Bioinspired Polydopamine Synthesis and Its Electrochemical

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Bioinspired Polydopamine Synthesis and Its Electrochemical Characterization María T. Corteś ,*,† Christian Vargas,† Diego A. Blanco,‡ Ingrid D. Quinchanegua,‡ Cristian Corteś ,†,‡ and Andres M. Jaramillo† †

Department of Chemistry, Universidad de los Andes, Bogotá 111711, Colombia Universidad Pedagógica Nacional, Bogotá 110221, Colombia

‡

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S Supporting Information *

ABSTRACT: The bioinspired polymer polydopamine is synthesized and deposited on semiconductive glass substrates under different synthesis conditions (pH and oxidant type). Electrochemical methods are used to verify polymer deposition on the substrate and to examine the charge transfer properties of the obtained coatings. The presence of aromatic rings from the polymer is assessed by Raman spectroscopy. The experiment was designed to fit within a physical chemistry course and can also be suitable for courses on polymers, materials science, and electrochemistry. KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Interdisciplinary/Multidisciplinary, Polymerization, Laboratory Instruction, Electrochemistry, Surface Science

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was initially studied in the 1960s.5 It was Messersmith et al. in 2007 who observed the mechanism of mussels’ strong adhesion to solid surfaces and later proposed polydopamine as a biomimicry high adhesion polymer.6 This observation revived the interest of the scientific community on the study of polydopamine properties. The presence of catechol groups in this bioinspired material favors immobilization, regulation, and detection properties of biomolecules. In addition, PDA is also useful for the incorporation of properties such as high hydrophobicity and/or electric resistivity in hybrid materials.4 In the experiment described here, polydopamine is synthesized by chemical oxidation of dopamine with three different oxidants (ammonium persulfate, oxygen, and sodium percarbonate). The effects of pH on the polymerization process are also tested. The PDA is obtained as a deposit on fluorine-doped tin oxide (FTO)-coated glass plates, which allows evaluation of its electrochemical properties and electric resistivity. The presence of the polymer is confirmed by Raman spectroscopy.

INTRODUCTION Materials that exhibit multiple desired properties can be designed from the combination of individual materials. In this line of work, properties such as hardness, electrical conductivity, and thermal resistance from a substrate material can be combined with additional desired properties like adhesion, electrical isolation, or chemical selectivity by modification of the substrate surface with another material.1,2 For example, the negatively charged polymer Nafion deposited on carbon fibers allows chemical selectivity of dopamine in the presence of ascorbic acid when using electrochemical detection in vivo. In this case, the electrical properties and small diameter of the fiber are combined with the anion exchange capacity of the polymer to allow selective dopamine detection in the brain of a mouse.3 Polydopamine (PDA) is a bioinspired polymeric material that has gained attention for surface modification due to its chemical properties (strong adhesion to many substrates, mild electrical conductivity at some synthesis conditions and electric isolation when synthesized at other conditions, hydrophobicity and amide chemistry, among others). Over the past 10 years, polydopamine has proven useful in many research fields, and in applications such as cancer therapy, wettability modulation, adhesion, catalysis, batteries, antimicrobial properties, capacitors, water treatment, and anticorrosion systems.4 Yet, its discovery is traced back to the beginning of the 20th century with the observation of neuromelanin (biologically formed PDA polymer) deposits in dopaminergic cells from Parkinson’s patients and the mechanism of neuromelanin formation which © XXXX American Chemical Society and Division of Chemical Education, Inc.

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LEARNING GOALS This experiment is appropriate for visualizing and understanding some general concepts on surface modification, catecholamine chemistry, surface electrochemical characterization, oxidation mediated polymerization, and bioinspired Received: June 14, 2018 Revised: April 16, 2019

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DOI: 10.1021/acs.jchemed.8b00432 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Proposed building blocks needed for PDA formation. PDA building blocks formed from dopamine oxidation and successive cyclization reactions and isomerizations (dopaminechrome and DHI). Note the UV−vis properties of them: two of the compounds do have color, and the polymer itself is black.4

materials. It was designed to fit within a physical chemistry course as well as courses on material science, polymers, biomaterials, and electrochemistry. The experiment reinforces key concepts related to the redox chemistry of dopamine, interfacial charge transfer, and pH-dependence of chemical reactions.

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EXPERIMENT OVERVIEW

Background

Polydopamine. We describe the synthesis of polydopamine (PDA) by chemical oxidation of dopamine using different oxidants at buffered pH solutions. To obtain a PDA film on a substrate, the concentration of dopamine must be higher than 2 mg/mL.7 The pH is also an important factor since the first step of polymerization involves the formation of dopaminochrome, the formation of which is favored at basic pH values.8 The chemical nature of the buffer and the type of chemical oxidant also contribute to the characteristics of PDA.4 When dopamine is added into an alkaline solution in the presence of an oxidant, the polymerization of dopamine oxidation products occurs, and a color change is observed from colorless to deep brown passing through orange, red, and burgundy colors which corresponds to the formation of different compounds such as the dopaminechrome (Figures 1 and 2). If a substrate is immersed into this solution, the surface is coated by PDA,6 and the coating thickness will depend on the concentration of dopamine and the polymerization time.7 Despite the fact that PDA can be produced in a simple process, the molecular mechanism behind its formation remains unknown due to a complex redox polymerization.4 However, there is little doubt that the initial driving force for PDA formation is the oxidation of dopamine at alkaline pH to produce dopamine-quinone. This compound undergoes intramolecular cyclization via a 1,4 Michael-type addition to yield leucodopaminechrome, which also suffers oxidation and rearrangement to form 5,6-dihydroxyindole (DHI)8 (Figure 1). In most existing theories of PDA formation, dopaminequinone and DHI are building blocks for PDA, and from them, various reaction pathways have been proposed.4 Vechia et al. proposed a model for the formation of PDA, which suggests that both covalent and noncovalent bond interactions occur and that many functional groups including planar indole units, amino groups, carboxylic acid groups, catechol or quinone functions, and indolic/catecholic π-

Figure 2. Dopamine oxidation with ammonium persulfate and PDA deposition on FTO glass. Dopamine (26.4 mM) was oxidized with ammonium persulfate (52.8 mM) in Tris buffer pH = 10. Top: Photographs of the DA solution were collected at times 0, 15, 45, 75, and 140 s after addition of the Tris buffer with the oxidant. Notice the changes in coloration which evidence the formation of dopaminechrome and other intermediates. Bottom: PDA-coated FTO glass after 24 h. Sodium persulfate was used as oxidant, and the pH was varied in the synthesis. From left to right: bare FTO, pH = 7.0, pH = 8.5, and pH = 10.

systems are integrated into polydopamine. Otherwise, it has been postulated that PDA is composed entirely of noncovalent assemblies of dopamine, dopamine-quinone, and DHI which are cross-linked primarily via strong, noncovalent forces. The proposed noncovalent and covalent pathways should not be seen as mutually exclusive, as it is possible that both “polymerization” and “self-assembly” contribute to PDA formation.4 The variety of functional groups contained in the PDA can explain its robust adhesion capability to virtually all types of surfaces and its versatility to be modified with other functionalities.9 Despite the complexity of this process, it is clear that the concentration of dopamine, solution pH, type of buffer, and oxidant influence the polymerization process,4 as is the case with others polymerization reactions.10 Cyclic Voltammetry. Cyclic voltammetry (CV) is an electrochemical technique where the potential at a working B




Experimental Details

electrode is changed linearly to a set value and then swept back. The summation of capacitive currents and Faradaic currents (when an electroactive species is present) is later registered at each potential value. Electrochemical reversibility refers to the electron transfer kinetics between the electrode and the electroactive species being measured. When the barrier to electron transfer is low (electrochemical reversibility), Nernstian equilibrium is established immediately upon any change in the applied potential. We recommend reading the work of Dempsey et al., who recently created an excellent introduction to cyclic voltammetry.11 Several potential applications of PDA are related to interfaces and charge transfer properties; therefore, the use of electrochemical techniques is very advisible to characterize PDA-coated surfaces.11−13 CV was used to observe the changes in the redox signals of an electrochemical probe (hexacyanoferrate(III)) when the PDA-coated FTO glass was employed as a working electrode in an electrochemical cell. The behavior of the cyclovoltammograms allows verification of the deposition in each synthesis, and the changes in the redox signals of the probe supply information about the charge transfer properties of the PDA. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) is an electrochemical technique in which a sinusoidal voltage is applied to the working electrode, and the respective currents are measured. The resistance to current flow is calculated as an impedance. Though resistances remain constant following Ohm’s law (V = IR) when sinusoidal perturbations are applied, the presence of capacitors or inductors in the system does vary with the frequency. One can visualize it in the case of capacitors which charge or discharge with a time constant (τ): at low frequencies there is enough time for the charge process to occur, but there is not enough time when high frequencies are applied. The Randles circuit, in memory of proponent John Edward Brough Randles, models the electrochemical interface in which an electroactive species is present in a solution and the double layer forms. In this circuit we find resistances, as solution resistance (Rs) and charge transfer resistance (Rct), and a double layer capacitor (Cdl). Spectra from this technique are typically displayed as a graphic of the imaginary impedance (Z″) versus the real impedance (Z′), called a Nyquist diagram. In this diagram, a semicircle is usually observed at high frequencies (left of x-axis); the points at which the impedance data curve cut the real impedance data (x-axis) represent resistance values. Since we are measuring impedance spectra in the presence of a redox probe (Fe(CN)63−/4‑), the most appropriate impedance parameter to evaluate the interfacial change, due to the presence of the PDA coating, is the charge transfer resistance (Rct). It is inversely proportional to the electron transfer rate and supplies information on the ease of electron transfer at the electrode interface. An estimate of the Rct value is determined by the diameter of the semicircle. The larger the circle is, the greater the surface resistance is.

The experimental work was designed to be accomplished in two lab sessions, with the students working in groups of two. The synthesis setup is conducted in a first lab session of 120 min; then, the polymerization process proceeded for 24 h, and in a second lab session, the PDA-coated FTO glass is characterized (approximately 120 min). The effects of the pH of the synthesis solution and of three types of oxidizing agents on the electrochemical characteristics of the PDA obtained are studied. In the first lab session, the students initiate the PDA synthesis using some of the conditions described herein; for instance, one group obtains PDA using ammonium persulfate as the oxidant at pH = 8.5, and another group uses sodium percarbonate at pH = 8.5, etc. Experimental details are given in the Supporting Information. In the second session, the students characterize the PDA-coated FTO glass by electrochemical techniques and Raman spectroscopy. For PDA synthesis and its deposition onto FTO glass, the students immerse the FTO glass in a beaker with Tris buffer solution containing dopamine and a given oxidant for 24 h. During this time, the PDA is formed by oxidative polymerization of dopamine, and the FTO glass surface is covered by the polymer observable by a dark color (Figure 2). In all cases, the PDA not only is deposited on the substrate to be coated but also is formed in the solution (as a precipitate and in the supernatant). However, when the particles produced in solution were put in contact with an identical substrate, no deposition occurred. Others authors have suggested that the presence of unoxidized dopamine or small oligomers thereof is necessary in the deposition process.14−17 After removing the PDA-coated FTO glass from the solution, it is rinsed with distilled water and air-dried. The PDA-coated glasses are characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to confirm PDA deposition and evaluate their charge transfer properties. The PDA-coated FTO glasses are further characterized by Raman spectroscopy to examine the presence of aromatic structures on the surface. See Supporting Information for further details.

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HAZARDS Students must wear safety glasses and laboratory coats at all times. Chemical waste should be disposed according to the MSDS specifications of each material. Dopamine hydrochloride is harmful if swallowed or by skin absorption. Ammonium persulfate, sodium percarbonate, and potassium hexacyanoferrate are hazardous in the case of skin contact, eye contact, ingestion, or inhalation. Ammonium persulfate and percarbonate are oxidizers and may be combustible at high temperatures. Sodium hydroxide and hydrochloric acid are corrosive and are irritants in the case of skin contact, eye contact, ingestion, or inhalation.

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RESULTS AND DISCUSSION The setup for the FTO glass coating with PDA is shown in Figure S1. FTO glass was chosen as a substrate for its low cost, good chemical stability, and electrical conductivity. Additionally, FTO is widely used in photocatalytic applications, which is one of the promising fields for PDA. All experiments resulted in the deposition of the PDA on the FTO glasses. The PDA coating adhesion on the substrate was not evaluated, but surfaces modified under the above-described experimental

Reagents

Dopamine hydrochloride (Sigma-Aldrich), Tris (tris(hydroxymethyl)aminomethane) (AlfaAesar) buffer (pH = 8.5, 7.0, 10.0), ammonium persulfate (AlfaAesar), sodium percarbonate (Sigma-Aldrich), potassium hexacyanoferrate(III) (99%, Sigma-Aldrich), potassium hexacyanoferrate(II) (98.5−102%, Sigma-Aldrich), acetone (99%,Alfa Aesar), and ethanol (ACS, 96%, AlfaAesar) were used. C




Figure 3. Effects of the pH on PDA formation using ammonium persulfate as the oxidant. Top: Stereoscope images of the PDA polymers synthesized under different pH conditions (pH = 7.0, 8.5, or 10.0). A 2× objective was employed. Bottom left: CVs of PDA-coated FTO glasses under the same synthesis conditions. A 0.01 M potassium hexacyanoferrate(III)/0.1 M KCl aqueous solution was used (5 cycles, 100 mV/s). A bare FTO glass plate is used as a control for comparison (black curve). Bottom right: Nyquist plots of the impedance measured in a solution containing 0.01 M potassium hexacyanoferrate(II) and 0.01 M potassium hexacyanoferrate(III) in phosphate buffered solution (PBS).

value for reversible reactions that transfer one electron (59 mV at 25 C), suggesting that the surface is somewhat resistive and that the equilibrium is not totally reversible. In the presence of the PDA coating, peak currents decrease, and peak separations increase. This is consistent with the formation of a partially blocking polymeric structure on the electrode surface in which ferricyanide ions hardly diffuse through free PDA spaces. This is in line with a lot of research done, which shows that PDA is an insulating, electrochemically inactive material. Notwithstanding, Zangmeister et al. observed electrochemical signals in absence of a redox probe on PDA films synthesized in short times (10 min), indicating that unoxidized dopamine was present in the polymer. However, when they evaluated the electrochemical activity of synthesized PDA for 16.5 h, they observed an intense decrease in those signals denoting the inactivity of this coating.19 Consistent with the color changes described, the most significant variation in the CVs is recorded when the PDA is deposited from the most basic solution tested (pH = 10). First, the redox signals of the probe strongly decrease, and second, the peak-to-peak potential separation (ΔEp) reached a maximum value at this pH (pH = 10). The greater the separation, the slower the electrochemical reaction on the surface. This is a consequence of charge transfer being impeded by higher resistivity of the surface. These findings indicate that either the PDA deposition on FTO glass occurs at a greater extent in solutions with higher pH or the chemical structure of the resulting polymer is more electrically insulating. Notice that the results obtained from cyclic voltammetry seem to be derived from a homogeneous layer, but the coatings themselves are heterogeneous (Figure 3 and Figure S7). These results may be interpreted as contradictory, though a thin homogeneous film is actually formed at the three different pH values, with further deposition of nonadherent brown and black polymers. Sonication was used to removed

conditions remained stable even after ultrasound treatment for 10 min (Figures S7 and S10). The PDA is characterized by its ability to coat a broad spectrum of solid surfaces. Synergistic salt displacement at solid and liquid interfaces by catechol and amine groups is one of the important mechanisms related to why PDA exhibits coating capability. Also, PDA layers utilize a variety of multiple binding mechanisms such as catechol− metal coordinations, electrostatic interactions, π−π interactions, hydrogen bonds, and covalent reactions depending upon the chemistry of material surfaces.18 Influence of Reaction Solution pH on Polydopamine Formation

Deprotonation of a dopamine amine group is a prerequisite for dopaminechrome formation and therefore polymer deposition. Consequently, the effect of pH during PDA formation is studied in this section. The electrochemical characterization (CV, EIS) of PDA-coated FTO glasses at different pH values shows changes in the charge transfer properties because of the coating, confirming surface modification with a more resistive material (Figure 3). This result is consistent with the fact that PDA is a cross-linked polymer with isolating characteristics. In all the syntheses the color of the solution intensified over time, possibly due to the generation of the oxidation intermediates. The color changes were more intense and faster at higher pH values, which indicates that basic pH favors polymerization (Figure S2). The CV of a solution of hexacyanoferrate(III) using bare FTO glass as working electrode shows redox signals typical of this electrochemical probe. Two well-defined peaks for both oxidation (positive currents) and reduction (negative currents) reactions are characteristic of reversible electrochemical reactions and demonstrate the electronic transfer of hexacyanoferrate with the FTO glass surface. The peak-topeak potential separation (ΔEp) is greater than the theoretical D




oxygen exhibits a ΔEp close to that of the bare FTO glass though the peak currents are diminished (Figure 5). The EIS results are congruent with the CV experiments as evidenced by the tendency of the charge transfer resistance values (Rct) (Figure 5 and Table S1).

nonadhering layers to expose the thin homogeneous PDA film. CV experiments showed that the electric insulating behavior is preserved as well as the Raman shifts (Figure S10). We recommend 10 min sonication in distilled water to avoid this confusion. The results from EIS are consistent with the previous CV data since the diameters of the semicircles (Rct) followed the order pH = 10.0 > 8.5 > 7.0 > bare FTO glass (Figure 3 and Table S1). This ordering is consistent with the peak-to-peak potential separations obtained from CV. Raman spectra are collected to verify FTO glass modification by the polymer. The PDA-coated FTO glasses show, in all cases, two bands at approximately 1374 and 1590 cm−1 from the stretching and deformation of aromatic rings20 (Figure 4).

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CONCLUSIONS Experiments based on catecholamine chemistry and the use of electrochemical techniques were described for teaching and practicing skills involved in the procedures of chemical synthesis, oxidative polymerization, solution preparation, chemical surface modification, and electrochemical characterization. The students gained knowledge on the characteristics of a bioinspired material such as PDA. Additionally, they were introduced to cyclic voltammetry and electrochemical impedance spectroscopy as methods for characterizing modified surfaces. The students enjoyed the simple activities to obtain a bioinspired material without the need for harsh reaction conditions, as well as the opportunity to characterize surfaces with electrochemical techniques.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00432. Instructor notes (PDF, DOCX) Student handout (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author Figure 4. Raman spectra of PDA films obtained at different pH conditions. The 50×/0.55 objective was used for the collection of the Raman spectra with the 532 nm laser and a 1800 grating. The spot size was thus constant in all experiments (laser spot diameter = 1.18 μm). The spectrum shown in the inset corresponds to bare FTO. Notice the absence of Raman signals in the 1200−2000 cm−1 region.

*E-mail: [email protected].

Influence of Oxidant Type on Polydopamine Formation

ACKNOWLEDGMENTS The authors thank the Universidad de los Andes, Chemistry Department. M.T.C. thanks the Faculty of Science (Project code INV-2017-51-1456) and the Banco de la Republica (Project code 4127) for providing funding. The authors also thank Ricardo Barrera for his help with the artwork and support.

ORCID

María T. Cortés: 0000-0002-7475-2083 Notes

The authors declare no competing financial interest.

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The study of the effects of chemical oxidant type on PDA synthesis is carried out at pH = 8.5. The ΔEp increased when PDA is deposited on FTO independently of the oxidant used, even though the insulation capacity of the produced polymers differs: percarbonate > ammonium persulfate > oxygen > bare FTO. The FTO glass coated with PDA synthesized with

Figure 5. Effects of the oxidant on PDA formation. PDA polymers were synthesized from a DA solution (26.4 mM) at pH 8.5 using different oxidants in a 1:2 molar relation (DA:oxidant). Left: CV using PDA-coated FTO in 0.01 M potassium hexacyanoferrate(III)/0.1 M KCl aqueous solution. Right: EIS spectra from the same coated electrodes. E



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(20) Ma, J. X.; Yang, H.; Li, S.; Ren, R.; Li, J.; Zhang, X.; Ma, J. Welldispersed graphene-polydopamine-Pd hybrid with enhanced catalytic performance. RSC Adv. 2015, 5 (118), 97520−97527.

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Bioinspired Polydopamine Synthesis and Its Electrochemical

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