Elsevier

Carbohydrate Polymers

Volume 175, 1 November 2017, Pages 530-537
Carbohydrate Polymers

Research Paper
Reduction of Au(III) by a β-cyclodextrin polymer in acid medium. A stated unattainable reaction

https://doi.org/10.1016/j.carbpol.2017.08.013Get rights and content

Highlights

  • β-Cyclodextrin polymer reduces Au(III) to Au0 in acid medium.

  • A mechanism to explain the low rate of reaction is proposed.

  • Primary alcohols are oxidized to carboxylic acid.

  • Gold nanoparticles are capped with dimers of the polymer.

Abstract

Gold nanoparticles (AuNPs) can be prepared from the reduction of Au(III) with cyclodextrins acting as both, reducing and capping agent. It has been stated that a basic medium (pH = 9–12) is a mandatory condition to achieve such reduction. We demonstrated, for the first time, the reduction of Au(III) by a crosslinked β-cyclodextrin-epichlorohydrin polymer (βCDP) in acid medium (pH ∼3). The coordination of Au(III) to the βCD in βCDP polymer required a βCD:[AuCl4] ratio of 4:1. The same ratio was necessary to achieve a 50% of the reduction of Au(III) to Au0 within the first 24 h of reaction. During this initial time, the reaction showed a concentration-dependent reduction rate while for longer times the reduction rate was diffusion-dependent. An overall mechanism to explain this dependency has been proposed. The 13C NMR spectrum identified the oxidation of the Csingle bondOH groups to carboxylic ones by recording a signal at 175.6 ppm. Gold nanoparticles cores (AuNPs) with a diameter of 34.2 ± 7.7 nm, determined by Transmission Electron Microscopy (TEM), was prepared by refluxing HAuCl4 in an aqueous solution of βCDP. The AuNPs core was capped by dimers of the βCDP polymer as determined by Dynamic Light Scattering measurements.

Introduction

Due to the unique optical, electronic, and chemical properties, gold nanoparticles (AuNPs) offer great potential applications in biomedicine (Gabas, Stepien, Moros, Mitchell, & Fuente, 2016; Peña-González et al., 2017), catalysis (Penhoat et al., 2016; Su, Li, Wang, Zhong, & Wang, 2016), sensor (Hassanzadeh, Khataee, Bagheri, & Lotfi, 2017; Jiang, Zhang, Jin, Wang, & Zhou, 2015), environmental (Dong et al., 2016; Sierra et al., 2016), electronics (Mthethwa, Tuncel, Durmuş, & Nyokong, 2013), among other fields. The preparation of AuNPs by chemical methods are preferred over the physical or biological ones owing to their versatility to control the shape, size and surface chemistry of the nanoparticles (Daniel & Astruc, 2004). Chemical methods consist on reducing a metal precursor salt, usually HAuCl4, with a reducing agent in the presence of a stabilizer under certain conditions of temperature, pH and concentration of the chemicals. Looking toward a chemically eco-friendly methodology, several occurring natural products, like polysaccharides, have been employed as reducing and stabilizing agents in the preparation of AuNPs (Iravani, 2011, Nazirov et al., 2016; Park, Hong, Weyers, Kim, & Linhardt, 2003).

Cyclodextrins (CD) are natural cyclic oligosaccharides, enzymatically produced mainly from starch, composed of 6, 7 or 8 glucopyranoside units (α, β, γCD, respectively) linked by α-(1  4) linkage (Villalonga, Cao, & Fragoso, 2007). The main feature of CDs relies on their unique structure that allows them to form stable inclusion complexes with a wide variety of guests containing hydrophobic moieties (Martin, Sánchez, Cao, & Rieumont, 2006). From the three commercially available CDs, βCD is the cheapest one but the less soluble in water (1.85 g at 25 °C) (Szejtli, 1998). This latter drawback can be overcome by crosslinking the βCD units with epichlorohydrin in basic medium to produce a highly water soluble βCD polymer (βCDP) (Martin et al., 2006; Renard, Deratani, Volet, & Sebille, 1997). Additionally, due to the cooperative effect of the βCD units in the polymer, the inclusion constant of the host-guest complex increases (Martin et al., 2006).

Under acid conditions (1 M HCl), the interaction of βCD with Au(III) has been studied and a formation constant of 2400 M−1 was reported (Koshima & Onishi, 1988). Au(III)-βCD interactions necessarily depend on the speciation of the metal ion. H[AuCl4] is most commonly used as the source of Au(III). Aqueous HAuCl4 solution consists of [AuClx(OH)4-x] species, where x  2 at low pH and x < 2 at high pH values (Wang, Qian, Bi, & Huang, 2009). The authors observed that the higher the pH, the larger the size and higher the polydispersity of the obtained AuNPs. According to cyclic voltammometric determinations, the reduction potential of [AuClx(OH)4-x] decreases with a decrease in the pH of the solution.

While some reports state that unmodified cyclodextrins do not suffer any reaction during the reduction of Au(III) (Liu, Male, Bouvrette, & Luong, 2003), more recent works indicate their capacity to reduce Au(III) and Ag(I) by the oxidation of the primary hydroxyl groups of glucopyranose (Devi & Mandal, 2013; Huang, Meng, & Qi, 2009; Pande et al., 2007). These reports clearly state that a high pH (9–12) is a mandatory condition to achieve the reduction of the metal ions. This reduction process should involve the interaction of Au(III) with βCD.

Recently, we reported the formation of gold nanoparticles (AuNPs), capped with a highly water-soluble β-cyclodextrin-epichlorohydrin polymer (βCDP), with citrate acting as reducing agent (Martin-Trasanco, Cao, Esparza-Ponce, García-Pupo, & Montero-Cabrera, 2015). We observed that even in the absence of citrate AuNPs were formed, a result that could be only attributed to an unexpected reducing activity of βCD over Au(III). The mentioned reduction occurred under a stated non-favourable condition since the addition of HAuCl4 decreases the pH value (∼3). This result motivated the study now presented on the capability of βCDP to reduce Au(III) in acid medium.

Section snippets

Experimental part

Tetrachloroauric acid in solution (wt 30%), HAuCl4, β-cyclodextrin hydrated, βCD (97%), and epichlorohydrin, epi, (98%) were purchased from Sigma-Aldrich and were used without further purification. D(+)-glucose anhydrous (>99%) was obtained from Merck. The β-cyclodextrin-epichlorohydrin polymer (βCDP, Mw  12 kDa, determined by size exclusion chromatography and 3.5 nm of hydrodynamic diameter) was prepared as previously reported (Martin et al., 2006). Freshly purified MilliQ water (18 mΩ/cm) was

Reductive capability of βCDP over Au(III)

In a recent work we encountered evidences of the formation of gold nanoparticles by refluxing HAuCl4 in a solution of a cross-linked β-cyclodextrin-epichlorohydrin polymer (βCDP) in the absence of trisodium citrate as reducing agent (Martin-Trasanco et al., 2015). The appearance of a bluish color solution, the presence of a broad band at 572 nm in the UV–vis spectrum as well as a population with a hydrodynamic diameter of 44 nm in the DLS profile, were some of the evidences that lead us to

Conclusions

For the first time, we have demonstrated the capability of a β-cyclodextrin-epichlorohydrin crosslinked polymer to reduce Au(III) to Au0 in acid medium (pH ̴3). A βCD:[AuCl4] ratio of 4:1 is necessary to reduce the 50% of the total Au(III) within the first 24 h, at 60 °C. Under these conditions the reaction occurs at a low rate. From these results we suggest a concentration-dependent step mechanism, during the first 24 h, and a diffusion-dependent one for longer time. The primary hydroxyl groups

Acknowledgements

This work was partially supported by the Fondo Nacional de Desarrollo Científico y Tecnológico [Grant FONDECYT-3150205] and by Millennium NucleusRC120001. The authors thanks the support of Dr. Carlos Ornélas, from the NANOTECH at Advanced Materials Research Center (CIMAV), in Chihuahua, Mexico and to the Dr. Cesar Zúñiga and Diego Oyarzún for their fruitful discussion during the work at the laboratory.

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