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Degradation of polydopamine coatings by sodium hypochlorite: A process depending on the substrate and the film synthesis method
Polymer Degradation and Stability 97 (2012) 1844e1849
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Degradation of polydopamine coatings by sodium hypochlorite: A process depending on the substrate and the film synthesis method Doriane Del Frari*, Jérôme Bour, Vincent Ball, Valérie Toniazzo, David Ruch Centre de Recherche Public Henri Tudor, Department of Advanced Materials and Structures, 66, rue de Luxembourg, L-4002 Esch-sur-Alzette, Luxembourg
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 March 2012 Received in revised form 6 April 2012 Accepted 10 May 2012 Available online 28 May 2012
Polydopamine coatings are promising new versatile coatings able to be deposited on almost all kinds of materials. Such films synthesized either by oxygenation or by electrochemistry on indium tin oxide (ITO) and glassy carbon substrates, were degraded by an oxidizing sodium hypochlorite solution. Films were successively immersed in a sodium hypochlorite solution (1 g/L) during different times. The characterizations of film degradation were made by XPS spectroscopy and AFM. They confirmed a homogeneous degradation of polydopamine, due to an oxidation reaction. The influence of the synthesis method and the nature of the substrate on the polydopamine degradation were also studied in this paper: coatings deposited on ITO by oxygenation degrade much faster than those deposited on glassy carbon or by electrochemistry. This suggests that the adhesion of the polydopamine films and their stability is markedly dependant on the used substrate (ITO vs. glassy carbon) as well as on the deposition method (oxygenation vs. electrochemistry), whereas the film thickness reached during deposition is almost substrate independent. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Degradation Polydopamine Oxygenation Electropolymerization Sodium hypochlorite
1. Introduction Fast and versatile coating technologies are required to protect the surface of materials against degradation events like corrosion or to confer to those materials some new surface functionalities. These new functionalities will allow for secondary chemical grafting reactions to be performed or to change the wettability or adhesive behavior of that material. For a coating method to be versatile it should be applicable on a large variety of materials, like noble metals, oxides and polymers, and preferentially the deposition should be performed in a cost effective manner and in environmentally friendly conditions, preferentially from an aqueous solution. Coating methods which fulfill simultaneously all these criteria are very rare with the notable exception of films having a composition and properties very close to those of melanin, the natural pigment and photoprotectant of the skin [1]. These films can be produced from solutions containing a catecholamine like dopamine [2] or norepinephrine [3] in basic aqueous solutions in the presence of dissolved oxygen which acts as an oxidant. The obtained coatings form films on almost all kinds of materials, even on teflon and on superhydrophobic substrates [4] in a one step manner. The thickness of the polydopamine coatings can be controlled by playing on the reaction time, on the nature of the used oxidant [5], on the * Corresponding author. Tel.: þ352 42 59 91 4638; fax: þ352 42 59 91 555. E-mail address: [email protected] (D. Del Frari). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.05.002
buffer used to control the pH of the solution [6] and on the concentration of dopamine [7]. Additionally, it is also possible to deposit polydopamine coatings on conductive substrates by means of electropolymerization [8]. This deposition method can only be applied on surfaces of metals or carbonaceous materials but it offers the advantage to be selective on the surface of the materials were the oxidation of the catecholamine takes place without oxidation in solution. Polydopamine coatings have a composition very close to that of eumelanin, a fascinating heterogeneous biomaterial [9]. They offer the advantage of easy post functionalization with molecules containing nucleophilic groups like thiols [2] or amines [10]. In addition, polydopamine coatings contain free radical groups, as natural melanin does, and sufficient chemical groups allowing to be oxidized and to simultaneously allow for the reduction of metal ions from solution and their subsequent deposition as nanoparticles [11]. In the case of polydopamine films decorated with silver nanoparticles, the obtained coatings display long term antimicrobial properties [12]. Among a plethora of possible applications, which have been reviewed recently [13,14], it has to be cited that polydopamine coatings can be used for the protection against corrosion and that they display a pH dependant permselectivity for ions [15] in accordance with their pH dependant surface potential [16]. Since melanin displays photoconductivity and antioxidative properties, it is of the highest interest to investigate its stability with time and in response to strong oxidants. The investigation of
D. Del Frari et al. / Polymer Degradation and Stability 97 (2012) 1844e1849
such degradation processes can be complicated from natural melanin, particularly due to the presence of proteins. The possibility to have melanin-like coatings on flat substrates allows to investigate such degradation processes in real time with the aid of surface sensitive analytical tools. In addition it is also mandatory to optimize cleaning methods of substrates coated with polydopamine films in order to regenerate the surface of such coatings. Since the adhesion and stability of polydopamine coatings may be dependent on the nature of the used substrate as well as on the used deposition method, it is the aim of this article to investigate the degradation of polydopamine coatings on two different substrates: indium tin oxide (ITO) and glassy carbon electrodes. These two substrates are conductive and well suited for the electrochemical deposition of polydopamine as well as for the following of film degradation by means of cyclic voltammetry. Indeed upon film degradation in the presence of a strong oxidant, as sodium hypochlorite, we expect that the permeability of the polydopamine coating will progressively increase. In addition ITO can be used as a substrate to follow the film degradation by means of X-ray photoelectron spectroscopy (XPS). Indeed, polydopamine films are black [17] and their absorbance will progressively decrease upon film degradation. Some clues into the film degradation mechanism will be obtained from atomic force microscopy (AFM) and XPS in order to investigate the change in film morphology and in chemical composition, respectively, during its degradation. Complementary, we will investigate, on the two substrates (ITO and glassy carbon), the influence of the deposition method, either via oxygenation of dopamine solutions or their electropolymerization, on the degradation kinetics in the presence of sodium hypochlorite. This oxidant was chosen because it is a major constituent in many cleaning and sterilization formulations. At purpose, we used two models substrates for the electrodeposition of polydopamine: glassy carbon and ITO, because they are of broad industrial use in microelectronics for instance. 2. Materials and methods 2.1. Substrates, synthesis and degradation A summary of the used substrates, their dimensions and the cleaning protocol applied before the deposition of polydopamine are presented in the Table 1. All solutions are prepared using ultrapure water from a Milli Q Plus water purification system (Millipore, Simplicity). The pH of the solutions is measured with a Mettler-Toledo pH-meter and adjusted by addition of hydrochloric acid solution (37%, Sigma Aldrich) or sodium hydroxide pellets (extra pure, Acros Organics). All experiments are performed at room temperature. Two methods are used to build melanin deposits from aqueous dopamine solutions: melanin formation is initiated by dopamine oxidation either by oxygen or by electrochemistry, as described in reference [18]. In the first method, by oxygenation, the substrate is immersed in a dopamine solution (2 g/L) in 50 mM tris(hydroxymethyl)Table 1 Nature, dimensions and cleaning of substrates. 1: CH Instruments, Austin, Texas, USA, CHI 104. 2: Indium tin oxide (glass slides for MALDI imaging e conductive ITO coating), Bruker.
Coating
AFM XPS
Nature
Size
Cleaning
Glassy carbon [1]
0.2 cm2
ITO [2] ITO [2] ITO [2]
8 cm2 / /
Polishing (1 mm): g-alumina powder with a particle diameter of 50 nm (Buehler, Lake Bluff, Illinois, USA) Ethanol in an ultrasonic bath during 10 min Ethanol in an ultrasonic bath during 10 min Ethanol in an ultrasonic bath during 10 min
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aminomethane (C4H11NO3, UltraPure, Euromedex), at pH ¼ 8.5, continuously aerated with an aquarium pump (Rena Air 50, Mars Fishcare). In the second method, by electrodeposition, the electrolyte is a dopamine solution (0.5 g/L) in 10 mM tris(hydroxymethyl)aminomethane, at pH ¼ 7.5. This solution was purged with N2 during the whole duration of the experiment. The electrochemical potentials of the working electrode was measured and expressed by reference to an Ag/AgCl electrode and the counter electrode was a platinum grid. The experiments were realized using a PARSTAT 2273 potentiostat/galvanostat (Princeton Applied Research). The coating process was realized by a voltamperometric technique. 100 cyclic voltammetry (CV) cycles were applied between 0.4 and 0.3 V/Ag/AgCl, at a scan rate of 10 mV/s [8]. After their synthesis, polydopamine films deposited on glassy carbon or ITO were immersed in a sodium hypochlorite solution (NaClO) during a predetermined time. Preliminary experiments have defined an optimum concentration of sodium hypochlorite equivalent to 1 g/L allowing for a fast degradation of polydopamine. A fast cleaning of the substrates is mandatory for practical applications. The hypochlorite ion, ClO, is well-known for its oxidative properties and it reacts following this equation:
ClO þ 2Hþ þ 2e /Cl þ H2 O
(1)
with a standard redox potential of 0.81 V vs. the normal hydrogen electrode. After immersion, the substrates are immediately rinsed with ultrapure water. 2.2. Characterization techniques Electrochemical characterizations were also made by the cyclic voltammetry (CV) technique. A CV cycle was realized between 0.1 and 0.6 V/Ag/AgCl, at a scan rate of 50 m V/s. For these experiments, the electrolyte is a solution of potassium hexacyanoferrate trihydrate at 1 mM (Sigma Aldrich) containing sodium nitrate at 0.15 M (Sigma Aldrich) and tris(hydroxymethyl)-aminomethane at 50 mM (Euromedex), at pH ¼ 7.5. The Atomic Force Microscopy (AFM) topographies of the films were acquired in the tapping mode with a Pico SPM microscope (Molecular Imaging) at a frequency of 1 Hz. Each image, with a resolution of 512 512 pixels, was acquired with a new pyramidal silicon tip. X-ray photoelectron spectroscopy (XPS) analyses were performed with a Hemispherical Energy Analyzer SPECS (PHOIBOS 150) employing a monochromatic Al Ka radiation (1486.74 eV) operating at 200 W with an anode voltage of 12 kV. The pressure in the analysis chamber was equal to 109 mbar. The pass energies were set to 80 eV and 20 eV for survey and higher resolution scans, respectively. The binding energy scale was calibrated from the carbon contamination using the C1s peak at 284.6 eV. Core peaks were analyzed using a nonlinear Shirley-type background. The peak positions and areas were optimized by a weighted least-square fitting method using 70% Gaussian and 30% Lorentzian line shapes. 3. Results and discussion 3.1. Degradation of films produced via oxygenation on ITO and on glassy carbon Polydopamine films were deposited on glassy carbon and ITO substrates, according to the oxygenation method, the reaction time being equal to 24 h, a time duration sufficient to reach a maximal thickness value of about 40 nm in these conditions (2 g/L in dopamine) according to the literature [2].
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3.1.1. Characterization by cyclic voltammetry The same mode of immersions with sodium hypochlorite (1 g/L) has been done on these kinds of coatings deposited either on ITO or on amorphous carbon. The glassy carbon electrode coated with polydopamine with oxygenation method was immersed for periods of 5 min with a total immersion time of 35 min (Fig. 1A). As polydopamine film is known to be non-conductive, the coated electrode not yet immersed in NaClO solution presents a CV without the appearance of faradic currents, synonymous of highly insulating and impermeable films. After successive immersions in the oxidative solution, the CVs acquired in the presence of 1 mM hexacyanoferrate redox probe show faradaic current densities more and more important. This shows that hexacyanoferrate anions again have access to the electrode surface. After a total immersion time of 20 min, a maximum of 30 mA/cm2 at a potential of 0.42 V/Ag/AgCl is reached. This value corresponds to an oxidation peak, accompanied by a reduction peak at around 0.05 V/Ag/AgCl. To compare with the cyclic voltammogram of a bare electrode, a CV of the substrate without a polydopamine coating was realized. The CVs in Fig. 1A show that the successive immersions of polydopamine films in the sodium hypochlorite solution show a progressive deterioration of the coating. After 20 min of immersion, all the cyclic voltammograms superimpose, proving that a steady-state is reached and that the oxidative solution has no longer influence on the coating after this immersion time. In a similar manner, the ITO substrate with polydopamine was immersed 5 times for a period of 15 s each time, yielding a total immersion time of 75 s. The non-immersed coating shows a similar curve to that the glassy carbon substrate, due to the presence of a polydopamine film on the electrode surface. However, in the presence of NaClO an oxidation peak appears around 0.10 V/Ag/AgCl, which shifts to anodic potentials after the first 15 s of immersion. There is an increase in current densities in the CV curves, with the appearance of an accentuated oxidation peak at 0.25 V/Ag/AgCl, while a reduction peak appears at 0.03 V/Ag/AgCl. The cyclic voltammetry of the bare substrate shows that these peaks should correspond to the ITO substrate. Cyclic voltammetry curves measured after the two others immersions are almost superimposable, regardless of the immersion time. Hence, the successive immersions of polydopamine films in NaClO at 1 g/L show a rapid degradation of the film.
We note that, compared with polydopamine films deposited on glassy carbon (Fig. 1A), those obtained on ITO slides deteriorate more quickly (Fig. 1B) during the immersions in the solution of sodium hypochlorite. This may well be due to a higher porosity of the films deposited on ITO than those deposited on amorphous carbon or to a lower adhesion of the coating on ITO than on amorphous carbon. A combination of both effects could also play a role. The adhesion of polydopamine coatings on different substrates as well as their porosity needs to be investigated. 3.1.2. Characterization by atomic force microscopy AFM topographies were acquired on ITO coated with polydopamine. Fig. 2A shows the surface of a sample not immersed in the solution of sodium hypochlorite, while Fig. 2B shows the surface of a sample immersed during 1 min, a time sufficient to induce considerable degradation of the polydopamine coating according to the CV (Fig. 1B). However according to the CV experiments some polydopamine should remain on the ITO substrate after 1 min of treatment in the 1 g/L NaClO solution. After immersion in the oxidizing solution, the root mean squared roughness of the surface of the polydopamine films markedly decreases. Indeed, the root mean squared roughness decreases from 30.6 to 2.42 nm, during the immersion of the substrate in the sodium hypochlorite during 1 min. It seems that the degradation of polydopamine films is due to a punctual erosion of the surface, mainly localized on the coatings peaks. Indeed, the surface topography of the polydopamine coating is grainy, and those grains seem to have disappeared after just 1 min of treatment in a 1 g/L NaClO solution. 3.1.3. Characterization by X-ray photoelectron spectroscopy XPS is used to determine the evolution of the chemical composition of the first 10 nm of the polydopamine at the interface, during the exposure of the coating to sodium hypochlorite solutions at 1 g/L. Table 2 shows the atomic composition of a polydopamine deposit obtained by oxygenation, on the same ITO electrode used for the electropolymerization of the film, before and after an immersion during 1 min in a sodium hypochlorite solution (1 g/L). Again this 1 min of reaction in the presence of NaClO is sufficient to degrade the coating in a significant but not in a quantitative manner (Figs. 1B and 2). Table 2 gathers composition values of the extreme surface of the substrates (about 10 nm sampling depth) obtained with high-resolution spectra in the C1s region of the XPS spectrum.
Fig. 1. Cyclic voltammograms during polydopamine degradation on glassy carbon (A) and ITO (B) by sodium hypochlorite immersion e the polydopamine films were produced by oxygenation.
D. Del Frari et al. / Polymer Degradation and Stability 97 (2012) 1844e1849
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Fig. 2. Surface topography of polydopamine films obtained according to method A, imaged by AFM, without degradation (A) and after 1 minute of degradation in the presence of NaClO at 1 g/L (B).
In the C1s peak, up to five components can be identified corresponding to aromatic carbons (CC/CH at photoelectron energy of 284.6 eV), oxygen or nitrogen substituted carbons (CO/CN at 286 eV), carbonyl groups (C¼O at 287.6 eV), carboxyl groups (OC¼O at 288.9 eV) and p e p* transitions at 291.1 eV. After an immersion of 1 min in the oxidizing solution, the following trends are observed: -
The CC/CH contribution decreases, The CO/CN peak intensity remains constant, The contribution of C¼O increases, Finally, the OC¼O contribution also increases.
The major change with respect to the unmodified films concerns the C¼O groups (Table 2). The XPS spectra confirm that the film undergoes strong oxidation during its exposure to sodium hypochlorite, as expected owing to the strong oxidizing power of ClO (Equation (1)). 3.2. Deposition process by electrochemistry Polydopamine films were deposited on glassy carbon and ITO, according to the electrodeposition method, hence by electrochemistry as detailed in reference [18] and [8]. This was done to investigate if the nature of the deposition method of the polydopamine film has an influence on its degradation kinetics. 3.2.1. Electropolymerization Fig. 3 shows the development of the cyclic voltammograms during polydopamine deposition by electrochemistry. These Table 2 High resolution C1s peak of melanin films with and without immersion in NaClO. Name
CeC/CeH CeO/CeN C¼O OeC¼O pep*
Quartz þ melanin
Quartz þ melanin after 1 min in NaHClO
Position
%At concentration
Position
%At concentration
284.6 286.0 287.6 289.0 291.1
62.7 26.7 6.6 2.6 1.5
284.6 286.0 287.6 288.8 291.2
54.7 27.0 11.7 4.5 2.1
syntheses were realized on glassy carbon substrates (Fig. 3A) and ITO (Fig. 3B). The potential was cycled between 0.4 V/Ag/AgCl and 0.3 V/Ag/AgCl at a scan rate of 10 mV/s in a deoxygenated solution. Only a few cycles over the total of 100 performed ones are represented in Fig. 4 for the seak of clarity. These experiments show that during the first CV cycles, the oxidation current is higher on glassy carbon than on ITO by more than an order of magnitude, meaning that the electron transfer from the electrode to the dopamine molecules is more efficient on the former material. This could be due to a higher amount of dopamine adsorbing on glassy carbon than on ITO or to a better electron transfer. Unfortunately we cannot address this point in the present investigation. When looking at CV curves obtained later on in the deposition process, it appears that the reduction in oxidation current is more pronounced on glassy carbon (Fig. 3A) than on ITO (Fig. 3B): this is a clear indication that the polydopamine coating over the glassy carbon electrode is either thicker or more compact than on ITO. To address this last point we wanted to investigate the film morphology on flat glassy carbon sheets (50 50 mm, purchased from Goodfellow) by means of AFM. Unfortunately those substrates could not be polished as nicely as the electrodes used in the CV experiments of Fig. 3A, which had the shape of disks 5 mm in diameter. We observed (data not shown) that, in contrast to the disk shaped electrodes, the polydopamine coatings on the glassy carbon squares were non reproducible, in relationship to the difficulty in reproducible polishing. Concerning the glassy carbon substrate, one strong oxidation peak (anodic peak a1 at a potential of 0.2 V/Ag/AgCl) and two weak reduction peaks (cathodic peaks c1 and c2 at 0.1 V/Ag/AgCl and at 0.32 V/Ag/AgCl) are visible in the first cycle. The peak currents decrease when the number of voltamperometric cycles increases. This decrease indicates that a compact layer of polydopamine forms at the surface of the working electrode progressively impeding the access of dopamine and its oxidation products to the electrode. Visually, coatings are black and homogeneous on the surface. Concerning experiments performed on ITO (Fig. 3B), the trend is similar to that obtained on glassy carbon. However, no peak is sufficiently well defined to compare the values of the oxidation and reduction peaks. Overall, a shift of potentials towards toward cathodic values is observed when the substrate is an ITO slide. This
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Fig. 3. Cyclic voltammograms during dopamine-melanin deposition on glassy carbon (A) and ITO (B).
means that the electron transfer between the dopamine solution and the electrode is substrate dependant. However, the same decrease of peak currents as on glassy carbon is observed, which confirms the formation of a compact layer of polydopamine on ITO. Table 3 summarizes the optimal intensity measured for the oxidation peak a1 and the reduction peak c1, according to the nature of the substrate. It appears that values of anodic currents, corresponding to the oxidation of dopamine, are largely higher on a glassy carbon substrate than on an ITO slide. Since intensity is proportional to the quantity of deposited coating, the glassy carbon substrate allows a better deposition of polydopamine, about 30 times more important than ITO. This is a surprising but nevertheless reproducible result. It shows that contrarily to the deposition method by oxygenation [2,3], via oxygenation of dopamine solutions, the deposition of polydopamine electrodeposition method is strongly substrate dependent. 3.2.2. Characterization of the polydopamine degradation process by CV Coatings of polydopamine deposited by electrochemistry are successively immersed in a solution of sodium hypochlorite (1 g/L). After each immersion, a characterization by voltammetry is made in the presence of a potassium hexacyanoferrate trihydrate (1 mM),
sodium nitrate (0.15 M) and tris(hydroxymethyl)-aminomethane (50 mM) solution. Fig. 4 presents the evolution of the cyclic voltammograms during polydopamine degradation, on glassy carbon (Fig. 4A) and ITO (Fig. 4B). Concerning the glassy carbon substrates, the polydopamine coatings were immersed 10 times for a period of 5 min each time, to reach a cumulative immersion time of 50 min. The polydopamine electrode which was not immerged in the NaClO solution before contact with the potassium hexacyanoferrate solution, displayed a pure capacitive CV which is characteristic of an impermeable coating. During the successive immersions in the solution of NaClO at 1 g/L, the CV curves present the appearance of an oxidation peak, which intensity increases with time until to reach a maximum of 25 mA/cm2 at a potential of 0.42 V/Ag/AgCl. As a point of comparison, a CV of the bare substrate was realized (black curve). A maximal current density, of 26 mA/cm2, is obtained at 0.33 V/Ag/AgCl. The successive immersions of polydopamine films in the NaClO solution show a progressive deterioration of the coating, until a limiting value. Indeed, after 40 min of immersion, there is a superposition of the CV curves, showing that a steady state on the surface is reached. However the curve of the coated electrode is never superimposed on that of the bare glassy carbon, this means
Fig. 4. Cyclic voltammograms during polydopamine degradation on glassy carbon (A) and ITO (B) by sodium hypochlorite e the polydopamine films were produced by electrochemistry.
D. Del Frari et al. / Polymer Degradation and Stability 97 (2012) 1844e1849 Table 3 Optimal anodic and cathodic intensities during the electropolymerization of polydopamine.
Oxidation Reduction
Glassy carbon
ITO
40 mA/cm2 7 mA/cm2
1.4 mA/cm2 0.5 mA/cm2
that the degradation of the polydopamine coating is not complete on glassy carbon. The ITO substrates coated with polydopamine were immersed 3 times for periods of 2, 2 and 5 min in the 1 g/L NaClO solutions. The non-immersed sample presents a CV curve similar to that of glassy carbon, due to the presence of the insulating film on the surface of the electrode. After the first 2 min of immersion in the NaClO solution, a stability of current densities of is observed in the CVs, with the appearance of an oxidation peak around 0.2 V Ag/AgCl, while a reduction peak appears to 0.03 V vs. Ag/AgCl. A CV of the bare ITO slide shows that these peaks correspond to the ITO response. This means that the polydopamine degradation is fast and almost quantitative on ITO (Fig. 4B), in contrast to glassy carbon where the degradation is slower and not quantitative (Fig. 4A). Hence, polydopamine films deposited by means of cyclic voltammetry on ITO degrade faster than on glassy carbon in the presence of sodium hypochlorite (1 g/L). After 2 min of incubation, a steady-state is obtained on ITO, while a coating deposited on glassy carbon requires 50 min of immersion to reach a steady CV curve. These results are quasi similarly to those observed during the film deposition obtained under oxygenation. However, the degradation of polydopamine films seems more complete on glassy carbon than on ITO. Indeed, the voltammetric curves of glassy carbon at the end of immersion are close to that on the bare substrate, while the difference is more pronounced on ITO. 4. Conclusions The main conclusion of this investigation is that regardless of the synthesis method, polydopamine coatings deposited on ITO degrade much faster than those deposited on glassy carbon. In addition to this qualitative finding, regardless of the nature of the substrate, coatings deposited by oxygenation degrade much faster than those deposited by electrochemistry. Overall, our investigation shows that the stability of polydopamine films in the presence of strong oxidants like NaClO strongly depends on the nature of the substrate as well as on the used deposition method. The differences in the deposition as well as in the degradation kinetics on both electrodes may well be due to the difference in the adsorption of dopamine (or the small oligomers thereof) or to the adhesion of polydopamine on the substrates. This has to be investigated in forthcoming investigations. On the ITO substrate, our XPS experiments showed that the degradation of polydopamine in the presence of NaClO is triggered by an increase in the oxidation state of the carbon atoms. This
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increase is probably associated with the appearance of more polar moieties whose solubility in water is higher, inducing an almost quantitative and fast degradation. This investigation, even if is of qualitative nature, highlights the need to investigate the interactions between polydopamine and different substrates from the point of view of adhesion as well as on the film’s nanostructure. Indeed both of these parameters could influence the stability of the polydopamine coating in the presence of strong oxidants. This work was performed in the framework of the FEDER program “Compétitivité régionale et emploi” 2007e2013, CAPTOCHEM project No 2009-02-039-35. References [1] Prota G. Melanins and melanogenesis. San Diego: Academic Press; 1992. [2] Lee H, Dellatore SM, Miller WM, Messersmith PB. Mussel-Inspired surface chemistry for multifunctional coatings. Science 2007;318(5849):426e30. [3] Kang SM, Rho J, Choi IS, Messersmith PB, Lee HJ. Norepinephrine: material independent, multifunctional surface modification reagent. Amer Chem Soc 2009;131(37):13224e5. [4] Kang SM, You I, Cho WK, Shon HK, Lee TG, Choi IS, et al. One-step modification of superhydrophobic surfaces by a mussel-inspired polymer coating. Angew Chem Int Ed 2010;49(49):9401e4. [5] Wei Q, Zhang F, Li J, Li B, Zhao C. Oxidant-induced dopaminepolymerization for multifunctional coatings. Polym Chem 2010;1(9):1430e3. [6] Bernsmann F, Ball V, Addiego F, Ponche A, Michel M, de Almeida Gracio JJ, et al. Dopaminemelanin film deposition depends on the used oxidant and buffer solution. Langmuir 2011;27(6):2819e25. [7] Müller M, Kebler B. Deposition from dopamine solutions at Ge substrates: an in situ ATR-FTIR study. Langmuir 2011;27(20):12499e505. [8] Li Y, Liu M, Xiang C, Xie Q, Yao S. Electrochemical quartz crystal microbalance study on growth and property of the polymer deposit at gold electrodes during oxidation of dopamine in aqueous solutions. Thin Solid Films 2006; 497(1e2):270e8. [9] 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(22):3914e21. [10] Lee H, Rho J, Messersmith PB. Facile conjugation of biomolecules onto surfaces via mussel adhesive protein inspired coatings. Adv Mater 2009;21(4):431e4. [11] Ball V, Nguyen I, Haupt M, Oehr C, Arnoult C, Toniazzo V, et al. The reduction of Agþ in metallic silver on pseudomelanin films allows for antibacterial activity but does not imply unpaired electrons. J Colloid Interface Sci 2011; 364(2):359e65. [12] Xu H, Shi X, Ma H, Lv Y, Zhang L, Mao Z. The preparation and antibacterial effects of dopa-cotton/AgNPs. Appl Surf Sci 2011;257(15):6799e803. [13] Ball V, Del Frari D, Michel M, Buehler MJ, Toniazzo V, Singh MK, et al. Deposition mechanism and properties of thin polydopamine films for high added value applications in surface science at the nanoscale. BioNanoSci 2012;2(1):16e34. [14] Lee BP, Messersmith PB, Israelachvili JN, Waite JH. Mussel-inspired adhesives and coatings. Ann Rev Mater Res 2011;41:99e132. [15] Yu B, Liu J, Liu S, Zhou F. Pdop layer exhibiting zwitterionicity: a simple electrochemical interface for governing ion permeability. Chem Commun 2010;46(32):5900e2. [16] Ball V. Impedance spectroscopy and zeta potential titration of dopa-melanin films produced by oxidation of dopamine. Colloids Surf A Physicochem Eng Aspects 2010;363(1e3):92e7. [17] Bernsmann F, Ponche A, Ringwald C, Hemmerlé J, Raya J, Bechinger B, et al. Characterization of dopaminemelanin growth on silicon oxide. J Phys Chem C 2009;113(19):8234e42. [18] Bernsmann F, Voegel JC, Ball V. Different synthesis methods allow to tune the permeability and permselectivity of dopamine-melanin films to electrochemical probes. Electrochem Acta 2011;56(11):3914e9.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Degradation of polydopamine coatings by sodium hypochlorite: A process depending on the substrate and the film synthesis method Doriane Del Frari*, Jérôme Bour, Vincent Ball, Valérie Toniazzo, David Ruch Centre de Recherche Public Henri Tudor, Department of Advanced Materials and Structures, 66, rue de Luxembourg, L-4002 Esch-sur-Alzette, Luxembourg
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 March 2012 Received in revised form 6 April 2012 Accepted 10 May 2012 Available online 28 May 2012
Polydopamine coatings are promising new versatile coatings able to be deposited on almost all kinds of materials. Such films synthesized either by oxygenation or by electrochemistry on indium tin oxide (ITO) and glassy carbon substrates, were degraded by an oxidizing sodium hypochlorite solution. Films were successively immersed in a sodium hypochlorite solution (1 g/L) during different times. The characterizations of film degradation were made by XPS spectroscopy and AFM. They confirmed a homogeneous degradation of polydopamine, due to an oxidation reaction. The influence of the synthesis method and the nature of the substrate on the polydopamine degradation were also studied in this paper: coatings deposited on ITO by oxygenation degrade much faster than those deposited on glassy carbon or by electrochemistry. This suggests that the adhesion of the polydopamine films and their stability is markedly dependant on the used substrate (ITO vs. glassy carbon) as well as on the deposition method (oxygenation vs. electrochemistry), whereas the film thickness reached during deposition is almost substrate independent. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Degradation Polydopamine Oxygenation Electropolymerization Sodium hypochlorite
1. Introduction Fast and versatile coating technologies are required to protect the surface of materials against degradation events like corrosion or to confer to those materials some new surface functionalities. These new functionalities will allow for secondary chemical grafting reactions to be performed or to change the wettability or adhesive behavior of that material. For a coating method to be versatile it should be applicable on a large variety of materials, like noble metals, oxides and polymers, and preferentially the deposition should be performed in a cost effective manner and in environmentally friendly conditions, preferentially from an aqueous solution. Coating methods which fulfill simultaneously all these criteria are very rare with the notable exception of films having a composition and properties very close to those of melanin, the natural pigment and photoprotectant of the skin [1]. These films can be produced from solutions containing a catecholamine like dopamine [2] or norepinephrine [3] in basic aqueous solutions in the presence of dissolved oxygen which acts as an oxidant. The obtained coatings form films on almost all kinds of materials, even on teflon and on superhydrophobic substrates [4] in a one step manner. The thickness of the polydopamine coatings can be controlled by playing on the reaction time, on the nature of the used oxidant [5], on the * Corresponding author. Tel.: þ352 42 59 91 4638; fax: þ352 42 59 91 555. E-mail address: [email protected] (D. Del Frari). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.05.002
buffer used to control the pH of the solution [6] and on the concentration of dopamine [7]. Additionally, it is also possible to deposit polydopamine coatings on conductive substrates by means of electropolymerization [8]. This deposition method can only be applied on surfaces of metals or carbonaceous materials but it offers the advantage to be selective on the surface of the materials were the oxidation of the catecholamine takes place without oxidation in solution. Polydopamine coatings have a composition very close to that of eumelanin, a fascinating heterogeneous biomaterial [9]. They offer the advantage of easy post functionalization with molecules containing nucleophilic groups like thiols [2] or amines [10]. In addition, polydopamine coatings contain free radical groups, as natural melanin does, and sufficient chemical groups allowing to be oxidized and to simultaneously allow for the reduction of metal ions from solution and their subsequent deposition as nanoparticles [11]. In the case of polydopamine films decorated with silver nanoparticles, the obtained coatings display long term antimicrobial properties [12]. Among a plethora of possible applications, which have been reviewed recently [13,14], it has to be cited that polydopamine coatings can be used for the protection against corrosion and that they display a pH dependant permselectivity for ions [15] in accordance with their pH dependant surface potential [16]. Since melanin displays photoconductivity and antioxidative properties, it is of the highest interest to investigate its stability with time and in response to strong oxidants. The investigation of
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such degradation processes can be complicated from natural melanin, particularly due to the presence of proteins. The possibility to have melanin-like coatings on flat substrates allows to investigate such degradation processes in real time with the aid of surface sensitive analytical tools. In addition it is also mandatory to optimize cleaning methods of substrates coated with polydopamine films in order to regenerate the surface of such coatings. Since the adhesion and stability of polydopamine coatings may be dependent on the nature of the used substrate as well as on the used deposition method, it is the aim of this article to investigate the degradation of polydopamine coatings on two different substrates: indium tin oxide (ITO) and glassy carbon electrodes. These two substrates are conductive and well suited for the electrochemical deposition of polydopamine as well as for the following of film degradation by means of cyclic voltammetry. Indeed upon film degradation in the presence of a strong oxidant, as sodium hypochlorite, we expect that the permeability of the polydopamine coating will progressively increase. In addition ITO can be used as a substrate to follow the film degradation by means of X-ray photoelectron spectroscopy (XPS). Indeed, polydopamine films are black [17] and their absorbance will progressively decrease upon film degradation. Some clues into the film degradation mechanism will be obtained from atomic force microscopy (AFM) and XPS in order to investigate the change in film morphology and in chemical composition, respectively, during its degradation. Complementary, we will investigate, on the two substrates (ITO and glassy carbon), the influence of the deposition method, either via oxygenation of dopamine solutions or their electropolymerization, on the degradation kinetics in the presence of sodium hypochlorite. This oxidant was chosen because it is a major constituent in many cleaning and sterilization formulations. At purpose, we used two models substrates for the electrodeposition of polydopamine: glassy carbon and ITO, because they are of broad industrial use in microelectronics for instance. 2. Materials and methods 2.1. Substrates, synthesis and degradation A summary of the used substrates, their dimensions and the cleaning protocol applied before the deposition of polydopamine are presented in the Table 1. All solutions are prepared using ultrapure water from a Milli Q Plus water purification system (Millipore, Simplicity). The pH of the solutions is measured with a Mettler-Toledo pH-meter and adjusted by addition of hydrochloric acid solution (37%, Sigma Aldrich) or sodium hydroxide pellets (extra pure, Acros Organics). All experiments are performed at room temperature. Two methods are used to build melanin deposits from aqueous dopamine solutions: melanin formation is initiated by dopamine oxidation either by oxygen or by electrochemistry, as described in reference [18]. In the first method, by oxygenation, the substrate is immersed in a dopamine solution (2 g/L) in 50 mM tris(hydroxymethyl)Table 1 Nature, dimensions and cleaning of substrates. 1: CH Instruments, Austin, Texas, USA, CHI 104. 2: Indium tin oxide (glass slides for MALDI imaging e conductive ITO coating), Bruker.
Coating
AFM XPS
Nature
Size
Cleaning
Glassy carbon [1]
0.2 cm2
ITO [2] ITO [2] ITO [2]
8 cm2 / /
Polishing (1 mm): g-alumina powder with a particle diameter of 50 nm (Buehler, Lake Bluff, Illinois, USA) Ethanol in an ultrasonic bath during 10 min Ethanol in an ultrasonic bath during 10 min Ethanol in an ultrasonic bath during 10 min
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aminomethane (C4H11NO3, UltraPure, Euromedex), at pH ¼ 8.5, continuously aerated with an aquarium pump (Rena Air 50, Mars Fishcare). In the second method, by electrodeposition, the electrolyte is a dopamine solution (0.5 g/L) in 10 mM tris(hydroxymethyl)aminomethane, at pH ¼ 7.5. This solution was purged with N2 during the whole duration of the experiment. The electrochemical potentials of the working electrode was measured and expressed by reference to an Ag/AgCl electrode and the counter electrode was a platinum grid. The experiments were realized using a PARSTAT 2273 potentiostat/galvanostat (Princeton Applied Research). The coating process was realized by a voltamperometric technique. 100 cyclic voltammetry (CV) cycles were applied between 0.4 and 0.3 V/Ag/AgCl, at a scan rate of 10 mV/s [8]. After their synthesis, polydopamine films deposited on glassy carbon or ITO were immersed in a sodium hypochlorite solution (NaClO) during a predetermined time. Preliminary experiments have defined an optimum concentration of sodium hypochlorite equivalent to 1 g/L allowing for a fast degradation of polydopamine. A fast cleaning of the substrates is mandatory for practical applications. The hypochlorite ion, ClO, is well-known for its oxidative properties and it reacts following this equation:
ClO þ 2Hþ þ 2e /Cl þ H2 O
(1)
with a standard redox potential of 0.81 V vs. the normal hydrogen electrode. After immersion, the substrates are immediately rinsed with ultrapure water. 2.2. Characterization techniques Electrochemical characterizations were also made by the cyclic voltammetry (CV) technique. A CV cycle was realized between 0.1 and 0.6 V/Ag/AgCl, at a scan rate of 50 m V/s. For these experiments, the electrolyte is a solution of potassium hexacyanoferrate trihydrate at 1 mM (Sigma Aldrich) containing sodium nitrate at 0.15 M (Sigma Aldrich) and tris(hydroxymethyl)-aminomethane at 50 mM (Euromedex), at pH ¼ 7.5. The Atomic Force Microscopy (AFM) topographies of the films were acquired in the tapping mode with a Pico SPM microscope (Molecular Imaging) at a frequency of 1 Hz. Each image, with a resolution of 512 512 pixels, was acquired with a new pyramidal silicon tip. X-ray photoelectron spectroscopy (XPS) analyses were performed with a Hemispherical Energy Analyzer SPECS (PHOIBOS 150) employing a monochromatic Al Ka radiation (1486.74 eV) operating at 200 W with an anode voltage of 12 kV. The pressure in the analysis chamber was equal to 109 mbar. The pass energies were set to 80 eV and 20 eV for survey and higher resolution scans, respectively. The binding energy scale was calibrated from the carbon contamination using the C1s peak at 284.6 eV. Core peaks were analyzed using a nonlinear Shirley-type background. The peak positions and areas were optimized by a weighted least-square fitting method using 70% Gaussian and 30% Lorentzian line shapes. 3. Results and discussion 3.1. Degradation of films produced via oxygenation on ITO and on glassy carbon Polydopamine films were deposited on glassy carbon and ITO substrates, according to the oxygenation method, the reaction time being equal to 24 h, a time duration sufficient to reach a maximal thickness value of about 40 nm in these conditions (2 g/L in dopamine) according to the literature [2].
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3.1.1. Characterization by cyclic voltammetry The same mode of immersions with sodium hypochlorite (1 g/L) has been done on these kinds of coatings deposited either on ITO or on amorphous carbon. The glassy carbon electrode coated with polydopamine with oxygenation method was immersed for periods of 5 min with a total immersion time of 35 min (Fig. 1A). As polydopamine film is known to be non-conductive, the coated electrode not yet immersed in NaClO solution presents a CV without the appearance of faradic currents, synonymous of highly insulating and impermeable films. After successive immersions in the oxidative solution, the CVs acquired in the presence of 1 mM hexacyanoferrate redox probe show faradaic current densities more and more important. This shows that hexacyanoferrate anions again have access to the electrode surface. After a total immersion time of 20 min, a maximum of 30 mA/cm2 at a potential of 0.42 V/Ag/AgCl is reached. This value corresponds to an oxidation peak, accompanied by a reduction peak at around 0.05 V/Ag/AgCl. To compare with the cyclic voltammogram of a bare electrode, a CV of the substrate without a polydopamine coating was realized. The CVs in Fig. 1A show that the successive immersions of polydopamine films in the sodium hypochlorite solution show a progressive deterioration of the coating. After 20 min of immersion, all the cyclic voltammograms superimpose, proving that a steady-state is reached and that the oxidative solution has no longer influence on the coating after this immersion time. In a similar manner, the ITO substrate with polydopamine was immersed 5 times for a period of 15 s each time, yielding a total immersion time of 75 s. The non-immersed coating shows a similar curve to that the glassy carbon substrate, due to the presence of a polydopamine film on the electrode surface. However, in the presence of NaClO an oxidation peak appears around 0.10 V/Ag/AgCl, which shifts to anodic potentials after the first 15 s of immersion. There is an increase in current densities in the CV curves, with the appearance of an accentuated oxidation peak at 0.25 V/Ag/AgCl, while a reduction peak appears at 0.03 V/Ag/AgCl. The cyclic voltammetry of the bare substrate shows that these peaks should correspond to the ITO substrate. Cyclic voltammetry curves measured after the two others immersions are almost superimposable, regardless of the immersion time. Hence, the successive immersions of polydopamine films in NaClO at 1 g/L show a rapid degradation of the film.
We note that, compared with polydopamine films deposited on glassy carbon (Fig. 1A), those obtained on ITO slides deteriorate more quickly (Fig. 1B) during the immersions in the solution of sodium hypochlorite. This may well be due to a higher porosity of the films deposited on ITO than those deposited on amorphous carbon or to a lower adhesion of the coating on ITO than on amorphous carbon. A combination of both effects could also play a role. The adhesion of polydopamine coatings on different substrates as well as their porosity needs to be investigated. 3.1.2. Characterization by atomic force microscopy AFM topographies were acquired on ITO coated with polydopamine. Fig. 2A shows the surface of a sample not immersed in the solution of sodium hypochlorite, while Fig. 2B shows the surface of a sample immersed during 1 min, a time sufficient to induce considerable degradation of the polydopamine coating according to the CV (Fig. 1B). However according to the CV experiments some polydopamine should remain on the ITO substrate after 1 min of treatment in the 1 g/L NaClO solution. After immersion in the oxidizing solution, the root mean squared roughness of the surface of the polydopamine films markedly decreases. Indeed, the root mean squared roughness decreases from 30.6 to 2.42 nm, during the immersion of the substrate in the sodium hypochlorite during 1 min. It seems that the degradation of polydopamine films is due to a punctual erosion of the surface, mainly localized on the coatings peaks. Indeed, the surface topography of the polydopamine coating is grainy, and those grains seem to have disappeared after just 1 min of treatment in a 1 g/L NaClO solution. 3.1.3. Characterization by X-ray photoelectron spectroscopy XPS is used to determine the evolution of the chemical composition of the first 10 nm of the polydopamine at the interface, during the exposure of the coating to sodium hypochlorite solutions at 1 g/L. Table 2 shows the atomic composition of a polydopamine deposit obtained by oxygenation, on the same ITO electrode used for the electropolymerization of the film, before and after an immersion during 1 min in a sodium hypochlorite solution (1 g/L). Again this 1 min of reaction in the presence of NaClO is sufficient to degrade the coating in a significant but not in a quantitative manner (Figs. 1B and 2). Table 2 gathers composition values of the extreme surface of the substrates (about 10 nm sampling depth) obtained with high-resolution spectra in the C1s region of the XPS spectrum.
Fig. 1. Cyclic voltammograms during polydopamine degradation on glassy carbon (A) and ITO (B) by sodium hypochlorite immersion e the polydopamine films were produced by oxygenation.
D. Del Frari et al. / Polymer Degradation and Stability 97 (2012) 1844e1849
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Fig. 2. Surface topography of polydopamine films obtained according to method A, imaged by AFM, without degradation (A) and after 1 minute of degradation in the presence of NaClO at 1 g/L (B).
In the C1s peak, up to five components can be identified corresponding to aromatic carbons (CC/CH at photoelectron energy of 284.6 eV), oxygen or nitrogen substituted carbons (CO/CN at 286 eV), carbonyl groups (C¼O at 287.6 eV), carboxyl groups (OC¼O at 288.9 eV) and p e p* transitions at 291.1 eV. After an immersion of 1 min in the oxidizing solution, the following trends are observed: -
The CC/CH contribution decreases, The CO/CN peak intensity remains constant, The contribution of C¼O increases, Finally, the OC¼O contribution also increases.
The major change with respect to the unmodified films concerns the C¼O groups (Table 2). The XPS spectra confirm that the film undergoes strong oxidation during its exposure to sodium hypochlorite, as expected owing to the strong oxidizing power of ClO (Equation (1)). 3.2. Deposition process by electrochemistry Polydopamine films were deposited on glassy carbon and ITO, according to the electrodeposition method, hence by electrochemistry as detailed in reference [18] and [8]. This was done to investigate if the nature of the deposition method of the polydopamine film has an influence on its degradation kinetics. 3.2.1. Electropolymerization Fig. 3 shows the development of the cyclic voltammograms during polydopamine deposition by electrochemistry. These Table 2 High resolution C1s peak of melanin films with and without immersion in NaClO. Name
CeC/CeH CeO/CeN C¼O OeC¼O pep*
Quartz þ melanin
Quartz þ melanin after 1 min in NaHClO
Position
%At concentration
Position
%At concentration
284.6 286.0 287.6 289.0 291.1
62.7 26.7 6.6 2.6 1.5
284.6 286.0 287.6 288.8 291.2
54.7 27.0 11.7 4.5 2.1
syntheses were realized on glassy carbon substrates (Fig. 3A) and ITO (Fig. 3B). The potential was cycled between 0.4 V/Ag/AgCl and 0.3 V/Ag/AgCl at a scan rate of 10 mV/s in a deoxygenated solution. Only a few cycles over the total of 100 performed ones are represented in Fig. 4 for the seak of clarity. These experiments show that during the first CV cycles, the oxidation current is higher on glassy carbon than on ITO by more than an order of magnitude, meaning that the electron transfer from the electrode to the dopamine molecules is more efficient on the former material. This could be due to a higher amount of dopamine adsorbing on glassy carbon than on ITO or to a better electron transfer. Unfortunately we cannot address this point in the present investigation. When looking at CV curves obtained later on in the deposition process, it appears that the reduction in oxidation current is more pronounced on glassy carbon (Fig. 3A) than on ITO (Fig. 3B): this is a clear indication that the polydopamine coating over the glassy carbon electrode is either thicker or more compact than on ITO. To address this last point we wanted to investigate the film morphology on flat glassy carbon sheets (50 50 mm, purchased from Goodfellow) by means of AFM. Unfortunately those substrates could not be polished as nicely as the electrodes used in the CV experiments of Fig. 3A, which had the shape of disks 5 mm in diameter. We observed (data not shown) that, in contrast to the disk shaped electrodes, the polydopamine coatings on the glassy carbon squares were non reproducible, in relationship to the difficulty in reproducible polishing. Concerning the glassy carbon substrate, one strong oxidation peak (anodic peak a1 at a potential of 0.2 V/Ag/AgCl) and two weak reduction peaks (cathodic peaks c1 and c2 at 0.1 V/Ag/AgCl and at 0.32 V/Ag/AgCl) are visible in the first cycle. The peak currents decrease when the number of voltamperometric cycles increases. This decrease indicates that a compact layer of polydopamine forms at the surface of the working electrode progressively impeding the access of dopamine and its oxidation products to the electrode. Visually, coatings are black and homogeneous on the surface. Concerning experiments performed on ITO (Fig. 3B), the trend is similar to that obtained on glassy carbon. However, no peak is sufficiently well defined to compare the values of the oxidation and reduction peaks. Overall, a shift of potentials towards toward cathodic values is observed when the substrate is an ITO slide. This
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Fig. 3. Cyclic voltammograms during dopamine-melanin deposition on glassy carbon (A) and ITO (B).
means that the electron transfer between the dopamine solution and the electrode is substrate dependant. However, the same decrease of peak currents as on glassy carbon is observed, which confirms the formation of a compact layer of polydopamine on ITO. Table 3 summarizes the optimal intensity measured for the oxidation peak a1 and the reduction peak c1, according to the nature of the substrate. It appears that values of anodic currents, corresponding to the oxidation of dopamine, are largely higher on a glassy carbon substrate than on an ITO slide. Since intensity is proportional to the quantity of deposited coating, the glassy carbon substrate allows a better deposition of polydopamine, about 30 times more important than ITO. This is a surprising but nevertheless reproducible result. It shows that contrarily to the deposition method by oxygenation [2,3], via oxygenation of dopamine solutions, the deposition of polydopamine electrodeposition method is strongly substrate dependent. 3.2.2. Characterization of the polydopamine degradation process by CV Coatings of polydopamine deposited by electrochemistry are successively immersed in a solution of sodium hypochlorite (1 g/L). After each immersion, a characterization by voltammetry is made in the presence of a potassium hexacyanoferrate trihydrate (1 mM),
sodium nitrate (0.15 M) and tris(hydroxymethyl)-aminomethane (50 mM) solution. Fig. 4 presents the evolution of the cyclic voltammograms during polydopamine degradation, on glassy carbon (Fig. 4A) and ITO (Fig. 4B). Concerning the glassy carbon substrates, the polydopamine coatings were immersed 10 times for a period of 5 min each time, to reach a cumulative immersion time of 50 min. The polydopamine electrode which was not immerged in the NaClO solution before contact with the potassium hexacyanoferrate solution, displayed a pure capacitive CV which is characteristic of an impermeable coating. During the successive immersions in the solution of NaClO at 1 g/L, the CV curves present the appearance of an oxidation peak, which intensity increases with time until to reach a maximum of 25 mA/cm2 at a potential of 0.42 V/Ag/AgCl. As a point of comparison, a CV of the bare substrate was realized (black curve). A maximal current density, of 26 mA/cm2, is obtained at 0.33 V/Ag/AgCl. The successive immersions of polydopamine films in the NaClO solution show a progressive deterioration of the coating, until a limiting value. Indeed, after 40 min of immersion, there is a superposition of the CV curves, showing that a steady state on the surface is reached. However the curve of the coated electrode is never superimposed on that of the bare glassy carbon, this means
Fig. 4. Cyclic voltammograms during polydopamine degradation on glassy carbon (A) and ITO (B) by sodium hypochlorite e the polydopamine films were produced by electrochemistry.
D. Del Frari et al. / Polymer Degradation and Stability 97 (2012) 1844e1849 Table 3 Optimal anodic and cathodic intensities during the electropolymerization of polydopamine.
Oxidation Reduction
Glassy carbon
ITO
40 mA/cm2 7 mA/cm2
1.4 mA/cm2 0.5 mA/cm2
that the degradation of the polydopamine coating is not complete on glassy carbon. The ITO substrates coated with polydopamine were immersed 3 times for periods of 2, 2 and 5 min in the 1 g/L NaClO solutions. The non-immersed sample presents a CV curve similar to that of glassy carbon, due to the presence of the insulating film on the surface of the electrode. After the first 2 min of immersion in the NaClO solution, a stability of current densities of is observed in the CVs, with the appearance of an oxidation peak around 0.2 V Ag/AgCl, while a reduction peak appears to 0.03 V vs. Ag/AgCl. A CV of the bare ITO slide shows that these peaks correspond to the ITO response. This means that the polydopamine degradation is fast and almost quantitative on ITO (Fig. 4B), in contrast to glassy carbon where the degradation is slower and not quantitative (Fig. 4A). Hence, polydopamine films deposited by means of cyclic voltammetry on ITO degrade faster than on glassy carbon in the presence of sodium hypochlorite (1 g/L). After 2 min of incubation, a steady-state is obtained on ITO, while a coating deposited on glassy carbon requires 50 min of immersion to reach a steady CV curve. These results are quasi similarly to those observed during the film deposition obtained under oxygenation. However, the degradation of polydopamine films seems more complete on glassy carbon than on ITO. Indeed, the voltammetric curves of glassy carbon at the end of immersion are close to that on the bare substrate, while the difference is more pronounced on ITO. 4. Conclusions The main conclusion of this investigation is that regardless of the synthesis method, polydopamine coatings deposited on ITO degrade much faster than those deposited on glassy carbon. In addition to this qualitative finding, regardless of the nature of the substrate, coatings deposited by oxygenation degrade much faster than those deposited by electrochemistry. Overall, our investigation shows that the stability of polydopamine films in the presence of strong oxidants like NaClO strongly depends on the nature of the substrate as well as on the used deposition method. The differences in the deposition as well as in the degradation kinetics on both electrodes may well be due to the difference in the adsorption of dopamine (or the small oligomers thereof) or to the adhesion of polydopamine on the substrates. This has to be investigated in forthcoming investigations. On the ITO substrate, our XPS experiments showed that the degradation of polydopamine in the presence of NaClO is triggered by an increase in the oxidation state of the carbon atoms. This
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increase is probably associated with the appearance of more polar moieties whose solubility in water is higher, inducing an almost quantitative and fast degradation. This investigation, even if is of qualitative nature, highlights the need to investigate the interactions between polydopamine and different substrates from the point of view of adhesion as well as on the film’s nanostructure. Indeed both of these parameters could influence the stability of the polydopamine coating in the presence of strong oxidants. This work was performed in the framework of the FEDER program “Compétitivité régionale et emploi” 2007e2013, CAPTOCHEM project No 2009-02-039-35. References [1] Prota G. Melanins and melanogenesis. San Diego: Academic Press; 1992. [2] Lee H, Dellatore SM, Miller WM, Messersmith PB. Mussel-Inspired surface chemistry for multifunctional coatings. Science 2007;318(5849):426e30. [3] Kang SM, Rho J, Choi IS, Messersmith PB, Lee HJ. Norepinephrine: material independent, multifunctional surface modification reagent. Amer Chem Soc 2009;131(37):13224e5. [4] Kang SM, You I, Cho WK, Shon HK, Lee TG, Choi IS, et al. One-step modification of superhydrophobic surfaces by a mussel-inspired polymer coating. Angew Chem Int Ed 2010;49(49):9401e4. [5] Wei Q, Zhang F, Li J, Li B, Zhao C. Oxidant-induced dopaminepolymerization for multifunctional coatings. Polym Chem 2010;1(9):1430e3. [6] Bernsmann F, Ball V, Addiego F, Ponche A, Michel M, de Almeida Gracio JJ, et al. Dopaminemelanin film deposition depends on the used oxidant and buffer solution. Langmuir 2011;27(6):2819e25. [7] Müller M, Kebler B. Deposition from dopamine solutions at Ge substrates: an in situ ATR-FTIR study. Langmuir 2011;27(20):12499e505. [8] Li Y, Liu M, Xiang C, Xie Q, Yao S. Electrochemical quartz crystal microbalance study on growth and property of the polymer deposit at gold electrodes during oxidation of dopamine in aqueous solutions. Thin Solid Films 2006; 497(1e2):270e8. [9] 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(22):3914e21. [10] Lee H, Rho J, Messersmith PB. Facile conjugation of biomolecules onto surfaces via mussel adhesive protein inspired coatings. Adv Mater 2009;21(4):431e4. [11] Ball V, Nguyen I, Haupt M, Oehr C, Arnoult C, Toniazzo V, et al. The reduction of Agþ in metallic silver on pseudomelanin films allows for antibacterial activity but does not imply unpaired electrons. J Colloid Interface Sci 2011; 364(2):359e65. [12] Xu H, Shi X, Ma H, Lv Y, Zhang L, Mao Z. The preparation and antibacterial effects of dopa-cotton/AgNPs. Appl Surf Sci 2011;257(15):6799e803. [13] Ball V, Del Frari D, Michel M, Buehler MJ, Toniazzo V, Singh MK, et al. Deposition mechanism and properties of thin polydopamine films for high added value applications in surface science at the nanoscale. BioNanoSci 2012;2(1):16e34. [14] Lee BP, Messersmith PB, Israelachvili JN, Waite JH. Mussel-inspired adhesives and coatings. Ann Rev Mater Res 2011;41:99e132. [15] Yu B, Liu J, Liu S, Zhou F. Pdop layer exhibiting zwitterionicity: a simple electrochemical interface for governing ion permeability. Chem Commun 2010;46(32):5900e2. [16] Ball V. Impedance spectroscopy and zeta potential titration of dopa-melanin films produced by oxidation of dopamine. Colloids Surf A Physicochem Eng Aspects 2010;363(1e3):92e7. [17] Bernsmann F, Ponche A, Ringwald C, Hemmerlé J, Raya J, Bechinger B, et al. Characterization of dopaminemelanin growth on silicon oxide. J Phys Chem C 2009;113(19):8234e42. [18] Bernsmann F, Voegel JC, Ball V. Different synthesis methods allow to tune the permeability and permselectivity of dopamine-melanin films to electrochemical probes. Electrochem Acta 2011;56(11):3914e9.