Japanese Journal of Applied Physics
Vol. 46, No. 8B, 2007, pp. 5614–5616#2007 The Japan Society of Applied Physics

Single Molecular Anatomy of Host–Guest Chemistry Based on Atomic ForceMicroscopy Study of Cyclodextrin–Ferrocene Molecular Interaction
Satoshi YASUDA1;5 , Yoshitaka O KUTSU1 , Iwao SUZUKI2 , Ken-ichi SHINOHARA3 ,Makoto K OMIYAMA4 , Osamu TAKEUCHI4 , and Hidemi S HIGEKAWA1 
1Institute of Applied Physics, 21st Century COE, CREST–JST, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai 980-8578, Japan3
School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan
4
Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan
5
Research Center for Advanced Carbon Materials, AIST, Tsukuba, Ibaraki 305-8565, Japan
2(Received January 19, 2007; revised April 5, 2007; accepted April 19, 2007; published online August 23, 2007)

The formation process of a typical host–guest compound, -cyclodextrin–ferrocene system, in various solutions was analyzed
on the basis of the results of unbinding-force measurement using atomic force microscopy (AFM). The ratio of the formation
numbers determined by AFM is consistent with the complexation constant obtained from the macroscopic analysis. We also
analyzed the unbinding force on the basis of the mechanism of surface tension, and solvophobic factors, the intrinsic
van der Waals interaction and the eﬀective length of surface tension for ferrocene, were evaluated at the single-molecule
level. [DOI: 10.1143/JJAP.46.5614]
KEYWORDS: cyclodextrin, self-assembled monolayer, AFM, unbinding force, complexation constant, solvophobic

1.

Introduction(a)

The understanding and control of the intermolecular
dynamics, particularly the molecular recognition processes,
have been fundamental subjects of biology in recent
decades, and are also the primary goals of the developmentof future nanoscale molecular science and technology. Since
most important systems are eﬀective in solvents, interactions
arising from solvent-related factors, such as the solvophobic
eﬀect, play important roles in intermolecular dynamics inaddition to intrinsic forces such as the van der Waals (vdW)
interaction and hydrogen bonding.1)
The development of atomic force microscopy (AFM) has
enabled the direct measurement of intermolecular interactions between individual molecules at the piconewton
level.2) Since the binding force can be measured at thesingle-molecule level, this technique has been used toinvestigate the molecular interaction on the basis of theanalysis of the force amplitude.
For example, by applying this technique to a combinationof -cyclodextrin (-CyD) and adamantane molecules, thecontributions of solvent-related factors have been distinguished from the intrinsic vdW force at the single-molecule
level.3) With respect to dynamic force spectroscopy (DFS),
which analyzes thermal activation processes of bondformation, the landscape of the interaction potential andthe lifetime of the bonding structures are evaluated.4)
From another point of view, since the bonds formed byseveral molecules are involved in the unbinding process, one
can analyze the number of bonds with respect to theindication of a binding probability of the host–guest system
at an equilibrium condition. In this study, we examined thevalidity of this approach using a -CyD [Fig. 1(a)] andferrocene complex system, the thermodynamic property ofwhich has been well studied macroscopically.5,6) The ratio of
the formation numbers determined by AFM was consistent
with the complexation constant obtained from the macroscopic analysis. Furthermore, the obtained unbinding forceE-mail address: hidemi@ims.tsukuba.ac.jp

(b)
OH

O
N
H

O
O
HO

(c)

OH

7

S S
SH

Fe

AFM tip: Ferrocene: β-CyDAu(111) surface

Fig. 1. Structures of (a) -cyclodextrin (CyD), (b) LP--CyD and 11ferrocenyl-1-undecanthiol, and (c) experimental setup.

was quantitatively analyzed, and the contributions ofsolvophobic factors, as well as the intrinsic vdW interaction,
were evaluated at the single-molecule level. After acomparison with the data, which were obtained for theadamantane molecule in previous work,3) it was demonstrated that the analysis based on surface tension can be used
to characterize the solvophobic eﬀect at the single-molecule
level.
2.

Experimental Methods

To form self-assembled monolayers (SAMs) of -CyD
molecules on a Au(111) surface, we used lipoamide (LP)-CyD molecules, which are -CyD molecules modiﬁed with alipoamide residue chemically bound to the primary hydroxyl
side. Scanning tunneling microscopy (STM) measurements
revealed that a LP--CyD monolayer was formed all over
the Au(111) surface as shown in Fig. 2(a), and densely
packed homogeneous LP--CyD molecules were conﬁrmed[Fig. 2(b)]. The formation process and the structure were
examined and conﬁrmed in detail using STM and cyclic
voltammogram measurements.7,8)5614

Jpn. J. Appl. Phys., Vol. 46, No. 8B (2007)

S. YASUDA et al.

(b)

(a)

Retraction

0.1nN

(a)

Adhesion Force

Fig. 2. STM images of LP--CyD molecules self-assembled on a Au(111)
surface (a) 75  75 nm2 , Vt ¼ þ1:0 V, It ¼ 0:3 nA, (b) 50  50 nm2 ,Vt ¼ þ1:0 V, It ¼ 0:15 nA.

F

2.5nm
Tip displacement

(c)

10
0

5

Counts15

Auto correlation (a.u.)

(b)

0

50

100

150

200

Adhesion force (pN)

0

50

100

150

200

Adhesion force (pN)

Fig. 3. (a) Typical force curve obtained for DMF. (b) Histogram derived
from the force measurement in DMF. (c) Autocorrelation analysis of thehistogram shown in (b).

Ferrocene molecules were chemically tethered on a goldcoated Si3 N4 AFM cantilever, using 11-ferrocenyl-1-undecanthiol (11-FUT) molecules, that is, ferrocene modiﬁedwith an undecanthiol chemically added to one cyclopentadienyl group. Schematic illustrations of the modiﬁed
molecules and the experimental setup are shown in
Figs. 1(b) and 1(c), respectively.
3.

Results and Discussion

Figure 3(a) is a typical force curve obtained in dimethylformamide (DMF) solution. Bond-breaking events occurred
step by step, because the tip–sample bonding consists ofmany individual host–guest bonds. For the analysis of thehost–guest interaction, the unbinding force in the ﬁnal step,
indicated by ‘‘F’’ in Fig. 3(a), was recorded. More than 300
force measurements were performed for methanol, DMF,
and water. A histogram of the unbinding forces measured for
DMF is shown in Fig. 3(b). As indicated by arrows, there isa certain periodic feature in the histogram, which is moreclearly conﬁrmed from an autocorrelation analysis of thehistogram, as shown in Fig. 3(c). This result indicates thatthe obtained unbinding forces are composed of integer
multiples of an elementary force for breaking a single -

CyD–ferrocene complex. Through a similar procedure, theunbinding forces in all solutions were determined. The
unbinding forces between ferrocene and -CyD molecules
measured in methanol, DMF, and water are 21  3, 26  3,
and 54  6 pN, respectively.9) The measurement error (12%)
mainly arises from that in the spring constant of thecantilever.10)
The unbinding force in water is close to that obtained inprevious work, 55  10 pN,9) and is smaller than that for thecase of the -CyD–adamantane complex, 97  12 pN.3) The
magnitude relationship of the unbinding force between CyD–ferrocene and -CyD–adamantane complexes is ingood agreement with the relationship of the free energy
macroscopically obtained for both cases, G ¼ 4:5 and6:2 kcal/mol, respectively.6,11) The analysis for the detailed treatment of the comparison will be our future work.
Ferrocene is known to dissolve well in polar DMF, which
results in a lower complexation. This higher dissolvability is
due to the high polarizability of ferrocene. In fact, according
to the macroscopic analysis, the complexation constants K offerrocene and CyD in water, DMF, and methanol are 2400,
67, and 125 mol1 (100 : 2:8 : 5:2), respectively. This eﬀect
on solvation is predicted to occur in AFM measurement. By
dividing the value of the adhesion force by the unbinding
force of a single -CyD and guest molecular complex, we
can obtain the number of bonded -CyD–ferrocene complexes in each measurement. Here, ‘‘adhesion force’’ is theaveraged value of the forces obtained from the bondbreaking events, where events occur as a single-step
unbinding process instead of the step by step shown in
Fig. 3(a). The ratio of the formation numbers determined forthe processes in water, DMF, and methanol is 100 : 3 : 17
[ð7800=54Þ : ð110=26Þ : ð510=21Þ], and the order of theintensity ratio components is consistent with that of K,
100 : 2:8 : 5:2. Here, the diﬀerence in the value for methanol
may be related to the diﬃculty in the macroscopic measurement of the complexation constant K in methanol due to thelow solubility.
The intermolecular interaction and solvophobic eﬀectwere then analyzed. How can we distinguish the solvophobic
eﬀect from other eﬀects? The solvophobic force is related to
the force required to form a space in a liquid to accommodate a guest molecule, and its strength is proportional to
the surface tension of the solvent.1) Although only on thebasis of the macroscopic physicochemical and thermodynamic measurements, the free energy for the formation of CyD and adamantane inclusion complex was found to
exhibit a linear relationship with respect to surface tension;that is, Gsolvent{solvent / ðv2=3 Þ, where  and ðv2=3 Þ are
the solvent surface tension and surface area change in thereaction, respectively.12) The surface tension contributes tothe stability of the complex.
From this standpoint, the obtained unbinding forces forthe ferrocene molecule were plotted as a function of surface
tension of the solvents macroscopically determined13)[Fig. 4-(I)]. Here, the results obtained for a combination of
-CyD and adamantane molecules in methanol-water solutions of various volume ratios with diﬀerent surface tensions
are presented together for comparison [Fig. 4-(II)].3)
The unbinding forces obtained for the combination of CyD and adamantane molecules linearly increase for an

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Jpn. J. Appl. Phys., Vol. 46, No. 8B (2007)

which is expected to occur in the measurement of the host–
guest system because of the solvation eﬀect.

20 40 60 80 100

(b)
(II)

0

Solvophobic
force

4.

(I)
Water
vdW
force

DMF
MeOH

0

Unbinding force (pN)(a)

S. YASUDA et al.

20

40

60

Effective
length80

Surface tension (mN/m)Fig. 4. (a) Unbinding forces obtained for (I) ferrocene and (II) adamantine
molecules,3) as a function of surface tension of solvents. (b) Intermolecular interactions.

increasing surface tension of the solvents. Since theobserved linear relationship is based on the measurementsof the intermolecular interaction at the single-molecule
level, the result indicates that the solvophobic eﬀect on asingle host–guest interaction can be analyzed using themechanism of surface tension at this level.3) From thegradient of the linear relationship shown in Fig. 4(a)-(II), thelength of this system related to surface tension [‘‘eﬀective
length’’, Fig. 4(b)] was estimated to be 1:07  0:13 nm,
which is consistent with the perimeter of the adamantane
molecule, 1:3 nm.
A similar analysis was performed for the -CyD–
ferrocene system. From the gradient of the linear relationship in Fig. 4(a)-(I), the eﬀective length of the ferrocene wasestimated to be 0:66  0:13 nm. The ferrocene and adamantane molecules are similar in size, and thus, the diﬀerence inthe eﬀective length may be related, for example, to thediﬀerence in the conformation and solvation eﬀects on theunbinding process. In this study, only three data points wereobtained because only three solutions are available to use in
the process of measuring the unbinding force for ferrocene.
Since it is diﬃcult to ﬁnd an adequate number of solutions,the use of surface tension in the analysis of the solvophobic
eﬀect would be advantageous at the single-molecule level.
The intrinsic vdW force, in this case obtained byextrapolating the linear relationship between the surface
tension and the unbinding force, is 5:2  5:9 pN (less than
10 pN). Clearly, the value is smaller than that obtained forthe case of the adamantane molecule, 18  7 pN. The ratioof the surface tensions macroscopically obtained for water,
DMF and methanol is 72 : 37 : 22 (¼ 100 : 51 : 31). In
contrast, the ratio of the solvophobic forces (unbinding
forces—vdW) for water, DMF and methanol obtained in thisexperiment is 49 : 21 : 16 (¼ 100 : 43 : 33), when the vdW
force is assumed to be 5 pN. The smaller ratio in DMF
measured from the unbinding force, unlike the macroscopic
value, can be related to the decrease in surface tension,

Conclusions

The formation process of a typical host–guest compound,-CyD–ferrocene system, was analyzed based on theunbinding-force measurement by AFM. The ratio of theformation numbers determined by AFM is consistent withthe complexation constant obtained from the macroscopic
analysis. The results demonstrate the potential of thismethod to provide more accurate understanding of theresults of the macroscopic analysis. Regarding the solvophobic eﬀect, the comprehensive results obtained for the
ferrocene and adamantane molecules suggest the importanceof using the eﬀective length for the analysis of the host–
guest interaction. Precise images of host–guest complexes
drawn at the molecular level are expected to pave the way
for a more straightforward molecular design.
Acknowledgements
This work was partially supported by a Grant-in-Aid for
Scientiﬁc Research from the Ministry of Education, Culture,
Sports, Science and Technology of Japan, and the Japan
Society for the Promotion of Science. We thank Ms. Rie
Yamashita for her help in preparing this paper.

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