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22690

J. Phys. Chem. B 2006, 110, 22690-22694

Physicochemical Properties and Sensing Ability of Metallophthalocyanines/Chitosan Nanocomposites Jose´ R. Siqueira, Jr.,† Luiz H. S. Gasparotto,‡ Frank N. Crespilho,§ Antonio J. F. Carvalho,| Valtencir Zucolotto,† and Osvaldo N. Oliveira, Jr.*,† UniVersidade de Sa˜ o Paulo, IFSC, CP 369, Sa˜ o Carlos, SP, 13560-970, Brazil, UniVersidade Federal de Sa˜ o Carlos, DQ, CP 676, Sa˜ o Carlos, SP, 13565-905, Brazil, UniVersidade de Sa˜ o Paulo, IQSC, Sa˜ o Carlos, SP, 13560-970, Brazil, and UniVersidade Federal de Sa˜ o Carlos, Sorocaba, SP, P. B. 3031, 18043-970, Brazil ReceiVed: July 31, 2006; In Final Form: August 25, 2006

Electroactive nanostructured films of chitosan (Ch) and tetrasulfonated metallophthalocyanines containing nickel (NiTsPc), copper (CuTsPc), and iron (FeTsPc) were produced via the electrostatic layer-by-layer (LbL) technique. The multilayer formation was monitored with UV-vis spectroscopy by measuring the increase of the Q-band absorption from metallophthalocyanines. Results from transmission and reflection infrared spectroscopy suggested specific interactions between SO3- groups from metallophthalocyanines and NH3+ from chitosan. The electroactive multilayered films assembled onto an ITO electrode were characterized by cyclic voltammetry, with Ch/NiTsPc films showing higher stability and well-defined voltammograms displaying reversible redox peaks at 0.80 and 0.75 V. These films could be used to detect dopamine (DA) in the concentration range from 5.0 × 10-6 to 1.5 × 10-4 mol L-1. Also, ITO-(Ch/NiTsPc)n electrodes showed higher electrocatalytic activity for DA oxidation when compared with a bare ITO electrode. On the other hand, only the Ch/FeTsPc and Ch/CuTsPc modified electrodes could distinguish between DA and ascorbic acid. These results demonstrate that versatile electrodes can be prepared by incorporation of different metallophthalocyanine molecules in LbL films, which may be used in bioanalytical applications.

1. Introduction Manipulation of novel materials at the molecular level is of interest for nanoscience and nanotechnology with production of tailored devices with optimized properties. Metallophthalocyanines (MPc) have long been used in various research areas due to their semiconductivity, high thermal stability, and welldefined redox activity.1 The latter allowed the use of MPc in technological applications, including sensing, modified electrodes for catalysis, optical memories, fuel and photovoltaic cells, electrochromic devices, light-emitting diodes (LEDs), and as photodynamic agents for cancer therapy.1-8 Metallophthalocyanines are normally used as thin films which may be fabricated using several techniques,2-8 including the layer-bylayer (LbL) method.9,10 In the LbL technique, species of opposite charge can be assembled in a layered structure by ionic interactions.6-16 The nanostructured nature of the LbL films allows the interactions between the components to be maximized,17 and this has generated a search for new materials to be combined aimed at optimized performance in tailored devices. One material that has been widely used in LbL films is chitosan, the N-deacetylated derivative of chitin (poly 2-acetamido-2deoxy-β-D-glucose), a natural polysaccharide found in crustacean shells.18 It presents an exceptional film-forming ability that is homogeneous and the films show very good mechanical * Corresponding author. E-mail: [email protected]. † Universidade de Sa ˜ o Paulo, IFSC, CP 369, Sa˜o Carlos. ‡ Universidade Federal de Sa ˜ o Carlos, DQ, CP 676, Sa˜o Carlos. § Universidade de Sa ˜ o Paulo, IQSC, Sa˜o Carlos. | Universidade Federal de Sa ˜ o Carlos, Sorocaba.

properties. An important characteristic is that chitosan is a polyelectrolyte soluble only under certain specific conditions. At low pH, chitosan is completely soluble, being insoluble in basic aqueous media. The latter allows its application in an aqueous environment without dissolution. The ability to form complexes with metals is also an important characteristic that makes chitosan one promising material for nanostructured sensors. Due to its polyelectrolyte characteristics, chitosan is suitable for LbL film fabrication,19-26 especially regarding the immobilization of biological species for sensing applications.23-26 In this paper, we report the fabrication of LbL films using chitosan (Ch) as a cationic polyelectrolyte and tetrasulfonated metallophthalocyanines of nickel (NiTsPc), copper (CuTsPc), and iron (FeTsPc) as anionic polyelectrolytes for dopamine (DA) detection. 2. Experimental Details Ni(II)TsPc, Cu(II)TsPc, and Fe(III)TsPc were purchased from Aldrich Co. and used without further purification. The metallophthalocyanine-anionic aqueous solutions were used at a concentration of 0.5 g L-1 and pH 4 or 7. Chitosan from shrimp was purchased from Galena Chemistry, Brazil, and had 120 Cps of viscosity and 85% de-acetylation. The concentration of the chitosan solution was set at 1 g L-1 and pH 4 or 7. Both solutions were prepared at room temperature under stirring. LbL films were fabricated by immersing the substrates alternately into the polycationic chitosan and anionic metallophthalocyanine solutions for 5 min. After each layer deposition, the films were rinsed in a washing solution with the same pH as the chitosan or metallophthalocyanine solution, and then they were dried under a N2 flow.

10.1021/jp0649089 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/12/2006

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Figure 2. UV-vis absorption spectra of NiTsPc, CuTsPc, FeTsPc, and chitosan aqueous solutions at pH 4. Figure 1. Chemical structures of the materials employed: (a) tetrasulfonated metallophthalocyanine (M ) Ni, Cu, and Fe), (b) chitosan, (c) dopamine, and (d) ascorbic acid.

The growth of the multilayers was monitored at every other deposited bilayer on hydrophilic glass by UV-vis spectroscopy using a Hitachi U-2001 spectrophotometer. Fourier transform infrared spectroscopy (FTIR) experiments were carried out in LbL films containing 25 bilayers on silicon substrates and Aucoated glass for transmission and reflection modes, respectively, using a Nicolet 470 Nexus spectrometer. Neat MPc and chitosan were analyzed as KBr pellets and cast films, respectively. Profilometry measurements were carried out using a Talystep Taylor-Hobson profilometer. An EG&G PAR M280 potentiostat was used for voltammetric analyses: the working and reference electrodes were the LbL films fabricated at pH 4 with different numbers of bilayers (5, 10, 15, and 20) on ITO (A ) 0.4 cm2) and a saturated Ag/AgCl electrode, respectively. A 1.0-cm2 platinum foil was employed as the counter electrode. The electrochemical measurements were carried out at room temperature and various scan rates (5, 10, 20, 30, 50, 75, and 100 mV s-1) using a 1.0 × 10-3 mol L-1 HCl solution as supporting electrolyte. Working electrodes containing 5 bilayers were used in the detection of dopamine (DA) (3-hydroxytyramine hydrochloride, C8H11NO2‚HCl) via cyclic voltammetry with the absence and presence of ascorbic acid (AA). The concentrations of DA ranged from 5.0 × 10-6 mol L-1 to 1.5 × 10-4 mol L-1. Before each measurement, N2 was bubbled in the electrolytic solution to eliminate dissolved O2. Figure 1 shows the chemical structures of the materials employed for LBL film fabrication as well as the analytes used for electrochemical detection. 3. Results and Discussion 3.1. Characterization of LbL Multilayers Using UV-Vis and FTIR Spectroscopy. Figure 2 shows the UV-vis spectra for NiTsPc, CuTsPc, FeTsPc, and Ch aqueous solutions used for film fabrication. The NiTsPc solution displayed an intense absorbance at 620 nm (Q-band) due to the dimeric species and a shoulder at 655 nm that is characteristic of the monomeric species.1 For CuTsPc solution, two absorption bands are observed at 610 and 690 nm, which are related to the dimeric and monomeric species, respectively.1 The FeTsPc solution presented an intense absorbance band at 635 nm, characteristic of the dimeric species, and a small shoulder at 675 nm due to the monomeric species.1 In summary, MPc assumed preferen-

Figure 3. Absorbance of the more intense dimeric bands vs number of bilayers for LbL films: (2) Ch/NiTsPc, (9) Ch/CuTsPc, and (b) Ch/FeTsPc.

tially the dimeric form in solution, under the conditions employed. The growth of the LbL films was monitored via UV-vis spectroscopy. A band shift was observed in the UV-vis spectra from LbL films in comparison to the solution (not shown). For NiTsPc there was a blue-shift of 5 nm, characteristic of H aggregates. For CuTsPc and FeTsPc we observed red-shifts of 7 and 10 nm, respectively, that is characteristic of J aggregates formation. The latter may be either an indicative of interactions occurring between chitosan and metallophthalocyanines or even aggregation of the metallophthalocyanine molecules in a face-to-face conformation in the LbL films.27 Figure 3 shows that the dimeric absorption bands increased linearly with the number of bilayers, indicating that a same amount of material was adsorbed at each deposition cycle.27,28 The average thickness of the bilayers, measured with a profilometer, was estimated at about 1 nm for Ch/NiTsPc and Ch/FeTsPc LbL films, in agreement with what has been reported by Zucolotto et al.27 for PAH/FeTsPc LbL films and by Lu¨tt et al.,29 who reported a thickness of 1 nm for each bilayer from PDAC/NiTsPc LbL films. For the Ch/CuTsPc films we obtained a thickness per bilayer of about 1.3 nm. FTIR experiments were carried out on neat Ch cast film, FeTsPc pellets, and on a 25-bilayer Ch/FeTsPc film deposited on a silicon substrate (Figure 4). For the sake of clarity, only the FeTsPc system is shown, since all systems behave similarly (see Supporting Information). The spectrum of the FeTsPc pellet displays an absorption band at 1033 cm-1, assigned to the stretching vibration mode from SO3- groups.27,28 This vibration mode was observed in all the metallophthalocyanines employed, as shown in Table 1. In the LbL films, the SO3- band is shifted to 1023 cm-1 (transmission mode, film fabricated at pH 4), 1027 cm-1 (reflection, pH 4), and 1025 cm-1 (reflection, pH 7). This

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Figure 4. FTIR spectra of cast chitosan film, FeTsPc KBr pellet, and 25-bilayer Ch/FeTsPc LbL film in transmission and reflection mode (at pH 4 and 7).

Figure 5. Cyclic voltammograms at different scan rates for a 5-bilayer Ch/NiTsPc LbL film. Inset: Linear relationship between anodic peak current and scan rate.

TABLE 1: Peak Assignments of the Stretching Vibration Mode of SO3- Groups Present in the Metallophthalocyanines stretching vibration mode of SO3- groups (cm-1) LbL film transmission mode

LbL film reflection mode

MTsPc

KBr pellet

pH 4

pH 4

pH 7

NiTsPc CuTsPc FeTsPc

1033 1033 1033

1024 1028 1023

1028 1030 1027

1026 1029 1025

shift is due to the electrostatic interaction between SO3- groups of the FeTsPc with NH3+ groups of the chitosan. As expected, the energy decreased because the interaction restricts the SO3degrees of freedom. Similar results were obtained for PAH/ FeTsPc27 and PANI/CuTsPc28 LbL films. This interaction may be associated with the nanostructured architecture of the LbL film, due to the intimate contact between the film components. It is also revealed in the spectra (reflection mode) of the films produced at different pH values. The FeTsPc spectrum displayed an absorption band at 1195 cm-1 attributed to SO3- stretching, which was shifted to 1178 cm-1 in the Ch/FeTsPc LbL film at pH 4, amounting to further evidence for the interaction between SO3- and NH3+. At pH 7, this band is poorly defined as the quantity of NH3+ is low at this pH. The spectra obtained in reflection and transmission modes in the ranges of 700-750 and 1100-1150 cm-1, assigned to the out-of-plane and in-plane C-H bending, respectively, are similar, suggesting that the MPc molecules are isotropically distributed in the films. 3.2. Electrochemical Characterization. Figure 5 shows cyclic voltammograms for a 10-bilayer Ch/NiTsPc LbL film deposited onto ITO at different scan rates. This electrode showed a well-defined electroactivity with a redox pair at 0.75 and 0.80 V attributed to the electrochemical conversion of the NiTsPc Pc unit [Ni(II)TsPc4-/Ni(II)TsPc6-].30 The anodic peak current (Ipa) varied linearly with the scan rate as shown in the inset of Figure 5, indicating the process is charge-transfer controlled.31 The voltammogram shape is not affected when the scan rate was varied, which indicates that the charge transfer among the MPc sites is fast and facilitated by the nanostructured configuration of the LbL film. Also important to note is the reversible character of the Ch/NiTsPc system, for the anodic and cathodic peak potentials show the same value at different scan rates. In addition, the difference between the anodic and cathodic potential peaks is lower than 57 mV, and the ratio between the anodic and cathodic peak currents is 1.0.32 It is worth mentioning that the stability of the Ch/NiTsPc system was verified by subjecting the film to several voltammetric cycles (not shown),

Figure 6. Cyclic voltammograms for Ch/NiTsPc LbL films containing different numbers of bilayers. Scan rate: 50 mV s-1.

and the peak currents remained the same, with no change in the voltammetric profile even after 20 cycles at 50 mV s-1. Also, chitosan did not interfere in the Faradaic current, which was consistent with the studies from Huguenin et al.33 In the Ch/CuTsPc LbL film, only the cathodic process was observed as the reaction Cu(II)TsPc2-/Cu(II)TsPc4- is irreversible.30 For the Ch/FeTsPc system, only a poorly defined cathodic process was seen as a result of the Fe(III)TsPc2-/ Fe(II)TsPc2process.1,34 The lower electroactivity is due to the aggregation of Fe(III)TsPc in acid solution,34 decreasing the availability of electroactive sites in the film. We also observed for the Ch/NiTsPc system that the chargetransport mechanism depends on the number of bilayers. Figure 6 shows cyclic voltammograms for Ch/NiTsPc electrodes containing different numbers of bilayers at 50 mV s-1. The redox pair is observed only for the electrodes with 5 and 10 bilayers. For electrodes containing 15 and 20 bilayers, the redox peaks are no longer defined and the anodic and cathodic currents do not increase with the number of bilayers. Therefore, as the distance between the conducting substrate and the electroactive sites of the film increases, as the films become thicker, electronhopping is hampered.31 3.3. Dopamine Detection with ITO Modified Electrodes with LbL Films. Ch/MTsPc LbL films deposited onto ITO were characterized using cyclic voltammetry in order to probe the possible use of such modified electrodes in detecting dopamine. Figure 7 depicts the cyclic voltammograms for bare ITO (a) and 5-bilayer LbL films from Ch/NiTsPc (b), Ch/CuTsPc (c), and Ch/FeTsPc (d) at various DA concentrations in a 1.0 × 10-3 mol L-1 HCl electrolytic solution. The redox pair is attributed to a two-electron oxidation/reduction process, where dopamine is oxidized to dopaminequinone.35 Comparing the voltammetric curves, one infers that only the Ch/NiTsPc sensors exhibit electrocatalytic effect for the oxidation of dopamine, since the anodic peak appears at a lower potential (0.78 V vs

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Figure 7. Cyclic voltammograms for (a) bare ITO and 5-bilayer LbL films from (b) Ch/NiTsPc, (c) Ch/CuTsPc, and (d) Ch/FeTsPc, at various DA concentrations in the range of 5.0 × 10-6 - 1.5 × 10-4 mol L-1 in 1.0 × 10-3 mol L-1 HCl solution. Scan rate: 50 mV s-1. (Note that the concentration used for bare ITO is higher than the highest concentration used for the modified electrodes.)

Figure 9. Cyclic voltammograms for various ratios between AA and DA for 5-bilayer LbL films from (a) Ch/CuTsPc at ν ) 100 mV s-1 and (b) Ch/FeTsPc at ν ) 20 mV s-1.

Figure 8. Linear calibration curves for DA detection for LbL films: (9) Ch/NiTsPc: Ipa (µA) ) 2.83 + 1.16 × 105 C, (b) Ch/CuTsPc: Ipa (µA) ) 3.75 + 0.77 × 105 C and (2) Ch/FeTsPc: Ipa (µA) ) 1.74 + 0.94 × 105 C. Scan Rate: 50 mV s-1.

TABLE 2: Detection Limits (DL) Exhibited by the Three Systems Employed electrodes

DL (mol L-1)

Ch/NiTsPc Ch/CuTsPc Ch/FeTsPc

4.88 × 10-5 9.74 × 10-5 3.70 × 10-5

Ag/AgCl). The oxidation potentials of dopamine on bare ITO, Ch/CuTsPc, and Ch/FeTsPc are practically the same (1.4 V), being higher than that for Ch/NiTsPc. In all experiments, the anodic peak current increased linearly with the concentration of DA in the range from 5.0 × 10-6 to 1.5 × 10-4 mol L-1, as shown in Figure 8. This is a positive feature, as a wide concentration range is usually required to avoid involved dilution steps. The Ch/NiTsPc film was the most sensitive electrode, since its linear calibration curve presented the highest angular coefficient value.36 The detection limits (DL) were obtained from calibration curves for dopamine as shown in Table 2. The DL values are similar to previous results for DA detection using PANI/ FeTsPc LbL28 or PANI LB37 modified electrodes. These results are promising for future bioanalytical studies, where more-sensitive electrochemical techniques32 would be applied to obtain lower detection limits. The selectivity of the electrodes to DA was investigated by adding different DA concentrations in a 1.5 × 10-4 mol L-1 ascorbic acid (AA) solution, since AA is a natural interferent for DA.38 Figure 9 shows cyclic voltammograms for a 5-bilayer

LbL film from Ch/CuTsPc (a) and Ch/FeTsPc (b) in electrolytic solutions containing different DA/AA proportions. Both electrodes displayed a selective behavior, being possible to distinguish between the AA and DA signal. The distance between AA and DA oxidation peaks was significantly large (0.56 and 0.51 V for Ch/CuTsPc and Ch/FeTsPc, respectively). Interestingly, the Ch/NiTsPc-modified electrodes could not distinguish between DA and AA, since only one peak was observed in the voltammograms collected in the presence of both analytes. The latter finding is probably due to the electrocatalytic effect observed in this system. 4. Conclusions Metallophthalocyanines have been assembled with chitosan in nanostructured films via the LbL technique. Vibrational analyses showed a specific NH3+-SO3- ionic interaction between chitosan and metallophthalocyanines. Such interaction may be associated with the nanostructured architecture of the LbL film, in which the contact between components can be maximized. The Ch/NiTsPc LbL film exhibited a high electrochemical stability and reversibility. Furthermore, Ch/CuTsPc and Ch/FeTsPc films were capable of distinguishing between DA and AA. One can therefore envisage the use of this approach of combining chitosan and metallophthalocyanines in LbL films for optimized bioanalytical applications. Acknowledgment. Financial support from CAPES, FAPESP, CNPq, and IMMP (Brazil) is gratefully acknowledged. Supporting Information Available: The FTIR spectra for NiTsPc and CuTsPc systems. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Leznoff, C. C.; Lever, A. B. P. PhthalocyaninessProperties and Applications; John Wiley & Sons: New York, 1989; Chapter 1.

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