Nanostructured monolayers on carbon substrates prepared by electrografting of protected aryldiazonium salts

Article abstract of DOI:10.1021/cm303844v

The electrogeneration of aryl radicals from protected diazonium salts combined with protection-deprotection steps was evaluated to design functional monolayers on carbon substrates with a well-controlled organization at the nanometric scale. The structure

Full text of DOI:10.1021/cm303844v

Article
pubs.acs.org/cm
Nanostructured Monolayers on Carbon Substrates Prepared by
Electrografting of Protected Aryldiazonium Salts
Yann R. Leroux and Philippe Hapiot*
Institut des Sciences Chimiques de Rennes, Universite
35042 Rennes Cedex, France
́
de Rennes 1, CNRS, UMR 6226 (Equipe MaCSE), Campus de Beaulieu,
S
* Supporting Information
ABSTRACT: The electrogeneration of aryl radicals from
protected diazonium salts combined with protection−depro-
tection steps was evaluated to design functional monolayers on
carbon substrates with a well-controlled organization at the
nanometric scale. The structure of the obtained monolayer is
adjusted by varying the size of the protecting group that is
introduced on the precursors (trimethylsilyl, triethylsilyl, and
tri(isopropyl)silyl were tested in the present study). After
deprotection, a robust ethynylaryl monolayer is obtained
whatever the substituent that serves as a platform to attach other functional groups by a specific “click chemistry” coupling step.
Electrochemical and structural analyses show that the organization of the attached monolayer is totally governed by the size of
the protecting group that leaves a footprint after removal but maintains a total availability of the immobilized functional groups.
Properties of the monolayer (charge transfer, permeation of molecules through the layer, density of functional groups) were
examined in combination with the performances for postfunctionalization taken with an alkyl-ferrocene derivative as an example
of the immobilized species.
KEYWORDS: modified organized surfaces, diazonium redox chemistry, carbon materials surfaces
proposed to limit the process to the formation of a single layer.
The first approach was the substitution of the aryldiazonium
ring with bulky groups6 or preorganized aromatic units like
calyx-arenes7 that impede the grafting of other radicals onto the
already attached aromatic rings. Another approach was the
postelimination of the extra-attached materials by breaking a
weak bond in the layer after its electrografting.8 Combining
these two strategies, we found that the bulky tri(isopropyl)silyl
group introduced on an ethynyl-diazonium salt limits the
grafting to the formation of an ultrathin layer.9 After
deprotection, we obtain an active monolayer of ethynylaryl
moieties that is covalently linked to the carbon substrate and
available for a postfunctionalization using the well-known
“click” coupling (Cu(I)-catalyzed Huisgen 1,3-dipolar cyclo-
addition).10 In this process, the deprotection leaves a
nanometric footprint on the layer because the protecting
trialkylsilyl group is bulkier that the protected entity. A fine
control of this phenomenon opens large possibilities to design
ordered hierarchical surface modifications. For example, we
could imagine building structured monolayers that would be
similar to those prepared with alkylthiol/gold surfaces11 but
with the advantages of covalent bonding and the possibility of a
much larger diversity of accessible substrates thanks to the
radical nature of the grafting reaction. Resulting pinholes could
INTRODUCTION
■
The preparation of robust modified surfaces with a molecular
organization remains of primary importance for many
applications, for example, in analytical chemistry or molecular
electronics.1 Carbon surfaces are receiving more and more
attention because of new possibilities of these materials.2 If an
organized layer could spontaneously be obtained on a gold
surface through the adsorption of long chain alkane-thiols,3 the
situation is more problematic when considering carbon
materials especially when a robust and organized chemical
modification of their surface is desired.2 The redox chemistry of
aryldiazonium salts permits the immobilization of functional
groups on almost any carbon materials such as, for example,
glassy carbon4 or graphene sheets.5 The electrografting method
relies on the generation of reactive phenyl radicals that are
produced by electrochemical reduction of the corresponding
aryl diazonium salts. Such radicals rapidly add on the surface
that serves as an electrode.4 On the basis of this technique, a
wide range of functional groups has already been immobilized
on carbon substrates.2,4 On one hand, the advantages of a
radical process are its versatility (it could be used with many
different materials interfaces) and the robustness of the
modification that is linked to the surface through a covalent
bond. On the other hand, the major drawback is the difficulty
to limit the efficiency of the radical grafting and the resulting
disorganization. Without special precautions, electrografting
leads to the formation of thick disorganized multilayer films.4
To circumvent this difficulty, different strategies have been
Received: November 28, 2012
Revised: January 17, 2013
Published: January 18, 2013
© 2013 American Chemical Society
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Scheme 1. Preparation of the Amine Precursors
Scheme 2. Preparation of the Aryl Diazonium Salt
THF:diisopropylamine (80:20 mL) gave the desired product in 78%
yield. NMR data are consistent with the already published procedure.3
1H NMR (300 MHz, CDCl3) δ 7.27 (d, J = 8.59 Hz, 2H), 6.57 (d, J =
8.59 Hz, 2H), 3.80 (s, 2H), 0.29 (s, 9H).
also be filled with a second functionality for preparing close
mixed binary layers.12,13
In this paper, we have examined how we could use this effect
to tune the molecular arrangement of the produced monolayer.
We used different alkylsilyl groups with increasing steric
obstructions. Three different substituents were introduced on
the ethynyl-aryldiazonium salts: trimethylsilyl (TMS), triethyl-
silyl (TES), and tri(isopropyl)silyl (TIPS) as protecting groups.
The electrochemistry of molecular redox probes in solution was
examined to characterize the permeability of the monolayers in
combination with the possibility of immobilizing functional
groups.
4-((Triethylsilyl)ethynyl)aniline. According to the general proce-
dure, 4-iodoaniline (1.0 g, 4.6 × 10−3 mol, 1 equiv), triethylsilylace-
tylene (0.900 mL, 5.0 × 10−3 mol, 1.1 equiv), tetrakis-
(triphenylphosphine)palladium(0) (264 mg, 2.283 × 10−4 mol, 5
mol %), and copper(I) iodide (44 mg, 2.283 × 10−4 mol, 5 mol %) in
THF:diisopropylamine (80:20 mL) gave the desired product in 79%
1
yield. H NMR (300 MHz, CDCl3) δ 7.28 (d, J = 8.56 Hz, 2H), 6.58
(d, J = 8.67 Hz, 2H), 3.78 (s, 2H), 1.03 (t, J = 7.52 Hz, 9H), 0.65 (q, J
= 8.01 Hz, 6H).
4-((Trimethylsilyl)ethynyl)benzenediazonium Tetrafluoroborate
+
+
(TMS-Eth-ArN2 ). Aryl diazonium ion TMS-Eth-ArN2 was produced
EXPERIMENTAL SECTION
1
from TMS-Eth-ArNH2. H NMR (300 MHz, CDCl3) δ 8.55 (d, J =
■
8.87 Hz, 2H), 7.77 (d, J = 9.10 Hz, 2H), 0.29 (s, 9H).
Chemicals. Commercially available reagents were used as received
without further purification. Aniline precursors were prepared using
the Sonogashira coupling reaction (see Scheme 1). All diazonium ions
were synthesized from the corresponding aniline using a published
procedure (see Scheme 2).9 4-((Triisopropylsilyl)ethynyl)-
4-((Triethylsilyl)ethynyl)benzenediazonium Tetrafluoroborate
+
+
(TES-Eth-ArN2 ). Aryl diazonium ion TES-Eth-ArN2 was produced
1
from TES-Eth-ArNH2. H NMR (300 MHz, CDCl3) δ 8.45 (d, J =
8.76 Hz, 2H), 7.74 (d, J = 8.67 Hz, 2H), 1.03 (t, J = 7.91 Hz, 9H), 0.69
(q, J = 7.91 Hz, 6H).
+
benzenediazonium tetrafluoroborate (TIPS-Eth-ArN2 ) was synthesis
and purified according to the already published procedure.9
(11-Azidoundecyl)ferrocene. (11-Azidoundecyl)ferrocene was syn-
thesized according published procedure.14 Tetrabutylammonium
fluoride, dimethylferrocene, ferrocene, decamethylferrocene, and
ferrocyanide were purchased from Alfa Aesar. Acetonitrile was 99.8%
anhydrous from Aldrich as well as tetrabutylammonium hexafluor-
ophosphate.
Electrochemical Setup and Surface Modification Proce-
dures. All electrochemical measurements were performed with an
Autolab PGSTAT 12 (Metrohm) and a conventional three-electrode
system, comprising the modified substrate as working electrode, a
platinum foil as the auxiliary electrode, and SCE electrode (Metrohm)
as reference. The GC electrodes were purchased from CH
Instruments, Inc. (TX, USA) as 2-mm-diameter rods. The electrodes
were polished successively with 1.0, 0.3, and 0.05 μm alumina slurry
made from dry alumina powder and Milli-Q water on microcloth pads
(CH Instruments, Inc., TX, USA). The electrodes were thoroughly
rinsed with Milli-Q water, acetone, and ethanol. Before derivatization,
General Procedure for the Synthesis of Aniline Derivatives. To a
stirring solution of 4-iodoaniline dissolved in a mixture of THF and
diisopropylamine was added the corresponding trialkylsilylacetylene
(1.1 equiv). After degassing with argon for 30 min, tetrakis-
(triphenylphosphine)palladium(0) (5 mol %) was added in one
portion. After 5 min, copper(I) iodide (5 mol %) was added, and the
solution was stirred at room temperature overnight under Ar. The
reaction mixture was filtered and washed with hexane. The combined
organic phase was washed with NH4Cl, brine, and water and dried
with MgSO4. Purification was performed on column chromatography
using silica gel and hexane:DCM (1:1).
4-((Trimethylsilyl)ethynyl)aniline. According to the general proce-
dure, 4-iodoaniline (1.0 g, 4.6 × 10−3 mol, 1 equiv), trimethylsilyla-
cetylene (0.715 mL, 5.0 × 10−3 mol, 1.1 equiv), tetrakis-
(triphenylphosphine)palladium(0) (264 mg, 2.283 × 10−4 mol, 5
mol %), and copper(I) iodide (44 mg, 2.283 × 10−4 mol, 5 mol %) in
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Scheme 3. Modification Procedure of the Carbon Surfaces Using Different Protecting Group Sizes
+
+
Figure 1. Si 2p XPS spectra of PPF carbon substrate modified with (1) TMS-Eth-ArN2 and (2) TES-Eth-ArN2 (a) before and (b) after
deprotection with nBu4NF solution.
the electrodes were dried with an argon gas stream. The preparation of
pyrolyzed photoresist film (PPF) followed methods described
previously.15 The PPF substrates were a gift from Prof. Alison
Downard (University of Canterbury, Christchurch, NZ) and were
squares of 15 × 15 mm2. Different carbon substrates (glassy carbon
(GC) and pyrolyzed photoresist film (PPF)) were considered as test
materials for the modification. PPF layers and GC present similar
reactivities, but PPF is more suitable for AFM experiments because of
a low roughness.2 Surface modification was performed in a solution of
10−2 mol L−1 aryldiazonium salt and 0.1 mol L−1 tetrabutylammonium
hexafluorophosphate (Bu4NPF6) in acetonitrile (ACN) using cyclic
voltammetry with a scan rate of 50 mV s−1 for five cycles between
+0.60 and −0.75 V. The electrodes were rinsed with a copious amount
of ACN and acetone, stirred 20 min in THF solution, and dried under
a stream of argon. The deprotection step consists of stirring the
modified surface 20 min in a THF solution containing 0.1 mol L−1
tetrabutylammonium fluoride (Bu4NF) as deprotecting agent. The
deprotection step is followed by a last cleaning of the sample.
protecting group moieties. The molecular volume is defined as the
volume inside a contour of 0.001 electrons/Bohr3 density without
additional correction. The conformation of the molecule was
optimized using DFT calculations at the B3LYP/6-31G* level. All
calculations were performed using the Gaussian 03 package with the
default parameters.16
RESULTS AND DISCUSSION
■
Electrografting of Carbon Surfaces. Scheme 3 presents
the general principle of the modification process. Briefly, the
carbon sample is used as an electrode and dipped in a solution
containing the protected aryldiazonium salt. The electrode
potential is cycled around the reduction threshold of the
aryldiazonium salts. After a careful cleaning, the sample is
treated using a solution of Bu4NF in THF to remove the
protecting group. In these conditions, the electrografting is a
self-inhibited process as the film blocks the electrode. The
interest is to provide well reproducible deposited layers. XPS
experiments were first made in order to confirm the effective
Modelization. The diameters d of the disk representing the
protecting group were evaluated by computing the molecular volume
of the corresponding tri(alkyl)silyl-acetylene representing the
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Figure 2. 2 × 2 μm topographic picture of PPF substrate modified with TES-Eth-ArN2+ showing a 0.4 × 2 μm scratch, after deprotection step and
line (black) and adjustment curve (red) of the AFM picture before (top) and after deprotection (bottom).
Figure 3. Cyclic voltammetry of ferrocene derivatives (10−3 mol L−1) in ACN (+0.1 mol L−1 NBu4PF6). Ferrocene (a), dimethylferrocene (b), and
decamethylferrocene (c) recorded on a blank GC electrode (black), after electrografting with TES-Eth-ArN2+ (red), and after deprotection (blue).
Scan rate = 0.1 V s−1.
deprotection of the different silyl protecting groups using
Bu4NF solution. Figure 1 shows Si 2p XPS spectra of a PPF
carbon substrates modified with aryl diazonium TMS-Eth-
thicknesses range from 0.8 to 2 nm based on repetitive
experiments.19 We noticed that at this step of the procedure,
the measured thickness depends on the cleaning of the surface.
It suggests the presence of loosely attached materials that also
limits the layer growing during the electrografting step.
However, after the deprotection step, all layers present similar
thicknesses (around 0.6 nm) with an excellent reproducibility
over several measurements and different samples. As seen in
Figure 2, the roughness of the modified sample was that of the
original PPF substrate. Such value agrees well with the
thickness expected for an ethynylaryl monolayer (around 0.66
nm)9 and is not dependent on the size of the used protecting
group. Interest in the deprotection is thus not only to reveal the
active ethynylaryl group but also to eliminate all loosely
attached materials. It is remarkable that a TMS group is large
enough to inhibit the attack of aryl radicals as we have observed
for the bulkier TIPS groups.9
+
+
ArN2 and TES-Eth-ArN2 before and after deprotection. In
Figure 1, two peaks could be identified at 101 and 102.5 eV.
The peak located at 101 eV is related to Si−C bonds and
ascribed to silicon in TMS and TES protection groups.17 No
particular precaution against O2 was taken explaining the
contribution at 102.5 eV in the Si 2p signal, due to the
formation of oxidized species. After deprotection, (Figure 1-
1b), the signal related to Si 2p (at 101 and 102.5 eV) is
considerably decreased. In Figure 1-2b, the signal at 101 eV
attributed to Si−C bonds is no longer visible. A contribution at
102.5 eV is still visible showing the presence of Si−O−C
bonds, which is probably due to fragments trapped in the
organic layer after the deprotection step.
Thicknesses of the layers were measured by AFM scratching
before and after the deprotection step18 (see Figure 2 for
To complete characterizations of the as-prepared layers,
permeation of molecules through the deposited layer was
investigated by examining the modifications of the electro-
chemical response of different redox probes in solution on the
modified surfaces. The voltammogram characteristics, such as
the shape or the variations of the peak current intensities,
+
sample modified with TES-Eth-ArN2 and Supporting
Information). To avoid possible bias, a statistical procedure
was devised to average the layer thickness over a large fraction
of the scratch area. At least 10 profile lines were taken to
calculate an average layer thickness. After electrografting, film
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4‑
Figure 4. Cyclic voltammetry of 10−3 mol L−1 Fe(CN)6 in H2O (+0.1 mol L−1 KCl) on a bare GC surface (black) and after modifications with
TMS-Eth-ArN2+ (a,b), TES-Eth-ArN2+ (c,d), and TIPS-Eth-ArN2+ (e,f); before (red) and after deprotection (blue). Experimental (a,c,d) and CEC
simulations (b,d,f). Scan rate = 0.1 V s−1.
provide valuable information about the global charge transfer
process (notably to discriminate between charge transfer by
tunneling through the layer or permeation through pinholes).20
A series of ferrocene derivatives with increasing sizes were first
considered (see Figure 3). Similar trends were observed for all
modified surfaces. Upon electrografting and after cleaning the
sample, the peak current becomes broader and its intensity
decreases showing a partial blockage of the electrode. The
inhibition increases with the size of the probe illustrating by an
S-shape voltammogram for the bulkiest ferrocene indicative of a
diffusion of the redox probe through pinholes.20
After deprotection and for all the ferrocene derivatives, the
voltammogram turns out to be indiscernible from that obtained
on the blanked carbon surface suggesting that the charge
transfer occurs through different pathways (via pinholes and
tunneling through the layer).20,21 This observation falls in line
with the AFM scratching data that conclude about a
subnanometric deposit layer allowing an efficient tunneling of
the charge besides permeation. Thus, ferrocene and its
derivatives are not sensitive enough to detect permeation and
the ferrocyanide/ferricyanide couple was chosen instead. As
discussed before, its inner sphere character permits the
observation of structural differences even for very thin modified
surfaces.11,22 After the electrografting step and for all modified
surfaces, ferrocyanide oxidation is totally inhibited (Figure 4,
red lines). After deprotection of the layer, ferrocyanide
oxidation reappears for the TES- and TIPS- prepared surfaces
(Figure 4c,e, blue lines) but remains inhibited for the TMS-
one (Figure 4a). The curves display a larger peak-to-peak
potential separation and a global decrease of the current
associated with a strong “S-shape” character. The voltammo-
grams were compared with simulations assuming a model of
interacting pinholes (interacting diffusing channels treated by
the CEC analogy).20a The experimental tendency is well
reproduced in the simulations confirming that the electro-
chemical response is for this probe governed by diffusion
channels. Notice that the blockage follows the size of the
protecting group used during the electro-grafting process;
inhibition is more pronounced for the TES- prepared surface
than for the TIPS- one, the oxidation of ferrocyanide on the
TMS- surface being totally inhibited.
Postfunctionalization Procedures. The “click” chemistry
reaction (Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition) is
nowadays often used to functionalize a wide range of surfaces
and is used here to verify the reactivity of the deprotected
acetylene moieties. The coupling was performed for each
modified and deprotected substrate with (11-azidoundecyl)-
ferrocene as a test system. The redox group serves as a
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Figure 5. Cyclic voltammogram in ethanol + 0.1 mol L−1 LiClO4 of modified-GC electrode after postfunctionalization with (11-
+
+
azidoundecyl)ferrocene using click chemistry coupling method on modified-GC electrode with (a) TMS-Eth-ArN2 , (b) with TES-Eth-ArN2 ,
and (c) with TIPS-Eth-ArN2 . Scan rate 0.1 V s−1. Inset: Variation of the anodic and cathodic peak current with the scan rate.
+
convenient label for measuring the initial amount of ethynylaryl
moieties. In a typical click chemistry procedure, a 1:1
water:THF solution of CuSO4 (10−2 mol L−1) and L(+)-
ascorbic acid (2 × 10−2 mol L−1) was stirred in the presence of
(11-azidoundecyl)ferrocene9 (10−5 mol L−1) and the modified
substrate. Figure 5 shows the electro-activity of the modified
electrode in a blank ethanolic solution containing only 0.1 mol
L−1 LiClO4 as supporting electrolyte.
The surface concentrations of active ferrocene centers (Γ)
were derived from these experiments using the slope of the
linear variation of the peak current Ip vs scan rate v according to
Γ = 4aRT/F2S. F is the Faraday constant, a the slope of Ip vs v
variation, and S the surface area of the sample. The surface S is
corrected from the roughness factor (S/Sgeom, where Sgeom is the
geometric surface area) taken as 2.48 for a glassy carbon
substrate. As seen in Table 1, all Γ values are on the order of
used protecting group. Finally, we could compare Γ variations
with the permeability seen by a ferro-/ferricyanide probe
(Figure 2) that is quantified by the apparent fractional coverage
ratio θCEC where θCEC is the value derived from the CEC
analogy.20a It illustrates that the porosity decreases suddenly
when passing from the TIPS- to the TES- layer. This behavior
is similar to what was observed for a nanostructured alkylthiol
monolayer on a gold surface.11 It was estimated that the passage
of the ferrocyanide couple is blocked when pinholes have
diameter lower than 0.5 nm, a value that is in good agreement
with our observations.
CONCLUSION
■
The electrografting of protected aryldiazonium salts is an
efficient process for the preparation of controlled structured
monolayers. It is noticeable that a trimethylsylil group is bulky
enough to protect the already attached aromatic groups from
the attack of aryl radicals and thus ensures the formation of
functional monolayer with high density of active sites.
However, the monolayer keeps a memory of the protecting
groups after its removal leading to the formation of molecular
pinholes. A thin control of this phenomenon opens large
possibilities to design ordered hierarchical surface modifications
at the molecular scale. Changing the size of the protecting
group controls the size of the pinhole as it could be
demonstrated by examining the permeation properties and
the density of immobilized active functions. Taking advantage
of the “click chemistry”, the layer could serve as a versatile
platform for immobilizing many functional entities with a
controlled spatial arrangement (density, spacing). Notice that
the procedure could easily be extended to different materials
substrates thanks to the radical nature of the electrografting
process.
Table 1. Characteristics of the Deprotected Monolayers
ab
,
ac
,
Γlabeling (relative to
TMS-)
ΓTheoretical (relative to
TMS-)
d
layer
θCEC
TMS-
TES-
3.3 (1)
3.3 (1)
0.995
0.98
0.72
2.5 (0.77)
2.7 (0.82)
TIPS-
2.0 (0.62)
2.3 (0.68)
a
b
Γ in 10−10 mol cm2. Taking 2.48 as roughness factor for a glassy
c
carbon surface. Estimated from the diameter of the equivalent sphere
of the protecting group TMS: 0.75 nm, TES: 0.84 nm, TIPS: 0.92 nm
d
and hexagonal compact packing. Using the CEC analogy.20a
several 10−10 mol cm−2 corresponding for a monolayer to high
densities of active ethynylaryl groups.9 Notice that Γ
considerably decreases when the size of the protecting group
increases. Γ values for the TES- and TIPS- prepared layers are
0.73 and 0.61 times lower than that of the TMS- one. Assuming
that the deposited aryl moieties are arranged in a hexagonal
close packing lattice (the optimal way to stack n discs of
diameter d), the surface concentration Γ in mol cm−2 could be
estimated using the following relation Γ = 1.92 × 10−8/d2
where d is in Å.9 The diameter d of the disk representing the
protecting group was evaluated by computing the molecular
volume of the corresponding tri(alkyl)silyl-acetylene represent-
ing the protecting group moieties. We obtained d = 0.758, 0.84,
and 0.92 nm respectively for the TMS, TES, and TIPS
protecting groups leading to the theoretical Γ shown in Table
1. It is remarkable that the experimental surface concentrations
show a good agreement with the calculated predictions that
assume control of the active function density by the size of the
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional electrochemical and AFM experimental data and
details about calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: philippe.hapiot@univ-rennes1.fr.
Notes
The authors declare no competing financial interest.
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attack (by H atom abstraction) the alkyl groups of the protecting
groups.
(20) (a) Amatore, C.; Saveant, J.-M.; Tessier, D. J. Electroanal. Chem.
́
1983, 147, 39. (b) Compton, R. G.; Menshykau, D. Langmuir 2009,
25, 2519.
(21) Outsphere redox probe like ferrocene are almost insensitive to
the presence of monolayer coverage. See for example: Lhenry, S.;
Leroux, Y. R.; Hapiot, P. Anal. Chem. 2012, 84, 7518 and references
therein.
ACKNOWLEDGMENTS
■
This work was supported in part by the Agence Nationale de la
Recherche. Contract ANR10-BLAN-714, Cavity-zyme(Cu)
project). Prof A.J. Downard (University of Canterbury,
Christchurch, NZ) is warmly thanked for providing us the
sample of PPF substrates. Dr. Fei Hui is thanked for his help in
the AFM scratching measurements.
(22) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958.
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(19) For TMS-Eth-Ar and TIPS-Eth-Ar layers, AFM scratching
measurements are consistent with monolayer formation. For the TES-
Eth-Ar layer, the measured thickness is larger than that expected for a
monolayer. This is more likely ascribed to the difficulty of removing all
adsorbed and loosely attached species during the rinsing process.
Another possible explanation could be that the aryl radicals slowly
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dx.doi.org/10.1021/cm303844v | Chem. Mater. 2013, 25, 489−495