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Versatile PISA templates for tailored synthesis of nanoparticles
Accepted Manuscript Versatile PISA Templates for Tailored Synthesis of Nanoparticles Yaoming Zhang, Zongyu Wang, Krzysztof Matyjaszewski, Joanna Pietrasik PII: DOI: Reference:
S0014-3057(18)31806-8 https://doi.org/10.1016/j.eurpolymj.2018.11.014 EPJ 8698
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
21 September 2018 3 November 2018 9 November 2018
Please cite this article as: Zhang, Y., Wang, Z., Matyjaszewski, K., Pietrasik, J., Versatile PISA Templates for Tailored Synthesis of Nanoparticles, European Polymer Journal (2018), doi: https://doi.org/10.1016/j.eurpolymj. 2018.11.014
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Versatile PISA Templates for Tailored Synthesis of Nanoparticles Yaoming Zhang1, Zongyu Wang2, Krzysztof Matyjaszewski2, 3 and Joanna Pietrasik1 1. Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland 2. Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States 3. Department of Molecular Physics, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland Abstract This paper demonstrates the universal character of nano-spheres, formed from block copolymers
of
(PDMAEMA-b-PSt)
poly(2-(dimethylamino)ethyl via
polymerization
methacrylate)-block-polystyrene
induced
self-assembly
(PISA),
as
multifunctional templates for tailored nanoparticles (NPs) synthesis. Due to the presence
of basic
amine
groups
located
within the
shell of spherical
PDMAEMA-b-PSt copolymers nano-objects, three types of inorganic precursors were loaded into the shells through electrostatic interactions. Both redox-type and sol-gel chemistry were used to successfully immobilize the in situ formed gold, or silica and titania NPs, respectively. The hybrids could be potentially used as catalysts or dielectric materials.
Keywords: Template, PISA, nanoparticles 1. Introduction Template assisted strategy has been widely applied to the synthesis of inorganic
1
nanoparticles (NPs), because the preformed templates not only confine the NPs morphology but also prevent particle aggregation, forming well-defined NPs. [1-7] Nanostructured block copolymers (BCPs) with precisely controlled morphologies and distributed functional groups have been extensively studied as templates for synthesis of a variety of inorganic NPs. [8-13] Generally, efficient selective particle precursor loading is a prerequisite for the templated synthetic method, typically through electrostatic interaction or coordination. [14, 15] Subsequently, the entrapped precursor reacts with an additional reducing agent or catalyst to induce a redox or hydrolysis process that leads to the formation of the targeted NPs. Unfortunately stringent conditions are generally required for preparation of nanostructured BCPs, such as very low polymer concentration and selective solvents. This limits the scale up for use of the nanostructured BCPs in template-assisted NPs synthesis. [16, 17] Polymerization induced self-assembly (PISA) is a recently developed strategy that yields nanostructured BCPs in situ in the polymerization mixture. [18-21] Various nanostructured BCPs with tunable size and morphology are formed at higher concentration of reagents, simply through variation of polymerization conditions. This significantly simplifies the preparation of well-defined nanostructures of BCPs and potentially broadens the number of applications of self-assembled BCP, including templates for NPs synthesis. [19, 22, 23] Analogous to conventional self-assembled BCPs nanostructures, PISA yields nanostructures with selective segment aggregation. After the precursor loading into targeted segments and immobilization of the NPs, hybrids are formed within the PISA nano-objects. NPs precursors immobilized within the shell of the nano-objects yield various inorganic NPs with hollow structures. [24, 25] The dense and compacted core structure impedes the penetration of precursor into the core segments within the PISA systems. This could be the reason for the predominant preparation of hollow hybrids. Some noble metal NPs, such as gold nanoparticles (AuNPs) [26, 27] and silver nanoparticles (AgNPs) [28, 29] were successfully immobilized within PISA formed nano-objects with various morphologies, including spheres, worm-like structures and 2
vesicles. Silica nanotubes with different size and thickness were also prepared within PISA formed nanorods. [30] The broad diversity of morphologies that are generated by PISA formed nano-objects can provide a useful tool for NPs synthesis, that also broadens the scope of applications for nanostructured BCPs. [24] Herein, PISA was applied to prepare nanospheres from poly(2-(dimethylamino)ethyl methacrylate)-block-polystyrene (PDMAEMA-b-PSt), with a PDMAEMA shell and a PSt core. The morphology was limited to spheres through the design of the degree of polymerization (DP) of PDMAEMA stabilizer. PDMAEMA, if needed, can be easily protonated for loading anions into the segment. On the other hand, the amine group is a good catalyst/reducing agent for certain reactions. [31-33] Therefore, the PDMAEMA phase on the shell of the nanospheres could serve as a multifunctional nanoreactor for fabricating hollow nanoparticles without necessitating use of external reducing agents. Besides, the arrangement and morphology of the NPs on the shell could be further tailored by tuning the ratio between precursor and the template. Equally important is the universal nature of the template that simplifies the procedures for NPs formation. To demonstrate the versatility of this strategy, the synthesized PISA nanospheres were used as efficient templates for the preparation of nanostructured gold nanoparticles (AuNPs), silica nanoparticles (SiO2NPs) and titania nanoparticles (TiO2NPs). Finally, individual AuNPs distributed on the shell of the template spheres, or a continuous layer of SiO2NPs or TiO2NPs with tunable thickness covering the spheres were obtained, respectively. Thus, the PISA formed PDMAEMA-b-PSt nanospheres are very useful templates for preparation of a broad range of inorganic NPs with significantly diminished the amount of additives which are currently required for NPs synthesis. 2. Experimental 2.1 Materials 2-(Dimethylamino)ethyl methacrylate (DMAEMA, Sigma-Aldrich) and styrene (St,
3
Sigma-Aldrich) were purified by passing through basic Al2O3 column for removal of the inhibitor prior to use. N, N’-Azobis(isobutyronitrile) (AIBN) was purified by recrystallization from ethanol. 4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPDB), tetrachloroauric(III) acid (HAuCl4, 200 mg/dL in deionized water), titanium(IV) bis(ammonium lactato)dihydroxide solution (TALH) ， tetraethyl orthosilicate (TEOS), sodium borohydride (NaBH4) and 4-nitrophenol (4-NP) were purchased from Sigma-Aldrich and used as received. The solvent tetrahydrofuran (THF), methanol (MeOH) and others without note were purchased from Poch, Poland and used as received. 2.2 Synthesis of PDMAEMA-CTA A typical synthetic procedure for preparation of a PDMAEMA macro chain transfer agent (macro-CTA), PDMAEMA-CTA is described below: a 25 mL Schlenk flask was charged with DMAEMA (4.0 g, 25.5 mmol), CPDB (59 mg, 0.21 mmol), THF (4 mL),
and
AIBN
(6
mg,
0.04
mmol)
with
the
molar
ratio
of
CPDB/DMAEMA/AIBN=1/125/0.2. First, the mixture was subjected to three freeze-pump-thaw cycles, and then the flask was purged with argon and sealed. The flask was then placed in a preheated oil bath at 70 oC and stirred until the desired conversion was reached. The polymerization mixture was added to excess of diethyl ether to precipitate the PDMAEMA-CTA, followed by filtration and vacuum drying at room temperature overnight, providing a pink colored PDMAEMA-CTA. 2.3 Synthesis of PDMAEMA90-b-PSt130 block copolymer by PISA. PDMAEMA-CTA (1.2672 g, 0.088 mmol), styrene (10 mL, 87.2 mmol), AIBN (2.86 mg, 0.0174 mmol) and 10 mL methanol were added to a dry 50 mL Schlenk flask. The mixture was degassed by three freeze-pump-thaw cycles. Subsequently, the flask was immersed in a thermostated oil bath at 65 oC until the target conversion was reached and a milky mixture was obtained. The reaction was stopped by cooling to room temperature and exposing to air. The dispersion was purified by dialysis against
4
MeOH/water to remove the remained monomers. The solids content of the samples was calculated by comparing the mass of the dispersions and dried samples. 2.4 Synthesis of NPs@PDMAEMA-b-PSt. The dispersion of PDMAEMAm-b-PStn polymer template was diluted with water to a certain concentration, and then the precursor of the NPs was added to the dispersion. The molar ratio of precursor/DMAEMA was adjusted individually as discussed in the R&D section. The reaction was mixed until the completion of NPs formation, and finally the NPs@PDMAEMA-b-PSt was obtained and used without further modification for the catalytic evaluations. 2.5 Characterization 1
H NMR. Proton nuclear magnetic resonance (1H NMR) determined monomer
conversion during the polymerizations. The 1H NMR spectra were recorded on a Bruker Avance DPX 250 MHz instrument, using either CDCl3 or DMSO-d6 as solvent. GPC. Gel permeation chromatography (GPC) provided molecular weights (MW) and molecular weight distributions (Ð). The GPC measurements were conducted on a Wyatt instrument equipped with two PSS columns and one guard column, light scattering (LS) and differential refractometer (RI) detectors. The measurements were conducted in DMF with 50 mM LiBr as eluent at a flow rate of 1 mL/min. Either linear PSt or poly(methyl methacrylate) (PMMA) standards were used for calibration, depending on the polymer composition. DLS. The dispersions of PDMAEMAm-b-PStn and NPs@PDMAEMA-b-PSt obtained from the reaction mixture were diluted to ~1 wt % by water or ethanol. The average hydrodynamic diameter of the nano-objects was measured by dynamic light scattering (DLS) using a Zetasizer Nano ZS90 instrument.
5
TEM. A 0.01wt % dispersion was dropped on the carbon-coated copper grid and allowed to dry to prepare the sample for TEM. A Jeol 1400 transmission electron microscopy (TEM) and Jeol 2000EX operated at 200kV provided the morphology of the particles. UV-Vis. UV-Vis spectra were performed using a Thermo Scientific Evolution 220 UV-Vis spectrophotometer with 2 nm resolution. Thermogravimetry (TGA). TGA was carried out with a Mettler Toledo instrument (TGA 2) with a heating rate of 10 oC min-1 under nitrogen atmosphere. X-ray diffraction (XRD). The crystal structure of the prepared titania was determined by XRD (PANalytical’s X’Pert-Pro MPD) using Cu Kα1 radiation (λ=1.5406Ǻ). Catalytic Characterization. A glass cuvette was charged with 2.5 mL of NaBH4 (0.01 M) solution and 100 µL of AuNPs@PDMAEMA160-b-PSt330 (DMAEMA:AuCl4= 4:1, concentration 1 mg/mL) solution. Then 25 µL 4NP (0.01 M) was added to the cuvette. The UV-Vis spectra of the solution were recorded as a function of reaction time.
3 Results and discussion 3.1 Synthesis of PDMAEMA-b-PSt by PISA The PDMAEMA-b-PSt block copolymer was prepared by chain extension of the PDMAEMA-CTAs by RAFT dispersion polymerization of St. Two macro-CTAs with different chain length of PDMAEMA, DP=90 and DP=160, were prepared and used for chain extension. As shown in Table 1, relatively narrow molecular weight distribution (Ð<1.37) indicated the well-controlled RAFT polymerization of both the macro-CTA and chain extended block copolymers. Linear polystyrene GPC standards were used for MW calculation of PDMAEMA-b-PSt. This could cause the MW 6
difference between GPC and NMR results. The inherent features of PISA [20, 34] indicate that when DP of PSt increased to certain value, the amphiphilic block copolymer started the self-assembly process and yielded nano-objects composed of an insoluble core-forming PSt block and a shell of soluble PDMEMA stabilizer. The order-order morphology evolution from spheres to worm-like structures or vesicles can be triggered by increasing the DP of the PSt segment. The morphology is also determined by other parameters. [35, 36] Herein, PDMAEMA with longer chain length was chosen as a stabilizer because the generation of spherical nano-objects was targeted. DLS monitored the change in size of the particles formed during polymerization. As shown in Fig. 1c, the hydrodynamic diameter increased with increasing DP of the PSt block. Specifically, when the DP of PSt increased from 130 to 423, the hydrodynamic diameter of PDMAEMA90-b-PSt increased from 55 nm to 277 nm. The PDMAEMA160-b-PSt formed nano-objects with diameter ranged from 60 nm to 227 nm. The narrow distribution of DLS curves (Fig. 1c and Table 1) indicated the in situ self-assembly yielded well-organized nano-objects. TEM confirmed this analysis. A mixture of nanospheres and sphere-aggregates was observed by TEM (Fig. 1). The primary spheres’ diameter of PDMAEMA160-b-PSt increased with the increasing DP of PSt (Table 1 and Fig. 1(d) (e) (f)). The size determined by TEM was smaller than by DLS, since DLS is measured in solution while TEM detects products in the dry state. Furthermore, the TEM images revealed a different trend of nano-objects self-assembly as a function of chain length of stabilizer, while comparing the morphology evolution of PDMAEMA90-b-PSt and PDMAEMA160-b-PSt. This could plausibly result from varied packing of polymer chains within single nano-object, affecting different swelling degree. PDMAEMA90-b-PSt130 formed primary spheres and the spheres trended to aggregate together, thus a growth in the hydrodynamic diameter of nano-objects was observed with the increasing DP of PSt. PDMAEMA160-b-PSt194 initially formed small spheres with diameter around 15 nm, which grew to larger diameter of 51 nm or 200 nm when the DP of PSt increased to
7
330 and then to 680, respectively. All self-assembled nano-objects were limited to spherical morphology even when the DP of PSt was increased to 680; this could be attributed to longer chain stabilizer. Thus, the increasing DP of PSt resulted in the formation of spheres with large size rather than order-order morphology transition. Table 1 Molecular characterization of PDMAEMA-CTA and PDMAEMA-b-PSt #
D90-CTA D160-CTA D90-St130 D90-St177 D90-St423 D160-St194 D160-St330 D160-St681 #
Mn (GPC) 9,100 13,200 53,580 57,700 88,700 39,600 60,970 83,750
Ð (GPC) 1.16 1.14 1.21 1.28 1.37 1.23 1.32 1.34
Mn (NMR) 14,050 24,760 27,560 41,130 58,000 44,930 59,080 95,580
D*, nm (DLS) 554 10414 27717 592 9715 2273
D**, nm (TEM) 53 52 55 15 51 200
Dm-Stn refers to PDMAEMAm-b-PStn. * the average diameter of spheres and
sphere-aggregates, ** the average diameter of the primary spheres, obtained by software ImageJ using several TEM images.
Fig. 1 TEM images of D90-St130 (a), D90-St177 (b), D160-St194 (d), D160-St330 (e) and
8
D160-St681 (f). (c) Representative DLS curves of the PISA formed nano-objects. To summarize, nanospheres with designed composition and tailorable size, containing the amine functional groups in the shell were successfully prepared by PISA. Furthermore, the formed nanospheres with different sizes were utilized to demonstrate multifunctional nature of the PDMAEMA-b-PSt nanospheres for preparation of well-defined nanoparticles through redox and sol-gel reactions. 3.2 Synthesis of AuNPs@PDMAEMA-b-PSt The templated NPs synthesis requires electrostatic interaction or coordination for the precursors loading. As reported previously [28], AgNPs were immobilized onto the shell of PAA-b-PSt (polyacrylic acid-block-polystyrene) spheres by using the electrostatic interaction between silver ions Ag+ and carboxylic acid group of PAA. Analogously, PDMAEMA-b-PSt could provide cations for loading anions, such as the conventional anionic precursor AuCl4 - for AuNPs synthesis, once DMAEMA units are protonated. [37, 38] Furthermore, the DMAEMA can play the role of reducing agent for the reduction of AuCl4 - to Au0. Herein, AuNPs@PDMAEMA-b-PSt were prepared based on the PISA synthesized template arbitrarily selected. The polymer template PDMAEMA-b-PSt water dispersion was first diluted to 250 mg/mL. Then a solution of HAuCl4 was added to the template dispersion, the molar ratio between AuCl4 - and DMAEMA was varied from 1:16 to 1:4. The resulting mixture was kept in the dark and stirred for 2 days to complete the reaction. Finally a purple dispersion was obtained. The UV-vis spectra (AuNPs@PDMAEMA160-b-PSt330, Fig. 2d) show the characteristic AuNPs plasmonic spectrum with the absorption at the peak of 525 nm, which indicates the formation of AuNPs. The TEM images (Fig. 2 a-c) show that the shell of the spheres is decorated with small nanoparticles of diameter around 2 nm, regardless of the variation of molar ratio between AuCl4 - and amine. However, the amount of free AuNPs outside the templates increased with the molar ratio AuCl4 /-RNHR-+.
Higher amount of gold precursor loading (AuCl4 -/DMAEMA > 4:1)
resulted in the precipitation of the AuNPs. This was also confirmed by TEM analysis
9
(not shown in this paper). This observation is different from the case of AgNPs@PAA-b-PSt synthesis, in which the size of stabilized AgNPs was controlled by the molar ratio between Ag+/-COO- from 3:1 to 1:3. [28] The relative high density of gold could be the reason that heavy AuNPs detached from the templates. After removing the free AuNPs from the dispersion by centrifugation, the catalytic performance of the nanospheres with a discrete layer of uniform small size AuNPs was investigated. The AuNPs@PDMAEMA160-b-PSt330 was evaluated as catalyst for reduction of 4-nitrophenol. The reaction reached high conversion in 40 min, as shown in Fig. 2e. The gradually decreasing intensity of the peak at 402 nm indicates the successful reduction, and confirms the catalytic activity of the synthesized AuNPs.
Fig. 2 TEM images of the AuNPs@PDMAEMA90-b-PSt330 with different molar ratio of DMAEMA/AuCl4-, (a) 16:1, (b) 8:1, and (c) 4:1. (d) UV-vis spectra of the AuNPs@PDMAEMA160-b-PSt330. (e) Catalytic performance of AuNPs, with time-dependent absorption spectra of 4-NP aqueous solution mixed with AuNPs@PDMAEMA160-b-PSt330 (DMAEMA/AuCl4 - = 4:1)
10
3.3 Synthesis of SiO2NPs@PDMAEMA-b-PSt
Fig. 3 TEM images of SiO2NPs@PDMAEMA90-b-PSt177 with different molar ratio of TEOS/DMAEMA, (a) (b) TEOS/DMAEMA =10, (c) (d) TEOS/DMAEMA =100. (e) Representative DLS curves of SiO2NPs@PDMAEMA-b-PSt. (f) TGA of SiO2NPs@PDMAEMA90-b-PSt177 (TEOS/DMAEMA =100) and PDMAEMA90-b-PSt177.
The PDMAEMA shell can also serve as a reactor for synthesis of metal oxide or transition metal oxide NPs based on sol-gel chemistry. For example, nanorods formed by PISA were reported as successful templates for synthesis of silica nanotubes. [30] Herein, the shell of PISA spheres was used as a nanoreactor for synthesis of silica nanoparticles (SiO2NPs) by hydrolysis of TEOS. Amine groups carry catalytic activity for this type of reaction. Before adding the precursor, the pH of the PDMAEMA90-b-PSt177 dispersion was adjusted to 3, by addition of 1M HCl solution. Then TEOS was added to the dispersion and the reaction mixed at room temperature for 2 days. The molar ratio between TEOS and protonated amine groups ranged from
11
10:1 to 100:1. The monomodal DLS curves and TEM images indicate that all precursors were loaded into the templates. The hydrodynamic diameter of the SiO2NPs@PDMAEMA-b-PSt were similar, they were at the range of 105 nm (Fig. 3e) even though the molar ratio of TEOS/DMAEMA increased from 10:1 to 100:1. TEM images
(Fig.
3c
and
Fig.
3d)
show
the
“raspberry”
structure
of
SiO2NPs@PDMAEMA-b-PSt when the molar ratio of TEOS/DMAEMA was 100:1. The yielded raspberry structure arose because the spherical templates were covered with a continuous layer of silica nanoparticles as the solids content increased. The inorganic content reached 77 wt % (Fig. 3f), comparing with the pure template with 2 wt % mass remaining after heating to 800 oC, demonstrating high capacity of inorganic NPs loading and high efficiency of the template. This is very different from the AuNPs@PDMAEMA-b-PSt system and could result from relative low density of the silica NPs, which remained on the template surface and did not detach from it. The surface activity of silica is significantly different in comparison to gold. On the contrary, if the molar ratio of TEOS/DMAEMA was only 10:1, the silica layer was barely detectable in TEM images (Fig. 3a and Fig. 3b). Therefore, it can be concluded that the thickness of the silica layer is easily tunable by adjusting the loaded amount of
the
TEOS
silica
precursor.
A porous
structure
expected
from
the
PDMAEMA90-b-St177-SiO2-100, could broaden potential applications of such systems, to include delivery or catalysis, as will be studied in the future. 3.4 Synthesis of TiO2@PDMAEMA-b-PSt
12
Fig. 4 TEM images of TiO2NPs@PDMAEMA-b-PSt with different molar ratio of TALH / DMAEMA (a)-(d). (a) PDMAEMA90-b-PSt177 (TALH/DMAEMA =100), (b) PDMAEMA160-b-PSt330 (TALH/DMAEMA =100), and (c) (d) PDMAEMA160-b-PSt681 (TALH/DMAEMA =50). (e) XRD of PDMAEMA160-b-PSt681 (TALH/DMAEMA =50). (f) TGA of TiO2NPs@PDMAEMA90-b-PSt423 (TALH/DMAEMA =50)
The PDMAEMA-b-PSt particles formed by PISA were also utilized as templates for synthesis of titania nanoparticles (TiO2NPs). The electrostatic interaction between titanium(IV) bis(ammonium lactato)dihydroxide (TALH) solution and DMAEMA were used for loading the TiO2 precursor [39] into the PISA formed nanoparticles. The PDMAEMA-b-PSt template was adjusted at a concentration of 0.5 mg/mL by addition of ethanol, then TALH was added to the dispersion, followed with 24h reflux at 95 oC to allow the hydrolysis and condensation of particle precursor to form TiO2NPs. Different templates with varied molar ratio of DMAEMA/TALH were applied for the synthesis of titania. The TiO2NPs@PDMAEMA-b-PSt, core-shell structures
shown
in
Fig.
4,
indicates
the
formation
of
nanostructured
TiO2NPs@PDMAEMA-b-PSt with varied diameter. The diameter depends on the 13
degree of polymerization of the templates and could be adjusted from 60 nm to 200 nm. Contrary to the raspberry structure of SiO2NPs@PDMAEMA-b-PSt, only compact layers of TiO2 were located within the shell of the templates, even with a molar ratio of TALH/DMAEMA of 100 or if the template with different chain length (DP) of stabilizer was used. The XRD result indicated the formation of anatase structure of the titania (Fig. 4e).[40] To conclude the high efficiency of the synthesized template was confirmed as shown in Fig. 4f, the inorganic content reached 39 wt % when the molar ratio of TALH/DMAEMA was 50. Such materials could be good candidates for high κ materials. Their dielectric performance will be studied in the future. 4 Conclusions Stable phase separated spherical block copolymers PDMAEMA-b-PSt with core-shell structure were prepared by PISA using a soluble PDMAEMA CTA as surfactant. The block copolymer nanospheres are effective templates for nanoparticles synthesis, specifically AuNPs, SiO2NPs and TiO2NPs, simply by utilizing the accessible amine functional groups distributed along the shell of PISA nano-objects. In one example, a layer of AuNPs, created by separated single nanoparticles, was immobilized within the shell, and they showed catalytic performance for the reduction of 4-nitrophenol. In contrast to the formation of distributed AuNPs, the continuous layers of silica and titania were created that covered the shell of the spheres. The thickness of the inorganic layer could be tuned by adjusting the molar ratio between TEOS/DMAEMA or TALH/DMAEMA. The advantage of this type of template is its multi-functionality, which dramatically simplifies the synthetic procedure currently employed for nanoparticles formation by reduction or sol-gel chemistry. We believe this study could pave the way towards large-scale preparation of nanoparticles.
14
Acknowledgement YZ, JP, KM acknowledge the financial support from National Science Center, Poland (via Grant UMO-2014/14/A/ST5/ 00204).
Data availability The raw data required to reproduce these findings cannot be shared at this time due to technical or time limitations
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[25] J. Chang, W. Zhang, C. Hong, Template-Directed Fabrication of Anatase TiO2 Hollow Nanoparticles and Their Application in Photocatalytic Degradation of Methyl Orange, Chin. J. Chem. 35(6) (2017) 1016-1022. [26] P.F. Shi, C.Q. Gao, X. He, P.C. Sun, W.Q. Zhang, Multicompartment Nanoparticles of Poly(4-vinylpyridine) Graft Block Terpolymer: Synthesis and Application as Scaffold for Efficient Au Nanocatalyst, Macromolecules 48(5) (2015) 1380-1389. [27] R. Bleach, B. Karagoz, S.M. Prakash, T.P. Davis, C. Boyer, In Situ Formation of Polymer-Gold Composite Nanoparticles with Tunable Morphologies, ACS Macro Lett. 3(7) (2014) 591-596. [28] Y. Zhang, P. Filipczak, G. He, G. Nowaczyk, L. Witczak, W. Raj, M. Kozanecki, K. Matyjaszewski, J. Pietrasik, Synthesis and characterization of Ag NPs templated via polymerization induced self-assembly, Polymer 129 (2017) 144-150. [29] W. Zhou, Q. Qu, W. Yu, Z. An, Single Monomer for Multiple Tasks: Polymerization Induced Self-Assembly, Functionalization and Cross-Linking, and Nanoparticle Loading, ACS Macro Lett. 3(12) (2014) 1220-1224. [30] W.J. Zhang, C.Y. Hong, C.Y. Pan, Fabrication and characterization of silica nanotubes with controlled dimensions, J. Mater. Chem. 2(21) (2014) 7819-7828. [31] M. Mullner, T. Lunkenbein, J. Breu, F. Caruso, A.H.E. Muller, Template-Directed Synthesis of Silica Nanowires and Nanotubes from Cylindrical Core-Shell Polymer Brushes, Chem. Mater. 24(10) (2012) 1802-1810. [32] J. Yuan, Y. Lu, F. Schacher, T. Lunkenbein, S. Weiss, H. Schmalz, A.H.E. Müller, Template-Directed Synthesis of Hybrid Titania Nanowires within Core−Shell Bishydrophilic Cylindrical Polymer Brushes, Chem. Mater. 21(18) (2009) 4146-4154. [33] Z.C. Zheng, A. Daniel, W. Yu, B. Weber, J. Ling, A.H.E. Muller, Rare-Earth Metal Cations Incorporated Silica Hybrid Nanoparticles Templated by Cylindrical Polymer Brushes, Chem. Mater. 25(22) (2013) 4585-4594. [34] N.J. Warren, S.P. Armes, Polymerization-Induced Self-Assembly of Block Copolymer Nano-objects via RAFT Aqueous Dispersion Polymerization, J. Am. Chem. Soc. 136(29) (2014) 10174-10185. [35] D. Zehm, L.P.D. Ratcliffe, S.P. Armes, Synthesis of Diblock Copolymer Nanoparticles via RAFT Alcoholic Dispersion Polymerization: Effect of Block Copolymer Composition, Molecular Weight, Copolymer Concentration, and Solvent Type on the Final Particle Morphology, Macromolecules 46(1) (2013) 128-139. [36] L.P.D. Ratcliffe, B.E. McKenzie, G.M.D. Le Bouedec, C.N. Williams, S.L. Brown, S.P. Armes, Polymerization-Induced Self-Assembly of All-Acrylic Diblock Copolymers via RAFT Dispersion Polymerization in Alkanes, Macromolecules 48(23) (2015) 8594-8607.
17
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18
Highlights:
Block copolymers can self-assembly into tailored nano-spheres
Obtained nano-spheres can be an efficient templates for nanoparticles synthesis
Amine groups of the nano-spheres are required for multifunctional nanoreactor
19
Graphical abstract
20
S0014-3057(18)31806-8 https://doi.org/10.1016/j.eurpolymj.2018.11.014 EPJ 8698
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
21 September 2018 3 November 2018 9 November 2018
Please cite this article as: Zhang, Y., Wang, Z., Matyjaszewski, K., Pietrasik, J., Versatile PISA Templates for Tailored Synthesis of Nanoparticles, European Polymer Journal (2018), doi: https://doi.org/10.1016/j.eurpolymj. 2018.11.014
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Versatile PISA Templates for Tailored Synthesis of Nanoparticles Yaoming Zhang1, Zongyu Wang2, Krzysztof Matyjaszewski2, 3 and Joanna Pietrasik1 1. Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland 2. Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States 3. Department of Molecular Physics, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland Abstract This paper demonstrates the universal character of nano-spheres, formed from block copolymers
of
(PDMAEMA-b-PSt)
poly(2-(dimethylamino)ethyl via
polymerization
methacrylate)-block-polystyrene
induced
self-assembly
(PISA),
as
multifunctional templates for tailored nanoparticles (NPs) synthesis. Due to the presence
of basic
amine
groups
located
within the
shell of spherical
PDMAEMA-b-PSt copolymers nano-objects, three types of inorganic precursors were loaded into the shells through electrostatic interactions. Both redox-type and sol-gel chemistry were used to successfully immobilize the in situ formed gold, or silica and titania NPs, respectively. The hybrids could be potentially used as catalysts or dielectric materials.
Keywords: Template, PISA, nanoparticles 1. Introduction Template assisted strategy has been widely applied to the synthesis of inorganic
1
nanoparticles (NPs), because the preformed templates not only confine the NPs morphology but also prevent particle aggregation, forming well-defined NPs. [1-7] Nanostructured block copolymers (BCPs) with precisely controlled morphologies and distributed functional groups have been extensively studied as templates for synthesis of a variety of inorganic NPs. [8-13] Generally, efficient selective particle precursor loading is a prerequisite for the templated synthetic method, typically through electrostatic interaction or coordination. [14, 15] Subsequently, the entrapped precursor reacts with an additional reducing agent or catalyst to induce a redox or hydrolysis process that leads to the formation of the targeted NPs. Unfortunately stringent conditions are generally required for preparation of nanostructured BCPs, such as very low polymer concentration and selective solvents. This limits the scale up for use of the nanostructured BCPs in template-assisted NPs synthesis. [16, 17] Polymerization induced self-assembly (PISA) is a recently developed strategy that yields nanostructured BCPs in situ in the polymerization mixture. [18-21] Various nanostructured BCPs with tunable size and morphology are formed at higher concentration of reagents, simply through variation of polymerization conditions. This significantly simplifies the preparation of well-defined nanostructures of BCPs and potentially broadens the number of applications of self-assembled BCP, including templates for NPs synthesis. [19, 22, 23] Analogous to conventional self-assembled BCPs nanostructures, PISA yields nanostructures with selective segment aggregation. After the precursor loading into targeted segments and immobilization of the NPs, hybrids are formed within the PISA nano-objects. NPs precursors immobilized within the shell of the nano-objects yield various inorganic NPs with hollow structures. [24, 25] The dense and compacted core structure impedes the penetration of precursor into the core segments within the PISA systems. This could be the reason for the predominant preparation of hollow hybrids. Some noble metal NPs, such as gold nanoparticles (AuNPs) [26, 27] and silver nanoparticles (AgNPs) [28, 29] were successfully immobilized within PISA formed nano-objects with various morphologies, including spheres, worm-like structures and 2
vesicles. Silica nanotubes with different size and thickness were also prepared within PISA formed nanorods. [30] The broad diversity of morphologies that are generated by PISA formed nano-objects can provide a useful tool for NPs synthesis, that also broadens the scope of applications for nanostructured BCPs. [24] Herein, PISA was applied to prepare nanospheres from poly(2-(dimethylamino)ethyl methacrylate)-block-polystyrene (PDMAEMA-b-PSt), with a PDMAEMA shell and a PSt core. The morphology was limited to spheres through the design of the degree of polymerization (DP) of PDMAEMA stabilizer. PDMAEMA, if needed, can be easily protonated for loading anions into the segment. On the other hand, the amine group is a good catalyst/reducing agent for certain reactions. [31-33] Therefore, the PDMAEMA phase on the shell of the nanospheres could serve as a multifunctional nanoreactor for fabricating hollow nanoparticles without necessitating use of external reducing agents. Besides, the arrangement and morphology of the NPs on the shell could be further tailored by tuning the ratio between precursor and the template. Equally important is the universal nature of the template that simplifies the procedures for NPs formation. To demonstrate the versatility of this strategy, the synthesized PISA nanospheres were used as efficient templates for the preparation of nanostructured gold nanoparticles (AuNPs), silica nanoparticles (SiO2NPs) and titania nanoparticles (TiO2NPs). Finally, individual AuNPs distributed on the shell of the template spheres, or a continuous layer of SiO2NPs or TiO2NPs with tunable thickness covering the spheres were obtained, respectively. Thus, the PISA formed PDMAEMA-b-PSt nanospheres are very useful templates for preparation of a broad range of inorganic NPs with significantly diminished the amount of additives which are currently required for NPs synthesis. 2. Experimental 2.1 Materials 2-(Dimethylamino)ethyl methacrylate (DMAEMA, Sigma-Aldrich) and styrene (St,
3
Sigma-Aldrich) were purified by passing through basic Al2O3 column for removal of the inhibitor prior to use. N, N’-Azobis(isobutyronitrile) (AIBN) was purified by recrystallization from ethanol. 4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPDB), tetrachloroauric(III) acid (HAuCl4, 200 mg/dL in deionized water), titanium(IV) bis(ammonium lactato)dihydroxide solution (TALH) ， tetraethyl orthosilicate (TEOS), sodium borohydride (NaBH4) and 4-nitrophenol (4-NP) were purchased from Sigma-Aldrich and used as received. The solvent tetrahydrofuran (THF), methanol (MeOH) and others without note were purchased from Poch, Poland and used as received. 2.2 Synthesis of PDMAEMA-CTA A typical synthetic procedure for preparation of a PDMAEMA macro chain transfer agent (macro-CTA), PDMAEMA-CTA is described below: a 25 mL Schlenk flask was charged with DMAEMA (4.0 g, 25.5 mmol), CPDB (59 mg, 0.21 mmol), THF (4 mL),
and
AIBN
(6
mg,
0.04
mmol)
with
the
molar
ratio
of
CPDB/DMAEMA/AIBN=1/125/0.2. First, the mixture was subjected to three freeze-pump-thaw cycles, and then the flask was purged with argon and sealed. The flask was then placed in a preheated oil bath at 70 oC and stirred until the desired conversion was reached. The polymerization mixture was added to excess of diethyl ether to precipitate the PDMAEMA-CTA, followed by filtration and vacuum drying at room temperature overnight, providing a pink colored PDMAEMA-CTA. 2.3 Synthesis of PDMAEMA90-b-PSt130 block copolymer by PISA. PDMAEMA-CTA (1.2672 g, 0.088 mmol), styrene (10 mL, 87.2 mmol), AIBN (2.86 mg, 0.0174 mmol) and 10 mL methanol were added to a dry 50 mL Schlenk flask. The mixture was degassed by three freeze-pump-thaw cycles. Subsequently, the flask was immersed in a thermostated oil bath at 65 oC until the target conversion was reached and a milky mixture was obtained. The reaction was stopped by cooling to room temperature and exposing to air. The dispersion was purified by dialysis against
4
MeOH/water to remove the remained monomers. The solids content of the samples was calculated by comparing the mass of the dispersions and dried samples. 2.4 Synthesis of NPs@PDMAEMA-b-PSt. The dispersion of PDMAEMAm-b-PStn polymer template was diluted with water to a certain concentration, and then the precursor of the NPs was added to the dispersion. The molar ratio of precursor/DMAEMA was adjusted individually as discussed in the R&D section. The reaction was mixed until the completion of NPs formation, and finally the NPs@PDMAEMA-b-PSt was obtained and used without further modification for the catalytic evaluations. 2.5 Characterization 1
H NMR. Proton nuclear magnetic resonance (1H NMR) determined monomer
conversion during the polymerizations. The 1H NMR spectra were recorded on a Bruker Avance DPX 250 MHz instrument, using either CDCl3 or DMSO-d6 as solvent. GPC. Gel permeation chromatography (GPC) provided molecular weights (MW) and molecular weight distributions (Ð). The GPC measurements were conducted on a Wyatt instrument equipped with two PSS columns and one guard column, light scattering (LS) and differential refractometer (RI) detectors. The measurements were conducted in DMF with 50 mM LiBr as eluent at a flow rate of 1 mL/min. Either linear PSt or poly(methyl methacrylate) (PMMA) standards were used for calibration, depending on the polymer composition. DLS. The dispersions of PDMAEMAm-b-PStn and NPs@PDMAEMA-b-PSt obtained from the reaction mixture were diluted to ~1 wt % by water or ethanol. The average hydrodynamic diameter of the nano-objects was measured by dynamic light scattering (DLS) using a Zetasizer Nano ZS90 instrument.
5
TEM. A 0.01wt % dispersion was dropped on the carbon-coated copper grid and allowed to dry to prepare the sample for TEM. A Jeol 1400 transmission electron microscopy (TEM) and Jeol 2000EX operated at 200kV provided the morphology of the particles. UV-Vis. UV-Vis spectra were performed using a Thermo Scientific Evolution 220 UV-Vis spectrophotometer with 2 nm resolution. Thermogravimetry (TGA). TGA was carried out with a Mettler Toledo instrument (TGA 2) with a heating rate of 10 oC min-1 under nitrogen atmosphere. X-ray diffraction (XRD). The crystal structure of the prepared titania was determined by XRD (PANalytical’s X’Pert-Pro MPD) using Cu Kα1 radiation (λ=1.5406Ǻ). Catalytic Characterization. A glass cuvette was charged with 2.5 mL of NaBH4 (0.01 M) solution and 100 µL of AuNPs@PDMAEMA160-b-PSt330 (DMAEMA:AuCl4= 4:1, concentration 1 mg/mL) solution. Then 25 µL 4NP (0.01 M) was added to the cuvette. The UV-Vis spectra of the solution were recorded as a function of reaction time.
3 Results and discussion 3.1 Synthesis of PDMAEMA-b-PSt by PISA The PDMAEMA-b-PSt block copolymer was prepared by chain extension of the PDMAEMA-CTAs by RAFT dispersion polymerization of St. Two macro-CTAs with different chain length of PDMAEMA, DP=90 and DP=160, were prepared and used for chain extension. As shown in Table 1, relatively narrow molecular weight distribution (Ð<1.37) indicated the well-controlled RAFT polymerization of both the macro-CTA and chain extended block copolymers. Linear polystyrene GPC standards were used for MW calculation of PDMAEMA-b-PSt. This could cause the MW 6
difference between GPC and NMR results. The inherent features of PISA [20, 34] indicate that when DP of PSt increased to certain value, the amphiphilic block copolymer started the self-assembly process and yielded nano-objects composed of an insoluble core-forming PSt block and a shell of soluble PDMEMA stabilizer. The order-order morphology evolution from spheres to worm-like structures or vesicles can be triggered by increasing the DP of the PSt segment. The morphology is also determined by other parameters. [35, 36] Herein, PDMAEMA with longer chain length was chosen as a stabilizer because the generation of spherical nano-objects was targeted. DLS monitored the change in size of the particles formed during polymerization. As shown in Fig. 1c, the hydrodynamic diameter increased with increasing DP of the PSt block. Specifically, when the DP of PSt increased from 130 to 423, the hydrodynamic diameter of PDMAEMA90-b-PSt increased from 55 nm to 277 nm. The PDMAEMA160-b-PSt formed nano-objects with diameter ranged from 60 nm to 227 nm. The narrow distribution of DLS curves (Fig. 1c and Table 1) indicated the in situ self-assembly yielded well-organized nano-objects. TEM confirmed this analysis. A mixture of nanospheres and sphere-aggregates was observed by TEM (Fig. 1). The primary spheres’ diameter of PDMAEMA160-b-PSt increased with the increasing DP of PSt (Table 1 and Fig. 1(d) (e) (f)). The size determined by TEM was smaller than by DLS, since DLS is measured in solution while TEM detects products in the dry state. Furthermore, the TEM images revealed a different trend of nano-objects self-assembly as a function of chain length of stabilizer, while comparing the morphology evolution of PDMAEMA90-b-PSt and PDMAEMA160-b-PSt. This could plausibly result from varied packing of polymer chains within single nano-object, affecting different swelling degree. PDMAEMA90-b-PSt130 formed primary spheres and the spheres trended to aggregate together, thus a growth in the hydrodynamic diameter of nano-objects was observed with the increasing DP of PSt. PDMAEMA160-b-PSt194 initially formed small spheres with diameter around 15 nm, which grew to larger diameter of 51 nm or 200 nm when the DP of PSt increased to
7
330 and then to 680, respectively. All self-assembled nano-objects were limited to spherical morphology even when the DP of PSt was increased to 680; this could be attributed to longer chain stabilizer. Thus, the increasing DP of PSt resulted in the formation of spheres with large size rather than order-order morphology transition. Table 1 Molecular characterization of PDMAEMA-CTA and PDMAEMA-b-PSt #
D90-CTA D160-CTA D90-St130 D90-St177 D90-St423 D160-St194 D160-St330 D160-St681 #
Mn (GPC) 9,100 13,200 53,580 57,700 88,700 39,600 60,970 83,750
Ð (GPC) 1.16 1.14 1.21 1.28 1.37 1.23 1.32 1.34
Mn (NMR) 14,050 24,760 27,560 41,130 58,000 44,930 59,080 95,580
D*, nm (DLS) 554 10414 27717 592 9715 2273
D**, nm (TEM) 53 52 55 15 51 200
Dm-Stn refers to PDMAEMAm-b-PStn. * the average diameter of spheres and
sphere-aggregates, ** the average diameter of the primary spheres, obtained by software ImageJ using several TEM images.
Fig. 1 TEM images of D90-St130 (a), D90-St177 (b), D160-St194 (d), D160-St330 (e) and
8
D160-St681 (f). (c) Representative DLS curves of the PISA formed nano-objects. To summarize, nanospheres with designed composition and tailorable size, containing the amine functional groups in the shell were successfully prepared by PISA. Furthermore, the formed nanospheres with different sizes were utilized to demonstrate multifunctional nature of the PDMAEMA-b-PSt nanospheres for preparation of well-defined nanoparticles through redox and sol-gel reactions. 3.2 Synthesis of AuNPs@PDMAEMA-b-PSt The templated NPs synthesis requires electrostatic interaction or coordination for the precursors loading. As reported previously [28], AgNPs were immobilized onto the shell of PAA-b-PSt (polyacrylic acid-block-polystyrene) spheres by using the electrostatic interaction between silver ions Ag+ and carboxylic acid group of PAA. Analogously, PDMAEMA-b-PSt could provide cations for loading anions, such as the conventional anionic precursor AuCl4 - for AuNPs synthesis, once DMAEMA units are protonated. [37, 38] Furthermore, the DMAEMA can play the role of reducing agent for the reduction of AuCl4 - to Au0. Herein, AuNPs@PDMAEMA-b-PSt were prepared based on the PISA synthesized template arbitrarily selected. The polymer template PDMAEMA-b-PSt water dispersion was first diluted to 250 mg/mL. Then a solution of HAuCl4 was added to the template dispersion, the molar ratio between AuCl4 - and DMAEMA was varied from 1:16 to 1:4. The resulting mixture was kept in the dark and stirred for 2 days to complete the reaction. Finally a purple dispersion was obtained. The UV-vis spectra (AuNPs@PDMAEMA160-b-PSt330, Fig. 2d) show the characteristic AuNPs plasmonic spectrum with the absorption at the peak of 525 nm, which indicates the formation of AuNPs. The TEM images (Fig. 2 a-c) show that the shell of the spheres is decorated with small nanoparticles of diameter around 2 nm, regardless of the variation of molar ratio between AuCl4 - and amine. However, the amount of free AuNPs outside the templates increased with the molar ratio AuCl4 /-RNHR-+.
Higher amount of gold precursor loading (AuCl4 -/DMAEMA > 4:1)
resulted in the precipitation of the AuNPs. This was also confirmed by TEM analysis
9
(not shown in this paper). This observation is different from the case of AgNPs@PAA-b-PSt synthesis, in which the size of stabilized AgNPs was controlled by the molar ratio between Ag+/-COO- from 3:1 to 1:3. [28] The relative high density of gold could be the reason that heavy AuNPs detached from the templates. After removing the free AuNPs from the dispersion by centrifugation, the catalytic performance of the nanospheres with a discrete layer of uniform small size AuNPs was investigated. The AuNPs@PDMAEMA160-b-PSt330 was evaluated as catalyst for reduction of 4-nitrophenol. The reaction reached high conversion in 40 min, as shown in Fig. 2e. The gradually decreasing intensity of the peak at 402 nm indicates the successful reduction, and confirms the catalytic activity of the synthesized AuNPs.
Fig. 2 TEM images of the AuNPs@PDMAEMA90-b-PSt330 with different molar ratio of DMAEMA/AuCl4-, (a) 16:1, (b) 8:1, and (c) 4:1. (d) UV-vis spectra of the AuNPs@PDMAEMA160-b-PSt330. (e) Catalytic performance of AuNPs, with time-dependent absorption spectra of 4-NP aqueous solution mixed with AuNPs@PDMAEMA160-b-PSt330 (DMAEMA/AuCl4 - = 4:1)
10
3.3 Synthesis of SiO2NPs@PDMAEMA-b-PSt
Fig. 3 TEM images of SiO2NPs@PDMAEMA90-b-PSt177 with different molar ratio of TEOS/DMAEMA, (a) (b) TEOS/DMAEMA =10, (c) (d) TEOS/DMAEMA =100. (e) Representative DLS curves of SiO2NPs@PDMAEMA-b-PSt. (f) TGA of SiO2NPs@PDMAEMA90-b-PSt177 (TEOS/DMAEMA =100) and PDMAEMA90-b-PSt177.
The PDMAEMA shell can also serve as a reactor for synthesis of metal oxide or transition metal oxide NPs based on sol-gel chemistry. For example, nanorods formed by PISA were reported as successful templates for synthesis of silica nanotubes. [30] Herein, the shell of PISA spheres was used as a nanoreactor for synthesis of silica nanoparticles (SiO2NPs) by hydrolysis of TEOS. Amine groups carry catalytic activity for this type of reaction. Before adding the precursor, the pH of the PDMAEMA90-b-PSt177 dispersion was adjusted to 3, by addition of 1M HCl solution. Then TEOS was added to the dispersion and the reaction mixed at room temperature for 2 days. The molar ratio between TEOS and protonated amine groups ranged from
11
10:1 to 100:1. The monomodal DLS curves and TEM images indicate that all precursors were loaded into the templates. The hydrodynamic diameter of the SiO2NPs@PDMAEMA-b-PSt were similar, they were at the range of 105 nm (Fig. 3e) even though the molar ratio of TEOS/DMAEMA increased from 10:1 to 100:1. TEM images
(Fig.
3c
and
Fig.
3d)
show
the
“raspberry”
structure
of
SiO2NPs@PDMAEMA-b-PSt when the molar ratio of TEOS/DMAEMA was 100:1. The yielded raspberry structure arose because the spherical templates were covered with a continuous layer of silica nanoparticles as the solids content increased. The inorganic content reached 77 wt % (Fig. 3f), comparing with the pure template with 2 wt % mass remaining after heating to 800 oC, demonstrating high capacity of inorganic NPs loading and high efficiency of the template. This is very different from the AuNPs@PDMAEMA-b-PSt system and could result from relative low density of the silica NPs, which remained on the template surface and did not detach from it. The surface activity of silica is significantly different in comparison to gold. On the contrary, if the molar ratio of TEOS/DMAEMA was only 10:1, the silica layer was barely detectable in TEM images (Fig. 3a and Fig. 3b). Therefore, it can be concluded that the thickness of the silica layer is easily tunable by adjusting the loaded amount of
the
TEOS
silica
precursor.
A porous
structure
expected
from
the
PDMAEMA90-b-St177-SiO2-100, could broaden potential applications of such systems, to include delivery or catalysis, as will be studied in the future. 3.4 Synthesis of TiO2@PDMAEMA-b-PSt
12
Fig. 4 TEM images of TiO2NPs@PDMAEMA-b-PSt with different molar ratio of TALH / DMAEMA (a)-(d). (a) PDMAEMA90-b-PSt177 (TALH/DMAEMA =100), (b) PDMAEMA160-b-PSt330 (TALH/DMAEMA =100), and (c) (d) PDMAEMA160-b-PSt681 (TALH/DMAEMA =50). (e) XRD of PDMAEMA160-b-PSt681 (TALH/DMAEMA =50). (f) TGA of TiO2NPs@PDMAEMA90-b-PSt423 (TALH/DMAEMA =50)
The PDMAEMA-b-PSt particles formed by PISA were also utilized as templates for synthesis of titania nanoparticles (TiO2NPs). The electrostatic interaction between titanium(IV) bis(ammonium lactato)dihydroxide (TALH) solution and DMAEMA were used for loading the TiO2 precursor [39] into the PISA formed nanoparticles. The PDMAEMA-b-PSt template was adjusted at a concentration of 0.5 mg/mL by addition of ethanol, then TALH was added to the dispersion, followed with 24h reflux at 95 oC to allow the hydrolysis and condensation of particle precursor to form TiO2NPs. Different templates with varied molar ratio of DMAEMA/TALH were applied for the synthesis of titania. The TiO2NPs@PDMAEMA-b-PSt, core-shell structures
shown
in
Fig.
4,
indicates
the
formation
of
nanostructured
TiO2NPs@PDMAEMA-b-PSt with varied diameter. The diameter depends on the 13
degree of polymerization of the templates and could be adjusted from 60 nm to 200 nm. Contrary to the raspberry structure of SiO2NPs@PDMAEMA-b-PSt, only compact layers of TiO2 were located within the shell of the templates, even with a molar ratio of TALH/DMAEMA of 100 or if the template with different chain length (DP) of stabilizer was used. The XRD result indicated the formation of anatase structure of the titania (Fig. 4e).[40] To conclude the high efficiency of the synthesized template was confirmed as shown in Fig. 4f, the inorganic content reached 39 wt % when the molar ratio of TALH/DMAEMA was 50. Such materials could be good candidates for high κ materials. Their dielectric performance will be studied in the future. 4 Conclusions Stable phase separated spherical block copolymers PDMAEMA-b-PSt with core-shell structure were prepared by PISA using a soluble PDMAEMA CTA as surfactant. The block copolymer nanospheres are effective templates for nanoparticles synthesis, specifically AuNPs, SiO2NPs and TiO2NPs, simply by utilizing the accessible amine functional groups distributed along the shell of PISA nano-objects. In one example, a layer of AuNPs, created by separated single nanoparticles, was immobilized within the shell, and they showed catalytic performance for the reduction of 4-nitrophenol. In contrast to the formation of distributed AuNPs, the continuous layers of silica and titania were created that covered the shell of the spheres. The thickness of the inorganic layer could be tuned by adjusting the molar ratio between TEOS/DMAEMA or TALH/DMAEMA. The advantage of this type of template is its multi-functionality, which dramatically simplifies the synthetic procedure currently employed for nanoparticles formation by reduction or sol-gel chemistry. We believe this study could pave the way towards large-scale preparation of nanoparticles.
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Acknowledgement YZ, JP, KM acknowledge the financial support from National Science Center, Poland (via Grant UMO-2014/14/A/ST5/ 00204).
Data availability The raw data required to reproduce these findings cannot be shared at this time due to technical or time limitations
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Highlights:
Block copolymers can self-assembly into tailored nano-spheres
Obtained nano-spheres can be an efficient templates for nanoparticles synthesis
Amine groups of the nano-spheres are required for multifunctional nanoreactor
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Graphical abstract
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