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Inorganic Chemistry Communications 8 (2005) 1075–1077

Oxidation of solid gold in chloroform solutionsof cetyltrimethylammonium bromide

Tom Mortier *, Andre Persoons, Thierry Verbiest

Department of Chemistry, Catholic University of Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium

Received 6 July 2005; accepted 20 August 2005Available online 12 October 2005

Abstract

Oxidation of solid gold was studied in chloroform solutions of cetyltrimethylammonium bromide (CTAB). We observed that, uponsonication, solid gold is oxidized and cetyltrimethylammonium tetrabromo aurate is formed. The Br� ions act as a nucleophile that low-ers the reduction potential of gold and molecular oxygen from the air acts as an oxidant. Addition of hydrazine to the sonicated solutionleads to the formation of gold nanoparticles dispersed in chloroform.� 2005 Elsevier B.V. All rights reserved.

Keywords: Gold; Cetyltrimethylammonium bromide; Sonication; Nanoparticles

Gold is one of the most stable coinage metals because itis not attacked by either oxygen or sulphur at any temper-ature [1]. The dissolution of gold is essential for its extrac-tion and recovery [2]. Two conditions have to be fulfilled todissolve bulk gold: an oxidizing agent and a suitable ligand.For example, gold will not dissolve appreciably in eitherhydrochloric or nitric acid alone, but dissolves in aqua re-gia to give tetrachloroauric (III) acid. In organic solutions,these conditions also have to be fulfilled.

The nobility of the coinage metals is due to the high po-sitive reduction potential value [3]. However, it is wellknown that these high redox potentials can be significantlylowered in the presence of a nucleophile. Furthermore, sizeeffects can also result in a lowering of the reduction poten-tial value making these metals more susceptible towardsoxidation [4]. In fact, it has been described that colloidalgold can be dissolved in water in the presence of a nucleo-phile and that gold (and other noble metals) will dissolve inpolar organic solvents in the presence of halogens and cor-responding halides [5–9].

In this communication, we describe the oxidation of so-lid gold in chloroform solutions of a simple surfactant

1387-7003/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2005.08.015

* Corresponding author.E-mail address: tom.mortier@fys.kuleuven.be (T. Mortier).

(cetyltrimethylammonium bromide, CTAB), with the aidof ultrasound. Furthermore, we show that this procedurecan form the basis for the preparation of gold colloids inorganic solvents.

Chloroform was chosen as a solvent because it dissolvesCTAB in high concentrations. Experiments were per-formed with CTAB concentrations of 10�3, 10�2 and10�1 M. In a typical experiment, a piece of solid gold(99.99% purity, approximately 1 g) was added to one ofthe CTAB solutions and ultrasonic vibrations were trans-mitted continuously into the solution by means of a Bran-son Sonifier 450 (Branson ultrasonic cooperation) for15 min. For the lowest concentrations of CTAB in chloro-form (10�3 M), the solution turned slightly yellow after15 min, while the chloroform solution with 0.1 M CTABbecame bright yellow. In the absence of sonication, no ob-servable reaction occurred, even after several days. The yel-low colour of the sonicated solution seems to indicate thata Au(III) compound is formed. UV/Vis absorption spec-troscopy shows a strong absorption at 387 nm with a dis-tinct shoulder at 458 nm, which is indicative of theformation of a Au(III) compound, more specifically theAuBr�4 anion. This spectrum is basically identical with thatof HAuBr4 solubilized in chloroform by CTAB (Fig. 1).Note also that the type of sonifier used is not crucial, but

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Fig. 2. UV/Vis spectrum from a sonicated solution after addition ofhydrazine hydrate.

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Fig. 1. UV/Vis spectra of a sonicated solution (0.1 M CTAB in CHCl3)(solid line) and HAuBr4 dissolved in a 0.1 M CTAB/CHCl3 solution.

1076 T. Mortier et al. / Inorganic Chemistry Communications 8 (2005) 1075–1077

the reaction time is dependent upon the sonification power.A simple ultrasonic bath for cleaning glassware is sufficientto oxidize the gold. In this case, however, the reaction timeis several hours.

We also monitored the formation of tetrabromoaurateas a function of time by UV/Vis absorption, by taking aUV/Vis spectrum every minute during the 15 min of soni-cation. During this time, the concentration of AuBr�4 wasfound to increase linearly with time during the time courseof the experiment, indicating zero order kinetics.

Since the formation of AuBr�4 from gold requires an oxi-dizing agent, probably molecular oxygen in air, we per-formed the same experiment under a molecular nitrogenatmosphere. In this case, no oxidation of gold was ob-served during the time course of the experiment, which sug-gests that molecular oxygen is indeed the oxidizing agent.This also indicates why sonication is absolutely necessary:during sonication the reaction mixture is continuously pro-vided with oxygen.

Fig. 3. AFM image of colloida

One of the advantages of this method is that it providesa unique and very simple procedure to transfer tetrabro-moaurate into organic solvents, leading to the possibilityof colloid formation in a variety of organic solvents. Whiletraditional methods for gold colloid formation are usuallywater-based, or require a two-phase method, the proceduredescribed below directly leads to colloid formation in theorganic solvent [10–12].

Upon addition of a small amount of hydrazine (40 ll) tothe sonicated gold/CTAB/chloroform solution, colloids areformed. The yellow solution initially becomes colourless,probably due to the formation of a Au(I)-complex, butafter 1 min the solution turns purple, indicative of colloidalgold formation. UV/Vis absorption spectra show a strongplasmon resonance at 535 nm (Fig. 2). Dynamic light scat-tering (DLS) experiments gave typical average diameters of28 nm, while atomic force microscopy (AFM) shows spher-ical particles with an average diameter of 17 nm but with asignificant size dispersity (17 ± 5 nm) (Fig. 3). The differ-

l particles (contact mode).

T. Mortier et al. / Inorganic Chemistry Communications 8 (2005) 1075–1077 1077

ence between DLS and AFM measurements can be attrib-uted to CTAB stabilizing the gold colloids in the solution.

In conclusion, we have shown that solid gold is effec-tively oxidized in concentrated chloroform solution ofcetyltrimethylammonium bromide with the aid of sonica-tion, leading to the formation of cetyltrimethylammoniumtetrabromo aurate. The Br� from CTAB is acting as anucleophile and oxygen in the air is responsible for theoxidation of the bulk gold. Moreover, this process can beutilized as a very easy preparation method for creating goldcolloids after adding a suitable reducing agent to theAuBr�4 /CTA

þ in chloroform system, whereby gold colloidsare manufactured and stabilized immediately in chloroform.

Acknowledgements

We thank the Fund for Scientific Research-Flanders(FWO-Vlaanderen, Grant G.0297.04), the Katholieke Uni-versiteit Leuven (Grant GOA/2000/03), and the BelgianGovernment (Grant IUAP P5/03) for financial support.

The research of T.M. was financed by a Ph.D. grant ofthe Institute for the Promotion of Innovation through Sci-ence and Technology in Flanders (IWT-Vlaanderen).

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