Facile synthesis of tunable silver nanostructures for antibacterial application using cellulose nanocrystals

Highlights

• Facile synthesis of Ag NPs using CNs as reductant and stabilizer.

• It is feasible to adjust the morphology of Ag nanostructures.

• The Ag dendritic nanostructure can be obtained using CNs as a reducing agent.

• The CNs/Ag nanohybrids exhibit an excellent antibacterial performance.

Abstract

In this study, we report a facile and environmentally friendly strategy for synthesis of well dispersed and stable silver nanostructures using cellulose nanocrystals in aqueous solution without employing any other reductants, capping or dispersing agents. Importantly, it is feasible to adjust the morphology of the silver nanostructures by varying the precursor AgNO₃ concentration. Silver nanospheres were formed when the AgNO₃ concentration was 0.4 mM, while the dendritic nanostructures predominated when the AgNO₃ concentration was increased to 250 mM. The antibacterial activity of the two different silver nanostructures against Escherichia coli and Staphylococcus aureus was characterized. Dendritic nanostructure showed a better antibacterial activity than that of silver nanosphere. The approach presented in this paper offers a very promising route to noble metal nanoparticles using renewable reducing agents.

Introduction

Noble metal nanoparticles, especially silver nanoparticles (Ag NPs) have attracted considerable attention due to their potential applications as electronic, optical, sensing, catalytic and importantly antibacterial materials. Silver has been utilized in antibacterial applications for thousands of years due to their broad-spectrum antimicrobial activities and high toxicity to different type of microorganisms (Kamel, 2012, Nassar and Youssef, 2012). However, among the conventional preparative methods, most of the reducing and stabilizing reagents used for the synthesis of Ag NPs are not environmentally benign. In view of the awareness toward green chemistry and sustainable strategy, the development of a simple and environment-friendly method for the synthesis of Ag NPs is necessary.

Biological synthesis of metal NPs using microorganisms, enzymes, and carbohydrate polymers have been suggested as possible eco-friendly alternatives to conventional petrochemical reductants-based methods (Iravani, 2011, Shaligram et al., 2009). A number of researchers have described the synthesis of various metal NPs using carbohydrate polymers, such as leaf broth (Shankar, Ahmad, Pasrichaa, & Sastry, 2003), hydroxypropyl starch (Hebeish et al., 2011), hydroxypropyl cellulose (Abdel-Halim & Al-Deyab, 2011) and bacterial cellulose (Yang, Xie, Deng, Bian, & Hong, 2012). Using renewable carbohydrate- or cellulose-based materials for the synthesis of NPs can be advantageous over microorganisms process because it eliminates the elaborate process of maintaining cell cultures and can also be suitably scaled up for large-scale NPs synthesis (Song, Jang, & Kim, 2009). Moreover, NPs produced by carbohydrate- or cellulose-based materials are more stable and the rate of synthesis is faster than that in the case of microorganisms (Iravani, 2011).

Cellulose nanocrystals (CNs), which can be isolated from a variety of natural sources such as cotton, tunicate, bacteria and wood pulp, has emerged as a new class of renewable carbohydrate polymers owing to its high stiffness, low density, well-defined size and morphology, controlled surface chemistry, environmental sustainability and anticipated low cost (Lam, Hrapovic, Majid, Chong, & Luong, 2012). The electron-rich feature of hydroxyl and sulfate ester groups on its surface endow CNs a good colloidal stability in water, which makes it well suitable for the preparation of metal NPs (Benaissi et al., 2010, Hirai et al., 1979, Lin et al., 2011). However, to date, the use of CNs in the biosynthesis of metal NPs is very limited. A small number of reports (Cirtiu et al., 2011, Lam et al., 2012, Shin et al., 2008) describe the formation of metal NPs using CNs as a stabilizing template, which inevitably involved the use of a chemical reducing agent. Recently, synthesis of platinum NPs using CNs as a reducing agent was reported by Benaissi et al. (2010). However, their results indicated that the reaction only proceeds when the water is in contact with supercritical carbon dioxide. The strict reaction condition hindered the large-scale application of this approach. To the best knowledge of the authors, no reports on the facile synthesis of metal NPs using CNs as reductant and stabilizer under mild conditions have been published.

Herein we present the successful synthesis of Ag NPs using CNs in aqueous solution without employing any other reductants, capping or dispersing agents under mild conditions. Furthermore, it is feasible to adjust the morphology of the Ag nanostructures by varying the precursor AgNO₃ concentration. We report for the first time that the dendritic Ag nanostructures can be obtained using CNs as a reducing agent. The antibacterial activity of Ag NPs with different nanostructures against Escherichia coli and Staphylococcus aureus was investigated. The green and environmentally benign approach developed in this paper offers a very promising route to the synthesis of other metal NPs, especially those to be used for antibacterial applications.