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From the paper in Philosophical Transactions entitled ‘Experimental relations of gold (and other metals) to light’ (1), based on his Bakerian lecture to the Royal Society in London on 5 February 1857, it is clear that Michael Faraday was fascinated by the ruby colour of colloidal gold. The objective of his investigations was to examine the interaction of light with metal particles, but much of this paper focused on various aspects of the formation, nature and properties of ruby gold, and these systematic studies and perceptive interpretations did in fact mark the birth of modern colloidal chemistry. This has now led to the strong emergence of the nanoscience of gold and nanotechnology (2), even although ruby glass was known to Faraday and had been used for centuries for stained glass windows, and ‘Purple of Cassius’, made from gold in the presence of tin, had been used for colouring glass and enamels since the seventeenth century (3). The work described in Faraday’s lecture fitted into his investigations into the relations between matter and electrical, magnetic, and optical phenomena (2). He was asking the question “to what extent experimental trials might be devised which, with their results and consequences, might contradict, confirm, enlarge, or modify... that wonderful production of the human mind, the undulatory theory of light...”. The action of light on particles which are small compared with the wavelength of light should produce interesting results, and gold sprang to mind because “known phenomena appeared to indicate that a mere variation in the size of its particles gave rise to a variety of resultant colours.” Although Faraday described work with a number of metals, the ruby colour produced in ‘solution’ by fine particles of gold which are “very minute in their dimensions”, and prepared via various practical approaches, was the main topic of his lecture. A typical preparation used an aqueous solution of a gold compound, e.g. NaAuCl 4 , and treated this with a reducing solution such as phosphorus in carbon disulfide in a two phase system. The yellow colour of sodium chloroaurate (NaAuCl 4 ) changes within minutes to the deep ruby colour of colloidal gold. Faraday concluded that the ruby fluid was gold dispersed in the liquid in a very finely divided metallic form not visible in any of the microscopes available in his day. Nearly 100 years later Turkevich et al (2, 4) used electron microscopic investigations to reveal that the ruby-coloured colloids made by Faraday’s preparative routes produce particles of gold with average sizes in the 6 ± 2 nm range. Faraday’s original gold colloids were very stable and at least one example survived until the second World War, when it was accidentally destroyed by enemy action, and an example is pictured in Figure 1. Gold Bulletin has monitored the development of uses for ruby gold via papers published by W.D. Mogerman (5), Leslie B. Hunt (3) and John Turkevich (4). Mogerman points out that the title for Faraday’s 1857 paper does not do full justice to its contents, but as usual, Faraday had very broad objectives in mind and he was clearly fascinated with the ruby gold ’suspensions’ and the various methods of obtaining them (the word ‘colloid’ was first coined only later by T.H.Graham in 1861 (2, 6). Yet the content of the lecture started from very small beginnings: in a strange statement written in a letter from Michael Faraday to his friend Prof C. F. Schönbein of Basle he wrote “I have been occupying myself with gold this summer; I did not feel headstrong enough for stronger things. This work has been of the Mountain and Mouse fashion; and if I ever publish it and it comes to your sight I dare say you will think so” (5). Hunt in his article entitled ‘The true story of the Purple of Cassius’, published in 1976 says “For the past 300 years the literature on gold, on glass and on ceramics has ascribed to Andrew Cassius the credit for discovering the purple preparation of colloidal gold and stannous hydroxide that bears his name

Michael Faraday's recognition of ruby gold: the birth of modern nanotechnology