Background art

Paints with antimicrobial properties are widely used, where the use of antimicrobials prolongs the life of the paint itself, especially by preserving water-based paints from mold attacks after opening and gives the surfaces to which said paints are applied antibacterial and antimicrobial properties for which there is a strong need. This need is strongly felt especially for painting facades, to prevent the onset of fungi and/or mold, as well as for painting interior environments such as hospitals, schools, premises for the preparation of food. Said paints are also widely used for painting objects and furnishings.

Paints with biocide additives are widely used, such as Carbendazim, Dichlofluanid, Isoproturon, Propiconazole, Tebuconazole, Terbutryn, Zinc pyrithione. Said active ingredients are also effective on particularly resistant bacteria, such as E. Coli. However, they have been shown to be responsible for the onset of bacterial resistance, especially in the case of spore-forming bacteria.

Alternatively, paints with photocatalytic properties have been used.

Among the most studied materials in photocatalysis, titanium dioxide, TiO2, has aroused the greatest interest, as it combines important features such as long-term stability and low toxicity for the biosphere, with good photocatalytic activity with respect to other semiconductors. The photocatalytic properties of TiO2 have been investigated in recent years on a wide range of both atmospheric and water pollutants: alcohols, halides, aromatic hydrocarbons. The studies carried out have given promising results for organic acids, dyes, NOx and other. These properties have been applied in the removal of bacteria and harmful organic materials in water and air, as well as from surfaces in particular places such as medical centers.

TiO2 exists in nature in amorphous and crystalline forms, the amorphous form is photocatalytically inactive. There are three natural crystalline forms of TiC : anatase, rutile and brookite. Anatase and rutile have a tetragonal structure, while the brookite structure is orthorhombic. Brookite is less common than the previous two crystal polymorphs and is much more difficult to obtain. Anatase and rutile are photocatalytically active, while brookite has never been tested for photocatalytic activity. Pure anatase is more active as a photocatalyst with respect to rutile, probably because it has a higher negative potential at the edge of the conductive band, which means higher photo-generation electron potential energy and also due to a higher number of -OH groups on the surface thereof.

The presence of SiO2, especially if mesoporous, to the extent of 5% mol, improves the photocatalytic activities due to an increase in surface area. A further methodology used to increase TiO2 activity relates to the possibility of intervening on the electronic levels of the semiconductor, decreasing the energy of the band-gap in order to be able to use light at a lower frequency, such as the Visible one, to promote the electrons from the valence band (BV) to the conduction band (BC). Such methodology sees the modification of the material through doping and consists of the introduction, during the synthesis step, of suitable precursors of elements capable of modulating the electronic properties thereof. The photocatalytic activity of anatase is strongly influenced by the dimension, but due to the toxicity shown by the nano dimension, many authors suggest the use of anatase in micrometric dimensions, although it decreases photocatalytic capacity.

From an electronic point of view, titanium dioxide is an n-type semiconductor; the Eg value of anatase is equal to 3.2 eV, that of rutile is 3.0 eV. From these values, it is clear from equation (1 ):

Eg=h v =hc/ A =1240/ A (1) that the anatase is "activated" by light having a wavelength A < 388 nm, i.e., from the UVA portion of the electromagnetic spectrum. In equation (1 ), h represents Planck’s constant, v the incident radiation frequency and c the speed of light in vacuum; the product he, constant term, is expressed in [eVxnm] and the wavelength A in nm.

Despite the advantages of the existing photocatalytic coatings based on TiO2, there is room for improvement in the art. In order to exert the effect thereof, some photocatalysts need to be pre-activated, for example by washing with water, adding a process step which makes the application of paints thus formulated inconvenient.

EP2188125B1 overcomes the pre-activation limit, describing an antibacterial photocatalytic paint comprising TiO2 in the form of anatase, having a crystalline particle size between 5 and 10 nm. However, the explosion in the use of nanomaterials has raised serious health and safety issues. One of these concerns the health effects of respirable nanoparticles.

US2006/0116279 describes an agglomerate comprising a larger particle (mother) which supports smaller particles (child). The smaller particles have an average diameter between 5 and 500 nm and contain TiO2 with photocatalytic activity. The agglomerate can be used in paints.

Therefore, there is a strong need to improve the photocatalysts currently available, in particular for the application thereof in the field of paints.

Description of the invention

In the present invention, with an innovative process which uses the physical conditions of an electrochemical process, a new nanostructured material with micrometric dimensions is obtained, which is suitable for producing crystals with dimensions, morphology, and structure such as to make the photocatalytic, anti-pollution, antibacterial and antiviral properties extraordinary.

Therefore, the photocatalytic additive according to the present invention is surprisingly capable of transforming inert substrates of any kind into a photocatalytic and antibacterial material by means of the biomimetic synthesis of nanosilicas functionalized with titanium-based precursors. Description of the drawings

Figure 1 : X-ray diffraction spectrum of the product according to the present invention.

Figure 2: (A) Scanning Electron Microscope (SEM) analysis of the product according to the present invention, representative images with two different magnifications; (B) representative spectrum obtained by subjecting the product according to the present invention to EDS spectroscopy (Energy Dispersive X-ray Spectrometry). Figure 3: Particle size analysis of the photocatalytic additive particles according to the present invention (gray line) and of the commercial anatase particles (gray line)

Figure 4: Representative graphs of the abatement of the bacterial load on plates painted and inoculated with E. Coli (A), P. aeuriginosa (B) or S. aureus (C).

Figure 5: (A) emission spectrum of the Philips PL-S 9W/2P BLB bulb used for radiating the samples; (B) emission spectrum of the 6500 K LED lighting system used for radiating the samples; concentration profiles for NO, NO2 and NOx (C) with UV radiation; (D) with visible radiation.

Detailed description

The present invention first relates to a photocatalytic additive comprising particles consisting of TiO2, at least one substrate and, optionally, one or more metals, where said particles have a size between 10 and 100 micrometers, and said constituents, that is said TiO2, said at least one substrate and, where present, said one or more metals, are organized together in a homogeneous network.

Said photocatalytic additive is a crystalline material which has the diffraction maxima characteristic of anatase (Figure 1 ).

In an embodiment, said photocatalytic additive is doped with one or more of the metals selected from the group comprising carbon (C), nitrogen (N), tungsten (W), iron (Fe), silver (Ag), copper (Cu), zinc (Zn), tin (Sn) and/or manganese (Mn).

In a further embodiment, said substrate comprises SiO2.

The SEM analysis of said additive loaded on a substrate according to the present invention shows that the same comprises clusters of substituted metal anatase microcrystals with a nanostructured hierarchical structure. A representative photo of an embodiment according to the present invention is shown in Figure 2A. The spectrum obtained with the EDS microanalysis (Figure 2B) of a sample according to an embodiment of the present invention shows that the elemental composition of the particles consists of Ti, Zn, Cu, Si. The images of Figure 2A show that said clusters consist of a homogeneous network, where the components of the particles, which the EDS microanalysis has shown to be, in the embodiment under analysis, Ti, Zn, Cu and Si, are complexed together in a network. Each area of a particle according to the present invention is similar to the other, i.e., the particles according to the present invention do not comprise areas in which a first component is concentrated, linked to areas in which a second component is concentrated but each particle consists of a structured network which sees the presence of the different components of the particle itself.

Therefore, the term "homogeneous network" means a structure comprising at least two components, where said at least two components are organized together so as to form a complex which does not make them separable and clearly distinguishable within said structure.

The particle size was analyzed by laser diffraction. In particular, Figure 3 shows the comparison data between particles according to the present invention (gray line) and commercial anatase particles (black line). The graph shows that the particle size distribution of the particles according to the present invention is between about 10 and about 100 micrometers, while the anatase particles according to the prior art see a distribution between 1 and 100 micrometers.

Advantageously, the particles according to the present invention, having a particle size distribution such as to considerably limit the presence of particles with a diameter lower than 10 pm, obviate the problems of the particles of the background art specifically linked to the nanometric dimensions of the particles.

The photocatalytic additive according to the present invention has shown advantageous antibacterial properties, as shown by the graphs of Figure 4. In fact, the photocatalytic additive has been shown to have a bactericidal effect on gram-positive and gram-negative bacteria. The photocatalytic additive according to the present invention has been shown to be capable of killing bacteria such as Escherichia, Pseudomonas and Staphylococcus.

Furthermore, the additive has amazing antiviral properties. In particular, the photocatalytic additive has been shown to be effective against Coronavirus.

Therefore, the present invention further relates to the use of the photocatalytic additive described herein for the abatement of the bacterial and/or viral load. Said use is particularly advantageous where said additive is used in coatings, such as paints, which cover the surface of interest.

In a further embodiment, said additive is advantageously used in cosmetic formulations, food compositions and textile items where it has proved useful in the antibacterial, antimicrobial, and antiviral function thereof.

The present invention further relates to a method for obtaining a photocatalytic additive comprising:

- providing an electrolytic cell;

- loading a substrate having a high surface area and free hydroxyl groups into said electrolytic cell;

- dripping the precursor of a semiconductor or the hydrolysis product thereof into said electrolytic cell;

- applying a voltage of 12-24 volts and a current of 0.01-0.10 A/cm ² to the electrodes of said electrolytic cell and keep stirring for a time between 1 and 8 hours, preferably between 2 and 4 hours, where, during said stirring, the polarity of said electrodes is frequently reversed;

- subjecting the preparation to heating at 120-140 °C, with a maximum pressure of 5 bar, for a time between 1 and 8 hours, preferably between 2 and 4 hours, obtaining said photocatalytic additive.

In an embodiment, the electrodes in said electrolytic cell comprising two electrodes preferably selected from Ti/Ti, Sn/Sn, Cu/Cu, Cu/Pt, Cu/Ag, Cu/Zn, Zn/Ag, Zn/Pt, Ag/Pt.

In an embodiment, said polarity reversal of the electrodes occurs with a frequency of about 1 or more times every 10 minutes, or 1 or more times every 15 minutes.

In an embodiment, said polarity reversal of the electrodes occurs with a frequency of about 1 time every 10 minutes, or 1 time every 15 minutes.

In an embodiment, said precursor is a titanium-based precursor, for example selected from the group comprising titanium (IV) oxysulfate, titanium tetrachloride, titanium tetraisopropoxide, titanium isopropoxide, titanium oxychloride.

In an embodiment, said precursor is subjected to acid or basic hydrolysis before being dripped into said electrochemical cell.

In an embodiment, said hydrolysis product of the titanium-based precursor is dripped into said electrochemical cell so as to obtain a suspension of Ti ions in a concentration between 5 - 15% (w/V), preferably about 12% (w/V).

In an embodiment, to obtain a photocatalytic additive doped with chemical elements or oxides, such as carbon (C), nitrogen (N), tungsten (W), iron (Fe), Silver (Ag), copper (Cu), Zinc (Zn), Tin (Sn), Manganese (Mn), said method further comprises the addition in said electrochemical cell of said metals, preferably in the form of hydroxides.

In a preferred form, Zn, Cu and/or Ag ions are added. In an embodiment, said Zn ions are added in an amount between 1 and 3% (w/V), preferably about 1 % (w/V), said Cu ions are added in an amount between 1 and 3% (w/V), preferably about 1 % (w/V), said Ag ions are added in an amount between 0.05 and 0.5% (w/V), preferably about 0.05% (w/V).

In an embodiment, said substrate comprises SiC>2, preferably it is a silicate, for example tetraethyl orthosilicate (TECS), or sodium silicate. In an embodiment, said substrate is bioglass, or is a xerogel or is a silica gel.

In the context of the present invention, bioglass means a bioceramic material which shows biocompatibility with cellular tissue and predisposition for the formation of a biological bond with bone tissue.

Advantageously, the gel form promotes the maintenance of the microparticles in suspension.

The present invention further relates to a photocatalytic additive obtained by the process according to the present invention.

The procedure according to the present invention advantageously allows enriching the hydrolysis product with Cu ²⁺ ions conveniently released by the electrode. In fact, the authors of the present invention have demonstrated that the polarity inversion of the electrodes makes the metal ions which interact with hydrolyzed Ti always available in suspension.

The extraordinary properties of the photocatalytic additive according to the present invention, illustrated in the following examples, given purely by way of example and not to be understood in any way as limiting the scope of the present invention, the scope of protection of which is defined by the claims, make it applicable both directly to the surfaces to be treated and as an additive in paints. Said antimicrobial and antibacterial properties have proved to be particularly advantageous both in conditions of natural or artificial light, and in conditions of darkness.

Said additive is conveniently stored and used in powder form. In an embodiment, said powder is rehydrated before use, for example before adding the additive in a paint.

The following examples have the sole purpose of better describing the invention, the scope of which is defined by the following claims.

Examples

Example 1 : obtaining a photocatalytic additive

A silica gel was loaded into an electrolytic cell comprising two Cu/Cu electrodes. The hydrolysis product of a titanium-based precursor was dripped therein, so as to have about 5% w/V of titanium ions in suspension. Zn and Ag ions were also added in the form of hydroxides to said electrolytic cell, so as to have a concentration of Zn ions equal to about 1 % w/V and a concentration of Ag ions equal to about 0.05% (p/V) in suspension.

A voltage of 12-24 volts and a current of 0.01-0.10 A/cm ² was applied to said electrodes, keeping under stirring for 3 hours. During said stirring, the polarity of said two electrodes was reversed every 12 minutes.

At the end of said stirring, the voltage was removed, and the product obtained was heated to about 130°C, at a maximum pressure of 5 bar, for 3 hours. The product obtained is the photocatalytic additive.

Example 2: obtaining paint comprising the photocatalytic additive

The photocatalytic additive obtained as in example 1 , called BLG6, was added to a water-based paint not comprising biocides in an amount equal to 5% w/V (the paint was called BLG-5%). In a further embodiment, the photocatalytic additive obtained as in example 1 is rehydrated and then added to the same paint as above in an amount equal to 5% w/V.

Example 3: antibacterial activity

The paint with photocatalytic additive obtained as in example 2, by adding 5% w/V of the additive according to the present invention to a water-based paint, BLG6-5%, was tested for antibacterial activity. As a control, the same paint not added with the photocatalytic additive according to the present invention was used. The samples consisted of a 50 x 50 mm steel plate on which a 1 mm thick film of the control or paint according to the invention was deposited.

The method for measuring the antibacterial activity of photocatalytic semiconductor materials ISO 27447:2019 was followed.

The radiation was provided by a LED or UV bulb 0.25 mW/cm ² , having the spectrum for an exposure time shown in Figure 5, of 4, 8 and 24 hours. Escherichia coli, Staphylococcus aureus and Pseudomonas aeuriginosa were used, inoculating 8.6 x 10 ⁵ CFU/ml

The results obtained following inoculation with E. Coli and exposure to LED light are shown in table 1 .

Table 1

LED light are shown in Table 2.

Table 2 light are shown in Table 3.

Table 3

The experiment was repeated, with the only variation being that the illumination was provided by UV light. The results are shown in Figure 4A for E. coli, in Figure 4B for P. aeuriginosa, in Figure 4C for S. aureus.

The data as a whole show that the paint added with the photocatalytic additive according to the present invention leads, in all the conditions tested, both after LED radiation and after UV radiation, to a complete killing of the 3 bacterial strains tested over the period of 24 hours.

Example 4: photocatalytic activity, abatement of nitric oxide

The NO abatement tests were performed with the tangential flow method, in accordance with the UNI 11484-2013 standard. The tests were carried out with a simplified procedure, i.e., when the stability condition of the concentrations measured under radiation was reached or the maximum radiation time of 180 minutes was reached, the flow velocity inside the reactor was not changed, thus ending the test under these conditions. The samples were studied under both UV and visible radiation. io The determination of the NO/NO2 content in the measurement flows was carried out by means of an APNA 370 chemiluminescence meter. The measurement reactor had an inner volume of 3.6 dm ³ . The mixing inside the reactor was ensured by a compact axial fan EBMPAPST 612 JH (dimensions 60x60x32 mm) which provides a nominal flow of 70 m ³ tr ¹ .

The UV radiation took place by means of a set of two Philips PL-S 9W/2P BLB fluorescent bulbs having a significant UV emission, the emission spectrum of which is shown in Figure 5A. The intensity of the radiation incident on the sample was 10 W rrr ² between 290 and 400 nm.

In the case of visible radiation, notwithstanding the UNI 11484 standard, a LED illuminator (6500 K) with no UV emission was used. The spectrum of such a source is shown in Figure 5B. The radiance on the sample surface was 250 W m ² between 400 and 800 nm.

The light intensity was evaluated by spectroradiometry using an Ocean Optics USB2000+UV-VIS spectrophotometer provided with an optical fiber with a diameter of 400 pm and a length equal to 30 cm provided with a cosine corrector (Ocean Optics CC-3-UV-T, PTFE optical diffuser, spectral range 200-2500 nm, outer diameter 6.35 mm, field of vision 180°). The spectroradiometer was calibrated with an Ocean Optics DH-2000-CAL Deuterium-Halogen Light Sources lamp for UV-Vis-NIR measurements which was itself calibrated in absolute radiance by the vendor (Radiometric Calibration Standard UV-NIR, calibration certificate # 2162).

The tested samples were two 10 cm x 10 cm x 9 mm ceramic tiles painted with BLG6-5% paint, as in example 2. One sample was used for the test with UV radiation, the other for the test with visible radiation. The samples were not exposed to any pre-treatment.

The evolution of the NO and NO2 concentrations during the test is shown in Figure 5, where panel C shows data with exposure to UV light, panel D shows data with exposure to visible light.

The tested samples showed a significant reduction of NO both under UV radiation and under visible radiation. In particular, in the tested conditions an NO reduction of 810 pg rrr ² tr ¹ was observed with UV radiation and 580 pg m’ ² h’ ¹ with visible radiation.

Example 5: antiviral activity

The paint with photocatalytic additive obtained as in example 2, by adding 5% w/V of the additive according to the present invention to a water-based paint, BLG6-5%, was tested for antiviral activity. The Plaque Reduction Neutralization Test (PRNT) against porcine coronavirus (PEDV) was conducted. The test was conducted as described in Wang S et al. J Immunol Methods 2005, 301 : 21-30. Briefly, PEDV was dripped onto the sample surface, and, after an incubation of 30 or 60 minutes, samples were collected in which the permanence of the virus in Vero cells was assessed. After incubation at 37°C for 3 days, the infected cells were fixed and labeled. The efficacy of the product was derived by observing the number of PEDV plaques in each sample, compared with the control (without BLG6 additive). The results show an efficiency of BLG6-5% equal to 89.58% after 30 minutes incubation and 99.21 % after 60 minutes incubation.

Example 6: particle size analysis

The particle size of the photocatalytic additive particles according to the present invention was analyzed by laser diffraction in accordance with ISO 13320: 2020. A Mastersizer 3000 (Malvern-Panalytical) was used. A sample of BGL6 particles, obtained as in example 1 , and for comparative purposes, a sample of commercial anatase nanoparticles were analyzed.

Nanoparticles according to the invention or control were dispersed in deionized water and the dispersion was added to the sampler. Measurements were taken after sonication with 100% internal ultrasound for 10 minutes. The analysis parameters are the following:

Calculation theory: Mie

Sample Refractive index: 1 .55 - (blue light: 1 .66)

Sample Absorbance index: 0.01

Dispersant Refractive index: 1.330

Darkness: 10-20 %

Circulation rate: 2000 rpm

Number of measurements: 6 Table 4 shows the results obtained.

Table 4

D(10): particle size expressed in pm below which 10% of the sample resides.

D(50): particle size expressed in pm below and above which 50% of the sample resides. Represents the median of the distribution.

D(90): particle size expressed in pm below which 90% of the sample resides.