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Morphology of adsorption anionic polyelectrolytes to mica substrate in different environment visualized by atomic force microscopy
Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 153–156
Morphology of adsorption anionic polyelectrolytes to mica substrate in different environment visualized by atomic force microscopy Feng Zhaoa,b,1 , Yu-Kou Dub , Ji’an Tanga,∗ , Xing-Chang Lia , Ping Yangb a
CAS Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b Chemistry and Chemical Engineering Department, Suzhou University, Su Zhou 215006, China Received 19 March 2004; accepted 13 September 2004 Available online 19 November 2004
Abstract We have observed the morphology of an anionic polyelectrolyte (partially hydrolyzed polyacrylamide) (HPAM) adsorbed to mica surface in different environment using atomic force microscopy with molecular resolution. HPAM molecules deposited on the mica surface from aqueous solution at natural pH were observed to form a nanostructured network. However, the nanostructured network-to-globule conformational transition on mica can be directly visualized when the mica substrate with the predeposited HAPM molecules was immersed into chloroform. Meanwhile, necklace-like globule conformation on mica can be obtained in the presence of NaCl. © 2004 Elsevier B.V. All rights reserved. Keywords: Morphology; Atomic force microscopy (AFM); Network-to-globule conformational; HAPM molecules
1. Introduction The structure and morphology of polymers at solid–liquid interface is of recent interest [1–3]. Knowledge about these properties is important for applications in coating, surface cleaning, dispersion, wettability, cosmetics, controlled release, biosensors, and biomaterials [4–6]. Furthermore, interface modification and assembly have been extended to the design and construction of nanostructured materials [2,7–9], which is very important for the miniaturization of electronic devices in modern industry. Various molecular objects of different shapes and functionalities are available for constituting nanostructure and controlling the molecular morphology on solid substrate visualized by AFM [3,10–13]. In many instances, studies have shown that conformational changes of polymers on solid surface were found to be strongly condition-dependent, such as ∗
Corresponding author. E-mail addresses: [email protected] (F. Zhao), [email protected] (J. Tang). 1 Tel.: +86 1082615871; fax: +86 1082612084. 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.09.030
the charge density of polymers [14–17], the ionic strength of solution [18] and the quality of the solvent [19], etc. Predominantly however these studies have focused on the application of cationic polyelectrolytes, such as poly(2-vinylpyridine) (P2VP) and polysterene/poly(2-vinylpyridine) (PS-P2VP) [10,18–21], poly(methacryloylocyethldimethy benzylammonium chloride) (PMB-Cl) [7], poly(styrene)-blockpoly(methylmethacrylate) (PS-b-PMMA) [3] and poly(2dimethylamino) ethylmethacrylate-block-met hy(methacrylate) (DMA-MMA) [13]. Because cationic polyelectrolytes can be strongly adsorbed to negatively charged solid substrate (Si and mica) owing to electrostatic attraction forces, giving high enough resolution of the AFM images. Another, relatively less is known the surface morphology of anionic polyelectrolytes [22–24]. It is hard to attach negatively charged polyelectrolytes on a negatively charged mica or Si surface because of electrostatic repulsive forces. However, under a relatively high polyelectrolytes concentration, van der Waals attraction to the adsorbing surface could overwhelm electrostatic repulsive forces and force the polymers onto the mica surface, which allows the imaging of single polymer chains [25].
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F. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 153–156
In this study, we present first visualization of conformations of anionic polyelectrolytes (partially hydrolyzed polyacrylamide) (HPAM) deposited on the mica substrate by AFM. Meanwhile, conformational transition can be investigated in different environment. 2. Experimental 2.1. Materials Partially hydrolyzed polyacrylamide (HPAM) was a commercial product of Pfizer Co., USA, its molecular weight is stated to be 16.15 × 106 g/mol with the degree of hydrolysis 26–28%. Sodium chloride (NaCl) and chloroform were all analytical reagent grade (Beijing Chemical Factory, China). All water used was doubly distilled. 2.2. Atomic force microscopy The adsorbed HPAM molecules were visualized in dry state with an AFM instrument Multimode (Digital Instruments, CA) using silicon ultralecers in a tapping mode. Each mica sample was freshly cleaved just before use. Measurements were performed in the temperature range 20 ± 1 ◦ C.
3. Results and discussion The freshly cleaved mica substrate was dipped into 0.01 mg/mL HPAM solution for 30 min, and then dried with a stream of nitrogen. The sample was imaged by AFM in air (Fig. 1). The mica substrate was covered with a nanostructured network. Cross-section profiles show a width of 45.6 ± 2.77 nm, indicating that the network was not formed with individual chains, but with aggregates of the HAPM chains.
Fig. 1. Representative AFM topography image (5 m × 4 m scan) of HPAM deposited on mica from 0.01 mg/mL solution.
The conformational charges of HAPM molecules deposited on mica can occur due to the weak interaction with the substrate. The following experiments proved that the various morphology of the adsorbed HPAM chain could be visualized. The above mica plate was again immersed into chloroform for 30 min and then imaged in dry state with AFM (Fig. 2a). We found large changes in molecular conformations. HPAM molecules deposited on mica feature a much more compact globular conformation than the corresponding conformations obtained without dipping into chloroform (see Fig. 1). Shape transition from network-like aggregate to single globular molecular conformation is induced due to the quality of the solvent itself. HPAM is a water-soluble polyelectrolyte. Chloroform is a very poor solvent for HPAM. When the mica plate predeposited with HPAM molecules is dipped into chloroform, HPAM chains have negative second virial coefficients, corresponding to an effective attraction between monomers. This short-range attractive (i.e., solvophobic interaction) induces the flexible HPAM chains to col-
Fig. 2. AFM topographic images (5 m × 5 m scan) (a, b) and cross-sections (c) of HPAM deposited on mica from chloroform.
F. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 153–156
lapse into compact globular conformation [26]. (In chloroform, HPAM is analogous to a neutral polymer.) Similar mechanisms have been reported [26–28]. Recently, Minko and coworkers [18,29] and Kirwan et al. [30] have reported the conformational transition of an adsorbed polyelectrolytes on mica surface to a compact globule in experiments. A similar collapse has been also reported by Dedinaite et al. [31] in which the association between the polyelectrolyte and the surfactant results in poor solvency conditions. In order to lower molecular conformation energy as much as possible, the single globular conformation of HPAM formed in terms of a collapse of single HPAM chains are being imaged in contrast to the network-like aggregates of HPAM. This point also can be verified by evaluating an experimental average volume. The theoretical calculations of single HPAM molecules are given by Vcalc = M/NA , where M is the molecular weight, ρ the density of HPAM, about 1.2 g cm−3 , and NA the Avogadro’s number. Thus the single HAPM molecular volume is determined: VHPAM,calc = (16 150 000/6.02 × 1.2) = 2.2 × 104 nm3 . We also determined the average volume of single HPAM molecules, VHPAM,exp in AFM experiment. After analyzing over 120 isolated HPAM molecules in Fig. 2a, the average height and diameter of HPAM molecules were estimated to be 5.21 ± 0.20, 77.5 ± 2.77 nm, respectively. Crosssection profiles clearly show that HPAM adsorbed to mica were visualized to form cone-like structure (Fig. 2b and c). To evaluate the volume of the visualized structure for simplicity we used the following formula for the conelike shape: Vcone = 1/3H(D/2)2 , where H and D are the height and diameter of structure observed with AFM. So VHPAM,exp = 0.82 × 104 nm3 . The value is reasonable as compared with the theoretical volume of HPAM. A little larger discrepancy could result from a tip-induced deformation of sample and the small number of HPAM molecules that were measured (N = 120) [32,33]. Therefore, this result shown supports that single HPAM molecules are being imaged. Only recently have Kirwan et al. [30] proposed the similar treatment procedure to distinguish single polymer molecules on mica, but polymers used in their experiments are still cationic polyelectrolytes. Furthermore, our experiments also provide a simple approach for the visualization of single anionic polymer molecules adsorbed to mica. This method does not require complicatedly and trivially modify a solid substrate to a positively charged surface to adsorb anionic polyelectrolytes [22,23]. The freshly cleaved mica plate was immersed into 0.01 mg/mL HPAM aqueous solution containing a NaCl concentration of 0.01 mol/L for 30 min, and dried with a stream of nitrogen, then imaged with AFM (Fig. 3). Compared to Fig. 1, Fig. 3 clearly confirms the existence of pear-necklace structures. And it is attributed to charges of electrostatic repulsion between the charged monomers along chains. Adding salt has an effect of screening the intrachain electrostatic repulsion and increasing influence of hydrophobic attraction. As a result, the HPAM adopt a more compact globule con-
155
Fig. 3. AFM images (5 m × 5 m scan) of HPAM deposited on mica from 0.01 mg/mL aqueous solution containing NaCl concentration of 0.01 mol/L.
formation [19,34]. Transition between stretched chain, subglobules (necklace-like) and large compact globules has predicted by Dobrynin et al. [26], Solis and Cruz Olvera [35], Lyulin et al. [27] and Micka et al. [28] in recent years. However, less direct visualization evidence for the conformational transition is presented [16,30]. Here, our experimental results are in good agreement with theoretical predictions. Due to the contraction of molecules, the height of HPAM films is increased (from 1.33 ± 0.10 nm in Fig. 1 to 44.6 ± 4.55 nm in Fig. 3). Meanwhile, adding salt has an enhancing effect of screening the electrostatic interaction and promote the adsorption [19], thus the surface density of molecules in Fig. 3 is larger than that of molecules in Fig. 1. In Fig. 3 larger globular structures are present as well (marked by red arrow). Two possibilities exist to account for this phenomenon: (i) locally over high NaCl concentration in solution makes the charged HPAM to be only weakly charged due to the screening of electrostatic repulsive forces along chains; (ii) due to the weak interaction with mica substrate, the adsorbed HPAM molecules are forced into this structure as a result of drying the sample prior to imaging.
4. Conclusion In this study, we first visualize the morphology of anionic polyelectrolytes (polyacrylamide) (HPAM) deposited on mica substrate. Meanwhile, the conformational transition from network-like aggregate structure to compact globules was imaged. The method offers the possibility of to easily distinguish individual HPAM molecules. In addition, the high enough resolution of the AFM images confirm the existence of necklace-like globules, supporting recently developed theoretical predictions.
References [1] J. Kumaki, Y. Nishikawa, T. Hashimoto, J. Am. Chem. Soc. 118 (1996) 3321.
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F. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 153–156
[2] V. Percec, C.H. Ahn, G. Ungar, D.J.P. Yeardley, M. Moller, S.S. Sheiko, Nature 391 (1998) 161. [3] J. Kumaki, T. Hashimoto, J. Am. Chem. Soc. 125 (2003) 4907. [4] F. Tiberg, J. Brinck, L. Grant, Curr. Opin. Colloid Interf. Sci. 4 (1999) 411. [5] A.B. Artyukhin, K.J. Burnham, A.A. Levchenko, R.V. Talroze, P. Stroeve, Langmuir 19 (2003) 2243. [6] G. Kramer, P. Somasundaran, Langmuir 18 (2002) 9357. [7] A. Kiriy, S. Minko, G. Gorodyska, M. Stamm, W. Jaeger, Nano Lett. 2 (2002) 881. [8] S. Minko, A. Kiriy, G. Gorodyska, M. Stamm, J. Am. Chem. Soc. 124 (2002) 10192. [9] R.D. Piner, J. Zhu, F. Xu, S. Hong, C.A. Mirkin, Science 283 (1999) 661. [10] A. Kiriy, G. Gorodyska, S. Minko, M. Stamm, C. Tsitsilianis, Macromolecules 36 (2003) 8704. [11] I.I. Potemkin, E.Y. Kramarenko, A.R. Khokhlov, R.G. Winkler, P. Reineker, P. Eibeck, J.P. Spatz, M. Moller, Langmuir 15 (1999) 7290. [12] J.P. Spatz, S. Sheiko, M. Moller, Macromolecules 29 (1996) 3220. [13] G.B. Webber, E.J. Wanless, S.P. Armes, F.L. Baines, S. Biggs, Langmuir 17 (2001) 5551. [14] U. Voigt, W. Jaeger, G.H. Findenegg, R. von Klitzing, J. Phys. Chem. B 107 (2003) 5273. [15] B. Schoeler, G. Kumaraswamy, F. Caruso, Macromolecules 35 (2002) 889. [16] P.M. Claesson, A. Dedinaite, O.J. Rojas, Adv. Colloid Interf. Sci. 104 (2003) 53. [17] O.J. Rojas, M. Ernstsson, R.D. Neuman, P.M. Claesson, Langmuir 18 (2002) 1604. [18] S. Minko, A. Kiriy, G. Gorodyska, M. Stamm, J. Am. Chem. Soc. 124 (2002) 3218.
[19] E. Poptoshev, B. Schoeler, F. Caruso, Langmuir 20 (2004) 829. [20] I.I. Potemkin, E.Yu. Kramarenko, A.R. Khokhlov, R.G. Winkler, P. Reineker, P. Eibeck, J.P. Spatz, M. Moller, Langmuir 15 (1999) 7290. [21] J.P. Spatz, S. Sheiko, M. Moller, Macromolecules 29 (1996) 3220. [22] M. Regenbrecht, S. Akari, S. Forster, R.R. Netz, H. Mohwald, Nanotechnology 10 (1999) 434. [23] M. Zhu, M. Schneider, G. Papastavrou, S. Akari, H. Mohwald, Langmuir 17 (2001) 6471. [24] L. Bastardo, A. Dedinaite, Langmuir 18 (2002) 9383. [25] A.V. Dobrynin, M. Rubinstein, J. Phys. Chem. B 107 (2003) 8260. [26] A.V. Dobrynin, M. Rubinstein, S.P. Obukhov, Macromolecules 29 (1996) 2974. [27] A.V. Lyulin, B. Dunweg, O.V. Borisov, A.A. Darinskii, Macromolecules 32 (1999) 3264. [28] U. Micka, C. Holm, K. Kremer, Langmuir 15 (1999) 4033. [29] A. Kiriy, G. Gorodyska, S. Minko, W. Jaeger, P. Stepanek, M. Stamm, J. Am. Chem. Soc. 124 (2002) 13454. [30] L.J. Kirwan, G. Papastavrou, M. Borkovec, S.H. Behrens, Nano Lett. 4 (2004) 149. [31] A. Dedinaite, P.M. Claesson, M. Bergstrom, Langmuir 16 (2000) 5257. [32] S. Gao, L. Chi, S. Lenhert, B. Anczykowski, C.M. Niemeyer, M. Adler, H. Fuchs, ChemPhysChem 6 (2001) 384. [33] E. Balnois, S. Stoll, K.J. Wilkinson, J. Buffle, Macromolecules 33 (2000) 7440. [34] S.L. Clark, M.F. Montague, P. Hammond, Macromolecules 30 (1997) 7237. [35] F.J. Solis, de la M. Cruz Olvera, Macromolecules 31 (1998) 5502.
Morphology of adsorption anionic polyelectrolytes to mica substrate in different environment visualized by atomic force microscopy Feng Zhaoa,b,1 , Yu-Kou Dub , Ji’an Tanga,∗ , Xing-Chang Lia , Ping Yangb a
CAS Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b Chemistry and Chemical Engineering Department, Suzhou University, Su Zhou 215006, China Received 19 March 2004; accepted 13 September 2004 Available online 19 November 2004
Abstract We have observed the morphology of an anionic polyelectrolyte (partially hydrolyzed polyacrylamide) (HPAM) adsorbed to mica surface in different environment using atomic force microscopy with molecular resolution. HPAM molecules deposited on the mica surface from aqueous solution at natural pH were observed to form a nanostructured network. However, the nanostructured network-to-globule conformational transition on mica can be directly visualized when the mica substrate with the predeposited HAPM molecules was immersed into chloroform. Meanwhile, necklace-like globule conformation on mica can be obtained in the presence of NaCl. © 2004 Elsevier B.V. All rights reserved. Keywords: Morphology; Atomic force microscopy (AFM); Network-to-globule conformational; HAPM molecules
1. Introduction The structure and morphology of polymers at solid–liquid interface is of recent interest [1–3]. Knowledge about these properties is important for applications in coating, surface cleaning, dispersion, wettability, cosmetics, controlled release, biosensors, and biomaterials [4–6]. Furthermore, interface modification and assembly have been extended to the design and construction of nanostructured materials [2,7–9], which is very important for the miniaturization of electronic devices in modern industry. Various molecular objects of different shapes and functionalities are available for constituting nanostructure and controlling the molecular morphology on solid substrate visualized by AFM [3,10–13]. In many instances, studies have shown that conformational changes of polymers on solid surface were found to be strongly condition-dependent, such as ∗
Corresponding author. E-mail addresses: [email protected] (F. Zhao), [email protected] (J. Tang). 1 Tel.: +86 1082615871; fax: +86 1082612084. 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.09.030
the charge density of polymers [14–17], the ionic strength of solution [18] and the quality of the solvent [19], etc. Predominantly however these studies have focused on the application of cationic polyelectrolytes, such as poly(2-vinylpyridine) (P2VP) and polysterene/poly(2-vinylpyridine) (PS-P2VP) [10,18–21], poly(methacryloylocyethldimethy benzylammonium chloride) (PMB-Cl) [7], poly(styrene)-blockpoly(methylmethacrylate) (PS-b-PMMA) [3] and poly(2dimethylamino) ethylmethacrylate-block-met hy(methacrylate) (DMA-MMA) [13]. Because cationic polyelectrolytes can be strongly adsorbed to negatively charged solid substrate (Si and mica) owing to electrostatic attraction forces, giving high enough resolution of the AFM images. Another, relatively less is known the surface morphology of anionic polyelectrolytes [22–24]. It is hard to attach negatively charged polyelectrolytes on a negatively charged mica or Si surface because of electrostatic repulsive forces. However, under a relatively high polyelectrolytes concentration, van der Waals attraction to the adsorbing surface could overwhelm electrostatic repulsive forces and force the polymers onto the mica surface, which allows the imaging of single polymer chains [25].
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F. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 153–156
In this study, we present first visualization of conformations of anionic polyelectrolytes (partially hydrolyzed polyacrylamide) (HPAM) deposited on the mica substrate by AFM. Meanwhile, conformational transition can be investigated in different environment. 2. Experimental 2.1. Materials Partially hydrolyzed polyacrylamide (HPAM) was a commercial product of Pfizer Co., USA, its molecular weight is stated to be 16.15 × 106 g/mol with the degree of hydrolysis 26–28%. Sodium chloride (NaCl) and chloroform were all analytical reagent grade (Beijing Chemical Factory, China). All water used was doubly distilled. 2.2. Atomic force microscopy The adsorbed HPAM molecules were visualized in dry state with an AFM instrument Multimode (Digital Instruments, CA) using silicon ultralecers in a tapping mode. Each mica sample was freshly cleaved just before use. Measurements were performed in the temperature range 20 ± 1 ◦ C.
3. Results and discussion The freshly cleaved mica substrate was dipped into 0.01 mg/mL HPAM solution for 30 min, and then dried with a stream of nitrogen. The sample was imaged by AFM in air (Fig. 1). The mica substrate was covered with a nanostructured network. Cross-section profiles show a width of 45.6 ± 2.77 nm, indicating that the network was not formed with individual chains, but with aggregates of the HAPM chains.
Fig. 1. Representative AFM topography image (5 m × 4 m scan) of HPAM deposited on mica from 0.01 mg/mL solution.
The conformational charges of HAPM molecules deposited on mica can occur due to the weak interaction with the substrate. The following experiments proved that the various morphology of the adsorbed HPAM chain could be visualized. The above mica plate was again immersed into chloroform for 30 min and then imaged in dry state with AFM (Fig. 2a). We found large changes in molecular conformations. HPAM molecules deposited on mica feature a much more compact globular conformation than the corresponding conformations obtained without dipping into chloroform (see Fig. 1). Shape transition from network-like aggregate to single globular molecular conformation is induced due to the quality of the solvent itself. HPAM is a water-soluble polyelectrolyte. Chloroform is a very poor solvent for HPAM. When the mica plate predeposited with HPAM molecules is dipped into chloroform, HPAM chains have negative second virial coefficients, corresponding to an effective attraction between monomers. This short-range attractive (i.e., solvophobic interaction) induces the flexible HPAM chains to col-
Fig. 2. AFM topographic images (5 m × 5 m scan) (a, b) and cross-sections (c) of HPAM deposited on mica from chloroform.
F. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 153–156
lapse into compact globular conformation [26]. (In chloroform, HPAM is analogous to a neutral polymer.) Similar mechanisms have been reported [26–28]. Recently, Minko and coworkers [18,29] and Kirwan et al. [30] have reported the conformational transition of an adsorbed polyelectrolytes on mica surface to a compact globule in experiments. A similar collapse has been also reported by Dedinaite et al. [31] in which the association between the polyelectrolyte and the surfactant results in poor solvency conditions. In order to lower molecular conformation energy as much as possible, the single globular conformation of HPAM formed in terms of a collapse of single HPAM chains are being imaged in contrast to the network-like aggregates of HPAM. This point also can be verified by evaluating an experimental average volume. The theoretical calculations of single HPAM molecules are given by Vcalc = M/NA , where M is the molecular weight, ρ the density of HPAM, about 1.2 g cm−3 , and NA the Avogadro’s number. Thus the single HAPM molecular volume is determined: VHPAM,calc = (16 150 000/6.02 × 1.2) = 2.2 × 104 nm3 . We also determined the average volume of single HPAM molecules, VHPAM,exp in AFM experiment. After analyzing over 120 isolated HPAM molecules in Fig. 2a, the average height and diameter of HPAM molecules were estimated to be 5.21 ± 0.20, 77.5 ± 2.77 nm, respectively. Crosssection profiles clearly show that HPAM adsorbed to mica were visualized to form cone-like structure (Fig. 2b and c). To evaluate the volume of the visualized structure for simplicity we used the following formula for the conelike shape: Vcone = 1/3H(D/2)2 , where H and D are the height and diameter of structure observed with AFM. So VHPAM,exp = 0.82 × 104 nm3 . The value is reasonable as compared with the theoretical volume of HPAM. A little larger discrepancy could result from a tip-induced deformation of sample and the small number of HPAM molecules that were measured (N = 120) [32,33]. Therefore, this result shown supports that single HPAM molecules are being imaged. Only recently have Kirwan et al. [30] proposed the similar treatment procedure to distinguish single polymer molecules on mica, but polymers used in their experiments are still cationic polyelectrolytes. Furthermore, our experiments also provide a simple approach for the visualization of single anionic polymer molecules adsorbed to mica. This method does not require complicatedly and trivially modify a solid substrate to a positively charged surface to adsorb anionic polyelectrolytes [22,23]. The freshly cleaved mica plate was immersed into 0.01 mg/mL HPAM aqueous solution containing a NaCl concentration of 0.01 mol/L for 30 min, and dried with a stream of nitrogen, then imaged with AFM (Fig. 3). Compared to Fig. 1, Fig. 3 clearly confirms the existence of pear-necklace structures. And it is attributed to charges of electrostatic repulsion between the charged monomers along chains. Adding salt has an effect of screening the intrachain electrostatic repulsion and increasing influence of hydrophobic attraction. As a result, the HPAM adopt a more compact globule con-
155
Fig. 3. AFM images (5 m × 5 m scan) of HPAM deposited on mica from 0.01 mg/mL aqueous solution containing NaCl concentration of 0.01 mol/L.
formation [19,34]. Transition between stretched chain, subglobules (necklace-like) and large compact globules has predicted by Dobrynin et al. [26], Solis and Cruz Olvera [35], Lyulin et al. [27] and Micka et al. [28] in recent years. However, less direct visualization evidence for the conformational transition is presented [16,30]. Here, our experimental results are in good agreement with theoretical predictions. Due to the contraction of molecules, the height of HPAM films is increased (from 1.33 ± 0.10 nm in Fig. 1 to 44.6 ± 4.55 nm in Fig. 3). Meanwhile, adding salt has an enhancing effect of screening the electrostatic interaction and promote the adsorption [19], thus the surface density of molecules in Fig. 3 is larger than that of molecules in Fig. 1. In Fig. 3 larger globular structures are present as well (marked by red arrow). Two possibilities exist to account for this phenomenon: (i) locally over high NaCl concentration in solution makes the charged HPAM to be only weakly charged due to the screening of electrostatic repulsive forces along chains; (ii) due to the weak interaction with mica substrate, the adsorbed HPAM molecules are forced into this structure as a result of drying the sample prior to imaging.
4. Conclusion In this study, we first visualize the morphology of anionic polyelectrolytes (polyacrylamide) (HPAM) deposited on mica substrate. Meanwhile, the conformational transition from network-like aggregate structure to compact globules was imaged. The method offers the possibility of to easily distinguish individual HPAM molecules. In addition, the high enough resolution of the AFM images confirm the existence of necklace-like globules, supporting recently developed theoretical predictions.
References [1] J. Kumaki, Y. Nishikawa, T. Hashimoto, J. Am. Chem. Soc. 118 (1996) 3321.
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F. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 153–156
[2] V. Percec, C.H. Ahn, G. Ungar, D.J.P. Yeardley, M. Moller, S.S. Sheiko, Nature 391 (1998) 161. [3] J. Kumaki, T. Hashimoto, J. Am. Chem. Soc. 125 (2003) 4907. [4] F. Tiberg, J. Brinck, L. Grant, Curr. Opin. Colloid Interf. Sci. 4 (1999) 411. [5] A.B. Artyukhin, K.J. Burnham, A.A. Levchenko, R.V. Talroze, P. Stroeve, Langmuir 19 (2003) 2243. [6] G. Kramer, P. Somasundaran, Langmuir 18 (2002) 9357. [7] A. Kiriy, S. Minko, G. Gorodyska, M. Stamm, W. Jaeger, Nano Lett. 2 (2002) 881. [8] S. Minko, A. Kiriy, G. Gorodyska, M. Stamm, J. Am. Chem. Soc. 124 (2002) 10192. [9] R.D. Piner, J. Zhu, F. Xu, S. Hong, C.A. Mirkin, Science 283 (1999) 661. [10] A. Kiriy, G. Gorodyska, S. Minko, M. Stamm, C. Tsitsilianis, Macromolecules 36 (2003) 8704. [11] I.I. Potemkin, E.Y. Kramarenko, A.R. Khokhlov, R.G. Winkler, P. Reineker, P. Eibeck, J.P. Spatz, M. Moller, Langmuir 15 (1999) 7290. [12] J.P. Spatz, S. Sheiko, M. Moller, Macromolecules 29 (1996) 3220. [13] G.B. Webber, E.J. Wanless, S.P. Armes, F.L. Baines, S. Biggs, Langmuir 17 (2001) 5551. [14] U. Voigt, W. Jaeger, G.H. Findenegg, R. von Klitzing, J. Phys. Chem. B 107 (2003) 5273. [15] B. Schoeler, G. Kumaraswamy, F. Caruso, Macromolecules 35 (2002) 889. [16] P.M. Claesson, A. Dedinaite, O.J. Rojas, Adv. Colloid Interf. Sci. 104 (2003) 53. [17] O.J. Rojas, M. Ernstsson, R.D. Neuman, P.M. Claesson, Langmuir 18 (2002) 1604. [18] S. Minko, A. Kiriy, G. Gorodyska, M. Stamm, J. Am. Chem. Soc. 124 (2002) 3218.
[19] E. Poptoshev, B. Schoeler, F. Caruso, Langmuir 20 (2004) 829. [20] I.I. Potemkin, E.Yu. Kramarenko, A.R. Khokhlov, R.G. Winkler, P. Reineker, P. Eibeck, J.P. Spatz, M. Moller, Langmuir 15 (1999) 7290. [21] J.P. Spatz, S. Sheiko, M. Moller, Macromolecules 29 (1996) 3220. [22] M. Regenbrecht, S. Akari, S. Forster, R.R. Netz, H. Mohwald, Nanotechnology 10 (1999) 434. [23] M. Zhu, M. Schneider, G. Papastavrou, S. Akari, H. Mohwald, Langmuir 17 (2001) 6471. [24] L. Bastardo, A. Dedinaite, Langmuir 18 (2002) 9383. [25] A.V. Dobrynin, M. Rubinstein, J. Phys. Chem. B 107 (2003) 8260. [26] A.V. Dobrynin, M. Rubinstein, S.P. Obukhov, Macromolecules 29 (1996) 2974. [27] A.V. Lyulin, B. Dunweg, O.V. Borisov, A.A. Darinskii, Macromolecules 32 (1999) 3264. [28] U. Micka, C. Holm, K. Kremer, Langmuir 15 (1999) 4033. [29] A. Kiriy, G. Gorodyska, S. Minko, W. Jaeger, P. Stepanek, M. Stamm, J. Am. Chem. Soc. 124 (2002) 13454. [30] L.J. Kirwan, G. Papastavrou, M. Borkovec, S.H. Behrens, Nano Lett. 4 (2004) 149. [31] A. Dedinaite, P.M. Claesson, M. Bergstrom, Langmuir 16 (2000) 5257. [32] S. Gao, L. Chi, S. Lenhert, B. Anczykowski, C.M. Niemeyer, M. Adler, H. Fuchs, ChemPhysChem 6 (2001) 384. [33] E. Balnois, S. Stoll, K.J. Wilkinson, J. Buffle, Macromolecules 33 (2000) 7440. [34] S.L. Clark, M.F. Montague, P. Hammond, Macromolecules 30 (1997) 7237. [35] F.J. Solis, de la M. Cruz Olvera, Macromolecules 31 (1998) 5502.