Binding of Aromatic Isocyanides on Gold Nanoparticle
Surfaces Investigated by Surface-Enhanced Raman
Scattering
SANG-W OO JOO,* W AN-JOONG KIM , WAN SOO YUN, SUNGU HWANG, and
INSUNG S. CHOI
Department of Chemistry, Soongsil University, Seoul 156-743, Korea (S.-W.J.); Department of Chemistry and School of Molecular
Science (BK21), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea (W.-J.K., I.S.C.);
Electronic Device Group, Korea Research Institute of Standards and Science, Daejeon 305-600, Korea (W.S.Y.); and School of
Free Major, Miryang National University, Miryang, Gyeongnam 627-702, Korea (S.H.)
The adsorption structure and binding of phenyl isocyanide (PNC),
2,6-dimethyl phenyl isocyanide (DM PNC), and benzyl isocyanide
(BZI) on gold nanoparticle surfaces have been studied by means of
surface-enhanced Raman scattering (SERS). PNC, DM PNC, and
BZI have been found to adsorb on gold assuming a standing geometry with respect to the surfaces. The presence of the n(CH) band
in the SERS spectra denotes a vertical orientation of the phenyl
ring of PNC, DMPNC, and BZI on Au. The lack of a substantial
red shift and signiŽ cant band broadening of the ring breathing
modes implied that a direct ring p orbital interaction with metal
substrates should be quite low. For PNC, the band ascribed to the
C–NC stretch ing vibration was found to almost disappear after adsorption on Au. On the other hand, the C–NC band rem ained quite
strong for DMPNC after adsorption. This result suggests a rather
bent angle of C–N[C: for the nitrogen atom of the NC binding
group on the surfaces, whereas a linear angle of C–N[C: should
be more favorable on gold surfaces due to an intramolecular steric
hindrance of its two methyl groups. SERS of BZI on gold nanopaticles also supports a bent angle of :C[N–CH 2 for its nitrogen
atom, suggesting a preference of sp 3 (or sp 2 ) hybridization for the
nitrogen atom.
Index Headings: Phenyl isocyanide; 2,6-Dimethyl phenyl isocyanide;
Benzyl isocyanide; Au; Adsorption; Surface-enhanced Raman scattering; SERS.
INT RODUCTIO N
In a theoretical work on self-assembled monolayers
(SAM s) of alkanethiolates on Au surfaces, sp 3 hybridization for the sulfur atom on the three-fold hollow sites
was predicted to be energetically most stable.1 Aromatic
thiols such as benzyl mercaptan and p-biphenylmethanethiol were reported to form closely packed and highly
ordered monolayers on gold while a lower surface coverage was form ed for thiophenol and p-biphenylthiol. 2
The C–Se bond of benzeneselenolate also appeared to be
rather parallel to the surface as indicated by a decrease
in its Raman intensity upon adsorption on Au.3
Although the structure and binding of thiolates have
been studied extensively by microscopic and spectroscopic tools,4 relatively little is known about the adsorption structures of aryl isocyanides. There have been several reports exploring the chemisorption of organoisocyanides on metal surfaces.5–8 We recently reported dissimilar adsorption orientations of p-biphenylisocyanide
Received 6 May 2003; accepted 13 October 2003.
* Author to whom correspondence should be sent. E-mail: sjoo@ssu.
ac.kr.
218
Volume 58, Number 2, 2004
on gold and silver nanoparticle surfaces. 9 The detailed
origins of its different adsorption characteristics on Ag
and Au have not been fully clariŽ ed.
The phenomenon of surface-enhanced Raman scattering (SERS) has become one of the most sensitive techniques in monitoring adsorbates on metal substrates at
the submonolayer coverage limit.10,11 In order to further
investigate interfacial structure and binding of self-assembly m onolayers prepared by aryl isocyanide, we performed a SERS study of phenyl isocyanide (PNC), 2,6dimethyl phenyl isocyanide (DMPNC), and benzyl isocyanide (BZI) on Au nanoparticle surfaces. DM PNC has
two methyl groups that m ay affect the orientation upon
adsorption on surfaces. The main purpose of this study
is to investigate different binding structures of arom atic
isocyanides on Au in uenced by an intramolecular steric
hindrance of the adsorbate.
EXPERIMENTAL
Sample Preparations. Phenyl isocyanide (PNC) was
prepared by a literature method.12 Phosphorus oxychloride (0.06 mol, 5.60 mL) dissolved in 40 mL of tetrahydrofuran (THF) was added to a stirred mixture of formanilide (0.02 mol, 2.29 g) and triethylamine (0.09 m ol,
12.54 m L) in 10 m L of THF at 278 8C over 30 m in. The
mixture was gradually warm ed to 0 8C overnight with
stirring and cooled to 215 8C. An ice-cold aqueous 50
mL solution of Na 2CO 3 (10%) was added dropwise to the
mixture and warmed up to room temperature. The organic phase was separated and extracted with an ice-cold
aqueous solution of Na 2CO 3 (10%), and then dried over
MgSO 4 . The solvent was evaporated and the crude product was chromatographed on a Florisil column using ethyl acetate and hexane with a ratio of 1:2 as an eluting
solution to yield PNC (1.73 g, 89%). The chemicals
DMPNC ($98%) and BZI (98%) were purchased from
Fluka and Sigma Aldrich, respectively, and used as received. The citrate-stabilized gold nanoparticle was synthesized by following the procedures in the literature:13
A portion of 133.5 mg of KAuCl 4 (Aldrich) was initially
dissolved in 250 m L of water, and the solution was
brought to boiling. A 25 mL solution of sodium citrate
(1%) was then added to the KAuCl4 solution under vigorous stirring, and boiling was continued for approximately 20 min. The resulting Au nanoparticle solution
was stable for several weeks. All the chemicals were re-
0003-7028 / 04 / 5802-0218$2.00 / 0
q 2004 Society for Applied Spectroscop y
APPLIED SPECTROSCOPY
F IG . 1.
(a) TEM image, (b) size distribution, (c) UV-Vis absorption spectrum, and (d ) XRD pattern of gold nanoparticles.
agent grade, and triply distilled water, of resistivity greater than 18.0 MV·cm, was used in making the aqueous
solutions.
Transmission Electron M icroscopy, UV-Vis, X-ray
Diffraction, and Raman M easurements. To estimate
the size of the gold nanoparticles, their transmission electron microscopy (TEM ) images were obtained with a
Tecnai F20 Philips m icroscope after placing a drop of
colloidal solution on a carbon-coated copper grid. UVVis absorption spectra of the colloidal solutions were obtained with a Shimadzu UV-1601PC spectrophotometer.
X-ray diffraction (XRD) m easurements were carried out
to examine the crystallinity of Au nanoparticles. The pattern was obtained at a scanning rate of 38 per minute by
a Rigaku D-M AX/IIIC diffractometer using Cu Ka radiation at 0.154 nm. Raman spectral measurements were
described in the previous literature.8,9
RESULTS AND DISCUSSION
Characterization of G old Nanoparticles. Figures 1a
and 1b show a representative TEM image and a size distribution of the prepared gold nanoparticles. The mean
particle diameter and its relative standard deviations were
obtained for each sample by counting at least 150 gold
particles. The statistical analysis revealed a size distribution of standard deviations in diameters, s ; 8.6% for
15.2 nm particles, respectively. Figure 1c shows the UVVis absorption spectrum of the gold nanoparticles. The
lmax value was found at 520 nm. The full width at halfmaximum for the samples was m easured to be 86 nm.
The X-ray diffraction pattern of the Au nanoparticles was
measured to examine the crystallinity of the nanoparticles
as shown in Fig. 1d. The (111), (200), (220), and (311)
planes represent a cubic structure of the metallic gold.
Raman Spectra of PNC. Figures 2a and 2b show the
ordinary Raman (OR ) spectrum of PNC in the liquid state
and the Au SERS spectrum, respectively. In order to obtain inform ation on the surface adsorption m echanism, it
is necessary to analyze spectral changes according to the
adsorption process. Consulting the earlier vibrational assignments,14 –16 we analyzed the Raman spectra in Fig. 2.
It was rather straightforward to correlate the OR bands
with the Au SERS bands. Their peak positions are listed
in Table I along with the appropriate vibrational assignments. Figure 2b shows the Au SERS spectrum taken
using a He–Ne laser at 632.8 nm.
The n(NC) stretching peak on Au was blue shifted by
62 cm 2 1 from that of a free state. Also, the bandwidth
APPLIED SPECTROSCOPY
219
TABLE I. Spectral data and vibrational assignments of PNC. a
a
b
F IG . 2. (a) OR spectrum of PNC in the liquid state and (b) SERS
spectrum of PNC in aqueous gold nanoparticle solution. The spectral
region between 2800 and 2400 cm 2 1 was omitted due to the lack of
any information. (c) n(CH) stretching region between 3200 and 2800
cm 2 1 .
(FW HM: full width at half-maximum) of the n(NC ) band
of a free state became broadened upon adsorption on the
gold surface. Such a higher blue shift indicated the donation from the N[C: group to the Au surface.8 As
shown in Fig. 2c, the benzene ring CH stretching band
was identiŽ ed very weakly at ;3080 cm 2 1 for PNC. This
may be due to a weak response of the charge-coupled
device (CCD) camera at the far wing of the He–Ne laser
excitation frequency of 632.8 nm.9 Our result supports a
rather vertical orientation of PNC on gold.17 It is noteworthy that the peak at ;620 cm 2 1 almost disappeared
in the Au SERS spectrum, as shown in Fig. 3. Although
it is tempting to assign this band as the n6b band of the
benzene ring mode, the peak position and the strong intensity in the OR spectrum may indicate that it is ascribed
to the C–NC band as referring from the peak positions
of the C–S and C–Se band. 3 The weakness of the band
in the SERS spectrum suggests a rather bent angle of C–
N[C: after adsorption on the gold surfaces.
Raman Spectra of DM PNC. Figure 4 shows the OR
and SERS spectrum of 2,6-dimethyl phenyl isocyanide
(DMPNC) on gold nanoparticle surfaces. Their peak positions are listed in Table II along with the appropriate
vibrational assignments. Our assignment for DMPNC is
mainly based on the case of meta-Di-‘‘light’’ in Ref. 14.
The observed n(NC) bands of DMBNC were blue shifted
in the Au SERS spectra by 56 cm 2 1 from that of the OR
220
Volume 58, Number 2, 2004
OR
Au SERS
3072
1596
1487
1188
1166
1029
1002
769
621
473
332
3080w
1592
1485
1194
1167
1021
1001
790
514
527
2126
621
2188
352
Assignmentb
In-plane
2 (A 1 ) n(C–H)
8a (A 1 ) n(C–C)
19a (A 1 ) n(C–C)
9a (A 1 ) b(C–H)
9b (B 2 ) b(C–H)
18a (A 1 ) b(C–H)
12 (A 1 ) Ring breathing
1 (A 1 ) a(C–C–C)
6b (B 2 ) a(C–C–C) and g(CC–H)
6a (A 1 ) a(C–C–C)
15 (B 2 ) b(C–H)
Out-of-plane
16b (B 1 ) g(C–C–C)
Substituents
n(NC)
n(C–NC)
Units in cm 2 1. w 5 weak.
Based on Ref. 14 in Wilson notation with symm etries based on C 2v
point group. The symm etry in parentheses corresponds to C 2v point
group. a and b, in-plane deformation; g, out-of-plane deformation; n,
stretching.
spectrum. As shown in Fig. 4c, the benzene ring CH
stretching band was identiŽ ed weakly at ;3050 cm 2 1 for
DMPNC. This result also supports a rather vertical orientation of DM PNC on gold.17 Different from the case
of PNC, the band at ;640 cm 2 1 rem ained quite strong
even after the adsorption on Au. This result may indicate
quite a linear angle of C–N[C: on the gold surface. This
may be presumably due to a steric hindrance of the methyl groups of DM PNC. It would not be favorable for the
C–N[C: angle of DM PNC to have a bent geometry on
the surface because of the methyl groups. This steric hindrance should lead to sp hybridization of the nitrogen
atom in the NC group. In order to check the adsorption
F IG . 3. n(C–NC) stretching region between 680 and 560 cm 2 1 in (a)
OR and (b) Au SERS spectrum for PNC.
F IG . 5. (a) OR spectrum of BZI in the liquid state and (b) SERS
spectrum of ;5 3 10 2 4 M BZI in aqueous gold nanoparticle solutions.
The spectral region between 2820 and 2300 cm 2 1 was omitted due to
the lack of any information.
F IG . 4. (a) OR spectrum of DMPNC in the liquid state and (b) SERS
spectrum of ;10 2 5 M DMPNC in aqueous gold nanoparticle solution.
The spectral region between 2900 and 2400 cm 2 1 was omitted due to
the lack of any information. (c) n(CH) stretching region between 3200
and 2800 cm 2 1.
TABLE II. Spectral data and vibrational assignments of DMPNC. a
OR
Au SERS
Assignmentb
characteristics of aromatic isocyanides, we performed
SERS of benzyl isocyanide (BZI) on gold nanoparticles.
Raman Spectra of BZI. Figure 5 shows the ordinary
Raman (OR) spectrum in the neat liquid state and the Au
SERS spectrum of BZI, respectively. Table III lists the
frequency positions of BZI on gold nanoparticle surfaces.
The concentration of BZI in the colloidal solution was
;5 3 10 2 4 M. The n(NC) stretching frequency at 2215
cm 2 1 in Fig. 5b is higher by 64 cm 2 1 than that at 2151
cm 2 1 of a free state in Fig. 5a. The absence of either a
TABLE III. Spectral data and vibrational assignments of BZI.a
In-plane
3074
3046
1604
1593
1269
1259
1175
1097
1082
995
365
497
463
2987
2950
2921
2856
2123
1468
1443
1386
1377
640
3046w
1590
1264
1264
1174
1089
377
497
2927
2865
2179
1463
1419
658
20b (B 2 ) n(C–H)
2 (A 1 ) n(C–H)
8b (B 2 ) n(C–C)
8a (A 1 ) n(C–C)
13 (A 1 ) n(C–C–NC)
3 (B 2 ) b(C–H)
9b (B 1 ) b(C–H)
18b (B 2 ) b(C–H)
18a (A 1 ) b(C–H)
12 (A 1 ) Ring breathing
9a (A 1 ) b(C–H)
Out-of-plane
16a (A 2 ) g(C–C–C)
16b (B 1 ) g(C–C–C)
Substituents
nas (CH 3) ip
nas (CH 3) op
ns (CH 3 )
ns (CH 3 )
n (NC)
das 1 (CH 3 )
das 1 (CH 3 )
ds1 (CH 3 )
ds1 (CH 3 )
n (C–NC)
Units in cm 2 1 . w, weak; ip, in-plane; op, out-of-plane; as, asymmetric;
s, symm etric.
b
Based on Ref. 14 in Wilson notation with symm etries based on C 2v
point group. The symm etry in parentheses corresponds to C 2v point
group. d, bending vibrations to which a well-deŽ ned plane cannot be
related.
a
OR
Au SERS
3061
1609
1588
1443
1203
1181
1157
1029
1003
601
814
474
272
3062
1603
1589
1433w
1199
745
401
224
746
2931
2151
1443
1203
798
619
1158
1029
1002
602
812
433
316
240
2215
1433w
1199
800
619
Assignmentb
In-plane
2 (A 1 ) n(C–H)
8a (A 1 ) n(C–C)
8b (B 2 ) n(C–C)
19b (B 2 ) n(C–C)
13 (A 1 ) n(C–C–CH 2 )
9a (A 1 ) b(C–H)
9b (B 2 ) b(C–H)
18a (A 1 ) b(C–H)
12 (A 1 ) Ring breathing
6b (B 2 ) a(C–C–C) and g(CC–H)
1 (A 1 ) a(C–C–C)
6a (A 1 ) a(C–C–C)
15 (B 2 ) b(C–H)
Out-of-plane
11 (B 1 ) g(C–H)
16a (A 2 ) g(C–C–C)
10b (B 1 ) g(C–C–N)
Substituents
nas (CH 2 )
n (NC)
bs (CH 2 ) (scissor, i.p.)c
gs (CH 2 ) (wag or twist, o.p.)d
ba s (CH 2 ) (rock, i.p.)c
n(C–NC)
Units in cm 2 1. w 5 weak.
Based on Ref. 14 in Wilson notation with symm etries based on C 2v
point group. The symm etry in parentheses corresponds to C 2v point
group.
c
i.p., in-plane.
d
o.p., out-of-plane.
a
b
APPLIED SPECTROSCOPY
221
TABLE IV. Relative enhancement factors of Au SERS bands of
PNC, DMPNC, and BZI.
Symmetry
typea
A1
F IG . 6. OR spectrum of BZI in (a) neat liquid state and (b) SERS
spectrum of ;5 3 10 2 4 M BZI in aqueous gold nanoparticles in the
spectral region between 3200 and 2800 cm 2 1 . The spectral region between 3200 and 2820 cm 2 1 in Fig. 5b was magniŽ ed by a factor of 5
for a better presentation.
substantial red shift or a signiŽ cant band broadening of
the ring breathing n12 mode at ;1000 cm 2 1 implied that
a direct ring p orbital interaction with the gold substrate
should be quite low. This result indicated that the adsorbate should have a rather vertical structure on Au. Figure
6 shows the spectral region between 3200 –2800 cm 2 1 for
BZI to exhibit a change of the nas(CH 2) and n(CH) bands
upon adsorption on gold. Regarding the geometry of the
benzene ring, the appearance of the CH stretching band
is identiŽ ed at ;3060 cm 2 1, suggesting that BZI has an
upright orientation on the Au surface, as discussed.17 It
is noteworthy that the nas(CH 2 ) stretching band of BZI at
;2930 cm 2 1 almost disappeared, whereas the n(CH) band
at ;3060 cm 2 1 remained conspicuous after adsorption on
gold nanoparticles, as shown in Fig. 6. This result suggests a rather parallel orientation of the asymmetric methylene stretching vibration in BZI on the gold surface.
This result supports a preference of sp 3 (or sp 2 ) hybridization for the nitrogen atom of BZI upon adsorption on
gold resulting from a bent angle of :C[N–CH 2 for its
nitrogen atom.
Plausible Orientations. The m ajor bands associated
with the ring vibrational m odes can be divided into four
symmetry species assuming C 2v symm etry. 14 The relative
intensities have been evaluated for the normal modes of
PNC, DM PNC, and BZI and are listed in Table IV. According to the electromagnetic (EM) theory on the SERS
selection rule,18,19 the in-plane vibration m odes of molecules adsorbed in a perpendicular orientation should be
more enhanced than the out-of-plane ones. For PNC and
DMPNC, although most ring m odes were found to belong to those of in-plane modes, the features at 460;530
cm 2 1 could be ascribed to the n16a and n16b out-of-plane
ring m odes. The weakness of the out-of-plane band suggests that the adsorbates should have a rather vertical
structure. A sm aller enhancement of the out-of-plane
mode of DMPNC may support a m ore vertical orientation
of its phenyl ring since it is likely that sp hybridization
of DMPNC m ay lead to a m ore vertical stance on the
surface. It seems, however, difŽ cult to discuss in detail
the differences in phenyl ring orientation of PNC,
222
Volume 58, Number 2, 2004
Tensor
elemen ts b
Normal
mode c
axx
ayy
azz
1
2
12
13
8a
9a
18a
19a
16a
10b
11
16b
6b
8b
9b
15
19b
A2
B1
axy
axz
B2
ayz
n(NC)
n(C–NC)
bs (CH 2 )
ba s(CH 2 )
gs (CH 2 )
Relative enhancemen t factor
PNC
DMPNC
0.341
0.229
1.69
1.44
7.00
2.92
1.06
8.61
7.48
7.00
0.335
2.94
BZI
4.27
3.88
6.21
15.9
7.00
7.33
0.205
4.83
8.56
4.89
6.89
0.830
3.41
1.54
2.35
0.418
5.47
9.20
4.75
2.59
4.59
18.6
7.12
4.59
7.92
15.9
Symmetry types corresponding to the C 2v point group.
Subscripts, i.e., x, y, and z, correspond to the conventional molecular
axes. The x-axis lies perpendicular to the ring, and the z-axis passes
through the NC group.
c
See Table I for vibrational assignment.
d
Normalized to 7.00 for the n8a band at ;1600 cm 2 1 .
a
b
DMPNC, and BZI on Au due to the lack of sufŽ cient
ring modes to analyze. Plausible orientations of PNC,
DMPNC, and BZI on gold nanoparticle surfaces are depicted in Fig. 7.
p-Biphenylisocyanide appeared to have a more vertical
stance on Ag than on Au from our previous study.9 This
result may indicate an sp hybridization for the nitrogen
atom on Ag, as in the case of the sulfur atom of alkanethiolates.1 It is not absolutely certain whether the bent
structure of both thiolate and isocyanide monolayers on
gold resulted from substrate-dependent properties of gold
nanoparticles. It is also a matter of conjecture as to how
polycrystalline surfaces of nanoparticles, as shown in
Fig. 1d, would affect the observed orientation. Since
SERS selection rules do not provide a precise interpretation of band intensities, we have currently been applying other spectroscopic techniques to reach a more consistent conclusion. Our future research will characterize
F IG . 7. Plausible orientations of (a) PNC, (b) DMPNC, and (c) BZI
on gold.
aromatic isocyanide-coated gold particles by combined
spectroscopic and microscopic m ethods. We plan to perform a molecular mechanics calculation to ensure that an
intramolecular steric hindrance from the two m ethyl
groups can affect the orientation on surfaces. Thermodynamic and electrochemical studies shall also be beneŽ cial to explain the different adsorption characteristics of
PNC, DMPNC, and BZI on Au surfaces.
CONCLUSION
The binding structure of phenyl isocyanide (PNC) and
2,6-dimethyl phenyl isocyanide (DMPNC) on gold nanoparticle surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). As evidenced
from the appearance of the ring CH stretching band, PNC
and DM PNC have a rather vertical stance on gold surfaces. The disappearance of the n(C–NC) band after adsorption on gold suggests a rather bent C–N[C: angle.
Thus, a bent conŽ guration would be a favorable hybridization state at the nitrogen atom. For DM PNC, the
strong intensity of the n(C–NC) band in the Au SERS
spectrum indicates that sp hybridization should be m ore
advantageous due to an intramolecular steric interaction
between the isocyanide and the two methyl groups. SERS
of BZI suggests a rather parallel orientation of the asymmetric methylene stretching vibration on the gold surface.
This is also thought to result from a bent angle of :C[N–
CH 2 for its nitrogen atom.
ACK NOW LEDGM ENTS
This work was supported by the Soongsil University Research Fund.
S.W.J. would like to thank Prof. Kwan Kim for his guidance on SERS
studies of self-assem bled monolayers prepared by aromatic isocyanides.
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APPLIED SPECTROSCOPY
223