While gold is known since a long time for its golden colour so appealing in jewellery, gold is also used since antiquity as red dye. For instance, Au NPs have been found in several red glasses or on porcelains decorated with red or pink enamels dating from ancient times, for instance, in Roman glassworks (such as the famous Lycurgus Cup dating from the fourth century CE) or in the Chinese “Famille Rose” porcelain dating from the eighteenth century [9–11]. Indeed, bulk gold reflects the yellow colour and appears golden-coloured, whereas at the nanoscale gold appears generally red. Michael Faraday, fascinated by this ruby colour of colloidal gold solutions, was the first scientist who has discovered that the optical properties of gold colloids differ from those of the corresponding bulk metal. He reported on the first synthesis of Au NPs in solution in 1857 . These colloids were obtained by reduction of gold salts by white phosphorous . Later, the specific colours of metal colloids were explained by Mie .
This phenomenon is due to the well-known plasmon resonance also called local surface plasmon resonance (LSPR), which can be explained by a confinement of the electromagnetic wave associated with the light inside the NPs. Indeed, when its wavelength is greater than the size of the NPs, the whole NP feels a uniform and oscillating electric field, and consequently electrons oscillate in phase. Nevertheless, this collective oscillation of the electrons is constrained by the reduced dimensions of the NP in which they are confined, leading to a significant absorption of the wavelengths around green. Then, NPs appear with the complementary colour, which is red.
This experiment introduces the audience into the idea that at the nanoscale, basic properties, such as colour, turn out to be very different from what happens in the “macro-world”. Therefore, nanoscience focuses on properties linked not only to a material but to its specific size. And sometimes, the changes of properties at the nanoscale have consequences visible at our human scale. In the present case, the assembly of gold atoms into NPs of ca. 1,000 atoms changes the colour of gold. This message is always puzzling for non-scientific audience.
Nowadays, among the different ways of synthesis of spherical Au NPs, Turkevich’s method is the most known and the easiest to perform . This method is based on the reduction of an Au(III) salt by sodium citrate, as described in the experimental part (experiment 1). The reaction starts by boiling a gold salt aqueous solution (pale yellow). A few minutes after the addition of sodium citrate, the mixture becomes firstly uncoloured, then grey, violet, and finally burgundy red (Fig. 2a). In this reaction, the sodium citrate is used not only as reducing agent, but also as stabilizing agent. Indeed, its stabilizing role restrains the growth of the NPs and controls the NP diameter. Thus, the size of the NPs can be modulated by changing the concentration of the gold salt solution and the ratio of the quantity of gold salt to the quantity of sodium citrate [16, 17]. The sizes of the NPs described in this experiment are 15 and 30 nm and has been verified by transmission electronic microscopy (TEM) (Fig. 2b)
Diffusion (or scattering) of light by the Au NPs can be evidenced with a laser beam pointed to the solution (Fig. 2c) and can be compared with a similar experiment using an aqueous solution containing an organic red dye (rhodamine or Congo red for instance) for which diffusion is not detectable (Fig. 2d). When light is scattered by single molecules such as dye molecule, the efficiency of the phenomenon is very low (usually quantified with the scattering cross section) and is barely visible by eyes (Fig. 2d). However, this cross section grows as the volume of the object, and with a particle of 30 nm whose diameter is 100 times larger than a molecule (volume 106 larger), the scattering process becomes easily visible as demonstrated in Fig. 2c. Moreover, Fig. 2c shows that if the laser wavelength is tuned to the plasmon resonance, the scattering is even stronger. The green laser beam is quickly damped when going through the NP suspension, whereas the red laser light is scattered less efficiently, but over a longer path.
Many other chemical methods of synthesis exist  and can lead to different shapes of NPs: nanorods [19, 20], nanopyramids , nanocubes, etc. Physical chemical syntheses such as photolytic and radiolytic methods can also be used, and generally lead to a better control of the size of the NPs [18, 22, 23]. Chemical and optical properties of Au NPs depend on their size, their shape, their aggregation state and their local environment. Such Au NPs have applications for example in catalysis (oxidation of CO), medicine (photothermal therapy and radiotherapy), plasmonics and electronics [9, 24, 25].