Fenton Polishing DOI: 10.1002/anie.201001449 A Golden Episode Continues Fentons Colorful Story** George Z. Chen* electrochemistry · Fenton’s reagent · gold · heterogeneous catalysis · nanoparticles In 1900, Henry J. H. Fenton, then a Fellow of the Royal Society working in the Chemical Laboratory of the University of Cambridge, completed and published the first part (or episode) of an interesting and colorful story [1a] that started before 1876 with his curiosity during undergraduate stud- ies. [1b] Fentons story was often told with organic colors as he described with passion: “When tartaric acid in aqueous solution interacts with certain oxidizing agents in presence of a trace of a ferrous salt, a solution is obtained which gives a beautiful violet color on the addition of caustic alkali”. [1c] Of various oxidizing agents tested, Fenton found that “hydrogen dioxide”, or hydrogen peroxide, (H 2 O 2 ) as it is now known, was particularly effective. [1c] The aqueous mixture of Fe 2+ and H 2 O 2 with or without other necessary additives has since been widely studied, and is nowadays called Fentons reagent. Interestingly, Fenton gave his 1900 paper the title, “The oxidation of organic acids in presence of ferrous iron. Part I”, thus indicating his belief that there should be at least a Part II of the story, or even more. However, possibly owing to his interest being shifted to other reactions, such as CO 2 reduction to formaldehyde in water, [1d] Fenton neglected to publish Part II of his work, although he did tell his color story again later. [1e] Nevertheless, the past 110 years have seen many of Fentons followers report new findings for various interests and purposes. [2–5] It is now generally accepted that the remarkable oxidizing activity of Fentons reagent results from the hydroxyl radicals (OHC and OOHC) produced through the following catalytic reactions in the mixture. Fe 2þ þ H 2 O 2 ¼ Fe 3þ þ OH  þ OHC ð1Þ Fe 3þ þ H 2 O 2 ¼ Fe 2þ þ H þ þ OOHC ð2Þ More complicated or slightly modified mechanisms of the Fenton reaction are present in the literature, depending on the substrate and reaction conditions. [2] Further, the concept and technologies have evolved into, for example, electro- Fenton, photo-Fenton, and hetero-Fenton processes in which the oxidizing agent is regenerated or supported on a solid substrate. [3a–c] An important addition to Fentons original work is the use of chelating agents, such as EDTA (ethyl- enediaminetetraacetic acid) and EDDS (ethylenediaminedi- succinic acid), to modulate the effectiveness of H 2 O 2 , or prevent Fe 2+ from hydrolytic precipitation in near neutral or alkaline solutions (pH > 5). [3d–f] Of particular relevance to the environmental impact of industrial processes, Fentons re- agent and its derivatives have been investigated and applied for the removal of organic, often toxic, pollutants from various waste waters and soils. [3a–c] Nonetheless, there is a common point in almost all past research on Fentons reagent—the target to be oxidized is typically an organic or biological substrate. The effects of this extraordinary reagent on inorganic substrates, particularly metals, have been largely neglected until recently. [2d, 4] One rare exception exploited the oxidizing power of Fentons reagent, in line with expectation, to create a thin biocompat- ible TiO 2 coating on the surface of a shape memory alloy, NiTi. [2d] Now, as shown in Figure 1, a surprising new study has demonstrated that Fentons reagent can attack gold, despite the fact that it is one of the most inert metals known. [4a] This intriguing finding was reported by Nowicka, Scholz, and co- workers. They discovered that the gold treated by Fentons reagent had lost its surface roughness and presented a more polished appearance. Figure 1. Comparison of the images obtained by atomic force micros- copy (extracted from ref. [4a]) of mechanically and Fenton polished gold, and schematic illustration of radical species (happy faces) interacting favorably with the defect sites (asperities) on the gold surface, and of the same radical species (unhappy faces) finding difficulties to interact with the Fenton polished smoother gold surface. [*] Prof. Dr. G. Z. Chen College of Chemistry and Molecular Sciences Wuhan University, Wuhan, 430072 (China) and Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division Faculty of Engineering, University of Nottingham Nottingham NG7 2RD (UK) Fax: (+ 44) 115-951-4171 E-mail: george.chen@nottingham.ac.uk Homepage: http://www.nottingham.ac.uk/ ~ enzgzc [**] Prof. F. Scholz and Prof. S. Fletcher are thanked for careful proof reading and constructive comments. A ngewandte Chemi e 5413 Angew. Chem. Int. Ed. 2010, 49, 5413 – 5415  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim