydrothermal process in acidic conditions? We thought the following reasons contribute to the successful production of H-PDA. First of all, a higher temperature accelerates the generation of PDA. Second, water at elevated temperatures and high pressures is a special medium for chemical reaction. The autoionization of water shows different equilibrium constants at high temperatures and high pressures. Self-ionization of water increases with increasing temperature and pressure; thus, the concentrations of hydronium and hydroxyl ions are higher in hydrothermal conditions. In the proposed formation mechanism of PDA, the oxidative polymerization of dopamine starts with the transformation of catechol moieties into the corresponding quinone moieties via an oxidation reaction. In these cascade reactions, the first step from dopamine to dopamine quinone via a two-electron oxidation process (removing two H+ and two electrons) requires a basic environment. More hydroxyl groups will assist the departure of hydrogen from dopamine molecules. Thus, the generation of oxidative polymerization of dopamine in a hydrothermal environment is somehow reasonable. On the other hand, the radical pathways may also involve in the oxidation and cyclization processes.68 Herlinger and co-workers have proposed that the reaction from dopaminonchrome to polymeric melanin may require a catalytic amount of OH• free radicals.38 A hydrothermal environment may benefit the generation of OH• free radicals, thus further promoting the polymerization of dopamine. Although we still have not yet fully understood the formation mechanism of PDA in a strong acidic environment (pH ∼ 1), the knowledge of different
formation processes of PDA will help us understand the construction of multi-level structures of PDA. In particular, it will also contribute to recognizing reactivity of organic molecules in a hydrothermal environment, which is somehow meaningful to know the origin of life.
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CONCLUSION In conclusion, we demonstrated a synthesis path of PDA via the hydrothermal process. For the first time, the pH value for selfpolymerization of dopamine is extended to a strong acidic environment (pH ∼ 1) without using special oxidants. The chemical property of PDA generated via the hydrothermal method is similar to that fabricated in a basic environment, which provides a good opportunity to synthesize the inorganic/ PDA hybrid nanostructure via the one-pot hydrothermal process.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02757. Supplementary figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 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. The authors thank Dr. Y. Liu (Peking University) for HRTEM characterization.
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REFERENCES
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