Patent Application
20210290503
The invention relates to a Janus particle comprising a core inorganic photocatalytic particle, a method of making said particle, and the use of said particle in sunscreen formulations.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
The creation of asymmetric, or Janus particles has been a field of interest since its conceptualisation by De Gennes in 1991. Such particles contain two (or more) distinct chemical or physical properties on their surface, or throughout their core. The asymmetry of such particles conveys directionality which facilitates its self-assembly depending on the environment it is placed in. In contrast, such directionality is typically absent in homogeneous particles.
Janus particles may be synthesised bottom-up, or existing particles may be modified to become Janus-like, i.e. via top-down method. Many methods to attain Janus particles have been conceived, and these particles may be comprised of polymeric or inorganic components, or sometimes both (Hu, J. et al., Chem. Soc. Rev. 2012, 41, 4356-4378). An intuitive modification method may be based on toposelective surface modification, and this is a process to make Janus particles amongst many others (Perro, A. et al., J. Mater. Chem. 2005, 15, 3745-3760). Various strategies have also been conceived, such as temporary masking, microcontact printing, partial contact with a reactive medium along an interface, and using reactive directional fluxes or fields.
Present technologies to make Janus particles largely involve masking, which protects one area of the particle while subjecting the other area to modification. A well-known technique involves paraffin wax masking, which is capable of producing gram quantities of Janus particles in a single batch (US 20080234394 A1). In this method, Pickering emulsions were made out of silica particles and paraffin wax, which functioned as a masking surface and enabled only the exposed areas of the particles to be modified by aminopropylsilane (APS). It was demonstrated that the incorporation and control of the concentration of the surfactants has to be precise as it could affect the amount of particle area subjected to modification (Jiang, S.; Granick, S., Langmuir 2008, 24, 2438-2445). However, such techniques necessitate the use of a mask, which is an additional component that has to be removed in the work-up.
A method that does not require an additional masking surface was conceived by Takahara et al., which involves particle-particle masking (Takahara, Y. K. et al., J. Am. Chem. Soc. 2005, 127, 6271-6275). This method relies on the agglomeration of silica particles in an immiscible water-toluene system to partially modify the exposed surface of the particles with n-octadecyltrimethoxysilane, while keeping the agglomerated surfaces within untouched. However, the addition of a defined, small amount of water to facilitate a tight aggregation of the silica particles requires precise control of the reaction conditions and is not feasible for large-scale production. Therefore, a main difficulty in the synthesis of Janus particles is to achieve large-scale production, with high yield and particle uniformity.
Given the above, there remains a need to develop new synthesis methods that produce Janus particles more efficiently with high yields, and at the same time achieve high particle uniformity and monodispersibility. More importantly, these methods have to be robust, cost-effective and versatile, such that Janus particles with various morphologies can be achieved easily.
As noted above, there remains a need for improved Janus particles, as well as methods of manufacture thereof.
Thus, in a first aspect of the invention, there is disclosed a Janus particle comprising:
In embodiments of the first aspect of the invention:
(i) the core inorganic photocatalytic particle may be selected from one or more of CeO2, SnO2, Nb2O5, WO3, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, TiO2, ZnO, and Cu2O (optionally wherein the core inorganic photocatalytic particle may be selected from one or more of TiO2, ZnO, and Cu2O (e.g. the core inorganic photocatalytic particle may be TiO2)) and/or the core inorganic photocatalytic particle may itself be a hybrid core-shell particle having a core selected from one or more of silica, a polymer, a ceramic, a metal and an alloy and a shell comprising one or more of CeO2, SnO2, Nb2O5, WO3, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, TiO2, ZnO, and Cu2O;
(ii) the low surface energy organic coating may be selected from one or more of a coating by a C8 to C26 alkylsilane, a fluoroalkylsilane and a C8 to C26 fatty acid covalently bound to the surface of the core inorganic photocatalytic particle (e.g. the low surface energy organic coating may be selected from octylsilane, hexadecylsilane, 1H,1H,2H,2H-perfluorodecylsilane covalently bound to the surface of the core inorganic photocatalytic particle, or it may be a C to C26 fluoroalkylsilane, such as 1H,1H,2H,2H-perfluorodecylsilane);
(iii) the first region may form from 30 to 70% of the surface area of the Janus particle and the second region forms from 70 to 30% of the surface area of the Janus particle, optionally wherein the first region may form from 40 to 60% of the surface area of the Janus particle and the second region forms from 60 to 40% of the surface area of the Janus particle, optionally wherein the first and second region form about 50% of the surface area of the Janus particle;
(iv) the Janus particles, when in the form of a compact film, have a water contact angle of from 30 to 140°.
In a second aspect of the invention, there is provided a method of manufacturing a Janus particle, the method comprising the steps of:
(a) providing an aqueous dispersion of precursor Janus particles comprising a core inorganic photocatalytic particle having a surface that is fully coated by a low surface energy organic coating that is susceptible to photo-degradation, which coating is covalently bound to the surface of the core inorganic photocatalytic particle; and
(b) subjecting the aqueous dispersion to irradiation by ultra-violet light for a period of time to provide Janus particles.
In embodiments of the second aspect of the invention:
(ai) the period of time may be from 30 minutes to 2 hours, such as 1 hour;
(aii) the ultra-violet light may be provided at a wavelength of from 100 to 400 nm, such as 365 nm;
(aiii) the ultra-violet light may be provided at an irradiance value of from 50 to 150 mW/cm2, such as from 70 to 140 mW/cm2 (e.g. 72 mW/cm2 or 136 mW/cm2);
(aiv) the precursor Janus particles may be prepared by reacting core inorganic photocatalytic particles with one or more of a C8 to C26 alkyltrialkoxysilane, a fluoroalkyltrialkoxysilane and a C8 to C26 fatty acid to provide the precursor Janus particles, optionally wherein the core inorganic photocatalytic particles are reacted with one or more of octyltriethoxysilane, hexadecyltrimethoxysilane, and 1H,1H,2H,2H-perfluoro-decyltriethoxysilane;
(av) the precursor Janus particles may be partially coated in a wax that covers from 30 to 70%, such as from 60 to 40% (e.g. 50%) of the surface area of the precursor Janus particle, optionally wherein after step (b) the wax is removed from the particles using a suitable organic solvent;
(avi) the precursor Janus particles are provided in the form of water-in-oil Pickering emulsion droplets that are dispersed into water to form the aqueous dispersion of precursor Janus particles, optionally wherein the low surface energy organic coating is a fluoroalkylsilane (e.g. 1H,1H,2H,2H-perfluorodecylsilane) covalently bound to the surface of the core inorganic photocatalytic particle;
(avii) the core inorganic photocatalytic particle may be selected from one or more of CeO2, SnO2, Nb2O5, WO3, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, TiO2, ZnO, and Cu2O optionally wherein the core inorganic photocatalytic particle may be selected from one or more of TiO2, ZnO, and Cu2O (e.g. the core inorganic photocatalytic particle is TiO2) and/or the core inorganic photocatalytic particle may itself be a hybrid core-shell particle having a silica core and a shell comprising one or more of CeO2, SnO2, Nb2O5, WO3, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, TiO2, ZnO, and Cu2O;
(aviii) the low surface energy organic coating may be selected from one or more of a C8 to C26 alkylsilane, a fluoroalkylsilane and a C8 to C26 fatty acid covalently bound to the surface of the core inorganic photocatalytic particle, optionally wherein the low surface energy organic coating is selected from octylsilane, hexadecylsilane, 1H,1H,2H,2H-perfluorodecylsilane covalently bound to the surface of the core inorganic photocatalytic particle (e.g. the low surface energy organic coating may be a C8 to C26 fluoroalkylsilane, such as 1H,1H,2H,2H-perfluorodecylsilane, covalently bound to the surface of the core inorganic photocatalytic particle).
In a third aspect of the invention, there is provided a sunscreen formulation comprising Janus particles as described in the first aspect of the invention, or any technically sensible combination of its embodiments, and a diluent, carrier or excipient.
It has been surprisingly found that Janus particles can be formed easily and cheaply, with consistent bulk properties that make them suitable for use in a range of different applications.
For example, in sun lotions or in other cosmetic formulations.
Thus, disclosed herein is a Janus particle comprising:
a core inorganic photocatalytic particle having a surface with a first region and a second region and an average diameter of from 50 nm to 10 μm; and
a low surface energy organic coating that is susceptible to photo-degradation, which coating is covalently bound to the surface of the core inorganic photocatalytic particle where present, wherein:
the low surface energy organic coating fully coats the entire surface of the first region and the second region is either uncoated or is coated by the low surface energy organic coating that has been partly or fully degraded, such that the first region displays hydrophobic/oleophilic or amphiphobic properties and the second region displays amphiphilic properties.
When used herein, a “Janus particle” refers to a particle that displays asymmetric surface properties. For example, the Janus particle may contain two regions displaying different surface properties, where the two regions may be of balanced surface area or one may be bigger than the other. In such particles, the surface of one region may display hydrophobic/oleophilic or amphiphobic properties, while the other region displays amphiphilic properties.
As will be appreciated, the first and second regions referred to above may have any suitable combination of surface areas—provided that they encompass the entire surface area of the Janus particle. For example, the first region may form from 30 to 70% of the surface area of the Janus particle, such that the second region forms from 70 to 30% of the surface area of the Janus particle. In additional or alternative embodiments, the first region may form from 40 to 60% of the surface area of the Janus particle, such that the second region forms from 60 to 40% of the surface area of the Janus particle. In particular embodiments that may be disclosed herein, the first and second regions may each form about 50% of the surface area of the Janus particle.
In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of” or synonyms thereof and vice versa.
When used herein, the term “inorganic photocatalytic particle” refers to an inorganic material in particulate form that exhibits photocatalytic activity upon exposure to light having an energy higher than a predetermined band gap. Any suitable inorganic material (or combination thereof) may be used in the current invention. Examples of suitable materials that may be used as the core particle of the Janus particle include one or more of CeO2, SnO2, Nb2O5, WO3, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, TiO2, ZnO, Cu2O.
In additional or alternate embodiments, the core particle of the Janus particle may be formed from a hybrid core-shell particle. In principle, when a hybrid core-shell particle is used as the core structure, any suitable solid material may be used as the core portion, as it is intended to act as a physical carrier of the photocatalytic shell. For example, the core portion may be selected from one or more of silica, a polymer, a ceramic, a metal and an alloy (e.g. the core portion may be selected from one or more of silica, a ceramic, a metal and an alloy). The shell of the hybrid core-shell particle may be selected from one or more of CeO2, SnO2, Nb2O5, WO3, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, TiO2, ZnO, and Cu2O. In more specific embodiments that may be mentioned herein, the hybrid core-shell particle may be silica surrounded by a shell comprising one or more of CeO2, SnO2, Nb2O5, WO3, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, TiO2, ZnO, and Cu2O. Unless otherwise stated, the metal oxides mentioned herein may be present in a single Janus particle (i.e. as an alloy) or in separate Janus particles (e.g. some Janus particles containing CuO and some containing NiO), whether as the whole of the particle or as the shell surrounding a silica core. In particular embodiments of the invention that may be disclosed herein, the core inorganic photocatalytic material may be selected from one or more of TiO2, ZnO, and Cu2O. For example, the core inorganic photocatalytic material may be TiO2.
Without wishing to be bound by theory, when the core portion of the hybrid core-shell particle is formed from a polymer, it is believed that the shell of the hybrid core-shell particle will shield the polymer in the core portion of said particle from the effects of UV light, such that the polymer will not be affected, at least with regard to the timescales contemplated with regard to the method of manufacture and resulting use of said materials herein.
The core inorganic photocatalytic particle may have any suitable size. For example, the core inorganic photocatalytic particle may have an average diameter of from 50 nm to 10 μm, such as from 75 nm to 5 μm.
When used herein, the term “low surface energy organic coating” is intended to refer to a material that provides a high contact angle >90° when a surface coated in said material is contacted by water. Any suitable low surface energy material that can form a covalent bond with the surface of the core inorganic photocatalytic particle may be used in the current invention, such that the low surface energy material becomes covalently bound to the surface of the core inorganic photocatalytic particle. For example, suitable compounds capable of forming a covalent bond with the core inorganic photocatalytic particle include, but are not limited to C8 to C26 alkyltrialkoxysilanes (e.g. octyltriethoxysilane and/or hexadecyltrimethoxysilane), fluoroalkyltrialkoxysilanes (e.g. 1H,1H,2H,2H-perfluoro-decyltriethoxysilane) and C8 to C26 fatty acids. The resulting low surface energy organic coating may therefore be formed by C8 to C26 alkylsilanes (e.g. octylsilane and/or hexadecylsilane), fluoroalkylsilanes or C8 to C26 fatty acids covalently bound to the surface of the core inorganic photocatalytic particles. As will be appreciated, one or more of the low surface energy organic coating compounds described hereinbefore may be used to form the low surface energy organic coating. In particular embodiments of the invention that may be mentioned herein the low surface energy organic coating may be a C6 to C26 fluoroalkylsilane, such as 1H,1H,2H,2H-perfluorodecylsilane, covalently bound to the surface of the core inorganic photocatalytic particle.
When used herein, a “degraded low surface energy organic coating” refers to a region of a Janus particle where a residue of the low surface energy organic coating remains on the surface of the Janus particle in the first region. Such residues remain on the surface of the core inorganic photocatalytic particle when the low surface energy organic coating is subjected to photodegradation, but not all of the covalently bound material is removed.
When used herein, “hydrophobic” refers to the property of a material to repel water (i.e. a material that displays a high contact angle (e.g. >90°) when water is dropped onto a surface composed of said material). When used herein, “oleophilic” refers to the property of a material to display a low contact angle (e.g. <90°) when an oil is dropped onto a surface composed of said material. In certain embodiments mentioned herein the terms “hydrophobic” and “oloephililc” may be used interchangeably. Examples of hydrophobic/oleophilic materials that may be mentioned herein include, but are not limited to surfaces formed by covalently bonding a fatty acid, octylsilane or hexadecylsilane to the surface of an inorganic particle of the type mentioned hereinbefore, such that these materials become covalently bound to the surface of the core inorganic photocatalytic particle.
When used herein, “amphiphobic” refers to the property of a material to display hydrophobic and oleophobic properties (i.e. high contact angles (e.g. >90°) when water or an oil (e.g. silicone oil) is dropped onto its surface. Examples of amphiphobic materials that may be mentioned herein include, but are not limited to surfaces formed by covalently bonding fluoroalkylsilanes (e.g. 1H,1H,2H,2H-perfluorodecylsilane) to the surface of an inorganic particle of the type mentioned hereinbefore, such that these fluoroalkylsilanes become covalently bound to the surface of the core inorganic photocatalytic particle.
When used herein, “amphiphilic” refers to the property of a material to display hydrophilic and oleophilic properties (i.e. low contact angles (e.g. <90°) when water or an oil (i.e. silicone oil) is dropped onto its surface. Examples of amphiphilic surfaces may refer to the bare inorganic photocatalytic particle surface or of said surface that is coated by the low surface energy organic coating that has been partly or fully degraded.
It will be appreciated that the above contact angles refer to the inherent properties of the individual materials, or specific combination of materials (such as the combination of covalently bound low surface energy molecules covering the surface of the core inorganic photocatalytic particle) used to form the first and second regions of the Janus particles disclosed herein. As such, it is to be expected that the overall properties of the Janus particles differ from the materials used to form the first and second regions on the surface of the Janus particle, as the Janus particle contains both regions. Suitable water contact angles for the Janus particles of the current invention, when presented in the form of a compact film as described in the examples, may be from 30 to 140°. For example, the water contact angle may be from 50 to 120°, such as from 80 to 110°.
For the avoidance of doubt, when a number of related numerical ranges are disclosed herein in relation to a specific feature of the invention, it is explicitly intended that the upper and lower limits of said ranges may be combined in any manner possible. For example, for the water contact angles disclosed herein, the following ranges are explicitly intended to be disclosed:
30 to 50°, 30 to 80°, 30 to 110°, 30 to 120°, 30 to 140°;
50 to 80°, 50 to 110°, 50 to 120°, 50 to 140°;
80 to 110°, 80 to 120°, 80 to 140°;
110 to 120°, 110 to 140°; and
120 to 140°.
The Janus particles disclosed herein may be made by any suitable method. Thus, there is also disclosed a method of manufacturing a Janus particle, the method comprising the steps of:
(a) providing an aqueous dispersion of precursor Janus particles comprising a core inorganic photocatalytic particle having a surface that is fully coated by a low surface energy organic coating that is susceptible to photo-degradation, which coating is covalently bound to the surface of the core inorganic photocatalytic particle; and
(b) subjecting the aqueous dispersion to irradiation by ultra-violet light for a period of time to provide Janus particles.
Unless otherwise specified below, where the materials and terms used above are identical to those used in relation to the Janus particles per se, then the same definitions apply. The core inorganic photocatalytic particles are the same as those defined hereinbefore.
The general process described above covers three possible processes that may be used to manufacture the Janus particles of the current invention. The first process simply involves the steps set out above, while the second process further partially coats the Janus precursor in a wax that covers from 30 to 70%, such as from 60 to 40% (e.g. 50%) of the surface area of the precursor Janus particle before step (b) above is conducted. The third process presents the precursor Janus particles as part of a Pickering emulsion droplet, obtained from a water-in-oil emulsion that is dispersed into water.
The precursor Janus particles for use in the first process referred to above may be prepared by reacting the core inorganic photocatalytic particles with any suitable material that can provide a low surface energy organic coating that is covalently bound to the surface of the core inorganic photocatalytic particles. For example, the core inorganic photocatalytic particles may be reacted with one or more of a C8 to C26 alkyltrialkoxysilane, a fluoroalkyltrialkoxysilane and a C8 to C26 fatty acid to provide the precursor Janus particles. In more particular embodiments, the core inorganic photocatalytic particles may be reacted with one or more of octyltriethoxysilane, hexadecyltrimethoxysilane, and 1H,1H,2H,2H-perfluoro-decyltriethoxysilane. Any suitable ratio of low surface energy organic coating to core inorganic photocatalytic particles may be used in this process. For example, the volume:volume ratio of low surface energy organic coating to core inorganic photocatalytic particles may be from 1:1 to 1:100, such as from 1:10 to 1:75, such as from 1:20 to 1:50, such as about 1:40 (e.g. the volume to volume ratio of 1H,1H,2H,2H-perfluoro-decyltriethoxysilane to TiO2 may be 1:40).
The reaction method to form the precursor Janus particles used directly in the first process may involve mixing a solution containing the core inorganic photocatalytic particles in a solvent with a solution containing the desired precursor of the low surface energy organic coating in a solvent. The resulting reaction solution may be mixed at room temperature or at elevated temperature for a suitable period of time until the core inorganic photocatalytic particles are fully covered by the low surface energy organic coating. Further details of how such reactions may be conducted (and the associated purification steps) are provided in the experimental section below.
It is noted that these precursor Janus particles described above are also used as a starting material for the second and third processes described in more detail below.
In the first of the processes described above, the process simply involves dispersing the precursor Janus particles described above directly into a dish of water, which is then subjected to UV light irradiation for a suitable period of time. This results in the direct formation of the Janus particles described hereinbefore and has the advantage of being a simple and straightforward process. It is noted that the precursor Janus particles float at the air-water interface and do not tend to significantly rotate, as evidenced by the differential properties of the Janus particles obtained in the experimental section below that make use of this method. Nevertheless, as the particles are able to rotate to a certain degree, it is noted that leaving the particles for an extended time under irradiation using the first process will remove the entire low surface energy organic coating. Further details of how the first process is conducted is provided in the examples section below.
For the second process, the precursor Janus particles prepared for direct use in the first process are partially covered in wax before they are added to a dish of water for the UV light irradiation step discussed above in the first process. This partial coating of wax may be achieved by dispersing the precursor Janus particles prepared for direct use in the first process in water held at an elevated temperature (i.e. a temperature that will melt the chosen wax) and then adding the chosen wax to the resulting mixture. Once the wax has melted, the mixture is subjected to vigorous mixing (e.g. from 10,000 to 20,000 rpm, such as 15,000 rpm) for a short period of time (e.g. from 1 to 5 minutes, such as 2 minutes) to provide a precursor Janus particle that is partially embedded in a wax droplet, leaving a portion of the low surface energy organic coating fully exposed. The resulting particles are then subjected to the same UV light irradiation process described above for the first process, but with stirring of the particles to ensure that the uncoated surface of the particles is fully degraded. Any suitable speed of stirring may be used in this process, such as (but not limited to) about 100 rpm. The wax may then be removed from the Janus particle by dissolution into a suitable organic solvent.
Any suitable wax may be used for the coating process described above. A suitable wax is one that protects the low surface energy organic coating of the precursor Janus particle that is covered by the wax from degradation by UV light during the irradiation step described herein. Thus, the second process has the advantage that if the particles are subjected to excessive irradiation, they will still provide Janus particles by virtue of the protective wax coating. The wax coating may cover from 30 to 70%, such as from 60 to 40% (e.g. 50%) of the surface area of the precursor Janus particle.
As indicated above, each wax droplet houses a number of partly-embedded precursor Janus particles. Ideally, the embedded precursor Janus particles form a monolayer on the surface of each wax droplet (which is ideally spherical or spheroidal). Whether such a monolayer is formed or not is influenced by the amount of low surface energy organic coating on the precursor Janus particles. A precursor Janus particle functionalised using a high concentration of low surface energy organic coating (e.g. a volume:volume ratio of low surface energy organic coating to core inorganic photocatalytic particles of about 1:4) is not optimal, as it will tend to agglomerate with other precursor Janus particles and inhibit the formation of spherical wax droplets with the desired monolayer of said precursor Janus particles. However, even when this occurs, the aggregated particles are still effectively masked by each other, and UV irradiation of such droplets should still produce Janus particles.
Further details of how the second process is conducted is provided in the examples section below.
For the third process, the precursor Janus particles prepared for direct use in the first process are used as Pickering emulsion stabilisers to form a water-in-oil emulsion. This may be achieved by first adding the precursor Janus particles that are used directly in the first process described above to an oil to form a mixture that is then subjected to vigorous mixing (e.g. from 10,000 to 20,000 rpm, such as 15,000 rpm), to which water is added in drop-wise fashion to form a water-in-oil mixture. Any suitable ratio of oil to water may be used (e.g. from 90:10 vol:vol oil to water to 50:50 vol:vol oil to water). Mixing may be continued for a period of time after all of the water has been added. The resulting emulsions provide droplets having a diameter of from 1 to 10 μm (e.g. 3.44 μm), where the precursor Janus particles act to stabilise the Pickering emulsion. In certain embodiments of the invention that may be disclosed herein, the low surface energy organic coating that is covalently bound to the surface of the core inorganic photocatalytic particle may be a fluoroalkylsilane (e.g. 1H,1H,2H,2H-perfluorodecylsilane), as this provides a particle that is amphiphobic, thereby encouraging the particles to align at the interface between the oil and water, which may result in a more stable Pickering emulsion.
In order to form the desired Janus particles, the Pickering emulsion formed above may be dispersed into a dish of water and subjected to UV light irradiation and stirring as described in the second process above.
Further details of how the third process is conducted is provided in the examples section below.
In all of the processes described above, the UV light irradiation may last for any suitable period of time. For example, the irradiation may be from 30 minutes to 2 hours, such as 1 hour. As will be appreciated, the amount of time allotted to the irradiation may vary depending on the irradiance value of the ultra-violet light. For example, the ultra-violet light may be provided at an irradiance value of from 50 to 150 mW/cm2, such as from 70 to 140 mW/cm2 (e.g. 72 mW/cm2 or 136 mW/cm2). When the irradiance value is low, the period of time may be higher and vice versa.
For the first process described above, it will be noted that the irradiation period of time should be sufficiently long to degrade a portion of the low surface energy organic coating, but not so long as to allow for complete degradation of the coating (i.e. the period of irradiation may be from 30 minutes to 2 hours, such as 1 hour). This is because the particles are free to rotate in the aqueous dispersion. In contrast, the period/irradiance value of UV light irradiation may not be so important for the second and third processes. For example, in the second process, part of the precursor Janus particle is protected from photodegradation by the presence of the wax coating. In the third process, the formation of an emulsion may restrict the ability of the precursor Janus particles to rotate, thereby minimising over-exposure of the surface of the Janus particles even when the irradiation period is longer than would be optimal for the first process. Nevertheless, it would be expected that the UV light irradiation period and irradiance value should be optimised in an industrial process in order to minimise energy wastage.
The UV light used in the processes above may have a wavelength of from 100 to 400 nm, such as from 300 to 395 nm, such as 365 nm.
The Janus particles disclosed herein may be particularly suitable for generating a self-assembled layer of particles for compositions that may be applied to an uneven surface, such as a skin. As such, the Janus particles may be particularly suitable for inclusion in cosmetic formulation, skin creams and sun lotions. Without wishing to be bound by theory, it is believed that the amphiphobic or hydrophobic/oleophilic face of the Janus particle is repelled by the skin and the rest of the formulation, so that the particles are driven upwards to the air-formulation interface, where they are held in place by the amphiphilic UV-treated face of the Janus particles, which serve to anchor the particles at the interface, consequently forming a desirable even layer of Janus particles at the air-formulation interface. As is well known, inorganic photocatalytic particles (e.g. TiO2) are commonly used in sunscreens and cosmetics to provide a degree of protection from UV-light, but the amount of the material used tends to be high. Unlike conventional inorganic particles, Janus particles form a layer at the air-formulation interface. As such, an advantage associated with the Janus particles of the current invention is that they may be used to provide increased sun protection factors in suitable formulations or may be added in reduced quantities compared to conventional inorganic particles used for such purposes, while retaining an equivalent sun protection factor. Details of this advantage are discussed in more detail in the examples section below.
Thus, the invention also discloses a sunscreen formulation comprising Janus particles as described herein and a diluent, carrier or excipient. Any suitable formulation of sunscreen may be used, wherein the Janus particles may form from 1 to 10 wt % of the formulation. Examples of suitable sunscreen formulations that may be adapted for use with the current invention include those disclosed in Wiechers, S. et al., Cosmetics & Toiletries 2013, 128 (5), 2-6.
Further aspects and embodiments of the invention are discussed in more detail below with reference to the examples.
Materials and Instruments
Anatase TiO2 (1171 titanium dioxide E 171) was obtained from KRONOS. Methanol was obtained from Fisher Chemical. Absolute ethanol and sodium hydroxide were obtained from Merck. 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS), hexadecyltrimethoxysilane (HDTMS), (3-aminopropyl)triethoxysilane 99% (APTES), toluene, 10 nm and 30 nm gold nanoparticles suspended in citrate buffer, copper (II) chloride dihydrate, polyvinylpyrrolidone (PVP, MW=40,000), L-ascorbic acid, and potassium bromide were obtained from Sigma Aldrich. Water used was purified water (Millipore, 18.2 M1 cm at 25° C.).
Contact Angle Measurement
Static contact angle measurements were taken using the contact angle goniometer OCA 20 (Dataphysics, Germany) and SCA 20 was used for analysis of the droplets. Approximately 2 mg of particles were placed on a glass slide, packed into a solid mound, and another glass slide was placed on top. The particles were manually hand-pressed into a consolidated film. All measurements were carried out at 25° C. in air, using 4 μL water as the test solvent. A minimum of three measurements was taken and their mean and standard deviation were reported.
Fourier-Transform Infrared Spectroscopy (FTIR)
Particles were ground and mixed with potassium bromide using a mortar and pestle, then pressed under a 10 ton force into a pellet measuring 13 mm in diameter. The pellet was analysed by FTIR (Frontier, Perkin Elmer) from 4000 cm−1 to 400 cm−1 with a resolution of 4 cm−1 with 32 scans.
Thermogravimetric Analysis (TGA)
TGA was performed on the particles using SDT Q600 (TA Instruments). Particles were loaded in an alumina crucible. Under a nitrogen flow of 100 mL/min, the particles were subjected to a temperature ramp of 20° C./min from 25° C. to 800° C. The data was analysed using TA Universal Analysis.
Field-Emission Scanning Electron Microscopy (FESEM)
Particles were drop cast from their suspending solution on an aluminum foil substrate. They were observed under FESEM (JEOL JSM-7600F) operating at an accelerating voltage of 10 keV in secondary electron imaging mode. Energy-dispersive X-ray spectroscopy (EDS) was performed at 20 keV using X-Max (Oxford Instruments), and the EDS data was analysed by INCA.
General Methods
Surface functionalisation of TiO2, Cu2O, ZnO, silica-TiO2 particles, glass substrates Typically, TiO2 particles (with average diameter of 155 nm) was dispersed in methanol with a concentration of 2 g/L. Separately, FAS was dissolved dropwise in methanol, followed by a slow addition of water into the solution to hydrolyse the FAS. The resulting 1% v/v FAS solution was stirred for an hour to complete the hydrolysis process. For example, the 1% v/v FAS solution can be first prepared by dissolving 0.5 mL of FAS in 48 mL of methanol, followed by the dropwise addition of 1.5 mL of water. To functionalise the metal oxide particles, the FAS solution was added dropwise to the particles in a pre-determined volume ratio. For example, the FAS solution was then added to the TiO2 solution in a 1:40 (FAS:TiO2) volume ratio, making a 60 mL solution in total. The resulting FAS-metal oxide dispersion was stirred for 1 hr, and then heated at 100° C. for 1 hr. The reaction scheme of this functionalisation is similar to the reported alkylsilane modification (G. L. Witucki, J. Coat. Technol. 1993, 65, 57). The as-synthesised particles were recovered by centrifugation and washed twice with methanol to remove any excess FAS, then left to dry overnight at room temperature.
The same procedure was used for functionalisation of: TiO2 particles with HDTMS; and Cu2O, ZnO and silica-TiO2 particles (TiO2 particles with a silica core) with FAS. All the particles (TiO2, ZnO, Cu2O, silica-TiO2) were first prepared in methanol at a concentration of 2 g/L, and 1% v/v FAS and HDTMS solutions were used. Different volume ratios of the particle suspension to the FAS/HDTMS solution were used for different particles. For example, a volume ratio of 1:40 was used for HDTMS:TiO2, 1:60 for FAS:ZnO, 1:100 for FAS:Cu2O, and 1:125 for FAS:silica-TiO2.
Glass substrates were cleaned with water and dried prior to FAS functionalisation. The substrates were submerged in methanol, followed by the dropwise addition of FAS solution. The resulting solution was stirred for 1 hr. The glass substrates were then removed from the solution and heated at 100° C. for 1 hr, before they were washed with methanol and dried.
Synthesis of Octahedral Cuprous Oxide Particles
Octahedral copper (1) oxide particles were synthesised following the procedure from Zhang et al. with slight modifications (D.-F. Zhang, et al., J. Mater. Chem. 2009, 19, 5220-5225). Briefly, at 55° C., 4.44 g of PVP was dissolved in 0.01 M aqueous solution of CuCl2. 2 M NaOH aqueous solution was added dropwise and stirred for 30 min, followed by the dropwise addition of 0.6 M ascorbic acid aqueous solution and stirred for 3 hrs. The recovered particles were washed with water and ethanol.
The preparation of the Janus particles of the current invention was carried out as shown in
In a petri dish containing water 14, the functionalised particles 12 were scattered over the surface of water and magnetically stirred for 30 min to disperse them across the air-water interface. The dish was placed directly under a Xe light source (Newport), at an irradiance at approximately 132 mW/cm2. Irradiation (16) of the particles was performed for 1- or 2-hr durations, producing Janus particles of varying wettability which is dependent on the irradiation duration. The surface exposed to the UV irradiation underwent photocatalytic decomposition of the hydrophobic silanes, resulting in the formation of hydrophilic surfaces 18. The resulting particles are denoted as FAS-TiO2-UV1h, FAS-TiO2-UV2h, HDTMS-TiO2-UV1h, and FAS-Cu2O-UV2h accordingly. FAS-functionalised glass substrates were placed directly under the light source for 2 hrs, and denoted as FAS-glass-UV2h.
The same procedure was also used on HDTMS-functionalised TiO2 particles, and for FAS-functionalised zinc oxide (ZnO) and 1 μm TiO2 particles with a silica core (silica-TiO2).
Results and Discussion
To allow the particles to align at the air-water interface prior to UV irradiation, they were first functionalised with FAS or HDTMS. These silanes were inherently superhydrophobic, owing to the fluorocarbon chains on FAS, and the long-chain alkylsilanes on HDTMS (E. Hoque, et al., J. Phys. Chem. C 2007, 111, 3956-3962). Theoretical calculations of the flotation behaviour of individual FAS-TiO2 particles were made based on the model by Kowalczuk et al. (P. B. Kowalczuk, et al., Colloids Surf. Physicochem. Eng. Aspects 2012, 393, 81-85). For FAS-TiO2, a range of floating positions at the interface was examined, revealing positive overall forces that indicated the dominance of the upward (floating) forces over the downward force. This was also observed experimentally: FAS- and HDTMS-functionalised particles had remained at the air-water interface even after a period of stirring.
The proposed mechanism for the degradation of FAS during UV irradiation is summarised in
The Janus particles of the current invention can also be prepared via UV irradiation of the particles assembled in Pickering emulsion in the presence of paraffin wax. The three main steps are as shown in
In the first step, FAS-TiO2 particles 12 (prepared in accordance to the general method described above) were dispersed on the surface of water 14 and heated to 75° C. This was then followed by the addition of paraffin wax flakes 22 (1:10 wax-water ratio by weight) into the mixture. The system was equilibrated at 75° C. for the wax to melt completely, then subjected to vigorous mixing using the T 18 digital Ultra-Turrax disperser (IKA) at 15,000 rpm for 2 mins. The resulting mixture was immediately left to cool, yielding wax droplets 23 that were recovered via filtration.
In the second step, the wax droplets 23 were dispersed in water 14, and then placed under a UV lamp 16 (wavelength 365 nm, irradiance of approximately 72 mW/cm2) for 1 and 2-hr durations, while the water phase was kept under constant stirring at 100 rpm. The surface exposed to the UV irradiation underwent photocatalytic decomposition of the hydrophobic silanes, resulting in the formation of hydrophilic surfaces 18. At the end of UV irradiation and in the final third step, the modified wax droplets 25 were filtered out and dispersed in xylene 24 to dissolve the wax core 22. The obtained asymmetric Janus particles 27 were recovered via centrifugation, then washed twice in methanol via centrifugation and redispersion, before being left to dry in ambient conditions overnight.
The same procedure was applicable and has been demonstrated on triethoxy(octyl)silane (OTES) modified TiO2 particles, FAS-treated 1 μm Sicastar™ spheres (micromod Partikeltechnologie GmbH) with silica-TiO2 core-shell structures.
The three types of wax droplets formed using the respective emulsifiers (FAS-TiO2, OTES-TiO2, FAS-silica TiO2) were filtered and dispersed over the surface of water, then placed under UV, which degraded the coating over the exposed particle surfaces. On the contrary, the surfaces buried in wax were not exposed to UV, and thus coatings on such surfaces were intact. The obtained asymmetric Janus particles, which consisted of particles containing partially coated FAS or OTES, were recovered by dissolving the wax in xylene.
The formed wax droplets 25 were observed under FESEM operating at 5 keV under secondary electron imaging mode (JEOL JSM-5500LV, JEOL JSM-6340F). Prior to visualisation, the wax droplets were dispersed on a conducting carbon tape, then sputtered with an approximately 10 nm layer of gold. The mean size of the wax droplets was counted by averaging the diameter of 20 different droplets.
The Janus particles of the current invention can also be prepared via UV irradiation of the functionalised particles assembled in a water-in-oil Pickering emulsion. The two main steps are as shown in
In the first step, water-in-oil (w/o) Pickering emulsions were made, using silicone oil 26 and water 14 in an oil:water volume ratio of mixture 90:10, and 1% weight of the FAS-functionalised particles 12 (relative to the entire system). The functionalised particles were first prepared in accordance to the general method described above. Typically, FAS-TiO2 was first dispersed in silicone oil 26 through sonication, then while under vigorous mixing using the T 18 digital Ultra-Turrax disperser (IKA) at 15,000 rpm, water 14 was added dropwise. The mixing proceeded for another 2 mins after all the water was added to complete emulsification. The resulting emulsions were viewed and captured under dark field optical microscopy (Olympus BX51, Infinity Analyze), and the droplet diameters were analysed and determined through ImageJ. Other volume ratios were also made, namely 10:90, 25:75, 50:50, and 75:25 (oil:water).
In the second step, the w/o emulsions were dispersed over a dish containing water 14, and then placed under a UV lamp 16 (wavelength 365 nm, irradiance of approximately 72 mW/cm2) for 30 minutes, while the water phase was kept under constant stirring at 100 rpm. The surface exposed to the UV irradiation underwent photocatalytic decomposition of the hydrophobic silanes, resulting in the formation of hydrophilic surfaces 18. After irradiation, the Janus particles 28 were recovered through centrifugation, then washed in xylene and methanol and left to dry in ambient conditions overnight.
Due to the high amphiphobicity of FAS, FAS-TiO2 particles naturally take the interface position between oil and water, and thus can act as Pickering emulsifiers. In an immiscible silicone oil-water system containing these particles, when energy was input into the system (in the form of high-energy mixing via the Ultra-Turrax), transient droplets form within the continuous (bulk) phase, and the particles take position at the droplet edges, eventually stabilising these droplets when an adequate droplet coverage is attained.
Both oil/water (o/w) and water/oil (w/o) emulsions were possible, though w/o emulsions were more favoured, giving rise to smaller-sized (and hence more stable) emulsion droplets. This is in line with Bancroft's rule; the slight preference of FAS for silicone oil over water resulted in silicone oil forming the continuous phase, while water formed the dispersed phase encapsulated in droplets. Phase inversion into o/w emulsions was also possible through limiting the amount of oil, leading to larger-sized emulsion droplets. All emulsions (w/o and o/w) also remained stable for at least a month, which also suggests the potential applications of FAS-TiO2 as emulsifiers beyond just employing them to create Janus particles.
The resulting Pickering w/o emulsions stabilised by FAS-TiO2 were observed under dark-field optical microscopy (
Irradiation of the emulsion droplets was carried out over the surface of water, which served to spread the emulsion droplets to allow for more efficient UV exposure. When irradiated by UV, the FAS coating over the exposed particles surfaces (those oriented outwards towards oil) were photodegraded, while the FAS coating on the unexposed surfaces (those oriented inwards towards water) intact. As a result, Janus-like particles consisting of TiO2 particles with FAS on one area were obtained (FAS-TiO2-UV).
For practical purposes, the bulk behaviour of the as-prepared Janus particles of the current invention was investigated. The overall wettability of the Janus particles was examined through static contact angles of water droplets atop powder films made out of the particles. Contact angles of the different substrate-silane combinations before and after irradiation were measured in accordance to the method described above.
Initial Studies
It was observed that surface functionalisation of the TiO2 particles with FAS rendered the particles amphiphobic, and their powder films exhibited high static contact angles of 144° against water (
As a control, the same procedure was carried out on a glass substrate (
Table 1 summarises the irradiation durations and the contact angle measurements of all experiments. Aside from the glass control, silica-TiO2, and Cu2O experiments, all other samples showed a significant decrease in contact angle within the specified UV irradiation period in comparison to samples prepared using the non-irradiated particles. As the significant changes were only observed in anatase TiO2 and ZnO (in which both are photocatalysts), this indicates that the presence of a photocatalyst is necessary for the accelerated photodegradation of the FAS. However, it was also observed that there was a decrease in contact angles for the glass control and silica-TiO2 particles after irradiation, probably due to the photodegradation of treated organic layer by UV, regardless of the presence of photocatalysts. The mechanism for such a photodegradation had been established (Ye, T. et al., J. Phys. Chem. B 2005, 109, 9927-9938). In the case of Cu2O, while there was no significant change in wettability, however, the gold NP attachment results (see Example 5) affirmed that the FAS layer had been degraded on its surface.
Subsequent Studies
The subsequent measurements of the static contact angles of the different substrate-silane combinations before and after irradiation are summarised in Table 2 and as shown in
The compacted Cu2O particles did not exhibit significant changes in contact angle after irradiation. This might be due to the octahedral shape of the particles, which maintain a higher degree of microscopic roughness in the powder film. Such a rough surface could maintain a high contact angle because of the trapped air on the surface structure, creating a heterogeneous surface following the Cassie-Baxter mechanism. Nevertheless, photocatalytic degradation of FAS on Cu2O had occurred and was verified by the gold labelling (see Example 5). Similarly, due to the microscopic roughness of the powder film, untreated Cu2O had an apparent contact angle of 67.1°, which was likely lowered by the roughness factor following the Wenzel equation.
For comparison, glass substrates were also treated with FAS and subjected to UV irradiation for 2 hrs. FAS treatment rendered the glass substrates superhydrophobic with a high contact angle of 119.1°. For smooth surfaces such as the glass substrates used here, 120° is the maximum contact angle attainable. Contrary to the FAS-TiO2 particles, even after irradiation of the glass substrates for 2 hrs, only a slight decrease in the contact angle was observed, which implies that most of the fluoroalkyl chains on the glass substrates were intact. This affirms that the presence of a photocatalyst accelerates the degradation of FAS to bring about larger changes in wettability. Nevertheless, degradation is still feasible for non-photocatalytic surfaces when subjected to a higher UV irradiance and/or longer irradiation durations.
The Pickering emulsification of wax droplets using FAS-TiO2 particles was achieved with droplet sizes having a mean diameter of 28 μm (
The OTES-TiO2 particles were also used for the formation of wax droplets (
In a similar procedure, FAS-silica TiO2 particles were also used as Pickering emulsifiers for the wax droplets. These particles gave contact angles of 145° against water and 45° against mineral oil (
After UV irradiation, the water contact angle of the recovered asymmetric Janus particles decreased. The FAS-TiO2 particles had a mean static contact angle of 120° after 1 hr of UV irradiation, and an angle of 53° after 2 hrs of irradiation. The mean static contact angles of OTES-TiO2 particles after irradiation for 1 hr and 2 hrs were 136° and 69°, respectively, and 127° and 112° respectively, for FAS-silica TiO2 (
For FAS-TiO2, the relatively lower contact angle of silicone oil (135.6°) compared to water (145.1°) indicated a slight preference of FAS-TiO2 for silicone oil over water. Consequently, water-in-oil emulsions were formed, following the Bancroft rule, which states that the phase in which the emulsifier is more soluble constitutes the continuous phase. In addition, the use of a large volume fraction of silicone oil (9:1 oil:water) also further promoted the formation of w/o emulsions. After 30 mins of UV irradiation, the water contact angle of FAS-TiO2-UV had decreased from 145.1° to 126.7° (Table 4), indicating less amounts of FAS on the surface of these particles. Silicone oil contact angles had decreased by a greater margin from 135.6° to 39.7°, though this was likely contributed by traces of silicone oil left on the particles. The static contact angles of silicone oil and water against various particle types are summarised in Table 4.
Fourier-Transform Infrared Spectroscopy (FTIR)
The Janus particles prepared from Example 1 were analysed by FTIR spectroscopy and their spectra are as shown in reported in
Relative to the particle bands, FAS and HDTMS peaks were expectedly weaker in intensity. FAS shows peaks in the 1100-1300 cm−1 region pertaining to C—F2 and C—F3 bonds (J.-D. Brassard, et al., Appl. Sci. 2012, 2, 453-464). The peaks at 1205 cm−1 and 1145 cm−1 (magnified in the insets of
Thermogravimetric Analysis (TGA)
TGA was performed on the as-prepared Janus particles of the current invention (of Examples 1-3) to provide a quantitative comparison of the surface coating (FAS and HDTMS) coating on the particles. At high temperatures, fluoroalkylsilanes thermally decompose at the methylene chain (T. Monde, et al., J. Ceram. Soc. Jpn. 1996, 104, 682-684), and similarly for alkylsilanes, this occurs at the CAC bond adjacent to the Si atom (G. J. Kluth, et al., Langmuir 1997, 13, 3775-3780). The amount of coating over the particles was inferred from weight losses occurring during the thermal decomposition in a nitrogen atmosphere.
Initial Analysis
In the initial studies, TGA was performed on the TiO2 particles before and after irradiation, and their respective weight losses are summarised in Table 5. The particles were subjected to a temperature ramp up to 800° C. in a nitrogen atmosphere. The respective weight loss curves with respect to temperature are shown throughout
In the case of the anatase TiO2 substrate, an average onset temperature of 438° C. and 401° C. was observed for FAS and HDTMS respectively. At such temperatures, the weight losses for both FAS and HDTMS were lower after they were UV treated when compared to their non-UV counterparts. This indicated quantitatively less amounts of FAS and HDTMS coating on the particles after UV. The results all agreed that UV irradiation had brought about a degree of photodegradation of the silane coating.
Subsequent Analysis
In subsequent TGA, untreated particles (both TiO2 and Cu2O), as well as FAS- and HDTMS-functionalised particles before and after UV irradiation were analysed. The results are summarised in Table 6, and the corresponding weight loss curves are as shown in
In all three cases, the onset temperature for FAS and HDTMS decomposition of the UV-irradiated particles were lower than their fully-functionalised counterparts. Lower weight losses were also observed for particles that were irradiated for all cases (FAS-TiO2-UV1h, HDTMS-TiO2-UV1h, and FAS-Cu2O-UV2h), which indicate less amounts of coating on particles that had been irradiated. This verifies that the irradiation process had partially degraded the coating on the particles. HDTMS-TiO2-UV1h also showed a proportionally larger weight loss than FAS-TiO2-UV1h, which was consistent with the relatively larger decrease in contact angle after 1 hr of irradiation.
Thermogravimetric analysis (TGA) performed on FAS-TiO2 and OTES-TiO2 particles gave quantitative insight into the amount of FAS and OTES coating present on the particle surfaces. Weight losses of the particles before and after 2-hr UV irradiation periods are summarised in Table 7, and their respective weight loss curves are shown in
Thermogravimetric analysis (TGA) was performed to assess quantitatively the amount of FAS on the particle surfaces. The weight losses occurring during the temperature ramp corresponded to the decomposition of materials throughout the process. Table 8 details the weight loss and their onset temperatures of the TGA performed, and
In order to visualise the distribution of coating over the particle surface after UV irradiation, the particles irradiated by UV were further modified with APTES and subsequently with gold nanoparticles (gold NPs). The schematic for this procedure is as shown in
Method (Gold Nanoparticle Labelling)
Typically, TiO2, FAS-TiO2, FAS-TiO2-UV1h, Cu2O, FAS-Cu2O, and FAS-Cu2O-UV2h particles were each treated with APTES following a similar surface functionalisation procedure. Untreated particles (TiO2, Cu2O) fully functionalised with APTES were used as the positive control sample, while FAS-functionalised particles not subjected to UV (FAS-TiO2, FAS-Cu2O) and functionalised with APTES were used as the negative control sample. A dispersion of particles (1 mg/mL) in ethanol was first prepared. 1% v/v APTES in ethanol was added dropwise into the dispersion, and the resulting solution was stirred for 1 hr and heated at 100° C. for 1 hr. The particles were recovered, washed twice in ethanol to remove excess APTES, and then dispersed in water. The gold nanoparticle (NP) solution (10 nm gold NP for TiO2 particles, 30 nm gold NP for Cu2O particles) was directly added to the aqueous particle solution in a pre-determined volume ratio. For example, 10 nm gold NP suspended in citrate buffer in a 1:4 (particle:gold NP) volume ratio was used. The system was sonicated at 37 kHz (Elmasonic S30H, Elma) for 1 hr and left to stand overnight. The recovered particles were then washed twice in water to remove un-adsorbed gold NPs. The particles were drop cast on aluminum foil, and then observed under FESEM (JEOL JSM-7600F) operating at an accelerating voltage of 10 keV in secondary electron imaging mode. As controls, untreated particles and particles fully coated with FAS were also treated with APTES and submerged in gold NP solution in the same procedure.
Other particles prepared from Examples 1-3 were treated using the same procedure described herein.
Results and Discussion
Under FESEM (
To affirm that the tiny particles on the surface of the larger particles were gold NPs, EDS was performed on FAS-TiO2-UV1h (
A similar gold NP attachment behaviour was observed in the case of Cu2O particles (
The silica-TiO2 particles were also subjected to gold NP labelling using 30 nm gold nanoparticles (
In the case of ZnO particles (
The gold NPs labelling procedure was also carried out on the Janus Particles of Example 2. To demonstrate the differences, two controls were used: FAS-TiO2 particles that had not been irradiated by UV were used as the negative control, while TiO2 particles fully treated with APTES (APTES-TiO2) were used as the positive control.
Both controls, as well as the FAS-TiO2 and OTES-TiO2 particles that have been irradiated for 1 hr were treated with APTES and gold NPs, and subsequently examined under SEM (
It was also observed that FAS-silica TiO2 particles had little or no gold NP on their surfaces (
To understand the behaviour of the as-synthesised Janus particles in water and oil, the oil/water partitioning of the irradiated particles was compared against non-functionalised and fully functionalised particles.
Method (Toluene/Water Partitioning)
In a glass container containing 1:1 volume ratio of toluene and water, approximately 2 mg of the particles was dispersed at the interface. The container was briefly sonicated at 37 kHz (Elmasonic S30H, Elma) for 10 s, manually shaken, and then left to stand for 15 min before photographs were taken.
Results and Discussion
Untreated TiO2, HDTMS-TiO2, and HDTMS-TiO2-UV1h particles were spread at the interface of an immiscible toluene/water mixture. Following disruption by sonication, untreated TiO2 dispersed into the bottom water phase, while HDTMS-TiO2 dispersed into the top toluene phase, verifying their respective hydrophilic and oleophilic characters (
According to Binks, Janus particles that contain equal areas of apolar (hydrophobic) and polar (hydrophilic) regions are both surface active and amphiphilic, and the two regions are characterised by the contact angles θA and θP, respectively (B. Binks, P. Fletcher, Langmuir 2001, 17, 4708-4710). The angle α indicates the position of the Janus boundary between the two regions, and ranges between 0° and 180° for Janus particles with varying ratios of both regions (
J is defined as the normalisation of the energy required to desorb the particle into the oil phase by that required to desorb it into the water phase. At J=1, maximum adsorption energy of the particle at the interface is attained, and thus is used to describe a perfectly balanced Janus particle. For HDTMS-TiO2-UV1h, J=5.90, which indicates a dominantly hydrophilic surface that agrees with the observed bulk contact angle. For homogeneous particles, α=θA=θP, and Eq. (1) may be reduced to:
HDTMS-TiO2 particles had a J value of 0.0060, while for untreated TiO2 particles J=1055.87, which agrees with the observed preference of HDTMS-TiO2 particles for the toluene phase and untreated TiO2 particles for the water phase. In contrast, a J value of 5.90 for HDTMS-TiO2-UV1h is much closer to 1, indicating higher adsorption energy to the interface. This agrees with their attachment to the toluene/water interface seen in
In the case of the FAS-treated particles, both FAS-TiO2 and FASTiO2-UV1h particles remained at the interface after disruption (
Similarly, both FAS-Cu2O and FAS-Cu2O-UV2h particles remained at the interface. While there was no discernible difference in the way they assembled at the interface, some FAS-Cu2O-UV2h particles had dispersed into the bottom water phase, which affirms their relative increase in wettability following UV irradiation (
In terms of stabilising emulsions, a Janus balance of 1, where desorption energies into the oil and water phase are equal, is most stable (S. Jiang, S. Granick, Janus balance of amphiphilic colloidal particles, AIP (2007)). Across the three types of Janus particles examined, FAS-TiO2-UV1h was closest to 1. FAS-Cu2O-UV2h has a J value of only 0.11 due to the microscopic roughness of the particles, which has overstated its contact angle, leading to an exaggerated energy required to desorb the particle into water. Conversely, the HDTMS-TiO2-UV1h had a J value much larger than 1. In all three cases, the particles remained at the interface for at least a month when left unperturbed, showing good stability.
The uneven topography of the skin makes it a challenge to attain an effective protection against UV, which requires an even application of sunscreen over the skin surface. TiO2 is widely used in sunscreen due to their UV-blocking properties when the particles are spread over the skin. To examine the efficacy of the Janus TiO2 particles of the current invention in sunscreen, the particles were incorporated into a formulation adapted from Wiechers et al. (Wiechers, S. et al., Cosmetics & Toiletries 2013, 128, 2-6).
For FAS-TiO2 Janus particles, having a partial FAS coating confers these particles with both amphiphilic (oil and water attracting) and amphiphobic (oil and water repelling) properties.
Such a configuration allows these particles to assemble favourably when incorporated in sunscreen as shown in
Method Table 10 lists the formulations used for the sunscreens. Different types of TiO2 particles were used to make sunscreens to assess the differences between them: untreated TiO2 particles (Untreated), fully coated FAS particles (FAS), FAS-Janus particles (FAS-Janus), fully coated OTES particles (OTES), OTES-Janus particles (OTES-Janus), and Blank representing no particles used. In Blank, water was added quantum satis to replace the weight percent of TiO2 particles. The procedure was as follows: all premixes (except Premix B) were first separately made through magnetic stirring at ambient conditions. In the main vessel, Premix A was heated to 80-85° C., Premix B (TiO2) was added and dispersed for 5 mins, then Premix C was added, stirring was increased, and continued for 15 mins. The mixture was left to cool with gentle stirring; once below 40° C., Premix D and Premix E was added in order. The final sunscreen was stirred for a further 15 mins to allow complete mixing.
To assess the Sun Protection Factor (SPF) of the sunscreen formulations, 20 mg of sunscreen was pipetted in a spiral fashion outwards, starting from the center, on a HelioScreen HelioPlate HD6 (Labsphere). The sunscreen was spread manually, with an index finger wearing a finger cot pre-wet with a small amount of sunscreen. The sunscreen was spread in a zigzag fashion from top to bottom, then the square plate was turned 90° clockwise and the motion was repeated. The spreading was performed for a total of 4 times, each starting from a different side of the square plate. The plate was left to dry for at least 15-30 mins before SPF measurements were taken. For each sunscreen, three different plates were used. The UV-2000S (Labsphere) was used to measure the SPF of each plate, using the COLIPA method and measuring five different spots on each plate.
Results and Discussion
In the initial studies, the sunscreen containing the FAS-Janus particle (at 5 wt. % particle loading) was compared against two controls: one made with untreated TiO2, and the other one made with TiO2 fully coated with FAS. They were evaluated based on their SPF rating and the results are as shown in (
In the subsequent studies, the SPF testing was extended to other sunscreens made with different particles (FAS, FAS-Janus, OTES and OTES-Janus, with 5 wt. % particle loading).
To evaluate further, different percent loading of TiO2 particles in the sunscreen and its impact on SPF was examined. In addition to 5.0 wt. % particle loading, 2.5% and 10.0% particle loading of TiO2 was examined, and water was added and reduced respectively, quantum satis. FAS, FAS-Janus, OTES, and OTES-Janus were used for these formulations. The resulting SPF values are shown in
To evaluate the assembly behaviour of the particles, the sunscreen formulations were applied on a synthetic skin substrate (VITROSKIN, IMS Inc.). To prepare the VITROSKIN for application, the VITROSKIN was cut into pieces measuring 6.2 cm×9.0 cm, and incubated for 24 hrs in a sealed chamber containing 15% weight glycerin in water at ambient temperature. 100 μL of each sunscreen was pipetted across a 6.2 cm×8.0 cm area on the VITROSKIN, and a finger cot was used to apply the sunscreen, first in a circular motion starting from one corner, then in a zigzag motion. The VITROSKIN was then left to dry in ambient conditions for 30 mins, before cutting into smaller samples to be viewed under FESEM (JEOL JSM-7600F) at 10 keV accelerating voltage, in secondary electron imaging mode. An approximately 5 nm of gold was sputtered over the samples prior to visualisation. Five different sunscreen formulations were examined: untreated, FAS, FAS-Janus, OTES, and OTES-Janus (at 5.0 wt. % particle loading).
Across formulations, sunscreen containing the untreated TiO2 particles (
As such, the use of the Janus particles of the current invention in sunscreen has been shown to provide an even application of sunscreen, which not only more effectively protects the skin from UV damage, but also potentially acts as a physical shield against small particulate matter in the environment. The penetration of dust and other pollutants common to urban areas into the skin may be prevented by the physical barrier imposed by the even spread of sunscreen.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10201806616T | Aug 2018 | SG | national |
| 10201902629Y | Mar 2019 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2019/050352 | 7/22/2019 | WO | 00 |
