Method for obtaining pore-expanded mesoporous silica nanoparticles

Abstract

The invention relates to a method for preparing pore-expanded mesoporous silica nanoparticles, comprising: - a first step involving preparing mesoporous silica nanoparticles using surfactants as templating agents; - and a second step of degradation of the silica nanoparticles using an amino tri-alcohol of general formula NR1OH R2OHR3OH, wherein R1, R2 and R3 represent identical or different organic groups, preferably hydrocarbon groups and particularly aliphatic chains with a chain length of less than four, and more preferably 2,2',2''-nitrilotriethanol or triethanolamine, the second step being carried out in water.

Description

[0001] PROCEDURE FOR OBTAINING MESOPORE SILICA NANOPARTICLES WITH EXPANDED PORES

[0002] Technical Sector

[0003] The present invention is in the field of the preparation of mesoporous silica materials.

[0004] State of the art

[0005] Siliceous mesostructured materials possess well-defined properties that make them unique. These include their ease of preparation and low cost, good biocompatibility, high surface area, modulation of both particle size and morphology, ordered pore structure, functionalizable surface, and high chemical and thermal stability, among others. This results in a wide range of applications, such as catalyst support, adsorption and separation, liquid chromatography, removal of contaminants from water, contrast agents in biomedicine, drug delivery, and more.

[0006] However, as with zeolites, the application of mesoporous materials is limited by pore size. Larger pores in the particles improve the capacity to hold active substances for controlled-release applications, increase diffusion rates through the pores, and broaden the range of molecules with which they can interact, facilitating interaction with enzymes and proteins—a capability typically limited by conventional mesoporous materials. The release of substances such as drugs, especially larger ones, is not as effective in small mesopores formed by surfactant micelles because diffusion processes depend on the cavity diameter. Furthermore, depending on the intended application of the final material, the pore diameter can be a limiting factor, as only molecules smaller than the mesopore can be introduced.Therefore, for certain applications we need the mesopore to have a larger diameter. Different strategies can be followed to obtain a larger mesopore:

[0007] Addition of a pore-expanding agent: This method involves the use of hydrophobic molecules, known as expanding agents, which are incorporated into the micelles, causing them to increase in size. Therefore, there is a combination of phases; that is, during synthesis, an organic phase and an aqueous phase are in contact. This can alter the hydrolysis and condensation processes of silica, resulting in a less homogeneous particle size distribution and affecting the degree of order in the mesostructure. These negative effects occur because silica formation happens simultaneously with the stabilization of the expanded micelles.The pore diameter can be increased by a couple of nanometers without disrupting the order of the mesostructure. However, if a much larger pore is desired by adding more expanding agent, the order of the pore system is completely lost, and in extreme cases, the structure will collapse. Furthermore, depending on the expanding agent used, a wide variety of pores can be produced. That is, a combination of pores with many different diameters can be found in the same product. Therefore, it is difficult to control the resulting pore size and ensure product homogeneity. Additionally, from a preparative standpoint, working in a multiphase system increases the number of synthesis steps and decreases reproducibility.

[0008] Mixture of surfactants:

[0009] By combining surfactants of different natures and sizes, a mixed micelle is formed. This micelle has properties intermediate between those of each surfactant individually. In other words, the size of this micelle falls between the sizes of the micelles formed by the surfactants alone. Typically, one of the surfactants used is a Pluronic, and the other is CTAB (cetylmethylammonium bromide), or a similar surfactant, as it aids in micelle formation. Depending on the molar ratio between the surfactants, the desired morphology can be obtained, or a wide variety of products can be produced. This synthesis allows for limited control over the resulting materials. Therefore, precise control over the molar ratio of the surfactants is essential to achieve the desired morphology.Pluronic can also inhibit particle formation, preventing them from growing and making them smaller compared to synthesis without it. This is because it can envelop the particles through hydrogen bonds, preventing further silica condensation onto them. This results in materials with low surface area and a less homogeneous pore distribution, with different pore families.

[0010] Bulkier Surfactants: Another strategy used to increase micelle size is the use of bulkier surfactants than CTAB. CTAB is typically the most commonly used surfactant in the synthesis of mesoporous siliceous materials, resulting in a pore diameter of 2.6 nm. With CTATos (cetyltrimethylammonium tosylate), larger pores are obtained, around 10 nm, but with less homogeneous pore distributions. Furthermore, the resulting materials have a low surface area, less than 600 m².2 / g. Bulkier surfactants can also be used in conjunction with CTAB. In this case, TOMAB (triocetylmethylammonium bromide) acts as an auxiliary agent, thus increasing the size of the micelles. Specifically, the pore diameter increases from 2.6 nm to 10.6 nm. However, these materials have a very low surface area, as it does not reach 100 m². 2 / gy the pores are more irregular.

[0011] Pseudomorphic transformations or chemical etching:

[0012] In this case, the process begins with solid, spherical silica particles, which are subjected to various procedures to impart the porosity they lacked initially. These procedures can be carried out in different ways, but all share the common characteristic of being highly aggressive to the particles. One method involves basic conditions, using a base such as NaOH, Na₂Cl₃, NaBH₄, or NH₃, and optionally a template agent, usually a surfactant. The entire process takes place in solution, and to further enhance the process, it is performed at high temperatures. In other cases, hydrothermal oxide (HF) is used, as the process is much faster, but it is more aggressive to the particles. Another method is acidic media (HCl or H₂SO₄), with or without hydrothermal treatment at 180 °C. Inorganic salts, such as NaCl or Na₂SO₄, are also added to aid the process. To prevent the entire surface of the particle from being attacked, protective coatings can be applied.They are usually made up of polymers, polyelectrolytes or surfactants, in this way the surface of the particle is partially protected against etching.

[0013] During the pseudomorphic transformation process, particle size increases because all the material removed to form the pore network is deposited on the particle surface. Furthermore, more material is typically added than the initial particles. The resulting cavities have large diameters, exceeding 30 nm, but mesopores with diameters around 2 nm are also formed. These materials have a low BET area; many do not exceed 300 m². 2The resulting mesopore volume is also very low. These syntheses make it difficult to have rigorous control over the resulting pore size and the final particle size. In other experiments, chemical etching is performed under basic conditions, but without surfactant, and the process is repeated several times consecutively. As before, they start with solid spherical particles and perform the etching using a 2.6 M ammonia solution, but they do not use hydrothermal treatment or heating. These conditions are slightly less aggressive. The problem they present is that they require the starting particle to have low lattice condensation. This favors etching compared to silica with a high degree of condensation. The resulting materials have a surface area of ​​less than 500 m². 2 / gy the pore diameter remains around 3 nm. Therefore, if we start with solid particles and under less aggressive conditions, we do not obtain an increase in size since the pore obtained has a similar size to that obtained using CTAB as a pore-directing agent.

[0014] Silica degradation:

[0015] Silica is chemically unstable in aqueous media or under biological conditions, contrary to common misconceptions. It can degrade to a certain point, which corresponds to the maximum solubility of the resulting degradation product, silicic acid. Solubility depends on the surrounding environment; in water at room temperature, its solubility ranges from 0.1 to 0.2 mg / mL.

[0016] Many studies have been conducted on the degradation of mesoporous silica particles, but all of these studies are aimed at biomedical applications because they will be used in drug delivery. Therefore, they are long-term studies that observe how the particles gradually degrade, becoming less dense, increasing in pore size, and losing their shape until many of them eventually collapse, resulting in small, shapeless fragments. Furthermore, the degradation media used are usually biological, such as PBS (phosphate-buffered saline), SIP (simulated intestinal fluid), SGF (simulated gastric fluid), SBF (simulated body fluid), etc. (ref. 1, 2, 3).

[0017] As an additional example, we can cite the article by Yamada, H.; Urata, C.; Aoyama, Y.; Osada, S.; Yamauchi, Y.; Kuroda, K. Preparation of Colloidal Mesoporous Silica Nanoparticles with Different Diameters and Their Unique Degradation Behavior in Static Aqueous Systems. Chem. Mater. 2012, 24 (8), 1462-1471. https: / / doi.Org / 10.1021 / cm3001688.

[0018] The document CHENG, Q., et al., (4) discloses a procedure for preparing nanoporous silica nanoparticles (MCM-41) comprising mixing previously synthesized mesoporous silica nanoparticles with a template surfactant, triethanolamine (TEA), in solution with ethanol, under vigorous stirring, at 0.1 MPa for 6 h at room temperature. In this way, the TEA is introduced into the pores of the silica, maintaining its structure. The final solid product is washed with ethanol, centrifuged to remove surface-adsorbed TEA, and dried at 50°C. e C.

[0019] According to the procedure followed in this document, no degradation or expansion of the mesopores occurs. The article does not indicate that an expanded pore material is obtained. Specifically, Figure 2 of this document shows a hysteresis loop that maintains a type IV isotherm with a single ramp around P / PO = 0.2–0.4, which does not correspond to the appearance of secondary pores, unlike the present invention.

[0020] The article by ZHANG, H., et al., (5) discloses a procedure for preparing mesoporous silica nanoparticles comprising mixing previously synthesized mesoporous silica nanoparticles with a template surfactant using dry toluene, maintaining the suspension for 20 h under reflux. The solid product is recovered by filtration and repeatedly washed with toluene and octane and dried overnight at 80 °C.

[0021] According to the procedure described in this document, there is no degradation or expansion of the mesopores. The article does not state that an expanded pore material is obtained; rather, the triethanolamine remains within the pores. Therefore, neither the procedure nor the objective of the present invention coincides with that of the present invention.

[0022] The article BELMABKHOUT, Y., et al., (6) discloses a procedure aimed at studying CO2 adsorption under various conditions, for which they prepare adsorbents comprising MCM-41 with expanded pores. However, they do not observe that different pore sizes are obtained. According to the document, amines are used to increase the mesopore size, but they do not degrade it. Instead, they are incorporated from the first stage of synthesis, along with the surfactants, to produce larger mesopores.

[0023] The degradation of nanoparticles in water, as carried out according to the present invention, involves: preservation of the spherical morphology of the particles and removal of some of the silica, resulting in the loss of some of the organization conferred upon the initial particles by the use of surfactant micelles as structural directing agents. According to the present invention, spherical silica mesoporous particles are obtained with a pore diameter adjustable between minimum values ​​greater than 2.5 nm and maximum values ​​up to 20 nm, preferably greater than 4.5 nm and maximum values ​​up to 20 nm, and more preferably greater than 6.5 nm and maximum values ​​up to 20 nm, with surface areas always greater than 600 m². 2 / g, preferably higher than 700 m 2 / g, and total pore volumes greater than 0.8 cm 3 / g, which implies a high host species carrying capacity. The procedure is simple, requiring only agitation at a controlled temperature, and is reproducible. By varying the degradation time, the pore size and volume can be modulated as needed.

[0024] Furthermore, homogeneous particles in size are obtained, not a variety of different materials.

[0025] The materials obtained according to the procedure of the present invention have a high surface area, greater than 600 m² 2 / g, in some cases up to 1150 m 2 / g, and the pores are regular.

[0026] With the procedure of the present invention, it has been possible to modulate the degradation of silica particles, and consequently their porosity, in a simple way by modifying synthesis variables (variation of the concentration of the medium and variation of the degradation time).

[0027] For this purpose, an amino alcohol, such as ethanolamine, was used because it maintains the pH of the medium under basic conditions and can also form complexes with silica. Furthermore, these degradation conditions are milder than those used in other known strategies.

[0028] The main advantage is that the procedure of the present invention allows for obtaining particles with a large mesopore size by emptying the material while maintaining very high surface area and pore volume. Furthermore, the procedure enables the production of highly homogeneous pore distributions with gradually adjustable size without the use of complex and expensive reagents, and through an easily scalable process. The increased pore size, along with the preservation of a high pore area and volume, allows for increased material loading in release processes, increased size of reactants and / or products in catalysis, protein capture, or protein inclusion to enhance enzymatic activity, etc.In small pore materials there is a clear limitation derived from their size, which prevents the capture or delivery of large molecules, typical in biomedical applications, such as many enzymes and proteins, among other species of interest.

[0029] References:

[0030] (1). Bhavsar, D.; Patel, V.; Sawant, K. Systematic Investigation of in Vitro and in Vivo Safety, Toxicity and Degradation of Mesoporous Silica Nanoparticles Synthesized Using Commercial Sodium Silicate. Microporous Mesoporous Mater. 2019, 284 (January), 343-352. https: / / doi.Org / 10.1016 / j.micromeso.2019.04.050.

[0031] (2) Travaglini, L; Picchetti, P.; Totovao, R.; Prasetyanto, EA; De Cola, L. Highly Degradable Imine-Doped Mesoporous Silica Particles. Mater. Chem. Front. 2019, 3 (1), 11 1-119. https: / / doi.org / 10.1039 / c8qm00438b.

[0032] (3) Hadipour Moghaddam, S. P.; Mohammadpour, R.; Ghandehari, H. In Vitro and in Vivo Evaluation of Degradation, Toxicity, Biodistribution, and Clearance of Silica Nanoparticles as a Function of Size, Porosity, Density, and Composition. J. Control. Release 2019, 311-312 (May), 1-15. https: / / doi.org / 10.1016 / j-jconrel.2019.08.028.

[0033] (4) CHENG, Q., “Structural and electrorheological properties of mesoporous silica modified with triethanolamine, Colloids and Surfaces A: Physicochemical and Engineering Aspects” 04 / 01 / 2008, Vol. 318, páginas 169-174, ISSN 0927-7757, <DOI: 10.1016 / j.colsurfa.2007.12.044>;

[0034] (5) ZHANG, H., et al., “Study of CO2 Capture Using Triethanolamine-modified Mesoporous Silica, Materials Science Forum”, 10 / 06 / 201 1 , Vol. 688, páginas 286-290, ISSN 0255-5476 (print), <DOI: 10.4028 / www.scientific.net / MSF.688.286>.

[0035] (6) BELMABKHOUT, Y., et al. "Effect of pore expansion and amine functionalization of mesoporous silica on CO2 adsorption over a wide range of conditions. Adsorption", 04 / 17 / 2009, Vol. 15, pages 318-328, ISSN 0929-5607 (print), ISSN 1572-8757 (electronic),<DOI: 10.1007 / s10450-009-9185-6> .

[0036] Detailed description of the invention

[0037] The present invention relates to a process for obtaining mesoporous silica nanoparticles with expanded pores and gradually adjustable size, comprising: - a first step comprising preparing mesoporous silica nanoparticles with the aid of surfactants as template agents,

[0038] - and a second stage of degradation of the mesoporous silica nanoparticles using an aminotriacohol of general formula NR 1 OH R 2 OHR 3 OH, in which R 1 R 2 and R 3They represent equal or different organic groups, preferably hydrocarbons and particularly aliphatic chains of chain length less than four and more preferably 2,2',2"-nitrilotriethanol or triethanolamine, wherein the second stage is carried out in water.

[0039] The fact that the second stage according to the procedure of the present invention is carried out in water means that both the prepared mesoporous silica nanoparticles are dispersed in water and the triaminoalcohol is dissolved in water. Furthermore, the presence of water in the medium is necessary for the hydrolysis reactions to take place, leading to the degradation and consequent increase in porosity of the silicas.

[0040] The invention process allows for obtaining mesoporous silica nanoparticles with a gradually expanding and adjustable pore size, meaning that the final nanoparticles are not restricted to specific sizes. The size of the second pore can be increased gradually and stopped at the desired point.

[0041] The mesoporous silica nanoparticles obtained in the first stage contain a single pore, which we call the "initial pore" or "first pore" (generated by the template effect of the surfactant micelles), which in the second stage can be lost, while a "second pore" is formed.

[0042] The amino alcohol, as the active agent in an aqueous medium during the second stage, allows for the generation of larger, more malleable mesopores. The amino alcohol penetrates the "first pore" and initiates an etching process (degradation or chemical attack) that extracts silica from the particles and generates the "second pore," which ultimately becomes the larger pore. Thus, the degradation process increases the size of the "first pore" and also generates a "second pore" larger than the first, which ends up being the dominant one. Therefore, the degradation process in the second stage of synthesis always produces an expansion of pore sizes, to a greater or lesser extent depending on the experimental conditions.

[0043] For this process to occur, the reaction medium in which the mesoporous silica nanoparticles obtained in the first stage and the aminotyl alcohol to be used in the second stage are suspended must be water. This allows the aminotyl alcohol to act as both a base and a ligand capable of interacting with the silicon atoms.

[0044] According to particular embodiments of the invention's procedure, the second stage comprises:

[0045] - mix a dispersion of silica nanoparticles with an aminotrial alcohol solution

[0046] - Keep the mixture agitated at a temperature between 10 and 50 e C, and preferably between 20 and 37 e C.

[0047] According to particular embodiments of the invention's procedure, the second stage comprises:

[0048] - mix a dispersion of silica nanoparticles with an aminotrial alcohol solution

[0049] - Keep the mixture agitated at a temperature between 10 and 50 e C for a period between 1 and 240 hours.

[0050] This interval between 1 and 240 hours corresponds to the degradation time.

[0051] The dispersion of the nanoparticles and the dissolution of the aminotrialcohol is in water.

[0052] According to particular embodiments of the invention's procedure, the second stage comprises:

[0053] - mix a dispersion of silica nanoparticles with an aminotrial alcohol solution

[0054] - Keep the mixture agitated at a temperature between 10 and 50 e C, and preferably between 20 and 37 e C, for a period between 1 and 240 hours, and preferably between 10 and 48 hours.

[0055] According to particular embodiments of the invention's procedure, the second stage comprises:

[0056] - disperse in water the mesoporous silica nanoparticles obtained in the first stage,

[0057] - apply ultrasound,

[0058] - Dissolve the aminotrialcohol in water,

[0059] - Mix the silica nanoparticle dispersion with the aminotrial alcohol solution, - Keep the mixture under agitation at a temperature between 10 and 50 e C, and preferably between 20 and 37 e C, for a period between 1 and 240 hours, and preferably between 10 and 48 hours.

[0060] Ultrasound is applied for a period of between 5 and 25 minutes, preferably between 10 and 20 minutes, and more preferably, 10 to 15 minutes.

[0061] The ratio of aminotrialcohol to silica nanoparticles can be between 1:1 and 250:1 by weight, preferably between 1:2.5 and 1:100 by weight.

[0062] The weight ratio of aminotrial alcohol and silica nanoparticles may be between 0.2 and 50 g of aminotrial alcohol, preferably triethanolamine or 2,2',2"-nitrilotriethanol, per 0.2 g of silica particles, and preferably between 0.5 and 20.

[0063] The dispersion ratio of nanoparticles to aminotrial alcohol solution, preferably triethanolamine or 2,2',2”-nitrilotriethanol, is between 50 and 300 mL of solution for 0.2 g of silica nanoparticles, and preferably between 100 and 200 mL for 0.2 g of silica nanoparticles.

[0064] After mixing the silica nanoparticle dispersion with the aminotrial alcohol solution, preferably triethanolamine or 2,2',2"-nitrilotriethanol, the pH is measured. The pH should be between 8.5 and 10.5, preferably between 9 and 10.

[0065] The mixture of the dispersion and the solution is kept under constant agitation at a speed between 250 and 400 rpm, preferably between 260 and 350 rpm, and more preferably between 280 and 310 rpm.

[0066] The procedure of the invention, according to particular embodiments, further comprises, after keeping the dispersion and dissolution mixture under agitation:

[0067] - to centrifuge,

[0068] - wash and

[0069] - dry the mesoporous silica nanoparticles with expanded and modular pore, obtained.

[0070] The spin cycle is performed at a speed between 10,000 rpm and 14,000 rpm, preferably between 12,000 and 13,500 rpm. The spin cycle lasts between 5 and 20 minutes, preferably between 10 and 15 minutes.

[0071] According to specific embodiments, the spin cycle is performed at a speed between 10,000 rpm and 14,000 rpm, preferably between 12,000 and 13,500 rpm, and for a time between 5 and 20 minutes, preferably between 10 and 15 minutes. More preferably, the spin cycle is performed at a speed between 12,000 and 13,500 rpm, and for a time between 10 and 15 minutes.

[0072] The washing can be done with water and then with alcohols, such as ethanol or methanol, or with acetone, and preferably with ethanol.

[0073] The drying process can be done, for example, in an oven, or in the open air.

[0074] The drying process can be done, for example, in an oven, at a temperature between 60 and 200 e C, preferably between 60 e C and 100 e C, more preferably between 75 e C and 80 eC. Air drying requires very long times of up to 2 days, for example, drying can be done at room temperature for a time between 3 hours and 2 days, preferably between 5 and 24 hours.

[0075] The particles obtained are spherical, similar to those synthesized in the first stage before treatment with aminotrial alcohol. Thus, no change in shape is observed after treatment with aminotrial alcohol, with the change in diameter being very small.

[0076] It is important to obtain individual particles of homogeneous size, and not large aggregates of them.

[0077] The final expanded pore mesoporous particles obtained in the second stage have a size between 80 and 800 nm and a mesopore size between values ​​greater than 2.5 nm and maximum values ​​up to 20 nm, preferably greater than 4.5 nm and maximum values ​​up to 20 nm, and more preferably greater than 6.5 nm and maximum values ​​up to 20 nm

[0078] The mesopore after the first stage is around 2.5 nm, as it is a pore generated by the template effect of the surfactant micelles. In the second stage, this mesoporous material is used as a starting point, and the pore size can be gradually expanded to 20 nm. The reaction medium in which the initial particles are suspended according to the present invention is always single-phase.

[0079] The present invention also relates to the use of the materials obtained in various industries, such as nanomedicine, sensor development, catalysis, and the biotechnology industry. This is particularly relevant in cases where conventional mesoporous silica materials are limited by particle size.

[0080] Description of the figures

[0081] Figure 1. TEM images of the particles obtained according to one embodiment of the invention. TEM micrographs of the degraded samples according to the second stage of the procedure, with different amounts of TEAH: d) 2.5, e) 5, f) 10 and g) 20 g.

[0082] Figure 2. Size distribution of the degraded samples with different amounts of TEAH: a) 0, b) 0.5, c) 1, d) 2.5, e) 5, f) 10 and g) 20 g.

[0083] Figure 3. SEM micrographs of the particles obtained according to one embodiment of the invention. Figure 3.a): Initial sample (material after the first stage). Figure 3.b): The sample from Figure 3a) after degradation with 20 g of TEAH in the medium

[0084] Figure 4. Low-angle diffractograms of the degraded samples with different amounts of TEAH.

[0085] Figure 5. a) Nitrogen adsorption-desorption isotherms and b) pore size distributions for samples degraded with different amounts of TEAH.

[0086] Figure 6. Representation of the change in the diameter of the second pore versus the amount of TEAH used in the degradation of the initial sample.

[0087] Figure 7. TEM micrographs of the samples degraded with 20 g of TEAH at different degradation times: 0, 3, 24, 48, 120 and 240 hours.

[0088] Figure 8. Size distribution of the degraded samples with 20 g of TEAH and different degradation times: 0, 3, 24, 48, 120 and 240 hours.

[0089] Figure 9. X-ray diffractograms of the samples obtained after different degradation times.

[0090] Figure 10. a) Nitrogen adsorption / desorption isotherms b) Pore distribution of the samples obtained after different degradation times. Figure 11. Representation of the change in the diameter of the second pore versus the amount of TEAH used in the degradation of the initial sample, according to example 2.

[0091] Figure 12. TEM micrographs of a) the initial sample and b) the degraded sample according to example 3.

[0092] Figure 13. Size distribution of a) the initial sample and b) the degraded sample according to example 3.

[0093] Figure 14. a) Nitrogen adsorption / desorption isotherms and b) pore distribution of each of the samples in example 3.

[0094] Figure 15. Low-angle X-ray diffractograms of the undegraded and degraded sample in ethanol medium and in the presence of TEAH from example 4.

[0095] Figure 16. a) Nitrogen adsorption / desorption isotherms and b) pore distribution of each of the samples in example 4.

[0096] EXAMPLES

[0097] The present invention is further illustrated by the following examples, which cannot be considered as a limitation of its scope.

[0098] EXAMPLE 1

[0099] Synthesis of mesoporous nanoparticles (starting material for the second stage): To synthesize the mesoporous silica nanoparticles with a spherical shape, we follow the following synthesis: 11 mL of tetraethyl orthosilicate (TEOS) and 23 mL of tetraethanolamine (TEAH) are heated under constant stirring to 140 °C, once that temperature is reached, it is allowed to cool to 120 °C and 4.5 g of cetylmethylammonium bromide (CTABr) are added.

[0100] The mixture is then cooled to 80 °C, and 450 mL of water and 250 mL of ethanol are added. The synthesis is then aged for 24 hours at 35 °C and 150 rpm. After this time, the product is centrifuged at 13,000 rpm for 10 minutes. The resulting product is washed with water and then with ethanol.

[0101] The sample is left to dry in an oven at 80 °C overnight, and the product is collected the following day. The final step in the synthesis is calcination, which exposes the pore network. This is achieved by calcining the sample for 6 hours at 550 °C with a ramp rate of 3 °C / min. After this process, the mesoporous silica nanoparticles resulting from the first stage of the procedure are ready for degradation.

[0102] Degradation in TEAH by vaping the concentrations and maintaining the degradation time constant:

[0103] The degradation process involved dispersing 0.2 g of sample (mesoporous silica nanoparticles obtained according to the preceding section) in 100 mL of water (15 minutes in an ultrasonic cleaner). Separately, the appropriate amount of TEAH was dissolved in 100 mL of water. Once one component was dispersed and the other dissolved, they were mixed and the pH was measured. The mixture was kept under constant stirring (300 rpm) at 37 °C for 24 hours. After this time, the pH of the dispersion was measured again, and the mixture was centrifuged at 13,000 rpm for 10 minutes. It was then washed with water and again with ethanol. The sample was left to dry overnight in an oven at 80 °C, and the following day the product was collected and weighed. Table 1 shows the conditions for each of the experiments performed.

[0104] Table 1. Conditions used in each of the experiments performed

[0105] We can see how, as the amount of TEAH in the medium increases, the percentage of degraded sample also increases. That is, the mass of the recovered sample decreases over time. Furthermore, the pH of the medium is also affected; at the end of the process, the pH has decreased compared to the initial pH. This is due to the redissolution and hydrolysis processes that the silica undergoes, releasing silicic acid, which is responsible for the pH drop in the degradation medium. In the TEM images (Figure 1), we can see that all the particles are spherical and become less dense as the concentration of TEAH in the medium increases, but without any significant changes in their diameter. This is corroborated by the size distribution of the samples (Figure 2), which shows that they are all very similar.Furthermore, the average diameter is similar in all cases, therefore we can conclude that there have been no significant changes in particle size after carrying out the degradation according to the procedure of the invention.

[0106] The SEM micrographs (Figure 3) show the initial sample (Figure 3a) and the sample after degradation with 20 g of TEAH in the medium (Figure 3b). Clear differences are observed between the two samples. Initially, small pores and a smoother surface are visible. However, with degradation, the pore size has increased and the surface has become rougher.

[0107] The low-angle diffractograms (Figure 4) of the samples, obtained through degradation processes, consistently maintain the intense peak characteristic of mesostructured siliceous materials with disordered hexagonal symmetry. Therefore, degradation does not significantly affect the order of the mesostructure.

[0108] Nitrogen adsorption / desorption allows us to verify and quantify the presence of pores. Nitrogen adsorption-desorption measurements were performed using a Micromeñtics ASAP-2020 instrument. Prior to measurement, the samples were degassed for 15 hours at 120 °C and a pressure of 10 -6Torr. The surface area can also be determined. In the nitrogen adsorption / desorption isotherms (Figure 5.a)), a first jump in the volume of adsorbed nitrogen can be observed in the undegraded sample (silica after the first preparation stage) between relative pressures of 0.2-0.4 (P / Po), which corresponds to the filling of the mesopores by capillary action (“first pore”). As the amount of TEAH in the degradation medium increases, this first jump shifts towards higher pressures, resulting in a larger pore (second pore).

[0109] But they still retain part of the “first pore” (generated by surfactant micelles), since that first jump to partial pressure values ​​in the 0.2-0.4 range (P / P oFor the most degraded samples, a second jump in the nitrogen adsorption-desorption isotherm is clearly visible, associated with the "second pore" (larger than the "first pore"). As the degree of etching progresses, the jump associated with the "second jump" becomes more pronounced, and finally, the jump linked to the "first pore" (at lower relative pressures) is significantly reduced and practically imperceptible in the most etched samples. This is clearer in the pore distribution graphs (Figure 5.b)). The undegraded sample has a single pore ("first pore"). However, as the silica nanoparticles degrade, this first pore decreases in size, and the formation of a larger mesopore ("second pore") is observed.

[0110] All the materials obtained in the experiments have a high surface area, being greater than 800 m² 2 / g. Table 2 shows all the results for BET area, volume and diameter of the “first pore” and the “second pore”.

[0111] Results of BET area, volume and diameter of the mesopore and second pore of each of the samples.

[0112] Table 2: Textural parameters determined from nitrogen adsorption-desorption isotherms

[0113] The surface area of ​​the particles decreases compared to the initial sample as they are degraded. This is because degradation causes the formation of a second pore, resulting in a loss of some surface area. The mesopore diameter increases slightly with degradation as it widens, but all materials maintain a constant mesopore volume. In contrast, the diameter and volume of the second pore increase as the amount of TEAH increases. Figure 6 clearly shows how the diameter of the second pore increases with increasing TEAH concentration.

[0114] EXAMPLE 2

[0115] Degradation in TEAH by varying the time and maintaining a constant concentration:

[0116] The degradation process involved dispersing 0.2 g of sample in 100 mL of water (15 minutes in an ultrasonic cleaner). Separately, 20 g of TEAH were dissolved in 100 mL of water. Once one component was dispersed and the other dissolved, they were mixed and the pH was measured. The mixture was kept under constant agitation (300 rpm) at 37 °C for the degradation time corresponding to each test. After this time, the pH of the dispersion was measured again, and the sample was centrifuged at 13,000 rpm for 10 minutes. It was then washed with water and again with ethanol. The sample was left to dry overnight in an oven at 80 °C, and the following day the product was collected and weighed. Table 3 shows the conditions for each of the experiments performed.

[0117] After degradation times exceeding 50 hours, the order of the mesostructure disappears. However, the important point is that the resulting expanded mesopore system is regular in terms of the size of the expanded pores.

[0118] Table 3: Conditions used in each of the experiments performed

[0119] As can be seen in the TEM micrographs (Figure 7), with increasing degradation time, the particles become progressively less dense, although they maintain their initial spherical morphology without significant changes in diameter (Figure 8). The low-angle diffractograms (Figure 9) of the samples, obtained through degradation processes, consistently exhibit the characteristic peak of mesostructured siliceous materials with disordered hexagonal symmetry. However, as degradation time increases, the peak becomes less intense, indicating a gradual loss of order in the original mesostructure. It is important to note, however, that this loss is not a negative aspect of the material. It is a natural evolution; as the level of degradation and silica loss increases, the mesopores grow in size and lose some of their organization.The decrease in peak intensity in the X-ray diffractograms discussed above is due to the combined effect of both phenomena.

[0120] Nitrogen adsorption / desorption allows us to verify the existence and quantify pores. The surface area can also be determined. In the nitrogen adsorption / desorption isotherms (Figure 10.a)), a jump in adsorption values ​​can be observed in the undegraded sample between relative pressures of 0.2–0.4 (P / P o ), which corresponds to the filling by capacity of the mesopores (“first pore”). As the degradation time increases, this pressure jump shifts towards higher pressures, resulting in a larger pore size (“second pore”). However, a small portion of the initial pore size is still retained, as this pressure jump is partially maintained between 0.2-0.4 (P / P oFor the samples with longer degradation times, this second, much more pronounced jump is clearly visible at relative pressures between 0.7 and 0.9 (P / Po) compared to the other samples, and the jump is practically imperceptible at lower relative pressures. This is clearer in the pore distribution graphs (Figure 10.b)). The undegraded sample has a single pore ("first pore"). However, as the samples degrade, this first pore decreases in size, and the formation of a second, larger mesopore is observed, which is the intended effect of the invention. This evolution is more pronounced than in the materials synthesized in Example 1. It is precisely this second, larger mesopore ("second pore") that provides the previously mentioned advantages to these silica particles.

[0121] All the materials obtained have a high surface area, exceeding 600 m²2 g. The greater degradation and removal of silica in the solids synthesized in Example 2, compared to those in Example 1, results in a decrease in surface area due to a more pronounced increase in the size of the larger mesopore. Table 4 shows all the BET area, volume, and diameter results for the mesopore and the second pore of each sample. Table 4.

[0122] The surface area decreases with degradation due to the formation of the second pore and the slight decrease in particle size. The diameter of the first pore increases slightly as degradation time increases, since it gradually widens. The second pore also increases in size with increasing degradation time because it is forming. Figure 11 clearly shows how the diameter of the second pore increases with degradation time.

[0123] EXAMPLE S

[0124] Degradation of porous mesoporous nanoparticles without TEAH:

[0125] With the aim of verifying whether aminoethanol has a complexing effect on silica, the degradation of a sample of mesoporous nanoparticles obtained in the first stage (according to the procedure indicated in Example 1 in the section called “synthesis of mesoporous silica nanoparticles” (starting material)) was carried out in a NaOH solution at a pH similar to that of the degradation with 20 g of TEAH described in Example 2.

[0126] In this case, 0.1032 g of a sample of mesoporous silica nanoparticles was dispersed in a NaOH solution for 15 minutes using ultrasound. The pH of the dispersion was 10.97. It was then subjected to constant stirring (300 rpm) for 24 hours at 37 °C. After this time, the pH was measured, yielding a value of 9.68, and the sample was centrifuged for 10 minutes at 13,000 rpm. The resulting product was washed with water and ethanol and then dried in an oven at 80 °C overnight. 0.0526 g of the initial sample was recovered.

[0127] The TEM micrographs (Figure 12) show that the particles have become slightly less dense, but not as much as with TEAH degradation. The nanoparticles have also retained their initial spherical morphology. Regarding the size distribution (Figure 13), the particles have become slightly smaller, but the changes are not very significant.

[0128] In mesoporous nanoparticles subjected to degradation without amino alcohol (using NaOH solution) part of the organization and pore system (“first pore”) of the starting material (starting material after the first synthesis stage) is lost, but a well-defined second pore is not generated.

[0129] With nitrogen adsorption / desorption measurements, we can analyze the evolution of porosity generated by treatment in NaOH medium. In the nitrogen adsorption / desorption isotherms (Figure 14.a)), a jump in adsorption can be observed in the undegraded sample between relative pressures of 0.2 and 0.4, which corresponds to the capillary filling of the mesopores (associated with the "first pore"). In the degraded sample, this jump is less pronounced (resulting in a less defined jump), and the range of relative pressures at which nitrogen is absorbed is broadened. A second jump, as observed in the samples degraded with TEAH, is not seen in this degraded sample. This is reflected in the pore distribution (Figure 14. b)) where it can be seen how the initial sample has a single pore and the peak is narrow, whereas for the degraded sample there is no longer a marked peak, but rather a wider band.This means that the initial pore has widened. It is also clear that there is no formation of a second pore; only the initial mesopore has widened. Therefore, the degradation that occurs due to the basic medium is not the same as that which occurs when using an amino alcohol such as TEAH.

[0130] EXAMPLE 4

[0131] Degradation of porous mesoporous nanoparticles with TEAH and without water:

[0132] With the aim of verifying the influence of the medium and especially the need for an aqueous medium for the chemical processes that lead to the degradation of silica to be operational, the degradation of a sample of mesoporous nanoparticles obtained in the first stage (according to the procedure indicated in Example 1 in the section called “synthesis of mesoporous silica nanoparticles” (starting material)) was carried out in an ethanol solution and using 20 g of TEAH, which corresponds to the largest quantities of this reagent used in Examples 1 and 2.

[0133] The degradation process involved dispersing 0.2015 g of sample in 100 mL of ethanol (15 minutes in an ultrasonic cleaner). Separately, 20.0173 g of TEAH were dissolved in 100 mL of ethanol. Once one component was dispersed and the other dissolved, they were mixed. The mixture was kept under constant agitation (300 rpm) at 37 °C for 24 hours. After this time, it was centrifuged at 13,000 rpm for 10 minutes. The mixture was then washed with ethanol. It was left to dry overnight in an oven at 80 °C, and the product was collected and weighed the following day. 0.1781 g of the initial sample was recovered.

[0134] In mesoporous nanoparticles subjected to degradation with aminotrial alcohol in an ethanol medium, some of the organization and order of the pore system (“first pore”) of the starting material is lost, as evidenced by the decrease in signal intensity at low angles in its diffractogram (Figure 15). However, a second pore is not generated, according to nitrogen adsorption / desorption measurements. In the nitrogen adsorption / desorption isotherms (Figure 16a), a jump in adsorption can be observed in the undegraded sample between relative pressures of 0.2–0.4, which corresponds to the capillary filling of the mesopores (associated with the “first pore”). In the degraded sample, this jump is attenuated (resulting in a less defined pore). In this degraded sample, a second jump is not observed, as was seen for the samples degraded with TEAH in aqueous medium (Examples 1 and 2).This is reflected in the pore distribution (Figure 16.b)), where the initial sample shows a single pore with a narrow peak, whereas the degraded sample no longer has a distinct peak, but rather a wider band of much lower intensity. This indicates that the initial pore has largely disappeared. It is also clear that no second pore forms. Therefore, the degradation that occurs in an ethanol medium (absence of water) does not generate an expanded pore system. This highlights the need for water for the chemical degradation generated by TEAH, through its moderately basic pH and its ability to interact with silicon centers.

Claims

CLAIMS 1. A process for preparing mesoporous silica nanoparticles with gradually expandable and adjustable pore size, comprising: - a first stage that involves preparing mesoporous silica nanoparticles using surfactants as template agents - and a second stage of degradation of the silica nanoparticles using an aminotriacohol of general formula NR 1 OH R 2 OHR 3 OH, in which R 1 R 2 and R 3 They represent equal or different organic groups, preferably hydrocarbons and particularly aliphatic chains of chain length less than four and more preferably 2,2',2"-n nitrilotriethanol or triethanolamine, wherein the second stage is carried out in water.

2. The process according to claim 1, wherein the second step comprises: - mix a dispersion of silica nanoparticles with an amino alcohol solution - Keep the mixture agitated at a temperature between 10 and 50 e C, and preferably between 20 and 37 e C, for a period between 1 and 240 hours.

3. The process according to claim 1, wherein the second step comprises: - disperse in water the mesoporous silica nanoparticles obtained in the first stage, - apply ultrasound, - Dissolve the amino alcohol in water, - mix the silica nanoparticle dispersion with the amino alcohol solution - Keep stirring at a temperature between 10 and 50 e C, preferably between 20 and 37 e C, for a period of between 1 and 240 hours, and preferably between 10 and 48 hours.

4. The process according to claim 3 wherein the ultrasound is applied for a time between 5 and 25 minutes, preferably between 10 and 20 minutes, and more preferably between 10 and 15 minutes.

5. The process according to claim 2 wherein the ratio of aminotrial alcohol and silica nanoparticles is between: 0.2 and 50 g of aminotrial alcohol per 0.2 g of silica, and preferably between 0.5 and 20 g of aminotrial alcohol / 0.2 g of silica.

6. The process according to claim 2 wherein the dispersion ratio of nanoparticles to aminotrialcohol solution is between: 50 and 300 mL of solution for 0.2 g of silica nanoparticles, and preferably between 100 and 200 mL / 0.2 g of silica.

7. The process according to claim 2 or 3, wherein the mixture of the dispersion and the solution is kept under constant agitation at a speed between 250 and 400 rpm, preferably between 260 and 350 rpm, and more preferably between 280 and 310 rpm.

8. The process according to claim 2 or 3, further comprising, after stirring the dispersion and dissolution mixture, - to centrifuge, - wash and - dry the mesoporous silica nanoparticles with expanded and modular pore, obtained.

9. The method according to claim 8, wherein the centrifugation is carried out at a speed between 10000 rpm and 14000 rpm, preferably between 12000 and 13500 rpm.

10. The method according to claim 8, wherein the centrifugation is carried out for a time between 5 and 20 minutes, preferably between 10 and 15 minutes.

11. The process according to claim 8, wherein the washing is carried out first with water and then with a solvent selected from ethanol, methanol and acetone.

12. The process according to claim 8, wherein the drying is carried out in an oven, at a temperature between 60 and 200 e C, preferably between 60 e C and 100 e C, more preferably between 75 e C and 80 e C.

13. The process according to claim 2, wherein drying is carried out at room temperature for a period of between 3 hours and 2 days, preferably between 5 and 24 hours.

14. Product obtained by the process defined in any one of the preceding claims, wherein the mesoporous silica nanoparticles with expanded pores obtained have a size of between 80 and 800 nm and a mesopore size, including the "first pore" and "second pore", of values ​​greater than 2.5 nm and up to 20 nm.

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