Fast assembling of magnetic iron oxide nanoparticles by microwave-assisted copper(I) catalyzed alkyne-azide cycloaddition (CuAAC)

Article abstract of DOI:10.1021/cm401326p

Two dimensional (2D) nanoparticles (NP) assemblies have become very attractive due to their original collective properties, which can be modulated as a function of the nanostructure. Beyond precise control on nanostructure and easy way to perform, fast assembling processes are highly desirable to develop efficient and popular strategies to prepare systems with tunable collective properties. In this article, we report on the highly efficient and fast 2D assembling of iron oxide nanoparticles on a self-assembled monolayer (SAM) of organic molecules by the microwave (MW)-assisted copper(I) catalyzed alkyne-azide cycloaddition (CuAAC) click reaction. Microwave irradiation favors a dramatic enhancement of the assembling reaction, which was completed with maximum density in NPs within one hour, much faster than the conventional CuAAC click reactions that require up to 48 h. Moreover, the MW-assisted click reaction presents the great advantage to preserve specific reactions between alkyne and azide groups at SAM and NP surfaces, respectively, and also to avoid undesired reactions. To the best of our knowledge, this is the first time this approach is performed to nanoparticles assembled on surfaces.

Full text of DOI:10.1021/cm401326p

Article
pubs.acs.org/cm
Fast Assembling of Magnetic Iron Oxide Nanoparticles by
Microwave-Assisted Copper(I) Catalyzed Alkyne−Azide
Cycloaddition (CuAAC)
Delphine Toulemon,† Benoît P. Pichon,*,† Cedric Leuvrey,† Spyridon Zafeiratos,‡ Vasiliki Papaefthimiou,‡
́
Xavier Cattoen,§,∥ and Sylvie Begin-Colin†
́
̈
†Institut de Physique et Chimie des Mater
́
iaux de Strasbourg, UMR 7504 (CNRS−UdS), 23, rue du Loess, BP 43, 67034 Strasbourg
Cedex 2, France
‡Institut de Chimie et Proced
Strasbourg Cedex 2, France
́
́ ́ ́
es pour l’Energie, l’Environnement et la Sante, UMR 7515 (CNRS−UdS), 25 rue Becquerel, 67087
§Institut Charles Gerhardt Montpellier, UMR 5253 (CNRS−UM2−ENSCM−UM1), 8 Rue de l’Ecole Normale, 34296 Montpellier
Cedex 5, France
S
* Supporting Information
ABSTRACT: Two dimensional (2D) nanoparticles (NP)
assemblies have become very attractive due to their original
collective properties, which can be modulated as a function of
the nanostructure. Beyond precise control on nanostructure
and easy way to perform, fast assembling processes are highly
desirable to develop efficient and popular strategies to prepare
systems with tunable collective properties. In this article, we
report on the highly efficient and fast 2D assembling of iron
oxide nanoparticles on a self-assembled monolayer (SAM) of
organic molecules by the microwave (MW)-assisted copper(I)
catalyzed alkyne−azide cycloaddition (CuAAC) click reaction.
Microwave irradiation favors a dramatic enhancement of the assembling reaction, which was completed with maximum density in
NPs within one hour, much faster than the conventional CuAAC click reactions that require up to 48 h. Moreover, the MW-
assisted click reaction presents the great advantage to preserve specific reactions between alkyne and azide groups at SAM and
NP surfaces, respectively, and also to avoid undesired reactions. To the best of our knowledge, this is the first time this approach
is performed to nanoparticles assembled on surfaces.
KEYWORDS: nanoparticle, assembly, click chemistry, CuAAC, microwave, magnetism, iron oxide
between terminal functional groups on both NPs and SAMs
surfaces meets most of these objectives. A crucial point is that
the assembling reaction has to proceed with high yield and high
rates.
INTRODUCTION
■
Owing to their great ability to modulate their physical
properties as a function of their nanostructure, 2D nanoparticle
(NP) assemblies have become a highly attractive field of
research for the development of new applications such as
biosensors for molecule detection, high density magnetic
storage, or magneto-resistive media.1−4 Indeed, collective
properties are ruled by the fine control on the preparation of
mono- and multilayer assemblies on surfaces.5−8 For instance,
assemblies with well-defined interparticle distances enable to
modulate the dipolar interactions between magnetic NPs and
thus the overall magnetic properties.9−11 Patterned NPs
assemblies on surfaces have been obtained in a very efficient
way by functionalizing substrates using self-assembled mono-
layers (SAMs) of organic molecules.12−17 Because of the
increasing interest for the assembly of NPs and their original
collective properties, easy-to-process methods are now required
to develop device miniaturization. Among current assembling
techniques, deposition of NPs driven by specific interactions
Over the past years, the copper(I) catalyzed azide−alkyne
cycloaddition (CuAAC) click reaction,18−21 which was
developed initially for small-molecule organic synthesis, has
proved to be a very simple and efficient method for surface
modification of substrates22,23 and nanoparticles.24 Very
recently, this approach has been reported by Kinge et al.25
and our group26 as a very useful tool to address the assembling
of magnetic NPs on SAMs by the irreversible covalent
formation of triazole linkages. Such a strategy strongly prevents
the formation of unspecifc NP assemblies, which may be driven
by dipolar interactions.26 Depending on the reaction time, we
Received: April 22, 2013
Revised: June 20, 2013
Published: June 20, 2013
© 2013 American Chemical Society
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purchased from Strem Chemicals. Oleic acid, methanesulfonyl
chloride, and 11-undec-1-ynol were purchased from Alfa Aesar.
Triethylamine was obtained from Fluka and docosene from Aldrich.
Synthesis of diethyl (12-bromododecyl)phosphonate. In a
round-bottomed flask equipped with a short distillation apparatus, a
mixture of 1,12-dibromododecane (65.6 g, 0.20 mol) and
triethylphosphite (5.7 mL, 33 mmol) was heated at 160 °C for 4 h.
The excess dibromododecane was then distilled off under vacuum,
then the crude product was purified by column chromatography on
silica gel (cyclohexane/ethyl acetate 3:1 to 1:1). Yield: 75%. 1H NMR
(CDCl3, 400 MHz): 4.09 (m, 4H); 3.40 (t, J = 7.0 Hz, 2H); 1.84 (m,
2H); 1.78−1.66 (m, 2H); 1.64−1.52 (m, 2H); 1.44−1.23 (m, 22H).
Synthesis of Diethyl (12-Azidododecyl)phosphonate. A
mixture of diethyl (12-bromododecyl)phosphonate (1.83 g, 4.7
mmol) and sodium azide (1.54 g, 23.8 mmol) in ethanol (96%, 10
mL) was refluxed for 40 h. After evaporation of the solvent, water was
added, and then, the mixture was extracted three times with
dichloromethane. The combined organic fractions were washed with
water then brine, and finally concentrated to afford diethyl (12-
azidododecyl)phosphonate (1.60 g, 97%). 1H NMR (CDCl3, 400
MHz): 4.09 (m, 4H); 3.25 (t, J = 7.0 Hz, 2H); 1.80−1.46 (m, 6H);
1.44−1.18 (m, 22H).
Synthesis of (12-Azidododecyl)phosphonic Acid. Trimethyl-
silyl bromide (1.97 g, 12.9 mmol) was added to a solution of diethyl
(12-azidododecyl)phosphonate (1.50 g, 4.7 mmol) in anhydrous
dichloromethane (8 mL) under argon. The solution was stirred
overnight at room temperature, and subsequently, the solvent was
removed by rotary evaporation. Water (10 mL) was then added, and
the mixture was stirred for two hours. The white solid was filtered off
and air-dried. (12-Azidododecyl)phosphonic acid (1.17 g, 93%) was
obtained as a white solid. 1H NMR (CDCl3, 400 MHz): 9.32 (br, 2H);
3.26 (t, J = 6.8 Hz, 2H); 1.80−1.67 (m, 2H); 1.65−1.52 (m, 4H);
1.40−1.22 (m, 16H). 13C NMR (CDCl3, 100 MHz): 51.4 (C−N3);
29.5−28.8; 26.7. 31P NMR (CDCl3, 400 MHz): 36.9.
have been able to tune the average interparticle distance so as
to produce either assemblies of quasi noninteracting NPs or
high density assemblies of NPs, which display collective
properties.26 Although this approach represents an important
step toward the assembling of NPs on SAMs in a controlled
fashion through specific interactions, it suffers from very slow
kinetics. Indeed, reaction times are longer (up to 48 h) than the
ones of reactions between alkyne and azide molecular
derivatives in solution and require large amounts of catalysts
(up to 10 times as much as the molar amount of azide and
alkyne groups). The assembling kinetics of NPs on SAMs
strongly depend on the probability that nanoparticles reach the
corresponding functional groups at the SAM surface. Several
parameters may slow down the kinetics of the reaction: (i) the
Brownian motion that rules the mobility of nanoparticles in
solution, (ii) the number of functional groups that are available
at the SAM surface, and (iii) the intermolecular interactions
between these groups that are favored by the tight packing of
molecules and thus reduce their reactivity. Although dense
monolayers of NPs can be obtained by specific interactions
using the CuAAC reaction, the assembly of NPs has been
reported to occur much faster by using other types of
SAMs.12,15,27 Therefore, a general and versatile strategy that
enables fast assembling of NPs on SAM surfaces still represents
an unfilled goal.
Recently, microwave (MW)-assisted synthesis has been
developed for CuAAC reactions. This approach is highly
appealing because of its major advantages such as decrease of
reaction time from hours to minutes, improved reaction yields,
absence of side products, and reproducibility. Originally
reported for molecular synthesis,21,28,29 MW-assisted CuAAC
reactions have very recently emerged for surface functionaliza-
tion. Gold29 and iron oxide30 NPs and SAMs on silicon
wafers31−33 have been functionalized by this method. All these
studies have demonstrated the acceleration of the CuAAC
reaction under exposure to MW irradiations from hours to
minutes. This has been established for relatively low power
(40−100 W), which resulted in mild to elevated temperatures
(70−150 °C). In all cases, these reactions featured a fast-
moving molecular derivative reacting with an immobile
substrate or a particle in slow-motion. It is worth noting that
these studies have been exclusively performed by using the
Sharpless conditions in the presence of Cu(II) catalyst and a
reducing agent (ascorbic acid) in aqueous and polar organic
solvent mixtures. Nevertheless, the alternative approach in the
presence of Cu(I) and triethylamine34 is much more suited for
NPs coated with hydrophobic molecules since such NPs form
unstable suspensions of aggregates in hydrophilic media.
Herein, we report on the development of a new way to
accelerate the assembling of slow-motion magnetic iron oxide
NPs on immobile SAMs by using the MW-assisted CuAAC
click reaction. The reaction proceeds under hydrophobic
conditions,29,30 and results in dramatically faster kinetics
compared to conventional heating, with an easy control of
surface coverage. This approach could be applied to the
assembly of a wide range of functionalized nanoparticles and
nano-objects. To the best of our knowledge, the benefits of
MW-assisted CuAAC click reaction has never been reported for
NPs assembling addressed by SAMs.
Synthesis of (11-Undec-1-ynyl)thiol. (11-Undec-1-ynyl)thiol
was synthesized from (11-undec-1-ynyl)thioacetate, which has been
synthesized following the synthesis pathway we have reported
previously.26 Then, 300 mg of (11-undec-1-ynyl)thioacetate were
dissolved in 20 mL of methanol. The solution was degassed
thoroughly and backfilled with N2. One milliliter of concentrated
HCl was added dropwise, and the entire mixture was refluxed under
N2 atm for 5 h. The reaction was then stopped by adding 20 mL of ice
cold water. The product was extracted twice with diethyl ether (20
mL), and the organic phase was washed twice with water (20 mL) and
1
dried over MgSO4. Rotary evaporation yielded a yellow oil. H NMR
(CDCl3, 400 MHz): 2.52 (q, J = 7.3 Hz, 2H); 2.18 (dt, J = 6.8 and 2.6
Hz, 2H); 1.94 (t, J = 2.6 Hz, 1H); 1.61 (m, 4H); 1.22−1.41 (m, 11H).
13C NMR (CDCl3, 100 MHz): 85.6 (C terminal C−H); 67.9 (C
alkyne); 39.1 (CH2−SH); 30.2−28.4 (6 CH2); 22.6 (CH2); 18.3
(CH2); 14.0 (CH2).
Synthesis of Azide-Terminated Iron Oxide Nanoparticles
(NP@N3). Iron stearate (Fe(stearate)2) (1.38 g, 2.2 × 10−3 mol) was
dissolved in docosene (20 mL) in the presence of oleic acid (1.24 g,
3.3 × 10−3 mol). The mixture was kept at 110 °C for at least 4 h to
avoid water residues and to completely dissolve the reactants. The
temperature was then carefully raised to reflux with a heating rate of 5
°C·min−1 and kept under reflux without stirring for 120 min under air.
After cooling to room temperature, the black suspension was washed
12 times with a mixture of hexane and acetone (v:v, 1:4) and
centrifuged (14 000 rpm, 10 min). The obtained nanoparticles coated
with oleic acid (NP@OA) were easily suspended in (THF) at a
concentration of 1.67 mg/mL. Oleic acid was subsequently replaced by
(12-azidododecyl)phosphonic acid (AP12N3) following direct ex-
change. A solution of AP12N3 (15 mg) in THF (10 mL) was added to
10 mL of the NP@OA suspension and stirred for 48 h at room
temperature. Free molecules were removed by ultrafiltration (using a
30 kD membrane, Millipore) in 60 mL of THF.
EXPERIMENTAL SECTION
Chemicals. Tetrahydrofurane (THF), methanol, and ethanol were
purchased from Carlo Erba and used as received. Iron stearate was
■
Preparation of Alkyne-Terminated Self-Assembled Mono-
layer (SAM-CC). Ion sputtered gold substrates were cleaned under
O2/H2 plasma for 2 min and were soaked in a 10 mM ethanolic
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solution of 11-(undec-1-ynyl)thiol at room temperature for 24 h. The
SAM was then rinsed with copious amounts of pure ethanol and used
directly after drying under air.
Nanoparticles Assembling. The assembling of nanoparticles by
microwave-assisted CuAAC click reaction was performed using an
Anton Paar microwave device (Monowave 300) with a 10 mL vial by
immersing SAM-CC (5 × 5 mm2) in 5 mL of a solution of NP@N3 in
THF (0.67 mg/mL). Then, 0.5 mL (3.70 mmol) of triethylamine and
6.5 mg (6.7.10−3 mmol) of CuBr(PPh3)3 were added. The MW
CuAAC reaction was performed under a maximum temperature of 100
°C, which was controlled by an infrared sensor, while the reaction vial
was cooled by compressed air flow. A maximum power of 50 W at a
frequency of 2.45 GHz was applied to the reaction medium. The
reaction time was varied from 2 min to 1 h. The power, temperature,
and pressure were recorded during the assembling reaction.
Figure 1. Schematic representation of the MW-assisted CuAAC click
reaction between azido-terminated NPs and alkyne-terminated SAMs,
which result in the formation of irreversible triazole linkages.
Characterization Techniques. Transmission electron microscopy
(TEM), high resolution TEM (HRTEM), and electron diffraction
(ED) were performed with a TOPCON model 002B TEM, operating
at 200 kV, with a point-to-point resolution of 0.18 nm. The size
distribution was calculated from the size measurement of more than
100 nanoparticles by using the Image J software. Granulometry
measurements were performed on a nanosize MALVERN (nano ZS)
apparatus for each NP suspension. Fourier transform infrared (FTIR)
spectroscopy was performed using Digilab Excalibur 3000 spectropho-
tometer (CsI beamsplitter) in the energy range 4000−400 cm−1.
Scanning electron microscopy (SEM) was performed using a JEOL
6700 microscope equipped with a field emission gun (SEM-FEG)
operating at an accelerating voltage of 3 kV. Atomic Force Microscopy
(AFM) was performed in the tapping mode using a Digital Instrument
3100 microscope coupled to a Nanoscope IIIa recorder. Collected data
were analyzed with Nanotec WSXM software.35 Polarization
modulation infrared reflection−absorption spectroscopy (PM-
IRRAS) was performed on gold substrates after being immersed in
thiol solution, using a IF66S Bruker spectrometer with a liquid
nitrogen-cooled mercury cadmium telluride (MCT) detector. The X-
ray photoelectron spectroscopy (XPS) measurements were carried out
in an ultrahigh vacuum (UHV) setup equipped with a VSW ClassWA
hemispherical electron analyzer (150 mm radius) with a multi-
channeltron detector. A monochromated AlKα X-ray source (1486.6
eV; anode operating at 240 W) was used as incident radiation. The
base pressure in the measurement chamber was ∼1 × 10−9 mbar. XP
spectra were recorded in the fixed transmission mode using pass
energy of 22 eV resulting in an overall energy resolution better than
0.4 eV. Prior to individual elemental scans, a survey scan was taken for
all the samples to detect all of the elements present. The CASA XPS
program with a Gaussian−Lorentzian mix function and Shirley
background subtraction was employed to deconvolute the XP spectra.
Magnetic curves were recorded at 300 and 5 K by applying a magnetic
field in the plane of the substrate by using a superconducting quantum
interference device (SQUID) magnetometer (Quantum Design
MPMS SQUID-VSM dc magnetometer).
thioacetate derivative.26 The assembling of thiol molecules as a
SAM was confirmed by the thickness measured by ellipsometry
(1.1 0.1 nm) and also by PM-IRRAS spectroscopy and XPS.
In contrast to the very few studies that report only on surface
functionalization by microwave-assisted CuAAC reaction from
Cu(II) catalyst associated to a reducing agent (ascorbic
acid),31−33 the assembling of NPs was performed by dipping
the SAM-CC in a mixture of the NP@N3 suspension in THF,
triethylamine, and the CuBr(PPh3)3 catalyst, which enable the
reaction to proceed in nonaqueous, aprotic solvents.34 The
CuAAC reaction was carried out in a sealed tube under
microwave irradiations with a maximum power of 50 W with
the aim not to exceed 100 °C. More details related to the
syntheses and assembling processes are available in the
experimental section of the Supporting Information.
The assembling of NP@N3 on SAM-CC was characterized
by scanning electron microscopy (SEM) (Figure 2a).
RESULTS AND DISCUSSION
■
The assembling process operating by click chemistry was
conducted between azide-terminated NPs (NP@N3) and
alkyne-terminated SAM (SAM-CC) (Figure 1). Iron oxide
NPs coated with oleic acid were prepared by the thermal
decomposition of iron stearate in docosene (bp 365 °C) and
were further functionalized by (12-azidododecyl)phosphonic
acid (AP12) following a ligand exchange procedure.9,26 NPs
exhibiting a narrow size distribution centered at 19.8 1.6 nm
were obtained and formed a highly stable suspension in
tetrahydrofurane (THF) according to TEM micrographs and
granulometry measurement, respectively. Prior to the assem-
bling reaction, the alkyne terminated SAM was prepared by the
adsorption of (11-mercaptoun)dec-1-yne in ethanol on a gold
substrate after plasma cleaning. This molecule was synthesized
by performing the acidic deprotection of the corresponding
Figure 2. (a) SEM image of NPs assembled on SAM after 1 h of MW-
assisted CuAAC click reaction. (b) Density in NPs assembled on SAM
as a function of the reaction time. (c) Height AFM images and (d)
cross-section profile corresponding to the line in panel c.
Representative micrographs recorded on different areas show
the high efficiency of the microwave-assisted CuAAC click
reaction. NPs cover the whole surface of the SAM with a
density of 1470
14 NP/μm2 after 1 h of reaction, which,
when taking into account the adsorption process, is rather close
to the maximum theoretical value (1820 NP/μm2) calculated
for compact assemblies. These results were confirmed by AFM
images and the corresponding cross-sectional profile, which
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shows that NPs were assembled in a well-defined and dense
monolayer (Figure 2c,d). In addition, extensive washings with
THF and exposure to ultrasounds do not affect the structure of
the NP assembly, which shows the irreversible formation of
highly stable covalent triazole linkages between NPs and the
SAM.
Although the assembling of NPs on SAM clearly
demonstrates that organic functionalities are preserved upon
the MW-assisted CuAAC reaction, we investigated the structure
of SAM and NPs before and after exposure to MW irradiation.
The reaction was performed without Cu catalyst to maintain
both SAM and NPs under the same conditions. All spectra
related to the following measurements can be seen in the
Supporting Information. The composition of SAM was
analyzed by XPS. Both C1s spectra exhibit a very similar
main peak at 284.6 eV with a slight asymmetry toward to the
high binding energy side. The main peak is related to sp3
carbon atoms in alkylene chains,36 while the peak asymmetry is
caused by shake up satellite peaks and/or contribution of sp1
(alkyne) species36 and carbon connected to oxygen species.
The S 2p spectra have been fitted by two S 2p3/2,1/2 doublets.
For each S 2p doublet, a 1.3 eV spin orbit splitting and a 2:1
intensity ratio between the 3/2 and 1/2 components was
used. The first doublet at a binding energy of 161.9 0.1 eV
(S 2p3/2) is related to thiol groups bound to the gold surface
(S−Au).37 The second doublet at a higher BE of 163.6 0.1
eV is attributed to some free thiol groups (unbound species).37
Moreover, the S/Au and S/C intensity ratios were calculated
0.09 0.01 and 0.1 0.01 and are practically unaffected by the
MW process, indicating that microwave irradiation does not
modify the composition of SAM.
The SAM structure was also investigated by PM IRRAS.
While νCC and νCC−H vibration modes33,36 corresponding to
alkyne terminal groups were not observed as reported
earlier,38,39 the νCH2 vibration modes are centered to 2918
and 2854 cm−1, which demonstrates that alkylene chains are in
the all-trans conformation and remain tightly packed.26
However, granulometry and FTIR spectroscopy have been
performed on NPs before and after exposure to MW
irradiation. Similar spectra proved the stability of azide
terminated NPs in the reaction medium.
Finally, magnetic measurements have been performed on the
assembly obtained after 1 h of MW-assisted CuAAC reaction.
Magnetization was recorded against a magnetic field in the −7
to +7 T range (Figure 3). Curves recorded at 300 and 5 K
display hysteresis loops with coercive fields (HC) of 50 and 400
Oe, respectively. These curves correspond to the ferrimagnetic
behavior of spinel iron oxide NPs with sizes of 20 nm, which
are featured by a rather high magnetic anisotropy in
comparison to smaller iron oxide NPs that are super-
Beyond the fact that NPs were assembled with a very high
density, a very important point is that the microwave-assisted
assembling reaction is dramatically faster than using conven-
tional heating conditions for which the maximum density was
obtained at 60 °C after 48 h.26 Because of the geometric
considerations, unreacted alkyne and azide groups are expected
to remain at the surface of SAM and NPs, respectively, after the
assembling reaction. Therefore, surface analysis methods such
as PM-IRRAS or XPS cannot reveal with certainty the
occurrence of the assembling reaction by CuAAC. With the
aim to gain better insight on this point, we performed
additional counter experiments that indirectly prove the
occurrence of the cycloaddition reaction. First, the reaction
was performed identically as the main experiment without
adding the Cu catalyst and resulted in the nondeposition of
NPs on SAM by SEM (see Supporting Information). Although
the microwave activation is highly efficient, it cannot overpass
the role of the Cu catalyst. Second, the reaction was performed
in the presence of methyl-terminated SAM, which also
prohibited the assembling of NPs. These two experiments
demonstrate the requirement of both Cu catalyst and alkyne
groups at SAM surface to assemble NP@N3 by the CuAAC
click reaction.
More details on the kinetics of the microwave-assisted
CuAAC reaction have been obtained by performing the
experiment for different durations. Reaction times shorter
than one hour resulted in lower NP densities (see Supporting
Information). Plotting the NP density as a function of time
shows the nonlinear increase of the kinetics (Figure 2b).
Indeed, the NP assembling does not happen within the first few
minutes. A minimum incubation time is necessary for iron
oxide NPs and gold surface to accumulate energy, which
induces local increase in temperature. Between 2 and 5 min, the
assembling reaction starts and the density increases faster to
reach 50% of the maximum value after 10 min. These results
are correlated to the increase in temperature, which reaches its
maximum (100 °C) only after 5 min of irradiation (see
Supporting Information). Although 100 °C is necessary to
obtain the maximum density in NPs after 1 h of reaction, the
click reaction can also occur at lower temperature. For longer
times, the kinetics decrease, which is related to the lower
amount of empty space at the SAM surface that NPs can
occupy. More generally, NPs that are suspended in solution are
submitted to the Brownian motion, which results on their
rather low mobility and statistic adsorption. Whatever the
efficiency of the microwave-assisted CuAAC reaction, the
probability of azide terminated NPs to react with free alkyne
groups at SAM surface decreases as long as the NP density
increases on SAM. In addition, the time needed (1 h) to reach a
maximum density is rather long in comparison to studies that
deal with the reaction of molecules with functional groups at
the SAM surface by using the same reaction, which can be
accomplished within a couple of minutes.28 This finding
supports the argument of the low mobility of NPs in solution in
comparison to molecules that are far smaller. Such an increase
of reaction time has been also noticed for increasing steric
hindrance of molecules.31
Figure 3. Magnetization recorded at 300 and 5 K against an applied
field for NPs assembled after 1 h of MW assisted CuAAC click
reaction. The inset depicts the full curves.
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paramagnetic.40 As a consequence, SEM shows NPs to
assemble in 1D chains after 10 min of reaction (see Supporting
Information). This behavior may be correlated to strong and
collective dipolar interactions, which may participate in the
assembling reaction. More generally, these results show that the
magnetic properties of these NPs are similar to others reported
in the literature40 and do not seem to be affected by the
microwave irradiations, even when the reaction is prolonged for
one hour.
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CONCLUSIONS
■
To summarize, the major finding of this study is the unseen and
very fast assembling of NPs on SAMs by performing the
microwave-assisted CuAAC click reaction. Beyond the fact of
formation of dense assemblies of NPs with a well-defined
monolayer structure, this process promotes the enhancement of
the kinetics of the reaction, reducing the reaction time from
days to minutes. While the elevation of temperature of the
reaction media is rather low in comparison to conventional
experimental conditions, the mechanism of the reaction is
expected to proceed through a local increase of the temperature
at the interface between iron oxide NPs and gold substrate.
This point is currently under investigation. Fortunately,
microwave irradiation does not affect the functionalities and
structures of both SAMs and NPs. Notably, the MW-assisted
CuAAC click reaction was demonstrated to be carried out for
the first time in the presence of a Cu(I) catalyst and alkyne-
thiolate SAMs on a gold surface, which enables this assembling
method to proceed for hydrophobic NPs as stable suspensions.
Other advantages of this assembling method are the absence of
side-products and that no purification process is required. Its
potential adaptability to a wide range of nanoparticles and
surfaces also renders it very promising to be used as a general
assembling technique.
̈
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ASSOCIATED CONTENT
* Supporting Information
■
S
Molecule synthesis. Structural analysis of nanoparticles before
and after exposure to microwave irradiation. SEM micrographs,
FTIR, PM-IRRAS, and XPS spectra. Experimental data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(B.P.P.) E-mail: benoit.pichon@unistra.fr. Tel: 0033 (0)3 88
10 71 33. Fax: 0033 (0)3 88 10 72 47.
■
Present Address
∥Institut Nee
́
l, UPR2940 CNRS/UJF, 25 rue des Martyrs,
38042 Grenoble, France.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
(31) Haensch, C.; Erdmenger, T.; Fijten, M. W. M.; Hoeppener, S.;
Schubert, U. S. Langmuir 2009, 25, 8019.
ACKNOWLEDGMENTS
Prof. Laurent Douce and Dr. Julien Fouchet for providing the
access to the microwave reactor and fruitful discussions.
■
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Chem. Commun. 2010, 46, 8416.
Funding was provided by Direction gen
́ ́
erale de l’armement
(DGA) and region Alsace.
́
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