Anal. Chem. 2007, 79, 608-611
Application of Raman Spectroscopy and Sequential Injection Analysis for pH Measurements with Water Dispersion of Polyaniline Nanoparticles Tom Lindfors* and Ari Ivaska
Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Åbo Akademi University, Biskopsgatan 8, 20500 Åbo/Turku, Finland
A novel method for pH measurements between pH 7.5 and 10.4 is reported in this paper. The method combines Raman spectroscopy and the automated sequential injection analysis system (SIA) and makes use of the acidbase properties of a commercially available water dispersion of polyaniline (PANI) nanoparticles with a mean particle size of 46 nm. The useful pH range of the PANI nanoparticles is broader than for conventional acid-base indicators, due to the 1:2 reaction stoichiometry of the proton-ligand complex of the nanoparticles. The pH measurements were conducted with the 633-nm laser excitation wavelength by calculating the difference between the Raman intensities of the primary and reference wavenumbers. In this study, the pH-sensitive CHdCH stretching band at 1422 cm-1 and C-H in-plane bending band of the quinoid form at 1163 cm-1 were chosen as the primary wavenumbers. The presented method is fast and allows pH to be measured with a precision of 0.2 pH unit. The advantage of the proposed method is that the hysteresis effect, which is usually observed for PANI films, can be overcome with the PANI nanoparticles, because a fresh nanoparticle solution is used in each measurement. It should be pointed out, that this work is a fundamental study showing the applicability of Raman spectroscopy in combination with the SIA technique for pH measurements in specific applications, where the conventional glass pH electrode cannot be used. It is well known that polyaniline (PANI), which is one of the most studied electrically conducting polymers (CP), is very pH sensitive.1,2 The chemical structure of the repeating unit of PANI is very similar to the structure of certain conventional acid-base indicators, like indophenols and variamine blues.3 The pH sensitivity of PANI originates from the emeraldine base (EB)/emeraldine salt (ES) transition of PANI1,2 and has usually been determined for thin electropolymerized PANI films, which have been prepared on conducting substrates, e.g., Au, Pt, or glassy carbon.1 However, * To whom correspondence should be addressed. E-mail: Tom.Lindfors@ abo.fi. Fax: +358-2-2154479. (1) Lindfors, T.; Ivaska, A. J. Electroanal. Chem. 2002, 531, 43-52. (2) Wallace, G. G.; Spinks, G. M.; Kane-Maguire, L. A. P.; Teasdale, P. R. Conductive Electroactive Polymers: Intelligent Materials Systems, 2nd ed.; CRC Press: Boca Raton, FL, 2003; pp 121-177. (3) Indicators; Bishop, E., Ed.; Pergamon Press: New York, 1972; pp 56-59.
608 Analytical Chemistry, Vol. 79, No. 2, January 15, 2007
the electropolymerized PANI films show a hysteresis effect, although it can to a certain extent be minimized by optimizing the film thickness and by performing the pH measurements only within a suitable pH range. For example, a slightly super-Nernstian pH response of 62.4 ( 0.9 mV/decade, with a quite low hysteresis effect, was obtained between pH 2 and 9 for thin PANI films prepared in HCl.1,4 On the other hand, a rather pronounced hysteresis effect was observed at pH of >9. The pH sensitivity of PANI can be suppressed by lipophilic additives5 or by Nsubstitution of the amine groups of the PANI backbone.4 PANI has previously been used as the transducer layer between the metal substrate and the poly(vinyl chloride) (PVC) membrane,6 as a membrane component dispersed in PVC7-9 or as a membrane material as such10,11 in different types of sensor and ion-selective electrode (ISE) applications. CP-based all-solidstate ISEs has recently been reviewed by Bobacka et al.12-14 We have also recently shown that pH can be measured with electropolymerized PANI films in combination with Raman spectroscopy between pH 3 and 6 with the 633-nm laser excitation wavelength. This is possible by monitoring the changes in the Raman intensity of the overlapping CHdCH and CdN stretching bands at 1439 cm-1, which are very pH sensitive, because the vibrations of the quinoid structures are strongly resonance enhanced by the 633-nm laser.15 The quinoid units originate from the ES to EB transition, which increases the content of quinoid units in the PANI film and vice versa. A hysteresis effect was also observed in the Raman-based pH measurements with the PANI films, which were prepared in hydrochloric acid. The small and mobile chloride counteranion makes PANI very pH sensitive, and (4) Lindfors, T.; Ivaska, A. J. Electroanal. Chem. 2002, 535, 65-74. (5) Lindfors, T.; Sandberg, H.; Ivaska, A. Synth. Met. 2004, 142, 231-242. (6) Lindfors, T.; Ivaska, A. Anal. Chem. 2004, 76, 4387-4394. (7) Bobacka, J.; Lindfors, T; McCarrick, M.; Ivaska, A.; Lewenstam, A. Anal. Chem. 1995, 67, 3819-3823. (8) Sjo ¨berg, P.; Lindfors, T.; Bobacka, J.; Lewenstam, A.; Ivaska, A. Anal. Chim. Acta 1999, 385, 163-173. (9) Lindfors, T.; Ervela¨, S.; Ivaska, A. J. Electroanal. Chem. 2003, 560, 69-78. (10) Lindfors, T.; Ivaska, A. Anal. Chim. Acta 2000, 404, 111-119. (11) Lindfors, T.; Ivaska, A. Anal. Chim. Acta 2001, 437, 171-182. (12) Bobacka, J.; Lindfors, T.; Lewenstam, A.; Ivaska, A. Am. Lab. 2004, (February), 13-20. (13) Bobacka, J. Electroanalysis 2006, 18, 7-18. (14) Bobacka, J. In Encyclopedia of Sensors; Grimes, C. A., Dickey, E. C., Pishko, M. V., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2006; Vol. 2, pp 279-294. (15) Lindfors, T.; Ivaska, A. J. Electroanal. Chem. 2005, 580, 320-329. 10.1021/ac061069d CCC: $37.00
© 2007 American Chemical Society Published on Web 12/15/2006
Figure 1. Experimental setup of the pH measurements with Raman spectroscopy and the SIA technique: C, carrier solution (deionized water); P, piston pump; V, valve; HC, holding coil; MV, multiport valve; PANI, PANI water dispersion (1:10); B, pH buffer solution (pH 7.0-10.4); Q, quartz capillary glass; DET, CCD detector; W, waste.
most of the film is already in the nonconducting EB form at pH 7. Optical methods for pH measurements, which are based on IR,16,17 UV-visible,18 and fluorescence19 spectroscopy, have been reported in the literature. Fiber-optic chemical sensors were recently reviewed by Wolfbeis.20 A new method based on optical pH measurements with a commercially available water dispersion of PANI nanoparticles21-24 was recently reported.25 This method combines UV-visible spectroscopy and the sequential injection analysis technique (SIA). The detection of pH between 6 and 10.5 was done in continuous mode at λ ) 800 nm. Alternatively, pH could be measured between pH 7.0 and 10.5 at a fixed wavelength of 400 or 580 nm in batch mode with a fiber-optic light guide. A 0.5H,L mixed protonation constant of log KH ) 4.4 (average proton 0.5L number, n ) 0.5) was calculated for the PANI nanoparticles from UV-visible data. The center of the pH transition interval (pHtrans) is located at pH 8.8 due to the reaction stoichiometry of 1:2 between the proton-ligand complex of the nanoparticles. The bulky poly(styrenesulfonic acid) counterion makes the PANI nanoparticles less pH sensitive than the PANI films prepared in HCl. This shifts the pH transition interval to higher pH values. The transition interval of the PANI nanoparticles, pHtrans ) (log 0.5H,L KH /n) ( 2 (n ) 0.5), is also broader than for conventional 0.5L acid-base indicators, pHtrans ) log KH,L HL ( 1, due to the reaction stoichiometry. In this paper, the PANI nanoparticles will be characterized with Raman spectroscopy between pH 2 and 12. A new method for pH measurements was developed based on the results of the Raman characterization. This novel and fast detection method makes use (16) Bae, I. T.; Scherson, D. A.; Yeager, E. B. Anal. Chem. 1990, 62, 45-49. (17) Zhang, S.; Soller, B. R.; Micheels, R. H. Appl. Spectrosc. 1998, 52, 400406. (18) Pringsheim, E.; Terpetschnig, E.; Wolfbeis, O. S. Anal. Chim. Acta 1997, 357, 247- 252. (19) Pringsheim, E.; Zimin, D.; Wolfbeis, O. S. Adv. Mater. 2001, 13, 819-822. (20) Wolfbeis, O. S. Anal. Chem. 2006, 78, 3859-3873. (21) Wessling, B.; Srinivasan, D.; Rangarajan, G.; Mietzner, T.; Lennartz, W. Eur. Phys. J. E 2000, 2, 207-210. (22) Wessling, B. Synth. Met. 2003, 135-136, 265-267. (23) Kahol, P. K.; Ho, J. C.; Chen, Y. Y.; Wang, C. R.; Neeleshwar, S.; Tsai, C. B.; Wessling, B. Synth. Met. 2005, 151, 65-72. (24) www.zipperling.de. (25) Lindfors, T.; Harju, L.; Ivaska, A. Anal. Chem. 2006, 78, 3019-3026.
of Raman spectroscopy in combination with the SIA technique. EXPERIMENTAL SECTION Chemicals. Water Dispersion of PANI and pH Buffer Solutions. The PANI water dispersion (D1012W1) (mean particle size 46 nm; pH 1.4) was obtained from Ormecon GmbH (Ammersbek, Germany). The size of 90% of the particles was
