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Langmuir 1998, 14, 1554-1559
Water-Soluble Poly(2-cinnamoylethyl methacrylate)-block-poly(acrylic acid) Nanospheres as Traps for Perylene Guochang Wang,† Fred Henselwood, and Guojun Liu* Department of Chemistry, The University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 Received July 10, 1997. In Final Form: December 5, 1997 Poly(2-cinnamoylethyl methacrylate)-block-poly(acrylic acid) (PCEMA-b-PAA) nanospheres are soluble in water. The lightly cross-linked PCEMA cores of these nanospheres can uptake perylene, a polycyclic aromatic hydrocarbon (PAH), from water. Reported are the rate of perylene incorporation into the nanosphere cores and the coefficient of perylene partition between the cores and water. Also the nanospheres were shown to precipitate completely with the addition of sufficient divalent cations such as Ca2+ and redisperse with EDTA which complexes with Ca2+. These nanospheres should be useful in concentrating PAHs present in trace amounts in water for chemical analysis or in the reclamation of waters contaminated by PAHs.
I. Introduction In a previous paper in this series,1 we described the synthesis of a poly(2-cinnamoylethyl methacrylate)-blockpoly(acrylic acid) (PCEMA-b-PAA) sample with 4.5 × 102 units of CEMA and 1.84 × 102 units of AA:
are the capacity of nanosphere 2 in uptaking perylene and the effectiveness of Ca2+ and EDTA in inducing nanosphere 2 precipitation and redispersion. Since the determination of K is not straightforward, we will describe this in detail in the next section. The experimental procedures will be presented in Section III. The results will reported in Section IV. Some conclusions are drawn in Section V. II. Perylene Partition Equilibrium
The polymer formed spherical micelles with PAA as the shell and PCEMA as the core in DMF/water with 20% DMF by volume. Photolyzing such micelles with UV light cross-linked the PCEMA cores to yield crosslinked micelles or nanospheres.2 These nanospheres (nanosphere 1) were stable in water and sorbed a large amount of organic compounds such as toluene and DMSO. More interestingly, the nanospheres could be precipitated out with the trapped organic compounds by the addition of divalent cations such as Ca2+ and be redispersed with the addition of a complexing reagent such as EDTA or a precipitant such as CO32- for Ca2+. We thus proposed the potential use of these nanospheres in concentrating organic compounds present in trace amounts in water for chemical analysis or in the reclamation of waters contaminated by organic compounds such as polycyclic aromatic hydrocarbons (PAHs). In this paper, we report the rate and equilibrium constant, K, for the uptake of perylene, one PAH, by nanosphere 1 in water. Due to the relatively short PAA block, nanosphere 1 was not stable in water for prolonged periods of time, for example weeks. We have thus prepared nanosphere 2 from a PCEMA-b-PAA sample with 3.6 × 102 units of CEMA and 5.6 × 102 units of AA. These nanospheres are directly soluble in water. Also reported † On leave from The Institute of Polymer Chemistry, Nankai University, Tianjin, China 300071.
(1) Henselwood, F.; Liu, G. Macromolecules 1997, 30, 488. (2) Guo, A.; Liu, G.; Tao, J. Macromolecules 1996, 29, 2487.
Literature Review. There have been many studies of the partition equilibrium of organic compounds between water and surfactant3-5 or block copolymer6-9 micellar cores. The tendency for an organic compound to get into micellar cores has been traditionally characterized by the partition coefficient
K)
cc cw
(1)
where cw and cc are the concentrations of the organic compound in water and in the micellar core at partition equilibrium. The methods used for determining cw and cc for evaluating K included electrochemical techniques,10-11 the differential spectroscopic method,12-14 radioactivity (3) Turro, N. J.; Gra¨tzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980, 19, 675. (4) Attwood, D.; Florence, A. T. Surfactant Systems. Their Chemistry, Pharmacy and Biology; Chapman and Hall: London, 1983. (5) (a) Tachiya, M. In Kinetics of Nonhomogeneous Processes; Freeman, G. R., Ed.; John Wiley & Sons: New York, 1987. (b) Barzykin, A. V.; Tachiya, M. Heterog. Chem. Rev. 1996, 3, 105. (6) Nagarajan, R. In Solvents and Self-Organization of Polymers; Webber, S. E., Munk, P., Tuzar, Z., Eds.; NATO ASI Series; Kluwer Academic Publishers: Dorfrecht, 1996. (7) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986, 2, 210. (8) Wilhelm, M.; Zhao, C.-L.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J.-L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033. (9) Prochazka, K.; Martin, T. J.; Munk, P.; Webber, S. E. Macromolecules 1996, 29, 6518. (10) Manabe, M.; Kawamura, H.; Kondo, S.; Kojima, M.; Tokunaga, S. Langmuir 1990, 6, 1596. (11) Mandal, A. B.; Nair, B. U. J. Phys. Chem. 1991, 95, 9008. (12) Loh, W.; Volpe, P. L. O. J. Colloid Interface Sci. 1992, 154, 369. (13) Kawamura, H.; Manabe, M.; Miyamoto, Y.; Fujita, Y.; Tokunaga, S. J. Phys. Chem. 1989, 93, 5536.
S0743-7463(97)00769-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/18/1998
Nanospheres as Traps for Perylene
Langmuir, Vol. 14, No. 7, 1998 1555
measurements,15 and fluorescence spectroscopy.3,5,8-9,16-21 Of the fluorescence techniques, the quenching method is popular.3,5,9,16-17 In such cases, a water-soluble quencher is frequently used. At sufficiently high quencher concentrations, the fluorescence of an organic chromophore in the aqueous phase is assumed to be completely quenched and that in the micellar cores is unaffected. By ratioing fluorescence intensities in the presence and absence of the quencher, cw and cc can be obtained. At a fixed fluorophore concentration, the addition of a surfactant which makes up the micelle normally increases the quantum yield of the chromophore, because fluorescence quenching by oxygen decreases in the core of a micelle and the rigidity in a micellar core also helps suppress some nonradiative decay processes of the chromophore. Following this fluorescence intensity increase with the addition of the surfactant, one should be able to determine the partition coefficient of the chromophore between water and the micellar cores. This has been done by Nowakowska et al.19-21 and Wilhelm et al.8 Our approach will be similar to those of Nowakowska et al or Wilhelm et al.8 but with differences in data treatment. For this, we will next show the derivation of an expression for perylene fluorescence intensity, I(R), at a constant perylene concentration as a function of nanosphere concentration, cN, in grams of nanospheres per unit volume of solvent. System Description. We assume that the hydrophobic cores of the nanospheres are not swollen by water and that the density of the core substance is F. Let the weight fraction of the core of the nanosphere be fc. The volume fraction R of the nanosphere core in the aqueous solution is
R ) cNfc/F
(2)
Since the corona of a micelle or a nanosphere is hydrophilic and the polymer density there is generally low,22 we assume the properties, such as solubility and fluorescence quantum yield, of perylene in the corona are the same as those in bulk water. The total volume fraction of bulk water and coronas is thus 1 - R. The perylene concentration is assumed to be low, and the flurophore is completely solubilized in water or in water and nanosphere mixtures. Let the quantum yield and molar extinction coefficient of perylene in water be φw and w and those in the nanosphere cores be φc and c. At partition equilibrium, the masses of perylene in the aqueous and hydrophobic phases are assumed to be mw and mc. Light Absorbed by the Illuminated Volume. Following the Beer-Lambert law, the amount of light absorbed, IA, by the illuminated volume of a sample should be
IA ) I0(1 - 10-(wmw+cmc)l/V0)
(3)
(14) Kawamura, H.; Manabe, M.; Tokunoh, T.; Saiki, H.; Tokunaga, S. J. Solution Chem. 1991, 20, 817. (15) Edwards, D. A.; Luthy, R. G.; Liu, Z. Environ. Sci. Technol. 1991, 25, 127. (16) Viseu, M. I.; Costa, M. B. Chem. Phys. Lett. 1990, 175, 43. (17) Auger, R. L.; Jacobson, A. M.; Domach, M. M. Environ. Sci. Technol. 1995, 29, 1273. (18) Abuin, E. B.; Lissi, E. A. J. Colloid Interface Sci. 1983, 95, 369. (19) Nowakowska, M.; White, B.; Guillet, J. E. Macromolecules 1989, 22, 2317. (20) White, B.; Nowakowska, M.; Vancso, G. J.; Guillet, J. E. Macromolecules 1991, 24, 2903. (21) Nowakowska, M.; White, B.; Guillet, J. E. Macromolecules 1990, 23, 3375. (22) Tuzar, Z.; Kratochvil, P. Surf. Colloid Sci. 1993, 15, 1.
where l is the length of the illuminated volume and V0 is the total volume of the sample. Of the absorbed light, the fraction absorbed by perylene solubilized in the aqueous phase is
ζw ≈
1 - 10-wlmw/V0 (1 - 10-wlmw/V0) + (1 - 10-clmc/V0)
(4)
for dilute solutions for which the absorbance is low. The fraction absorbed by fluorophores solubilized in the hydrophobic phase is ζc ) 1 - ζw. Fluorescence Intensity from the Illuminated Volume. The fluorescence intensity from the illuminated volume should be
I(R) ≈ IA(φwζw + φcζc)
(5)
where “≈” instead of “)” was used to justify the omission of a proportionality constant. Most PAHs have low solubility in water, and their absorbances in water with l typically around 0.5 cm (for a 1.00-cm cell) are close to 0. The exponential terms in eqs 3 and 4 can thus be approximated by the first-order terms upon their expansion. Inserting eqs 3 and 4 into eq 5 and simplifying yield
I(R) ≈ φwwmw + φccmc
(6)
where constants like V0, l, and I0 have been omitted. By mass balance, we have
m 0 ) mw + m c
(7)
where m0 is the total mass of fluorophore in the sample. According to eq 1,
K)
mc/R mw/(1 - R)
(8)
Insertion of eqs 7 and 8 into eq 6 and rearrangement yield
I(R) )
1-R KR I + I KR + 1 - R ∞ KR + 1 - R 0
(9)
where I∞ and I0, proportional to φccm0 and φwwm0, are the fluorescence intensities of the sample when all the fluorophores are in the hydrophobic (R ) 1) and aqueous (R ) 0) phases, respectively. Correlating the experimental I(R) data with eq 9 should yield I∞/I0 and K. III. Experimental Section Fluorescence Measurements. All fluorescence measurements were carried out on a Photon Technology International Alpha Scan system equipped with a 75-W xenon lamp. Excitation and emission spectra were reported as they were recorded without correcting for wavelength-dependent lamp emission efficiency and photomultiplier tube response. Excitation spectra were obtained by monitoring emission at 475 nm. Emission spectra were obtained by exciting at 410 nm. For following the kinetics of perylene insertion into nanosphere or micelle cores, a bandpass filter with the transmission efficiency of 18% at 410 nm was added to the excitation side. Preparation of Aqueous Perylene Solution. Solid perylene was sonicated in 250 mL of water for 0.5 h. The mixture was then allowed to settle over a 3-day period. After the mixture was centrifuged at 4.5 × 103 rpm for 0.5 h, the solid particles were removed by filtration once through a filter paper and once through two nylon filters (pore size ) 0.45 µm, Chromatography Specialties) connected in series. The filtrate was diluted to a final volume of 300 mL. By comparing the fluorescence intensity of this sample with that of a sample at 2.0 × 10-8 M, prepared by adding a known amount of an acetone solution of perylene into water
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Table 1. Characteristics of the PCEMA-b-PAA Sample Used
Table 2. Characteristics of the Nanospheres Used
nanosphere
n/ma
fc
10-4M h w (g/mol)a
10-2n
10-2m
nanosphere
CEMA conv (%)
h w/g mol-1 10-5M from LS
RG/nm from LS
1 2
2.5 0.65
0.90 0.70
13.1 13.4
4.5 3.6
1.84 5.6
1 2
15 20
75
34
a The values of n/m and M h w were obtained by NMR and light scattering, respectively, as described in ref 1.
(acetone content was less than 0.1% of the final volume), we obtained a perylene concentration of 1.9 × 10-8 M for the aqueous sample. Despite the fact that the perylene concentration is higher than the reported solubility of 2 × 10-9 M,23 the solution appeared homogeneous, because the fluorescence intensities of the samples taken from the top and bottom of the container were the same. Nanosphere Solution. Nanosphere 1, 25.0 mg, was dissolved by heating in 5.0 mL of DMSO at 80 °C overnight. After cooling, 10 mL of water was added. The solution mixture was then transported into a dialysis tube (Spectrum Medical Industries, Inc.) and dialyzed in a 1000-mL beaker against distilled water for 3 days. The distilled water was replenished constantly by leaving a small stream running. The dialyzed solution was then transferred to a volumetric flask and diluted to a final volume of 50.0 mL. Nanosphere 2 was soluble in warm water, and its solutions were prepared directly in water. Partition Equilibrium Studies. Nanosphere solutions at different concentrations were prepared by diluting their bulk solutions. For partition equilibrium studies, 1.00 mL of a nanosphere solution was mixed with 2.00 mL of the aqueous perylene solution. The mixture was stirred for 3-5 days before fluorescence intensity measurement. In all cases, 1.00 mL of a nanosphere solution was always mixed with 2.00 mL of water to yield a control. The perylene fluorescence intensity was then obtained by subtracting the fluorescence intensity of the control from that of the perylene/nanosphere solution. Perylene Pickup Kinetics. After mixing 1.00 mL of a nanosphere 1 solution with 2.00 mL of the perylene solution, the fluorescence intensity change at 475 nm was continuously monitored for 10-12 h with the excitation wavelength fixed at 410 nm. Nanosphere Capacity Measurement. We followed the method of Nawakowska et al.19 for evaluating the capacity of nanosphere 2 in uptaking perylene. Different volumes of a perylene solution in acetone, 2.10 mg/L, were added to clean sample vials. The acetone was evaporated, and to each vial was added 4.00 mL of a nanosphere 2 solution at cN ) 6.8 × 10-2 mg/mL. The vials were subsequently capped, wrapped in aluminum foil, and stirred for 3 weeks. Perylene fluorescence intensity was measured after the samples were centrifuged at 1500 rpm for 10 min to precipitate perylene particles not solubilized. Nanosphere 2 Precipitation. A solution of the nanospheres saturated with perylene was centrifuged, and 2.50 mL of the supernatant was taken. To it was added different volumes of a 0.100 M CaCl2 solution in 10-µL intervals. After each addition, the sample was stirred for 2 min, centrifuged for 10 min, and measured for its fluorescence intensity.
IV. Results and Discussion Samples Used. The procedures used for preparing and characterizing PCEMA-b-PAA and their nanospheres have been described in detail in a previous paper.1 Both types of nanospheres have narrow size distributions, but nanosphere 2 is directly soluble in water due to the longer PAA block. The PCEMA cross-linking densities for nanospheres 1 and 2 are ∼15% and ∼20%, respectively. Other characteristics of the polymer and the nanospheres are shown in Tables 1 and 2. Fluorescence Properties of Perylene. Illustrated in Figure 1 are the raw fluorescence emission and excitation spectra of perylene in water and in an aqueous (23) Pearlman, R.; Yalkowsky, S. H. J. Phys. Chem. Ref. Data 1984, 13, 555.
a
Rh/nm from LSa 42 ( 2 55 ( 2
Determined in DMSO.
Figure 1. Uncorrected excitation (left) and emission (right) spectra of perylene in water (lower curves) and in an aqueous nanosphere 1 solution (upper curves) with cN ) 2.09 × 10-5 g/mL. Excitation spectra were obtained by monitoring emission at 475 nm.
Figure 2. Uncorrected excitation (left) and emission (right) spectra of nanosphere 1. The excitation spectrum was obtained for a sample with cN ) 1.67 × 10-4 g/mL and with the emission wavelength set at 475 nm. The emission spectra (from bottom to top) were obtained with the excitation wavelength fixed at 410 nm for samples with cN ) 0.130 × 10-5, 0.52 × 10-5, and 2.09 × 10-5 g/mL, respectively.
nanosphere 1 solution with cN ) 2.09 × 10-5 g/mL. In the aqueous nanosphere solution, the perylene emission intensity increased. This is expected because perylene should have a higher fluorescence quantum yield in the nanosphere cores than in water. Both the excitation and emission spectra varied when the solvating medium changed from water to the dilute nanosphere solution. First, the spectra red-shifted. Then, the relative intensities of the different peaks changed. Our investigation revealed that this band-shape change was largely caused by the interference from water and nanosphere background emission or scattering. Background Correction. Since our main interest was in fluorescence emission intensities, we did not attempt the background correction for the excitation spectra. That both water and nanospheres contributed to emission and absorption is obvious from Figure 2. In pure water, the
Nanospheres as Traps for Perylene
Figure 3. Emission spectrum of perylene in an aqueous nanosphere 1 solution with cN ) 2.09 × 10-5 g/mL (top) and that of the nanosphere solution (bottom). The difference of the two (middle) was integrated between 435 and 600 nm to obtain the fluorescence intensity for perylene.
Figure 4. Perylene fluorescence intensity versus nanosphere 1 concentration at a constant perylene concentration of 1.25 × 10-8 M. From bottom to top: cN ) 0, cN ) 0.13 × 10-5, cN ) 0.52 × 10-5, and 2.09 × 10-5 g/mL. All spectra are shown after background correction.
peak at 475 nm was derived from Raman scattering.24 In nanosphere solutions, the emission consisted of the water Raman scattering peak and a broad band of the nanospheres centered around 500 nm. The procedure for correcting for background emission is illustrated in Figure 3. The emission spectrum of perylene was obtained by subtracting that of a control from that of a perylene/nanosphere solution, where the control consisted of a nanosphere solution of the same concentration as that in the perylene/nanosphere mixture. This procedure worked well, because perylene emission spectra obtained from this procedure all had intensity values close to zero at 580 nm even at cN ) 1.6 × 10-4 g/mL (Figure 4). After background correction, the relative intensities for three emission peaks of perylene remained approximately constant with the solvating medium change (Figure 4). The red shift for the 440-nm peak in water was 8.5 nm, and that for the 468-nm peak was 9.5 nm. Partition Coefficient. The fluorescence spectra of perylene at different nanosphere 1 concentrations cN were obtained and corrected for background emission. The cN values were then used to calculate R using eq 2 and fc ) 0.90 by assuming F ) 1.0 g/cm3. Perylene fluorescence intensity was obtained as the integrated area under the
Langmuir, Vol. 14, No. 7, 1998 1557
Figure 5. Plot of perylene fluorescence intensity (after background subtraction) as a function of the core volume fraction, R, for nanosphere 1 (b). The solid curve represents the best fit to experimental data by eq 9. The perylene concentration remained constant at 1.25 × 10-8 M in all cases.
spectrum between 435 and 600 nm. Since the fluorescence spectra shifted when the solvating medium changed from water to nanosphere cores (Figure 4), fluorescence intensities evaluated this way may have some uncertainty. Illustrated in Figure 5 are the I(R) versus R data for perylene. Correlating the data with eq 9 yielded K ) 3.3 × 105, which agrees well with 3 × 105, a value determined by Wilhelm et al.8 for pyrene partition between water and the PS cores of PS-b-PEO micelles. The significance of this K value is that 3.0 × 10-6/0.90 or 3.3 × 10-6 g of the nanospheres in 1 mL of water can extract half of the perylene from water, no matter how low the original perylene concentration is. Fitting the I(R) data also yielded I∞/I0 ) 7.0. Theoretically, this ratio is equal to cφc/(wφw). Since the excitation wavelength used was always 410 nm, the position of the perylene absorption peak maximum in water, and the maximum red-shifted to 413 nm when perylene was solubilized in the nanosphere cores, we expect c/w to be slightly less than 1.0 and φc/φw to be somewhat larger than 7.0. Patterson and Vieil25 determined lifetimes of ∼0.5 ns and 5.0 ( 0.2 ns for perylene in water and in the core of micelles of a small molecule surfactant. Our φc/φw value of 7 is in good agreement with the lifetime ratio determined by Patterson and Vieil. The correlation coefficient generated from fitting I(R) data by eq 9 was 0.993. This and the reasonable K and I∞/I0 values suggest the validity of our data treatment method and the assumptions made in this treatment. Rate for Perylene Partition. Having established the thermodynamic feasibility of using the nanospheres as traps for perylene, we next went to examine the kinetic feasibility. Illustrated in Figure 6 is the increase in the perylene fluorescence intensity at 475 nm with time after mixing with nanosphere 1. Under identical instrumental settings and an equal perylene concentration, the fluorescence intensity of a perylene solution in water did not increase significantly with continuous radiation by the excitation source. The increase in the fluorescence intensity of perylene after the addition of nanospheres suggested the gradual insertion of perylene into the cores of the nanospheres. The fluorescence intensity increase data ∆I(t) could be fitted by
∆I(t) ) a0 - a1 exp(-t/τ1) - a2 exp(-t/τ2)
(10)
A measure of the rate of perylene incorporation was (24) For concept on Raman scatteing, see, for example: Atkins, P. Physical Chemistry, 5th ed.; Freeman: New York, 1994.
(25) Patterson, L. K.; Vieil, E. J. Phys. Chem. 1973, 77, 1191.
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Figure 6. Fluorescence intensity of a perylene solution in water with continuous irradiation by the excitation source under identical instrumental settings and equal perylene concentrations (bottom). The fluorescence intensity of perylene increased after the addition of a nanosphere 1 (top) sample at the aftermixing concentration of 5.6 × 10-5 g/mL. Table 3. Parameters Generated from Fitting Kinetic Data of Perylene Uptake by PCEMA-b-PAA Nanospheres 102cN/ (mg/mL)
a1
10-3τ1/s
a2
10-4τ2/s
10-4〈τ〉/s
Rval
0.67 1.11 1.90 5.6
0.24 0.26 0.31 0.21
1.84 1.44 1.69 3.6
0.73 0.72 0.63 0.77
3.4 1.55 1.58 2.20
2.60 1.18 1.11 1.81
0.998 0.999 0.999 1.000
obtained from the average partition time:
〈τ〉 )
a1τ1 + a2τ2 a 1 + a2
(11)
We measured the partition time at four different nanosphere concentrations. The shortest 〈τ〉 value was 1.11 × 104 s or 3.1 h. The surprising result is that 〈τ〉 did not decrease monotonically with increasing cN (Table 3). An explanation of this observation may require the solution of the kinetic equation for such a system. Chromophores can be partitioned between nanospheres and water and among different nanospheres. Fortunately, the change in the fluorescence intensity of perylene is only sensitive to the occurrence of the following processes: k1
z Nn+1 Nn + P y\ (n + 1)k -1
(12)
where Nn stands for a nanosphere with n perylene (P) molecules and k1 and k-1 denote the rate constants for perylene insertion and rejection, respectively. A coefficient of n + 1 was added before k-1, as perylene molecules have been assumed not to interact with one another and n + 1 independent perylene molecules inside a micelle would be n + 1 times more likely to exit than a trapped single molecule. The general kinetic equation for such a system is then
d[Nn] ) k+[Nn-1][P] - nk-[Nn] - k+[Nn][P] + dt n ) 0, 1, ... (13) (n + 1)k-[Nn+1] with [N-1] ) 0. Unfortunately, the simultaneous equations given by eq 13 are difficult to solve analytically. Direct observation of chromophore partition equilibrium using the method described here has not been reported.
Figure 7. Increase in perylene fluorescence intensity as a function of the amount of perylene added, mPe, to 4.00 mL of a nanosphere 2 solution at cN ) 6.8 × 10-2 mg/mL. The maximum amount of perylene the nanospheres can uptake is determined from the crossing point between the straight lines describing intensity versus perylene amount data at high and low perylene contents.
Previous methods for determining the micelle entrance and exit rates of chromophores involved fluorescence and phosphorescence quenching.3 The method described here has not been used, because the rate for chromophore entrance into or exit from conventional micelles was very fast. Capacity of Nanosphere 2. Plotted in Figure 7 is the variation in perylene fluorescence intensity as a function of the amount of perylene added to 4.00 mL of a nanosphere 2 solution at cN ) 6.8 × 10-2 mg/mL. The fluorescence intensity increased sharply initially with the addition of perylene. The increase rate slowed down, and the intensity value eventually leveled off. The initial fluorescence intensity increase with increasing chromophore concentration is expected due to their solubilization into the nanosphere cores. (Perylene solubility in water is ∼2 × 10-9 M.23) The leveling off of the fluorescence intensity after a sufficient amount of perylene is added is reasonable as well. The nanospheres have only a certain capacity. Once saturated, they cannot uptake more perylene regardless of how much perylene is added. The excess perylene will exist as microcrystals in the water, which settle upon centrifugation and do not contribute to a further fluorescence intensity increase. At the intermediate stage, the fluorescence intensity increased more slowly with the addition of perylene, possibly for two reasons. First, the partition coefficient K of the nanospheres may decrease at higher perylene loadings. Second, the uptake of many perylene molecules by the same nanosphere may decrease the perylene fluorescence quantum yield due to “concentration quenching” or excimer formation. Excimer formation was, however, not observed in this study, probably due to the restricted perylene motion in the nanosphere cores and the still low perylene concentrations in the cores. An experiment which eliminated the excimer formation concern has been to precipitate out the nanospheres by Ca2+ addition and to extract the perylene out of the nanospheres with an organic solvent before fluorescence intensity was analyzed.26 Also illustrated in Figure 7 is our method for determining the capacity of the nanospheres. The maximal amount of perylene the nanospheres can uptake is determined to be 0.77 µg from the crossing point between the straight lines describing intensity versus perylene amount data (26) Henselwood, G.; Liu, G. Submitted to J. Appl. Polym. Sci.
Nanospheres as Traps for Perylene
Figure 8. Decrease in perylene fluorescence intensity as a function of the amount of CaCl2 added to 2.50 mL of a perylenesaturated nanosphere 2 solution at cN ) 6.8 × 10-2 mg/mL.
at high and low perylene loadings. Since the total amount of nanospheres present was 0.272 mg, the capacity of the nanospheres is 2.83 mg/g and the capacity of the nanosphere cores is 4.0 mg/g (fc ) 0.70). Using the capacity of 4.0 mg/g and assuming a density of 1.0 g/cm3 for the core, we obtained a perylene concentration of 1.59 × 10-2 M in the nanosphere cores. If the solubility of perylene in water is 2 × 10-9 M,23 the use of eq 1 yields a perylene partition coefficient of 8 × 106, which is considerably larger than what we determined using eq 9. This difference may well derive from the uncertainty in the solubility value of perylene, because we found that a solution at a perylene concentration of 1.9 × 10-8 M was homogeneous and showed no excimer peaks characteristic of perylene aggregation in water. The capacity of nanosphere 1 was not determined, because these spheres could not stay dispersed for longer than 1 or 2 weeks. Nanosphere 2 Precipitation Induced by Ca2+ Addition. We titrated 2.50 mL of a perylene-saturated nanosphere 2 solution at cN ) 6.8 × 10-2 mg/mL. Using data of Table 1, we obtained a mole number of 7.1 × 10-7 and a molar concentration of 2.84 × 10-4 M for the acrylic acid units. Illustrated in Figure 8 is the decrease in perylene fluorescence intensity as a function of the amount of CaCl2 added. The first striking feature of this plot is the near complete disappearance of the perylene fluorescence upon the addition of sufficient Ca2+. The complete precipitation of the nanospheres with trapped perylene at a AA concentration of 2.84 × 10-4 M or an estimated nanosphere concentration of ∼10-9 M suggests the tremendous precipitation power of Ca2+. This also explains why the PAA nanochannels in polymer thin films prepared by us completely closed by the addition of CaCl2.27 Then, the amount of Ca2+ required for reaching the titration end point, defined as the point at which perylene fluorescence intensity decreased by half, is 3.5 µmol. This corresponds to a Ca2+ concentration of 1.4 × 10-3 M in 2.50 mL of nanosphere solution, which is 4.9 times the molar concentration of AA. The high molar equivalent of Ca2+ required is expected, as the nanosphere concentration was very low. Also, the sufficiently high Ca2+ concentration required would make the nanospheres stable in natural water where some divalent cations are present. Exactly how the variation in the nanosphere concentration, the length of the PAA block, the Ca2+ concentration, and the charge number of the cations affect nanosphere stability will be discussed in a future paper. (27) Liu, G.; Ding, J. Adv. Mater. 1998, 10, 69.
Langmuir, Vol. 14, No. 7, 1998 1559
Figure 9. Comparison of the perylene fluorescence spectra of a 2.50-mL perylene-saturated nanosphere 2 solution at cN ) 6.8 × 10-2 mg/mL (s), after the addition of 80 µL of a 0.100 M CaCl2 solution (- - -), and after the addition of 120 µL of a 0.100 M EDTA solution (- ‚ - ‚ -). The intensities of the latter two spectra were corrected for the dilution due to the addition of CaCl2 and EDTA solutions.
Redispersion of Precipitated Nanospheres by the Addition of EDTA. Illustrated in Figure 9 is the comparison between perylene fluorescence spectra before Ca2+ addition, after Ca2+-induced nanosphere precipitation, and after nanosphere redispersion by the addition of EDTA. To induce precipitation in a 2.50-mL nanosphere 2 solution at cN ) 6.8 × 10-2 mg/mL, 80 µL of a 0.100 M CaCl2 solution was used. To redisperse the nanospheres, we injected 120 µL of a 0.100 M EDTA solution. After such a cycle, perylene fluorescence intensity was completely recovered within experimental error. The complete signal disappearance and then recovery suggests that perylene always stayed with the nanospheres. Also, it is possible to insert a step between the nanosphere precipitation and redispersion steps so that one can extract the PAHs out of the nanospheres using an organic solvent. After the removal of the trapped PAHs, the nanospheres can be reused by dispersion with EDTA, or better with CO32-, which precipitates Ca2+. V. Conclusion Nanosphere 1 from PCEMA-b-PAA has been shown to uptake perylene, a PAH, with a large partition coefficient of 3.3 × 105 and a reasonably fast rate. The capacity of perylene uptake by nanosphere 2 was determined to be 2.83 mg/g. Due to the relatively low capacity, the nanospheres should be particularly useful in removing trace amounts of PAHs from contaminated waters. To induce the precipitation of a 6.8 × 10-2 mg/mL solution of nanosphere 2, the concentration of Ca2+ required was 1.4 × 10-3 M, which is slightly above the abundance level of divalent cations in natural water. Once precipitated, the nanospheres can be redispersed with the use of EDTA, which complexes with Ca2+. Acknowledgment. The authors acknowledge the Research Grant Program of the Natural Sciences and Engineering Research Council of Canada and the University of Calgary for financial support of this research. G.W. would like to express his gratitude to the National Science Foundation of China for supporting his overall research program in Tianjin, China. LA970769Y
