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

Abstract

The invention relates to a process for preparing a dichromatic material, in the form of a translucent film, comprising monodisperse nanoparticles formed of gold and optionally of a noble metal chosen from platinum, palladium, silver and copper, and at least one organic macromolecule chosen from proteins, polysaccharides and synthetic polymers; said dichromatic material; and the uses thereof.

Claims

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.sub.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.sup.−4 to 5×10.sup.−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.sup.10 to 5.0×10.sup.10 NPs/cm.sup.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.sub.0 comprises from 10.sup.−4 to 5×10.sup.−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.sub.0 comprising NPs, said colloidal suspension S.sub.0 comprising from 10.sup.−4 to 5×10.sup.−1 mol/l of gold, with a solution S.sub.0′ comprising said organic macromolecule, said solution S.sub.0′ comprising from 10.sup.−3 to 5 mol/l of organic macromolecule, it being understood that: the volume ratio: volume of the colloidal suspension S.sub.0/volume of the solution S.sub.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.sub.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.sup.−5 mol/l to 10.sup.−2 mol/l, and B) a step of preparing said colloidal suspension S.sub.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.sub.1) of heating to boiling point a solution S.sub.1 comprising at least one gold salt in which the gold is in the (+III) or (+I) oxidation state, the molar concentration of [Au.sup.3+] or [Au.sup.+] gold ions in said aqueous solution S.sub.1 ranging from 5×10.sup.−5 mol/l to 10.sup.−2 mol/l, a substep A.sub.2) of mixing the solution S.sub.1 from substep A.sub.1) with a solution S.sub.2 comprising at least one reducing agent and optionally a stabilizer, the molar concentration of reducing agent in said solution S.sub.2 [Reducing agent] ranging from 5×10.sup.−5 mol/l to 5×10.sup.−1 mol/l, it being understood that the molar ratio: number of moles of reducing agent/number of moles of Au.sup.3+ or Au.sup.+ gold ions ranges from 0.1 to 20, a substep A.sub.3) of maintaining the heating to boiling point of the mixture from substep A.sub.2) until the gold (III) or the gold (I) has been completely reduced to gold (0), and a substep A.sub.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.sub.1) of heating to boiling point a solution S.sub.3 comprising at least one gold salt in which the gold is in the (+III) or (+I) oxidation state, the concentration of [Au.sup.3+] or [Au.sup.+] gold ions in said solution S.sub.3 ranging from 10.sup.−4 mol/l to 0.1 mol/l, a substep B.sub.2) of mixing the solution S.sub.3 from substep B.sub.1) with the colloidal suspension comprising seeds as obtained in step A) or in substep A.sub.4), it being understood that the molar ratio defined by the number of moles of Au.sup.3+ or Au.sup.+ gold ions of the solution S.sub.3/the number of moles of Au.sup.0 gold of the colloidal suspension of gold seeds ranges from 5 to 1000, a substep B.sub.3) of mixing a solution S.sub.4 comprising at least one reducing agent and optionally a stabilizer with the mixture from substep B.sub.2), the concentration of reducing agent in said solution S.sub.4 [Reducing agent] ranging from 2×10.sup.−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.sup.3+ or Au.sup.+ gold ions ranges from 0.1 to 10, a substep B.sub.4) of maintaining the heating to boiling point of the mixture from substep B.sub.3) until the gold (III) or the gold (I) has been completely reduced to gold (0), and a substep B.sub.5) of cooling to ambient temperature in order to obtain said colloidal suspension S.sub.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.sup.10 to 5.0×10.sup.10 NPs/cm.sup.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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a shows the gold seeds obtained at the end of step A) by transmission electron microscopy (TEM), from example 1 in accordance with one embodiment;

(2) FIG. 1b shows a UV-visible absorption spectrum of such a colloidal suspension, obtained through a 1 mm thick cuvette, from example 1 in accordance with one embodiment;

(3) FIG. 2a shows the gold nanoparticles obtained at the end of step B) by transmission electron microscopy (TEM), from example 1 in accordance with one embodiment;

(4) FIG. 2b shows a UV-visible spectrum of these NPs. They have a diameter of 85 nm approximately, from example 1 in accordance with one embodiment;

(5) FIGS. 3a and 3b show the film which is blue in transmission (FIG. 3a) and orange in backscattering (FIG. 3b), from example 1 in accordance with one embodiment;

(6) FIGS. 4a and 4b show the film which is blue in transmission (FIG. 4a) and orange in backscattering (FIG. 4b), from example 2 in accordance with one embodiment;

(7) FIG. 5a shows said gold nanoparticles by transmission electron microscopy (TEM), from example 2 in accordance with one embodiment; and

(8) FIG. 5b shows the film which is violet both in transmission and in backscattering; from example 2 in accordance with one embodiment.

DETAILED DESCRIPTION

Examples

(9) The raw materials used in the examples are listed below: potassium tetrachloroaurate KAuCl.sub.4, purity 99.995%, Sigma-Aldrich, Milli-Q ultra pure distilled water, resistivity 18.2 MΩ.Math.cm, Merck Millipore, trisodium citrate dihydrate, purity 99%, Sigma-Aldrich, polyvinyl alcohol, purity 99%, molar mass 86 000 g/mol, Sigma-Aldrich, gelatin powder, 24350.262, VWR, Prolabo.

(10) Unless otherwise indicated, all the materials were used as received from the manufacturers.

Example 1

Manufacture of a Dichromatic Material in Accordance with the Invention and as Obtained According to the Process in Accordance with the Invention

1.1 Preparation of the Aqueous Colloidal Suspension of Gold Seeds [Step A) of the Process in Accordance with the Invention]

(11) An aqueous solution comprising 9.45 mg of potassium tetrachloroaurate in 100 ml of ultra pure distilled water was prepared and brought to boiling. The concentration of [Au.sup.3+] gold ions in said aqueous solution was 0.25 mmol/l.

(12) An aqueous solution comprising 100 mg of trisodium citrate in 10 ml of ultra pure distilled water was prepared. The concentration of trisodium citrate in said aqueous solution was 34 mmol/l.

(13) 5 ml of the aqueous sodium citrate solution were added to the 100 ml of aqueous tetrachloroaurate solution.

(14) The resulting mixture was maintained at boiling until the gold (III) was completely reduced to gold (0) (the solution becomes red), then it was cooled to ambient temperature.

(15) An aqueous colloidal suspension of gold seeds in spherical form and having a diameter of 15 nm approximately was obtained and comprised 2.4×10.sup.−4 mol/l of gold.

(16) FIG. 1a shows the gold seeds obtained at the end of step A) by transmission electron microscopy (TEM). They have a diameter of 15 nm approximately.

(17) The images obtained by transmission electron microscopy in the present invention were produced using a microscope sold under the trade name 100 CX II by JEOL.

(18) FIG. 1b shows a UV-visible absorption spectrum of such a colloidal suspension, obtained through a 1 mm thick cuvette. This spectrum was obtained with a CARY 5000 spectrometer from Agilent.

1.2 Preparation of the Aqueous Colloidal Suspension of Gold Nanoparticles [Step B) of the Process in Accordance with the Invention]

(19) An aqueous solution comprising 3.78 mg of potassium tetrachloroaurate in 10 ml of ultra pure distilled water was prepared and brought to boiling. The concentration of [Au.sup.3+] gold ions in said aqueous solution was 1 mmol/l.

(20) 1 ml of the aqueous colloidal suspension of gold seeds as prepared in example 1.1 above was added to the aqueous tetrachloroaurate solution.

(21) The molar ratio: number of moles of Au.sup.3+ gold ions of the aqueous solution/number of moles of Au.sup.0 gold of the aqueous solution of gold seeds was 41.7.

(22) 150 μl of the aqueous sodium citrate solution previously prepared in example 1.1 were added to the aqueous solution of tetrachloroaurate and gold seeds.

(23) The resulting mixture was maintained at boiling until the gold (III) was completely reduced to gold (0) (the solution becomes brown/violet), then it was cooled to ambient temperature.

(24) An aqueous colloidal suspension of monodisperse gold nanoparticles in spherical form and having a diameter of 85 nm approximately was obtained and comprised 0.92 mmol/l of gold.

(25) FIG. 2a shows the gold nanoparticles obtained at the end of step B) by transmission electron microscopy (TEM).

(26) FIG. 2b shows a UV-visible spectrum of these NPs. They have a diameter of 85 nm approximately.

1.3 Preparation of a Dichromatic Material in Accordance with the Invention

(27) An aqueous solution comprising 4.4 g of polyvinyl alcohol PVA in 100 ml of ultra pure distilled water was prepared.

(28) The molar concentration of PVA in the aqueous solution was 1 mol/l.

(29) Next, in a crystallizing dish having a diameter of 3 cm, 2 ml of the aqueous colloidal suspension of gold nanoparticles as prepared in example 1.2 above was mixed with 3 ml of said aqueous PVA solution. The height H of the resulting colloidal suspension in the crystallizing dish was 8 mm.

(30) The concentration of gold in the mixture was 0.37 mmol/l.

(31) The molar concentration of PVA in the mixture was 0.6 mol/l.

(32) Next, the mixture was placed in an oven at 70° C. until the water had completely evaporated (10 hours approximately).

(33) The film obtained had a thickness E of 90 μm approximately.

(34) The film had a volume density of 1.2×10.sup.11 NPs/cm.sup.3 for a 90 μm film thickness E, which corresponds to a volume density of 1.08×10.sup.10 NPs/cm.sup.3 for a 1 mm film thickness E.

(35) The film obtained had an optical transmission coefficient of 32% approximately, measured by a conventional UV-visible spectrometer at 490 nm.

(36) FIG. 3 shows the film which is blue in transmission (FIG. 3a) and orange in backscattering (FIG. 3b).

Example 2

Manufacture of a Dichromatic Material in Accordance with the Invention and as Obtained According to the Process in Accordance with the Invention

(37) An aqueous solution comprising 10 g of gelatin in 20 ml of ultra pure distilled water was prepared.

(38) Next, in a crystallizing dish having a diameter of 3 cm, 3 ml of the aqueous colloidal suspension of gold nanoparticles as prepared in example 1.2 above was mixed with 2 ml of said aqueous gelatin solution. The height H of the resulting colloidal suspension in the crystallizing dish was 5 mm.

(39) The concentration of gold in the mixture was 0.37 mmol/l.

(40) Next, the mixture was placed in an oven at 70° C. until the water had completely evaporated (10 hours approximately).

(41) The film obtained had a thickness E of 2 mm approximately.

(42) The film had a volume density of 1.1×10.sup.10 NPs/cm.sup.3 for a 2 mm film thickness E, which corresponds to a volume density of 2.2×10.sup.10 NPs/cm.sup.3 for a 1 mm film thickness E.

(43) The film obtained had an optical transmission coefficient of 20% approximately, measured by a conventional UV-visible spectrometer at 490 nm.

(44) FIG. 4 shows the film which is blue in transmission (FIG. 4a) and orange in backscattering (FIG. 4b).

Comparative Example 3

Manufacture of a Material not in Accordance with the Invention

(45) An aqueous solution comprising 6.8 mg of potassium tetrachloroaurate in 20 ml of ultra pure distilled water was prepared and brought to boiling. The concentration of [Au.sup.3+] gold ions in said aqueous solution was 0.9 mmol/l.

(46) An aqueous solution comprising 100 mg of trisodium citrate in 10 ml of ultra pure distilled water was prepared. The concentration of trisodium citrate in said aqueous solution was 34 mmol/l.

(47) 0.8 ml of the aqueous sodium citrate solution were added to the 20 ml of aqueous tetrachloroaurate solution.

(48) The resulting mixture was maintained at boiling until the gold (III) was completely reduced to gold (0) (the solution becomes brown-red), then it was cooled to ambient temperature.

(49) An aqueous colloidal suspension of gold nanoparticles was obtained and comprised 0.86 mmol/l of gold.

(50) FIG. 5a shows said gold nanoparticles by transmission electron microscopy (TEM). They are roughly spherical and have a diameter of 30 nm approximately.

(51) An aqueous solution comprising 4.4 g of polyvinyl alcohol PVA in 100 ml of ultra pure distilled water was prepared.

(52) The molar concentration of PVA in the aqueous solution was 1 mol/l.

(53) Next, in a crystallizing dish having a diameter of 3 cm, 2 ml of the aqueous colloidal suspension of gold nanoparticles as prepared above was mixed with 3 ml of said aqueous PVA solution. The height H of the resulting solution in the crystallizing dish was 5 mm.

(54) The concentration of gold in the mixture was 0.35 mmol/l.

(55) The molar concentration of PVA in the mixture was 0.6 mol/l.

(56) Next, the mixture was placed in an oven at 70° C. until the water had completely evaporated (10 hours approximately).

(57) The film obtained had a thickness E of 90 μm approximately.

(58) The film obtained had an optical transmission coefficient of 40% approximately, measured by a conventional UV-visible spectrometer at 490 nm.

(59) FIG. 5b shows the film which is violet both in transmission and in backscattering. The dichromatic effect is not therefore observed.