Fig. 1 shows typical cyclic voltammogram (CV) responses and
UV-visible spectra for the in situ substituent-assisted diazotization of
4-ABSA. CVs recorded in HCl-free solution, 30 min after NaNO2
addition, show typical I–E patterns as for the in situ diazotization in
strong acidic conditions (Fig. S1†), except a lower intensity in current.
The first CV recorded at a GC electrode shows an irreversible wave
located at ꢀ0.54 V that corresponds to the reduction of the in situ
produced diazonium salt to form the substituted phenyl radical,
which passivates the carbon surface. Concomitantly, after NaNO2
addition, a peak at 270 nm emerges on the UV spectrum that can be
attributed to the diazonium ion, which rapidly reacts with the initial
primary amine to provide a triazene (lmax ¼ 358 nm). These
assignments were confirmed by UV performed in 0.5 M HCl
(Fig. S2†) and by NMR experiments in D2O (Fig. S3†).
conditions used. Downard and co-workers12 have recently claimed
that two distinct mechanisms are responsible for the grafting at
a glassy carbon surface: a first potential-dependent process gives the
radical aryl and a second potential-independent step extends the film
growth by chemical grafting onto a primer layer. For the grafting at
activated carbon substrates, the mechanism is less clear and no
evidence for radical arylation at the surface has been reported.13 In all
ꢁ
cases, less than a monolayer was obtained and, as Belanger suggests,
the grafting is blocked as soon as the available reactive sites at surface
are saturated.7 In the latter study, surface reactions, implying
oxygenated functionalities, are suspected. So the chemical grafting is
probably the result of complex surface chemistry processes, implying
dediazoniation products (phenyl cations or aryl radical), azo-
coupling reactions and surface functionalities.
In pure water, it is assumed that the sulfonic acid protonates the
amino group,10 after which the diazonium ion can be generated
(Scheme 1).
Assuming the inclination of amine-containing compounds to react
with carbon,14 we postulate that the higher blocking effect for GC
spontaneously modified in HCl-free solution results in a chemical
cooperation of the starting amine and diazo functionalities, which
implies different active sites at the carbon surface. To validate our
assumptions, activated carbon (Norit-S50) was spontaneously func-
tionalized in various conditions either from in situ produced 4-diaz-
obenzene sulfonic acid (4-DBSA) or from previously synthesized
4-DBSA (Table 1).
Because only one proton per arylamine compound can be
provided, the incomplete diazotization reaction can be combined
with the subsequent N-azo coupling due to the nucleophilicity of
primary arylamines, to form the symmetric 1,3-diaryltriazene product
(Fig. S3†). During the first 30 min, the pH of the solution changes
from 2.8 to 5.3 and remains constant beyond. So, the pH is low
enough to avoid C-coupling, but not too low for limiting the
decomposition of the triazene that would regenerate the starting
products.11 As indicated by NMR experiments in D2O (Fig. S3†),
after a few minutes at room temperature, the self-diazotization
reaction was optimal and the solution becomes slightly reddish one
hour after sodium nitrite was added. It must be noted that, in our
conditions, three days after the diazotization started, the UV-visible
spectrum shows an absorption peak assigned to the diazonium salt.
So the solution obtained in pure water contains at once amine, azo
and triazene functionalities, being suitable for surface derivatization
in mild conditions.
Highest values of the percentage of sulphur were obtained when
the diazotization was promoted by the acidic substituent in pure
water (NS1) and when the carbon product was prepared from the
synthesized diazonium salt (NS4). On the other hand, the nitrogen
content was higher for NS1 compared to all other carbon products.
First, these results show that for different conditions in solution, the
surface concentration of benzene sulfonic acid groups tends to the
same value, suggesting that the grafting mechanism is governed by
heterogeneous factors. Second, the highest nitrogen percentage for
NS1 is evidence for the concomitant participation of amine and diazo
compounds in the grafting process based on the substituent-assisted
diazotization procedure. It is noteworthy that the simpler diazotiza-
tion strategy reported is more efficient for the grafting onto activated
carbon than the conventional in situ diazotization reaction in HCl
solution and as much as with synthesized diazonium salt. In the latter
case, only the nitrogen content changes, indicating that the grafting
occurs through the amine and the diazonium ion when the diazoti-
zation is substituent-assisted.
Cyclic voltammetry was used to investigate the blocking properties
of modified GC electrodes in the presence of ferri-ferrocyanide in
water (Fig. S4†). For electrografting, strong blocking effects against
redox reaction were obtained whatever the conditions in which the
diazonium salts were produced (in acidic condition or in water).
Intriguingly, for electroless grafting, the redox reaction of Fe(CN)3ꢀ/4ꢀ
was more suppressed at a surface modified from 4-ABSA diazotized in
acid-free solution. These changes in blocking properties are evidence
that electrografting and grafting occur through different mechanisms.
For electrografting, the carbon electrode serves as the cathode and
it is well assumed that the neutral aryl radical formed by homolytic
dediazoniation is responsible for the arylation of the substrate. For
chemical grafting, the mechanism remains elusive. Mono- or multi-
layers can be obtained, depending on the substrate and to the
NS1 and NS2 carbon powders were studied by X-ray photoelec-
tron spectroscopy (Fig. 2). Both carbons show a N 1s peak in the
vicinity of 405–406 eV, due to NO2 groups that stem from the use of
sodium nitrite. An additional nitrogen peak, located at 401.4 eV, was
observed in NS1 and NS2 carbons with an atomic ratio depending on
the in situ procedure. This N1s peak, characteristic of ammonium
cations, is more intense for the NS1 carbon because the amino group
was fully protonated in strong acidic conditions.15
Moreover, for NS1 a nitrogen peak at 400 eV mainly contributes
to the N1s core level spectrum, unlike NS2 for which the major
contribution is located at 399.4 eV. The two latter peaks are evidence
that the grafting occurs through different routes depending on the
procedure. In the presence of HCl, an azo bond can be suspected,16
whereas in water, it could be assumed that the unprotonated amine
contributes to the grafting. So, in the absence of strong acidic
conditions, the grafting at the surface seems to take advantage of the
incomplete diazotization reaction by combining amine and diazo-
nium salts for optimal grafting at the surface.
Scheme 1 Reaction systems for the diazotization of sulfanilic acid in
acidic solution and in water.
12222 | J. Mater. Chem., 2011, 21, 12221–12223
This journal is ª The Royal Society of Chemistry 2011
Lebegue, Estelle
