ANTIMICROBIAL CELLULOSE-BASED MATERIAL

Abstract

A use, as antimicrobial material, of semicrystalline nano- and/or microfibrillated cellulose to which silver nanoparticles are attached, the silver nanoparticles being present in a weight amount strictly greater than 1% and strictly less than 20% relative to the total weight of said cellulose, the material being obtained with a process including at least one step of microwave irradiation of an aqueous dispersion of semicrystalline nano- and/or microfibrillated cellulose supplemented with at least one silver salt, in the presence of a reducing agent. An antimicrobial composition including such an antimicrobial material and to the application thereof for forming an antimicrobial film or coating, in particular for an article for food-processing.

Claims

1. A process for antimicrobial treatment using semicrystalline nano- and/or microfibrillated cellulose to which silver nanoparticles are attached, said silver nanoparticles being present in a weight amount strictly greater than 1% and strictly less than 20% relative to the total weight of said cellulose, said material being obtained with a process comprising at least one step of microwave irradiation of an aqueous dispersion of semicrystalline nano- and/or microfibrillated cellulose supplemented with at least one silver salt, in the presence of a reducing agent.

2. The process according to claim 1, wherein the amount of silver nanoparticles is greater than or equal to 2% by weight, relative to the total weight of said cellulose.

3. The process according to claim 1, wherein the amount of silver nanoparticles is less than or equal to 15% by weight, relative to the total weight of said cellulose.

4. The process according to claim 1, wherein the amount of silver nanoparticles is between 2% and 10% by weight, relative to the total weight of said cellulose.

5. The process according to claim 1, wherein the amount of silver nanoparticles is between 2% and 5% by weight, relative to the total weight of said cellulose.

6. The process according to claim 1, wherein the silver salt is silver nitrate (AgNO3).

7. The process according to claim 1, wherein the reducing agent is chosen from hydrazine, N,N-diethylhydroxylamine, urea, thiourea and mixtures thereof.

8. The process according to claim 1, wherein the aqueous dispersion used for preparing said antimicrobial material comprises a content of semicrystalline nano- and/or microfibrillated cellulose of between 1% and 5% by weight, relative to the total weight of the aqueous dispersion.

9. The process according to claim 1, wherein the aqueous dispersion used for preparing said antimicrobial material comprises a content of semicrystalline nano- and/or microfibrillated cellulose of between 2% and 4% by weight, relative to the total weight of the aqueous dispersion.

10. The process according to claim 1, wherein the aqueous dispersion used for preparing said antimicrobial material comprises a content of semicrystalline nano- and/or microfibrillated cellulose of between 2% and 3% by weight, relative to the total weight of the aqueous dispersion.

11. The process according to claim 1, wherein the preparation of the semicrystalline nano- and/or microfibrillated cellulose to which silver nanoparticles are attached comprises at least the steps consisting in: (i) providing an aqueous dispersion of semicrystalline nano- and/or microfibrillated cellulose; (ii) adding at least one silver salt with stirring until said silver salt has totally dissolved; (iii) adding a reducing agent to the mixture thus obtained; and (iv) subjecting said mixture to microwave irradiation under conditions conducive to the attachment of said silver nanoparticles to the nano- and/or microfibrillated cellulose.

12. The process according to claim 1, wherein the microwave irradiation has energy of between 200 and 1000 W.

13. The process according to claim 1, wherein said semicrystalline nano- and/or microfibrillated cellulose to which silver nanoparticles are attached forms all or part of an antimicrobial film or coating.

14. The process according to claim 13, wherein said semicrystalline nano- and/or microfibrillated cellulose to which silver nanoparticles are attached forms all or part of an antimicrobial film or coating at the surface of a substrate, said substrate being a film.

15. The process according to claim 14, wherein said film is of cellulose-based nature.

16. An antimicrobial composition comprising at least: semicrystalline nano- and/or microfibrillated cellulose to which silver nanoparticles are attached in a weight amount strictly greater than 1% and strictly less than 20% relative to the total weight of said cellulose, obtained according to the process defined according to claim 1; and at least one compound, termed mechanical reinforcement, capable of improving the mechanical properties of a coating formed from said composition.

17. The composition according to claim 16, in which said mechanical reinforcement(s) is (are) chosen from polymer nanoparticles.

18. The composition according to claim 17, wherein said polymer nanoparticles are nanoparticles of natural or synthetic latex, polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and mixtures thereof.

19. The composition according to claim 16, wherein said mechanical reinforcement(s) is (are) present in a content of between 0.5% and 5% by weight relative to the total dry weight of antimicrobial composition.

20. The composition according to claim 16, said composition comprising one or more surfactant(s).

21. The composition according to claim 20, comprising non-ionic surfactant(s).

22. The composition according to claim 20, wherein said surfactant(s) is/are chosen from polyoxyethylene sorbitan monooleate, polyoxyethylene sorbitan monolaurate, octoxinol 10, polyethylene-polypropylene glycol, n-dodecyl--D-maltoside (DDM), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate and mixtures thereof.

23. The composition according to claim 16, said composition being in liquid form or in solid form.

24. The composition according to claim 16, said composition comprising one or more aqueous solvents.

25. An antimicrobial film or coating formed from a composition as defined according to claim 16.

26. An antimicrobial film or coating according to claim 25, said film or coating having a thickness of between 1 and 500 m.

27. A process for preparing an antimicrobial film or coating as defined according to claim 25, comprising at least one step (a) of applying, via the aqueous route, a composition, and a drying step (b) suitable for evaporating off said aqueous solvent(s).

28. The process according to claim 27, wherein said composition is applied in step (a) by dip-coating, by spraying, spin depositing, spray-coating, inkjet coating, spin-coating, slot die coating, coating by impregnation, flow-coating, depositing by dipping or by screen printing, depositing by size-press, bar-coating.

29. The process that is of use for conferring antimicrobial properties on a substrate, comprising at least the application, at the surface of said substrate, of a film or coating as defined according to claim 25.

30. The process according to claim 29 wherein said substrate is a film or paper of cellulose-based nature.

31. An article, comprising a substrate coated with at least one antimicrobial film or coating as defined according to claim 25.

32. An article according to claim 31, said article being for food processing.

33. An article according to claim 31, wherein said substrate is a food film based on cellulose.

Description

FIGURES

[0128] FIG. 1: illustration of the two known types of semicrystalline nanocelluloses CNC and NMFC;

[0129] FIG. 2: X-ray diffraction spectra obtained for a sample of nanocellulose, before and after anchoring of silver nanoparticles (5% by weight) according to the process of example 1;

[0130] FIG. 3: 3d Ag XPS spectrum obtained for a sample of nanocellulose after anchoring of silver nanoparticles according to the process of example 1;

[0131] FIG. 4: observation of the antibacterial efficacy of the samples of nanocellulose treated according to examples 1 and 2, with respect to Gram-positive bacteria (Bacillus subtilis);

[0132] FIG. 5: observation of the antibacterial efficacy of the samples of untreated nanocellulose and of the samples of nanocellulose incorporating silver nanoparticles (5% by weight) and gold nanoparticles (5% by weight), prepared according to examples 1 and 2, with respect to Gram-positive bacteria (Bacillus subtilis) and Gram-negative bacteria (Escherichia coli).

EXAMPLES

Example 1

[0133] Semicrystalline Fibrillated Nanocellulose Incorporating Silver Nanoparticles

[0134] The fibrillated nanocellulose (5 ml at a w/v concentration of 2.2 g/100 ml (%), i.e. 110 mg) is diluted in deionized water (30 ml) with vigorous stirring. 8.7 mg of AgNO.sub.3 are added to the dispersion.

[0135] After obtaining total dissolution of the silver salt still with vigorous magnetic stirring, 1 drop of hydrazine (strong reducing agent) is added to the reaction medium. A color change is observed, typical of the formation of (d.sup.0) metal nanoagglomerates. The solution is then transferred into a Teflon minireactor and is subjected to microwave irradiation (1 minute at 750 W). After the treatment, the dispersion of nanocellulose to which (d.sup.0) silver nanoparticles are attached is recovered and is directly applicable.

[0136] According to the same protocol, various samples of nanocellulose incorporating variable amounts of silver nanoparticles (silver weight percentages of 0.1%, 0.5%, 1%, 2%, 3%, 4% and 20% relative to the dry weight of cellulose) were prepared by varying the amount of silver salt introduced.

[0137] Sample Analysis

[0138] The analysis of the samples by scanning electron microscopy SEM (LEO 1530 microscope, Electron Microscopy Ltd) and by transmission electron microscopy TEM (OSIRIS 1, Tecnai) makes it possible to confirm the presence of silver nanoparticles distributed in the nanocellulose.

[0139] An X-ray diffraction analysis (Bruker D8-Advance, copper XR source) of the nanocellulose, before and after anchoring of the silver nanoparticles (5% by weight) according to the process described above (FIG. 2) makes it possible to verify that the nanocellulose retains the same semicrystalline structure before and after attachment of the silver nanoparticles.

[0140] The silver nanoparticles are of face-centered cubic type.

[0141] An XPS analysis (X-ray photoelectron spectroscopy, VersaProbe II, Phi) makes it possible to verify that the nanoparticles created in the nanocellulose are indeed metal nanoparticles, since zero silver Ag.sup.0 is detected (FIG. 3). There is no longer any silver salt, but indeed silver metal nanoparticles.

[0142] Finally, observations by SEM of the samples during the preparation of the nanocellulose, prior and subsequent to the microwave-irradiation step, make it possible to observe a better dispersion of the silver nanoparticles in the samples after microwave irradiation. On the other hand, many agglomerates can be observed in the materials that have not been subjected to microwave irradiation, located more particularly at the surface of the samples.

[0143] Analysis of the Antibacterial Efficacy of the Samples

[0144] The bacteriostatic efficacy was tested on two types of bacteria: Gram-positive bacteria (Bacillus subtilis) and Gram-negative bacteria (Escherichia coli).

[0145] 750 l of bacteria (Bacillus subtilis or Escherichia coli) precultured in rich liquid medium (LB=Luria Bertani) having an OD.sub.600 nm=0.7-0.8 are uniformly inoculated on a petri dish (LB-agar=Luria Bertani-agar). After 30 minutes, the solid samples of nanocellulose (in the form of solid pellets) are deposited on the inoculated petri dishes. The whole is then incubated at 37 C. for 24 h to 72 h.

[0146] Conclusion

[0147] FIG. 4 is an image of the Gram-positive bacteria (Bacillus subtilis) cultures observed 24 hours after depositing of the samples of nanocellulose incorporating varied contents of silver nanoparticles.

[0148] Observation of the cultures shows an area of inhibition (clear ring characteristic of the absence of bacteria) around the samples of nanocellulose incorporating 2%, 3%, 4% and 5% by weight of silver.

[0149] Similar results are obtained for the cultures of Gram-negative bacteria (Escherichia coli).

[0150] Thus, the nanocelluloses prepared according to the invention, incorporating more than 1 by weight of silver nanoparticles, act as antibacterial material with respect to Gram-positive and Gram-negative bacteria.

Example 2 (Counter Example)

[0151] Semicrystalline Fibrillated Nanocellulose on which Gold Nanoparticles are Anchored

[0152] By way of comparison, nanocellulose incorporating gold nanoparticles in a proportion of 5% by weight are prepared according to the same protocol as that described above in example 1.

[0153] The fibrillated nanocellulose (5 ml at a w/v concentration of 2.2 g/100 ml (%), that is to say 110 mg) is diluted in deionized water (30 ml) with vigorous stirring. 9.5 mg of HAuCl.sub.4 are added to the dispersion.

[0154] After obtaining total dissolution of the gold salt still with vigorous magnetic stirring, 1 drop of hydrazine (strong reducing agent) is added to the reaction medium. A color change is observed, typical of the formation of (d.sup.0) metal nanoagglomerates. The solution is then transferred into a Teflon minireactor and is subjected to microwave irradiation (1 minute at 750 W). After the treatment, the dispersion of nanocellulose to which (d.sup.0) gold nanoparticles are attached is recovered and is directly applicable.

[0155] Analysis of the Antibacterial Efficacy

[0156] The bacteriostatic efficacy was tested on two types of bacteria; Gram-positive bacteria (Bacillus subtilis) and Gram-negative bacteria (Escherichia coli), as described in example 1.

[0157] Observation of the cultures, 24 hours after introduction of the nanocellulose incorporating gold nanoparticles, shows the absence of any area of inhibition, which indicates that it does not have antibacterial properties, unlike the sample of nanocellulose incorporating one and the same content of silver nanoparticles (FIG. 5).

Example 3

[0158] Antibacterial Compositions

[0159] Antibacterial compositions, incorporating, in addition to the nanocellulose treated according to the invention, mechanical reinforcements (PVP, PVA, latex beads) and optionally a detergent (Triton X 100), were prepared as described below.

[0160] A first series of samples (4 different samples) is prepared by adding, to the dispersion of nanocellulose incorporating 5% by weight of silver nanoparticles, obtained at the end of the process described in example 1, 5% by weight respectively of polyvinylpyrolidone (PVP), of polyvinyl alcohol (PVA) or of latex beads of 100 nm or of 2 m.

[0161] A second series of samples (4 different samples) is prepared by adding, to the dispersion of nanocellulose incorporating 5% by weight of silver nanoparticles, obtained at the end of the process described in example 1, 5% by weight of each of the reinforcements described above for the first series of samples and 5% by weight, relative to the total weight of nanocellulose, of Triton X 100 detergent.

[0162] The antibacterial efficacy of the various compositions thus prepared was tested on the Gram-positive and Gram-negative bacteria, as described in example 1 above.

[0163] The compositions of the first series comprising, in addition to the nanocellulose, mechanical reinforcements, without addition of Triton X 100, show an antibacterial efficacy, after 24 hours of incubation, that is lower than that observed for the dispersion of nanocellulose integrating 5% by weight of silver and free of mechanical reinforcements. In fact, an area of inhibition (ring) around the deposited samples that is less transparent can be observed, this being a sign that not all of the bacteria were destroyed.

[0164] The compositions of the second series incorporating reinforcements and supplemented with Triton X 100 show an antibacterial efficacy, after 24 hours, that is comparable to that observed for the dispersion of nanocellulose integrating 5% by weight of silver in the absence of mechanical reinforcement. In fact, an area of inhibition around the deposited samples that is transparent can be observed; no bacterium remains.

[0165] 48 hours and 72 hours after incubation, all the samples of the first and second series of nanocellulose incorporating 5% by weight of Ag and mechanical reinforcements, with or without the additional presence of detergent, show a transparent area of inhibition, and thus have good antibacterial activity.

REFERENCES

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