Appeared earlier in TG Magazine 22-3 of April 2019.
Michael Faraday (1751-1867) was fascinated by gold when he found out that ultrathin sheets kept their yellowness upon light reflection as for normal gold but looked green upon transmission. Moreover, when etching the sheets to clean them he found the cleaning liquid to turn ruby red. This urged him to perform an impressive series of experiments the results of which are summarized in his Bakerian lecture to the Royal Society in 1857 [1]. He argued that the ruby red light was due to the strongly wavelength dependent scattering of light that we nowadays know as the Faraday-Tyndall effect. The same effect is also responsible for the blue color of the sky.
My first exposure to colloidal gold – metallic gold particles of submicron size – was in the late 70’s when working as an electronics engineer in the pre-clinical cytology laboratory of the Leiden academic hospital. The gold particles were used for the detection of tropical infections such as schistosomiasis (also known as bilharzia) in what is known as the Enzyme-Linked Immuno-Sorbent Assay (ELISA). My involvement [2] was the automated readout of trays with cups, one for each essay, in such a way that it could also be used in the lesser equipped laboratories in the tropic areas where the infections were frequently found. The gold particles, coated with a special protein, were responsible for the amplification of signal used for the detection of reaction product from the immune-reaction.
Later on, during my PhD-studies at the Lorentz Institute for Theoretical Physics, I learned more about the properties of submicron-sized metallic particles. Some of my colleagues studied the optical properties of gold nanoparticles when deposited, a bit like truncated spheres, on sapphire. This was done under the supervision of Jan Vlieger and Dick Bedeaux who later on summarized these and many more findings in their monograph on the Optical Properties of Surfaces [3]. The scattering of gold particles that Faraday observed is due to the fact that the surface plasmon resonance causes a sharp absorption peak for light in the visible regime. Other metal particles in that size range do that similarly but due to material properties of gold this happens to be exactly in the visible part of the spectrum. As it is an interfacial property, it critically depends on surface coverage as employed in the ELISA, on the precise shape of the particles as studied by my colleagues and on the close vicinity of a surface.
Although Faraday was smart enough to identify the ruby red color of the cleaning liquid being due to small dispersed gold particles, the willful synthesis of colloidal gold is far from trivial. Yet, it was one of the most challenging areas in alchemy to produce the Elixer of Life, i.e. potable gold; it was searched for during many centuries. Because of gold’s sheer indestructibility, alchemists ascribed a great therapeutic value to Aurum Potabile (drinkable gold). Only in the late 19th century systematic synthesis routes became available through the work of – amongst others –the 1925 Chemistry Nobel Prize winner Richard Zsigmondy.
It was not before the work of our late TG-Member of Honour, professor Gert Frens that controlled nanoparticle synthesis became feasible. As a consequence most Chemical Engineering students that studied in Delft between 1988 and 2002 were exposed to this synthesis route under the guidance of Nico van Westen, the physical chemistry laboratory technician of those days. Frens’ Nature paper [4] on this topic is still heavily being cited and techniques like the one he proposed are used as of today. The reason for this is most probably not in the many samples of Aurum Potabile that are on offer today and sell at rates of the order of 10 euro per ml; it is supposed to heal all illnesses! Despite the high price, the turnover is relatively little. It is also not in the ELISA applications for tropical deceases and others pharmaceutical applications. It is the pregnancy test that is based on ELISA that makes most of the money!
Much later, we ourselves turned our interest in the synthesis of metal nanoparticles. In contrast to the many synthesis routes available, our aim was in making fine, uniformly sized nanoparticles at high yields. Conventional routes provide nanoparticles at relatively low concentration, because at higher concentrations the particles aggregate into larger structures yielding non-uniform particles. While preparing a manuscript [5] on our high yield synthesis of uniformly sized gold nanoparticles, a discussion on the stability time scale of colloidal dispersions developed in which it seemed appropriate to mention the world record in this: the more than 150 year stability of the gold sols prepared by Michael Faraday. For the manuscript at hand, a primary source was needed to refer to but whatever we could find; they were all – at best – secondary sources: information collected by others. One of the more explicit sources, the web site of a well-known, British manufacturer of colloid scientific equipment, mentions the Science Museum in London. Many other sites and documents do likewise.
After sending an electronic request to the conservator of the museum, the following answer was received “The situation is a little bit complex. Until 1999 we had a Faraday exhibit which displayed gold films deposited on watch glasses made by Faraday alongside a tall vessel containing colloidal gold prepared according to the Zsigmondy’s method which otherwise had nothing to do with Faraday.” The interesting consequence of this statement by the conservator of the Science Museum could be that there are quite a few false statements about and very likely even pictures of vessels not older than a few years instead of the 155 as claimed!
A further message from the conservator of the Science Museum revealed that some gold sols, of which pictures circulate the internet, could be from the Royal Institution (Ri), also in London. The confirmation came from the Curator of Collections who stated that “They are on permanent display within the Michael Faraday Museum area of the Ri, on the lower ground floor of the building, within the only section of Faraday’s original laboratory that still exists.” In addition, pictures were sent of which one accompanies this article and demonstrates the Tyndall effect that betrays colloidal dispersions. So, the gold sols made by Faraday are indeed in London but not in the often mentioned Science Museum but in the museum of the Royal Institution. We are happy to have spent some time finding out the truth about these gold sols and not to have merely repeated a false statement.
References
- M. Faraday, Philos. Trans. R. Soc. London, 147 (1857) 145 – 181.
- A.M. Deelder, G. Koper, R. de Water, et al., Automated measurement of immune galactosidase reactions with a fluorogenic substrate by the aperture defined microvolume measurement method and its potential application to Schistosoma mansoni immune diagnosis. J Immunol. Methods 36 (1980) 269-83.
- D. Bedeaux and J. Vlieger, Optical Properties of Surfaces, Imperial College Press, London 2002.
- G. Frens, Controlled Nucleation for the Regulation of Particle Size in Monodisperse Gold Suspensions.
- K.N.K. Kowlgi, G.J.M. Koper, S.J. Picken, U. Lafont, L. Zhang, B. Norder, Synthesis of magnetic noble metal (nano)particles, Langmuir 27 (2011) 7783–7787.