Interaction between Dendrons Directly Studied by Single-MoleculeForce Spectroscopy†

Weiqing Shi, Yiheng Zhang, Chuanjun Liu, Zhiqiang Wang, and Xi Zhang*

Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, TsinghuaUniVersity, Beijing 100084, PR China, and Key Lab for Supramolecular Structure and Materials, College

of Chemistry, Jilin UniVersity, Changchun 130012, PR China

ReceiVed June 15, 2007. In Final Form: July 30, 2007

In this article, we have investigated the interaction between two poly(benzyl ether) dendrons directly by single-molecule force spectroscopy. For this purpose, one dendron was immobilized on an AFM tip through a poly(ethyleneglycol) (PEG) spacer, and the other dendron was anchored on a gold substrate as a self-assembled monolayer. Twodendrons approached and then interacted with each other when the AFM tip and the substrate moved close together.The rupture force between dendrons was measured while the AFM tip and the substrate separated. PEG as a flexiblespacer can function as a length window for recognizing the force signals and avoiding the disturbance of the interactionbetween the AFM tip and the substrate. The interaction between two first-generation dendrons is measured to be about224 pN at a force loading rate of 40 nN/s. The interaction between second- and first-generation dendrons rises to 315pN at the same loading rate. Such interactions depend on the force loading rate in the range of several to hundredsof nanonewtons per second, indicating that the rupture between dendrons is a dynamic process. The study of theinteraction between surface-bound dendrons of different generations provides a model system for understanding thesurface adhesion of molecules with multiple branches. In addition, this multiple-branch molecule may be used to mimicthe sticky feet of geckos as a man-made adhesive.

Introduction

The rapidly developing atomic force microscopy (AFM)-basedsingle-molecule force spectroscopy (SMFS) technique providesa platform for measuring minute force and distance, thus openinga new horizon of single-molecule chemistry and physics.1-4 Forinstance, SMFS allows measurements of the elastic propertiesand conformational changes of single polymers. The elasticityof a single polymer chain can be influenced by its side-groupproperties and interaction with solvents.5-8 Some polymerspresent force-extension curves with characteristic features dueto their special structures. For example, the unfolding of a doubleor triple helix shows a plateau profile,9,10 and the chair-to-boatconformation transition of individual glucopyronose ringsprovides a shoulder plateau as a fingerprint on force-extensioncurves.11,12 More importantly, SMFS can provide abundantinformation for inter- and intramolecular interactions at a single-

molecule level, which is not easily available by conventionalmethods.1-4,13-15 Until now, various inter- and intramolecularinteractions have been detected by SMFS, including covalentbonding,16 hydrophobic interactions,17-20 coordination bond-ing,21-23 host-guest interactions,24-26 multiple hydrogen bond-ing,27 π-π interactions,28 intercalation interactions of DNA andsmall molecules,29 and charge-transfer interactions.30,31 Oneexample is to combine SMFS and a specially prepared linearsegmented copolymer for directly measuring the desorption forceper polystyrene (PS) segment adsorbed on the PS substrate in

† Part of the Molecular and Surface Forces special issue.* Corresponding author. E-mail: xi@mail.tsinghua.edu.cn. Tel:+86-

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Ed. 2000, 39, 3213-3237.(3) Zhang, W. K.; Zhang, X.Prog. Polym. Sci.2003, 28, 1271-1295.(4) Liu, C. J.; Shi, W. Q.; Cui, S. X.; Wang, Z. Q.; Zhang, X.Curr. Opin. Solid

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10.1021/la701784b CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 08/29/2007

water in order to understand the hydrophobic interaction.17 Thedynamic properties of the desorption force can be investigatedby changing the force loading rate. Hence, SMFS becomes apowerful tool to impart physical insight to the principle ofinteractions among molecules and to provide information aboutthe driving forces of assembled molecular systems as well.

Dendrimers, a type of regular-branched molecule, haveattracted an increasing amount of attention because of their well-defined architectures and many unique properties.32-36Researchhas ranged from the synthesis of dendrons to different methodsof functionalizing their focal points, branching units, andperipheries. Dendrimers are promising building blocks for self-organization in the bulk and at interfaces.37-44The self-assembledmonolayer (SAM) of a series of poly(benzyl ether) dendronswith thiol groups shows patterned stripes with nanometer-sizedfeatures and long-range order. The different periphery structuresof dendron thiols lead to different surface self-organizationmorphologies.40,41Another kind of polyether dendrimer with acarboxyl-terminal group is used to fabricate a multilayer filmbecause the formation of hydrogen bonding can stabilize themultilayer structure.45 Therefore, dendrimers are interesting notonly because of their new properties and applications but alsobecause of their interactions among assembled dendrimers, thatis, the driving force for dendrimer aggregation.

In this article, we have designed a system to explore theinteraction between poly(benzyl ether) dendrons directly usingSMFS in an attempt to gain new insight into the principle ofdendron self-organization in the bulk as well as adhesion atinterfaces. Furthermore, the surface adhesion of molecules withmultiple branches is generally utilized in nature. For example,a gecko’s feet, which are made up of a large number of microscalesetae, can support the animal while it rapidly climbs a smoothsurface and even a vertical wall.46 Therefore, we hope that thestudy of the interaction between surface-bound dendrons ofdifferent generations can provide a model system for under-standing thesurfaceadhesionofmoleculeswithmultiplebranches.

Experimental Section

Materials and Instruments. Poly(ethylene glycol) (PEG) waspurchased from Acros. The molecular mass of PEG is 6660 g/mol,and the polydispersity (PD) index is 1.15. 1-Ethyl-3-(3-dimethyl-aminopropyl) carbodiimide (EDC) was purchased from ShanghaiSanjie Biochemistry Technology,N-hydroxy succinimide (NHS)was purchased from Alfa Aesar, and 3-aminopropyltriethoxysilane

(APTES) was purchased from Fluka. Other chemical reagents werepurchased from Beijing Chemical Reagent Company. The solventsused were all freshly distilled.

1H NMR spectra were recorded in CDCl3 solution on a JEOLJNM-ECA300 (300 MHz) spectrometer.

Synthesis of Dendrons and Polymers.Synthesis of1 (G1-SH).Frechet-type dendron bromides G1-Br (n ) 1, 2) were synthesizedaccording to the literature43,47-49and were well characterized duringevery step. The dendron thiol was prepared according to Scheme1a. G1-Br (0.2300 g, 0.4700 mmol) and thiourea (0.1140 g, 1.4099mmol) were added to 25 mL of tetrahydrofuran (THF). The mixturewas refluxed for 4 h under argon protection. NaOH (1 M, 10.0 mL)was then added to the mixture and refluxed for 2 h, and followedby acidification with∼10.0 mL of HCl (1 M) to produce the dendronthiol that was finally purified by silica gel column chromatographywith dichloromethane as the eluent. The evaporation of the solventgave the G1-SH white powder.

Synthesis of2 (G1-PEG-OH).Polymers2 and3 were preparedaccording to Scheme 1b. A mixture of PEG (3.6799 g, 0.6133 mmol)and NaH (0.2 g, 8 mmol) was stirred in dry THF at 40°C with Arprotection for 1 h. Then, G1-Br (0.1004 g, 0.2051 mmol) was addedto the mixture, which was stirred under Ar for 6 h atroom temperature.

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Scheme 1. Synthesis Routes for (a) G1-SH and (b) PEG-G1

Interaction between Dendrons Langmuir, Vol. 24, No. 4, 20081319

After evaporation of the solvent, the residue was precipitated inton-hexane. The crude product was purified by silica gel columnchromatography with 10:1 CH2Cl2/methanol as the eluent. Theevaporation of the solvent gave the viscous target compound.

Synthesis of3 (PEG-G1).A mixture of 2 (1.0 g, 0.1563 mmol)and NaH (0.3 g, 12 mmol) was stirred in dry THF under 40°C withAr protection for 1 h. Then Br-CH2-Ph-COOCH3 (0.1145 g,0.4998 mmol) was added to the mixture, which was stirred underAr for 6 h, and after evaporation of the solvent, the residue wasprecipitated inton-hexane. The crude product was refluxed in aNaOH (1 M) and THF mixed solution under Ar for 10 h. After theevaporation of THF, the residue was acidified to pH 7 with HCl (1M). The mixture was extracted with CH2Cl2, and the organic layerwas dried with anhydrous Na2SO4. The evaporation of the solventled to the viscous target compound used to modify the AFM tip.PEG-G2 was synthesized in a similar way.

The structures of the dendrons are characterized by1H NMR andgel permeation chromatography (GPC) with polystyrene as thestandard.

G1-SH: 1H NMR δ 3.42 (s, 2H, ArCH2SH), 5.02 (s, 2H, ArCH2O),5.05 (s, 4H, ArCH2O), 6.53 (s, 2H, ArH), 7.22-7.40 (m, 15H, ArH).

G1-PEG-OH: 1H NMR δ 3.38 (t, 2H, G1CH2OH), 3.53-3.73(m, around 550H, CH2CH2O), 3.87 (s, 2H, CH2O), 4.43 (s, 2H,ArCH2OH), 5.01 (s, 2H, ArCH2O), 5.08 (s, 4H, ArCH2O), 6.63 (s,2H, ArH), 7.24-7.40 (m, 15H, ArH).

G2-PEG-OH: 1H NMR δ 3.41 (t, 2H, G2CH2OH), 3.59-3.75(m, around 550H, CH2CH2O), 3.88 (s, 2H, CH2O), 4.83 (s, 6H,ArCH2O), 4.99 (s, 12H, ArCH2O), 6.65 (s, 2H, ArH), 6.76 (s, 4H,ArH), 6.79 (s, 2H, ArH), 7.18-7.35 (m, 45H, ArH).

PEG: GPCMn 9544, PD 1.15.PEG-G1: GPCMn 10 367, PD 1.17.PEG-G2: GPCMn 12 288, PD 1.28.SMFS Experiment. AFM Tip Modification.2,21 The cantilever

was incubated in a toluene solution of APTES for 2 min. Then thecantilever was washed in turn with toluene, ethanol, and water.Between each step, it was dried with filter paper. Finally, the cantileverwas dried in an oven at 70°C for 2 h. After cooling to roomtemperature, the cantilever was incubated in a solution of NHS (6mM), EDC (20 mM), and PEG-G1 (20 or 10 mM for PEG-G2) indimethyl sulfoxide (DMSO) for 4 min. Then the cantilever waswashed with DMSO and water three times and dried with filterpaper. Finally, it was dried in a desiccator overnight.

Force Measurement.The force measurements were carried outon a Molecular Force Probe 3D (Asylum Research, Santa Barbara,CA). V-shaped Si3N4 cantilevers from Veeco (Santa Barbara, CA)were used. The spring constants of the cantilevers were in the rangeof 0.01-0.03 N/m according to the measurement of their thermalfluctuation.50,51

Characterization of G1-SH SAM.Electrochemical Measurementof G1-SH. Electrochemical measurements were performed using apotentiostat (Autolab PGSTAT12, The Netherlands) at roomtemperature. A bare gold electrode (model CHI 101, 2 mm diameter)was used as the working electrode. The counter electrode wasplatinum, and the reference electrode was a Ag/AgCl (saturatedKCl) electrode. The gold electrode underwent pretreatment to obtaina mirrorlike surface according to the literature.52Then the monolayerwas formed by placing the bare gold electrode in a 1 mMsolutionof G1-SH in THF overnight at room temperature. The gold electrodemodified with G1-SH SAM was rinsed with absolute THF and driedwith high-purity nitrogen. The electrochemical property for G1-SHSAM on gold was studied by cyclic voltammetry (CV) in a mixedsolution of 2 mM K3[Fe(CN)6] and 0.1 M KCl. The potential wasscanned from-0.05 to 0.55 V at a scan rate of 0.10 V s-1.

Scanning Tunneling Microscopy (STM).STM measurements werecarried out with a commercial instrument (Digital Instruments,Multimode Nanoscope IV) at room temperature in air. STM tipswere prepared from a Pt-Ir (90:10) wire (0.25 mm diameter) by themechanical cutting method. STM images were taken in constant-current mode at a scaning rate of 1 Hz and a resolution of 512 pixels× 512 pixels. Tunneling parameters of around 750 mV (bias voltage)and 350 pA (current setpoint) were used for taking images.

Results and Discussion

To measure the interaction between two poly(benzyl ether)dendrons of different generations, we immobilized one dendron,namely, PEG-Gn (n ) 1, 2), onto the AFM tip through a flexiblePEG chain and a modified gold substrate with the G1-SH SAM,as shown in Scheme 2. In the SMFS experiment, with themovement of the piezo tube, the dendron-modified AFM tip wasbrought into contact with the SAM of the dendron thiol. Thedendron on the AFM tip adsorbed to the G1-SH SAM, forminga PEG molecular bridge between the AFM tip and the substrate.Upon separating the AFM tip from the substrate, the molecularbridge was stretched, and the cantilever was deformed. Finally,the molecular bridge was broken by further stretching, and thecantilever suddenly recovered. After we recorded the movementof the piezo and the deformation of the cantilever, the data wereconverted into a force versus extension curve, in brief, a forcecurve. Measurements were carried out in chloroform (CHCl3),which is a good solvent for both PEG-Gn and G1-SH. According

(50) Butt, H. J.; Jaschke, M.Nanotechnology1995, 6, 1-7.(51) Hutter, J. L.; Bechhoefer, J.ReV. Sci. Instrum.1993, 64, 1868-1874.(52) Jiang, Y. G.; Wang, Z. Q.; Xu, H. P.; Chen, H.; Zhang, X.; Smet, M.;

Dehaen, W.; Hirano, Y.; Ozaki, Y.Langmuir2006, 22, 3715-3720.

Scheme 2. Schematic Illustration of the Force Measurement between PEG-Gn (n ) 1, 2) and the G1-SH Monolayer UsingSMFS

1320 Langmuir, Vol. 24, No. 4, 2008 Shi et al.

to the rupture force on each force curve, we can obtain thestatistical force value of the rupture between the dendronsmodified on the AFM tip and on the substrate, respectively. PEGacting as a flexible spacer can function as a length window fordifferentiating the force signals based on the extension lengthas well as for avoiding the disturbance of the nonspecificinteraction between the AFM tip and the substrate.28,29Further-more, the single-chain elongation of PEG can ensure that onlyone dendron on the AFM tip ruptures from the surface-boundSAM dendron.

Typical force curves obtained from the SMFS experimentswith different modified AFM tips and G1-SH SAM substratesare shown in Figure 1. The force curves show some similarfeatures. The peak signal on each force curves is “clean”,indicating that there is no other influence during the polymerchain stretching. The force value increases monotonically withthe extension of the polymer chain and then drops to zero suddenlywhen a rupture point is reached. The contour length of the polymerchain is varied because PEG is polydisperse. These force curvescan be fitted by a modified freely jointed chain (M-FJC) model,which is a classical empirical model for a single polymer chain.1,3,4

In the M-FJC model, which is based on the extended Langevinfunction shown below, a single polymer chain is treated as manyindependent segments. The segments are freely jointed (i.e., thereis no restriction on their spatial distribution) and can be stretchedunder stress.

In eq 1,x represents the extension of a polymer chain (end-to-end distance. In SMFS experiments,x is the section spanningthe AFM tip and the substrate,F is the applied force upon anindividual polymer chain,Lc is the contour length of the polymerchain, the Kuhn length (lk) is the length of the statisticallyindependent segment,n is the number of segments being stretched,which equalsLc/lk, kB is the Boltzmann constant, andT is theKelvin temperature. The deformability of segments is character-ized by the segment elasticityKs. The elasticity of a single polymerchain is controlled by both entropy and enthalpy. We used theM-FJC model to fit all of the force curves, and the fittingparameters are the Kuhn length (0.63( 0.10 nm) and the segmentelasticity (6.5( 1.0 N/m). These fitting parameters with narrowdistributions are similar to the reported data for single-chainelongation of the PEG homopolymer,53,54indicating that the force

signal originates from the elongation of the single PEG chain.In addition, the force curves provide the stretching length of thePEG chain. The length values of PEG elongation are around 50nm, as shown in Figure 2. These lengths agree well with thecontour length of PEG that we used in the experiment (∼56 nm),which is further evidence for single-PEG-chain elongation.

The rupture event on a force curve of single-PEG-chainstretching should be ascribed to the disassociation of PEG-G1on the tip from the G1-SH monolayer on the gold substrate. Theinteraction between PEG-G1 and G1-SH is noncovalent bonding,consequently acting as the weakest link along the whole stretchedmolecular bridge in the SMFS experiment. The most probablerupture force is 224( 62 pN by Gaussian fitting, as shown inFigure 3c, with a loading rate of 40( 8 nN/s. The loading rateof the rupture force was calculated from the 20 points on theforce versus time curve just before the rupture point. A linearleast-squares fitting of these data points afford the force loadingrate. Because poly(benzyl ether) has no charges or permanentdipole, the interaction between two dendrons cannot be a charge-charge or dipole-dipole interaction. However, a van der Waalsinteraction, which acts among all atoms and molecules, shouldobviously contribute to the interaction of the dendrons. In addition,considering the phenyl groups acting on each other when theposition is suitable, there should be aπ-π interaction betweentwo dendrons as well.

Because the interaction between the dendrons is induced fromthe van der Waals andπ-π interactions, we wondered whetherthe strength of the interactions would be increased while thegeneration of the dendrons increased to second generation. Toaddress this issue, we immobilized second-generation dendronPEG-G2 onto an AFM tip by using the same method describedabove. The monolayer on the substrate interacting with PEG-G2was still G1-SH SAM. The most probable rupture force betweenPEG-G2 and the G1-SH monolayer in CHCl3 is 315( 79 pN,as measured by SMFS at the same loading rate of 40( 8 nN/s.As expected, the interaction between PEG-G2 and G1-SH islarger than that between PEG-G1 and G1-SH. Because the SAMis the same in the interaction, the enhanced rupture force shouldbe induced from PEG-G2 instead of PEG-G1 on the AFM tip.In other words, the strength of the interaction between dendronsincreases with increasing dendron generation. The interactionbetween the two dendrons should originate mainly from van derWaals andπ-π interactions in terms of the chemical structure.Therefore, the more the atoms and phenyl groups that take partin the interaction, the larger the van der Waals andπ-πinteractions and, consequently, the larger the rupture force. Itshould be pointed out that the rupture force does not show multipleincreases from G1 to G2. This disproportion is reasonable if weconsider the topological structure of the dendrons. On the one

(53) Zou, S.; Scho¨herr, H.; Vancso, G. J.Angew. Chem., Int. Ed. 2005, 44,956-959.

(54) Oesterhelt, F.; Rief, M.; Gaub, H. E.New J. Phys.1999, 1, 6.1-6.11.

Figure 1. Typical force curves of PEG-G1 on the G1SH-Ausubstrate. The fitting curve generated by the M-FJC model is shownas a dashed line for one of the curves.

x(F) ) [coth(FlkkBT) -

kBT

Flk](Lc + nFKs

) (1)

Figure 2. Distributions of the rupture length of PEG-G1 and G1-SH in CHCl3 at a loading rate of 40 nN/s.

Interaction between Dendrons Langmuir, Vol. 24, No. 4, 20081321

hand, the interaction between two dendrons should increase withthe increase in the generation of dendrons. On the other hand,the dendron modified on the AFM tip cannot guarantee full contactwith the surface-anchored dendron on the substrate because ofthe topological structure, especially for the second-generationdendron. The wide distribution of the force value is also evidenceof incomplete contact between the dendrons on the tip and thegold substrate. These two factors may work together, resultingin an enhanced but not simply proportional interaction for thedendrons’ increase from G1 to G2.

We have performed force spectroscopy under a series of forceloading rates in order to understand the dynamic property of theinteraction between dendrons. It is found that there is a forceloading rate (rf) dependence of the rupture force betweendendrons. For a given loading rate, the rupture force has a certainvalue, for example, PEG-G1 interacting with G1-SH SAM asshown in Figure 3a-e. Such a force value varies with the loadingrate. The most probable rupture force of PEG-G1 on G1-SHSAM is 188( 50 pN at a lowerrf of 6.6 ( 0.9 nN/s, and the

force increases to 356( 104 pN at anrf of 110( 20 nN/s. Thelinear dependence of the rupture force on log(rf), as shown inFigure 3f, indicates that the rupture event is a dynamic process.

Both van der Waals andπ-π interactions are weak inter-molecular interactions. How can such weak interactions providea force of several hundred piconewtons between individualdendron molecules? The strength of several kinds of weakinteractions can be enhanced by a cooperative effect. Such acooperative effect exists widely in the interaction betweenenzymes and substrates, which is based on combined intermo-lecular interactions. In addition, the combined interaction can beused to understand why a gecko can move rapidly up a smooth,vertical surface. Each gecko’s toe has millions of setae, and eachseta is branched into hundreds of 200 nm spatulae that makeintimate contact with a variety of surface profiles.55,56 Thecombined surface area of billions of spatulae can maximize van

(55) Russell, A. P.J. Zool.1975, 176, 437-476.(56) Ruibal, R.; Ernst, V.J. Morphol.1965, 117, 271-293.

Figure 3. (a-e) Histograms of the rupture forces between the PEG-G1 and G1-SH monolayers under different force loading rates. (f) Loadingrate dependence of PEG-G1 rupture from the G1-SH SAM surface. The fitting curve for the linear dependence of the rupture force on log(r f)is shown as a solid line.

1322 Langmuir, Vol. 24, No. 4, 2008 Shi et al.

der Waals interactions to generate large adhesive and shearforces.46,57There is increasing interest in creating new types ofadhesives by mimicking the gecko mechanism, such as micro-fabricated dense arrays of polyimide hairs.58 Considering thestructure of dendrons that is similar to that of gecko’s spatulae,we should expect that dendrons with multiple branches mayprovide a model system for enhancing the intermolecularinteraction leading to the mimicry of gecko setae.

To provide information on whether G1-SH is well immobilizedon a gold substrate, we have employed an electrochemicalinstrument and the STM method. During the electrochemicalmeasurement, we compared the properties of a bare gold electrodeand a G1-SH SAM-coated gold electrode. In the case of a baregold electrode, a pair of easily reversible waves with a smallpeak potential separation of 0.076 V on the CV curve wasobserved, as shown in Figure 4a. Upon monolayer formation ona gold surface, we observed a decrease in the current response.Therefore, the CV data suggest that a G1-SH monolayer witha blocking effect of electron transfer was formed on the gold

surface.52 With the STM experiment, the flat morphology onAu(111) was observed directly, indicating that the SAM of G1-SH is uniform, as shown in Figure 4b.

Conclusions

We have directly investigated the rupture force betweendendrons by SMFS, which reflects the dissociation of dendronsfrom each other. The interaction between dendrons increaseswith the increase in the generation of the dendrons. The forceloading rate dependence of the rupture force indicates athermodynamic pulling property. This line of research can providea new insight into the self-organization of dendron in the bulkas well as at the interface. In addition, mimicking the micro“animal foot” achieving new manmade adhesives by using thesemultiple-branch molecules is greatly anticipated.

Acknowledgment. We thank the Natural Science Foundationof China (20474035) and the National Basic Research Program(2007CB808000) for financial support. We also thank ProfessorZhishan Bo (Institute of Chemistry, CAS, China) and Dr. HuapingXu for helpful discussions.

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(57) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.;Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J.Proc. NatlAcad. Sci. U.S.A.2002, 99, 12252-12256.

(58) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov,A. A.; Shapoval, S. Y.Nat. Mater.2003, 2, 461-463.

Figure 4. (a) CV curves of a bare Au electrode (black) and G1-SH (gray) on the Au electrode. The counter electrode was platinum, andthe reference electrode was Ag/AgCl electrode with the Fe(CN)6

3-/Fe(CN)64- system used as a redox probe. Potential scanning is between-0.05 and 0.55 V at a scan rate of 0.10 V s-1. (b) STM image of the G1-SH SAM. The scale is 130 nm× 130 nm, and the height bar forthe image is 2 nm.

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