Topographical and photophysical properties of poly(amidoamine) dendrimers with ionic surfactants

Topographical and photophysical properties of poly(amidoamine) dendrimers with ionic surfactants

Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 181–190 Topographical and photophysical properties of poly(amidoamine) dendrimers with ...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 181–190

Topographical and photophysical properties of poly(amidoamine) dendrimers with ionic surfactants Mandeep Singh Bakshi a,∗ , Aman Kaura a , Rohit Sood a , Gurinder Kaur b , Tomokazu Yoshimura c , Kanjiro Torigoe c , Kunio Esumi c a

Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India Department of Physics, Guru Nanak Dev University, Amritsar 143005, Punjab, India c Department of Applied Chemistry and Institute of Colloid and Interface Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601 Japan

b

Received 21 December 2004; received in revised form 11 June 2005; accepted 15 June 2005

Abstract The atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been performed on the poly(amidoamine) dendrimers of second generation (2G) and its fluoroderivative (2D) at room temperature. Both studies have demonstrated that 2G and 2D exist in large aggregates on solid surface. The presence of ionic surfactants facilitates their solubilization in micellar phase resulting in the aggregates of much smaller dimensions. The aqueous bulk properties of 2G and 2D both in the absence as well as in the presence of ionic surfactants (i.e. dodecyltrimethylammonium bromide (DTAB), dimethylene bis (dodecyldimethylammonium bromide) (12-2-12), and sodium dodecyl sulfate (SDS)) have been carried out with the help of pyrene fluorescence and turbidity (τ) measurements. From the variation of I1 /I3 pyrene intensity and the τ of these aqueous solutions, it has been found that both DTAB and 12-2-12 interact with the surface groups of 2G and 2D favorably in basic medium, while SDS has been found to interact with that in acidic medium. Apart from this, interactions of cationic surfactants have been found to be stronger with 2D in comparison to 2G, while reverse has been observed in the case of SDS. © 2005 Elsevier B.V. All rights reserved. Keywords: Poly(amidoamine) dendrimers; Ionic surfactant; Topographical and photophysical properties; Dendrimer–surfactant aggregates; pH effect

1. Introduction During the last decade, the poly(amidoamine) dendrimers (PAMAM) have fetched a considerable importance due to its readily water soluble nature. The physicochemical aspects of these macromolecules have been evaluated with a variety of techniques [1–13]. The contrasting spherical nature of these polymers from that of conventional linear polymers have generated tremendous scope in evaluating the fundamental properties both in the homogeneous as well as in the heterogeneous media [14–18]. Several studies have reported their interactions with conventional ionic surfactants and found ∗

Corresponding author. E-mail address: ms [email protected] (M.S. Bakshi).

0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.06.059

that anionic surfactants interact much strongly than cationic surfactants [19,20]. Apart from this, the nanopaticles formation in the presence of these PAMAM have also been reported [21] even without the use of reductant. The PAMAM macromolecules have been found to reduce the metal salt to generate metal nanoparticles loaded with dendritic macromolecules. In the present study, we have selected second generation of PAMAM (2G) and its fluoroderivative (2D) (Scheme 1) to study their interactions with different kinds of surfactants such as sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and dimethylene bis (dodecyldimethylammonium bromide) (12-2-12) in neutral, acidic, and basic media. Both 2G and 2D are expected to aggregate in aqueous phase and their aggregation processes have been studied with the help of AFM and TEM measurements.

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Scheme 1.

In the solution phase, this aggregation behavior is expected to influence their interactions with present ionic surfactants and hence, experiments have been performed in acidic, basic, and neutral media. The selection of different ionic surfactants would help us to evaluate the dependence of PAMAM–surfactant interactions on the basis of different nature of surfactant head groups as well as hydrophobic tails.

2. Experimental 2.1. Materials Poly(amidoamine) dendrimer (2G) was synthesized by means of procedure described by Tomalia et al. [22] where ethylenediamine was used as a nitrogen core and fully characterized by IR, 1 H NMR, 13 C NMR, and mass spectroscopy

before use. The fluoroderivative (2D) was synthesized as reported [23] in the literature and was fully characterized by 1 H NMR. The molecular structures of both 2G and 2D have been shown in Scheme 1. Dimethylene bis (dodecyldimethylammonium bromide) was synthesized according to the method reported elsewhere [24]. Dodecyltrimetylammonium bromide was synthesized as follows: 1-bromododecane was refluxed (80 ◦ C) in the presence of 5–10% excess trimethylamine in dry ethanol for 48 h. The excess trimethylamine was used to ensure the completion of reaction. The surfactant thus synthesized was recrystallised several times from acetone. The purity of the surfactant was checked by 1 H NMR using a Brucker AC 200E instrument. 1 H NMR (CDCl3 ) δH : 3.60 (2H, t, –NCH2 –), 3.49 (9H, s, –NCH3 ), 1.43 (20H, m, –(CH2 )10 –), 0.90 (3H, t, –CH3 ). Sodium dodecyl sulfate 99%, from Aldrich, used as received.

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2.2. AFM studies

3. Results

AFM observations were carried out using tapping mode operation with a Nanoscope III Multimode scanning probe microscope from Digital Instruments (Santa Barbara, CA) using a “D” vertical engage scanner. Silicon micro-cantilever (SI-DF40, manufacturer specifications: spring constant 42 N m−1 , resonance frequency 250–390 KHz) obtained from Seiko Instrument Inc. (Japan) was used for imaging. All images were flattened and dendrimer height and width dimensions (x, y, and z) were measured by random cross section analysis in the x–y plane of surface. All measurements have been performed on mica surface.

3.1. Characterization of 2G and 2D by AFM and TEM

2.3. TEM measurements TEM observations were made for the samples dried on copper-grid coated with collodion. A Hitachi H-9000 NAR was used at an accelerating voltage of 200 kV. 2.4. Fluorescence measurements The pyrene fluorescence of the micelle formation process of each surfactant in the absence as well as presence of aqueous PAMAM solutions was measured by using Hitachi 2500 fluorescence spectrophotometer at 25 ◦ C. Similar experiments were also done by keeping constant surfactant concentration and varying that of PAMAM. It is well known [25] that the ratio of first to third vibronic peaks (I1 /I3 ) of pyrene emission spectrum at 385 nm indicates the polarity of the medium in which it is dissolved. The excitation wavelength of pyrene was, λ = 335 nm, and the pyrene concentration was around 10−6 mol dm−3 . 2.5. Turbidity (τ) measurements The τ measurements were carried out using a NepheloTurbidity meter model CL 52D, after allowing sufficient time for equilibration. This instrument works on the basis of scattering of light by the colloidal particles. The light coming from the light source is focused on the colloidal solution by passing it through a pair of lenses. The scattered light at a right angle to the incident light is detected by a photo multiplier tube, which gives the τ of the solution in arbitrary nephelo turbidity units. 2.6. Sample preparation All the fluorescence and τ measurements of different surfactants have been carried out in aqueous PAMAM solutions keeping [PAMAM] = 0.125 g dm−3 . A concentrated surfactant solution was successively added to aqueous PAMAM and both fluorescence and transmittance titrations were performed by covering both the pre- and post-micellar regions. All surfactant stock solutions were kept overnight to give maximum time for stabilization.

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Several attempts were made to determine the AFM images of aqueous 2G at various concentrations, but none gave clear organized structures due to unknown reasons. On the other hand, clear AFM images of 2D were obtained and Fig. 1a shows a typical example of 1.7 wt.% 2D aqueous solution on mica. One can see several irregular structures where the average size is equal to 36 nm. It is well known [26–28] that because PAMAM aggregates in aqueous phase, 2D as a derivative of PAMAM is also expected to exist in aggregated structures. The surface topographical behavior of both 2G and 2D was also studied with the help of TEM. Fig. 2a shows a typical TEM micrograph of aggregated assemblies of aqueous 2G 0.4 wt.% solutions. The dark dots shown in Fig. 2a are in fact large assemblies of 2G aggregates with size approximate 100 nm. Similar TEM micrograph of 2D is shown in Fig. 2b, where the size of aggregate is approximate equal to 35 nm, which is close to the average size demonstrated by AFM (Fig. 1a). Both AFM and TEM micrographs suggest that both 2G and 2D are favorable in the aggregated structures in aqueous phase though the size of 2G aggregates seems to be greater than that of 2D aggregates. Both techniques were also applied to present 2G/2D + surfactant mixtures, but no clear topographical structure were obtained from AFM studies. However, again several small aggregates of 2D + DTAB aqueous mixtures (DTAB = 0.3 wt.%) can be seen in Fig. 2c with approximate size of 5–10 nm. Similar aggregates along with much smaller size can be observed for the mixtures of 2D + SDS (SDS = 2.5 wt.%) in Fig. 2d. The topographical behavior of this mixture on mica (Fig. 1b) shows numerous small particles resulting upon the complexation of 2D with SDS in aqueous phase. It is to be mentioned here that this mixture showed the insoluble salt formation upon mixing both components. Therefore, both AFM and TEM studies were performed after filtering the solution and clear aqueous phase was used for these studies. It suggests that the presence of surfactant significantly reduces the aggregation of dendritic molecules, which might be due to the solubilization of the latter in the micellar phase. 3.2. Fluorescence and transmittance behavior 3.2.1. 2G/2D + water binary mixtures Figs. 3 and 4 demonstrate the pyrene fluorescence and τ behaviors, respectively, both for the aqueous solutions of 2G and 2D in different media. One can see that the increase in [2G/2D] leads to a continuous decrease in I1 /I3 values in all cases of different pH solutions (Fig. 3). It has already been reported [20] that the decrease in I1 /I3 intensity of pyrene in aqueous PAMAM is generally attributed to the quenching of excited state pyrene by PAMAM macromolecules. Therefore, a decrease in I1 /I3 ratio can be related to an increase

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Fig. 1. (a) AFM image of aqueous 2D of 1.7 wt.% aqueous solution on mica. (b) AFM image of aqueous 2D + SDS of 2.5 wt.% aqueous solution on mica.

in quenching of pyrene with the increase in [2G/2D]. Apart from this, the presence of different media does not significantly influence this behavior, which is quite evident from the identical nature of all curves in Fig. 3a–c. On the other hand, the variation of τ versus [2G/2D] is much interesting (Fig. 4). The increase in [2G] leads to an instant increase in

τ in the form of shallow maximum, which tends to a smaller constant value thereafter almost in all cases. The magnitude of increase in τ in aqueous 2G at the maximum, increases from acidic to basic media, while τ of aqueous 2D solution instead of increasing, it decreases initially (especially at pH 4.75) and then tending to a constant value at higher [2D]. At

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Fig. 2. (a) TEM micrograph of aggregated assemblies of aqueous 2G 0.4 wt.% solution. (b) TEM micrograph of aggregated assemblies of aqueous 2D 0.4 wt.% solution. (c) TEM micrograph of aggregates of 2D + DTAB aqueous mixtures having DTAB = 0.3 wt.% solution. (d) TEM micrograph of aggregates of 2D + SDS aqueous mixtures having SDS = 2.5 wt.% solution.

pH 7 and 9.2, though there is an initial little decrease in the τ, the values tend to remain mostly constant. Since, the τ can be related to the scattering of light from colloidal particles, therefore, higher τ values can be attributed to the presence of larger particles. Thus, it demonstrates that aqueous 2G in all the media exist in much aggregated form rather than 2D. The turbidity of 2G and 2D are larger at pH 7 than other pH, namely 2G forms larger aggregates at pH 7. The later information is not very clear for aqueous 2D aggregates. Hence, collectively it can be said that the decrease in I1 /I3 pyrene intensity with the increase in [2G/2D] is practically related to self-aggregation of 2G/2D, though which is much prominent in the case of 2G and is evident from τ behavior. Thus, the larger aggregates entrap greater number of excited state pyrene molecules and hence, cause higher quenching. These results are very much in line with AFM and TEM studies (Figs. 1a and 2, respectively), where larger aggregates of 2G have been shown especially by TEM.

3.2.2. 2G/2D + surfactant + water ternary mixtures Figs. 5 and 6 show the I1 /I3 pyrene fluorescence plots of micelle formation of DTAB and 12-2-12, respectively, at constant amounts of [2G/2D] in different media. Both figures demonstrate a clear micelle formation process of these surfactants in the absence of 2G/2D in different media. In the presence of constant [2G/2D], the I1 /I3 plots for both surfactants run quite close to each other especially in the acidic and neutral media (Figs. 5a, b, and 6a, b), and the I1 /I3 values are much smaller than in the absence of 2G/2D. The critical micelle concentration (cmc) values thus obtained from the break in the curve (Fig. 5a) for present surfactants both in the absence as well as presence of 2G/2D have been listed in Table 1. The decrease in I1 /I3 value in the presence of 2G/2D can obviously be related to the quenching of pyrene as reported in previous section. At pH 9.2 (Figs. 5c and 6c), the earlier insignificant difference between I1 /I3 curves for 2G and 2D, shows a large difference with higher I1 /I3 value in

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Fig. 3. (a–c) Plots of pyrene intensity ratio (I1 /I3 ) of 2G and 2D in acidic (pH 4.75), neutral (pH 7), and basic (pH 9.2) media, respectively. Table 1 Values of critical micelle concentration, cmc, × 10−4 /mol dm−3 of DTAB and 12-2-12 in the presence of 2G/2D (g dm−3 ) from fluorescence measurements in different media PAMAM

[PAMAM] (g dm−3 )

cmc (DTAB)

cmc (12-2-12)

0 0.125 0.125

120 ± 12 80 ± 8 80 ± 8

3.5 ± 0.35 1.5 ± 0.15 1.0 ± 0.10

0 0.125 0.125

50 ± 5 40 ± 4 38 ± 3.8

1.0 ± 0.10 0.5 ± 0.05 0.5 ± 0.05

0 0.125 0.125

70 ± 7 60 ± 6 60 ± 6

2.0 ± 0.2 1.5 ± 0.15 1.5 ± 0.15

pH 4.75 2G 2D pH 7 2G 2D pH 9.2 2G 2D

Fig. 4. (a–c) Plots of turbidity of 2G and 2D in acidic (pH 4.75), neutral (pH 7), and basic (pH 9.2) media, respectively.

aqueous 2G rather than 2D. It demonstrates that both DTAB and 12-2-12 interact with 2G/2D in basic medium and interactions were not significant in acidic and neutral media. It seems that least protonation of 2G and 2D in basic medium makes feasible the electrostatic interactions between the surface groups of dendritic molecules and cationic head groups. A close inspection of Figs. 5c and 6c also suggests that the I1 /I3 curves for both cationic surfactants are lowest in aqueous 2D. The results are further explained on the basis of τ measurements. Figs. 7–9 show the τ behaviors of SDS, DTAB, and 12-212, respectively, in different media. Fig. 7a and b indicate that there is a significant increase in τ of SDS with the increase in

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Fig. 5. (a–c) Plots of pyrene intensity ratio (I1 /I3 ) of micelle formation of DTAB at constant amounts of 2G/2D = 0.125 g dm−3 in acidic (pH 4.75), neutral (pH 7), and basic (pH 9.2) media, respectively.

[SDS] both in aqueous 2G and 2D, and the τ values tend to become constant at higher [SDS] after running through strong maxima. No such behavior is observed in basic medium and the τ value remains practically constant. The τ is much higher in aqueous 2G rather than in aqueous 2D (Fig. 7a and b). This behavior suggests that the turbid solution appeared at the maximum in each τ curve is due to the strong complexation, and redissolves in the micellar phase with the further increase in [SDS]. The latter effect reduces the τ value. On the other hand, Figs. 8 and 9 for both DTAB and 12-2-12, respectively, indicate that τ is insignificant in acidic medium

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Fig. 6. (a–c) Plots of pyrene intensity ratio (I1 /I3 ) of micelle formation of 12-2-12 at constant amounts of 2G/2D = 0.125 g dm−3 in acidic (pH 4.75), neutral (pH 7), and basic (pH 9.2) media, respectively.

(Figs. 8a and 9a), while it is relatively much higher in basic medium with mostly intermediate values in the case of neutral medium. Apart from this, the τ for both DTAB and 12-2-12 is much higher in aqueous 2D rather than in aqueous 2G. This behavior is contrary to that observed for SDS (Fig. 7a and b). A relative comparison among the transmittance values at the maxima in the case of all surfactants demonstrate that much stronger complexation can be seen in the SDS + 2G/2D mixtures in comparison to that cationic + 2G/2D due to the significantly higher τ in the former case (Fig. 7a).

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Fig. 7. (a–c) Plots of turbidity of micelle formation of SDS at constant amounts of 2G/2D = 0.125 g dm−3 in acidic (pH 4.75), neutral (pH 7), and basic (pH 9.2) media, respectively.

Fig. 8. (a–c) Plots of turbidity of micelle formation of DTAB at constant amounts of 2G/2D = 0.125 g dm−3 in acidic (pH 4.75), neutral (pH 7), and basic (pH 9.2) media, respectively.

4. Discussion

solubilization of dendritic phase in the micellar phase. There are several studies reported [26–28] in the literature suggesting the self-aggregation behavior of PAMAM, which reduces in the presence of ionic surfactants. The results from both AFM and TEM have been further supported by pyrene fluorescence and τ studies in the aqueous phase. Both studies (Figs. 3 and 4, respectively) fully support the self-aggregation of 2G/2D in the aqueous phase. The τ studies clearly differentiate the greater size of 2G aggregates from that of 2D, and the trend remains almost same even in different media. The larger size of the former can be attributed to the better probability

The results presented from various techniques can be explained as follows. The AFM and TEM studies clearly indicate the presence of 2G/2D dendritic macromolecules in the form of aggregated assemblies (Figs. 1a and 2). The aggregates of 2G are much larger in size (Fig. 2a) in comparison to that of 2D (Fig. 2b) at comparable concentrations. TEM studies further indicate that 2D aggregates split into smaller ones in the presence of DTAB (Fig. 2c), while more favorably in the presence of SDS (Fig. 2d), suggesting the

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resulting in insignificant interactions with DTAB and 12-212. On the other hand, in basic medium (Figs. 5c and 6c), the I1 /I3 curves for both surfactants in aqueous 2G lie higher to that of 2D, which can be attributed to the difference in the 2G/2D–cationic surfactant interactions. The latter seems to be greater in the case of 2D rather than 2G due to the possible esterification of ester groups of the former in basic medium, which would further facilitates the electrostatic interactions between cationic head groups and predominantly electronegative acidic carboxylate surface groups. This would result in an increase in the τ value significantly in aqueous 2D rather than 2G (Figs. 8c and 9c). These results are based on the fact that the degree of quenching of pyrene by 2G and 2D is almost identical in all media (Fig. 3), and hence, the difference in I1 /I3 curves represented in Figs. 8c and 9c can only be explained on the basis of a difference in the interactions of 2G and 2D with the surfactants. On the other hand, a significantly higher τ for SDS in aqueous 2G than 2D especially in the acidic and neutral media can be related to the stronger interactions of anionic head group with relatively much protonated amine terminated surface groups of 2G rather than less protonated ester terminated surface groups of 2D. It is to be mentioned here that unfortunately we could not compare the τ results of SDS (Fig. 7) with that of fluorescence due to the appearance of cloudiness especially in neutral and acidic media. 5. Conclusions

Fig. 9. (a–c) Plots of turbidity of micelle formation of 12-2-12 at constant amounts of 2G/2D = 0.125 g dm−3 in acidic (pH 4.75), neutral (pH 7), and basic (pH 9.2) media, respectively.

of H-bonding among the surface groups of different amine terminated 2G in comparison to that of ester terminated surface groups of 2D. Again, these studies have been applied to evaluate the comparative behavior of DTAB, 12-2-12, and SDS in aqueous dendritic solutions. Figs. 5 and 6 for DTAB and 12-2-12, respectively, indicate that there is no difference among the I1 /I3 curves for both surfactants in acidic and neutral media suggesting the presence of 2G/2D–DTAB/12-2-12 interactions if exist, remain identical with both the dendritic macromolecules. This can be attributed to the protonation of amine and ester terminated surface groups of 2G and 2D

The present study concludes that the topographical behavior of 2G and 2D PAMAM exist in the aggregated form. The aggregates of 2G are much larger in dimensions than that of 2D. These aggregates split into smaller ones in the presence of ionic surfactants due to their solubilization in micellar phase. In the aqueous bulk phase, the dendrimer–surfactant interactions have been governed by pH of the solution. The cationic surfactants such as DTAB and 12-2-12 have been found to interact favorably with both 2G and 2D dendritic molecules in basic medium while anionic surfactant like SDS interacts favorably with 2G/2D in acidic medium. Acknowledgments Financial assistance from DST Project [No. SP/S1/H22/2001] is thankfully acknowledged. A. Kaura thanks the CSIR, New Delhi, for a Junior Research Fellowship. References [1] G. Caminati, N.J. Turro, D.A. Tomalia, J. Am. Chem. Soc. 112 (1990) 8515. [2] M.F. Ottovani, C. Turro, N.J. Turro, S.H. Bossmann, D.A. Tomalia, J. Phys. Chem. 100 (1996) 13667. [3] M.F. Ottovani, N.J. Turro, S. Jockusch, D.A. Tomalia, J. Phys. Chem. 100 (1996) 13675.

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