Application of sol-gel methods to obtain silica materials decorated with ferrocenyl-ureidopyrimidine moieties. Preparation of hollow spheres and modification of a carbon electrode

The solid materials were characterized 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.

Microporous and Mesoporous Materials 308 (2020) 110380 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso 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 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: skodane@almos.uni-pannon.hu (R Skoda-Fă oldes) https://doi.org/10.1016/j.micromeso.2020.110380 Received March 2020; Received in revised form May 2020; Accepted June 2020 Available online July 2020 1387-1811/© 2020 The Authors Published by Elsevier Inc This is (http://creativecommons.org/licenses/by-nc-nd/4.0/) an open access article under the CC BY-NC-ND license 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: cm3 of concentrated nitric acid was added to 20 mg of the samples and the mixture was boiled for 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 = 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 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− 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− 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.) 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 H and 13C NMR spectra were recorded in CDCl3 on a Bruker Avance 400 spectrometer at 400 and 100 MHz, respectively using a 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 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) mmol of ferrocene-carboxaldehyde (1), mmol of acetophenone and 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) mmol of 3-(ferrocenyl)-1-phenylprop-2-en-1-one (2), mmol guanidine carbonate, and mmol NaOH were dissolved in 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 S.J Keszei et al Microporous and Mesoporous Materials 308 (2020) 110380 Scheme Synthesis of ferrocenyl-ureidopyrimidine Reaction conditions: i: acetophenone, NaOH, room temperature, 24 h; ii: guanidine carbonate, THF, NaOH, reflux, 16 h; iii: (EtO)3Si(CH2)3NCO, 100 ◦ C, 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 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) 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, H); 3.46–3.41 (m, 2H); 1.84–1.76 (m, 2H); 1.18 (t, J = 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: ml for Sil3 and 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 in 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 ml water were added A spectral graphite working electrode (OD = mm), Ag/AgCl reference electrode and Pt counter electrode were immersed into the solution and cathodic potentials were applied for 60 (− 1200 mV for 30 and − 900 mV for 30 min) E-Sil2 — E-Sil5: A solution of 0.1 mmol of in 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 ml water were added A spectral graphite working electrode (OD = mm), Ag/AgCl reference electrode and Pt counter electrode were immersed into the solution and − 1300 mV potential was applied for 15 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, ml ethanol and 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 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, ml ethanol, mg NaOH, 36.5 mg CTAB (0.1 mmol) and 182 μl TEOS (0.8 mmol) were transferred to a Schlenk tube and ml of distilled water was added The mixture was stirred at a temperature of 50 ◦ C for h and the product was isolated by vacuum S.J Keszei et al Microporous and Mesoporous Materials 308 (2020) 110380 (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 and the spectra of the modified organosilicas (Figure S11 and S12) Two N–H stretching vibrations can be identified in the IR spectrum of at 3423 and 3246 cm− The difference between the two frequencies shows that the structure of 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 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 in all of the infrared spectra The medium intensity peaks in the region of 1250-1175 cm− 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 Cyclic voltammogram of (1 mM in acetonitrile, supporting electrolyte: tetrabutylammonium-perchlorate (0.1 M) on a glassy carbon electrode, scan rate: 100 mV/s) 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 (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 from alkenyl ketone via ring-closure with guanidine-carbonate, and acyla­ tion of the amino group of 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 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 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 (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 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 13 C NMR spectrum of S.J Keszei et al 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 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 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 29Si CP MAS NMR spectra of Sil1 and Sil2, measured at 79 MHz (spinning rate: 5000 Hz) 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 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 SEM images of Sil1 S.J Keszei et al Microporous and Mesoporous Materials 308 (2020) 110380 Fig SEM images of Sil2 Fig 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 S.J Keszei et al Microporous and Mesoporous Materials 308 (2020) 110380 Fig 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 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] 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 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 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: 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 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) 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 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 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 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] References [27] [1] F.A Larik, A Saeed, T.A Fattah, U Muqadar, P.A Channar, Recent advances in the synthesis, biological activities and various applications of ferrocene derivatives, Appl Organomet Chem 31 (2017), https://doi.org/10.1002/aoc.3664 [2] D Astruc, Why is ferrocene so exceptional? 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https://doi.org/10.1007/s10562-011-0753-5 11 ... M Haroon, R.S Ullah, A Nazir, T Elshaarani, M Usman, S Fahad, F Haq, Research advances in the synthesis and applications of ferrocenebased electro and photo responsive materials, Appl Organomet... the aldol reaction of acetophenone and ferrocene-carboxaldehyde, formation of aminopyrimidine from alkenyl ketone via ring-closure with guanidine-carbonate, and acyla­ tion of the amino group 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, ml ethanol and mg NaOH was transferred to a Schlenk tube and 150 μl of distilled water was added It was stirred at a temperature of