Electroactive LbL films of metallic phthalocyanines and poly(0-methoxyaniline) for sensing
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- Santos, A.C., Zucolotto, V., Constantino, C.J.L. et al. J Solid State Electrochem (2007) 11: 1505. doi:10.1007/s10008-007-0338-9
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Multilayered nanostructured films have been widely investigated for electrochemical applications as modified electrodes, including the layer-by-layer (LbL) films where properties such as thickness and film architecture can be controlled at the molecular level. In this study, we investigate the electrochemical features of LbL films of poly(o-methoxyaniline; POMA) and tetrasulfonated phthalocyanines containing nickel (NiTsPc) or copper (CuTsPc). The films displayed well-defined electroactivity, with redox pairs at 156 and 347 mV vs SCE, characteristic of POMA, which allowed their use as modified electrodes for detecting dopamine and ascorbic acid at concentrations as low as 10−5 M.
KeywordsLbL filmsDopamineAscorbic acidSensorsPhthalocyanines
Nanostructured thin films have been widely used in sensing, especially because of the possibility of combining materials in a synergistic way where specific properties may be controlled at the molecular level [1, 2]. Organic–inorganic hybrid films are particularly interesting for sensing, as parameters such as the electronic properties of each material can be tuned to maximize interactions between film and analyte. This is the case of conducting polymers assembled with porphyrins and phthalocyanines used as modified electrodes for electrocatalysis and sensors [3, 4]. The activity of modified electrodes of polyaniline (PANI) and polypyrrole (PPY) containing metallic phthalocyanines in oxygen reduction, for example, was investigated by Coutanceau et al. . They compared oxygen reduction rates between conducting polymers electrochemically synthesized in the presence of cobalt (CoTsPc) or iron (FeTsPc) tetrasulfonated phthalocyanines and a bare platinum electrode. Using a similar approach, Fuentes et al.  found that the control over film thickness was crucial to optimize the electrocatalytic properties of modified electrodes of PPY incorporating nickel tetrasulfonated phthalocyanine (NiTsPc). The modified electrodes displayed an electrocatalytic effect on the anodic oxidation of propylgalate in comparison to bare platinum electrodes.
Another efficient strategy to produce multilayered hybrid films is through the electrostatic layer-by-layer technique (LbL) , which is further advantageous owing to the experimental simplicity and possible assembly of materials directly from their aqueous solutions [7, 8]. In a previous study, our group produced LbL films made with parent PANI and phthalocyanines for dopamine (DA) and ascorbic acid (AA) sensing . In this paper, we extend this investigation to analyze the physicochemical properties of LbL films of poly(-o-methoxyaniline; POMA) alternated with tetrasulfonated phthalocyanines containing nickel (NiTsPc) or copper (CuTsPc). The films were used as modified electrodes and displayed well-defined electroactivity, allowing their use as sensors for DA and AA.
O-methoxyaniline (Aldrich) was distilled twice under a vacuum and stored in the dark at −10 °C. Hydrochloric acid (Vetec) and ammonium persulfate [(NH)4S2O8, Vetec] were used as purchased. POMA in its emeraldine base form was chemically synthesized using the method described in . The POMA powder was dissolved in dimethylacetamide (DMAc) under stirring for 12 h. The POMA/DMAc solution was filtered and slowly added to an HCl solution, pH 2.5, and then diluted to 2.0 g/l. The final pH was ca. 2.8. The procedures to obtain water-soluble POMA were adapted from those described in . Poly(vinyl sulfonic acid; PVS), NiTsPc, and CuTsPc were purchased from Aldrich and used as received. The PVS and phthalocyanine anionic solutions were used at a concentration of 0.5 g/l and pH 2.5. All solutions were prepared with pure water from a Millipore system with a resistivity of 18.0 mΩcm.
POMA/NiTsPc and POMA/CuTsPc LbL films containing up to 20 bilayers were deposited onto hydrophilic glass, ITO-covered glass, and gold-covered glass substrates. The deposition of multilayers was carried out by immersing the substrates alternately into the polycationic (POMA) and anionic solutions for 5 min. After each deposition step, the films were rinsed in the washing solution (at pH 2.5) and dried with a N2 flow. Film growth was monitored using UV-VIS absorption spectroscopy (Hitachi U-3000 spectrometer). Fourier transform infrared absorption spectroscopy (FTIR) analyses were carried out in films deposited onto Au substrates using a Nicolet 470 Nexus spectrometer, with the sample chamber purged with N2 gas. The Raman scattering (Stokes) was obtained using the 514.5-nm laser line with a Renishaw micro-Raman system In-Via, which is equipped with a Leica microscope (DMLM series) whose 50×-microscope objective lens was used to focus the laser beam onto a spot of ca. 1.0 μm2.
Cyclic voltammograms were collected with LbL films deposited onto ITO using a potentiostat Autolab PGSTAT 30 Eco Chemie and a 10.0-ml, 3-electrode electrochemical cell. The reference electrode was an Hg/HgCl/KCl(sat.) (SCE); a 2.0-cm2 platinum foil was used as auxiliary electrode, and the working electrode was the POMA/Phthalocyanine LbL film containing 3, 5, 10, 15, and 20 bilayers deposited onto ITO. The electrochemical response of POMA/NiTsPc films was studied as a function of the number of POMA/NiTsPc bilayers, pH of electrolytic solution, and scan rate. Detection of DA and AA was carried out in a HCl 0.1-M solution at concentrations varying from 1.41 × 10−5 mol/l to 2.04 × 10−4 mol/l at 50 mV/s. Cyclic voltammograms were also recorded in the presence of 6.7 × 10−5 mol/l of DA and AA.
Results and discussion
The current density increased linearly with the scan rate in the range from 10 to 100 mV/s, as shown in Fig. 4c. In addition, the potentials at which the redox processes occur depend on the scan rate, especially for the first anodic process for the POMA/NiTsPc LbL film, shifting from +125 mV at 25 mV/s to +195 mV at 100 mV/s. This caused an overlap between the first and second anodic processes. We selected the rate of 50 mV/s for the remaining experiments because it led to a considerable current density and a good definition of the peaks. Therefore, for optimizing the parameters concerning the detection tests with DA and AA, we selected a 20-bilayer POMA/NiTsPc LbL film in HCl 0.1 M (pH 1) and a scan rate of 50 mV/s.
The shift in the oxidation peak for more positive potentials and the increased current—caused by cycling with DA or AA—were maintained when the measurements were taken again in 0.1-M HCl solutions with no DA or AA. In contrast, the response of the PAH/NiTsPc LbL films (Fig. 8b) was reversible. Upon cycling in HCl, these films displayed redox peaks typical of NiTsPc at 578 and 830 mV for oxidation and at 740 and 540 mV vs SCE for reduction. In the presence of DA or AA, these processes disappear, and an oxidation peak is seen at 693 mV, which is accompanied by a large increase in current. As the cycling was performed again in a 0.1-M HCl solution without DA or AA, the original electrochemical response was recovered. Furthermore, important to note is that the electrocatalytic effect towards DA oxidation was higher for POMA-NiTsPc electrodes (Fig. 5) in comparison to the PAH-NiTsPc modified electrodes.
Nanostructured thin films containing Ni or Cu tetrasulfonated phthalocyanines were assembled in a multilayered fashion in conjunction with POMA. The films displayed a well-defined electroactivity, with redox pairs at 156 and 347 mV vs ESC characteristic of POMA. As expected, POMA/NiTsPc films presented a high electrochemical stability, which allowed their use as modified electrodes for detecting DA and AA at concentrations down to 10−5 M. Despite the high sensitivity exhibited by the POMA/NiTsPc system, the electrodes did not show reversibility because of absorption of DA or AA at the electrodes.
This work was supported by FAPESP, FAPEPI, CNPq, CAPES and IMMP/MCT (Brazil).