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Charge interaction of low generation dendrimers during zeolite formation
Journal of Non-Crystalline Solids 357 (2011) 771–774
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Charge interaction of low generation dendrimers during zeolite formation D. Lombardo a,⁎, L. Bonaccorsi b, A. Longo c, E. Proverbio b, P. Calandra a a b c
CNR–IPCF, Istituto per i Processi Chimico Fisici-Sez. Messina, C.da Papardo (Salita Sperone) Messina, I-98158, Italy Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, S. Agata (Messina), I-98166, Italy CNR–ISMN, Istituto per lo studio dei Materiali Nanostrutturati-Sez. Palermo, Via Ugo La Malfa 153, I-90146, Palermo, Italy
a r t i c l e
i n f o
Article history: Received 16 April 2010 Received in revised form 2 July 2010 Available online 9 September 2010 Keywords: Self-assembly; Complex systems; Zeolites; Dendrimers; Nanostructures
a b s t r a c t A Small Angle X-ray Scattering (SAXS) investigation has been performed to study the self-assembly process generated by the addition of a low generation G1.5 polyamidoamine dendrimer during the zeolite LTA formation process. The presence of a condensed cationic Na+ charge all around the dendrimers, which is responsible for the intense electrostatic interparticle interaction potential, stimulate the condensed growth of the zeolitic phase onto the dendrimer substrate. The screening produced by the zeolite grown on the dendrimer surface promote also an entanglement process between the primary units with the formation of large clusters as evidenced by Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray (EDX) microprobe spectroscopy experiments. The current investigation emphasizes the potentials of using hybrid organic–inorganic networks in the synthetic construction of functional materials with high-order structures for application in material science and biotechnology. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently there has been increasing interest in the study of nanoparticles with novel structural and topological features suitable for application in material science and biotechnology [1,2]. Particularly interesting in this respect are a class of highly branched biopolymers called dendrimers [3]. This class of macromolecules present a regularly branched structure obtained by iterative, controlled, reaction sequences [4]. The ability to functionalize the terminal groups of the dendrimers allow them to be used in a wide range of applications such as molecular recognition, signal processing as well as for binding various targeting or guest molecules [5–7]. Particularly interesting in this respect is also the construction of supra-molecular hybrid organic–inorganic nanostructures based on microporous and nanoporous materials [8–10]. In this paper, we describe the synthesis of spherical nanoaggregates derived by the growth of the aluminosilicate components on a low generation G1.5 carboxyl-terminated polyamidoamine (Pamam) dendrimer during the zeolite LTA synthesis. More specifically we evidence that spherical porous nanoparticle were formed when the charged terminal groups of the dendrimer irreversibly anchor the zeolite formation to the dendrimer surface. Such selfassembly recently attracted a sensitive attention in material science [11–15] due also for the important implications in the improvement of the models and theoretical approaches describing the relevant
⁎ Corresponding author. Tel.: +39 090 39762222. E-mail address: [email protected] (D. Lombardo). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.07.033
assembly mechanism and processes involved in material and life science [16–20]. 2. Experimental 2.1. Sample preparation PAMAM dendrimers of generation G=1.5 (Mw =2935 g/mol) were purchased from Aldrich Chemical Co. As schematically shown in the inset of Fig. 1, the investigated system consists of a tetrafunctional ethylenediamine core [NNCH2CH2Nb] and [–CH2CH2(C=O)NHCH2CH2Nb] spacers and is terminated at the final generation with 16 sodium carboxylate terminal groups (COO− Na+) in average. The dendrimers were dispersed in deionised water, while the obtained solutions were filtered with Teflon filters (filter diameter was D=0.02 μm). The solutions were also checked by dynamic light scattering prior to SAXS measurements to remove the presence of possible aggregates in the system. The zeolite synthesis mixtures, prepared according the standard procedure [21–23], had the following molar ratio: 2.0 NaO2:1 Al2O3:1.9 SiO2:65 H2O. All aluminosilicate reactants were preliminary mixed and diluted in water (with a dilution factor 20 in weight) and then added with water solution of generation G1.5 Pamam dendrimers at the concentration of C=1 wt.% [Pamam G1.5/AS]. 2.2. Characterization The Small Angle X-ray Scattering (SAXS) experiments have been performed by using a laboratory instrumentation consisting of a Philips PW X-ray generator (providing Cu Kα, Ni-filtered X-ray
772
D. Lombardo et al. / Journal of Non-Crystalline Solids 357 (2011) 771–774
Assuming the dendrimer solution as a monodisperse system, the SAXS scattering intensity I(q) can be expressed as a product of the form factor P(q), which contains information on the shape and dimension of the scattering particles and the structure factor S(q) describing the interparticle interaction [24,25] 2
IðqÞ = NðΔρÞ P ðqÞSðqÞ
ð1Þ
where N is the number density of the particles, and Δρ = (ρ − ρ0) is the so-called “contrast” (i.e., the difference between the scattering length density of the particle ρ and that of the solvent ρ0). In the dilute region the interparticle interaction can be neglected (i.e., S(q) ≈ 1), so that the analysis of scattering intensity I(q) can furnish direct information of morphological features of the scattering particles. The expression of the sphere form factor P(q) = [3 J1(qR)/(qR)]2 (where R is the sphere radius and J1(x) = [sin(x) − xcos(x)]/x2 is the first-order spherical Bessel function) has been used to fit our data in the concentration range between 0.5 ≤ c ≤ 1.5 wt.%, where interparticles interference effects are assumed to be negligible [24,25]. Results of the fitting, as presented in Fig. 1B for the sample at concentration C = 1 wt.%, show that the scattering data are well reproduced in all the q range. Information about the dendrimer radius of gyration Rg has also been obtained from the slope of lnI(q) vs. q2 in the so-called Guinier region (i.e. for qRg bb 1), where the particle form factor can be expressed as q2 R2g P ðqÞ ≅ P ð0Þ exp − 3
Fig. 1. (A) SAXS profiles of water solution of G1.5 PAMAM dendrimers for two different concentrations. A schematic representation of a four-armed PAMAM dendrimers of generation G1.5 is given in the inset below. The precursor generations, G0 and G1, are identified to emphasize the iterative process of dendrimer synthesis. (B) SAXS form factor data analysis for the water solution of Pamam dendrimers generation G1.5 at the concentration of C = 1 wt.%. A summary of the obtained results for the different dendrimer concentrations is presented in the inset above.
radiation of wavelength 1.5418 Å) with a Kratky-type small angle camera in the “finite slit height geometry” equipped with step scanning motor and scintillator counter as detector. All measurements were carried out at the temperature of T = 25 °C. The scattering data were normalized with respect to transmission and were corrected by the empty cell and solvent contribution. The Electron Microscopy was performed using a scanning electron microscope JEOL 5600LV operated at 10 kV in low-vacuum condition. The microscope was equipped with a back-scattered electron detector and an EDS electronic microprobe (SEMQuant Oxford).
3. Results and discussion In order to obtain valuable information about the structure and interaction of our investigated system a set of SAXS measurements has been carried out at the temperature of T = 25 °C. In Fig. 1A the SAXS spectra of the solely G1.5 polyamidoamine dendrimers in water solution are presented for the two different concentrations of C = 5 wt.% of and C = 10 wt.%. The figure evidence the presence of a pronounced interference peak which is indicative of a long-range structural order in the system due to interparticle interaction in solution.
! ð2Þ
The results of the form factor data analysis for all the studied concentrations are summarized in the above inset of Fig. 1B. The obtained results in the dilute regime furnish an average values of Rg = 9.4 Å for the sphere radius and of Rg = 7.6 Å for the radius of gyration (reported in the above inset of Fig. 1B as dot dashed line). At higher concentration (i.e. for C ≥ 2 wt.%) a possible source of uncertainty in the determination of Rg is connected with the presence of the structure factor S(q) contribution to the SAXS spectra. On the other hand the information about the particle radius R, at the higher concentrations, can be retrieved as a result in connection with the analysis of the structure factor S(q). In order to obtain information about the inter-dendrimer interaction potential the charged dendrimers has been approximated as impenetrable spheres of radius R whose charge Ze is distributed on the surface. Those spheres are immersed in the uniform neutralising background of the solvent molecules which participates with its dielectric constant ε (ε = 78 for water) and which produces also a screening effect in the system. According to this model the repulsive potential between two identical spherical objects (macro-ions) of diameter σ = 2R placed at a distance r (centre to centre distance) is given by [26,27] U ðr Þ =
−κðr−σ Þ ðZ0 eÞ2 e 2 r 4πεð1 + κσ Þ
ð3Þ
Here κ is the Debye–Huckel inverse screening length which is determined, at a given temperature T, by the ionic strength I of the solvent (in mol/l), according to the following relation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 8πe2 Na I κ= εKB T × 103
ð4Þ
Here e is the unit of electron charge, KB the Boltzmann constant, Na the Avogadro number. Moreover a hard sphere type repulsive component for the potential has been adopted to represent the close contact inter-dendrimer interaction. In our specific case we have
D. Lombardo et al. / Journal of Non-Crystalline Solids 357 (2011) 771–774
investigated the observed inter-dendrimer structure factors S(q), obtained by SAXS experiments, in terms of the hypernetted chain approximation scheme (HNC) [27–29]. In Fig. 2 the numerical structure factor S(q) computed according the HNC scheme has been compared with the experimental structure factor obtained from SAXS spectra for the sample at concentration of C = 10 wt.%. From the figure it is clear how the adopted model reproduce quite satisfactorily the experimental results with an average dendrimer effective charge of Zeff = 10 ± 1 (in unit of electron charge |e|). From the finding our obtained results we can deduce that the carboxylate terminal groups of Pamam half integer dendrimers in water solution are partially dissociated (COO− Na+). In particular an average number of Zeff = 10 over the Zend = 16 total carboxylic endgroups per dendrimer (for the G = 1.5 generation) realize this ionization. This correspond to a rate of ionization near to Zeff/Zend = 0.6. In order to obtain direct information of the morphological features during self-assembly process all aluminosilicate reactants were preliminary mixed and diluted in water (with a dilution factor 20 in weight) and then added with a water solution of generation G1.5 Pamam dendrimers at the concentration of C = 1 wt.% [AS/Pamam]. Results of the dimension analysis as a function of the elapsed time after the mixing are reported in Fig. 3, which indicates that the radius of gyration Rg slightly increases with the reaction time, starting from Rg = 7.6 Å (for solely Pamam dendrimer in water solution) up to Rg = 9.4 Å (18 h after the mixing) and Rg = 9.8 Å (40 h after the mixing). This thickening of the original particles is caused by the condensed growth of the zeolitic phase onto the dendrimer substrate. The dendrimer carboxylate end groups, which are responsible for the long-range electrostatic interparticle interaction potential, create a diffuse layer of condensed Na + ions in the neighborhood of the dendrimer surface which act as the effective structure-directing agent. Rearrangement of aluminosilicates around hydrated Na+ cations, in the form of polyhedral cage structures, favorite a condensed growth of the zeolitic phase onto the dendrimer substrate. Due to the highly ramified nature of the dendrimer the possible Na + ions migration in the interior of the dendrimers may favorite the growth of the zeolite phase in the interior of the dendrimer. In this respect it is possible to state that the peculiar cage structure formation does not ensure a facile removal of the nanotemplate. On the other hand, relevant advantages of the choice of the dendrimers rely on the possibility to choice size and composition of the template substrate by changing number, typology, or spatial distribution of the terminal groups. In this sense the zeolites cage structure, acting as a selective barrier, may be used to promote selective reactions or incorporation of
Fig. 2. Comparison between the experimental structure factor S(q) (for the sample at dendrimer concentrations of C = 10 wt.%) and the one computed by means of the adopted model. The concentration dependence of the rate of ionization Zeff/Zend is reported in the inset.
773
Fig. 3. Analysis of the radius of gyration Rg as a function of the elapsed time after the mixing of the main components. The symbols represent the sample for solely Pamam dendrimers (full circle), 18 h after the mixing (hollow circles) and 40 h after the mixing (hollow triangles).
key features of selected enzymes or metal complexes thus mimicking the enzyme functions. The screening of the repulsive interaction promotes also an entanglement process between the primary units, with the formation of large clusters of the order of one micron as evidenced by the Scanning Electron Microscopy (SEM) investigation (Fig. 4A). The back-scattered SEM image of the system, 7 days after the mixing, confirmed a condensation of the aluminosilicate components on the large aggregates surfaces as proved by the EDX microprobe
Fig. 4. Scanning electron microscopy images of the generated spherical supramolecular assembly for the Pamam G1.5/AS system investigated.
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D. Lombardo et al. / Journal of Non-Crystalline Solids 357 (2011) 771–774
zeolite LTA formation. This study, while confirming the main trends of our previous Dynamic Light Scattering (photon correlation spectroscopy) investigation in a bigger generation (G3.5) Pamam dendrimer [30], show the benefits of using a dendrimer nanotemplate for the nanoparticles formation process in zeolite syntheses especially for what concern the possibility to choose the size and composition of dendrimer substrate. Further investigations of these systems in solution are underway in order to discover the main feature which drives the size and morphology of the condensed surface structures as a function of the dendrimer typology and effective surface charge.
References
Fig. 5. X-ray diffraction pattern of the synthesis mixture after three weeks reaction at room temperature.
analysis (inset of Fig. 4A). Larger irregular aggregates of the order of few microns was also detected as reported by the scanning electron microscopy (SEM) image in Fig. 4B. The confirmation of the nature of the condensate phase is finally given by the X-ray diffraction experiment. The clear synthesis solution was left reacting at room temperature for 3 weeks and then was dried at T = 40 °C for 24 h. The collected solid, dried again at T = 80 °C for 12 h, was checked by XRD powder diffraction (Fig. 5) that confirmed, through the analysis of the position of the diffraction peaks, the formation of crystalline LTA zeolite. It is worth noticing that the screening of Na+ counterions, although producing a reduction in the effective surface charge with respect the number of terminal groups (ionization ratio less than one), does not compromise the colloid stability of dendrimers in solution. Up to date, in fact, no aggregation has been observed in dendrimer systems in solution, even with the addition of a given salt amount to the system [31]. On the other it is possible to assume that entanglement is favoured by reduction of the charge density at the surface of the nanoparticles cause by the progressive growth of the zeolitic phase onto the dendrimer substrate. 4. Conclusion In this paper we describe the charge interaction and formation of porous aggregates derived by the growth of zeolite LTA on a low generation G1.5 polyamidoamine (Pamam) dendrimer acting as nano-template. By using the liquid integral equation theory for charged systems in solution we estimated the presence of an average effective charge of Z = 10e in the surface of the dendrimer (over 16 total chargeable groups present in the dendrimer). Those charges, which are responsible of the long-range electrostatic interparticle interaction, act as the effective structure-directing agent for the
[1] L.E. Foster, Nanotechnology: Science, Innovation, and Opportunity, Prentice Hall, 2005. [2] H.S. Nalwa, Encyclopedia of Nanoscience and Nanotechnology, American Scientific Publishers, 2004. [3] D.A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder, P. Smith, Polym. J. 17 (1985) 117. [4] D.A. Tomalia, A.M. Naylor, W.A. Goddard III, Angew. Chem. Int. Ed Engl. 29 (1990) 138. [5] M.X. Tang, C.T. Redemann, F.C. Jr, Szoka, Bioconjugate Chem. 7 (1997) 703. [6] M. Zhao, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 120 (1998) 4877. [7] L. Balogh, R. Valluzzi, K.S. Laverdure, S.P. Gido, G.L. Hagnauer, D.A. Tomalia, J. Nanopart. Res 1 (1999) 353. [8] P. Judenstein, C. Sanchez, J. Mater. Chem. 6 (1996) 511. [9] S.H. Jhung, J.-H. Lee, P.M. Forster, G. Frey, A.K. Cheetham, J.-S. Chang, Chem. Eur. J. 12 (2006) 7899. [10] P.C. Angelomé, M.C. Fuertes, G.J.A.A. Soler-Illia, Adv. Mater. 18 (2006) 2397. [11] Y. Yang, A.J. Heeger, Nature 372 (1994) 344. [12] L. He, V.M. Garamus, S.S. Funari, M. Malfois, R. Willumeit, B. Niemeyer, J. Phys. Chem. B 106 (2002) 7596. [13] A. Mazzaglia, N. Angelini, R. Darcy, D. Lombardo, N. Micali, M.T. Sciortino, V. Villari, L. Monsú Scolaro, Chem. Eur. J. 9 (2003) 5762. [14] F. Lafleche, D. Durand, T. Nicolai, Macromolecules 36 (2003) 1331. [15] A. Mazzglia, N. Angelini, D. Lombardo, N. Micali, S. Patane, V. Villari, L. Monsù Scolaro, J. Phys. Chem. B 109 (2005) 7258. [16] F. Gröhn, G. Kim, B.J. Bauer, E.J. Amis, Macromolecules 34 (2001) 2179. [17] F. Mallamace, R. Beneduci, P. Gambadauro, D. Lombardo, S.H. Chen, Physica A 302 (2001) 202. [18] G.M. Whitesides, B. Grzybowski, Science 295 (2002) 2418. [19] D. Lombardo, A. Longo, R. Darcy, A. Mazzaglia, Langmuir 20 (2004) 1057. [20] D. Chun, F. Wudl, A. Nelson, Macromolecules 40 (2007) 1782. [21] R.M. Barrer, Synthesis of zeolites, in: B. Drzaj, S. Hocevar, S. Pejovnik (Eds.), Zeolites, Elsevier Science Publishers BV, Amsterdam, 1985. [22] D. Breck, Zeolite Molecular Sieves, Structure, Chemistry and Use, New York, Wiley, 1974. [23] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. [24] L.A. Feign, D.I. Svergun, Structure Analysis by Small-Angle X-ray and Neutron Scattering, Plenum Press, New York, 1987. [25] O. Glatter, O. Kratky, Small-Angle X-ray Scattering, Academic Press, London, 1982. [26] E.J.W. Verwey, J.T.G. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. [27] R.J. Hunter, Foundations of Colloid Science, vols. I–II, Oxford University Press, NewYork, 1986. [28] J.P. Hansen, I.A. McDonald, Theory of Simple Liquids, Academic Press, New-York, 1986. [29] L. Belloni, in: Lindner, Zemb (Eds.), Neutron X-ray and Light Scattering, Elsevier Science Publishers (B.V.), Amsterdam, 1991. [30] L. Bonaccorsi, D. Lombardo, A. Longo, E. Proverbio, A. Triolo, Macromolecules 42 (2009) 1239. [31] G. Nisato, R. Ivkov, E.J. Amis, Macromolecules 32 (1999) 5895.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Charge interaction of low generation dendrimers during zeolite formation D. Lombardo a,⁎, L. Bonaccorsi b, A. Longo c, E. Proverbio b, P. Calandra a a b c
CNR–IPCF, Istituto per i Processi Chimico Fisici-Sez. Messina, C.da Papardo (Salita Sperone) Messina, I-98158, Italy Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, S. Agata (Messina), I-98166, Italy CNR–ISMN, Istituto per lo studio dei Materiali Nanostrutturati-Sez. Palermo, Via Ugo La Malfa 153, I-90146, Palermo, Italy
a r t i c l e
i n f o
Article history: Received 16 April 2010 Received in revised form 2 July 2010 Available online 9 September 2010 Keywords: Self-assembly; Complex systems; Zeolites; Dendrimers; Nanostructures
a b s t r a c t A Small Angle X-ray Scattering (SAXS) investigation has been performed to study the self-assembly process generated by the addition of a low generation G1.5 polyamidoamine dendrimer during the zeolite LTA formation process. The presence of a condensed cationic Na+ charge all around the dendrimers, which is responsible for the intense electrostatic interparticle interaction potential, stimulate the condensed growth of the zeolitic phase onto the dendrimer substrate. The screening produced by the zeolite grown on the dendrimer surface promote also an entanglement process between the primary units with the formation of large clusters as evidenced by Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray (EDX) microprobe spectroscopy experiments. The current investigation emphasizes the potentials of using hybrid organic–inorganic networks in the synthetic construction of functional materials with high-order structures for application in material science and biotechnology. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently there has been increasing interest in the study of nanoparticles with novel structural and topological features suitable for application in material science and biotechnology [1,2]. Particularly interesting in this respect are a class of highly branched biopolymers called dendrimers [3]. This class of macromolecules present a regularly branched structure obtained by iterative, controlled, reaction sequences [4]. The ability to functionalize the terminal groups of the dendrimers allow them to be used in a wide range of applications such as molecular recognition, signal processing as well as for binding various targeting or guest molecules [5–7]. Particularly interesting in this respect is also the construction of supra-molecular hybrid organic–inorganic nanostructures based on microporous and nanoporous materials [8–10]. In this paper, we describe the synthesis of spherical nanoaggregates derived by the growth of the aluminosilicate components on a low generation G1.5 carboxyl-terminated polyamidoamine (Pamam) dendrimer during the zeolite LTA synthesis. More specifically we evidence that spherical porous nanoparticle were formed when the charged terminal groups of the dendrimer irreversibly anchor the zeolite formation to the dendrimer surface. Such selfassembly recently attracted a sensitive attention in material science [11–15] due also for the important implications in the improvement of the models and theoretical approaches describing the relevant
⁎ Corresponding author. Tel.: +39 090 39762222. E-mail address: [email protected] (D. Lombardo). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.07.033
assembly mechanism and processes involved in material and life science [16–20]. 2. Experimental 2.1. Sample preparation PAMAM dendrimers of generation G=1.5 (Mw =2935 g/mol) were purchased from Aldrich Chemical Co. As schematically shown in the inset of Fig. 1, the investigated system consists of a tetrafunctional ethylenediamine core [NNCH2CH2Nb] and [–CH2CH2(C=O)NHCH2CH2Nb] spacers and is terminated at the final generation with 16 sodium carboxylate terminal groups (COO− Na+) in average. The dendrimers were dispersed in deionised water, while the obtained solutions were filtered with Teflon filters (filter diameter was D=0.02 μm). The solutions were also checked by dynamic light scattering prior to SAXS measurements to remove the presence of possible aggregates in the system. The zeolite synthesis mixtures, prepared according the standard procedure [21–23], had the following molar ratio: 2.0 NaO2:1 Al2O3:1.9 SiO2:65 H2O. All aluminosilicate reactants were preliminary mixed and diluted in water (with a dilution factor 20 in weight) and then added with water solution of generation G1.5 Pamam dendrimers at the concentration of C=1 wt.% [Pamam G1.5/AS]. 2.2. Characterization The Small Angle X-ray Scattering (SAXS) experiments have been performed by using a laboratory instrumentation consisting of a Philips PW X-ray generator (providing Cu Kα, Ni-filtered X-ray
772
D. Lombardo et al. / Journal of Non-Crystalline Solids 357 (2011) 771–774
Assuming the dendrimer solution as a monodisperse system, the SAXS scattering intensity I(q) can be expressed as a product of the form factor P(q), which contains information on the shape and dimension of the scattering particles and the structure factor S(q) describing the interparticle interaction [24,25] 2
IðqÞ = NðΔρÞ P ðqÞSðqÞ
ð1Þ
where N is the number density of the particles, and Δρ = (ρ − ρ0) is the so-called “contrast” (i.e., the difference between the scattering length density of the particle ρ and that of the solvent ρ0). In the dilute region the interparticle interaction can be neglected (i.e., S(q) ≈ 1), so that the analysis of scattering intensity I(q) can furnish direct information of morphological features of the scattering particles. The expression of the sphere form factor P(q) = [3 J1(qR)/(qR)]2 (where R is the sphere radius and J1(x) = [sin(x) − xcos(x)]/x2 is the first-order spherical Bessel function) has been used to fit our data in the concentration range between 0.5 ≤ c ≤ 1.5 wt.%, where interparticles interference effects are assumed to be negligible [24,25]. Results of the fitting, as presented in Fig. 1B for the sample at concentration C = 1 wt.%, show that the scattering data are well reproduced in all the q range. Information about the dendrimer radius of gyration Rg has also been obtained from the slope of lnI(q) vs. q2 in the so-called Guinier region (i.e. for qRg bb 1), where the particle form factor can be expressed as q2 R2g P ðqÞ ≅ P ð0Þ exp − 3
Fig. 1. (A) SAXS profiles of water solution of G1.5 PAMAM dendrimers for two different concentrations. A schematic representation of a four-armed PAMAM dendrimers of generation G1.5 is given in the inset below. The precursor generations, G0 and G1, are identified to emphasize the iterative process of dendrimer synthesis. (B) SAXS form factor data analysis for the water solution of Pamam dendrimers generation G1.5 at the concentration of C = 1 wt.%. A summary of the obtained results for the different dendrimer concentrations is presented in the inset above.
radiation of wavelength 1.5418 Å) with a Kratky-type small angle camera in the “finite slit height geometry” equipped with step scanning motor and scintillator counter as detector. All measurements were carried out at the temperature of T = 25 °C. The scattering data were normalized with respect to transmission and were corrected by the empty cell and solvent contribution. The Electron Microscopy was performed using a scanning electron microscope JEOL 5600LV operated at 10 kV in low-vacuum condition. The microscope was equipped with a back-scattered electron detector and an EDS electronic microprobe (SEMQuant Oxford).
3. Results and discussion In order to obtain valuable information about the structure and interaction of our investigated system a set of SAXS measurements has been carried out at the temperature of T = 25 °C. In Fig. 1A the SAXS spectra of the solely G1.5 polyamidoamine dendrimers in water solution are presented for the two different concentrations of C = 5 wt.% of and C = 10 wt.%. The figure evidence the presence of a pronounced interference peak which is indicative of a long-range structural order in the system due to interparticle interaction in solution.
! ð2Þ
The results of the form factor data analysis for all the studied concentrations are summarized in the above inset of Fig. 1B. The obtained results in the dilute regime furnish an average values of Rg = 9.4 Å for the sphere radius and of Rg = 7.6 Å for the radius of gyration (reported in the above inset of Fig. 1B as dot dashed line). At higher concentration (i.e. for C ≥ 2 wt.%) a possible source of uncertainty in the determination of Rg is connected with the presence of the structure factor S(q) contribution to the SAXS spectra. On the other hand the information about the particle radius R, at the higher concentrations, can be retrieved as a result in connection with the analysis of the structure factor S(q). In order to obtain information about the inter-dendrimer interaction potential the charged dendrimers has been approximated as impenetrable spheres of radius R whose charge Ze is distributed on the surface. Those spheres are immersed in the uniform neutralising background of the solvent molecules which participates with its dielectric constant ε (ε = 78 for water) and which produces also a screening effect in the system. According to this model the repulsive potential between two identical spherical objects (macro-ions) of diameter σ = 2R placed at a distance r (centre to centre distance) is given by [26,27] U ðr Þ =
−κðr−σ Þ ðZ0 eÞ2 e 2 r 4πεð1 + κσ Þ
ð3Þ
Here κ is the Debye–Huckel inverse screening length which is determined, at a given temperature T, by the ionic strength I of the solvent (in mol/l), according to the following relation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 8πe2 Na I κ= εKB T × 103
ð4Þ
Here e is the unit of electron charge, KB the Boltzmann constant, Na the Avogadro number. Moreover a hard sphere type repulsive component for the potential has been adopted to represent the close contact inter-dendrimer interaction. In our specific case we have
D. Lombardo et al. / Journal of Non-Crystalline Solids 357 (2011) 771–774
investigated the observed inter-dendrimer structure factors S(q), obtained by SAXS experiments, in terms of the hypernetted chain approximation scheme (HNC) [27–29]. In Fig. 2 the numerical structure factor S(q) computed according the HNC scheme has been compared with the experimental structure factor obtained from SAXS spectra for the sample at concentration of C = 10 wt.%. From the figure it is clear how the adopted model reproduce quite satisfactorily the experimental results with an average dendrimer effective charge of Zeff = 10 ± 1 (in unit of electron charge |e|). From the finding our obtained results we can deduce that the carboxylate terminal groups of Pamam half integer dendrimers in water solution are partially dissociated (COO− Na+). In particular an average number of Zeff = 10 over the Zend = 16 total carboxylic endgroups per dendrimer (for the G = 1.5 generation) realize this ionization. This correspond to a rate of ionization near to Zeff/Zend = 0.6. In order to obtain direct information of the morphological features during self-assembly process all aluminosilicate reactants were preliminary mixed and diluted in water (with a dilution factor 20 in weight) and then added with a water solution of generation G1.5 Pamam dendrimers at the concentration of C = 1 wt.% [AS/Pamam]. Results of the dimension analysis as a function of the elapsed time after the mixing are reported in Fig. 3, which indicates that the radius of gyration Rg slightly increases with the reaction time, starting from Rg = 7.6 Å (for solely Pamam dendrimer in water solution) up to Rg = 9.4 Å (18 h after the mixing) and Rg = 9.8 Å (40 h after the mixing). This thickening of the original particles is caused by the condensed growth of the zeolitic phase onto the dendrimer substrate. The dendrimer carboxylate end groups, which are responsible for the long-range electrostatic interparticle interaction potential, create a diffuse layer of condensed Na + ions in the neighborhood of the dendrimer surface which act as the effective structure-directing agent. Rearrangement of aluminosilicates around hydrated Na+ cations, in the form of polyhedral cage structures, favorite a condensed growth of the zeolitic phase onto the dendrimer substrate. Due to the highly ramified nature of the dendrimer the possible Na + ions migration in the interior of the dendrimers may favorite the growth of the zeolite phase in the interior of the dendrimer. In this respect it is possible to state that the peculiar cage structure formation does not ensure a facile removal of the nanotemplate. On the other hand, relevant advantages of the choice of the dendrimers rely on the possibility to choice size and composition of the template substrate by changing number, typology, or spatial distribution of the terminal groups. In this sense the zeolites cage structure, acting as a selective barrier, may be used to promote selective reactions or incorporation of
Fig. 2. Comparison between the experimental structure factor S(q) (for the sample at dendrimer concentrations of C = 10 wt.%) and the one computed by means of the adopted model. The concentration dependence of the rate of ionization Zeff/Zend is reported in the inset.
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Fig. 3. Analysis of the radius of gyration Rg as a function of the elapsed time after the mixing of the main components. The symbols represent the sample for solely Pamam dendrimers (full circle), 18 h after the mixing (hollow circles) and 40 h after the mixing (hollow triangles).
key features of selected enzymes or metal complexes thus mimicking the enzyme functions. The screening of the repulsive interaction promotes also an entanglement process between the primary units, with the formation of large clusters of the order of one micron as evidenced by the Scanning Electron Microscopy (SEM) investigation (Fig. 4A). The back-scattered SEM image of the system, 7 days after the mixing, confirmed a condensation of the aluminosilicate components on the large aggregates surfaces as proved by the EDX microprobe
Fig. 4. Scanning electron microscopy images of the generated spherical supramolecular assembly for the Pamam G1.5/AS system investigated.
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zeolite LTA formation. This study, while confirming the main trends of our previous Dynamic Light Scattering (photon correlation spectroscopy) investigation in a bigger generation (G3.5) Pamam dendrimer [30], show the benefits of using a dendrimer nanotemplate for the nanoparticles formation process in zeolite syntheses especially for what concern the possibility to choose the size and composition of dendrimer substrate. Further investigations of these systems in solution are underway in order to discover the main feature which drives the size and morphology of the condensed surface structures as a function of the dendrimer typology and effective surface charge.
References
Fig. 5. X-ray diffraction pattern of the synthesis mixture after three weeks reaction at room temperature.
analysis (inset of Fig. 4A). Larger irregular aggregates of the order of few microns was also detected as reported by the scanning electron microscopy (SEM) image in Fig. 4B. The confirmation of the nature of the condensate phase is finally given by the X-ray diffraction experiment. The clear synthesis solution was left reacting at room temperature for 3 weeks and then was dried at T = 40 °C for 24 h. The collected solid, dried again at T = 80 °C for 12 h, was checked by XRD powder diffraction (Fig. 5) that confirmed, through the analysis of the position of the diffraction peaks, the formation of crystalline LTA zeolite. It is worth noticing that the screening of Na+ counterions, although producing a reduction in the effective surface charge with respect the number of terminal groups (ionization ratio less than one), does not compromise the colloid stability of dendrimers in solution. Up to date, in fact, no aggregation has been observed in dendrimer systems in solution, even with the addition of a given salt amount to the system [31]. On the other it is possible to assume that entanglement is favoured by reduction of the charge density at the surface of the nanoparticles cause by the progressive growth of the zeolitic phase onto the dendrimer substrate. 4. Conclusion In this paper we describe the charge interaction and formation of porous aggregates derived by the growth of zeolite LTA on a low generation G1.5 polyamidoamine (Pamam) dendrimer acting as nano-template. By using the liquid integral equation theory for charged systems in solution we estimated the presence of an average effective charge of Z = 10e in the surface of the dendrimer (over 16 total chargeable groups present in the dendrimer). Those charges, which are responsible of the long-range electrostatic interparticle interaction, act as the effective structure-directing agent for the
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