nanospheres via microemulsion self-assembling

Accepted Manuscript Facile fabrication of silver decorated polyarylene ether nitrile composites micro/ nanospheres via microemulsion self-assembling Dawei Zhang, Ruoyu Zhang, Xiaohong He, Kun Jia, Xiaobo Liu PII:

S1359-8368(18)32461-2

DOI:

10.1016/j.compositesb.2018.08.046

Reference:

JCOMB 5876

To appear in:

Composites Part B

Received Date: 4 August 2018 Accepted Date: 12 August 2018

Please cite this article as: Zhang D, Zhang R, He X, Jia K, Liu X, Facile fabrication of silver decorated polyarylene ether nitrile composites micro/nanospheres via microemulsion self-assembling, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.08.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile fabrication of silver decorated polyarylene ether nitrile composites micro/nanospheres via microemulsion self-assembling

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Dawei Zhang1, Ruoyu Zhang1, Xiaohong He1, Kun Jia1∗, Xiaobo Liu1*. Research Branch of Advanced Functional Materials, School of Materials and Energy,

University of Electronic Science and Technology of China, Chengdu 611731, PR China

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Abstract

Although soft micro/nanoreactors obtained from amphiphilic block co-polyolefin

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are intensively used for metallic nanoparticle synthesis, the intricate crosslinking after self-assembling of copolymers is an indispensable step. In this work, an amphiphilic block copolymer made of aromatic macromolecular with rigid backbone structures (i.e. polyarylene ether nitrile, PEN) is explored to formulate a robust micro/nano-reactor for direct silver nanoparticles (Ag NPs) synthesis without any crosslinking step. We show

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that the polymeric micro/nanoreactors can be fabricated in both microemulsion and reverse microemulsion system. More importantly, Ag NPs with modulated morphology and optical properties can be in-situ synthesized using these PEN micro/nanoreactors,

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which is due to the coordination between Ag+ and pendant cyano/sulfonate groups of PEN. Interestingly, the anisotropic polygonal plate-like hybrid nanostructures are

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obtained via the optimization of nucleation and growth kinetics of Ag NPs using PEN nanoreactor in the water-in-oil W/O reverse microemulsion. Based on these results, the present work would open the way for the facile fabrication of soft micro/nanoreactors for plasmonic nanostructures synthesis using amphiphilic block-copolymers with rigid molecular structures.

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Corresponding authors, Tel: +86 28 83207326; Fax: +86 28 83207326

Email address: [email protected] (Kun Jia), [email protected] (Xiaobo Liu) 1

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Keywords: nanoreactors; microemulsion; amphiphilic polyarylene ether nitriles; silver nanoparticles; microsphere

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Introduction Micro/nanoreactors derived from amphiphilic materials have witnessed increasing research interests for recent decades due to their unique features of self-assembly, space restriction, reaction templating effect and so on [1-4]. Especially, a large amount of

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different amphiphilic polymers have been employed to fabricate versatile micro/nanoreactor for diverse applications including medicine, catalysis, biochemical

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sensing and nanoparticles synthesis [5-8]. As a typical example, plasmonic nanostructures made of noble metal (gold, silver, platinum) have been synthesized and organized in the presence of variety polymeric micro/nanoreactors for biochemical sensing as well as advanced optical devices development[9-13]. Generally, the micro/nano-reactor used for plasmonic nanoparticles synthesis is fabricated via the

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self-assembling of amphiphilic copolymer either in microemulsion or reverse microemulsion system in the majority of published work, because these block copolymers contain single type of metallic ions coordination groups, which should be

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placed in a specific position of micro/nano-reactors for further plasmonic nanoparticle nucleation and growth [14]. In this sense, the amphiphilic polymer that can be used to

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construct plasmonic nanoparticles synthesis nanoreactors in both microemulsion and reverse microemulsion system is rarely reported. As one type of high performance thermoplastics, polyarylene ether nitrile (PEN)

has been intensively employed as an outstanding polymeric matrix for variously advanced engineering composites. In recent years, we have found that the PEN exhibits intrinsic fluorescence emission and can serve as an effective capping agent to synthesize silver nanoparticles (Ag NPs) with modulated morphology as well as optical properties[15], which is due to the coordination between silver ions with 2

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pendant cyano groups of PEN [16]. Very recently, we have synthesized a novel amphiphilic block co-polyarylene ether nitrile (amPEN) containing pendant sulfonate groups, and further combined with semiconductor quantum dots to fabricate a

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sensitive fluorescent sensor for silver ions detection [17], which again confirm the strong interaction between silver ions and pendant cyano/sulfonate groups of amPEN. Inspired by these previous works, we believe that amPEN would be an ideal candidate to construct versatile micro/nano-reactors for Ag NPs synthesis.

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In this work, we firstly synthesized the amphiphilic block co-polyarylene ether nitrile (amPEN), which was subsequently self-assembled into nanostructures that can

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be further employed as the versatile micro/nanoreactors for in-situ formation of silver nanoparticles (Ag NPs) via both oil-in-water (O/W) microemulsion and water-in-oil (W/O) reverse microemulsion protocol. Finally, the morphology and optical properties of obtained Ag NPs conjugated polymeric microspheres (Ag-PMS) can be modulated in terms of the concentration ratio of amPEN/silver ions, reductant

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type/concentration, reaction time as well as temperature.

Experimental section

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Materials

Bisphenol A(BPA, AR), potassium carbonate (K2CO3, AR), silver nitrate (AgNO3,

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AR), 1,1,2,2-tetrachloroethane (AR), sodium borohydride (NaBH4, AR), N-methyl pyrrolidone (NMP,AR), N,N-Dimethylformamide (DMF, AR), ethanol (AR), sodium dodecyl sulfate(SDS, AR), dichloromethane (DCM, AR), tetrahydrofuran (THF, AR) and hexadecyl trimethyl ammonium bromide (CTAB, AR) were all received from Kelong reagents Co. Ltd. Toluene (AR) was purchased from J&K Scientific Ltd (Beijing, China). Hydroquinone monosulfonic acid potassium salt (SHQ, AR) and 2, 6-difluorobenzonitrile (DFBN, AR) were obtained from Sigma Aldrich. Deionized water was obtained from Milli-Q water filtration station. 3

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Synthesis of amphiphilic block co-polyarylene ether nitrile (amPEN) The amphiphilic block co-polyarylene ether nitrile (amPEN) was obtained from the block copolymerization of a hydrophobic oligomer A and a hydrophilic oligomer B.

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The nucleophilic substitution was conducted on oligomer A and oligomer B between DFBN/BPA and DFBN/SHQ, respectively. Moreover, the controlled stoichiometric ratios of monomers for oligomers synthesis assured that oligomer A and oligomer B was end-capped with phenolic hydroxyl and fluoro group, respectively. Specifically,

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the oligomer A was obtained via the reaction of BPA (14.383 g, 63 mmol), DFBN (3.346 g, 60 mmol), K2CO3 (12.749 g, 92.25 mmol), NMP (36 mL) and toluene (12 mL)

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which was heated to 140 °C and maintained for 3 h, while the similar preparation was conducted on oligomer B using SHQ (13.680 g, 60 mmol), DFBN (8.763 g, 63 mmol) and K2CO3 (12.749 g, 92.25 mmol) dispersed in the mixture of NMP (36 mL) and toluene (12 mL). Then the two oligomers were completely mixed after freely cooled, followed by gradually heating to 175 °C for 5 h to obtain the crude product, and the

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excess reactants were removed thorough washing with hot ethanol and deionized water for three times. Lastly, the purified white powder of amPEN was obtained after drying at 80 °C in a vacuum oven.

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The synthesis of Ag-PMS via O/W microemulsion

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A mixture of 30 mg sodium dodecyl sulfate (SDS) or 20 mg hexadecyl trimethyl ammonium bromide (CTAB) was added into 10 mL deionized water, followed by addition of different volume of 0.1 M aqueous solution of AgNO3. When the surfactant was completely dissolved, 5 mg amPEN dissolved in 900 µL dichloromethane (DCM) and 100 µL tetrahydrofuran (THF) was added and stirring at room temperature for 12 h. Next, the stirred vial was partially opened to slowly evaporate organic solvents within 12 h. The acquired emulsion was centrifuged and washed using pure water for 4 times to remove excess uncoordinated Ag+. The white pellets from the final centrifugation 4

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was dispersed in 10 mL deionized water and different volume of 0.1 M aqueous solution of sodium borohydride (NaBH4) was introduced to reduce coordinated silver ions and reacted for 12 h at room temperature with stirring, finally resulting to the

The synthesis of Ag-PMS via W/O reverse microemulsion

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formation of Ag-PMS.

A mixture of 5mg amPEN and 20 mg hexadecyl trimethyl ammonium bromide

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(CTAB) was dissolved in 10 mL 1,1,2,2-tetrachloroethane. The mixture was stirred (speed of 1200 rpm) at room temperature, followed by addition of different volume of

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0.1 M AgNO3 aqueous solution. After stirring for 24 h, excess AgNO3 aqueous solution was removed and sodium borohydride dissolved in either DMF or H2O with a concentration of 0.1M was introduced into stirred mixture at room temperature to initiate the formation of Ag NPs inside polymeric nanoreactors within 12 h. Next, the excess amount of water was added into W/O reverse microemulsion to extract the

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uncoordinated silver ions, and then the Ag-PMS was finally obtained after repeating 5 times of water extraction. Characterization

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The UV-Vis spectra and fluorescence emission spectra of hybrid colloids solution were recorded by TU 1901 UV-Vis spectrophotometer (Persee Co. Ltd.) and a F98-Pro

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fluorescent spectrophotometer (Lengguang Tech.), respectively. The surface morphology of Ag-PMS was characterized using a scanning electron microscope (SEM, JEOL, JSM-6490LV).

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Results and discussion Scheme 1 illustrates schematically the evolution process of two different types of Ag NPs conjugated polymer microspheres (Ag-PMS). The coordination of silver ions

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with sulfonate groups of amPEN is proposed and the structure of Ag-PMS obtained from different methods is displayed. The amPEN block copolymer consists of hydrophilic segments and hydrophobic segments, the hydrophilic segments containing

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silver-coordination groups should be presented on the microsphere surface in microemulsion while embedded inside the polymeric microspheres in reverse

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microemulsion. Therefore, it is able to modulate the specific position and morphology of Ag NPs on polymeric microspheres via modulation of experimental parameters,

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such as nanoreactor morphology, reductant concentration as well as reaction time.

Scheme 1 Schematic illustration showing the mechanism for the synthesis of Ag NPs conjugated polymeric microspheres (Ag-PMS) using amPEN as nanoreactor both in microemulsion and reverse microemulsion.

We have found that the silver ions had strong interaction with the amphiphilic 6

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co-PEN due to the presence of nitrile and sulfonate groups according to our previous work[15,17], thus the silver ions and amPEN concentration should be the essential parameters to determine the morphology and plasmonic properties of synthesized

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hybrid nanoparticles. It is clear from Fig.1a that the localized surface plasmon resonance (LSPR) peak around 400 nm corresponding for Ag NPs with enhanced intensity and blue-shifted wavelength is detected as the increasing of AgNO3 concentration, while the wavelength of absorption bands around 250 nm and 320 nm

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corresponding to π-π transition of amPEN is independent of silver ions concentration, which implies that the initial concentration of Ag+ indeed plays an important role in of

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NP

morphology

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has

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the

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determination

morphology/properties of amPEN micro/nanoreactor. In addition, we find that the optical properties of Ag-PMS are dependent on the amPEN concentration according to Fig.1b, where the LSPR peak with highest absorption intensity is recorded from sample synthesized using a medium concentration of amPEN. Accordingly, 73 µmol of AgNO3

experiments.

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and 5 mg/mL amPEN are fixed as the optimized concentration for the following Meanwhile,

the

morphology

of

nanoreactors

obtained

from

self-assembling of amphiphilic block copolymers is believed to highly dependent on

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the surfactants involved in microemulsion, which would further determine the efficiency of functional nanoparticles synthesis using polymer nanoreactors. It is clear from Fig.1c that CTAB is a more effective surfactant to enable the synthesis of

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Ag-PMS compared to previously used SDS, since the absorption intensity corresponding to amPEN and Ag NPs both enhanced as the increasing of CTAB concentration, and the blue-shifted LSPR wavelength of Ag NPs at higher CTAB concentration proves that the CTAB also plays an important role in morphology modulation of Ag NPs. Finally, the influence of reaction time onto the optical properties of Ag-PMS is evaluated as shown in Fig.1d, and it is found that the LSPR band with smaller full width at half maximum (FWHM), enhanced intensity as well as 7

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unchanged wavelength was detected for the Ag-MPS samples synthesized in the initial 5 hours, while the prolonged reaction time results to further increasing of adsorption intensity along with a broaden peak. It is assumed that uniform small sized Ag NPs

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seeds are generated upon addition of reducing agent in the initial stage, while the further growth of Ag NPs would be restricted by the amPEN nanoreactors, thus the

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wider size distribution of Ag NPs is obtained after longer reaction time.

Figure 1 The UV-Vis absorption spectra of Ag-PMS synthesized using different concentrations of AgNO3 (a), amPEN (b), surfactants (c) and reaction time (d) in an oil-in-water O/W microemulsion system.

In addition to the modulation of Ag NP size via silver ions concentration, we discovered that the overall size of Ag-PMS can be easily modulated via the stirring rate during O/W microemulsion synthesis. Fig.2a and Fig.2c demonstrate the SEM 8

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morphology of two types of Ag-PMS synthesized under same parameter (temperature, time and feeding ratio to reactants) except for the stirring rate. It is clear that the hybrid microspheres with smooth surface morphology are obtained from both cases,

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and there is no obvious rupture observed from synthesized Ag-PMS, which could be attributed to the rigid backbone nature of amPEN block copolymer. In terms of size distribution, the size of Ag-PMS synthesized at higher stirring speed is in the range from 500 nm to 7 µm and the majority Ag-PMS is larger than 1 µm according to

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Fig.2b. While Ag-PMS synthesized at lower stirring speed has narrower size distribution from 200 nm to 1.3 µm and the average size is around 400 nm as shown

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in Fig.2d. In short, the slower stirring rate would contribute to generation of small

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sized Ag-PMS with more uniform size distribution.

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Figure 2 The SEM morphology and corresponding particle size distribution histogram of Ag-PMS synthesized at a stirring speed of 1200 r/min (a, b) and 900 r/min (c, d) in an oil-in-water O/W microemulsion system.

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On the contrary to microemulsion protocol, the silver coordinating sulfonate groups should be embedded inside the polymeric microspheres obtained via water-in-oil (W/O) reverse microemulsion system, thus the morphology and optical properties of Ag-PMS can not only be modulated via the variation of feeding ratio of

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reactants but also can be tuned by manipulating the diffusion process of silver ions and reductants into polymeric microspheres reactors. In this work, the immiscible

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solvents of H2O and tetrachloroethane as well as surfactant of CTAB have been employed to construct the thermodynamically stable W/O reverse microemulsion system, where the silver ions are basically localized inside the hydrophilic pocket of polymer micro/nanoreactor. Therefore, the diffusion of NaBH4 reductant into amPEN micro/nanoreactor should be an essential factor to govern the nucleation and growth

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of Ag NPs inside polymer microspheres. For this reason, two parallel groups of experiments have been designed by reducing different concentrations of silver ions via the same reductant of NaBH4 dissolved in either DMF or H2O. As shown in

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Fig.3a, when the reductant is dissolved in DMF, the typical LSPR wavelength of Ag NPs is constantly blue-shifted with the increasing of AgNO3 concentration, while the

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minimum FWHM of Ag NPs, corresponding to an optimized size distribution, is detected at a medium concentration (36.5 µmol) of AgNO3. On the contrary, the Ag NPs with a nearly constant LSPR wavelength as well as much narrower plasmonic band are obtained by using the aqueous solution of NaBH4 as reductant according to Fig.3b. The dramatically different plasmonic properties of Ag-PMS synthesized using DMF and aqueous solution of reductant could be rationalized as follows. Firstly, it is well-known that the morphology of Ag NPs is determined by the nucleation and growth process, since the silver ions precursors are introduced into reverse 10

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microemulsion system prior to addition of NaBH4 reductant, thus the same concentration of silver ions supposed to be captured inside the amPEN micro/nanoreactor in both cases, but the NaBH4 reductant dissolved in DMF solution

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should penetrate into the amPEN micro/nanoreactor much easier due to the good compatibility between DMF and tetrachloroethane, while the NaBH4 aqueous solution is dispersed like a micelle in a W/O reverse microemulsion, thus the controlled diffusion of reductant micelle into hydrophilic pocket of amPEN micro/nanoreactor

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allows a slower nanoparticles nucleation and growth, which contributes to generation of more uniform Ag NPs. Secondly, the dissolving/swelling of amPEN

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micro/nanoreactor by the aggressive DMF solvent could also result to the wide size distribution of Ag NPs as recorded in Fig.3a. In order to further prove the Ag NPs formation inside the amPEN micro-reactor, the fluorescence emission spectroscopy of obtained Ag-PMS solution is recorded. It is clear from Fig.3c that intrinsic fluorescence emission (-420 nm) of amPEN is strongly quenched by the generated Ag

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NPs because of the metal nanoparticles mediated energy transfer (NMET) even in the case using lowest concentration of Ag+ precursor. In addition, we have previously discovered that the fluorescence silver nanoparticles can be obtained by using PEN

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copolymer as capping agent thanks to the coordination between silver ions and pendant cyano groups, thus we have conducted the Ag-PMS synthesis using DMF

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solution of NaBH4 in an ice-bath to decline the Ag NPs formation process and recorded the temporal evolution of fluorescence emission. As shown in Fig.3d, when the reaction time increases, the emission peak corresponding to silver nanoparticles at 480 nm emerges along with the gradual quenching of PEN emission at 450 nm. In short, the fluorescence emission spectroscopy shown in Fig.3c and Fig.3d unambiguously proves the presence of Ag NPs in the amPEN microspheres.

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Figure 3 The UV-Vis absorption spectra of Ag-PMS synthesized using different concentrations of AgNO3 reduced by NaBH4 dissolved in DMF (a) and H2O (b), the fluorescence emission spectra of Ag-PMS reduced by NaBH4 dissolved in DMF at room temperature (c), temporal

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evolution of emission spectra of Ag-PMS synthesized with 18.5 µmol AgNO3 in an ice bath (d),

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all the synthesis are conducted in a water-in-oil W/O reverse microemulsion system.

Given that the diffusion process of NaBH4 into amPEN micro-reactor is

dependent on the temperature, the morphology and optical properties of synthesized Ag-PMS should be modulated by different temperature. As shown in Fig.S1 in supporting information, the rapid formation of Ag NPs is observed instantly upon the addition of Ag+ precursors since the LSPR wavelength is independent of reaction time no matter NaBH4 is dissolved in DMF or H2O. Therefore, we have synthesized the Ag-PMS at much lower temperature in an ice-bath to monitor their temporal evolution 12

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of optical properties. Taking the sample synthesized using 36.5 µmol AgNO3 and DMF solution of NaBH4 as an example, it is found that the LSPR wavelength of Ag NPs is red-shifted from 385 nm to 415 nm with the increasing of reaction time and the

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majority of wavelength shift is recorded within 1 h according to Fig.4a. This result indicates that the diffusion of NaBH4 into amPEN micro-reactors is strongly dependent on the temperature, which can be explored to modulate the morphology and plasmonic properties of Ag NPs. However, the LSPR band of Ag NPs synthesized

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using aqueous solution of NaBH4 is independent of reaction time according to Fig.4b. Since the Ag NPs is obtained after the penetration of NaBH4 micelles into hydrophilic

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pocket of amPEN, which is a thermodynamic stable process independent of temperature, thus the nucleation and growth of Ag NPs in ice bath are similar to those at room temperature. In order to further confirm the reproducibility of optical properties of Ag NPs, we have repeated the Ag-PMS synthesis involving aqueous solution of NaBH4 four times, and the LSPR peak of obtained Ag NPs is highly

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overlapped as shown in Fig.4c, which again proves that the advantages of “reductant micelle” mediated diffusion protocol in terms of morphology and optical properties modulation. Moreover, the fluorescence emission spectra of Ag-PMS synthesized

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using different concentration of AgNO3 and aqueous solution of NaBH4 in the ice bath are recorded in Fig.4d, it is clear that the fluorescence emission corresponding to amPEN at ~450 nm is gradually quenched along with the increasing of reaction time,

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while the weak emission band detected at ~480 nm of Ag NPs is observed after longer reaction.

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Figure 4 The UV-Vis absorption spectra of Ag-PMS synthesized using 36.5µmol of AgNO3 in ice bath with NaBH4 dissolved in DMF (a) and H2O (b), absorption spectra from 4 repeated synthesis using 55.0µmol of AgNO3 with NaBH4 dissolved in H2O (c) and its corresponding

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fluorescence emission spectra during different reaction time (d) in a water-in-oil W/O reverse

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microemulsion system.

Finally, the fine morphology of Ag-PMS synthesized in the water-in-oil W/O

reverse microemulsion system is characterized with SEM. Taking the samples synthesized at same AgNO3 concentration of 36.5 µM but with different reductant solvents as the example, it is clear from Fig.5a that the Ag-PMS with larger size (~600-700 nm) and irregular shape are obtained when the reductant NaBH4 was dissolved in DMF, while the small sized Ag-PMS (300~400 nm) of spherical morphology are observed for the sample synthesized using aqueous solution of 14

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NaBH4 (see Fig.5b). The different morphology in these two cases is due to the different diffusion process of NaBH4 to amPEN micro/nano-reactors, which is consistent with the LSPR spectra evolution shown in Fig.4 as well. Interestingly, the

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anisotropic Ag-PMS is obtained by using a lower concentration of AgNO3 (18.5 µM) according to Fig.5c, especially for the polygonal plate like nanoparticles as shown in the enlarged image of Fig.5d. The controlled diffusion of reductant micelle and coordination of sulfonate group to Ag+ could be the possible reasons for the

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the future to elucidate the specific mechanism.

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generation of anisotropic AgNO3, more detailed experiments would be conducted in

Figure 5 The SEM morphology of Ag-PMS synthesized using 36.5 µmol of AgNO3 reduced by NaBH4 dissolved in DMF (a), H2O (b) and lower concentration (18.5 µmol) of AgNO3 with NaBH4 dissolved in H2O (c) in water-in-oil W/O reverse microemulsion, shown in (d) is 15

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the higher magnification image of highlighted area in panel (c).

Conclusion

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In this work, the amphiphilic block co-polyarylene ether nitrile (amPEN) has been synthesized and subsequently used as micro/nano-reactors for silver nanoparticles synthesis via both microemulsion and reverse microemulsion protocol. We have found that the morphology and optical properties of obtained Ag NPs conjugated polymeric

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microspheres (Ag-PMS) can be effectively modulated by adjusting the silver ions precursor concentration, surfactant type/concentration, stirring speed and reaction time

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in both protocols. Furthermore, we have discovered that the water-in-oil reverse microemulsion serves as an ideal micro-environment to formulate polymeric micro/nanoreactor containing hydrophilic pocket, which in turn provides additional pathway (i.e. diffusion of reductant micelles into micro/nanoreactor) to tune the silver nanoparticles nucleation and growth phases, finally leading to the generation of

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anisotropic plasmonic nanostructures. Although more detailed work is still required to elucidate the morphology evolution of Ag-PMS, the present work would open a new way to fabricate novel anisotropic silver nanostructures on the basis of self-assembles

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of rigid rod-like amphiphilic polyarylene ethers copolymer.

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Acknowledgment

The authors gratefully thank the financial support from the National Natural

Science Foundation of China (Project 51403029), the Fundamental Research Funds for the Central Universities (ZYGX2016J040) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars from State Education Ministry (LXHG5003).

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