Monolayers and Langmuir–Blodgett films of Fe2+-mediated polyelectrolyte with viologen derivatives as linkers at the air–water interface

Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 561–569

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Monolayers and Langmuir–Blodgett films of Fe2+ -mediated polyelectrolyte with viologen derivatives as linkers at the air–water interface Shengsheng Zhang, Hong-Lei Wang, Meng Chen, Dong-Jin Qian ∗ Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China

a r t i c l e

i n f o

Article history: Received 22 January 2011 Received in revised form 14 April 2011 Accepted 8 May 2011 Available online 13 May 2011 Keywords: Viologens Monolayers Langmuir–Blodgett films Electrochemistry

a b s t r a c t Monolayer behaviors of newly synthesized viologens (Vs) containing tolunitrile substituents have been investigated at the air–water interfaces. Surface pressure–area isotherms indicated that these viologens could not form stable monolayers on the surfaces of the subphases of pure water, Fe(BF4 )2 and anionic polyelectrolyte of PSS (PSS: poly(styrenesulfonic acid-o-maleic) acid), while they could form insoluble monolayers on the surface of the subphase containing mixtures of Fe(BF4 )2 and PSS, which was attributed to the formation of Fe–V metal–organic polyelectrolytes based on an interfacial coordinative reaction between the Fe2+ ions and viologen derivatives. With the use of Langmuir–Blodgett (LB) method, monolayers of Fe–V polyelectrolytes were transferred on the substrate surfaces. X-ray photoelectron spectra indicated that the LB film was composed of the elements of S, C, N, O and Fe, suggesting the formation of Fe–V/PSS hybrid multilayers. Cyclic voltammograms of the viologens revealed two reversible redox couples in the solutions, corresponding to two electron transfers of V2+ ↔ V+ • and V+ • ↔ V0 , respectively. For the indium tin oxide electrode covered by the LB films of Fe–V/PSS, only one broad redox couple was recorded in the potential range of 0 and −1.0 V vs Ag/AgCl, which was attributed to the first redox couple of V2+ ↔ V+ • . The charger transfer process of viologens in the solutions and Fe–V/PSS multilayers was investigated by the potential chronocoulometry method. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Viologens (Vs) are a group of electroactive organic electrolytes, which exist in three main oxidation states, V2+ ↔ V+• ↔ V0 , together with a color change after the first reductive reaction [1,2]. These redox reactions are highly reversible, especially for the first one, the most important feature of which is the applications as electron transfer mediator to various biological molecules and electron acceptors [3,4]. For instance, methyl viologen, benzyl viologen and poly(viologen) have been widely used as electron mediators for hydrogenase in the catalytic interconversion of protons and hydrogen gas [5–7]. Moreover, the oxidized form of viologens (V2+ ) is colorless, while their reduced form (V+• ) is blue, leading to the particular interests in electrochemistry, analytical chemistry and potential applications as electrochromic devices. Because of these desirable characteristics, viologen derivatives have been extensively investigated in the solutions and on the immobilized electrodes by the casting and molecular assembly techniques [8,9], in the diads or triads with other molecules (such as porphyrins and

∗ Corresponding author. Tel.: +86 21 65643666; fax: +86 21 65643666. E-mail address: [email protected] (D.-J. Qian). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.05.016

Ru complexes) [10,11], as well as in the hybrids with mesoporous, inorganic nanomaterials and carbon nanotubes [12]. From the structural point of view, viologens are composed of one bipyridinium core and two alkylated substituents, which can be designed as a functional unit to bind other molecules or anchor on electrode surfaces for the development of chemically modified electrodes and electrochromic display devices [13,14]. When the alkylated substituents contain a thiol or silane, the viologen derivatives can form self-assembled monolayers on gold or indium tin oxide (ITO) electrode surfaces [15,16]. When alkylated substituents are long alkyl chains, the obtained amphiphilic viologens can form stable monolayers at the air–water interface and be deposited on the substrate surfaces to form Langmuir–Blodgett (LB) films [17,18]. Because viologens are positively charged electrolytes, the obtained monolayers are a positively charged layer, which can be used as a support for the formation of hybrid supramolecular materials with negatively charged polyelectrolytes or proteins such as hydrogenase [19,20]. Besides the modified electrodes, viologens can also be immobilized on the surfaces of porous or nanostructural materials. For example, with the use of mesoporous silica as a support, porphyrin and viologens/titania have been assembled to form composite films, which could initiate a one electron reduction of methyl viologen ions accompanied by the simultaneous decomposition of

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without further purification. Ultrapure water (18.2 M cm) was prepared with a Rephile filtration unit (China). 2.2. Synthesis of viologen derivatives

Fig. 1. Structure of the viologen derivatives synthesized in the present work.

porphyrins within the mesoporous silica channel [21]. Moreover, viologen-modified porous polymeric microspheres or nanoparticles were developed for a rapid response as sensors for chemicals and enzymes [22,23]. Our previous work revealed that viologens could also be immobilized on the surfaces of carbon nanotubes with or without the addition of negatively charged polyelectrolytes, based on an interlayer electrostatic interaction [24,25]. The carbon nanotubes could act as nanowire (mediator) to improve the electron transfer between electrode surfaces and viologens. In the present work, four viologens containing tolunitrile substituent (Fig. 1) have been synthesized and characterized. It has been known that the ligands with the tolunitrile or nitrilic substituent can coordinate with transition metal ions to form various inorganic complexes, coordination polymers or supramolecular building blocks [26,27]. We are interested in the construction of coordinative polyelectrolyte films at interfaces, based on an interfacial coordination reaction between multidentate ligands (as linkers) on the water surface and transition metal ions (as connectors) in the subphases. Examples include Pd2+ -/Cd2+ -mediated multiporphyrin arrays, Hg2+ -/Ag+ -mediated coordination polymer nanocrystals, nanotubes and nanocombs [28,29]. More recently, we have reported construction of metal-mediated viologen-like polyelectrolyte multilayers at interfaces with well electrochromic response upon the applied potential of −1.1 V vs Ag/AgCl [30]. Here, it is expected that the transition metal ions could coordinate with tolunitrile substituents of the viologens (similar to bisdentate ligand), resulting in the formation of metal–viologen polyelectrolytes. Our results revealed that Fe2+ ions could react with the synthesized viologens to form stable monolayers at the air–water interface together with the co-existence of negatively charged polymer, the hybrid monolayer of which could be deposited on the substrate surfaces by using the LB technique. Electrochemical properties for the viologens in the 10 mM KCl electrolyte solutions and the LB films were investigated. 2. Experimental 2.1. Materials ␣-Bromo-o-tolunitrile, ␣-bromo-m-tolunitrile and iron (II) tetrafluoroborate hexahydrate (97%) were purchased from Acros Organics. 4,4 -bipyridyl was from Shanghai Chemical Reagent Co. Chloroform and acetonitrile used as spreading solvents were from Fisher Chemicals Co. Poly(styrenesulfonic acid-o-maleic) acid (PSS) was from Aldrich Chemical Co. All chemicals were used as received

N,N -di(m-tolunitrile)-4,4’-bipyridium dibromide (V1A) and N,N -di(o-tolunitrile)-4,4 -bipyridium dibromide (V2A) shown in Fig. 1 were synthesized by refluxing mixtures of ␣-bromo-otolunitrile or ␣-bromo-m-tolunitrile and 4,4 -bipyridyl (molar ratio, 5:2) in acetonitrile for 10 h. The precipitate was filtered and well washed by plenty of acetonitrile to remove unreacted reactants and dried under vacuum at room temperature. Anal. 1 H NMR (D2 O, ppm). For V1A, 9.11 (4H), 8.49 (4H), 7.82 (4H), 7.75 (2H), 7.61 (2H), 5.90 (4H). For V2A, 9.12 (4H), 8.51 (4H), 7.86 (2H), 7.77 (2H), 7.64 (4H), 6.11 (4H). Calcd for C26 H20 N4 Br2 (V1A): C, 56.73%; H, 4.00%; N, 10.18%. Found: C, 56.81%; H, 3.87%; N, 10.40%. Calcd for C26 H20 N4 Br2 (V2A): 56.73%; H, 4.00%; N, 10.18%. Found: C, 56.78%; H, 3.91%; N, 10.46%. The viologens of bishexafluorophosphate salts (V1B and V2B, Fig. 1) were obtained by reprecipitation of V1A and V2A in an aqueous solution of 0.35 M NH4 PF6 . The precipitate was filtered and washed with plenty of water, ethanol and dried under vacuum at room temperature. Anal. 1 H NMR (CD3 CN, ppm). For V1B, 9.02 (4H), 8.42 (4H), 7.83 (4H), 7.78 (2H), 7.63 (2H), 5.84 (4H). For V2B, 8.90 (4H), 8.79 (4H), 7.88 (2H), 7.78 (2H), 7.59 (4H), 5.98 (4H). Calcd for C26 H20 N4 P2 F12 (V1B): C, 45.88%; H, 3.23%; N, 8.24%. Found: C, 45.02%; H, 3.36%; N, 8.19%. Calcd for C26 H20 N4 P2 F12 (V2B): C, 45.88%; H, 3.23%; N, 8.24%. Found: C, 46.01%; H, 3.23%; N, 8.32%. 2.3. Monolayers and Langmuir–Blodgett films of Fe2+ -mediated polyelectrolytes Monolayers of the viologens of V1B and V2B and their Fe2+ mediated polyelectrolytes of Fe-V1B and Fe-V2B were prepared by spreading a dilute (∼5 × 10−5 M) V1B or V2B solution in mixed solvents of chloroform and acetonitrile (ratio 4:1, v/v) onto the surfaces of the subphases of pure water, Fe(BF4 )2 , PSS and mixtures of Fe(BF4 )2 –PSS. The surface pressure–area (–A) isotherm measurements and LB film transfer were performed using a KSV 5000 minitrough (KSV Instrument Co., Finland) operated at a continuous speed for two barriers of 10 cm2 /min at 25 ◦ C. The accuracy of the surface pressure measurement was about 0.03 mN/m. Transfer of the monolayers of Fe–V1B and Fe–V2B polyelectrolytes onto the solid plates was done by a vertical dipping method at 20 mN/m. For every transfer, the dipping speed was 2 mm/min. 2.4. Measurements The UV–vis absorption spectra of the viologens in the solutions and transferred LB films were measured using a Shimadzu UV-1601 UV–vis spectrophotometer. Fourier transform infrared spectra (FITR) were recorded on a Nicolet NEXUS470 infrared spectrophotometer. X-ray diffraction for the LB films on the quartz substrate surface was carried out with a Rigaku D/max-␥B diffractometry in transition mode and Cu K␣ radiation. The scan range of 2 was 10–60◦ with a step interval of 0.02◦ . X-ray photoelectron spectra (XPS) for the LB films on the quartz substrate surfaces were recorded using a PHI 5000C ESCA system, with nonmonochromatized Mg K␣ X-rays as the excitation source. The system was carefully calibrated by Fermi-edge of nickel, Au 4f2/7 and Cu 2p2/3 binding energy. Pass energy of 70 eV and step size of 1 eV were chosen when taking spectra. In the analysis chamber pressures of 1–2 × 10−7 Pa were routinely maintained. The binding energies obtained in the XPS analysis were corrected by referencing the C1s peak to 284.60 eV.

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1.5

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Fig. 2. Absorption spectra for (A) V1A and (B) V2A in aqueous solution, as well as (C) V1B and (D) V2B in acetonitrile solution.

2.5. Electrochemistry

3.1. Spectral features of viologen derivatives

Electrochemical behaviors of viologen derivatives were investigated in the solutions and Fe–V LB films by using an electrochemical analyzer (CHI 601b). A Pt wire and Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. Freshly prepared glass carbon (GC) was used as the working electrode for the measurements in the solutions. For the experiments in the LB films, the ITO electrodes modified by one layer of Fe–V1B and Fe–V2B polyelectrolytes were investigated in the 10 mM KCl electrolyte solution. An initial potential of 0 V was applied for 2 s, and subsequently cyclic scans to a final potential of −1.0 or −1.2 V were done for 10 cycles. The CV curves and data of potentials and redox current intensities reported in the present work were the 10th cycle. Chronocoulomograms were measured by setting a fixed initial potential Ei and several final potential E in the solutions and LB films. The charge Q(t) following each potential jump Ei → E was recorded versus the time t elapsed from the instant of the jump for 250 ms, after which the potential was stepped back to Ei . The Ei was set to 0 V, and E was range from −0.1 to −0.6 or −0.8 V. All electrochemical measurements were performed in Ar atmosphere at room temperature.

Fig. 2 shows absorption spectra for the viologens of V1A and V2A in water as well as V1B and V2B in acetonitrile solutions. Similar curves were obtained, which revealed two main absorption bands at the wavelengths of about 262 and 230 nm. The band at 262 nm was attributed to the –* transition of 4,4 -alklated bipyridinium core, and that at about 230 nm to the absorption of the substituents of tolunitrile. In acetonitrile, the band at 230 nm became a shoulder one (not so clear as that in the aqueous solutions), which was due to the absorption of the solvent molecules below 230 nm. FTIR spectra of four viologens were much similar (figures not shown) and revealed several absorption peaks characteristic of the viologens. As an example, the main absorption peaks for the V1A are as follows: C–H, 3029 (m), 2983 (vs), 2941 (m), 2864 (w), 1447 (m), 1359 (m); C N, 2239 (s); C C 1553 (m) and C N 1508 (m), 1634 (vs). Co-existence of C N and C N vibration peaks supported the formation of viologen products.

3. Results and discussion Since the viologens of V1A and V2A could be well dissolved in water, to prepare their insoluble monolayers, the anionic counter ions of Br− in the V1A and V2A were replaced by PF6 − resulting in the formation of the viologens of V1B and V2B (Fig. 1), which were not soluble in water but well dissolved in acetonitrile. This treatment was similar to our previous work for the synthesis of viologen thiols and viologen-like ligands [16,30]. During experiments, we found that, after reprecipitate from an aqueous solution of V1A and V2A in the 0.35 M NH4 PF6 solution, pure pale products of V1B and V2B could be obtained without further purification. The as-prepared V1B and V2B in mixed solvents of acetonitrile and chloroform were spread at the air–water interface for the formation of Fe2+ -mediated polyelectrolyte frameworks.

3.2. Monolayer behaviors of viologen derivatives Fig. 3 shows –A isotherms for the monolayers of V1B and V2B on the surfaces of the subphases of 0.1 mM Fe(BF4 )2 , 0.1 mg/ml PSS and mixtures of Fe(BF4 )2 –PSS (0.1 mM Fe(BF4)2 and 0.1 mg/ml PSS) at 25 ◦ C. These curves revealed the following features. Firstly, almost no surface pressure increase was recorded for the viologens spread on the surfaces of the subphases of Fe(BF4 )2 and PSS (even increasing their concentrations), which indicated that viologens could not form stable monolayers in these cases. This may be because the viologens are charged electrolytes; they can slightly dissolve into water though the solubility is very low. Secondly, the surface pressures could reach to 20–30 mN/m, when the viologens were spread on the subphase surface of mixtures of Fe(BF4 )2 –PSS, which was attributed to the formation of insoluble monolayers after an interfacial coordinative reaction between Fe2+ ions and viologens. Our previous studies have revealed that Fe2+ ions could coordinate with bisterpyridine ligands to form coordination polyelectrolyte monolayers at interfaces [30]. Here, it is suggested that the Fe2+ ions were coordinated with the tolunitrile sub-

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C(1s)

30

Surface Pressure (mN/m)

(A)

N(1s)

20

S(2p) Fe(2p) 10

0 0.0

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408 708 714 720

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Fig. 4. X-ray photoelectron spectra for the transferred Fe–V1B/PSS LB films.

10

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Area (nm2/molecule) Fig. 3. –A isotherms for the viologens of (A) V1B and (B) V2B on the surfaces of the subphases of (· · ·) 0.1 mM Fe(BF4 )2 , (- - -) 0.1 mg/ml PSS and (—) mixtures of Fe(BF4 )2 –PSS at 25 ◦ C.

stituents of viologens resulting in the formation of monolayers of Fe–V monolayers at the interface. Such kind of Fe2+ -nitrilic coordination binding has been widely used for the construction of three-dimensional building blocks in coordination chemistry [26,27]. Because the produced Fe–V monolayers are polyelectrolyte, they can slightly dissolve in aqueous solution. Thus, to avoid the dissolution of the Fe–V polyelectrolytes in water, anionic polyelectrolyte of PSS was added in the subphase before the monolayer spreading. The produced Fe–V/PSS hybrid monolayer was hardly soluble into water and floated at the air–water interface, so the surface pressure was largely increased after compression. Such kind of monolayer formation method has been often used for the preparation of monolayers of polyelectrolytes and proteins. For instance, it has been previously revealed that the positively charged poly-l-lysine or CaCl2 in subphases could stabilize the monolayer formation of hydrogenase [31]. The anionic polyelectrolyte of PSS was also often used as an alternative layer for the preparation of LB films of charged species or layer-by-layer (LBL) multilayers of proteins and viologen polyelectrolytes [32,33]. 3.3. Langmuir–Blodgett films of Fe–V polyelectrolytes Monolayers of the V1B and V2B on the Fe(BF4 )2 –PSS (0.1 mM Fe(BF4 )2 and 0.1 mg/ml PSS) subphase surface were deposited on quartz and ITO substrate surfaces at 20 mN/m. Based on the transfer ratio, Z-type LB films were obtained, which was reasonable because the monolayers were composed of positively charged Fe–V polyelectrolytes and negatively charged PSS, resulting in a multilayer closely packed with an interlayer electrostatic interaction, just like the layer-by-layer multilayers of charged proteins and polyelectrolytes [34,35].

3.3.1. UV–vis and FITR spectra UV–vis absorption spectra (figures not shown) were measured for five layers of the Fe–V/PSS LB films after the deposition, which revealed a broad absorption band in the range of 200–280 nm. This was in agreement with the spectral features of viologens in the solutions, and indicated a successful transfer of the Fe–V/PSS polyelectrolyte monolayer from the air–water interface to the quartz substrate surface. The FTIR spectra for the LB films on the CaF2 substrate surfaces indicated a small shift for the C N vibration peak (from 2239 cm−1 to 2235 cm−1 ), which may be ascribed to the formation of the Fe–V polyelectrolytes. 3.3.2. X-ray photoelectron spectra Element compositions for five layers of Fe–V polyelectrolyte LB films were detected by using the XPS spectra, which showed several peaks in the binding energy from 190 to 800 eV. As shown in Fig. 4 (Fe–V1B as an example), except for the Si and O elements (partly) from the quartz substrate, four elements of S(2p), C(1s), N(1s) and Fe(2p) were detected from the LB films of Fe–V/PSS hybrids, among which C (partly) and N were from the ligand of viologens, S, C (partly) and O (partly) were from the anionic polymer of PSS, and Fe was from the connector of inorganic salt of Fe(BF4 )2 . These data confirmed that (1) PSS was adsorbed by the Fe–V monolayer and co-existed in the LB films, and (2) the metal ions of Fe2+ were coordinated with the viologen ligands and formed Fe–V polyelectrolyte monolayers at interface. It is noted that no XPS peaks corresponding to the smaller anionic ions from inorganic salt of BF4 − were

Intensity (a.u.)

Surface Pressure (mN/m)

(B)

5

10

15

20

25

2θ Fig. 5. XRD spectrum for the transferred Fe–V1B/PSS LB films.

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Fig. 6. Cyclic voltammograms for the viologens of (A) V1A, (B) V2A in the 10 mM KCl electrolyte solution at scan rate of 0.05 V/s. Concentration of viologen: (a) 3.6 × 10−3 M, (b) 1.8 × 10−3 M, (c) 0.9 × 10−3 M and (d) 0.36 × 10−3 M.

detected, which could be attributed to that the smaller anionic ions were replaced by the larger anionic polymer of PSS during the formation of the monolayers and deposition of the LB films of Fe–V/PSS hybrids.

and one layer of PSS. The spectral features, data of XPS and XRD confirmed the formation of organized layered LB films of the Fe–V/PSS hybrid multilayers. 3.4. Electrochemistry

3.3.3. X-ray diffraction Fig. 5 shows an X-ray diffraction spectrum for ten layers of Fe–V polyelectrolyte LB films deposited on the quartz substrate surface at 20 mN/m. This spectrum revealed several regular diffraction peaks (e.g., at the angles of 2 of 6.6, 12.9 and 19.2◦ ) between 5 and 20◦ , which suggested that the molecules of Fe–V polyelectrolyte and PSS were in an ordered arrangement in the LB films. Based on the Bragg’s equation, the layer thickness was estimated to be about ˚ which was composed of one layer of Fe–V polyelectrolyte 13.4 A,

It has been well known that viologens have three oxidation states in the potential range of 0 and −1.2 V vs Ag/AgCl. The first redox process is a highly reversible electron transfer between the di-cationic viologen (V2+ ) and mono-cationic viologen (V+• ) species. To reveal electrochemical behaviors of the present viologen derivatives, we measured their CV curves in the 10 mM KCl electrolyte solutions at various concentrations and in the LB films of Fe–V/PSS hybrids, which could provide information on the redox

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potentials of viologens in solutions and LB films, reversibility of the viologen redox reactions, as well as on the effects of concentrations, film thickness of the modified electrode on the viologen redox reaction. 3.4.1. Cyclic voltammograms in aqueous solutions Since the viologens of V1A and V2A could dissolve in water, we measured their CV curves in the 10 mM KCl electrolyte solutions at different concentrations (3.6 × 10−3 to 3.6 × 10−4 M). These curves are shown in Fig. 6, which revealed following features. Firstly, two couples of redox waves were recorded for both viologens, with the cathodic potentials (Epc ) falling into the range of −0.5 to −1.0 V, and the anodic potentials (Epa ) of −0.9 to −0.3 V vs Ag/AgCl, respectively. These two couples of waves corresponded to the one electron transfer process of V2+ ↔ V+• and V+• ↔ V0 , respectively. For the V1A, the two redox waves centered at about −0.47 and −0.90 V, while for the viologen of V2A, the two redox waves centered at about −0.40 V, and −0.54 V, respectively. Both the curve shapes and potential position closely depended on the structure of viologens. Secondly, compared with the first redox couple, the relative current intensity of the second couple of viologen V2A was stronger than that of the viologen V1A, which also indicated that the position of the tolunitrile substituent had large effect on the electrochemical behaviors of the viologens. Thirdly, Table 1 summarized the peak potentials of the first redox couple at different concentrations together with the peak potential difference E (E = Epc − Epa ). From the Table, we can see that, with decreasing the viologen concentrations from 3.6 × 10−3 M to 3.6 × 10−4 M, the peak separation E became smaller, which may be attributed to well reversible redox reactions occurred at the lower concentration of viologens. Previous study has revealed that viologens may form dimmer when they were reduced, especially in higher concentration solutions and in closely packed thin films [1,36]. This means that dimmers may form in the solutions at higher concentrations of V1A or V2A after reduction, resulting in smaller peak separation. Moreover, viologens are more easily moved and exchange electrons with electrode surface in the dilute solution, which also leads to the reversible redox reaction and a smaller peak separation. Except for the peak separation, intensity of the peak current also closely related to the viologen concentrations in the electrolyte solution. Taking the reduction process of V2+ → V+• as an example, we calculated the current intensity for the V1A and V2A in the solutions at various concentrations, a plot of which was shown in Fig. 7. We can find that, for both viologens, the current intensity was increased with increasing the viologen concentrations, indicating

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Fig. 7. Plots of the current intensity for the first reductive peak to the viologen concentrations (, V1A; 䊉, V2A) in the 0.1 mol/l KCl electrolyte solution at scan rate of 0.05 V/s.

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Potential ( V vs Ag /AgCl ) Fig. 8. Cyclic voltammograms for the viologens of (A) V1A, (B) V2A in the 10 mM KCl electrolyte solution at various scan rates (from inside to outside, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 V/s).

that more viologens became electroactive and exchanged electrons with the electrode surfaces at higher concentrations. However, only linear increase was recorded for the viologen of V2A within the concentrations from 3.6 × 10−3 M to 3.6 × 10−4 M, which may be ascribed to the reason that viologen of V1A, having a tolunitrile substituent in the meta-position of the bipyridium core, may easily form dimmer during the redox process. Based on the previous work [1,36], we suggested that there existed a reduction reaction of 2V1A2+ → (V1A+• )2 during the dimmer formation. Fig. 8 shows the CV curves for the viologens of V1A and V2A in the 10 mM KCl electrolyte solutions at different scan rates, which indicated that the redox current intensity increased with increasing the scan rates. We calculated the current intensity for each curve, and found that the plot of current intensity was proportional to the potential scan rates (figures not shown), which was in agreement with the equation of ip = n2 A T F2 /(4RT), where n is the number of electrons,  T is total electroactive coverage, F is the Faraday constant, A is the electrode surface, and  is the potential scan rate [37,38]. It has been suggested that, for a film-modified electrode, a product of D/d2 , where D is the charge transport diffusion coefficient,  is the experimental time scale (related to the time for the potential scan to traverse the wave) and d is the polymer film thickness, can be used to describe the charge transfer processes for a reversible reaction. When D/d2  1, all electroactive sites in the film are in equilibrium with the electrode potential. That is, the film is so thin compared to the diffusion layer. On the other hand, when D/d2 1, that is, the film is rather thick, the current intensity is proportional to the root of the scan rate. Here, the electroactive species of V1A and V2A in the electrolyte solutions were randomly adsorbed on the electrode surface; they may form a very thin layer, thus resulting in the current intensity proportional to the scan rate.

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Table 1 Peak potentials of anionic (Epa ) and cathodic (Epc ) electrodes and potential difference (Ep ) at various concentrations in the 10 mM KCl electrolyte solutions at scan rate of 0.05 V/s. Concentration (mM)

V1A

3.6 1.8 0.9 0.36

V2A

Epc

Epa

Ep

Epc

Epa

Ep

−0.543 −0.553 −0.546 −0.535

−0.398 −0.396 −0.425 −0.434

0.145 0.157 0.121 0.101

−0.487 −0.493 −0.49 −0.491

−0.33 −0.313 −0.345 −0.406

0.157 0.18 0.145 0.085

3.4.2. Cyclic voltammograms of Fe2+ –viologen polyelectrolyte LB films Fig. 9 shows the CV curves for the ITO electrodes modified by one layer of Fe–V polyelectrolytes in the 10 mM KCl electrolyte solutions at different scan rates. Different from those observed in aqueous solutions, here, the CV curves revealed one couple of broad bands in the potential range of 0 and −1.0 V vs Ag/AgCl. This band centered at about −0.38 V and corresponded to the first redox couple of the viologens. The peak separation of Ep was about 0.17 V, larger than those observed in the KCl electrolyte solutions. This difference may be attributed to the film resistance caused by the closely packed arrangement of the LB films, where the viologen molecules may not so easily exchange electrons with the electrode surface as that in the solutions because of the co-existed layer of polymer PSS. The peak current intensity was also proportional to the potential scan rates in both cases, which is in agreement with the equation of ip = n2 A T F2 /(4RT). It has been pointed out that, for a filmmodified electrode, a product of D/d2 , where D is charge transport diffusion coefficient,  is the experimental time scale (related to the time for the potential scan to traverse the wave) and d is polymer film thickness, can be used to describe the charge transfer processes

100

Current Intensity (μA)

(A) 50

0

-0.2

-0.4

-0.6

-0.8

-1.0

Potential (V vs Ag/AgCl) 40

Current Intensity (μA)

(B) 20

0

-20

-40 0.0

3.4.3. Chronocoulometric properties of viologens in solutions Charge transfer behaviors of the viologens in solutions and the Fe–V LB films were investigated by the potential step chronocoulometry method. This method has been often used to determine the charge transfer diffusion coefficient for the diffusion-like propagation of charge through the modified electrode [37,40]. Fig. 10 shows a series of Q(t)–t curves for the viologens of V1A and V2A in the 10 mM KCl electrolyte solutions. The potential jump was from an initial potential of 0 V to several final potential Es . Es was ranged from −0.1 to −0.6 V. These curves revealed following features. Firstly, at the less negative E value, E > −0.2 or −0.3 V, at which the viologens were still not electroactive, Q(t) increased quickly at the initial several ms because of the flow of the capacitive current that was required to charge the electrode surface. Then, Q(t) almost did not increase with time because the viologens were electroinactive, that is, no redox reaction occurred. Secondly, when the value of E became more negative, E ≈ Epc or E < Epc , Q(t) showed a quick increase at initial 50–70 ms and then slow increase. This means that the electroreduction rate of viologens was quick at the initial time and then became more stable. A Cottrell plot of the quantity of electricity Q(t) to the root of time (t1/2 ) can provide charge transfer properties in the electrolyte solutions, according to the Cottrell equation. Q (t) =

-50

-100 0.0

for a reversible reaction [38,39]. As having been stated that, when D/d2  1, all electroactive sites in the film are in equilibrium with the electrode potential. In this case, the film modified is also so thin compared to the diffusion layer, which results in the peak current intensity proportional to the scan rate.

-0.2

-0.4

-0.6

-0.8

-1.0

Potential (V vs Ag/AgCl) Fig. 9. Cyclic voltammograms for the ITO electrodes covered by one layer of (A) Fe–V1B and (B) Fe–V2B polyelectrolytes in the 10 mM KCl electrolyte solution at scan rates from 0.05 to 0.5 V/s.

2nFAC(Dt) 1/2 1/2

where n is electron number, F is Faraday constant, A is the electrode area, C is the concentration of electroactive species, and D is the “diffusion coefficient”. According to the Cottrell equation, slope of the curve in the plot of Q(t) to t1/2 equals 2nFACD1/2 /1/2 . Fig. 11 shows the curves of Q(t) to t1/2 for the viologens of V1A and V2A in the 10 mM KCl electrolyte solutions for the potential jumps from 0 V to −0.1 ∼ −0.6 V. These curves revealed two types of slopes; a quick increase at the initial time (e.g., before 0.25 s1/2 for the potential jump from −0.1 to −0.6 V) and then followed with a slow increase. Based on the initial slope of these curves, the diffusion coefficient D was calculated to be in the range of 1.5–1.7 × 10−8 cm2 /s. 3.4.4. Chronocoulometric properties of the Fe–V LB films Fig. 12 shows a series of Q(t)–t curves for the ITO electrodes modified by one layer of Fe–V polyelectrolytes in the 10 mM KCl electrolyte solutions. The potential jump was from an initial potential of 0 V to several final potential Es . Es was ranged from −0.1 to −0.8 V. The curves indicated a similar Q(t)–t curves to those for the viologens in the KCl electrolyte solutions, except for the potentials shifted to lower position. That is, the reductive process almost completed for the jump from 0 V to −0.8 V, which was in agreement

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1.5

Q(t) (μC)

(A)

(B)

1.0

0.5

0.0 0.00

0.05

0.10

0.15

0.20

0.25

0.00

0.05

0.10

t (s)

0.15

0.20

0.25

t (s)

Fig. 10. Q(t)–t curves for the viologens of (A) V1A and (B) V1B in the 10 mM KCl electrolyte solution. The curves were obtained by stepping the potential from a fixed initial value Ei to final values E from 0 V to −0.6 V.

1.5

1.5

(B)

(A) 1.0

0.5

0.5

Q(t) (μC)

1.0

0.0 0.0

0.1

0.2 1/2

t

0.3

0.4

0.5

0.0 0.0

0.1

0.2

1/2

0.3

1/2

(s )

t

0.4

0.5

1/2

(s )

Fig. 11. Cottrell plots for the viologens of (A) V1A and (B) V1B in the 10 mM KCl electrolyte solution.

50

(B)

(A)

Q(t) (μC)

40

30

20

10

0 0.00

0.05

0.10

0.15

0.20

0.25 0.00

0.05

0.10

t (s)

0.15

0.20

0.25

t (s)

Fig. 12. Q(t)–t curves for the LB films of (A) Fe–V1B/PSS and (B) Fe–V2B/PSS hybrids in the 10 mM KCl electrolyte solution. The curves were obtained by stepping the potential from a fixed initial value Ei to final values E from 0 V to −0.2, −0.4, −0.6, and −0.8 V.

with the peak position of the reductive reaction and attributed to the increase of film resistance in the transferred LB films of the Fe–V polyelectrolytes. Theoretically, the diffusion coefficient in the LB films could also be calculated based on the Cottrell equation. However, because the concentration of viologens in the LB films was unknown, it was difficult to obtain the D value in the LB films according the slope

of the curve of Q(t)–t1/2 . Thus, we did not provide the curves of Q(t)–t1/2 for the Fe–V polyelectrolytes. 4. Conclusions We have demonstrated preparation of the Fe2+ -mediated viologen polyelectrolyte LB films at the air–water interface. Based on

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an interfacial coordination reaction and electrostatic interaction, Fe–viologen polyelectrolyte LB films were stabilized and transferred on substrate surfaces. Electrochemical studies revealed two reversible redox couples in the 10 mM KCl electrolyte solutions with their relative intensity and curve shapes closely related to the position of substituent of viologens, which corresponded to the two electron transfer process of viologens. The reversible redox process was also observed in the LB films of the Fe–viologen polyelectrolytes but the potentials shifted to more negative position due to the film resistance. The present method provided a facile route to prepare organic–inorganic polymeric hybrid films of electroactive materials for the chemically modified electrodes, which could be developed as an electron mediator of enzymes (such as hydrogenase) and as redox-based molecular devices. Acknowledgement The authors are grateful for the National Science Foundation of China (21073044). References [1] C.-L. Bird, A.T. Kuhn, Electrochemistry of the viologens, Chem. Soc. Rev. 10 (1981) 49–82. [2] K. Hoshino, Y. Oikawa, I. Sakabe, T. Komatsu, Reversible polycolour change of viologens from violet through transparent to white, Electrochim. Acta 55 (2009) 165–170. [3] C.S. Kim, S. Lee, L.L. Tinker, S. Bernhard, Y.L. Loo, Cobaltocene-doped viologen as functional components in organic electronics, Chem. Mater. 21 (2009) 4583–4588. [4] T. Masuda, M. Irie, K. Uosaki, Photoswitching of electron transfer property of diarylethene–viologen linked molecular layer constructed on a hydrogenterminated Si (1 1 1) surface, Thin Solid Films 518 (2009) 591–595. [5] O.A. Zadvorny, A.M. Barrows, N.A. Zorin, J.W. Peters, T.E. Elgren, High level of hydrogen production activity achieved for hydrogenase encapsulated in sol–gel material doped with carbon nanotubes, J. Mater. Chem. 20 (2010) 1065–1067. [6] D.J. Qian, A.R. Liu, C. Nakamura, S.O. Wenk, J. Miyake, Photoinduced hydrogen evolution in an artificial system containing photosystem I, hydrogenase, methyl viologen and mercaptoacetic acid, Chin. Chem. Lett. 19 (2008) 607–610. [7] D.J. Qian, S.O. Wenk, C. Nakamura, T. Wakayama, N. Zorin, J. Miyake, Photoinduced hydrogen evolution by use of porphyrin, EDTA, viologens and hydrogenase in solutions and Langmuir–Blodgett films, Int. J. Hydrogen Energy 27 (2002) 1481–1487. [8] D. Quan, W. Shin, A nitrite biosensor based on Co-immobilization of nitrite reductase and viologen-modified chitosan on a glass carbon electrode, Sensors 10 (2010) 6241–6256. [9] N.S. Lee, H.K. Shin, Y.S. Kwon, B.J. Lee, Characterization of the electrical and optical properties of viologen devices using chlorophyll a as an electron excimer, Ultramicroscopy 110 (2010) 650–654. [10] W. Lww, S.K. Min, G. Cai, Y. Hwang, S. Baek, B.W. Cho, S.H. Lee, S.H. Han, Enhanced photocurrent in the RuL2 (NCS)2 /di-3-aminopropyl)–viologen/Au nanoparticles/ITO system: use of Au nanoparticles for retardation of the back electron transfer reaction, J. Nanosci. Nanotechnol. 9 (2009) 6934–6937. [11] M. Ogawa, B. Balan, G. Ajayakumar, S. Masaoka, H.B. Kraatz, M. Muramatsu, S. Ito, Y. Nagasawa, H. Miyasaka, K. Sakai, Photoinduced electron transfer in tris(22 -bipyridine)ruthenium(II)–viologen dyads with peptide backbones leading to long-lived charge separation and hydrogen evolution, Dalton Trans. 39 (2010) 4421–4434. [12] A.F. Liu, S.H. Han, H.W. Che, L. Hua, Fluorescent hybrid with electron acceptor methylene viologen units inside the pore walls of mesoporous MCM-48 silica, Langmuir 26 (2010) 3555–3561. [13] M.A. Saab, R. Abdel-Malak, J.F. Wishart, T.H. Ghaddar, Photocurrent generation in layer-by-layer assembled dentrimers with ruthenium tris-bipyridine peripheral groups and a viologen-like core, Langmuir 23 (2007) 10807–10815. [14] L.P. Zhao, K.G. Neoh, E.T. Kang, Nanoscale metal coatings and dispersions prepared using viologen systems, Langmuir 19 (2003) 5137–5144. [15] X. Liu, K.G. Neoh, E.T. Kang, Viologen-functioalized conductive surfaces: physiochemical and electrochemical characteristcs and stability, Langmuir 18 (2002) 9041–9047.

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