Fullerene modification of gold electrodes and gold nanoparticles based on application of aromatic thioacetate-functionalized C60

Article abstract of DOI:10.1039/c4ra12139b

A new procedure for the synthesis of S-acetyl-derivatized fullerene is described. The deprotected aromatic S-acetyl-derivatized fullerene was employed for efficient modification of a gold electrode and gold nanoparticles. The proposed deposition procedure of fullerene derivative on Au surfaces allows the control of the thickness of the self-assembled fullerene layer. The modified surfaces were characterized by electrochemical methods and X-ray photoelectron spectroscopy (XPS). Functionalized fullerenes largely retain favorable redox electronic properties, behaving as an electron sink and revealing 4 reversible sequential 1e electrode processes at slightly more negative potentials than those observed for unsubstituted C₆₀. For the first time, fullerene-capped gold nanoparticles were obtained by a two-step ligand exchange procedure involving substitution of alkanethiol with thiolated fullerene derivative following the synthesis of alkanethiol capped nanoparticles. The ligand exchange procedure was very efficient-the number of fullerene moieties per single gold nanoparticle was found to be as high as 30. This journal is

Full text of DOI:10.1039/c4ra12139b

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Two types of nanocomposites were synthesized: Au electrode covered with self-assembled in situ
deprotected aromatic S-acetyl-derivatized C60 fullerenes and Au nanoparticles decorated with high number
of C60 fullerenes.
1
11x64mm (300 x 300 DPI)

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Fullerene Modification of Gold Electrodes and Gold
Nanoparticles Based on Application of Aromatic
Thioacetate-Functionalized C60
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
a
Piotr Piotrowski , Joanna Pawłowska , Jan Pawłowski , Lidia Jagoda Opuchlik ,
a
a*
Received 00th January 2012,
Accepted 00th January 2012
Renata Bilewicz , Andrzej Kaim
DOI: 10.1039/x0xx00000x
A new procedure for the synthesis of Sꢀacetylꢀderivatized fullerene is described. The
deprotected aromatic Sꢀacetylꢀderivatized fullerene was employed for efficient modification
of gold electrode and gold nanoparticles. The proposed deposition procedure of fullerene
derivative at Au surfaces allows the control of the thickness of the selfꢀassembled fullerene
layer. The modified surfaces were characterized by electrochemical methods and Xꢀray
photoelectron spectroscopy (XPS). Functionalized fullerenes largely retain favorable redox
electronic properties, behaving as an electron sink and revealing 4 reversible sequential 1e
electrode processes at slightly more negative potentials than those observed for
unsubstituted C . For the first time, fullereneꢀcapped gold nanoparticles were obtained by a
www.rsc.org/
6
0
two ꢀ step ligand exchange procedure involving substitution of alkanethiol with thiolated
fullerene derivative following the synthesis of alkanethiol capped nanoparticles. The ligand
exchange procedure was very efficient – the number of fullerene moieties per single gold
nanoparticle was found to be as high as 30.
free thiols which are readily oxidized, the thioesters are much more
stable and can be easily converted into thiols when needed.
Introduction
7
Functionalization of gold nanoparticles (AuNPs) with C60 fullerene
has been recently subject of a number of reports but usually alkyl
Fullerenes and their derivatives attract reasonable attention due to
their unique electronic properties including reversible reduction with
8ꢀ12
1
thiols were employed while aromatic moieties on the gold surface
up to six electrons per single C60 molecule. This gives them huge
13,14
2
were attached to only indirectly.
potential as building blocks for the construction of nanodevices
In the present paper, we describe the synthesis of a new C60 fullerene
functionalized with a short aromatic thioacetate linker. Following
deprotection, the fullerene derivative is used to selfꢀassembly into
densely packed fullerene layer on Au (111) surface of gold electrode
or gold nanoparticles. Two variants, including the in situ procedure
for the deprotection of the thioacetylꢀderivatized fullerene by
pyrrolidine were elaborated (Fig. 1). Both approaches can be of
general interest because of their simplicity and lack of side reactions
with the fullerene cage.
Electronic properties of the novel C60 derivative bound to both gold
electrode and AuNPs were investigated using the electroanalytical
methods. The gold nanoparticles showed narrow size distribution,
well defined composition and high fullerene content, calculated to be
ca. 30 fullerene substituents per single AuNP.
including wellꢀordered monolayers or thin films for practical
applications in photovoltaic cells, field effect transistors, sensors,
molecular wires or optoelectronics.
3
,4
The thiol moiety is the most widely used anchoring group for the
modification of metals surfaces. Although the literature on thiol selfꢀ
assembly on gold surface is very extensive and these structures have
been thoroughly investigated, thiol functionalized fullerenes
5
applications were limited to alkyl derivatives. Due to the conductive
and electronic properties, the aromatic thiols are more interesting for
6
future implementations but reports on the subject are scarce even
though linker’s structure and length may improve the electrical
contact between the fullerene moiety and the gold surface. In this
paper we focused on short aromatic linker originating from
thioacetate protected thiol functionalized fullerene. In contrary to
Fig. 1. Scheme for preparation of thioacetylꢀfunctionalized C60 fullerene films on the gold surface.
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+
Yield: 8%, the mass spectrum (ESIꢀMS) showed a [M+Na] peak at
9
50.7 (Figure S2, Electronic Supplementary Information); IR (KBr
Experimental section
ꢀ
1
disk) νmax(cm ) 2946.1, 2841.4, 2777.9, 1708.0, 1488.3, 1461.2,
1
6
423.3, 1332.0, 1177.8, 1114.3, 1089.3, 1016.4, 944.6, 766.2, 728.0,
Reagents
08.6, 552.4, 526.2, see (Figure S6, Electronic Supplementary
1
Acetyl chloride, dimethylformamide (DMF) and pyrrolidine Information); H NMR (200 MHz; CDCl ; TMS) 2.41 (3H, s), 2.83
3
were
obtained
from
Alfa
Aesar.
Hydrogen (3H, s), 4.26ꢀ4.31 (1H, d, J=9.4Hz), 4.97 (1H, s), 4.97ꢀ5.02 (1H, d,
tetrachloroaurate(III), 4ꢀ(methylthio)benzaldehyde, sodium J=10.1Hz), 7.46ꢀ7.51 (2H, d, J=8.5Hz), 7.88ꢀ7.93 (2H, d, J=7.4Hz)
1
3
tertꢀbutyl
sulfide
(
t
BuSNa),
N
ꢀmethylglycine, ppm (Figure S9, Electronic Supplementary Information); C NMR
tetrabutylammoniumhexafluorophosphate
(TBAHFP), (125 MHz; CDCl ) 30.30, 40.29, 69.19, 70.14, 83.02, 128.17,
3
tetraoctylammoniumbromide (TOAB) and toluene were bought 135.70, 135.96, 138.58, 139.93, 141.90, 140.02, 140.05, 142.07,
from SigmaꢀAldrich. Dichloromethane, ethanol, ethyl acetate, 142.12, 142.16, 142.26, 142.60, 144.43, 145.21, 145.30, 145.34,
methanol,
nꢀhexane, acetonitrile (AN) and sodium sulfate were 145.37, 145.56, 145.57, 145.97, 145.99, 146.17, 146.31, 147.34,
purchased from POCh (Poland). Fullerene C , 99.9%, was 152.92, 153.10, 153.94, 156.11, 193.93 ppm (Figure S10, Electronic
6
0
obtained from MER Corp. DMF and toluene were dried and Supplementary Information).
purified before use, according to standard procedures, other
solvents were analytical grade reagents and were used as
received.
Deprotection of fullerene thioacetate
Synthesis of fullerene thioacetate
ꢀ(Sꢀacethylthio)benzaldehyde was synthesized using the
combination and modification of two methods described before.
To a suspension of tBuSNa (2.82 g; 25 mmol) in dry DMF (35mL),
a solution of 4ꢀ(methylthio)benzaldehyde (1.9g; 12.5 mmol) in DMF
At the very beginning we discarded reported sodium
1
8
19
hydroxide and sodium thiomethoxide methods that required
5,16 use of ethanol and methanol respectively, solvents which
synthesized fullerene compound was insoluble in. However,
aromatic thioacetate group present in some nonꢀrelalated
compounds was previously deprotected also in solvents suitable
for most fullerene derivatives using tetrabutylammonium
4
1
(
5 mL) was injected via a syringe while stirring vigorously. Reaction
o
was kept under argon atmosphere at 153 C for 4h and suspension
became dark brown. It was then cooled to 0 C and acetyl chloride
20
20,21
o
hydroxide (TBAH) or aliphatic amines, i.e. triethylamine,
.
Seemingly, synthesized fullerene thioacetate should follow the
same route. Unexpectedly, we observed difficulties that turned
out to be inherent to aromatic tioacetate fullerene derivatives
while trying to apply these common deprotection procedures.
Despite good solubility of fullerene thioacetate in
tetrahydrofuran the use of TBAH solution was unable to
deprotect ATF-C60 because brown precipitate formed soon
after addition of tetrabytulammonium hydroxide solution. The
probable explanation for this fact can be that TBAH is well
(4.3mL; 60mmol) was added in strong Ar stream. After 45min, the
resulting suspension was poured into 400 mL of water, followed by
addition 80ml of CH Cl , resulting two phases were stirred for 30
2
2
min. and the organic layer was separated, extracted with water
2x10mL) and dried over anhydrous sodium sulfate. After
(
evaporation of the solvent under reduced pressure the purification
was accomplished by means of flash column chromatography (3:1 nꢀ
hexane: ethyl acetate) to give desired product as white solid.
o
Yield: 78%, mp 46 C, The mass spectrum (ESIꢀMS) showed a
22
+
known catalyst of fullerene hydroxylation and some insoluble
[
M+H] peak at 181.0 (Figure S1, Electronic Supplementary
ꢀ
1
fullerenol was formed instead of desired free thiol. The second
Information); IR (KBr disk) νmax(cm ) 2972.7, 2547.0, 1698.0,
20,21
studied deprotection agent was triethylamine;
which can
1
1
593.4, 1561.1, 1424.2, 1400.1, 1365.7, 1309.4, 1291.6, 1278.1,
also react with C60 fullerene as reported previously by Kajii et
174.9, 1126.2, 1016.3, 941.1, 859.6, 810.4, 712.1, 625.3, 537.1
23
1
al. and in our case even large excess of Et N in toluene did
3
(Figure S5, Electronic Supplementary Information); H NMR (200
not deprotect ATF-C60 to form free thiol. After that, we tried to
MHz; CDCl ; TMS) 2.47 (3H, s), 7.58ꢀ7.61 (2H, d, J=8.4Hz), 7.89ꢀ
24
3
use a modified procedure given by Yelm who applied
7
.92 (2H, d, J=8.3Hz), 10.04 (1H, s) ppm (Figure S7, Electronic
1
3
pyrrolidine to deprotect nonꢀfullerene aromatic thioacetates in
solvents suitable for fulleropyrrolidines. Despite the fact that
Supplementary Information); C NMR (125 MHz; CDCl ) 30.53,
3
1
30.02, 134.52, 135.44, 136.47, 191.46, 192.31 ppm (Figure S8,
25
fulleropyrrolidines are less reactive than pure C60 fullerene we
Electronic Supplementary Information).
were still considering possible sideꢀreactions of pyrrolidine
Preparation of aromatic thioacetate functionalized C60 fullerene
26,27
17
with C60 fullerene cage under aerobic conditions
and the
(
ATF-C ) was accomplished using the modified Prato synthesis.
60
chance that heat also may promote reaction of secondary
A stirred solution of C60 fullerene (144 mg, 0.2mmol), 4ꢀ(Sꢀ
acethylthio)benzaldehyde (36 mg, 0.2 mmol) and Nꢀmethylglycine
28
amines with fullerene, so the deprotection was finally done
under inert gas atmosphere and without heating. This time
results were fully satisfying, thiolate anion of deprotected ATFꢀ
C60 has been detected by means of ESIꢀMS (Figure S3,
Electronic Supplementary Information). Thus, pyrrolidine at
room temperature and in inert gas atmosphere was found to be
the best deprotection agent for synthesized fullerene
(89 mg, 1mmol) in dry toluene (130 mL) was heated at reflux under
argon atmosphere for 8 h. The reaction mixture was then cooled,
filtered to remove insoluble precipitate and the solvent was removed
under reduced pressure. The residue was chromatographed (2:1 nꢀ
hexane: toluene) to give pure ATF-C60 as dark brown powder.
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thioacetate. However, in situ deprotection with gold plate we inferred that this ligandꢀexchange reaction was not accompanied
immersed into a solution of ATF-C60 with 20eq. of pyrrolidine by side effects, such as irreversible aggregation or significant gold
led to formation of multilayer with remaining traces of amine core size modifications. The clear burgundy solution was rotary
whereas removal of the pyrrolidine excess before selfꢀassembly evaporated, and the blackish solid was dissolved in a small volume
allowed producing stable monolayers on both gold electrode of toluene (max. 2 mL). In the next step, the NPs were precipitated
and nanoparticles. In typical procedure to a 0.5mM solution of using ethanol and centrifuged 2 h later (13 000 rpm, 5 min). The
ATF-C60 in toluene (10mL) 7.1 mg (8.3µL, 0.1mmol) of precipitate was dissolved in toluene, sonicated and the suspension
pyrrolidine was added. Resulting mixture was stirred for 3h at was centrifuged again at 13 000 rpm for 20 minutes The toluene
room temperature under argon atmosphere. Then, mixture was washing process was repeated several times until the supernatant
concentrated under reduced pressure, removing the pyrrolidine fluid became colorless. Resulting fullerene functionalized
at the same time, and 10 mL of methanol was added. Precipitate nanoparticles were characterized by UVꢀVis, DLS and XPS
was centrifuged (10 000 rpm, 5 min), washed with methanol measurements.
and monitored by TLC and ESIꢀMS for complete removal of
formed Nꢀacetylpyrrolidine (detected in methanol solution by
ESIꢀMS, see Figure S4, Electronic Supplementary Information)
and any remaining traces of pyrrolidine (usually not present in
Characterization methods and Instrumentation
supernatant). Then it was dissolved in 10ml of toluene to form
ESIꢀMS spectra were acquired on a Micromass LCT ESIꢀTOF mass
stock solution
S and proceeded to selfꢀassembly.
spectrometer equipped with an orthogonal electrospray ionization
1
13
source. H NMR and C NMR spectra were recorded on Varian
Unity Plus 200 and 500 MHz spectrometers, respectively, using
Self-assembly of thioacetate derivative on surface of gold
electrode
CDCl as a solvent. The infrared experiments were carried out using
3
the Nicolet 8700.
Attempts to obtain selfꢀassembled monolayer (SAM) of fullerene
thioacetate at the gold electrode surface without previous
deprotection procedure failed, so we can assume that from the
mixture of deprotected fullerene derivative and fullerene thioacetate
only the first one should deposit on Au surface. Selfꢀassembly of
deprotected ATF-C60 on gold electrode was done by dipping the
gold substrates into the solution S diluted 100x with toluene or 1,2,4ꢀ
trichlorobenzene for 24h at RT. Resulting films were washed with
the corresponding solvent and dried under stream of nitrogen. After
successful approach with chemisorption of C60 derivative on the
surface of gold electrode we decided to synthesize gold
nanoparticles covered with C60 using deprotected fullerene
thioacetate as a ligand.
XPS measurements were carried out using a VG ESCALAB 210
electron spectrometer equipped with an Al Kα source (1486.6 eV).
XPS data were calibrated using the binding energy of C1s 285.0 eV
as the internal standard.
The morphology and size of obtained gold nanoparticles were
obtained using a Libra TEM (Carl Zeiss) operating at 120 kV.
UVꢀVis spectra were collected from 300 to 700 nm using Evolution
6
0 UVꢀVis Spectrophotometer from Thermo Scientific and a quartz
glass cuvette.
The Dynamic Light Scattering (DLS) measurements were performed
using Zetasizer Nano ZS from Malvern Instruments.
Thermogravimetric analysis (TG) was performed under high purity
nitrogen atmosphere using TA Instruments Q50 Thermal
Gravimetric Analyzer with heating rate of 5K/min.
Synthesis of gold nanoparticles
CV (Cyclic Voltammetry) and DPV (Differential Pulse
Voltammetry) experiments were done using the Autolab potentiostat
(ECO Chemie, Netherlands) in a three electrode arrangement, with a
silver/silver chloride (Ag/AgCl) electrode as the reference, platinum
foil as the counter and the glassy carbon electrode (GCE, BAS, 3mm
diameter) or the modified gold electrodes (gold (200nm) evaporated
on glass with a preꢀlayer of 2ꢀ4 nm of chromium (Gold Arrandee,
Gmbh) as the working electrodes. The reference electrode was
separated from the working solution by an electrolytic bridge filled
with 0.1M TBAHFP/toluene/AN (4:1) solution. The reference
potential electrode was calibrated using the ferrocene (Fc) redox
process in the same TBAHFP/toluene/AN solution, which was also
used as the supporting electrolyte. Argon was used to deaerate
solution and an argon blanket was maintained over the solution
during the experiments. All experiments were carried out at 25°C.
The Atomic Force Microscopy (AFM) images were acquired using
Agilent Technologies 5500AFM (Agilent Technologies, Santa Clara,
CA, USA). The nonꢀcontact mode was used for imaging. Ultra sharp
probes were purchased from NANOSENSORS (SSSꢀNCLR with 2
nm tip radius). Single gold monocrystal was applied as substrate for
AFM measurements. The atomic flat Au (111) surface at
monocrystal surface was used for image acquisition. Substrate
before fullerene adsorption and AFM imaging was flame annealed
and cleaned in piranha solution (concentrated H SO /30% H O 3:1
Gold NPs were prepared using a modified BrustꢀSchiffrin method
29
reaction. An aqueous solution of hydrogen tetrachloroaurate (III)
30 mL, 30 mM) was extracted with 80 mL of tetraoctylammonium
(
bromide (2.19 g, 4 mmol) solution in toluene. Most of the
tetrachloroaurate ions were transferred to the organic layer as the
ꢀ
6
yellow aqueous layer faded until colorless. 1.44 ꢁL (8.3ꢂ10 M) of 1ꢀ
octanethiol was added to the organic phase, and the mixture was
reduced with freshly prepared aqueous solution of sodium
borohydride (378mg, 200 mmol in 25 mL of deionized water). A
solution of borohydride was very slowly added under vigorous
stirring. After further stirring for 3 h, the organic phase was
separated, evaporated to 10 mL in a rotary evaporator, and mixed
with 400 mL of absolute ethanol to precipitate NPs. The mixture was
kept for 12 h at ꢀ4 °C. The dark brown precipitate was sonicated for
6
0 s and centrifuged (5 min, 13 000 rpm). Again, the precipitate was
dissolved in a small amount of toluene (10 mL), precipitated with
ethanol (400 mL), and centrifuged. Finally, all samples were
dissolved in 10 mL of toluene. Obtained nanoparticles were
characterized by TEM, UVꢀVis and DLS measurements.
AuNPs Ligand-Exchange Reaction
2
4
2
2
An organic ligand deprotected ATF-C60 toluene solution S (10mL)
was added to the toluene solution of octanethiol stabilized gold NPs
v/v) for 30 minutes and rinsed with MilliꢀQ ultrapure water. AFM
images of fullerene derivativeꢀmodified Au substrate were recorded
after its immersion in fullerene solutions for 24 hours.
(200 ꢁL), and the reaction proceeded for ca. 72 h upon mixing at
room temperature. As no precipitation and/or color change occurred,
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Results and discussion
TEM measurements
Fig. 3. UVꢀVis spectra of bare octanethiol AuNPs, ATF-C60
and ATF-C @AuNPs collected from 300 to 700 nm.
One or two droplets of asꢀprepared nanoparticle solutions
diluted with toluene to a very small concentration were placed
on a standard copper grid (400ꢀmesh) coated with carbon. The
6
0
grid was left overnight to dry. The TEM image of the Dynamic Light Scattering
synthesized AuNPs is shown in Fig. 2. The dispersed
nanoparticles have an average diameter of 4.72 nm.
Size of single fullerene thioacetate molecule was theoretically
estimated from fully optimized geometry structure using the
30ꢀ33
DFT/B3LYP/6ꢀ31g*,
approach to be 1.37nm. Dynamic Light
Scattering measurements (Figure S11, Electronic Supplementary
Information) sized the bare octanethiolꢀAuNPs as 4.71nm which is
in excellent agreement with TEM results (4.72nm). AuNPs coated
with fullerene derivative indicated the average diameter of 7.27nm
(Figure S12, Electronic Supplementary Information). Substracting
the size of fullerene coating which increases the whole size of
nanoparticle by 2.74nm, the average size of coated nanoparticle can
be assessed to be 4.53nm, thus slightly less than that from DLS
measurements. This induces that we clearly obtain a monolayer and
the orientation of fullerene substituents is probably not perpendicular
to the AuNP surface but slightly tilted from the surface normal.
FTIR spectroscopy of SAMs
Fig. 4. shows FTIR spectra of both ATF-C60 and its
deprotected SAMs on the surface of gold electrode.
Deprotection of ATF-C60 and its deposition on Au surface
causes several changes in the infrared spectrum. The intense
ꢀ
1
signal at 1708 cm attached to the C=O stretching mode almost
completely disappeared confirming the efficient deprotection of
thioacethyl group and removal of Nꢀacetylpyrrolidine. Four
characteristic fulleropyrrolidine bands are visible in both
spectra and their positions are similar. Bands at 1113ꢀ1114
Fig. 2. TEM image of AuNPs with average size of 4.72 nm
obtained at 120 kV.
−
1
−1
cm and 1423ꢀ1424 cm are associated with C60 fullerene
cage and contribution of pyrrolidine ring is visible in the range
UV-Vis Spectroscopy
−
1 34
of 2750ꢀ3000 cm .
UVꢀVis spectroscopy was used to control the formation of
ATF-C -AuNPs by observation of a peak referring to the
6
0
maximum of absorption at 522 nm. Data were collected from
00 to 700 nm for 3 different species: the starting solution of
3
AuNPs (20 µL of NPs diluted 40x with toluene), the solution of
gold nanoparticles modified with fullerene derivative (250 µL
of modified NPs diluted 3.2x with toluene) and ATF-C60
solution in toluene (C = 0.0176 mg/mL). As we can observe in
Fig. 3. fullerene functionalized nanoparticles retain peak
coming from AuNPsꢀoctanethiol (λmax = 522 nm) and show
additional one referring to the maximum of absorption at 336
nm for C60 derivative ATF-C60 indicating the successful
1
2
exchange of the ligand.
Fig. 4. IR spectra of ATF-C60 in KBr pellet (A) and the
reflectance spectrum of the thin film obtained after its
deprotection and selfꢀassembly from solution
S diluted 100x
with 1,2,4ꢀtrichlorobenzene (B).
Atomic Force Microscopy
The surface morphology of deprotected ATF-C60 layer deposited, on
the Au(111) surface, from pyrrolidine free solution S diluted 100x
4
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with two different solvents: 1,2,4ꢀtrichlorobenzene (Fig.5A) and
toluene (Fig.5B) was studied by AFM. Both images provide a
precise insight into the surface morphology and allow for assessing
the influence of the solvents used for the selfꢀassembly on the
topography of the films of deprotected thiolated fullerene derivatives
deposited on gold. In both cases, the gold surface is covered by small
granular aggregates. Analysis of the AFM images enables us to
evaluate the average size of these grains. Diameter of the smallest
fullerene aggregates was estimated to be in the range of 65ꢀ80 nm in
the case of toluene and 10ꢀ15 nm for 1,2,4ꢀtrichlorobenzene. In
addition, there are more morphological differences between the films
obtained in toluene and trichlorobenzene solutions. Namely, the Fig. 6. Partial XPS spectra of the S2p and N1s regions for films
aggregated grains in the Figure 5B have no clearly marked spherical
shapes and exhibit a tendency to form large domains with diameters
up to ca. 200 nm which are visible as bright spots. Comparing
images in Figure 5A and 5B it is obvious that the morphology of
fullerene aggregates depends strongly on the solvent properties.
Smaller fullerene grains obtained in case of 1,2,4ꢀtrichlorobenzene
solution are not surprising, considering dielectric constant, ε of each
solvent and the higher solubility of fullerene derivatives in 1,2,4ꢀ
obtained using in situ ATF-C60 deprotection method.
Results for the N1s region were deconvoluted into two peaks (Fig.
6
B). According to literature data the bigger one observed at 399.7
4
0
eV is assigned to nitrogen atoms from fulleropyrrolidine with
atomic percent value of 81 at%. Second peak with the lower binding
41
energy of 398.4 eV is assigned to pyrrolidine , indicating that amine
remains in obtained film (19 at%). However, taking into account the
small molecular mass of pyrrolidine in comparison to the
functionalized fullerene molecule the overall weight percent of both
components are 98 wt% for fullerpyrrolidine and only 2 wt% for
pyrrolidine. These results show that in situ deprotection of aromatic
thioacetate can be used whenever the thickness of the film and
35
trichlorobenzene, both factors favor the formation of smaller
3
6
aggregates as clearly demonstrated in the presented figures.
Obtained AFM results confirm earlier reports that fullerene
derivatives can not selfꢀassemble into ordered monomolecular
37,38
layers.
remaining small amount of amine
requirements.
still meet application
In the S2p region of the XPS spectra of the thin film obtained using
pyrrolidine free solution S of deprotected ATF-C60 dilluted 100x
with 1,2,4ꢀtrichlorobenzene (Fig. 7A) one doublet at 161.9eV and
1
63.1eV is observed, with expected 2:1 ratio. This results show that
all sulfur atoms in the investigated sample are bound to gold surface.
Fig. 5. Nonꢀcontact AFM images of selfꢀassembled films of
deprotected ATF-C60 on Au(111) from solution diluted 100x
with A) 1,2,4ꢀtrichlorobenzene, B) toluene.
S
XPS Spectroscopy Results
Xꢀray photoelectron spectroscopy was used to assess the
composition of nanomaterials obtained from selfꢀassembly of in situ
deprotected thioacetate, chemisorption after removal of pyrrolidine
and Nꢀacetyl pyrrolidine as well as of nanoparticles functionalized
with C60 fullerenes. XPS spectra of all investigated samples show the
presence of gold, sulfur, nitrogen, carbon while the nanoparticles
sample also contained bromine atoms. Binding energies obtained
from XPS spectrum confirmed the presence of functionalized
fullerenes on the surface of gold electrode and gold nanoparticles
attached to Au by the sulfur atoms.
Fig. 7. Partial XPS spectra of the S2p and N1s regions for films
obtained using pyrrolidine free solution of deprotected ATF-
C60. S2p :S2p intensity ratio 2:1 and splitting of 1.2 eV.
3/2
1/2
The N1s photoelectron peak appears at 399.8eV (Fig. 7B) and we
can assign it to the fulleropyrrolidine nitrogen atoms. As a result, the
pyrrolidine free mixture lead to deposition of functionalized
fullerenes without any impurities and formation of thin film on the
gold electrode surface.
The S2p region of spectrum obtained for the in situ deprotection
sample (Fig. 6A) shows two doublets, each with area ratios of 2:1
and splitting of 1.2 eV. First one is observed at 161.9 eV and 163.1
eV assigned to S2p3/2 and S2p1/2 respectively, this binding energy
Results obtained for AuNPs covered with functionalized fullerenes
reveal a doublet in the S2p region (Fig. 8A) (2.0 at% overall atoms).
It was deconvoluted into two peaks centered at 162.0 and 163.2eV
corresponding to S2p3/2 and S2p1/2 atoms, confirming that all ATF-
C60 molecules are bound to the surface of gold nanoparticles by Sꢀ
Au bonds and, no fullereneꢀgold interaction is observed. A doubleꢀ
peak observed in the N1s scanning range (Fig. 8B) from 396 to 404
eV was deconvoluted with very good correlation in two peaks with
binding energies 399.3 eV and 402.1 eV associated with
3
9
value is typical for the S–Au bond and it was found to be 53 at% of
all sulfur atoms in the sample. Second doublet at 163.8 eV for S2p3/2
and 165.0 eV for S2p corresponds to surface unbound sulfur
1
/2
7
atoms and equals remaining 47 at%. These results are in good
agreement with AFM data suggesting that the proposed in situ
method results in thick multilayer formation.
42
fulleropyrrolidine and quaternary ammonium nitrogen respectively
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(38 at% and 62 at% of all nitrogen atoms, 0.8 at% and 1.3 at%
overall atoms). XPS spectrum of the Br3d region confirms the
presence of bromine from tetraoctylammonium salt with intensity
1
.1 at% overall atoms which is in good correlation with results
obtained for their counter tetrabutylammonium ion nitrogen atoms
1.3%). Results presented above allow to calculate ATF-C60
(
:
TOAB: octanethiol ratio on AuNPs surface from corresponding
signals (0.8at%: 1.3at%: (2% of total S amount ꢀ 0.8 at% S from
ATF-C )), that is exactly 1:1.625:1.5.
6
0
Fig. 9. Results of TG analysis obtained for the AuNPs covered
with deprotected ATF-C60
.
Fig. 8. Partial XPS spectra of the S2p and N1s regions for gold
nanoparticles functionalized with deprotected ATF-C60
.
Electrochemical behavior of thioacetate derivative in the
solution
In summary, thioacetate group is not observed according to
XPS results for all samples, which is confirmed by lack of
4
3,44
S2p3/2 signal at around 163.eV, typical for SꢀC bond.
. Thus
Electrochemical properties of C60 and thioacetate derivative ATF-
C60 in 0.1M TBAHFP toluene/acetonitrile solution were investigated
using voltammetry (CV and DPV). The fullerene moiety preserves
sequential oneꢀelectron reversible redox processes characteristic of
confirming the efficiency of deprotection and selective
deposition of free thiol instead of Sꢀacetyl functionalized
fullerene from any remaining traces of protected compound.
Moreover, the absence of peaks above 166 eV for S2p region
indicates that the obtained fullereneꢀgold nanocomposites are
free from oxidized sulfur species usually found at higher
1,47
the extended πꢀelectron system (Fig.10 A). The oneꢀtoꢀone ratio
of CV peak currents and the close to 60 mV peakꢀtoꢀpeak separation
confirm the reversibility. The thioacetate derivative exhibits the
voltammetric behavior similar to that of C60, however, the peak
potentials are shifted to more negative values due to the presence of
linker, and the peak currents are much smaller indicating smaller
diffusion coefficients of the derivative compared to that of
unmodified fullerene.
Four reversible peaks were also recorded in the DPV
experiment. The fullerene centered processes appear again at
more negative potentials, compared to of C60 (Fig. 10B). Such
shifts are due to the presence of a substituent containing an
electron donating moiety, in this case the aromatic ring. As the
result, the fullerene derivative becomes a weaker electron
acceptor than the unsubstituted molecule, however the
reversibility of each of the first 4 steps of the electrode
processes is retained. All peak currents of the thioacetate
derivative are smaller than those of the unsubstituted fullerene
reflecting the smaller diffusion coefficients ꢀ the molecule is
larger due to the presence of the substituent.
4
5
binding energies.
Thermogravimetric analysis
TG analysis results (Fig. 9.) showed that 8.5% of total mass of
nanoparticles is organic coating and remaining 91.5% was
clearly visible pure Au. Assuming the spherical shape of
nanoparticles, the average number of gold atoms per AuNP
(
N ) was calculated according to Eq. (1) proposed by Huo et
Au
4
6
al. whereas the number of fullerene ligands per nanoparticle
N ) was determined using XPS results by Eq. (2) (where
(
L
%
wtorg and %wtAu are thermogravimetrically determined
percentage mass values for organic coating and Au core
respectively, D is the average diameter of AuNPs in nm, ML
and Moct are molecular masses of corresponding ATF-C60 and
octanethiol thiolate anions, MAu is atomic weight of gold and
MTOAB stands for the molecular mass of tetraoctylammonium
bromide) and was found to be ca. 30. The remaining organic
mass at the AuNP surface consists of tetraoctylammonium ions
and octanethiol molecules. Note, that the left factor in the Eq.
(
2) corresponds to the total mass of organic coating and the
right one is connected to the fullerene content.
ꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁꢂꢃ ꢄ ꢅꢆ. ꢇꢈꢉꢆꢊꢋꢅ
(1)
(2)
%
ꢍꢎꢏꢐꢑ∙ꢒꢂꢃ∙ꢅꢆ.ꢇꢈꢉꢆꢊꢋꢅ
ꢓ
ꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ ꢄ
∙ ꢓ∙ꢒ
ꢌ
%
ꢍꢎꢂꢃ
ꢌ
ꢔꢓ.ꢉꢊꢕ∙ꢒꢖꢗꢂꢘꢔꢓ.ꢕ∙ꢒꢏꢙꢎ
Fig. 10. Voltammograms of (black line) C60 and (red line)
compound ATF-C60 in 0.1 M TBAHFP/ toluene/acetonitrile
(4:1), (A) CV, v = 100 mV/s; (B) DPV, tp= 3 ms, ꢃE = 50 mV
ꢇ.ꢕ∙ꢓꢈꢚ.ꢆ∙ꢅꢆ.ꢇꢈꢉꢆꢊ∙ꢛ.ꢚꢊꢅ
ꢓ
ꢀ
ꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ ꢄ
∙
ꢌ
ꢈꢓ.ꢕ
ꢓ∙ꢇꢇꢛ.ꢈꢔꢓ.ꢉꢊꢕ∙ꢕꢛꢉ.ꢚꢔꢓ.ꢕ∙ꢓꢛꢕ.ꢅ
(
characteristic parameters of the voltammetric behavior of
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ ꢄ ꢊꢈ. ꢇꢉ
ꢜ
6
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4RA121 B
ART39ICLE
2
compound ATF-C60 are given in Electronic Supplementary
C @AuNPs and pure fullerene derivative is linear with R = 0.982
60
Information Table S1).
and 0.991, respectively (Figure 13). It indicates that the process is
controlled by diffusion.
Application of deprotected thioacetate derivative ATF-C60
for Au electrode and AuNPs modification
The C60 thioacetate derivative following its deprotection was selfꢀ
assembled on the gold electrode surface. The cyclic voltammogram
of the gold electrode modified with selfꢀassembled monolayer of C60
derivative recorded in acetonitrile containing 0.1M TBAHFP is
shown on Fig.11 (A).
Fig. 12. Voltammograms of (black line) ATF-C60 and (red line)
the C60 derivative tethered gold nanoparticles (ATF-
C @AuNPs) in solution 0.1 M TBAHFP/ toluene/acetonitrile
6
0
(
4:1), CV, v=100 mV/s.
Fig. 11. Cyclic voltammogram of deprotected C60 derivative
ATF-C60 (A) selfꢀassembled monolayer recorded in 0.1 M
TBAHFP solution in AN, v = 1V/s and (B) desorption of
adsorbed monolayer in 0.5M KOH in water, v=50mV/s.
The two redox peaks observed in Figure 11A, correspond to the first
and second redox step of the immobilized fullerene derivatives. The
redox potentials are close to the values reported for fullerene lipoic
acid derivative or fullerene with long alkane thiol chain selfꢀ
4
8,49
assembled on the gold substrates.
The peak currents increase
linearly with scan rate as predicted for the surface immobilized
species. The surface concentration of the deprotected ATF-C60
derivative was calculated based on Eq. (3):
1/2
Fig. 13. Dependence of peak current versus v for ATF-
C @AuNPs (A) and the fullerene derivative ATF-C60 in a
6
0
solution 0.1 M TBAHFP/ toluene/acetonitrile (4:1) (B).
ꢝ ꢄ ꢞ⁄ꢟꢜꢂ
(3)
where n is the number of exchanged electrons, A is the electrode
2
ꢀ2
surface area in cm , Γ is the surface concentration in mol cm , Q is
Conclusions
the charge under the peak in C. The surface concentration of the
fullerene is 7.1ꢂ10 mol cm .
ꢀ
11
ꢀ2
A new procedure for efficient modifiction of gold surfaces with
fullerenes has been elaborated. Firstly, a new aromatic thioacetate
functionalized fullerene derivative ATF-C60 was synthesized in a
two step procedure and was thoroughly characterized using a wide
range of analytical techniques. The electrochemical behavior of the
thioacetate functionalized fullerene derivative was evaluated in
toluene/acetonitrile solution and 4 reversible sequential 1e electrode
processes were resolved both in the cyclic and differential pulse
voltammograms
In the next step, the newly synthesized molecules following suitable
deprotection, were used for the preparation of the C60 fullerene–
modified gold electrode, and C60 fullereneꢀmodified gold
nanoparticles. The proposed deprotection procedure for Sꢀacetylꢀ
derivatized aromatic fullerene is very efficient and allows the control
of the thickness of the selfꢀassembled fullerene layer providing
complete removal of the excessive pyrrolidine and resulting Nꢀ
acetylpyrrolidine. Otherwise, traces of the deprotection reagents can
promote multilayer formation as found from the XPS data.
ΓCV is calculated from the charge of the fullerene reduction peak
while Γdes is obtained from the charge of the reductive desorption
peak of the compound from the Au surface. Assuming that fullerene
2
occupies on the surface ca. 95Å , the surface concentration should be
ꢀ
10
ꢀ2
1
.75ꢂ10
mol cm . The surface concentration Γ
is slightly
des
ꢀ
11
ꢀ2
smaller (8.3ꢂ10 mol cm ) than the value corresponding to a
monolayer. The values of ΓCV are similar to those of Γdes
.
Figure 12 compares the cyclic voltammograms of ATF-C60 and of
the tethered gold nanoparticles, ATF-C @AuNPs. It can be seen
6
0
that there are four pairs of voltammetric waves for the ATF-C60
solution (red curve), which are all close to reversible. For the ATF-
C @AuNPs, a series of two wellꢀdefined voltammetric peaks at ꢀ
60
0
0
.56 and ꢀ0.95V can be found within the potential range of ꢀ1.4 to
V. These signals can be ascribed to the successive 1ē reduction
0
/ꢀ1
ꢀ1/ꢀ2
steps of the C
molecules, i.e., C60
and C60
couples,
The capacitance is much larger for the ATF-
C @AuNPs compared to the faradaic process. Interestingly the
6
0
4
7,50,51
respectively.
60
shift of peak potentials indicates stronger electron acceptor
properties of the fullerenes when bound to gold nanoparticles
compared to those of ATF-C60 dissolved in the solution. The
The described twoꢀstep approach proved to be an efficient way for
the preparation of fullerene modified gold nanoparticles.
Interestingly, the gold nanoparticles coverage is higher than
achieved using oneꢀstep procedures. DLS and TEM measurements
1
/2
dependence of peak current versus v
in both cases ATF-
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show that octanethiolꢀAuNPs of 4.71nm diameter following ligand
exchange procedure gave fullerene modified AuNPs of 7.27nm
diameter with high population of 30 bound fullerene moieties per
one nanoparticle. Anchoring the fullerene on gold nanoparticle lead
also to improved acceptor properties of the fullerene moieties that
was confirmed by easier uptake of electrons from the electrode in the
electrochemical experiments.
16 A. Pinchart, C. Dallaire and M. Gingras, Tetrahedron Lett.,
998, 39, 543ꢀ546.
1
1
1
7
8
M. Prato and M. Maggini, Acc. Chem. Res., 1998, 31, 519ꢀ526.
A. Van den Hoogenband, J.H.M. Lange, R.P.J. Bronger, A.R.
Stoit and J.W. Terpstra, Tetrahedron Lett. 2010, 51, 6877ꢀ
6881.
1
2
9
0
Wallace, O.B.; Springer, D. M. Mild, Tetrahedron Lett. 1998,
39, 2693–2694.
Acknowledgements
We gratefully acknowledge the financial support from the
National Science Centre (Projects: Piotr Piotrowski
H. Valkenier, E.H. Huisman, P.A. van Hal, D.M. de Leeuw,
R.C. Chiechi and J.C. Hummelen, J. Am. Chem. Soc., 2011,
2
2
012/07/N/ST5/01966 and Joanna Pawłowska
011/03/N/ST5/04825). Theoretical calculations were
133, 4930ꢀ4939.
2
2
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A. Shaporenko, M. Elbing, A. Blaszczyk, C. von Hanisch, M.
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performed using the computational resources of the
Interdisciplinary Center for Mathematical and Computational
Modeling at Warsaw University (Grant G15ꢀ11).
K. Kokubo, S. Shirakawa, N. Kobayashi, H. Aoshima and T.
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