Mild Synthesis of Copper Nanoparticles with Enhanced Oxidative

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Mild Synthesis of Copper Nanoparticles with Enhanced Oxidative Stability and Their Application in Antibacterial Films Liangzhen Tang, Li Zhu, Fu Tang, Chuang Yao, Jie Wang, and Lidong Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02470 • Publication Date (Web): 13 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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Mild Synthesis of Copper Nanoparticles with Enhanced Oxidative Stability and Their Application in Antibacterial Films Liangzhen Tang,#,a Li Zhu,#,b Fu Tang,*,a Chuang Yao,c Jie Wang,a and Lidong Li*,a aState

Key Laboratory for Advanced Metals and Materials, School of Materials Science

and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China bDepartment

of Otolaryngology, Peking University Third Hospital, Beijing100191,

China cKey

Laboratory of Extraordinary Bond Engineering and Advance Materials

Technology (EBEAM) of Chongqing, Yangtze Normal University, Chongqing 408100, China.

ABSTRACT Copper nanoparticles possess unique physical and chemical properties; however, their application is often restricted owing to their tendency to oxidize. In this work, we prepared copper nanoparticles with enhanced oxidative stability via a simple and lowcost method, where a modified starch was used as an environmentally friendly reducing agent and biocompatible polyethyleneimine was used as a stabilizer. The prepared copper nanoparticles could be stored in air for at least 6 months without any oxidation in a dried state. Interestingly, our synthesis could even be performed at room temperature with a longer reaction time. We used various characterization methods to

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study the reaction mechanism. The prepared copper nanoparticles were further uniformly doped into an agar film and this composite showed excellent bacterial killing efficiency owing to the antibacterial properties of the copper nanoparticles. Our composite film shows potential for various clinical applications, such as wound dressing materials.

INTRODUCTION Metal nanoparticles have drawn considerable attention for their unique physical and chemical properties owing to quantum confinement effects.1-3 Various methods have been proposed to synthesize metal nanoparticles. Among these methods, chemical reduction is a common approach owing to its effectiveness in designing and synthesizing nanoparticles in a controllable manner.4, 5 These processes often involve the use of reducing agents that provide electrons to reduce metal salts into metal species. Capping agents are also essential in the process to control and stabilize the obtained metal nanoparticles. However, most reported synthesis methods rely on hazardous reagents, such as sodium borohydride6, 7 and hydrazine,8, 9 which are harmful to the environment. Although, some environmentally friendly agents, such as glucose,10 Lascorbic acid11 and amino acid,12 have been developed for syntheses of metal nanoparticles, more cost-effective and environmental friendly methods are still highly desired. Compared with other noble metals (such as silver and gold), copper (Cu) is considerably cheaper but possesses comparable electrical, optical, catalytic, thermal

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and antibacterial properties. Thus, Cu nanoparticles show promise for applications in flexible electronics, transparent conductors, anti-microbial agents and catalysts.13-15 However, synthesis of copper nanoparticles is more challenging than synthesis of silver and gold nanoparticles owing to the lower standard redox potential of copper nanoparticles16 and their tendency to rapidly oxidize.17-20 Considerably less research has focused on fabrication of Cu nanoparticles compared with the extensive studies on silver and gold nanoparticles, and there is a lack of methods involving nontoxic agents. Furthermore, applications of Cu nanoparticles are greatly limited by their inherent tendency to oxidize in air. Strategies to stabilize Cu nanoparticles by introducing capping agents or stabilizers have been proposed21, 22; however, the additional steps make these synthesis processes laborious and time consuming. More simple and efficient methods are needed to promote more widespread applications of Cu nanoparticles. Recently, biocompatible macromolecules23-25 have been used in the fabrication of metal nanoparticles owing to their advantages in terms of renewability, biocompatibility, biodegradability and diversity of structures. The main role of biocompatible macromolecules in the synthesis process is to stabilize the nanoparticles and prevent their aggregation. Some bio-macromolecules have been reported to be weak reducing agents owing to functional groups (such as hydroxyl groups). For example, bull serum albumin was used to synthesize gold nanoclusters, where it acted as both a template and reducing agent.26 DNA has been used to template and direct the growth of silver and gold nanostructures.27-29 Polysaccharides (such as cellulose,30 3


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chitosan31 and alginate32 ), having many functional groups on their backbones, have also been used as environmentally friendly reducing agents and/or stabilizers for the formation of metal nanoparticles. Among these, starch is an abundant polysaccharide that can be easily obtained from plants such as potato and corn. Starch is widely used as a source of renewable fuel and material.33 The biocompatible and cost-effective characteristics of starch, together with its reducing properties make it an attractive material for metal nanoparticle preparation. Raveendran, el al.34 developed a method for synthesizing Ag nanoparticles using starch as a stabilizer and glucose as a reducing agent. Our group35 previously reported a green and in situ reduction method to synthesize Ag nanoparticle-containing hybrid hydrogels with modified starch in the hydrogel as the reductant. Herein, we propose a low-cost and effective method based on low-toxic reagents for synthesis of Cu nanoparticles with enhanced oxidative stability. The synthesis process was illustrated in Scheme 1. A modified dialdehyde starch (DAS) was used here as the reducing agent. A biocompatible polyelectrolyte, polyethyleneimine (PEI), was used to stabilize the Cu nanoparticles. This reaction required no high temperature and could even be performed at room temperature by increasing the reaction time. The prepared Cu nanoparticles showed good stability when stored in air for more than 6 months without any oxidation. The prepared Cu nanoparticles were further doped into a polysaccharide film. Owing to the antibacterial properties of Cu nanoparticles, this composite film exhibited excellent bacterial killing efficiency, demonstrating potential applications in various clinical fields, particularly as a wound dressing material. 4


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Scheme 1. The synthesis process of Cu nanoparticles and chemical structures of DAS and PEI. EXPERIMENTAL SECTION Materials and Measurements. Corn starch, sodium periodate, ethylene glycol, sodium hydroxide (NaOH), urea, copper sulfate pentahydrate, polyethyleneimine (PEI, Mn = 60,000), agarose were purchased from Sigma-Aldrich and were used without further purification, unless otherwise noted. Distilled water was used throughout the experiments. Escherichia coli (TOP10) were purchased from Beijing Bio-Med Technology Development Co., Ltd. Ultraviolet−visible (UV−vis) absorption spectra were collected on a Hitachi U3900H spectrophotometer. Transmission electron microscope (TEM) and highresolution TEM (HRTEM) images were obtained on a JEM 2010 instrument. Fieldemission scanning electron microscope (SEM) images were obtained with a Zeiss Supra 55. X-ray diffraction (XRD) patterns were measured with a Rigaku SmartLab Xray diffractometer. FTIR spectra were measured from samples as KBr pellets using a Bruker VERTEX 70v spectrometer. The ζ-potentials of the nanoparticles were 5


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measured by dynamic light scattering (DLS) with a Malvern Zetasizer Nano ZS90 at room temperature. Preparation of Dialdehyde Starch. Corn starch and sodium periodate (1:0.5 weight ratio) were dissolved in water in a flask with vigorous mechanical stirring at 37 °C for 12 h. Then, the reaction was quenched by the addition of 5 mL of ethylene glycol. The resulting slurry was filtered and washed with deionized water four times before drying in a vacuum freeze drier for 3 days. After grinding in a mortar, a dialdehyde starch (DAS) powder was obtained. Synthesis of Cu Nanoparticles. A 3-g portion of NaOH, 2 g of urea and 1 g of DAS were mixed and added into 45 mL of deionized water with mechanical stirring. This mixture was cooled to −20 °C for 12 h, and thawed under stirring at room temperature. Finally, a 2 wt% DAS solution was obtained. To synthesize the Cu nanoparticles, 5 mL of the DAS solution was first added to a glass vial, followed by addition of 1 mL of copper sulfate pentahydrate solution (15 mM) and 1mL of PEI (1 mg/mL) solution under magnetic stirring. The reaction mixture was bubbled with argon and then incubated at 50 °C for 3 h. After washing and centrifugation three times, Cu nanoparticles were obtained and finally re-dispersed in 1 mL of distilled water. Preparation of Cu Nanoparticle-Embedded Agar Film. The Cu nanoparticleembedded agar film (Cu-agar film) was prepared according to a reported method with some modifications.36 Typically, 1.5 g of agar and 1 g of glycerol were first added into 100 mL of distilled water. The mixture was stirred and boiled until a homogeneous solution was obtained. Then, 9 mL of the hot agar solution was mixed with 1 mL of the 6


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Cu nanoparticle solution and poured into a culture dish. The mixture was cooled and the Cu-agar film was gradually formed after drying in a vacuum oven. Antibacterial Properties of Cu Nanoparticle-Embedded Agar Film. The antibacterial properties of the Cu-agar film were assessed against E. coli bacteria. To prepare bacterial solutions, a single colony of E. coli on a solid Luria−Bertani (LB) agar plate was transferred to 10 mL of liquid LB culture medium in the presence of 50 μg/mL ampicillin and grown at 37 °C for 6 h. The bacteria were collected by centrifugation at 7000 rpm for 2 min and washed twice with PBS. The supernatant was removed and the remaining bacteria were resuspended in PBS and diluted to an optical density of 1.0 at 600 nm (OD600 = 1.0). For the colony formation assays, the bacterial suspensions (2 mL, 9.0 × 108 colony-forming unit (CFU)/ mL) and the film (2 cm × 1 cm) were incubated at 37 °C for 45 min. The bacterial solutions were then diluted 106fold with PBS and 100 μL portions of the diluted bacteria were spread on the solid LB agar plate. Colonies were formed after 12-h incubation at 37 °C and the number of CFUs was counted. For comparison, agar films without Cu nanoparticles were prepared and incubated with bacteria under the same conditions. Computational Details. The adsorption of primary, secondary and tertiary amine groups (three typical moieties of PEI) was modelled on a slab model of a Cu (111) surface with a 20 Å vacuum layer. The geometric structures and adsorption energies were calculated by density functional theory (DFT) with the Perdew–Burke–Ernzerhof (PBE) energy functional and a plane-wave pseudopotential formalism. The calculations presented here were performed including gradient corrections via the generalized 7


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gradient approximation (GGA) with the CASTEP computer code in materials studio. RESULTS AND DISCUSSION Synthesis and Characterization of Copper Nanoparticles. We synthesized Cu nanoparticles via a simple and mild method, introducing a modified starch as both the reducing agent and stabilizer. Cationic polyelectrolyte, PEI, was used to further stabilize the Cu nanoparticles. The starch was modified with aldehyde groups via an oxidation process and dissolved in a NaOH/urea system to form a transparent and light yellow-coloured stock solution. In a typical preparation process, 1 mL of copper sulfate pentahydrate solution, 5 mL of the DAS solution and 1 mL of PEI solution were added into a glass vial under mild stirring. Then the mixture was purged with argon and incubated at 50 °C for 3 h. As the reaction proceeded, the mixture showed a gradual colour change from light yellow to reddish brown. After centrifugation and washing steps, a reddish-brown solution was obtained (see photograph in the inset of Figure 1a). The light absorption of the Cu nanoparticles was studied by UV-vis photo spectrometry and these results (Figure 1a) showed that the precipitates exhibited a maximum absorption peak at approximately 592 nm, which is a characteristic surface plasmon resonance peak of Cu nanoparticles.11 This result suggested that we had successfully prepared Cu nanoparticles by this method.

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Figure 1. (a) UV−vis absorption spectrum, (b) SEM image, (c) TEM image and (d) High-magnification TEM image of the prepared Cu nanoparticles. Inset of (a) shows a photograph of the Cu nanoparticle solution. The morphology of the prepared Cu nanoparticles was studied by SEM and TEM, respectively. Figure 1b and 1c show that the Cu nanoparticles were nearly spherical and with diameters of approximately 50-100 nm. A high-magnification TEM image of a single nanoparticle, shown in Figure 1d revealed that there was a layer (light grey shell) adsorbed around the surface of the prepared Cu nanoparticle. To investigate the composition of this layer, we performed FTIR spectroscopy on the prepared Cu nanoparticles. Figure 2a shows the intense adsorption peaks at 3435 cm-1 (O-H stretching), 2918 cm-1 (C-H stretching) and 1621 cm-1 (O-H deformation) in the FTIR spectrum of the prepared Cu nanoparticles. We compared these features with the spectrum of free DAS, which had peaks at 3448 cm-1, 2931 cm-1 and 1645 cm-1, and assigned these peaks to O-H, C-H and O-H vibrations, respectively. We attributed the 9


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slight shift of the peak positions to adsorption of DAS at the surfaces of the Cu nanoparticles through unpaired electrons in the -OH groups, which usually results in the formation of vibration bands at a lower wavelength than that of their free states.37 We observed a new peak at 1666 cm-1 in comparing the FTIR spectra of the Cu nanoparticles with and without PEI. We considered this feature to be related to vibration of -NH2 groups of PEI absorbed to the Cu nanoparticles surfaces.38 These results demonstrated the existence of DAS and PEI layers absorbed on the surfaces of Cu nanoparticles.

Figure 2. (a) FTIR spectra of starch, DAS, PEI, nanoparticles prepared without PEI and with 1 mg PEI in the range of 4000-400 cm-1, (b) FTIR spectra of nanoparticles prepared without PEI and with 1 mg PEI in the range of 2000-1250 cm-1, (c) FTIR spectra of starch and DAS in the range of 2000-1250 cm-1, (d) ζ-potential of Cu nanoparticles in different pH environments. To further verify that the surface layer was composed of DAS and PEI, we also measured ζ-potentials of the Cu nanoparticles under different pH environments. As shown in Figure 2d, the ζ-potential of the Cu nanoparticles showed a positive value under acidic conditions (pH 3.0 and 5.0). However, after dispersion in neutral and alkali solutions, the ζ-potential changed to a negative value. We attributed the positive charge 10


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to protonation of amino groups on the PEI main chain and the negative charge was likely cause by deprotonation of carboxyl groups in DAS. Changes in the ζ-potential further confirmed the existence of DAS and PEI on the surfaces of Cu nanoparticles. Mechanism of Nanoparticles Formation. Starch is a polysaccharide, which is abundant in plants and contains a large amount of hydroxyl groups on its backbone. Although there have been reports on the use of polysaccharide hydroxyl groups to prepare metal nanoparticles,39-41 these usually involve high reaction temperatures and long reaction times, which are undesirable in terms of energy saving. Aldehyde groups are more reactive than hydroxyl groups in terms of their ability to act as reducing agents, and might facilitate the formation of Cu nanoparticles at lower temperatures and shorter reaction time. Thus, we prepared a dialdehyde starch via a highly specific periodate oxidation of raw starch, cleaving the C2-C3 bond of the anhydroglucose unit with simultaneous oxidation of vicinal 2,3-hydroxyl groups to aldehyde groups.42 FT-IR spectroscopy of the resulting product (Figure 2c) showed that a new peak appeared at 1730 cm-1, which could be assigned to C=O stretching vibrations.43 This result revealed that aldehyde groups were successfully introduced into the backbone of starch. By mild heating of the reaction mixture, copper nanoparticles were obtained via the aldehyde reduction. However, almost no Cu nanoparticles (Figure S1) were generated when we substituted the aldehyde-modified starch with unmodified starch, as no characteristic surface plasmon resonance peak of Cu nanoparticles could be found when the reaction solution was analyzed by UV-vis spectra (Figure S1). A very weak absorption peak at around 630 nm could be attributed to the absorption of the copper ion-PEI complex.44 11


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Photograph of the reaction solution was shown in the inset of Figure S1. And the slight blue color from the copper ions further indicating that formation of Cu nanoparticles could hardly happen at the absence of aldehyde groups in our particle synthesis method. The alkali environment offered by the dissolved starch system and NaOH/urea solution, was another important factor that facilitated the fabrication of Cu nanoparticles. Owing to extensive inter- and intra- molecular hydrogen bonding, the modified starch was difficult to completely dissolve in water. Therefore, a NaOH/urea solution, which is usually used to dissolve cellulose,45 was used to dissolve the modified starch. This strong alkali environment effectively lowered the redox potential of the copper salt reduction reaction.16 As a result, Cu nanoparticles were easily obtained at lower temperatures. Interestingly, in our system, Cu nanoparticles were obtained even at room temperature. As shown in Figure S2, characteristic absorption peaks of Cu nanoparticles were detected after incubating the reaction mixture at room temperature for 20 h. However, no visible characteristic absorption peaks of Cu nanoparticles were detected when the pH of the reaction mixture was lower than 12 (as shown in Figure S3) even after incubating the reaction mixture at 50 ℃ for 3 h. These results indicate that an alkali environment is essential for forming Cu nanoparticles at low temperatures. Oxidative Stability of the Prepared Cu Nanoparticles. Oxidative stability is important for further applications of Cu nanoparticles. We studied the antioxidation properties of the prepared Cu nanoparticles by X-ray diffraction (XRD). Figure 3a shows the XRD pattern of the Cu nanoparticles stored in ambient atmosphere after 1, 3 12


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and 6 months, respectively. Three diffraction peaks at 2θ = 43.4°, 50.5° and 74.2° were clearly observed from all of the XRD patterns, which we indexed to the (111), (200) and (220) crystal planes of Cu nanoparticles, respectively (JCPDS: 04-0836). No other peaks indexed to Cu2O or CuO were detected. This result indicates that little oxidation of the Cu nanoparticles occurred during storage in air for at least 6 months. This is a promising result, because it has previously been widely reported that Cu nanoparticles tend to oxidize in air after a short time.46, 47 We speculated that the polymer layers on the Cu nanoparticles surfaces improved the nanoparticle stability to oxidation.

Figure 3. (a) XRD patterns of the as-prepared Cu nanoparticles and the same sample stored in ambient atmosphere after 1, 3, 6 months, (b) UV-vis absorption spectrum and (c) XRD pattern of nanoparticles prepared without PEI. To verify our speculation and further study the role of DAS and PEI, we conducted a control experiment that without addition of PEI. We found that, in the absence of PEI, a yellow solution (inset of Figure 3b) instead of a reddish brown solution was obtained after centrifugation and washing steps. No typical absorption peaks from Cu nanoparticles were detected, as shown by the UV-vis absorption spectrum in Figure 3b. We analyzed the precipitates of the yellow solution by XRD characterization and the obtained XRD patterns are shown in Figure 3c. The main diffraction peaks were indexed to the (110), (111), (200), (220) and (311) crystal planes of Cu2O (JCPDS: 0513


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0667), respectively, except for two peaks that we indexed to the crystal planes of Cu (111) and (200). This result indicates that, in the absence of PEI, Cu2O nanoparticles were the main product of the reaction. Thus, the Cu nanoparticles oxidized rapidly in the absence of PEI. The good oxidative stability of our Cu nanoparticles can be attributed to the presence of PEI on their surfaces. Because the three typical moieties of PEI, i.e., primary amine, secondary amine and tertiary amine groups, form strong chemical bonds with Cu nanoparticles surfaces through Cu-N coordination.48,

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investigate interactions between PEI and Cu, we studied the adsorption properties of primary (Figure 4a), secondary (Figure 4b) and tertiary amine (Figure 4c) groups of PEI with a Cu (111) surface through density functional theory. The geometric structures and adsorption energies calculation results were shown in Figure 4. We found that all these molecules bonded to the surface through the nitrogen atom, with Cu-N bond lengths of 2.027, 2.133 and 2.193 Å, respectively. The according adsorption energies are −0.561, −0.549 and −0.294 eV, indicated that the strength of chemisorption increased in the order of primary amine > secondary amine > tertiary amine groups. These results confirmed the strong interaction of PEI and Cu nanoparticles. Owing to the high molecular weights of PEI, it likely forms a strong protective film that can block reactive species from close contact with the surface of Cu nanoparticles to effectively suppress oxidation.

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Figure 4. The adsorption properties calculation of (a) primary amine, (b) secondary amine and (c) tertiary amine groups of PEI with Cu (111) surface. Cu Nanoparticle-Embedded Agar Film and Its Antibacterial Properties. Cu nanoparticles have been reported to possess good antibacterial properties,15 hence, we mixed them with agar to form a Cu-nanoparticle embedded composite film and studied its antibacterial properties. The UV−vis absorption spectrum of this film (shown in Figure 5a) showed a maximum absorption peak at about 647 nm, which we attributed to the surface plasmon absorption of Cu nanoparticles embedded in the agar film. The absorption of Cu nanoparticles was red-shifted by 55 nm comparing with that of the Cu nanoparticle solutions. This red shift could be attributed to an increase of the refractive index around the Cu nanoparticles after becoming embedded in the agar film. We studied the microstructure of the resulting film by SEM observations. As shown in Figure 5c, Cu nanoparticles were clearly observed and distributed uniformly in the network of the agar film. Conversely, the control experiment of the bare agar film (Figure 5b) exhibited a much smoother surface. These results indicate the successful fabrication of a Cu-agar film.

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Figure 5. (a) UV-vis absorption spectrum of agar film and Cu-agar film, SEM images of (b) bare agar film and (c) Cu-agar film. Inset of (a) shows a photograph of the bare agar film and Cu-agar film. To evaluate the antibacterial properties of the Cu-agar film, bacterial suspensions (2 mL, 9.0 × 108 colony-forming unit (CFU)/ mL) and the film (2 cm × 1 cm) were incubated at 37 °C for 45 min. Then the bacterial solutions were diluted and 100 μL portions of the diluted bacteria were spread on the solid LB agar plate. After incubation at 37 °C for 12 h, the number of CFUs were counted. These results and corresponding photographs of the CFUs are shown in Figure 6. The bare agar film without Cu nanoparticles was studied as a control. As expected, the bare agar film showed little antibacterial activity compared with the blank experiment. However, the Cunanoparticle-modified agar showed a sharp decrease in bacterial viability and nearly no CFUs were found in the incubation plate, suggesting excellent bacterial killing efficiency of the Cu-agar film. This could be due to the copper ions which migrate from the nanoparticles that effectively killed the bacterial nearby the Cu-agar film.[50]

Figure 6. (a) Bacterial viability and (b-d) photographs of E. coli colonies grown on 16


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agar plates. Viability ratio data were obtained by counting the CFUs in (b-d). Error bars represent the standard deviation. CONCLUSION We present a simple and cost-effective strategy for synthesis of Cu nanoparticles under mild conditions. No toxic agents or high temperature conditions were needed for this reaction. Our synthesized and dried Cu nanoparticles were stable to oxygen in air for more than 6 months. Dialdehyde groups of the modified starch and the alkali environment offered by the dissolved starch system were responsible for the easily formation of Cu nanoparticles. We attribute the improved oxidative stability to the strong chemical bonds between Cu-N, which was induced via interactions of polyethyleneimine and Cu nanoparticles that blocked reactive species from close contact with the surface of Cu nanoparticles, thus effectively suppressing oxidation. We used the antibacterial properties of the Cu nanoparticles to form Cu-nanoparticlemodified agar films, which exhibited excellent bacterial killing efficiency, demonstrating potential for various clinical applications, such as wound dressing materials.

ASSOCIATED CONTENT Supporting Information UV-vis absorption spectra and photograph of reaction mixture using unmodified starch as reductant; UV-vis absorption spectra and photographs of reaction mixtures versus reaction times; UV-vis absorption spectra and photographs of reaction mixtures under 17


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different pH environment. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L.L.); [email protected] (F. T.) Author Contributions #L.T.

and L.Z. contributed equally to this study.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673022) and the State Key Laboratory for Advanced Metals and Materials (2018Z18).

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Mild Synthesis of Copper Nanoparticles with Enhanced Oxidative

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