Langmuir 1999, 15, 3035-3037
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Spectral Fluctuation and Heterogeneous Distribution of Porphine on a Water Surface Yao Qun Li SEDC Laboratory of Analytical Science, Department of Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China
Maxim. N. Slyadnev,† Takanori Inoue,‡ Akira Harata, and Teiichiro Ogawa* Department of Molecular and Material Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan Received July 15, 1998. In Final Form: March 1, 1999 Using a confocal fluorescence microscope, we found a significant temporal fluctuation in the fluorescence spectrum of porphine molecules on the water surface. The spectra fluctuated when the accumulation time was 300 ms and the pH was within a certain range. No fluctuation was observed with a longer accumulation time (36 s) or on a solid surface. The present observation indicates that the distribution of porphine on the water surface is heterogeneous and that the Brownian motion of the surface species induces spectral fluctuation. A neutral form of porphine produces domains at a higher surface coverage.
Introduction The surface of a liquid has unique properties which differ from those in the bulk.1,2 Phenomena occurring at interfaces play an important role in chemistry and biology and are thus of great scientific interest. The ionization potential of perylene obtained at the air-water interface indicates that the water surface is less polar than the bulk.3 The pH levels of the surface and the bulk are different.2 Moreover, the density, the dielectric properties, and even the molecular arrangement at the surface of a solution may be different from those in the bulk.1 These changes should occur within an interfacial region a few nanometers deep.1 Therefore, it is necessary to clarify the unique properties of the surface in order to understand the chemical behavior at the surface of a solution. Fluorescence spectra measured on the water surface have interesting features.4-6 Some phthalocyanine complexes and fluorescein molecules show a spontaneous aggregation on the water surface,4,5 and merocyanine shows domains of monomers and aggregates on the water surface.6 Although the surface heterogeneity in the adsorbate distribution has been recognized for a solid surface,7 it has not been investigated in detail on the water surface. To our knowledge, there has been no publication on spectral fluctuations from a large number of molecules on a liquid surface. We have observed the fluorescence spectra of a porphine at the air-water interface using a confocal fluorescence * To whom correspondence should be addressed. † On leave from St. Petersburg State University, Department of Chemistry, Universitetskij prospekt 2, 198904 Petergoff, St. Petersburg, Russia. ‡ Present address: Department of Applied Chemistry, Oita University, Tannohara, Oita 870-1192, Japan. (1) Benjamin, I. Chem. Rev. 1996, 96, 1449. (2) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343. (3) Ogawa, T.; Chen, H. T.; Inoue, T.; Nakashima, K. Chem. Phys. Lett. 1994, 229, 328. (4) Matsuzawa, Y.; Seki, T.; Ichimura, K. Thin Solid Films 1997, 301, 162. (5) Dutta, A. K.; Salesse, C. Langmuir 1997, 13, 5401. (6) Kajikawa, K.; Takezoe, H.; Fukuda, A. Chem. Phys. Lett. 1993, 205, 225. (7) Wintterlin, J.; Vo¨lkening, S.; Janssens, T. V. W.; Zambelli, T.; Ertl, G. Science 1997, 278, 1931.
microscope to evaluate their unique spectral fluctuations. This fluctuation on the water surface has been ascribed to cluster formation of the neutral porphine and an inhomogeneous distribution (domain formation) of solutes. Experimental Section The confocal microspectroscope, which was constructed in our group, has been described elsewhere.8 In brief, a cw Ar+ laser (Lexel 95-4; 1.7 W for 488 nm) beam was focused on the sample surface, and the typical laser power used was 100 mW. The fluorescence excited at 488 nm was collected by the same objective lens [0.25 NA (numerical aperture)]. The optics were based on a normal microscope (Nikon Labophot 2). Among three objective lens (10x/0.25 NA, 20x/0.4 NA, and 40x/0.65 NA) tested to see the effect of the spot size, the objective with 0.25 NA was used for most of this work in order to measure at a lower spatial resolution (10-100 µm in diameter) to overcome any effects due to evaporation of water and mechanical instability.8 A fiber bundle (Hamamatsu A6400; 177 µm × 40 fibers) served as a set of flexible pinholes, and the fluorescence spectra were measured using a monochromator equipped with an intensified CCD camera (Hamamatsu PMA-100). The fiber bundle provided a spatial separation on the CCD plane between fluorescence from the focused region and scattered light.8 By integrating the selected pixel area on the CCD plane, the Raman scattered light and other background from bulk water were reduced significantly, and the sensitivity of measurements was enhanced substantially. 2,3,7,8,12,13,17,18-Octaethyl-21H,23H-porphine (OEP) was used as a model compound. A specific amount of OEP, dissolved in benzene, was spread at the water surface in a quartz container with a diameter of 2.6 cm under the conditions at which benzene rapidly evaporates. Water was distilled, deionized, filtered and redistilled in a quartz vessel, and its pH was adjusted using a phosphate buffer (sodium phosphate and either HCl or NaOH). Although the phosphate buffer does not have enough buffer capacity at pH ) 1, the amount of OEP added was small, and pH was maintained. Because OEP is insoluble in water and stays only on the water surface, the surface coverage was estimated directly using the amount of OEP spread on the water and the area of the water surface. OEP was spread onto a quartz surface in an identical procedure. (8) Li, Y. Q.; Sasaki, S.; Inoue, T.; Harata, A.; Ogawa, T. Appl. Spectrosc. 1998, 52, 1111.
10.1021/la980896+ CCC: $18.00 © 1999 American Chemical Society Published on Web 04/27/1999
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Figure 1. Sequence of normalized fluorescence spectra of OEP molecules. The accumulation time was 300 ms. a, surface coverage: 4.3 × 105 molecules/µm2; pH 1. b, surface coverage: 8.6 × 103 molecules/µm2; pH 6. c, same as a), however, the vertical axis is not normalized to show the intensity fluctuation clearly.
Results and Discussion Surfaces have unique properties which differ from those in bulk liquid.1 Changes in surface properties occur within an interfacial region a few nanometers deep.1 To obtain information from the whole of the interfacial region, however, we carried out the present experiments in a relatively low spatial resolution. The constant evaporation of the water would make it difficult to carry out a high spatial-resolution experiment on the water surface. Typical fluorescence spectra on the water surface are shown in Figure 1. In this case, accumulation time was 300 ms. Tens of spectra were successively collected for an identical sample, and 10 typical ones are shown for each case in Figure 1. The measurements were repeated several times under both identical and spectrally varied conditions. The spectra in Figure 1a were taken for a surface coverage of 4.3 × 105 molecules/µm2 at pH 1, and the intensity of each spectrum was normalized; this surface coverage corresponded to 2.3 nm2 per molecule. The spectra in Figure 1b were taken for a surface coverage of 8.6 × 103 molecules/µm2 at pH 6, which was much smaller than that in Figure 1a and the intensity of each spectrum was normalized. The spectra in Figure 1c are the same as those in Figure 1a; intensity is shown as observed, and a typical baseline is also shown. The spectra show spontaneous fluctuations under these experimental con-
Letters
ditions. Parts a and b of Figure 1 indicate fluctuations in spectrum, and part c indicates the fluctuation in intensity. The peak at 632 nm fluctuates much more than that at 602 nm. We observed the spectral fluctuations with time over a wide range of surface coverages [(4.3-4300) × 103 molecules/µm2] within a specific pH range. However, due to poor SN ratio, these fluctuations are not reproducible below 1 × 103 molecules/µm2. The spectral fluctuation was evident as long as the aqueous subphase was within a suitable pH range: pH 3-6 for a surface coverage of 8.6 × 103 molecules/µm2 and pH 1-3 for a surface coverage of 4.3 × 105 molecules/µm2. The coverage was calculated assuming that all the molecules were on the surface. However, it may actually be slightly lower because ionized species may partly dissolve in bulk water. When the accumulation time was changed, it was found that the more apparent fluctuations were observed for the least accumulation time. However, the signal-to-noise ratio was reduced as the accumulation time decreased. The accumulation time of 300 ms was selected as a compromise. Spectra obtained with each of the three objective lenses showed essentially identical spectral fluctuations. Because water evaporates at a rate of approximately 2 µm/min8 and a lens of a higher spatial resolution requires a more frequent realignment of the microscope, we used a lens of NA ) 0.25 for most of this work to eliminate the need of aligning the confocal microscope system and to exclude any possibility of fluctuation with mechanical instability. We did not observe such spectral fluctuation from OEP molecules on a quartz plate, even though the surface concentration, the accumulation time, and the optics were identical to those used in Figure 1. This finding indicates that the fluctuation of the spectrum is characteristic of the water surface. The liquid surface vibrates thermodynamically for a few angstroms.1 This length is much smaller than the spatial resolution of our apparatus. Furthermore, the fact that the spectral fluctuation was observed only within a certain pH range indicated that the fluctuation did not have a mechanical origin. The origin of fluctuation was not local heating by laser irradiation, either. The heat absorbed on the surface was estimated to be at most 0.16 mW at 4.3 × 105 molecules/ µm2. The local temperature raise was estimated to be at most 4.2 °C by numerical integration of the heat conduction equation,9 being thus negligible. The two pyrrolenine nitrogen atoms of OEP (Figure 2a) are capable of accepting protons. The fluorescence spectrum of OEP varies with the pH of the solution because of protonation equilibrium,10,11 as shown in Figure 2b,c. The maximum peak intensity was normalized. Accumulation time was 36 s, with the exception of the two spectra in part c of Figure 2 at pH 1 and pH 2, which were acquired in 90 s. No fluctuation was observed when the spectra were measured with such a long accumulation time. The neutral free base of OEP (unprotonated form, PH2) may become a monocation (monoprotonated form, PH3+) or a (9) The calculation was carried out by Y. Takatou and Prof. N. Imaishi by using the following heat conduction equation in the cylindrical coordinate (z, r, φ), assuming an axial symmetry where the laser irradiated a circular region of 100 µm:
FCp ∂T ∂2T 1 ∂T ∂2T ∂T ) 2+ + 2, )0 λ ∂τ r ∂r ∂φ ∂r ∂z where T is the temperature, F is the density, Cp is the specific heat, and λ is the heat conductivity. (10) Hambright, P. Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 238. (11) Papkoisky, D. B.; Ponomarev, G. V.; Wolfbeis, O. S. J. Photochem. Photobiol., A 1997, 104, 151.
Letters
Figure 2. Fluorescence spectra of OEP molecules on the water surface at different pHs. The peak intensity was normalized. The accumulation time was 36 s, except the two spectra in c with pH 1 and pH 2, which were acquired in 90 s. a, Structure of OEP. b, surface coverage: 8.6 × 103 molecules/µm2. c, surface coverage: 4.3 × 105 molecules/µm2.
dication (diprotonated form, PH42+), and the monocation is unstable.10,11 The emission maximum of PH2 was observed at 632 nm, that of PH3+ at 623 nm, and those of PH42+ at 602 and 662 nm. A comparison of Figure 2b,c indicates that OEP is more protonated at lower pH and surface coverage and that PH2 appears in the higher pH region at a lower surface coverage. It has been demonstrated that the air-water interface favors the neutral over the charged form in acid-base equilibrium.2,4 However, no varying degree of protonation has been shown with the surface coverage at the air-water interface. This change in the OEP acid-base equilibrium differs from the usual behavior in the bulk solution, as will be explained later. The spectral peaks shown in Figure 1 were found to be identical to those observed in Figure 2. The ratio of PH2 and PH42+ changed randomly at the higher coverage (pH 1), and that of PH2 and PH3+ changed randomly at the lower coverage (pH 6). The spectral fluctuation observed at the higher surface coverage can be explained as follows. The neutral form, PH2, forms domains (or clusters) on the water surface, and their collective motion (Brownian motion) within the interfacial region may induce a spectral intensity fluctuation. A careful examination of the spectral intensity in Figure 1a indicated that the fluctuation mainly was due to the intensity jump of PH2 at 632 nm, with the intensity of PH3+ or PH42+ showing only a small fluctuation. This finding indicates that ionic species distributed more uniformly on the water surface than neutral species. Mutual repulsion due to their charge prevented them from
Langmuir, Vol. 15, No. 9, 1999 3037
producing clear domains as the neutral species. The domains of PH2 were stable and floated on the water surface.4 When one of them came into the viewing region of the microscope, an intense signal of PH2 appeared. It has been reported that a large number of molecules exhibit a collective-diffusion behavior due to micro-Brownian motion.12,13 Accordingly, relative coverage of each species on the water surface fluctuated in a microscopic region in a short time scale, and this fluctuation was averaged for a longer accumulation time. A careful examination of the spectral intensity in Figure 1b indicated that there was some fluctuation even at a lower surface concentration, although this fluctuation was not as evident as that shown in Figure 1a. Two peaks appeared at 623 (PH3+) and 632 nm (PH2) in the spectra with some intensity fluctuation. There was no intense signal at 632 nm, and thus there were no noteworthy clear domains of PH2 as in the case of Figure 1a. The spectral intensity fluctuation would indicate both the existence of a surface region (domain) with more neutral or more monoprotonated forms on the surface and the fluctuation of the relative coverage of each species on the water surface. Thus, the distribution of the solute on the water surface is heterogeneous even at a lower surface coverage. There is additional evidence for the domain formation. There are clear differences between the spectra shown in b and c of Figure 2: the peak at 632 nm (monomer) is more intense at a lower pH and a high concentration (Figure 2c). This finding indicates not only that domain (clusters) formation of the neutral species is more dominant at higher concentrations but also that such domain formation affects the equilibrium at the air-water interface. If no domain were formed, the spectra at the same pH should be identical at the two concentrations. There are four species which should be taken into consideration with respect to equilibrium: PH2, PH3+, PH42+, and the PH2 cluster (domain). Thus, as surface coverage increases, the equilibrium between the neutral form (PH2) and ionic species (PH3+ or PH42+) is shifted toward the neutral form, which prefers to stay in domains. Although surface heterogeneity in adsorbate distribution has been known to occur on a solid surface,7 it has not been investigated in detail on the water surface. The present technique will offer new insight into the dynamical and photochemical properties of molecules on the water surface. Acknowledgment. JSPS postdoctoral fellowships of Japan Society for the Promotion of Science to Y.Q.L. and M.N.S. are gratefully acknowledged. The authors thank Y. Takatou and Prof. N. Imaishi of Department of Applied Science for Electronics and Materials of Kyushu University for their calculation on the temperature rise in the irradiation region. Y.Q.L. also wishes to thank Prof. Sunao Yamada for helpful discussions. This work was supported by a Grant-in-Aid for Scientific Research (No. 08455389) and two Grants-in-Aid for a JSPS scientist (Y.Q.L.) from the Ministry of Education, Science and Culture of Japan. LA980896+ (12) Castro, A.; Bhattacharyya, K.; Eisenthal, K. B. J. Chem. Phys. 1991, 95, 1310. (13) Zhao, X.; Goh, M. C.; Subrahmanyan, S.; Eisenthal, K. B. J. Chem. Phys. 1990, 94, 3370.
