Properties of Electropolymerized Dopamine and Its Analogues

Aug 23, 2018 - Monomers with two hydroxyl groups (except Adrenalin) resulted in films that were thicker (∼10–15 nm) than the one with only one hyd...
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Properties of Electropolymerized Dopamine and Its Analogues Shengxi Li,† Huifeng Wang,‡ Megan Young,‡ Fujian Xu,§,∥ Gang Cheng,*,‡ and Hongbo Cong*,† †

Department of Chemical and Biomolecular Engineering, University of Akron, Akron, Ohio 44325, United States Department of Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States § Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical Technology), Ministry of Education, Beijing 100029, China ∥ Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, China Langmuir Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/24/18. For personal use only.

‡

ABSTRACT: This work reports a study of electropolymerization kinetics, film thickness, stability, and antifouling properties of polydopamine (PDA) and its three analogues: poly(3-(3,4-dihydroxyphenyl)-L-alanine) (PL-DOPA), poly(5-hydroxytryptophan) (PL-5-HTP), and poly(Adrenalin) (PAdrenalin). It was observed that the number of the hydroxyl groups on the benzene ring and the type (primary vs secondary) of amine group significantly affect the electropolymerization kinetics and thus the thickness of the obtained polymer films. Monomers with two hydroxyl groups (except Adrenalin) resulted in films that were thicker (∼10−15 nm) than the one with only one hydroxyl group (PL-5-HTP) (∼5−8 nm) under similar conditions. Adrenalin containing a secondary amino group could not be deposited onto the ITO substrate, while the other three compounds containing a primary amino group completely covered the ITO. The PDA films had better electrochemical stability than the other films. No film showed stable antifouling surfaces against protein.

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INTRODUCTION Dopamine (DA) is a naturally existing molecule that plays an important role in the body.1,2 It functions as a neurotransmitter in the brain and chemical messenger outside the brain. Recent studies discovered that DA is a critical component for the extraordinary adhesive properties of mussels.3−8 Messersmith and colleagues reported that polydopamine (PDA) film can be formed from dopamine monomer (e.g., dopamine hydrochloride) under alkaline conditions (pH > 7.5) on a variety of materials surfaces, including metals, ceramics, and plastics, through a spontaneous polymerization process with oxygen as the oxidant.3 The PDA films have many advantages, such as being able to be deposited on many types of substrates and its possession of useful functional groups (e.g., catechol, amine, and imine) that can be used to design hybrid materials. Therefore, PDA has been rapidly adapted into a wide range of applications in the fields of chemical, medical, biological, and material sciences.9−11 Besides oxidation-polymerization, an alternative way to achieve polymerization of DA monomers is by electrochemical methods, such as cyclic voltammetry.12−14 As compared to self-polymerization, the electropolymerization of dopamine results in a higher deposition rate,15 which can be © XXXX American Chemical Society

achieved in acidic solutions, suitable for alkaline-sensitive materials,16 and also avoids aggregation by increasing the surface roughness.17 The PDA films obtained from the electropolymerization method have been used for surface modification of a variety of biomaterials.18−21 Besides DA, its structure analogues are expected to have extra desired properties, because the surface chemistry plays a key role in the manufacture, modification, and performance of a material or device. Despite extensive research on the polymerization of DA, there is a lack of understanding on how the molecular structure of DA and its analogues affects the electropolymerization kinetics and the properties of the resultant films. In this study, we systematically investigated the electropolymerization behavior of dopamine hydrochloride with three analogues: 3-(3,4-dihydroxyphenyl)-L-alanine (LSpecial Issue: Zwitterionic Interfaces: Concepts and Emerging Applications Received: May 2, 2018 Revised: August 3, 2018

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cross-section of the films. The thickness measurements were performed on the PDA, PL-DOPA, and PL-5-HTP films obtained using both CV and galvanostatic methods. Protein Adsorption Study. The protein adsorption property of the polymer films was studied using an electrochemical quartz crystal microbalance (eQCM) system from Gamry. The polymer films were predeposited onto the Au quartz crystal electrodes (Gamry) using the galvanostatic method. A constant current density of 20 μA/cm2 was applied to the Au electrodes until high potential values (∼1 V) were reached, indicating a full coverage of the polymer films. Mass information on the deposited films was obtained from the frequency shift. After electrodeposition, the coated Au quartz crystal electrodes were rinsed with DI water and dried under compressed Ar (99.998%) for later protein adsorption test. Prior to the protein adsorption test, all samples were immersed in PBS (pH 7.4) overnight (>12 h) to reach an equilibrium state. The protein adsorption measurement was conducted in a flow cell connected to a peristaltic pump with a flow rate of 0.1 mL/min. When a stable baseline was established in PBS flow for at least 10 min, a fibrinogen solution with a concentration of 1 mg/mL in PBS was passed through the flow cell for at least 20 min. Then, the flow solution was changed back to PBS for another 20 min. The frequency shifts (mass change) of each sample were monitored using eQCM.

DOPA), 5-hydroxytryptophan (L-5-HTP), and (±)-epinephrine hydrochloride (Adrenalin). The surface morphology, film thickness, electrochemical stability, and protein adsorption properties of all resultant polymeric films were evaluated and compared.

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EXPERIMENTAL SECTION

Chemicals. Dopamine hydrochloride and three analogues, 3-(3,4dihydroxyphenyl)-L-alanine (L-DOPA), 5-hydroxytryptophan (L-5HTP), and (±)-epinephrine hydrochloride (Adrenalin), were purchased from Sigma-Aldrich (St. Louis, MO). Figure 1 presents the chemical structures of these four compounds.

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Figure 1. Chemical structures of (a) DA, (b) L-DOPA, (c) L-5-HTP, and (d) Adrenalin.

RESULTS AND DISCUSSION The cyclic voltammograms of DA and L-DOPA show similar oxidation and reduction peaks, indicating similar electropolymerization behavior (Figure 2a−d). A major difference between these two compounds is the deposition rates of the polymer films. For the experiments with VS = 0.5 V, the anodic peak current at ∼380 mV decreased much quicker for DA (Figure 2a) than L-DOPA (Figure 2c), which can also be seen from the peak current profiles shown in Figure 3. This phenomenon indicates that the electropolymerization rate of DA was faster than that of L-DOPA. At the end of 50 cycles, the anodic current on the PDA-coated ITO surface became extremely small (Figure 2a), suggesting a nearly complete insulation of the ITO electrode by the polymer film. In contrast, an anodic current of approximately 80 μA (Figure 2c) was observed on the PL-DOPA-coated ITO surface after 50 cycles. When the switching potential was increased from 0.5 to 1.0 V, an additional oxidation peak was observed at approximately 0.8 V (Figure 2b) for DA, which suggests further oxidation of the polymeric film. The corresponding reduction peak was found at ∼0 V. The second anodic peak (∼0.8 V) also appeared in the L-DOPA case (Figure 2d), indicating similar electrochemical behavior between DA and LDOPA. For the PL-DOPA-coated ITO, the anodic current reached approximately zero after 50 cycles of CV scan with VS = 1 V (Figure 2d). Therefore, when more charge was passed through the ITO electrode (VS = 1.0 V vs VS = 0.5 V), a completely covered PL-DOPA film could form on ITO electrodes. In summary, the results shown in Figure 2a−d indicate that DA has a faster deposition rate than L-DOPA, possibly because of the lower steric hindrance of DA. Unlike DA and L-DOPA, the cyclic voltammograms of L-5HTP on ITO electrodes did not show any noticeable anodic peaks (Figure 2e,f). In addition, the anodic current was about 1 order of magnitude lower than that for PDA and PL-DOPA. Therefore, PL-5-HTP could not be readily deposited onto ITO electrodes through the electropolymerization process, and the film of PL-5-HTP was expected to be much thinner than PDA and PL-DOPA. It is hypothesized that the molecular structure of L-5-HTP plays a key role, possibly because of the presence of only one hydroxyl group on its benzene ring. While PDA,

K3[Fe(CN)6], K4[Fe(CN)6]·3H2O, KCl, KNO3, and NaCl were purchased from Fischer Scientific. Phosphate-buffered saline (PBS) and Tris(hydroxymethyl)aminomethane (Tris) were purchased from Sigma-Aldrich (St. Louis, MO). Human fibrinogen (Fg) was purchased from Calbiochem (San Diego, CA). All chemicals were used as received without further purification. Deionized (DI) water used in all experiments was purified using a Milli-Q Direct-Q3 Ultrapure Water system (Millipore, Billerica, MA) with a resistivity of 18.2 MΩ·cm. Electropolymerization. Both cyclic voltammetry (CV) and galvanostatic methods were used for electropolymerization in a three-electrode electrochemical cell with a Gamry Interface 1000E potentiostat. The potential values were measured against a silver/ silver chloride (Ag/AgCl in 3 M KCl) reference electrode, and a platinum mesh was used as counter electrode. The electropolymerization experiments were conducted in 5 mM of the four compounds (Figure 1) in Tris-buffered saline (TBS) solution (25 mM Tris, 140 mM NaCl, and 3 mM KCl at pH 7.4).13 During the electropolymerization process, deoxygenation was achieved by purging pure Ar gas (99.998%) before (1 h) and during the experiments. In the CV method, the electrodeposition was studied on ITO electrodes (∼1 cm2) by successive sweeps of 50 cycles. The CV scans started at −0.5 V, scanned anodically to a switching potential (VS, + 0.5 V and +1.0 V) and then reversed cathodically to −0.5 V at the scan rate of 20 mV/s. In the galvanostatic method, a constant current density of 20 μA/cm2 (iapplied) was applied to ITO electrodes (∼1 cm2) until high potential values (∼2 V) were reached, indicating a full coverage of the polymer films. Electrochemical Stability Study. The electrochemical stability of the polymer films was studied with CV scans (5000 cycles) between −0.5 and 1.2 V using the same electrochemical setup. The electrolyte was 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] and 0.5 M KNO3.13 Before and after the stability tests, electrochemical impedance spectroscopy (EIS) was performed at the open-circuit potential with 10 mV amplitude AC perturbation in the frequency range from 10−2 to 105 Hz. This was done to compare the conductivity/resistivity and thus the relative thickness (quantitatively) of the polymer films. Morphology Characterization. Atomic force microscopy (AFM, Bruker Multimode 8) was used to characterize the surface morphology and film thickness. For thickness measurements, the polymer films were electrochemically deposited onto ITO electrodes with glass substrates and then scratched using a scalpel to reveal the B

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believed to be caused by the secondary amino group of Adrenalin. Film thickness is one of the key parameters that determine the stability and durability of polymeric materials. The thicknesses of the polymeric films deposited using the CV method (Figure 2) were obtained through AFM measurements and are presented in Figure 4. At VS = 0.5 V, the film thickness

Figure 4. AFM measurements of the thickness of the polymeric films (PDA, PL-DOPA, and PL-5-HTP) obtained using the CV method (50 cycles) with different switching potentials (0.5 and 1 V).

was 16.6 ± 1.0, 14.3 ± 0.4, and 7.4 ± 0.2 nm for PDA, PLDOPA, and PL-5-HTP, respectively. At higher switching potential of VS = 1 V, the corresponding film thickness increased to 21.5 ± 0.5, 15.3 ± 0.3, and 8.2 ± 0.7 nm. The PDA films are thicker than the PL-DOPA films for both conditions, while the PL-5-HTP films were much thinner than the PDA and PL-DOPA films. This agrees with the CV results shown in Figure 2. Figure 5 presents the AFM images of the surfaces of the polymeric films. The PDA films had relatively higher surface roughness compared to the PL-DOPA and PL5-HTP films, probably because of its larger thickness. For all films, surface roughness increased slightly when the switching potential increased from 0.5 to 1 V. Before the electrochemical stability test (CV), EIS spectra were obtained in 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] and 0.5 M KNO3 solution. Figure 6a suggests that the film resistance increased with switching potential: 4.61 × 106 Ω·cm2 (VS = 1 V) versus 2.42 × 105 Ω·cm2 (VS = 0.5 V) for PDA, and 7.37 × 105 Ω·cm2 (VS = 1 V) versus 5.65 × 104 Ω·cm2 (VS = 0.5 V) for PL-DOPA. Under the assumption of uniform conductivity throughout the thickness of the film, higher resistance can be interpreted as higher thickness, as confirmed by AFM measurements (Figure 4). In addition, the PDA films had larger resistance than the PL-DOPA films for both cases, which might suggest that the PDA films were thicker than the PLDOPA films when they were deposited under the same conditions. It should be noted that the resistance of the PLDOPA film obtained with larger CV scan range (VS = 1 V) is of the same order of magnitude compared to the PDA film obtained with smaller scan range (VS = 0.5 V). This comparison suggests that although the deposition rate of PLDOPA was lower than that of PDA, the passage of sufficient charge could have resulted in comparable film thickness. Different from the EIS spectra of the PDA-coated and PLDOPA-coated ITO electrodes, PL-5-HTP-coated ITO electrode had an inductive loop in the low-frequency range (Figure 6a), which was possibly caused by the adsorption of [Fe(CN)6]3− and/or [Fe(CN)6]4−.

Figure 2. Cyclic voltammograms of electropolymerization of PDA (a and b), PL-DOPA (c and d), PL-5-HTP (e and f), and PAdrenalin (g and h) onto ITO electrodes (5 mM in TBS with deoxygenation). Two different switching potentials were investigated, VS = 0.5 V (a, c, e, and g) and VS = 1.0 V (b, d, f, and h). CV cycle numbers are given in the legend.

Figure 3. Anodic peak current values obtained from the CV graphs at approximately 380 mV shown in Figure 2a,c,e as a function of CV cycles (switching potential VS = 0.5 V).

PL-DOPA, and PL-5-HTP films could be deposited on ITO surfaces, no or negligible PAdrenalin film formed on ITO electrodes, as verified by the relative high anodic current (100−200 μA) even after 50 cycles (Figure 2g,h). This result is C

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Figure 5. AFM images of the polymeric films (PDA, PL-DOPA, and PL-5-HTP) obtained using the CV method (50 cycles) with different switching potentials (0.5 and 1 V). The 50 nm scale bar applies to all images.

Figure 6b shows the stability test results of the PDA, PLDOPA, and PL-5-HTP polymer films. The anodic peak currents at ∼400 mV were plotted against the number of CV cycles, up to 5000. The results indicate that the PDA films were more stable than PL-DOPA films, while the PL-5-HTP film had the least stability, possibly because of its least thickness. In addition, for both PDA and PL-DOPA, the films obtained using a larger CV scan range (VS = 1 V) had better stability than those obtained using a smaller scan range (VS = 0.5 V). The EIS spectra obtained after the stability test further confirmed these findings (Figure 6c). The PDA film obtained at VS = 1 V had the largest resistance, while the PL-5-HTP film (VS = 1 V) and the thinner PL-DOPA film (VS = 0.5 V) had EIS spectra similar to that of a bare ITO electrode. By comparison of the EIS spectra before (Figure 6a) and after (Figure 6c), the resistance of all films decreased after the stability test, as expected. The stability tests suggest that the PDA and PL-DOPA films with larger resistance had better stability, possibly because of the denser and thicker coverage. The electrodeposition of PDA, PL-DOPA, PL-5-HTP, and PAdrenalin films on ITO substrate was also conducted using the galvanostatic method by applying a constant current density of 20 μA/cm2. The potential evolution in Figure 7a shows that the four compounds behaved differently in the electrodeposition process. Films with nearly complete electrical insulation, as evidenced by the high potential of the electrodes (i.e., ∼2 V), could form on the ITO surfaces in L-5-HTP, DA,

Figure 6. EIS spectra of the PDA, PL-DOPA, and PL-5-HTP polymer film-coated ITO electrodes before (a) and after (c) stability tests. The films were deposited using the CV method with different switching potentials (0.5 and 1 V). (b) Anodic current values (at ∼400 mV) as a function of CV cycles during the stability tests.

and L-DOPA solutions but not in Adrenalin solution even after 24 h. In addition, the potential of the ITO electrode rose quickly ( PDA > PL-5-HTP. The AFM measurements of the film thickness of these three polymeric films confirmed this trend (Figure 7b). The PLDOPA film (17.8 ± 0.4 nm) was close to, but slightly thicker than the PDA film (15.7 ± 0.6 nm). In contrast, the PL-5-HTP film thickness was only 5.1 ± 0.8 nm. The AFM images of the polymeric films (Figure 7c) indicate that all obtained films were relatively uniform and the surface roughness increased with increasing film thickness. A comparison between Figure 7b and Figure 4 suggests that the film thickness is comparable between PDA and PL-DOPA films; however, the electrochemical method and its parameters may have some influence D

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Figure 7. (a) Comparison of the potential profiles of the ITO electrodes during the electrodeposition of PDA, PL-DOPA, PL-5HTP, and PAdrenalin polymer films using the galvanostatic method (iapplied = 20 μA/cm2); (b) thickness and (c) surface morphology of the PDA, PL-DOPA, and PL-5-HTP films by AFM measurements. The 50 nm scale bar applies to all images in part c.

on the final film thickness. In contrast, the PL-5-HTP film thickness is the smallest regardless of the method used, which suggests the possible effect of molecular structures. The polymer films obtained using the galvanostatic method were also tested in solutions with 5 mM K3[Fe(CN)6]/ K4[Fe(CN)6] and 0.5 M KNO3 for stability evaluation. The EIS data obtained before the stability test (Figure 8a) show that the PDA film had a larger resistance than the PL-DOPA film, while the PL-5-HTP film had an extremely small film resistance. Figure 8b indicates that the PDA film had a better stability than the PL-DOPA film, while the PL-5-HTP film had the worst stability. Therefore, the films with larger film resistance showed better stability during the CV tests. The EIS spectra of the films after stability tests (Figure 8c) indicate a trend similar to that observed in Figure 8b. A comparison between Figures 6 and 8 suggests the same trend of electrochemical stability (PDA > PL-DOPA > PL-5- HTP) regardless of the electrochemical methods used (CV vs galvanostatic). The electropolymerization was also conducted on the Au quartz crystal electrodes (eQCM) using the galvanostatic method (iapplied = 20 μA/cm2) to obtain the mass information on the PDA, PL-DOPA, and PL-5-HTP films. Figure 9a shows the potential profiles of the Au electrodes during electropolymerization and demonstrates the increase of potential upon a nearly complete coverage of the polymer films. The potential profiles show similarity to those in Figure 7a, and the time to reach high potentials increases in the order PL-5-HTP < PDA < PL-DOPA. However, the maximum potential reached on the Au substrate (∼1 V) was much lower than that on the ITO substrate (∼2 V), suggesting the effect of substrate materials. The obtained frequency shift data was converted to mass gain using the Sauerbrey equation, Δf = Δm·Cf, where the calibration factor, Cf, equals 226 Hz·cm2/μg for the 10 MHz AT-cut quartz crystal.22 The calculated mass gain results (Figure 9b) show that the mass of the resultant films was in the order PL-DOPA > PDA > PL-5-HTP, similar to the AFM thickness measurements (Figure 7b). It should be noted that the largest thickness of the PL-DOPA film was

Figure 8. EIS spectra of the PDA, PL-DOPA, and PL-5-HTP polymer film-coated ITO electrodes before (a) and after (c) stability tests. The films were deposited using the galvanostatic method (iapplied = 20 μA/ cm2). (b) Anodic current values (at ∼400 mV) as a function of CV cycles during the stability tests.

Figure 9. (a) Potential profiles of the Au quartz crystal electrodes during the electropolymerization process (PDA, PL-DOPA, and PL-5HTP) using the galvanostatic method (iapplied = 20 μA/cm2); (b) the corresponding mass gain profiles of the Au quartz crystal electrodes.

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only one hydroxyl group (i.e., PL-5-HTP) under the same conditions. The amino group (primary vs secondary) was also found to influence the electrochemical deposition behavior of the chemicals. The Adrenalin containing the secondary amino group could not be deposited onto the ITO substrate, while the other three compounds containing a primary amino group covered the ITO surface completely. The PDA films had better electrochemical stability than the PL-DOPA films obtained under the same conditions. Furthermore, the PDA film had a fouling surface toward protein, while the electrodeposited PLDOPA films exhibited both fouling and antifouling surfaces. The PL-5-HTP film showed an unstable protein adsorption. The results of this study may potentially provide guidance for the selection of catechol compounds for surface modification purposes and lay the foundation for developing more robust surface modification processes of catechol compounds.

attributed to the largest amount of charge, resulting from the longest polymerization time. However, the PDA film had the fastest deposition rate as indicated by the largest slope of its mass gain curve shown in Figure 9b. This result agrees with the anodic peak current trend shown in Figure 3 for deposition using the CV method. The Au quartz crystal electrodes with predeposited PDA, PL-DOPA, and PL-5-HTP films were then subjected to the protein adsorption study. Figure 10 shows the frequency

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: (+01) 330-972-8491. Fax: (+01) 330-972-5856. *E-mail: [email protected]. ORCID

Fujian Xu: 0000-0002-1838-8811 Gang Cheng: 0000-0002-7170-8968 Hongbo Cong: 0000-0001-5263-6623

Figure 10. Fibrinogen adsorption measurements on the Au crystal quartz electrodes (eQCM) with predeposited polymer films using the galvanostatic method (iapplied = 20 μA/cm2).

Notes

The authors declare no competing financial interest.

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change results by fibrinogen adsorption. Because the change in mass is proportional to the frequency change, the PDA film exhibited a typical fouling surface. The frequency change occurred rapidly after the PDA film was in contact with the protein and finally stabilized at around −620 Hz. The PLDOPA films in one case showed a typical antifouling surface because the frequency change of the PL-DOPA-coated Au electrode was only −20 Hz after 3000 s (not shown). However, repeated experiments (>3) indicated that the PLDOPA films had fouling surfaces (−660 Hz in Figure 10). Different from the PDA and PL-DOPA films, the PL-5-HTPcoated Au electrode did not show a stable mass increase even after 5000 s. Instead, the frequency change of this electrode kept fluctuating with a magnitude up to −190 Hz, which indicates its unstable antifouling feature or incomplete/uneven surface coverage. The results suggest that PL-DOPA and PL-5HTP, in their zwitterionic state, may have resistance to nonspecific protein adsorption. The zwitterionic nature of these two compounds can be attributed to the presence of both amine groups and carboxyl groups in their structures.23−28 Because of the excellent antifouling properties and biocompatibility of these zwitterionic materials, they have been considered as a new generation of antifouling materials for many biomedical applications.23,29−32

ACKNOWLEDGMENTS This work is supported by U.S. National Science Foundation (DMR-1454837, DMR-1206923) and Natural Science Foundation of China (NSFC- 51528301).

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REFERENCES

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CONCLUSIONS In this study, we systematically investigated the effect of molecular structure on the electropolymerization kinetics and film properties of PDA, PL-DOPA, PL-5-HTP, and PAdrenalin. We discovered that the number of hydroxyl groups on the benzene ring significantly affected the kinetics of the electropolymerization process and thus the thickness of the obtained polymer films. Chemicals with two hydroxyl groups (except Adrenalin) resulted in thicker films than those with F

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DOI: 10.1021/acs.langmuir.8b01444 Langmuir XXXX, XXX, XXX−XXX