Surface functionalisation of cellulose with noble metals nanoparticles through a selective nucleation

Abstract

Films and fibres of cellulose were functionalised with silver and gold nanoparticles directly generated on the surface. The present method lays on the cellulose surface pre-modification, replacing some of the hydroxyl groups by amino functions, which selectively act as the seed coordination sites. Nanoparticles can then grow through the interaction with aqueous dilute solutions of AgNO₃ or NaAuCl₄ at room temperature, without adding any reducing agent. The procedure offers the advantage of limiting the generation of nanoparticles to the cellulose surface, leaving the dispersion medium completely exempt of them. The cellulose pre-modification also ensures chemical anchoring of the nanoparticles to the surface, avoiding any risk of particle desorption and extending the lifetime of the resulting hybrid materials. Evidence of the formation of the NPs on the cellulose surfaces was supported by FE-SEM, AFM, XPS and XRD. The method is simple and reproducible.

Introduction

Metallic nanoparticles have been the focus of intensive research in the past decades and opened the way toward the preparation of advanced functional materials having applications in different areas including electronics (Maier et al., 2001), namely sensing devices with novel functionalities, catalysis (Campbell, Parker, & Starr, 2002), and biomedicine (Daniel & Astruc, 2004). Developing methods for immobilizing such nanoparticles onto surfaces will be very useful for many of these applications. Of particular significance are electrically conducting substrates, optical sensing devices and specific membranes. Several approaches have been developed for the immobilisation of metal NPs on a variety of substrates, such as dendrimers (Esumi et al., 2004, Scott et al., 2005), latex particles (Agrawal et al., 2007, Agrawal et al., 2008), and microgels (Malynych et al., 2002, Pich et al., 2005, Rubio-Retama et al., 2007). Very recently, platinum nanoparticles have been synthesised on cellulose using reducing agents (Benaissi, Johnson, Walsh, & Thielemans, 2010). In this work, we introduce a simple, reproducible and economic method to grow metal NPs directly on cellulose substrates.

The use of cellulose, either as a reductant or stabiliser, in the synthesis of NPs was the subject of several studies. Silver, gold, and platinum nanoparticles were synthesised through hydrothermal reduction by cellulose (transparent nanoporous cellulose gel was used as supporting medium) or by adding a reductant (Cai, Kimura, Wada, & Kuga, 2009). Silver NPs were deposited on the surface of cellulose microfibrils, previously oxidised by periodate (Wu, Kuga, & Huang, 2008). In another study, silver NPs were prepared with carboxymethyl cellulose sodium that effectively works as both reducing and stabilizing reagent (Chen, Wang, Zhang, & Jin, 2008). Actually, the porous texture, characteristic of cellulose matrix and fibres, has been found to work as a template for metal nanosized particles and has been shown to be a useful support material for different metal nanoparticles of platinum, palladium, silver, and copper (He et al., 2003, Vainio et al., 2007). Among the metal NPs, silver and gold have been the most studied mainly due to the well-known wide range of applications. The antibacterial activity of silver is being explored commercially in applications such as antibacterial textiles, to prevent infections and to treat burn injuries (Taylor, Ussher, & Burrel, 2005). Immobilisation of Ag nanoparticles on bacterial cellulose fibrils was achieved using triethanolamine as chelating-reducing agent (Barud et al., 2008). Silver nanoparticles impregnated into bacterial cellulose, using different reducing agents, were prepared by immersing bacterial cellulose in a silver nitrate solution (Maria et al., 2009). Cellulose filter paper grafted with acrylamide followed by entrapment of silver nanoparticles results in development of a biomaterial with a fair biocidal action against, for instance, Escherichia coli. It can be also used as an antibacterial packaging material to prevent food stuff from bacterial infection (Tankhiwale & Bajpai, 2009). Gold NPs have multiple applications namely in domains as electronics and catalysis or for biomedical applications including labelling, drug delivery, heating and sensing (Sperling, Gil, Zhang, Zanella, & Parak, 2008).

In a previous work, we have shown that selective growth of Ag NPs could be generated on an amine modified cellulose film using, for this purpose, DMSO as a reducing agent. The cellulose films are an excellent model to study the NPs production in heterogeneous phase. Now, we extend our investigation toward a simpler, more economical and ecological method using aqueous solutions, as precursors, without the need of DMSO. The NPs could be selectively generated on the cellulose surface thanks to the presence of amino groups appended on the surface and acting as nucleating sites, and the hydroxyl groups of the anhydroglucosic unit (AGU) of the cellulose acting as reducing agents. For selective implantation of the nanoparticles, the prerequisite step consists always on the modification of the cellulose film surface through a derivatisation with di- or triaminoalkanes (Ferraria, Boufi, Battaglini, Botelho do Rego, & Vilar, 2010). In this case, one of the amine functions, or even two, in the case of the triamine, acts as an anchoring group to the carbonyldiimidazol activated cellulose film through a carbamate linkage. The remaining terminal amine groups stay available for further reaction, which, in this case, is the nucleation of silver or gold nanoobjects. This can be applied either on films, or even on fibres or other cellulosic materials.

The potentialities of the applications and the interesting science associated to the surface chemistry of the most abundant polymer on Earth, led us to investigate the possibility of grafting silver and gold NPs on its surface. The build-up of these hybrid materials, where NPs are chemically linked to cellulose may be particularly useful in different fields, as referred above, namely in the production of innovative products in paper and textile engineering. Comparatively to other methods, this one achieves undoubtedly, a higher resistant attachment of the NPs to the cellulosic substrates and assures longer lifetimes to the material. Moreover, it employs methods obeying the “green chemistry twelve principles” (http://www.epa.gov/greenchemistry/).