Preparation of mono-dispersed silver nanoparticles assisted by chitosan-g-poly(ɛ-caprolactone) micelles and their antimicrobial application
Graphical abstract
Introduction
Recently nanoparticles have been widely studied since the nanotechnology endows traditional materials, especially noble metals (e.g. Au, Ag, Pt and Pd), with novel functions and unique properties [1], [2]. Owing to the small size effect and high surface area, metal nanoparticles exhibit special optical, thermal, mechanical and magnetic properties. Therefore, great efforts have been made to take the new properties for further applications, including catalysis, optical sensor and data storage [3], [4], [5]. However, these novel characteristics are closely related to their size and shape. In order to produce stable nanoparticles, many chemical and physical methods have been developed. Generally, there are two main approaches to prepare metal nanoparticles [6]. One is the physical route such as laser ablation, evaporation/condensation and irradiation with visible light or ultraviolet, etc. [7]. The other is the chemical route which utilizes reducing agent (e.g. citrate, sodium borohydride and ascorbate, etc.) to favor the formation of small metal clusters or aggregates [8].
Among several noble metal nanoparticles, silver nanoparticles (silver NPs) have attracted special attention because of its superior antibacterial characteristics as compared to bulk silver [9]. Besides, silver NPs have also found applications in fields of catalyst, optical data storage, electroluminescent displays, medical diagnosis and biomedical imaging [10]. However, the high surface energy caused by small size is unfavorable to the stability of silver NPs [11]. In order to prevent this issue, several approaches have been proposed and could be summarized into two general categories, electrostatic stabilization and steric stabilization. The former method is to make use of anionic species (e.g. halides, carboxylates) to coordinate with NPs via the formation of electrical multilayer, while steric stabilization is achieved by the presence of bulky materials to prevent the diffusion of nanoparticles [12]. In recent years, polymer-assisted fabrication has become a promising way to enhance or control spatical distribution of NPs. For example, Tarasankar et al. apply the TX-100 (poly(oxyethylene)isooctyl phenyl ether) micelles as template/capping agents to control the size and shape of gold NPs [13]. Xia et al. exploit poly(vinyl pyrrolidone) as stabilizer to generate silver nanowires by a solution-phase method [14]. Moreover, different polymer matrixes have been widely explored as stabilizer or scaffold for immobilization, such as dendrimers and block copolymer micelles [15], [16].
Chitosan is an eco-friendly natural material, second only to cellulose in annual production. Due to its unique properties (e.g. biocompatibility, biodegradability, non-toxicity and antibacterial), it has been extensively explored in various fields including cosmetics, food, medicine and tissue engineering [17]. Importantly, the amine groups on the backbone of chitosan could favor the stabilization of silver NPs [18]. However, the strong inter- and intra-molecular hydrogen bonding make it undissolved in most common solvents, which brings the difficulty of preparing silver NPs. To overcome this issue, graft modification is adopted [19]. For example, Hu and his co-workers [20] prepared a series of chitosan-g-poly(L-lactide) (CS-g-PLLA) and chitosan-g-poly(D-lactide) (CS-g-PDLA) copolymers via “graft onto” method, which were able to dissolve in water and various common organic solvents (e.g. dichloromethane, tetrahydrofuran and dimethylformamide). Huang and Duan applied chitosan-based graft copolymers as drug delivery systems by self-assembling into core–shell micelles or as scaffolds by spinning into nanofibers [21], [22]. However, few researches have been focused on its stabilization with metal NPs.
In this study, CS-g-PCL was synthesized by grafting poly(ɛ-caprolactone) onto phthaloyl-protected chitosan, followed by deprotection of phthaloyl groups to regenerate free amino groups. Then the chemical modified polymers were self-assembled into micelles with poly(ɛ-caprolactone) segment as core and chitosan as shell. Thereafter, the obtained micelles were used as stabilizers to prepare silver nanoparticles triggered by UV light. This method required no reducing agent or any manipulative skill, and it is reproducible.
Section snippets
Materials
Chitosan (CS) powder (125 mPa s) was purchased from Zhejiang Aoxing Biotechnology Co. Ltd. (China) and treated in a 40 wt% NaOH solution for 2 h at 100 °C twice. The degree of deacetylation (DD) was 98% and the viscosity-average molecular weight was 1.03 × 105 Da according to the reference [23]. ɛ-Caprolactone (ɛ-CL, Aldrich), benzyl alcohol and N,N-dimethylformide (DMF) were dried over CaH2 and distilled under a reduced pressure. Phthalic anhydride was dried under vacuum after recrystallizing from
Synthesis and characterization of CS-g-PCL
The graft copolymers were synthesized via protected/deprotected method to maintain free primary amino groups in the backbone of polysaccharide (Fig. 1). First, Chitosan was reacted with phthalic anhydride to protect free amino groups. In addition, the protection could make the product soluble in DMF, which was convenient for the following “graft onto” reaction. The PCL segment was subsequently coupled with phthaloylchitosan via the OH/COOH groups. Finally, polymers with free amino groups were
Conclusions
To summarize, CS-g-PCLs were synthesized via chemical coupling by means of protection/deprotection to obtain graft polymers with free amino groups. Thereafter, amphiphilic polymers with different grafting degree of PCL segments were self-assembled into micelles with uniform core–shell structures. The mean size of polymer micelles decreased with the increase of PCL. Silver nanoparticles were prepared in the presence of polymer micelles. Different time scales and silver ions/polymer ratios were
Acknowledgments
This research was supported by the Fundamental Research Funds for the Central Universities (WD0913008), The National Natural Science Foundation of China (21274039), Basic Research Key Program Project of Commission of Science and Technology of Shanghai (12JC1403000, 12JC1403100) and “Shu Guang” Project of Shanghai Municipal Education Commission.
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