Kinetics of deposition and stability of pyrocatechol –FeIII coordinated films

Materials Science and Engineering C 72 (2017) 620–624

Contents lists available at ScienceDirect

Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Kinetics of deposition and stability of pyrocatechol –FeIII coordinated films Claire Meyer a, Florian Ponzio b, Eric Mathieu b, Vincent Ball b,c,⁎ a b c

Université de Strasbourg, Faculté de Chimie, 1 rue Blaise Pascal, France Unité Mixte de Recherche 1121, Institut National de la Santé et de la Recherche Médicale, 11 rue Humann, 67085 Strasbourg Cedex, France Université de Strasbourg, Faculté de Chirurgie Dentaire, 8 rue Sainte Elisabeth, 67000 Strasbourg, France

a r t i c l e

i n f o

Article history: Received 22 August 2016 Received in revised form 24 November 2016 Accepted 27 November 2016 Available online 2 December 2016 Keywords: Pyrocatechol Metal coordination Films Stability

a b s t r a c t Metal coordination between polyphenols and metal cations like Fe3+ allows to produce conformal homogeneous and robust coatings on a vast variety of materials. The deposition kinetics and the stability of the obtained films are however only poorly investigated. In the present article it is shown that rough, granular but pinhole free coatings up to 50 nm in thickness can be obtained in a one pot manner using pyrocatechol (Pyr)/Fe3+ mixtures at different stoichiometries (with Fe3+/Pyr ratios equal to 0.55 or 1.10) provided the deposition time is extended up to 24 h. More importantly, we show that these films are dissolved upon oxidation of Pyr in cyclic voltammetry experiments. When the films deposited during short durations are rinsed with buffer and subsequently re-exposed to Pyr containing solution, they undergo partial dissolution most probably through a ligand exchange process. Such a dissolution process does not occur anymore in the same conditions, when the deposition time is increased above 5 h. All Pyr-Fe3+ based films can be stabilized by a post-deposition of a polyelectrolyte multilayer film based on the alternated adsorption of poly(allylamine hydrochloride) and the sodium salt of poly(styrene sulfonate). The deposition of 5 bilayers of these polyelectrolytes allows suppressing the dissolution of Pyr-Fe3+ based films produced for short deposition times © 2016 Elsevier B.V. All rights reserved.

1. Introduction Polyphenol based coatings are now becoming a popular coating method owing to their versatility. Indeed it has shown that a mixture of tannic acid and iron(III) chloride leads to a conformal coating on almost all kinds of substrates [1]. The use of urushiol based lacquers to paint ceramics and cultural objects goes back to the neolithic age in China and allowed affording extremely robust coatings still present and almost unaltered 5000 years after their application [2]. In this case the stabilization of the coating is due to laccase catalysed oxidation of urushiol using dissolved oxygen as the oxidant [3,4]. Nowadays polyphenol-metal cation based coordinated amorphous films find numerous applications essentially as antibacterial coatings in order to protect ship hulls [5], a major technical challenge in order to reduce biofilm formation at the origin of huge friction between the ship and sea water and hence to save considerable amounts of energy. In addition, the antibacterial paintings on ship hulls shouldn't be too toxic to the environment, a major problem particularly in the see water of harbors. For this reason, polyphenols extracted from trees seem extremely ⁎ Corresponding author at: Unité Mixte de Recherche 1121, Institut National de la Santé et de la Recherche Médicale, 11 rue Humann, 67085 Strasbourg Cedex, France. E-mail address: [email protected] (V. Ball).

http://dx.doi.org/10.1016/j.msec.2016.11.123 0928-4931/© 2016 Elsevier B.V. All rights reserved.

promising but significant research effort has to be devoted to increase their long term stability which is not comparable to that of oriental (urushiol based) lacquers. Since the smallest possible polyphenol usable to produce metal cation based coordination coatings, seems to be pyrocatechol, namely 1,2-dihydroxyphenol [6], it seems mandatory to investigate the stability of those coatings in the presence of water, under oxidative conditions (applying a potential difference between a film on a conductive substrate and a reference electrode) and upon exposure to a solution of pyrocatechol, namely the ligand used to build up the film. Hence we are asking the question if ligand exchange could contribute to film erosion which seems a priori surprising owing to high stability constant of polyphenol-metal cation complexes of the order of 1036– 1039 [7]. It is the main aim of this paper to demonstrate that the stability of pyrocatechol based films obtained upon complexation with Fe3+ cations when exposed to a pyrocatechol solution of the same concentration as that used to build up the film is dependent on the deposition time. Namely, the Pyr-FeIII films are dissolving after low deposition times but remain stable after long (higher than about 5 h of deposition) times when put in the presence of a pyrocatechol solution at 2 mg·mL−1. These findings imply a time dependent change in the coordination mode of the films which are certainly out of equilibrium at short deposition times. In the following we will use the following nomenclature for those films: Pyr-FeIII(XmM)-Yh where X is the

C. Meyer et al. / Materials Science and Engineering C 72 (2017) 620–624

concentration of the iron(III) nitrate salt to induce the coating and Y is the deposition time expressed in hours respectively. In all our experiments the pyrocatechol concentration was kept constant and equal to 2 mg·mL−1, namely 18.2 mM. The deposition experiments were performed in the presence of 50 mM sodium acetate buffer at pH = 5.0. We also propose a simple strategy to slow down the erosion of the coatings produced at small deposition times by coating them with polyelectrolyte multilayer based films [8] using poly(allylamine hydrochloride) (PAH) as the polycation and the sodium salt of poly(4-styrene sulfonate) as the polyanion. Such (PAH-PSS)n multilayers are known to be very compact and impermeable to anions [9] as well as to small molecules. We show indeed that a (PAH-PSS)5 polyelectrolyte multilayer film totally inhibits the dissolution of Pyr-FeIII(10 mM)-2 h coatings when put in the presence of pyrocatechol solutions at 2 mg·mL−1. Complementarily, but as a new information we show that PyrFeIII(XmM)-Yh coatings can reach very high thickness values, of the order of 50 mm, after 24 h of deposition in a one pot manner, i.e. without refreshing the pyrocatechol-Fe3+ mixture. 2. Materials and methods All the solutions were freshly prepared before the beginning of each new experiment using distilled and deionized water from a Milli RO+ system (resistivity 18.2 MΩ·cm). Pyrocatechol (ref. C9510) and iron(III) nitrate nonahydrate (Fe(NO3)3·9H20, ref. 31233) were purchased from Sigma-Aldrich and used as received. These compounds were dissolved in a 50 mM sodium acetate buffer with a pH adjusted to 5.0 with concentrated hydrochloric acid. The pH was measured with a calibrated pH meter in the pH range between 4.0 and 7.0 (Hannah Hi221pH meter). In all experiments the concentration of pyrocatechol was constant and equal to 2 mg·mL−1 (18.2 mM). To that aim a volume V of iron(III) nitrate nonahydrate at X (X = 10 or 20) mM was injected in an identical volume V of pyrocatechol at 4 mg·mL−1 under strong agitation with a magnetic stirrer. This defined time t = 0 of the deposition experiment. The samples to be coated, either quartz (Thuet, Blodelsheim, France) or silicon slides ((100) from Siltronix Archamps, France) where hanging vertically in the initially prepared pyrocatechol solution. The substrates were cleaned just before the beginning of each deposition experiment in an ethanol bath under sonication, with a 2% (v/v) Hellmanex solution (Hellma, Germany) during 30 min, rinsed with distilled water, immersed briefly in a 1 M HCl solution, rinsed with distilled water, dried under a stream of filtered air and finally put during 30 min in an air plasma produced in a PDC-32G-2 plasma cleaner (Harrick Plasma, USA). Immediately after this cleaning process, the used substrates were perfectly hydrophilic as evaluated by the total spreading of 5 μL water droplets. The reaction time Y was varied between 0.25 and 24 h in this investigation The UV–vis spectra of the Pyr-FeIII(XmM)-Yh coatings deposited on quartz slides were measured after water rinse, drying with a filtered air stream using a single beam Xenius spectrophotometer (Safas, Monaco) in the wavelength range between 250 and 700 nm. Prior to that the reference spectrum was acquired with an identical freshly cleaned quartz slide. The thickness of the Pyr-FeIII(XmM)-Yh films was determined from single wavelength ellipsometry measurements (PZ2000, Horiba France, Longjumeau) at a fixed angle of incidence (70°) and a constant wavelength (632.8 nm). The ellipsometric angles ψ and Δ were transformed in thickness assuming a homogeneous and isotropic later on top of SiO2 layer (coming from the spontaneous oxidation of silicon) with a constant thickness of 2 nm. The refractive index of the Pyr-FeIII(XmM)-Yh layer was assumed constant and equal to 1.73 ± 0.02 i. The real part of the refractive index was taken equal to that of polydopamine films and the imaginary part of the refractive index reflects the light absorption by the film due to metal to ligand charge transfer in the pyrocatechol-Fe3 + complexes [6]. The given thickness values are the

621

average ± one standard deviation over 5 measurements taken along the main axis of the silicon wafers. The morphology of the Pyr-FeIII(10 mM)-5 h coatings was investigated by means of scanning electron microscopy using a Quanta 250 FEG (FEI, Eindhoven, Netherlands) under an electron acceleration potential of 15 kV. Before imaging the deposits, they were sputter coated with a thin carbon-palladium conductive layer. The cyclic voltametry experiments were performed with a three electrode electrochemical set up (CHI 604 B, CHInstruments, Austin, Texas) using an amorphous carbon working electrode (ref. CH104, CH Instruments, 2 mm in diameter), an Ag/AgCl reference electrode (ref.CH 111, CH Instruments) and a platinum wire (ref. 115, CH Instruments) as the auxiliary electrode. The working electrode was intensively polished on a silicon carbide disk and then on Al2O3 slurries with a grain size of 1 and 0.1 μm (Escil, France). The working electrode was then sonicated for 5 min in two successive water baths. After that, a CV curve was recorded between −0.6 and + 1.0 V (vs Ag/AgCl) forth and back at a scan rate of 100 mV·s−1 in the presence of sodium acetate buffer and then in the same buffer containing 1 mM K4Fe(CN)6. This measurement allowed checking the efficiency of the previously described cleaning method: the experiment was pursued only if the difference between the oxidation peak potential and the reduction peak potential of hexacyanoferrate was smaller than 100 mV and if the oxidation and reduction current maxima differed by less than 20%. If this was the case, the electrodes were dipped in a freshly prepared mixture of pyrocatéchol (final concentration: 2 mg·mL−1) and iron (III) nitrate (final concentration: 10 mM) for 5 h. The working electrode was then intensively rinsed with sodium acetate buffer. Then n CV cycles were performed at different scan rates between − 0.6 and + 1.0 V vs Ag/ AgCl. Such a curve constitutes the capacitive current on the PyrFeIII(10 mM)-5 h film. Then the three electrodes were immersed in the 1 mM K4Fe(CN)6 solution to measure the Faradic current on the film. Some of the Pyr-FeIII(10 mM)-Yh coatings where further modified with PAH (ref 283215 from Sigma-Aldrich, Mw = 15.000 g·mol−1) and PSS (ref. 243501, from Sigma Aldrich, Mw = 70.000 g·mol− 1) based multilayer films. The polyelectrolytes were dissolved at a concentration of 1 mg·mL−1 in the 50 mM sodium acetate buffer. The polyelectrolyte adsorption lasted over 5 min and each adsorption step was separated from the next one (with an oppositely charged polyelectrolyte) by buffer rinse during 1 min

3. Results and discussion The deposition of Pyr-FeIII(10 mM) films was first followed as a function of the deposition time by means of ellipsometry and UV–vis spectroscopy, the adsorption substrates being silicon wafers and quartz plates respectively. The data are represented in Fig. 1. In the study performed by Caruso et al. [6], the pyrocatechol/FeIII ratio was maintained constant and equal to 1. In addition the contact time between the pyrocatechol and iron(III) chloride was limited to a few minutes, yielding very thin coatings about 10 nm in thickness. Our data are consistent with those findings (Fig. 1A) but also show that much thicker films, up to 55 nm in thickness can be reached after prolonged contact time between the pyrocatechol/Fe3 + mixture and the substrates. In reference [6], thicker films were obtained in a multistep process, refreshing the pyrocatechol/Fe3+ mixture. The present investigation shows that such a multistep process may not be required to obtain coatings 10s of nm in thickness in a one pot manner. However we observe that prolonged reaction time leads to the formation of huge precipitates in the solution which sediment rapidly at the bottom of the reaction beaker as soon as the magnetic stirring is stopped. This may present a major drawback for the coating of particles, since the pyrocatechol/Fe3+ precipitates may adsorb on these particles producing heterogeneous and irregular coatings. This drawback is limited here

622

C. Meyer et al. / Materials Science and Engineering C 72 (2017) 620–624

A 60

B

0.75

50

0.45

30

A

Thickness (nm)

0.60

40

0.30

20 10

0.15

0

0.00

0

5

10

15

20

25

30

200

300

time (h)

400

500

600

700

λ (nm)

Fig. 1. A: Evolution of the thickness, obtained by ellipsometry, of Pyr-FeIII(XmM) coatings as a function of time for X = 10 (●) and X = 20 mM ( ). B: UV–vis spectra of Pyr-FeIII(10 m) ) of contact with a quartz slide. The insets represent pictures of dried films at the end of the deposition step. films after 1.5 h (—) and 23 h (

because the substrates were maintained vertically along the walls of the reaction beaker. UV–vis spectroscopy (Fig. 1B) confirms that the deposition of the Pyr-FeIII coatings is progressive because its absorbance increases slowly up to long contact times between the quartz slide and the pyrocatecholFe3+ containing solution. Note that the spectrum after 23 h of deposition displays a band peaking at 650–680 nm which is typical for metal to ligand charge transfer of the iron(III)-pyrocatechol complex, in agreement with the findings of reference [6]. The occurrence of such charge transfer bands is apparent after about 2 h of deposition (see Fig. 6) but no visible bands between 650 and 680 nm are observed at shorter deposition times perhaps because their intensity is not significantly higher than the signal/noise level of the used spectrophotometer. It has to be noted that the pyrocatechol/Fe3+ stoichiometry plays a marked role in the film deposition: thicker coatings are obtained particularly at longer deposition times when the concentration of iron(III) nitrate was of 10 mM, corresponding to a Pyr/Fe3+ initial stoichiometry of 0.55, close to 0.5, a situation more favorable to get thicker coatings than the stoichiometry of 1.10 corresponding to iron(III) nitrate at 20 mM (Fig. 1A). The morphology of the coatings was then investigated by means of scanning electron microscopy (Fig. 2) after 5 h of deposition. The films

appear grainy with some larger deposits decorating the underlying film. The occurrence of such larger grains increases with increasing the deposition time (data not shown) in agreement with the formation of precipitates in solution. These films were particularly difficult to image by Atomic Force Microscopy in the dry state and in contact mode because they were particularly sticky to the tip. Such an adhesive behavior will be the subject of a forthcoming investigation. The SEM topography displayed in Fig. 2 does not allow to certify that the Pyr-FeIII(10 mM)-5 h constitutes an homogeneous pinhole free film. To that aim we performed some cyclic voltametry measurements on a Pyr-Fe(III)-5 h coating deposited on a polished amorphous carbon electrode 2 mm in diameter, hence exhibiting at least 3.14 mm2 of contact area with the aqueous solution. This solution contained 1 mM potassium hexacyanoferrate: it was found that the CV curve in the presence of the redox probe was almost undistinguishable from the CV curve obtained in the same conditions but in the absence of the redox probe (i.e. in the presence of sodium acetate buffer only), Fig. 3.

3e-5

2e-5

I (A)

1e-5

0

-1e-5

-2e-5

-3e-5 -1.0

-0.5

0.0

0.5

1.0

E (V vs Ag/AgCl)

Fig. 2. SEM topography of a Pyr-FeIII(10 m) film after 5 h of contact with a silicon wafer.

Fig. 3. Cyclic voltametry (CV) of an amorphous carbon electrode put in contact with 1 mM potassium hexacyanoferrate in the presence of 50 mM sodium acetate buffer (—) and the same electrode after 5 h of contact with a pyrocatechol (2 mg·mL−1) and Fe(NO3)3 (10 mM) solution without ( ) and with potassium hexacyanoferrate ( ). The CV scans were all performed at a sweep rate of 100 mV·s−1.

C. Meyer et al. / Materials Science and Engineering C 72 (2017) 620–624

However a marked reversible oxidation-reduction current of hexacyanoferrate is measured on a pristine electrode. This shows that the Pyr-FeIII(10 mM)-5 h film is impermeable to the used redox probe and constitutes hence an homogeneous pin hole free coating. A careful inspection of Fig. 3 shows nevertheless that the current in the presence of the redox probe (blue curve) is even smaller than in the absence of the redox probe (red curve), the so called capacitive current of the coating. This observation incited us to perform additional CV scans on freshly prepared Pyr-FeIII(10 mM)-5 h coatings at a scan rate of 100 mV·s−1 in order to check for the film stability. A stable coating should display an unchanged capacitive current (in the absence of an extrinsically added redox probe) after repeated potential scans. This is however not the case here (Fig. 4): after 10 successive potential scans (between −0.6 and +1.0 V versus Ag/AgCl), the capacitive current is very close to that measured on the pristine uncoated working electrode and has markedly decreased with respect to the current measured during the first scan, hence confirming what was suspected in Fig. 3: the Pyr-FeIII(10 mM)-5 h coatings (as well as the coatings obtained for longer deposition times, data not shown) are not stable upon oxidation-reduction cycles. It is also observed in Fig. 4 that the first CV scan performed on the Pyr-FeIII(10 mM)-5 h coatings is markedly different from the CV scan performed at the same potential scan rate in the presence of a pyrocatechol solution at 0.2 mg·mL− 1. The film displays a peak at around +0.6 V vs Ag/AgCl as the pyrocatechol solution but also an additional peak close to +0.25 V vs Ag/AgCl reflecting the different nature of the film versus the pyrocatechol only containing solution, most likely due to the presence of Fe3 + cations. Indeed the presence of exclusively Fe3+ cations and the absence of Fe2+ cations in the Pyr-FeIII films has been demonstrated by means of X ray photoelectron spectroscopy [6]. This finding shows that the cohesion of the Pyr-FeIII films is due exclusively to coordination chemistry and not to redox processes. It constitutes also the basis for a reasonable explanation for the film erosion upon application of succesive potential scans in CV experiments (Fig. 4): upon oxidation of the catechols, quinone groups are formed and those chemical functionalities present a much reduced affinity for metal cations than their catechol counterparts [10]. We then wondered if the Pyr-FeIII(10 mM)-Yh films are stable in the presence of sodium acetate buffer and the same buffer with added pyrocatechol at 2 mg·mL−1, hence the same ligand as that incorporated in the film. In the presence of sodium acetate buffer, not film erosion

623

was found up to 48 h of contact both by ellipsometry and by UV–vis spectroscopy (data not shown). However in the presence, of pyrocatechol, some marked film erosion was found only for coatings produced at short deposition times, less than 5 h (Fig. 5). After this critical time, the film were stabilized for at least 24 h with less than 10% of the material eroded. Such a film stability is of course mandatory for biological applications in the case of coatings containing highly toxic Fe3 + cations. Some preliminary evaluations show nevertheless good biocompatibility of the Pyr-FeIII coatings [11]. The experiments presented here are an extreme case, not encountered in biological experiments were the as deposited film is exposed again to a solution containing its constituting ligand. The obtained results are however interesting from a fundamental point of view: the stability of the Pyr-Fe(III)-Yh coatings with respect to their constituting ligand, pyrocatechol, is increasing with the deposition time. We suggest that this effect originates from a slow change of the coordination mode between the cations and the ligand. There is some evidence in the literature for such changes [12] but this will be the subject of future investigations using EXAFS spectroscopy to investigate the coordination sphere around Fe3+ cations. The film erosion process was also confirmed by UV–vis spectroscopy for deposition times shorter than 5 h, Fig. 6. About 66% of the film was eroded after 5 h of contact with a 2 mg·mL−1 pyrocatechol solution, in excellent agreement with the film thickness data obtained from ellipsometry. We then thought about a strategy to stabilize the Pyr-FeIII(10 mM) films obtained for deposition times lower than 5 h. A straightforward strategy would be to use polyelectrolyte multilayer films obtained through the alternated deposition of oppositely charged polyelectrolytes [8]. Among the known combinations of polycations and polyanions able to produce highly compact films with low porosity (on the order of the nm) are PAH and PSS as the polycation and polyanion respectively. We started the film deposition with the polycation PAH, trusting on a negative zeta potential of (− 43 ± 6) mV for the Pyr-FeIII(XmM) coatings [6]. The deposition of 3 (PAH-PSS) bilayers, about 15 nm in thickness in these conditions [13] is already sufficient to significantly reduce the film erosion to 33% (Fig. 7) compared to 66% erosion in the absence of a polylectrolyte based capping layer (Fig. 6) The stabilization effect is even increased, to become almost quantitative when the capping layer is made of 5 (PAH-PSS) based bilayers (Fig. 8)

4e-5

1.0

Relative film thickness

3e-5 2e-5

I (A)

1e-5 0 -1e-5

0.8

0.6

0.4

0.2 -2e-5 -3e-5 -0.6 -0.4 -0.2 0.0

0.0 0.2

0.4

0.6

0.8

1.0

1.2

E (V vs Ag/AgCl) Fig. 4. Capacitive CV curve on a pristine amorphous carbon electrode ( ) and on the same electrode after 5 h of contact with a pyrocatechol solution at −1 2 mg·mL + 10 mM Fe(NO3)3, buffer rinse and performing the first CV scan (—) and after the 10th CV scan (—). CV of a pyrocatechol solution at 0.2 mg·mL−1 ( ) on a pristine amorphous carbon electrode.

0

5

10

15

20

25

t (h) Fig. 5. Variation of the relative film thickness, as measured by ellipsometry, of PyrFeIII(10 mM)-Yh films deposited on silicon slide for different durations (2 h: —○—, 3 h: , 5 h: , 12 h: and 23 h: ) and subsequently put in the presence of a 2 mg·mL−1 pyrocatechol solution (in the presence of 50 sodium acetate buffer at pH = 5.0).

624

C. Meyer et al. / Materials Science and Engineering C 72 (2017) 620–624

1.2

0.18

0.25

0.14 A 280nm

Absorbance

0.20

0.15

0.12 0.10 0.08 0.06

0.10

0.04 0

1

2

3

4

5

6

t (h)

0.05

0.00 300

400

500

600

700

Relative absorbance (λ=280 nm)

0.16

1.0

0.8

0.6

0.4

0.2 0

λ (nm)

4. Conclusions The deposition of Pyr-FeIII(XmM)-Yh films yields to films about 50 nm thick when the deposition time Y is increased up to 24 h in a one pot manner without the need to refresh the pyrocatéchol-Fe3 + mixture. The thickness of the films is also dependent on the pyrocatechol/Fe3+ stoichiometry, but more experiments need to be performed in this sense. The most interesting findings of this investigation are: i) that the Pyr-FeIII(10 mM) films undergo decomposition upon the application of potential cycles in cyclic voltametry experiments ii) that they undergo important erosion when put in contact with a freshly prepared pyrocatechol solution only when the deposition time of the films is shorter than 5 h. iii) that even the films deposited at short deposition times can be stabilized against pyrocatechol induced dissolution by capping them

0.18 0.16 0.14 A280nm

0.14

Absorbance

0.12 0.10

0.12 0.10 0.08

0.08

0.06

0.06

0.04 0

1

2

0.04

3

4

5

6

t (h)

0.02 0.00 300

400

500

2

3

4

5

6

t (h)

Fig. 6. UV vis spectra of a Pyr-FeIII(10 mM)-2 h film (—) and of the same film put in contact with a 2 mg·mL−1 pyrocatechol containing 50 mM sodium acetate buffer during 0.5 ( ), 1 ( ), 2 ( ) and 5 ( ) h. The inset displays the reduction of the absorbance at 280 nm as a function of contact time with the pyrocatechol solution.

0.16

1

600

700

λ (nm) Fig. 7. UV vis spectra of a Pyr-FeIII(10 mM)-2 h film (—) and of the same film after coating with a (PAH-PSS)3 multilayer film ( ) and the same film subsequently put in contact with a 2 mg·mL−1 pyrocatechol containing 50 mM sodium acetate buffer during 0.5 (—), 1 ( ), 2 ( ) and 5 ( ) h. The inset displays the reduction of the absorbance at 280 nm as a function of contact time with the pyrocatechol solution.

Fig. 8. Relative absorbance of Pyr-FeIII(10 mM) films deposited during 2 h on a quartz slide before exposure to a 2 mg·mL−1 containing pyrocatechol solution as a function of time in the absence (—●—) of polyelectrolyte multilayer capping film and in the presence of (PAH-PSS)3 ( ) or (PAH-PSS)5 ( ) capping film. The relative absorbance is obtained by dividing the absorbance of each film after a time duration t in the pyrocatechol solution by the absorbance of the same film before being put in contact with the pyrocatechol solution.

with (PAH-PSS)n films, an optimal stabilization being obtained after the deposition of n = 5 bilayers. Finding (ii) suggests that the coordination mode of Fe3 + changes progressively with time in the Pyr-FeIII(10 mM) film. This fascinating observation requires more detailed mechanistic investigations from a structural point of view. References [1] H. Ejima, J.J. Richardson, K. Liang, J.P. Best, M.P. van Koeverden, G.K. Such, J. Cui, F. Caruso, One-step assembly of coordination complexes for versatile film and particle engineering, Science 341 (2013) 154–157. [2] J. Kamanotani, Urushi (oriental lacquer)-a natural aesthetic durable and future promising coating, Prog. Org. Coat. 26 (1995) 163–195. [3] T. Yoshida, R. Lu, S. Han, K. Hattori, T. Katsuda, K. Takeda, K. Sugitomo, M. Funaoka, Laccase catalysed polymerization of lignocatechol and affinity on proteins of resulting polymers, J. Polym. Sci. A Polym. Chem. 47 (2009) 824–832. [4] W. Bai, L. Cai, D. Zhuo, Y. Xu, H. Xue, Q. Chen, J. Lin, Resurrection of dead lacquer-cupric potassium chloride dihydrate (K2CuCl4·2H20) used as the mimic laccase, Prog. Org. Coat. 77 (2014) 431–438. [5] N. Bellotti, C. Deyà, B. del Amo, R. Romagnoli, Antifouling paints with zinc “tannate”, Ind. Chem. Eng. Res. 49 (2010) 3386–3390. [6] M.D.A. Rahim, K. Kempe, M. Müllner, H. Ejima, Y. Ju, M.P. van Koeverden, T. Suma, J.A. Braunger, M.G. Leeming, B.F. Abrahams, F. Caruso, Surface confined amorphous films from metal coordinated simple phenolic ligands, Chem. Mater. 27 (2015) 5825–5832. [7] A.K.L. Yuen, G.A. Hutton, A.F. Masters, T. Maschmeyer, The interplay of catechol ligands with nanoparticulate iron oxides, Dalton Trans. 41 (2012) 2545–2559. [8] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (1997) 1232–1237. [9] S. Han, B. Lindholm-Sethson, Electrochemistry at ultrathin polyelectrolyte films selfassembled at planar gold electrodes, Electrochim. Acta 45 (1999) 845–853. [10] J. Yu, W. Wei, E. Danner, J.N. Israelachvili, J.H. Waite, Effects of interfacial redox in mussel adhesive proteins on mica, Adv. Mater. 23 (2011) 2362–2368. [11] M.A. Rahim, H. Ejima, K.L. Cho, K. Kempe, M. Müllner, J.P. Best, F. Caruso, Coordination-driven multistep assembly of metal-polyphenol films and capsules, Chem. Mater. 26 (2014) 1645–1653. [12] E. Mentasti, E. Pelizzetti, Reactions between iron(III) and catechol (Odihydroxybenzene). Part I: equilibria and kinetics of complex formation in aqueous acid solution, J. Chem. Soc. Dalton Trans. (1973) 2605–2608. [13] G. Ladam, P. Schaad, J.-C. Voegel, P. Schaaf, G. Decher, F. Cuisinier, In situ determination of the structural properties of intially deposited polyelectrolyte multilayers, Langmuir 16 (2000) 1249–1255.