Assembly of gold nanoparticles on functionalized Si(100) surfaces through pseudorotaxane formation

Article abstract of DOI:10.1002/chem.201204318

The assembly of gold nanoparticles (AuNPs) on a hydrogenated Si(100) surface, mediated by a series of hierarchical and reversible complexation processes, is reported. The proposed multi-step sequence involves a redox-active ditopic guest and suitable calix[n]arene-based hosts, used as functional organic monolayers of the two inorganic components. Surface reactions and controlled release of AuNPs have been monitored by application of XPS, atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM) and electrochemistry. Copyright

Full text of DOI:10.1002/chem.201204318

FULL PAPER
DOI: 10.1002/chem.201204318
Assembly of Gold Nanoparticles on Functionalized Si
Pseudorotaxane Formation
ACHTUNGERTN(NUNG 100) Surfaces through
Alice Boccia,[a] Fabio DꢀOrazi,[a] Elena Carabelli,[b] Rocco Bussolati,[b] Arturo Arduini,[b]
Andrea Secchi,*[b] Andrea G. Marrani,[a] and Robertino Zanoni*[a]
Abstract: The assembly of gold nanoparticles (AuNPs) on a hydrogenated SiACTHNUTRGNE(UNG 100)
surface, mediated by a series of hierarchical and reversible complexation processes,
is reported. The proposed multi-step sequence involves a redox-active ditopic
guest and suitable calix[n]arene-based hosts, used as functional organic monolayers
of the two inorganic components. Surface reactions and controlled release of
AuNPs have been monitored by application of XPS, atomic force microscopy
(AFM), field-emission scanning electron microscopy (FESEM) and electrochemis-
try.
Keywords: calixarenes
chemistry · gold · nanoparticles ·
surface chemistry
·
electro-
Introduction
cal bonds.[12–14] The combination of a stable anchoring bond
at the interface with the supramolecular interaction offered
by the attached receptors leads to specific recognising sys-
tems, which can be considered prototypical of more complex
inorganic–organic hybrids.[15–17] In the latter context, we
have shown that ditopic bis-pyridinium-based guests are
able to promote the self-assembly of calix[4]arene-protected
gold nanoparticles (AuNPs) in low polar solvents.[18] More
recent efforts have been aimed at bringing the properties of
molecular-machine prototypes to the interface by anchoring
calix[6]arene-based pseudorotaxanes onto inorganic surfaces
through their axial component.[14,19] On this basis, we report
herein the reversible assembly of gold nanoparticles on an
The anchoring of nanoparticles at specific surface sites is a
process of large basic and technological importance, since it
can modulate relevant properties such as those exhibited by
nanowires, semiconductors, nanocapacitors and nanomag-
nets. Consider, for example, the social impact of the devel-
opment of new flash memories based on metallic nanoparti-
cles, devices that are nano-miniaturised, that have low
power consumption and could store an entire database in
ultra-small spaces.[1] Moreover, an important step toward
the generation of novel integrated devices is based on the
postulated possibility to transport light under the diffraction
limit by using ordered arrays of close-spaced metal nanopar-
ticles placed on technologically relevant surfaces such as sili-
con (plasmonic waveguides).[2–4] Silicon is an attractive inor-
ganic platform, as it offers the possibility to make robust
SiACHTNUTRGNEUNG(100) surface mediated by a series of hierarchical com-
plexation processes. These last involve a redox-active ditopic
guest and suitable calix[n]arene-based hosts that are used as
functional organic monolayers of the two inorganic compo-
nents.
À
and durable devices by forming stable Si C covalent
bonds.[5,6] Several reports on organic monolayers formed
À
through a covalent Si C bond have been published so far by
several research groups,[7–9] including ours.[10,11]
Results and Discussion
Supramolecular interactions have now started to diffuse
into the realm of Si-anchored receptors, thus extending the
control of reversibility that is typical for this type of chemi-
Design and synthesis: Calix[6]arene Cx6 (see Scheme 1) was
used for the covalent functionalisation of hydrogenated Si-
AHCTUNGTREG(NNNU 100) surfaces. This compound was identified as a good can-
didate for Si coating because it presents three w-unsaturated
C11 alkyl chains on its lower rim. Such long arms act as an-
choring points that favour an upward orientation of the aro-
matic cavity of Cx6, which can thus retain its recognition
properties.
[a] Dr. A. Boccia, Dr. F. DꢀOrazi, Dr. A. G. Marrani, Prof. R. Zanoni
Dipartimento di Chimica
Universitꢁ degli Studi di Roma “La Sapienza”
Piazzale Aldo Moro 5, 00185 Roma (Italy)
E-mail: robertino.zanoni@uniroma1.it
[b] Dr. E. Carabelli, Dr. R. Bussolati, Prof. A. Arduini, Prof. A. Secchi
Dipartimento di Chimica
Calix[6]arene Cx6 was synthesised in three steps starting
from known compounds (see Scheme 1). Calix[6]arene 1
was alkylated to its phenolic groups with the undec-10-enyl
tosylate 2 in 70% yield. The three nitro groups of 3 were
then quantitatively reduced to amino groups in ethyl acetate
under reflux conditions by using SnCl2·2H2O as the regiose-
Universitꢁ degli Studi di Parma
Parco Area delle Scienze 17/A, 43124 Parma (Italy)
E-mail: andrea.secchi@unipr.it
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204318.
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vent the calix[6]arene macrocycle adopts a cone conforma-
tion on the NMR spectroscopic timescale. Indeed the six
bridging methylene protons resonate as a typical AX system
of two doublets at d=4.72 and 3.68 ppm with a coupling
constant (J) of 15 Hz. The three w unsaturations give rise to
a characteristic system of two doublets at d=5.05 and
4.5 ppm and a multiplet centred at d=5.8 ppm and thus in-
directly confirmed the effectiveness of the regioselective re-
duction of the nitro groups with respect to the alkene termi-
nations. As typical of these type of calix[6]arene
“wheels”,[20,21] the three methoxy groups present in the
lower rim of the macrocycle experience a large shielding
effect from the aromatic rings that bear the phenylurea
groups, thus resonating at high fields (d=2.87 ppm).
Grafting and characterisation of calix[6]arenes on the Si-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
before use in the anchoring process, conducted in Schlenk
tubes. The method adopted for the surface anchoring re-
quires visible-light activation (see the Supporting Informa-
tion).[10,11,22–24] This extra-mild approach preserves the integ-
rity of the molecular species and effectively limits the sur-
face oxidation of the silicon substrate.
Scheme 1. Reagents and conditions: i) K2CO3, CH3CN, reflux, 7 d;
ii) SnCl2·2H2O, AcOEt, reflux, 36 h; iii) C6H5NCO, CH2Cl2, RT, 3 h.
lective reducing agent. Reaction of the amino groups of 4
with phenylisocyanate yielded Cx6 in 20% overall yield. The
1H NMR spectra of Cx6 dissolved in C6D6 (see Figure S1 in
the Supporting Information) shows evidence that in this sol-
The characterisation of the functionalised Si surface (Si/
Cx6) was carried out through XPS measurements (see
Figure 1 and the Supporting Information). The Si 2p region
of the XPS spectra (see Figure 1a) shows, in addition to the
Figure 1. a) Si 2p and b) N 1s XPS regions of Si/Cx6 surface. N 1s XPS region after complexation reaction of Si/Cx6 surface with c) viologen DOV, d) bi-
functional guest 8 and e) after the hierarchical assembly of AuNPs NP (Cx4) (dꢀ5 nm).
(Cx4) (dꢀ5 nm). f) Au 4f XPS region of Si/Cx6/8/NP
A
ACHTUNGTRENNUNG
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Gold Nanoparticle Assembly
FULL PAPER
Table 1. XPS relative quantitative ratios (Æ10%) and binding energies (BE) for the Si/Cx6 surface as prepared and after complexation reactions at the
interface.[a]
Si 2p
N 1s
Au 4f
Designation
C/N
17 (18.5) 0.037
N/Si NH/N+ Au/Si Au/N(tot) Au/N+ BE [eV] Assignment BE [eV] Assignment BE [eV] Assignment
ACHTUNGRTENNUGN
Si/Cx6
–
–
–
–
–
–
–
–
–
99.7
99.9
103.5
Si bulk
401.0
401.0
NH
–
–
–
À
À
Si C/Si H
SixOy
Si/Cx6/DOV
Si/Cx6/8
17 (17.1) 0.042 2.9 (3)
99.7
99.9
103.5
Si bulk
NH
N+
–
À
À
Si C/Si H 403.2
SixOy
22 (15.9) 0.021 1.9 (2)
0.030 2.0 (2)
–
99.7
103.6
Si bulk
SixOy
400.8
403.0
NH
N+
–
–
Si/Cx6/8/NP
Si/Cx6/8/NP
(Cx4)[b] 38
0.009 0.28
1.3
8.7
99.7
103.6
Si bulk
SixOy
400.7
403.2
NH
N+
85.0
AuI
(Cx4)[c] 35
0.072 1.8 (2)
0.19
2.6
99.7
103.8
Si bulk
SixOy
400.9
403.0
NH
N+
84.3
85.1
86.5
Au0
AuI
AuIII
Si/Cx6/NP
(Cx4)[c]
14
0.054 1.8 (2)
0.005 0.10
–
99.7
103.5
Si bulk
SixOy
400.8
NH
84.4
85.4
86.3
Au0
AuI
AuIII
[a] Theoretical values in parentheses. [b] Mean core size of 1 nm as determined by TEM measurements. [c] Mean core size of 5 nm.
main 2p3/2,1/2 spin–orbit doublet due to bulk Si, a second dou-
blet with its main component located at 99.9 eV, assigned on
gold nanoparticles (AuNPs) on the silicon substrate. To this
end, the new supramolecular linker 8 was devised and syn-
thesised in 50% overall yield (see Scheme 2). With respect
to DOV, the redox-active 4,4’-bipyridinium core of 8 has
been linked through a flexible dodecyl chain to another pyr-
idinium ring. This latter structural motif has been indeed
successfully used for the supramolecular grafting of calix[4]-
arene-protected AuNPs onto inorganic surfaces.[27]
the basis of the literature[25] to the superficial Si (Si H2 ter-
À
À
minations) and the Si C components. A small broad feature
centred at 103.5 eV is due to a very limited oxidation of the
Si surface that occurs upon reaction with Cx6. The N 1s
region (see Figure 1b) presents an NH peak component at
401 eV. This peak is diagnostic of the effective anchoring of
Cx6 onto the Si surface.
The structural integrity of the attached monolayer and the
covering degree of the Si surface was evaluated considering
the relative quantitative ratios of diagnostic elements (see
Table 1 and the Supporting Information). The calculated C/
N ratio is, within the experimental error, in good agreement
with the stoichiometry of Cx6, thus confirming the mildness
of the photoactivation process employed.
The functionalised Si/Cx6 surface was then exposed to a
solution of 1,1’-dioctyl-4,4’-bipyridinium diiodide (DOV) in
95:5 CH2Cl2/CH3OH. Indeed, it is known that this redox-
active viologen salt forms very stable and electrochemically
switchable pseudorotaxane complexes with calix[6]arene
macrocycles similar to those linked on the Si substrate.[26]
The simultaneous presence in the N 1s region of two peak
components (see Figure 1c) due to the NH groups of Cx6
and the N+ from the pyridine rings of 1, respectively, indi-
cates the establishment of the pseudorotaxane complex to
an extent that was quantified by taking the experimental rel-
ative atomic ratios between the two N 1s components. The
experimental NH/N+ ratio matched the value expected
from the pseudorotaxane stoichiometry (see Table 1).
The previous findings prompted us to explore the possibil-
ity of exploiting a series of successive and hierarchical com-
plexation processes to promote the reversible assembly of
Scheme 2. Reagents and conditions: i) pyridine, CH3CN, reflux, 48 h;
ii) CH3CN, reflux, 48 h.
To demonstrate the feasibility of the whole process, a
simple experiment was devised (see Scheme 3): the Si/Cx6
surface was exposed through a dip-coating process to a solu-
tion of guest 8 in 95:5 CH2Cl2/CH3OH (see the Supporting
Information for the experimental details). XPS measure-
ments (see Figure 1d and the Supporting Information) veri-
fied that the outcome of the threading reaction at the inter-
face was almost quantitative (see the NH/N+ ratio in
Table 1).
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A. Secchi, R. Zanoni et al.
Figure 2. a) AFM image (needle sensor, in an ultra-high vacuum) show-
ing a portion (300ꢃ300 nm) of the Si/Cx6/8 surface after its exposure to a
solution of approximately 5 nm NPACHTNUGTRENUNG(Cx4) AuNPs; b) enlarged image
(150ꢃ150 nm) of the same area; c) FESEM image of the same sample
taken at 600 KX magnification (500ꢃ375 nm2); d) enlarged image (190ꢃ
190 nm) of the same area showing a hexagonal motif in the packing sym-
metry.
Scheme 3. Schematic representation of the formation of pseudorotaxane
complexes at the interface between a Si(100) surface coated with calix[6]-
arene-based “wheels” Cx6 and the viologen-based “axle” (8) present in
AHCTUNGTRENNUNG
After XPS characterisation, the Si/Cx6/8/NPACTHNUTRGNENG(U Cx4) (d
ꢀ5 nm) system was investigated through atomic force mi-
croscopy (AFM) and field-emission scanning electron micro-
scopy (FESEM) measurements. In Figure 2a and b, a region
of the Si surface (300ꢃ300 nm) is reported along with a de-
tailed view (150ꢃ150 nm), which was analysed in ultra-high
vacuum by needle-sensor AFM. In the image, the resolution
of the instrument used for the measurements allowed us to
detect structures with a size of approximately 10 nm, which
is compatible with NPs approximately 5 nm in size protected
by an organic layer approximately 2 nm thick. The average
height calculated over the pixels of the whole image is
0.90 nm with a root-mean-square (RMS) roughness of
0.27 nm. The measured height value is lower than the over-
CH2Cl2/MeOH.
The complexed Si/Cx6/8 surface was successively dipped
into a CH2Cl2 solution that contained AuNPs (NPACTHUNRGTNEUNG(Cx4))
protected with the thiolate calix[4]arene ligand Cx4 (see
Scheme 3) and with a mean core size of 1 nm. XPS measure-
ments (see the Supporting Information) of the Si/Cx6/8/NP-
A
ratio equal to the theoretical one, thereby confirming that
the Cx6/8 assembly remained unchanged after grafting of
NPACHTUNGTRENNUNG(Cx4).
The core size of the nanoparticles used in the previous ex-
periment was small enough for a surface-sensitive technique
such as XPS to produce a meaningful relative quantification
of the elements present in the assembled layers on the Si
substrate. On the other hand, these AuNPs were too small
for an accurate morphological investigation of the surface
and, most importantly, they were not imbued with plasmonic
properties. To overcome this problem, the assembly experi-
ment was performed by exposing the Si/Cx6/8 surface to a
all diameter of the NPACTHNGUTERNNU(G Cx4) unit, thereby suggesting that
NPs are organised in self-assembled aggregates, probably re-
flecting the morphology underneath. The latter aspect pre-
vents an accurate determination of the overall self-assem-
bled monolayer (SAM) thickness. Height- and phase-con-
trast AFM images acquired in ambient air in tapping mode
(see the Supporting Information) also confirm the presence
of particles with a diameter of approximately 10 nm. High-
resolution FESEM images (see Figure 2c,d) evidence a high
coverage of dispersed NPs organised in aggregates with a
hexagonal motif of packing symmetry. The origin of such a
motif should be found in non-specific electrostatic interac-
tions. Single NPs can be clearly identified and a narrow size
distribution centred at 10 nm can be estimated, in agree-
ment with AFM images.
solution of nanoparticles NPACHTNUGRTENUNG(Cx4) with a mean core size of
5 nm in CH2Cl2. XPS characterisation confirmed the results
obtained with the smaller AuNPs as to the NH/N+ ratio
(see Figure 1e,f, Table 1, and the Supporting Information),
whereas a large increase in the Au/Si ratio was found rela-
tive to the case of Si/Cx6/8/NP
(Cx4) (dꢀ1 nm), which was
expected on the basis of the larger number of Au atoms in
To verify the role played by the calix[6]arene and the di-
topic guest 8 units in the assembly of nanoparticles NPACHTUNGTRENNUNG(Cx4),
the NP, along with a moderate decrease in the C/N ratio.[28]
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Gold Nanoparticle Assembly
FULL PAPER
separate bare hydrogenated Si and Si/Cx6 surface were
Figure 3) show a single reversible one-electron wave, proba-
bly associated with one of the reduction processes of the
three N+ groups of 8 in the supramolecular surface assem-
bly.[20] The formal potentials (E8) recorded, estimated as the
mean value between the anodic (Ea) and the cathodic (Ec)
peak potentials, were À0.514, À0.550 and À0.407 V in vol-
tammograms in Figure 3a–c, respectively. The three samples
dipped into a solution of approximately 5 nm NP
CH2Cl2. In principle, the resulting systems should not be
covered at all by NP(Cx4), the interaction between the two
ACHTUNGTNER(NUGN Cx4) in
AHCTUNGTRENNUNG
parts being totally non-specific in this case. The comparison
of the two Au/Si ratios (see Table 1) clearly indicates that
the amount of NPs deposited on the substrate is more than
an order of magnitude higher when NP
G
showed CV waves with a peak potential splitting (DEpp =
with bare hydrogenated Si than with Si/Cx6. This is probably
EaÀEc) of 0.041, 0.028 and 0.003 V, in the series shown in
Figure 3a–c. These values are very close to the 0 V splitting
predicted for an ideal redox species confined at the surface
of an electrode[29,30] and they indicate that the electron-
transfer rate across the surface-bound molecular layer is
faster than the 0.8 VsÀ1 scan rate used here.
Due to the establishment of a surface photopotential of
unknown value upon irradiation with white light, no definite
correspondence can be drawn between the values of formal
potentials in Ref. [20] (obtained from closely comparable
systems in solution) and those reported here. Nevertheless,
given that a single reduction of
À
due to the relevant hydrophobic interaction between the H
Si surface and the organic fraction of AuNPs. On the other
hand, the Au/Si ratio found in the Si/Cx6/NP
G
system demonstrates that the presence of the ditopic guest 8
is mandatory for the construction of the supramolecular as-
sembly presented here.
Electrochemical studies at the interface: Electrochemical in-
vestigations were carried out by means of cyclic voltamme-
try (CV) on the Si/Cx6/8 (Figure 3a) and Si/Cx6/8/NPACTHNUTRGNENUG(Cx4)-
the axle unit is sufficient to
induce the disassembly of one
or more units of the whole as-
sembly,[26] one could infer that
the reduction–oxidation process
evidenced by the CV can be as-
sociated with a reversible “un-
threading–rethreading” move-
ment of either 1) the unit NP-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
sembly out and into the Si/Cx6
surface species. On this basis, to
induce the release of AuNPs
from the supramolecular sur-
face assembly, the two Si/Cx6/8/
NP
core sizes of 5 and 1 nm) were
polarised at potential of
ACHTUNGTREN(NUGN Cx4) samples (AuNP mean
a
À0.7 V for 2 min, then washed
in CH3CN and examined with
XPS again. In both samples, a
sizeable decrease in intensity of
the Au 4f signal was detected,
Figure 3. Cyclic voltammograms of a) Si/Cx6/8; and Si/Cx6/8/NPACHTNUGRTENUNG(Cx4) with NPs with a mean core size of
b) 5 nm and c) 1 nm. Measurements were run in CH3CN+0.1m TBAP at a 0.8 VsÀ1 scan rate under illumina-
tion with white light. On the left is a schematic representation of the electrochemical disassembly process.
functionalised surfaces, for which the code NP
(Cx4) identi-
with a change in the Au/Si ratio from 0.0090 to 0.0046 and
from 0.19 to 0.046 for the Au NPs with mean core sizes of 1
and 5 nm, respectively. This behaviour shows that the de-
tachment of AuNPACTHNUTRGNE(UNG Cx4) units can be obtained. Its extent
could probably be modulated by controlling both the polari-
sation potential and time.
fies nanoparticles with a mean core size of either 5 or 1 nm
(Figure 3b and c, respectively). Measurements were run in a
nitrogen-purged CH3CN/tetrabutylammonium perchlorate
(TBAP) solution under illumination with a low-intensity hal-
ogen lamp. In the dark no current signal was detected upon
potential scanning, owing to the low concentration of nega-
tive charge carriers in the n-doped semiconductor. Under
white-light irradiation, a significant photocurrent, depending
on the photon flux, was detected as a consequence of the
generation of electrons in the conduction band, which pro-
moted a reduction process at the semiconductor surface. All
the cyclic voltammograms of the three samples (see
Conclusion
The synthesis and characterisation of novel hybrid systems
on the basis of a supramolecular surface assembly of AuNPs
onto an SiACTHNUTRGNEUG(N 100) surface have been performed. AuNPs have
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A. Secchi, R. Zanoni et al.
48 h. After cooling to room temperature, the solvent was evaporated to
dryness under reduced pressure and the residue was dissolved in ethyl
acetate. Pure compound 6 (95%; 1.1 g) was recovered from the solution
by suction filtration as small white crystals. M.p. 52.2–56.38C; 1H NMR
(300 MHz, MeOD): d=9.05 (d, 3J=7 Hz, 2H), 8.63 (t, 3J=7 Hz, 1H),
8.15 (t, J=7 Hz, 2H), 4.67 (t, J=7 Hz, 2H), 3.45 (t, J=7 Hz, 2H), 2.1–
2.0 (m, 2H), 1.8–1.7 (m, 2H), 1.5–1.3 ppm (m, 14H); 13C NMR
(100 MHz, MeOD): d=145.5, 144.5, 128.1, 61.74, 48.0, 47.9, 47.6, 47.3,
47.0, 33.1, 32.6, 31.1, 29.1, 29.0, 28.7, 28.4, 27.7, 25.8 ppm; MS-ESI: m/z
(%): 326.2 (100) [MÀBr], 328.2 (97) [MÀBr].
been attached to and electrochemically detached from a Si-
ACHTUNGTRENNUNG(100) surface that presents functional groups able to bind
the nanoparticles through a hierarchical multi-step sequence
of reactions. The proposed method involves suitable calix-
[n]arene-based hosts, which have been used as the interface
species for the assembly of the two inorganic moieties, and a
bipyridinium-based redox-active ditopic guest. Surface reac-
tions have been monitored at their various steps by applica-
tion of XPS, AFM, FESEM and electrochemistry.
3
3
3
Synthesis of 8: In a sealed glass autoclave kept under a nitrogen atmos-
phere, a solution of 6 (0.5 g, 1.2 mmol) and N-pentyl-4(4’-pyridyl) pyridi-
nium bromide (7; 0.7 g, 1.2 mmol) in acetonitrile (20 mL) was heated to
reflux for 48 h. After cooling to room temperature, the resulting hetero-
geneous solution was diluted with ethyl acetate (15 mL) and filtered to
recover 8 (0.6 g, 74%) as a yellow solid. M.p. 76.4–79.38C; 1H NMR
(400 MHz, [D6]DMSO): d=9.49 (d, J=5.6 Hz, 4H), 9.32 (d, J=6.8 Hz,
2H), 8.87 (d, J=6.8 Hz, 4H), 8.7 (m, 1H), 8.18 (t, J=7 Hz, 2H), 4.75 (t,
J=7 Hz, 4H), 4.65 (t, 2H, J=7 Hz), 2.0–1.8 (m, 26H), 0.88 ppm (t, J=
7 Hz, 3H); 13C NMR (100 MHz, MeOD): d=149.3, 145.9, 144.9, 128.5,
127.1, 61. 6, 61.4, 31.1, 30.8, 29.1, 28.8, 28.7, 27.9, 25.8, 21.9, 14.1 ppm;
MS-ESI: m/z (%): 237 (100) [MÀ3Br] (z=2).
Experimental Section
Materials and synthetic methods: All solvents were dried using standard
procedures. All other reagents were of reagent-grade quality obtained
from commercial suppliers and were used without further purification.
NMR spectra were recorded at 400 and 300 MHz for 1H and 100 and
75 MHz for 13C. Melting points are uncorrected. Chemical shifts (d) are
expressed in ppm by using the residual solvent signal as internal refer-
ence. Mass spectra were recorded in ESI mode. Calix[6]arene 1,[31]
undec-10-enyl tosylate (2),[32] N-pentyl-4(4’-pyridyl) pyridinium bromide
Preparation of hydrogenated silicon: SiACTHUNRGTNE(NUG 100) wafers of 400 mm thickness,
n-doped (phosphorus-doped, single-side polished, 10–50 WcmÀ1 resistivi-
ty) and with areas of approximately 1 cm2 were first washed in boiling
1,1,2-trichloroethane for 10 min and subsequently in methanol at room
temperature with sonication for 5 min. They were then oxidised in H2O2/
HCl/H2O (2:1:8) at 353 K for 15 min, rinsed copiously with water, etched
with 10% aqueous HF for 10 min, rinsed with water again, dried under a
stream of N2 and immediately used in the functionalisation process.
(7)[33] and the calix[4]arene-protected gold nanoparticles NP
ACTHNUTRGENUGN(Cx4) with
mean core sizes of 1[34] and 5 nm[18] were synthesised according to pub-
lished procedures.
Synthesis of 3: This compound was synthesised by using an improved
published procedure.[35] In a sealed glass autoclave, kept under a nitrogen
atmosphere, a suspension of K2CO3 (1 g, 7 mmol) in a solution of cal-
ix[6]arene 1 (2 g, 2 mmol) and 2 (2.3 g, 7 mmol) in acetonitrile (150 mL)
was heated at reflux for 3 days. After this period, the apparatus was
cooled to room temperature and the solution was poured into a beaker
that contained a mixture of HCl (10%, 100 mL) and ethyl acetate. The
organic phase was separated, dried on Na2SO4 and evaporated to dryness
under reduced pressure. The oily residue was purified by column chroma-
tography on silica gel (n-hexane/ethyl acetate 9:1) to yield 3 as a yellow-
ish solid (75%). M.p. 138–1408C; 1H NMR (300 MHz, CDCl3): d=7.67
(brs, 6H), 7.21 (brs, 6H), 5.9–5.7 (m, 3H), 5.0–4.9 (m, 6H), 4.6–4.1 (m,
6H), 3.83 (brs, 6H), 3.7–3.4 (m, 6H), 2.86 (s, 9H), 2.1–2.0 (m, 6H), 1.85
(brs, 6H), 1.6–1.0 ppm (m, 53H); 13C NMR (75 MHz, CDCl3): d=146.8,
143.6, 139.1, 127.8, 127.3, 123.1, 114.1, 77.4, 73.9, 59.9, 34.2, 33.7, 31.4,
31.2, 30.9, 30.5, 30.2, 29.4, 29.2, 29.0, 28.8, 26.0 ppm; MS-ESI: m/z: 1461.7
[M+Na+].
Photoimmobilisation of calix[6]arene Cx6 on hydrogenated silicon:
Freshly etched Si samples were always used for anchoring. The function-
alisation experiments on the surface-activated samples were carried out
using standard preparative Schlenk-line procedures. After functionalisa-
tion, all samples were subjected to the same cleaning procedure, which
consisted of two sonication cycles (5 min each) with dichloromethane
and drying in a stream of N2. Hydrogen-terminated SiACHTUNTRGNEU(GN 100) wafers were
dipped in solutions of calix[6]arene Cx6 (10 mm) in toluene in Schlenk
tubes under a N2 atmosphere and subjected to a 35 mWcmÀ2 visible irra-
diation for 4 h from a quartz–iodine lamp. The molecular solution was
previously deoxygenated by three freezing–pumping–annealing cycles
and kept under N2 to prevent silicon from undergoing surface oxidation.
Complexation reactions on Si/Cx6 surface: The feasibility of the thread-
ing reaction between the calix[6]arene-based “wheels” Cx6 placed on the
silicon surface and viologen-based “axles” was investigated in 9:1 di-
chloromethane/methanol, since it is known that the formation of these
types of pseudorotaxane complexes is favoured in low-polar solvents.[21]
Methanol was used as co-solvent to favour the solubility of the viologen
axles in dichloromethane. In particular, a freshly prepared Si/Cx6 surface
was dipped in a 1 mm solution of axle (DOV or 8), which was prepared
by dissolving the salt in the minimum amount of methanol and then di-
luting with nine parts of dichloromethane. After 30 min the hydrogenated
Synthesis of Cx6: SnCl2·2H2O (1.4 g, 6 mmol) was added to a solution of
calix[6]arene 3 (0.6 g, 0.4 mmol) in ethyl acetate (50 mL). The resulting
heterogeneous mixture was heated at reflux for 36 h, cooled to room
temperature and then quenched by addition of a saturated solution of
Na2CO3 (50 mL). The separated organic phase was washed with water
until neutrality, dried over anhydrous Na2SO4 and evaporated to dryness
under reduced pressure. The recovered amino compound 4 (0.5 g) was
then dissolved in dry CH2Cl2 (100 mL) and treated with phenyl isocya-
nate (0.12 g, 1 mmol). After stirring at room temperature for 5 h, the sol-
vent was evaporated to dryness under reduced pressure. Purification of
the oily residue by column chromatography on silica gel (n-hexane/ethyl
acetate 7:3) yielded calix[6]arene Cx6 (0.4 g, 55%) as a yellowish solid.
M.p. 143–1458C; 1H NMR (400 MHz, C6D6): d=7.4 (s, 6H), 7.2 (d, 3J=
SiACTHNUTRGNEUGN(100) wafer was removed from the solution of the viologen and then
copiously rinsed with dry dichloromethane. After drying with an N2
stream, the “complexed” silicon surfaces Si/Cx6/DOV and Si/Cx6/2 were
submitted to XPS analysis.
3
Supramolecular anchoring of calix[4]arene-protected AuNPs on silicon:
The calix[6]arene-loaded silicon surface previously treated with the di-
topic guest 8 (Si/Cx6/8) was dipped in a solution of calix[4]arene-protect-
ed AuNPs with a mean core size of either 1 or 5 nm in dichloromethane.
After 30 min, the treated SiACHTNUTRGNE(NUG 100) surfaces were rinsed with pure dry di-
chloromethane, dried with an N2 stream and submitted to XPS, AFM,
FESEM and electrochemistry measurements.
7.8 Hz, 6H), 7.0 (brs, 3H), 6.98 (t, J=7.8 Hz, 6H), 6.9 (brs, 3H), 6.8–6.7
(m, 9H), 5.8 (ddt, 3J1(trans) =17 Hz, 3J2(cis) =10 Hz, 3J3 =7 Hz, 3H), 5.05 (d,
3
2
3J1(trans) =17 Hz, 3H), 5.00 (d, J2(cis) =10 Hz, 3H), 4.72 (d, J=15 Hz, 6H),
3
2
3
3.94 (t, J=6 Hz, 6H), 3.68 (d, J=15 Hz, 6H), 2.9 (brs, 9H), 2.0 (q, J=
7 Hz, 6H), 1.9–1.8 (m, 6H), 1.6–1.5 (m, 6H), 1.4–1.3 ppm (m, 57H);
13C NMR (100 MHz, C6D6): d=155.2 (2 resonances), 154.2, 151.7, 146.6,
138.9, 138.8, 136.0, 133.7, 133.6, 128.7, 123.0, 122.0, 120.2, 114.2, 72.9,
60.1, 34.0, 33.9, 31.4, 31.2, 30.6, 29.6, 29.5, 29.2, 29.0, 26.4 ppm; MS-ESI:
m/z (%): 1744 (100) [M+Na+].
XPS measurements: XPS measurements were performed using a modi-
fied Omicron NanoTechnology MXPS system with a monochromatic X-
ray source (Omicron XM-1000) and an Omicron EA-127 energy analy-
ser. The experimental conditions adopted were excitation by AlKa pho-
tons (hn=1486.7 eV), which were generated operating the anode at 14–
Synthesis of 6: In a sealed glass autoclave kept under a nitrogen atmos-
phere, a solution of 1,12-dibromo dodecane (5; 5 g, 15 mmol) and pyri-
dine (0.2 g, 2.7 mmol) in acetonitrile (50 mL) was heated at reflux for
8004
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Chem. Eur. J. 2013, 19, 7999 – 8006

Gold Nanoparticle Assembly
FULL PAPER
15 kV, 10–20 mA. No charging of the samples was experienced during
measurements. All the photoionisation regions were acquired using an
analyser pass energy of 20 eV, except the survey scan, which was record-
ed at a 50 eV pass energy. A take-off angle of 118 with respect to the
sample surface normal was adopted. The measurements were performed
at room temperature and the base pressure in the analyser chamber was
about 2ꢃ10À9 mbar during the spectral detection. The binding energy
(BE) of the Si 2p3/2 bulk component at 99.7 eV was used as an internal
standard reference for the BE scale (accuracy of Æ0.1 eV). All measure-
ments were conducted in the least possible time after sample preparation.
No sizeable sign of degradation upon extended acquisition times under
the X-rays was observed. The effects on quantitative analysis possibly
due to photoelectron diffraction at preferential directions of electron col-
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AFM and FESEM measurements: The morphology of the hybrid assem-
blies obtained after deposition of NPs with a mean core diameter of
5 nm was investigated under ultra-high vacuum (UHV) conditions by a
needle-sensor atomic force microscope (VT-AFM, Omicron NanoTech-
nology), which was attached to a UHV chamber in which XPS measure-
ments were also run (see below). The silicon nitride micro-fabricated tip
had a nominal curvature radius of <10 nm and a resonance frequency of
997.500 kHz. The values of average z-height and of root-mean-square
(rms) roughness (defined as the standard deviation from the average z-
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Technology) and WSxM software. A Veeco Multimode atomic force mi-
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were collected using the tapping mode and a silicon tip. The drive fre-
quency was 260 kHz, the drive amplitude was about 180 mV, the voltage
was 1 V and the scan rate was 0.5–1 Hz. The same system was studied
also with a FESEM instrument (Auriga Zeiss) equipped with an EDS
probe. Metallization of samples was not required.
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À
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tact to it. The electrode setup was obtained by pressing the Si crystal
against an O ring, sealing a small aperture in the polytetrafluoroethylene
(PTFE) cell, thereby exactly defining the electrode area (0.3 cm2). The
electrochemical properties of the supramolecular assemblies covalently
bound to n-SiACHTUNGTRENNUNG(100) surfaces were explored by cyclic voltammetry in 0.1m
NBu4ClO4 (tetrabutylammonium perchlorate, or TBAP) in dry CH3CN.
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purging the solution with nitrogen and using an Autolab electrochemical
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counter electrode was a platinum coil wire, and a silver wire immersed in
0.01m AgNO3/0.1m TBAP in CH3CN, separated from the main solution
by a porous fritted glass plus agar plug, served as a reference electrode.
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were acquired at a 0.8 VsÀ1 scan rate under illumination with a low-in-
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Acknowledgements
The authors thank Universitꢁ La Sapienza and the Centro di Ricerca per
le Nanotecnologie applicate allꢀIngegneria della Sapienza (CNIS) in
Rome, Italy, for access to the AFM and FESEM facilities of the SNN
Lab. They also thank Centro Interdipartimentale di Misure—G. Casnati
in Parma, Italy, for the NMR spectroscopic measurements.
Chem. Eur. J. 2013, 19, 7999 – 8006
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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137.
Received: December 4, 2012
Revised: March 13, 2013
Published online: April 18, 2013
8006
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 7999 – 8006