Pure hybrid nanofibers made of 4,4 ′ -bis(triethoxysilyl)-1,1 ′ -biphenyl and the way of their production

Introduction

A commonly used term for hybrid organosilane nanofibers belongs to those types of nanofibers which are composed of various combinations of organic polymers[1]; combinations of the alkoxides containing organic polymers [2–4] and/or inorganic nanofibers dopped with various types of surfactants[5–9]. So-called “hybrid organosilane nanofibers” which are already described in the literature have been mainly prepared through self-assembly techniques in the presence of various additives [5–7,10–12] not the electrospinning technique. These types of nanofibers have several disadvantages such as low mechanical resistance or decomposition with the change of the environment in which they have been formed[5,6,8,9,13,14]. They also need to be prepared with the presence of cationic surfactants[15–17] and/or various types of gelators[18,19]. According to the literature, hybrid organic–inorganic nanofibers may be formed from various types of organosilanes[20–22], particularly from their bridged organo-mono-silylated analogs with the support of organic polymers, which often remain inside the organosilane framework after their preparation via an electrospinning technique[21,23–25]. Mentioned types of hybrid nanofibers are tested for various applications such as water/air filtration[26,27], optoelectronics and sensors[28,29], energetics[30], medicine[31], and/or catalysis[32]. Their biggest disadvantage lays just in the use of inorganic and organic components separately during the process of their preparation. However, this problem may be effectively eliminated by the use of hybrid organic–inorganic precursors particularly bridged organo-bis-silylated precursors[33,34] in which the organic and inorganic parts are interconnected via strong covalent bonds between carbon and silicon. Until 2021, only five solitary research papers regarding the preparation of the purely hybrid organic-inorganic organosilane nanofibers were published.[35–39] Mentioned nanofibers were prepared by three independent groups Tao et al.[35], Schramm et al.[36,37], and Máková et al.[38,39]. The fibrous materials prepared by Tao et al.[35] were mainly composed of an inorganic component based on tetraethyl orthosilicate (TEOS). Their hybrid nanofibers contained only 10⁻⁴ mol % of a porphyrin bridged organosilane unit compared to TEOS. On the contrary, Schramm et al.[36] used slightly different processes for the formation of hybrid organosilane fibers. These fibers were prepared by co-condensation of (3-triethoxysilylpropyl)succinic anhydride (TESP-SA) with either tetraethoxysilane (TEOS) or methyltriethoxysilane (MTEOS) which were hydrolyzed and reacted subsequently with (3-amino)propyltriethoxysilane (APTES). However, thus prepared sol had to be adjusted before electrospinning with a non-ionic surfactant Triton X-100. Moreover, this surfactant remained inside the prepared hybrid fibers. The third group, Máková et al.[38,39], successfully prepared the first types of hybrid organic–inorganic nanofibers in a significantly higher molar ratio of hybrid precursors inside the silica dioxide matrix compared to the other two research groups[35–37]. They were able to use hybrid precursors based on 1,4-bis(triethoxysilyl)benzene and triethoxyphenylsilane, with the molar ratio reaching up to 40 mol % inside the hybrid matrix[39]. Similarly to this, they also prepared hybrid nanofibers made of synthesized organo-bis-silylated precursor (1S,2S)-cyclohexane-1,2-diamine with the molar ratio in the organosilica framework up to 43 mol %[38]. Both types of these nanofibers were successfully tested for biomedical purposes[38,39]. Hybrid organo-bis-silylated precursor 4,4^′-bis(triethoxysilyl)-1,1^′-biphenyl (BTEBP) is a commercially available precursor. Moreover, this precursor has been used in applications which are already patented (CN110723738(A), 24.1.2020; CN106610397(A) and CN106610394 (A), 3.5.2017; CN106353389(A), 25.1.2017, as well as in the research papers[5,14,29,34,40]. Patents regarding refer to this precursor in various forms such as aerogel for the fields of heat preservation materials, heat insulation materials, waste gas treatment materials, wastewater treatment materials, and catalysts, or in the form of the sensor for electrochemical detection of dopamine, and/or ascorbic acid by using biphenyl organic mesoporous material-doped carbon paste electrode. The above-mentioned research papers present 4,4^′-bis(triethoxysilyl)-1,1^′-biphenyl in various forms including nanoparticles used as drug delivery systems for doxorubicin,[40] composites showing fluorescence due to the existence of biphenyl chromophores in the stable organosilica framework[34], thin films exhibiting very smooth surface without cracks and excellent mechanical behavior determined for applications as acoustic wave sensors and biosensors[29], or mesoporous nanofibers and/or nanotubes prepared with a help of chiral low molecular weight gelators (LMWGs)[5], which are sensitive to pH value, temperature, and polarity of solvents. The optical activity of these fibers is attributed to the π–π stacking and the chiral conformation of present aromatic groups, thus they can potentially be used as a chiral stationary phase for enantioseparation[5]. However, the preparation of such types of nanofibers based on the 4,4^′-bis(triethoxysilyl)-1,1^′-biphenyl by sol–gel process together with electrospinning technique has not been described yet. Herein, we briefly report the currently patented process leading to formation of the first purely hybrid organic–inorganic nanofibers composed of solely the 4,4^′-bis(triethoxysilyl)-1,1^′-biphenyl (BTEBP) hybrid precursor. Moreover, these nanofibers have been successfully prepared using a combination of the sol–gel process together with the electrospinning techniques. The uniqueness of these purely hybrid fiber mats lays mainly in the interconnection of organic and inorganic molecules connected via strong silicon–carbon (Si–C) covalent bond[41,42]. This feature gives them completely new possibilities of use based on the interconnection of both components in a hybrid matrix. Thus prepared nanofibers were successfully characterized via various types of techniques including Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). These are the first nanofibers, which were prepared without the presence of additives and in high molar content of hybrid precursor reaching the 100 mol %.

Materials and methods

Preparation of the hybrid BTEBP nanofibers

The spinnable sols used in this study were prepared analogically via the sol–gel process as described elsewhere[38,39]. Bridged organo-bis-silylated precursor (BTEBP), ethanol, deionized water, and HCl were added into a 50 mL round bottom flask and stirred at 700 rpm/30 min/r.t. The molar ratio between deionized water and organosilane precursor was kept low at 2, the molar ratio between ethanol and organosilane high at 99.3. The pH of the sol was adjusted with HCl to ≈2. Finally, the sol was heated up to 90°C in an oil bath at 350 r/min and refluxed up to 24 h. The dynamic viscosity was measured before the electrospinning and appropriately adjusted via distillation of solvent. Hybrid fibers were prepared via electrospinning technique, known worldwide as a Nanospider (Elmarco s. r.o.; CZ) industrial roller electrospinning machine was used to examine the fabrication of the nanofibers on an industrial scale. This technique was used to examine the fabrication of the nanofibers on an industrial scale. All of the vital parameters regarding the usage of Nanospider are summarized in Table 1.

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Table 1.

Parameters of the electrospinning technique using Nanospider^TM.

Distance between electrode and collector (mm)	Voltage (kV)		Speed of cartridge (mm/s)	Temperature (°C)	Humidity (%)
	Electrode	Collector
160	<40	>–20	<340	<24	>30

Results and discussion

Changes in the structure of the sol during the sol–gel process were observed using FTIR spectroscopy. The hydrolytic and condensation reactions were evaluated by focusing on the finger print region from 1500 to 450 cm⁻¹. In this area there can be observed several strong to medium absorption peaks describing in particular the bonds between silicon, carbon and oxygen in various forms. As expected, Figure 1 shows the growth in interconnections of silica units into the network by forming polysiloxane matrix linked through the Si–O–Si bonds. Those units prevail over various not completely hydrolyzed non-bridging units such as alkoxides, silanols, or defected SiO⁻. The prevailing formation of the siloxane structure can be also proven by a considerable increase in the absorption peak of siloxane units (bridging oxygens; antisymmetric stretching vibration of Si–O–Si) in the region at around 1200–1000 cm⁻¹ and an accompanying decreasing intensity in the absorption of silanol groups between 1000–900 cm⁻¹ (in plane or symmetric stretching vibrations of non-bridging oxygen units). This phenomenon marks out the ongoing hydrolysis/condensation reactions and can be explained as the structural transformation of Si–O–R/Si–OH/Si–O^- units into Si–O–Si units. This is accompanied by a significant decrease in broad absorption bend characteristic for water in the area 3500–3000 cm⁻¹[39,46–50].OPEN IN VIEWER

The structure of prepared fibers was observed using Raman spectroscopy with an aim to confirm that no shift in chemical nature of the aromatic organic linker (biphenyl unit). The spectra of hybrid fibers with above-mention organic unit (Figure 2) is characterized by a strong peak at ∼1600 cm⁻¹ which corresponds to the double bond C=C and several corresponding rather weak bends down to 1300 cm⁻¹ (all stretching vibrations of C=C). The medium sharp absorption peak at around ∼1280 cm⁻¹ is attributed to the deformation vibration of a 1,4-disubstituted aromatic compounds (Si-(CH₂)-Ar). The peak at ∼1130 cm⁻¹ belongs to the Si-O-C bond stretching vibrations, probably coming from remaining unhydrolyzed alkoxy functionalities. In the spectra, there can be also seen several weak peaks located in the range from 3100 to 2800 cm⁻¹. Mentioned peaks are typical for alkyl/alkenyl chain modes (-C-H; =C-H; -CH₃; -CH₂-; etc.). In the finger print region[51–54], rather weak Si-C stretching vibration modes are assigned to peak at around 800 cm⁻¹. Those findings are in a good agreement with the observation in the interval at around 1650–1400 cm⁻¹ using FTIR spectroscopy (Figure 1) belonging to the stretching vibration of C=C units, which remain unchanged at the beginning as well as at the end of the sol–gel processing[54]. Thus, the organic biphenyl units stay covalently interlinked in the organosilane matrix and no organic cleavage has occurred.OPEN IN VIEWER

Morphology and diameter distribution of the formed BTEBP fibers were observed using scanning electron microscopy. As can be seen in the Figure 3(a), BTEBP fibers are morphologically compact and homogenous with an average size of 665.5 nm. The key parameter in the forming of smooth fibers without any structural inhomogeneities by using electrospinning techniques is viscosity of the spinning solution. It was observed that too low viscosity of the polymer leads to forming fibers with spherical shapes distributed in the structure along the fibers (Figure 3(b)). Figure 4 illustrates the average diameter of prepared BTEBP nanofibers spun via DC electrospinning technique (Nanospider^™ NS 1WS500U).OPEN IN VIEWER

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Figure 4.

Distribution of nanofibers diameters.

Thermal behavior of the hybrid organosilane fibers (Figure 5) was studied using thermogravimetric analysis coupled with FTIR spectroscopy. BTEBP fibers exhibit three weight losses. Two minor losses (approximately 0.05 and 7.03 wt.%) are assigned to the evaporating of adsorbed water and to the dissociation of ethanol or ethoxy groups releasing from the not-completely polymerized network. The main weight loss (31.66 wt.%) occurs at a maximum 526.74°C. According to FTIR spectroscopy, it is attributed to the decomposition of the organic molecules (aromatic structures) present in the hybrid organosilica framework.OPEN IN VIEWER

Conclusion

Thus prepared hybrid organosilane nanofibers are the first purely hybrid organosilane materials prepared without the support of organic polymers, additives, and/or various types of surfactants. Their uniqueness lays particularly in their applicability in the various fields of material science such as optoelectronics, catalysis or medicine. The above-described nanofibers are composed only of organo-bis-silylated precursor 4.4^′-bis(triethoxysilyl)-1.1^′-biphenyl (BTEBP) using one pot synthesis. These are the first types of hybrid organosilane nanofibers prepared by this method. BTEBP nanofibers have a smooth surface with a diameter of around 600 nm and show thermal stability reaching up to 400°C. According to the preliminary results, these hybrid nanofibers seem to be a promising material particularly for optoelectronic and/or biomedical applications.