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Interaction forces between metal-chelating lipid monolayers measured by colloidal probe atomic force microscopy
Colloids and Surfaces A: Physicochemical and Engineering Aspects 146 (1999) 329 – 335
Interaction forces between metal-chelating lipid monolayers measured by colloidal probe atomic force microscopy Ryo Ishiguro a,1, Darryl Y. Sasaki b, Charles Pacheco b, Kazue Kurihara a,* a
Institute for Chemical Reaction Science, Tohoku Uni6ersity, Katahira, 980 -8577 Sendai, Japan b Sandia National Laboratory, Albuquerque, NM 87185, USA Received 14 July 1998; accepted 15 September 1998
Abstract Surface forces between LB films of metal-chelating lipids in water have been studied using colloidal probe atomic force microscopy. The LB films of an amphiphile functionalized by the iminodiacetic acid group were prepared on hydrophobic glass substrates. The electric double layer repulsion operated between these LB film surfaces changed depending on pH reflecting the different protonation states of the iminodiacetic acid groups. The titration curve of the iminodiacetic acid monolayer was obtained from the force profiles. The Cu2 + complexation process was also monitored by measuring the force profiles at various Cu2 + ion concentrations. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Interaction forces; Lipid monolayers; Colloidal probe atomic force microscopy
1. Introduction The surface forces measurement is widely accepted as an essential tool for investigating intermolecular and surface interactions [1]. An instrument called ‘surface forces apparatus’ (SFA) has been used for a long time for measuring interaction force – distance profiles at a molecular resolution, though it limits the variation of sample surfaces practically to mica and modified-mica. * Corresponding author. Tel./fax: + 81-22-2175673; e-mail: [email protected]. 1 Present Address: Department of Bimolecular Science, Faculty of Engineering, Gifu University, Yanagido, Gifu 5011193, Japan.
Colloidal probe atomic force microscopy (AFM), introduced by Ducker et al. more recently [2,3], has removed this limitation, and enabled us to directly measure interactions between various surfaces such as glass [2,3], and other inorganic [4] and organic [5] substances in a quantitative manner. Adsorption of a trivalent cage surfactant onto glass surfaces has been studied by the same technique [6]. It is, however, still needed to develop methods of preparing well-defined functionalized surfaces for further utilization of the colloidal probe AFM in a wide range of materials science. Langmuir–Blodgett (LB) deposition have been employed in SFA measurements for modifying mica in a desired manner by lipid bilayers [7], polypeptide and polyelectrolyte brushes [8,9] as well as nucleic acid bases [10,11].
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R. Ishiguro et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 329–335
In the present study, we demonstrate that LB modification is also feasible for the colloidal probe AFM using LB forming lipid functionalized by the iminodiacetic acid group (DSIDA, Fig. 1). Interactions between DSIDA monolayers have been studied in respect of different protonation states of the iminodiacetate group and of chelate complex formation with Cu2 + ion. This monolayer is attractive because two-dimensionally organized layers of poly(histidine)-tagged (His-tag) proteins can be formed on Cu2 + -DSIDA layers [12,13]. Importance of preparing oriented protein layers is apparent for investigating specific and nonspecific interactions of proteins by the surface forces measurement. Here, in order to form foundation for such a future protein study, we have performed surface forces measurement between DSIDA monolayers at various pHs and Cu2 + concentrations.
2. Experimental The preparation of DSIDA lipid was described previously [12]. Dioctadecyl dimethylammonium bromide (DODA) was purchased from Sogo Pharmaceutical and used as received. Other reagents were analytical grade. Forces between glass surfaces were measured on an atomic force microscope SPA300 (Seiko Instruments) following the procedures by Ducker et al. (Fig. 2) [2,3]. A soda lime glass sphere of 5–10 mm radius (R) (Polyscience) was used as a
Fig. 1. Structural formula of DSIDA: I, II, and III represent different protonation states of the iminodiacetate group.
Fig. 2. Schematic drawings of the experimental set-up: surface forces measurement system employing the atomic force microscope (a), and sample surfaces (b). The glass surfaces are driven by the piezoelectric devise. Surface force, F, is given as the product of spring constant of cantilever, k, and cantilever deflection, DD, which is obtained from the direction of the laser light reflected on the back of the cantilever. The laser light is detected by the four-sectored photo diode.
colloidal probe and was attached to the top of a rectangular cantilever of 100 mm length (RC800PSA, Olympus) by epoxy resin (epikote 1004, Shell), and force–distance profiles between the glass sphere and a glass plate in aqueous solutions were recorded. The glass plate was mounted on the scanner table. Prior to force measurement or Langmuir–Blodgett deposition, the glass sphere before gluing to the cantilever was cleaned in a hot mixture of concentrated sulfuric acid and 30% H2O2 (4:1, v/v), then attached to the cantilever, and exposed to a water plasma of 20 W, 13.56 MHz at 0.6 Torr, 25°C for 3 min to produce silanol groups on the glass surface, as well as the glass plate. The aqueous phase was exchanged using the peristaltic pump. The spring constant (k) of each cantilever was determined by the resonance method [14], and found to lie around 1 N m − 1. The measured force (F) was normalized by a radius of the sphere to
R. Ishiguro et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 329–335
give F/R which is proportional to the interaction energy between flat surfaces (Gf) by F/R =2pGf [1]. Computer controlled film balance systems (FSD 50, USI System; and NL-BIO20-MWC, Nippon Laser & Electronics) were used for measuring surface pressure (p)-molecular area (A) isotherms at 20.090.1°C, and for Langmuir– Blodgett deposition. The solvents used for spreading monolayers were a mixture of ethanol/dichloromethane/benzene (2:1:7, v/v) for DODA and chloroform for DSIDA. To render the glass surfaces hydrophobic, the DODA monolayer was transferred onto both of the glass plate and the sphere, attached to a cantilever, from the pure water subphase at p= 35 mN m − 1 in the upstroke vertical mode at a transfer rate of 10 mm min − 1. Subsequently, DSIDA monolayer was transferred on the hydrophobic DODA surface at p=40 mN m − 1 from the pure water subphase in the down-stroke mode at a rate of 10 mm min − 1. The p-A isotherm of DSIDA monolayer on pure water (Fig. 3) exhibited that the monolayer at 40 mN m − 1 was in the solid condensed phase of which limiting molecular area extrapolated to the zero pressure was 0.3989 0.003 nm2 per molecule. The transfer ratios of amphiphiles were found to be 1.0 90.1 for DODA on glass and 0.8 90.1 for DSIDA on hydrophobic glass plates.
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Fig. 4. The Force profile between unmodified glass surfaces in 10 − 3 M NaBr (a), and one between hydrophobic, DODA modified glass surfaces in pure water (b). The arrow represents the position where the surface jumps into contact, and the solid line is drawn to guide the eye.
The transmission IR spectra were recorded on Perkin Elmer FT-IR system 2000, equipped with a TGS detector under dried air flow. The resolution was 4 cm − 1, and 50 scans were collected for each spectrum. DSIDA monolayers prepared at air water interface were transferred (one layer) on CaF2 plate p= 40 mN m − 1. 3. Results and discussion
3.1. Interaction between glass surfaces
Fig. 3. The surface pressure− molecular area (p− A) isotherm of DSIDA lipid on pure water at 20.0 90.1°C.
Surface force measured between plasma-treated glass surfaces in 10 − 4 –10 − 1 M NaBr aqueous solution at pH 5.6 exhibited electric double layer repulsion described by an exponential function, of which decay length was in fair agreement with the Debye length calculated for a corresponding salt concentration. No adhesive force was observed. A typical force profile is shown in Fig. 4(a) for a NaBr concentration of 10 − 3 M, where an experimental and a calculated decay length are 9.9 and 9.6 nm, respectively. These results accord well with previous report on glass surfaces [3].
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3.2. Interaction between hydrophobic DODA surfaces Hydrophobic modification of both glass surfaces with DODA monolayers altered their interaction to long range attraction extending to a distance of 40 nm in pure water (Fig. 4(b)). Similar long range attraction was observed in water between hydrophobic surfaces of LB films of ammonium amphiphiles by SFA [14], supporting that glass surfaces were covered sufficiently by the LB films. The pull-off force between hydrophobic DODA surfaces was 79924 mN m − 1, which was considerably smaller than that observed between hydrophobic mica surfaces by SFA, 200 –300 mN m − 1 [15,16]. We do not know clear explanation for this difference at the moment.
3.3. Interaction between DSIDA monolayers Fig. 5 shows the force – distance curves between the DSIDA surfaces in 10 − 4 and 10 − 3 M NaBr aqueous solutions at pH 7.5. Repeating measurement cycles at the same point on the surface, which accompanied a number of surface contacts, did not alter the force profiles, demonstrating that
Fig. 5. The force profiles between DSIDA surfaces in 10 − 3 and 10 − 4 M NaBr aqueous solutions at pH 7.5 (NaOH). The solid curves are the electrostatic double layer forces calculated at constant surface potential with the parameters: for 10 − 4 M NaBr, 80 (surface potential)= − 49 mV, k − 1 (decay length) = 28.5 nm; for 10 − 3 M, 80 = − 24 mV, k − 1 =8.3 nm.
DSIDA monolayers were stable enough for the forces measurement. The observed forces were well fitted to the electric double layer forces. The decay lengths were obtained to be 28.5 nm at 10 − 4 M NaBr and 8.3 nm at 10 − 3 M NaBr, which agreed well with the Debye lengths (k − 1) for corresponding salt concentrations, 30.5 nm and 9.6 nm, respectively. The surface potential 80 was calculated from the double layer forces assuming the constant surface potential to be − 49 mV for 10 − 4 M NaBr and −24 mV for 10 − 3 M NaBr [17]. Pull-off forces at pH 7.5 was small and about 1 mN m − 1.
3.4. pH dependence of DSIDA interactions The electric double layer force between DSIDA monolayers changed depending on pH of the aqueous phase (10 − 4 M NaBr) as shown in Fig. 6(a). Here, the pH value was adjusted by adding appropriate amount of NaOH or HNO3. Under pH 6.0 no appreciable repulsion was observed, whereas the electric double layer repulsion emerged at pH 6.5 and increased with increasing pH. No pull-off force was observed at pH 9.5. Pull off forces at pH 7.5 and 5.5 were obtained to be 1.0 9 0.5 mN m − 1 and 4 93 mN m − 1, respectively. The latter value contained relatively large error because the pull-off forces deviated considerably from one spot to another at this pH. The pull-off force between neutral DSIDA monolayers which displays larger adhesive force seems more sensitive to changes in surface contact modes possibly caused by different orientation of surface chemical groups, heterogeneity of monolayers, and difference in surface roughness. In order to examine the pH dependence in detail, the surface charge density of the DSIDA monolayer surface, estimated from a measured force profile by fitting to a theoretical one assuming the constant potential [16], was plotted as a function of pH of the aqueous bathing solution. The charge density changed stepwise with rising pH (Fig. 6(b)): it increased sharply till pH 7.5, and exhibited plateau between 7.5 and 8.5, then rose again beyond 8.5. The iminodiacetic acid group is known to form various deprotonation species: species I, II and III in Fig. 1 [18,19].
R. Ishiguro et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 329–335
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Fig. 6. Force profiles between DSIDA monolayer surfaces in 10 − 4 M NaBr aqueous solutions at various pH’s adjusted by addition of NaOH or HNO3 (a); the surface charge density of the DSIDA surface as a function of pH (b). Arrow represent the position where the surface jumps into contact, and solid lines are drawn to guide the eye. The charge density was calculated from the surface potential obtained by fitting the force profile. The marks of I, II, III indicate formation of each dissociation state of DSIDA shown in Fig. 1.
Therefore, the multiple step change of Fig. 6b can be attributable to dissociation of I to II below pH 7.5, stable formation of II between pH 7.5–8.5, and further dissociation of II to III beyond 8.5. The pK1 of DSIDA monolayer in 10 − 4 M NaBr likely lies around 6.5 and pK2 is around 9. The pK1 value of iminodiacetic acid in water has been reported to be 3.5 [18]. Considerable shift of pKs of monolayers from those in solutions has been reported to occur in many systems [8]. The species II of iminodiacetic acid has been known to be stable at a certain range of pH (4.5 – 8.5) in water [19], and similarly the species II is stable at pH 7.5–8.5 in the case of DSIDA monolayers.
3.5. Interactions between DSIDA monolayers in the presence of Cu 2 + The iminodiacetate group is known to form a chelate complex with Cu2 + ion. Force profiles measured between DSIDA monolayers in the presence of CuCl2 at pH 7.5 are illustrated in Fig. 7a. The dependence of surface charge density on CuCl2 concentration is shown in Fig. 7b. The double layer force decreased with increasing the CuCl2 concentration from 10 − 7 to 10 − 5 M owing
to possible chelation of the iminodiacetate group with Cu2 + . Pull-off force remained practically constant, 0.99 0.6 mN m − 1. The most pronounced drop of the repulsion was detected between 10 − 7 and 10 − 6 M CuCl2, which was consistent with the concentration range of CuCl2 found for clear change in the p-A isotherm of DSIDA (data is not shown). Transmission infrared spectra of the LB films of DSIDA transferred on the CaF2 plate from the aqueous solutions containing CuCl2 exhibited appearance of the peak at 1655 cm − 1 (chelated COO) with the expense of one at 1594 cm − 1 (ionized COO), which indicated that chelation proceeded indeed at CuCl2 concentrations between 10 − 7 and 10 − 6 M (Fig. 8) [18]. The force profiles between DSIDA in 10 − 5 M CuCl2 was maintained even after the water phase was replaced by pure water demonstrating the Cu2 + ion binding was irreversible.
4. Summary This paper describes that the surface forces measurement is useful to monitor various chemi-
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Fig. 7. Forces profiles between DSIDA monolayer surfaces in aqueous solutions containing various concentrations of CuCl2 at pH 7.5 (a); and the surface charge density of the DSIDA surfaces as a function of CuCl2 concentration. The density was calculated from the surface potential obtained by fitting the force profile.
cal processes occurring on functionalized surfaces of metal-chelating lipid monolayers. Feasibility of LB modification for colloidal probe atomic force microscopy will broaden the potential of force measurements. The Cu2 + -DSIDA monolayer is known to bind specially modified poly(histidine) tagged proteins through specific interaction between the poly(histidine) and the Cu2 + iminodi-
Fig. 8. Transmission FT-IR spectra of DSIDA LB films (one layer) on CaF2 plates transferred from the aqueous phase containing various concentrations of CuCl2 at pH 7.5.
acetate group. A study on interactions between protein layers bound to Cu2 + -DSIDA monolayers is in progress at our laboratory.
References [1] J.N. Israelachvili, Intermolecular and Surface Forces, 2nd edition, Academic Press, New York, 1992. [2] W.A. Ducker, T.J. Senden, R.M. Pashley, Nature 353 (1991) 239. [3] W.A. Ducker, T.J. Senden, R.M. Pashley, Langmuir 8 (1992) 1831. [4] I. Larson, C.J. Drummond, D.Y. Chan, F. Grieser, J. Am. Chem. Soc. 115 (1993) 11885. [5] Y. Q. Li, N.J. Tao, J. Pan, A.A. Garcia, S.M. Lindsay, Langmuir 9 (1993) 637. [6] M.E. Karaman, R.M. Pashley, N.K. Bolonkin, Langmuir 11 (1995) 2872. [7] J. Marra, J.N. Israelachvili, Biochemistry 24 (1985) 4608. [8] K. Kurihara, T. Kunitake, N. Higashi, M. Niwa, Langmuir 8 (1992) 2087. [9] T. Abe, K. Kurihara, N. Higashi, M. Niwa, J. Phys. Chem. 99 (1995) 1820. [10] P. Berndt, K. Kurihara, T. Kunitake, Langmuir 11 (1995) 3083. [11] K. Kurihara, T. Abe, N. Nakashima, Langmuir 12 (1996) 4053. [12] D.R. Shnek, D.W. Pack, D.Y. Sasaki, F.H. Arnold, Langmuir 10 (1994) 2382. [13] K. Ng, D.W. Pack, D.Y. Sasaki, F.H. Arnold, Langmuir 11 (1995) 4048.
R. Ishiguro et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 329–335 [14] J.P. Cleveland, S. Manne, O. Bocek, R.K. Hansma, Rev. Sci. Instrum. 62 (1993) 403. [15] P.M. Claesson, H.K. Christenson, J. Phys. Chem. 92 (1988) 1650. [16] K. Kurihara, T. Kunitake, J. Am. Chem. Soc. 114 (1992) 10927.
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[17] D.Y.C. Chan, R.M. Pashley, L.R. White, J. Colloid Interface Sci. 77 (1) (1980) 283. [18] K. Ueno, Chelate Chemistry, Nankodo, Tokyo, 1969. [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th edition, Wiley, New York, 1997.
Interaction forces between metal-chelating lipid monolayers measured by colloidal probe atomic force microscopy Ryo Ishiguro a,1, Darryl Y. Sasaki b, Charles Pacheco b, Kazue Kurihara a,* a
Institute for Chemical Reaction Science, Tohoku Uni6ersity, Katahira, 980 -8577 Sendai, Japan b Sandia National Laboratory, Albuquerque, NM 87185, USA Received 14 July 1998; accepted 15 September 1998
Abstract Surface forces between LB films of metal-chelating lipids in water have been studied using colloidal probe atomic force microscopy. The LB films of an amphiphile functionalized by the iminodiacetic acid group were prepared on hydrophobic glass substrates. The electric double layer repulsion operated between these LB film surfaces changed depending on pH reflecting the different protonation states of the iminodiacetic acid groups. The titration curve of the iminodiacetic acid monolayer was obtained from the force profiles. The Cu2 + complexation process was also monitored by measuring the force profiles at various Cu2 + ion concentrations. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Interaction forces; Lipid monolayers; Colloidal probe atomic force microscopy
1. Introduction The surface forces measurement is widely accepted as an essential tool for investigating intermolecular and surface interactions [1]. An instrument called ‘surface forces apparatus’ (SFA) has been used for a long time for measuring interaction force – distance profiles at a molecular resolution, though it limits the variation of sample surfaces practically to mica and modified-mica. * Corresponding author. Tel./fax: + 81-22-2175673; e-mail: [email protected]. 1 Present Address: Department of Bimolecular Science, Faculty of Engineering, Gifu University, Yanagido, Gifu 5011193, Japan.
Colloidal probe atomic force microscopy (AFM), introduced by Ducker et al. more recently [2,3], has removed this limitation, and enabled us to directly measure interactions between various surfaces such as glass [2,3], and other inorganic [4] and organic [5] substances in a quantitative manner. Adsorption of a trivalent cage surfactant onto glass surfaces has been studied by the same technique [6]. It is, however, still needed to develop methods of preparing well-defined functionalized surfaces for further utilization of the colloidal probe AFM in a wide range of materials science. Langmuir–Blodgett (LB) deposition have been employed in SFA measurements for modifying mica in a desired manner by lipid bilayers [7], polypeptide and polyelectrolyte brushes [8,9] as well as nucleic acid bases [10,11].
0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 8 ) 0 0 8 0 0 - 0
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R. Ishiguro et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 329–335
In the present study, we demonstrate that LB modification is also feasible for the colloidal probe AFM using LB forming lipid functionalized by the iminodiacetic acid group (DSIDA, Fig. 1). Interactions between DSIDA monolayers have been studied in respect of different protonation states of the iminodiacetate group and of chelate complex formation with Cu2 + ion. This monolayer is attractive because two-dimensionally organized layers of poly(histidine)-tagged (His-tag) proteins can be formed on Cu2 + -DSIDA layers [12,13]. Importance of preparing oriented protein layers is apparent for investigating specific and nonspecific interactions of proteins by the surface forces measurement. Here, in order to form foundation for such a future protein study, we have performed surface forces measurement between DSIDA monolayers at various pHs and Cu2 + concentrations.
2. Experimental The preparation of DSIDA lipid was described previously [12]. Dioctadecyl dimethylammonium bromide (DODA) was purchased from Sogo Pharmaceutical and used as received. Other reagents were analytical grade. Forces between glass surfaces were measured on an atomic force microscope SPA300 (Seiko Instruments) following the procedures by Ducker et al. (Fig. 2) [2,3]. A soda lime glass sphere of 5–10 mm radius (R) (Polyscience) was used as a
Fig. 1. Structural formula of DSIDA: I, II, and III represent different protonation states of the iminodiacetate group.
Fig. 2. Schematic drawings of the experimental set-up: surface forces measurement system employing the atomic force microscope (a), and sample surfaces (b). The glass surfaces are driven by the piezoelectric devise. Surface force, F, is given as the product of spring constant of cantilever, k, and cantilever deflection, DD, which is obtained from the direction of the laser light reflected on the back of the cantilever. The laser light is detected by the four-sectored photo diode.
colloidal probe and was attached to the top of a rectangular cantilever of 100 mm length (RC800PSA, Olympus) by epoxy resin (epikote 1004, Shell), and force–distance profiles between the glass sphere and a glass plate in aqueous solutions were recorded. The glass plate was mounted on the scanner table. Prior to force measurement or Langmuir–Blodgett deposition, the glass sphere before gluing to the cantilever was cleaned in a hot mixture of concentrated sulfuric acid and 30% H2O2 (4:1, v/v), then attached to the cantilever, and exposed to a water plasma of 20 W, 13.56 MHz at 0.6 Torr, 25°C for 3 min to produce silanol groups on the glass surface, as well as the glass plate. The aqueous phase was exchanged using the peristaltic pump. The spring constant (k) of each cantilever was determined by the resonance method [14], and found to lie around 1 N m − 1. The measured force (F) was normalized by a radius of the sphere to
R. Ishiguro et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 329–335
give F/R which is proportional to the interaction energy between flat surfaces (Gf) by F/R =2pGf [1]. Computer controlled film balance systems (FSD 50, USI System; and NL-BIO20-MWC, Nippon Laser & Electronics) were used for measuring surface pressure (p)-molecular area (A) isotherms at 20.090.1°C, and for Langmuir– Blodgett deposition. The solvents used for spreading monolayers were a mixture of ethanol/dichloromethane/benzene (2:1:7, v/v) for DODA and chloroform for DSIDA. To render the glass surfaces hydrophobic, the DODA monolayer was transferred onto both of the glass plate and the sphere, attached to a cantilever, from the pure water subphase at p= 35 mN m − 1 in the upstroke vertical mode at a transfer rate of 10 mm min − 1. Subsequently, DSIDA monolayer was transferred on the hydrophobic DODA surface at p=40 mN m − 1 from the pure water subphase in the down-stroke mode at a rate of 10 mm min − 1. The p-A isotherm of DSIDA monolayer on pure water (Fig. 3) exhibited that the monolayer at 40 mN m − 1 was in the solid condensed phase of which limiting molecular area extrapolated to the zero pressure was 0.3989 0.003 nm2 per molecule. The transfer ratios of amphiphiles were found to be 1.0 90.1 for DODA on glass and 0.8 90.1 for DSIDA on hydrophobic glass plates.
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Fig. 4. The Force profile between unmodified glass surfaces in 10 − 3 M NaBr (a), and one between hydrophobic, DODA modified glass surfaces in pure water (b). The arrow represents the position where the surface jumps into contact, and the solid line is drawn to guide the eye.
The transmission IR spectra were recorded on Perkin Elmer FT-IR system 2000, equipped with a TGS detector under dried air flow. The resolution was 4 cm − 1, and 50 scans were collected for each spectrum. DSIDA monolayers prepared at air water interface were transferred (one layer) on CaF2 plate p= 40 mN m − 1. 3. Results and discussion
3.1. Interaction between glass surfaces
Fig. 3. The surface pressure− molecular area (p− A) isotherm of DSIDA lipid on pure water at 20.0 90.1°C.
Surface force measured between plasma-treated glass surfaces in 10 − 4 –10 − 1 M NaBr aqueous solution at pH 5.6 exhibited electric double layer repulsion described by an exponential function, of which decay length was in fair agreement with the Debye length calculated for a corresponding salt concentration. No adhesive force was observed. A typical force profile is shown in Fig. 4(a) for a NaBr concentration of 10 − 3 M, where an experimental and a calculated decay length are 9.9 and 9.6 nm, respectively. These results accord well with previous report on glass surfaces [3].
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3.2. Interaction between hydrophobic DODA surfaces Hydrophobic modification of both glass surfaces with DODA monolayers altered their interaction to long range attraction extending to a distance of 40 nm in pure water (Fig. 4(b)). Similar long range attraction was observed in water between hydrophobic surfaces of LB films of ammonium amphiphiles by SFA [14], supporting that glass surfaces were covered sufficiently by the LB films. The pull-off force between hydrophobic DODA surfaces was 79924 mN m − 1, which was considerably smaller than that observed between hydrophobic mica surfaces by SFA, 200 –300 mN m − 1 [15,16]. We do not know clear explanation for this difference at the moment.
3.3. Interaction between DSIDA monolayers Fig. 5 shows the force – distance curves between the DSIDA surfaces in 10 − 4 and 10 − 3 M NaBr aqueous solutions at pH 7.5. Repeating measurement cycles at the same point on the surface, which accompanied a number of surface contacts, did not alter the force profiles, demonstrating that
Fig. 5. The force profiles between DSIDA surfaces in 10 − 3 and 10 − 4 M NaBr aqueous solutions at pH 7.5 (NaOH). The solid curves are the electrostatic double layer forces calculated at constant surface potential with the parameters: for 10 − 4 M NaBr, 80 (surface potential)= − 49 mV, k − 1 (decay length) = 28.5 nm; for 10 − 3 M, 80 = − 24 mV, k − 1 =8.3 nm.
DSIDA monolayers were stable enough for the forces measurement. The observed forces were well fitted to the electric double layer forces. The decay lengths were obtained to be 28.5 nm at 10 − 4 M NaBr and 8.3 nm at 10 − 3 M NaBr, which agreed well with the Debye lengths (k − 1) for corresponding salt concentrations, 30.5 nm and 9.6 nm, respectively. The surface potential 80 was calculated from the double layer forces assuming the constant surface potential to be − 49 mV for 10 − 4 M NaBr and −24 mV for 10 − 3 M NaBr [17]. Pull-off forces at pH 7.5 was small and about 1 mN m − 1.
3.4. pH dependence of DSIDA interactions The electric double layer force between DSIDA monolayers changed depending on pH of the aqueous phase (10 − 4 M NaBr) as shown in Fig. 6(a). Here, the pH value was adjusted by adding appropriate amount of NaOH or HNO3. Under pH 6.0 no appreciable repulsion was observed, whereas the electric double layer repulsion emerged at pH 6.5 and increased with increasing pH. No pull-off force was observed at pH 9.5. Pull off forces at pH 7.5 and 5.5 were obtained to be 1.0 9 0.5 mN m − 1 and 4 93 mN m − 1, respectively. The latter value contained relatively large error because the pull-off forces deviated considerably from one spot to another at this pH. The pull-off force between neutral DSIDA monolayers which displays larger adhesive force seems more sensitive to changes in surface contact modes possibly caused by different orientation of surface chemical groups, heterogeneity of monolayers, and difference in surface roughness. In order to examine the pH dependence in detail, the surface charge density of the DSIDA monolayer surface, estimated from a measured force profile by fitting to a theoretical one assuming the constant potential [16], was plotted as a function of pH of the aqueous bathing solution. The charge density changed stepwise with rising pH (Fig. 6(b)): it increased sharply till pH 7.5, and exhibited plateau between 7.5 and 8.5, then rose again beyond 8.5. The iminodiacetic acid group is known to form various deprotonation species: species I, II and III in Fig. 1 [18,19].
R. Ishiguro et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 329–335
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Fig. 6. Force profiles between DSIDA monolayer surfaces in 10 − 4 M NaBr aqueous solutions at various pH’s adjusted by addition of NaOH or HNO3 (a); the surface charge density of the DSIDA surface as a function of pH (b). Arrow represent the position where the surface jumps into contact, and solid lines are drawn to guide the eye. The charge density was calculated from the surface potential obtained by fitting the force profile. The marks of I, II, III indicate formation of each dissociation state of DSIDA shown in Fig. 1.
Therefore, the multiple step change of Fig. 6b can be attributable to dissociation of I to II below pH 7.5, stable formation of II between pH 7.5–8.5, and further dissociation of II to III beyond 8.5. The pK1 of DSIDA monolayer in 10 − 4 M NaBr likely lies around 6.5 and pK2 is around 9. The pK1 value of iminodiacetic acid in water has been reported to be 3.5 [18]. Considerable shift of pKs of monolayers from those in solutions has been reported to occur in many systems [8]. The species II of iminodiacetic acid has been known to be stable at a certain range of pH (4.5 – 8.5) in water [19], and similarly the species II is stable at pH 7.5–8.5 in the case of DSIDA monolayers.
3.5. Interactions between DSIDA monolayers in the presence of Cu 2 + The iminodiacetate group is known to form a chelate complex with Cu2 + ion. Force profiles measured between DSIDA monolayers in the presence of CuCl2 at pH 7.5 are illustrated in Fig. 7a. The dependence of surface charge density on CuCl2 concentration is shown in Fig. 7b. The double layer force decreased with increasing the CuCl2 concentration from 10 − 7 to 10 − 5 M owing
to possible chelation of the iminodiacetate group with Cu2 + . Pull-off force remained practically constant, 0.99 0.6 mN m − 1. The most pronounced drop of the repulsion was detected between 10 − 7 and 10 − 6 M CuCl2, which was consistent with the concentration range of CuCl2 found for clear change in the p-A isotherm of DSIDA (data is not shown). Transmission infrared spectra of the LB films of DSIDA transferred on the CaF2 plate from the aqueous solutions containing CuCl2 exhibited appearance of the peak at 1655 cm − 1 (chelated COO) with the expense of one at 1594 cm − 1 (ionized COO), which indicated that chelation proceeded indeed at CuCl2 concentrations between 10 − 7 and 10 − 6 M (Fig. 8) [18]. The force profiles between DSIDA in 10 − 5 M CuCl2 was maintained even after the water phase was replaced by pure water demonstrating the Cu2 + ion binding was irreversible.
4. Summary This paper describes that the surface forces measurement is useful to monitor various chemi-
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Fig. 7. Forces profiles between DSIDA monolayer surfaces in aqueous solutions containing various concentrations of CuCl2 at pH 7.5 (a); and the surface charge density of the DSIDA surfaces as a function of CuCl2 concentration. The density was calculated from the surface potential obtained by fitting the force profile.
cal processes occurring on functionalized surfaces of metal-chelating lipid monolayers. Feasibility of LB modification for colloidal probe atomic force microscopy will broaden the potential of force measurements. The Cu2 + -DSIDA monolayer is known to bind specially modified poly(histidine) tagged proteins through specific interaction between the poly(histidine) and the Cu2 + iminodi-
Fig. 8. Transmission FT-IR spectra of DSIDA LB films (one layer) on CaF2 plates transferred from the aqueous phase containing various concentrations of CuCl2 at pH 7.5.
acetate group. A study on interactions between protein layers bound to Cu2 + -DSIDA monolayers is in progress at our laboratory.
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