Article pubs.acs.org/Langmuir
Design and Fabrication of a New Class of Nano Hybrid Materials based on Reactive Polymeric Molecular Cages De Suo Zhang,†,§ Xiang Yang Liu,*,‡,⊥,§ Jing Liang Li,§ Hong Yao Xu,⊥ Hong Lin,† and Yu Yue Chen*,†
Langmuir 2013.29:11498-11505. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/06/19. For personal use only.
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National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou 215123, P.R. China ‡ Research Institute for Biomimetics and Soft Matter, College of Materials, Xiamen University, Xiamen 361005, P.R. China ⊥ College of Material Science and Engineering, Donghua University, Shanghai 201620, P.R. China § Department of Physics and Department of Chemistry, Faculty of Science, National University of Singapore, Singapore 117542 ABSTRACT: This paper describes a strategy of fabricating a new class of nano hybrid particles in terms of the “nanocages” of reactive molecular matrices/networks. The concept is to design molecular matrices functionalized with particular reactive groups, which can on-site synthesize and fix nanoparticles at the designated positions of the molecular networks. The cages of the molecular networks impose the confinement and protection to the nanoparticles so that the size and the stability of nano hybrid particles can be better controlled. To this end, polyamide network polymers (PNP) were synthesized and adopted as the reactive molecular cages for the control of silver nanoparticles formation. It follows that the silver nano hybrid particles fabricated by this method have an average diameter of 4.34 nm much smaller than any other or similar methods ie by a hyperbranched polyamide polymer (HBPA). As per our design, the size of the silver nano hybrid particles can also be tuned by controlling the molar ratio between silver ions and the functional groups in the polymeric matrices. The silver nano hybrid particles reveal the substantially enhanced stability in aqueous solutions, which gives rise to the long stable performance of localized surface plasmon resonance. As the nano hybrid particles display long eminent nanoeffects, they exert broad implications for a wide range of applications such as biomedicine, catalysis, and optoelectronics.
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applications.3,4 The unique properties of nanoparticles, such as physical, chemical and biomedical properties, are affected by their size and the stability.5,6 In this context, to control the size, size distribution, and morphology of the synthesized nanoparticles is very desirable. This is particularly challenging for synthesis in aqueous solutions.7,8 Because of the high surface energy, nanoparticles tend to aggregate in aqueous solutions. The common solution is to capture the nanoparticles with protective surfactants or polymers during the nanoparticle preparation.9−11 Compared with surfactants and linear polymers, dendrimers and hyperbranched polymers with numerous interior cavities and functional groups have been extensively used as efficient templates to synthesize nanoparticles.12−18 They offer a better control over size, shape, and size distribution of nano hybrid particles. Nevertheless, the stability of the nanoparticles remains to be improved. In this paper, we put forward a generic strategy for the design and fabrication of nano hybrid materials by a so-called molecular reactive caging approach, which will give rise to
INTRODUCTION In the area of materials science and engineering, three trends are the key major research focuses:1,2 The ultra performance materials refer to those having some extraordinary properties. The materials entirely or partially appear to be super hard, superhydrophobic, superhydrophilic, superconducting, etc. Multifunctional materials correspond to those having more than one major in-use properties/functions. The fluorescence silks are one of the examples. Smart and responsive materials are those that respond to some external stimuli, in the way that some particular properties of the materials change drastically and/or in opposite to conventional materials. Under some external simulating, the color, optical properties, conductivity, etc., of the materials change correspondingly. Hybrid materials play an increasingly important role in today’s science and technology.2 Subject to the structure characteristics, the aforementioned three trends of research can be implemented much easier in supramolecular materials. In this regard, we will demonstrate that based on the understanding of the formation mechanism, one can design and fabricate a new class materials of multiple functions. The controllable nano hybrid materials represent a useful platform that can be tailored to offer tremendous potential © 2013 American Chemical Society
Received: June 18, 2013 Revised: August 4, 2013 Published: August 27, 2013 11498
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source for the reduction process.23,24 After reduction, the silver nanoparticles can be formed in the meshes/cages by the amine groups, which can achieve the functionalization for the nano hybrids. As the silver nanoparticles are confined in the meshes of the polymers, their growth will be physically restricted by the meshes so that the size and size distribution can be effectively controlled. In addition, the entrapment of silver nanoparticles in the meshes of polymeric networks will physically prevent their aggregation. To illustrate the advantages of the polymer cage network, hyperbranched polyamide (HB-PA) was also synthesized for the preparation of silver nano hybrid particles. The size and stability of the nano hybrid particles prepared and kept in the solutions of PNP and HB-PA were compared.
the synthesis of nano hybrid materials with controllable size and long stability in aqueous media. In this regard, molecular matrices/networks (ie. polymers) with three-dimensional “nanocage” structures are synthesized. In order to assemble nano hybrid particles at the specific positions of the molecules, some reactive functional groups are incorporated into the molecular matrices/networks. As these groups serve as the reactive sites for the formation of nano hybrid particles, the nano hybrid particles form on-site, and can be fixed to the specific positions of the cages/networks. Furthermore, the nano hybrid particles can be entrapped in the designated positions of the molecular cages of the networks. The volume confinement effect of the networks enables us to control the size of nano hybrid particles. As the molecular matrices provide a physical barrier for the nanoparticle aggregation, the stability of the nano hybrid particles can be greatly enhanced in aqueous media. To implement this strategy, we synthesized the reactive molecular matrices/networks by a refined tech of synthesizing hyperbranched polymer. In this research, polyamide network polymers (PNP) were synthesized based on the synthesis tech of hyperbranched polyamide (HB-PA).19 PNP will then be adopted as reactive molecular matrices/networks to control the formation of silver nano hybrid particles. The choice of silver nano hybrid particles is based on the following consideration: (1) they have numerous important applications, such as anti bacteria, catalysis, and optical sensing;20−22 (2) the poor stability in aqueous solutions.
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EXPERIMENTAL SECTION
Materials. Diethylenetriamine, methyl acrylate, and silver nitrate were purchased from Sigma-Aldrich. Methanol was purchased from Merck. All chemicals were used as received. Deionized water (18 MΩ cm) was used in the preparation of all samples. Synthesis of Polymers. HB-PA was synthesized by the same method as reported earlier (Scheme 2a).19 The solution of methyl acrylate (0.5 mol) in methanol (100 mL) was added dropwise into diethylene triamine (0.5 mol) under nitrogen gas protection and magnetic stirring, and cooled with ice bath. Then the mixture was removed out of the ice bath to continue the reaction at room temperature for 4h to form AB2 type monomer with AB type monomer as a byproduct (A stands for −CO2CH3 group, B stands for −NH2 group). Then the mixture was transferred to an eggplantshaped flask with an automatic rotary vacuum evaporator. After removing the methanol under low pressure, the temperature was raised to 150 °C using an oil bath, and left for 4 h until the honeylike HB-PA was obtained. As each −NH2 could react with −CO2CH3, every two AB2 monomers could react with each other to produce more terminated amine groups. The polymer finally took shape just like a ball with numbers of −NH2 outside. AB hardly forms several line branches. When the molar ratio was 1.5:1 for methyl acrylate and diethylenetriamine, keeping the other conditions unchanged, three types of monomers AB2, A2B, and A3 could be produced at first. Since methyl acrylae was overdosed, a handful of A2B and A3 were configurated. In the system, the ratio of reactive group A and B is 1:1, leaving both −NH2 and −CO2CH3 groups at the outside part of the polymer. These groups ensure the reaction to proceed further to form ring-shaped structure one by one. Therefore, the PNP (Scheme 2b) was configurated with a great number of meshes/cages. Preparation of Silver Nano Hybrid Particles. PNP and HB-PA were dissolved in deionized water to prepare a stock solution with a polymer concentration of 100 g/L. Solutions with different polymer concentrations were prepared by adding different amounts of stock solution to 50 mL of deionized water. Then 0.5 mL of AgNO3 solution (0.1 M) was added dropwise into the solutions and stirred constantly at room temperature. The silver ions were captured by amine groups of PNP or HB-PA. The mixtures were then boiled. Heating was stopped when the solutions turned yellow in color. The hot solutions were then left to cool. Deionized water was added to compensate the loss of water and maintain the total volume. Characterization. Fourier transform infrared spectra (FT-IR) of monomers, synthesized polymers and silver nano hybrid particles were measured by a Nicolet 380 FT-IR spectrophotometer (Thermo electron corporation, USA). Nuclear magnetic resonance (1H NMR) spectra of polymers were collected by a Varian Inova NMR spectrometer (USA) at 400 MHz using D2O as a solvent at room temperature. Gel permeation chromatography (GPC) was carried out on a Waters 600 apparatus with column (TSKgel2000 SW XL 300 × 7.8 mm; Waters, USA) and a Waters 2410 ultraviolet detector using phosphate buffer solution (pH 7.0) as the eluent at a flow rate of 0.50 mL min−1. The molecular weights of polymers were calculated based on narrow polypeptide standards.
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PRINCIPLES OF CONSTRUCTING REACTIVE POLYMERIC MOLECULAR CAGES The first step of our approach is to design reactive molecular matrices/networks. Such a matrix/network illustrated in Scheme 1 has meshes/cages of a certain size and functional/ Scheme 1. Illustration of Size-Controlled Synthesis of Silver Nano Hybrid Particles in Reactive Polymeric Molecular Nanocages
reactive groups. According to the principles outlined above, the amine groups will be the reactive functional groups to be built into the molecular matrices/networks. In this context, we adopt diethylenetriamine and methyl acrylate to synthesize the reactive molecular matrices/networks. The molecule of PNP contains numerous secondary and tertiary amine groups, as well as some primary amine groups at the peripheral region (shown in Scheme 2b). These amine groups are able to attract silver ions and reduce them to generate silver nanoparticles in the three-dimensional polymeric matrices/networks. They can potentially provide electron 11499
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Scheme 2. Schematic Representation of the Synthesis of (a) HB-PA and (b) PNP
of HB-PA (Scheme 2a) was monitored by FTIR and 1H NMR. FTIR spectra of AB2 and AB type monomers mixture and the HB-PA are shown in Figure 1A. In the spectra of the monomers, the absorption at 1729.3 cm−1 corresponds to the CO stretch of ester bond (−CO2CH3). After HB-PA was produced, the CO stretch of ester bond disappeared and the absorption of CO stretch in amide bond (−CONH−) at 1638.2 cm−1 appears, demonstrating the transformation of monomers to hyperbranched polymers. The structure of HBPA was also confirmed by 1H NMR spectroscopy (Figure 1B). To fabricate network polymers according to Scheme 2b, we introduced BA2 monomers in addition to AB2 monomer in the system at the same molar ratio, A and B reacts with each other and the branches cross-linked to form three-dimensional network architecture, which is the so-called reactive molecular cage. The FTIR spectra of monomers and PNP are shown in Figure 2A. The absorption at 1729.8 cm−1 corresponding to the CO stretch of ester bond (−CO2CH3) also displayed in the spectra of the AB2, A2B, and A3 monomers mixtures. After PNP was produced, the absorption of CO stretch in amide bond (−CONH−) appeared at 1636.5 cm−1. However, the CO stretch of ester bond did not disappeared completely. The 1H NMR spectra of PNP also show a peak of 1H in −CH3 (Figure 2B), which is located at the peripheral region of the polymer. All the analyses confirm the successful synthesis of PNP. The molecular weights of HB-PA and PNP were determined by GPC. The results were listed in Table 1. The weight-average (Mw) and the number-average (Mn) molecular weight of HBPA were 6955 and 2576 respectively, and the polydispersity index was 2.7. During the synthesis of PNP, due to more methyl acrylate took part in the reaction, −CO2CH3 also
To avoid the aggregation, the synthesized nanoparticles were not separated from the unreacted/excess polymer using such as centrifugation and they were characterized directly after preparation. The extinction spectra of silver nano hybrid particles solutions were measured with a Cary 50 Bio UV−visible spectrophotometer (Varian, USA) at room temperature in a quartz cuvette with a path length of 1 cm. The nano hybrid particles were examined with a transmission electron microscope (TEM, JEOL 3010) operated at an accelerating voltage of 300 kV. Samples were prepared by placing small drops of silver nano hybrid particles solution on carbon-coated copper grids, and allowing the solvent to slowly evaporate at room temperature. The size distribution of silver nanoparticles was measured with a high performance particle sizer (HPPS) (Malvern, UK) at 25 °C. The pH values of nano hybrid aqueous solutions were recorded by a JENWAY 4330 conductivity and pH meter. The zeta potential of silver nano hybrid particles was studied by a ZEN 3600 zetasizer (Malvern, UK). X-ray diffraction (XRD) of silver nano hybrid particles was characterized with an X′pert pro diffractometer (Philips, Holland) utilizing a Cu Kα X-ray light source at a voltage of 40 kV and a current of 30 mA. The scanning rate used was 5.0° min−1 over the range of 2θ = 30−90°. X-ray photoelectron spectroscopy (XPS) measurements were carried out on VG ESCALAB MkII with an Al Kα X-ray light source. To calibrate the surface charging effects, all binding energies were referenced to the C1s hydrocarbon peak at 284.6 eV.
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RESULTS AND DISCUSSION Synthesis of Polymers. Normally, the hyperbranched polymers were synthesized by polymerization of the AB2 type monomer in one-step. The progressive reaction of group A of one monomer with group B of another monomer eventually results in the formation of three-dimensional dendritic architecture, with a number of group Bs located at the periphery of the polymer at the end of reaction.14 The synthesis 11500
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Table 1. Molecular Weights of HB-PA and PNP sample
Mw
Mn
Mw/Mn
HB-PA PNP
6955 15714
2576 8725
2.7 1.8
respectively. The molecular weight polydispersity index of PNP reduced to 1.8, which indicated that PNP has a relatively narrow molecular weight distribution. Silver Nano Hybrid Particles. 1. Silver/HB-PA Nano Hybrid Particles. The silver/HB-PA nano hybrid particles were prepared by a one-step reaction based on the reduction of silver ions. The ratio between the metal ions and the capping agents are crucial in determining the size of the metal nanoparticles.15 In this work, the molar ratio of silver ions to nitrogen atoms of the polymer backbone was controlled to be 1:1 and 1:5. For the future reference, PNP (1:1) and HB-PA (1:1) refers equal molar ratio of silver ions to amine groups of the PNP and HBPA polymer respectively, and PNP (1:5) and HB-PA (1:5) means the molar ratio of silver ions to amine groups is 1:5 in PNP and HB-PA polymer, respectively. First, hyperbranched polymer (HB-PA) was employed to synthesize silver nanoparticles and the stability of the synthesized silver/HB-PA nano hybrid particles was investigated. The unique localized surface plasmon resonance (LSPR) is a special property of silver nanoparticles, which is derived from the interaction of light with metal nanoparticles when conduction electrons oscillate locally around nanoparticles at a certain frequency. As the LSPR is associated with the size and shape of Ag nanoparticles, it can usually be employed to monitor the formation and the aggregation of silver nanoparticles.25 Due to the limited polymer when the silver nano hybrid particles aqueous solution was prepared by HB-PA (1:1), less protection was performed for nanoparticles, so that black deposition generated directly during the preparation process. While synthesized by HB-PA (1:5) it could exhibit golden yellow color emission owning to the LSPR property of silver nanoparticles. Therefore, the stability of these silver/HB-PA (1:5) nano hybrid particles was further investigated. Figure 3 reveals the LSPR spectra of silver/HB-PA (1:5) nano hybrid particles obtained at different times. Only one
Figure 1. (A) FTIR spectra of (a) AB2 and AB type monomers mixture and (b) HB-PA, and (B) 1H NMR spectrum of HB-PA.
Figure 2. (A) FTIR spectra of (a) AB2, A2B, and A3 monomers mixture and (b) PNP; and (B) 1H NMR spectrum of PNP.
Figure 3. Extinction spectra of silver nano hybrid particles prepared by HB-PA (1:5) obtained at different time storage (0 min, 20 min, 40 min, 1 h, 1.5 h, 2 h, 3.5 h, 7 h, 1 day, 2 days, 1 week). Inset is a photo of silver/HB-PA (1:5) nano hybrid particles aqueous solution showing LSPR emission color under indoor light (left to right: initial color after synthesis and the color after 7 h placement).
located periphery of polymer, which enhanced the chance of reaction with monomers. Therefore, the molecular weight of PNP increased. Its Mw and Mn were 15 714 and 8725, 11501
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size of silver nanoparticles with narrow distribution. Silver nanoparticles after 1 week of storage aggregated with a size distribution from around 30 to 200 nm (Figure 4b). All of above results indicated that HB-PA is unable to stabilize silver nanoparticles and their LSPR properties. HB-PA has many primary amine groups at the periphery of its molecule. The amine groups can capture the silver ions and reduce them to generate silver nanoparticles at the periphery of the polymer which cannot be protected by HB-PA efficiently. Some silver nanoparticles generated in the internal area between the branches of the hyperbranched polymer molecule also cannot be protected very well because of the unenclosed structure. Hence they tend to aggregate to form larger silver nanoparticles as shown in Figures 3 and 4. As a result, deposition of particles on to the bottom of glass vials was also observed. 2. Silver/PNP Nano Hybrid Particles. In contrast to silver nanoparticles synthesized by HB-PA, the nanoparticles obtained by PNP showed tremendous stability including their particle size and LSPR properties. The silver/PNP nano hybrid particles solutions have a light yellow emission color that was still observed after being stored for 6 months. The LSPR spectra of PNP, PNP (1:1), and PNP (1:5) are displayed in Figure 5A. All the solutions have been adjusted to the same concentration. The PNP solution (a) has a peak at 299 nm only, whereas silver/PNP nano hybrid particles solutions (b and c) have a strong LSPR extinction peak in the range of 394− 406 nm in addition to the peak at 299 nm. The LSPR extinction peak of silver nanoparticles shifts from 406 to 394 nm with a decrease in intensity, when the ratio of silver ions to amine groups changes from 1:5 to 1:1. This can be attributed to the fact that the amine groups are partially protonated in the polymer aqueous solution, losing their reducing capability while increasing the alkalescence of the solution. In addition, not all the remaining amine groups participate in the reduction process. Hence only a fraction of silver ions were reduced to silver nanoparticles when the ratio silver ions to amine groups is 1:1, contributing to a decline in its extinction peak. It is also responsible for the lighter shade of color. The shift in LSPR extinction peak is accredited to the incomplete reduction of adsorbed silver ions on the surfaces of the nanoparticles.26 The silver nanoparticles prepared by PNP were further examined by TEM and their size distributions were measured by a HPPS. Images c and e in Figure 4 show the TEM images and particle size distributions of silver nanoparticles synthesized by PNP (1:1) and PNP (1:5), respectively. When the ratio of silver ions to amine groups was 1:1, silver nanoparticles were larger, with an average size of 13.23 nm because of the relatively high concentration of silver ions. When the ratio was adjusted to 1:5, fewer silver ions were entrapped into the nanocages, hence fewer silver atoms were available for the growth of silver nanoparticles. The silver nanoparticles were confined by the polymeric nanocages and thus prevented to grow into bigger silver nanoparticles. Therefore, smaller nanoparticles with a narrow size distribution were obtained. The maximal size is 7 nm and the average size of the nanoparticles is 4.34 nm. The stability of silver nanoparticles synthesized by PNP was monitored by a UV−visible spectrophotometer. The LSPR spectra of silver nanoparticles monitored during a period of 6 months are shown in panels B and C in Figure 5. No other LSPR extinction peak of silver nanoparticles can be observed with time go on. The foremost change observed in the spectra was the increase of intensity of the peak around 400 nm. The
extinction peak around 410 nm was observed at the early stage. A second peak at around 600 nm appeared after 2 h. The intensity of this peak increases and shifts to a longer wavelength with time goes on. Concurrently, the intensity of the peak at 410 nm gradually decreases. The LSPR extinction peaks of nanosized metal particles and bulk material are different and they have been proved to depend on their size, aggregation, and shape.25,26 An extinction peak around 600 nm appears in Figure 3 illustrating that silver nanoparticles in solution gradually aggregated. Both the size and quantity of silver nanoparticles affects the intensity and position of LSPR extinction peak.26 Thus the peaks around 600 nm increase in intensity and broaden in succession with red shift. The LSPR emission color of the silver nano hybrid particles aqueous solution turned from golden yellow to dark green within 7 h after reduction (inset of Figure 3), which also verified the aggregation of nanoparticles to form larger ones. Images a and b in Figure 4 show the TEM images and the particle size distributions of silver nanoparticles that were prepared by HB-PA (1:5) freshly and after 1 week storage, respectively. It follows that most nanoparticles synthesized by HB-PA were around 10 nm with a small amount in the range of 20−100 nm, which indicated HB-PA could not controlled the
Figure 4. TEM images and the particle size distribution of silver nanoparticles: (a) freshly prepared by HB-PA (1:5) and (b) after 1 week storage; (c) freshly prepared by PNP (1:1) and (d) after 6 months storage; (e) freshly prepared by PNP (1:5) and (f) after 6 months storage. 11502
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are due to the confinement of the molecular nanocages, which not only controlled the particle size but also furthest prevented their aggregation. Here the behavior of aggregation for forming bigger nanoparticles was determined by two factors: (1) zeta potential of nanoparticle and (2) protection by polymer. The zeta potentials of silver nanoparticles synthesized by PNP (1:1), PNP (1:5) and HB-PA (1:5) are shown in Table 2. The silver Table 2. Zeta Potential and pH Value of Silver Nanoparticle Synthesized by PNP and HB-PA sample
PNP (1:1)
PNP (1:5)
HB-PA (1:5)
pH value zeta potential
9.06 10.81
9.53 17.93
9.72 29.05
nano hybrid particles aqueous solutions display alkalescence because of the protonation of amine groups in the polymers. Because of the protection of protonated polymer on silver nanoparticles, the zeta potential of the hybrid particles shows a positive value. Because HB-PA has more primary amine groups in the structure, which is easier to being protonated, the zeta potential of silver nano hybrid particles prepared by HB-PA is higher than that synthesized by PNP. The pH value of the silver/HB-PA nano hybrid particles aqueous solution is also higher accordingly. However, the silver nanoparticles synthesized by HB-PA are not stable as shown above. In contrast, those nanoparticles synthesized by PNP exhibited no change in the LSPR emission color after a long storage of 6 months and underwent at most minor aggregation. This indicates that the protection function of polymer is more important than the zeta potential in terms of maintaining the stability of the nanoparticles. PNP can confine the silver nanoparticles in the nanocages and the amine groups in the backbone of the polymer bind the nanoparticle to prevent it from moving out of the cages which was verified with the FTIR result (Figure 6).
Figure 5. (A) Extinction spectra of (a) PNP, (b) PNP (1:1) and (c) PNP (1:5). Inset in (A) is a photo of silver/PNP nano hybrid particles solutions showing LSPR emission color. (B) Time-dependent (0, 1, 2, 3, 4, 5, 6 months) Extinction spectra of silver nano hybrid particles solutions prepared by PNP (1:1) and (C) PNP (1:5).
changes of extinction intensity are attributed to the following two reasons. The first is that the unreduced Ag+ in the solution can form more silver nanoparticles during the storage which in turn cause the arising of the peak. The second is the aggregation of silver nanoparticles. Accompanied with the increase of the peak, the values of λmax also show a red shift. The LSPR extinction peak positions of PNP (1:1) and PNP (1:5) shift from 394 to 405 nm and from 406 to 421 nm, respectively. This also indicates that the silver nanoparticles underwent some degree of aggregation. Therefore TEM and HPPS were used to characterize the size and size distribution of silver nanoparticle after having been stored for 6 months. After 6 months, the average size of silver nanoparticles synthesized by PNP (1:1) and PNP (1:5) changed from 13.23 to 18.83 nm and from 4.34 to 8.99 nm, respectively (Figure 4). The minor aggregation of silver nanoparticles prepared by PNP
Figure 6. FT-IR spectra of (a) PNP and (b) silver/PNP nano hybrid.
HB-PA has many primary amine groups residing outside the polymer which leads to the production of nanoparticles generated at the exterior of the polymer. In addition, the incompact structure of HB-PA will allow the silver nanoparticle generated in the interior of the polymer to escape. HB-PA offers a minimal protection over the silver nanoparticles. Figure 6 shows the comparison of the FTIR spectra between 4000 and 400 cm−1 of the pure PNP and silver/PNP nano hybrid. The band positions and their assignments are listed in Table 3. Compare the FTIR spectra of pure polymer and silver/PNP nano hybrid, the bands positions of PNP show an apparent shift. The band position at 3065.3 cm−1, correspond11503
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polymer and the oxygen atoms presented in the native oxide layer of copper grids. In the spectrum, a strong silver peak indicates that the nanoparticles are silver. Figure 7b shows the XRD spectrum of silver nano hybrid particles. All the peaks at 2θ values are 38.1, 44.3, 64.4, 77.5, and 81.6, representing the 111, 200, 220, 311, and 222 Braggs reflections of the face centered cubic (fcc) structure of silver respectively, which are in excellent agreement with the values reported in literature.28,29 This also substantiates the formation of silver nanoparticles. XPS was utilized to analyze the chemical state of silver nanoparticles. Panels c and d in Figure 7 show the wide scan and Ag3d spectra of silver nanoparticles, respectively. In accordance with the standard ones, the wide scan spectra have the peaks of Ag4d, Ag4p, Ag4s, Ag3d, Ag3p, and C1s (from pollution). High-resolution narrow scan (Ag 3d) (Figure 7d) conveys the binding energy peaks at 367.96 and 373.97 eV, corresponding to the Ag 3d5/2 and Ag 3d3/2 respectively, which presents an unambiguous evidence for the formation of Ag0 particles.30,31
Table 3. FT-IR Bands for PNP and Silver/PNP Nano Hybrid band position (cm−1) assignment
free PNP
Silver/PNP nano hybrid
amide A amide B CH2 asymmetric stretching CH2 symmetric stretching amide I amide II CH2 scissoring CH2 wagging amide III
3269.1 3065.3 2927.9 2816.6 1636.5 1545.6 1431.4 1358.5 1273.8
3268.3 3080.8 2927.2 2849.8 1626.8 1551.1 1436.9 1358.4 1326.4
ing to amide B (NH stretching vibration modes) of PNP, shifted to higher regions (3080.8 cm−1) after the formation of silver nanoparticles. The band position of CH2 symmetric stretching is at 2816.6 cm−1. It shifted to 2849.8 cm−1. The bands position of amide I and III of PNP are at 1636.5 and 1273.8 cm−1. Amide I moved to lower regions (1626.8 cm−1) and amide III to higher regions (1326.4 cm−1). All of these variations imply the interaction between amide groups and silver nanoparticles as well as the conformational change of PNP after the formation of silver nanoparticles in the polymer matrix.14,27 This interaction is another reason behind the stability of the silver nano hybrid particles aqueous solution. To verify that silver nanoparticles were synthesized, we carried out energy-dispersive X-ray spectroscopy (EDX) and XRD. As expected, EDX spectrum (Figure 7a) shows strong copper and carbon peaks. Copper arises from the supporting copper grids and carbon from the supporting carbon film in the copper grids together with the component of polymer. The small oxygen peak in the spectrum is probably from both the
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CONCLUSIONS Silver nano hybrid particles were conveniently obtained by a one-step method based on the reduction of silver nitrate with PNP and HB-PA. The results have demonstrated that nanoparticles achieved in PNP are far smaller in size with a narrower distribution and better stability than those formed in HB-PA. HB-PA has many amine groups outside the polymer, which induces nanoparticle formation at the exterior of polymer and, in combination with the incompact structure of the polymer, leads to the formation of bigger and unstable silver nanoparticles. In contrast, PNP has its special network that can act as reactive polymeric molecular nanocages for the formation of silver nanoparticles. Because of the confinement by the
Figure 7. (a) EDX spectrum, (b) XRD spectrum, (c) XPS wide scan spectrum, and (d) XPS Ag3d spectrum of the silver/PNP nano hybrid particles. 11504
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molecular nanocages, size and size distribution of silver nanoparticles were well-controlled, and agglomeration was prevented. FT-IR results confirmed the interaction between silver nanoparticles and polymer molecules, which also contributes to the stability of polymer/silver nanoparticle hybrids. EDX, XRD, and XPS analysis verified the generation of Ag0 nanoparticles.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (X.Y.L.);
[email protected] (Y.Y.C.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National High Technology Research and Development Program of China (No. 2012AA030313), Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 11KJB540002), Suzhou City Key Technology R&D Program (No. ZXS2012008), and Singapore MOE AcRF Tier 1 funding (No. R-143-000-497-112).
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dx.doi.org/10.1021/la4023085 | Langmuir 2013, 29, 11498−11505
