Journal of Colloid and Interface Science 459 (2015) 29–35
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Composite films of polydopamine–Alcian Blue for colored coating with new physical properties Florian Ponzio a,b, Jérôme Bour c, Vincent Ball a,b,⇑ a
Institut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche, 11 rue Humann, 67085 Strasbourg Cédex, France Université de Strasbourg, Faculté de Chirurgie Dentaire, 8 rue Sainte Elisabeth, 67000 Strasbourg, France c Luxembourg Institute of Science and Technology, ZAE Robert Steichen, 5 rue Bommel, L4940 Hautcharage, Luxembourg b
g r a p h i c a l a b s t r a c t Polydopamine + Alcian blue based coangs
Increase in the reaction time
a r t i c l e
i n f o
Article history: Received 23 June 2015 Revised 2 August 2015 Accepted 3 August 2015 Available online 4 August 2015 Keywords: Dopamine Eumelanins Alcian Blue Composite films
a b s t r a c t Polydopamine (PDA) coatings appear as a universal functionalization methodology allowing to coat the surface of almost all kinds of known materials with a conformal, stable, robust and reactive material. Relatively few investigations were dedicated to the incorporation of other molecules in PDA coatings during their deposition from dopamine solutions under oxidative conditions. Herein we rely on the assumption that the basic building blocks of PDA could be porphyrin like tetramers (as well as higher order oligomers) of 5,6-dihydroxyindole and we investigate the influence of a cationic Cu(II) phtalocyanine, namely Alcian Blue (AB), on the deposition kinetics and on the properties of PDA films. We demonstrate that AB is indeed incorporated in the PDA films to yield a composite PDA–AB coating displaying the optical features of both PDA and AB. The amount of incorporated dye depends on its concentration in solution. The obtained PDA–AB films have a smaller thickness than their related PDA counterparts, a different morphology and a higher permeability to the anionic hexacyanoferrate redox probe. In addition, the incorporation of AB in the films is not homogeneous through their thickness as inferred by means of X-ray photoelectron spectroscopy. The reason for this interesting finding is discussed on the basis of the interactions between AB and PDA as well as on the basis of the structure of PDA films. Ó 2015 Elsevier Inc. All rights reserved.
⇑ Corresponding author at: Institut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche, 11 rue Humann, 67085 Strasbourg Cédex, France. E-mail address: [email protected] (V. Ball). http://dx.doi.org/10.1016/j.jcis.2015.08.006 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Polydopamine films produced from dopamine containing solutions in the presence of an oxidant have become a one pot versatile
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coating methodology able to functionalize any kind of materials [1,2]. The functionalization of surfaces with those dopamine– eumelanin like coatings from oxygenated dopamine solutions, or in the presence of other oxidants offer fascinating possibilities in biomaterials science [3,4], for energy conversion devices [4,5] and for bioelectronics [6]. In addition to the simplicity of the method, polydopamine (PDA) can be deposited at the solid–liquid [1], liquid–liquid [emulsion] and liquid–air interfaces [7,8]. It is also possible to post functionalize these coatings making PDA a central player in a broad range of applications. However the exact structure of PDA is still unknown [9]. Recent experimental [10] and computational studies [11] report that PDA and related materials are probably not of polymeric nature but rather supramolecular aggregates of small oligomers of 5,6-dihydroxyindole (DHI), most probably a DHI tetramer as reported by Chen et al. [11]. Most published studies were focused on PDA as raw material and on the possibilities for post functionalization [12,13] but only a few were devoted to the elaboration of a composite in order to specifically tune the optical properties of PDA which is a black material. Indeed the direct addition of another compound in the dopamine solution can change the self-assembly process of PDA. It has been found that poly(vinyl alcohol) (PVA) allows to significantly decrease the size of eumelanin in a 5,6-dihydroxyindole solution [14]. It has been postulated that the adsorption of PVA on the surface of the particles rich in hydroxyl groups allow for their steric stabilization. Several other polymers like poly(ethylene glycol) and branched poly(N-isopropyl acrylamide) [15] are integrated in the PDA coatings when present in the dopamine solution. Poly(N-vinyl pyrrolidone) allows for an almost complete inhibition of PDA deposition at solid–liquid interfaces [16]. In these investigations the optical properties of PDA were however not affected. The tuning of PDA optical properties is of great interest in chemical, biological and materials science applications. Indeed, the optical absorption spectrum of the brown-black PDA is almost featureless [17], with however a small residual peak around 290 nm due to the presence of small oligomers of DHI [18]. The incorporation of a dye in PDA is expected to change its hue. In this context we decided to investigate how the addition of a dye will affect the self-assembly of PDA and if it will be able to change the coloration of the coating and its optical properties. Recently, So et al. [19] investigated the coloration of PDA coatings with rhodamine B on different fabrics. In the present investigation, we decided to select the structure of the dye assuming that PDA is made of the assembly of a DHI tetramer [11]. It has already been shown by Sarna et al. that eumelanins, structurally close to PDA [8], strongly interact with cationic porphyrins [20] provided in solution to interact with already formed eumelanin. We hence selected a candidate dye on the basis of its structural similarity with the porphyrin like DHI tetramers: namely a cationic copper phtalocyanine, Alcian Blue (AB, Scheme 1). This compound should interact strongly with PDA by p-stacking but also by means of electrostatic interactions, PDA being negatively charged at all pH
values above 4 [21]. Herein we report the formation of PDA–AB composite films with a blue color which intensity depends on the reaction time and the concentration of AB initially present in the dopamine solution. The permeability for hexacyanoferrate anions, the thickness, the morphology, the absorption spectrum and the composition of these films were then characterized respectively by cyclic voltammetry, ellipsometry, Atomic Force Microscopy, UV–Vis spectroscopy and X-ray Photoelectron Spectroscopy. 2. Materials and methods 2.1. Chemicals Dopamine hydrochloride (Product No: H8502, CAS: 62-31-7) and Alcian Blue (A4045) were purchased from Sigma–Aldrich. Tris(hydroxymethyl) aminomethane (Product No:200923-A, CAS: 77-86-1) was obtained from EURODEMEX (Schiltigheim, France). All these chemicals were used as received. 2.2. Polydopamine and polydopamine–Alcian Blue films Dopamine hydrochloride at 2 mg mL 1 (10.6 mM) was dissolved in Tris buffer at 50 mM, pH = 8.5 with oxygen as the oxidant. Alcian Blue (AB) at 0.05, 0.1 or 0.2 mM was dissolved in Tris buffer at 50 mM, pH = 8.5 under strong agitation (300 rpm with a magnetic stirrer) overnight. The solubility of AB was limited to 0.2 mM in these conditions. At higher concentrations, small AB particles remained in solution and could not be dissolved. Even if the solubility of AB can be increased somewhat by the addition of other solvents (ethanol) we decided not to add such solvents to the synthesis medium because they could by themselves modify the deposition of PDA films. Hence we limited our investigation to AB concentrations lower or equal to 0.2 mM. Dopamine was then added to the AB containing solutions, corresponding to the beginning of PDA formation. The dopamine concentration was the same in all experiments and equal to 2 mg mL 1 (10.6 mM). The adsorption substrates were dipped in the dopamine + AB solutions for different durations up to 24 h. They were then rinsed thoroughly with pure water and dried under a nitrogen flux before characterization. Some control experiments were performed in the absence of AB, yielding PDA coatings. The films prepared from dopamine–AB mixtures will be called of the PDA–AB type. We performed some additional experiments in which the dopamine solution was refreshed every 2 h. In the first series of these experiments each dopamine solution at 2 mg mL 1 contained 0.2 mM AB and in the second series only the first deposition was made in the presence of dopamine + AB (0.2 mM), the subsequent depositions were made only in the presence of dopamine but in the absence of AB. The two series of experiments were compared by measuring the spectra of the films after every 2 h deposition cycle. These experiments were aimed to simulate the influence of a disappearance of AB in the dopamine solution, due to its incorporation in the film, on the optical properties of the films. 2.3. Characterization of PDA and PDA–AB films
Scheme 1. Chemical structure of Alcian Blue.
The Si wafers (Siltronix France) and quartz slides (Thuet, Blodelsheim, France) needed for the experiments were cleaned with the following treatment: Hellmanex at 2% v/v during half an hour, distilled water, 0.1 M HCl, water, NaClO at 1 g/L, and finally with distilled water. For the cyclic voltammetry (CV) experiments, freshly polished amorphous carbon electrodes (ref. 104 from CH Instruments, Austin, Texas) were hanged vertically in the dopamine + AB solution and were removed from the solution at
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regular time intervals, rinsed with water and dried with nitrogen. The deposition of PDA and PDA–AB on amorphous carbon electrodes was investigated indirectly by its influence on the oxidation/reduction current of K4Fe(CN)6 (ref. 9387, Sigma–Aldrich) dissolved at 1.0 mM in a 150 mM NaCl solution. The CV experiments were performed with a conventional three electrode set up (CH Instruments 604, Austin, Texas) as described elsewhere [22]. The reference and counter electrodes were an Ag/AgCl electrode and a Pt wire respectively, all from CH Instruments. UV–Vis spectra were acquired using a double beam UV–mc2 spectrophotometer (SAFAS, Monaco). The spectra were acquired using a cleaned quartz slide as the reference. The absorbance values correspond to slides coated on their both faces. The absorption spectra of the polydopamine coated quartz slides were recorded between 200 and 600 nm with a spectral resolution of 1 nm. The thickness of the PDA and PDA–AB films were measured with an AUTO SE spectroscopic ellipsometer (Horiba, France) operating in the wavelength range between 450 and 900 nm and at a constant incidence angle of 70°. The ellipsometric angles w(k) and D(k) were then fitted with a three layer model: a stack of semi-infinite silicon, a 2 nm thick SiO2 layer and a topmost PDA layer which was modeled with a semiconductor dispersion curve. We fixed the complex refractive index of the PDA layer at 1.73 + 0.02 i at k = 632.8 nm according to the measurement of the real part of the refractive index and the absorption coefficient of polydopamine [22].
The thickness of the films was also measured by Atomic Force Microscopy (AFM). AFM images were taken on a Bioscope Catalyst (Bruker) in the scan assist air mode with a MLCT cantilever (spring constant k = 0.02 N m 1 as given by the furnisher). The thickness was determined on 20 regularly spaced line scans spanning the region between the film and the pristine Si. The samples were needle scratched before imaging. Each data point was acquired on an independently prepared film. The surface chemical composition of the PDA and PDA–AB films were analyzed by X-ray photoelectron spectroscopy (Hemispherical Energy Analyzer SPECS, PHOIBOS 150) with a monochromatic Al Ka (1486.7 eV) source operating at 200 W with an anode voltage of 12 kV.
3. Results and discussion PDA deposited on interfaces and produced in solution is characterized by a featureless and broadband absorption throughout the entire UV–Vis range [17,23]. On the other hand, dissolved AB displays a blue color and shows three maximum absorptions at 260, 336 and 600 nm which is typical for phtalocyanins. Assuming that PDA will interact strongly with AB by p-stacking and electrostatic interactions and because of their structural similarity, AB is expected to be incorporated in PDA films and to change the optical properties of the resulting coatings.
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Fig. 1. (a) Digital pictures of glass slides after being put in dopamine–AB blends with AB concentrations of 0.05, 0.1 and 0.2 mM after 1, 3, and 24 h of reaction. The dopamine concentration was the same, 10.6 mM, in all experiments. (b) UV–Vis spectra taken on quartz slides put in dopamine–AB mixtures after 3 h of reaction and increasing AB concentration from 0.05 to 0.2 mM as indicated in the inset. (c) UV–Vis spectra taken on quartz slides put in dopamine–AB mixtures after 24 h of reaction and increasing AB concentration from 0.05 to 0.2 mM as indicated in the inset.
F. Ponzio et al. / Journal of Colloid and Interface Science 459 (2015) 29–35
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(A)
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Cu/C
Once the AB solutions and dopamine were mixed they became dark-brown because of the oxidation of dopamine. Pictures of the coatings obtained for the dopamine–AB mixtures at AB concentrations of 0.05, 0.1, 0.2 mM and after three different times of reactions are shown in Fig. 1a. From the pictures obtained after 1 and 3 h of reaction, it is clear that the blue coloration of the films increases. Even for the PDA–AB films produced at 0.2 mM in AB and after 24 h of reaction some blue color is still visible even if the black coloration dominates. This is not the case for the films produced in the presence of the two smaller dye concentrations where the blue coloration is not perceptible anymore by eye after 24 h of reaction. Hence, it is apparent that for the short reaction time, AB is present in or on the surface of the films because of the blue color but for 24 h hours of reaction the samples are brown suggesting a predominance of PDA like structures and properties. In a control experiment, we showed that a quartz slide dipped for 24 h in the AB solution at 0.2 mM and rinsed with water in a manner similar to the coatings obtained from PDA–AB blends, did not display the characteristic absorption bands of AB. This means that the blue hue and permanent coloration observed after short reaction times from dopamine–AB mixtures is the result of strong interactions between PDA and AB forcing the dye to be deposited at the surface. In an additional experiment PDA was deposited during 3 h on a quartz plate in the absence of AB, rinsed with water and characterized by means of UV–Vis spectroscopy. This slide was then immersed in an AB solution at 0.2 mM during 1 h, rinsed, dried and characterized again by UV–Vis spectroscopy. This time some small but detectable bands at 336 and 600 nm characteristic of AB were visible on the spectrum (data not shown). This experiment shows that AB interacts with PDA but not with quartz since no bands due to AB are apparent on unmodified quartz. This visual impression is correlated by the UV–Vis data of the PDA–AB films for the three different concentrations in AB and after 3 or 24 h of reaction (Fig. 1b and c). The spectra of the films obtained after 3 h of reaction display a peak at k = 336 nm due to the presence of AB in the film. The same holds true for the Q band of AB close to 600 nm. The intensity of the peak at 336 nm increases with the AB concentration for a given deposition time of 3 h. When the reaction time is increased to 24 h, the characteristic peaks of AB at k = 336 nm and 600 nm, is no longer apparent on the absorption spectra of the films. This means that additional incorporation of AB is not possible anymore at long time of reaction or that the coloration of AB is masked by intense absorption of the PDA component. To further show the incorporation of AB in the films, XPS spectroscopy was performed in order to identify the presence of Cu atoms. Typical XPS spectra are shown in Fig. 1 of the Supplementary data file. Fig. 2 represents the time evolution of the C/N and Cu/C ratios for the PDA and PDA–AB 0.1 mM films. This plot shows that for the PDA–AB 0.1 mM the Cu/C ratio is decreasing with time of reaction and cannot be seen anymore after 24 h of reaction. However, on the same film, the C/N ratio is increasing and for the long time of reaction is close to the C/N ratio of the PDA film. Control experiments have shown that the Cu content in PDA films is very small yielding Cu/C ratios of the order of 10–5, hence two orders of magnitude less than for the PDA–AB films at small reaction times. In addition, Table 1 demonstrates an increase in Cu/N ratio on the surface of the films when the AB concentration in solution increases at a constant reaction time of 3 h. After 24 h of reaction the XPS spectra show that the PDA–AB and PDA films have almost an identical composition and that Cu is no longer detectable in the top most part of the PDA–AB films. Such films are about 30 nm thick in the dry state (XPS is a high
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t (h) Fig. 2. (A) Time evolution of the Cu/C ratio for the PDA–AB ( ) and the PDA films (j) as determined by means of XPS. (B) Time evolution of the C/N ratios for the PDA–AB ( ) and for the PDA films (j). The PDA-A films were produced in the presence of 0.1 mM in AB, the polydopamine concentration was of 10.6 mM in all cases. Each point corresponds to an individual experiment for an independently coated silicon wafer. The lines are only aimed to guide the eye.
Table 1 Evolution of Cu/C and C/N ratios as a function of the AB concentration in the composite PDA–AB films. The film deposition time was constant and equal to 3 h. Sample
Cu 2p3/2 position (eV)
C 1s position (eV)
N 1s position (eV)
Cu/C
C/N
PDA–AB 0.05 mM 3 h PDA–AB 0.1 mM 3 h PDA–AB 0.2 mM 3 h
934.5 934.0 934.3
286.7 282.2 284.6
399.5 399.7 398.9
0.0008 0.0022 0.0027
6.8875 6.4149 6.9175
vacuum technique) whereas the sampling depth of the X-rays employed is only a few nm. This means that no Cu is present in the top most part of the PDA–AB films and hence that AB is only present in the deepest part of the film, close to its interface with the solid substrate. An alternative explanation is that AB present in the initial steps of the deposition process, as exemplified by means of UV–Vis spectroscopy (Fig. 1), and XPS spectroscopy (Fig. 2), progressively leaches out from the coating during prolonged deposition time in contact with Tris buffer. This assumption was however ruled out by the following experiment: a PDA–AB coating obtained after 3 h of dopamine oxidation in the presence of 0.1 mM AB, was removed from the solution, rinsed and immersed in Tris buffer (without present dopamine and AB). No release of AB was observed for weeks, meaning that the interactions between AB and PDA are strong enough to imped the desorption of the dye in the solution. Hence during the last steps of the film deposition, no AB is incorporated in the film and all the
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the probe to reach the electrode surface on a macroscopic surface typically 1 mm2 in surface area. PDA by itself is not conductive enough to allow for the oxidation of the selected redox probe by single contact with the PDA film [21]. The PDA and PDA–AB (produced in the presence of 0.1 mM AB) films coated for 3, 6, and 24 h were tested for their permeability to the hexacyanoferrate anions. Fig. 4a shows that after 3 h of reaction the PDA film already suppressed the faradic currents of hexacyanoferrate anions, in agreement with previous investigations [24]. However for the PDA–AB film (Fig. 4b) the oxidation and reduction peak of the hexacyanoferrate anions are still present after 3 h of deposition in the presence of 0.1 mM AB. This demonstrates that PDA–AB film keeps some permeability for Fe(CN)46 anions whereas for similar film thicknesses the PDA films are already totally impermeable. This is due either to the presence of a different internal porosity in the PDA–AB films or to the presence of channels, not detectable from the AFM topographies of PDA–AB films, contacting the solution and the pristine electrode. After 6 h of reaction, the hexacyanoferrate peaks cannot be seen anymore even in the PDA–AB films but a peak at around 0.6 V, already observed after 3 h of deposition, is still present and attributed to AB. Indeed CV experiments of AB solutions display an oxidation peak at the same potential (data not shown). Finally after 24 h of reaction, the CV of the PDA–AB films is very similar to the one of PDA in the presence of Fe(CN)46 anions in the solution. These findings are in qualitative agreement with the UV–Vis
experimental data show that after prolonged deposition time, the film properties (the thickness and morphology excepted) are closer to that of pure PDA than to that of the PDA–AB composite (which is blue, rich in Cu and electroactive). In addition the composition of the extreme surface of the films becomes similar for the PDA and the PDA–AB coatings as inferred from the XPS data (Fig. 2). The changes in optical properties and in composition are not the only differences between the PDA–AB and PDA coatings. Indeed the thicknesses of the PDA–AB films is lower than the thickness of the PDA ones (Fig. 3a) for all the investigated deposition times. After 24 h of reaction the PDA–AB films are (30 ± 4) nm thick and the PDA ones are (50 ± 5) nm thick. This difference in thickness becomes higher when the reaction time increases (Fig. 3a). This is a proof that the presence of AB modifies the deposition of PDA films. In addition the morphologies of the PDA films are different when they are produced from dopamine solutions in the presence of AB. Both the PDA and PDA–AB films have a granular structure but the PDA–AB films (Fig. 3c) display a bigger grain size and are rougher compared to the PDA ones (Fig. 3b). The thickness difference between the PDA and the PDA–AB films was confirmed by means of spectroscopic ellipsometry (Fig. 2 of the Supplementary Data section). The difference in morphology and thickness between the films incited us to investigate the permeability of the films for an electrochemical probe like hexacyanoferrate anions. Such an experiment allows to detect the presence of nanopores, sufficient for
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Fig. 3. (a) Evolution of the thickness of the PDA (d) films and of the PDA–AB 0.1 mM ( ) films determined by AFM on needle scratched films. (b) Typical AFM topographical image of a PDA films after 24 h of reaction with a root mean squared roughness of 8 nm and (c) typical AFM topographical image of PDA–AB films with a root mean squared roughness of 11 nm. Both films characterized by AFM were obtained after 24 h of reaction. Each point in part a corresponds to an individual sample and the thickness was obtained from 20 profiles perpendicular to scratched lines. They are affected by a relative standard deviation of about 10%.
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Fig. 4. CV experiments performed at a potential scan rate of 100 mV s 1 on working electrodes coated during 3, 6, and 24 h with PDA films (a) and PDA–AB films (b). The later were produced in the presence of 0.1 mM AB. The redox probe was potassium hexacyanoferrate at 1 mM in the presence of 50 mM Tris + 150 mM NaCl.
Long deposion mes
Inial deposion mes
Simultaneous deposion of AB and PDA
All AB available is incorporated, PDA deposion connues
Fig. 5. Schematic representation of the deposition of PDA–AB films with a progressive removal of AB from the solution whereas PDA continues to deposit.
experiments where it was shown that the optical properties of the PDA–AB and PDA films become almost undistinguishable at long reaction times whereas they were markedly different at the initial steps of the deposition (Fig. 1b and c). To explain all these findings, and in particular the reduction in the blue coloration (Fig. 1) as well as the reduction in the Cu/C ratio (Fig. 2) when the film grows, two assumptions are formulated: (i) all the initially available AB is completely consumed during the first hours of reaction (ii) small oligomers of oxidized dopamine, formed essentially at the beginning of the oxidation process are needed to form the PDA–AB composite, AB being not incorporated anymore at longer reaction times when the small oligomers of oxidized dopamine are consumed. In order to confirm one of these assumptions, UV–Vis spectroscopy was performed on a solution of PDA–AB 0.2 mM at certain time of reaction. The signal of AB could not be detected even after 30 min of reaction meaning that all of it is already consumed. This experiment explains why the AB is only visible at short reaction times in the film. The mechanism of dye incorporation in the PDA films is represented in Fig. 5: the incorporation of AB in the initial steps of the film deposition before it is totally depleted from the solution is able to explain all our experimental findings, and in particular a progressive decrease in the Cu content measured by means of XPS when the reaction time increases. An additional series of experiments was performed to simulate a progressive decrease in the AB concentration on the optical spectra of the obtained films. To that aim the dopamine solution was
refreshed every 2 h, the films were rinsed and blown dry before spectral characterization. In the first series of experiments the dopamine solutions also contained AB at 0.2 mM in each deposition step. The incorporation of AB in the film increased progressively (Fig. 3 in the Supplementary Data) as exemplified by the increase in the absorbance at 336 nm and more particularly by the persistence of strong blue coloration and a high A336nm/ A500nm ratio (AB does almost not absorb at 500 nm at which PDA strongly absorbs) when additional deposition steps are performed (Fig. 6). On the other hand when AB was added only in the first PDA deposition step but not in the two next ones, the absorbance at 336 nm increased less (Fig. 4 in the Supplementary Data section) than in the first series of experiments, the final film appeared less blue and the A336nm/A500nm ratio decreased rapidly. This shows that a progressive reduction in the AB fed in the dopamine solution allows to produce a film that appears progressively less blue in agreement with the model proposed in Fig. 5. The fact that the film thickness and morphology is affected by the presence of AB as well as the total consumption of the dye from the solution point to very strong interactions between AB and PDA. Such interactions need to be investigated from a structural point of view in future studies. Unfortunately, as already explained, the dye concentration cannot be increased above 0.2 mM since AB is not soluble anymore in water. Otherwise, we could have investigated if increased AB concentration would allow for its incorporation in the top most part of the films. Another possibility to overcome the quantitative depletion of AB from the reaction mixtures consists to perform the deposition of the composite films in a multistep manner [25] using freshly prepared dopamine–AB mixtures, as shown in Fig. 6.
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7
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t (h) Fig. 6. Evolution of the A336nm/A500nm absorbance ratio for films deposited on quartz slides when AB (at 0.2 mM) was added in each freshly prepared dopamine solution (2 mg mL 1) ( , curve A) or when AB was added only during the first deposition step whereas the two last deposition steps were performed in the absence of AB (j, curve B). The optical pictures on the right show the difference in the film color after the 3 deposition cycles shown on curves A and B.
4. Conclusion It has been shown that it is possible to form a composite film of PDA–AB by adding AB in a solution of PDA. The addition of AB allows changing the optical properties of the PDA coating by rending it blue at short reaction times of 3 and 6 h. By changing the reaction time and the concentration of AB it is possible to tune the color of the film when the concentration of AB is increased. The PDA–AB films finally become black-brown like PDA films for long time of reaction. The incorporation of AB also changes the permeability, the morphology and thickness of the film. However this incorporation is not uniform in the film and the copper present in AB is not visible anymore after 24 h of reaction as shown by the XPS data. This gradient of incorporation arises from the fact that all the AB is consumed during the first hour of reaction. So by adding a compound that can interact strongly with PDA it is possible to form a composite film and in this case to change the optical properties of the PDA films. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.08.006. References [1] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007) 426– 430. [2] D.R. Dreyer, D.J. Miller, B.D. Freeman, D.R. Paul, C.W. Bielawski, Chem. Sci. 4 (2013) 3796–3802. [3] M.E. Lynge, R. van der Westen, A. Postma, B. Städler, Nanoscale 3 (2012) 4916– 4928.
[4] V. Ball, D. Del Frari, M. Michel, M.J. Buehler, V. Toniazzo, M.K. Singh, J. Gracio, D. Ruch, BioNanoSci 2 (2012) 16–34. [5] H.J. Nam, J. Cha, S.H. Lee, W.J. Yoo, D.-Y. Jung, Chem. Comm. 50 (2014) 1458– 1461. [6] M. Ambrico, P.F. Ambrico, A. Cardone, N.F. Della Vecchia, T. Ligonzo, S.R. Cicco, M. Mastropasqua Talamo, A. Napolitano, V. Augeli, G.M. Farinola, M. d’Ischia, J. Mater. Chem. C 1 (2013) 1018–1028. [7] F. Ponzio, P. Payamyar, A. Schneider, M. Winterhalter, J. Bour, F. Addiego, M.-P. Krafft, J. Hemmerlé, V. Ball, J. Phys. Chem. Lett. 5 (2014) 3436–3440. [8] S. Hong, C.F. Schaber, K. Dening, E. Appel, S.N. Gorb, H. Lee, Adv. Mater. 26 (2014) 7581–7587. [9] M. d’Ischia, A. Napolitano, V. Ball, C.-T. Chen, M. Buehler, Acc. Chem. Res. 47 (2014) 3541–3550. [10] N.F. Della Vecchia, R. Avolio, M. Alfè, M.E. Errico, A. Napolitano, M. d’Ischia, Adv. Funct. Mater. 23 (2013) 1331. [11] C.-T. Chen, C. Chuang, J. Cao, V. Ball, D. Ruch, M.J. Buehler, Nat. Comm. 5 (2014) 4859. [12] H. Lee, J. Rho, P.B. Messersmith, Adv. Mater. 21 (2009) 431–434. [13] H.O. Ham, Z. Liu, K.H. Aaron Lau, H. Lee, P.B. Messersmith, Angew. Chem. Int. Ed. 50 (2011) (2011) 732–736. [14] M. Arzillo, G. Mangiapia, A. Pezzella, R.K. Heenan, A. Radulescu, L. Paduano, M. d’Ischia, Biomacromolecules 13 (2012) 2379–2390. [15] Y. Zhang, B. Thingholm, K.N. Goldie, R. Ogaki, B. Städler, Langmuir 28 (2012) 17585–17592. [16] Y. Zhang, B.M. Teo, A. Postma, F. Ercole, R. Ogaki, M. Zhu, B. Städler, J. Phys. Chem. B 117 (2013) 10504–10512. [17] P. Meredith, B.J. Powell, J. Riesz, S.P. Nighswander-Rempel, M.R. Pederson, E.G. Moore, Soft Matter 2 (2006) 37–44. [18] F. Ponzio, V. Ball, Colloids Surf. A: Physicochem. Eng. Aspects 443 (2014) 540– 543. [19] V.L.L. So, L. He, J.H. Xin, RSC Adv. 4 (2014) 41081–41086. [20] D. Wróbel, A. Planner, I. Haniz, A. Wilgus, T. Sarna, J. Photochem. Photobiol. B: Biol. 41 (1997) 45–52. [21] V. Ball, Colloids Surf. A: Physicochem. Eng. Aspects 363 (2010) 92–97. [22] F. Bernsmann, A. Ponche, Ch. Ringwald, J. Hemmerlé, J. Raya, B. Bechinger, J.-C. Voegel, P. Schaaf, V. Ball, J. Phys. Chem. C 113 (2009) 8234–8242. [23] P. Meredith, T. Sarna, Pig. Cell Res. 19 (2006) 572–594. [24] F. Bernsmann, J.-C. Voegel, V. Ball, Electrochim. Acta 56 (2011) 3914–3919. [25] F. Bernsmann, V. Ball, F. Addiego, A. Ponche, M. Michel, J.J. de Almeida Gracio, V. Toniazzo, D. Ruch, Langmuir 27 (2011) 2819–2825.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Composite films of polydopamine–Alcian Blue for colored coating with new physical properties Florian Ponzio a,b, Jérôme Bour c, Vincent Ball a,b,⇑ a
Institut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche, 11 rue Humann, 67085 Strasbourg Cédex, France Université de Strasbourg, Faculté de Chirurgie Dentaire, 8 rue Sainte Elisabeth, 67000 Strasbourg, France c Luxembourg Institute of Science and Technology, ZAE Robert Steichen, 5 rue Bommel, L4940 Hautcharage, Luxembourg b
g r a p h i c a l a b s t r a c t Polydopamine + Alcian blue based coangs
Increase in the reaction time
a r t i c l e
i n f o
Article history: Received 23 June 2015 Revised 2 August 2015 Accepted 3 August 2015 Available online 4 August 2015 Keywords: Dopamine Eumelanins Alcian Blue Composite films
a b s t r a c t Polydopamine (PDA) coatings appear as a universal functionalization methodology allowing to coat the surface of almost all kinds of known materials with a conformal, stable, robust and reactive material. Relatively few investigations were dedicated to the incorporation of other molecules in PDA coatings during their deposition from dopamine solutions under oxidative conditions. Herein we rely on the assumption that the basic building blocks of PDA could be porphyrin like tetramers (as well as higher order oligomers) of 5,6-dihydroxyindole and we investigate the influence of a cationic Cu(II) phtalocyanine, namely Alcian Blue (AB), on the deposition kinetics and on the properties of PDA films. We demonstrate that AB is indeed incorporated in the PDA films to yield a composite PDA–AB coating displaying the optical features of both PDA and AB. The amount of incorporated dye depends on its concentration in solution. The obtained PDA–AB films have a smaller thickness than their related PDA counterparts, a different morphology and a higher permeability to the anionic hexacyanoferrate redox probe. In addition, the incorporation of AB in the films is not homogeneous through their thickness as inferred by means of X-ray photoelectron spectroscopy. The reason for this interesting finding is discussed on the basis of the interactions between AB and PDA as well as on the basis of the structure of PDA films. Ó 2015 Elsevier Inc. All rights reserved.
⇑ Corresponding author at: Institut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche, 11 rue Humann, 67085 Strasbourg Cédex, France. E-mail address: [email protected] (V. Ball). http://dx.doi.org/10.1016/j.jcis.2015.08.006 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Polydopamine films produced from dopamine containing solutions in the presence of an oxidant have become a one pot versatile
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coating methodology able to functionalize any kind of materials [1,2]. The functionalization of surfaces with those dopamine– eumelanin like coatings from oxygenated dopamine solutions, or in the presence of other oxidants offer fascinating possibilities in biomaterials science [3,4], for energy conversion devices [4,5] and for bioelectronics [6]. In addition to the simplicity of the method, polydopamine (PDA) can be deposited at the solid–liquid [1], liquid–liquid [emulsion] and liquid–air interfaces [7,8]. It is also possible to post functionalize these coatings making PDA a central player in a broad range of applications. However the exact structure of PDA is still unknown [9]. Recent experimental [10] and computational studies [11] report that PDA and related materials are probably not of polymeric nature but rather supramolecular aggregates of small oligomers of 5,6-dihydroxyindole (DHI), most probably a DHI tetramer as reported by Chen et al. [11]. Most published studies were focused on PDA as raw material and on the possibilities for post functionalization [12,13] but only a few were devoted to the elaboration of a composite in order to specifically tune the optical properties of PDA which is a black material. Indeed the direct addition of another compound in the dopamine solution can change the self-assembly process of PDA. It has been found that poly(vinyl alcohol) (PVA) allows to significantly decrease the size of eumelanin in a 5,6-dihydroxyindole solution [14]. It has been postulated that the adsorption of PVA on the surface of the particles rich in hydroxyl groups allow for their steric stabilization. Several other polymers like poly(ethylene glycol) and branched poly(N-isopropyl acrylamide) [15] are integrated in the PDA coatings when present in the dopamine solution. Poly(N-vinyl pyrrolidone) allows for an almost complete inhibition of PDA deposition at solid–liquid interfaces [16]. In these investigations the optical properties of PDA were however not affected. The tuning of PDA optical properties is of great interest in chemical, biological and materials science applications. Indeed, the optical absorption spectrum of the brown-black PDA is almost featureless [17], with however a small residual peak around 290 nm due to the presence of small oligomers of DHI [18]. The incorporation of a dye in PDA is expected to change its hue. In this context we decided to investigate how the addition of a dye will affect the self-assembly of PDA and if it will be able to change the coloration of the coating and its optical properties. Recently, So et al. [19] investigated the coloration of PDA coatings with rhodamine B on different fabrics. In the present investigation, we decided to select the structure of the dye assuming that PDA is made of the assembly of a DHI tetramer [11]. It has already been shown by Sarna et al. that eumelanins, structurally close to PDA [8], strongly interact with cationic porphyrins [20] provided in solution to interact with already formed eumelanin. We hence selected a candidate dye on the basis of its structural similarity with the porphyrin like DHI tetramers: namely a cationic copper phtalocyanine, Alcian Blue (AB, Scheme 1). This compound should interact strongly with PDA by p-stacking but also by means of electrostatic interactions, PDA being negatively charged at all pH
values above 4 [21]. Herein we report the formation of PDA–AB composite films with a blue color which intensity depends on the reaction time and the concentration of AB initially present in the dopamine solution. The permeability for hexacyanoferrate anions, the thickness, the morphology, the absorption spectrum and the composition of these films were then characterized respectively by cyclic voltammetry, ellipsometry, Atomic Force Microscopy, UV–Vis spectroscopy and X-ray Photoelectron Spectroscopy. 2. Materials and methods 2.1. Chemicals Dopamine hydrochloride (Product No: H8502, CAS: 62-31-7) and Alcian Blue (A4045) were purchased from Sigma–Aldrich. Tris(hydroxymethyl) aminomethane (Product No:200923-A, CAS: 77-86-1) was obtained from EURODEMEX (Schiltigheim, France). All these chemicals were used as received. 2.2. Polydopamine and polydopamine–Alcian Blue films Dopamine hydrochloride at 2 mg mL 1 (10.6 mM) was dissolved in Tris buffer at 50 mM, pH = 8.5 with oxygen as the oxidant. Alcian Blue (AB) at 0.05, 0.1 or 0.2 mM was dissolved in Tris buffer at 50 mM, pH = 8.5 under strong agitation (300 rpm with a magnetic stirrer) overnight. The solubility of AB was limited to 0.2 mM in these conditions. At higher concentrations, small AB particles remained in solution and could not be dissolved. Even if the solubility of AB can be increased somewhat by the addition of other solvents (ethanol) we decided not to add such solvents to the synthesis medium because they could by themselves modify the deposition of PDA films. Hence we limited our investigation to AB concentrations lower or equal to 0.2 mM. Dopamine was then added to the AB containing solutions, corresponding to the beginning of PDA formation. The dopamine concentration was the same in all experiments and equal to 2 mg mL 1 (10.6 mM). The adsorption substrates were dipped in the dopamine + AB solutions for different durations up to 24 h. They were then rinsed thoroughly with pure water and dried under a nitrogen flux before characterization. Some control experiments were performed in the absence of AB, yielding PDA coatings. The films prepared from dopamine–AB mixtures will be called of the PDA–AB type. We performed some additional experiments in which the dopamine solution was refreshed every 2 h. In the first series of these experiments each dopamine solution at 2 mg mL 1 contained 0.2 mM AB and in the second series only the first deposition was made in the presence of dopamine + AB (0.2 mM), the subsequent depositions were made only in the presence of dopamine but in the absence of AB. The two series of experiments were compared by measuring the spectra of the films after every 2 h deposition cycle. These experiments were aimed to simulate the influence of a disappearance of AB in the dopamine solution, due to its incorporation in the film, on the optical properties of the films. 2.3. Characterization of PDA and PDA–AB films
Scheme 1. Chemical structure of Alcian Blue.
The Si wafers (Siltronix France) and quartz slides (Thuet, Blodelsheim, France) needed for the experiments were cleaned with the following treatment: Hellmanex at 2% v/v during half an hour, distilled water, 0.1 M HCl, water, NaClO at 1 g/L, and finally with distilled water. For the cyclic voltammetry (CV) experiments, freshly polished amorphous carbon electrodes (ref. 104 from CH Instruments, Austin, Texas) were hanged vertically in the dopamine + AB solution and were removed from the solution at
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regular time intervals, rinsed with water and dried with nitrogen. The deposition of PDA and PDA–AB on amorphous carbon electrodes was investigated indirectly by its influence on the oxidation/reduction current of K4Fe(CN)6 (ref. 9387, Sigma–Aldrich) dissolved at 1.0 mM in a 150 mM NaCl solution. The CV experiments were performed with a conventional three electrode set up (CH Instruments 604, Austin, Texas) as described elsewhere [22]. The reference and counter electrodes were an Ag/AgCl electrode and a Pt wire respectively, all from CH Instruments. UV–Vis spectra were acquired using a double beam UV–mc2 spectrophotometer (SAFAS, Monaco). The spectra were acquired using a cleaned quartz slide as the reference. The absorbance values correspond to slides coated on their both faces. The absorption spectra of the polydopamine coated quartz slides were recorded between 200 and 600 nm with a spectral resolution of 1 nm. The thickness of the PDA and PDA–AB films were measured with an AUTO SE spectroscopic ellipsometer (Horiba, France) operating in the wavelength range between 450 and 900 nm and at a constant incidence angle of 70°. The ellipsometric angles w(k) and D(k) were then fitted with a three layer model: a stack of semi-infinite silicon, a 2 nm thick SiO2 layer and a topmost PDA layer which was modeled with a semiconductor dispersion curve. We fixed the complex refractive index of the PDA layer at 1.73 + 0.02 i at k = 632.8 nm according to the measurement of the real part of the refractive index and the absorption coefficient of polydopamine [22].
The thickness of the films was also measured by Atomic Force Microscopy (AFM). AFM images were taken on a Bioscope Catalyst (Bruker) in the scan assist air mode with a MLCT cantilever (spring constant k = 0.02 N m 1 as given by the furnisher). The thickness was determined on 20 regularly spaced line scans spanning the region between the film and the pristine Si. The samples were needle scratched before imaging. Each data point was acquired on an independently prepared film. The surface chemical composition of the PDA and PDA–AB films were analyzed by X-ray photoelectron spectroscopy (Hemispherical Energy Analyzer SPECS, PHOIBOS 150) with a monochromatic Al Ka (1486.7 eV) source operating at 200 W with an anode voltage of 12 kV.
3. Results and discussion PDA deposited on interfaces and produced in solution is characterized by a featureless and broadband absorption throughout the entire UV–Vis range [17,23]. On the other hand, dissolved AB displays a blue color and shows three maximum absorptions at 260, 336 and 600 nm which is typical for phtalocyanins. Assuming that PDA will interact strongly with AB by p-stacking and electrostatic interactions and because of their structural similarity, AB is expected to be incorporated in PDA films and to change the optical properties of the resulting coatings.
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Fig. 1. (a) Digital pictures of glass slides after being put in dopamine–AB blends with AB concentrations of 0.05, 0.1 and 0.2 mM after 1, 3, and 24 h of reaction. The dopamine concentration was the same, 10.6 mM, in all experiments. (b) UV–Vis spectra taken on quartz slides put in dopamine–AB mixtures after 3 h of reaction and increasing AB concentration from 0.05 to 0.2 mM as indicated in the inset. (c) UV–Vis spectra taken on quartz slides put in dopamine–AB mixtures after 24 h of reaction and increasing AB concentration from 0.05 to 0.2 mM as indicated in the inset.
F. Ponzio et al. / Journal of Colloid and Interface Science 459 (2015) 29–35
1e-1
(A)
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Cu/C
Once the AB solutions and dopamine were mixed they became dark-brown because of the oxidation of dopamine. Pictures of the coatings obtained for the dopamine–AB mixtures at AB concentrations of 0.05, 0.1, 0.2 mM and after three different times of reactions are shown in Fig. 1a. From the pictures obtained after 1 and 3 h of reaction, it is clear that the blue coloration of the films increases. Even for the PDA–AB films produced at 0.2 mM in AB and after 24 h of reaction some blue color is still visible even if the black coloration dominates. This is not the case for the films produced in the presence of the two smaller dye concentrations where the blue coloration is not perceptible anymore by eye after 24 h of reaction. Hence, it is apparent that for the short reaction time, AB is present in or on the surface of the films because of the blue color but for 24 h hours of reaction the samples are brown suggesting a predominance of PDA like structures and properties. In a control experiment, we showed that a quartz slide dipped for 24 h in the AB solution at 0.2 mM and rinsed with water in a manner similar to the coatings obtained from PDA–AB blends, did not display the characteristic absorption bands of AB. This means that the blue hue and permanent coloration observed after short reaction times from dopamine–AB mixtures is the result of strong interactions between PDA and AB forcing the dye to be deposited at the surface. In an additional experiment PDA was deposited during 3 h on a quartz plate in the absence of AB, rinsed with water and characterized by means of UV–Vis spectroscopy. This slide was then immersed in an AB solution at 0.2 mM during 1 h, rinsed, dried and characterized again by UV–Vis spectroscopy. This time some small but detectable bands at 336 and 600 nm characteristic of AB were visible on the spectrum (data not shown). This experiment shows that AB interacts with PDA but not with quartz since no bands due to AB are apparent on unmodified quartz. This visual impression is correlated by the UV–Vis data of the PDA–AB films for the three different concentrations in AB and after 3 or 24 h of reaction (Fig. 1b and c). The spectra of the films obtained after 3 h of reaction display a peak at k = 336 nm due to the presence of AB in the film. The same holds true for the Q band of AB close to 600 nm. The intensity of the peak at 336 nm increases with the AB concentration for a given deposition time of 3 h. When the reaction time is increased to 24 h, the characteristic peaks of AB at k = 336 nm and 600 nm, is no longer apparent on the absorption spectra of the films. This means that additional incorporation of AB is not possible anymore at long time of reaction or that the coloration of AB is masked by intense absorption of the PDA component. To further show the incorporation of AB in the films, XPS spectroscopy was performed in order to identify the presence of Cu atoms. Typical XPS spectra are shown in Fig. 1 of the Supplementary data file. Fig. 2 represents the time evolution of the C/N and Cu/C ratios for the PDA and PDA–AB 0.1 mM films. This plot shows that for the PDA–AB 0.1 mM the Cu/C ratio is decreasing with time of reaction and cannot be seen anymore after 24 h of reaction. However, on the same film, the C/N ratio is increasing and for the long time of reaction is close to the C/N ratio of the PDA film. Control experiments have shown that the Cu content in PDA films is very small yielding Cu/C ratios of the order of 10–5, hence two orders of magnitude less than for the PDA–AB films at small reaction times. In addition, Table 1 demonstrates an increase in Cu/N ratio on the surface of the films when the AB concentration in solution increases at a constant reaction time of 3 h. After 24 h of reaction the XPS spectra show that the PDA–AB and PDA films have almost an identical composition and that Cu is no longer detectable in the top most part of the PDA–AB films. Such films are about 30 nm thick in the dry state (XPS is a high
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t (h) Fig. 2. (A) Time evolution of the Cu/C ratio for the PDA–AB ( ) and the PDA films (j) as determined by means of XPS. (B) Time evolution of the C/N ratios for the PDA–AB ( ) and for the PDA films (j). The PDA-A films were produced in the presence of 0.1 mM in AB, the polydopamine concentration was of 10.6 mM in all cases. Each point corresponds to an individual experiment for an independently coated silicon wafer. The lines are only aimed to guide the eye.
Table 1 Evolution of Cu/C and C/N ratios as a function of the AB concentration in the composite PDA–AB films. The film deposition time was constant and equal to 3 h. Sample
Cu 2p3/2 position (eV)
C 1s position (eV)
N 1s position (eV)
Cu/C
C/N
PDA–AB 0.05 mM 3 h PDA–AB 0.1 mM 3 h PDA–AB 0.2 mM 3 h
934.5 934.0 934.3
286.7 282.2 284.6
399.5 399.7 398.9
0.0008 0.0022 0.0027
6.8875 6.4149 6.9175
vacuum technique) whereas the sampling depth of the X-rays employed is only a few nm. This means that no Cu is present in the top most part of the PDA–AB films and hence that AB is only present in the deepest part of the film, close to its interface with the solid substrate. An alternative explanation is that AB present in the initial steps of the deposition process, as exemplified by means of UV–Vis spectroscopy (Fig. 1), and XPS spectroscopy (Fig. 2), progressively leaches out from the coating during prolonged deposition time in contact with Tris buffer. This assumption was however ruled out by the following experiment: a PDA–AB coating obtained after 3 h of dopamine oxidation in the presence of 0.1 mM AB, was removed from the solution, rinsed and immersed in Tris buffer (without present dopamine and AB). No release of AB was observed for weeks, meaning that the interactions between AB and PDA are strong enough to imped the desorption of the dye in the solution. Hence during the last steps of the film deposition, no AB is incorporated in the film and all the
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the probe to reach the electrode surface on a macroscopic surface typically 1 mm2 in surface area. PDA by itself is not conductive enough to allow for the oxidation of the selected redox probe by single contact with the PDA film [21]. The PDA and PDA–AB (produced in the presence of 0.1 mM AB) films coated for 3, 6, and 24 h were tested for their permeability to the hexacyanoferrate anions. Fig. 4a shows that after 3 h of reaction the PDA film already suppressed the faradic currents of hexacyanoferrate anions, in agreement with previous investigations [24]. However for the PDA–AB film (Fig. 4b) the oxidation and reduction peak of the hexacyanoferrate anions are still present after 3 h of deposition in the presence of 0.1 mM AB. This demonstrates that PDA–AB film keeps some permeability for Fe(CN)46 anions whereas for similar film thicknesses the PDA films are already totally impermeable. This is due either to the presence of a different internal porosity in the PDA–AB films or to the presence of channels, not detectable from the AFM topographies of PDA–AB films, contacting the solution and the pristine electrode. After 6 h of reaction, the hexacyanoferrate peaks cannot be seen anymore even in the PDA–AB films but a peak at around 0.6 V, already observed after 3 h of deposition, is still present and attributed to AB. Indeed CV experiments of AB solutions display an oxidation peak at the same potential (data not shown). Finally after 24 h of reaction, the CV of the PDA–AB films is very similar to the one of PDA in the presence of Fe(CN)46 anions in the solution. These findings are in qualitative agreement with the UV–Vis
experimental data show that after prolonged deposition time, the film properties (the thickness and morphology excepted) are closer to that of pure PDA than to that of the PDA–AB composite (which is blue, rich in Cu and electroactive). In addition the composition of the extreme surface of the films becomes similar for the PDA and the PDA–AB coatings as inferred from the XPS data (Fig. 2). The changes in optical properties and in composition are not the only differences between the PDA–AB and PDA coatings. Indeed the thicknesses of the PDA–AB films is lower than the thickness of the PDA ones (Fig. 3a) for all the investigated deposition times. After 24 h of reaction the PDA–AB films are (30 ± 4) nm thick and the PDA ones are (50 ± 5) nm thick. This difference in thickness becomes higher when the reaction time increases (Fig. 3a). This is a proof that the presence of AB modifies the deposition of PDA films. In addition the morphologies of the PDA films are different when they are produced from dopamine solutions in the presence of AB. Both the PDA and PDA–AB films have a granular structure but the PDA–AB films (Fig. 3c) display a bigger grain size and are rougher compared to the PDA ones (Fig. 3b). The thickness difference between the PDA and the PDA–AB films was confirmed by means of spectroscopic ellipsometry (Fig. 2 of the Supplementary Data section). The difference in morphology and thickness between the films incited us to investigate the permeability of the films for an electrochemical probe like hexacyanoferrate anions. Such an experiment allows to detect the presence of nanopores, sufficient for
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Fig. 3. (a) Evolution of the thickness of the PDA (d) films and of the PDA–AB 0.1 mM ( ) films determined by AFM on needle scratched films. (b) Typical AFM topographical image of a PDA films after 24 h of reaction with a root mean squared roughness of 8 nm and (c) typical AFM topographical image of PDA–AB films with a root mean squared roughness of 11 nm. Both films characterized by AFM were obtained after 24 h of reaction. Each point in part a corresponds to an individual sample and the thickness was obtained from 20 profiles perpendicular to scratched lines. They are affected by a relative standard deviation of about 10%.
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Fig. 4. CV experiments performed at a potential scan rate of 100 mV s 1 on working electrodes coated during 3, 6, and 24 h with PDA films (a) and PDA–AB films (b). The later were produced in the presence of 0.1 mM AB. The redox probe was potassium hexacyanoferrate at 1 mM in the presence of 50 mM Tris + 150 mM NaCl.
Long deposion mes
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All AB available is incorporated, PDA deposion connues
Fig. 5. Schematic representation of the deposition of PDA–AB films with a progressive removal of AB from the solution whereas PDA continues to deposit.
experiments where it was shown that the optical properties of the PDA–AB and PDA films become almost undistinguishable at long reaction times whereas they were markedly different at the initial steps of the deposition (Fig. 1b and c). To explain all these findings, and in particular the reduction in the blue coloration (Fig. 1) as well as the reduction in the Cu/C ratio (Fig. 2) when the film grows, two assumptions are formulated: (i) all the initially available AB is completely consumed during the first hours of reaction (ii) small oligomers of oxidized dopamine, formed essentially at the beginning of the oxidation process are needed to form the PDA–AB composite, AB being not incorporated anymore at longer reaction times when the small oligomers of oxidized dopamine are consumed. In order to confirm one of these assumptions, UV–Vis spectroscopy was performed on a solution of PDA–AB 0.2 mM at certain time of reaction. The signal of AB could not be detected even after 30 min of reaction meaning that all of it is already consumed. This experiment explains why the AB is only visible at short reaction times in the film. The mechanism of dye incorporation in the PDA films is represented in Fig. 5: the incorporation of AB in the initial steps of the film deposition before it is totally depleted from the solution is able to explain all our experimental findings, and in particular a progressive decrease in the Cu content measured by means of XPS when the reaction time increases. An additional series of experiments was performed to simulate a progressive decrease in the AB concentration on the optical spectra of the obtained films. To that aim the dopamine solution was
refreshed every 2 h, the films were rinsed and blown dry before spectral characterization. In the first series of experiments the dopamine solutions also contained AB at 0.2 mM in each deposition step. The incorporation of AB in the film increased progressively (Fig. 3 in the Supplementary Data) as exemplified by the increase in the absorbance at 336 nm and more particularly by the persistence of strong blue coloration and a high A336nm/ A500nm ratio (AB does almost not absorb at 500 nm at which PDA strongly absorbs) when additional deposition steps are performed (Fig. 6). On the other hand when AB was added only in the first PDA deposition step but not in the two next ones, the absorbance at 336 nm increased less (Fig. 4 in the Supplementary Data section) than in the first series of experiments, the final film appeared less blue and the A336nm/A500nm ratio decreased rapidly. This shows that a progressive reduction in the AB fed in the dopamine solution allows to produce a film that appears progressively less blue in agreement with the model proposed in Fig. 5. The fact that the film thickness and morphology is affected by the presence of AB as well as the total consumption of the dye from the solution point to very strong interactions between AB and PDA. Such interactions need to be investigated from a structural point of view in future studies. Unfortunately, as already explained, the dye concentration cannot be increased above 0.2 mM since AB is not soluble anymore in water. Otherwise, we could have investigated if increased AB concentration would allow for its incorporation in the top most part of the films. Another possibility to overcome the quantitative depletion of AB from the reaction mixtures consists to perform the deposition of the composite films in a multistep manner [25] using freshly prepared dopamine–AB mixtures, as shown in Fig. 6.
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t (h) Fig. 6. Evolution of the A336nm/A500nm absorbance ratio for films deposited on quartz slides when AB (at 0.2 mM) was added in each freshly prepared dopamine solution (2 mg mL 1) ( , curve A) or when AB was added only during the first deposition step whereas the two last deposition steps were performed in the absence of AB (j, curve B). The optical pictures on the right show the difference in the film color after the 3 deposition cycles shown on curves A and B.
4. Conclusion It has been shown that it is possible to form a composite film of PDA–AB by adding AB in a solution of PDA. The addition of AB allows changing the optical properties of the PDA coating by rending it blue at short reaction times of 3 and 6 h. By changing the reaction time and the concentration of AB it is possible to tune the color of the film when the concentration of AB is increased. The PDA–AB films finally become black-brown like PDA films for long time of reaction. The incorporation of AB also changes the permeability, the morphology and thickness of the film. However this incorporation is not uniform in the film and the copper present in AB is not visible anymore after 24 h of reaction as shown by the XPS data. This gradient of incorporation arises from the fact that all the AB is consumed during the first hour of reaction. So by adding a compound that can interact strongly with PDA it is possible to form a composite film and in this case to change the optical properties of the PDA films. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.08.006. References [1] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007) 426– 430. [2] D.R. Dreyer, D.J. Miller, B.D. Freeman, D.R. Paul, C.W. Bielawski, Chem. Sci. 4 (2013) 3796–3802. [3] M.E. Lynge, R. van der Westen, A. Postma, B. Städler, Nanoscale 3 (2012) 4916– 4928.
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