The present invention generally relates to durable antimicrobial coatings and preparations thereof. In particular, the invention teaches a stabilized nanodispersion used for the preparation of durable and wear resistant transparent antimicrobial coating and preparation method thereof.
Repulsive electrostatic double-layer forces are responsible for the stabilization of charged colloidal particles in the presence of adsorbed polyelectrolytes of opposite and high line charge densities. Stability of metal nanoparticles (NPs) in suspensions depends on several factors, e.g pH, ionic strength or particle surface chemistry. The effect of the pH strongly influences the zeta potential of the NPs, i.e. it affects the stability of the suspension. Determining the zeta potential is a key factor in the study of colloidal stability.
The surface charge of the particles can be described by the value of the zeta potential. The point of zero charge (PZC) is the value where the zeta potential is close to zero and the particles agglomerate easily. If the zeta potential is a high value (positive or negative) the stability of the dispersion is increased, due to the electrostatic repulsion of the particles.
In aqueous dispersion metal oxides (e.g. TiO2 or ZnO) hydrolyse and hydroxide layers are formed on the surface. Change in the pH of the solution can result in the protonation or deprotonation of the surface groups and this process can increase or decrease the particles charge. In a complex dispersion where more than one nanoparticle or e.g polymers are present, they can interact with each other, which can cause agglomeration and sedimentation. Titanium-dioxide is charged strongly positive under pH 6, and strongly negative above pH 7. The PZC value of TiO2, where the charge of the particles is close to zero is reached around 6.2±0.1. This shows that the surface charge depends on the pH value. In the case of zinc-oxide the surface charge is positive under pH=9.4, due to the Zn2+(aq) and Zn(OH)+(aq) ions present. Above pH 10 charge inversion occurs as the Zn(OH)2(s), Zn(OH)3−(aq) and Zn(OH)42−(aq) become dominant in the system, and the surface charge changes to negative. The PZC value of ZnO is found to be at pH 9.4.
The term photocatalyst is a combination of two words: photo related to photon and catalyst, which is a substance altering the reaction rate in its presence. Therefore, photocatalysts are materials that change the rate of a chemical reaction on exposure to light. This phenomenon is known as photocatalysis. Photocatalysis includes reactions that take place by utilizing light and a semiconductor. The substrate that absorbs light and acts as a catalyst for chemical reactions is known as a photocatalyst. All the photocatalysts are basically semiconductors. Photocatalysis is a phenomenon, in which an electron-hole pair is generated on exposure of a semiconducting material to light. The photocatalytic reactions can be categorized into two types on the basis of appearance of the physical state of reactants: (i) Homogeneous photocatalysis: When both the semiconductor and reactant are in the same phase, i.e. gas, solid, or liquid, such photocatalytic reactions are termed as homogeneous photocatalysis. (ii) Heterogeneous photocatalysis: When both the semiconductor and reactant are in different phases, such photocatalytic reactions are classified as heterogeneous photocatalysis. The energy difference between the valence band (HOMO) and the conduction band (LUMO) are known as the band gap (Eg). On the basis of band gap, the materials are classified into three basic categories: (1) metal or conductor: Eg<1.0 eV; (2) semiconductor: Eg<1.5-3.0 eV, (3) insulator: Eg>5.0 eV.
Semiconductors are capable of conducting electricity even at room temperature in the presence of light and hence, work as photocatalysts. When a photocatalyst is exposed to light of the desired wavelength (sufficient energy), the energy of photons is absorbed by an electron (e−) of valence band and it is excited to conduction band. In this process a hole (h+) is created in valence band. This process leads to the formation of photo-excitation state, and e− and h+ pair is generated. This excited electron is used for reducing an acceptor in which a hole is used for oxidation of donor molecules. The importance of photocatalysis lies in the fact that a photocatalyst provides both oxidation as well as a reduction environment and that too, simultaneously. The fate of the excited electron and hole is decided by the relative positions of the conduction and valence bands of the semiconductor and the redox levels of substrate.
There are four ways in which the semiconductor and the substrate interact with each other depending upon the relative positions of the valence and conduction bands and the redox levels. The four different combinations are: (1.) Reduction of substrate takes place, when the redox level of substrate is lower than the conduction band of the semiconductor. (2.) Oxidation of substrate takes place, when the redox level of the substrate is higher than the valence band of the semiconductor. (3.) Neither oxidation nor reduction is possible, when the redox level of the substrate is higher than the conduction band and lower than the valence band of the semiconductor. (4.) Both reduction and oxidation of the substrate take place, when the redox level of the substrate is lower than the conduction band and higher than the valence band.
Photocatalysts may be used for antifouling, antifogging, conservation and storage of energy, deodorization, sterilization, self-cleaning, air purification, wastewater treatment, etc. Semicoductors act as sensitizers for photoredox processes due to their electronic structure. Some semiconductors are able to photocatalyze the complete mineralization of many organic pollutants like aromatics, halo hydrocarbons, insecticides, pesticides, dyes, and surfactants.
Metal oxides have a wide range of application to solve environmental problems and in electronics due to their capacity to form charge carriers when they are exposed to light. Metal oxides have properties like: required electronic structure, light absorption properties, charge transport characteristics, Semiconductor-mediated photocatalysis has gained huge attention as it helps to overcome the problem related to fast charge recombination.
Binary oxides photocatalyst can be categorized into three categories: 1. titanium dioxide, 2. zinc oxide, and 3. other metal oxides like molybdenum oxide, vanadium oxide, indium oxide, tungsten oxide, and cerium oxide.
Titanium dioxide is a promising material for photocatalytic applications due to its availability, low cost, and non-toxic nature. However, due to its wide band gap (Eg=3.2 eV for anatase), and fast recombination rate of photogenerated electron and hole pairs, it needs high-energy (UV) photons to be activated and has a low quantum efficiency.
ZnO nanostructures have been shown to be prominent photocatalyst candidates to be used in photodegradation owing to the facts that they are low-cost, non-toxic and more efficient in the absorption across a large fraction of the solar spectrum compared to TiO2. Comparable to TiO2, ZnO is an n-type semiconductor oxide but has not been well investigated in previous studies. ZnO has been proposed as an alternative photocatalyst to TiO2 as it possess same band gap energy but exhibits higher absorption efficiency across a large fraction of the solar spectrum when compared to TiO2.
During the illumination of the most commonly applied heterogenous photocatalyst particles, reactive oxygen species with high oxidative potential are formed. These species include superoxide radicals (O2−), hydroxyl radicals (HO·) and hydrogen peroxide (H2O2): with the help of these, the photocatalyst-containing surfaces can oxidize organic pollutants and eliminate microorganisms.
During practical applications, it is of crucial importance to provide proper adhesion between the photocatalyst particles and everyday substrate materials, such as glass, textile, paper, ceramics or wall and plastic surfaces. Polymer-based coatings are the state-of-the-art composites of this field, having the highest versatility. Polymers are esteemed photocatalyst binders as they have adjustable elasticity, low density, price and excellent impact-resistance.
The foregoing invention teaches a process for the preparation of kinetically stable semiconductor/polymer nanodispersion whereby the aqueous stability of the dispersed negatively charged photocatalyst particles and anionic polyelectrolyte latex particles are ensured by the high surface charge. These identical negative surface charge caused electrostatic repulsive forces between the dispersed particles are induced by adjusting the pH. These repulsive forces keep the particles in well-dispersed form in the aqueous medium Consequently, the thin photocatalytically active antimicrobial coating prepared from this electrostatically stabilized homogeneous aqueous dispersion by spray-coating method contains well-dispersed and evenly distributed surface photocatalyst particles in polymer binder layer and thus it provides homogeneous, transparent and mechanically stable photoreactive and antimicrobial thin film on arbitrary surfaces. Moreover, the homogeneous surface distribution of the non-aggregated photocatalyst particles ensures that the light induced photocatalytic process effectively takes place and enough reactive oxygen species (ROS) generate to exert their photooxidation ability.
The invention may take physical form in certain parts and arrangement of parts, some embodiments of which will be described in the specification and illustrated in accompanying drawings which form a part hereof, wherein, when referring to the drawings, the inventor identifies the following components thereto,
As mentioned before, stability of semiconductor nanoparticles in aqueous media depends on several factors, e.g. pH, ionic strength or particle surface chemistry. Thus, the effect of the pH strongly influences the zeta potential of the particles, which is also affects the stability of the suspension.
The particle size and zeta potential values of the synthetized latex nanoparticles and photocatalyst particles (0.01% aqueous dispersion) were determined by dynamic light scattering (DLS) at different pH1 values with a Zetasizer Nano ZS ZEN 4003 apparatus (Malvem Ins., UK) equipped with a He—Ne laser (λ=633 nm). The measurements were performed at 25±0.1° C. Size distribution measurements by DLS were carried out in triplicate, and average values are reported. Error bars refer to the standard deviation.
To study the morphology and aggregation state of the photocatalyst nanoparticles, transmission electron microscopy (TEM) measurements were performed using a FEI Tecnai G2 20 X-TWIN microscope with a tungsten cathode operated at 200 kV. For TEM measurements, 10 μL of aqueous nanodispersion was dropped on a grid (carbon film with 200 Mesh coper grids (CF200-Cu, Electron Microscopy Sciences, USA) and dried at room temperature. The above presented results were also supported by the results shown in
Similarly. the corresponding zeta-potential values of ZnO (
The above presented surface charge and colloidal stability was also affected the particle size of the semiconductor photocatalysts and polymer dispersion.
The next important issue is how the presence of polymer in the composite layer affects the photocatalytic properties of the TiO2 and ZnO particles.
Incorporation of the photocatalysts particles into the polymer matrix resulted in a ˜50% decrease in the photocatalytic reaction rates, compared to pure TiO2, since the polymer partially covered the photocatalyst particles. Next it was also studied how many radicals are formed on the photoreactive surface depending on the light intensity since considering the potential antimicrobial application of the photoreactive coating, it is also an important question.
The amount of hydroxyl radicals was measured from the reaction of luminol and hydrogen peroxide. The results were calculated from the chemiluminescence (CL) data with Sirius L Single Tube luminometer (Berthold Detection Systems, Hungary). Six milligrams of luminol was diluted in 1 mL of sodium hydroxide (0.1 M) and filled out to 20 mL with distilled water. The nanohybrid films were immersed in 40 mL of distilled water, then illuminated and shaken continuously during the experiment using a magnetic stirrer. Samples were taken after 60 min of illumination, 100 μL of the samples was added to 100 μL of luminol solution, and the intensity of the chemiluminescence was measured immediately with the luminometer (Hirakawa, T., and Nosaka, Y. (2002). Properties of O2-And OH Formed in TiO2 Aqueous Suspensions by Photocatalytic Reaction and the Influence of H2O2 and Some Ions. Langmuir 18, 3247-3254. doi:10.1021/la015685a). Based on the previously determined calibration curve (0-5 mM), the concentration of OH radicals is directly proportional to the measured RLU values as follows: CH2O2 (mM)=measured RLU value/41866, R2=0.9977. For quantitative characterization of the free radical concentration from the RLU data, the calculated equivalent concentration of H2O2 (mM) is displayed as a function of illumination time with the used light source (15 W low pressure mercury lamp (LightTech, Hungary) with characteristic emission wavelength at λmax=435 nm) at 25.0±0.5° C. The distance of the light source from the nanohybrid films was systematically changed in order to determine how the surface reactive oxygen species concentration changes with increasing distance from the light source. As it can be seen the measured light intensity is inversely proportional to the square of the distance from the source (
In order to mimic the effect of practical use and the wear resistance of the photoreactive coating the composite layer was also prepare on the surface of glass plate and the coating was wiped with a cloth (
The antibacterial tests were carried out according to EN ISO 27447:2009 standard. For the evaluation of the surviving bacteria the washing technique was used, because the counting of the survival bacteria is more accurate with this method. Before the microbiological measurements nanohybrid films on glass samples were activated by UV-irradiation for an hour (lightsource: LightTech GCL307T5U/Cell lamp λ=250 nm) to increase the surface concentration of the photocatalyst particles in the surface region of the nanohybrid films. 1×105-5×105 cfu/mL bacterial suspensions were spread uniformly (0.1 mL) on the surface of the nanohybrid films (2.5×2.5 cm2) and covered with the top of Petri dish during the experiment, to avoid water vapour evaporation which can modify results. During the microbiological measurements the glass samples with the nanohybrid films were illuminated with visible-light (light source: LED lamp 7W; λ=405 nm), exposure times were 0, 4 and 24 h. During the experiments the distance of the light source from the nanohybrid films was 35 cm. The light intensity on the surface of the nanohybrid films was measured with a power meter (Thorlabs GmbH, Germany). After different illumination periods the inoculated nanohybrid films were placed into anew sterile Petri-dish by sterile tweezers and the inoculums were washed out from the activated nanohybrid films with 3 mL sterile physiological saline water to regain all surviving bacteria from the uneven surface of the samples. Bacterial suspensions with survival bacteria were streaked (0.1 mL) uniformly on the Mueller-Hinton (Oxoid, Hampshire, UK) media. After the incubation time (37° C.; 24 h) the antibacterial activity was evaluated by counting colony forming units (cfu/mL) with BZG40Colony Counter (WTWGmbH, Germany). The number of colony forming units were converted to the cell number of the survival bacteria per milliliter of the original inoculums on the nanohybrid films. The result on
The poly(MMA-MAA) latex particles (
During the synthesis of the aqueous photoreactive coating material (
0.2 g P25 TiO2 was added to 996.8 mL of water. Next, 2.73 g 30% aqueous poly(MMA-MAA) latex particles dispersion (Example 1) was added to the photocatalyst dispersion and the pH was set to >9, preferably 11 by the addition of NaOH. The obtained dispersion was sonicated and, if necessary, the pH was set again for >9, preferably 11.
0.2 g ZnO was added to 996.8 mL of water. Next, 2.73 g 30% aqueous poly(MMA-MAA) latex particles dispersion (Example 1) was added to the photocatalyst dispersion and the pH was set to >9, preferably 11 by the addition of NaOH. The obtained dispersion was sonicated and, if necessary, the pH was set again for >9, preferably 11.
Beside the anionic polyacrylate binder describe in Example 1 other negatively charged synthetic- or natural polyanions and their derivatives are also suitable for the preparation of photoreactive coating material such us polyacrylic acid, sodium-polyacrylate, anionic polyacrylamide, sodium-poly(styrene sulfonate), alginate, carboxymethyl cellulose, etc. During the synthesis of the aqueous photoreactive coating material suitable for the preparation of homogenous and mechanically stable composite thin films, first 0.16 g P25 TiO2 and 0.04 g ZnO was added to 996.8 mL of water. Next, negatively charged synthetic- or natural polyanions listed above was also added to the photocatalyst dispersion in an amount that the photocatalyst/polymer mass ratio will be from 0.2/0.8 to 0.8/0.2 and the pH was set to 11 by the addition of NaOH. The obtained dispersion was sonicated and, if necessary, the pH was set again for 11.
The hybrid layers consist of photocatalysts particles and polyacrylate binder was prepared by applying the spray-coating technique on the substrate surface. During the preparation process, the aqueous suspension obtained in Example 2-6 was evenly sprayed on the substrate surfaces (˜1 L/15 m2) from a distance of 15-30 cm using a R180 type Airbrush spray gun at an operating pressure of 3 bar.
During the surface coating process, the aqueous suspension obtained in Example 2-6 was evenly sprayed on the substrate surfaces (˜1 L/15 m2) from a distance of 15-30 cm using a Graco HVLP type air assisted spray gun at an operating pressure of 1-2 bar.