Covalently attached metalloporphyrins in LBL self-assembled redox polyelectrolyte thin films

Electrochimica Acta 53 (2008) 5215–5219

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Covalently attached metalloporphyrins in LBL self-assembled redox polyelectrolyte thin films R.R. Carballo a , V. Campodall’ Orto a,1 , J.A. Hurst a , A. Spiaggi a , C. Bonazzola b , I.N. Rezzano a,∗,1 a Departmento de Qu´ımica Anal´ıtica y Fisicoqu´ımica, Facultad de Farmacia y Bioqu´ımica, Universidad de Buenos Aires, Jun´ın 956, CP 1113 Buenos Aires, Argentina b INQUIMAE, Departamento de Qu´ımica Inorg´ anica, Anal´ıtica y Qu´ımica F´ısica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabell´ on 2, Ciudad Universitaria, CP 1428 Buenos Aires, Argentina

a r t i c l e

i n f o

Article history: Received 14 December 2007 Received in revised form 31 January 2008 Accepted 13 February 2008 Available online 19 February 2008 Keywords: Formylporphyrin LBL Polyelectrolyte films Catalysis

a b s t r a c t A formylporphyrin has been covalently bound to Poly (Allylamine Hydrochloride) (PAH) and electrostatically self-assembled polyelectrolyte films, containing the attached metalloporphyrin, have been constructed. The UV–vis absorption band at 390 nm has been followed as core porphyrin marker. The reflection–absorption IR spectra of the gold films modified with layer-by-layer (LBL) polyelectrolytes were recorded after 6 and 12 layers. Characteristic infrared absorbance bands of porphyrin, PAH and PVS became more evident on increasing the number of bilayers. The absorption bands at 750, 1214 and 2960 cm−1 , attributed at (S–O), s (SO3 − ) and ( NH2 + ), respectively, showed a linear growth (R2 > 0.99) with the number of adsorbed layers. A lower correlation coefficient was observed for the band at 1585 cm−1 attributed to Fe-protoporphyrin. In order to evaluate the electron transfer (ET) rate, the Ep of the [Fe(CN)6 ]4− /[Fe(CN)6 ]3− couple in solution was measured after covering the electrode. A proportional increase of the Ep with the number of layers is observed up to the 4th layer. After the second bilayer, the magnitude of the peak separation is highly related to the charge of the topmost layer. The method allowed controlling the film thickness via the number of deposited layers (LBL). The electrode described, resulted in a good catalyst for O2 reduction and sulfite oxidation. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction The redox chemistry of metalloporphyrins has been intensively studied for many years. These compounds have been employed in sensing and catalytic applications due to their rich physicochemical properties [1,2], which can be modulated by the environment when confined in ordered molecular assemblies [3]. With this idea, a variety of immobilizing methods have been described, electropolimerization [4,5], chemisoption [6] and self-assembled monolayers [7,8]. In the last decade, the electrostatically self assembled LBL deposition of polyelectrolytes of opposite charge provided a versatile method to obtain films electrically connected to an electrode surface [9–13]. The robust and homogeneous growth of films allows controlling the interatomic distances and the amount of deposited material therefore leading to chemical sys-

∗ Corresponding author. Tel.: +54 1149648263; fax: +54 1149648263. E-mail address: [email protected] (I.N. Rezzano). 1 CONICET permanent staff. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.02.036

tems with highly ordered mono or multilayers onto macro, micro-and nano-electrodes [14]. Poly (allylamine hydrochloride) (PAH), poly(sodium-4-styrensulfonate) (PSS) or polyvinylsulfonate (PVS) [15] combined with redox mediators (osmium, ferrocene [16,17]) have been extensively studied in LBL self-assembled films formation. In the case of metalloporphyrins as active materials, electroactive monolayer films have been prepared by the self-assembly of thiol derivatives of porphyrins adsorbed onto Au surfaces. More recently, positively charged tetrakis(Nmethylpyridyl)porphyrinatocobalt [18], and negatively charged tetrakis(4-carboxyphenyl)porphyrin [19] were prepared by a layerby-layer deposition (LBL) by alternative deposit with platinum nanoparticles (PtNPs)) pyrenetetrasulfonic acid (PTSA), respectively. In the present paper, we report the synthesis of PAH derivatized with Fe-protoporphyrin via a Schiff base intermediate. This redox polymer was immobilized by electrostatic LBL deposition onto a gold surface. Thus, this method allowed controlling the film thickness via the number of deposited layers (LBL). The spectroscopic analysis along with the catalytic properties of this films, are reported.

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solution was then purified by dialysis against water for three days. IR spectra showed an intense band at 1600–1580 cm−1 (C N) and the disappearance of bands at 1660–1770 cm−1 (C O). 2.3. Electrode preparation Gold electrodes were thoroughly cleaned by immersion in hot 1:1 nitric–sulfuric acid for several hours. Clean electrodes were further rinsed with Milli-Q water. Initially, the gold electrode was immersed in a MPS solution for 30 min followed by rinsing with deionized water. After thiol adsorption, the first polycation layer was deposited on a thiol-modified Au substrate by immersion in PAH-FeP solution for 15 min. The next and subsequent layers were deposited onto the modified surface by alternate immersion for 15 min in a solution of the respective polyanion (PVS) and polycation (PAH-FeP). 2.4. Apparatus

Scheme 1. Schematic representation of the alternate electrostatic self assembly procedure onto gold electrodes. (1) Structure of formylporphyrin and (2) structure of redox polymer.

2. Experimental 2.1. Chemicals and solutions Osmium tetroxide was purchased from Fluka (Buchs, Switzerland). Analytical grade 3-mercapto-1-propanesulfonic acid sodium salt, MPS (Aldrich), poly (vinyl sulfonic acid) sodium salt, PVS (Aldrich), poly(allylamine hydrochloride), PAH (Aldrich) and sodium periodate (Aldrich) were used as supplied. Aqueous solution of redox polymer and PVS 10 mM, were employed for the film preparation by electrostatic adsorption. 2.2. Synthesis of the redoxpolymer, poly(allylamine hydrochloride) Fe-porphyrin (PAH-FeP) The 2,4-diformyldeuteroporphyrin IX (1) (Scheme 1) was obtained from the oxidation of the vinyl groups of protoporphyrin IX. It was oxidized with osmium tetroxide to the 2,4-di (␣,␤dihidroxy)ethyldeuteroporphyrin, which on reaction with sodium periodate formed 2,4-diformyldeuteroporphyrin IX. [20]. The product was characterized by mass spectra [FABS m/z: 595 (M+)] and 1 H NMR (300 MHz, ppm: ı 9.35 (s, 1Hmeso ), 9.56 (s, 1Hmeso ), 9.65 (s, 1Hmeso ), 10.05 (s, 1Hmeso ), 10.99 (s, 1H, CHO), 11.06 (s, 1H, CHO). PAH (15 mg) was dissolved in dry methanol by addition of 0.04 ml triethyl amine. A solution of diformylporphyrin (15 mg) in methanol was added dropwise within 1 h. The mixture was stirred for another hour at room temperature. Finally, an excess of sodium borohydride was added after cooling the solution in an ice bath and stirring continued for 1 h. Methanol was removed under reduced pressure and the residue dissolved in water and lyophilized to dryness. The residue was dissolved in acetic acid/pyridine and mixed with a methanolic saturated solution of FeSO4 [21]. This polymer

Cyclic voltammetries (CVs) were performed with a specially built potentiostat (microprocessor controlled), with digital signal generator for implementation of different electrochemical techniques. Au working electrodes (0.36 cm2 area), a Ag/AgCl reference electrode (BAS) and a platinum wire auxiliary electrode were used for voltammetric experiments of electrodeposition. FTIR spectroscopy was used in transmission (TIR) and reflection absorption spectroscopy (IRRAS) modes. TIR spectra were recorded using a Nicolet 510P spectrometer provided with a DTGS detector and a Balston 75-45 purge gas generator. IRRAS spectra were recorded with a Nicolet Nexus 670 and a Whatman 75-65 purge gas generator, equipped with a liquid N2 cooled MCTA detector and Ca2 F windows. Reflection–absorption spectra were obtained for gold films modified with LBL polyelectrolyte using a specially built reflectance setup with 80◦ angle of incidence and a bare gold surface as background (reference). All spectra were collected at 4 cm−1 spectral resolution using 100 scans and presented without smoothing correction. 3. Results The LBL self-assembled polyelectrolyte films have been constructed as depicted in Scheme 1. The target redox polymer, PAH-FeP (1), has been successfully made by the synthesis of a Schiff base, between PAH and the formyl porphyrin (2) as described in the experimental section. The introduction of the central metal Fe(II), was performed after reduction of the Schiff base to the corresponding amine and was followed by UV–vis spectroscopy. Fig. 1 shows this result comparing the UV–vis spectra of PAH and PAH-FeP at acid aqueous solutions (pH 2), wherein the presence of Fe-porphyrin

Fig. 1. UV–vis spectra in aqueous solution pH 2 of PAH-FeFP (a) and PAH (b).

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Fig. 3. Ep [Fe(CN)6 ]4− /[Fe(CN)6 ]3− vs. number of layers in 0.1 M sodium phosphate buffer (pH 7.0) at () 50 mV/s; (*) 100 mV/s.

Fig. 2. A reflection–absorption spectra of the gold films modified with LBL polyelectrolyte: after (a) 6 and (b) 12 bilayers of PAH-FeFP. B FTIRRAS of 12 layers of Au/MPS/PAH-FeP(d) and Au/MPS/PAH (e).

moiety could be easily confirmed by the absorption band at 390 nm, typical of porphyrin ring. The Q bands at 540 and 630 nm, indicate the coordination of metals. The porphyrin concentration in the redox polymer was estimated as 7 × 10−2 mg/mg of polymer, by the spectrophotometric assay, assuming the absorption coefficient of the porphyrin in the polymer equal to the free formylporphyrin. Scheme 1(a) shows the electrostatic layer by layer immobilization process of the polyelectrolyte films over the thiol (MPS) modified gold electrode, by sequential immersion in solutions of polycation (PAH-FeP) and polyanion polyvinylsulfonate (PVS). Reflection–absorption spectra were obtained for gold films modified with LBL polyelectrolyte in order to follow the immobilization process. Previously, the transmission (TIR) spectra of the polyanion and polycation samples included in KBr pellets were performed to determine frequency position and relative intensity of the characteristics bands in each structure. Typical absorption bands of PAH are ( NH) 3450–3300 cm−1 ; ( NH2 + ) 3000–2000 cm−1 , ( CH) and (CH); ı( NH) 1650–1550 cm−1 ; ı( NH2 + )1600–1460 cm−1 [20]. For the Fe-porphyrin moiety [21] (C N) 1600–1580 cm−1 ; ıas (CH3 )1470–1430 cm−1 ; ıs (CH3 )1395–1365 cm−1 ; ı( CH) −1 1005–985 cm and (C N)1690–1580 cm−1 must be considered. Finally, the sulfonate group in PVS showed the intense absorption bands as (SO3 − ) and s (SO3 − ) 1213 cm−1 , and 1040 cm−1 , respectively, and (S–O) at 870–690 cm−1 . The consecutive deposition of layers showed the characteristic FTIRRAS peak for mercaptopropanesulfonate-modified gold surface (Au/MPS) at 1074 cm−1 , which was shifted at 1052 cm−1 after adsorption of the first polycation (PAH) layer. Also, a new broad band (around 1200 cm−1 ) became apparent after the first polyanion (PVS) deposition [22]. The inclusion of porphyrin moiety masked the characteristic peaks of the polyelectrolyte FTIRRAS, but showed a differential broad band near 1600 cm−1 (Fig. 4B). The reflection-absorption spectra of the gold films modified with LBL polyelectrolyte can be observed in Fig. 2A. The spectra were recorded after 6 and 12 bilayer deposition steps. It is

easy to notice that the characteristic infrared absorbance bands of porphyrin, PAH and PVS, become more evident as increasing the number of bilayers. This result indicates that the stepwise deposition of the PAH-FeP occurred irrespective of the high content of bulky aromatic ring of the porphyrin moiety. The absorption bands at 750, 1214 and 2960 cm−1 , attributed at (S–O), s (SO3 − ) and ( NH2 + ) showed a linear growth (R2 > 0.99) with the number of adsorbed layers. On the other hand, the band at 1585 cm−1 assigned to (N C) in Fe-porphyrin, showed a lower correlation coefficient (R2 > 0.97) probably due to anisotropic effects of the porphyrin in the LBL polyelectrolyte multilayer. 3.1. Electrochemical characterization The insulating properties of adsorbed thiol monolayers of different thicknesses have been reported by comparing electron-transfer rates to solution redox couples [22]. In our case, the electrochemical behavior of the [Fe(CN)6 ]4− /[Fe(CN)6 ]3− redox system was examined with the LBL electrode, prepared with different number of layers. The cyclic voltammetry technique was used to access the electrochemical reactivity of the electrode surface by comparing the peak-to-peak separation. The Ep of the external couple allowed inferring the rate of electron transfer at the electrode/solution interface (Fig. 3). In all the cases, even number of layers correspond to LBL electrodes ended with PAH-FeP positively charged, and the odd numbers are assigned to LBL electrodes ended with anionic PVS layer. Table 1 summarizes this description up to eight bilayers. It is worth noting that a significant potential shift in the couple of [Fe(CN)6 ]4− /[Fe(CN)6 ]3− was observed with the covered electrode. The Ep , always resulted in larger values than the expected for an ideal one-electron redox system, indicating repulsive interaction between the redox sites [23]. In Fig. 3, a proportional increase of the Ep with the number of layers is observed up to the 4th bilayer, this more sluggish electron Table 1 Composition of the LBL electrodes according to the number of layers Number of layers

Description of layer deposited

0 1 2 3 4 5 6 7 8

Au Au/MPS Au/MPS/PAH-FeP Au/MPS/PAH-FeP/PVS Au/MPS/[PAH-FeP]2 /[PVS]1 Au/MPS/[PAH-FeP]2 /[PVS]2 Au/MPS/[PAH-FeP]3 /[PVS]2 Au/MPS/[PAH-FeP]3 /[PVS]3 Au/MPS/[PAH-FeP]4 /[PVS]3

Schematic representation of the composition of the LBL electrodes, according to the number of layers.

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3.2. Catalytic activity In order to evaluate the electrocatalytic activity of this electrode, we explored the reduction of dioxygen with the LBL electrode modified with six bilayers. An interesting structural aspect of metalloporphyrins is their tendency to include axial ligands which strongly influence their redox chemistry. In fact, the different biological functions of heme enzymes are ruled by the environment [27,28]. In this line of research, it has been reported that N-donor ligands such as ethanolamine, glycine, pyridine, ammonia and imidazole are readily coordinated by Fe-porphyrin in N-acetyl microperoxidase-11 [29], whereas no evidence of coordination was observed in ferric myoglobin. With these ideas in mind, we compared the catalytic oxygen reduction in the presence of 2-methylimidazole and ammonia, these results are depicted in Fig. 4. In our system, the presence of ammonia increased the catalytic current up to 200 ␮A whereas the 2-methylimidazole decreased the overvoltage in 40 mV. These catalytic effects were only observed for oxygen reduction and clearly indicate that the presence of N-donor ligands strongly influences the heme redox potential and the O O bond cleavage. A qualitative correlation between reduction potential of metal centre and the ability of the ligand to carry a negative charge has been described for heme enzymes [28]. Thus, one can speculate that 2methylimidazole group increases the Fe3+/Fe2+ reduction potential of the redox center and facilitates the formation of Fe4+ O intermediate. On the other hand, the binding of primary amines to Fe(III) porphyrins has been carefully studied. It was demonstrated that there is a linear dependence of the affinity constant and the pKa

Fig. 4. Voltamperometric response of O2 reduction obtained with six bilayers LBL electrode (up) and bare Au electrode (down) in the presence of: (a) PAH-FeFP without ligands; (b) PAH-FeFP with NH3 0.05 M. (c) PAH-FeFP 2-methylimidazole 0.05 M; buffer phosphate 50 mM pH 4.30 as supporting electrolyte. Scan rate: 50 mV s−1 .

transfer process at the electrode surface with increasing thickness of the deposited material is consistent with the interfacial electron tunneling between the electrode and the redox centre, described for alkanethiols [24], and with a small amount of metalloporphyrin in the film. It is interesting to observe that after the 4th bilayer, with increased concentration of redox sites, the magnitude of peak separation is highly related to the charge of the topmost layer. A decrease of Ep of [Fe(CN)6 ]4− /[Fe(CN)6 ]3− occurs at four and six deposited layers, which corresponds to multilayers ended in PAH-FeP. Thus, the nature of charge of the latest layer clearly influences the redox behavior of the film. This conclusion is coincident with previous reports for other systems LBL systems, PAH-ferrocene [25] and PAHOs [26], where inactive but positively charged polyelectrolytes were adsorbed on top of a redox multilayer. Based on the estimated electron transfer rate, we decided to prepare the six bilayers electrode for catalytic experiments. The surface coverage of the LBL electrode was estimated from  av (mol cm−2 ) =

Q AneNA

(1)

in which e is the electron charge (1.60 × 10−19 C) and NA the Avogadro constant (6.02 × 1023 mol−1 ). The charge density Q/A was determined by integration of the cathodic/anodic peaks of the redox couple of the metalloporphyrin divided by A (0.36 cm2 ) [23]. The calculated  av for six bilayers is 1.45 × 10−8 mol cm−2 .

Fig. 5. Voltametric response to sulfite (up) and nitrite (down) oxidation with: (—) base line (a and d); clean Au (b and e); EAuMPS/[PAHFeFP(+)6 /PVS(−)5 ] (c and f). Scan rate: 50 mV s−1 .

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tional increase of the resistance of the film with the number of layers can be observed up to the 4th monolayer. This behavior is not affected for addition of ligands. The latest layer defines the effective redox sites in the LBL. The six layers deposited LBL electrode was employed for catalytic reduction of O2 and SO2 . Addition of ligands such as ammonia increased the catalytic response (up to 200 ␮A) whereas the imidazole decreases the overpotential in 40 mV. The presence of ligands only affected the catalytic oxygen reduction. Finally the catalytic response increased linearly with SO2 concentration. Acknowledgements Financial support from University of Buenos Aires (UBACyT B062) and CONICET (PIP5021) are gratefully thanked. Romina Carballo also thanks CONICET for a doctoral fellowship. References Fig. 6. Current response vs. [SO2 ] for: () bare electrode y = 0.0149x + 0.0244, R2 = 0.9972 and () six bilayers, LBL modified electrode y = 0.0283 + 0.0446, R2 = 0.9935.

of the conjugated acid of the primary amine [30]. In our case, the increase of ip for oxygen reduction correlates with a higher number of active redox centers, suggesting that the inclusion of ammonia as axial ligand promotes the required geometry to bind oxygen. To evaluate further catalytic applications, we also explored the sulfite and nitrite oxidation. The voltammetric response for both substrates can be observed in Fig. 5, Polycrystalline gold electrodes exhibit intrinsic electrocatalytic response towards the oxidation of aqueous SO2 and NO2 − in mild acidic media. The oxidation reaction starts at potentials slightly higher than 0.6 V (SO2 ) and 0.9 V (NO2 − ) displaying characteristic voltammetric peak for irreversible processes whose maximum rate is diffusion limited. The catalytic sulfite oxidation was significantly enhanced with the six bilayers LBL electrode, Fig. 5(up, f). In this case, −160 mV overpotential and a 100% increase of current intensity can be observed. A less dramatic response was obtained for nitrite oxidation where a slightly peak current increase (0.45 mA) and a small shift to lower potentials (40 mV) were found. None of these reactions was affected by the presence of N-donor ligands, which reinforces the idea that only redox sites for oxygen reduction are sensitive to axial ligation. Fig. 6 shows the increasing peak current response with increasing sulfite concentrations, in a linear range of 4–25 ppm of SO2 and calculated sensitivity of 28.3 ␮A ppm−1 . 4. Conclusions As far as we know, this is the first LBL self-assembled polyelectrolyte films modified electrodes constructed with a covalently attached Fe(III)porphyrin. The target molecule, PAH-FeP, has been successfully made by a intermediate Schiff base, between PAH and the formyl porphyrin (1). The rate of ET was inferred from the cyclic voltammetry of [Fe(CN)6 ]4− /[Fe(CN)6 ]3− after covering the electrode. A propor-

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