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Thermal phase separation of aqueous triethylamine (TEA) solutions (TEA wt % ) 6.5-6.7 in H2O) was induced by irradiating a focused 1064-nm laser beam ...
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Anal. Chem. 2005, 77, 6055-6061

Laser-Induced Liquid-to-Droplet Extraction of Chlorophenol: Photothermal Phase Separation of Aqueous Triethylamine Solutions Noboru Kitamura,* Momoko Yamada, Shoji Ishizaka, and Kumiko Konno

Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

Thermal phase separation of aqueous triethylamine (TEA) solutions (TEA wt % ) 6.5-6.7 in H2O) was induced by irradiating a focused 1064-nm laser beam (spot size ∼1 µm) under an optical microscope, and this produced a single micrometer-sized TEA droplet as demonstrated by in situ Raman microspectroscopy. Since H2O absorbs 1064-nm light, heat is generated at the focal spot of the incident laser beam, giving rise to photothermal phase separation of the aqueous TEA solution. The TEA droplet produced by phase separation was trapped simultaneously by the incident laser beam. In the presence of p-chlorophenol (CP) in an aqueous TEA solution, laserinduced photothermal phase separation and simultaneous TEA droplet formation brought about extraction/concentration of CP from the surrounding solution phase to the TEA droplet (∼15-µm diameter and 1.7-pL volume). Raman microspectroscopy demonstrated that the distribution coefficient of CP (KD) between the solution phase and the single TEA droplet was KD(drop) ) ∼21, while that in a bulk TEA/H2O system was KD(bulk) ) 4.7. The larger KD(drop) value as compared to KD(bulk) was discussed in terms of radiation pressure exerted on CP in the TEA droplet. Thermal phase transition/separation of a solution system has been employed in separation and analytical sciences. A typical example is extraction/concentration of a solute upon the thermal phase transition of an aqueous poly(N-isopropylacrylamide) (PNIPAM) solution.1-4 It is known that an aqueous PNIPAM solution exhibits a lower critical solution temperature (LCST) at ∼32 °C, below which the polymer is hydrated and the solution is clear, while the solution becomes turbid above LCST owing to dehydration of the polymer and subsequent precipitation of PNIPAM microparticles.5 Therefore, a water-soluble hydrophobic solute such as porphyrin derivatives,1 metal chelates,2-4 acridine orange,6 and so forth7 solubilized homogeneously in the solution below LCST can be extracted and concentrated into the hydro* Corresponding author. E-mail: [email protected]. (1) Saitoh, T.; Ohkubo, S.; Matsubara, C. Chem. Lett. 1999, 151-152. (2) Saitoh, T.; Ohyama, T.; Takamura, K. Sakurai, T.; Kaise, T.; Matsubara, C. Anal. Sci. 1997, 13, 1-4. (3) Fujinaga, K.; Yamato, Y.; Seike, Y.; Okumura, M. Anal. Sci. 1997, 13, 141144. (4) Saitoh, T.; Ohyama, T.; Sakurai, T. Kaise, T.; Takamura, K.; Suzuki, Y.; Matsubara, C. Talanta 1998, 46, 541-550. 10.1021/ac050822k CCC: $30.25 Published on Web 08/20/2005

© 2005 American Chemical Society

phobic environments of the PNIPAM particles upon thermal phase transition. Another interesting example is thermal phase separation of a perfluorinated solvent/organic solvent system.8 In the case of a perfluoromethylcyclohexane (3 mL)/n-hexane (3 mL)/ toluene (1 mL) mixture, the solution is separated into two phases at room temperature, while that becomes miscible above 36.5 °C. Such a system has been applied to extraction-free synthetic reactions known as a fluorous biphase system.8 Potential applications of thermal phase transition/separation behavior of a solution system provide therefore a novel means in separation and analytical sciences. Thermal phase transition/separation of a certain solution system can be also induced by laser irradiation. In the case of an aqueous (H2O) PNIPAM solution, irradiation of a focused CW 1064-nm laser beam under an optical microscope can induce the thermal phase transition of the solution.9 As shown in Figure 1, H2O as a medium absorbs incident 1064-nm laser light through the overtone band of the O-H stretching vibration mode, and this results in heat generation in the vicinity of the focal spot of the laser beam. As a result, a single micrometer-sized PNIPAM particle is produced at the focal spot of the laser beam. The laserinduced method has a high potential, since a thermal phase transition can be induced in minute space. On the other hand, spectroscopic and electrochemical studies on microdroplets in solution have been reported.10-17 As an example, we introduced a laser trapping-microanalysis technique (5) Heskins, M.; Guillet, J. E. J. Macromol. Sci., Chem. 1968, 1441-1455. (b) Yamamoto, I.; Iwasaki, K.; Hirotsu, S. J. Phys. Soc. Jpn. 1989, 58, 210-215. (c) Winnik, F. M. Macromolecules 1990, 23, 233-242. (d) Winnik, F. M. Polymer 1990, 31, 2125-2134. (e) Chee, C. K.; Rimmer, S.; Soutar, I.; Swanson, L. Polymer 1997, 38, 483-486. (f) Wu, C.; Wang, X. H.; Phys. Rev. Lett. 1998, 80, 4092-4094. (g) Wang, X. H.; Wu. C. Macromolecules 1999, 32, 4299-3401. (6) Kitamura, N.; Hosoda, Y.; Iwasaki, C.; Ueno, K.; Kim, H.-B. Langmuir 2003, 19, 8484-8489. (7) Matsubara, C.; Kikuchi, N.; Denpouya, I.; Takamura, K. Chem. Lett. 1993, 849-850. (b) Matsubara, C.; Izumi, S.; Takamura, K.; Yoshioka, H.; Mori, Y. Analyst 1993, 118, 553-556. (8) Curran, D. P. Angew. Chem., Int. Ed. 1998, 37, 1174-1196. (b) Horva´th, I. T. Acc. Chem. Res. 1998, 31, 641-650. (c) Dobbs, A. P.; Kimberley, M. R. J. Fluorine Chem. 2002, 118, 3-17. (d) Zhang, W. Chem. Rev. 2004, 104, 2531-2556. (9) Ishikawa, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Chem. Lett. 1993, 481-484. (b) Ishikawa, M.; Misawa, H.; Kitamura, N.; Fujisawa, R.; Masuhara, H. Bull. Chem. Soc. Jpn. 1996, 69, 59-66. (10) Nakatani, K.; Chikama, K.; Kitamura, N. In Advances in Photochemistry; Neckers, D. C., Volman, D. H., von Bu ¨ nau, G., Eds.; Wiley-Interscience: New York, 1999; Vol. 25, pp 173-233. (b) Kitamura, N.; Kitagawa, F. J. Photochem. Photobiol. C: Photochem. Rev. 2003, 4, 227-247.

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Figure 1. Near-IR absorption spectra of H2O, D2O, and triethylamine.

to study chemical characteristics of individual oil droplets in solution.10 It is worth noting that such experiments are the indispensable basis for studying size-dependent chemical/physical processes in or across single microdroplet/solution interfaces. By a strict definition, however, such an experimental mode is not a “single-particle measurement” but is a “particle-resolved mode”, since a sample solution involves a large number of droplets. As another approach to droplet measurements, a single droplet can be hung in a solution at the end of a glass tube,13 while the droplet is connected with the inner solution in the glass tube. In both cases, chemical/physical responses of a single droplet by an external stimulus cannot be followed in a quantitative manner owing to disturbance by untrapped droplets or the solution (11) Barnes, M. D.; Ng, K. C.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 1993, 65, 2360-2365. (b) Barnes, M. D.; Kung, C.-Y.; Whitten, W. B.; Ramsey, J. M.; Arnold, A.; Holler, S. Phys. Rev. Lett. 1996, 76, 3931-3934. (12) Lu, H.; Gratzl, M. Anal. Chem. 2000, 72, 1569-1575. (b) Tohda, K.; Lu, H. Umezawa, Y.; Gratzl, M. Anal. Chem. 2001, 73, 2070-2077. (c) Yoshida, M.; Tohda, K.; Gratzl, M. Anal. Chem. 2003, 75, 6133-6140. (13) Nakatani, K.; Negishi, T. Anal. Sci. 2001, 17, 1109-1111. (b) Nakatani, K.; Noguchi, T.; Negishi, T. Anal. Sci. 2002, 18, 533-536. (c) Negishi, T.; Nakatani, K. Phys. Chem. Chem. Phys. 2003, 5, 594-598. (d) Chikama, K.; Negishi, T.; Nakatani, K. Bull. Chem. Soc. Jpn. 2003, 76, 295-299. (e) Chikama, K.; Negishi, T.; Nakatani, K. Anal. Chim. Acta 2004, 514, 145150. (f) Nakatani, K.; Yamashita, J.; Negishi, T.; Osakai, T. J. Electroanal. Chem. 2005, 575, 27-32. (g) Negishi, T.; Nakatani, K. Anal. Chem. 2005, 77, 1807-1812. (14) Gatherer, R. D. B.; Reid, J. P. Chem. Phys. Lett. 2002, 357, 153-160. (b) Gatherer, R. D. B.; Sayer, R. M.; Reid, J. P. Chem. Phys. Lett. 2002, 366, 34-41. (c) Sayer, R. M.; Gatherer, R. D. B.; Gilham, R. J. J.; Reid, J. P. Phys. Chem. Chem. Phys. 2003, 5, 3732-3739. (d) Sayer, R. M.; Gatherer, R. D. B.; Reid, J. P. Phys. Chem. Chem. Phys. 2003, 5, 3740-3747. (15) Wadhawan, J. D.; Wain, A. J.; Compton, R. G. ChemPhysChem 2003, 4, 1211-1215. (b) Wain, A. J.; Lawrence, N. S.; Greene, P. R.; Wadhawan, J. D.; Compton, R. G. Phys. Chem. Chem. Phys. 2003, 5, 1867-1875. (c) Davies, T. J.; Brookes, B. A.; Compton, R. G. J. Electroanal. Chem. 2004, 566, 193216. (16) Musick, J.; Popp, J. Phys. Chem. Chem. Phys. 1999, 1, 5497-5502. (b) Schlu ¨ cker, S.; Roman, V.; Kiefer, W.; Popp, J. Anal. Chem. 2001, 73, 31463152. (17) Schwell, M.; Baumga¨rtel, H.; Weidinger, I.; Kra¨mer, B.; Vortisch, H.; Wo¨ste, L.; Leisner, T.; Ru ¨ hl, E. J. Phys. Chem. A 2000, 104, 6726-6732. (b) Santesson, S.; Andersson, M.; Degerman, E.; Johansson, T.; Nilsson, J.; Nilsson, S. Anal. Chem. 2000, 72, 3412-3418. (c) Magome, N.; Kohira, M. I.; Hayata, E.; Mukai, S.; Yoshikawa, K. J. Phys. Chem. B 2003, 107, 3988-3990. (d) Nichkova, M.; Feng, J.; Sanchez-Baeza, F.; Marco, M.-P.; Hammock, B. D.; Kennedy, I. M. Anal. Chem. 2003, 75, 83-90. (e) He, M.; Sun, C.; Chiu, D. T. Anal. Chem. 2004, 76, 1222-1227.

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connected with the droplet. A droplet injection method has been also reported.12,13 However, simultaneous manipulation of an injected droplet in solution (i.e., levitation) is in general very difficult. In such a case, therefore, a nonspherical droplet attached on the bottom of a sample cell or the surface of an electrode has been studied. An introduction of a single spherical microdroplet and its manipulation in solution is thus a technically difficult task. If laser-induced photothermal phase transition/separation is applicable to a water/oil system under an optical microscope, a single microdroplet (oil or water) could be produced arbitrarily in the liquid phase (water or oil, respectively). Therefore, such a system will be applied to studying various chemical and physical responses or phenomena of single microdroplets in solution. For further advances in laser applications in analytical and separation sciences, such a study is worth exploring in detail by means of laser-induced photothermal phase separation (LIPS). Since water/oil phase separation is the fundamental basis for solvent extraction, the phase diagrams of water/oil systems as a function of both a volume ratio and temperature have been so far accumulated. At certain water/oil volume ratios, in practice, various systems show thermal phase separation at around room temperature:18,19 triethylamine/water,20 1-butanol/water,18 2-butanol/ water,18,21,22 and so forth.23 In the case of an aqueous triethylamine (TEA) solution, furthermore, Mukai et al. reported recently on LIPS of the system by irradiating a focused CW 1064nm laser beam under an optical microscope.24 They also demonstrated that the single TEA droplet produced upon phase separation was optically trapped simultaneously by the 1064-nm laser beam. Hobley et al. reported the dynamics of LIPS of an aqueous TEA solution as well.25 Therefore, a TEA/H2O system is very suitable to demonstrate an analytical application of LIPS of a liquid/liquid system in minute dimension. In this paper, we report general features of LIPS of an aqueous TEA solution (i.e., TEA microdroplet formation) and simultaneous confocal Raman microspectroscopic analysis of the produced droplet. Furthermore, we demonstrate that p-chlorophenol (CP) solubilized homogeneously in the solution before laser irradiation is extracted and concentrated very efficiently into the single TEA microdroplet produced by LIPS. We also evaluated the extraction/ concentration efficiency of CP for the single picoliter TEA droplet (15-µm diameter and 1.7-pL volume) on the basis of in situ Raman microspectroscopy. Very efficient extraction/concentration of CP into the TEA droplet upon LIPS of the system was then discussed in terms of radiation pressure exerted on CP in the droplet. EXPERIMENTAL SECTION Laser Trapping-Confocal Raman Microspectroscopy System. Figure 2 shows the experimental setup for LIPS of an (18) Letcher, T. M.; Siswana, P. M. Fluid Phase Equil. 1992, 74, 203-217. (19) Kazakov, S. V.; Chernova, N. I. Chem. Eng. Commun. 2003, 190, 213-235. (20) Atkins, P. W. Physical Chemistry; 6th ed.; Oxford University Press: Oxford, 1998; Chapter 8. (21) Aizpiri, A. G.; Chazarra, P.; Rubio, R. G.; Peo`a, M. D. Chem. Phys. 1990, 146, 39-45. (22) Monroy, F.; Casielles, A. G.; Aizpiri, A. G.; Rubio, R. G.; Ortega, F. Phys. Rev. B 1993, 47, 630-637. (23) Aizpiri, A. G.; Correa, J. A.; Rubio, R. G.; Peo`a, M. D. Phys. Rev. B 1990, 41, 9003-9012. (24) Mukai, S.; Magome, N.; Kitahata, H.; Yoshikawa, K. Appl. Phys. Lett. 2003, 83, 2557-2559. (25) Hobley, J.; Kajimoto, S.; Takamizawa, A.; Ohta, K.; Trang-Cong, Q.; Fukumura, H. J. Phys. Chem. B 2003, 107, 11411-11418.

Figure 2. Block diagram of the experimental setup. Key: DM, dichroic mirror; PH, pinhole; NF, notch filters.

aqueous TEA solution and simultaneous laser trapping-confocal Raman microspectroscopy of single TEA microdroplets.26,27 A 1064nm laser beam from a CW Nd3+:YAG laser (Spectron, SL-902T) was used as a light source for both LIPS and laser trapping. The 1064-nm laser beam and a 488-nm laser line from an Ar+ laser (300 mW, Coherent, Innova 70) as an excitation source for Raman scattering were introduced coaxially into an inverted optical microscope (Nikon, Eclipse E300) by using a dichroic mirror (DM) and irradiated to a sample solution through an oil immersion objective lens (×100, NA ) 1.30). In the actual experiments, 8.4 µL of a sample solution was poured onto a slide glass and covered with a coverslip as a sample cell. Raman scattering light was passed through a pinhole (diameter 100 µm) for the confocal arrangement. Incident (488 and 1064 nm) and Rayleigh scattering light were removed by passing two holographic notch filters (Kaiser Optics). Raman scattering light was then detected by a cooled CCD detector (Andor Tech.) equipped with a polychromator (1200 grooves/mm). The spatial resolutions along the lateral and vertical directions of the system were 0.5 and 2.4 µm, respectively, and the spectral resolution was 2.0 cm-1. Other Measurements. UV-visible and near-infrared absorption spectroscopies were conducted by using U-3300 and U-4100 spectrophotometers (both Hitachi Co. Ltd.), respectively. The phase diagram of an aqueous TEA solution as a function of a TEA wt % in H2O and temperature (T) was prepared by observing the T dependence of the phase separation behavior of the solution by naked eyes. The temperature of an aqueous TEA solution at a given TEA wt % in a glass sample tube was controlled by a thermostat water bath. The distribution coefficient (KD) of CP between TEA and H2O was determined as follows. An aqueous CP solution (8.5 × 10-4 M, 20 mL) was mixed vigorously with TEA (20 mL), and the mixture was allowed to stand overnight at 23 °C. After reaching a distribution equilibrium, the concentration of CP in the water phase ([CP]w) was determined photometrically at 299 nm (molar absorptivity 2.3 × 103 M-1 cm-1). Experiments analogous with those mentioned above were also performed at 35 °C. These experiments demonstrated that the KD value was almost constant in the T range of 23-35 °C: KD ) [CP]TEA/[CP]w ) 4.7. (26) Tsuboi, Y.; Nishino, M.; Sasaki, T.; Kitamura, N. J. Phys. Chem. B 2005, 109, 7033-7039. (27) Experimental setup for optical trapping of single microparticles; see also refs 10 and 28.

Figure 3. Optical micrographs of the LIPS behavior of an aqueous TEA solution (TEA wt % ) 6.5 in H2O) before (a, t ) 0) and during irradiation of a focused 1064-nm laser beam (b-e, at P1064 ) 1.5 W and 23 °C), while (g) and (h) show those after switching-off the laser beam (e ) f). The four lines seen in each photograph are a positional gauge and have no physical meaning.

Chemicals. TEA (GR grade 99%) and CP (reagent grade 95%), both supplied from Wako Pure Chemicals Co. Ltd., were used without further purification. For near-IR absorption and Raman spectroscopies on a neat solution, TEA or CP was purified by distillation over an appropriate drying reagent.29 Water was deionized and distilled before use (Advantech Toyo, GSR-200). RESULTS AND DISCUSSION General Features of Laser-Induced Photothermal Phase Separation of an Aqueous TEA Solution. Figure 3 shows a typical example of the optical micrographs of the phase separation behavior of an aqueous TEA solution (i.e., droplet formation, TEA wt % ) 6.5 in H2O at 23 °C) during focused 1064-nm laser irradiation (spot size ∼1 µm and laser power (P1064) ) 1.5 W). Before laser irradiation (a, t ) 0), the solution was completely homogeneous, while a single minute droplet appeared at the focal spot upon laser irradiation (b, t ) 3 s). Prolonged laser irradiation gave rise to the increase in the droplet diameter (c and d) and its diameter (d) reached a constant value at t ) 15 s (d ∼ 15 µm, e). After switching-off the laser beam, the droplet disappeared very quickly within 2 s (f () e) ∼ h). Droplet formation/disappearance upon switching on/off the laser beam was highly reversible. Figure 4 shows the Raman spectra of the solution before (a, t ) 0) and after 1064-nm laser irradiation (b, t ) 15 s). Since the solution before laser irradiation is water-rich, the strong Raman band responsible for the O-H stretching mode of H2O is observed at ν (wavenumber) ) 3200-3700 cm-1. At t ) 15 s, on the other hand, the O-H stretching band disappears almost completely and the peaks ascribed to the C-H stretching modes of TEA are observed in the ν region of 2800-3000 cm-1.30 This demonstrates explicitly that the droplet produced by 1064-nm laser irradiation is composed of TEA. (28) Misawa, H.; Koshioka, M.; Sasaki, K.; Kitamura, N.; Masuhara, H. J. Appl. Phys. 1991, 70, 3829-3836. (29) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980. (30) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John-Wiley & Sons: New York, 1991.

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Figure 4. Raman spectra of an aqueous TEA solution (TEA wt % ) 6.5 in H2O) before (a) and after 1064-nm laser irradiation (b, t ) 15 s at P1064 ) 1.5 W and 23 °C). The insets represent the relevant optical micrographs of the solution.

Focused 1064-nm laser irradiation to a TEA/H2O solution under an optical microscope certainly can induce LIPS and single TEA droplet formation. As characteristics of LIPS, furthermore, since the refractive index of TEA (n ) 1.40) is higher than that of water (n ) 1.33),31 the TEA droplet produced is trapped simultaneously by the 1064-nm laser beam.28 This is shown clearly by the fact that the TEA droplet produced always sits at the focal spot during laser irradiation (Figure 3b-e), while that shifts from the focal spot upon switching-off the laser beam, though the image in Figure 3g is not necessarily clear enough. The present results agree very well with those reported by Mukai et al.,24 although they have not reported spectroscopic analysis of the produced droplet. Optical trapping of the TEA droplet also made it possible to conduct precise and accurate Raman spectroscopy of the droplet. It is worth emphasizing that phenomena analogous with those in Figure 3 cannot be observed for a TEA/D2O mixture (TEA wt % ) 6.5). As seen clearly in Figure 1, H2O absorbs 1064-nm light as described before, while both D2O and TEA are transparent at 1064 nm, indicating that the results in Figure 3 are induced by the photothermal effects through absorption of 1064-nm light by H2O and subsequent heat generation at the focal spot of the laser beam. To confirm further the point, we studied a TEA wt % dependence of the phase separation temperature (Tp)-phase diagram in Figure 5. The closed rectangle in the figure represents Tp determined experimentally at a given TEA wt % in H2O, and the solid curve shows the binodal line of the solution, below which the solution is one phase while the two phases are separated at a given T above the curve. The data demonstrate that the solution is always homogeneous below 18 °C irrespective of a TEA wt % (i.e., Tp > 18 °C), while Tp increases sharply with decreasing (90%). Since the present experiments at 23 °C are conducted at TEA wt % ) 6.5, Tp is estimated to be ∼26 °C. The phase diagram in Figure 5, therefore, indicates that a laser-induced T jump with several degrees centigrade should induce phase separation of the system: shown by the dotted vertical arrow in Figure 5. We assume here that the photon energy of a focused 1064nm laser beam (spot size 1 µm) absorbed by a spherical H2O (31) Organic Solvent, 3rd ed. Technique of Chemistry; Riddick, J. A., Bunger, W. B., Eds.; Wiley-Interscience: New York, 1970; Vol. II.

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Figure 5. Phase diagram of a TEA/H2O system. See also the main text.

Figure 6. Laser power (P1064) dependence of the equilibrium TEA droplet diameter (closed circles) or volume (open circles) produced by LIPS of an aqueous TEA solution (TEA wt % ) 6.5 in H2O at 23 °C).

droplet (radius (R) ) 0.5 µm) is converted to heat (Q) and that local heat is equilibrated by the surrounding solution phase. In such a case, the relevant temperature increase in the droplet (∆T) is estimated by the following equation,9

∆T ) Q/4πκR

(1)

where κ is the thermal conductivity of water (0.6 W m-1 K-1). The Q value can be calculated on the basis of the absorbance of H2O at 1064 nm (5.6 × 10-6 for 1-µm optical path length; see also Figure 1) and P1064. At P1064 ) 1.5 W, ∆T was then estimated to be ∼5 °C for a 1-µm3 H2O droplet. Under the present experimental conditions at 23 °C, therefore, irradiation of the 1064-nm laser beam certainly can induce LIPS of the TEA/H2O solution. Laser Power Dependence of TEA Droplet Size. According to eq 1, it is predicted that ∆T increases with P1064 through an increase in Q. This is verified by the laser power dependence of the TEA droplet size produced by LIPS. Figure 6 shows the P1064 dependence of the equilibrated TEA droplet diameter (d at t > 15 s). The data demonstrated clearly that d increased with increasing P1064 and became as large as ∼20 µm at P1064 ) 2.0 W. In the initial stage of laser irradiation, H2O absorbs incident light and this gives rise to phase separation of the system as described above. Upon TEA droplet formation, however, TEA itself is transparent at 1064 nm (Figure 1), so that heat generation in the droplet is not expected except for that by a small amount of H2O

Figure 7. Raman spectra of an aqueous TEA/CP solution (TEA wt % ) 6.7 in H2O, [CP] ) 0.11 M, 23 °C) before (a) and after 1064-nm laser irradiation (b, t ) 15 s at P1064 ) 1.5 W). The insets represent the relevant optical micrographs of the solution. The arrow shows the peak responsible for CP: ν ) 3072 cm-1.

dissolved in the TEA droplet. Despite this, the diameter of a TEA droplet increases with an increase in P1064. The results will be explained as follows. In the experiments, since the focused laser beam is irradiated on the solution with a large cone angle (∼120°) due to the use of a large NA () 1.30) objective lens, water is heated over a volume larger than 1 µm3, in particular, that just below and above the focal spot (i.e., along the laser beam axis). The larger the volume heated, the lower is the cooling rate by the surrounding solution phase, since the surface area/volume ratio of a droplet decreases with increasing R. Since ∆T becomes higher with increasing P1064, the heating rate of water surpasses the cooling rate and local heat should propagate from the focal spot toward the surrounding solution phase. Upon prolonged laser irradiation, therefore, the TEA droplet size increases and reaches a constant value: d in a heating-cooling equilibrium at given P1064. This is one possible reason for the P1064 dependence on d in Figure 6. Furthermore, single TEA droplet formation should generate a concentration gradient of TEA in the solution phase around the TEA droplet. Thus, TEA molecules dissolved in the solution phase are likely to transport toward the TEA droplet, and this leads to coalescence of the molecules into the droplet. We suppose that this also plays a role partly in determining the P1064 dependence of the TEA droplet-size. Laser-Induced Water-to-TEA Droplet Extraction/Concentration of p-Chlorophenol and Simultaneous Raman Spectroscopic Analysis. An aqueous TEA solution (6.7 wt %) containing CP (0.11 M) was irradiated by the focused 1064-nm laser beam to induce LIPS, similar to the experiments shown in Figure 3. Before phase separation, TEA/CP/H2O is a homogeneous mixture. As shown in Figure 7a, in practice, the Raman spectrum of the solution before laser irradiation was best characterized by the strong O-H stretching band of H2O observed in ν ) 3200-3700 cm-1 similar to the relevant spectrum in Figure 4, and the C-H stretching bands responsible for both TEA (28003000 cm-1) and CP were almost discernible. Upon laser irradiation, on the other hand, CP solubilized in the solution is likely to distribute to the single TEA droplet produced by LIPS, as expected from the distribution coefficient of CP determined in a bulk TEA/ H2O system: KD(bulk) ) [CP]TEA/[CP]w ) 4.7 (23-35 °C). As

shown in Figure 7b, the Raman spectrum of the solution during laser irradiation (i.e., that of the TEA droplet) exhibited the strong peak at ν ) 3072 cm-1 (shown by the arrow in the figure), in addition to those of TEA at 2878, 2933, and 2965 cm-1.32 The peak observed at 3072 cm-1 can be assigned confidently to the C-H stretching mode of CP as judged from the Raman spectrum of neat CP: see Supporting Information, Figure S1. Therefore, the present results in Figure 7 demonstrate explicitly laser-induced photothermal TEA droplet formation and simultaneous distribution of CP into the optically trapped TEA droplet. It is worth noting that the Raman scattering intensity at ν ) 3200-3700 cm-1 during laser irradiation in Figure 7b is somewhat stronger than the relevant spectrum in Figure 4 without CP. The results are readily understood by the contribution of the O-H stretching band of CP to the observed spectrum (Figure S1), which also supports distribution of CP to the TEA droplet. For further quantitative discussion, we evaluated the distribution coefficient of CP between the TEA droplet and the surrounding solution phase on the basis of the ratio of the Raman scattering intensity of CP at ν ) 3072 cm-1 (ICP) to that of TEA at ν ) 2933 cm-1 (ITEA): KD(drop) ) [CP]drop/[CP]soln ≈ ICP/ITEA. The relationship between ICP/ITEA and [CP] prepared by separate bulk experiments was satisfactorily linear (ICP/ITEA ) 0.20[CP]; see Supporting Information Figure S2), and the ICP/ITEA value of 0.46 in Figure 7b indicated that the CP concentration distributed to the single TEA droplet was [CP]drop ) 2.30 M (TEA droplet diameter (volume) ) 15 µm (1.7 pL)).33 Since the concentration of CP in an aqueous TEA solution before droplet formation is [CP]soln ) 0.11 M, the KD(droplet) is calculated to be ∼21. Knowing KD(bulk) ) 4.7 (23-35 °C), the KD(drop) value is ∼4.5 times larger than KD(bulk), demonstrating that CP is extracted and concentrated very efficiently from the surrounding solution phase into the single TEA droplet. The results of KD(drop) . KD(bulk) are unique and important characteristics of the present experiments. In a distribution equilibrium, KD(drop) should be in principle equal to KD(bulk). The temperature in the TEA droplet could be higher (>26 °C as judged from the Tp value at TEA wt % ) 6.56.7) than the surrounding solution (23 °C). Although CP is transparent at 1064 nm, incident light will be absorbed by H2O distributed partly in the TEA droplet, and this will contribute more or less heat generation. Nevertheless, since KD(bulk) is almost constant at 4.7 in the T range of 23-35 °C, T effects on KD will not explain such a high KD(drop) value. On the other hand, the presence of CP (0.11 M) in the aqueous TEA solution will cause a change in the mutual solubility between TEA and water as compared to that without CP, and this might influence KD(drop). Nonetheless, the ∼4.5 times increase in KD in the droplet system (32) The Raman band observed at around ν ) 2800 cm-1 in Figure 4b was obscured in Figure 7b. It has been reported that the band at ν ) 2800 cm-1 is responsible for the C-H stretching mode of the methylene group in TEA, and the band is under the influence of hyperconjugation between σ* of the C-H bond and the lone pair electron on the N-atom (see ref 25). In the presence of CP, since TEA and CP produces a hydrogen bond in the droplet, this will cause the changes in the Raman shift and scattering intensity of the C-H band. (33) At [CP] ) 0.15 M, experiments analogous with those in Figure 7 demonstrated that the [CP] distributed into the single TEA droplet produced by laser irradiation was 0.28 M. The value corresponds to KD(drop) ∼ 19, which agrees very well with that at [CP] ) 0.11 M (KD(drop) ∼ 21) within an experimental error.

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as compared to that in the bulk solution will not be explained by the solubility change alone. It is worth noting that Ajito et al. reported that the KD value of p-nonylphenol (NP) between single toluene droplets and the surrounding water phase (KD(drop) ) [NP]drop/[NP]water) evaluated by laser trapping-Raman microspectroscopy is larger than that determined in the relevant bulk system, and KD(drop) increases with a decrease in d.34 They reported that the droplet size effects on KD(drop) were attributed to the increase in the surface area/volume (A/V) ratio of the droplet with a decrease in d, by which adsorption of NP on the droplet surface was facililated.34a For water-to-droplet distribution of a solute, it is certainly true that the rate becomes faster with decreasing d as demonstrated experimentally by our research group.35 However, KD(drop) should be in principle independent of d (i.e., volume). Although the present experimental observations agree very well with those by Ajito et al., the principal reason for KD(drop) > KD(bulk) is worth discussing in some more detail. Roles of Radiation Pressure in Extraction/Concentration of p-Chlorophenol into Single TEA Droplets. We suppose that radiation pressure generated by 1064-nm laser irradiation plays an important role in determining KD(drop). Beside radiation pressure as the driving force for optical trapping of microparticles (i.e., Mie scattering), it is known that radiation pressure is also exerted upon Rayleigh particles whose size is smaller than the wavelength of incident light, Rayleigh scattering.36 The radiation pressure exerted on a Rayleigh particle (F) is given by36

1 ∂ F ) R∇E2 + R (E × B) 2 ∂t

(2)

where E and B are the electric field strength and the magnetic flux density of incident light, respectively. R is a polarizability of the particle and is given by

(na/nb)2 - 1 R ) 4πbr3 (na/nb)2 + 2

(3)

where r is the radius of the particle. na and nb are the refractive indices of the particle and the surrounding medium, respectively, and b is the dielectric constant of the medium. The first term in eq 2 is an electrostatic force acting on the dipole in the inhomogeneous electric field, called a gradient force. At na > nb, the gradient force attracts the particle to the focal point of incident light. The second term is called a scattering force, which pushes the particle along the beam direction. Since the gradient force is usually much stronger than the scattering force, the radiation pressure works for trapping Rayleigh particles in the vicinity of the focal point. In practice, it has been reported that molecules themselves are laser trapped in the vicinity of the focal point of incident light. As an example, Hotta et al. demonstrated that (34) Ajito, K.; Morita, M.; Torimitsu, K. Anal. Chem. 2000, 72, 4721-4725. (b) Ajito, K.; Torimitsu, K. Trends Anal. Chem. 2001, 20, 255-262. (35) Kogi, O.; Kim, H.-B.; Kitamura, N. Anal. Chim. Acta 2000, 418, 129-135. (b) Nakatani, K.; Uchida, T.; Kitamura, N.; Masuhara, H. J. Electroanal. Chem. 1994, 375, 383-386. (36) Shen, Y. R. The Principles of Nonlinear Optics; Wiley-Interscience: New York, 1984.

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swelled micelles ( KD(bulk). We suppose that the results reported by Ajito et al. described above might be also explained along with analogous context with those mentioned above: na(p-nonylphenol) ) 1.513 > nb(toluene) ) 1.489.39,41 It is worth pointing out, furthermore, that since the present Raman microspectroscopy is conducted in the confocal arrangement, information about the central region of the droplet should be emphasized. Therefore, the present KD(drop) value will be more or less overestimated, which will be another reason for the KD(drop) > KD(bulk). CONCLUSIONS Laser-induced photothermal phase separation of aqueous TEA solutions (TEA wt % ) 6.5-6.7) producing single TEA micro(37) Hotta, J.; Sasaki, K.; Masuhara, H. J. Am. Chem. Soc. 1996, 118, 1196811969. (38) Kitamura, N.; Sekiguchi, N.; Kim, H.-B. J. Am. Chem. Soc. 1998, 120, 19421943. (39) The Merck Index, 8th ed.; Stecher, P. G., Ed.; Merck & Co., Inc.: Rahway, NJ, 1968. (40) As one of the reviewers has pointed out, when we use a solute with a refractive index smaller than that of TEA, concentration effects of the solute to a TEA droplet will not be observed. Such an experiment will prove a role of radiation pressure in the extraction/concentration phenomena observed in the present study. Experiments are now in progress, and the results will be reported in a separate publication. (41) As judged from Figure 5 in ref 34a, KD(drop) for extraction of NP to a single toluene droplet (d ) 15 µm) is ∼3 times larger than the relevant KD(bulk) value. The KD(drop)/KD(bulk) ratio observed in the present study (∼4.5) is larger than the results by Ajito et al. Since the experimental conditions are different (incident laser power and wavelength, KD(bulk), and so forth), we cannot compare directly the two results. If the difference in KD(drop)/ KD(bulk) between the two experiments is meaningful, the results will be explained by the larger refractive index difference in the present system (na ) 1.558 and nb ) 1.400) as compared to that in a NP/toluene system (na ) 1.513 and nb ) 1.489) as predicted from eqs 2 and 3.

droplets and simultaneous trapping of the droplet by 1064-nm laser irradiation was very successful. In the presence of CP, furthermore, it was shown that CP was extracted and concentrated into the TEA droplet with KD(drop) being ∼4.5 times higher than KD(bulk). We also demonstrated the important roles of radiation pressure exerted on CP molecules in extraction/concentration of CP into the TEA droplet. In the present experiments, we employed an aqueous TEA solution as a phase separation medium. Furthermore, since TEA acted as a fluorescence quencher to various excited-state species, we could not use fluorescence spectroscopy for detecting an extracted solute. Therefore, we selected CP as a solute and Raman spectroscopy for detection. On the basis of such a combination, we succeeded in proving our idea of LIPS and simultaneous extraction/concentration of CP. Nevertheless, the detection limit of CP by the present method is limited to the Raman activity of the molecule, and unfortunately, the present results do not imply trace analysis of CP. Despite this, the results of KD(drop) . KD(bulk) are highly potential as a new and novel approach to simultaneous extraction/concentration/analysis of a ultratrace amount of an analyte. Since we have already confirmed LIPS of a butanol/H2O mixture and simultaneous fluorescence detection of an extracted/concentrated analyte,42 we believe that the present idea could be extended to ultratrace analysis of various analytes. Besides analytical sciences, furthermore, the LIPS method will be also applicable to a fluorous biphase system, laser-induced

fluorous biphase separation. Finally, it is worth noting that we have so far reported chemical and physical processes across single microdroplet/solution interfaces on the basis of a laser trappingmicroanalysis technique.10 The present results suggest that our previous research results on single microdroplets also might be under the influence of radiation pressure. However, since laser power employed in the previous studies (several hundreds of mW) is weaker than the present one (P1064 ) 1.5 W), effects of radiation pressure on our previous observations will not be so large as compared to those on the present results. Systematic and careful studies, including P1064 dependences of observed phenomena, will reveal further characteristics of chemical reactions proceeding in single microdroplet/solution systems.

(42) Kitamura, N.; Konno, K. Unpublished results.

AC050822K

ACKNOWLEDGMENT N.K. expresses thanks for a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and technology (MEXT) of the Japanese Government for the support of the research: 13853004 and 14050001 (Priority Research Area 417). SUPPORTING INFORMATION AVAILABLE Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review May 11, 2005. Accepted July 24, 2005.

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