Electrostatic Nature and Ionic Strength Independence at pH 5

(1)

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/273831923

Layer-by-layer deposition of tannic acid and Fe 3+ cations is of electrostatic nature but almost ionic strength independent at pH 5

Article in Journal of Colloid and Interface Science · June 2015

DOI: 10.1016/j.jcis.2015.03.009

CITATIONS

47

READS

353 2 authors, including:

Ball Vincent

University of Strasbourg 255PUBLICATIONS 11,197CITATIONS

SEE PROFILE

All content following this page was uploaded by Ball Vincent on 27 April 2019.

The user has requested enhancement of the downloaded file.

(2)

Layer-by-layer deposition of tannic acid and Fe

³⁺

cations

is of electrostatic nature but almost ionic strength independent at pH 5

Christian Ringwald

^a

, Vincent Ball

^a,b,^⇑

aInstitut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche 1121, Faculté de Médecine, 11 rue Humann, 67085 Strasbourg Cedex, France

bFaculté de Chirurgie Dentaire, 8 rue Sainte Elisabeth, 67000 Strasbourg, France

g r a p h i c a l a b s t r a c t

PEI-(TA-Fe

³⁺

)

₆

ﬁlms: extremely strong electrostac

Fe

³⁺

TA

PEI

a r t i c l e i n f o

Article history:

Received 4 February 2015 Accepted 6 March 2015 Available online 14 March 2015

Keywords:

Tannic acid Fe³⁺cations

Coordination driven self-assembly Inﬂuence of the ionic strength

a b s t r a c t

The step-by-step assembly of tannic acid (TA) and of Fe³⁺cations allows to produce films of controlled thickness using exclusively small multivalent ions. In the present investigation, it is shown that even if electrostatic interactions are dominant over ligand to metal charge transfer interactions in stabilizing such films, those electrostatic interactions display a small sensitivity to concentration in NaCl used as a supporting electrolyte as well as to the concentration in sodium acetate in the absence of NaCl. This finding highlights the strong stability of the films obtained through the step-by-step deposition of TA and Fe³⁺cations. Complementarily, the films made from 6 deposition cycles of TA and Fe³⁺cations do not form Prussian Blue when put in contact with hexacyanoferrate anions. This shows that Fe³⁺is so tightly bound to the film that it is not able to form a coordination polymer with Fe(CN)64anions.

1. Introduction

Tannic acid (TA) is a polyphenol known for its antibacterial and antioxidant properties[1]. This multivalent molecule is present in plant leafs, among other polyphenols, like in the green tea and is a

powerful drug[2]. TA displays also a large repertoire of possible binding partners, like polymers through hydrogen bonding [3], proteins[4]and metal cations through coordination bonds[5,6].

This is possible owing to the multiple hydroxyl groups present on the surface of TA acting as hydrogen bong donors as long as the pH is lower than the average pKa of TA which lies around pH = 8.5[7]. At pH values higher than the average pKa, TA behaves as a rigid polyanion and can form polyelectrolytes complexes with polycations like quaternized poly(4-vinylpyridine)[7].

⇑ Corresponding author at: Institut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche 1121, Faculté de Médecine, 11 rue Humann, 67085 Strasbourg Cedex, France.

E-mail address:[email protected](V. Ball).

Contents lists available atScienceDirect

Journal of Colloid and Interface Science

w w w . e l s e v i e r . c o m / l o c a t e / j c i s

(3)

All the three modes of interactions of TA, electrostatic, hydrogen bonding and metal coordination can be exploited to deposit TA on surfaces as well as to produce colloidal particles[8]. The electrostatic binding mode has been recently employed in environmental applications to concentrate metal cations in multilayer films made from the step-by-step alternated deposition of poly(ethylene imine) and TA[9]. In addition, multilayered films made from the alternated adsorption of TA and weak or strong polycations can be decomposed when the pH of the solution in contact with the films is lowered to a point where TA looses all its negative charges [10]. On the other hand, the alternate deposition of TA and polymers carrying hydrogen bond acceptor groups like poly (N-vinylpyrrolidone) and poly(N-vinylcaprolactam) is possible as long the pH of the solutions in which the polymer and TA are dissolved is lower than the average pKa value of TA[7]. At pH values higher than the pKa of TA, the hydrogen bond interactions cannot stabilize the (polymer-TA)n(wherendenotes the number of deposition cycles) films anymore and those films undergo decomposition.

Concerning the metal coordination abilities of TA, it has been demonstrated that nanometer thick films can be deposited on almost all kinds of surfaces from freshly prepared mixtures of TA and Fe³⁺ containing solutions[11]. Multilayered films based on the alternate deposition of tannic acid and Fe³⁺cations can not only be obtained in such a one pot manner, where TA–Fe³⁺aggregates are deposited, but also obtained in a step-by-step deposition manner. Such films are obtained through ‘‘Coordination driven multi- step assembly’’ (CDMA) [12]. It is the aim of this article to investigate the importance of ionic strength, through a change in the concentration in NaCl or in sodium acetate (in the absence of added NaCl) as the supporting electrolyte, on the deposition of such PEI-(TA–Fe³⁺)n films where poly(ethylene imine) (PEI) acts as an anchoring layer. Such deposits are fascinating because they are not obtained through the alternated deposition of polymers, proteins, DNA or nanoparticles but exclusively from multivalent ions. Films obtained through the repeated or simultaneous deposition of multivalent ions are relatively rare in the literature[13,14].

The combination of TA and Fe³⁺cations is particularly interesting because when the deposition is performed in acidic conditions, which is required because of the lack of stability of Fe³⁺cations in neutral or basic solutions, TA is uncharged. However examina- tion of the UV–vis spectra of the CDMA ﬁlms[12]reveal that the maximal absorption of TA occurs atkmax= 323–325 nm which corresponds to the deprotonated form of TA. Hence the obtained (TA–

Fe³⁺)n films are stabilized by electrostatic interactions and the negative charge on TA occurs most certainly from a pKa shift, which is a common phenomenon when weak polyelectrolytes are employed in polyelectrolyte multilayered films[15,16]. As an additional information, we will also investigate the interactions of PEI- (TA–Fe³⁺)_nfilms with a redox probe like Fe(CN)₆⁴which is known to form an inorganic coordination polymer with Fe³⁺cations[13], namely Prussian Blue. It will be shown that even Fe³⁺ cations deposited in the last step of the film are not accessible to Fe(CN)64anions. This experiment shows again the high strength of the interactions between TA and Fe³⁺cations.

2. Materials and methods

All solutions were prepared from distilled and double de-ion- ized water (Milli Q+, Millipore,

q

= 18.2 MXcm). The buffer solution was made from 50 mM sodium acetate (Sigma–Aldrich, ref.

236500) at pH 5.0, the pH being adjusted with concentrated hydrochloric acid. NaCl was added to the acetate buffer solutions at different concentrations between 0 and 2 mol L¹ in order to investigate the inﬂuence of this parameter on the growth of PEI- (TA–Fe³⁺)nﬁlms. Some additional experiments were performed in

either 5 mM or 25 mM sodium acetate buffer (with pH adjusted to 5.0) in the absence of NaCl in order to reduce the ionic strength and to increase the range of electrostatic interactions. Taking the pKa value of acetic acid into account (pKa = 4.75) a solution with C mM sodium acetate contributes to 0.62C to the ionic strength of the solution. Overall the Debye length of the solution was chan- ged from 0.21 nm (in the presence of 50 mM sodium acetate + 2000 mM in NaCl) to 5.4 nm (in the presence of 5 mM sodium acetate). It was not possible to further decrease the concentration of the sodium acetate buffer because the iron nitrate solution (at 1 mg mL¹ie 2.5 mM) could otherwise not be buffered anymore.

Tannic acid (ref. 403040,MW= 1701.20 g mol¹), iron III nitrate nonahydrate (Fe(NO3)39H20, ref: 31233) and potassium hexacyanoferrate (K4Fe(CN)6, ref P9387) were purchased from Sigma–

Aldrich and used without further purification. Poly(ethylene imine) (ref. P3143,Mw= 7.510⁵g mol¹as provided by the fur- nisher, Sigma–Aldrich) was dissolved at 1 mg mL¹in each of the used acetate buffers and was used as an efficient adhesion primer for the deposition of films obtained in a step-by-step manner as in many other investigations[15].

The adsorption substrates were silica coated quartz crystals (ref: QSX 303, Q Sense, Sweden) for monitoring film deposition by means of quartz crystal microbalance with dissipation monitoring (QCM-D), quartz plates (Thuet, Blodelsheim) to follow the film deposition by means of UV–vis spectroscopy, silicon wafers (Siltronix, Archamps) to measure the film thickness by means of Atomic Force microscopy. These substrates were cleaned with ethanol, a commercial detergent formulation during 30 min (Hellmanex at 2% v/v in water, Hellma Gmbh, Germany), rinsed with distilled water, 1 mol L¹ HCl, and distilled water again.

These cleaning steps were performed just before the beginning of a deposition experiment.

The electrochemical behavior of the PEI-(TA–Fe³⁺)nﬁlms was investigated by means of cyclic voltammetry using amorphous carbon (ref: CH 104, CH Instruments, Austin, Texas) as the working electrode. The permeability of the same ﬁlms to hexacyanoferrate anions was also investigated by means of cyclic voltammetry (CV).

The working electrode for the CV experiments were polished suc- cessively with 1 and 0.1

l

m alumina powders (Escil, France) and sonicated in a distilled water bath for two times. Each sonication step at a frequency of 135 kHz lasted over 2 min. This cleaning step was performed just before the deposition of the polyelectrolyte multilayer ﬁlm. The cleaning efﬁciency of the electrode was checked by means of CV in the presence of 1 mM K4Fe(CN)6 in the presence of 50 mM sodium acetate + NaCl (which concentration is the experimental parameter) and a potential scan rate of 100 mV s¹. The electrode was considered satisfactory when the oxidation and reduction peak currents were equal within 5% and when the potential difference between the oxidation and reduction peak was lower then 80 mV (it should ideally be of 59 mV for a reversible one electron process at 25°C). Otherwise, the electrode was discarded and polished again.

The PEI-(TA/Fe³⁺)nfilms were deposited on the adsorption substrate, either silica coated quartz crystals, quartz plates or amorphous carbon electrodes by first adsorbing the PEI polycation [15]. The deposition process was followed by rinse with the buffer solution, by the adsorption of TA, buffer rinse, adsorption of the Fe³⁺cations and a final rinse with the buffer solution. The optimal adsorption times were determined by following the deposition kinetics by means of QCM-D: the adsorption of a species was considered to be achieved when the frequency changes of the quartz crystal were lower than the thermal drift, ie about 0.05 Hz min¹. The deposition of TA and Fe³⁺was performedntimes to obtain a film withn‘‘deposition cycles’’. Those QCM-D experiments were performed with a E1 device from QSense (Göteborg, Sweden).

120 C. Ringwald, V. Ball / Journal of Colloid and Interface Science 450 (2015) 119–126

(4)

The solutions were injected by means of a peristaltic pump at a constant ﬂow rate of 250

l

L min¹. The reduced frequency changes,Dfm/

m

, as well as the dissipation changes,DDm, at the 3rd (

m

= 3), 5th (

m

= 5) and 7th (

m

= 7) overtone were followed as a function of time. In the case where the reduced frequency changes overlap and if the (dimensionless) dissipation changes are small (see Results and discussion for a longer discussion) the ﬁlms can be considered as rigid and the Sauerbrey[17] equation (Eq.(1)) can be used to calculate the surface coverage in adsorbed molecules:

C¼ C:DFm

m

^ð1Þ

where the proportionality constantCdepends on the shear modulus and density of quartz and is given equal to 17.7 ng cm²Hz¹by QSense. The QCM-D data were also plotted in the form ofDDversus DFn/nplots to give some insights in the mechanical properties and in the homogeneity of the PEI-(TA–Fe³⁺)_nﬁlms[18].

UV–vis spectroscopy experiments were performed with a single beam Xenius spectrophotometer from Safas, Monaco. The reference spectra were taken with the pristine quartz slide just before the beginning of the coating process.

The thickness of dried PEI-(TA–Fe³⁺)nﬁlms was measured by means of AFM, which constitutes an absolute measurement provided the piezoelectric ceramic on which the sample is glued is well calibrated. AFM topographic images were obtained in the contact mode and in the dry state using a Nanoscope IV microscope (Bruker, Germany). The used cantilevers were of MSCT type with a nominal spring constant of 0.2 N m¹. Topographies were acquired over 10

l

m10

l

m surface areas after repetitive scans at a frequency of 1 Hz and a resolution of 512512 pixels and the lowest possible deflection set point to ensure minimal damage to the investigated films. The thickness of the films was determined by measuring height changes in the direction perpendicular to lines needle scratched in the film just before imaging. The given thickness corresponds to the average of 20 line profiles ± one standard deviation on images acquired over 20

l

m20

l

m area. The image acquisition was performed using Nascope614r1 as the software.

The cyclic voltammetry (CV) experiments were performed in a three electrode classical conﬁguration using an amorphous carbon electrode, an Ag/AgCl (ref. 111, CH Instruments) and a Pt wire (ref.

115, CH Instruments) as the working electrode, the reference and the auxiliary electrode respectively. The CV curves were measured by cycling the potential (vs. Ag–AgCl) between0.80 V and 0.80 V.

The as prepared PEI-(TA–Fe³⁺)6films were incubated with 50 mM sodium acetate buffer containing the same NaCl concentration as used during the deposition of the film and at least 3 CVs at a scan rate of 100 mV s¹were recorded. Finally the PEI-(TA–Fe³⁺)6films were incubated with a 1 mM K₄Fe(CN)₆solution in the corresponding acetate containing NaCl buffer during 30 min and CV curves at 100 mV s¹were then acquired to investigate if Fe(CN)64anions can permeate through PEI-(TA–Fe³⁺)6films to be oxidized at the amorphous carbon electrode’s surface.

3. Results and discussion

QCM-D experiments show that the adsorption kinetics of TA and of Fe³⁺ cations is pretty fast and ﬁnished in less than 5 min (Fig. 1A and B). This result obtained in the presence of 50 mM sodium acetate buffer is consistent with the data obtained by Rahim et al. [12] where TA was dissolved in water at different pH values and Fe³⁺cations were dissolved at a constant pH value of 3. In this previous investigation it was found that the amount of deposited material, ﬁlms containing TA and Fe³⁺ cations;

increases with the pH of the TA solutions, ie when the degree of

ionization of TA increases[12]. This points to the importance of electrostatic interactions to form stable TA–Fe³⁺containing ﬁlms.

In this investigation we aim to get an idea of the stability of those electrostatic interactions. To that aim we performed deposition experiments of PEI-(TA–Fe³⁺)nﬁlms at constant pH (5.0 where TA is unprotonated in solution) but at different concentrations in the supporting electrolyte, NaCl. Additional experiments were also performed in the absence of NaCl and at lower concentration in sodium acetate as explained in the Materials and Methods section.

Coming back to our QCM-D experiments, the frequency changes measured at the different overtones

m

of the fundamental reso- nance frequency of the quartz crystal (

m

= 3, 5, 7. . .) almost overlap even if the dissipation changes become rather high (Fig. 1B). This is rather surprising because the close coincidence of the frequency changes (DFm/

m

) incites to use the Sauerbrey equation (Eq.(1)) to calculate the amount of deposited species per unit surface area of the quartz crystal whereas the high dissipation changes (DDm) (Fig. 1B) indicate that the ﬁlms dissipate a high fraction of the mechanical energy provided by the oscillating crystal, meaning that the ﬁlms are not rigid which precludes the use of the Sauerbrey equation. To be more quantitative we plotted the ratio DD_m/(DF_m/

m

) as a quantitative criterion for the use of the Sauerbrey equation[19]. In all cases and along the duration of all experiments, theDDm/(DFm/

m

) ratio was found to be smaller than 410⁷Hz¹the maximal value considered to be acceptable for the use of the Sauerbrey equation (Fig. S1 of the Supplementary data). In addition after the deposition of PEI, the DDm/(DFm/

m

)

t (min)

0 20 40 60 80 100 120

t (min)

0 20 40 60 80 100 120

Fn/n (Hz)

-600 -500 -400 -300 -200 -100 0

Dn (10-6 )

0 5 10 15 20 25 30 35

A

B

Fig. 1.Reduced frequency (A) and dissipation changes (B) undergone by a silica coated quartz crystal during the deposition of a PEI-(TA–Fe³⁺)nﬁlm. The experiment was performed in the presence of 50 mM sodium acetate buffer at pH = 5.0 without added NaCl. The blue and red arrows correspond to the injection of TA and iron nitrate respectively. The black arrow correspond to the injection of PEI. The beginning of the rinsing steps with buffer, separating two adsorption steps, are not indicated for reasons of clarity. The data are plotted for the third ( ), ﬁfth ( ) and 7th ( ) overtone of the quartz crystal.

(5)

values remain almost constant showing that (DDm) is proportional DFm/

m

.

By using the Sauerbrey equation and assuming the mass density of the ﬁlm to be equal to 1.3 g cm³yields a ﬁlm thickness that increases linearly with the number of deposition cycles (Fig. S2 of the Supplementary data) and a thickness increment of about 12 nm per deposition cycle. This value will be compared later on to the thickness increment per layer pair obtained by means of AFM.

A linear growth of the ﬁlm with a calculated thickness of about (60 ± 20) nm after 5 deposition cycles is found not only in the presence of 50 mM sodium acetate buffer in the absence of added NaCl but also at higher NaCl concentrations up to 2 mol L¹.Fig. S3 of the Supplementary datashows the change in dissipation plotted versus the opposite of the frequency changes at the 3rd, 5th and 7th overtone of the quartz crystal for an experiment performed in the presence of 500 mM NaCl. The slope of these curves amount to710⁸Hz¹ almost the same value as for the experiment performed in the absence of NaCl (610⁸Hz¹). This strongly suggests that the deposition of PEI-(TA–Fe³⁺)_n ﬁlms is weakly affected by the salt concentration.

To confirm this trend, the deposition of the PEI-(TA–Fe³⁺)nfilms in the presence of 50 mM acetate buffer (with different NaCl concentrations) or by decreasing the concentration in sodium acetate (in the absence of added NaCl) was then investigated by means of UV–vis spectroscopy. It first appears that at all concentrations in the supporting electrolyte (hence at all values of the ionic strength in the range from 3.2 to 2032 mM), the spectra of the films display a maximal absorbance at a wavelength of 323–325 nm (Fig. 2A).

This corresponds to the deprotonated form of TA at pH higher than the average pKa (about 8.5) in solution. The deposition experiments were performed from sodium acetate buffers at pH 5.0 where TA displays a maximal absorbance atk= 275 nm in solution (Fig. S4 in the Supplementary data). This means that TA undergoes a strong pKa shift when it interacts ﬁrst with PEI and then with Fe³⁺cations (the pH of the solutions decreased from 5.0 for the pure buffer to 4.7 when iron nitrate was dissolved at a concentration of 1 mg mL¹). This is in agreement with previous investigations [12]. The slope of the absorbance increase (atk= 325 nm) versus the number of deposition cycles is linear for all the investigated values of the ionic strength (Fig. 2A). More surprising, but in

perfect agreement with the QCM-D experiments, the ﬁlm deposition is almost independent on the ionic strength in the solution (Fig. 2B). A closer analysis of the absorption spectra of the ﬁlms shows that the absorbance increase atk= 550 nm with the number of deposition cycles is much smaller than at 325 nm. The band cen- tered atk= 550 nm for metal–tannic acid complexes is generally attributed to ligand to metal charge transfer interactions [20].

Such interactions seem hence to be weak during the build-up of PEI-(TA–Fe³⁺)n films confirming the importance of electrostatic interactions in the cohesion of the films. In addition, as shown in Fig. 2B, the ratio between the absorbance at 550 and at 325 nm is constant and equal to about 0.2 in the range of explored ionic strengths.

An analysis of the scratch heights obtained through AFM imaging (representative data are shown inFig. 3) reveals that the thickness increment per deposition cycle amounts to (5 ± 0.5) nm independently of the ionic strength between 3.2 and 2032 mM (Fig. 4). These data are in perfect agreement with the data obtained by means of UV–vis spectroscopy (Fig. 2). In addition, the thickness increment of about 5 nm per deposition cycle is much higher than expected on the basis of the size of TA (1.851.651.01 nm)[10]

and the hydration radius of Fe³⁺showing that TA probably form aggregates during its interactions with already deposited Fe³⁺

cations. Note that the thickness increment per deposition cycle measured directly by AFM is smaller than the value calculated from the QCM-D experiments (about 12 nm per deposition cycle).

This is not unexpected because the AFM measurements were car- ried in the dry state whereas the films were characterizedin situ in the presence of buffer by means of QCM-D. The mass sensed by QCM-D is hence that of an hydrated film, and the thickness is calculated from the obtained surface coverage making an assump- tion on the value of the mass density. This mass density was assumed to be equal to 1.3 g cm³which may induce a huge uncertainty in the film thickness. Assuming that the mass density of the film could vary between 1.2 and 1.6 g cm³, a range of values rea- sonable for materials based on organic molecules, water and tran- sition metal cations, the relative uncertainty on the film thickness will lie around 30%. Nevertheless if the thickness of the hydrated films should be the same as that obtained from AFM height measurements, the mass density of the films should be equal to 3.12 g cm³ which is highly unrealistic for a film containing a

A B

(nm)

300 400 500 600 700 800

Abasorbance

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

n deposition cycles

0 2 4 6 8 10

A

-0.2 0.0 0.2 0.4 0.6 0.8

I (mM)

10 100 1000

Slope

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

Fig. 2.A: Absorption spectra of PEI-(TA–Fe³⁺)nfilms taken after 2 ( ), 4 ( ), 6 ( ) and 8 ( ) deposition cycles. The films were deposited from 50 mM sodium acetate buffer (pH = 5.0) in the presence of 500 mM NaCl. The inset represents the absorbance increase atk= 325 nm, (s, slope = 0.0938 per deposition cycle,r²= 0.998) and atk= 550 nm (d, slope = 0.0239 per deposition cycle,r²= 0.988). The inserted photograph corresponds to a dried film obtained after adsorption of PEI and the subsequent deposition of (TA–Fe³⁺)8. B: Evolution of the slope of the absorbance of PEI-(TA–Fe³⁺)nfilms atk= 325 nm (s) and atk= 550 nm (d) from experiments as those displayed in part A as a function of the ionic strength. Each point corresponds to an individual experiment as displayed in part A. The dashed lines are aimed to guide the eye. The experiments labelled with ( ,k= 325 nm and ,k= 550 nm) were performed in the presence of 25 mM sodium acetate buffer without additional NaCl.

(6)

x (µm)

0.0 0.5 1.0 1.5 2.0 2.5

z (nm

-10 0 10 20 30 40

NaCl 500 mM

x (µm)

0 1 2 3 4

z (nm)

-20 -10 0 10 20 30 40 50

NaCl 750 mM

x (µm)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

z (nm)

-10 0 10 20 30 40 50

NaCl 1000 mM

x (µm)

0 1 2 3 4

z (nm)

-10 0 10 20

30

NaCl 200mM

Fig. 3.AFM images of PEI-(TA–Fe³⁺)6films in regions were the films have been needle scratched as a function of the NaCl concentration added in the 50 mM sodium acetate buffer (pH = 5.0) used to prepare the films. Typical line profiles taken from the images on the left row are represented in the right row.

(7)

rather hydrophilic molecule like TA and hydrated cations.

Assuming a mass density of 1.3 g cm³is realistic for a ﬁlm containing water and an organic molecule like TA.

The root mean squared roughness of the films (Fig. 5) as well as their thickness (Fig. 4) follow the same trend as the absorbance increase per deposition cycle, namely an almost independence on the ionic strength used during the deposition. Analysis of the surface topography (Fig. 5) also reveals that the films are of grainy structure and cover the substrate homogeneously in the case of films made from 6 deposition cycles of TA and Fe³⁺ cations.

However the line proﬁles were acquired with cantilever tips hav- ing a radius of curvature of about 10–20 nm meaning that smaller cavities reaching the substrate from the ﬁlms surface cannot be detected.

To further conﬁrm the full coverage of the substrate, the permeability of the ﬁlms ending with the deposition of Fe³⁺cations was investigated by means of cyclic voltammetry using an anionic redox probe, hexacyanoferrate anions. This redox probe has the additional advantage to strongly interact with Fe³⁺ cations [13]

to form Prussian blue, an inorganic network easily detectable by UV–vis spectroscopy as well as by CV. The ﬁrst CV scans were performed in the absence of the redox probe and some irreversible oxidation peaks were found in the ﬁrst oxidation cycle (Fig. 6), with no corresponding reduction peaks and a disappearance of the oxidation peaks during the second CV cycle. The CV of a I (mM)

1 10 100 1000

Thickness increment per deposition cycle (nm)

0 2 4 6 8

Fig. 4.Thickness increment per deposition cycle of PEI-(TA–Fe³⁺)n films as determined from AFM height profiles as those shown inFig. 3as a function of the ionic strength (including the contribution of the 50 mM sodium acetate buffer (pH = 5.0) and of the added NaCl). The experiments labelled with filled blue circles ( ) were performed in the presence of 5 and 25 mM sodium acetate buffer without additional NaCl. The error bars correspond to one standard deviation over 20 lines as those shown in the right panel ofFig. 3.

A: NaCl 0 mM

RMS=21.3 nm B: NaCl 500 mM

RMS= 22.2 nm

C: NaCl 1000mM D RMS=20.1 nm

I (mM)

10 100 1000

RMS (nm)

0 10 20 30 40

Fig. 5.Typical surface topographies of PEI-(TA–Fe³⁺)6 films deposited on silicon slides from 50 mM sodium acetate buffer (pH = 5.0) in the presence of different concentrations in NaCl (A–C) and (D) evolution of the root mean squared roughness of those films (as determined on 10lm10lm images) as a function of the ionic strength. The experiments labelled with filled blue circles ( ) were performed in the presence of 5 and 25 mM sodium acetate buffer without additional NaCl. The dashed line is aimed to guide the eye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(8)

1 mg mL¹ TA solution at pH 5.0 displays the same behavior (Fig. S5 of the Supplementary data). This means that there are some TA molecules able to undergo oxidation and accessible to the electrode in the PEI-(TA–Fe³⁺)n films. When these films are then put in contact with a 1 mM potassium hexacyanoferrate solution (dissolved in the same sodium acetate + NaCl buffer used to build up the film), no oxidation/reduction peaks due to Fe(CN)64

are detected. This means that the PEI-(TA–Fe³⁺)6 films remain stable when put in contact with Fe(CN)₆⁴anions and in addition that these films are impermeable to the redox probe, confirming on a molecular scale the expected prediction from AFM imaging.

The same trend is found for ﬁlms prepared in the presence of 200, 500, 750 or 1000 mM in NaCl (only the data acquired in the presence of 1000 mM in NaCl are shown inFig. 6).

To conﬁrm the non-accessibility of Fe³⁺cations present in the ﬁlms to Fe(CN)64 anions in solution, we performed a control experiment by means of UV–visible spectroscopy: a PEI-(TA–

Fe³⁺)6film was put in the presence of a 1 mM K4Fe(CN)6solution during 30 min (in the presence of 50 mM sodium acetate buffer + 2 mol L¹in NaCl). The film was then rinsed with distilled water and its absorption spectrum was compared to that of the same film before its immersion in the K₄Fe(CN)₆ solution. No observable spectral change was found (Fig. S6 in the Supplementary data) confirming the result obtained by cyclic voltammetry.

All these ﬁndings show that even if the deposition of PEI-(TA–

Fe³⁺)n films implies some electrostatic interactions (TA is in the partially deprotonated form in the film,Fig. 2andFig. S4 in the Supplementary data) these interactions are insensitive to the ionic strength of the solution when the ionic strength is increased from 3.2 to 2032 mM. With exception of the films obtained by the sequential deposition of poly(4-styrene sulfonate) (PSS) and poly (allylamine) (PAH)[21], such a finding is rare. Indeed, the thickness (or the deposited mass per unit area) of the so called polyelectrolyte multilayer films usually increases when the ionic strength increases, passes through a maximum before strongly decreasing to zero when the deposition is performed at higher ionic strengths [22,23]. This commonly observed trend simply traduces that an increase in ionic strength reduces the intramolecular electrostatic

repulsions in the polyelectrolyte chains forcing them to adopt a more loopy conformation. This results in a higher thickness increment per layer pair in comparison to the value obtained at low ionic strength were the chains are in a rigid conformation and tend to adsorb in a flat orientation on the substrate. However when the ionic strength increases further, the intermolecular attractive interactions between oppositely charged species are screened with the consequence of a strong decrease in film deposition. There is indeed a competition between polyelectrolyte–polyelectrolye and polyelectrolyte–small ions interactions in the deposition of polyelectrolyte multilayer films[24]. In the case of films made from the step-by-step deposition of tannic acid and Fe³⁺ cations one may however expect a different evolution of the film thickness (after a given number of deposition steps) with the concentration of the supporting electrolyte because of the rigid nature of TA.

The fact that esteriﬁed gallic acid units are covalently bound to a central glucose unit makes the conformation of this molecule almost salt concentration independent. One would hence expect a continuous decrease in the amount of deposited material with an increase in the NaCl concentration, but not the occurrence of a maximum as for ﬁlms made from the alternated deposition of oppositely charged polyelectrolytes. Indeed, the interactions between deprotonated TA and Fe³⁺cations are so strong, that that they are almost ionic strength independent up to 2 mol L¹in NaCl (Figs. 2 and 4). In future studies, the interactions between TA and Fe³⁺cations will be investigated in solution using isothermal titration microcalorimetry and conductimetry as a function of the ionic strength. The aim of these experiments will be to investigate if the complexation between TA and Fe³⁺cations will be accompanied by counterion release as the interpolyelectrolyte complexation does.

4. Conclusions

In the present investigation it is shown that the deposition of PEI-(TA–Fe³⁺)nfilms is almost independent on the ionic strength up to more than 2 mol L¹. This shows that the interactions between TA and Fe³⁺, even if the electrostatic contribution is important, are extremely strong and almost insensitive to the ionic strength of the solutions. These interactions will be quantified in future studies by means of isothermal titration calorimetry. The present investigation suggests that PEI-(Ta–Fe³⁺)nfilms are chemi- cally very robust as long as the pH of the solution remains higher than 3. These films dissolve however very rapidly when put in contact with highly acidic solutions owing to the force protonation of TA.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.jcis.2015.03.009.

References

[1]M. Akagawa, K. Suyama, Eur. J. Biochem. 268 (2001) 1953–1963.

[2]E. Haslam, J. Nat. Prod. 59 (1996) 205–215.

[3]A.R. Patel, J. Seijen Ten-Hoorn, J. Hazekamp, T.B.J. Blijdenstein, K.P. Velikov, Soft Matter. 9 (2013) 1428–1436.

[4]M. Liu, Y. Zhnag, Q. Yang, Q. Xie, S. Yao, J. Agric. Food Chem. 54 (2006) 4087–

4094.

[5]B.H. Cruz, J.M. Diaz-Cruz, C. Ariño, M. Esteban, Electroanalysis 12 (2000) 1130–

1137.

[6]A.T. Iffat, Z.T. Maqsood, N. Fatima, J. Chem. Soc. Pak. 27 (2005) 174–177.

[7]I. Eral-Unal, S.A. Suhkhisvili, Macromolecules 41 (2008) 3962–3970.

[8]A.R. Patel, J. Seijen Ten-Hoorn, P.C.M. Heussen, E. Drost, J. Hazekamp, K.P.

Velikov, J. Coll. Interf. Sci. 374 (2012) 150–156.

[9]V. Ball, Coll. Interf. Sci. Comm. 3 (2014) 1–4.

[10] T. Shutava, M. Prouty, D. Kommireddy, Y. Lvov, Macromolecules 38 (2005) 2850–2858.

E (V vs Ag/AgCl)

-0. 8 -0. 4 0.0 0.4 0.8

I (A)

-1.5e-5 -1.0e-5 -5.0e-6 0.0 5.0e-6 1.0e-5 1.5e-5 2.0e-5

Fig. 6.Cyclovoltamograms (CVs) performed at a scan rate of 100 mV s¹for a 1 mM K4Fe(CN)6solution on a freshly polished amorphous carbon electrode ( ), for a PEI-(TA–Fe³⁺)6film after the first ( ) the 2nd ( ) and the third ( ) CV scan and of the same film put in contact with a 1 mM K4Fe(CN)6solution ( ).

All experiments were performed in the presence of 50 mM sodium acetate buffer (pH = 5) in the presence of 1000 mM in NaCl.

(9)

[11]H. Ejima, J.J. Richardson, K. Liang, J.P. Best, M.P. van Koeverden, G.K. Such, J. Cui, F. Caruso, Science 341 (2013) 154–157.

[12]Md.A. Rahim, H. Ejima, K.L. Cho, K. Kempe, M. Müllner, J.P. Best, F. Caruso, Chem. Mater. 26 (2014) 1645–1653.

[13]M. Pyrasch, B. Tieke, Langmuir 17 (2001) 7706–7707.

[14]M. Lefort, G. Popa, E. Seyrek, R. Szamocki, O. Felix, J. Hemmerlé, L. Vidal, J.-C.

Voegel, F. Boulmedais, G. Decher, P. Schaaf, Angew. Chem. Int. Ed. 49 (2010) 10110–10113.

[15]K. Apaydin, A. Laachachi, J. Bour, V. Toniazzo, D. Ruch, V. Ball, Colloids Surf. A Physicochem. Eng. Aspects 415 (2012) 274–280.

[16]A.F. Xie, S. Granick, Macromolecules 35 (2002) 1805–1813.

[17]G. Sauerbrey, Z. Phys. 155 (1959) 206–222.

[18]F. Höök, M. Rodahl, P. Brzezinski, B. Kasemo, Langmuir 14 (1998) 729–734.

[19]I. Reviakine, D. Johansmann, R.P. Richter, Anal. Chem. 83 (2011) 8838–8848.

[20]P.K. South, D.D. Miller, Food Chem. 63 (1998) 167–172.

[21]H. Mjahed, J.-C. Voegel, A. Chassepot, B. Senger, P. Schaaf, F. Boulmedais, V.

Ball, J. Coll. Interf. Sci. 346 (2010) 163–171.

[22]S.A. Suhkhisvili, E. Kharlampieva, V. Izumrudov, Macromolecules 39 (2006) 8873–8881.

[23]V. Ball, Macromol. Chem. Phys. 216 (2015) 85–94.

[24]D. Volodkin, R. von Klitzing, Curr. Opin. Coll. Interf. Sci. 19 (2014) 25–31.

View publication stats