ANTI-MICROBIAL COATING

Abstract

The present invention relates to an antimicrobial coating of a substrate, the coating being obtained by applying the coating on a surface of the substrate by means of an electrostatic spraying method, and the coating comprising at least one metal oxide and/or at least one metal salt.

Furthermore, the present invention relates to an electrostatic spraying method for coating at least one substrate with an antimicrobial coating.

In addition, the present invention relates to a use of a coating material for producing an antimicrobial coating on a surface of a substrate, with the coating comprising at least one metal oxide and/or at least one metal salt.

Claims

1. An antimicrobial coating of a substrate, wherein the coating is obtained by applying the coating on a surface of the substrate by means of an electrostatic spraying method, and wherein the coating comprises at least one metal oxide and/or at least one metal salt.

2. The antimicrobial coating according to claim 1, wherein the coating comprises at least one complex compound.

3. The antimicrobial coating according to claim 1, wherein the structure of the metal oxide is described by the formula A.sub.cO.sub.d, wherein A is selected from the elements of group 4 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen, wherein c and d, independently of each other, can assume a value between 0 and 24.

4. The antimicrobial coating according to claim 3, wherein the structure of the metal oxide is described by the formula AO.sub.2, wherein A is selected from the elements of group 4 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen, the metal oxide being in particular TiO.sub.2, ZrO.sub.2 or HfO.sub.2.

5. The antimicrobial coating according to claim 1, wherein the structure of the metal oxide is described by the formula Me.sub.eO.sub.d, wherein Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen, wherein d and e, independently of each other, can assume a value between 0 and 24.

6. The antimicrobial coating according to claim 2, wherein the structure of the complex compound is described by the formula A.sub.cB.sub.dX.sub.nMe.sub.eB.sub.f or X.sub.nMe.sub.eB.sub.f, wherein A is selected from the elements of group 4, B is selected from the elements of group 15 or 16, X is selected from the elements of groups 5, 7, 8, 9, 10, 11, 12, 13, 14, from the lanthanides or the actinides, and Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature), and wherein c, d, n, e and f, independently of one another, can assume a value between 0 and 24.

7. The antimicrobial coating according to claim 6, wherein the structure of the complex compound is described by the formula AO.sub.2X.sub.nMeO.sub.4 or X.sub.nMeO.sub.4, wherein A is selected from the elements of group 4, X is selected from the elements of groups 5, 7, 8, 9, 10, 11, 12, 13, 14, from the lanthanides or the actinides, and Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen, wherein n can assume a value between 0 and 24, and the complex compound comprising in particular molybdates, tungstates or chromates.

8. The antimicrobial coating according to claim 2, wherein the structure of the complex compound is described by the formula AO.sub.2Me.sub.eO.sub.d, wherein A is selected from the elements of group 4, Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen, wherein d and e, independently of each other, can assume a value between 0 and 24.

9. The antimicrobial coating according to claim 1, wherein the structure of the metal oxide and/or of the metal salt is described by the formula AO.sub.2XBO.sub.3 or XBO.sub.3, wherein A is selected from the elements of group 4, X is selected from the elements of groups 5, 7, 8, 9, 10, 11, 12, 13, 14, from the lanthanides or the actinides, and B is selected from the elements of group 15 or 16 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen, and the metal oxide and/or the metal salt being in particular TiO.sub.2AgNO.sub.3 or AgNO.sub.3.

10. The antimicrobial coating according to claim 1, wherein the coating is designed in the form of a matrix structure which comprises a plurality of islands spaced apart from one another, and wherein the islands have a diameter in a range in particular from about 0.1 m to about 500 m, and wherein the islands are each spaced apart from one another in accordance with their diameter.

11. The antimicrobial coating according to claim 10, wherein the islands comprise TiO.sub.2 and ZnMoO.sub.4 or wherein that the islands have a surface which is formed like a pan with a central region and an edge region rising radially outwards with respect thereto or wherein the islands have a convex surface which is formed with a central region and an edge region that flattens out radially outwards with respect thereto or wherein the surface of the islands has a wrinkled structure, the wrinkles each having a width of in particular about 10 m, so that the surface of the islands of the matrix structure is enlarged.

12. (canceled)

13. (canceled)

14. (canceled)

15. The antimicrobial coating according to claim 1, wherein the surface of the coating has hydrophilic properties.

16. The antimicrobial coating according to claim 15, wherein the antimicrobial properties of the coating are available independently of light incidence.

17. The antimicrobial coating according to claim 15, wherein the antimicrobial properties of the coating are enhanced by light incidence.

18. An electrostatic spraying method for coating at least one substrate, comprising at least the following steps: providing a substrate; coating the substrate with an aqueous solution or suspension in droplet form by the electrostatic spraying method, the aqueous solution or suspension comprising at least one metal oxide and/or at least one metal salt soluble therein, whereby the aqueous solution or suspension has antimicrobial properties; and forming a solid, antimicrobial coating on the substrate in the form of a matrix structure by evaporation of the aqueous and/or liquid phase from the aqueous solution or suspension, so that the metal oxide and/or the metal salt is/are contained in the matrix structure of the coating.

19. The electrostatic spraying method according to claim 18, wherein the metal oxide, before addition to the aqueous solution or suspension, is present in the form of nanoparticles with an average size of in particular smaller than about 100 nm, and wherein the aqueous solution or suspension has a pH value of in particular smaller than or equal to approximately 6.8.

20. The electrostatic spraying method according to claim 19, wherein the metal oxide is comprised in the aqueous solution or suspension in a range in particular from about 0.005% to about 20%.

21. The electrostatic spraying method according to claim 20, wherein the aqueous solution or suspension comprises at least one complex compound.

22. The electrostatic spraying method according to claim 21, wherein, at least during the process of coating the substrate, the substrate is electrically positively or negatively charged and the droplets of the aqueous solution or suspension are electrically positively or negatively charged.

23. A method of using a coating material for producing an antimicrobial coating on a surface of a substrate, wherein the coating comprises at least one metal oxide and/or at least one metal salt.

24. The use according to claim 23, wherein the coating is an antimicrobial coating and/or the coating is obtained by an electrostatic spraying method.

25. (canceled)

26. (canceled)

Description

[0047] Further details and advantages of the invention will now be explained in more detail by means of the exemplary embodiments shown in the drawings wherein:

[0048] FIG. 1 shows in an enlarged SEM illustration a top view of a first exemplary embodiment of an antimicrobial coating according to the invention;

[0049] FIG. 2 shows in two enlarged illustrations in each case a top view of the first exemplary embodiment of the coating according to FIG. 1;

[0050] FIG. 3 shows in two enlarged perspective illustrations the hydrophilic properties of the first example of the coating according to FIG. 1;

[0051] FIG. 4 shows a general tabular characterization of the antimicrobial effectiveness (according to ISO 22196) of antimicrobial coatings;

[0052] FIG. 5 shows a tabular illustration of the antimicrobial effectiveness of further exemplary embodiments of an antimicrobial coating according to the invention;

[0053] FIG. 6 shows two further tabular illustrations of the antimicrobial effectiveness of further exemplary embodiments of an antimicrobial coating according to the invention;

[0054] FIG. 7 shows a further tabular illustration of the antimicrobial effectiveness of further exemplary embodiments of an antimicrobial coating according to the invention;

[0055] FIG. 8 shows a further tabular illustration of the antimicrobial effectiveness of further exemplary embodiments of an antimicrobial coating according to the invention;

[0056] FIG. 9a shows a schematic illustration of an exemplary embodiment of an electrostatic spraying method for obtaining an antimicrobial coating according to the invention;

[0057] FIG. 9b is an enlarged illustration of an island;

[0058] FIG. 10 shows a diagram of a comparison of the temporal germ reduction of an exemplary embodiment of the coating of the invention according to FIG. 5 in darkness and light;

[0059] FIG. 11 is a bar chart of the antimicrobial effectiveness of an exemplary embodiment of the coating of the invention according to FIG. 5 for a 2-fold, 3-fold and 4-fold coating of a Petri dish;

[0060] FIG. 12 is a bar chart with a comparison of the temporal germ reduction of three exemplary embodiments of the coating of the invention according to FIG. 5 in darkness and light;

[0061] FIG. 13a is a diagram of a comparison of the temporal germ reduction of two exemplary embodiments of the coating of the invention according to FIG. 5 in darkness and light;

[0062] FIG. 13b shows selected data points from FIG. 13a in tabular illustration;

[0063] FIG. 14a is a tabular illustration of the antimicrobial effectiveness of an exemplary embodiment of the coating according to FIG. 5 against the germ Staphylococcus aureus;

[0064] FIG. 14b shows a temporal reduction development of the germ Aspergillus fumigatus on an uncoated Petri dish and one coated with an exemplary embodiment of the coating according to FIG. 5;

[0065] FIG. 15 shows a temporal reduction development of the germ Candida albicans on an uncoated petri dish and one coated with an exemplary embodiment of the coating according to FIG. 5; and

[0066] FIG. 16a is a bar chart with a comparison of the temporal germ reduction of E. coli for six exemplary embodiments of the coating of the invention according to FIG. 5 and FIG. 6 in darkness.

[0067] FIG. 1 shows an enlarged plan view of a first embodiment of an anti-microbial coating 10 of a substrate 12 according to the invention.

[0068] The anti-microbial coating 10 of the substrate 12 is obtained by applying the coating 10 to a surface 14 of the substrate 12 by means of an electrostatic spray method.

[0069] The coating 10 contains at least one metal oxide.

[0070] The structure of the metal oxide is described by the formula A.sub.cO.sub.d.

[0071] Accordingly, A is selected from the elements of group 4 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen.

[0072] Furthermore, the indices c and d can independently of each other have a value between 0 and 24.

[0073] The structure of the metal oxide is described even more specifically by the formula AO.sub.2.

[0074] Here, A is also selected from the elements of group 4 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen.

[0075] The metal oxide is particularly TiO.sub.2 (or ZrO.sub.2 or HfO.sub.2).

[0076] The coating 10 according to FIG. 1 is formed in the form of a matrix structure in which the TiO.sub.2 is contained.

[0077] According to FIG. 1, this matrix structure has several islands 16 spaced apart to one another.

[0078] These islands 16 have a diameter in a range particularly from about 2 m to about 100 m.

[0079] In addition, the islands 16 are each spaced apart according to their diameter.

[0080] Besides TiO.sub.2, the islands 16 also contain AgNO.sub.3.

[0081] FIG. 2 shows two further enlarged representations of a respective plan view of the first embodiment of the coating according to FIG. 1.

[0082] A SEM analysis of an island shows a flat pan in the central area 18 with a clear elevation at the edges of the TiO.sub.2-islands.

[0083] Accordingly, the islands 16 have a surface that is pan-like with a central area 18 and an edge area 20 radially outwardly elevating thereto.

[0084] In a further embodiment (not shown in the figures) the islands 16 have a convex surface.

[0085] This surface is also formed with a central area and an edge area radially outwardly flattening thereto.

[0086] Furthermore, FIG. 2 shows the surfaces of the islands 16, which have a furrowed structure.

[0087] The furrowed structure is especially formed in the edge area 20 of the individual islands 16.

[0088] The furrows 22 each have a width of about 2 m, so that the surface of the islands 16 of the matrix structure is enlarged.

[0089] FIG. 3 also shows in two enlarged perspective views the hydrophilic properties of the first embodiment of coating 10 according to FIG. 1.

[0090] In this sense, the two representations of FIG. 3 show a comparison of an uncoated surface 12 (left) and a coated surface 12 (right).

[0091] The surface of coating 10 with the hydrophilic properties (right) has a clearly recognisable hydrophilic effect.

[0092] This is particularly evident in the visible flattening of the water droplet shape.

[0093] FIG. 4 shows a tabular characterization of the anti-microbial effectiveness of anti-microbial coatings in general.

[0094] The anti-microbial or anti-bacterial effectiveness of different coatings can be classified according to FIG. 4 as none, slight, significant and strong.

[0095] The reduction factor R.sub.L is used to quantify the anti-microbial effectiveness.

[0096] This reduction factor R.sub.L can be represented by the following mathematical relationship: R.sub.L=log (A/B).

[0097] Here, A corresponds to an average value of so-called colony forming units (CFU) per ml on a reference surface without anti-microbial coating.

[0098] Consequently, B corresponds to an average value of colony forming units (CFU) per ml on a reference surface with an anti-microbial coating according to the present invention.

[0099] The colony-forming units (CFU) can also be interpreted as the specific total germ count per ml.

[0100] The internationally recognized JIS test (Japanese Industrial Standard Test, JIS Z 2801), which corresponds to ISO standard 22196 in Europe, is used for objective assessment of the germ reducing effect of surfaces.

[0101] Thereby, Petri dishes coated with the test substance are first wetted with a germ suspension (e.g. E. coli or Staphylococcus aureus), covered with a foil and then incubated at 35 C. and 95% humidity.

[0102] Here, the experiments can be performed in the dark or under defined lighting conditions (e.g. by means of LED light at 1600 lux).

[0103] At the end of the incubation, the number of surviving germs is determined and a reduction factor R.sub.L is calculated as described above.

[0104] FIG. 5 shows a tabular representation of the anti-microbial effectiveness of further embodiments of an anti-microbial coating 10 according to the invention.

[0105] The anti-microbial effectiveness against E. coli bacteria is shown in FIG. 5 for a duration of 5 min in darkness and under defined light conditions at 1600 lux.

[0106] The embodiments of the respective anti-microbial coating 10 according to the invention as shown in FIG. 5 essentially have the same structural (macroscopic) and functional features as the embodiments shown in FIGS. 1 and 2.

[0107] Merely the following differences shall be discussed:

[0108] The structure of the metal oxide and metal salt or only the metal salt contained in the anti-microbial coating is generally described by the formula AO.sub.2XBO.sub.3 or XBO.sub.3 according to FIG. 5.

[0109] Wherein A is selected from the elements of group 4, X is selected from the elements of group 11, and B is selected from the elements of group 15 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen.

[0110] Particularly, the metal oxide and the metal salt are TiO.sub.2AgNO.sub.3 or the metal salt is AgNO.sub.3.

[0111] FIG. 6 shows a further tabular representation of the anti-microbial effectiveness of further embodiments of an anti-microbial coating 10 according to the invention.

[0112] The anti-microbial effectiveness against E. coli bacteria is shown in FIG. 6 for a duration of 5 min, 1 h and 24 h under defined light conditions at 1600 lux.

[0113] The embodiments of the respective anti-microbial coating 10 shown in FIG. 6 essentially have the same structural (macroscopic) and functional features as the embodiments shown in FIGS. 1 and 2.

[0114] Merely the following differences shall be discussed:

[0115] The coating 10 contains at least one complex compound.

[0116] The structure of the complex compound is generally described by the formula A.sub.cB.sub.dX.sub.nMe.sub.eB.sub.f or X.sub.nMe.sub.eB.sub.f.

[0117] Wherein A is selected from the elements of group 4, B is selected from the elements of group 15 or 16, X is selected from the elements of groups 5, 7, 8, 9, 10, 11, 12, 13, 14, the lanthanoids, or the actinides and Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature).

[0118] In addition, c, d, n, e and f can independently of each other take a value between 0 and 24.

[0119] Particularly, the structure of the complex compound is described by the formula AO.sub.2X.sub.nMeO.sub.4 or X.sub.nMeO.sub.4

[0120] Wherein A is selected from the elements of group 4, X is selected from the elements of groups 5, 7, 8, 9, 10, 11, 12, 13, 14, the lanthanoids, or the actinides, and Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen.

[0121] In addition, n can have a value between 0 and 24.

[0122] According to FIG. 6, the complex compound contains particularly molybdates or tungstates.

[0123] The molybdates comprise particularly (NH.sub.4).sub.6Mo.sub.7O.sub.24, Na.sub.2MoO.sub.4, Ag.sub.2MoO.sub.4, Al.sub.2(MoO.sub.4).sub.3, CeMoO.sub.4, CoMoO.sub.4, CuMoO.sub.4, Fe-III-MoO.sub.4, MnMoO.sub.4, NiMoO.sub.4 or ZnMoO.sub.4.

[0124] The anti-microbial coating 10 can be formed either from these molybdates or from a compound of these molybdates with TiO.sub.2.

[0125] As further shown in FIG. 6, the anti-microbial coating 10 can also include Mo.sub.03 or a compound of TiO.sub.2 and Mo.sub.03 instead of the molybdates.

[0126] The tungstates, on the other hand, comprise particularly Na.sub.2WO.sub.4, AgWO.sub.4, A.sub.1WO.sub.4, CeWO.sub.4, CoWO.sub.4, CuWO.sub.4, Fe-III-WO.sub.4, MnWO.sub.4, NiWO.sub.4 or ZnWO.sub.4.

[0127] The anti-microbial coating 10 can either be formed from these tungstates or from a compound of these tungstates with TiO.sub.2.

[0128] As can be additionally seen in FIG. 6, the anti-microbial coating 10 can also include WO.sub.3 or a compound of TiO.sub.2 and WO.sub.3.

[0129] FIG. 7 shows a further tabular representation of the anti-microbial effectiveness of further embodiments of an anti-microbial coating 10 according to the invention.

[0130] The anti-microbial effectiveness against E. coli bacteria is shown in FIG. 7 for a duration of 1 h and 24 h under defined light conditions at 1600 lux.

[0131] The embodiments of the respective anti-microbial coating 10 shown in FIG. 7 essentially have the same structural (macroscopic) and functional features as the embodiments shown in FIGS. 1 and 2.

[0132] Merely the following differences shall be discussed:

[0133] The representation in FIG. 7 particularly serves to show the difference in the anti-microbial effectiveness of the anti-microbial coating, which on the one hand is formed from a tungstate and on the other hand is formed from this tungstate in combination with TiO.sub.2.

[0134] The tungstates according to FIG. 7 include particularly AgWO.sub.4, AlWO.sub.4, CeWO.sub.4, CuWO.sub.4, or ZnWO.sub.4 or these tungstates in combination with TiO.sub.2.

[0135] In addition, the second last or last line of the table shown in FIG. 7 shows another metal oxide and another complex compound.

[0136] The structure of this metal oxide is described by the formula Me.sub.eO.sub.d.

[0137] Wherein Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen.

[0138] In addition, d and e can independently of each other have a value between 0 and 24.

[0139] The metal oxide as shown in FIG. 7 is particularly WO.sub.3.

[0140] The structure of the complex compound, however, is described by the formula AO.sub.2Me.sub.eO.sub.d.

[0141] Wherein A is selected from the elements of group 4, Me is selected from the elements of group 6 of the periodic table of the elements (IUPAC nomenclature) and O is the element oxygen.

[0142] In addition, d and e can independently of each other take a value between 0 and 24.

[0143] The complex compound according to FIG. 7 is particularly WO.sub.3*TlO.sub.2.

[0144] FIG. 8 shows a further tabular representation of the anti-microbial effectiveness of further embodiments of an anti-microbial coating 10 according to the invention.

[0145] The anti-microbial effectiveness against E. coli bacteria is shown in FIG. 8 for a duration of 1 h under defined light conditions at 1600 lux.

[0146] The embodiments of the respective anti-microbial coating 10 shown in FIG. 8 essentially have the same structural (macroscopic) and functional features as the embodiments shown in FIGS. 1 and 2.

[0147] Merely the following differences shall be discussed:

[0148] The representation in FIG. 8 serves particularly to show the difference in the anti-microbial effectiveness of this anti-microbial coating 10 with different compositions.

[0149] This coating 10 includes particularly ZnCrO.sub.4, ZnMoO.sub.4 or ZnWO.sub.4.

[0150] On the one hand, chromium oxide has a strong toxic effect.

[0151] But, the composition of zinc chromate (ZnCrO.sub.4) according to K.sub.2CrO.sub.4+Zn(NO.sub.3).sub.2-->ZnCrO.sub.4+2 KNO.sub.3 should complete the principle of the anti-microbial effect of metal acids in the form of MeXO.sub.4 of group 6 of the periodic table of the elements (IUPAC nomenclature).

[0152] FIG. 9a shows a schematic representation of an embodiment of an electrostatic spray method for obtaining an anti-microbial coating 10, 10, 10, 10, 10 according to the invention.

[0153] The electrostatic spray method for coating at least one substrate 12 comprises the following steps: [0154] providing a substrate 12; [0155] coating the substrate 12 with an aqueous solution or suspension 24 in droplet form by the electrostatic spray method, the aqueous solution or suspension 24 containing at least one metal oxide and/or at least one metal salt soluble therein, whereby the aqueous solution or suspension 24 has anti-microbial properties; and [0156] Formation of a solid, anti-microbial coating 10, 10, 10, 10, 10 on the substrate 12 in the form of a matrix structure by evaporation of the aqueous and/or liquid phase from the aqueous solution or suspension 24, so that the metal oxide and/or the metal salt are contained in the matrix structure of the coating 10, 10, 10, 10, 10.

[0157] Before addition to the aqueous solution or suspension 24, the metal oxide is present in the form of nanoparticles with an average size of less than or equal to about 10 nm.

[0158] The aqueous solution or suspension 24 has a pH value of less than or equal to about 1.5.

[0159] Further, the metal oxide is contained in the aqueous solution or suspension 24 in a range, particularly, of about 0.1% to about 2%.

[0160] The aqueous solution or suspension 24 may also contain a complex compound.

[0161] In FIG. 9a it is also shown that during the coating of substrate 12 the microdroplets with the TiO.sub.2 dissolved therein are electrically positively charged.

[0162] FIG. 9b shows an enlarged representation of an island 16 in this respect.

[0163] Particularly, it shows a 500-fold magnification of TiO.sub.2 (mass concentration: 15 g/L) therein after drying under the light microscope.

[0164] It may be intended that several droplets of 26 can be combined to form a large structure.

[0165] The effectiveness of the respective embodiment of the coating 10, 10, 10, 10, 10 according to the invention can now be described as follows on the basis of several experimental results: [0166] In all experiments executed, water-soluble nano titanium dioxide (average particle size of less than or equal to about 10 nm) is used in an aqueous solution 24 with a nitrate content of about 28% (pH=about 1.5).

[0167] The moisture content is 2%.

[0168] Ultimately, this behaviour determines the basic idea of converting water-soluble TiO.sub.2 after application in the form of small droplets 26 with the electrostatic spray method described above into a solid matrix into which both soluble and insoluble (complex) compounds with a germ reducing effect can be introduced.

[0169] The deposited microdroplets 26 evaporate very quickly at room temperature within only 1-2 min and leave a transparent TiO.sub.2 matrix in the form of small islands 16 (cf. FIGS. 1 and 2).

[0170] Another important aspect that guided to the idea of working with water-soluble titanium dioxide as the basic matrix is the property of this oxide, as described in the literature, that after deposition from the aqueous-acidic environment (pH<about 6.8) it retains this positive charge even in the dry state.

[0171] Consequently, species in the form of TiO(H.sup.+)Ti as well as OTi.sup.+O occur.

[0172] Compounds with a permanent positive charge (e.g. quaternary ammonium compounds such as PHMB) are known to energize bacteria having a negative polar outer shell and thus preventing them from being transported back into the ambient air.

[0173] In addition, the positive charge leads to a structural change of the bacterial membrane and a dysfunction of the ion channels.

[0174] Therefore, cell homeostasis is brought out of balance and the microorganism dies.

[0175] As a result of the hydrophilic properties of the TiO.sub.2 matrix (see FIG. 3), such hydrophilic surfaces of the substrate 12 have the advantage, contrary to hydrophobic surfaces, that they bind bacteria on the surface and prevent back transfer away from the coating surface.

[0176] They also facilitate cleaning, as a monomolecular water layer is formed between the dirt (i.a. cell debris) and the surface.

[0177] This property is an important first step towards improved room hygiene.

[0178] In this context, FIG. 10 shows a diagram with a comparison of the temporal germ reduction of E. coli for an embodiment of the coating according to the invention pursuant to FIG. 5 in the form of TiO.sub.2.

[0179] To determine the experimental results for the germ reduction in the TiO.sub.2 matrix as shown in FIG. 10, a suspension of about 15 g/L of dissolved TiO.sub.2 (pH=about 1.5) was sprayed twice onto Petri dishes and incubated with E. coli bacteria (JIS test Z 2801 or ISO standard 22196) for 0 to 24 hours in the dark as well as under defined LED lighting conditions (1600 lux=white office light).

[0180] The result confirms a two-phase reduction with a rapid loss of vitality within the first hour and a further slow, essentially linear reduction between 1 h and 24 h.

[0181] The two curves also show the same progression under dark and light conditions and lead to a strong effectiveness after 24 h (R.sub.L>3.5).

[0182] It can therefore be concluded that under both conditions the anti-microbial properties of the coating 10 are present independently of UV light incidence.

[0183] In methods with underlying electron transfer (redox reaction) or electron excitation (photocatalysis) a rapid death of the microorganisms would be expected.

[0184] Especially in the latter method, a curve progression would be expected which differs significantly from that of the dark reaction.

[0185] In this respect, experiments with e.g. potassium iodide starch used therein at 1600 lux on surfaces coated with TiO.sub.2 give no indications of the formation of a blue iodine-starch complex according to the reaction: 2 J.sup.+2 h.sup.+--->J.sub.2 (h.sup.+: electron hole); J.sub.2+starch--->J.sub.2 starch (blue).

[0186] Furthermore, FIG. 11 shows a bar chart of the anti-microbial effectiveness of an embodiment of the inventive coating 10 according to FIG. 5 for 2-times, 3-times and 5-times coating of a Petri dish.

[0187] The anti-microbial coating contains a matrix of TiO.sub.2 and silver nitrate TiO.sub.2*AgNO.sub.3.

[0188] The anti-microbial effectiveness of the coating against E. coli bacteria is shown in FIG. 11 after an incubation duration of 24 h under defined light conditions at 1600 lux.

[0189] In the first step, silver changes the tertiary structure of the bacterial outer membrane.

[0190] This increases their permeability, whereupon sulphur-containing enzymes of the respiratory chain and proteins responsible for DNA replication are inactivated consequently and the microorganism consequently dies.

[0191] Since this leads to a complete standstill of the cell homeostasis, which is important for survival, silver is not suspected of forming resistance.

[0192] For the TiO.sub.2*AgNO.sub.3 matrix described here, 500 mg AgNO.sub.3 were dissolved in a TiO.sub.2 suspension (about 15 g/L) and applied to square aluminium plates (11 cm2) by using electrosprays (experimental set-up not shown in the attached figures).

[0193] Since it is important to achieve a long-lasting anti-microbial effect by introducing silver ions, in a first study Petri dishes coated several times (2-times, 5-times, 10-times) with TiO.sub.2*AgNO.sub.3 rested for 2 days and were mixed with hydrochloric acid after decanting the water.

[0194] In none of the cases silver chloride (AgCl) could be detected here.

[0195] Furthermore, it is shown that the structure of the deposited TiO.sub.2*AgNO.sub.3 matrix is stable against 1000-times wiping with an anti-septic cloth (ethanol, benzalkonium chloride).

[0196] Thus, e.g. with a cleaning of the surface of the anti-microbial coating once a day, a lifetime of about three years can be achieved.

[0197] Already first experiments showed a strong antibacterial effectiveness (R.sub.L>3) of the TiO.sub.2*AgNO.sub.3 matrix against E. coli bacteria after 24-hour incubation according to the JIS test (see FIG. 11).

[0198] Subsequently, the strong anti-microbial and anti-bacterial effectiveness was confirmed for a period of 30 min to 24 h.

[0199] In addition, FIG. 12 shows two bar diagrams with a comparison of the temporal germ reduction of a coating containing TiO.sub.2*AgNO.sub.3 and its individual components according to FIG. 5 in darkness and brightness.

[0200] A detailed investigation of TiO.sub.2*AgNO.sub.3 and its individual components shows after 5 min incubation that the strong effectiveness of TiO.sub.2*AgNO.sub.3 in the dark (R.sub.L=3.31.0) is dominated by the anti-microbial effectiveness of the silver cations (R.sub.L=4.3).

[0201] TiO.sub.2 itself shows a significant effectiveness at this time (R.sub.L=2.1).

[0202] Even if the evaluation according to colony forming units per ml (CFU/ml) gives the impression that TiO.sub.2 develops a stronger effectiveness under light than in the dark, the result of the R.sub.L value at 1600 lux (R.sub.L=2.10.9) does not show a clear tendency.

[0203] For a better understanding of the mechanism of action, two kinetics each of the anti-microbial coating with TiO.sub.2 alone and in combination with TiO.sub.2*AgNO.sub.3 therefore were carried out under different light conditions according to FIG. 13a.

[0204] Thus, FIG. 13a shows a diagram of a comparison of the temporal germ reduction of these two embodiments of the inventive coating 10 according to FIG. 5 in darkness and brightness (about 1600 Lux).

[0205] The evaluation is shown in percentages for a better overview.

[0206] According to FIG. 13a, 100% correspond to the respective starting concentration (about 510.sup.5 CFU).

[0207] The combination of TiO.sub.2*AgNO.sub.3 shows a very strong germ reduction within the first 5 minutes by up to >99.99%.

[0208] Remarkable is the very strong germ reduction at 1600 lux of 98.1% after 1 min and 99.859% after 3 min.

[0209] The respective lower reduction numbers of this TiO.sub.2*AgNO.sub.3 composition matrix at 1 min (71.3%) and after 3 min (88.1%) in the darkness provide a first proof of the involvement of a light-dependent mechanism of action.

[0210] The anti-microbial properties of this coating 10 therefore may be enhanced by UV light incidence.

[0211] On the other hand, with TiO.sub.2, the picture is not so clear.

[0212] Here, after 30 minutes, a clear difference in the reduction of germs at 1600 lux of 82% (cf. darkness: 68%) can be seen.

[0213] The reason for these fluctuations is due to the JIS test, which ultimately does not allow absolute numbers but a classification into not, slightly, significantly and strongly effective.

[0214] For compounds with effectiveness in the range 1.0<R.sub.L<3.0 the observed fluctuations are strongest, while the strongly effective AgNO.sub.3 provides reproducible R.sub.L values in the range 4.0-4.3.

[0215] For further illustration, FIG. 13b additionally shows the data points shown in FIG. 13a in tabular form.

[0216] The existing germ reducing property of TiO.sub.2*AgNO.sub.3 can be explained as follows

[0217] Firstly, via the proven hydrophilicity and the positive charge of the metal oxide TiO.sub.2 germs can be energized and retained.

[0218] In the second step, cations from the TiO.sub.2 matrix can change the tertiary structure of the bacterial outer membrane in such a way that this membrane becomes porous and the bacteria dies.

[0219] Secondly, cationic silver has a very high oxidation potential and is able to attack the outer membrane of the microorganisms by fast electron transfers, whereby additional sulphur-containing enzymes are chemically inactivated.

[0220] These are very fast methods in terms of time, which lead to a rapid death of the bacteria.

[0221] In further in-vitro experiments a strong effectiveness of TiO.sub.2*AgNO.sub.3 against the Gram-positive germ Staphylococcus aureus could be proven.

[0222] In this respect, FIG. 14a shows a tabular representation of the anti-microbial effectiveness of an embodiment of the coating 10 according to FIG. 5 against a Staphylococcus aureus germ.

[0223] The anti-microbial effectiveness of this coating 10 against the Staphylococcus aureus germ is shown in FIG. 14a after an incubation period of 24 hours under defined light conditions of about 1600 lux with R.sub.L=4.1.

[0224] For further evaluation of the potential effectiveness of TiO.sub.2*AgNO.sub.3 against the colonization of mould and yeast fungi, this combination with the anti-microbial coating was tested against these pathogenic germs under real conditions.

[0225] At this, coated and uncoated Petri dishes are dry-contaminated and the growth of germs is checked by means of a Contact-Slides method (RODAC method).

[0226] FIG. 14b shows in this respect a temporal reduction of an Aspergillus fumigatus germ on an uncoated Petri dish and a Petri dish coated with an embodiment of the coating 10 according to FIG. 5.

[0227] The anti-microbial coating in FIG. 14b shows a TiO.sub.2*AgNO.sub.3 matrix structure, as also shown in FIGS. 11 to 14a.

[0228] In a 24 h study, TiO.sub.2*AgNO.sub.3 coated Petri dishes (right figure in FIG. 14b) show a significant growth control within the first 4 h and a clear reduction of germs after 24 h compared to the uncoated reference Petri dish (left figure in FIG. 14b).

[0229] FIG. 15 shows a temporal reduction of a Candida albicans germ on an uncoated Petri dish and a Petri dish coated with an embodiment of the coating 10 as shown in FIG. 5.

[0230] The anti-microbial coating 10 in FIG. 15 also shows a TiO.sub.2*AgNO.sub.3 matrix structure as also shown in FIGS. 11 to 14b.

[0231] Also, during a 24 h study, the Petri dishes coated with TiO.sub.2*AgNO.sub.3 (right figure in FIG. 15) showed a strong reduction of Candida albicans germ by up to 4 logarithmic levels (R.sub.L=3.7) already after 4 hours incubation compared to the uncoated reference Petri dish (left figure in FIG. 14b).

[0232] The anti-microbial coating 10 of substrates according to FIGS. 11 to 15 with TiO.sub.2*AgNO.sub.3 is conceivable e.g. in outdoor areas (e.g. building walls, surfaces of public transport vehicles or road surfaces).

[0233] Therefore, the toxicological behaviour of this coating towards Daphnia (water flea) as well as Artemia nauplii (brine shrimp) was investigated.

[0234] Here, Petri dishes were coated 0-, 2-, 5- and 10-times with TiO.sub.2*AgNO.sub.3 and these aquatic organisms were cultivated for three days therein. As a result, these animals show the same vitality in the coated plates as in the uncoated ones.

[0235] After finishing the experiments, the biological matrix was filtered off and hydrochloric acid was added to the clear aqueous solution.

[0236] However, no formation of silver chloride according to the reaction Ag.sup.++Cl.sup.->AgCl could be observed.

[0237] This means that the silver ions are retained in the TiO.sub.2 matrix.

[0238] FIG. 16a also shows a bar chart with a comparison of the temporal E. coli germ reduction of six embodiments of the inventive coating 10, 10 according to FIG. 5 and FIG. 6 after 5 min incubation time in the darkness.

[0239] In this respect, the anti-microbial coating contains a combination of the TiO.sub.2 matrix with oxides and salts of group 6 (IUPAC nomenclature) of the periodic table of the elements.

[0240] Since chromium compounds are characterized by a very pronounced toxicity, the focus was on the oxides and salts of the elements molybdenum (Mo) and tungsten (W).

[0241] Their redox potential and acidic properties are mentioned as possible mechanisms of action.

[0242] In the search for alternatives to the soluble silver nitrate (AgNO.sub.3), initial experiments were therefore carried out with the slightly soluble zinc molybdate (ZnMoO.sub.4), which are shown in FIG. 16a.

[0243] The zinc molybdate (ZnMoO.sub.4) was applied alone (about 5.0 g/L) and in combination with the TiO.sub.2 matrix described above (about 15 g/L) to a substrate 12 via electrospray and tested against E. coli.

[0244] Z.sub.nMoO.sub.4 (R.sub.L=3.2) shows a weaker germ reducing effect compared to AgNO.sub.3 (R.sub.L=4.3), but a stronger germ reducing effect compared to TiO.sub.2 (R.sub.L=2.1).

[0245] Interestingly, the combination TiO.sub.2*ZnMoO.sub.4 is significantly more effective (R.sub.L=4.1) than the two individual components TiO.sub.2 and ZnMoO.sub.4.

[0246] This strong effectiveness cannot be increased even by adding AgNO.sub.3 to the TiO.sub.2*ZnMoO.sub.4 matrix.

[0247] Additional tests in combination with another germ Staphylococcus aureus confirm the germ reducing effectiveness of ZnMoO.sub.4 and TiO.sub.2*ZnMoO.sub.4 in the JIS test at 1600 lux light incidence.

[0248] Due to the anti-microbial potential of the substance class of molybdates as well as molybdenum oxide, the following compounds were each sprayed onto a substrate 12 in the form of an anti-microbial coating as described above and tested against E. coli bacteria within 24 hours incubation time and a light incidence of 1600 lux.

[0249] The corresponding molybdates and the molybdenum oxide and their anti-microbial effectiveness after 1 h and 24 h can be taken from FIG. 6.

[0250] In addition, ammonium heptamolybdate (NH.sub.4).sub.6Mo.sub.7O.sub.24 was tested, which has an anti-microbial effectiveness of 1.4 after 1 h and 4.3 after 24 h under these experimental conditions.

[0251] The ammonium heptamolybdate (NH.sub.4).sub.6Mo.sub.7O.sub.24 used for the synthesis has a significant effectiveness already after 1 h.

[0252] However, the sodium molybdate (Na.sub.2MoO.sub.4), which is also highly soluble, shows no anti-microbial effect even after 24 h (see FIG. 6).

[0253] In addition to the zinc molybdate (ZnMoO.sub.4) as described above, silver molybdate (Ag.sub.2MoO.sub.4) could be determined as a further strongly germ-reducing compound.

[0254] In analogy to the molybdates and the molybdenum oxide, the corresponding tungstates and tungsten oxide were synthesized and tested against E. coli in the JIS test at 1600 lux.

[0255] The starting material for the syntheses was sodium tungstate, which reacts with the soluble salts (chloride, nitrate, sulphate) of aluminium, cerium, cobalt, copper, nickel, manganese, silver and zinc to form slightly soluble salts of the form X.sub.nWO.sub.4.

[0256] The corresponding tungstates and the tungsten oxide and their anti-microbial effectiveness after 1 h and 24 h can be taken from FIGS. 6 and 7.

[0257] In analogy to sodium molybdate, tungsten molybdate shows no antimicrobial effectiveness after an incubation period of 1 h and 24 h as well.

[0258] With the exception of manganese tungstate, all other tungstates and even the tungsten oxide have a significant to strong effectiveness against E. coli.

[0259] In analogy to zinc molybdate, zinc tungstate (ZnWO.sub.4) alone and in the combination TiO.sub.2*ZnWO.sub.4 shows a strong antimicrobial effectiveness within 24 h.

[0260] These are also observed for silver tungstate (AgWO.sub.4), aluminium tungstate (AlWO.sub.4), cerium tungstate (CeWO.sub.4), copper tungstate (CuWO.sub.4) and for their respective combination with the TiO.sub.2 matrix.

[0261] Furthermore, tungsten oxide also has this strong effectiveness as well as its combination with the TiO.sub.2 matrix.

[0262] When combining the mixed suspensions of metal tungstate and TiO.sub.2, it is noticeable that the partly very colourful tungstates together with TiO.sub.2 form a colourless complex.

[0263] As examples, the combination of TiO.sub.2 with CeWO.sub.4 (yellow) and CuWO.sub.4 (green) are shown here.

[0264] These observations lead to the assumption that the partially positively charged TiO.sub.2 crystals form a complex of the form O(Ti).sup.+ . . . .sup.W(O.sub.4) or O(Ti).sup.+ . . . .sup.OW(O.sub.3) with the negatively charged tungstate anion.

[0265] Possibly a three-center complex may also be formed between the positively charged TiO.sub.2, the positive metal cation (e.g. Ce.sup.2+) and the tungstate anion.

[0266] In any case, the electronic states change in such a way that the colourfulness of the original tungstates is lost.

[0267] The yellow tungsten oxide (WO.sub.3) also leads to a colourless suspension with TiO.sub.2 through complexation.

[0268] In summary, it can be stated that due to the anti-microbial effectiveness of complexes of the type TiO.sub.2*X.sub.nMeO.sub.4 (Me=Cr, Mo or W; X=Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Re, Os, Ir, Pt, Au, Hg, as well as Ce and the lanthanides; n=0-24), they can be applied by means of electrospray to all types of surfaces and develop an anti-microbial effectiveness.

[0269] Such use of a coating material can thus be provided for producing an anti-microbial coating 10, 10, 10, 10, 10 as described above on a surface 14 of a substrate 12, said coating 10 containing at least one metal oxide and/or metal salt as described above.

[0270] The coating 10, 10, 10, 10, 10 is thereby obtained by an electrostatic spray method as described above.

[0271] The surface 14 of the coating 10, 10, 10, 10, 10 may be a work surface or may be in contact, at least temporarily, with ambient air, fluids or liquids.

[0272] Furthermore, TiO.sub.2*X.sub.nMeO.sub.4 can be added to lacquers and paints (e.g. anti-fouling) in the form of the suspension or as a solid after drying, thus giving them anti-microbial properties.

[0273] In this case, a TiO.sub.2*X.sub.nMeO.sub.4 complex is added to the coating material, which is especially designed as an anti-fouling lacquer or anti-fouling paint, in the form of a suspension or as a solid after drying.

[0274] The colourless complexes of the form TiO.sub.2*X.sub.nMeO.sub.4 can be incorporated into plastics (e.g. silicone, PU, etc.) or building materials (e.g. cement), which thus become anti-microbial.

[0275] Both the molybdates X.sub.nMoO.sub.4 and the tungstates X.sub.nWO.sub.4 are characterized by a very poor solubility.

[0276] These compounds show a strong precipitating effect in the suspensions with TiO.sub.2, which makes storage in an aqueous medium difficult and possibly leads to the fact that not always the correct concentration is transferred with the electrospray.

[0277] Both in the synthesis of molybdenum oxide from ammonium heptamolybdate and in the preparation of tungsten oxide from sodium tungstate under acidic conditions, it has been noticed that the resulting oxides are difficult to filter due to their gel-like character.

[0278] However, this observation helped to generate a suitable suspension for the above-mentioned poorly soluble compounds.

[0279] If the acidic TiO.sub.2 nano-suspension (pH=1.5) is first mixed with 50-150 mg ammonium heptamolybdate, visible streaks of TiO.sub.2 . . . MoO.sub.3 or MoO.sub.3*(H.sub.2O).sub.n are formed.

[0280] If ZnMoO.sub.4 is now added, it remains stable in abeyance over a longer period of time without precipitating.

[0281] This opens up new approaches for the representation with mixed components, which contain MoO.sub.3, WO.sub.3 and/or the above-mentioned salts in addition to the parent matrix TiO.sub.2.

[0282] With the help of these findings for improving the overall formulation, new combinations of TiO.sub.2 with further poorly soluble metal oxides (AgO, CuO, SiO.sub.2, ZnO) or matrix crosslinkers (Na.sub.2SiO.sub.4, Na.sub.2[B.sub.4O.sub.5(OH).sub.4]) are opened up.

[0283] These could have a positive effect on the anti-microbial effectiveness as well as on the age resistance and robustness of the deposited TiO.sub.2 matrix.

[0284] As an alternative to TiO.sub.2 it could be proven, for example, that the water-soluble nano-zirconium oxide ZrO.sub.2 can also be applied to a transparent matrix comparable to TiO.sub.2 (same group in periodic table of the elements) by means of the electrostatic spray method.

[0285] In an exemplary experiment on the transferability of the TiO.sub.2 matrix principle to ZrO.sub.2, 15 g/L nano-ZrO.sub.2 were dissolved with 0.5 g AgNO.sub.3 and tested, after spraying on, against E. coli for 1 h in the JIS test (1600 lux).

[0286] The effectiveness of this combination here is R.sub.L=4.3 (strong).

[0287] This proofed that ZrO.sub.2 can be used successfully as a replacement for TiO.sub.2 or in combination with it.

[0288] Similarly, hafnium oxide as a group relative of the IV. subgroup (Ti, Zr, Hf) should be usable.

LIST OF REFERENCE SYMBOLS

[0289] 10 Antimicrobial coating [0290] 12 Substrate [0291] 14 Surface of the substrate [0292] 16 Island [0293] 18 Central region [0294] 20 Edge region rising towards outside [0295] 22 Wrinkles [0296] 24 Aqueous solution or suspension [0297] 26 Droplet [0298] 10 Antimicrobial coating [0299] 10 Antimicrobial coating [0300] 10 Antimicrobial coating [0301] 10 Antimicrobial coating