SILICA COATING ON NANOPARTICLES

Abstract

This invention relates to a method for synthesizing a SiO2-coated nanoparticle, the method comprising the step of reacting a hydroxyl-functionalised silane with a nanoparticle in a substantially aqueous phase under conditions to induce silanization of the nanoparticle. The method enables silanization of the nanoparticle in aqueous phase that is substantially free of organic solvents.

Claims

1. A method for synthesizing a SiO.sub.2-coated nanoparticle, the method comprising steps of: reacting a hydroxyl-functionalised silane having the following formula (I)
[Math. 1]
R.sup.1(CH.sub.2).sub.nSi(OH).sub.3Formula (I) wherein R.sup.1 is selected from the group consisting of amino, glycidyl, mercapto and any mixture thereof; and n is an integer from 1 to 10; with a nanoparticle in a substantially aqueous phase and subsequently adding an aqueous ammonia solution under conditions to induce silanization of the nanoparticle.

2. The method according to claim 1, wherein the substantially aqueous phase comprises less than 5% (v/v) organic solvent or is substantially free of organic solvent.

3. (canceled)

4. The method according to claim 1, wherein the substantially aqueous phase is substantially water.

5. (canceled)

6. The method according to claim 1, wherein R.sup.1 is mercapto.

7. The method according to claim 1, wherein n is 3.

8. The method according to claim 1, wherein the hydroxyl-functionalised silane is formed by the hydrolysis of a silane precursor.

9. The method according to claim 8, wherein the hydrolysis is performed by mixing the silane precursor with substantially water to form the hydroxyl-functionalised silane.

10. The method according to claim 8, wherein the silane precursor has the following formula (II):
[Math. 2]
R.sup.1(CH.sub.2).sub.nSi(OR.sup.2).sub.3Formula (II) wherein R.sup.1 is selected from the group consisting of amino, glycidyl, mercapto and mixture thereof; R.sup.2 is an optionally substituted alkyl group; and n is an integer from 0 to 10.

11. The method according to claim 10, wherein R.sup.1 is mercapto.

12. The method according to claim 10, wherein R.sup.2 is methyl.

13. The method according to claim 10, wherein n is 3.

14. method according to claim 10, wherein the silane precursor is (3-mercaptopropyl)trimethoxysilane.

15. The method according to claim 1, wherein the nanoparticle is a metal nanoparticle or a metal oxide nanoparticle.

16. The method according to claim 15, wherein the metal is a transition metal.

17. The method according to claim 16, wherein the metal is selected from the group consisting of titanium, iron, platinum, copper, silver, gold, zinc and any mixture thereof or the metal oxide is selected from the group consisting of iron oxide, titanium oxide, zinc oxide and any mixture thereof.

18. (canceled)

19. The method according to claim 1, wherein the conditions to induce silanization of the nanoparticle comprises the use of a base or ammonia.

20. (canceled)

21. The method according to claim 1, wherein the reaction proceeds at a pH of greater than 7.

22. The method according to claim 1, wherein the reaction is performed at room temperature.

23. The method according to claim 1, wherein the reaction is performed for a duration of 2 to 4 hours.

24. The method according to claim 1, comprising the step of purifying the SiO.sub.2-coated nanoparticle.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0068] The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0069] FIG. 1 is a schematic illustration showing the synthetic step for coating SiO.sub.2 on metal NPs and metal oxide NPs in aqueous medium.

[0070] FIG. 2 is a TEM image of SiO.sub.2-coated Ag NPs. Scale bar represents 100 nm.

[0071] FIG. 3 is a TEM image of SiO.sub.2-coated Pt NPs. Scale bar represents 100 nm.

[0072] FIG. 4 is a TEM image of SiO.sub.2-coated Au NPs. Scale bar represents 100 nm.

[0073] FIG. 5 is a TEM image of SiO.sub.2-coated ZnO NPs. Scale bar represents 100 nm. The inset shows a magnified view of the SiO.sub.2-coated ZnO NPs with a scale bar of 20 nm.

[0074] FIG. 6 is a TEM image of SiO.sub.2-coated TiO.sub.2 NPs. Scale bar represents 100 nm.

[0075] FIG. 7 is a graph showing the EDX spectra of (a) coreless SiO.sub.2 NPs and (b) SiO.sub.2-coated Ag NPs. The insets are corresponding TEM images.

[0076] FIG. 8 is a graph showing the EDX spectrum of SiO.sub.2-coated ZNO NPs. The inset is its corresponding TEM images.

[0077] FIG. 9 is a graph showing the FTIR spectra of ZnO NPs before and after SiO.sub.2 coating. The siloxane bond is clearly observed after SiO.sub.2 coating.

[0078] FIG. 10 refers to .sup.29Si NMR spectra of (a) MPTMS monomer, (b) hydrolyzed MPTMS monomer, (c) SiO.sub.2 formed after 1 hour of reaction time, and (d) SiO.sub.2 formed after 3 hours of reaction time.

EXAMPLES

[0079] Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1

Synthetic Procedure

[0080] Silver nitrate (AgNO.sub.3, 99.5%), tetrachloroauric acid trihydrate (HAuCl.sub.4.3H.sub.2O, 99.9%), hexachloroplatinic acid hexahydrate (H.sub.2PtCl.sub.6.6H.sub.2O, 37.5%), 3-(mercaptopropyl)trimethoxysilane (MPTMS, 95%), and sodium citrate dihydrate (99%) were used as received from Sigma-Aldrich Ammonia solution (30 wt%) and absolute ethanol (99%) were also used as received from Mallinckrodt and Honeywell, respectively. Deionized water was used throughout all experiments.

[0081] Metal NPs and Metal Oxide Nanoparticles

[0082] Ag, Pt and Au NPs were selected to represent metal NPs and ZnO and TiO.sub.2 NPs were selected to represent metal oxide NPs.

[0083] Preparation of 50 nm Ag NPs

[0084] Typically, an aqueous solution of sodium citrate (10 mL, 40 mM) was added dropwise into a boiling aqueous solution of silver nitrate (490 mL, 1 mM) in a period of 2 min under vigorous stirring. After boiling for 1 h, the reaction solution was allowed to cool to room temperature followed by centrifugation at 500 rpm for 1 h to remove larger Ag NPs. After collecting by centrifugation, 50 nm Ag NPs were redispersed in 500 mL of deionized water for further use. The particle concentration of 50 nm Ag NPs in solution was 6.610.sup.10 particles per mL, based on the ICP-AES measurement.

[0085] Preparation of 50 nm Au NPs

[0086] A sodium citrate aqueous solution (5 mL, 40 mM) was quickly added into a boiling tetrachloroauric acid aqueous solution of (50 mL, 1 mM) under vigorous stirring. After boiling for 15 min, 15 nm Au NPs were produced and the reaction system was allowed to cool to room temperature. For the seeded growth to produce 50 nm Au NPs, a tetrachloroauric acid aqueous solution (125 mL, 0.3 mM) was heated to boiling, followed by injecting 1.125 mL of the as-prepared seed solution and 0.56 mL of the sodium citrate aqueous solution (40 mM) under vigorous stirring. After boiling for 30 min, an additional 5 mL of the sodium citrate aqueous solution (40 mM) was added, and the mixture was further refluxed for 1 h to stabilize the resulting 50 nm Au NPs. After collecting by centrifugation, 50 nm Au NPs were redispersed in 130 mL of deionized water for further use. The particle concentration of 50 nm Au NPs in solution was 6.410.sup.10 particles per mL, based on the ICP-AES measurement.

[0087] Preparation of 50 nm Pt NPs

[0088] A sodium citrate aqueous solution (5 mL, 40 mM) was quickly added into a boiling hexachloroplatinic acid aqueous solution of (50 mL, 1 mM) under vigorous stirring. After boiling for 1 h, 3 nm Pt NPs were produced and the reaction system was allowed to cool to room temperature. For the seeded growth to produce 50 nm Pt NPs, a hexachloroplatinic acid aqueous solution (125 mL, 0.3 mM) was heated to boiling, followed by injecting 1.1 mL of the as-prepared seed solution and 0.5 mL of the sodium citrate aqueous solution (40 mM) under vigorous stirring. After boiling for 4 h, an additional 0.5 mL of the sodium citrate aqueous solution (40 mM) was added, and the mixture was further refluxed for 2 h to stabilize the resulting 50 nm Pt NPs. After collecting by centrifugation, 50 nm Pt NPs were redispersed in 10 mL of deionized water for further use. The particle concentration of 50 nm Pt NPs in solution was 3.010.sup.10 particles per mL, based on the ICP-AES measurement.

[0089] The ZnO and TiO.sub.2 nanoparticles were used as-received from Sigma-Aldrich.

[0090] General Synthetic Procedure

[0091] Preparation of Ag@SiO.sub.2 NPs as an Example:

[0092] Experimentally, 100 L of MPTMS was added in 30 mL of deionized water, and the turbid mixed solution was shaken at 300 rpm for 1 h until a transparent solution was obtained at room temperature. Then, 3 mL of the pre-hydrolyzed MPTMS aqueous solution was mixed with 2 mL of the as-prepared stock solution of colloidal Ag NPs to form a clear solution (pH=5.5) followed by adding 20 L of ammonia solution (30 wt %) to form a basic solution (0.12 wt % ammonia concentration, pH=10.2) for silica coating. The mixture was continuously shaken at 300 rpm for different times (from 1 to 5 h) for silica coating, followed by centrifugation at 3000 rpm for 10 min to collect Ag@SiO.sub.2 NPs. The collected Ag@SiO.sub.2 NPs were purified by washing with deionized water via redispersion-centrifugation for three rounds. The obtained Ag@SiO.sub.2 NPs were redispersed in 2 mL of deionized water for further use. Following the same procedure, Au@SiO.sub.2 and Pt@SiO.sub.2 NPs were also prepared, purified and redispersed in 2 mL of deionized water.

[0093] The SiO.sub.2-coated metal NPs and metal oxide NPs are prepared by the aqueous-phase procedure as shown in FIG. 1. Initially, the metal and/or metal oxide NPs are dispersed in deionized water with mechanical stiffing, followed by the addition of an aqueous solution of pre-hydrolyzed 3-(mercaptopropyl)trimethoxysilane (MPTMS). Pre-hydrolyzed MPTMS is prepared by dissolving MPTMS in deionized water under continuous shaking until a transparent solution is obtained. The mixture is continuously stirred at room temperature, and the pH of the mixture is measured to be approximately pH 5.5. Subsequently, an aqueous ammonia solution (30 wt %) is added to the mixture to initiate the SiO.sub.2 formation. The final ammonia concentration is approximately 0.1 to 0.2 wt %. At this point, the pH of the mixture is measured to be approximately 10. The pH of the mixture is adjusted to approximately 10 by adding ammonica. It takes 5 to 10 minutes for the silica coating to start, with a thin layer of silica forming quickly, and the reaction is stirred for 3 hours to obtain a complete, thick SiO.sub.2 coating around the metal and/or metal oxide NPs. The process is carried out in one-pot and at room temperature. The resulting SiO.sub.2-coated metal NPs and metal oxide NPs are collected by centrifugation and purified by washing with deionized water via redispersion centrifugation. The obtained SiO.sub.2-coated metal NPs and metal oxide NPs are redispersed and stored in deionized water for further use.

Example 2

Characterization of the NPs

[0094] Transmission Electron Microscopy (TEM) Images

[0095] Transmission electron microscopy (TEM) images of the SiO.sub.2-coated Ag, Pt and Au NPs are shown in FIGS. 2, 3 and 4 respectively. FIG. 2 shows SiO.sub.2 coated Ag NPs having an Ag particle size of approximately 50 nm and a SiO.sub.2 layer thickness of approximately 30 to 40 nm. FIG. 3 shows SiO.sub.2 coated Pt NPs having a Pt particle sized of approximately 50 nm and a SiO.sub.2 layer thickness of approximately 10 to 15 nm. FIG. 4 shows SiO.sub.2-coated Au NPs having an Au particle size of approximately 50 nm and a SiO.sub.2 layer thickness of 50 to 70 nm.

[0096] TEM images of SiO.sub.2-coated ZnO and TiO.sub.2 NPs are shown in FIGS. 5 and 6 respectively. FIG. 5 shows SiO.sub.2-coated ZnO NPs having a particle size of approximately 100 to 150 nm and a SiO.sub.2 layer thickness of approximately 5 to 10 nm. FIG. 6 shows SiO.sub.2-coated TiO.sub.2 NPs having a TiO.sub.2 particle size of approximately 150 to 200 nm and a SiO.sub.2 layer thickness of approximately 10 to 15 nm.

[0097] All the TEM images show successful SiO.sub.2 coating, as observed from the uniform SiO.sub.2 layer around the entire NP. The images indicate effective and direct SiO.sub.2 coating on the surfaces of metal NPs and metal oxide NPs without any requirement of surface pre-treatment or modification.

[0098] Energy Dispersive X-ray (EDX) Analysis

[0099] The energy dispersive X-ray (EDX) analysis is used to investigate the major chemical elements of the SiO.sub.2 coated NPs. THE EDX spectrum of the SiO.sub.2 coated Ag NPs (FIG. 7) reveals the presence of oxygen, silicon and sulfur elements by peaks at 0.53, 1.74 and 2.3 keV, respectively, as the key constituents of the coated thiol-functionalized SiO.sub.2 shell. These peaks are present in both the coreless SiO.sub.2 NPs as well as the SiO.sub.2-coated Ag NPs. The presence of the Ag core for the SiO.sub.2 coated Ag NPs is confirmed by additional Ag-L peaks for L.sub., L.sub. and L.sub.2 at 2.98, 3.15 and 3.34 keV, respectively. It should be noted that the carbon peaks appear in both spectra due to the scattering caused by carbon tape used to mount the samples on the holder. In the same manner, the EDX spectrum of the SiO.sub.2-coated ZnO NPs (FIG. 8) confirms the existence of all the key constituents of both ZnO core and coated SiO.sub.2 shell. Again, the carbon peak in the spectrum appears due to the scattering caused by carbon tape used to mount the sample on a holder.

[0100] Fourier Transform Infrared (FTIR) Spectroscopy

[0101] Fourier transform infrared (FTIR) spectroscopy is further used to reveal the existence of the SiO.sub.2 shell formed. The FTIR spectrum of the SiO2-coated ZnO NPs (FIG. 9) clearly reveals the presence of a dominant peak corresponding to the siloxane bond (SiOSi) indicating the successful coating on the surface of the NPs.

Example 3

[0102] Investigation of the Mechanism

[0103] The mechanism of SiO.sub.2 coating on the surface of NPs in aqueous solution using MPTMS as the SiO.sub.2 precursor is investigated by using .sup.29Si nuclear magnetic resonance (NMR) spectroscopy. The .sup.29Si NMR spectra of MPTMS monomer, hydrolyzed MPTMS monomer and SiO.sub.2 formed are compared in FIG. 10. The SiO.sub.2 coating mechanism is described in combination with the .sup.29Si NMR results. T.sub.in represents the number of connectivity (n) of silicon atoms to form a siloxane bond (SiOSi).

[0104] Hydrolysis

[0105] Initially, an oil-in-water emulsion is formed when MPTMS monomer (FIG. 10a) is added to water, and the solution appears turbid due to MPTMS droplets. After shaking for about 1 to 2 hours at room temperature, the droplets gradually disappear, this being accompanied with the disappearance of turbidity. This occurs due to the hydrolysis of the three methoxy (OCH.sub.3) groups in the MPTMS molecule (HS(CH.sub.2).sub.3Si(OCH.sub.3).sub.3) to hydroxyl (OH) groups, resulting in the formation of the hydrolyzed MPTMS monomer (represented as the T.sub.0 species in FIG. 10b). The hydrolysis of MPTMS is experimentally revealed by the shift of the .sup.29Si NMR peak from 42.50 ppm for the non-hydrolyzed MPTMS monomer (FIG. 10a) to 40.26 ppm for the hydrolyzed MPTMS monomer (FIG. 10b).

[0106] Condensation and Cross-linking

[0107] The thiol tail group (SH) of the hydrolyzed MPTMS monomer can form strong chemical bonds with the surface of the metal and/or metal oxide NPs through chemisorption. The SH groups are bound to the metal NPs and metal oxide surfaces whereas the silanol groups (SiOH) are arranged outward from the surface for further condensation with the other hydrolyzed MPTMS monomer molecules. After the surface chemisorption of the hydrolyzed MPTMS monomer, condensation of silanol groups (SiOH+SiOH.fwdarw.SiOSi+H.sub.2O) takes place by the addition of ammonia as a cross-linking catalyst for siloxane bond (SiOSi) formation to produce dimers, represented as the T.sub.1 species in FIG. 10c. The further condensation of monomers and dimers forms short-chain and then long-chain polymers (T.sub.2 and T.sub.3 species in FIG. 10d) on the surface of the metal NPs and metal oxide NPs. The successive cross-linking of long-chain polymers yields an SiO.sub.2 shell around the metal NPs and metal oxide NPs. In the meantime, residual thiol groups are grafted on the SiO.sub.2 surface, resulting in a thiol-functionalized NP core-SiO.sub.2 shell structure. The condensation and cross-linking of the hydrolyzed MPTMS monomer molecules to form SiO.sub.2 on the surface of metal NPs and metal oxide NPs are revealed by the changes of the .sup.29Si NMR peaks. At an initial stage of condensation (i.e. after 1 hour of reaction time) shown in FIG. 10c, the peak intensity of the hydrolyzed MPTMS monomer (T.sub.0) chemisorbed on the surface of NPs at 40.26 ppm decreases, and a new peak for dimers (T.sub.1) at 49.43 ppm is observed. With the increase in the reaction time to 3 hours, the further condensation and cross-linking of monomers and dimers lead to two new peaks at 58.95 ppm for short-chain polymers (T.sub.2) and 67.95 ppm for long-chain polymers (T.sub.3) as shown in FIG. 10d. The broadness of these two peaks indicates a mixture of polymers with different chain lengths, revealing the extensive formation of SiO.sub.2 around the NPs.

INDUSTRIAL APPLICABILITY

[0108] The method may be useful in making SiO.sub.2-coated metal NPs and metal oxide NPs for use in applications where combined photocatalytic self-cleaning properties and UV-blocking properties are required. The SiO.sub.2-coated metal NPs and metal oxide NPs made by the method may be mixed with monomers to form polymer coatings on various surfaces. The SiO.sub.2-coated metal NPs and metal oxide NPs made by the method may be useful in coating glass windows, buildings and cars. The SiO.sub.2-coated metal NPs and metal oxide NPs made by the method may also be useful in water and air purification applications, as antibacterials, in coatings and paints, in diagnostic materials, sensors, cosmetic and as catalysts.

[0109] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.