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J. Phys. Chem. 1996, 100, 1494-1497
Micrometer Size Effect on Dye Association in Single Laser-Trapped Water Droplets Hiroshi Yao, Yasuhisa Inoue, Hiroshi Ikeda, Kiyoharu Nakatani, Haeng-Boo Kim, and Noboru Kitamura* Department of Chemistry, Faculty of Science, Hokkaido UniVersity, Kita-ku, Sapporo 060, Japan ReceiVed: August 14, 1995; In Final Form: October 25, 1995X
Molecular association of malachite green (MG) in single micrometer-sized water droplets dispersed in din-butyl phthalate was studied by means of scanning laser manipulation and absorption spectroscopy to elucidate characteristic features of chemical processes at the droplet/oil interface. At a high MG concentration, the droplet showed a new absorption band around 580 nm in addition to the monomer band at 615 nm. Analyses of the concentration-dependent absorption spectrum of MG in the droplet indicated that the band at 580 nm could be assigned to that of the MG dimer. The dimer formation of MG in the water droplet was shown to be facilitated with decreasing the droplet diameter, demonstrating the micrometer size effect on dye association. A role of the water/oil interface for the MG dimer formation is discussed.
Introduction A laser trapping method based on radiation force of light can be applied to control Brownian motion of single microparticles in solution and has been shown to have a high potential for spectroscopic investigations of various microparticles.1,2 Laser trapping of a particle by a focused laser beam is successful when the refractive index of a particle (np) is higher than that of the surrounding medium (ns): np > ns. Under such a refractive index condition, refraction of a laser beam by a particle generates an attractive radiation force, by which the particle is trapped in the vicinity of the focal spot of the laser beam. By combination of the method with a spectroscopic or electrochemical technique, studies on chemical processes across a particle/solution interface such as mass transfer and electron transfer have been conducted for individual oil droplets or polymer particles in a relatively low refractive index medium.1,3-5 In the case of np < ns, contrarily, the situation is the reverse of that of np > ns and a particle experiences repulsive radiation force. As an example, a water droplet (np ) 1.33)6 cannot be optically trapped in common organic solvents (ns > 1.33) by a single focused laser beam. In order to trap and manipulate low refractive index particles in a relatively higher refractive index medium, a scanning laser manipulation technique has been developed.1,7 By the scanning method, a focused laser beam is repetitively scanned around a particle to produce a “photon cage”, in which the particle receives repulsive force from all directions and is trapped at the center of the photon cage. Furthermore, modulation of the photon cage in space enables one three-dimensional manipulation of the particle. Indeed, it has been reported that the method is applied to manipulate a water droplet in liquid paraffin (ns ) 1.46-1.47).7 Nevertheless, no spectroscopic study has been explored for particles with np < ns since elaborated experiments are necessary to perform this. Among various low refractive index particles, spectroscopic investigations of micrometer water droplets dispersed in an oil are of primary importance to understand characteristic features of interfacial phenomena at the water/oil boundary and to get an inside look at chemical reactions in water-in-oil emulsions. Simultaneous manipulation and spectroscopic analyses of individual water droplets will be also applicable to microtitration of ultratrace amounts of various ions. Therefore, we attempted X
Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-1494$12.00/0
simultaneous scanning laser manipulation and absorption spectroscopy of single water droplets in an organic solvent. In this article, we report molecular association of a malachite green (MG) dye in micrometer-sized water droplets dispersed in din-butyl phthalate and demonstrate for the first time that the dimer formation of MG is dependent on the droplet diameter. The role of the water droplet/oil interface for MG dimer formation is also discussed. Experimental Section Apparatus. An experimental setup reported previously8 was used with some modifications to perform simultaneous scanning laser manipulation and microspectroscopy of single water droplets (Figure 1). A 1064-nm beam from a CW Nd3+:YAG laser (Spectron; SL902T) as a trapping light source was introduced to an optical microscope (Nikon; Optiphoto 2) and focused to a ∼1 µm spot through an oil immersion objective lens (×100, NA ) 1.3). In order to produce a “photon cage” in a sample solution, the laser beam was modulated spatially by computer-controlled galvano mirrors (GSI, G325DT; Marubun, TI-325 controller; NEC, PC9801VX) set between the laser and the microscope as illustrated in Figure 1. Laser power was adjusted to 2.5 W throughout the experiments. Although we have not determined the laser power under the microscope, it has been reported that 94% of incident laser power is limited by the optics outside and inside of the microscope for an analogous setup,9 so that actual laser power irradiated to a sample solution will be on the order of ∼150 mW. A Xe white light beam was introduced to the microscope coaxially with the laser beam and irradiated to the center of an optically-trapped water droplet to conduct absorption spectroscopy. For precise absorption measurements, the Xe beam diameter in a sample solution was set at ∼2 µm by the use of a pinhole and appropriate optics. Transmitted light passed through the dye/water droplet (intensity, I), was reflected by a beam splitter set under the microscope stage, and was analyzed by a polychromator (Oriel; Multispec 257)-multichannel photodetector (Princeton Instruments; ICCD-576E/G) combination. The incident light intensity of the Xe beam (I0) was determined under the same conditions without the dye in a droplet. Absorbance of the dye in the droplet was calculated on the basis of I and I0 determined for similar-sized droplets. A sample solution placed between two glass plates was set on the © 1996 American Chemical Society
Size Effect on Dye Association
Figure 1. Block diagram of a scanning laser manipulation-microspectroscopy system.
J. Phys. Chem., Vol. 100, No. 5, 1996 1495
Figure 3. Absorption spectra of single MG/water droplets. The individual droplets in DBP were trapped under similar experimental conditions with those for Figure 2. Absorbance of each spectrum was normalized to that of the droplet with d ) 20 µm.
Figure 4. Absorption spectra of homogeneous dilute (5.5 × 10-3 mM; solid curve) and concentrated (10 mM; broken curve) aqueous MG solutions. See also text and ref 12. Figure 2. Absorption spectra of single Ru(bpy)32+ (20 mM)/water droplets. The droplet dispersed in DBP was trapped in the photon cage produced by circular scanning of the focused laser beam at a repetition rate of ∼30 Hz. The inset shows a relationship between absorbance at 452 nm and the droplet diameter.
microscope stage and experiments were performed at ambient temperature. Conventional absorption spectroscopy was made by a Shimadzu UV-200S spectrometer. Chemicals. Tris(2,2′-bipyridine)ruthenium(II) dichloride hexahydrate (Ru(bpy)32+) and malachite green oxalate (MG) were purchased from Aldrich and Kanto Chemicals, respectively, and used without further purification. Pure water was obtained by an Aquarius GSR-200 (Advantec Co. Ltd.). Din-butyl phthalate (DBP; GR grade, Tokyo Kasei) as an oil was used as received. Water-in-oil emulsions were prepared by dispersing a DBP-saturated aqueous dye solution in watersaturated DBP with a volume ratio (water/DBP) of 1/20. In order to prevent photodegradation of MG, pH of the aqueous solution was adjusted to ∼5 with HCl, and absorption spectroscopy was performed for freshly-prepared emulsions. For preparation of dye-free water droplets, a small amount of poly(vinyl alcohol) as a surfactant was dissolved in the water phase. Results and Discussion Microspectroscopy of a Single Laser-Trapped Water Droplet. We conducted laser trapping-absorption spectroscopy of single Ru(bpy)32+ (20 mM)/water droplets in DBP to check performance of the present experimental setup. A single Ru(bpy)32+/water droplet was trapped in the photon cage produced by circular scanning of the focused laser beam (repetition rate ) ∼30 Hz), and the absorption spectrum of Ru(bpy)32+ in the droplet was measured as shown in Figure 2. The S/N value of the spectrum was not necessarily good, since the experiment
was performed by a single-shot measurement.10 Nevertheless, the spectral band shape of Ru(bpy)32+ coincided very well with that observed for the relevant bulk diluted solution. Also, we confirmed a good linear relationship between absorbance at the maximum wavelength (λmax ) 452 nm) and the droplet diameter (d ) 15-35 µm). Assuming d to be the optical path length, the molar extinction coefficient () of Ru(bpy)32+ at 452 nm was determined to be 1.65 × 104 M-1 cm-1, which was slightly larger than but in fairly good agreement with the reported value (1.47 × 104 M-1 cm-1).11 The linear relationship between absorbance and d indicates correct response of the present experimental setup. Absorption Spectrum of Malachite Green in a Single Water Droplet. Figure 3 shows absorption spectra of individual MG/water droplets in DBP ([MG] ) 5-12.5 mM). Although the diameter of the droplet ranged between 20 and 25 µm, absorbance of each spectrum in Figure 3 was normalized to that of the droplet with d ) 20 µm for comparison. A close inspection of the spectra indicated that a new absorption band appeared around 580 nm in addition to the monomer band at 615 nm. Furthermore, the ratio of absorbance of the new band to that of the monomer band increased with an increase in the MG concentration. Spectroscopic properties of MG in concentrated solutions have been rarely reported probably due to the very large value at the maximum wavelength (M615 ) 8.1 × 104 M-1 cm-1).12 In order to assign the new band around 580 nm, therefore, we conducted spectroscopic measurements for both homogeneous concentrated (10 mM in a thin-layer cell, 10-20 µm thickness13) and diluted (5.5 × 10-3 mM in a 10 mm optical path length cuvette) aqueous MG solutions. As summarized in Figure 4, the concentrated MG solution exhibited the peaks around 615 and 580 nm, while the diluted sample showed a single peak at 615 nm. The results demonstrate that
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Yao et al.
Figure 5. Zanker analysis of the concentration-dependent absorption spectrum of MG. The MG concentration in the water droplet was varied from 5.0 to 12.5 mM. The solid line represents that with the slope value of 2 (correlation coefficient ) 0.86).
Figure 6. Absorption spectra of the MG (10 mM)/water droplets with various diameters. The individual droplet in DBP was trapped in the photon cage (repetition rate ) ∼30 Hz).
the band at 580 nm observed for the MG/water droplets is inherent in a concentrated MG solution, and absorption at 580 nm is concluded to be originated from molecular association of MG in water. Molecular Association of MG in a Single Water Droplet. Molecular association of MG in the water droplets can be discussed on the basis of the Zanker analysis14 of the concentration-dependent absorption spectrum. When the monomer and the n-mer of MG are in equilibrium, the relevant association constant, KAS, can be given as in eq 1,
KAS ) X/{nC0n-1(1 - X)n}
(1)
where C0 and X represent the initial concentration of MG and the fraction of n-mer in the total mole number of MG, respectively. n is the association number of MG. At a certain wavelength, both monomer and n-mer absorb, so that an observed molar extinction coefficient () is expressed as
) Xn/n + (1 - X)M
(2)
In eq 2, M and n are the molar extinction coefficients of the monomer and the n-mer of MG, respectively. Combining eqs 1 and 2 under the assumption of /M >> n/(nM), we obtained eq 3,
log[C0(1 - /M)] ) log(CKAS) + n log(C0/M)
(3)
C ) nn/(n - n/M)n-1
(4)
where
If n is much smaller than M at a given wavelength, C equals n in eq 4 and, therefore, the KAS and n values can be determined by the intercept and slope values of a log[C0(1 - /M)] vs log(C0/M) plot, respectively, as indicated by eq 3. Knowing M615 to be 8 × 104 M-1 cm-1 as determined from the absorption spectrum of MG in the dilute solution (Figure 4),15 the present spectroscopic data were analyzed on the basis of eq 3 under the assumption of n
