Unraveling the reaction mechanism of silver ions reduction by chitosan from so far neglected spectroscopic features
Introduction
Hybrid systems combining chitosan with metal nanoparticles, like silver nanoparticles (Ag NP), have been studied for over a decade, finding applications, in catalysts (Calo et al., 2004; Rajesh, Sujanthi, Kumar, & Venkatesan, 2015), chemiluminescent sensors and biosensors (Wang & Cui, 2008; Zhang et al., 2011), optical sensors or waveguides (Mironenko, Modin, Sergeev, Voznesenskiy, & Bratskaya, 2014), bactericide nanocomposites, or phototherapeutic agents (Sanpui, Murugadoss, Prasad, Ghosh, & Chattopadhyay, 2008; Boca et al., 2011), just to mention a few.
Chitosan is a biocompatible and biodegradable, naturally occurring polysaccharide produced by deacetylation of chitin. Scheme 1 shows its chemical structure, with some degree of acetylation (chitin-like).
Chitosan is known to be soluble in (slightly acidic) aqueous solutions, being also a chelating agent that can be used as both a stabilizing and a reducing agent of metal ions (Pestov, Nazirov, Modin, Mironenko, & Bratskaya, 2015; Huang & Yang, 2004). On the other hand, silver nanoparticles have, among other advantages, unique antimicrobial properties that have led to their success in biomedical applications (Rai, Yadav, & Gade, 2009). However, metal NP are prone to aggregation unless stabilized.
Recently, in our group, silver and gold metallic NP were synthesized in situ on aminated cellulose ultrathin films and fibers (Ferraria, Boufi, Battaglini, Botelho do Rego, & Vilar, 2010; Boufi et al., 2011). It was shown that Ag and Au NP nucleate and grow directly at the surface of cellulose previously functionalized with amine groups. Most importantly, the surface was efficiently modified after interaction with metal salts without adding any external reducing agent and the reaction took place selectively and exclusively at the surface, leaving the liquid phase exempt from NP.
The most common method used to synthesize Ag NP in the presence of chitosan is the chemical reduction of the metal salt using reducing agents such as NaBH4, citrate, ascorbic acid or a combination of different reagents (Huang, Yuan, & Yang, 2004; Potara, Gabudean, & Astilean, 2011). However, some of the reagents used may have several harmful effects on the environment and human health. Alternatively, several processes involving non-toxic reducing agents have been proposed. For instance, the use of glucose followed by a microwave thermal treatment (Bozanic, Trandafilovic, Luyt, & Djokovic, 2010) takes advantage of the presence of the hemiacetal (cyclic form)/aldehyde (open form) moiety. In another approach, Rodríguez-Argüelles et al. were able to synthesize tiny Ag NP (<1.7 nm) on a chitosan-tripolyphosphate (TPP) nanocomposite, after a thermal treatment (Rodríguez-Argüelles, Sieiro, Cao, & Nasi, 2011). Photon-assisted reduction methods have also been reported, in particular by γ-rays (Chen, Song, Liu, & Fang, 2007) or UV-irradiation (Reicha, Sarhan, Abdel-Hamid, & El-Sherbiny, 2012).
In most of these studies, the efficiency of the Ag0 nanoparticles synthesis is attested by its typical surface plasmon resonance absorbance close to 400 nm, XRD patterns and TEM images. For instance, Wei et al. synthesized chitosan-stabilized Ag NP by heat treatments and using only chitosan as reducing and stabilizing agent, a green chemistry approach identical to the one described here. Systems were studied mainly by FTIRS, TEM and UV–vis absorption spectroscopy (Wei, & Qian, 2008). However, no quantitative data treatment is performed. Actually, in most publications, in spite of hypothesizing about the probable intermediate species involved in the silver reduction mechanism, just a few show experimental evidence supporting the existence of those species at some point of the reaction or under given conditions. The exception is the Reicha and collaborators’ study that shows spectral features assigned to Ag+-Chitosan complexes formed by an electrochemical process before the UV irradiation.
Here, Ag NP were generated, in situ, from the interaction between a silver salt and the chitosan surface, under a mild thermal treatment, and in the absence of any additional reducing agent, stabilizer, template or activating irradiation. The hypothesis here stated is that a suitable combination of the quantitative UV/Vis absorption spectra with X-ray Photoelectron Spectra (XPS) data treatments will clarify the correlation (if any) between the NP yield and the chitosan and silver nitrate concentrations (both absolute and relative).
The present work aims: to unravel the mechanism of silver reduction detecting the chemical species produced at the early stages of the interaction between the silver salt and chitosan, with the help of a detailed data treatment of both UV/Vis absorption and XPS; to identify the final oxidation states of silver by an appropriate XPS data treatment (none of the studies reported on these hybrid systems ever made a suitable oxidation state identification/quantification, even those where XPS analysis was used) and, in this way, to evaluate the reduction extent of Ag+ to Ag0. System characterization also shed some light on the chitosan oxidation mechanism. The characterization was complemented by DLS and AFM measurements.
Section snippets
Materials
Chitosan low molecular weight with Mw 50,000–190,000 Da and a degree of deacetylation ≥75% (according to the Aldrich specification sheet), estimated by XPS to be ∼79%, was used. For the work here reported, the chitosan used for all experiments came from exactly the same pack, therefore the Mw and the degree of deacetylation is kept constant. Silver nitrate 99.9999% trace metals basis and acetic acid ACS reagent ≥99.7% were purchased from Aldrich and used without further purification. Acetone,
UV–vis absorption studies
Fig. 1 shows the UV–vis absorption spectra for the aqueous chitosan (Chi) solution 7 mg/mL, for the aqueous AgNO3 4 × 10−2 M, and for two mixtures of Chi and AgNO3, one freshly prepared and another after 48 h at room temperature. For both, [chitosan] = 7 mg/mL and [AgNO3] = 4 × 10−2 M.
Chitosan solution spectrum displays an increasing absorption from around 590 nm to around 300 nm, reaching a plateau which extends till ∼260 nm and increasing again for lower wavelengths reaching saturation below 250 nm. The AgNO3
Conclusions
This green chemistry process does not involve the use of any toxic chemicals, it is cost-effective and environment friendly. Moreover, it does not require the use of further stabilizers, chitosan playing also that role. In this work, we put in evidence that the products of oxidation in this process are carbonyl groups issued from the oxidation of alcohol and or glycosidic groups, present in the chitosan chains, fully explaining, qualitatively and quantitatively the UV–vis absorption band
Acknowledgements
The authors acknowledge funding from Fundação para a Ciência e a Tecnologia (FCT): A. P. Carapeto Grant SFRH/BD/75734/2011, A. M. Ferraria Grant SFRH/BPD/108338/2015 and project UID/NAN/50024/2013. We also acknowledge North Atlantic Treaty Organization (NATO) SFP project 984842 (CATALTEX) for the AFM (Innova, BRUKER) acquisition.
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