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Synthesis of core-shell titanium dioxide nanoparticles with water-soluble shell of poly(methacrylic acid)
E. L. Krasnopeeva1'2, E. Yu. Melenevskaya1, O. A. Shilova2, A. M. Nikolaev2, A. V. Yakimansky1
1 Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia 2Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia
opeeva@gmail.com
PACS 61.46.+w, 68.47.Pe DOI 10.17586/2220-8054-2021-12-3-336-345
The article is devoted to the covalent modification of the surface of titanium dioxide nanoparticles with polymethacrylic acid using the method of controlled atom transfer radical polymerization. For the first time, core-shell titanium dioxide nanoparticles with water-soluble shell of poly(methacrylic acid) were obtained.
Keywords: modification of nanoparticles, titanium dioxide, controlled radical polymerization, polymethacrylic acid, core-shell nanoparticles. Received: 2 December 2020 Revised: 9 April 2021
1. Introduction
In the recent years, nanosized titania and titania-based nanocomposites have become increasingly popular materials in the industrial-scale manufacturing of many products, agriculture, environmental science, and power engineering due to their photocatalytic [1-6], photoluminescent [7,8], semiconductor [9-11], pigment [12], and biological [13-15] properties. In particular, titanium dioxide nanoparticles are widely used in agriculture for the photocatalytic decomposition of pesticides [16-31], solar energy-assisted photocatalytic water purification (review [16] and references therein), plant disease control [32-34]. One of the most important application areas of TiO2 nanoparticles in agriculture is treatment of crop seeds aimed at accelerating seed germination and crop vegetation [35-40]. The possibility of this practical application of titanium dioxide is related to its capability for photocatalytic activation of oxygen, which yields reactive oxygen species such as peroxide and hydroxide radicals and ions. As a result, stress tolerance of seeds is enhanced, their ability to absorb water and oxygen increase [36]. Besides, treatment of seeds with titanium dioxide nanoparticles leads to enhanced nitrogen binding by plants, increase in chlorophyll and protein contents in plants [38].
In order to increase efficiency of TiO2-based photocatalytic systems, it is necessary to use this substance in the form of stabilized aqueous dispersions (which are generated, for instance, upon interaction with triethanolamine [41]), or in the form of core-shell nanoparticles with TiO2 core and a water-soluble shell. The most promising synthetic method for these core-shell structures consists in modifying titanium dioxide nanoparticle surfaceswith covalently grafted chains of water-soluble hydrophilic polymers. To date, a relatively large number of works devoted to modification of TiO2 nanoparticles with various polymers have been published. The most common method is controlled atom transfer radical polymerization (ATRP) of vinyl monomers involving a-Br-ester groups immobilized on nanoparticle surface [42-54]. In most cases, this immobilization is preceded by modification of TiO2 surface with amino groups in the process of treatment of nanoparticles with 3-aminopropyltriethoxysilane [42,48-51, 53] or dopamine [43]. Then, the obtained nanoparticles are treated with bromoanhydrides of 2-bromopropionic or 2-bromo-isobutyric acids [42,43,48-51,53], which results in immobilization of ATRP-initiating a-Br-ester groups on nanoparticle surface. In some cases, TiO2 nanoparticles are immediately treated with 2-Br-isobutyroyl bromide [44].
It should be noted that there are virtually no literature data on modification of TiO2 nanoparticle surfaces with water-soluble polymers. A small number of published papers report the synthesis of titanium dioxide and zinc oxide nanoparticles modified with poly(N,N-diethylaminoethyl methacrylate), which were used as nanocontainers for anticancer preparations [44]. Titanium dioxide nanoparticles modified with ionized poly(styrene sulfonic acid) were used as ion exchangers for Ag+ ; the products were intended for applications as TiO2/Ag binary nanocomposite photocata-lysts [45] and one of components of proton-conducting membranes [54]. There are also papers reporting the synthesis of TiO2 nanoparticles modified with poly(oligoethylene glycol) methacrylate [47] and poly(N-isopropyl acrylamide); the latter demonstrate self-flocculation in aqueous medium at temperatures above low critical solution temperature of poly(N-isopropyl acrylamide) [51,52]. In addition, block copolymers of styrene and acrylic acid were used as micellar nanoreactors for the synthesis of TiO2 nanoparticles and stabilization of TiO2 aqueous dispersions [55].
In the present work, we synthesized core-shell TiO2 nanoparticles modified with water-soluble poly(methacrylic acid) for the first time. The two-stage grafting method described above was used for immobilization of 2-Br-isobutyrate groups on the surface of titanium dioxide; the method involved successive treatment of nanoparticles with 3-aminopropyltriethoxysilane and 2-Br-isobutyroyl bromide. Then, the obtained TiO2-based macroinitiators were used for polymerization of tert-butyl methacrylate (TBMA) by ATRP. The prepared core-shell TiO2 @PTBMA nanoparticles with grafted PTBMA chains were subjected to acidic hydrolysis; ester groups of the grafted chains were converted to carboxylategroups, and TiO2@PMAA core-shell nanoparticles with poly(methacrylic acid) shells were obtained. In order to determine lengths of the grafted chains, they were isolated by selective alkaline hydrolysis of ester bonds that connect polymer chains to TiO2 surface; molecular masses and molecular mass distributions were estimated by size exclusion liquid chromatography (SEC).
The structure of nanoparticles at all stages of synthesis was confirmed by IR spectroscopy and solid state 13C NMR spectroscopy.
2. Experimental: materials
3-Aminopropyltriethoxysilane (99 %, Acros Organics), N,N-dimethylformamide (DMF) (reagent grade, Vekton, Russia), and N, N, N', N", N"-pentamethyldiethylenetriamine (PMDETA) (99 %, Aldrich) were used without additional purification. Methylene chloride (reagent grade, Vekton, Russia) and toluene (analytical grade, Vekton, Russia) were dried by heating over anhydrous calcium hydride (99.9 %, Aldrich), then distilled under vacuum. The ionic liquid (1-butyl-3-methylimidazolium chloride (BMIMCl) (>98.0 % (Aldrich)), glacial acetic acid (reagent grade, Vekton, Russia), trifluoroacetic acid (99 %, Aldrich), copper (I) bromide (99.999 %, Aldrich), 2-bromo-isobutyroyl bromide (98 %, Aldrich) were used without additional purification. Tert-butyl methacrylate (TBMA) (98 %, Aldrich) was twice distilled under vacuum before use, commercial TiO2 rutile nanopowder (Plasmotherm) was used for the modification.
3. Experimental: methods 3.1. TiO2 nanoparticles
SEM image of particles is presented in Fig. 1. According to these data, the particles have the size of 100 - 300 nm, being characterized by the specific surface area (SSA) of ^ 13 m2/g and specific pore volume of 0.044 cm3/g [56]. X-Ray diffraction of particles is presented in Fig. 2.
Fig. 1. SEM image of TiÜ2 nanoparticles
3.2. Synthesis of TiO2 nanoparticles modified with 3-aminopropyltriethoxysilane (APTES)
Titanium dioxide particles (0.73 g) were annealed under vacuum at 120 °C for 4 h in order to remove physically adsorbed water [57]. Then the product was cooled down to room temperature, and DMF (10 mL) was recondensed into the reaction vessel, and the mixture was subjected to ultrasound treatment for 30 min. Then the mixture was degassed thrice, and argon was passed through the system; APTES (0.5 mL) was added at cooling in an ice bath and at constant stirring. The mixture was left to stand for 24 h at constant agitation under anargon atmosphere at room temperature.
Fig. 2. X-Ray diffractogram of the TiO2
The particles were isolated by centrifugation, washed thrice with DMF and thrice with methylene chloride. Yield: 0.74 g.
3.3. Synthesis of TiO2-APTES-BiB macroinitiators based on TiO2-APTES
Titanium dioxide nanoparticles modified with APTES (TiO2-APTES, 0.5 g) were heated at 110 °C under vacuum for 2 h. Dry toluene (25 mL) was recondensed into the reaction flask; the mixture was degassed thrice, andargon was passed through the system; then 2-bromo-isobutyroyl bromide (BiB, 2 mL) was added. The reaction mixture was exposed at room temperature and constant stirring for 24 h. The product was precipitated into deionized water and washed thrice with subsequent centrifugation. The final product was obtained in the form of white powder. Yield: 0.83 g.
3.4. Synthesis of TiO2-APTES-BiB-PTBMA core-shell nanoparticles with poly(tert-butylmethacrylate) (PTBMA) shells
The initiator (0.2 g) was put into the reaction flask together with 10 g of the ionic liquid (butyl-methyl imidazolium chloride), heated to 80 °C at constant stirring, degassed and subjected to ultrasound treatment for 30 min. Then, argon was passed through the flask; CuBr (0.063 g) and PMDETA (0.7 mL) were added. Then preliminarily distilled TBMA was added in the amounts indicated below. The mixture was exposed at 80 °C with constant agitation for 24 h. The synthesized particles were precipitated into excess distilled water and dried in air. The final product was obtained in the form of white powder.
The following samples containing various amounts of PTBMA were synthesized by addition of 7.5, 2.5, and 1 mL of TBMA: TiO2-APTES-BiB-PTBMA-1, TiO2-APTES-BiB-PTBMA-2, and TiO2-APTES-BiB-PTBMA-3, respectively.
3.5. Elimination of grafted PTBMA chains by alkaline hydrolysis of TiO2 -APTES-BiB-PTBMA core-shell nanoparticles
The particles were put into 2 wt. % solution of KOH in methanol at 65 °C and exposed for 16 h. Linear PTBMA was precipitated into the methanol:water mixture and dried until constant weight was achieved.
3.6. Synthesis of TiO2-APTES-BiB-PMAA core-shell nanoparticles with poly(methacrylic acid) (PMAA) shells
The synthesized TiO2-APTES-BiB-PTBMA-1, TiO2-APTES-BiB-PTBMA-2, and TiO2-APTES-BiB-PTBMA-3 samples were dissolved in minimal amount of methylene chloride; trifluoroacetic acid (small excess) was added, then the mixture was exposed for 24 h at room temperature and constant stirring. The formed precipitate was washed with dichloromethane until neutral pH was reached, and then dried in air. Titanium dioxide particles modified with water-soluble poly(methacrylic acid) chains of various lengths (TiO2-APTES-BiB-PMAA-1, TiO2-APTES-BiB-PMAA-2, and TiO2-APTES-BiB-PMAA-3) were obtained.
3.7. X-Ray diffraction
X-Ray diffraction was carried out using Rigaku MiniFlexll diffractometer with CuKa radiation in the 20 range 5-75 °, with step 0.02° at 5 °/min speed. Crystal scattering domain (CSD) length was calculated with Debye-Scherrer formula using 110 reflections.
3.8. IR spectroscopy
IR Fourier transform spectra were obtained with the aid of an IRAffinity-1S spectrometer (Shimadzu) equipped with a Quest attenuated total reflectance attachment (Specac) (diamond prism, spectral range 7800 - 400 cm-1).
3.9. NMR spectroscopy
Solid state 13C NMR spectra were recorded using a Bruker Avance 400 SB instrument (1H: 400 MHz).
3.10. Size exclusion chromatography
The samples were analysed with the aid of an Agilent-1260 Infinity chromatography setup equipped with two columns packed with PLgel MIXED-C sorbent (column dimensions: 7.5 x 300 mm; sorbent particle size: 5 ^m). Elution proceeded in isocratic regime; dimethylformamide was used as an eluent. Molecular masses and polydisper-sity parameters were measured by multidetector gel permeating chromatography. Characteristics of polymers were determined from the data of refractometric and viscosimetric detectors used in combination with a light scattering detector, which makes it possible to manage without calibration standards.
3.11. Scanning electron microscopy
The SEM studies were performed with the aid of a Carl Zeiss N Vision 40 instrument at an accelerating voltage of 1 kV and magnification of up to x 300000.
3.12. Thermogravimetry
A DTG-60 setup (Shimadzu, Kyoto, Japan) was used for thermogravimetric (TGA) analysis to determine the amount of OH-groups chemically bound to the surface of TiO2 nanoparticles, the samples being heated to 600 °C at a rate of 5 °C/min in air flow (80 mL/min).
4. Results and discussion
TiO2 nanoparticles were studied by the TGA method in order to determine the amount of surface OH-groups bound covalently to Ti atoms. According to the procedure described in [57], the following equation was used to this aim:
#OH/nm2 = 0.625 x
2 (AWti ) Na
(1)
SSA x MWh2 o '
where AWTl ^t2 is the weight loss fraction upon heating the sample from T1 = 120 °C (the annealing temperature at which physically adsorbed water is removed) to T2 = 600 ° C (the temperature at which no chemically bound OH-groups remain on the surface of titania nanoparticles [57]), NA is Avogadro number, MWH2O is the molecular weight of water. As may be found from the TGA curve presented in Fig. 3, AWTl^T2 « 0.0092. Then, according to formula (1) with SSA = 13 m /g, the surface density of chemically bound OH-groups, #OH/nm2 = 29.4 nm . This value corresponds to ^ 3.822 • 1020 chemically bound surface OH-groups per 1 g of TiO2 nanoparticles.
Fig. 3. TGA curve of the powder of TiO2 nanoparticles
The scheme of the synthesis of TiO2-APTES-BiB-PMAA core-shell particles with titanium dioxide cores and poly(methacrylic acid) shells included several stages; the synthesis scheme is presented in Fig. 4.
At the first stage, titanium dioxide nanoparticles were treated with APTES in order to graft primary amino groups onto nanoparticles' surface. The taken amount of APTES (0.5 g) corresponds to an approximately 5-fold molar excess
Fig. 4. Scheme of the synthesis of TiO2-APTES-BiB-PMAA core-shell particles with titanium dioxide cores and poly(methacrylic acid) shells
of APTES with respect to chemically bound surface OH-groups in the taken amount of annealed TiO2 nanoparticles (0.73 g). The presence of amino groups on the surface of the synthesized particles is confirmed by comparison of 13C NMR spectra of the initial titanium dioxide nanoparticles (Fig. 5a) and the nanoparticles treated with APTES (Fig. 5b). The spectrum of the initial nanoparticles presented in Fig. 5a indicates that they have pure surface, i.e. there are no carbon-containing groups or compounds on TiO2.
Meanwhile, the spectrum of TiO2-APTES nanoparticles contains the signals attributed to carbon atoms of amino-propyl groups near 10, 25 and 45 ppm. In addition, the presence of surface amino groups is confirmed by comparison of IR spectra of the initial TiO2 nanoparticles before and after treatment with APTES (Fig. 6). The following bands appear in the spectra of APTES derivatives: (i) wide bands in the region 1560 - 1570 cm-1 attributed to deformation vibrations of primary amino groups [58]; (ii) C-N stretching vibration band of APTES at 1323 cm [58]; (iii) the bands near 1140 cm-1 assigned to valence vibrations of C-N bond in primary amines [59].
At the second stage, surface amino groups of TiO2-APTES nanoparticles were acylated with 2-bromo-isobutyroyl bromide to graft 2-bromo-isobutyrate groups onto nanoparticles' surface; these fragments are able to initiate controlled atom transfer radical polymerization. 13C NMR spectrum of the resulting TiO2-APTES-BiB nanoparticles is presented in Fig. 5c. Comparison of this spectrum with NMR spectrum of TiO2-APTES (Fig. 5b) demonstrates that the former-contains new signals attributed to methyl carbon atoms (32 ppm), quaternary carbon atoms (52 ppm), and carbonyl atoms in 2-bromo-isobutyrate fragments (170 - 180 ppm). Thus, the 13C NMR spectroscopy data confirm successful preparation of TiO2-APTES-BiB nanoparticles with surface 2-bromo-isobutyrate groups.
FIG. 5. Solid state 13C NMR spectra of the initial titanium dioxide (a), TiO2-APTES (b), TiO2-APTES-BiB (c), TiO2-APTES-BiB-PTBMA (d) and TiO2-APTES-BiB-PMAA particle (e)
1560-1570
1140
2
1600
1400
Wave number, cm"1
1200
1000
Fig. 6. IR spectra of the initial titanium oxide (1) and TiO2-APTES nanoparticles (2)
The synthesized TiO2-APTES-BiB nanoparticles were used in polymerization of tert-butyl methacrylate according to ATRP mechanism. The TiO2-APTES-BiB-PTBMA nanoparticles were synthesized; their 13C NMR spectrum (Fig. 5d) virtually coincides with that of poly(tert-butyl methacrylate). It includes signals related to all PTBMA carbon atoms: methylene group (^ 55 ppm) and a-carbon atom (^ 45 ppm) of PTBMA backbone, methyl group at a-carbon (^ 18 ppm), signals of carbonyl atoms (^ 180 ppm), quaternary carbon atoms 80 ppm) and methyl carbon atoms (^ 28 ppm) of side ester groups. It should be noted that signals of TiO2-APTES-BiB macroinitiator cannot be registered due to its extremely low amount in TiO2-APTES-BiB-PTBMA samples.
The grafted PTBMA chains were eliminated from TiO2-APTES-BiB-PTBMA nanoparticles in the process of alkaline hydrolysis, which destructs the ester bonds connecting polymer chains to macroinitiator fragments. Conditions of alkaline hydrolysis were selected so as to prevent transformation of side ester fragments of PTBMA [60]. The isolated linear PTBMA was analyzed by multidetector size exclusion liquid chromatography with the so-called triple detection (a combined analysis of the data provided by refractometric, viscosimetric and light scattering detectors [61]). It was found that the isolated PTBMA chains have the following characteristics: Mn = 70000, Mw = 120000, Mw /Mn = 1.7. Thus, the PTBMA chains grafted onto the surface of titanium dioxide nanoparticles are long (polymerization degree about 500) and have relatively narrow molecular mass distributions.
At the last synthesis stage, TiO2-APTES-BiB-PTBMA nanoparticles were subjected to "dry" acidic hydrolysis that affected side ester groups of PTBMA chains. Hydrolysis proceeded under the action of anhydrous trifluoroacetic acid and was accompanied by release of gaseous isobutylene and transformation of ester groups into carboxylic fragments. As a result, we obtained TiO2-APTES-BiB-PMAA nanoparticles with water-soluble poly(methacrylic acid) shells, their polymerization degree being the same as that of the initial PTBMA chains. 13C NMR spectrum of these nanoparticles given in Fig. 5e confirms the presence of poly(methacrylic acid) in the shell. Thus, the spectrum contains the signals attributed to methylene group atoms (^ 55 ppm) and a-carbon atoms(^ 45 ppm) of PMAA backbone, the signals of methyl group at a-carbon atom (^ 20 ppm) and carboxylic carbon atom (^ 185 ppm). At the same time, there are no peaks related to quaternary carbon atoms and methyl carbon atoms of side ester groups, which indicates complete transformation of TiO2-APTES-BiB-PTBMA into TiO2-APTES-BiB-PMAA.
According to the polymerization degree of ^ 500 of PMAA chains in the shell of TiO2-APTES-BiB-PMAA particles, the shell is ^ 125 nm thick, taking into account that the monomer unit length of PMAA side chains is ^ 0.25 nm. The UV spectrum of the synthesized TiO2-APTES-BiB-PMAA nanoparticles in water solution is presented in Fig. 7.
As seen from Fig. 7, the spectrum shows a typical absorption of TiO2 cores below 400 nm [62].
5. Conclusions
Modification of nanoparticles surface with polymers is of great interest for various practical applications. The
obtained TiO2-APTES-BiB-PMAA core-shell nanoparticles with a water-soluble shell can be used, in particular, for
treatment of agricultural crop seeds with the purposes of enhancing their germination and increasing growth rate.
Each nanoparticle contains photocatalytically active core of titanium dioxide that increases photosynthesis efficiency
and poly(methacrylic acid) shell providing solubility of the product in water. It should be noted that the synthesis of
titanium dioxide nanoparticles with grafted poly(methyl methacrylate) chains has been described in the literature [63].
However, water-soluble shells of poly(methacrylic acid) were grafted onto the surface of TiO2 nanoparticles for the
first time.
0,0 -I-,-,-,-,-,-,-,-,-,-,-,-1
200 300 400 500 600 700 800 Wavelength, nm
Fig. 7. UV spectrum of TiO2-APTES-BiB-PMAA nanoparticles in water
Acknowledgements
The work is supported by the Russian Science Foundation (Project No. 19-13-00442). References
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