Process for preparing a dichromatic material in the form of a film

1. Process for preparing a dichromatic material in the form of a dichromatic translucent film, said dichromatic material comprising monodisperse nanoparticles formed of gold and optionally of a noble metal chosen from platinum, palladium, silver and copper (NPs), and at least one organic macromolecule chosen from proteins, polysaccharides and synthetic polymers, said process comprising the following steps:

i) a step of mixing at least one organic macromolecule chosen from proteins, polysaccharides and synthetic polymers with a colloidal suspension S 0 of NPs, and ii) a step of drying the mixture from step i), in order to form a dichromatic material in the form of a translucent film deposited on a support, wherein:

the NPs are in spherical form and have a diameter ranging from 70 to 100 nm,

the molar concentration of gold in the mixture from step i) ranges from 10 −4 to 5×10 −1 mol/l,

the molar concentration of macromolecule in the mixture from step i) ranges from 0.001 to 5 mol/l,

the film has a volume density ranging from 0.5×10 10 to 5.0×10 10 NPs/cm 3 for a 1 mm film thickness E, and

said NPs imparting said material with a dichromatic optical effect producing two different colours, one colour when light transmits from behind the material and a different colour when light contacts the material from the same side as an observer, such that said material produces a one first colour in transmission when light from a behind the material passes through said material and one second colour in backscattering when light from the opposite direction reflects off of said material from said same side as said observer, and wherein said first and said second colours are complimentary and produce together a neutral grey colour.

2. Process according to claim 1 , wherein the film has a thickness E ranging from 10 μm to 10 mm.

3. Process according to claim 1 , wherein the support is a transparent or translucent substrate.

4. Process according to claim 3 , wherein the transparent or translucent substrate is made of glass, glass-ceramic, ceramic, plastic or cellulose paper.

5. Process according to claim 1 , wherein the colloidal suspension S 0 comprises from 10 −4 to 5×10 −1 mol/l of gold.

6. Process according to claim 1 , wherein step i) is carried out by mixing in a container comprising at least one inner surface suitable for receiving said mixture:

said colloidal suspension S 0 comprising NPs, said colloidal suspension S 0 comprising from 10 −4 to 5×10 −1 mol/l of gold, with

a solution S 0 ′ comprising said organic macromolecule, said solution S 0 ′ comprising from 10 −3 to 5 mol/l of organic macromolecule,

it being understood that:

the volume ratio: volume of the colloidal suspension S 0 /volume of the solution S 0 ′ ranges from 0.1 to 100, and

the resulting suspension has a height H in the container ranging from 0.1 to 30 mm.

7. Process according to claim 1 , wherein said process additionally comprises, between step i) and step ii), a step i-1) of applying the mixture from step i) to said support.

8. Process according to claim 6 , wherein the inner surface of said container acts as support for step ii) and step ii) is carried out by directly drying said container comprising said mixture.

9. Process according to claim 1 , wherein step ii) is carried out at a temperature ranging from 50° C. to 150° C.

10. Process according to claim 1 , wherein said process additionally comprises, after step ii), a step iii) of heat treatment at a temperature ranging from 20° C. to 200° C.

11. Process according to claim 1 , wherein said process additionally comprises, after step ii) or after step iii), a step iv) during which the film of dichromatic material is removed or unstuck from said support.

12. Process according to claim 1 , wherein the colloidal suspension S 0 is prepared beforehand according to the following steps:

A) a step of preparing a colloidal suspension comprising seeds of a metal chosen from gold, platinum, palladium, copper and silver, said metal seeds being in spherical form and having a diameter ranging from 1 to 30 nm, said aqueous colloidal suspension comprising a molar concentration of metal ranging from 5×10 −5 mol/l to 10 −2 mol/l, and

B) a step of preparing said colloidal suspension S 0 from the colloidal suspension comprising seeds from step A).

13. Process according to claim 12 , wherein step A) is carried out by the Turkevich-Frens method, by radiolysis or by irradiation.

14. Process according to claim 12 , wherein the seeds are gold seeds and step A) comprises:

a substep A 1 ) of heating to boiling point a solution S 1 comprising at least one gold salt in which the gold is in the (+III) or (+I) oxidation state, the molar concentration of [Au 3+ ] or [Au + ] gold ions in said aqueous solution S 1 ranging from 5×10 −5 mol/l to 10 −2 mol/l,

a substep A 2 ) of mixing the solution S 1 from substep A 1 ) with a solution S 2 comprising at least one reducing agent and optionally a stabilizer, the molar concentration of reducing agent in said solution S 2 [Reducing agent] ranging from 5×10 −5 mol/l to 5×10 −1 mol/l, it being understood that the molar ratio: number of moles of reducing agent/number of moles of Au 3+ or Au + gold ions ranges from 0.1 to 20,

a substep A 3 ) of maintaining the heating to boiling point of the mixture from substep A 2 ) until the gold (III) or the gold (I) has been completely reduced to gold (0), and

a substep A 4 ) of cooling to ambient temperature in order to obtain said colloidal suspension comprising gold seeds.

15. Process according to claim 12 , wherein the seeds are gold seeds and step B) comprises:

a substep B 1 ) of heating to boiling point a solution S 3 comprising at least one gold salt in which the gold is in the (+III) or (+I) oxidation state, the concentration of [Au 3+ ] or [Au + ] gold ions in said solution S 3 ranging from 10 −4 mol/l to 0.1 mol/l,

a substep B 2 ) of mixing the solution S 3 from substep B 1 ) with the colloidal suspension comprising seeds as obtained in step A) or in substep A 4 ), it being understood that the molar ratio defined by the number of moles of Au 3+ or Au + gold ions of the solution S 3 /the number of moles of Au 0 gold of the colloidal suspension of gold seeds ranges from 5 to 1000,

a substep B 3 ) of mixing a solution S 4 comprising at least one reducing agent and optionally a stabilizer with the mixture from substep B 2 ), the concentration of reducing agent in said solution S 4 [Reducing agent] ranging from 2×10 −5 mol/l to 2 mol/l, it being understood that the molar ratio: number of moles of reducing agent/number of moles of Au 3+ or Au + gold ions ranges from 0.1 to 10,

a substep B 4 ) of maintaining the heating to boiling point of the mixture from substep B 3 ) until the gold (III) or the gold (I) has been completely reduced to gold (0), and

a substep B 5 ) of cooling to ambient temperature in order to obtain said colloidal suspension S 0 .

16. Dichromatic material in the form of a translucent film as obtained according to the process defined in claim 1 , said dichromatic material comprising NPs and at least one organic macromolecule chosen from proteins, polysaccharides and synthetic polymers, wherein said material has two complementary colours distinct to the naked eye, one colour in transmission and one colour in backscattering and in that:

the NPs are in spherical form and have a diameter ranging from 70 to 100 nm, and

said film has a volume density ranging from 0.5×10 10 to 5.0×10 10 NPs/cm 3 for a 1 mm film thickness E.

17. Material according to claim 16 , wherein said material has a thickness E ranging from 10 μm to 10 mm.

18. A dichromatic material in the form of a translucent film as defined in claim 16 , wherein said material is a coating a transparent or translucent substrate.

19. A dichromatic material according to claim 18 , wherein the transparent or translucent substrate is made of glass, glass-ceramic, ceramic, plastic or cellulose paper.

20. A dichromatic material in the form of a translucent film as defined in claim 16 , is a means of authentication, decoration or protection.