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Grafting of chains organo-silane on silica surface: a quantum chemical investigation
Chemical Physics Letters 400 (2004) 353–356 www.elsevier.com/locate/cplett
Grafting of chains organo-silane on silica surface: a quantum chemical investigation A. Dkhissi a
a,*
, A. Este`ve a, L. Jeloaica a, D. Este`ve a, M. Djafari Rouhani
a,b
Laboratoire dÕAnalyse et dÕArchitecture des Syste`mes, CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse, France b Laboratoire de Physique des Solides, 118 Route de Narbonne, 31062 Toulouse, France Received 15 September 2004; in final form 14 October 2004
Abstract Theoretical calculations have been carried out on the grafting of two chains organo-silane compounds on SiO2 hydroxylated solid surfaces. Considering two different silylated coupling agents, two grafting stable complexes are obtained. These complexes are stabilized by two interactions: (i) the chain is grafted to the cluster with a covalent bond SiAOASi; (ii) the chain interacts with the cluster via an hydrogen bond HAO O in the other side of the chain. The electronic, geometrical and vibrational properties of these systems are analysed. These results give new insight about the grafting of long chains organo-silane on silica surfaces. 2004 Elsevier B.V. All rights reserved.
1. Introduction There have been increasing activities in the development of bioactive and biocompatible nanomaterials for a variety of applications. For this purpose, inorganic nanocrystals and nanoparticules are bioconjugated with the attachment of DNA [1], peptides [2], and proteins [3]. It is well known that in order to have reproducible DNA chips, the main difficulty is the control of the chemical properties of the modified substrates. Therefore, in practice, modifying the surfaces and depositing DNA molecules is generally performed with the grafting of long polymers chains that recognizes the biomolecules and also stabilizes the nanoparticles. It is usually accepted that the use of long-chains alkyl-trichlorosilanes are suitable to produce a highly oriented film in which the chains are nearly perpendicularly oriented to the silica surface [4]. Note that the direct conjugation of biological species to inorganic nanoparticles has been already demonstrated possible very recently [5]. Our pre*
Corresponding author. Fax: +33 561336208. E-mail address: [email protected] (A. Dkhissi).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.116
liminary aim is to investigate the basic atomistic mechanisms that govern the grafting procedure via quantum calculations. In a second step, this effort could eventually allow us to propose new simulation tools at the mesoscopic scale dedicated to the experimental process calibration. We have thus started our study with a very simple case, since we perform our calculations with quantum chemical calculations which are computational costly. The interaction between different silylated agents, with only few alkyl chains, (trichlorosilanes (Structure 1) and trihydroxysilanes (Structure 2) and Si2O7H6 molecule is studied. The Si2O7H6 molecule is considered as a model of the substrate. These silyl coupling agents, while being the suitable one for DNA chip applications [6], were synthesized by the BennetauÕs group [7]. The hydroxyl groups (OH)3 in the case of trihydroxysilane molecules are generated from a previous hydrolysis of (Cl)3 termined chain under water exposure. Therefore, chlorine atoms should never remain on the molecule, however, a comparative study of the two monolayers adsorpted on silica surface is very interesting. So, two stable grafting complexes are obtained and the calculations indicate that, for each complex, the chain is grafted
354
A. Dkhissi et al. / Chemical Physics Letters 400 (2004) 353–356
in one way to the cluster with a strong covalent bond SiAOASi, and interacts by electrostatic forces in the other side to the cluster by moderated hydrogen bond O HAO. In the present Letter, we will focus our analysis on these two interaction behaviour. Note that all calculations are performed with DFT/ B3LYP method combined with a standard basis set 6-31 + G** by means of the GAUSSIAN 98 package.
2. Methodology The structure and vibrational properties of each system have been calculated with the density functional theory using the combined BeckeÕs three parameter exchange functional and the gradient-corrected functional of Lee, Yang and Parr [8,9]. All calculations have been performed with the standard basis set 6-31 + G**. The choice of this basis set is based on the consideration that in order to obtain reliable properties of intermolecular systems, it is essential to employ basis sets that possesses sufficient diffuseness and angular flexibility [10]. Further this basis set is sufficient to predict reliable properties for intermolecular systems, as demonstrated in [11]. The IR frequencies and intensities have been computed using analytical derivative procedures implemented in the GAUSSIAN 98 program [12].
Fig. 1. Structure of trichlorosilanes.
3. Results and discussion The grafting between the two organo-silane (Figs. 1 and 2) and the cluster can be written as the following reactions: Chain ð1Þ þ Si2 O7 H6 ! complex ð1Þ þ HCl
ð1Þ
Chain ð2Þ þ Si2 O7 H6 ! complex ð2Þ þ H2 O
ð2Þ
Complex (1) and complex (2) are described in Figs. 3 and 4.
4. Energetic and structural properties The main structural parameters of complexes (1) and (2) are summarized in Table 1. These systems are stabilised by two interactions. The chains are covalently bonded to the cluster with SiAOASi bond with an angles of 158.0 and 167.6 for complexes (1) and (2), respectively. The elongation of the bond length of SiAO is about 0.007 and 0.025 ˚ with respect to the same bond length of the systems A before grafting. This difference suggest that the complex (2) is more stable than (1). The second interaction is an electrostatic force defined by one hydrogen bond OAH O. The predicted H-bond angles are nearly lin-
Fig. 2. Structure of trihydroxysilanes.
ear of 172.8 and 176.4 of complex (1) and (2), respectively. The intermolecular distance R(OAO) in the case of complex (2) is slightly shorter than in complex (1)
A. Dkhissi et al. / Chemical Physics Letters 400 (2004) 353–356
355
Table 1 Selected structural data for complexesa Complex (1)
Complex (2)
Covalent bond: SiAOASi R(SiAO) Æ(SiOSi)
1.638 (1.645)b 158.0
1.620 (1.645) 167.6
Hydrogen bond: OAH H R(O O) R(OAH) Æ(OHO) R(O O) R(OAH) Æ(OHO)
2.856 0.975 (0.965) 172.8 – – –
2.836 0.977 (0.965) 176.4 2.851 0.975 172.4
General parameters Æ(SiOSi) cluster Æ(SiOC) chain
139.9 (145.5) 150.9 (146.0)
146.3 (145.5) 129.2 (147.4)
˚ ) and angles in (). Distances are in (A Numbers in parentheses correspond to the bond length of the free system. a
Fig. 3. Structure of the interaction between trichlorosilanes and Si2O7H6 molecule.
Fig. 4. Structure of the interaction between trihydroxysilanes and Si2O7H6 molecule.
˚ compared to 2.856 A ˚ ). These values are in good (2.836 A agreement with the mean of experimental data in the case of moderated hydrogen bonded complexes [13]. The elongation of bond lengths of (OH) is nearly similar for each complex. Furthermore in the case of complex (2), there is another intermolecular hydrogen bond between the chain and the cluster. The predicted values for this interaction are summarised in Table 1. The hypothetical local presence of an other hydroxyl group near the grafting site induces a multi-point intermolecular interaction and therefore, a non perpendicular positioning of the grafted molecule). However, in the case of complex (1), an intramolecular hydrogen bond (in the substrate) is obtained after complexation.
b
In the complex (1), the trichlosilane chain remains nearly linear after grafting. For this purpose, the angle Æ(SiOC) varies from 150.9 to 146.0, while in the case of complex (2), the same angle in the trihydroxysilane chain changes largely from 129.2 to 147.4. The binding energies of the two complexes (1) and (2) are 8.97 and 11.83 kcal/mol, respectively. The interaction energies are low due to a large deformation of the Si2O7H6 molecule. The difference in the interaction energies of the two grafting systems is due, in a large part, to the presence of an extra intermolecular interaction in the complex (2) compared to complex (1). This results confirms the changes (after grafting) in the structural parameters obtained for the two systems. Note that the corrected BSSE is not taken into account in the calculations as we have demonstrated very recently [14] that this correction with 6-31 + G** basis set is not important. Another important physical property in the case of grafting on the surface is the charge transfer between the molecule and the surface. Here, our calculation indicates that a partial electron transfer between the cluster and the chains is transferred, especially in the case of the chemisorption of the trihydroxysilanes on the cluster which shows a partial negative charge (0.42 e) located on the cluster and a partial positive charge on the chain.
5. Vibrational analysis Table 2 summarizes the theoretical frequencies and their intensities for the main important and intense frequencies (most characteristic modes). The calculated infrared spectrum indicates that the SiAO stretching frequency in the grafted region evolves from 940 to 1155 and 1157 cm1 for the two complexes (1) and (2),
356
A. Dkhissi et al. / Chemical Physics Letters 400 (2004) 353–356
Table 2 Vibrational analysis of the complexes Complex (1) Frequency m(SiO) grafted m(SiO) cluster m(C@O) m(OH) bonded m(OH) bonded m(OH) bonded (intramolecular)
1155 1076 1820 3670 – 3788
Complex (2) Intensity
Frequency
Intensity
(220)
669 984 271 724
(102)
207
1157 1100 1816 3632 3658 –
689 696 294 372 949 –
(215)a (43)
(217) (19) (258) (232)
a Frequency shift: the shifts are given with respect to the monomers or units before complexation. For the Si2O7H6 monomer, the frequencies m(OH) and m(SiAO) are the average of the all stretching frequencies.
respectively. This is due to the decrease in SiAO bond length for each complex with respect to the system after grafting. These important frequency shifts are significant and should help experimental investigations, especially infrared spectroscopy, to analyse the SiAO stretching frequency before and after grafting without confuse. In contrast to the SiAO stretching frequency, the formation of hydrogen bond OAH O, decreases the frequency of the OAH bond stretching. The frequency shifts calculated are about 220 and 258 cm1 for the two complexes. Indeed this result confirms that the complex (2) is more stable than complex (1). Based on the demonstrated accuracy of the DFT methodology to predict the vibrational frequencies, we believe that our theoretical results obtained here are reliable enough to help experimental groups to interpret their experimental spectra.
6. Conclusions By using quantum chemistry, we have studied the grafting of two chains organo-silanes on the Si2O7H6 molecule. Two slightly stronger systems are obtained
(complexes (1) and (2)). These systems are stabilised by different interactions: a covalent bond SiAOASi in one side and by hydrogen bonds O HAO in the other side. On the basis of both energetical and vibrational properties, we can expect that the grafting is slightly more suitable with trihydroxysilanes than trichlorosilanes. On-going research is now engaged to ameliorate the surface description and to investigate the self-stabilization of two precursor molecules on the substrate. These data are the first results of our project, which needs to study a real cluster of silica.
Acknowledgement The work is supported by the European Commission ÔATOMCADÕ (Project: HPRN-CT-1999.00048).
References [1] J.J. Storhoff, C.A. Mirkin, Chem. Rev. 99 (1999) 1849. [2] S.R. Whaley, D.S. English, E.L. Hu, P.F. Barbara, A.M. Belcher, Nature 405 (2000) 665. [3] W.C.W. Chan, S. Nie, Science 281 (1998) 2016. [4] N. Navarre, F. Choplin, J. Bousbaa, B. Bennetau, L. Nony, J.P. Aime, Langmuir 17 (2001) 4844. [5] M.J. Meziani, Y.P. Sun, J. Am. Chem. Soc. 125 (2003) 8015. [6] B. Bennetau, J. Bousbaa, F. Choplin, J.P. Cloarec, J.R. Martin, E. Souteyrand, Fr. Demand No. 00 00697 (2000). [7] B. Bennetau, J. Bousbaa, F. Choplin, Fr. Demand No. 00 00695 (2000). [8] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [9] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [10] G. Chalasinski, M. Szczesniak, Chem. Rev. 94 (1994) 1723. [11] A. Dkhissi, L. Adamowicz, G. Maes, J. Phys. Chem A 104 (2000) 2112. [12] M.J. Frisch et al., GAUSSIAN 98, Revision A.7, Gaussian Inc., Pittsburgh, PA, 1998. [13] A. Held, W.D. Pratt, J. Am. Chem. Soc. 115 (1993) 9708. [14] A. Dkhissi, L. Adamowicz, G. Maes, Chem. Phys. Lett. 324 (2000) 127.
Grafting of chains organo-silane on silica surface: a quantum chemical investigation A. Dkhissi a
a,*
, A. Este`ve a, L. Jeloaica a, D. Este`ve a, M. Djafari Rouhani
a,b
Laboratoire dÕAnalyse et dÕArchitecture des Syste`mes, CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse, France b Laboratoire de Physique des Solides, 118 Route de Narbonne, 31062 Toulouse, France Received 15 September 2004; in final form 14 October 2004
Abstract Theoretical calculations have been carried out on the grafting of two chains organo-silane compounds on SiO2 hydroxylated solid surfaces. Considering two different silylated coupling agents, two grafting stable complexes are obtained. These complexes are stabilized by two interactions: (i) the chain is grafted to the cluster with a covalent bond SiAOASi; (ii) the chain interacts with the cluster via an hydrogen bond HAO O in the other side of the chain. The electronic, geometrical and vibrational properties of these systems are analysed. These results give new insight about the grafting of long chains organo-silane on silica surfaces. 2004 Elsevier B.V. All rights reserved.
1. Introduction There have been increasing activities in the development of bioactive and biocompatible nanomaterials for a variety of applications. For this purpose, inorganic nanocrystals and nanoparticules are bioconjugated with the attachment of DNA [1], peptides [2], and proteins [3]. It is well known that in order to have reproducible DNA chips, the main difficulty is the control of the chemical properties of the modified substrates. Therefore, in practice, modifying the surfaces and depositing DNA molecules is generally performed with the grafting of long polymers chains that recognizes the biomolecules and also stabilizes the nanoparticles. It is usually accepted that the use of long-chains alkyl-trichlorosilanes are suitable to produce a highly oriented film in which the chains are nearly perpendicularly oriented to the silica surface [4]. Note that the direct conjugation of biological species to inorganic nanoparticles has been already demonstrated possible very recently [5]. Our pre*
Corresponding author. Fax: +33 561336208. E-mail address: [email protected] (A. Dkhissi).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.116
liminary aim is to investigate the basic atomistic mechanisms that govern the grafting procedure via quantum calculations. In a second step, this effort could eventually allow us to propose new simulation tools at the mesoscopic scale dedicated to the experimental process calibration. We have thus started our study with a very simple case, since we perform our calculations with quantum chemical calculations which are computational costly. The interaction between different silylated agents, with only few alkyl chains, (trichlorosilanes (Structure 1) and trihydroxysilanes (Structure 2) and Si2O7H6 molecule is studied. The Si2O7H6 molecule is considered as a model of the substrate. These silyl coupling agents, while being the suitable one for DNA chip applications [6], were synthesized by the BennetauÕs group [7]. The hydroxyl groups (OH)3 in the case of trihydroxysilane molecules are generated from a previous hydrolysis of (Cl)3 termined chain under water exposure. Therefore, chlorine atoms should never remain on the molecule, however, a comparative study of the two monolayers adsorpted on silica surface is very interesting. So, two stable grafting complexes are obtained and the calculations indicate that, for each complex, the chain is grafted
354
A. Dkhissi et al. / Chemical Physics Letters 400 (2004) 353–356
in one way to the cluster with a strong covalent bond SiAOASi, and interacts by electrostatic forces in the other side to the cluster by moderated hydrogen bond O HAO. In the present Letter, we will focus our analysis on these two interaction behaviour. Note that all calculations are performed with DFT/ B3LYP method combined with a standard basis set 6-31 + G** by means of the GAUSSIAN 98 package.
2. Methodology The structure and vibrational properties of each system have been calculated with the density functional theory using the combined BeckeÕs three parameter exchange functional and the gradient-corrected functional of Lee, Yang and Parr [8,9]. All calculations have been performed with the standard basis set 6-31 + G**. The choice of this basis set is based on the consideration that in order to obtain reliable properties of intermolecular systems, it is essential to employ basis sets that possesses sufficient diffuseness and angular flexibility [10]. Further this basis set is sufficient to predict reliable properties for intermolecular systems, as demonstrated in [11]. The IR frequencies and intensities have been computed using analytical derivative procedures implemented in the GAUSSIAN 98 program [12].
Fig. 1. Structure of trichlorosilanes.
3. Results and discussion The grafting between the two organo-silane (Figs. 1 and 2) and the cluster can be written as the following reactions: Chain ð1Þ þ Si2 O7 H6 ! complex ð1Þ þ HCl
ð1Þ
Chain ð2Þ þ Si2 O7 H6 ! complex ð2Þ þ H2 O
ð2Þ
Complex (1) and complex (2) are described in Figs. 3 and 4.
4. Energetic and structural properties The main structural parameters of complexes (1) and (2) are summarized in Table 1. These systems are stabilised by two interactions. The chains are covalently bonded to the cluster with SiAOASi bond with an angles of 158.0 and 167.6 for complexes (1) and (2), respectively. The elongation of the bond length of SiAO is about 0.007 and 0.025 ˚ with respect to the same bond length of the systems A before grafting. This difference suggest that the complex (2) is more stable than (1). The second interaction is an electrostatic force defined by one hydrogen bond OAH O. The predicted H-bond angles are nearly lin-
Fig. 2. Structure of trihydroxysilanes.
ear of 172.8 and 176.4 of complex (1) and (2), respectively. The intermolecular distance R(OAO) in the case of complex (2) is slightly shorter than in complex (1)
A. Dkhissi et al. / Chemical Physics Letters 400 (2004) 353–356
355
Table 1 Selected structural data for complexesa Complex (1)
Complex (2)
Covalent bond: SiAOASi R(SiAO) Æ(SiOSi)
1.638 (1.645)b 158.0
1.620 (1.645) 167.6
Hydrogen bond: OAH H R(O O) R(OAH) Æ(OHO) R(O O) R(OAH) Æ(OHO)
2.856 0.975 (0.965) 172.8 – – –
2.836 0.977 (0.965) 176.4 2.851 0.975 172.4
General parameters Æ(SiOSi) cluster Æ(SiOC) chain
139.9 (145.5) 150.9 (146.0)
146.3 (145.5) 129.2 (147.4)
˚ ) and angles in (). Distances are in (A Numbers in parentheses correspond to the bond length of the free system. a
Fig. 3. Structure of the interaction between trichlorosilanes and Si2O7H6 molecule.
Fig. 4. Structure of the interaction between trihydroxysilanes and Si2O7H6 molecule.
˚ compared to 2.856 A ˚ ). These values are in good (2.836 A agreement with the mean of experimental data in the case of moderated hydrogen bonded complexes [13]. The elongation of bond lengths of (OH) is nearly similar for each complex. Furthermore in the case of complex (2), there is another intermolecular hydrogen bond between the chain and the cluster. The predicted values for this interaction are summarised in Table 1. The hypothetical local presence of an other hydroxyl group near the grafting site induces a multi-point intermolecular interaction and therefore, a non perpendicular positioning of the grafted molecule). However, in the case of complex (1), an intramolecular hydrogen bond (in the substrate) is obtained after complexation.
b
In the complex (1), the trichlosilane chain remains nearly linear after grafting. For this purpose, the angle Æ(SiOC) varies from 150.9 to 146.0, while in the case of complex (2), the same angle in the trihydroxysilane chain changes largely from 129.2 to 147.4. The binding energies of the two complexes (1) and (2) are 8.97 and 11.83 kcal/mol, respectively. The interaction energies are low due to a large deformation of the Si2O7H6 molecule. The difference in the interaction energies of the two grafting systems is due, in a large part, to the presence of an extra intermolecular interaction in the complex (2) compared to complex (1). This results confirms the changes (after grafting) in the structural parameters obtained for the two systems. Note that the corrected BSSE is not taken into account in the calculations as we have demonstrated very recently [14] that this correction with 6-31 + G** basis set is not important. Another important physical property in the case of grafting on the surface is the charge transfer between the molecule and the surface. Here, our calculation indicates that a partial electron transfer between the cluster and the chains is transferred, especially in the case of the chemisorption of the trihydroxysilanes on the cluster which shows a partial negative charge (0.42 e) located on the cluster and a partial positive charge on the chain.
5. Vibrational analysis Table 2 summarizes the theoretical frequencies and their intensities for the main important and intense frequencies (most characteristic modes). The calculated infrared spectrum indicates that the SiAO stretching frequency in the grafted region evolves from 940 to 1155 and 1157 cm1 for the two complexes (1) and (2),
356
A. Dkhissi et al. / Chemical Physics Letters 400 (2004) 353–356
Table 2 Vibrational analysis of the complexes Complex (1) Frequency m(SiO) grafted m(SiO) cluster m(C@O) m(OH) bonded m(OH) bonded m(OH) bonded (intramolecular)
1155 1076 1820 3670 – 3788
Complex (2) Intensity
Frequency
Intensity
(220)
669 984 271 724
(102)
207
1157 1100 1816 3632 3658 –
689 696 294 372 949 –
(215)a (43)
(217) (19) (258) (232)
a Frequency shift: the shifts are given with respect to the monomers or units before complexation. For the Si2O7H6 monomer, the frequencies m(OH) and m(SiAO) are the average of the all stretching frequencies.
respectively. This is due to the decrease in SiAO bond length for each complex with respect to the system after grafting. These important frequency shifts are significant and should help experimental investigations, especially infrared spectroscopy, to analyse the SiAO stretching frequency before and after grafting without confuse. In contrast to the SiAO stretching frequency, the formation of hydrogen bond OAH O, decreases the frequency of the OAH bond stretching. The frequency shifts calculated are about 220 and 258 cm1 for the two complexes. Indeed this result confirms that the complex (2) is more stable than complex (1). Based on the demonstrated accuracy of the DFT methodology to predict the vibrational frequencies, we believe that our theoretical results obtained here are reliable enough to help experimental groups to interpret their experimental spectra.
6. Conclusions By using quantum chemistry, we have studied the grafting of two chains organo-silanes on the Si2O7H6 molecule. Two slightly stronger systems are obtained
(complexes (1) and (2)). These systems are stabilised by different interactions: a covalent bond SiAOASi in one side and by hydrogen bonds O HAO in the other side. On the basis of both energetical and vibrational properties, we can expect that the grafting is slightly more suitable with trihydroxysilanes than trichlorosilanes. On-going research is now engaged to ameliorate the surface description and to investigate the self-stabilization of two precursor molecules on the substrate. These data are the first results of our project, which needs to study a real cluster of silica.
Acknowledgement The work is supported by the European Commission ÔATOMCADÕ (Project: HPRN-CT-1999.00048).
References [1] J.J. Storhoff, C.A. Mirkin, Chem. Rev. 99 (1999) 1849. [2] S.R. Whaley, D.S. English, E.L. Hu, P.F. Barbara, A.M. Belcher, Nature 405 (2000) 665. [3] W.C.W. Chan, S. Nie, Science 281 (1998) 2016. [4] N. Navarre, F. Choplin, J. Bousbaa, B. Bennetau, L. Nony, J.P. Aime, Langmuir 17 (2001) 4844. [5] M.J. Meziani, Y.P. Sun, J. Am. Chem. Soc. 125 (2003) 8015. [6] B. Bennetau, J. Bousbaa, F. Choplin, J.P. Cloarec, J.R. Martin, E. Souteyrand, Fr. Demand No. 00 00697 (2000). [7] B. Bennetau, J. Bousbaa, F. Choplin, Fr. Demand No. 00 00695 (2000). [8] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [9] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [10] G. Chalasinski, M. Szczesniak, Chem. Rev. 94 (1994) 1723. [11] A. Dkhissi, L. Adamowicz, G. Maes, J. Phys. Chem A 104 (2000) 2112. [12] M.J. Frisch et al., GAUSSIAN 98, Revision A.7, Gaussian Inc., Pittsburgh, PA, 1998. [13] A. Held, W.D. Pratt, J. Am. Chem. Soc. 115 (1993) 9708. [14] A. Dkhissi, L. Adamowicz, G. Maes, Chem. Phys. Lett. 324 (2000) 127.