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J. Phys. Chem. B 2008, 112, 2930-2936
Discogen-DNA Complex Films at Air-Water and Air-Solid Interfaces Alpana Nayak and K. A. Suresh* Raman Research Institute, SadashiVanagar, Bangalore 560 080, India ReceiVed: October 17, 2007; In Final Form: December 10, 2007
We have studied films of an ionic discogenic (discotic mesogenic) molecule (pyridinium salt tethered with hexaalkoxytriphenylene (PyTp)) and DNA complex at air-water (A-W) and air-solid interfaces. We have formed an PyTp monolayer on an aqueous subphase containing a small amount of DNA to obtain a PyTpDNA complex at the A-W interface. Compared to the pure PyTp monolayer, the PyTp-DNA complex monolayer exhibits a higher collapse pressure and lower limiting area, indicating condensation and better stability. A Brewster angle microscope was used for in situ observation of the morphology of the film at the A-W interface. The PyTp-DNA complex films on silicon wafers were prepared using the LangmuirBlodgett (LB) technique. We find that several tens of layers of the PyTp-DNA complex monolayer can be transferred with good efficiency. Fourier transform infrared spectroscopy studies confirm the presence of DNA in the LB films of the PyTp-DNA complex. Nanoindentation measurements using atomic force microscope reveal that the PyTp-DNA complex films are about two times harder as compared to the pure PyTp films.
1. Introduction The interaction of DNA with molecules in a monolayer at an air-water (A-W) interface (Langmuir monolayer) has received considerable attention in recent years, with a view to understanding templated supramolecular organization as well as the transfer of DNA across biological bilayer membranes in gene therapy.1-3 A variety of cationic surfactants, such as linear and branched polymer, glycopeptides, and dendrimers, as well as lipid membranes, have been shown to be capable of complexing with DNA.4,5 In particular, the cationic lipids complexed with DNA are promising nonviral carriers of DNA vectors for gene therapy.6 Langmuir-Blodgett (LB) films are useful for immobilization of nucleic acids (DNA, polynucleotides) on solid supports in the designing of nucleic acid-based biosensors.7 The ability of double-stranded DNA to act as a conduit for electron transfer over a long range and the corresponding inability of single-stranded DNA to do the same have been the basis for DNA hybridization biosensors.8 The immobilization of DNA molecules on a solid substrate by means of electrostatic interactions has a clear advantage, as compared with chemical bonding. Although there have been many studies on the formation of DNA-cationic lipid complexes, there have been no reports on the interaction of DNA with cationic discotic molecules at the A-W interface. Recently, Cui et al. reported DNA complexes with cationic discotic surfactants in the bulk.9 The interaction of DNA with a discotic surfactant in the bulk is different from such an interaction at the A-W interface. In this article, we report our studies on the films of a discogen (discotic mesogen, pyridinium salt tethered with hexaalkoxytriphenylene (PyTp)) and DNA complex at both A-W and airsolid (A-S) interfaces. We find that, compared to the pure PyTp monolayer, the PyTp-DNA complex monolayer exhibits a higher collapse pressure, indicating better stability. In addition, the PyTp-DNA complex monolayer does not revert from the collapsed state, unlike that of a pure PyTp monolayer.10 Atomic * Corresponding author. Tel: +91-80-28382924. Fax: +91-80-23610492. E-mail:
[email protected].
Figure 1. Surface pressure (π)-area per molecule (Am) isotherms of PyTp-DNA complex for different concentrations of DNA in the subphase.
force microscope (AFM) studies show that the PyTp-DNA complex films transferred onto silicon wafers are quite compact. AFM has also been used to study the mechanical properties of the film surfaces on the nanometer scale by nanoindentation.11 We have used the nanoindentation technique to study qualitatively the hardness of pure PyTp and PyTp-DNA complex films at the nanometer scale. We find that the PyTp-DNA complex films are harder than the pure PyTp films. 2. Experimental Section A pyridinium salt tethered with a hexaalkoxytriphenylene (PyTp) molecule with bromide as counterion was used to form the Langmuir monolayer. The material was synthesized12 by S. K. Pal and S. Kumar. 1H NMR, 13C NMR, IR, and UV spectroscopy and elemental analysis indicated high purity (99%) of the material. For their complexation with DNA, sodium salts of DNA (Sigma-Aldrich) were dissolved in the ultrapure water
10.1021/jp710084q CCC: $40.75 © 2008 American Chemical Society Published on Web 02/19/2008
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J. Phys. Chem. B, Vol. 112, No. 10, 2008 2931 experiments were carried out at room temperature (25 °C). The details of surface manometry, BAM, hydrophilic and hydrophobic substrate preparation, and AFM have been described in an earlier paper.10 3. Results and Discussion
Figure 2. Variation of compressional elastic modulus (|E|) with area per molecule (Am) for the pure PyTp monolayer and the PyTp-DNA complex monolayer with a 10-8 M concentration of DNA in the subphase.
subphase. This is a double-stranded DNA with approximate molecular weight of 1.3 × 106 (∼2000 bp). The surface pressure (π)-area per molecule (Am) isotherm for the complex monolayer was obtained by spreading a 0.236 mM concentration solution of PyTp in chloroform (HPLC grade) on the subphase containing DNA at various concentrations. The surface pressure measurements with time were carried out to investigate the stability of the PyTp-DNA complex monolayer. A Brewster angle microscope (BAM) was used for in situ observation of the morphology of the complex film at the A-W interface. The LB technique was employed to transfer films of the PyTp-DNA complex onto hydrophilic and hydrophobic substrates at different surface pressures, and the films were studied employing AFM. The AFM images were acquired using the AC mode in ambient condition. Scratching with the AFM tip was performed to find the thickness of the compact films. AFM was also used for nanoindentation to study the hardness of the pure film and the complex film qualitatively. In this technique, AFM was used as a depth sensing instrument. Here, the cantilever deflection gives the load applied on the tip, and the relative motion between the tip and the sample gives the indentation. The advantage of using AFM as a nanoindenter was that we could observe the deformation (elastic or inelastic) by imaging the area before and after indentation. Silicon tips with a spring constant, kc, of 5 N/m and resonance frequency of 160 kHz were used. All measurements were carried out inside an environmental chamber in which dry nitrogen gas was circulated to avoid capillary condensation of water at the contact point between the tip and the surface. Transmission Fourier transform infrared (FTIR) spectroscopy (Shimadzu 8400 FTIR) was also carried out on these LB films to confirm the presence of DNA. All the
3.1. Surface Manometry. The π-Am isotherms for the PyTp molecule with different molar concentrations of DNA in the subphase are shown in Figure 1. The collapse pressure increases and the limiting area per molecule (A0) of the isotherms decreases with the increase in the concentration of DNA in the subphase. However, beyond 10-8 M concentration of DNA, there was no further change in the isotherm. The decrease in the A0 value and the increase in the collapse pressure as compared to the pure PyTp monolyer10 indicates condensation and enhanced stability of the PyTp-DNA complex monolayer.13 For the 10-8 M concentration, the collapse pressure was about 58 mN/m, which is 25% higher as compared to the pure PyTp monolayer. The isotherm exhibited a clear slope change at 0.95 nm2/molecule, indicating a phase transformation. The A0 value was 1.9 nm2/molecule for the first transformation and 0.9 nm2/ molecule for the second transformation. Comparing with the molecular dimensions, we suggest that the molecules adopt the edge-on conformation with a value of 1.9 nm2/molecule, corresponding to an expanded phase, and a value of 0.9 nm2/ molecule, corresponding to a condensed phase. The isocycles performed by compression and expansion of the PyTp-DNA complex monolayer at the condensed phase showed a negligible amount of hysteresis. However, if the isocycles were performed after the complex monolayer reached the collapsed state, an appreciable amount of hysteresis was observed, and the monolayer did not exhibit reversibility on expansion. This observation was unlike the pure PyTp monolayer, which showed completely reversible collapse.10 This indicated a stable complex formation between DNA and the cationic PyTp molecules at the A-W interface. The surface pressure measurements with time (t) were carried out to further investigate the stability of the PyTp-DNA complex monolayer. The complex monolayer was compressed to a particular surface pressure (π0), after which the surface pressure was recorded as a function of time at constant area. The π-t decay curves of the PyTp-DNA complex monolayer show that the complex monolayer was fairly stable with time (see the Supporting Information). The compressional elastic modulus (|E|) was calculated using the expression
|E| ) Am
( ) dπ dAm
(1)
Figure 3. Brewster angle microscope images of PyTp-DNA complex monolayer at (a) Am ) 0.85 nm2 and (b) Am ) 0.35 nm2, for a 10-8 M concentration of DNA in the subphase. The scale bar in each image represents 500 µm.
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Figure 4. AFM topography images of a monolayer of the PyTp-DNA complex transferred at (a) 5 mN/m and (b) 35 mN/m onto hydrophilic silicon substrates. The respective height profiles corresponding to the white lines on the images are shown below.
Figure 5. Schematic representation of the PyTp-DNA complex monolayer in the edge-on configuration formed at the A-W interface by electrostatic interaction. (a) Loosely packed molecules at an A0 value of 1.9 nm2/molecule. (b) Compactly packed molecules at an A0 value of 0.9 nm2/molecule.
where dπ/dAm is the change in surface pressure with area per molecule. The variation of |E| with Am obtained from the isotherms of a pure PyTp monolayer and a PyTp-DNA
complex monolayer are shown in Figure 2. The |E| value showed a maximum of 283 mN/m at an Am of 0.78 nm2/ molecule for the PyTp-DNA complex monolayer. The maximum value of |E| for a pure PyTp monolayer was 83 mN/m at an Am of 0.87 nm2/molecule. Interestingly, the maximum |E| value attained by the complex monolayer was more than three times the value attained by the pure PyTp monolayer, indicating a much better packing of molecules in the complex monolayer as compared to that of a pure PyTp monolayer. We find a sharp change in the value of |E| at an Am of 0.95 nm2/molecule for the complex monolayer, indicating a phase transition. The phase above an Am of 0.95 nm2/molecule has a much lower value of |E| as compared to the phase below this Am. This suggests that the complex monolayer undergoes a phase transition in the edgeon configuration from an expanded phase to a condensed phase. 3.2. Brewster Angle Microscopy. The BAM images of the PyTp-DNA complex monolayer with 10-8M concentration of DNA in the subphase are shown in Figure 3. Similar to the pure PyTp monolayer,10 the intensity in the BAM image increases gradually upon compression, as shown in Figure 3a. But in the collapsed state, the 3D domains were markedly different. Pure PyTp film showed small crystalline domains filling the whole surface of water in a meshlike texture. On the contrary, the PyTp-DNA complex film showed long, threadlike 3D crystals (Figure 3b) at collapse. On expansion, these threadlike domains continued without change, and the monolayer did not revert back to its original state. This confirms the irreversibility seen in the π-Am isotherm of the PyTp-DNA complex monolayer from the collapsed state to the monolayer state. These results, together with surface manometry, indicated the stable complex formation between the DNA molecules and the cationic discotic PyTp molecules at the A-W interface. Here, the interaction is mainly electrostatic between the pyridinium group of discotic mesogen and the phosphate group of DNA, which play an important role in the formation and stability of the complex films. 3.3. Atomic Force Microscopy. 3.3.1. Topography. For AFM studies, we have transferred films by theLB technique onto both hydrophilic and hydrophobic silicon substrates.10 If the substrate is hydrophilic, the first monolayer is transferred as the substrate is raised through the subphase, and subsequently, a monolayer
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J. Phys. Chem. B, Vol. 112, No. 10, 2008 2933
Figure 6. AFM topography images for two layers of a PyTp-DNA complex film transferred at 35 mN/m onto hydrophobic silicon substrates. (a) Morphology of the film surface showing roughness. (b) Scratched film to measure the thickness. The respective height profiles corresponding to the white lines on the images are shown below.
is deposited on each traversal of the substrate. Thus, a multilayer structure containing only an odd number of layers can be produced. However, if the substrate is hydrophobic, a monolayer will be deposited as it is first lowered into the subphase. This leads to a multilayer film structure containing an even number of monolayers.14 We have prepared films containing an odd number of layers on hydrophilic silicon substrates and films containing an even number of layers on hydrophobic silicon substrates. For both types of substrate, the transfer efficiency was close to unity. AFM images of the PyTp-DNA complex monolayer transferred onto hydrophilic silicon substrates are shown in Figure 4. The film transferred at a π of 5 mN/m (Figure 4a) showed a height of 1.4 ( 0.4 nm with reference to the substrate. Comparing with the molecular dimension, we find that this value lies between the values expected for a face-on and an edge-on configuration of molecules in the film. The film morphology depicts heterogeneity with streaklike features (length 200-300 nm, breadth 20-30 nm, and height 0.9 nm). These may be due to the DNA strands complexed with the PyTp molecules in the film. The morphology of the film transferred at a π of 35 mN/m (Figure 4b) was compact and homogeneous with some small voids. The film showed a height of 3 ( 0.2 nm, which is more than the film height of pure PyTp in the edge-on configuration by nearly 1 nm.10 This extra thickness of 1 nm can be attributed to the presence of DNA in the film, but the value is markedly smaller than the geometrical size of the cylindrical DNA (diameter 2 nm). By employing neutron and X-ray reflectivity techniques, Wu et al. have reported a lipid-DNA LB monolayer film to be thicker than the pure lipid film by ∼0.9 nm.15 Kago et al. have confirmed by a direct in situ X-ray reflectivity technique that the structure and conformation of the DNA molecule in the lipid-DNA complex deposited on a solid substrate are largely different from its conformation at the A-W interface. They found the thickness of the DNA layer in the lipid-DNA complex film at the A-W interface to be ∼2.52.8 nm, which is comparable to the diameter of a DNA molecule, whereas when those films were deposited on the solid substrate, this thickness decreased to about 1.1 nm, which is too small compared to the geometry of the DNA molecule.16 Our observation of the extra thickness of 1 nm is in good
agreement with these reports and confirms the PyTp-DNA complex formation. The π-Am isotherm and the calculated |E| value indicate a phase transformation from an expanded phase to a condensed phase. The A0 value obtained in the expanded phase was 1.9 nm2/molecule; in the condensed phase, 0.9 nm2/molecule. Additionally, the AFM topography images showed a height of ∼1.4 nm for the film transferred at expanded phase and a height of ∼3 nm for the film transferred at condensed phase. Comparing all of these results with the molecular dimension,10 we infer that the face-on configuration was suppressed. The phase transition was from loosely packed molecules in the edge-on configuration to a compactly packed edge-on configuration. On the basis of these results, we schematically represent the configuration of the molecules at the A-W interface as shown in Figure 5. Figure 6 represents AFM topography for two layers of the PyTp-DNA complex LB film deposited at 35 mN/m on a hydrophobic silicon substrate. As compared to the two layers of pure PyTp film10 which had regular rectangular voids, the morphology of the PyTp-DNA complex film with two layers was more compact, with some circular holes (Figure 6a) of different depth. Some of these holes exhibited a depth of ∼2 nm. These holes might have developed in the film deposition process due to the drainage or evaporation of the entrapped water molecules within the films. To obtain the actual film thickness, we have scratched the film using an AFM tip in contact mode and then imaged the scratched region in noncontact mode using the same tip (Figure 6b). The film was scratched in different directions to obtain the correct thickness of the film with better reproducibility. The thickness of the film obtained by height profile across the scratch was ∼5 ( 0.5 nm. We have studied the morphology and thickness of the complex films transferred up to 20 layers. The AFM images of the complex films with 3, 12, and 20 layers are shown in the Supporting Information. Scratching using the AFM tips became difficult for higher layers because of a large accumulation of material at the edges. For this reason, scratching was not performed for films with more than 20 layers, although it was possible to transfer even 50 layers. In the case of the pure PyTp film, it was not possible to transfer more than two layers efficiently. Interestingly, we find
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that the DNA complexation facilitated the film deposition of several tens of layers with high transfer efficiency. The ability to transfer several layers gives an excellent scope for device applications such as transistors. Moreover, immobilization of nucleic acids onto solid supports by the LB technique is a useful approach in designing nucleic acid based biosensors.7-8 The morphology of the complex film with 20 layers showed threadlike structures aligned in the film deposition direction. These threadlike structures appeared for films with more than five layers and grew in number with increasing layers. This may be due to the strong Π-Π stacking interaction between the discotic cores, bringing multiple DNA molecules together to form these threadlike bundles.17 The typical dimensions of these bundles were ∼80 nm in width, 1.5-3.0 µm in length, and 10-12 nm in height. The notable variation in the dimensions of these DNA bundles suggests partial coiling up or self-folding of the DNA.18 Possible explanations for the alignment of these structures in the film deposition direction may be parallel alignment of the DNA strands at the A-W interface and an effect created as a result of the receding meniscus force during the transfer process. Our observation is consistent with the reports on the orientation of DNA strands along the dipping direction of LB films.19 3.3.2. Nanoindentation. Cantilever deflection (d) versus piezo displacement (z) trace-retrace curves were obtained for a freshly cleaned silicon surface (Figure 7a), two layers of pure PyTp film surface (Figure 7b), and two layers of PyTp-DNA complex film surface (Figure 7c) at a load of 9 nN. The load, F, applied by the cantilever tip to the surface was computed from the cantilever spring constant, kc, using Hooke’s law,
F ) kcd
(2)
The indentation depth, δ, is given by
δ)z-d
(3)
where z is the piezo displacement and d is the deflection of the cantilever. Due to thermal drift in the detection system, as well as to the stresses in the cantilever, the deflection of the free cantilever was not equal to zero. Therefore, it was necessary to subtract the deflection offset from all the deflection values. All the plots in Figure 7 are shown after the offset correction. From these trace and retrace curves, the force of adhesion, jump-to-contact (point at which tip-sample contact occurs, marked A in Figure 7), and jump-off-contact (point at which tip-sample contact breaks off, marked B in Figure 7) were obtained. The amount of force just before the jump-off-contact gives a measure of the maximum tip-sample adhesion. The tip-sample adhesion for both the pure PyTp and the PyTpDNA complex film was 3.75 nN, which is one-half the value of adhesion for a clean hydrophilic silicon surface (7.5 nN). This difference may be attributed to the fact that the AFM tips made up of silicon were hydrophilic due to the presence of oxide layer but the PyTp and PyTp-DNA complex film surfaces were hydrophobic. This reduces the tip-sample adhesion significantly.20-22 The retraction curves suggest that the adhesive interaction between the tip and the film surface did not allow the tip to break away from the surface abruptly, but rather, produced a more gradual response. From a set of 10 measurements at different regions of the films (of which Figure 7b and c are typical examples), the mean values of the jump-to-contact and jump-off-contact distances were 7 ( 2 and 18 ( 5 nm, respectively.
Figure 7. Cantilever deflection (d) versus piezo displacement (z) curves obtained at a force of 9 nN for (a) a reference silicon surface, (b) two layers of a pure PyTp film surface, and (c) two layers of a PyTpDNA complex film surface. The arrows in the figure indicate the piezo traveling direction. Slopes of the deflection curves above zero are given in the figure. Jump-to-contact is marked as A and jump-off-contact is marked as B.
In addition, we have extracted information on the hardness of the films from the deflection-displacement curves, depending on the indentability of the tip into the film surface at a given load.23 Figure 7a shows that the slope of the cantilever deflection vs piezo displacement is equal to 1. Because silicon is an infinitely stiff surface, as compared to the cantilever stiffness, the tip followed the piezo displacement. On the soft film surface, the tip indented the film surface, and the cantilever deflection was smaller than the piezo vertical displacement, yielding a corresponding slope lower than 1. For the pure film (Figure 7b), a slope of 0.68 was obtained, whereas for the complex film, the value was 0.73 at the same load of 9 nN. The corresponding indentation depths, δ, of the films calculated from these curves were 1.3 nm for the pure film and 0.8 nm for the complex film
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J. Phys. Chem. B, Vol. 112, No. 10, 2008 2935 the transfer of DNA molecules from the A-W to the A-S interface by the LB process. Judging from the absorption band at 1223 cm-1 (antisymmetric stretching vibration of PO2-) in the PyTp-DNA complex FTIR spectrum, we infer that DNA has a B-form conformation.7,9 Figure 8b shows the FTIR spectrum of the PyTp-DNA complex LB film in the range 1600-1720 cm-1 deconvoluted into individual absorption bands. Interestingly, the peak at 1710 cm-1 was broadened and shifted to 1705 cm-1. In addition, a peak at 1684 cm-1, which is close to 1690 cm-1, appeared. This indicates that the basepair stacking in DNA molecules was partially disturbed due to complexation with the PyTp molecules. This may be caused by the replacement of intramolecular nucleic base-base hydrogen bonds by intermolecular nucleic base-PyTp hydrogen bonds. The positively charged pyridinium group of the PyTp molecule interacts electrostatically with the negatively charged phosphate (PO2-) groups of the DNA backbone, forming stable bonds in the process of PyTp-DNA complex formation. These interactions provide stability to the multilayer films of the complex. 4. Conclusions
Figure 8. FTIR absorption spectrum of PyTp-DNA complex LB film with 19 layers (a) in the range 900-1800 cm-1. (b) The spectrum in the range 1600-1720 cm-1 (open circles) is deconvoluted into individual absorption bands (dotted lines).
at the same load. Less indentation on a complex film surface than that on a pure film surface for a given load suggests that the surface of the complex film is harder than the pure film. Unequal slopes during trace and retrace suggests that the deformation is inelastic and does not recover fully during retrace. Typical AFM images showing the topography of the indents formed on a pure film and on a complex film are shown in the Supporting Information. The hardness value obtained by nanoindentation is defined as the load divided by the projected area of the indent. However, because the indent was not uniform, it became difficult to measure the area accurately. Thus, the relative hardness can be determined qualitatively.24,25 Our experiments yielded a hardness value of 3.58 MPa for the complex film and 1.79 MPa for the pure film. This shows that the PyTp-DNA complex film is about twice as hard as the pure PyTp film. The hardness of films can be important in surface mechanical property studies of materials and has great potential in understanding the role of the interface in defect production and migration.20 3.4. FTIR Spectroscopy. Transmission Fourier transform infrared spectroscopy was used to study the PyTp-DNA complex LB film transferred onto a hydrophilic silicon substrate. Figure 8a shows the FTIR absorption spectra of the PyTpDNA complex LB film with 19 layers on a hydrophilic silicon substrate. (The spectra of pure PyTp and pure DNA are shown in the Supporting Information.) In the spectrum of pure DNA, we find the absorption bands at 1710, 1648, 1223 , and 969 cm-1 (see Supporting Information), which indicates the regular, double-helix structure of DNA.7,9,26 The spectrum of the PyTpDNA complex film (Figure 8a) possessed characteristic absorption bands of both pure PyTp and pure DNA with certain modifications, as expected due to complexation. This confirms
We have for the first time shown the supramolecular complexation between DNA and a cationic discotic mesogen (discogen), at both A-W and A-S interfaces. The PyTp-DNA complex was formed at the A-W interface primarily due to the electrostatic interaction between the pyridinium group of the triphenylene molecule and the phosphate group of DNA. The formation of the DNA complex has enhanced the transfer efficiency over several tens of layers, whereas for the pure PyTp system, it was not possible to transfer more than two layers. FTIR spectroscopy confirms the presence of DNA in the LB films of the PyTp-DNA complex and suggests that the double helical structure gets partially disturbed due to the complexation of DNA with the PyTp molecules. AFM studies on the LB films showed that the PyTp-DNA complex monolayer was thicker than the pure PyTp monolayer by ∼1 nm. This thickness is reasonable for a DNA layer in a dried state and comparable to the reported values using neutron and X-ray reflectometry. The nanoindentation studies using AFM showed that the PyTpDNA complex film was about twice as hard as the pure PyTp film. The ability of the PyTp-DNA complex to form multilayers on a substrate by the LB technique may find application in the fabrication of devices such as thin film transistors and nucleic acid-based biosensors. Acknowledgment. We thank Santanu Kumar Pal and Sandeep Kumar for providing us the pyridinium salt tethered with hexaalkoxytriphenylene (PyTp) material for our experiments. Supporting Information Available: Surface pressure (π)time (t) relaxation isotherms of the PyTp-DNA complex monolayer at constant area for various surface pressures. AFM topography images for 3, 12, and 20 layers of the PyTp-DNA complex film. AFM topography images for three layers of pure PyTp film showing desorption. Typical AFM 3D images showing the topography of the indents formed on two layers of a pure PyTp film surface and two layers of a PyTp- DNA complex film surface. FTIR absorption spectrum of monolayer of pure PyTp LB film. FTIR absorption spectrum of pure DNA solution in water. This material is available free of charge via the Internet at http://pubs.acs.org.
2936 J. Phys. Chem. B, Vol. 112, No. 10, 2008 References and Notes (1) Radler, J. O.; Koltover, I.; Salditt, T.; Safinya, C. R. Science 1997, 275, 810. (2) Putnam, D. Nat. Mater. 2006, 5, 439. (3) Guillot-Nieckowski, M.; Joester, D.; Stohr, M.; Losson, M.; Adrian, M.; Wagner, B.; Kansy, M.; Heinzelmann, H.; Pugin, R.; Deiderich, F.; Gallani, J. L. Langmuir 2007, 23, 737. (4) Frances, M. P. W.; Dorothy, L. R.; Marcel, B. B. Biochemistry 1996, 35, 5756. (5) Sastry, M.; Ramakrishnan, V.; Pattarkine, M.; Gole, A.; Ganesh, K. N. Langmuir 2000, 16, 9142. (6) Lasic, D. D.; Templeton, N. S. AdV. Drug DeliVery ReV. 1996, 20, 221. (7) Sukhorukov, G. B.; Montrel, M. M.; Petrov, A. I.; Shabarchina, L. I.; Sukhorukov, B. I. Biosens. Bioelectron. 1996, 11, 913. (8) Gooding, J. J.; King, G. C. J. Mater. Chem. 2005, 15, 4876. (9) Cui, L.; Miao, J.; Zhu, L. Macromolecules 2006, 39, 2536. (10) Nayak, A.; Suresh, K. A.; Pal, S. K.; Kumar, S. J. Phys. Chem. B 2007, 111, 11157. (11) Tranchida, D.; Piccarolo, S.; Soliman, M. Macromolecules 2006, 39, 4547. (12) Kumar, S.; Pal, S. K. Tetrahedron Lett. 2005, 46, 4127. (13) Matti, V.; Saily, J.; Ryhanen, S. J.; Holopainen, J. M.; Borocci, S.; Mancini, G.; Kinnunen, P. K. J. Biophys. J. 2001, 81, 2135.
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