Molecular recognition by self-assembled monolayers of cyclodextrin on Ag

Article abstract of DOI:10.1021/jp963916u

Self-assembled monolayer (SAM) of a thiolated cyclodextrin (CD) derivative was prepared, and stereoselective inclusional complexation of optically active azo dyes (p- or o-methyl red conjugates with chiral 1-phenyl-ethylamine, p- or o-MR-PEA) with both fr

Full text of DOI:10.1021/jp963916u

J. Phys. Chem. B 1997, 101, 6855-6860
6855
Molecular Recognition by Self-Assembled Monolayers of Cyclodextrin on Ag†
Hiroyuki Yamamoto, Yasushi Maeda, and Hiromi Kitano*
Department of Chemical and Biochemical Engineering, Toyama UniVersity, Toyama 930, Japan
ReceiVed: NoVember 25, 1996; In Final Form: June 17, 1997X
Self-assembled monolayer (SAM) of a thiolated cyclodextrin (CD) derivative was prepared, and stereoselective
inclusional complexation of optically active azo dyes (p- or o-methyl red conjugates with chiral 1-phenyl-
ethylamine, p- or o-MR-PEA) with both free CD and the SAM of the CD derivative on silver surface was
investigated by using resonance Raman (RR) and surface-enhanced resonance Raman spectroscopy (SERRS),
respectively. Characteristic molecular vibrations of functional groups of MR-PEA were used to describe
the complexation. To our knowledge, this is the first report which compares the complexations of free and
surface-confined CD by Raman spectroscopy. Association constants for the complexation (K) were determined
from the relative intensity of Raman scattering of the dye complexed with CD. The K values for the
complexation between MR-PEA and free CD determined by Raman spectroscopy were quite similar to
those measured by UV-visible spectroscopy, which confirmed the validity of Raman spectroscopy for the
analysis of the inclusion phenomena. Though the K values of the SAM system were much larger than that
of the free system, similar stereoselectivity was observed in both of the systems. The R enantiomer of o-MR-
PEA was more preferentially included both by free and surface-confined CD than the S-enantiomer, whereas
a slight selectivity was observed for p-MR-PEA. The importance of the position of optically active carbon
atom with respect to the azobenzene moiety, which penetrated into the cavity of CD, was suggested. Our
findings show that the enantioselectivity of CD is preserved after immobilization onto solid surface.
Introduction
absorption spectroscopy,13 and Raman spectroscopy,14 etc.
Among them IR and Raman spectroscopies can provide very
Enzymes accelerate chemical reactions highly efficiently and
selectively via enzyme-substrate complex. Cyclodextrins
(CDs) which have a hydrophobic cavity whose dimensions are
depth ) 9 Å and diameters ) 5, 7, and 8.5 Å for R-, â-, and
γ-CD, respectively, have been investigated as an enzyme model
because they form inclusional host-guest complexes with
various substrates resulting in significant catalytic effects in
many chemical reactions.1 Regio- and stereoselective molecular
recognition of CDs have been expected because the cavity of
CDs themselves is surrounded by chiral sugars. Unmodified
CDs, however, have a limit in the inclusional force and
selectivity, and chemical modification of CDs with various
functional groups has been carried out to improve those
properties.2 CD derivatives which have responsiveness to
external stimuli,3 and supramolecules such as catenane4 and
rotaxane (nanotube of CD)5 were also prepared. Furthermore,
modified or unmodified CDs have been applied to carriers of
gas and liquid chromatography6 or a drug delivery system.7
Recently, some research groups prepared self-assembled
monolayers (SAMs) from thiolated CD derivatives on metal
surfaces, and molecular recognition by surface-confined CDs
was investigated.8 SAMs of thiols and disulfides are spontane-
ously composed supramolecular assemblies on silver or gold
surfaces via S-Ag or S-Au bonds, respectively.9 The SAMs
having various functionalities are expected to be applied to
electronic devices and microprintings, and SAMs with long alkyl
chains and hydrophilic surface are noted as biomembrane
mimetics.10
useful information because vibrational spectroscopy is sensitive
to orientation and conformation of molecules. Substances
adsorbed on metal surfaces are known to give rise to a strong
Raman scattering (so-called surface-enhanced Raman scattering,
SERS), when the surfaces are excited with a light in resonance
with surface plasmon polariton of the metal.15 SERS-active
surfaces applied thus far are roughened electrode, island film
and colloid, of gold, silver, and other metals.10,14,15 Furthermore,
a chromophore-containing substance causes surface-enhanced
resonance Raman scattering (SERRS), which is much stronger
than SERS, when the substance is excited at the resonance
wavelength of the chromophore.15 Furthermore, SERS and
SERRS have a great advantage in the analysis of fluorescent
molecules because of its ability to quench fluorescence, which
causes trouble in normal Raman spectroscopy.16
In the previous study, we investigated complexation of azo
dye by SAM of CD derivative8a as examples of analyses of
molecular recognition at interface by SER(R)S. In the present
work, we prepared SAM of a thiolated R-CD, 6-(2-mercapto-
ethylamino)-6-deoxy-R-CD (MEA-R-CD), on Ag and examined
the stereoselective complexation between optically active azo
dyes and the R-CD derivative by SERRS. The use of the guest
molecules that have both asymmetric carbon and a chromophoric
group enables us to investigate the chiral complexation phe-
nomenon in detail by spectroscopic methods.
Experimental Section
Structure of SAMs and adsorbed molecules on the metal
surfaces can be analyzed by using scanning tunneling micros-
copy,11 atomic force microscopy,12 infrared (IR) reflection
Materials. R-Cyclodextrin (R-CD), (R)-(+)- and (S)-(-)-
1-phenylethylamine ((R)- and (S)-PEA) were purchased from
Wako Pure Chemicals (Osaka, Japan). p- and o-methyl red (p-
and o-MR) were purchased from Tokyo Chemical Co. (Tokyo,
Japan) and Nacalai Tesque (Kyoto, Japan), respectively. Other
reagents were commercially available. Milli-Q grade water was
used for sample solutions.
* To whom all correspondence should be addressed.
† Presented at the 45th Annual Meeting of the Society of Polymer
Science, Japan, at Nagoya, in May 1996.
X Abstract published in AdVance ACS Abstracts, August 1, 1997.
S1089-5647(96)03916-8 CCC: $14.00 © 1997 American Chemical Society

6856 J. Phys. Chem. B, Vol. 101, No. 35, 1997
Yamamoto et al.
SCHEME 1. Preparation of 6-((2-Mercaptoethyl)amino)-
6-deoxy-r-cyclodextrin (MEA-r-CD)
Ag colloids were prepared by reduction of AgNO3 with
NaBH4 at 0 °C for 1 h, and finally pH of the suspension was
adjusted to 7.35 with HCl. The final concentration of Ag was
3.13 × 10-4 mol/L. The absorption maximum of the colloids
was 393 nm. Using a dynamic light-scattering method (DLS-
7000DL, Otsuka Electronics, Hirakata, Japan; light source, Ar
laser 488.0 nm), the average hydrodynamic diameter of the Ag
particles was estimated to be 64 nm.
Modification of Cyclodextrin (Scheme 1). 6-((2-Mercap-
toethyl)amino)-6-deoxy-r-cyclodextrin (MEA-r-CD), the thi-
olated r-CD, was prepared by a reduction with mercaptoethanol
at room temperature overnight after a substitution of tosylated
r-CD (degree of substitution of tosyl groups; 4) with excess
amount of cystamine in pyridine at 40 °C for 2 days. After
evaporation of the solvent, an oily reaction mixture was put
into cold acetone, and a white precipitate was obtained. The
precipitate was dissolved in MeOH:water ) 3:1 and recrystal-
lized in cold acetone. The precipitate was finally filtrated and
dried in vacuo.
Figure 1. Resonance Raman spectra of 10 µM p-MR-(R)-PEA under
various conditions. Excited at 488.0 nm: (a) 0.1 N HCl, (b) HEPES
buffer (pH 7.35), and (c) 0.1 N NaOH.
28
o-MR-(R)-PEA: Yield: 230 mg (33.1%). [R]D ) -1056
mL dm-1 g-1 (in solvent D). ꢀmax ) 23 200 cm-1 M-1 (445
nm, in solvent D). IR (KBr, cm-1) 1652 (νCdO, amide I), 1554
(δNH, amide II). Anal. Calcd for C23H24N4O: C, 74.20; H,
6.50; N, 15.00. Found: C; 74.42, H; 6.56, N; 15.02.
o-MR-(S)-PEA: Yield: 148 mg (21.3%). [R]D28 ) 1085 mL
dm-1 g-1 (in solvent D). ꢀmax ) 22 900 cm-1 M-1 (445 nm,
in solvent D). IR (KBr, cm-1) 1652 (νCdO, amide I), 1554 (δNH
,
amide II). Anal. Calcd for C23H24N4O: C, 74.20; H, 6.50; N,
15.00. Found: C; 74.67, H; 6.59, N; 15.06.
Spectroscopic Measurements. Raman spectra were re-
corded on a Raman spectrophotometer (NR-1100, Japan Spec-
troscopic Co., Tokyo, Japan; light source, argon laser 488.0 nm)
with a band resolution of 5 cm-1 or less in a 300 µL quartz
cell. The observation cell was thermostated by a Peltier device
(RT-IC, Japan Spectroscopic Co.).
UV-visible spectra were recorded on a UV-visible spec-
trophotometer (Ubest 35, Japan Spectroscopic Co.) equipped
with a quartz cell thermostated by a Peltier device (EHC-363,
Japan Spectroscopic Co.).
Preparation of Sample Solutions. To prepare SAM on Ag,
MEA-R-CD dissolved in water was added to colloidal Ag and
kept at ambient temperature for 1 h or more. A dye solution
(0.1 mL, 270 µM in MeOH) was added to a buffer solution
containing R-CD (2.6 mL) or a suspension of Ag colloid
modified with MEA-R-CD for the measurements of the free
CD system and the SAM system, respectively. Acidic and
neutral solutions were prepared with a CH3COONa-HCl (pH
3.00, 20 mM) and (2-hydroxyethyl)piperazine-N ′-2-ethane-
sulfonic acid (HEPES, pH 7.35, 10 mM) buffer, respectively.
Yield 975 mg (86.5%). 1H NMR (400 MHz, DMSO-d6) δ
5.26 (6H, C(1)H), 2.53 (8H, CH2SH), 2.43 (16H, CH2NH). Anal.
Calcd for (MEA)4-R-CD: C, 43.70; H, 6.67; N, 4.63. Found:
C, 43.47; H, 6.81; N, 4.68.
Syntheses of Optically Active Azo Dyes. Four chiral azo
dyes, methyl red-1-phenylethylamine conjugates (MR-PEAs),
were synthesized by coupling of p- or o-MR with chiral PEA
in chloroform at 0 °C for 3 h and at room temperature overnight
by using dicyclohexylcarbodiimide and 1-hydroxybenzotriazole
as condensing agents. Dicyclohexylurea was filtrated off, and
the filtrate was washed carefully with water, 10% NaHCO3,
0.1 N HCl, and water in this order. The washed organic phase
was dried with anhydrous Na2SO4 overnight. After Na2SO4 was
filtrated off, the solvent was evaporated. The crude product
was purified by column chromatography (Silica Gel 60 (Merck),
mobile phase; solvent A (mentioned below) for p-MR-PEA
and solvent B for o-MR-PEA).
Results and Discussion
All of the dyes were characterized by elemental analysis,
optical rotatory power, and IR and UV-vis spectroscopy.
Solvent systems used were solvent A CHCl3:MeOH ) 9.4:0.6,
B CHCl3:MeOH ) 9.5:0.5, C CHCl3:MeOH ) 9.8:0.2, and D
CHCl3.
Characterization of Optically Active Azo Dyes. Charac-
terization of functional groups of p- and o-MR-PEAs in
aqueous solutions of various pHs was carried out by using
resonance Raman (RR) spectroscopy. Peaks attributable to the
quinoid form (CdC stretching vibration (νCdC) and N-N
stretching vibration (νN-N)) were observed under acidic condi-
tions, and those of the azo form (NdN stretching vibration
(νNdN) and φ-Nd stretching vibration (νφ-Nd)) were observed
at neutral and alkaline conditions (Figure 1).17 The pKa’s of
the azo groups (pKa(NdN)) of p- and o-MR-PEA obtained by
pH titration using UV-visible spectroscopy were 4.0 and 4.2,
respectively. These results suggest that these dyes exist as the
azo form in neutral and alkaline solutions (pH > pKa(NdN)),
and the protonation of azo group in acidic solutions (pH < pKa-
(NdN)) produces the quinoid form (Figure 2).17
28
p-MR-(R)-PEA: Yield: 1.025 g (72.4%). [R]D ) -138
mL dm-1 g-1 (in solvent C). ꢀmax ) 15 200 cm-1 M-1 (437
nm, in solvent C). IR (KBr, cm-1) 1633 (νCdO, amide I), 1560
(δNH, amide II). Anal. Calcd for C23H24N4O‚1/3H2O: C, 73.08;
H, 6.59; N, 14.80. Found: C, 72.75; H, 6.70; N, 14.31.
p-MR-(S)-PEA: Yield: 992 mg (70.6%). [R]D28 ) 142 mL
dm-1 g-1 (in solvent C). ꢀmax ) 15 400 cm-1 M-1 (437 nm, in
solvent C). IR (KBr, cm-1) 1633 (νCdO, amide I), 1560 (δNH
,
amide II). Anal. Calcd for C23H24N4O‚1/3H2O: C, 73.08; H,
6.59; N, 14.80. Found: C, 73.98; H, 6.56; N, 14.69.

Self-Assembled Monolayers of Cyclodextrin on Ag
J. Phys. Chem. B, Vol. 101, No. 35, 1997 6857
of MR-PEAs, which showed the most prominent peak shift
by the complexation with R-CD under neutral conditions, for
the evaluation of K of MR-PEAs. Stoichiometry of the
inclusional complexation was 1:1, too. The K values obtained
by using RR spectroscopy were close to those determined by
UV-vis method which has been often used to analyze inclu-
sional phenomena of CD (Table 1).1,20 Therefore, the estimation
of the association constants by Raman spectroscopy is accept-
able.
The associations of R-CD with both p- and o-MR-PEA were
pH dependent, and the K values in acidic solutions were larger
than those in neutral solutions. This may be mainly due to
difference in the flexibility of the azo and quinoid forms. The
quinoid form (dominant in acidic solutions) is considered to be
more flexible than the azo-form because the degree of rotational
freedom around azonium bond (dN-NH-) is larger than that
of azo bond (-NdN-). Accordingly, the quinoid form may
penetrate into the cavity of R-CD more deeply and fit the inner
wall of the CD cavity better than the azo form does. Association
constant is the ratio of reaction rates for an association to a
dissociation. The larger K value at acidic pH may be due to
slower dissociation rate induced by the deeper inclusion.
Figure 2. Schematic drawing of the structure changes of aminoazo-
type dye induced by the environmental changes.
The stereoselectivity was judged from the ratio of the K values
for R and S enantiomers (R/S in Table 1). o-MR-PEA showed
noticeable stereoselectivity at neutral pH and the selectivity
slightly decreased under acidic conditions (R/S ) 1.86 (pH 7.35)
f 1.68 (pH 3.00)). The reduction of the selectivity of o-MR-
PEA may be due to the increase in the nonspecific interaction
of the MR moiety relative to the stereospecific interaction of
the chiral PEA moiety. As for p-MR-PEA, the stereoselectivity
was not observed at neutral pH (R/S ) 1.06 (pH 7.35)), whereas
at acidic pH a slightly selective complexation with the S
enantiomer was realized (R/S ) 0.76 (pH 3.00)). p-MR-PEA
is presumed to be included more deeply, and the effect of the
chiral center becomes more significant under acidic conditions.
In short, when the inclusional force is strong, p-MR-PEA shows
the enhanced stereoselectivity, whereas o-MR-PEA does not.
Recently, stereoselective complexation of R- and â-CD with
chiral isomers was examined by titration calorimetry.21 Ste-
reoselectivity was observed in the complexations with ephedrine
and pseudoephedrine,21b whereas no noticeable selectivity was
detected for phenylalanine and chiral aliphatic alcohols.21a It
is generally agreed that the existence of two bulky substituents
on an asymmetric carbon atom is necessary for the stereose-
lective complexation with unmodified CDs and that spatial
arrangements of the substituents are important for the selectivity.1b
The present results are not inconsistent with the former one.
Figure 3. Double-reciprocal plot of [I(νφ-Nd)/I(νφ(PEA))] versus the
concentration of R-CD obtained by resonance Raman technique in CH3-
COONa-HCl buffer (pH 3) at 25 °C. [I(νφ-Nd)/I(νφ(PEA))] is the
difference in normalized intensities of νφ-Nd between systems with
R-CD and without R-CD: (a) o-MR-PEA, (b) p-MR-PEA, (b) R
enantiomer, and (O) S enantiomer.
The color of the solution of the dyes changed from red to
yellow by an addition of R-CD solution under acidic conditions
(pH 3.0). At the same time, in RR spectra of the solutions, the
peak intensities at 1630 cm-1 (νCdC) and 1194 cm-1 (νN-N
)
decreased, and those at 1405 cm-1 (νNdN) and 1152 cm-1
(νφ-Nd) increased with increasing concentration of R-CD. These
spectral changes were considered to arise from the environ-
mental change around the azo group of the dye molecules by a
complexation in the hydrophobic cavity of R-CD. Conse-
quently, the phenomenon observed suggests structural change
of the dye from the quinoid to azo form due to the inclusion of
the azonium group (dN-NH-) by R-CD (Figure 2).8a,18 These
spectral properties were commonly observed for all four dyes
examined here.
Evaluation of the Association Constants by Using Reso-
nance Raman Technique. The Raman spectra of MR-PEA
drastically changed by the complexation with R-CD at acidic
pH. The νφ-Nd band, which showed a pronounced spectral
change with the inclusion by R-CD, was used to evaluate the
relative population of complexed MR-PEA to free one. The
intensity of the peak at 1025 cm-1 (attributable to a benzene
ring of phenylethylamino group (νφ(PEA))) was used as an internal
standard, and the reciprocal of the relative intensity of νφ-Nd
band [I(νφ(PEA))/I(νφ-Νd)] was plotted against the reciprocal
of the concentration of R-CD in Figure 3. A linear relationship
in the wide concentration range of R-CD suggests that the
stoichiometry of the inclusional complex between each azo dye
and R-CD is 1:1. The association constants (K) of the
complexation of these chiral azo dyes with R-CD were obtained
from the x intercept of the plot.
Thermodynamic Analysis. The temperature dependence of
the inclusional complexation was examined by UV-vis spec-
troscopy at pH 7.35 (Table 1). The K values for both p- and
o-MR-PEA decreased with the rise of temperature. Further-
more, the values of R/S for o-MR-PEA increased with the rise
of temperature, whereas the stereoselectivity for p-MR-PEA
did not change. The enhancement of selectivity for o-MR-
PEA at higher temperature can also be rationalized with the
similar mechanism stated above, that is, the reduction of the
nonspecific MR moiety-CD interaction which relatively weak-
ens the specific interaction of the chiral moiety improves the
stereoselectivity.
Thermodynamic parameters for the inclusional complex
formation were given in Table 2. With respect to p-MR-PEA,
the driving force of the inclusional complexation is the large
negative ∆H (∆H ) -34 kJ/mol) for both of the optical isomers.
On the other hand, the association of o-MR-PEA with R-CD
is governed by entropy change (∆S ) 13 J/K mol) for the R
On the other hand, some peaks coming from azo dyes such
as methyl red and methyl orange were reported to shift by the
complexation with CDs at neutral pH.19 We used νφ-Nd band

6858 J. Phys. Chem. B, Vol. 101, No. 35, 1997
Yamamoto et al.
TABLE 1: Association Constants (K) and the Stereoselectivity (R/S) for the Complexation of p-MR-PEA and o-MR-PEA with
Free r-CD and MEA-r-CD on Ag
p-MR-PEA
K (M-1
o-MR-PEA
K (M-1
)
)
pH
temp. (°C)
(R)
58 ( 23
68 ( 8
30 ( 4
18 ( 4
15 ( 4
12 ( 3
(S)
R/Sd
(R)
(S)
R/Sd
free R-CD 3.00a
25
25
15
25
25
35
25
73 ( 22
90 ( 9
30 ( 9
17 ( 8
14 ( 5
12 ( 4
0.79 ( 0.37 138 ( 21
0.76 ( 0.10 140 ( 9
1.01 ( 0.29
1.03 ( 0.21
1.06 ( 0.23
1.02 ( 0.38
84 ( 12
83 ( 7
1.64 ( 0.28
1.68 ( 0.15
1.47 ( 0.27
1.85 ( 0.42
1.86 ( 0.32
1.92 ( 0.41
3.00b
7.35a
7.35a
7.35b
7.35a
91 ( 12
84 ( 21
90 ( 11
76 ( 7
62 ( 11
45 ( 16
48 ( 8
40 ( 10
SAMc
7.35
(20 ( 0.2) × 104e (23 ( 0.1) × 104 e 0.86 ( 0.08f
(2.0 ( 0.1) × 104 (1.3 ( 0.2) × 104 1.61 ( 0.36
a Measured by UV-vis. b Measured by RR. c Measured by SERRS with SAM of MEA-R-CD on colloidal Ag. [MEA-R-CD] ) 16 µM. d Ratio
of the association constants (K) for R and S enantiomers. e The values of 1/P estimated from Freundlich isotherm are shown. f The ratio of 1/P for
R and S enantiomers is shown.
TABLE 2: Thermodynamic Parameters for the
Complexation of p-MR-PEA and o-MR-PEA with r-CD in
10 mM HEPES Buffer (pH 7.35)
p-MR-PEA
(R) (S)
o-MR-PEA
(R)
(S)
∆H (kJ/mol) -33.9 ( 2.5 -34.2 ( 2.9 -6.6 ( 3.8
∆S (J/K mol) -90 ( 1.3 -90 ( 1.4 13 ( 2.1
-16.7 ( 5.8
-26 ( 3.0
∆G (kJ/mol) -7.0 ( 0.46 -7.0 ( 1.04 -11.0 ( 0.52 -9.0 ( 0.76
enantiomer and governed by enthalpy change (∆H ) -16.7
kJ/mol) for the S enantiomer. As was often observed in the
complexation of CDs with guest molecules, a linear relationship
(the compensation effect) between ∆H and ∆S is present, and
the slope of the straight line (isoequilibrium temperature) is 275
K, which is good agreement with the literature values (26522
and 274 K21a).
Figure 4. Schematic drawings of the inclusional complex of MR-
PEA and R-CD: (a) o-MR-(S)-PEA, (b) p-MR-(S)-PEA.
The thermodynamic analysis of the complexation of CD has
been performed by many research groups.21-23 The formation
of the inclusional complex of CD is considered to include the
contribution of various factors such as van der Waals interaction,
release of the water molecules from the cavity (high-energy
water), the break of a water cluster, and the conformational
change of the CD ring. The large favorable ∆S indicates that
an apolar binding has a large contribution to the stability of the
complex formation between o-MR-(R)-PEA and R-CD in the
same way as the complexation between R-CD and hydrophobic
molecules such as 1-adamantanecarboxylate.24 One of reasons
for the favorable ∆S of o-MR-PEA is a large break of water
cluster around the guest molecule in the course of complexation
compared with p-MR-PEA. On the other hand, the unfavorable
∆S for p-MR-PEA is due to an absence of interaction between
the benzene ring of phenylethylamino group and R-CD rim. The
decrease in the degree of rotational freedom of PEA moiety of
p-MR-PEA might give a small negative ∆S, whereas a
favorable ∆S due to the break of water cluster around PEA
moiety of p-MR-PEA would be much smaller than that of
o-MR-PEA. These factors (the break of a water cluster and
the change in degree of freedom) are considered to lead to the
small negative or favorable ∆S for o-MR derivatives and the
large negative ∆S for p-MR derivatives.
Mechanism of the Stereoselectivity in the Inclusional
Complex Formation of CD. Dye-R-CD inclusional com-
plexes were schematically drawn in Figure 5. X-ray crystal-
lographic study on R-CD-methyl orange complex showed that
the azobenzene moiety can pass through the cavity of R-CD,
and the azo group is located near the center of the cavity.25
Electronic spectra indicated that this is also true in aqueous
solutions. Taking Raman spectral change of MR-PEA induced
by the complexation with R-CD into consideration, the MR
moiety of MR-PEA seems to interact with the cavity of R-CD
Figure 5. Raman spectra of MEA-R-CD excited at 488.0 nm (a) solid
state and (b) SAM on colloidal Ag (pH 7.35).
dominantly. The asymmetric carbon of p-MR-PEA which has
a linear configuration is, therefore, considered to exist at the
position where it feels little attractive or repulsive force from
R-CD. On the other hand, o-MR-PEA has a bent form, and
the PEA moiety seems to locate on the edge of R-CD.
Therefore, the chiral PEA moiety of o-MR-PEA is able to
interact with the CD rim. Of course, R-CD is chiral and the
interaction could be stereoselective. Further investigation is
necessary to decide the interacting points on both CD and MR-
PEA for the occurrence of stereoselectivity. The favorable ∆S,
however, suggests that the selectivity to the R enantiomer of
o-MR-PEA by R-CD resulted from more desirable interaction
between the phenyl group and the R-CD rim than that of the S
enantiomer. The water cluster around the PEA moiety would
be destroyed by contact with the CD rim, and therefore, the
effective break of water cluster of the R enantiomer compared

Self-Assembled Monolayers of Cyclodextrin on Ag
J. Phys. Chem. B, Vol. 101, No. 35, 1997 6859
Figure 7. Langmuir and Freunflich plots for the adsorption isotherms
of MR-PEA to SAM of MEA-R-CD on Ag colloid at 25 °C. The
relative intensities of the νφ(PEA) band of MR-PEA to the intensity of
νC-S band of SAM (I) are plotted. (a) Langmuir plot for o-MR-(R)-
PEA, (b) Frendlich’s plot for p-MR-(R)-PEA.
Figure 6. SERRS spectra of p- and o-MR-PEA included into SAM
of MEA-R-CD on colloidal Ag (pH 7.35) excited at 488.0 nm, 25 °C;
[dye] ) 4 µM. (a) p-MR-(R)-PEA and (b) o-MR-(R)-PEA.
with the S enantiomer in the course of formation of the
inclusional complex with R-CD may lead to the favorable ∆S.
p-MR-PEA showed a sigmoidal shape and the adsorption did
not obey Langmuir’s isotherm (data not shown).
Langmuir’s isotherm is
Molecular Recognition by SAM of CD at the Solid-
Liquid Interface. The SAM of R-CD was prepared from
MEA-R-CD to investigate the molecular recognition at the
solid-liquid interface. In Figure 5, SERS spectrum of SAM
of MEA-R-CD on Ag was shown with Raman spectra of MEA-
R-CD at the solid state. Key features of SERS are a shift of
the C-S stretching vibration band (νC-S) at a region of 600-
750 cm-1 and disappearance of a S-H stretching vibration band
(νS-H) observed at about 2580 cm-1 in the solid state. These
features are reported to be characteristic of the cleavage of S-H
bonds followed by formation of S-Ag bonds.26 These results
suggested that MEA-R-CD was adsorbed to the Ag surface
via a S-Ag bond.
We investigated the stereoselective complexation of o- and
p-MR-PEA by the SAM composed of MEA-R-CD on the
surface of colloidal Ag. Addition of the dye solution to the
colloidal Ag modified with MEA-R-CD gave rise to Raman
bands from MR-PEA together with SERS of MEA-R-CD
(Figure 6). The concentration of the dye was too low to observe
RR scattering from free dye in the solution with our experi-
mental setup. Moreover as described before MR-PEA exists
as the quinoid form in the solution whose pH is lower than
pKa(NdN) of the dye, whereas MR-PEA complexed with CD
exists as the azo form. Characteristic bands for the azo form
observed in the Raman spectrum of MR-PEA in MEA-R-
CD/Ag system at pH 3.0 strongly suggested the bands were
due to SERRS from MR-PEA included in the cavity of surface-
confined CD. Our electrochemical study on the same system
showed that surface coverage of MEA-R-CD was fairly good
(97% of the calculated maximum density)27 and SERRS from
MR-PEA which directly adsorbed to Ag surface might be
negligible. After all, Raman bands of the dye observed in Figure
6 were considered to at least mainly, if not exclusively, contain
SERRS from the dye which was included by CD bound to Ag
surface.
na/bL ) KC/(1 + KC)
(1)
where na is the number of adsorbed molecules, bL is the number
of sites for adsorption, K is the association constant, and C is
the concentration of MR-PEA added to the system.
In this case na is given by I/R (I: intensity of νφ(PEA) band
normalized by νC-S from SAM), and therefore
1
I
1
1 1
1
)
+
(2)
(
)
R KbL C bL
The association constant is obtained from the x intercept of
1/I versus 1/C plots (Figure 7a).
Complexation of p-MR-PEA with surface-confined R-CD
obeyed Freundlich’s equation (eq 3):
na ) KFC1/n
(3)
F
where KF and nF are constants. Substitution of na by I/R gives
1
nF
log I ) log KF + log R + log C
(4)
The x intercept of log I versus log C plots gives a constant as
eq 6 (Figure 7b):
1/P ) 10n ‚log(K R)
(5)
F
F
Though 1/P given by eq 6 is not equal to the association
constant, this value can be used to estimate relative stereose-
lectivity.
The equilibrium constants for the association of MR-PEAs
and SAM of MEA-R-CD are given at the bottom of Table 1.
The K value of o-MR-PEA with CD of SAM system is much
larger than that of the free system. Taking the chemical
structures into consideration, MR-PEAs are considered to
penetrate into the cavity of surface-confined R-CD toward the
Ag surface from the dimethylamino group side. Electrostatic
interaction between the dimethylamino group of the dye and
colloid surface may strongly stabilize the complex. Therefore,
in the molecular recognition by the SAM composed of CD on
the Ag surface, the association constant is much larger than
that of free CD. In addition, the high surface concentration of
CD may induce a change in local environment and have some
effects on the association.
When the dye concentration was increased in the region where
the contribution of RR scattering from free dye can be ignored,
the intensities of SERRS from the dye increased correspond-
ingly, probably because the number of dye molecules that were
included in the cavity of CD to give SERRS increased. With
the assumption that the intensities of SERRS per one MR-
PEA molecule (R) is constant, which may be influenced by the
distance from the surface and orientation of axis of the dye
molecule to surface normal, the total intensities of SERRS (I)
are proportional to the number of adsorbed molecules. When
the relative intensities of the νφ(PEA) band of the dye normalized
by the intensity of νC-S band of SAM were plotted against the
concentrations of the dyes, o-MR-PEA obeyed the adsorption
isotherm of the Langmuir type (eq 1), whereas the plots for
The stereoselectivity (R/S) was slightly decreased compared
with the free system for o-MR-PEA, whereas the stereoselec-
tivity of p-MR-PEA with surface-confined CD was higher than

6860 J. Phys. Chem. B, Vol. 101, No. 35, 1997
Yamamoto et al.
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that with free CD probably because of the deeper inclusion and
the enhancement of nonspecific interaction with Ag surface.
These are probable reasons for the increase in the stereoselec-
tivity of p-MR-PEA (the deeper inclusion promotes the contact
of chiral center with the CD rim) and reduction in that of
o-MR-PEA (the enhanced nonspecific interaction weakens the
specific effect of chiral center) on Ag surfaces.
In conclusion, investigation of the inclusional complexation
of free and surface-confined CD was carried out by Raman
spectroscopy for the first time. To study the stereoselectivity
of the molecular recognition by CD, optically active azo dyes
were used. The association constants for free CD and MR-
PEA obtained by Raman spectroscopy were almost the same
with those obtained by UV-vis spectroscopy. The association
constants between surface-confined CD and MR-PEA were
measured by surface-enhanced resonance Raman spectroscopy.
o-MR-PEA formed more a stable inclusional complex than
p-MR-PEA (regioselective) due to the contribution of the
destruction of water cluster around PEA moiety. The R
enantiomer of o-MR-PEA associated with both free and
surface-confined CD more preferentially than did the S enan-
tiomer (stereoselective). Furthermore, the stereoselectivity for
o-MR-PEA decreased at interfaces due to the enhancement of
the nonspecific interaction, whereas that for p-MR-PEA
increased by the contribution of interaction with the chiral center
which was induced by the deeper inclusion.
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Although the stereoselectivity observed here was not satis-
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SAM of CD derivatives as stereoselective chemical sensor.
Acknowledgment. This work was supported by Grant-in-
Aid (08246222, 09240213, 09232223) from Ministry of Educa-
tion, Science and Culture, Japan. Y.M is grateful for the financial
support from the Nissan Science Foundation (Tokyo).
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