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Fluorescent dye N,N′-dioctadecylrhodamine as a new interfacial acid–base indicator
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Spectrochimica Acta Part A 69 (2008) 1125–1129
Fluorescent dye N,N-dioctadecylrhodamine as a new interfacial acid–base indicator N.O. Mchedlov-Petrossyan a,∗ , N.A. Vodolazkaya a , O.N. Bezkrovnaya b , A.G. Yakubovskaya a , A.V. Tolmachev b , A.V. Grigorovich a b
a V.N. Karazin Kharkov National University, 61077 Kharkov, Ukraine Institute for Single Crystals, National Academy of Sciences of Ukraine, 61001 Kharkov, Ukraine
Received 15 November 2006; received in revised form 13 June 2007; accepted 15 June 2007
Abstract This paper reports the spectral properties and protolytic behavior of the fluorescent dye N,N -dioctadecylrhodamine on the micelle/water and microdroplet/water interfaces as well as in Langmuir–Blodgett films soaked into aqueous media. Long hydrocarbon chains provide similar orientation of its cation and zwitterion, with the dissociating group (COOH → COO− ) directed toward the bulk (aqueous) phase. Both absorption and fluorescence of the dye can be used for monitoring electrical surface potentials and for determination of bulk pH. © 2007 Elsevier B.V. All rights reserved. Keywords: N,N -Dioctadecylrhodamine; Vis-spectroscopy; Fluorescence; Interfacial acid–base indicator; Surfactant micelles; Langmuir–Blodgett films
1. Introduction In this paper, we propose a hydrophobic rhodamine dye N,N dioctadecylrhodamine as a colored and fluorescing interfacial acid–base indicator. The extensive development of physical chemistry of interfaces requires new effective reporter molecules (molecular probes) for checking interfacial acidity, electrical potentials, polarity, etc. A lot of such probes are based on well-known dyes and luminophores, which are functionalized by introducing special groups, e.g., long hydrocarbon tails [1]. Such modifications (hydrophobization) ensure the fixation of the probes at due places, for instance, on water/micelle or water/air interface. Rhodamine dyes belong to mostly used molecular probes owing to their fluorescent properties [1–3]. The behavior of several hydrophobic rhodamines, especially their photophysical properties, is described in micelles [4], water/air and water/hydrocarbon interfaces [5–8], and in Langmuir–Blodgett films [9–14]. However, application of rhodamines as interfacial acid–base indicators is practically unknown [15]. The aim of ∗
Corresponding author. E-mail address: [email protected] (N.O. Mchedlov-Petrossyan). 1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.06.011
this paper is to report the means of rhodamine luminophores in this field of surface chemistry. The key characteristic of the acid–base indicator situated on the micelle/water interface is the so-called ‘apparent’ ionization constant, Kaa (pKaa = − log Kaa ). As a rule, it is obtained using absorption or emission spectra of the indicator, accompanied by determination of the pH value of the bulk (aqueous) phase with glass electrode [15–19]. The concentration of the reporter molecule is very small (as a rule, 10−5 M), and the probability of appearance of two molecules in one and the same micelle is close to zero. According to the well-known electrostatic model, the relation between the pKaa value of the probe on the charged micellar interface and the local electrical surface potential is as follows (Eq. (1)) [15–20]: pKaa = pKai −
ΨF 2.303RT
(1)
Here Kai is the so-called ‘intrinsic’ ionization constant, Ψ the local electrical surface potential (as a rule, regarded as the potential of Stern layer), F the Faraday constant, R the gas constant, and T is the absolute temperature. The pKaa values of indicators may be used for estimation of the electrical potential of the charged surface, Ψ , if correct evaluation of pKai is possible [15–20]. The same approach can be used for examination of vesicles [21], droplets of microemulsions
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N.O. Mchedlov-Petrossyan et al. / Spectrochimica Acta Part A 69 (2008) 1125–1129
Chart 1.
[22], polyelectrolytic biomolecules [23], or charged monolayers [24]. Recently we proposed the well-known fluorescent dye rhodamine B as an acid–base probe for micelle/water interface [15]. However, the above rhodamine dye is completely bound only by micelles of anionic surfactants and (at high enough surfactant concentrations) by those of nonionic ones [15]. In the case of cationic surfactants, the binding of the dye is incomplete. In this letter, we communicate the results of examination of another rhodamine dye, N,N -dioctadecylrhodamine (DODR) in form of HR+ X− (X− = Cl− or ClO4 − ) which is tightly embedded in the micelles of any colloidal surfactants due to its long hydrophobic tails:
The existence of two long hydrocarbon chains allows to expect the position of the probe on the surface of any ionic micelle as shown in Chart 1. Now we report the pKaa values of DODR in micellar solutions of different type of surfactants, in microemulsions, and in the Langmuir–Blodgett films and demonstrate the possibility of Ψ estimation with this reporter molecule. The visible spectra (Fig. 1) demonstrate that the formation of colorless lactone in such water-rich systems as direct micelles of surfactants are, is less probable [25]. So, we attribute the ionization constant to the (cation zwitterion + H+ ) equilibrium. 2. Experimental The dye in form of chloride and perchlorate samples was synthesized according to the described procedure [26] and kindly gifted to us by Dr. V.I. Alekseeva, Research Insti-
Fig. 1. Absorption (1 and 2) and emission (1 and 2 ) spectra of N,N -dioctadecylrhodamine acid–base couple in micellar solution of Ncetylpyridinium chloride (0.01 M), ionic strength 0.05 M (NaCl): cationic form (1 and 1 ) and neutral form (2 and 2 ).
tute of Organic Intermediates and Dyes, Moscow, Russia. The surfactant samples of laurylsulfate, n-C12 H25 OSO3 − Na+ , N-octadecylpyridinium chloride, n-C18 H37 NC5 H5 + Cl− , Noctadecylpyridinium bromide, N-cetylpyridinium chloride, n-C16 H33 NC5 H5 + Cl− , and stearic acid were from Fluka (purity 99%). The samples of the purified nonionic surfactant nC12 H25 (OC2 H4 )12 OH and of oxyethylated anionic surfactant n-C12 H25 (OC2 H4 )3 OSO3 − Na+ were kindly put to our disposal by Dr. E.M. Gluzman and Dr. Yu.M. Bochkaryev, respectively. Hydrochloric, acetic and phosphoric acids, borax,
KOH, potassium chloride, sodium bromide, sodium chloride were analytical-grade reagents. Organic solvents were purified according to standard procedures. Absorption spectra of the dye solutions were measured using SF-46 apparatus (Russia) and Specord M 40 (Germany), against solvent blanks. pH determinations were performed with a glass electrode in a cell with liquid junction, with a P 37-1 potentiometer and pH-121 pH-meter (Russia). The fluorescence spectra were determined on Hitachi F 4010 fluorometer (Japan). The fluorescence decay was studied on a nanosecond pulse fluorometer, and then mathematically treated with the phase plane method [15]. Measurements were performed in a two-sectional Langmuir trough (6.5 cm × 60 cm), constructed in Research Institute of Organic Intermediates and Dyes, Moscow, Russia, which was used to obtain Langmuir–Blodgett films. The surface pressure, π, was measured with a Wilhelmy balance (±0.3 mN m−1 ). The dye was dissolved in stock aqueous surfactant solutions and diluted together with the surfactant. All spectra
N.O. Mchedlov-Petrossyan et al. / Spectrochimica Acta Part A 69 (2008) 1125–1129
were referenced against solvent blanks. Suitable pH values of solutions were created by HCl (pH ≤ 3.5) and by mixtures of sodium acetate (0.01 M) with acetic acid; higher pH values were obtained using phosphoric acid + NaOH mixtures, borate buffer solutions, and diluted sodium hydroxide. The required ionic strength was maintained by NaCl additions. Oilin-water microemulsions were prepared using benzene as oil, N-cetylpyridinium chloride or sodium laurylsulfate as surfactant, and n-pentanol as co-surfactant, as described earlier [22]. The pKaa values were determined spectrophotometrically at 25.0 ± 0.1 ◦ C according to the standard procedure [15–19]; in the case of N-octadecylpyridinium chloride micelles the temperature was 30 ◦ C, and the corresponding pH values of standard buffer solutions were used for graduation of the cell. The working concentrations of DODR in micellar solutions were (1–1.5) × 10−6 M, optical path lengths as a rule 5 cm. The differences of absorbances at 510 and 540 nm, or 520 and 550 nm were utilized for calculations because the dependence of such function versus pH is distinctly expressed. This procedure allowed to determine the pKaa values with satisfactory accuracy, notwithstanding the relatively poor resolution of the bands of the equilibrium species (Fig. 1). The samples of DODR chloride and perchlorate exhibited identical pKaa values. For preparing monolayers on water subphase, solutions of a surfactant (stearic acid or N-octadecylpyridinium bromide) with DODR were prepared in trichloromethane with concentration 10−3 and (0.5–3) × 10−4 M, respectively. The monolayers were spread onto twice-distilled water (pH 6.0) at 20 ◦ C. The state of monolayers on the water surface was studied by constructing compression isotherms, π versus A (A—area per molecule, ˚ 2 ). The dye-containing monolayers obtained on pure subphase A were deposited on glass and quartz supports. The plates with the deposited dye-containing multilayers were placed for 5 min in aqueous buffer solutions and HCl solutions with known pH values, and the spectra were recorded after drying several minutes on air. The films without dye, containing the same amounts of surfactants, were prepared in the same manner and used as
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blanks in the spectroscopic measurements. In experiments with multilayers, both the pH and spectral measurements were made at 20 ◦ C. 3. Results and discussion The determined pKaa values are compiled in Table 1. The dye molecules are certainly isolated from each other in micelles because under experimental conditions the number of micelles is (10–100)-fold or even higher than that of dye molecules. The stability of solutions confirms the complete binding of the waterinsoluble dye to micelles. The data presented in Table 1 demonstrate an adequate response of the pKaa values of DODR to the charge and Ψ sign of the micellar surface. Let us use these results for Ψ estimation of versatile interfaces by the relation Ψ = 59.16(pKai − pKaa ) for 25 ◦ C [derived from Eq. (1)]. The mostly used approach is based on using pKaa values in micelles of nonionic oxyethylated surfactants (here: 4.15) as the pKai value in ionic micelles [15–17,19,20]. Doing so, one can determine the Ψ values of +104 and +98 mV in micelles of N-octadecylpyridinium chloride and N-cetylpyridinium chloride, respectively, while in N-cetylpyridinium-based microdroplets Ψ = +96 mV. The value +64 mV for water/micelle interface at high bulk concentration of NaCl (0.40 M) demonstrates the strong screening of surface charge of cationic micelles by the electrolyte. In anionic surfactant micelles and microdroplets, the Ψ values are equal to −63 and −50 mV, respectively. Fig. 2 reflects the response of relative intensity of emission, Irel , on the variations of bulk pH in micellar solutions of nC12 H25 (OC2 H4 )3 OSO3 − Na+ . Analogous curves, with different pH values of inflection points, have been obtained in other lyophilic colloidal systems (not shown here). The small (and well reproducible) increase in emission intensity at pH < 3 is probably caused by some rearrangements of micelles, as a result of ion exchange between Na+ and H3 O+ (or H5 O2 + , etc.) cations in the micellar palisade.
Table 1 The pKaa values of the cation HR+ of N,N -dioctadecylrhodamine in different systems as obtained vis-spectroscopically, 25 ◦ C System
n-C18 H37 NC5 H5 + Cl− micellesa n-C16 H33 NC5 H5 + Cl− micellesb n-C16 H33 NC5 H5 + Cl− micellesb ‘Cationic’ microemulsionc Langmuir–Blodgett film based on n-C18 H37 − NC5 H5 + Br−d n-C12 H25 (OC2 H4 )12 OHb n-C12 H25 (OC2 H4 )3 OSO3 − Na+b Sodium laurylsulfateb ‘Anionic’ microemulsiong a b c d e f g
Bulk electrolytic background (M)
0.05 (NaCl + HCl) 0.05 (NaCl + HCl) 0.40 (NaCl + HCl) 0.05 (NaCl + HCl) 0.003–0.03 (HCl) 0.05 (NaCl + buffer)e 0.05 (NaCl + buffer)e 0.05 (NaCl + buffer)e 0.05 (NaCl + buffer)e
λabs max (nm)
pKaa
HR+
R±
533 533 533 532 550 531 532 532 532
528 528 528 527 542 525 528 528 528
Surfactant concentration 0.003 M, 30 ◦ C. Surfactant concentration 0.01 M. Benzene + n-C16 H33 NC5 H5 + Cl− + 1-pentanol, molar ratio 1:1:4, total volume fraction of the dispersed phase: 1.0%. 80 monolayers, dye fraction 5.6 mole%; 20 ◦ C. Acetate buffer solution. From emission intensity vs. pH dependence: 5.21 ± 0.08. Benzene + n-C12 H25 OSO3 − Na+ + 1-pentanol, molar ratio 4:1:7, total volume fraction of the dispersed phase: 1.0%.
2.40 2.48 3.06 2.53 2.4 4.15 5.22 5.21 5.00
± ± ± ± ± ± ± ± ±
0.02 0.14 0.02 0.07 0.2 0.02 0.03f 0.09 0.02
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Fig. 3. The compression isotherms of stearic acid monolayers with 0 mole% (1), 0.5 mole% (2), 1 mole% (3) and 2 mole% (4) of N,N -dioctadecylrhodamine; dye was used in form of perchlorate; aqueous subphase, pH 6.0. Fig. 2. The dependence of N,N -dioctadecylrhodamine fluorescence intensity at 547 nm on pH in micellar solutions of n-C12 H25 (OC2 H4 )3 OSO3 − Na+ (0.01 M); ionic strength 0.05 M (NaCl).
The excited state values, pKaa∗ , can be estimated by using the F¨orster–Weller cycle. For example, for the aforesaid micellar system, pKaa∗ = 5.52, while the pKaa value calculated using the Irel versus pH dependence (5.22 ± 0.08) is close of that obtained using absorbance versus pH curve (5.21 ± 0.03, Table 1). Indeed, the fluorescence lifetime values, τ, of cationic and zwitterionic forms of DODR coincide (τ ≈ 4 ns), being practically equal in cationic and anionic surfactant systems. Therefore the HR+ and R± species emit preferably before the equilibrium in the excited state is reached. Hence, pH1/2 values obtained from emission data coincide with pKaa in the ground state. The results obtained demonstrate that DODR can be applied for fluorometric estimation of the pH values. Especially, such a response of fluorescence on bulk acidity can be exploited in optical pH sensors basing on self-assembled systems. DODR does not form any stable monomolecular layer on the aqueous subphase. However, it was possible to retain the dye on water/air interface by using amphiphilic matrixes, namely by preparing mixed (DODR + surfactant) monolayers. These observations are in accord with the known fact for a similar dye, N,N -dioctadecylrhodamine B [9]. As surfactants, stearic acid and N-octadecylpyridinium bromide were used. Compression curves of mixed (stearic acid + DODR) monolayers are presented in Fig. 3. The values of limiting area per molecule, Sm , in (stearic acid + DODR) mixtures increase as compared with the pure matrix, thus indicating the presence of the hydrophobic dye in ˚ 2 for pure stearic acid, while for films. For example, Sm = 22.5 A dye contents of 0.49 and 2.0 mole%, the Sm values are equal to ˚ 2 , respectively. At the same time, the observed col24 and 27 A lapse pressure decreases from 56 mN m−1 in pure C17 H35 COOH monolayers to 55–53 mN m−1 at smallest dye contents, and drops up to π = 36 mN m−1 for 2 mole% of DODR in mixed monolayers. The (DODR + surfactant) monolayers were used for preparation of Langmuir–Blodgett dye-containing multi-layer films.
Both vis-absorption and emission spectra of such films (Fig. 4) demonstrate response to the pH values of the bulk aqueous phase, which they were soaked into. Namely, the λabs max values (±1 nm) in the both types of (DODR + surfactant) mixed (60–80)-monolayer films, with dye content from 2 to 7 mol%, equal to 550 and 542 nm as measured after soaking into aqueous solutions with pH 1.5 and 6.0, respectively. They correspond to the dye species HR+ and R± , respectively; the λem max values (±2 nm) are 584 and 579 nm, respectively, both in DODR + n-C17 H35 COOH and in DODR + N-octadecylpyridinium bromide films. The spectra are typified in Fig. 4; the 60-layer film was prepared at pH 6 and π = 30 mN m−1 film and sustained during 5 min in water at pH 5.8 (1 and 1 ) and in aqueous HCl solution with pH 0.9 (2 and 2 ). All the compression curves of monolayers, as well as vis-absorption and emission spectra of dye-containing Langmuir–Blodgett films were numerously reproduced. The spectrum of ‘acidic’ multilayer turns to that typical for ‘neutral’
Fig. 4. Normalized absorption (1 and 2) and emission (1 and 2 ) spectra of neutral (1 and 1 ) and cationic (2 and 2 ) N,N -dioctadecylrhodamine species in stearic acid Langmuir–Blodgett multilayers with dye content 2 mole%.
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colloidal surfactant aqueous solutions and microemulsions. The ‘apparent’ pKaa values can be converted into the (local) interfacial electrical potentials. 2. The dependence of light emission of the dye bound to micelles versus pH of the bulk water provides the possibility of using DODR also as a fluorescent interfacial indicator. 3. Langmuir–Blodgett surfactant-based DODR-containing multilayers are fabricated. The absorption and emission of these films are pH-sensitive. This can be used for creating sensor devices. Acknowledgement Fig. 5. The dependence of N,N -dioctadecylrhodamine absorption in n-C18 H37 − NC5 H5 + Br− Langmuir–Blodgett multilayers at λ = 550 nm and of λabs max values on pH of the aqueous phase; n = 80. The dye was used in form of chloride.
one after dipping into the buffer solution with corresponding pH, and is restored after putting into 0.1 M HCl aqueous solution. Gradual and reversible changes of absorbance and band position of these films along with acidity alteration of aqueous solutions, into which they are soaked, are depicted in Fig. 5. The multilayers were prepared at pH 6 and π = 25 mN m−1 , dye content 5.6 mole%. The absorbance versus pH curve can be also used for pKaa estimation, in the same manner as it was made in surfactant micelles. Furthermore, the Ψ value of 102 mV can be calculated for cationic surfactant films. The indicator immobilized in the wetted films can be regarded as placed into a water-organic mixed solvent or into self-assembled aggregate. However, the real pH values in the location sites of the indicator species in the wetted films can evidently differ from pH values of the aqueous buffers used in calculations of pKaa , and the latter are conventional, the more so, that the difference may be in general case inconstant. Moreover, the shape of the absorption curves in multilayers (Fig. 4) indicates the possibility of dye aggregation. Using the ratio of absorbances at the short-wavelength shoulder and at the principle maxima as a criterion of dimer/monomer ratio [11], one can compare the values determined in stearic acid and N-octadecylpyridinium films, 0.70 ± 0.15 and 0.73 ± 0.10, respectively, with those in micellar solutions of various surfactants, 2.9–3.5 (for HR+ ) and 3.0–3.6 (for R± ). This is not surprising, taking into account the very high DODR content in the films, contrary to that in surfactant micelles. Hence, the equilibrium in the above dye-containing Langmuir–Blodgett films is rather complicated. Nevertheless, the pH-response of these multilayers can be used in sensor devices. The purpose of this communication was only to demonstrate the possibilities of DODR as an acid–base indicator for examination of various interfaces. The detailed consideration of the pKaa values in a lot of other colloidal systems, as well as comparative analysis of the data, obtained with DODR and with other indicators, will be given elsewhere. 4. Conclusions 1. The fluorescent dye N,N -dioctadecylrhodamine, DODR, is proposed as an interfacial acid–base indicator for different
The authors are grateful to Dr. V.I. Alekseeva (Research Institute of Organic Intermediates and Dyes, Moscow, Russia) for her gift of N,N -dioctadecylrhodamine accompanied with a lot of valuable information about its properties. References [1] R.P. Haugland, Handbook of Fluorescent Probes and Research Products, ninth ed., Molecular Probes, Inc., Eugen, OR, 2002. [2] P. Reungpatthanaphong, S. Dechsupa, J. Meesungnoen, C. Loetchutinat, S. Mankhetkorn, J. Biochem. Biophys. Methods 57 (2003) 1. [3] L. Cavallo, M.H. Moore, J.E.T. Corrie, F. Fraternali, J. Phys. Chem. A 108 (2004) 7744. [4] H.L. Tavenier, F. Laine, M.D. Fayer, J. Phys. Chem. A 105 (2001) 8944. [5] S. Tsukahara, Y. Yamada, H. Watarai, Langmuir 16 (2000) 6787. [6] X.-Y. Zheng, A. Harata, T. Ogawa, Chem. Phys. Lett. 316 (2000) 6. [7] A. Miura, N. Tamai, Chem. Phys. Lett. 328 (2000) 23. [8] O. Slyadneva, A. Harata, Y. Hatano, J. Colloid Int. Sci. 260 (2003) 142. [9] N. Yonezawa, M. Ohno, S. Suga, K. Wataya, H. Hada, Nippon Kagaku Kaishi, J. Chem. Soc. Jpn. (1) (1987) 116. [10] N. Tamai, T. Yamazaki, I. Yamazaki, Chem. Phys. Lett. 147 (1988) 25. [11] P. Ballet, M. Van der Auweraer, F.C. De Schryver, H. Lemmetyinen, E. Vuorimaa, J. Phys. Chem. 100 (1996) 13701. [12] S.N. Shtykov, B.N. Klimov, G.Yu. Naumenko, G.V. Melnikov, T.D. Smirnova, E.G. Glukhovskoi, T.Yu. Rusanova, D.A. Gorin, Zh. Fiz. Khim. 73 (1999) 1689. [13] V.I. Alekseeva, N.Kh. Ibraev, E.A. Lukyanets, L.E. Marinina, L.P. Savvina, D.Zh. Satybaldina, Zh. Fiz. Khim. 73 (1999) 2217. [14] K.-I. Ishibashi, O. Sato, R. Baba, D.A. Tryk, K. Hashimoto, A. Fujishima, J. Colloid Int. Sci. 233 (2001) 361. [15] N.O. Mchedlov-Petrossyan, N.A. Vodolazkaya, A.O. Doroshenko, J. Fluoresc. 13 (2003) 235. [16] N. Funasaki, J. Phys. Chem. 83 (1979) 1998. [17] G.V. Hartland, F. Grieser, L.R. White, J. Chem. Soc., Faraday Trans. 83 (1987) 591. [18] N.O. Mchedlov-Petrossyan, A.V. Plichko, A.S. Shumakher, Chem. Phys. Rep. 15 (1996) 1661. [19] L.P. Loginova, L.V. Samokhina, N.O. Mchedlov-Petrossyan, V.I. Alekseeva, L.P. Savvina, Colloids Surf. A 193 (2001) 207. [20] F. Grieser, C.J. Drummond, J. Phys. Chem. 92 (1988) 5580 (and references cited therein). [21] G.P. Gorbenko, N.O. Mchedlov-Petrossyan, T.A. Chernaya, J. Chem. Soc., Faraday Trans. 94 (1998) 2117. [22] N.O. Mchedlov-Petrossyan, Yu.V. Isaenko, N.V. Salamanova, V.I. Alekseeva, L.P. Savvina, Zh. Anal. Khim. 58 (2003) 1140. [23] J.V. Moller, U. Kragh-Hansen, Biochemistry 14 (1975) 2317. [24] J.G. Petrov, D. M¨obius, Langmuir 6 (1990) 746. [25] N.O. Mchedlov-Petrossyan, V.I. Kukhtik, V.D. Bezugliy, J. Phys. Org. Chem. 16 (2003) 380. [26] I.S. Ioffe, I.F. Gofman, M.Z. Zalmanovich, Zh. Org. Khim. 1 (1965) 584.
Spectrochimica Acta Part A 69 (2008) 1125–1129
Fluorescent dye N,N-dioctadecylrhodamine as a new interfacial acid–base indicator N.O. Mchedlov-Petrossyan a,∗ , N.A. Vodolazkaya a , O.N. Bezkrovnaya b , A.G. Yakubovskaya a , A.V. Tolmachev b , A.V. Grigorovich a b
a V.N. Karazin Kharkov National University, 61077 Kharkov, Ukraine Institute for Single Crystals, National Academy of Sciences of Ukraine, 61001 Kharkov, Ukraine
Received 15 November 2006; received in revised form 13 June 2007; accepted 15 June 2007
Abstract This paper reports the spectral properties and protolytic behavior of the fluorescent dye N,N -dioctadecylrhodamine on the micelle/water and microdroplet/water interfaces as well as in Langmuir–Blodgett films soaked into aqueous media. Long hydrocarbon chains provide similar orientation of its cation and zwitterion, with the dissociating group (COOH → COO− ) directed toward the bulk (aqueous) phase. Both absorption and fluorescence of the dye can be used for monitoring electrical surface potentials and for determination of bulk pH. © 2007 Elsevier B.V. All rights reserved. Keywords: N,N -Dioctadecylrhodamine; Vis-spectroscopy; Fluorescence; Interfacial acid–base indicator; Surfactant micelles; Langmuir–Blodgett films
1. Introduction In this paper, we propose a hydrophobic rhodamine dye N,N dioctadecylrhodamine as a colored and fluorescing interfacial acid–base indicator. The extensive development of physical chemistry of interfaces requires new effective reporter molecules (molecular probes) for checking interfacial acidity, electrical potentials, polarity, etc. A lot of such probes are based on well-known dyes and luminophores, which are functionalized by introducing special groups, e.g., long hydrocarbon tails [1]. Such modifications (hydrophobization) ensure the fixation of the probes at due places, for instance, on water/micelle or water/air interface. Rhodamine dyes belong to mostly used molecular probes owing to their fluorescent properties [1–3]. The behavior of several hydrophobic rhodamines, especially their photophysical properties, is described in micelles [4], water/air and water/hydrocarbon interfaces [5–8], and in Langmuir–Blodgett films [9–14]. However, application of rhodamines as interfacial acid–base indicators is practically unknown [15]. The aim of ∗
Corresponding author. E-mail address: [email protected] (N.O. Mchedlov-Petrossyan). 1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.06.011
this paper is to report the means of rhodamine luminophores in this field of surface chemistry. The key characteristic of the acid–base indicator situated on the micelle/water interface is the so-called ‘apparent’ ionization constant, Kaa (pKaa = − log Kaa ). As a rule, it is obtained using absorption or emission spectra of the indicator, accompanied by determination of the pH value of the bulk (aqueous) phase with glass electrode [15–19]. The concentration of the reporter molecule is very small (as a rule, 10−5 M), and the probability of appearance of two molecules in one and the same micelle is close to zero. According to the well-known electrostatic model, the relation between the pKaa value of the probe on the charged micellar interface and the local electrical surface potential is as follows (Eq. (1)) [15–20]: pKaa = pKai −
ΨF 2.303RT
(1)
Here Kai is the so-called ‘intrinsic’ ionization constant, Ψ the local electrical surface potential (as a rule, regarded as the potential of Stern layer), F the Faraday constant, R the gas constant, and T is the absolute temperature. The pKaa values of indicators may be used for estimation of the electrical potential of the charged surface, Ψ , if correct evaluation of pKai is possible [15–20]. The same approach can be used for examination of vesicles [21], droplets of microemulsions
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Chart 1.
[22], polyelectrolytic biomolecules [23], or charged monolayers [24]. Recently we proposed the well-known fluorescent dye rhodamine B as an acid–base probe for micelle/water interface [15]. However, the above rhodamine dye is completely bound only by micelles of anionic surfactants and (at high enough surfactant concentrations) by those of nonionic ones [15]. In the case of cationic surfactants, the binding of the dye is incomplete. In this letter, we communicate the results of examination of another rhodamine dye, N,N -dioctadecylrhodamine (DODR) in form of HR+ X− (X− = Cl− or ClO4 − ) which is tightly embedded in the micelles of any colloidal surfactants due to its long hydrophobic tails:
The existence of two long hydrocarbon chains allows to expect the position of the probe on the surface of any ionic micelle as shown in Chart 1. Now we report the pKaa values of DODR in micellar solutions of different type of surfactants, in microemulsions, and in the Langmuir–Blodgett films and demonstrate the possibility of Ψ estimation with this reporter molecule. The visible spectra (Fig. 1) demonstrate that the formation of colorless lactone in such water-rich systems as direct micelles of surfactants are, is less probable [25]. So, we attribute the ionization constant to the (cation zwitterion + H+ ) equilibrium. 2. Experimental The dye in form of chloride and perchlorate samples was synthesized according to the described procedure [26] and kindly gifted to us by Dr. V.I. Alekseeva, Research Insti-
Fig. 1. Absorption (1 and 2) and emission (1 and 2 ) spectra of N,N -dioctadecylrhodamine acid–base couple in micellar solution of Ncetylpyridinium chloride (0.01 M), ionic strength 0.05 M (NaCl): cationic form (1 and 1 ) and neutral form (2 and 2 ).
tute of Organic Intermediates and Dyes, Moscow, Russia. The surfactant samples of laurylsulfate, n-C12 H25 OSO3 − Na+ , N-octadecylpyridinium chloride, n-C18 H37 NC5 H5 + Cl− , Noctadecylpyridinium bromide, N-cetylpyridinium chloride, n-C16 H33 NC5 H5 + Cl− , and stearic acid were from Fluka (purity 99%). The samples of the purified nonionic surfactant nC12 H25 (OC2 H4 )12 OH and of oxyethylated anionic surfactant n-C12 H25 (OC2 H4 )3 OSO3 − Na+ were kindly put to our disposal by Dr. E.M. Gluzman and Dr. Yu.M. Bochkaryev, respectively. Hydrochloric, acetic and phosphoric acids, borax,
KOH, potassium chloride, sodium bromide, sodium chloride were analytical-grade reagents. Organic solvents were purified according to standard procedures. Absorption spectra of the dye solutions were measured using SF-46 apparatus (Russia) and Specord M 40 (Germany), against solvent blanks. pH determinations were performed with a glass electrode in a cell with liquid junction, with a P 37-1 potentiometer and pH-121 pH-meter (Russia). The fluorescence spectra were determined on Hitachi F 4010 fluorometer (Japan). The fluorescence decay was studied on a nanosecond pulse fluorometer, and then mathematically treated with the phase plane method [15]. Measurements were performed in a two-sectional Langmuir trough (6.5 cm × 60 cm), constructed in Research Institute of Organic Intermediates and Dyes, Moscow, Russia, which was used to obtain Langmuir–Blodgett films. The surface pressure, π, was measured with a Wilhelmy balance (±0.3 mN m−1 ). The dye was dissolved in stock aqueous surfactant solutions and diluted together with the surfactant. All spectra
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were referenced against solvent blanks. Suitable pH values of solutions were created by HCl (pH ≤ 3.5) and by mixtures of sodium acetate (0.01 M) with acetic acid; higher pH values were obtained using phosphoric acid + NaOH mixtures, borate buffer solutions, and diluted sodium hydroxide. The required ionic strength was maintained by NaCl additions. Oilin-water microemulsions were prepared using benzene as oil, N-cetylpyridinium chloride or sodium laurylsulfate as surfactant, and n-pentanol as co-surfactant, as described earlier [22]. The pKaa values were determined spectrophotometrically at 25.0 ± 0.1 ◦ C according to the standard procedure [15–19]; in the case of N-octadecylpyridinium chloride micelles the temperature was 30 ◦ C, and the corresponding pH values of standard buffer solutions were used for graduation of the cell. The working concentrations of DODR in micellar solutions were (1–1.5) × 10−6 M, optical path lengths as a rule 5 cm. The differences of absorbances at 510 and 540 nm, or 520 and 550 nm were utilized for calculations because the dependence of such function versus pH is distinctly expressed. This procedure allowed to determine the pKaa values with satisfactory accuracy, notwithstanding the relatively poor resolution of the bands of the equilibrium species (Fig. 1). The samples of DODR chloride and perchlorate exhibited identical pKaa values. For preparing monolayers on water subphase, solutions of a surfactant (stearic acid or N-octadecylpyridinium bromide) with DODR were prepared in trichloromethane with concentration 10−3 and (0.5–3) × 10−4 M, respectively. The monolayers were spread onto twice-distilled water (pH 6.0) at 20 ◦ C. The state of monolayers on the water surface was studied by constructing compression isotherms, π versus A (A—area per molecule, ˚ 2 ). The dye-containing monolayers obtained on pure subphase A were deposited on glass and quartz supports. The plates with the deposited dye-containing multilayers were placed for 5 min in aqueous buffer solutions and HCl solutions with known pH values, and the spectra were recorded after drying several minutes on air. The films without dye, containing the same amounts of surfactants, were prepared in the same manner and used as
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blanks in the spectroscopic measurements. In experiments with multilayers, both the pH and spectral measurements were made at 20 ◦ C. 3. Results and discussion The determined pKaa values are compiled in Table 1. The dye molecules are certainly isolated from each other in micelles because under experimental conditions the number of micelles is (10–100)-fold or even higher than that of dye molecules. The stability of solutions confirms the complete binding of the waterinsoluble dye to micelles. The data presented in Table 1 demonstrate an adequate response of the pKaa values of DODR to the charge and Ψ sign of the micellar surface. Let us use these results for Ψ estimation of versatile interfaces by the relation Ψ = 59.16(pKai − pKaa ) for 25 ◦ C [derived from Eq. (1)]. The mostly used approach is based on using pKaa values in micelles of nonionic oxyethylated surfactants (here: 4.15) as the pKai value in ionic micelles [15–17,19,20]. Doing so, one can determine the Ψ values of +104 and +98 mV in micelles of N-octadecylpyridinium chloride and N-cetylpyridinium chloride, respectively, while in N-cetylpyridinium-based microdroplets Ψ = +96 mV. The value +64 mV for water/micelle interface at high bulk concentration of NaCl (0.40 M) demonstrates the strong screening of surface charge of cationic micelles by the electrolyte. In anionic surfactant micelles and microdroplets, the Ψ values are equal to −63 and −50 mV, respectively. Fig. 2 reflects the response of relative intensity of emission, Irel , on the variations of bulk pH in micellar solutions of nC12 H25 (OC2 H4 )3 OSO3 − Na+ . Analogous curves, with different pH values of inflection points, have been obtained in other lyophilic colloidal systems (not shown here). The small (and well reproducible) increase in emission intensity at pH < 3 is probably caused by some rearrangements of micelles, as a result of ion exchange between Na+ and H3 O+ (or H5 O2 + , etc.) cations in the micellar palisade.
Table 1 The pKaa values of the cation HR+ of N,N -dioctadecylrhodamine in different systems as obtained vis-spectroscopically, 25 ◦ C System
n-C18 H37 NC5 H5 + Cl− micellesa n-C16 H33 NC5 H5 + Cl− micellesb n-C16 H33 NC5 H5 + Cl− micellesb ‘Cationic’ microemulsionc Langmuir–Blodgett film based on n-C18 H37 − NC5 H5 + Br−d n-C12 H25 (OC2 H4 )12 OHb n-C12 H25 (OC2 H4 )3 OSO3 − Na+b Sodium laurylsulfateb ‘Anionic’ microemulsiong a b c d e f g
Bulk electrolytic background (M)
0.05 (NaCl + HCl) 0.05 (NaCl + HCl) 0.40 (NaCl + HCl) 0.05 (NaCl + HCl) 0.003–0.03 (HCl) 0.05 (NaCl + buffer)e 0.05 (NaCl + buffer)e 0.05 (NaCl + buffer)e 0.05 (NaCl + buffer)e
λabs max (nm)
pKaa
HR+
R±
533 533 533 532 550 531 532 532 532
528 528 528 527 542 525 528 528 528
Surfactant concentration 0.003 M, 30 ◦ C. Surfactant concentration 0.01 M. Benzene + n-C16 H33 NC5 H5 + Cl− + 1-pentanol, molar ratio 1:1:4, total volume fraction of the dispersed phase: 1.0%. 80 monolayers, dye fraction 5.6 mole%; 20 ◦ C. Acetate buffer solution. From emission intensity vs. pH dependence: 5.21 ± 0.08. Benzene + n-C12 H25 OSO3 − Na+ + 1-pentanol, molar ratio 4:1:7, total volume fraction of the dispersed phase: 1.0%.
2.40 2.48 3.06 2.53 2.4 4.15 5.22 5.21 5.00
± ± ± ± ± ± ± ± ±
0.02 0.14 0.02 0.07 0.2 0.02 0.03f 0.09 0.02
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Fig. 3. The compression isotherms of stearic acid monolayers with 0 mole% (1), 0.5 mole% (2), 1 mole% (3) and 2 mole% (4) of N,N -dioctadecylrhodamine; dye was used in form of perchlorate; aqueous subphase, pH 6.0. Fig. 2. The dependence of N,N -dioctadecylrhodamine fluorescence intensity at 547 nm on pH in micellar solutions of n-C12 H25 (OC2 H4 )3 OSO3 − Na+ (0.01 M); ionic strength 0.05 M (NaCl).
The excited state values, pKaa∗ , can be estimated by using the F¨orster–Weller cycle. For example, for the aforesaid micellar system, pKaa∗ = 5.52, while the pKaa value calculated using the Irel versus pH dependence (5.22 ± 0.08) is close of that obtained using absorbance versus pH curve (5.21 ± 0.03, Table 1). Indeed, the fluorescence lifetime values, τ, of cationic and zwitterionic forms of DODR coincide (τ ≈ 4 ns), being practically equal in cationic and anionic surfactant systems. Therefore the HR+ and R± species emit preferably before the equilibrium in the excited state is reached. Hence, pH1/2 values obtained from emission data coincide with pKaa in the ground state. The results obtained demonstrate that DODR can be applied for fluorometric estimation of the pH values. Especially, such a response of fluorescence on bulk acidity can be exploited in optical pH sensors basing on self-assembled systems. DODR does not form any stable monomolecular layer on the aqueous subphase. However, it was possible to retain the dye on water/air interface by using amphiphilic matrixes, namely by preparing mixed (DODR + surfactant) monolayers. These observations are in accord with the known fact for a similar dye, N,N -dioctadecylrhodamine B [9]. As surfactants, stearic acid and N-octadecylpyridinium bromide were used. Compression curves of mixed (stearic acid + DODR) monolayers are presented in Fig. 3. The values of limiting area per molecule, Sm , in (stearic acid + DODR) mixtures increase as compared with the pure matrix, thus indicating the presence of the hydrophobic dye in ˚ 2 for pure stearic acid, while for films. For example, Sm = 22.5 A dye contents of 0.49 and 2.0 mole%, the Sm values are equal to ˚ 2 , respectively. At the same time, the observed col24 and 27 A lapse pressure decreases from 56 mN m−1 in pure C17 H35 COOH monolayers to 55–53 mN m−1 at smallest dye contents, and drops up to π = 36 mN m−1 for 2 mole% of DODR in mixed monolayers. The (DODR + surfactant) monolayers were used for preparation of Langmuir–Blodgett dye-containing multi-layer films.
Both vis-absorption and emission spectra of such films (Fig. 4) demonstrate response to the pH values of the bulk aqueous phase, which they were soaked into. Namely, the λabs max values (±1 nm) in the both types of (DODR + surfactant) mixed (60–80)-monolayer films, with dye content from 2 to 7 mol%, equal to 550 and 542 nm as measured after soaking into aqueous solutions with pH 1.5 and 6.0, respectively. They correspond to the dye species HR+ and R± , respectively; the λem max values (±2 nm) are 584 and 579 nm, respectively, both in DODR + n-C17 H35 COOH and in DODR + N-octadecylpyridinium bromide films. The spectra are typified in Fig. 4; the 60-layer film was prepared at pH 6 and π = 30 mN m−1 film and sustained during 5 min in water at pH 5.8 (1 and 1 ) and in aqueous HCl solution with pH 0.9 (2 and 2 ). All the compression curves of monolayers, as well as vis-absorption and emission spectra of dye-containing Langmuir–Blodgett films were numerously reproduced. The spectrum of ‘acidic’ multilayer turns to that typical for ‘neutral’
Fig. 4. Normalized absorption (1 and 2) and emission (1 and 2 ) spectra of neutral (1 and 1 ) and cationic (2 and 2 ) N,N -dioctadecylrhodamine species in stearic acid Langmuir–Blodgett multilayers with dye content 2 mole%.
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colloidal surfactant aqueous solutions and microemulsions. The ‘apparent’ pKaa values can be converted into the (local) interfacial electrical potentials. 2. The dependence of light emission of the dye bound to micelles versus pH of the bulk water provides the possibility of using DODR also as a fluorescent interfacial indicator. 3. Langmuir–Blodgett surfactant-based DODR-containing multilayers are fabricated. The absorption and emission of these films are pH-sensitive. This can be used for creating sensor devices. Acknowledgement Fig. 5. The dependence of N,N -dioctadecylrhodamine absorption in n-C18 H37 − NC5 H5 + Br− Langmuir–Blodgett multilayers at λ = 550 nm and of λabs max values on pH of the aqueous phase; n = 80. The dye was used in form of chloride.
one after dipping into the buffer solution with corresponding pH, and is restored after putting into 0.1 M HCl aqueous solution. Gradual and reversible changes of absorbance and band position of these films along with acidity alteration of aqueous solutions, into which they are soaked, are depicted in Fig. 5. The multilayers were prepared at pH 6 and π = 25 mN m−1 , dye content 5.6 mole%. The absorbance versus pH curve can be also used for pKaa estimation, in the same manner as it was made in surfactant micelles. Furthermore, the Ψ value of 102 mV can be calculated for cationic surfactant films. The indicator immobilized in the wetted films can be regarded as placed into a water-organic mixed solvent or into self-assembled aggregate. However, the real pH values in the location sites of the indicator species in the wetted films can evidently differ from pH values of the aqueous buffers used in calculations of pKaa , and the latter are conventional, the more so, that the difference may be in general case inconstant. Moreover, the shape of the absorption curves in multilayers (Fig. 4) indicates the possibility of dye aggregation. Using the ratio of absorbances at the short-wavelength shoulder and at the principle maxima as a criterion of dimer/monomer ratio [11], one can compare the values determined in stearic acid and N-octadecylpyridinium films, 0.70 ± 0.15 and 0.73 ± 0.10, respectively, with those in micellar solutions of various surfactants, 2.9–3.5 (for HR+ ) and 3.0–3.6 (for R± ). This is not surprising, taking into account the very high DODR content in the films, contrary to that in surfactant micelles. Hence, the equilibrium in the above dye-containing Langmuir–Blodgett films is rather complicated. Nevertheless, the pH-response of these multilayers can be used in sensor devices. The purpose of this communication was only to demonstrate the possibilities of DODR as an acid–base indicator for examination of various interfaces. The detailed consideration of the pKaa values in a lot of other colloidal systems, as well as comparative analysis of the data, obtained with DODR and with other indicators, will be given elsewhere. 4. Conclusions 1. The fluorescent dye N,N -dioctadecylrhodamine, DODR, is proposed as an interfacial acid–base indicator for different
The authors are grateful to Dr. V.I. Alekseeva (Research Institute of Organic Intermediates and Dyes, Moscow, Russia) for her gift of N,N -dioctadecylrhodamine accompanied with a lot of valuable information about its properties. References [1] R.P. Haugland, Handbook of Fluorescent Probes and Research Products, ninth ed., Molecular Probes, Inc., Eugen, OR, 2002. [2] P. Reungpatthanaphong, S. Dechsupa, J. Meesungnoen, C. Loetchutinat, S. Mankhetkorn, J. Biochem. Biophys. Methods 57 (2003) 1. [3] L. Cavallo, M.H. Moore, J.E.T. Corrie, F. Fraternali, J. Phys. Chem. A 108 (2004) 7744. [4] H.L. Tavenier, F. Laine, M.D. Fayer, J. Phys. Chem. A 105 (2001) 8944. [5] S. Tsukahara, Y. Yamada, H. Watarai, Langmuir 16 (2000) 6787. [6] X.-Y. Zheng, A. Harata, T. Ogawa, Chem. Phys. Lett. 316 (2000) 6. [7] A. Miura, N. Tamai, Chem. Phys. Lett. 328 (2000) 23. [8] O. Slyadneva, A. Harata, Y. Hatano, J. Colloid Int. Sci. 260 (2003) 142. [9] N. Yonezawa, M. Ohno, S. Suga, K. Wataya, H. Hada, Nippon Kagaku Kaishi, J. Chem. Soc. Jpn. (1) (1987) 116. [10] N. Tamai, T. Yamazaki, I. Yamazaki, Chem. Phys. Lett. 147 (1988) 25. [11] P. Ballet, M. Van der Auweraer, F.C. De Schryver, H. Lemmetyinen, E. Vuorimaa, J. Phys. Chem. 100 (1996) 13701. [12] S.N. Shtykov, B.N. Klimov, G.Yu. Naumenko, G.V. Melnikov, T.D. Smirnova, E.G. Glukhovskoi, T.Yu. Rusanova, D.A. Gorin, Zh. Fiz. Khim. 73 (1999) 1689. [13] V.I. Alekseeva, N.Kh. Ibraev, E.A. Lukyanets, L.E. Marinina, L.P. Savvina, D.Zh. Satybaldina, Zh. Fiz. Khim. 73 (1999) 2217. [14] K.-I. Ishibashi, O. Sato, R. Baba, D.A. Tryk, K. Hashimoto, A. Fujishima, J. Colloid Int. Sci. 233 (2001) 361. [15] N.O. Mchedlov-Petrossyan, N.A. Vodolazkaya, A.O. Doroshenko, J. Fluoresc. 13 (2003) 235. [16] N. Funasaki, J. Phys. Chem. 83 (1979) 1998. [17] G.V. Hartland, F. Grieser, L.R. White, J. Chem. Soc., Faraday Trans. 83 (1987) 591. [18] N.O. Mchedlov-Petrossyan, A.V. Plichko, A.S. Shumakher, Chem. Phys. Rep. 15 (1996) 1661. [19] L.P. Loginova, L.V. Samokhina, N.O. Mchedlov-Petrossyan, V.I. Alekseeva, L.P. Savvina, Colloids Surf. A 193 (2001) 207. [20] F. Grieser, C.J. Drummond, J. Phys. Chem. 92 (1988) 5580 (and references cited therein). [21] G.P. Gorbenko, N.O. Mchedlov-Petrossyan, T.A. Chernaya, J. Chem. Soc., Faraday Trans. 94 (1998) 2117. [22] N.O. Mchedlov-Petrossyan, Yu.V. Isaenko, N.V. Salamanova, V.I. Alekseeva, L.P. Savvina, Zh. Anal. Khim. 58 (2003) 1140. [23] J.V. Moller, U. Kragh-Hansen, Biochemistry 14 (1975) 2317. [24] J.G. Petrov, D. M¨obius, Langmuir 6 (1990) 746. [25] N.O. Mchedlov-Petrossyan, V.I. Kukhtik, V.D. Bezugliy, J. Phys. Org. Chem. 16 (2003) 380. [26] I.S. Ioffe, I.F. Gofman, M.Z. Zalmanovich, Zh. Org. Khim. 1 (1965) 584.