Microporous and Mesoporous Materials 308 (2020) 110380

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Microporous and Mesoporous Materials
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Application of sol-gel methods to obtain silica materials decorated with
ferrocenyl-ureidopyrimidine moieties. Preparation of hollow spheres and
modification of a carbon electrode
´s Jakab d, Lívia Nagy e,
Soma J. Keszei a, P´eter Pekker b, Csaba Feh´er a, Szabolcs Balogh c, Miklo
ăldes a, *
Rita Skoda-Fo
a

University of Pannonia, Institute of Chemistry, Department of Organic Chemistry, Egyetem u. 10, P.O.Box 158, Veszpr´em, H-8200, Hungary
Research Institute of Biomolecular and Chemical Engineering, NANOLAB, University of Pannonia, Egyetem u. 10, P.O.Box 158, Veszpr´em, H-8200, Hungary
c
Institute of Chemistry, NMR Laboratory, University of Pannonia, Egyetem u. 10, Veszpr´em, H-8200, Hungary
d
Institute of Materials Engineering, University of Pannonia, Egyetem u. 10, Veszpr´em, H-8200, Hungary
e
J´
anos Szent´
agothai Research Center, University of P´ecs, Ifjús´
ag útja 20, P´ecs, H-7624, Hungary
b

A R T I C L E I N F O

A B S T R A C T

Keywords:
Ferrocene
Immobilization
Hollow sphere
Sol-gel method
Electrode-modification

The application of a sol-gel method, starting from N-(4-ferrocenyl-6-phenylpyrimidin-2-yl)-N’-(3-(triethoxysilyl)
prop-1-yl)-urea (4), tetraethoxysilane and a structure directing agent, led to the formation of hollow spherical
particles with organic moieties concentrated on the inner surface. A similar sol-gel electrodeposition technique
was used for the modification of the surface of a spectral graphite electrode. The solid materials were charac­
terized by solid phase NMR and IR measurements, thermal analysis, SEM and TEM. The functional group
attached to the ferrocene core offers the possibility to form H-bonds with various guest molecules that makes it a
potential electrochemical sensor.

1. Introduction
The discovery of ferrocene opened a new area of research in the field
of organometallic chemistry [1–3]. The parent compound and its de­
rivatives may serve as catalysts [4,5] or ligands in homogeneous cata­
lytic reactions [6–8], can be built into drugs to enhance or modify
biological activity [9–13] or can be used as electrochemical sensors
[14], ion receptors [15], biosensors [16,17] or molecular machines [18]
due to the favourable redox properties of the ferrocene core.
In most of the applications mentioned above, the immobilization of
ferrocene on a solid carrier may broaden the utility of the original
molecule. Thin films modified with ferrocene [19], ferrocene-based
polymers [20] or ferrocene-functionalized graphene nanotubes [21]
can serve as biosensors. Moreover, incorporation of ferrocene into
polymers leads to redox-controllable materials [22], that can be used in

drug-delivery systems [23], redox-responsive polymer gels [24], artif­
ical molecular receptors [25] and in several other electrochemical ap­
plications, (e.g. electrosynthesis, supercapacitors and redox flow
batteries) [26].
A broad range of solid carriers was decorated with ferrocene

derivatives, such as graphene or carbon nanotubes [21], organic poly­
mers [22,23,27] and nanoparticles [19].
Besides post-synthetic modification methods [28], silica-based ma­
terials can be obtained by the polymerization of a silica precursor (e.g.
TEOS (tetraethoxysilane) or TMOS (tetramethoxysilane)) in the pres­
ence of a ferrocene derivative to confine the latter in the silica network
[29–31].
A further possibility for the preparation of modified silicas is the direct
copolymerization of the organic modifier bearing trialkoxysilyl moieties
with silica precursors [32,33]. The reaction can be catalyzed by added acid or
base, resulting in the condensation of the SiOR groups. Similarly,
co-condensation of the silica precursor with the appropriately modified
ferrocene derivative (1,1-bis(trimethoxysilyl)ferrocene [34,35], (ferroce­
nylmethyl)dimethyl(ω-trimethoxysilyl)alkylammonium hexafluorophosph
ate [36] or N-(3-trimethoxysilylpropyl)-ferrocenylacetamide [37]) led to the
ferrocene-modified gel that could be deposited on the surface of an electrode
by drop coating [34,35] or spin casting [36].
Silica materials with ordered structure can be obtained by the sol-gel
method by adding different template molecules into the reaction
mixture [38]. The methodology was also used successfully for the

* Corresponding author.
E-mail address: (R. Skoda-Fă
oldes).

/>Received 3 March 2020; Received in revised form 6 May 2020; Accepted 2 June 2020
Available online 9 July 2020
1387-1811/© 2020 The Authors.
Published by Elsevier Inc.
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S.J. Keszei et al.

Microporous and Mesoporous Materials 308 (2020) 110380

preparation of ferrocene-modified silicas [39–41] via the

co-condensation of 1,10-bis[2-(triethoxylsilyl)ethyl]ferrocene [39,40]
or 4-triethoxysilyl-3′ -ferrocenylazobenzene [41] and TEOS under acidic
[39] or basic conditions [40,41], using cetyltrimethylammmonium
bromide (CTAB) [40,41] or Pluronic P123 [39] as the
structure-directing agent.
Sol-gel electrodeposition is a special case of direct synthesis for
creating thin silica films on the surface of an electrode. The main point of
this method is to apply cathodic or anodic potential on the reaction
mixture to obtain the catalytically active H+/OH− species by electro­
lyzing the water content of the solution. During the immobilization, a
potential gradient is created in the solution that results in the formation
of the organosilica in the vicinity of the electrode [42,43].
The capability of urea to form multiple hydrogen bonds with com­
plementary anions as well as with neutral molecules makes it a popular
functionality in sensors and supramolecular assemblies [44–46].
Recently, the synthesis of various new 2-ureido-4-ferrocenyl pyrimidine
derivatives was developed in our group [47]. These compounds were
shown to bind 2,6-diaminopyridine [47] and to form complexes with
anions of strong acids after protonation [48]. Both procedures could be
followed by NMR and cyclic voltammetry, so these materials are
promising candidates to be the building blocks for sensors.
As a continuation of our work, the preparation of silica materials
incorporating the 2-ureido-4-ferrocenyl pyrimidine moiety is reported
in the present paper. A sol-gel methodology was used not only for the
construction of an ordered structure but also for the deposition of a
ferrocene-silica hybrid material on a spectral graphite electrode.

(AMETEK) was used at 5.0 kV for imaging and 25.0 kV for elemental
mapping.
The Sil2 sample for transmission electron microscopy (TEM) was

prepared by depositing a drop of aqueous suspension of sediment par­
ticles on copper grids covered by lacey carbon amorphous support film.
TEM analyses were performed using a Talos F200X G2 instrument
(Thermo Fisher), operated at 200 kV accelerating voltage, equipped
with a field-emission gun and a four-detector Super-X energy-dispersive
X-ray spectrometer, and capable of working in both conventional TEM
and scanning transmission (STEM) modes. Bright-field (BF) images were
obtained in TEM mode. STEM high-angle annular dark-field (HAADF)
images were collected both for characterization and for mapping
elemental compositions by coupling STEM imaging with energydispersive X-ray spectrometry (EDS).
Thermogravimetric analysis was carried out on a Netsch TG 209
instrument, using 15 ◦ C/min heating speed.
The Fe content of the modified silica phases was determined by ICPOES using a SpectroFlame Modula E (Spectro) atomic absorption spec­
trometer. The samples were prepared for analysis according to the
following method: 4 cm3 of concentrated nitric acid was added to 20 mg
of the samples and the mixture was boiled for 6 h.
Cyclic voltammetry experiments were performed on a Radiometer
Analytical PST-006 potentiostat (4 in solution; E-Sil1) or on a Radi­
ometer Analytical PGZ-301 potentiostat (E-Sil2—E-Sil5) with a con­
ventional three-electrode configuration, consisting of a spectral graphite
or glassy carbon working electrode (OD = 3 mm), a platinum wire
auxiliary electrode, and Ag/AgCl reference electrode.
Procedure for the cyclic voltammetry experiment in organic me­
dium: 6.03 mg (0.01 mmol) of 4 was dissolved in dry acetonitrile,
containing 349.1 mg (1 mmol) of tetrabutylammonium-perchlorate,
which served as supporting electrolyte. The solution was bubbled with
argon to remove dissolved gas residuals and to ensure inert atmosphere
during measurements. The working electrode was wet polished on 0.5
μm alumina slurry or emery paper grade 500, after each measurement.
Cyclic voltammograms were recorded with a scan rate of 0.1 V s− 1.

Procedure for cyclic voltammetry experiments with a modified
working electrode (E-Sil1, E-Sil2) in aqueous medium: 149.1 mg KCl (2
mmol) was dissolved in 10 ml of distilled water and the solution was
transferred to the electrochemical cell. Procedure for cyclic voltammetry
experiments with a modified working electrode (E-Sil1-E-Sil5) in
organic medium: 341.9 mg TBAClO4 (1 mmol) was dissolved in 10 ml of
CH2Cl2 and the solution was transferred to the electrochemical cell. The
mixtures were bubbled with argon to remove dissolved gas residuals and
to ensure inert atmosphere during measurements.. Cyclic voltammo­
grams were recorded with multiple scan rates in a range of 0.05–0.5 V
s− 1
CAUTION: TBAClO4 may cause skin, eye, and respiratory irritation
and may intensify fire. (Keep away from combustible materials. Avoid
breathing dust. Wear eyeshields, protective gloves, and a full-face par­
ticle respirator.)

2. Experimental
2.1. Reagents and materials
Starting materials were purchased from commercial sources and
were used without further purification.
2.2. Methods and apparatus
The reactions leading to compounds 2–4 were followed by thin layer
chromatography.
1
H and 13C NMR spectra were recorded in CDCl3 on a Bruker Avance
400 spectrometer at 400 and 100 MHz, respectively using a 5 mm SB
BBO probehead with Z-gradient. Chemical shifts (δ) are reported in ppm
relative to CHCl3 (7.26 and 77.00 ppm for 1H and 13C, respectively) and
acetone (2.05 and 206.26 ppm for 1H and 13C, respectively). Solid phase
13

C- and 29Si CP MAS NMR measurements were carried out on a Bruker
Avance 400 spectrometer using a 4 mm MAS probehead at spinning
speeds up to 10 kHz in order to differentiate spinning sidebands from
isotropic shifts.
IR spectra were obtained using a Thermo Nicolet Avatar 330 FT-IR
instrument. Samples were prepared as KBr pellets.
Scanning electron microscopy measurements of Sil1 and Sil2 sam­
ples were performed by a Philips/FEI XL 30 environmental scanning
electron microscope. Observation by ESEM was carried out in high
vacuum with an accelerating voltage of 25.0 kV.
SEM measurements of Sil3 —Sil6 were accomplished by a FEI/
ThermoFisher Apreo S scanning electron microscope in scanning
transmission (STEM) and scanning (SEM) mode. Observation by both
STEM and SEM was carried out in high vacuum with an accelerating
voltage of 30.0 kV (STEM) and 2.0 kV (SEM). Samples of Sil3 and Sil4
for scanning electron microscopy were prepared by depositing a drop of
aqueous suspension of sediment particles on copper grids covered by
lacey carbon amorphous support film.
The surface of the modified (E-Sil1) spectral graphite electrode was
tested by scanning electron microscopy coupled with energy-dispersive
x-ray spectroscopy (SEM/EDS). Apreo SEM (ThermoFisher Apreo S
scanning electron microscope) equipped with Octane Elect Plus EDS

2.3. Synthetic procedures
2.3.1. Synthesis of 3-(ferrocenyl)-1-phenylprop-2-en-1-one (2)
1 mmol of ferrocene-carboxaldehyde (1), 1 mmol of acetophenone
and 2 mmol of NaOH were mixed in a round bottomed flask and the
mixture was stirred at room temperature for 24 h. The product was
isolated by column chromatography (silica, eluent: toluene) [49]. NMR
data were identical to those reported before [50]. Yield: 94%. Rf: 0.57

(silica, toluene:ethyl acetate 25:1).
2.3.2. Synthesis of 2-amino-4-ferrocenyl-6-phenylpyrimidine (3)
1 mmol of 3-(ferrocenyl)-1-phenylprop-2-en-1-one (2), 2 mmol
guanidine carbonate, and 1 mmol NaOH were dissolved in 3 ml THF in a
Schlenk tube equipped with a reflux condenser and a balloon on the top.
The reaction mixture was refluxed for 16 h in argon atmosphere. The
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Microporous and Mesoporous Materials 308 (2020) 110380

Scheme 1. Synthesis of ferrocenyl-ureidopyrimidine 4. Reaction conditions: i: acetophenone, NaOH, room temperature, 24 h; ii: guanidine carbonate, THF, NaOH,
reflux, 16 h; iii: (EtO)3Si(CH2)3NCO, 100 ◦ C, 3 h.

product was isolated by column chromatography (silica, eluent:
toluene/EtOAc, 8:1) [47,49]. Yield: 53%. Rf: 0.29 (silica, toluene:ethyl
acetate 8:1). The 1H NMR was identical to that reported before [51].

filtration to produce 161.6 mg of Sil2. CTAB content was removed by
Soxhlet extraction, using dichloromethane. Iron content by ICP-OES:
10.87 mg/g, corresponding to 0.19 mmol 4/g solid material. 13C CPMAS NMR (100 MHz) δ: 170.66; 158.18; 135.49; 128.86; 106.46;
79.97; 70.69; 54.08; 43.92; 30.16; 23.39; 14.58; 10.45 ppm. 29Si CP
MAS NMR (79 MHz) δ: − 56.89, − 65.67, − 101.11, − 110.97 ppm. IR
(KBr) cm-1: 3424; 3260; 3088; 2926; 2859; 1686; 1590; 1582; 1529;
1197; 1079; 955; 790; 771; 690.

2.3.3. Synthesis of N-(4-ferrocenyl-6-phenylpyrimidin-2-yl)-N’-(3(triethoxysilyl)prop-1-yl)-urea (4)
0.1 mmol 2-amino-4-ferrocenyl-6-phenylpyrimidine (3) and 0.7

mmol TESPI (3-(triethoxysilyl)propyl isocyanate) were mixed and stir­
red at a temperature of 100 ◦ C for 3 h, under argon atmosphere. The
product was isolated by column chromatography (silica, eluent:
toluene/EtOAc, 8:1).
Yield: 66%. Rf: 0.60 (silica, toluene-ethyl-acetate 2:1).
1
H NMR (400 MHz, acetone-d6) δ 9.50 (brs, 1H); 8.44 (s, 1H);
8.22–8.20 (m, 2H); 7.72 (s, 1H); 7.57–7.54 (m, 3H); 5.18 (t, J = 1.9 Hz,
2H); 4.59 (t, J = 1.9 Hz, 2H); 4.15 (s, 5H); 3.82 (q, J = 7.0 Hz, 6 H);
3.46–3.41 (m, 2H); 1.84–1.76 (m, 2H); 1.18 (t, J = 7 Hz, 9H); 0.78–0.74
(m, 2H) ppm. 13C NMR (100 MHz, acetone-d6) δ 169.87; 164.39; 158.93;
154.77; 137.09; 131.26; 129.16; 127.49; 106.07; 80.48; 71.67; 70.35;
68.59; 58.35; 42.68; 24.05; 18.18; 8.16 ppm. IR (KBr) cm− 1: 3423; 3246;
3084; 2975; 2925; 2884; 1683; 1590; 1581; 1527; 1106; 1079; 957; 774.

2.4.3. Preparation of Sil3 and Sil4
Sil3 and Sil4 were prepared by methods identical to the preparation
of Sil2 but by adding different amounts of water: 2 ml for Sil3 and 8 ml
for Sil4, resulting in the isolation of 87.4 mg (Sil3) and 163.4 mg (Sil4)
solid material.
2.4.4. Preparation of Sil5 and Sil6
Sil5 and Sil6 were prepared by methods identical to the preparation
of Sil2 but in the absence of CTAB in case of Sil5 and in the absence of
TEOS in case of Sil6, resulting in the isolation of 114.4 mg (Sil5) and 5.9
mg (Sil6) solid material.
2.4.5. Modification of a spectral graphite electrode
E-Sil1: A solution of 0.5 mmol of 4 in 8 ml ethanol was transferred to
the electrochemical cell and 173 mg CTAB (0.5 mmol), 443 μl TEOS (2
mmol), 202.2 mg KNO3 (2 mmol) and 2 ml water were added. A spectral
graphite working electrode (OD = 3 mm), Ag/AgCl reference electrode

and Pt counter electrode were immersed into the solution and cathodic
potentials were applied for 60 min (− 1200 mV for 30 min and − 900 mV
for 30 min).
E-Sil2 — E-Sil5: A solution of 0.1 mmol of 4 in 8 ml ethanol was
transferred to the electrochemical cell and 173 mg CTAB (0.5 mmol),
TEOS (89 μl (0.4 mmol) for E-Sil2, 22 μl (0.1 mmol) for E-Sil3, 177 μl
(0.8 mmol) for E-Sil4 and 266 μl (1.2 mmol) for E-Sil5), 202.2 mg KNO3
(2 mmol) and 2 ml water were added. A spectral graphite working
electrode (OD = 3 mm), Ag/AgCl reference electrode and Pt counter
electrode were immersed into the solution and − 1300 mV potential was
applied for 15 min.

2.4. Preparation of ferrocene—silica materials (condensation of N-(4ferrocenyl-6-phenylpyrimidin-2-yl)-N’-(3-(triethoxysilyl)prop-1-yl)-urea
(4))
2.4.1. Preparation of Sil1
A mixture of 0.1 mmol of 4, 3 ml ethanol and 5 mg NaOH was
transferred to a Schlenk tube and 150 μl of distilled water was added. It
was stirred at a temperature of 50 ◦ C for 8 h and the solid material was
isolated by vacuum filtration to produce 41.9 mg of Sil1. Iron content by
ICP-OES: 110.41 mg/g, corresponding to 1.97 mmol 4/g solid material.
13
C CP MAS NMR (100 MHz) δ: 167.76; 158.07; 136.69; 129.30;
79.61; 70.65; 60.99; 44.09; 24.40; 15.14; 10.97 ppm. 29Si CP MAS NMR
(79 MHz) δ: − 59.21, − 67.68 ppm. IR (KBr) cm-1: 3436; 3340; 3084;
2974; 2929; 2884; 1687; 1591; 1581; 1531; 1263; 1196; 1129; 1037;
850; 776; 687.
2.4.2. Preparation of Sil2
A mixture of 0.1 mmol of 4, 8 ml ethanol, 5 mg NaOH, 36.5 mg CTAB
(0.1 mmol) and 182 μl TEOS (0.8 mmol) were transferred to a Schlenk
tube and 4 ml of distilled water was added. The mixture was stirred at a

temperature of 50 ◦ C for 8 h and the product was isolated by vacuum
3


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(4) with TEOS in the presence of CTAB as structure directing agent led to
Sil2. According to ICP-OES measurements the iron-content of the
functionalized silicas was found to be 110.41 mg/g (1.97 mmol immo­
bilized 4/g) and 10.87 mg/g (0.19 mmol immobilized 4/g) for Sil1 and
Sil2, respectively.
3.1.1. Infrared studies
The presence of the 2-ureido-4-ferrocenyl pyrimidine functionality
in the solid materials is clearly indicated by the similarity of the infrared
spectrum of monomer 4 and the spectra of the modified organosilicas
(Figure S11 and S12). Two N–H stretching vibrations can be identified in
the IR spectrum of 4 at 3423 and 3246 cm− 1. The difference between the
two frequencies shows that the structure of 4 is defined by an intra­
molecular hydrogen bond between one pyrimidine nitrogen atom and an
NH group of urea (Scheme 1) similarly to other ferrocenylureidopyrimidines reported before [47,48]. Although the N–H stretch­
ing vibrations can also be observed in the spectra of Sil1 and Sil2, the
bands overlap with the Si–OH stretching vibrations in the same region.
The presence of ureidopyrimidine 4 is clearly indicated by the appear­
ance of ferrocene C–H stretching (3084 cm− 1), alkyl C–H stretching
(2975, 2925 and 2884 cm− 1) and carbonyl stretching (1683 cm− 1) vi­
brations of 4 in all of the infrared spectra. The medium intensity peaks in
the region of 1250-1175 cm− 1 can be assigned to the CH2 wagging vi­
brations of the Si-propyl groups, although they are obscured by the

strongly absorbing siloxane Si–O stretching vibrations (1150-1000
cm− 1).

Fig. 1. Cyclic voltammogram of 4 (1 mM in acetonitrile, supporting electrolyte:
tetrabutylammonium-perchlorate (0.1 M) on a glassy carbon electrode, scan
rate: 100 mV/s).

3. Results and discussion
3.1. Synthesis of silica materials modified by ferrocene
In order to obtain silica materials with covalently bound ferrocene
moieties, a triethoxysilyl group was introduced into the 2-ureido-4-fer­
rocenyl pyrimidine skeleton. Ureidopyrimidine 4 (Scheme 1) was syn­
thesized by a modification of the procedure reported earlier [47] in
three steps, involving the aldol reaction of acetophenone and
ferrocene-carboxaldehyde, formation of aminopyrimidine 3 from
alkenyl ketone 2 via ring-closure with guanidine-carbonate, and acyla­
tion of the amino group of 3 with 3-(triethoxysilyl)propyl isocyanate.
The structure of the product (4) was proved by NMR and IR measure­
ments. Its cyclic voltammetric behaviour was similar to other 2-urei­
do-4-ferrocenyl pyrimidine derivatives synthesized in our group.
Cyclic voltammogram of 4 in acetonitrile shows well-defined and
reproducible anodic and cathodic peaks related to the Fc/Fc+ redox
couple with quasireversible behavior (Fig. 1). The value of the peak
separation potential is ΔEp = (Epa–Epc) = 90 mV, greater than 56.5 mV,
expected for an ideal reversible system at exactly 25 ◦ C [52].
Two different sol-gel methods were used to synthesize ferrocene
containing silica materials. Sil1 was prepared by the direct condensation
of ureidopyrimidine 4, while the co-condensation of the same derivative

Fig. 2.


13

3.1.2. NMR studies
The immobilized derivatives Sil1 and Sil2 were characterized by
13
C- and 29Si CP MAS NMR spectra. Based on the 13C spectrum of the
monomer 4, the main peaks in the 13C CP MAS NMR spectra of the solid
materials could easily be identified (Fig. 2). The signals in the region of
65–80 ppm in the spectra of the organosilicas can be attributed to the
ferrocene moiety and are in good accordance with the singlets in the 13C
NMR spectrum of 4 (at 70.35 ppm for the carbons of the unsubstituted
cyclopentadienyl ring and at 68.59 ppm, 71.67 ppm and 80.48 ppm for
those of the substituted one). Similarly, the broad signals between 125
ppm and 135 ppm can be assigned to the carbons of the aromatic ring,
with the corresponding singlets at 127.49 ppm, 129.16 ppm, 131.26
ppm and 137.09 ppm in the spectrum of 4. The quaternary carbons of
the pyrimidine ring appear at 164.39 ppm, 158.93 ppm and 154.77
ppm, which support the presence of the same moiety in the solid ma­
terials with signals between 155 ppm and 170 ppm. The three methylene
carbons of the propyl group can be found at 8.16 ppm, 24.05 and 42.68
ppm both in the spectra of the silica derivatives and in that of monomer

C CP MAS NMR spectra, measured at 100 MHz of Sil1 and Sil2 (spinning rate: 10000 Hz) and solvent phase
4

13

C NMR spectrum of 4.



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Microporous and Mesoporous Materials 308 (2020) 110380

developed probably due to the application of the silica precursor and
CTAB template.
3.1.4. TEM studies
The structure of Sil2 was also studied using transmission electron
microscopy to provide further information about the ordered silica
framework. TEM and STEM analysis showed the presence of hollow
spherical silica particles (Fig. 6). Increased magnification allowed us to
identify a uniform porosity in the spheres with an average pore diameter
of 2.97 nm (Fig. 6D).
Mapping of the elemental composition of Sil2 confirmed the previ­
ous assumption of hollow spheres, since the intensity of X-rays charac­
teristic of Si (as well as that of Fe and C) showed a local minimum along
the diameter of the spheres (Fig. 7, Figure S13).
This measurement not only showed the incorporation of the ferro­
cene labelled heterocycle, but also provided information about the
location of organic moieties inside the inorganic material. Carbon and
iron distributions suggest that beside a moderate amount of 4 inside the
pores, the majority of the ferrocenyl-ureidopyrimidine is concentrated
on the inner surface of the silica particles (Fig. 7, Figure S13). This is also
indicated by the differences in the maxima of the silicon and iron in­
tensity profiles along the sphere diameter of a Sil2-sphere (Fig. 7,
Figure S13).
Hollow spheres are usually prepared using different templating
strategies, by coating polymeric core materials or by using emulsions,
micelles or gas bubbles as templates [54–56]. CTAB is widely used as a

template in microemulsion templating methodologies. In the present
case, the ability of the urea functionality to develop H-bonds with anions
[57] such as bromide, may induce an interaction between monomer 4
and the headgroups of CTAB micelles, leading to the formation of a
ferrocene-rich layer on the surface of the micelles. The outer silica layer,
lacking ferrocene and formed by the condensation of TEOS, stabilizes
the hollow particles. In earlier studies it was found that the ratio of water
and the organic co-solvent is the key factor in forming hollow sphere
silicas in the presence of CTAB [58]. In ethanol—water mixtures, the less
polar ethanol can seep into the CTAB micelles forming an ethanol-rich
phase inside the micelle and a water-rich phase outside the micelle.
To prove the validity of this model in our case, organosilicas were pre­
pared by using different ethanol/water mixtures (ethanol/water = 2:1

Fig. 3. 29Si CP MAS NMR spectra of Sil1 and Sil2, measured at 79 MHz
(spinning rate: 5000 Hz).

4. The presence of ethoxysilyl moieties can be recognized by the
methylene and methyl carbons at 58.35 and 18.18 ppm also in the
spectra of the solid materials. This shows that condensation is not fully
completed.
The peak at 30.13 ppm in the 13C CP MAS spectrum of Sil2 is
probably related to some residual CTAB used as structure directing agent
that could not be fully removed even by multiple washings with
dichloromethane.
The multiple peaks around − 65 ppm, which can be seen in the 29Si
CP MAS spectra of both Sil1 and Sil2 (Fig. 3) represent silicon attached
to organic moieties, such as (Si(OSi)3R); (Si(OSi)2ROEt) or (Si(OSi)R
(OEt)2) [53]. The signals at lower chemical shift present in the spectrum
of Sil2, refer to (Si(OSi)3OH) and (Si(OSi)4) groups around − 101 ppm

and − 108 ppm, respectively.
3.1.3. SEM studies
The structural properties of Sil1 and Sil2 were investigated by SEM
(Figs. 4 and 5). As it was expected, no ordered structure could be seen in
Sil1, however in case of Sil2 spherical silica particles could be observed,

Fig. 4. SEM images of Sil1.
5


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Microporous and Mesoporous Materials 308 (2020) 110380

Fig. 5. SEM images of Sil2.
Fig. 6. HAADF images of the silica particles in
sample Sil2. Hollow spherical morphology in the
low-magnification images (A and B) and a uni­
form porosity in the high-magnification image
(C) are clearly visible. (Pores appear as small
dark dots in C). The fast Fourier transform (FFT)
image (D) of the high-magnification HAADF
image (C) allows us to measure the pore size
more accurately. (Fast Fourier transformation
converts the typical spatial frequencies (sizes)
noticeable in a non-periodic real image to rings
with the radius proportional to the real size.) The
radius of the highest intensity part of the ring (as
marked in D) and the width of the ring suggest
that the most common pore size is 2.97 nm and

pore sizes range from 2.7 to 3.3 nm, respectively.

6


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Fig. 7. STEM-EDS elemental maps and their line profile evaluation (calculated from the map data along the dashed line in the HAADF image), obtained from a
spherical silica particle of Sil2. In the elemental maps the distributions of chemical elements composing the material can be visualized. The visible intensity in the
map is proportional to the counts on the EDS detector (after background correction and peak deconvolution) and can vary with thickness and the changes of the
chemical composition. In this silica sphere the intensity distribution of the Si map is proportional to the thickness of the particle, with the intensity increasing from
the edge along the radius (showing maxima in the shell at the positions marked with A and D on the line profile), and dropping in the inner part (roughly parallel to
the intensity changes in the HAADF image), confirming the hollow spherical morphology. The intensity maxima in the Fe and C maps clearly coincide with the inner
surface of the shell (marked with B and C on the line profile), exposing the location of the organic moieties inside the sphere.

(Sil2), 4:1 (Sil3) and 1:1 (Sil4). Hollow silica spheres were obtained
only in the presence of a larger amount of water (Sil2 and Sil4)
(Figures S14-S15). The presence of a sufficient amount of water may
result in the formation of ethanol droplets inside the CTAB micelles
leading to the formation of hollow spheres. Although smaller particles
were obtained in mixtures with lower water-content (Sil3), the size and
size distribution of hollow spheres (Sil2 and Sil4) could not be
controlled by the change in the composition of the solvent mixture. The
presence of both CTAB and TEOS is critical to obtain hollow spheres. The
absence of CTAB leads to the formation of solid particles (Sil5,
Figure S16), while only fragments of particles could be isolated in the
absence of TEOS (Sil6, Figure S17).


3.2. Preparation of ferrocene-modified electrodes
Based on the encouraging results in the incorporation of compound 4
in silica materials, the possibility of its immobilization on the surface of
a spectral graphite electrode was investigated.
In order to achieve a better binding, a sol-gel electrodeposition
process was applied here. The parameters were designed based on lit­
erary examples [42], taking into consideration the effect of deposition
potential and time on hydrolysis and condensation reactions and the
effect of supporting electrolyte.
Electrode modification was first attempted by using a similar reac­
tion mixture as it was explained in section 2.4.1 (4, H2O, EtOH), i. e.
without an added silica precursor or structure directing agent. Despite
the expectations, the presence of the ferrocene functionalized film could
not be proved, since no reversible redox peaks could be observed in
cyclic voltammetry measurements.
In the next experiment (E-Sil1), both a silica precursor (TEOS) and a
structure directing agent (CTAB) were added to the reaction mixture,
while all the other parameters of the method were unchanged. The
presence of the film coating on the surface was proved by parallel cyclic
voltammetry experiments in the same electrolyte (KCl/H2O, 0.2 M),
using the modified electrode as the working electrode or a same, but
unmodified electrode for a blank measurement (Fig. 9). On the cyclic
voltammogram obtained by the modified electrode, the ferrocene/fer­
rocenium conversion can clearly be seen, although a shift towards the

3.1.5. Thermogravimetric measurements
DTG analysis was also carried out to compare silica materials Sil1
and Sil2 (Fig. 8). Water content of Sil1 was eliminated in two steps (112
◦
and 154 C), while the entire water content of Sil2 departed at the same

temperature. According to the literature it can be assumed that the
decay of the ferrocene containing ureidopyrimidine takes place between
200 and 300 ◦ C, in two stages (250 and 304 ◦ C for Sil1; 270 and 328 for
Sil2) [59]. The weight loss between 400 and 600 ◦ C can be associated
with the decomposition of the organic linker as well as a further
condensation of residual silanol groups [60].

7


S.J. Keszei et al.

Microporous and Mesoporous Materials 308 (2020) 110380

Fig. 10. Cyclic voltammograms obtained by the modified spectral graphite
electrode (E-Sil1) in organic (solid line) and in aqueous (dashed line) electro­
lyte; 100 mV/s).

Fig. 8. DTG analysis of Sil1 and Sil2.

Fig. 11. SEM analysis of the modified electrode surface of E-Sil1.

reaction.
The modified electrode (E-Sil1) was also studied in an organic
electrolyte (TBAClO4/CH2Cl2, 0.1 M) similarly to former solution phase
experiments for the detection of 2,6-diaminopyridine [47]. The anodic
and cathodic peaks could be identified, however flat peaks (Epa = 1010
mV and Epc = 15 mV) and even greater peak separation (ΔE = 995 mV)
could be observed than that in the aqueous experiment (Fig. 10).
SEM analysis was also carried out to characterize the surface coating

on the working electrode. The presence of the modified silica film of
similar structure as Sil2 could clearly be detected (Fig. 11) and a further
confirmation was also provided by the elemental map, obtained by
energy-dispersive X-ray spectroscopy (Figure S18).
Besides carbon (68%) (which is mainly related to the electrode ma­
terial), 11% of oxygen and 13% of silicon is present on the surface,
suggesting the formation of the silica thin film. The elemental map also
ensured the presence of the ferrocenyl-ureidopyrimidine moiety, since
6% of nitrogen and 1% of iron could also be detected.
Characterization of the CV response shapes at different scan rates
and Fe/Si levels was not possible by using the original electrodeposition
method (E-Sil1), due to the uncertainty of determination of peak cur­
rents and peak potentials of the flat peaks. The issues could be overcome
by the modification of the electrodeposition experiments to produce
thinner coatings on the electrode (E-Sil2—E-Sil5).

Fig. 9. Cyclic voltammograms obtained by the bare spectral graphite electrode
(E-Sil1) (dashed line) and the modified spectral graphite electrode (solid line)
in H2O/0.2 M KCl; 100 mV/s).

negative potentials (Epa: 340 mV; Epc: 5 mV) was observed compared to
the CV of 4, presumably as a result of immobilization.
The cyclic voltammogram showed flat, but reproducible anodic and
cathodic peaks related to the Fc/Fc+ redox couple with a one-electron
transfer quasi-reversible behaviour (Fig. 9). The value of the peak sep­
aration potential is ΔEp = (Epa–Epc) = 335 mV, much greater than that
observed for monomer 4, but can be explained by the thick modified
silica film on the surface of the electrode (caused probably by the
extended electrodeposition time), which may slow down the electrode
8



S.J. Keszei et al.

Microporous and Mesoporous Materials 308 (2020) 110380

Fig. 12. Cyclic voltammograms obtained by the bare spectral graphite electrode (dashed line) and the modified spectral graphite electrode, E-Sil2 (solid line) (left)
and differences in voltammetric response of E-Sil1 (dashed line) and E-Sil2 (solid line) (right) in CH2Cl2/0.1 M TBAClO4; 100 mV/s).

4. Conclusions
A ferrocenyl ureido-pyrimidine derivative bearing a (triethoxysilyl)
propyl side chain (4) was synthesized in a three step procedure. The
introduction of the reactive triethoxysilyl functionality made it possible
to incorporate the ferrocene derivative into organosilicas as well as to
prepare an electrochemically active organosilica thin film on the surface
of an electrode.
A hybrid material with a highly ordered structure could be prepared
in one step by the co-condensation of the ferrocene derivative 4 with a
silica precursor (TEOS) in the presence of a structure directing agent
(CTAB). This method led to the formation of hollow spherical particles
with ferrocenyl-ureidopyrimidine moieties concentrated on the inner
surface. Recently, ferrocene-containing hollow mesoporous silicas were
reported to bear flame retardant properties [61] or to serve as
redox-responsive drug delivery vehicles [62], so further investigations
concerning the interactions of the silica material with guest molecules
are in progress.
Immobilization of the ferrocene derivative on the surface of spectral
graphite electrode was carried out by a sol-gel electrodeposition tech­
nique in one step, which is — to the best of our knowledge — the first
attempt to prepare a ferrocene functionalized silica film on the surface of

an electrode with a direct condensation instead of a post-synthetic
modification of a silica layer. Also, the material was shown to retain
the electrochemical behaviour of the ferrocene precursor which may
make the present work a good starting point for the development of
electrochemical sensors.

Fig. 13. Cyclic voltammograms, obtained by different Fe/Si ratios (E-Sil2—ESil5) in CH2Cl2/0.1 M TBAClO4; 100 mV/s).

Although the peak currents decreased significantly compared to ESil1, voltammetric response of E-Sil2 showed more easily evaluable
anodic (Epa = 810 mV) and cathodic (Epc = 540 mV) peaks, with
decreased peak separation (ΔE = 270 mV) (Fig. 12).
The electrochemical behavior of the modified electrode (E-Sil2) was
monitored by cyclic voltammetry experiments at different scan rates
(Figure S19). The effect of the Fe/Si ratio during electrodeposition was
studied by the preparation of different modified electrodes (E-Sil2—ESil5) and CV experiments in dichloromethane (TBAClO4/CH2Cl2, 0.1 M,
100 mV/s). Increasing the amount of the silica precursor during elec­
trodeposition experiments had almost no effect on the cyclic voltam­
metry response up to the ratio of Fe:Si = 1:8, but a further increase
resulted in a drastic decrease in the peak currents. The results suggest
that the silica precursor TEOS, added to the reaction mixture in large
excess, can lead to a lower amount 4 immobilized on the surface
(Fig. 13, Figure S20).

CRediT authorship contribution statement
´ter Pekker: Inves­
Soma J. Keszei: Investigation, Visualization. Pe
´r: Methodology. Szabolcs Balogh:
tigation, Visualization. Csaba Fehe
s Jakab: Investigation, Visualization. Lớvia Nagy:
Investigation. Miklo

ă ldes: Resources, Writing - re­
Validation, Supervision. Rita Skoda-Fo
view & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
9


S.J. Keszei et al.

Microporous and Mesoporous Materials 308 (2020) 110380

the work reported in this paper.
[21]

Acknowledgment

[22]

This work was supported by the GINOP-2.3.2-15-2016-00049 grant.
L. Nagy acknowledges the support of the National Research, Develop­
ment and Innovation Office (Budapest, Hungary) under grant K125244.
TEM/SEM studies were performed at the electron microscopy labo­
ratory of the University of Pannonia, established using grant no. GINOP2.3.3-15-2016-0009 from the European Structural and Investments
Funds and the Hungarian Government.

[23]

[24]


Appendix A. Supplementary data

[25]

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.micromeso.2020.110380.

[26]

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