Structure and dynamics in alkanethiolate monolayers self-assembled on gold nanoparticles: A DSC, FT-IR, and deuterium NMR study

Article abstract of DOI:10.1021/ja963571t

Gold nanoparticles, ca. 30 ? in diameter, have been derivatized with specifically deuterated (position 1 and positions 10 to 13) and perdeuterated (positions 2 to 18) octadecanethiols (C₁₈SH). The phase behavior of the octadecanethiolate monolayers chemisorbed onto the colloidal gold surface was characterized by differential scanning calorimetry (DSC). The DSC thermograms show that the C₁₈SH-derivatized Au nanoparticles undergo distinct phase transitions which can be associated with the reversible disordering of the alkyl chains. Despite the highly curved geometry of these Au particles, there is a remarkable degree of conformational order in the alkanethiolate chains and the thermotropic behavior of the thiol-modified gold nanoparticles is very similar to that of conventional, planar self-assembled monolayers. Both the peak maximum temperature and the enthalpy associated with the DSC transition strongly parallel those of the gel-to-liquid crystalline transition of n-diacylphosphatidylcholine lipid bilayer membranes of equivalent chain length. Restricted chain mobility due to covalent bonding of the sulfur head group to the gold surface does not affect the cooperativity of the transition in terms of the transition temperature and enthalpy. Local chain ordering and dynamics in the deuterated C₁₈S/Au nanoparticles have been probed using variable-temperature solid-state deuterium NMR spectroscopy and transmission FT-IR spectroscopy. The temperature dependence of the symmetric CD₂ stretching frequency has confirmed that the DSC-detected phase transition involves a thermally-induced change from a predominantly all-trans conformation to a chain disordered state. A comparison of the thermal behavior of d₃₅-C₁₈S/Au and 10, 10, 11, 11, 12, 12, 13, 13-d₈-C₁₈S/Au shows that disordering originates in the chain terminus region and propagates toward the middle of the chain as the temperature increases. Studies of 1,1-d₂C₁₈S/Au show that the disorder does not extend to the tethered sulfur headgroup. Deuterium NMR spectroscopy specifically establishes that chain melting arises from an increased frequency of gauche bonds in the alkanethiolate chains. The ²H NMR line shapes further indicate that the tethered alkanethiolate chains are undergoing rapid trans-gauche bond isomerization and axial chain rotation.

Full text of DOI:10.1021/ja963571t

2682
J. Am. Chem. Soc. 1997, 119, 2682-2692
Structure and Dynamics in Alkanethiolate Monolayers
Self-Assembled on Gold Nanoparticles: A DSC, FT-IR, and
Deuterium NMR Study
Antonella Badia,† Louis Cuccia, Linette Demers, Fred Morin, and
R. Bruce Lennox*
Contribution from the Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West,
Montreal, Quebec, Canada H3A 2K6
ReceiVed October 14, 1996X
Abstract: Gold nanoparticles, ca. 30 Å in diameter, have been derivatized with specifically deuterated (position 1
and positions 10 to 13) and perdeuterated (positions 2 to 18) octadecanethiols (C18SH). The phase behavior of the
octadecanethiolate monolayers chemisorbed onto the colloidal gold surface was characterized by differential scanning
calorimetry (DSC). The DSC thermograms show that the C18SH-derivatized Au nanoparticles undergo distinct phase
transitions which can be associated with the reversible disordering of the alkyl chains. Despite the highly curved
geometry of these Au particles, there is a remarkable degree of conformational order in the alkanethiolate chains and
the thermotropic behavior of the thiol-modified gold nanoparticles is very similar to that of conventional, planar
self-assembled monolayers. Both the peak maximum temperature and the enthalpy associated with the DSC transition
strongly parallel those of the gel-to-liquid crystalline transition of n-diacylphosphatidylcholine lipid bilayer membranes
of equivalent chain length. Restricted chain mobility due to covalent bonding of the sulfur head group to the gold
surface does not affect the cooperativity of the transition in terms of the transition temperature and enthalpy. Local
chain ordering and dynamics in the deuterated C18S/Au nanoparticles have been probed using variable-temperature
solid-state deuterium NMR spectroscopy and transmission FT-IR spectroscopy. The temperature dependence of the
symmetric CD2 stretching frequency has confirmed that the DSC-detected phase transition involves a thermally-
induced change from a predominantly all-trans conformation to a chain disordered state. A comparison of the thermal
behavior of d35-C18S/Au and 10,10,11,11,12,12,13,13-d8-C18S/Au shows that disordering originates in the chain
terminus region and propagates toward the middle of the chain as the temperature increases. Studies of 1,1-d2-
C18S/Au show that the disorder does not extend to the tethered sulfur head group. Deuterium NMR spectroscopy
specifically establishes that chain melting arises from an increased frequency of gauche bonds in the alkanethiolate
chains. The 2H NMR line shapes further indicate that the tethered alkanethiolate chains are undergoing rapid trans-
gauche bond isomerization and axial chain rotation.
Introduction
for thiol self-assembly (21.4 Å2/molecule).4 These alkanethiol-
derivatized Au nanoparticles are isolated as black solids whose
texture ranges from a wax to fine crystals, depending on the
alkane chain length. They are air-stable and soluble in
chloroform, toluene, hexanes, and other nonpolar solvents. The
solvent can be evaporated, and the particles can be repeatedly
redissolved in organic solvent without metallization occurring.
Most importantly, the high surface area of these nanoparticles
(i.e. ca. 100 m2/g) offers access to much larger quantities of
the monolayer surfactant, rendering the self-assembled thiol
monolayer amenable to structural characterization by techniques
conventionally used for bulk materials. NMR spectroscopy
(both solution1,2,5,6 and solid state2), transmission FT-IR
spectroscopy,1,7-9 and differential scanning calorimetry1,5 are
of particular note.
We have recently investigated the phase properties of self-
assembled alkanethiolate (RS) monolayers on metal surfaces
Via the synthesis of alkanethiol-stabilized gold clusters.1,2 This
system was chosen because of its unique potential to serve as
a highly dispersed analogue to self-assembled monolayers
(SAMs) formed on planar metal surfaces. Transmission electron
microscopy established that the nanoparticles have average Au
core diameters ranging from 20 to 30 Å, with a typical size
polydispersity of 30%.1 Elemental analysis results for an
octadecanethiol-derivatized Au cluster showed that the gold core
(median diameter of 22.4 Å), modeled as a sphere, contains
∼346 Au atoms and ∼102 octadecanethiolate chains, resulting
in a surface Au-to-thiolate ratio of 2.2.1,3 This leads to a higher
alkanethiolate surface density on the nanoparticles (15.4 Å2/
molecule) as compared to the flat Au(111) surface widely used
(4) Camillone, N.; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys.
1993, 98, 4234-4245.
* Author to whom correspondence should be addressed.
† Present address: Max-Planck-Institut fu¨r Polymerforschung, Postfach
3148, D-55021 Mainz (FRG).
(5) Terrill, R. H.; Postlethwaite, A.; Chen, C.-h.; Poon, C.-D.; Terzis,
A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.;
Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R.
W. J. Am. Chem. Soc. 1995, 117, 12537-12548.
X Abstract published in AdVance ACS Abstracts, February 15, 1997.
(1) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox,
R. B. Chem. Eur. J. 1996, 2, 359-363.
(6) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am.
Chem. Soc. 1996, 118, 4212-4213.
(2) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L.
Langmuir 1996, 12, 1262-1269.
(3) If these C18S/Au nanoparticles are modeled as truncated octahedra
containing 459 Au atoms (so-called Magic number) in the Au core,16 the
elemental analysis results yield ∼136 C18S chains per particle and hence a
surface Au-to-thiolate ratio of 1.8.
(7) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J.
Chem. Soc., Chem. Commun. 1994, 801-802.
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Soc., Chem. Commun. 1995, 1655-1656.
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3604-3612.
S0002-7863(96)03571-8 CCC: $14.00 © 1997 American Chemical Society

Alkanethiolate Monolayers Self-Assembled on Au Nanoparticles
J. Am. Chem. Soc., Vol. 119, No. 11, 1997 2683
We find that SAMs on colloidal Au particles show strong
parallels to the monolayers formed at planar surfaces,1,9,10 given
the excellent correspondence in chain ordering/disordering
phenomena in these two systems. The CH stretching vibrations
in the FT-IR spectra of RS/Au nanoparticles (where R ) C8-
C20) indicate that for R g C16, the alkyl chain exists predomi-
nantly in an extended, all-trans conformation at 25 °C.1,11
Moreover, a recent, detailed FT-IR study by Hostetler et al.9
reports that analogous to conventional RS/Au SAMs, long chain
RSHs self-assembled on the Au nanoparticles possess a high
percentage of end-gauche defects, no detectable internal gauche
conformers, and a small amount of near surface gauche defects.
Solid-state 13C NMR spectroscopy further reveals the coexist-
ence of motionally restricted all-trans chains with a smaller
population of liquid-like conformationally disordered chains in
C18S/Au at 25 °C.2 Calorimetry,1,5 variable-temperature FT-
IR spectroscopy,1 and solid-state 13C NMR studies1,2 have
established that a chain melting transition occurs in these
alkylated metal colloids. The transition temperature associated
with each chain (for R ) C12-C20) closely parallels those found
in electrochemical studies of planar RS/Au SAMs,10 X-ray
diffraction studies of fatty acid Langmuir-Blodgett monolayer
films,12 and calorimetry studies of phospholipid bilayer mem-
branes.13 Melting transitions are highly sensitive to the degree
of chain organization. That the thermotropic behavior of the
RS/Au nanoparticles is very similar to that of highly ordered,
2D self-assembled systems tells us that there is a surprisingly
high degree of chain conformational order in these coated
colloids. Differences between the strictly planar and colloid
systems do arise however due to the high radius of curvature
of the nanoparticles and the presence of single-crystal facets
on the surface.5,8,14-18 In planar RS/Au SAMs, the alkyl chain
packing density and organization are largely dictated by the
underlying metal lattice and extensive, chain-chain van der
Waals interactions.4,19 How chain ordering arises in the RS/
Au colloids is not immediately apparent given that the length
of the tethered RS chain and the Au core diameter are similar
and the relation between the extended chain conformation and
the highly curved geometry of these particles. Transmission
electron microscopy of (sub)monolayer films,1,15,16 powder
X-ray diffraction of thick film samples,15,16,20 and density
measurements of the solid material5 reveal that adjacent Au
particles are separated by approximately one chain length, and
not by the expected two chain lengths. Based on this observa-
tion, we proposed that long range ordering arises from an
interdigitation of chain domains on neighboring particles.1 An
alternative packing configuration, involving the interdigitation
of individual chains,5 is not likely given the large ratio of chain
end area to Au-S head group area of 14:1.1 Recent molecular
dynamics (MD) simulations of dodecanethiol-derivatized Au
nanoparticles support our chain packing model.15,17 More
specifically, the MD simulations predict that the dodecane-
thiolate monolayer forms oriented molecular domains or “bundles”
of conformationally ordered chains at temperatures below the
phase transition temperature. Furthermore, the C12S/Au nano-
particles in these simulations assemble into 3D superlattices
through the “interlocking” of molecular bundles on neighboring
particles and not Via the interdigitation of individual chains.17
Despite the rapid advances made in characterizing these
interesting materials, several important issues (such as chain
dynamics, structure, and orientation) have not been directly
addressed to date. To this end, we now report the use of
deuterium labeling to probe both the local ordering and chain
dynamics of alkanethiols adsorbed to the surface of these gold
nanoparticles. Solid-state deuterium NMR spectroscopy, in
conjunction with FT-IR spectroscopy and differential scanning
calorimetry (DSC), is used to study the thermodynamics of
2
RSH-derivatized Au nanoparticles. In the past, solid-state H
NMR has been extensively used to probe conformational order
and molecular motion in lipid membranes,21-24 spherical sup-
ported lipid bilayers,25,26 rigid-rod polymers with alkyl side
groups,27-29 alkylammonium layer-structure compounds,30 and
alkylsilyl-modified silica gel chromatographic stationary
2
phases.31,32 Solid-state H NMR spectroscopy offers distinct
advantages over 13C and 1H NMR for probing the structure and
dynamics of ordered systems. The peaks observed in 2H NMR
originate from the nuclear quadrupole coupling of the deuteron
with the electric field gradient of the C-2H σ-bond. An axially
symmetric quadrupolar interaction dominates the 2H NMR
spectrum so that the line shapes are simple and solely dependent
upon the amplitude and symmetry of the molecular motions.
The peak shapes reflect intramolecular dynamics and there are
no complex contributions from chemical shift anisotropy and
heteronuclear dipolar interactions which greatly increase the
complexity of 13C and 1H NMR spectra in regards to analyzing
motional phenomena. Furthermore, since the relaxation of the
2H nucleus is dominated by the quadrupolar interaction, there
is no need to establish relaxation mechanisms.23,32-34 More
importantly, the low natural abundance of deuterium (i.e.
0.015%) eliminates background contributions from unlabeled
(21) Griffin, R. G. Methods Enzymol. 1981, 72, 108.
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(24) Blume, A.; Hubner, W.; Muller, M.; Bauerle, H. D. Ber. Bunsenges.
Phys. Chem. 1989, 93, 964-973.
(10) Badia, A.; Back, R.; Lennox, R. B. Angew. Chem., Int. Ed. Engl.
1994, 33, 2332-2335.
(11) Demers, L.; Badia, A.; Lennox, R. B. Unpublished results.
(12) Riegler, J. E. J. Phys. Chem. 1989, 93, 6457-6480.
(13) Marsh, D. Handbook of Lipid Bilayer Membranes; CRC Press: Boca
Raton, 1991, pp 135-150.
(14) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys.
Chem. 1995, 99, 7036-7041.
(15) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar,
I.; Wang, Z. L.; Cleveland, C.; Luedtke, W. D.; Landman, U. In Proceedings
of the NATO AdVanced Research Workshop on “The Chemical Physics of
Fullerenes 10 (and 5) Years Later”; Kluwer Academic Publishers: Varenna,
Italy, 1995; pp 475-490.
(16) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar,
I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. C.; Luedtke, W. D.;
Landman, U. AdV. Mater. 1996, 8, 433-437.
(17) Luedtke, W. D.; Landman, U. J. Phys. Chem. 1996, 100, 13323-
13329.
(25) Naumann, C.; Brumm, T.; Bayerl, T. M. Biophys. J. 1992, 62,
1314-1319.
(26) Bayerl, T. M.; Bloom, M. Biophys. J. 1990, 58, 357-362.
(27) Spiess, H. W. Ber. Bunsenges. Phys. Chem. 1993, 97, 1294-1305.
(28) Kricheldorf, H. R.; Wutz, C.; Probst, N.; Domschke, A.; Gurau,
M. In Multidimensional Spectroscopy of Polymers: Vibrational, NMR, and
Fluorescence Techniques; Urban, M. W., Provder, T., Eds.; American
Chemical Society: Washington, DC, 1995; pp 311-332.
(29) van Genderen, M. H. P.; Pfaadt, M.; Moller, C.; Valiyaveettil, S.;
Spiess, H. W. J. Am. Chem. Soc. 1996, 118, 3661-3665.
(30) Jurga, S.; Macho, V.; Huser, B.; Spiess, H. W. Z. Phys. BsCondensed
Matter 1991, 84, 43-49.
(31) Kelusky, E. C.; Fyfe, C. A. J. Am. Chem. Soc. 1986, 108, 1746-
1749.
(18) Lebuis, A. M., X-ray Diffraction Facility, Department of Chemistry,
McGill University, Dec. 1996, personal communication.
(19) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43,
437-463.
(20) Lebuis, A. M., X-ray Diffraction Facility, Department of Chemistry,
McGill University, Nov. 1995, personal communication.
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6358.
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16, 492-496.
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Canada, 1983; pp 73-137.

2684 J. Am. Chem. Soc., Vol. 119, No. 11, 1997
Badia et al.
material so that the dynamics of a single chain position or
residue can be exclusively observed in selectively labeled
molecules.
Experimental Section
Materials. d35-Stearic acid (98 atom % D) and deuterium gas
(99.99%) were purchased from Cambridge Isotope Laboratories, Inc.
(Andover, MA). 10,12-Octadecadiynoic acid was obtained from
Farchan Laboratories, Inc. (Gainesville, FL). Stearic acid (lab-grade),
from Anachemia Chemicals Ltd. (Montreal, Canada), was recrystallized
from hexanes before use. n-Octadecanethiol (98%, Aldrich Chemical
Co., Milwaukee, WI) was recrystallized three times from ethanol. Tris-
(triphenylphosphine)rhodium(I) chloride (Wilkinson’s catalyst), lithium
aluminum hydride, lithium aluminum deuteride (98 atom % D),
hydrogen tetrachloroaurate(III) trihydrate, and tetraoctylammonium
bromide were all purchased from Aldrich. Sodium thiosulfate was from
A&C American Chemicals Ltd. (Montreal, Canada) and 48% HBr was
from Fluka Corp. (Ronkonkoma, NY).
2
Quadrupolar splitting in H NMR is especially sensitive to
motions and orientations at different positions in a molecule.
For the 2H nucleus with spin I ) 1 and an asymmetry parameter
η ) 0 for the C-2H bond, the allowed energy transitions are
-1 T 0 and 0 T 1, giving rise to a doublet in the NMR
spectrum with a peak separation given by
∆υQ ) 3/4(e2qQ/h)(3 cos2 θ - 1)
(1)
where ∆υQ is the quadrupolar splitting or peak separation in
frequency units, (e2qQ/h) is the quadrupolar coupling constant
(168 kHz for an aliphatic sp3 C-2H bond), and θ is the angle
between the principal axis of the electric field tensor of the 2H
nucleus and the magnetic field, Ho.23,34 For a rigid polycrys-
talline solid, all values of θ are possible and a so-called rigid
powder pattern or Pake pattern is obtained. In this case, the
separation between the peak maxima (θ ) 90°) is ca. 130 kHz
and the separation between the steps at the wings (θ ) 0°) is
twice this value.23,34 In the case of lipid membranes, organic
thin films, and rigid-rod polymers with flexible side chains, there
is considerable intramolecular motion due to such processes as
methyl group rotation, trans-gauche bond isomerization, axial
rotation, lateral diffusion of molecules, or flipping of a phenyl
ring about its C2 axis. Molecular motions that have correlation
d35-Octadecanethiol. d35-Stearic acid (1.10 g, 3.45 mmol) was
dissolved in 60 mL of dry THF. This solution was added Via a dropping
funnel to a three-neck round-bottom flask equipped with a reflux
condenser and a calcium carbonate drying tube and containing LiAlH4
(1.04 g, 27.5 mmol) dissolved in 125 mL of dry THF. The reaction
mixture was refluxed for ca. 12 h. The reaction vessel was cooled in
an ice bath and distilled water was carefully added dropwise to the
reaction mixture to destroy the residual LiAlH4. A 4-mL aliquot of 6
N HCl solution was added and reflux was continued for an additional
12 h. Hexanes (100 mL) were added with stirring to the cooled reaction
mixture. The remaining white precipitate was removed by filtration
at this stage and the mixture was poured into a separatory funnel. The
organic layer was separated from the aqueous layer. The aqueous phase
was further extracted with 2 x 50 mL of hexanes. The combined
organic extracts were washed with 2 × 100 mL of distilled water and
then dried over anhydrous magnesium sulfate. The solvent was
removed by rotary evaporation to yield 0.980 g (93%) of d35-octadecanol
(i.e. CD3(CD2)16CH2OH).
The perdeuterated octadecanol (0.980 g, 3.21 mmol) was added to
a mixture of 50 mL of 48% HBr and 100 µL of concentrated H2SO4.
The mixture was refluxed for 8 h. The HBr mixture was diluted with
25 mL of distilled water and 50 mL of hexanes was added with stirring.
The hexanes phase was separated from the aqueous HBr phase, and
then the HBr phase was further extracted with 2 × 50 mL of hexanes.
The hexanes extracts were pooled together, washed with 2 × 75 mL
of water, and dried over anhydrous magnesium sulfate. The solvent
was removed by rotary evaporation and the residue purified by column
chromatography (silica gel, n-hexane) to yield 0.986 g (84%) of d35-
octadecyl bromide.
2
times of ca. 10-7-10-4 s23,33 can be studied by H NMR, a
much longer time scale than most other spectroscopic tech-
niques. Motions that are slow on the NMR time scale are
expected to have a negligible influence on the 2H NMR
spectrum. Fast motions (t , 10-6 s), on the other hand, cause
the (3 cos2 θ-1) term in eq 1 to be averaged with time, resulting
in a decrease of the quadrupolar splitting and/or changes in the
peak line shape.23,34 Thus, in most cases, the 2H NMR spectral
line shapes and the spin-lattice (T1) relaxation times provide
incisive information regarding molecular orientation as well as
the type and the rate of motion that a particular 2H-labeled site
is undergoing.23,34 For this reason, FT-IR and 2H NMR
spectroscopy can give complementary information on the
The octadecyl bromide (0.980 g, 2.66 mmol) was dispersed in 50
mL of absolute ethanol. A solution of Na2S203‚5H20 (0.660 g, 2.66
mmol) in 10 mL of water was added to the octadecyl bromide and the
reaction was refluxed for 6 h. The resulting Bunte salt was hydrolyzed
with HCl (9.0 mL of 6 N HCl) by refluxing for 4 h under a nitrogen
atmosphere. The cooled reaction mixture was poured with stirring into
a two-phase mixture of 10 mL of distilled water and 25 mL of hexanes.
The organic layer was separated from the aqueous layer, and the
aqueous phase was then further extracted with 2 × 25 mL of hexanes.
The combined hexanes extracts were washed with 2 × 35 mL of water
and dried over anhydrous magnesium sulfate. The solvent was removed
by rotary evaporation and the residue was recrystallized from ethanol
to yield pure d35-octadecanethiol (0.768 g, 90%). TLC (silica gel,
n-hexane): Rf 0.7; 1H NMR (499.84 MHz, CDCl3) δ 2.52 (s, CH2SH);
2H NMR (76.73 MHz, CHCl3) δ 0.83 (s, 3D, CD3), 1.19 (s, 30D,
2
structure and dynamics of H-labeled systems, given that FT-
IR spectroscopy probes a time scale which is less than 10-10
s.35
In this study, n-octadecanethiols deuterated at different
positions along the alkyl chain were used to derivatize the
surface of Au nanoparticles. The molecules synthesized in-
clude the following: CD3(CD2)16CH2SH (positions 2 to 18
deuterated), CH3(CH2)16CD2SH (position 1 deuterated), and
CH3(CH2)4CD2CD2CD2CD2(CH2)9SH (positions 10 to 13 deu-
terated).
2
Solid-state H NMR spectroscopy is used to show that the
DSC-detected phase transition is a thermally-induced chain
melting process. FT-IR spectroscopy establishes that the chain
melting begins at the chain terminus and propagates toward the
middle of the chain with increasing temperature. The excellent
correspondence observed between the thermal properties of RSH
SAMs on planar and colloidal Au surfaces suggests that the
nanoparticle solid-state NMR results are applicable to conven-
tional SAMs.
CD3(CD2)15), 1.53 (s, 2D CD2CH2SH). MS (EI) m/z 321 [M]•+
.
1,1-d2-Octadecanethiol. Stearic acid (2.00 g, 7.03 mmol) was
converted to 1,1-d2-octadecanol (1.78 g, 93%) by reduction with LiAlD4
as described above. Bromination followed by Bunte salt formation
and hydrolysis using the procedures previously described yielded 1,1-
d2-octadecanethiol (1.36 g, 67% with respect to stearic acid). TLC
1
(silica gel, n-hexane): Rf 0.7; H NMR (499.84 MHz, CDCl3) δ 1.59
(t, 2H, CH2CD2SH), 1.37 (m, 2H, CH2CH2CD2SH), 1.26 (s, 28H,
CH3(CH2)14), 0.88 (t, 3H, CH3); 2H NMR (76.73 MHz, CHCl3) δ 2.49
(s, CD2SH); MS (EI) m/z 288 [M]•+
.
(35) Bloom, M.; Thewalt, J. Chem. Phys. Lipids 1994, 73, 27-38.
(36) C14S/Au nanoparticles synthesized using a 3:1 mole ratio of AuCl4
-
10,10,11,11,12,12,13,13-d8-Octadecanethiol. 10,12-Octadecadiyn-
oic acid (1.00 g, 3.62 mmol) was dissolved in 10 mL of freshly distilled,
argon saturated THF in a pressure reactor. Wilkinson’s catalyst (100
to C14H29SH had a mean diameter of 33 ( 7 Å. A partial elemental analysis
(Galbraith Laboratories) yielded 73.51% Au and 3.21% S, resulting in a
surface Au-to-thiol ratio of 2.1.

Alkanethiolate Monolayers Self-Assembled on Au Nanoparticles
J. Am. Chem. Soc., Vol. 119, No. 11, 1997 2685
mg) was added and the reactor was evacuated and flushed with argon
three times. The reactor was then flushed with D2 gas and filled to a
pressure of 1 psi. The reaction mixture was stirred for ∼19 h after
which the reactor was refilled with additional D2 gas. The reaction
mixture was stirred for an additional 3 days. The solvent was removed
by rotary evaporation and the residue chromatographed (silica gel, 80:
20 (v/v) hexanes-ethyl acetate). Recrystallization from hexanes yielded
pure 10,10,11,11,12,12,13,13-d8-octadecanoic acid (0.495 g, 47%). mp
67-69 °C; 1H NMR (499.84 MHz, CDCl3) δ 2.35 (t, 2H, CH2COOH),
1.63 (m, 2H, CH2CH2COOH), 1.25 (s, 20H, CH3(CH2)4(CD2)4(CH2)6
maintained at each temperature for 20 min before a spectrum was
acquired. Background spectra of the clean KBr disc were collected at
the same temperatures and subtracted from the sample spectra.
Differential Scanning Calorimetry. DSC experiments were con-
ducted with a Perkin-Elmer DSC-7 instrument calibrated for temperature
and peak area by means of indium and octadecane standards for the
-40 to 100 °C temperature range or indium and cyclohexane for runs
of -100 to 100 °C. Thermograms were run on samples of ∼10 mg of
C18S/Au particles in sealed aluminum pans under a purging atmosphere
of either nitrogen (-40 to 100 °C) or helium (-100 to 100 °C) gas at
heat-cool rates of 5 or 10 °C/min.
2
CH2CH2COOH), 0.88 (t, 3H, CH3); H NMR (76.73 MHz, CHCl3) δ
0.93 (s, (CD2)4).
2
2
Solid-State H NMR Spectroscopy. Samples for H NMR were
prepared by packing a shortened 5-mm NMR tube with 150-180 mg
10,10,11,11,12,12,13,13-d8-Octadecanoic acid (0.485 g, 1.66 mmol)
was reduced to the alcohol (0.364 g, 79%) using LiAlH4 as described
previously. Bromination with 48% HBr followed by Bunte salt
formation and hydrolysis yielded 10,10,11,11,12,12,13,13-d8-octa-
decanethiol (0.268 g, 70% with respect to d8-octadecanol). TLC (silica
gel, n-hexane): Rf 0.7; 1H NMR (499.84 MHz, CDCl3) δ 2.51 (t, 2H,
CH2SH), 1.59 (m, 2H, CH2CH2SH), 1.37 (m, 2H, CH2CH2CH2SH),
1.25 (s, 20H, CH3(CH2)4(CD2)4(CH2)6CH2CH2CH2SH), 0.88 (t, 3H,
CH3); MS (EI) m/z 294 [M]•+
2
of dry H-labeled C18S/Au powder and sealing the tube with Teflon
tape. 2H NMR spectra were recorded on a Chemagnetics 300-MHz
spectrometer operating at a frequency of 46.045 MHz. A quadrupole
echo sequence (90°(x-τ1-90°y-τ2)37 with transmitter blanking was
employed and a 500-kHz spectral window was scanned. τ1 and τ2 were
40 and 20 µs, respectively. Spectra were acquired at both low and
high temperature with pulse delays ranging from 0.5 to 5 s. Since the
longer pulse delay times produced spectra that were identical to the
ones acquired using short delays, all spectra were acquired with a pulse
delay of 0.5 s to shorten the acquisition time. Depending on the signal-
to-noise ratio, between 512 and 29 000 transients were accumulated.
After data accumulation, the FID was left-shifted by the correct number
of points so that for each spectrum, the FID was Fourier transformed
starting at the echo maximum. Heating and cooling of the NMR sample
was accomplished using air, cooled by passage through an acetone/
dry ice bath, and a calibrated temperature control unit (Chemagnetics).
A 30 min sample equilibration time was allowed at each temperature
C18SH-Derivatized Gold Nanoparticles. Gold nanoparticles de-
rivatized with 10,10,11,11,12,12,13,13-d8-octadecanethiol, 1,1-d2-
octadecanethiol, and d35-octadecanethiol were prepared following the
method of Brust et al.1,7 The syntheses were carried out using 1.8
mmol of HAuCl4‚3H2O and a modified AuCl4-:C18SH mole ratio of
3:1, instead of 1:1.7 The 3:1 AuCl4-:RSH ratio was used because it
produces fully covered Au nanoparticles (∼30 Å in diameter) and
eliminates the exhaustive (and exhausting!) washing/extraction process
required to remove the large amounts of residual thiol and disulfide
present in the original preparation.1,36 For the synthesis of Au
nanoparticles coated with 10,10,11,11,12,12,13,13-d8-octadecanethiol,
only one-third of the octadecanethiol surfactant molecules were
deuterated. For the other syntheses, the deuterated octadecanethiols
were used undiluted. The C18S/Au colloids were purified as described
previously and dried under vacuum.1 Complete removal of unbound
thiol was verified by TLC (silica gel, n-hexane) and by 1H NMR (499.84
MHz) and/or 13C NMR (125.70 MHz) spectroscopy (i.e. 40 mg of C18S/
2
before the start of spectral acquisition. For each H-labeled C18S/Au
sample, an initial spectrum was acquired at 25 °C and then spectra
were collected at progressively lower temperatures. The cooled sample
was then gradually reheated past 25 °C and spectra were collected at
higher temperatures. After heating past the melting transition, the
sample was cooled to 25 °C and a spectrum was acquired to check if
any thermally-induced changes had occurred.
1
Au colloid in 0.7 mL of d6-benzene).1 The H and 13C NMR signals
Results and Discussion
of C18S chains bound to the Au nanoparticles are considerably
broadened as compared to those of free C18SH. The R, â, and γ
methylene proton signals at ∼2.5, ∼1.6, and ∼1.4 ppm, respectively,
are not observable for the C18S/Au nanoparticles.1 The solution-state
13C NMR spectrum is characterized by broad peaks at 13.9 (C18), 22.7
(C17), 31.9 (C16), 30.9 (C4-C14), and 29.5 ppm (C15). The 13C
resonances due to C1, C2, and C3 are not observed for the Au-tethered
C18S chains.1,2 With 1.8 mmol of AuCl4- as the starting material, 350-
380 mg of purified C18S/Au powder was obtained.
Transmission Electron Microscopy. Samples for TEM were
prepared by dipping standard carbon-coated (200-300 Å), Formvar-
covered copper grids (200 mesh) into a dilute hexane solution of the
RS/Au nanoparticles for several seconds. The TEM grids were
withdrawn from the solution and allowed to dry under ambient
atmosphere for 5 min. Phase contrast images were obtained with a
top-entry Phillips EM 410T electron microscope operated at an
accelerating voltage of 80 keV. Micrographs were obtained at
magnifications of 6 × 104 and 8 × 104. The size distributions of the
C18S/Au nanoparticles were determined from the diameters of at least
250 particles located in a representative region of the micrographs using
a video image analysis software program (SigmaScan Pro3, Jandel
Scientific). The particle sizes obtained are the following: 1,1-d2-C18S/
Au 27 ( 8 Å; 10,10,11,11,12,12,13,13-d8-C18S/Au 29 ( 8 Å; d35-C18S/
Au 27 ( 6 Å.
DSC Measurements. The phase behavior of thiol-deriva-
tized Au nanoparticles has been characterized by DSC in terms
of the temperature and the enthalpy of the phase transition.1,5
The DSC thermograms clearly show that the RS/Au particles
(where R ) C12-C20), in the solid state, undergo distinct phase
transitions which can be associated with reversible disordering
of the alkyl chains.1 Both the peak maximum temperature and
the enthalpy associated with the DSC transition were found to
increase with increasing chain length.1,5 This trend strongly
parallels that seen in other materials undergoing gel-to-liquid
crystalline transitions, such as diacylphospholipid vesicles. In
fact, the DSC peak maximum temperature and enthalpy
measured for C12S/Au, C14S/Au, C16S/Au, and C18S/Au are very
close to the main transition temperature and enthalpy of
n-diacylphosphatidylcholines of equivalent chain length.1,5,38
A
comparison5 to the thermotropic behavior of pure hydrocarbons
is not appropriate since the enthalpies of the RSH-derivatized
(37) Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P.
Chem. Phys. Lett. 1976, 42, 390-394.
(38) The DSC peak maximum temperatures measured for the RS/Au
colloids1 Vs the DSC peak maximum temperature measured for n-
diacylphosphatidylcholines13 of equivalent chain length (value in paren-
theses) are the following: C12, 3 °C (-1.1 ( 0.4 °C),; C14, 22 °C (23.5 (
0.4 °C); C16, 41 °C (41.4 ( 0.5 °C); C18, 51 °C (55.1 ( 1.5 °C), and C20,
64 °C (64.5 ( 0.5 °C). The enthalpies measured for the RS/Au nanopar-
ticles1,5 in kJ per mol of RS chains Vs the enthalpies reported for the gel-
to-liquid crystalline transitions of n-diacylphosphatidylcholines13 (value
given in parentheses in kJ per mol per chain) are the following: C12, 6.4 (
2.5 (6.1 ( 3.4); C14, 10 (12.4 ( 1.4); C16, 13.8 ( 0.8 (17.4 ( 1.6); and
C18, 21 (21.2 ( 1.8). A melting temperature of 7 °C (280 K) and an enthalpy
of 20 kJ per mol of thiol molecules have been predicted for C12S/Au
nanoparticles from caloric curves produced by MD simulations.17
Infrared Spectroscopy. The C18S/Au colloidal particles were
deposited dropwise onto a KBr disc from a concentrated hexane
solution. Evaporation of the solvent resulted in a uniform film. Infrared
spectra were acquired using a Perkin-Elmer FT-IR Microscope Model
16PC (MCT detector) equipped with a Mettler FP 52 hot stage for
variable-temperature experiments (10-100 °C). Spectra were collected
in the transmission mode with an unpolarized beam, at a resolution of
2 cm-1 with 128 scans, and a spectral window of 4000-600 cm-1
The IR sample was purged continuously with dry nitrogen and
.

2686 J. Am. Chem. Soc., Vol. 119, No. 11, 1997
Badia et al.
be due to the fairly large particle size distribution which can
affect the alkyl chain packing. While these factors can
contribute somewhat to the peak broadening, a possible effect
of the restricted chain mobility (due to covalent bonding of the
sulfur to the Au) on the phase transition must also be considered.
In recent years, model polymer membranes have been
designed to obtain lipid bilayers which exhibit thermal stabilities
superior to those of biomembranes.41-43 Such stable membranes
are of interest for studies of cell-cell interaction and as drug
carriers. For this purpose, a C18 diacyl lipid analogue has been
synthesized with a polymerizable methacryloylic moiety at-
tached to the quaternary ammonium head group.41-43 Poly-
merization restricts the lateral mobility of the lipid head group
in the bilayer and is reported to significantly reduce the
cooperativity of the gel-to-liquid crystalline phase transition.
This restricted head group mobility causes the polymer mem-
brane to exhibit a broader gel-to-liquid crystalline transition than
the monomer membrane (i.e. the peak width at half maximum
is 0.7 °C for the monomer membrane and 2.1 °C for the polymer
membrane).42 Furthermore, the peak maximum temperature is
depressed by 20 °C and the enthalpy is reduced by a factor of
2 compared to the monomer bilayer.42 As mentioned earlier,
the DSC peak maximum temperatures and enthalpies measured
for C12S/Au, C14S/Au, C16S/Au, and C18S/Au nanoparticles are
however close to the values of the main transition for n-
diacylphosphatidylcholine lipids. Thus, even though these lipid
molecules have relatively unrestricted lateral mobility whereas
pinning of the sulfur to the Au is expected to restrict the
alkanethiolate chain mobility, there is no effect manifested in
terms of either the RS/Au transition temperature or enthalpy.
Melting of the RS/Au nanoparticles also introduces the pos-
sibility that rotational and/or lateral motion of the entire particle
sets in. The onset of an isotropic peak in the 2H NMR spectra
at higher temperatures (Vide infra) may suggest rotation or
tumbling of the entire Au particle. MD simulations of C12S/
Au nanoparticles in fact predict that these clusters have high
surface mobilities at temperatures above Tm, with a calculated
surface diffusion constant (1.5 × 10-5 cm2/s) which is close to
that of bulk liquid alkanes.17 The simulations also show that
the diffusive motion of the particles occurs through coupled
lateral rotation and translation (“pivoted-slip”).17 This rotation
can be formally equivalent to the 2D diffusion available to lipid
molecules on melting. If this were the case, then the DSC peak
broadness may be a manifestation of the particle size distribution
in an indirect way. That is, the particle tumbling rates may be
size dependent. The alkyl chains (in this model) must melt
before the tumbling (or entropy) issue becomes important. The
width of the RS/Au DSC peak and the similarity in the enthalpy
values of the RS/Au nanoparticles and n-diacylphosphatidyl-
choline multilamellar vesicles are sensitive indicators of the
cooperative nature of the order-disorder process.44 As such,
the broad transitions observed for both planar and colloidal RS/
Au SAMs are most probably due to the complex inter-
relationship between the particle and RS chain domain sizes
and their polydispersities.
Figure 1. DSC endotherms of 1,1-d2-C18S/Au, 10,10,11,11,12,12,13,13-
d8-C18S/Au, and d35-C18S/Au powders. Scan rate is 5 °C‚min-1 for 1,1-
d2-C18S/Au and 10,11,12,13-d8-C18S/Au and 10 °C‚min-1 for d35-C18S/
Au.
Au nanoparticles38 are much smaller than the melting heats
reported for pure alkanes.39,40 Instead, the alkyl chains are
clearly in a gel state, as in lipid membranes, rather than the
crystalline state found in pure hydrocarbons.
Figure 1 shows the DSC endotherms of the various deuterated
C18S/Au samples. The peak maximum temperature for both
the 1,1-d2-C18S/Au and d35-C18S/Au is 48 °C, while the peak
maximum temperature for the 10,10,11,11,12,12,13,13-d8-C18S/
Au is 52 °C. A peak maximum temperature of 51 °C was
previously reported for the fully hydrogenated C18S/Au.1 The
main phase transition temperatures of perdeuterated phospho-
lipids26 are depressed by ∼4-5 °C compared to the fully
hydrogenated analogue. Such a definite conclusion cannot be
made in our case for the d35-C18S/Au because the peak
maximum temperature observed by DSC varies by 2-3 °C from
one RS/Au preparation to another.
It is important to note that whereas the gel-to-liquid crystalline
transitions in lipid bilayer membranes occur over ca. 5 °C, the
transition for the RS/Au colloidal system is relatively broad and
occurs over a ca. 30 °C temperature range for each sample.
Furthermore, the DSC peaks of the deuterated C18S/Au samples
are preceded by a sloping or curved baseline extending down
to -100 °C, which is best seen in the endotherm of d35-C18S/
Au (Figure 1). This may reflect either the existence of a
disordered liquid-like chain population or the occurrence of
chain motional processes which accompany the phase transition.
As suggested earlier, the broadness of the transition may be
caused by poor bulk crystallization of the RS/Au nanoparticles.1,2
Recrystallization and thermal annealing do not however sig-
nificantly sharpen the DSC peak. The broad transition may also
Bloom and Bayerl also report significant differences in the
DSC phase behavior of single lipid bilayers supported on glass
microspheres (SSVs) as compared to multilamellar lipid vesicles
(MLVs).26 For instance, the SSV does not exhibit a chain tilt/
untilt pretransition, the main transition temperature is lowered
(41) Ebelhauser, R.; Spiess, H. W. Makromol. Chem., Rapid Commun.
1984, 5, 403-411.
(39) The melting points and enthalpies for pure hydrocarbons are the
following: C12H26, -9.6 °C (36.6 kJ/mol); C14H30, 5.9 °C; C16H34, 18.2
°C; C18H38, 28.2 °C (61.4 kJ/mol).40
(40) CRC Handbook of Chemistry and Physics, 70th ed.; Weast, R. C.,
Ed.; CRC Press, Inc.: Boca Raton, FL, 1989-90.
(42) Ebelhauser, R.; Spiess, H. W. Ber. Bunsenges. Phys. Chem. 1985,
89, 1208-1214.
(43) Ebelhauser, R.; Spiess, H. W. Makromol. Chem. 1987, 188, 2935-
2949.
(44) Blume, A. Biochemistry 1980, 19, 4908-4913.

Alkanethiolate Monolayers Self-Assembled on Au Nanoparticles
J. Am. Chem. Soc., Vol. 119, No. 11, 1997 2687
by 2 °C over that of the MLV, the half-width of the main
transition peak is a factor of 4 broader for the SSV than for the
MLV, the enthalpy of the main transition is 25% lower for the
SSV, and the hysteresis observed between the melting and
crystallization temperature values is larger.26 Bloom and Bayerl
attribute these changes to quasi-2D melting phenomena. In
MLVs, interbilayer interactions also increase the cooperativity
of the phase transition. This is interesting because fatty acid
Langmuir-Blodgett (LB) monolayers on silicon wafers also
show broad chain disordering transitions as detected by electron
diffraction.12 The gradual chain disordering occurs over a ca.
20 °C temperature range and the temperatures are very similar
to those of equivalent chain length RS/Au SAMs.1 Since the
fatty acid chains are only physisorbed to the underlying silicon,
the gradual chain melting process observed for these L-B
monolayer films cannot be a manifestation of restricted chain
mobility due to strong head group-substrate interaction. Thus,
it may well be that the broad chain order-disorder phase
transitions observed for self-assembled alkanethiol monolayers
on planar and colloidal Au surfaces are due to melting in quasi-
2D systems.
10,10,11,11, 12,12,13,13,
FT-IR Spectroscopy. DSC establishes that a phase transition
occurs in alkanethiol monolayers self-assembled at colloidal Au
surfaces. Although the transition can be characterized in terms
of temperature, enthalpy, and cooperativity, no direct informa-
tion can be obtained from DSC about the conformational state
of the alkyl chains. FT-IR spectroscopy is a powerful technique
for probing the conformations and orientations of alkyl chains
adsorbed to surfaces.45-51 In particular, the frequencies of the
methylene symmetric and antisymmetric vibrational stretches
are known to be related to the population of trans and gauche
conformers.25,46,47,49,52 Variable-temperature transmission FT-
IR spectroscopic studies show that these alkylated nanoparticles
undergo a transition from a highly chain-ordered state to a chain-
disordered state at the phase transition temperatures measured
by DSC.1 However, no insight was gained concerning where
the disordering starts in the alkyl chain and how far along the
chain the disorder propagates. FT-IR studies of deuterated lipids
in multilamellar vesicles52 and supported on spherical micro-
beads25 have shown that the temperature dependence of the
symmetric CD2 stretching vibration depends on the average
number of gauche conformers per lipid acyl chain. The
symmetric CD2 stretching frequency (∼2090 cm-1) increases
by ∼5 cm-1 during the gel-to-liquid crystalline phase transi-
tion.25 The antisymmetric CD2 stretch (∼2190 cm-1) also
increases with temperature, but the shift is less pronounced.52
By deuterating the C18SH molecule at specific sites along the
chain, the thermally-induced onset of local disorder can be
monitored Via the CD2 stretching vibrations.
1,1
Figure 2 tracks the CD2 symmetric (υsym(CD2)) and antisym-
metric (υasym(CD2)) stretches of the deuterated C18S/Au samples
as a function of temperature. In Figure 3A, the peak position
of υsym(CD2) is plotted as a function of temperature for d35-
(45) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am.
Chem. Soc. 1987, 109, 3559-3568.
(46) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990,
93, 767-773.
(47) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Electron Spectrosc.
Relat. Phenom. 1990, 54/55, 1143.
(48) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990,
112, 558-569.
(49) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. J. Vac. Sci.
Technol. A 1995, 13, 1331-1336.
Figure 2. Variable-temperature transmission FT-IR spectra of the CD2
stretching region (2300-2000 cm-1) for the deuterated C18S/Au
nanoparticles; (A) 10,10,11,11,12,12,13,13-d8-C18S/Au synthesized
using 1/3 labeled C18SH and 2/3 unlabeled C18SH, (B) d35-C18S/Au, and
(C) 1,1-d2-C18S/Au. The dotted lines are visual aids.
(50) Ulman, A. AdV. Mater. 1991, 3, 298-303.
(51) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82,
2136-2140.
(52) Mendelsohn, R.; Davies, M. A.; Brauner, J. W.; Schuster, H. F.;
Dluhy, R. A. Biochemistry 1989, 28, 8934-8939.

2688 J. Am. Chem. Soc., Vol. 119, No. 11, 1997
Badia et al.
gradual appearance of gauche conformational chain defects upon
heating, instead of the dramatic population increase expected
from a first-order melting process. By contrast, the shift in υsym
-
(CD2) for d8-C18S/Au shown in Figure 3A specifically reflects
the temperature dependence of the trans and gauche bond
population at positions 10-13 (i.e. in the middle of the alkyl
chain) and a sharp transition is observed. Molecular dynamics
simulations of C15S chains absorbed onto a Au(111) surface in
fact predict that at 27 °C, the percentage gauche defect
concentration is approximately 4.4% and that all of the gauche
defects are located at the chain termini.55 At this temperature,
the middle of the chain is predicted to be essentially defect free.
The FT-IR results described here for 10,10,11,11,12,12,13,13-
d8-C18S/Au and d35-C18S/Au confirm this prediction. As the
temperature is raised past 27 °C in the MD calculations, the
development of gauche bonds is concentrated to the middle of
the C15S chains and a low gauche defect population is observed
near the sulfur head group.55 These predictions are, again, in
excellent agreement with the thermal behavior observed here
for 10,10,11,11,12,12,13,13-d8-C18S/Au and 1,1-d2-C18S/Au
(Vide infra).
The variable-temperature FT-IR spectra of 1,1-d2-C18S/Au
nanoparticles (Figure 2C) show a υsym(CD2) peak (∼2123 cm-1
)
which is shifted by ∼15 cm-1 compared to the bulk thiol
(∼2139 cm-1). A weak, broad υasym(CD2) peak at ca. 2200
cm-1 is also observed and represents a shift of ∼20 cm-1 from
that of 1,1-d2-C18SH.56,57 Interestingly, SERS58 studies of planar
RS/Au SAMs report a ca. 20 cm-1 decrease in the υ(C-S)
frequency upon bonding of the thiol to the Au surface due to
weakening of the C-S bond. Besides the ca. 15-cm-1 shift in
the υsym(CD2) peak observed for the carbon adjacent to the sulfur
head group, it is important to note that its position does not
shift over the temperature range of 10-90 °C (Figure 3B). This
strongly suggests that local order is maintained at the carbon
closest to the Au surface during the chain disordering transition
described above.
Figure 3. The peak position of the symmetric CD2 stretch as a function
of temperature for (A) 10,11,12,13-d8-C18S/Au and d35-C18S/Au and
(B) 1,1-d2-C18S/Au. The error bars represent a 0.5-cm-1 uncertainty in
the peak position values.
C18S/Au and 10,10,11,11,12,12,13,13-d8-C18S/Au. The value
of the υsym(CD2) peak frequency for the 10,10,11,11,12,12,13,
13-d8-C18S/Au nanoparticles exhibits a sharp discontinuity at
the DSC detected phase transition temperature, ∼50 °C. By
contrast, the d35-C18S/Au sample shows only a gradual chain
disordering transition. A similar trend is observed for the
Transmission FT-IR spectroscopy thus definitively establishes
that the DSC-detected phase transition arises from an alkyl chain
disordering process which increases the number of gauche
conformers in the chain. One can envisage that specific
deuteration along the length of the alkanethiol chain will allow
one to study the onset of chain disordering at highly-defined
regions in an assembly of alkyl chains.
2H NMR Spectroscopy. IR spectroscopy probes molecular
vibrations occurring on a time scale shorter than 10-10 s, so
that the IR measurement is essentially a “snapshot” of the
different bond conformations existing at a given instant averaged
υ
asym(CD2) peak. Furthermore at 10-15 °C, υsym(CD2) for
10,10,11,11,12,12,13,13-d8-C18S/Au occurs at ∼2084 cm-1
,
while υsym(CD2) for d35-C18S/Au is ∼2088 cm-1. This clearly
indicates that at 10-15 °C, the population of gauche conformers
is larger in the perdeuterated C18S/Au material than for
10,10,11,11,12,12,13,13-d8-C18S/Au. Obviously, in d35-C18S/
Au, there are contributions from gauche CD2s located in the
C14-C17 region, even 35 °C below the DSC-detected phase
transition temperature. Helium diffraction53,54 and surface IR
studies46,47 of long-chain alkanethiol SAMs on planar Au
confirm that gauche conformational defects are concentrated
at the chain termini at temperatures below 27 °C. These chain
end gauche defects persist even at -73 °C.46,47 Thus, the ca.
4-cm-1 shift in peak frequency of the symmetric CD2 stretch
(υsym) of d35-C18S/Au relative to that of 10,10,11,11,12,12,13,
13-d8-C18S/Au is definitely a result of the disordered state of
the chain ends. Since only a gradual shift in υsym(CD2) is
observed for d35-C18S/Au with increasing temperature, it is most
probable that a weighted average of the trans and gauche bond
populations along the entire chain length is reported in this
experiment. In variable-temperature surface IR studies of planar
alkanethiol monolayers, Dubois and Nuzzo46,47 observed the
(55) Mar, W.; Klein, M. L. Langmuir 1994, 10, 188-196.
(56) (a) It is noteworthy that for the bulk 1,1-d2-C18SH, the υasym(CD2)
peak is less intense than the υsym(CD2) peak, while in the d35-C18SH, the
υasym(CD2) peak is more intense than the υsym(CD2) peak. The assignment
of the 2120-cm-1 peak in the spectrum of 1,1-d2-C18S/Au to υsym(CD2) is
reasonable since (i) there are no interfering C-H or C-C vibrations in the
2000-2300-cm-1 region, (ii) deuteration is at a single position, and (iii)
given the relative intensities of the symmetric and antisymmetric CD2
stretches in bulk 1,1-d2-C18SH. Refer to the spectra included in the
Supporting Information. (b) The complementary υ(CH2) data (2800-3000
cm-1) for d35-C18S/Au (i.e. hydrogenated at carbon 1) contain significant
contributions from CHD due to a 2% isotopic hydrogen impurity.57 For
the C18 chain, there are 0.7 CHD and 2 CH2 groups per molecule. These
isolated CHD groups cause site-group splitting of the C-H stretches so
that no direct comparison can be made with the peak frequencies of fully
hydrogenated C18S/Au. Refer to the spectra included in the Supporting
Information. No site-group splitting of the C-H stretches is however
observed for 10,10,11,11,12,12,13,13-d8-C18S/Au due to the insignificant
amount of isolated CHD groups (<0.001) compared to CH2 groups (29).
(57) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J.
Phys. Chem. 1984, 88, 334-341.
(53) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem.
Phys. 1989, 91, 4421-4423.
(54) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.;
Scoles, G. J. Chem. Phys. 1991, 94, 8493-8502.
(58) Bryant, M. E.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284-
8293.

Alkanethiolate Monolayers Self-Assembled on Au Nanoparticles
J. Am. Chem. Soc., Vol. 119, No. 11, 1997 2689
10,10,11,11, 12,12,13,13,
Figure 4. 2H NMR spectra of 10,10,11,11,12,12,13,13-d8-C18S/Au.
out over the area of irradiation.35 IR spectroscopy thus reveals
the equilibrium trans and gauche bond populations as a function
of temperature, but does not provide any insight about the
motional processes which accompany chain disordering. For
Figure 5. Computationally “de-Paked” spectra overlaid on the 2H NMR
powder patterns obtained at -55 and 25 °C for 10,10,11,11,12,12,13,13-
d8-C18S/Au. The “de-Paking” process filters out all but the 0° orientation
from the powder pattern.
2
this reason, the long time scale probed by H NMR (typically
10-6-10-4 s)35 has been exploited to study the temperature
dependence of the alkanethiolate chain dynamics.
2
The variable-temperature H NMR spectra of 10,10,11,11,
12,12,13,13-d8-C18S/Au are displayed in Figure 4. At -55 °C,
a static Pake pattern with a quadrupolar splitting of 118 kHz is
observed, clearly indicating that there is little motion of the
C-2H bonds at positions 10-13 of the bound C18S chain. As
the 10,10,11,11,12,12,13,13-d8-C18S/Au sample is heated, the
2H NMR spectra exhibit motional averaging of the line shape.
2
At -30 °C, the H resonance appears as a “flat-topped” peak,
a line shape which has been attributed to a slow, restricted trans-
gauche bond isomerization.33,59,60 Figure 4 shows that with
further increases in temperature, a number of the C18S chains
exhibit a motion that is characterized by rounded triangular
Figure 6. Plot of the full width at half height (fwhh) of the broad
spectral component in the powder pattern of 10,10,11,11,12,12,13,13-
d8-C18S/Au Vs temperature (T). Tm refers to the DSC peak maximum
temperature.
2
patterns such as that observed in the H NMR spectrum at 0
°C. In the temperature range of ca. 0 to 50 °C, the spectral
line shapes consist of a narrower triangular component super-
imposed on a much broader component. The “de-Paked”
spectrum at 25 °C (Figure 5), for example, clearly shows that
the complex line shape of the powder pattern actually consists
of two sets of two peaks. The outer set of peaks has a static
quadrupolar splitting of 240 kHz (i.e. θ ) 0°) and the inner
set has a quadrupolar splitting of ∼40 kHz. The inner set of
peaks reflects disordered RS chains that are not subject to true
isotropic motion because they are tethered to the Au surface.
More importantly, with increasing temperature, the relative
contribution of the triangular peak increases. The full width at
half height (fwhh) of the broad spectral component in the
powder pattern is plotted as a function of temperature in Figure
6. The graph reveals that from -55 to 45 °C, the fwhh remains
fairly constant at 110-130 kHz. Around the phase transition
temperature of 50 °C, the broad component abruptly disappears
and the 2H NMR spectra at 55 and 65 °C exhibit solely a narrow
isotropic peak. The gauche conformer population largely
determines the width of the broad spectral component.42 More
precisely, when the gauche bond population equals the trans
population, no quadrupole splitting remains.42 This detail clearly
explains the disappearance of the broad spectral component in
the 2H NMR spectrum between 50 and 55 °C. (Figure 4). The
absence of a narrow isotropic peak at temperatures below the
phase transition temperature supports our previous suggestion
that the chains exist in ordered domains or bundles and that the
Au particles close-pack Via the interdigitation of these ordered
chain bundles1 (Scheme 1). The interdigitation of individual
chains on adjacent Au particles would not be sufficient to fill
(59) Huang, T. H.; Skarjune, R. P.; Witterbort, R. J.; Griffin, R. G.;
Oldfield, E. J. Am. Chem. Soc. 1980, 102, 7377-7379.
(60) Hentschel, D.; Sillescu, H.; Spiess, H. W. Macromolecules 1981,
14, 1605-1607.

2690 J. Am. Chem. Soc., Vol. 119, No. 11, 1997
Badia et al.
Scheme 1. A Schematic 2D Representation of the RS/Au
Scheme 2. The Types of Chain Dynamic Processes
2
Nanoparticle Packing Structure in the Solid Statea
Suggested by the H NMR Line Shapes of the Deuterated
C18S/Au Nanoparticlesa
a In this description, domains or bundles of ordered alkylthiolate
chains on a given Au particle will interdigitate into the chain domains
of neighboring particles in order to compensate for the substantial
decrease in the chain density which occurs toward the methyl chain
end. Chains with large populations of gauche bonds may arise from
(i) those which occupy interstitial regions in the particle lattice and
cannot efficiently overlap with adjacent chains or from (ii) chains
residing at domain boundaries.
the large void volume created by the 14:1 ratio of area available
to the chain end to Au-S head group area imposed by the
curvature of the nanoparticle. Individually interdigitated chains
would be disordered far below the observed phase transition
temperature. The υ(CD2) peak positions (Figures 2 and 3A)
observed for 10,10,11,11,12,12,13,13-d8-C18S/Au at ca. 35 °C
below the phase transition temperature also point to the existence
of ordered chain domains.
a These processes involve trans-gauche bond isomerization and
pseudorotational motion of individual chain segments about the long
axis of the alkanethiolate molecule.
RS molecule.42 Thus, at higher temperatures, the motional
behavior of the RS chain segments is expected to include both
axial rotation and rapid bond isomerization, such that the
complex, rounded triangular line shapes should be describable
by a six-site jump model.32,42 In fact, molecular dynamics
simulations by Mar and Klein55 predict that the formation of
internal gauche defects for densely packed C15S chains is
coupled to the onset of axial chain rotation.55 Although these
qualitative comparisons with the spectral line shapes of other
tethered chain systems is insightful, detailed line shape simula-
Both the flat-topped and triangular peak shapes observed in
2
the H NMR spectra of 10,10,11,11,12,12,13,13-d8-C18S/Au
have been observed in studies of amorphous polymers,33,59,60
liquid crystals,61 lipid bilayers,21 polymer model membranes,41-43
and alkylsilyl-modified silica gel.32 These line shapes are
associated with the conformational exchange of alkyl chains
between two unequally populated trans and gauche sites through
the reorientation of individual C-2H bonds around the tetra-
hedral angle (109.5°). These bond reorientation processes have
been modeled using both two-site and six-site “jump” mod-
els,32,33,42,59,61 and the resulting simulated line shapes (i.e. flat-
topped Vs triangular peak) are reported to be dependent upon
both the relative trans and gauche bond populations and the
conformational exchange (or jump) rates. For the RS/Au
nanoparticles, the population of trans and gauche conformers
is controlled by the temperature. By analogy to the chain
dynamics reported for similar tethered alkyl chain systems,32,42
a motional model emerges for the RS chains involving both
trans-gauche bond isomerization and axial chain rotation
(Scheme 2).
2
tions of the H NMR spectra of 10,10,11,11,12,12,13,13-d8-
C18S/Au are presently underway to more precisely define the
types and rates of chain motion. At the present time, we are
focusing on the broader, qualitative information derived from
2
the temperature dependence of the H NMR spectra, and not
on more refined kinetic information. The latter requires a
detailed analysis of the chain conformation kinetics and evolves
into issues of model evaluation.
The fact that a rounded triangular peak component is already
observed in 10,10,11,11,12,12,13,13-d8-C18S/Au at 0 °C (Figure
4) suggests that there exists a significant population of C18S
molecules with a large number of gauche defects in the middle
of the chain at 50 °C below the Tm (DSC peak maximum
temperature). This likely arises from a distribution of chain
geometries where large gauche bond populations exist in chains
which occupy interstitial regions in the particle lattice.1 These
chains cannot efficiently interdigitate or bundle with adjacent
chains to form ordered domains (Scheme 1). Chains residing
at domain boundaries are also expected to be disordered at these
low temperatures.1 In fact, Reven and co-workers have shown
through 13C spin-lattice relaxation measurements of C18S/Au
that at 25 °C below the Tm, a population (74%) of motionally
restricted all-trans chains coexists with a smaller population
(26%) of liquid-like disordered chains.2 In addition, the 13C
chemical shift of the interior methylene carbons and the reduced
In this model, the chains are densely packed at temperatures
far below the phase transition temperature. Less space is
available for molecular reorientations and trans-gauche bond
isomerization is restricted. This results in a slow conformational
exchange rate and the resulting flat-topped spectral line shapes
should be describable by a simple two-site jump model.32,42
A
low gauche conformer population is expected at these temper-
atures and the overall orientation of the chains can be maintained
by the formation of kinks and jogs.42 As the number of chain
gauche conformers increases with temperature, the chain free
volume should increase. This causes the pseudo-rotational
motion of individual chain segments about the long axis of the
(61) Liesen, J.; Boeffel, C.; Spiess, H. W.; Yoon, D. Y.; Sherwood, M.
H.; Kawasumi, M.; Percec, V. Macromolecules 1995, 28, 6937-6941.
1
methylene H proton line widths observed in two-dimensional

Alkanethiolate Monolayers Self-Assembled on Au Nanoparticles
J. Am. Chem. Soc., Vol. 119, No. 11, 1997 2691
Figure 7. 2H NMR spectra of d35-C18S/Au.
wideline separation (WISE) NMR experiments,62,63 suggest that
the conformationally ordered C18S chains are undergoing large-
amplitude motions about their long axis at room temperature.2
The temperature-dependent behavior of the 2H NMR spectra
of d35-C18S/Au (Figure 7) is distinctly different from that of
10,10,11,11,12,12,13,13-d8-C18S/Au, but the spectral line shapes
are similar. This suggests that the motions of the chain segments
are the same but that their onset occurs at different temperatures
for specific regions of the chain. For instance, the spectral line
shape of d35-C18S/Au exhibits a rounded triangular peak in the
center of the static Pake pattern at -55 °C (Figure 7). Since
10,10,11,11,12,12,13,13-d8-C18S/Au shows only a rigid Pake
pattern at this temperature (Figure 4), the triangular peak must
arise from C-2H bond motions at positions 14-18 along the
chain. This behavior is expected given that 13C spin-lattice
relaxation measurements2 indicate that molecular mobility
increases toward the untethered chain ends which have a higher
population of gauche conformers.46,47,53,54 Moreover, it is in
accord with the FT-IR correlations discussed above. It is
particularly noteworthy that in contrast to the 2H NMR spectra
obtained for multilamellar vesicles of perdeuterated lipids,21,26,37
distinct quadrupolar splittings from individual methylene and
methyl resonances are not observed for d35-C18S/Au. The
different splittings observed for the perdeuterated lipids arise
from the existence of a flexibility gradient along the lipid chain
which changes the characteristics of the molecular motions.21,22
However, Bayerl and Bloom have shown that as the surface
curvature increases for lipid bilayers supported on 0.25-µm-
sized beads (SSVs), the powder spectrum is broadened and the
individual methylene quadrupolar splittings become indistin-
guishable.26 The RS/Au nanoparticle would contain an extreme
case of surface curvature (i.e. a radius of 15 Å Vs 2500 Å in
the Bayerl and Bloom SSVs), so that the powder patterns
obtained for d35-C18S/Au most likely reflect the extended chain
geometry and large curvature of the metal particle.
Figure 8. 2H NMR spectra of 1,1-d2-C18S/Au.
spectroscopy. Further experiments may help to examine this
possibility.
2
Finally, the H NMR spectra of 1,1-d2-C18S/Au are shown
in Figure 8. Unlike, the spectra of 10,10,11,11,12,12,13,13-
d8-C18S/Au and d35-C18S/Au, those of 1,1-d2-C18S/Au are rigid
Pake patterns with a quadrupolar splitting of 120 kHz at
temperatures up to 65 °C. This establishes that, at least on the
2
microsecond timescale probed by H NMR spectroscopy: (i)
the C1-C2 bond does not experience a measurable degree of
trans-gauche bond isomerization, (ii) it is unlikely that the
alkanethiolate chain undergoes axial rotation about the C-S
bond, and (iii) there is no diffusion of the RS-Au species on
the Au particle surface. The surface migration of thiolate-Au
species has been observed in UHV-STM studies64-66 over
periods of minutes to hours. The rigid Pake patterns observed
here for 1,1-d2-C18S/Au do not conclusively reveal whether 2D
diffusion of the RS chains, with or without attached Au atoms,
occurs on the particle surface. At 65 °C, a narrow isotropic
peak is observed in the center of the rigid Pake pattern, whose
contribution to the total spectrum is ∼10%. The intensity/
contribution of this peak does not change with a further increase
in temperature. We believe this isotropic peak is due to melting
of free or loosely bound thiol whose motions become isotropic
once the entrapping matrix has become disordered.
Conclusions
RS/Au nanoparticles present themselves as an interesting class
of materials given their now obvious parallels to planar SAMs
and the fact that their physical properties are strongly dependent
upon the structure and properties of the alkanethiolate chains.
We have demonstrated in the foregoing that the DSC-detected
phase transition is in fact a thermally induced chain melting
process. Chain melting begins at the chain terminus region and
propagates toward the center of the chain as the temperature
increases. FT-IR spectroscopy of specifically deuterated samples
establishes that at 35 °C below the calorimetrically-determined
phase transition temperature, many of the chain termini are
highly disordered while there is a low gauche defect density in
the middle of the chain and near the sulfur head group. 2H
It is important to acknowledge that the narrow isotropic peak
observed at temperatures above 50 °C (T ) Tm) for both d35-
C18S/Au and 10,10,11,11,12,12,13,13-d8-C18S/Au may also be
associated with rotation or tumbling of the entire Au particle
within the ensemble of Au particles. Since the diameter of our
Au colloids is the same as the length of the RS chains, Au
particle tumbling would presumably be accessible once the alkyl
chains have melted, but would not be observable by FT-IR
(64) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966-10970.
(65) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979-
983.
(66) Stranick, S. J.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Phys.
Chem. 1994, 98, 11136-11142.
(62) Chin, Y.-H.; Kaplan, S. Magn. Reson. Chem. 1994, S53.
(63) Clauss, J.; Schmidt-Rohr, K.; Adam, A.; Boeffel, C.; Spiess, H. W.
Macromolecules 1992, 25, 5208.

2692 J. Am. Chem. Soc., Vol. 119, No. 11, 1997
Badia et al.
NMR spectroscopy, as a powerful probe of chain order and
dynamics, establishes that chain melting arises from an increased
frequency of gauche bonds in the Au-tethered alkanethiol chains.
The excellent correspondence observed between the properties
of the planar surface and nanoparticle systems through FT-IR,
X-ray diffraction, and calorimetry (differential scanning and
electrochemical) experiments suggests that the nanoparticle
solid-state NMR (2H and 13C) conclusions are transferable to
the planar RS/Au SAMs.
dePaking software and for his advice on spectral manipulation.
The authors are grateful to Professor Linda Reven (McGill
University) for insightful discussions and a critical reading of
the manuscript. Professor Royce Murray (University of North
Carolina at Chapel Hill) is thanked for providing preprints of
his work in this area.
Supporting Information Available: Transmission FT-IR
spectra (3200-1800 cm-1) of bulk CD3(CD2)16CH2SH (d35-
C18SH), CH3(CH2)16CD2SH (1,1-d2-C18SH), and the corre-
sponding thiol-derivatized Au nanoparticles are given (7 pages).
See any current masthead page for ordering and Internet access
instructions.
Acknowledgment. The authors wish to thank NSERC
(Canada) and FCAR (Quebec) for financial support. This
research was funded in part by a Grant-in-Aid of Research from
Sigma Xi to A.B. The authors would like to thank Professor
Michel Lafleur (Universite´ de Montre´al) for providing the
JA963571T