BACKGROUND OF THE DISCLOSURE The present disclosure relates to functionalized hollow silica particles. More particularly, the disclosure relates to functionalized silica particles with substantially impervious silica shells.

Nano core/shell particles are submicroscopic colloidal systems composed of a solid or liquid core surrounded by a thin polymer or inorganic shell. This solid or liquid core is removed to form hollow nanospheres. Such core/shell systems may be prepared by deposition of the shell material onto a template particle, wherein the shell material can be either organic, inorganic, or hybrid. The selective removal of the core (template) material without disturbing the shell generates hollow particles. In many applications of hollow nanoparticles, for example in cases where they are used as drug delivery agents, or catalyst support, it is desired for the porosity and surface area of the material to be high, in order to ensure the delivery of the host molecule, or enough surface area for efficient catalysis. In applications in which delivery of voids is important for the application, such as in photonic band gap materials, thermal insulation materials or coatings, it is desirable that the porosity of the shell is minimized, to ensure the integrity of the central void of the hollow particle. Therefore a need exists for synthetic methods for substantially impervious hollow particles. SUMMARY OF THE DISCLOSURE

The disclosure provides functionalized hollow silica nanospheres, and allows for control of the porosity and surface area of the silica shells, providing access to substantially impervious hollow particles with tunable surface properties. The surface properties of the particles are tuned through grafting with a variety of functionalized alkoxysilanes. In a first aspect, the functionalized hollow silica particle comprises:

(i) a core/shell silica particle comprising a template core particle and a silica treatment, more typically a coating, wherein the core/shell silica particle has an outer surface; and wherein the silica treatment is prepared using a water-based silica precursor; and

(ii) a functionalized surface on the core-shell silica particle, wherein the functionalized surface is prepared using sulfonic acid, phosphonic esters, carboxylic acid, amines, epoxides, boronic acids or quarternary amines, and wherein the template core particle is removed before or after functional ization.

The template core particle from the core/shell silica particle may be removed before or after functionalization, more typically before

functionalization. If removed after functionalization, it is important the core removal does not damage the functionalized surface. For example, with a-methyl styrene, core removal is at low temperatures so no harm is done to the functionalized surface if core removal is achieved after

functionalization. Also, acid- or base-labile core materials that can be removed by hydrolysis can be removed before or after functionalization.

More typically the silica treatment is substantially impervious. By 'substantially impervious' we mean the surface area and porosity of the silica shell, typically walls, has to be tuned. Whether the silica shell is adequate can be determined by comparing the surface area of the particles with calculated surface area of a smooth sphere of the same diameter. Typically, we consider the shell substantially impervious if its surface area does not surpass about 130% of the calculated surface area of a smooth sphere of the same dimensions, i.e., it is about 30% or less higher than the surface of the core/shell silica particle compared to the calculated surface area of a smooth sphere of the same diameter, more typically about 125% of the smooth sphere surface area, and still more typically about 120% of the smooth sphere surface area of the same dimensions. Addition of various amounts of the silica precursor will lead to more or less porous silica layers, which can lead to control of the porosity and surface area of the particles. Further, the silica precursor may be added in stages to modulate the porosity of the particles as well as their surface. Lastly, calcination at temperatures higher than about 500 °C may decrease the porosity and surface area of the particles without increasing the thickness of the wall.

In the first aspect, the process for preparing the core/shell silica particle comprising a template core particle and a silica treatment comprises: a) providing a template core particle, more typically prepared using emulsion polymerization;

b) coating the template particle with a water-based silica precursor such as sodium silicate, potassium silicate, ammonium silicate or pre-formed silicic acid, more typically sodium silicate or potassium silicate; and

c) maintaining the pH at about 2 to about 10 to form a silica

treatment, typically a coating, on the template particle.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the process for making hollow silica particles, as described in Examples 3 and 5.

Figure 2 shows TEM images of hollow silica particles obtained by a procedure described in Example 3.

Figure 3 shows functionalization of hollow silica particles with

phosphonate ester through use of diethyl-(2-

(triethoxysilyl)ethyl)phosphonate, as described in Example 7.

Figure 4 shows the scheme for converting the phosphonate ester- functionalized silica from Figure 3, Example 7 into phosphonic acid- functionalized silica particles through hydrolysis of phosphonic ester groups, as described in Example 8. Figure 5 shows functionalization of hollow silica particles with (3- glycydoxypropyl)trimethoxysilane, as described in Examples 9 and 10.

Figure 6 shows the process for further functionalization of the epoxy silyl- funtionalized silica with glycine, to generate carboxyl group-functionalized silica particles through an amine linkage, as described in Example 9.

Figure 7 shows the process for further functionalization of the epoxy silyl- funtionalized silica with thioglycolic acid, to generate carboxyl group- functionalized silica particles through a thioether linkage, as described in Example 10. Figure 8 shows the process for functionalization of hollow silica particles with (diethoxyphosphoryl)methyl-2-((triethoxysilyl)ethyl)carbamat e, as described in Example 1 1 .

Figure 9 shows the TEM image of sample described in Example 5.

DETAILED DESCRIPTION OF THE DISCLOSURE

In this disclosure "comprising" is to be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but does not preclude the presence or addition of one or more features, integers, steps, or components, or groups thereof. Additionally, the term "comprising" is intended to include examples encompassed by the terms "consisting essentially of and "consisting of." Similarly, the term "consisting essentially of is intended to include examples encompassed by the term "consisting of."

In this disclosure, when an amount, concentration, or other value or parameter is given as either a range, typical range, or a list of upper typical values and lower typical values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or typical value and any lower range limit or typical value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the disclosure be limited to the specific values recited when defining a range.

In this disclosure, terms in the singular and the singular forms "a," "an," and "the," for example, include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to "hollow particle", "the hollow particle", or "a hollow particle" also includes a plurality of hollow particles.

A process is provided for preparing the hollow inorganic particles, typically hollow silica, through intermediacy of template particles, onto which the shell material is deposited, to generate core/shell particles. The process of silica deposition is such that it allows tuning of the surface area and porosity of the silica shells, thereby allowing for synthesis of impervious core/shell particles. The core material is removed to generate hollow particles. The resulting hollow particles are then functionalized with a variety of alkoxysilanes to generate functionalized hollow particles, with tunable porosity and surface area.

The particles, such as template particles, described herein are between about 100 to about 900nm in size, more typically between about 100 and about 600nm, and still more typically between about 150 and about 300nm. The disclosure describes the process for hollow silica particles with tunable porosity and surface area, as well as the functional group on the hollow particles' surface. Particles of about 50 to about 100nm in size may be used provided they do not have a surfactant layer on the outer surface of the particle. The functionalized hollow silica particle comprises:

(i) a core/shell silica particle comprising a template core particle and a silica treatment, more typically a coating, wherein the core/shell silica particle has an outer surface; and wherein the silica treatment is prepared using a water-based silica precursor; and

(ii) a functionalized surface on the core/shell silica particle, wherein the functionalized surface is prepared using sulfonic acid, phosphonic esters, carboxylic acid, amines, epoxides, boronic acids or quarternary amines, and wherein the template core particle is removed before or after functionalization.. The template core particle may be removed before or after functionalization, more typically before functionalization. If removed after functionalization, it is important the core removal does not damage the functionalized surface. For example, with a-methyl styrene core removal is at low temperatures so no harm is done to the functionalized surface if core removal is achieved after functionalization. Also acid- or base-labile core materials that can be removed by hydrolysis can be removed before or after functionalization.

The core/shell silica particle comprising a template core particle and a silica treatment, typically a coating, is prepared by a process comprising: (a) providing a template core particle, more typically prepared using emulsion polymerization;

(b) coating the template particle with a water-based silica precursor such as sodium silicate, potassium silicate, ammonium silicate or pre-formed silicic acid, more typically sodium silicate or potassium silicate; and

(c) maintaining the pH at about 2 to about 10 to form a silica

treatment on the template particle.

The template particle or core is prepared using typically an organic monomer which is polymerized to generate template particles, or dispersed in water to generate template particles of the appropriate size. Some monomes for the template include styrene, methyl methacrylate, polyacrylic acid, a-methylstyrene, lactic acid, formaldehyde, or copolymers like Surlyn® (copolymer of ethylene and methacrylic acid) more typically styrene, methyl methacrylate, α-methylstyrene, Surlyn®, and still more typically methyl methacrylate, styrene, or polyacrylic acid. Similarly, a group of two monomers can be chosen for a copolymerization, such as a variety of diacids and dialcohols for polyester polymers (like polyethylene terephthalate, PET), diacids and diamides for various polyamides (like Nylon 6,6, or other Nylons), etc. Typically, the particle size of the template is tunable, and the particle size distribution of the template particles achieved is narrow, which is advantageous. For example, preparation of the template particle or core by emulsion polymerization is achieved by emulsification of the water-insoluble monomer or a mixture of in water, and polymerized using radical polymerization conditions. Radical initiators such as potassium- or ammonium persulfate, and 2,2-azobis(2- methylpropionamidine) hydrochloride (AIBA) can be used. Surfactant can also typically be used. Some examples of suitable surfactants include sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), poly-(vinylpyrrolidinone) PVP, etc. In some cases, silyl group-containing monomers, such as 3-(trimethoxysilyl)propylmethacrylate can be used, in order to facilitate the silica deposition in the subsequent step. In order to perform the polymerization, the reaction temperature is kept between about 25 and about 100°C, more typically about 45 to about 90°C, still more typically about 55°C to about 75°C.

Alternately, the template particle or core may be inorganic, for example calcium carbonate, or other inorganic particles onto which silica can be deposited.

The template particle or core is then coated with a shell material to generate a core/shell particle. To generate a silica treatment or shell, at least one water-based silica precursor is used. Some examples of water- based silica precursors include sodium silicate, potassium silicate, ammonium silicate or pre-formed silicic acid, more typically sodium silicate or potassium silicate. When using organic siloxanes, the reaction is typically done in a dilute ethanol/water ammonia solution, with or without sonication. Typically, the suspension of template particles in a dilute ethanol/water ammonia solution is treated with a water-based silica precursor, such as sodium- or potassium silicate, the template particles are suspended in water, and the silicate agent is added either drop-wise, over a period of time, or all at once. The pH is maintained at about 2 to about 10, more typically about 5 to about 9 to form a silica layer on the recyclable template particle and the reaction times are held between about 1 to about 24 hours, more typically about 1 .5 to about 18 hours, still more typically about 2 to about 12 hours. This results in the deposition of a silica treatment or shell on the recyclable template particle or core.

The core/shell particles are separated or removed from the aqueous solution by centrifugation or filtration, more typically by

centrifugation. Typically, in order to form impervious silica shells, surface area and porosity of the silica walls have to be tuned. Whether the silica shell is adequate can be determined by comparing the surface area of the particles with calculated surface area of a smooth sphere of the same diameter. Typically, we consider the shell impervious if its surface doesn't surpass about 130% of the calculated surface area of a smooth sphere of the same dimensions, i.e., it is about 30% or less higher than the surface of the core/shell silica particle prior to functionalization, more typically about 125% of the smooth sphere surface area, and still more typically about 120% of the smooth sphere surface area of the same dimensions. Addition of various amounts of the silica precursor will lead to more or less porous silica layers, which can lead to control of the porosity and surface area of the particles. Further, the silica precursor may be added in stages to modulate the porosity of the particles as well as their surface. Lastly, calcination at temperatures higher than about 500°C can decrease the porosity and surface area of the particles without increasing the thickness of the wall.

The core may then be removed before or after grafting of a variety of alkoxysilanes onto the surface of the silica particles to form hollow silica particles having a functionalized surface. In one embodiment, removal of the template may be achieved through calcination, namely heating the material to about 300 to about 800 °C, more typically about 400 C to about 600 °C, and most typically about 450 to about 550 °C. In the case of CaCO3/silica core/shell particles, hollow particles are typically obtained through reaction with acid. If the core is made of recyclable material, it may be recycled either through thermal depolymerization, or acid- or base hydrolysis. In a specific embodiment, core materials made out of poly-(a- methylstyrene), PMMA, various polyamides, as well as styrene are depolymerized at increased temperatures, with the temperatures of depolymerization varying with the polymer used. Some suitable

temperature ranges include about 250 to about 450°C, more typically about 275 to about 400°C, still more typically from about 290 to about 325°C, to generate hollow particles as well as core monomer. For example, poly(methylmethacrylate)@silica core/shell particles can be heated above around about 300°C to generate methyl methacrylate monomer and hollow silica particles. Further, poly(a- methylstyrene)@silica can be heated to about above 60°C to generate hollow silica particles and a-methylstyrene monomer. Alternatively, acid- or base-labile core materials can be hydrolyzed instead of thermally depolymerized to generate hollow particles with possibility of monomer recycling. Polymers such as Delrin® (polyacetal), poly(lactic acid), as well as other polyesters can be depolymerized through acid hydrolysis. For example, treating polyacetal@silica with acid should generate hollow silica as well as aldehyde monomer that can be recycled in template particle synthesis. Similarly, polyesters or polyamides from core/shell particles can be recycled in the same fashion to generate diacid/dialcohol (diacid/diamine) monomer couples as well as hydroxylic or amino acids as monomers (like in the case of polylactic acid, for example). When the core is calcium carbonate, it is removed by acid treatment, which generates hollow particles.

The functionalized surface on the silica particle may be prepared using sulfonic acid, phosphonic esters, carboxylic acids, amines, epoxides, boronic acids, quaternary amines, etc. Grafting of a variety of alkoxysilanes onto the surface of the hollow silica particles provides functionalized hollow silica particles. A large spectrum of functionalities can be introduced onto the silica surface, for example silyl phosphonates, phosphonic acids, amines, alcohols, epoxides, carboxylic acids, thiols, thioethers, carbamates, isocyanates, quarternary ammonium ions, etc. The grafting process includes mixing the grafting agent with silica particles, with or without the solvent, with optional heating of the material, in the temperature range about 25 to about 150°C, more typically about 60 to about 130°C, still more typically about 80 to about 120°C, with or without the application of vacuum, in order to remove the volatile byproducts, like water or alcohols. In one embodiment of the disclosure, the hollow silica particles may be functionalized with

(diethoxyphosphoryl)methyl-2-((triethoxysilyl)ethyl)carba mate, introducing phosphonate functionality on the surface. In another embodiment of the disclosure, the silica particles may be functionalized with diethyl [2- (triethoxysilyl)ethyl]phosphonate to generate phosphonate-functionalized silica particles. Then, in another embodiment, phosphonate ester functionality on the surface of the silica particles may be hydrolyzed to generate phosphonic acid-functionalized hollow silica particles. In another embodiment of the disclosure, silica particles may be treated with (3- glycidopropyl)trimethoxysilane, to generate epoxy functionality on the silica surface. The epoxy silica may be then treated, in one embodiment of the disclosure, with glycine, to introduce carboxylic acid functionality through an amine linkage on the particle. In another, the epoxy silica may be treated with thioglycolic acid to introduce the carboxylic functionality through a thioether group.

APPLICATIONS These inorganic hollow particle dispersions are useful as hiding or opacifying agents in coating and molding compositions. They are also useful as photonic band gap or thermal insulation materials.

EXAMPLES

Example 1 : Polystyrene template particle synthesis To a 2L four-neck round bottom flask, equipped with a mechanical stirrer, thermometer, a reflux condenser, and a nitrogen inlet, was added styrene (18mL, 157.1 mmol), and 600mL of degassed water. Polyvinylpyrrolidinone, PVP (100mg) solution in 100mL of degassed water was then added. The resulting mixture was stirred at room temperature for 15min. The mixture was degassed by bubbling nitrogen for 20min. To the reaction was then added a degassed solution of 2,2-azobis(2- methylpropionamidine) hydrochloride, AIBA(100mg, 1 .1 mmol) in 100mL water, and the reaction was heated to 70°C overnight. Particle size analysis of the resulting suspension revealed particles with average particle size of 250nm.

Example 2: Preparation of PMMA recyclable template particle To a three-necked 250ml_ round bottom flask with 100.0 ml_ water was added methyl methacrylate (9.5g, 94.89mmol), 2- (methacryloxy)ethyltrimethylammonium chloride (0.125g of 80% aqueous solution, mmol), ethylene glycol dimethacrylate (0.4g, mmol), and AIBA (0.1 g, mmol). Trimethoxysilyl propyl methacrylate (0.5 g 2.01 mmol) was then added. The mixture was degassed by purging N2 for 10min, and then heated to 70°C under nitrogen overnight, to generate a white slurry of PMMA particles. Particle size analysis of the resulting suspension revealed particles with average particle size of 280nm.

Example 3: Preparation of PMMA silica core/shell particle, and its conversion to a hollow silica particle

To a 100ml three-necked round bottom flask, equipped with an overhead stirrer and a reflux condenser was added 25ml_ of PMMA suspension from Example 2, and the pH of the mixture was adjusted to 9 with addition of aqueous NaOH. To this mixture was added sodium silicate (4.72g of 26.5wt% aqueous solution, diluted to 10ml_ with water), and HCI (10ml_ of 1 .40M solution), using two syringe pumps

simultaneously, to maintain the pH in the 8-9 range. The rate of addition was 2ml_/h. After the addition, the temperature of the reaction was increased to 80°C, and the mixture was left stirring overnight. After that time, the mixture was cooled down, and the solids isolated by

centrifugation, and washed with water and ethanol, to generate a while solid (2.73g), whose TEM confirmed the core/shell structure. 500mg of this sample was placed in a tube furnace and heated to 500°C at a 1 °C/min rate, and kept at 500°C for ten hours. Upon cooling, 233mg of white solid was obtained, whose TEM images confirmed the hollow structure. Example 4: Preparation of a PMMA particle

To a three-necked round bottom flask, equipped with a reflux condenser, nitrogen inlet, an overhead stirrer, and a thermometer, was added water (720ml_), acetone (480ml_), and methyl methacrylate (14.2g, 0.149mol), and the mixture was degassed with nitrogen for 1 h. The reaction mixture was heated to 68°C, the 2,2'-azobis(2- methylpropionamidine)dihydrochloride was added as a solution in water (0.20g, 0.720mmol in 30.0 mL water), and the reaction mixture was left stirring at 68°C for 15 minutes. After that time, 3- (trimethoxysilyl)methacrylate (4.1 g, 0.0166mol) was added via syringe, and the reaction mixture left stirring for 45 minutes, and then cooled to room temperature. The particle size of the polymer particles generated was measured to be 190nm.

Example 5: Preparation of a PMMA silica core/shell particle, and its conversion to a hollow particle The material obtained in Example 4 (150ml_ of the suspension) was taken and concentrated in vacuo to remove acetone, until the weight of the mixture reached 100g. The material was then transferred to a 1 L three neck round bottom flask, equipped with an overhead stirrer and pH probe inlet. The pH of the mixture was adjusted to 9.5, and the mixture heated to 80°C. Through use of a dual syringe pump, sodium silicate (7.08 mL of 26.2% solution in water, diluted further to 20mL total volume), and HCI (2.01 mL, diluted to 20mL with water) were added to the flask over 5h. During that time, pH of the reaction was monitored, and maintained at 9.5 with addition of either acid (dilute HCI), or base (dilute NaOH). After the addition, the reaction was left stirring at 90°C overnight, then centrifuged and washed to isolate the solids (2.78g of core/shell particles was obtained). The solid material (1 .49g) was then placed in a tube furnace, and heated to 500°C at a rate of 1 .5°C/min, and then held at that temperature for 12h to generate hollow particles (550 mg).

Example 6: Preparation of PS/silica core/shell particle, and its conversion to a hollow silica particle To a three-necked 250ml_ round bottom flask, equipped with an addition funnel and a reflux condenser is added 25ml_ of polystyrene slurry from Example 1 , following with sodium silicate solution (26.5 wt%, 4.72g). The resulting mixture is heated to 80°C for three or more hours, and the solids are isolated by centrifugation. Upon washing, the core/shell particles are placed inside a tube furnace and heated to 500°C to generate hollow silica particles.

Example 7: Grafting the HSP with phosphonate ester

Solid hollow particles (10g) are dispersed in 300ml_ dimethyl formamide (DMF). To this suspension is added a diethyl-(2- (triethoxysilyl)ethyl)phosphonate (10ml_, 31 .Ommol ), and the mixture heated to 120°C overnight. The resulting material is centrifuged to remove the DMF solvent, and washed with ethanol. The presence of grafting groups can be measured by TGA and ESCA.

Example 8: Phosphonic acid-functionalized particles To a 1 L round bottom flask, equipped with an addition funnel and a reflux condenser are added 25.6g of phosphonate ester-functionalized particles (Example 5) and 400ml_ dichloromethane, and the mixture is kept under nitrogen. To the mixture is added trimethylsilyl bromide (75ml_), dropwise, via an addition funnel. Upon addition, the mixture is heated to reflux for 18h. The mixture is then cooled to room temperature and the volatiles removed in vacuo. To the residue is then added 150ml_ of methanol, and 50ml_ of dichloromethane, and the mixture left stirring at room temperature overnight. The silica material is then centrifuged to remove from the solvent and excess reagents, and washed with water and methanol. ToF SIMS data can confirm the presence of phosphonic acid functionality, and disappearance of phosphonic ester functionality. Example 9: Epoxide-functionalized hollow silica particles, followed by reaction with glycine

To a mixture of 10mL of DMF and 500uL of triethylamine (TEA) is added 38.5mg of hollow silica particles, and the mixture is sonicated in an ultrasound (US) bath for ~15min. To the mixture is then added 1 ml_ of (3- glycydoxypropyl)trimethoxysilane, and the mixture is then heated to 120 C. After three or more hours, the mixture is cooled to r.t., and 24mg glycine in 200uL of water is added to the mixture, which is left stirring overnight. The sample is isolated by centrifuging and washing the solids with ethanol twice, and drying the solids. The material can be analyzed by ESCA to confirm the presence of nitrogen atoms on the silica surface.

Example 10: Epoxide-functionalized hollow silica particles, followed by reaction with thioglycolic acid

To 10ml_ of DMF is added 36.5mg of hollow silica particles, and the mixture is sonicated in a US bath for ~15min. To the mixture is then added 1 ml_ of (3-glycydoxypropyl)trimethoxysilane, and the mixture is heated to 120°C overnight. The mixture is then cooled to r.t., and, 500μΙ_ of thioglycolic acid is added, and the mixture left stirring for two days. The sample is isolated by centrifuging and washing the solids with ethanol twice, and drying the solids. The material can be analyzed by ESCA to confirm the presence of sulfur atoms on the silica surface.

Example 1 1 : Phosphonic ester (carbamate)

Solid hollow particles (1g) are dispersed in dilute ammonia solution (7wt%, 20ml_), with sonication. The resulting suspension is added to 30ml_ DMF, and the water was removed in vacuo. To this suspension is added (diethoxyphosphoryl)methyl-2-((triethoxysilyl)ethyl)carbamat e (1 g, 2.49mmol), and the mixture is then heated to 120 °C overnight. The resulting material is centrifuged to remove the DMF solvent, and washed with ethanol. The presence of grafting groups can be measured by TGA and ESCA.