Article pubs.acs.org/Langmuir
UV-Irradiation-Induced Templated/In-Situ Formation of Ultrafine Silver/Polymer Hybrid Nanoparticles as Antibacterial Mengjun Chen, Yining Zhao, Wantai Yang, and Meizhen Yin* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Beijing University of Chemical Technology, 100029 Beijing, China S Supporting Information *
ABSTRACT: Two types of facile approaches toward ultrafine Ag/ polymer hybrid nanoparticles (NPs) within 10 nm are introduced. Template and in-situ formation method are developed by photoreduction based on inverse microemulsion (IME) polymerization of N,N-dimethylacrylamide (DMAA). The template method refers to the usage of size-varied polymeric PDMAA NPs as templates for the preparation of Ag/PDMAA hybrids with desired morphology and optical property. To avoid the self-seeding nucleation of free Ag+ in the solution, in-situ formation method is developed by introducing AgNO3 during IME polymerization, in which product hybrids could be obtained via autoprecipitation in large scale. Additionally, the produced Ag/PDMAA hybrids show high antibacterial performance.
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INTRODUCTION Up to date, the synthesis of metallic nanoparticles, especially silver nanoparticles (Ag NPs), attracts a great interest for their widespread applications in the broad areas of science and technology.1−6 Compared with bulk metallic substance, Ag NPs present great advantages of brightness, low toxicity, low cost, and photostability. However, Ag NPs usually suffer from small size effect and tend to aggregate.7,8 Numerous methods have been exploited for the fabrication of stable and monodisperse Ag NPs with size-dependent optical property, indicating the great challenge in precise synthesis of Ag NPs with controllable size and narrow size distribution.9 Several publications account for the synthesis of watersoluble Ag NPs by chemical reduction.10−13 However, the utilization of a strong reducing agent, such as NaBH4, led to the formation of large and nonfluorescent Ag NPs.14,15 By contrast, photoreduction method circumvents these issues and offers a facile procedure for Ag NP synthesis.16,17 In these related reports, a template is generally essential to indirectly control the morphologies of product NPs.18−20 To synthesize ultrafine Ag NPs with precise control over the size distribution, the inverse microemulsion (IME) has been rapidly developed recently.21−23 However, ultrasmall-sized Ag NPs prepared via IME generally suffer from serious conglutination without protecting agents; thus, single product NP could not exhibit clear outline to be distinguished from each other.24,25 Therefore, cross-linked polymerization, i.e. IME polymerization, was introduced herein for precisely adjusting the morphologies of the produced Ag NPs. In this paper, with the combination of advantages of IME polymerization and photoreduction, two strategies toward ultrafine hybrid Ag NPs, i.e. template and in-situ formation © 2013 American Chemical Society
method, were presented. They were both developed based on the modified procedures of IME polymerization of hydrophilic monomer N,N-dimethylacrylamide (DMAA).26 Either the generated PDMAA NPs or the inverse micelles were employed as reactive places, and the subsequent ultraviolet (UV) light irradiation led to the formation of Ag/PDMAA hybrids via photoreduction. In addition, size-controllable synthesis of such Ag/PDMAA hybrids was realized by taking advantages of the feature of IME system. These procedures offered facile strategies toward synthesis of ultrafine sub-10 nm Ag/ PDMAA hybrids with controllable sizes, which also generated the desired optical and antibacterial properties.
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EXPERIMENTAL SECTION
Synthesis of Ultrafine Polymeric PDMAA Nanoparticles. General Procedure. The IME polymerization of DMAA emulsified by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was carried out in nhexane by using N,N′-methylenebis(acrylamide) (MBA) as the crosslinking agent, with redox initiated system of ammonium peroxodisulfate (APS)/N,N,N′,N′-tetramethylethylenediamine (TMEDA). AOT (5.6 g, 12.6 mmol) was dissolved in n-hexane (20 mL). To the solution, DMAA (557 mg, 5.6 mmol) and MBA (45 mg, 0.3 mmol) were added, which formed an anhydrous IME system. When extra water was added into the system, an aqueous solution of the monomer (DMAA) was used in IME. After well dispersed by ultrasound, the mixture was then stirred at 25 °C for 30 min under a N2 atmosphere. Subsequently, APS (100 mg/mL, 10 μL) and TMEDA (10 μL) were added to initiate the polymerization. The polymerization was carried out at 35 °C for 12 h. Finally extra solvent (n-hexane) was added to Received: October 29, 2013 Revised: November 28, 2013 Published: December 5, 2013 16018
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Figure 1. Schematic process of template method for synthesis of Ag/PDMAA hybrid nanoparticles. accelerate the precipitation of the product particles. The white solid product was collected by centrifugation, rinsed with n-hexane for several times, and dried under vacuum. By adjusting the water content in the system, PDMAA NPs ranging in size from 2.6 to 10 nm can be obtained. Synthesis of Ag/PDMAA Hybrids with Polymeric PDMAA Nanoparticles as Templates. The above-synthesized template polymeric PDMAA NPs (8 mg) were dispersed in 10 mL of ethanol, while 200 μL of aqueous AgNO3 (0.04 mol/L) was dropped into the dispersion. The system was stirred for 30 min under a N2 atmosphere and then was exposed under UV-light irradiation (25 W/m2) for various time. In-Situ Formation of Ag/PDMAA Hybrids via Inverse Microemulsion Polymerization. AOT (5.6 g, 12.6 mmol) was dissolved in n-hexane (20 mL). To the solution DMAA (557 mg, 5.6 mmol) and MBA (45 mg, 0.3 mmol), varied amounts of aqueous AgNO3 (0.04 mol/L) were added to adjust the w (n(water)/ n(surfactant)) of the system. After being well dispersed by ultrasound, the mixture was then stirred at 25 °C for 30 min under a N2 atmosphere. Subsequently, APS (100 mg/mL, 10 μL) and TMEDA (10 μL) were added to initiate the polymerization. The polymerization was carried out at 35 °C for 12 h. Finally, extra solvent (n-hexane) was added to accelerate the precipitation of the product particles. The brown solid product was collected by centrifugation, rinsed with nhexane for several times, and dried under vacuum. Antibacterial Activity of Ag/PDMAA Hybrids. The dynamic shake flask method was used to assess the antibacterial ability of the Ag/PDMAA hybrid NPs. A colony of Gram-negative E. coli bacteria was cultivated in Luria−Bertani broth (containing 10 g/L peptone, 10 g/L sodium chloride, and 5 g/L yeast extract) at 37 °C, shaking at 160 rpm for 24 h. The bacteria were diluted with 0.1 mol/L phosphate buffer solution (PBS, pH = 7) to the desired concentration. The asprepared Ag/PDMAA hybrids (10 mg/mL in PBS, pH = 7) were immersed in the bacterial suspension with the volume ratio of 2% and then shaken at 37 °C for 2 h. A certain amount of suspension was taken, diluted appropriately, and plated on L-agar plates for 48 h incubation. Theoretically, each surviving bacterium develops into a distinct colony after incubation, and the number of viable bacterium was then determined as colony forming units (CFU), thus providing a direct measure of bacterial viability.
All the above-mentioned materials and apparatus were treated with high-temperature sterilization at 121 °C for 30 h to ensure the aseptic condition.
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RESULTS AND DISCUSSION Template Method for the Synthesis of Ag NPs. The ultrafine size-controllable PDMAA NPs were prepared via IME as described in the Experimental Section (Figure 1). The obtained PDMAA NPs with certain sizes were dispersed in ethanol and used as hydrophilic templates, while aqueous AgNO3 was added as Ag source. The dispersion was degassed with N2 for 30 min and then exposed under UV-light irradiation (25 W/m2) for photoreduction to form Ag/ PDMAA hybrids. In hydrophilic PDMAA NPs, the cross-linked network structures possessed rich N atom, which showed specific adsorption to silver ion (Ag+). Because the lone pair electrons of the N atom could occupy the empty orbit of Ag+, the coordination bond between N−Ag thus formed.27−29 Longtime stirring ensured the complete combination of Ag+ and the template PDMAA NPs. UV-light irradiation induced the reduction of Ag+ into silver elementary substance (Ag(0)), which occurred either on the surface or inside of the template polymeric NPs (Figure 1). UV-induced reduction avoids the use of chemical reducing agent and offers an environment friendly way to produce ultrafine Ag NPs. The process can be facilely performed and easily controlled by varying irradiation time. To investigate the effect of UV irradiation time on the formation of Ag hybrid NPs, we modified the procedure by exposing the solution under UV-light irradiation (25 W/m2) for certain illumination duration. UV−vis spectra further demonstrated the differences caused by time-varied UV irradiation. As shown in Figure 2, absorbance spectrum of the Ag hybrid NPs displays a maximum at 425 nm after 10 min irradiation, which is attributed to the formation of Ag(0).30,31 With increasing irradiation duration, the absorbance intensities increase obviously; meanwhile, the maximum absorption peak shows a 16019
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system. Simultaneously, the original colorless solution appeared to be slightly brown, suggesting the successful photoreduction of Ag+. No suspension or precipitation was observed after longterm storage. The solution was directly dropped onto a carboncoated copper grid for the high resolution-transmission electron microscope (HRTEM) observation (Figure 3b). Ultrafine Ag/ PDMAA hybrid NPs exhibited similar morphologies and shapes but exhibited legible outline compared with the original template PDMAA NPs (Figure 3a). As described in Figure 1, after specific adhesion between PDMAA and Ag+, the photoreduced Ag(0) grew on the surface of (or inside) the template NPs. It can be proved by the slight increase in the sizes of Ag/PDMAA hybrids (d = 3.6 nm) compared with those of the templates (d = 2.6 nm). The composition of Ag/PDMAA hybrid NPs was characterized by HRTEM/EDS. According to the results shown in Figure 3c, the elements of C, O, and N were assigned to the template PDMAA NPs. The strong Ag signal indicated the successful formation of Ag/PDMAA hybrids. In addition, the peak of Cu was attributed to the carbon-coated copper grid that we used for HRTEM observation. Because of the optical properties of Ag NPs, the produced Ag/PDMAA hybrids can also be characterized by fluorescence microscopy. After UV
Figure 2. UV-absorbance of Ag/PDMAA hybrid NPs (dtemplate = 2.6 nm) under different UV irradiation times. Solvent: ethanol. Concentration (Ag/PDMAA hybrid NPs) = 0.8 mg/mL.
slight red-shift to 435 nm at the irradiation duration of 30 min. This is explained by the growing sizes of the reduced Ag NPs that results in the size-dependent optical properties. Under further UV exposure, the absorbance remained unchanged, indicating the complete reduction of the aqueous Ag+ in the
Figure 3. (a) HRTEM images of PDMAA nanoparticles produced via anhydrous IMP (d = 2.6 nm); scale bar is 10 nm. (b) Ag/PDMAA hybrid NPs obtained via template method (d = 3.6 nm); scale bar is 10 nm. (c) Fourier diffraction pattern analyzed on the inset HRTEM image of (b). (d) HRTEM/EDS of the Ag/PDMAA hybrid NPs. 16020
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Figure 4. Fluorescence microscope images of (a) template PDMAA NPs (2.6 nm) and (b) Ag/PDMAA hybrid NPs (dtemplate = 2.6 nm). (c) Fluorescence spectrum of Ag/PDMAA hybrid NPs (dtemplate = 2.6 nm).
Figure 5. Schematic process of in-situ formation method toward Ag/PDMAA hybrid NPs via inverse microemulsion polymerization.
NPs indeed acted as the templates for the formation of ultrafine hybrid NPs. In short, the polymeric-NP-template method has been successfully applied to produce Ag/PDMAA hybrids. The facile approach is easily performed to produce the Ag/PDMAA hybrids with desired optical properties, and their size controlling can be realized by varying the sizes of the employed template PDMAA NPs. However, some dispersed Ag+ still remained in the solution without attaching to the templates. The self-seeding nucleation of free Ag+ might occur in the solution without templates. Especially, when the procedure is expected to be performed on a larger-scale or with higher Ag content, the influence of free Ag+ cannot be ignored. Since this template procedure had to be performed on a small scale, and the produced Ag/PDMAA hybrids cannot be conveniently extracted from the system due to the ultrafine sizes and their great dispersibility in ethanol; thus further application of such hybrids is limited. To resolve the issues, a new strategy toward
photoreduction, the ethanol solution containing product hybrids was directly dropped onto a coverslip for fluorescence microscopic observation. As shown in Figure 4a,b, Ag/PDMAA hybrids exhibited obvious fluorescence compared with the nonblooming original template PDMAA NPs. Since each of these hybrid NPs contains tens of thousands of Ag(0) on the surface or inside, the emitted fluorescence signal could be observed. Fluorescence property of such hybrid NPs was also measured in solution. As shown in Figure 4c, the emission maximum peak was observed at 550 nm, upon excitation at 435 nm. In order to further confirm that the template method is applicable for various PDMAA NPs, we employed larger PDMAA NPs (d = 6.4 nm) as templates to synthesize Ag/ PDMAA hybrids. Larger PDMAA template NPs were prepared by increasing water content in IME polymerization system. As shown in HRTEM images (Figure S1, Supporting Information), the produced hybrid NPs also matched well with the original template PDMAA NPs, which proved that PDMAA 16021
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Ag/PDMAA hybrids was designed according to the feature of IME system and called as in-situ formation method. In-Situ Formation of Ag/PDMAA Hybrids via Inverse Microemulsion Polymerization. Based on the general process of producing size-varied PDMAA NPs, modification was developed by introducing aqueous AgNO3 during the IME. Because of the feature of IME system, the aqueous AgNO3 can be incorporated only inside the inverse micelles rather than in the organic continuous phase. Then aqueous AgNO3 can be bounded within the network structure in the polymeric NPs until the polymerization finished. Further in-situ photoreduction can be realized under UV exposure. The process is described as shown in Figure 5. Thus, disturbance of free Ag+ on the formation of Ag/PDMAA hybrids can be significantly avoided in this way. We have reported the autoprecipitation of PDMAA NPs in the IME system, and the mechanism was well interpreted in the previous communication.26 Similarly, the autoprecipitation of produced Ag/PDMAA hybrids was observed as well in the reaction system by in-situ formation method (Figure S2). As discussed above, the product Ag/PDMAA hybrids obtained via the template method were not easy to be extracted from the system. By comparison, in-situ formation method shows great advantages in the feasibility for large-scale production and facile extraction process of Ag/PDMAA hybrids. With typical procedure, the Ag/PDMAA NPs synthesis was preformed under w (n[water]/n[AOT]) = 5. For comparison, bare PDMAA NPs were prepared as blank experiment by altering aqueous AgNO3 with same amount of deionized water. The obtained hybrid NPs were characterized by IR (Figure S3). It showed that the organic chemical compositions of the obtained Ag/PDMAA hybrid NPs were almost as same as those of bare PDMAA NPs, indicating the successful polymerization. The product Ag/PDMAA hybrid NPs were dispersed in water and dropped onto a copper grid for HRTEM observation. It showed great improvement in the monodispersity of Ag/PDMAA hybrids. Compared with the bare PDMAA NPs prepared under the same w, the Ag/PDMAA hybrids exhibited a slight increase in size, which might be owing to the extra space occupied by the nucleation of Ag NPs. HRTEM/ EDS was also performed to characterize the composition of the hybrid NPs. The results showed the successful encapsulating and in-situ reduction of Ag (Figure 6c). In addition, the obtained Ag/PDMAA hybrids generated similar optical properties as those prepared via the template method (data not shown). It is well-known that the sizes of product polymeric NPs were determined by w (n(water)/n(surfactant)) of the IME system.32,33 When keeping the surfactant amount as a constant, product sizes could be controlled by adjusting the water content in the system, and this has been discussed in our previous work.26 In this in-situ synthesis method, size controlling of Ag/PDMAA hybrids could also be realized by altering amount of aqueous AgNO3. Adjusting w of the system was achieved by varying aqueous AgNO3 amount. The sizes of Ag/PDMAA hybrids and the bare PDMAA NPs under the same w were summarized in Table 1. The results showed that the sizes of product Ag/PDMAA hybrids grew along with an increase of the aqueous AgNO3 amount. Finally, size controllable synthesis was realized. Generally, by employing in-situ formation method, monodisperse Ag/PDMAA hybrids could be obtained via autoprecipitation. Because the Ag sources remained in the inverse
Figure 6. (a) HRTEM images of PDMAA NPs produced via IME polymerization under w (n(water)/n(surfactant) = 5 (w was adjusted by deionized water); scale bar is 10 nm. (b) Ag/PDMAA hybrid NPs obtained by in-situ formation method under w = 5 (w was adjusted by aqueous AgNO3 (0.04 mol/L)); scale bar is 20 nm. (c) HRTEM/EDS of the Ag/PDMAA hybrid NPs.
Table 1. Summary of Number-Average Diameters of Bare PDMAA NPs and Ag/PDMAA Hybrid NPs under Different wa wa (mol/mol)
dPDMAAb (nm)
dAg/PDMAAc (nm)
0 (anhydrous) 1 5 10 15
2.6 4.5 6.4 8.4 10.2
6.3 7.9 11.6 12.9
a w represents the mole ratio of water to surfactant (AOT) (n[water]/ n[AOT]). bdPDMAA represents the average diameter of PDMAA NPs prepared under different w. cdAg/PDMAA represents the average diameter of Ag/PDMAA hybrid NPs prepared under different w. In all cases, the diameters of the particles were read directly from HRTEM images by averaging 50 particles.
micelles during the whole IME procedure, it avoids the selfseeding nucleation of free Ag+. Thus, compared with smallscaled template method, the in-situ formation method could realize scale-up production of Ag/PDMAA hybrids and the Ag content could be easily tuned by adjusting AgNO3 aqueous amount. In addition, size controlling of product hybrid NPs could be achieved by varying amount of aqueous AgNO3 in the system. Antibacterium of Ag/Hybrid NPs. We investigated the antibacterial activity of Ag/PDMAA hybrids obtained via in-situ formation method and template method. Antibacterium of Ag/ Hybrid NPs was assayed by contacting with viable Gram16022
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Figure 7. Photographs of L-agar plates onto which E. coli suspension of PBS (pH = 7) contained (a) Ag/PDMAA hybrids obtained from in-situ formation method (hybrids content: 10 mg/mL × 2% = 0.2 mg/mL) and (b) no hybrids were deposited and incubated for 48 h.
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negative bacteria E. coli in suspension. Figure 7 shows the digital photographs of L-agar plates incubated with bacteria from suspensions, treated either with the as-produced Ag/ PDMAA hybrids through in-situ formation method (Figure 7a) or without NPs (Figure 7b). Obviously, by treated with 2% of the produced Ag/PDMAA hybrids at a concentration of 10 mg/mL, nearly no colonies were developed on the plate incubated with bacteria suspension, while the control plate was covered with significant bacterial colonies (1.04 × 106 CFU). The antibacterium performance of Ag/Hybrid NPs obtained from the template method is presented in the Supporting Information. The antibacterial assay with E. coli demonstrated that the Ag/PDMAA hybrids showed desirable antibacterial activity, and the high antibacterial performance supports the successful synthesis of Ag/PDMAA hybrids.
ASSOCIATED CONTENT
S Supporting Information *
HRTEM images and antibacterial perform of Ag/PDMAA hybrids obtained via template method; digital photograph of the autoprecipitated product and IR spectra of Ag/PDMAA hybrids obtained via in-situ formation method. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (M.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the financial support from the “National Science Foundation of China” (No. 21174012, 51103008, and 51221002) and the “New Century Excellent Talents Award Program, Ministry of Education, China” (NCET-10-0215) and the Doctoral Program of Higher Education Research Fund (20100010120006, 20120010110008).
CONCLUSIONS In summary, two strategies were developed for the synthesis of ultrafine Ag/PDMAA hybrid NPs (2−10 nm), i.e., template and in-situ formation methods. The template method employed hydrophilic PDMAA NPs obtained via IME polymerization as templates to anchor Ag+ (inside or onto the surface of the particles), and the following UV irradiation induced the photoreduction of Ag+ into Ag(0). The product hybrid NPs matched well with the morphologies of the original polymeric templates and exhibited fluorescent properties. Ag/ PDMAA hybrids with varied sizes can be obtained by employing PDMAA template NPs with corresponding diameters. However, the Ag/PDMAA hybrids derived from template method suffered from the interference of the free Ag+ and the small-scale production owing to the complicated extraction process. Such issues were improved by developing in situ formation method benefiting from the feature of IME. The aqueous Ag+ was bonded in the inverse micelles during the polymerization of DMAA, and thus the self-seeding nucleation of free Ag+ was avoided. Owing to autoprecipitation, the product Ag/PDMAA hybrids prepared through in-situ formation method can be extracted from the reaction system in a facile way; thus, large-scale production can be realized. By varying amount of aqueous Ag+, size controlling of the product hybrids could be realized. In addition, the generated Ag/ PDMAA hybrids showed great performance in antibacterium.
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