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A Study of Molecular Adsorption of Bromothymol Blue by Optical Waveguide Spectroscopy Zhi-mei Qi,†,‡ Naoki Matsuda,*,† Jose Santos,† Kiminori Itoh,§ Akiko Takatsu,| and Kenji Kato| Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan, Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan, and Metrological Laboratory of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8563, Japan Received August 16, 2002. In Final Form: November 8, 2002 Optical waveguide spectroscopy of aqueous bromothymol blue (BTB) solutions confirmed that neutral BTB molecules could easily adsorb from both acidic and basic solutions onto hydrophobic surfaces and that the molecular BTB adsorption from basic solution, causing transient nonequilibrium between the molecular and anionic BTB concentrations, was accompanied with the conversion of BTB- anions into BTB molecules in the solution.
Optical waveguide (OWG) spectroscopy as a powerful technique for in situ detection of molecular interactions at the liquid/solid interface is of considerable interest.1-7 An OWG spectrometer constructed with a white light source, a waveguide transducer, and a spectrally resolved photodetector can provide much information such as amount, orientation, and conformation of the adsorbate at the interface as well as the adsorption kinetics of the analyte. A broadband OWG spectrometer also allows for simultaneous investigation of different chromophores having different behaviors at the interface. In this Letter, typical OWG spectroscopy of multiple chromophores was performed using aqueous bromothymol blue (BTB) solutions as samples that contain both chromophores of yellow BTB molecules and blue BTB- anions. Anions of BTB and other pH indicator dyes have been recognized as surfaceinactive chromophores for glass plates and metal oxide thin films whose surfaces are generally negatively charged. As a result, basic solutions of these dyes are often used as bulk absorbents for characterizing the evanescent absorbance sensitivity of OWG sensors.7-10 However, no * To whom corresonpdence should be addressed. E-mail:
[email protected] † Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology. ‡ On leave from the State Key Laboratory of Transducer Technology, Institute of Electronics, Academia Sinica, Beijing 100080, People’s Republic of China. § Graduate School of Environment and Information Sciences, Yokohama National University. | Metrological Laboratory of Japan, National Institute of Advanced Industrial Science and Technology. (1) Piraud, C.; Mwarania, E.; Wylangowski, G.; Wilkinson, J.; O’Dwyer, K.; Schiffrin, D. J. Anal. Chem. 1992, 64, 651. (2) Kato, K.; Takatsu, A.; Matsuda, N. Chem. Lett. 1995, 437. (3) Ross, Susan E.; Seliskar, Carl J.; Heineman, William R. Anal. Chem. 2000, 72, 5549. (4) Bradshaw, J. T.; Mendes, S. B.; Saavedra, S. S. Anal. Chem. 2002, 74, 1751. (5) Qi, Z.-M.; Matsuda, N.; Yoshida, T.; Asano, H.; Takatsu, A.; Kenji, K.; Opt. Lett. 2002, 27, 2001. (6) Santos, J. H.; Matsuda, N.; Qi, Z.-M.; Takatsu, A.; Kato, K. IEICE Trans. Electron. 2002, E85-C, 1275. (7) Doherty, W. J.; Donley, C. L.; Armstrong, N. R.; Saavedre, S. S. Appl. Spectrosc. 2002, 56, 920. (8) Degrandpre, M. D.; Burgess, L. W.; White, P. L.; Goldman, D. S. Anal. Chem. 1990, 62, 2012. (9) Tsunoda, K.; Itabashi, H.; Akaiwa, H. Bull. Chem. Soc. Jpn. 1992, 65, 581.
Figure 1. Schematic diagram of the OWG spectrometer used.
information on interfacial behaviors of the neutral dye molecules coexisting in aqueous solutions with their anions could be available in the reported works perhaps due to incapabilities of the sensors generally working at a single wavelength of 633 nm. The present study not only characterized the OWG used but also revealed the dependence of interfacial behavior of molecular BTB on the surface properties and the solution pH. A somewhat similar study was reported by Eisenthal and co-workers that focused on investigation of the behaviors of phenols and anilines at the air/water interface by a technique of the second harmonic generation.11,12 In earlier work,13 pure BTB thin films with a refractive index of 1.69 at 633 nm have been fabricated by vacuum evaporation and spincoating techniques and used for optical sensing of ammonia gas. This study demonstrated that the immersion of hydrophobic glass slides into aqueous BTB solutions could result in formation of molecular BTB monolayers onto the substrates. Figure 1 shows the configuration of the OWG spectrometer used. White light from a 150 W xenon lamp passing through a glass fiber (200 µm in diameter with (10) Kuhn, K. J.; Burgess, L. W. Anal. Chem. 1993, 65, 1390. (11) Bhattacharyya, K.; Castro, A; Sitzmann, E. V.; Eisenthal, K. B. J. Chem. Phys. 1988, 89, 3376. (12) Castro, A.; Bhattacharyya, K.; Eisenthal, K. B. J. Chem. Phys. 1991, 95, 1310. (13) Qi, Z.-M.; Yimit, A.; Itoh, K.; Murabayashi, M.; Matsuda, N.; Takatso, A.; Kato, K. Opt. Lett. 2001, 26, 629.
10.1021/la0264217 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/04/2002
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Figure 2. Absorbance spectra of the acidic and basic BTB solutions measured with the hydrophilic and hydrophobic glass waveguides (A, acidic; B, basic; Hl, hydrophilic; Hb, hydrophobic; gray line, TE mode; black line, TM mode).
a 100-µm core) is coupled into a slab waveguide by focusing the light on a 5-mm 45° LaSF8 prism via a lens. With another prism and fiber the light is coupled out of the waveguide and then conducted to a CCD detector (S-2300, Soma Optics, Ltd., Tokyo, Japan). A computer connected to the CCD detector is used for data collection and processing. A Teflon plate with a rectangular hole (2 cm × 0.5 cm × 1 cm) as a sample cell is mounted onto the waveguide. A polarizer is fixed in front of the input prism coupler to select transverse electric (TE) and transverse magnetic (TM) polarization states of the guided light. Slab waveguides were fabricated by immersing soda-lime glass slides (76 mm × 26 mm × 1 mm) into molten KNO3 at 400 °C for 30 min. After K+-Na+ ion exchange the glass waveguides are highly hydrophilic. Some of them were modified to be hydrophobic by dipping the waveguides into the absolute toluene containing 1% (wt/wt) octadecyltrichlorosilane for 1 h. The silanized waveguides have a water contact angle of >70°. The glass waveguides as transducers of the spectrometer give a transmission spectrum ranging from 450 to 800 nm. The acidic (0.08 mM, pH 4.80) and basic (0.96 mM, pH 9.18) BTB solutions were prepared using deionized water and tetraborate buffer solution as solvents, respectively. An absorption spectrum of a solution sample can be determined from the equation A ) log(IR/Is), where IR and IS are output intensities at a fixed wavelength measured with the solvent and its solution in the cell, respectively. Figure 2 shows absorption spectra of the acidic (A) and basic (B) BTB solutions obtained with the hydrophilic (Hl) and hydrophobic (Hb) glass waveguides. In the A-Hl spectra the TE- and TM-polarized absorbances are nearly zero in the wavelength range 470-720 nm. However, the A-Hb spectra exhibit relatively large absorbances at wavelengths below 520 nm, which, compared with the A-Hl spectra, reveals that the detected absorbances are attributed to absorption of the molecular BTB adlayer rather than that of the bulk solution. In the present case the bulk solution absorption could not be detected due to a low bulk concentration as well as a weak evanescent field. From the findings it is concluded that neutral BTB
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molecules can be immobilized from aqueous BTB solution onto hydrophobic rather than hydrophilic glass. This is because the attractive force between the hydrophobic surface and the hydrophobic groups (i.e., benzene rings and -CH3) of the BTB molecule is larger than the solvation force between water and its hydrophilic groups such as -SO3-, -Br, and -OH. In addition, it was observed from the A-Hb spectra that at a fixed wavelength ( 0, of potassium ion exchanged waveguides induced by compressive stress in the ionexchanged layers.14 In the wavelength range 470-520 nm, the TE- and TMpolarized absorbances are present in the B-Hb spectra but disappear in the B-Hl spectra. On the basis of the findings above, this significant difference results from the molecular BTB adsorption from the basic solution onto the hydrophobic waveguide. By use of the HendersonHasselbalch equation: pH ) pKa + log(CB/CA), where pKa for BTB in aqueous solution is 7.1 and CA and CB are bulk concentrations of the BTB molecule and its anion. CA in the basic BTB solution is calculated to be 8 µM, which is 10 times as low as that in the acidic BTB solution. However, the detected absorbances at wavelengths e520 nm in the A-Hb and B-Hb spectra are relatively close to each other. To confirm this extraordinary result, the absorbance spectra of several samples having the same pH values (4.80 or 9.18) but different dye concentrations were investigated using the silanized glass waveguides. The filled circles in Figure 3 show the TM-polarized absorbances at 470 nm as a function of the calculated CA in pH 9.18 BTB solutions. The absorbance rapidly increases with increasing CA up to 2 µM and then moves toward saturation with further increase in CA. The filled squares in Figure 3 show the case of pH 4.80 BTB solutions, and two lines represent the best fits to the Langmuir isotherm. At the same CA the absorbance of the basic BTB solution is much larger than that of the acidic one, suggesting that the number of neutral BTB molecules adsorbed from the basic solution is much greater than that from the acidic one in the case of the same CA. The difference between molecular BTB adsorption from the acidic and basic solutions is considered to result from the conversion of BTB- anions into BTB molecules in the basic solution. It is known from the Henderson-Hasselbalch equation that CA and CB in a given aqueous BTB solution are determined by the solution pH. Neutral BTB molecules are the predominant chromophore in acidic BTB solutions, and thus the molecular BTB adsorption from acidic solutions should have a negligible effect on the equilibrium between CA and CB. It is not such a case for basic BTB solutions (14) Qi, Z.-M.; Itoh, K.; Murabayashi, M. Appl. Opt. 2000, 39, 5750.
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Figure 4. 1/A as a linear function of 1/C. Figure 3. Measured absorbance at 470 nm versus the calculated CA in pH 9.18 and 4.80 BTB solutions. Lines show the best fits to the Langmuir adsorption isotherm.
where neutral BTB molecules become the minority chromophore. The molecular BTB adsorption from basic solutions will decrease CA to some extent. To keep the equilibrium between CA and CB in basic BTB solutions with fixed pH values, the molecular BTB adsorption will be accompanied by spontaneous conversion of BTB- anions into BTB molecules in the solutions. As a result, a large number of BTB- anions in basic solutions, which served as a source to generate neutral BTB molecules for adsorption onto the hydrophobic waveguide, resulted in the increased absorbances in the wavelength range 470520 nm. The molecular BTB adsorption onto hydrophobic glass would make the pKa value of the dye at the interface smaller than that in aqueous solution. The molecular BTB adsorption onto hydrophobic glass was found to obey the Langmuir isotherm. With the surface concentration CS in mol/cm2 and its maximum CSmax, the Langmuir adsorption equation can be expressed as CS/ CSmax ) C/(C + R), where C is the bulk concentration and R ) 55.5 exp(∆Gads/RT).12 From the equation it is seen that 1/CS linearly depends on 1/C. The pseudo-Beer law noted below indicates that the absorbance A is proportional to CS. Therefore, a linear dependence of 1/A on 1/C exists for the Langmuir adsorption. Using the measured absorbances in Figure 3, we did find that 1/A was a linear function of 1/C (here C ) CA + CB). From two straight lines in Figure 4 ∆Gads is derived to be -8.02 and -8.07 kcal/mol for molecular BTB adsorption from pH 9.18 and 4.80 solutions, respectively. As expected, both energies of adsorption are approximately equal, independent of the solution pH. A linear relationship also exists between 1/A and 1/CA, from which ∆Gads is calculated to be different for different pH (-10.81 and -8.07 kcal/mol for pH ) 9.18 and 4.80). A comparison between two calculations suggests that not CA but C determines ∆Gads and CS of BTB molecules adsorbed onto a given hydrophobic substrate. After the BTB solution was removed from the sample cell, the hydrophobic waveguide surface without retaining any solution drop was exposed to air. Such a waveguide, in the presence of ammonia gas, could absorb the guided light in the wavelength range 520-680 nm (see Figure 5) due to deprotonation of the BTB molecules immobilized onto the waveguide. This is more evidence of the existence of the molecular BTB adlayer on the hydrophobic glass surface. The molecular BTB adsorption onto the silanized TiO2 film was also observed by the OWG spectroscopy performed with a TiO2 film coated glass waveguide. The
Figure 5. Absorption spectrum (a) of the molecular BTB adlayer formed from pH 4.80 BTB solution onto the hydrophobic glass waveguide and spectral response (b) of the adlayer to ammonia gas.
OWG spectroscopy also confirmed the molecular adsorption of thymol blue (TB) from aqueous solutions onto hydrophobic substrates. The molecular adsorption from a given BTB or TB solution was found to highly depend on the waveguide surface hydrophobicity that had a bad reproducibility and consequently resulted in difficulty in accurate measurement. The potassium ion exchanged glass waveguides used above have a narrow spectral bandwidth that limits the measured absorbances of the molecular BTB adlayer to wavelengths g470 nm. To achieve a complete electron spectrum of the molecular BTB adlayer, the waveguide transducer of the OWG spectrometer was changed into a 50-µm-thick glass plate that could transmit light at wavelengths down to 360 nm (see ref 5 for details). Before use, the plate was silanized in hexamethyldisilazane vapor at 90 °C for 1 h. Figure 6a shows the measured TEpolarized absorption spectrum of the basic BTB solution (1.10 mM, pH 9.18). The spectrum includes two bands, and the left band with a peak at 440 nm is attributed to absorption of the molecular BTB adlayer. For comparison, an absorption spectrum of the bulk solution measured with a conventional UV-vis spectrometer is also shown in Figure 6a (the BTB solution was diluted in order to obtain a measurable spectrum). The molecular BTB adsorption onto the hydrophobic glass plate makes the evanescent absorption spectrum quite different in shape from the bulk solution spectrum. Figure 6b shows the evanescent absorbance at 440 nm versus the measuring time (the CCD detector recorded the absorption spectrum once every second). The steady-state absorbance was
Letters
Figure 6. Absorption spectra (a) of pH 9.18 BTB solutions measured with the OWG and conventional UV-vis spectrometers and (b) dependence of the evanescent absorbance at 440 nm on the measuring time. The waveguide used for OWG spectroscopy was a 50-µm-thick glass plate with a silanized surface.
detected immediately as the solution sample was introduced into the cell within 1 s from t ) 0. It indicates that the molecular BTB adsorption on the hydrophobic glass surface is very fast. A thin-film glass plate serving as the waveguide transducer allows us to estimate the surface concentration by use of the waveguide theory and the pseudo-Beer law:5 A ) ηcL, where η is the optical power fraction in the adlayer, is the molar absorptivity, L is the interaction path length, and c ) CS/d (d is the adlayer thickness). In the present case, L is 2 cm and for molecular BTB is 1.638 × 104 M-1 cm-1 at 440 nm. To calculate η, the BTB molecule having a stereo structure is treated as a ball with a diameter of 1 nm and the effective refractive index
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of the guided light in the glass plate was measured to be ∼1.43.5 On the basis of the TE mode in a four-layer slab waveguide consisting of the air substrate (n ) 1), the 50µm glass plate (n ) 1.53), the 1-nm BTB adlayer (n ) 1.69), and the water superstrate (n ) 1.333), η in the molecular BTB adlayer is calculated to be 2.1324 × 10-5 at 440 nm. With a measured absorbance of A ) 0.44 at 440 nm, CS is determined to be 6.3 × 10-11 mol/cm2, which occupies 38% of a closely packed BTB monolayer. This is the largest surface concentration for the thin-film glass plate used because the dye concentration of 1.10 mM exceeds the value required for saturated adsorption of BTB molecules from pH 9.18 solution (see Figure 3). Such a low surface concentration suggests a low degree of silanization of the glass surface. A comparison between the measured and calculated evanescent absorbances at 617 nm demonstrated that the calculation of CS above was rational and accurate. At 617 nm the evanescent power fraction in water superstrate was calculated to be 2.0272 × 10-3 and for anionic BTB is 3.958 × 104 M-1 cm-1. CB in the solution sample used was derived to be 1.0922 mM. Therefore, the calculated absorbance at 617 nm is A ) 0.1752. From Figure 6a the measured evanescent absorbance at 617 nm is A ) 0.1728, which is approximately equal to the calculated value. Note that calculating CS would be difficult for potassium ionexchanged glass waveguides due to difficulty in accurate determination of refractive index and depth of the ionexchanged layer. In conclusion, the dependence of the interfacial behavior of neutral BTB molecules on both the surface property and the solution pH has been demonstrated by the OWG spectroscopy of aqueous BTB solutions. In the meantime, the study shows the outstanding applicability of the OWG spectroscopy for simultaneous characterization of multiple chromophores having different properties at the liquid/ solid interface. Note Added after ASAP Posting. This article was released ASAP on 12/4/2002. Changes made after posting were as follows: last page of paper, first paragraph, line 6, equation c ) CSd was changed to c ) CS/d; last page of paper, first paragraph, line 18, the value 6.3 × 10-9 was changed to 6.3 × 10-11. The correct version was posted on 12/12/2002. LA0264217
