Method for producing dendritic fibrous structure silica nanoparticles in an aqueous environment without adding any organic solvent, PORE modifier or organic catalyst

Abstract

The invention relates to a method for producing dendritic fibrous structure silica nanoparticles comprising : providing a solution comprising water, a templating agent, a silica precursor and an inorganic catalyst; heating the solution in a closed tube or vessel at a temperature between 150 °C and 200°C, preferably 150°C, at a pressure between 2 and 5 bar; removing the templating agent by calcination or by acid solution extraction or by heat treatment.

Description

[0001] METHOD FOR PRODUCING DENDRITIC FIBROUS STRUCTURE

[0002] SILICA NANOPARTICLES IN AN AQUEOUS ENVIRONMENT WITHOUT ADDING ANY ORGANIC SOLVENT , PORE MODIFIER OR

[0003] ORGANIC CATALYST

[0004] The present invention relates to a method of developing silica nanoparticles with a radial fibrous structure .

[0005] Mesoporous silica nanoparticles (MSNs) "are solid materials , composed of a porous honeycomb-like structure consisting of hundreds of empty channels that are capable of absorbing or encapsulating relatively large amounts of molecules" as described in Pednekar , P . P . , Godiyal , S . C . , Jadhav, K. R. & Kadam, V. J . Chapter 23 - Mesoporous silica nanoparticles : a promising multifunctional drug delivery system, in Nanostructures for Cancer Therapy (eds . Ficai , A. & Grumezescu , A. M. ) 593-621 (Elsevier , 2017 ) .

[0006] Although MSNs possess a high surface area , pore accessibility is limited, causing limitations in mass transport . More recently, a new type of MSN has been reported, the so-called "dendritic" mesoporous silica nanoparticles (DMSN) characterized by a radial pore structure branching from the center of the particle .

[0007] DMSn have attracted strong interest in many sectors due to their particular open-pore structure , which gives them unique properties as intelligent drug delivery systems , catalysts , and actuators . In particular , unlike traditional mesostructures , this new type of "dendritic" porous material is characterized by having the pore entrance larger than the overall pore size. In addition, the pore size distribution can be wide, especially when the pore sizes are very large, allowing for the storage and release of a greater variety of molecules, such as proteins and DNA / RNA (Xu, C. , Lei, C . , Wang, Y. & Yu, C. Dendritic Mesoporous Nanoparticles: Structure, Synthesis and Properties. Angew Chem Int Ed 61, e202112752 (2022)) .

[0008] In the latest recent review Zhang, S. et al. Dendritic mesoporous silica nanoparticles for enzyme immobilization. Green Chemical Engineering S2666952823000365 (2023) , the authors classify the synthesis approaches used to produce DMSN into three types: (i) the emulsion synthesis method, (ii) the biphase stratification method and (iii) the homogeneous synthesis method.

[0009] The first method involves the use of organic pore modifiers, such as ethyl ether, n-octane, toluene, cyclohexane etc. , which act as pore enlarging agents .

[0010] The biphase approach is based on an oil-water system where, generally, the upper oil phase contains a source of silicon with hydrophobic organic solvent, while the lower aqueous phase contains a surfactant as a templating agent and a basic catalyst.

[0011] The homogeneous method, on the other hand, is based on an aqueous phase system and can be divided into two types. The first is a surfactant-assisted method, involving a cationic surfactant and counteranions, such as salicylate anion and fluorocarbon anions (Zhang, K. et al. Facile Large- Scale Synthesis of Monodisperse Mesoporous Silica Nanospheres with Tunable Pore Structure. J. Am. Chem. Capital 135, 2427-2430 (2013)) . The second type involves the typical Stober system without the addition of surfactants, by adding resorcinol and formaldehyde to the aqueous solution containing ethanol and ammonia.

[0012] Fig. 1 shows the list of approaches adopted and reported in the literature for the synthesis of DMSN divided into the three methods of classification mentioned above.

[0013] In a common synthesis, the use of a surfactant (cetyl trimethylammonium bromide (CTAB) , cetyl trimethylammonium tosylate (CTAC) , cetyl pyridinium bromide (CPB) ) as a templating agent and the addition of hydrophobic pore modifiers (often an oil) are critical for the formation of DMSN. Several water-oil emulsion systems are reported in the literature as a method to obtain DMSN with radial pores: wa ter-ether (Du, X. et al. Developing functionalized dendrimer-like silica nanoparticles with hierarchical pores as advanced delivery nanocarriers. Adv Mater 25, 5981-5985 (2013) and Du, X. & He, J. Amino-functionalized silica nanoparticles with center-radially hierarchical mesopores as ideal catalyst carriers. Nanoscale 4, 852-859 (2012)) , water-octane (Nandiyanto, A. B. D. , Kim, S.-G. , Iskandar, F. & Okuyama, K. Synthesis of spherical mesoporous silica nanoparticles with nanometer-size controllable pores and outer diameters. Microporous and Mesoporous Materials 120, 447-453 (2009)) , water- cyclohexane (Polshettiwar , V. , Cha, D. , Zhang, X. & Basset, J. M. High-Surface-Area Silica Nanospheres (KCC-1) with a Fibrous Morphology. Angew Chem Int Ed 49, 9652-9656 (2010)) , water-clorobenzene (Xu, C. et al. Core-Cone Structured Monodispersed Mesoporous Silica Nanoparticles with Ultra-large Cavity for Protein Delivery. Small 11, 5949-5955 (2015)) .

[0014] Each of these emulsion systems is accompanied by a different mechanism to explain the formation of DMSN. However, the formation of DMSN in the oil synthesis system is generically described as a controlled and dynamic cooperative assembly process, involving surfactant, co-surfactant (agent with the function of enlarging the pores) , silica precursor (TEOS, TMOS) and, possibly, a catalyst.

[0015] An important subcategory OF DMSn is dendritic fibrous nanoparticles (DFNPs) characterized by a distinct structure exhibiting high center-radial fibrous pore density and high surface area. According to a recent review (Wang, Y . , Du, X. , Liu, Z . , Shi, S. & Lv, H. Dendritic fibrous nano-particles (DFNPs) : rising stars of mesoporous materials. J. Mater. Chem. A 7, 5111-5152 (2019)) , published in a high-impact journal, "DFNPs with three-dimensional (3D) hierarchical center-radial pores have unique structural features including open-pore nanocannels, highly accessible interior spaces, large pore volumes, significantly greater surface area, etc. , compared to conventional mesoporous materials." For these reasons , DFNPs have attracted considerable attention thanks to this specific architecture, which shows intrinsic structural superiorities for absorbent materials and release systems due to their considerably larger surface area (compared to an MSN / DMSN) . Dendritic fibrous nanoparticles are characterized by a high-density structure of radial pores that appear as fibers on TEM images. Hence the definition / choice of "fibrous", as reported in Wang, Y . , Du, X. , Liu, Z. , Shi, S. & Lv, H. Dendritic fibrous nano-particles (DFNPs) : rising stars of mesoporous materials. J. Mater. Chem. A 7, 5111-5152 (2019) .

[0016] Since 2010, two different techniques, considered easy, have been discovered to synthesize DFNPs. The first concerns the use of an emulsion system for the synthesis of DFNPs, which are referred to as KCC-18. The second technique used an ethyl ether emulsion, which may be considered an evolution of the biphase oil-water system, to fabricate hierarchically mesoporous silica nanoparticles.

[0017] In 2014, Moon, D.-S. & Lee, J.-K. Formation of Wrinkled Silica Mesostructures Based on the Phase Behavior of Pseudoternary Systems. Langmuir 30, 15574-15580 (2014) described the role of phase in microemulsion systems on product structure. In this work, internal morphologies and interparticle connective structures were modified using various types of microemulsion systems as a structure formation model. This system, called Winsor III, provides for the possibility of controlling the structure of DMSN by modulating the water-surfactant- oil mixing ratio and adding various co-solvents.

[0018] An improvement for DFNP production has recently been published (Liu, X. et al. Tunable synthesis of dendritic fibrous nano silica using 1 -pentanol -water microemulsion at low oil to water ratio. Nanotechnology 33, 325601 (2022)) . In this work, the authors successfully synthesized DFNP using a water- CTAB- 1 -pentanol -ethanol -tetramethylbenzidine ( TMB) microemulsion system, wherein ethanol exists as a cosolvent , TMB as a pore enlarging agent . In these conditions , "the Water-CTAB-l-pentanoethanol-TMB microemulsion system can form a bicontinuous microemulsion phase in a low 0 / W volumetric ratio (oil / water) which is of enormous importance for large-scale industrialization" .

[0019] Considering the above , it is evident that the mechanism of formation of the DFNPs is based on the presence of a biphasic solution in which an organic compound, often toxic to human health and the environment , is necessary for the formation of the characteristic porous structure of the DFNPs . Furthermore , the known synthesis protocols provide for reaction times ranging from a minimum of two hours to a maximum of 24 .

[0020] The object of the present invention is to provide a new, more ecological and faster method to develop silica nanoparticles with radial fibrous structure .

[0021] According to embodiments of the present invention , a method for producing silica nanoparticles with dendritic fibrous structure comprises : providing a solution comprising water , a templating agent , a silica precursor and an inorganic catalyst ; heating the solution in a closed tube or vessel or reaction flask at a temperature between 150 ° C and 200 ° C , preferably 150° C , at a pressure between 2 and removing the templating agent by calcination or by acid solution extraction or by heat treatment.

[0022] More specifically, the method comprises: a) providing a templating agent in an aqueous solution, such as, for example, cetyl trimethylammonium bromide (CTAB) , cetyl trimethylammonium tosylate (CTAC) , cetyl pyridinium bromide (CPB) ; b) adding an inorganic catalyst agent, such as for example sodium hydroxide (NaOH) , potassium hydroxide (KOH) , ammonium hydroxide (NH4OH) ; c) keeping the solution under agitation, for example at a temperature between [50-80] °C, preferably 80 °C, for 5-20 minutes, preferably 20 minutes ; d) placing the solution in a tube, reaction flask or vessel, such as, for example, a Teflon container for a microwave reactor; e) adding to the solution a silica precursor, such as for example tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) , gradually (dropwise) ; f) closing the reaction tube / vessel / flask; g) placing the reaction tube / vessel / flask in a pressurized oven or microwave reactor, maintaining the pressure, for example, within the range [2, 5] bar ; h) heating the solution for a period of time, for example about 5 minutes, to reach the temperature in the desired range; i) continuing heating, under moderate agitation, to maintain the temperature in the desired range for a period of time, for example between 5 and 10 minutes ; j) collecting the nanoparticles contained in the resulting solution by centrifugation; k) washing the collected nanoparticles by carrying out one or more resuspension and centrifugation cycles with a washing mixture, such as water and ethanol ; l) removing the templating agent by calcination or by acid solution extraction or by heat treatment, for example at a temperature of 250°C.

[0023] The problems of the prior art discussed above are solved by the invention by means of a synthesis protocol that does not require the addition of organic pore modifiers or the use of organic catalysts, but is based on the use of a closed, high temperature and pressure container to obtain the fibrous structure. By means of these latter technical features, the time needed for the synthesis is reduced to 5 / 10 minutes. Therefore, the synthesis method according to the embodiments of the present invention makes it possible to provide a quick and easy way to obtain silica nanoparticles with a dendritic fibrous structure that is highly sought after in various fields of science due to its unique structural characteristics, i.e. larger pore volumes, more open channels, more accessible inner space, etc. than conventional MSNs as, for example, described in Polshettiwar , V. Dendritic Fibrous Nanosilica: Discovery, Synthesis, Formation Mechanism, Catalysis, and CO2 Capture-Conversion. Acc. Chem. Res. 55, 1395- 1410 (2022) . The features of the invention and the advantages deriving therefrom will become more apparent from the description of non-limiting embodiments , illustrated in the attached drawings , wherein :

[0024] Fig . 1 shows a list of approaches adopted for the synthesis of DMSN .

[0025] Fig . 2 shows a schematic drawing of a process commonly used to obtain MSN .

[0026] Fig . 3 is a TEM image of DFNPs obtained by using the protocol according to embodiments of the present invention with a reaction time of 10 min .

[0027] Fig . 4 shows TEM images of MSN (a) and DFNP (b) obtained by modifying the synthesis parameters (temperature and pressure) of the protocol according to the invention .

[0028] Fig . 5 is a TEM image of DFNPs obtained by using the protocol according to embodiments of the present invention with a reaction time of 5 min .

[0029] Fig . 6 reports the size distribution of the DFSNs obtained using the protocol according to the invention : average size 260 nm, standard deviation 60 nm.

[0030] Fig . 7 is a SEM image of the obtained DFNPs .

[0031] Fig . 8 is a higher magnification view of the previous figure .

[0032] Fig . 9 shows the calculation of pore size distribution by FFT and line profile .

[0033] Fig . 10 shows the calculation of the pore size distribution across the SEM image . The average pore size is 3. 11 nm, while the standard deviation is 0 .47 nm.

[0034] Fig . 11 shows the N2 adsorption-desorption isotherms of the obtained DFNPs . Fig . 12 shows the calculation of the pore size distribution across the SEM image obtained by doubling the catalyst content .

[0035] Figures 13 and 14 show SEM images of DFNPs obtained using potassium hydroxide and ammonium hydroxide as the inorganic catalyst , respectively .

[0036] Fig . 15 shows the SEM image of indefinitely shaped silica nanoparticles obtained by reaching the desired temperature of 150 ° C in a time greater than 30 minutes .

[0037] Fig . 16 shows the SEM image of nanoparticles obtained at 150 ° C in a refluxing flask at atmospheric pressure .

[0038] The invention described here relates to a new synthesis protocol for developing dendritic fibrous silica nanoparticles (DFNPs) . This method involves synthesis in aqueous solution and without the use of oil , organic solvent , organic catalyst , pore modifiers or co-surfactants to fabricate the dendritic pore structure of DFNPs .

[0039] The structure of the radial pores of the DFNPs is due to the high temperature ( 150 ° -200 ° C) , the higher pressure (2 / 5 bar) than the atmospheric pressure , in a confined and enclosed space , which can be obtained both with a microwave reactor in closed Teflon tubes and in a closed vessel heated with a plate . This approach makes it possible to quickly and easily manufacture DFNPs with high surface area , which are highly appreciated in many fields of science thanks to their open dendritic fibrous morphology, which significantly increases their accessibility . Some non-limiting examples of applications include distribution systems , smart materials for the development of coatings for cultural heritage conservation or for anti-corrosion purposes .

[0040] The starting point is a synthesis method commonly used for the production of MSN , such as the MCM-41 process , for example described in Catalano , F . & Pompa , P . P . Design Rules for Mesoporous Silica towards the Nanosize : A Systematic Study . ACS Appl . Mater . Interfaces 11 , 47237-47246 (2019) .

[0041] The synthesis of mesoporous silica nanoparticles according to this process , exemplified in Fig . 2 , starts from a surfactant .

[0042] Surfactants are amphiphilic molecules that spontaneously self-assemble in aqueous solution in various ordered and disordered phases . Under certain conditions , one-dimensional structures in the form of long , flexible worm-like micelles can develop . Cetyl trimethylammonium bromide (CTAB) is one of the most widely used surfactants .

[0043] By adding a silica precursor , such as tetraethyl orthosilicate (TEOS) , it is possible to obtain the desired porous silica structure (after removal of the micellar skeleton with a calcination or purification process in acidic solution) thanks to the presence of a catalyst , such as NaOH , KOH , NH4OH , which allows the silica precursor to hydrolyze and condense on the micelles .

[0044] The inventors have surprisingly found that by varying the pressure and temperature of this known process , DFNPs with high pore density structure can be obtained . The formation of the dendritic structure is due to the high temperature and pressure in a confined, closed and sealed space and not to the use of additional reagents (organic pore modifiers or organic catalysts) during the synthesis process as in the known art. In these particular conditions, the CTAB is organized into a radial fibrous structure.

[0045] Therefore, using the same reagents it is possible to produce both MSN (as reported in the literature) and DFNP. The modification of the pore structure is obtained by varying the temperature and pressure: for MSN 60 °C and Patm in an open container, while for DFNP 150-200°c and pressure between 2 and 5 bar in a closed container.

[0046] In more detail, a certain amount of CTAB is added to a beaker containing Milli-Q water. The ratio of CTAB to solvent may range from 1:200 to 1:2000, preferably 1:1000 (w / w) .

[0047] Then, a certain amount of NaOH (2M) is added to the aqueous solution and stirred at a temperature comprised between 50 and 80°C, preferably 80 °C over 5-20 minutes, preferably 20 minutes.

[0048] The solution is then placed into a Teflon microwave tube. A certain amount of TEOS is added dropwise into the solution without agitation. The ratio between TEOS and the NaOH solution (2M) can be comprised in a range between 0.5:1 and 30:1, preferably 1.5:1 (v / v) .

[0049] The ratio of TEOS to CTAB may be in a range from 1:1 to 100:1, preferably 10:1 (w / w) .

[0050] Then the solution is placed in the microwave reactor in a closed vessel, the temperature is increased to a value comprised between 150 and 200 °C, preferably 150 °C in about 5 minutes and maintained in this temperature range for 5-10 minutes under moderate agitation at a pressure comprised between 2 and 5 bar .

[0051] The term moderate agitation in chemical synthesis refers to a rate of agitation that is sufficient to keep the reactants uniformly mixed without causing excessive turbulence or excessive splashing. In a liquid solution, moderate agitation creates a steady flow and slight vortex without causing splashing or bubble formation. Moderate agitation depends on the materials and equipment, such as the size and shape of the magnetic bar, the shape of the container and the volume of the solution .

[0052] In the specific example described above, moderate agitation is obtained, for example, using a Milestone Ethos Up microwave, 100 ml Teflon tube filled with 40 ml of solution and 50% agitation with a 2 cm cylindrical anchor.

[0053] The nanoparticles of the resulting opaque solution are then collected by centrifugation and washed extensively by several cycles of resuspension and centrifugation with a mixture of water and ethanol . The CTAB within the pores can be removed by calcination or acid solution extraction, as described in Xu, C. , Lei, C. , Wang, Y. & Yu, C. Dendritic Mesoporous Nanoparticles: Structure, Synthesis and Properties. Angew Chem Int Ed 61, e202112752 (2022) e Catalano, F. & Pompa, P. P. Design Rules for Mesoporous Silica toward the Nanosize: A Systematic Study. ACS Appl . Mater. Interfaces 11, 47237-47246 (2019) , or with heat treatment (required temperature 250 ° C) .

[0054] The CTAB within the pores may, for example , be removed by acid solution extraction with reflux of the DFNPs suspended in a solution of EtOH and 37% hydrochloric acid (HC1 ) at 60 ° C for 24 hours .

[0055] The reaction protocol is based on environmentally friendly materials and is waterbased . In fact , it uses CTAB and NaOH , which are overall less toxic than other reagents commonly used in the synthesis of DMSN .

[0056] Using non-organic and greener materials , formulations reduce environmental impact and align with sustainable practices . These characteristics make the formulations risk-free both for the environment and for the operators , reducing the risks related to the handling of toxic products .

[0057] The closed reaction space may be obtained using a microwave-assisted synthesis . This approach is considered an efficient synthetic method as it makes it possible to reduce the time required to reach the synthesis temperature , reducing the energy required for the reaction . In addition , the use of microwave- assisted synthesis makes it possible to achieve high levels of reproducibility and scalability thanks to the very short reaction time (5 / 10 minutes) which makes it possible to produce a large amount of material per day . It was calculated that the production capacity with the current laboratory configuration could be higher than 0 .5 grams of DFNP in 10 minutes in half a litre of reaction solution . This result is easily scalable to achieve semi- industrial or industrial production through the use of machinery in series or with a higher capacity .

[0058] This advantage is particularly relevant for industrial applications , where the ability to quickly produce a high quantity of material is essential to reduce production costs .

[0059] Let ’ s now look at a specific example of DFNP synthesis in accordance with the teachings of the present invention .

[0060] As seen above , the protocol is based on the use of reagents commonly used for MSN synthesis that achieve a different pore structure (similar to a conical / conical pore fiber) due to the use of higher temperatures and pressures in a closed vessel .

[0061] The protocol requires two steps / stages .

[0062] Step 1 : 0 . 04 g of CTAB is added to 40 ml of Milli-Q water in a beaker . Then 0 .292 mL of NaOH (2M) is added to the aqueous solution and stirred at 80 ° C for 20 minutes .

[0063] Step 2 : The solution is placed in a Teflon microwave tube , 0 .416 mL of TEOS is added dropwise into the solution without agitation . Then the Teflon tube is closed with a Teflon cap and placed inside the microwave reactor . The temperature is increased to 150 ° C in 5 minutes and maintained at this temperature for 5 / 10 minutes under moderate agitation .

[0064] The nanoparticles in the resulting opaque solution are then collected by centrifugation and washed extensively by one or more resuspension and centrifugation cycles , for example with a mixture of water and ethanol . The CTAB within the pores may be removed by calcination , by acid solution extraction , or by heat treatment , for example at a temperature of 250 ° C .

[0065] Transmission electron microscope (TEM) analysis (Fig . 3) demonstrated that DFNPs have a size of about 250 nm and a high pore density with a diameter of about 4 nm (FIG . 3b) .

[0066] The reagents and their concentrations are those typically used to manufacture conventional mesoporous materials (MSN) as , for example , described in the aforementioned article by Catalano , F . & Pompa , P . P . Design Rules for Mesoporous Silica toward the Nanosize : A Systematic Study . ACS Appl . Mater . Interfaces 11 , 47237-47246 (2019) . The idea behind the invention is to produce DFNPs with high density porous structure by exploiting the high temperature and pressure achievable in a closed container .

[0067] Fig . 4 shows the two different pore structures that can be obtained with the above protocol using the same reagents , but modifying the synthesis parameters . In the first case , MSNs were obtained at 60 ° C and Patm using an open vessel and a heating plate (Fig . 4a) , in the second case the DFNPs were (unexpectedly) obtained at 150 ° C and 2 / 5 bar using a Teflon tube and a microwave reactor without changing the concentration of the solvent or adding additional reagents (such as organic pore modifiers) (Fig . 4b) .

[0068] Fig . 5 is a TEM image of DFNPs obtained with 5 min reaction time .

[0069] Fig . 6 shows the size distribution of the DFSNs obtained . The images obtained from SEM analysis (Fig . 7 and 8) make it possible to observe the morphology of the DFNPs and to appreciate the distribution of the pores that homogeneously cover the surface of the nanoparticles. These DFNPs were obtained using sodium hydroxide as the inorganic catalyst. Figures 13 and 14 show the DFNPs obtained with the same reaction parameters using potassium hydroxide and ammonium hydroxide as the catalyst, respectively.

[0070] The pore size distribution was measured using three methods: SEM images, line profile and Fast Fourier Transform (FFT) . The relevant results are shown in Fig. 9 and 10.

[0071] The physical properties of DFNPs were obtained using N2 adsorption-desorption measurements . The measured specific surface area is 998 m2 / g and the pore volume is 1.0 cm3 / g. The table below summarizes the main information. An excellent result has been achieved, among the highest values reported in the literature, ranging from 500 to 1000 cm3 / g (Lim, S.W. , Jang, HG. , Sim, HI. et al. Preparation of dandelion -type silica spheres and their application as catalyst supports. J Porous Mater 21, 797-809

[0072] (2014)) . Fig. 11 shows the N2 adsorption-desorption isotherms of the synthesized DFNPs.

[0073] The pore size obtained using the N2 adsorptiondesorption measurement is in close agreement with that calculated using the other methods above.

[0074] The same analysis carried out on MSNs synthesized according to the known art, demonstrates that the MSNs have a lower specific surface area (736 m2 / g) than the DFNPs obtained with the method that is the object of the present invention . The summary of the results obtained for the MSNs is given in the table below .

[0075] By doubling the catalyst content , dendritic nanoparticles with smaller sizes and larger surface area ( 1100 m2 / g) can be synthesized as shown in the following table and in Fig . 12 .

[0076] All this using known and commonly used reagents to produce a classical mesoporous structure without the use of any organic solvent, pore modifier or organic catalyst .

[0077] Figures 15 and 16 show the important role played by temperature and pressure in the process of forming the DFNPs according to the invention .

[0078] The inventors have , in fact , been able to verify how using the same reaction ingredients , but by modifying pressure and temperature , different particles are obtained .

[0079] In particular , with pressure between 2 and 5 bar and heating up to 150 ° C , by raising the temperature in a time of 5-10 minutes, dendritic particles of defined shape are obtained.

[0080] With pressure between 2 and 5 bar and heating up to 150°C, raising the temperature in a time of 30 minutes, no more particles (of any kind) are obtained but indefinite shapes as shown in Fig. 15.

[0081] At atmospheric pressure (1 bar under standard conditions) or slightly higher, but less than 2 bar, raising the temperature to 150°C in times even longer than 30 minutes, however, classic mesoporous particles are obtained as shown in Fig. 16.

[0082] The project from which this patent application is derived is partially funded by the European Union ’ s research and innovation programme under the GoGreen Grant Agreement 101060768.

Claims

CLAIMS1. Method for producing dendritic fibrous structure silica nanoparticles comprising: providing a solution comprising water, a templating agent, a silica precursor and an inorganic catalyst; heating the solution in a closed tube or vessel at a temperature between 150 °C and 200°C, preferably 150°C, at a pressure between 2 and 5 bar; removing the templating agent by calcination or by acid solution extraction or by heat treatment, for example at a temperature of 250°C.

2. Method according to Claim 1, comprising: a) providing a templating agent in an aqueous solution ; b) adding an inorganic catalyst agent; c) keeping the solution under agitation; d) placing the solution in a tube, vessel, or reaction flask such as, for example, a Teflon mi cr owave tube ; e) adding a precursor of the silica dropwise; f) closing the reaction tube / vessel / flask; g) placing the tube / vessel / flask in a pressurized microwave oven or reactor; h) heating the solution to reach the temperature in the range; i) continuing the heating to maintain the temperature in the range for a period of time under moderate agitation; j) collecting the nanoparticles contained in the resulting solution by centrifugation; k) washing the collected nanoparticles by carrying out one or more resuspension andcentrifugation cycles with a washing mixture , such as water and ethanol ;1 ) removing the templating agent by calcination or by acid solution extraction or by heat treatment , for example at a temperature of 250 ° C .3 . Method according to Claim 1 or 2 , wherein the templating agent is selected from the group comprising : Cetyl trimethylammonium Bromide (CTAB) , Cetyl trimethylammonium Tosylate (CTAC) , Cetylpyridinium Bromide (CPB) .4 . Method according to any one of the preceding claims , wherein the inorganic catalyst agent is sodium hydroxide (NaOH) , potassium hydroxide (KOH) or ammonium hydroxide (NH4OH) .5 . Method according to any one of the preceding claims , wherein the silica precursor is tetraethyl orthosilicate ( TEOS) or tetramethyl orthosilicate (TMOS) .6 . Method according to any one of the preceding claims , wherein the temperature is brought to the value in the range of about 5 minutes and maintained in the range for a period of between 5 and 10 minutes .7 . Method according to any one of Claims 2 to 6 , wherein the agitation of step c) takes place at a temperature comprised between 50 ° C and 80 ° C , preferably 80 ° C , for 5-20 minutes , preferably 20 minutes .8 . Method according to any one of the preceding claims , wherein the ratio of the templating agent to water is in a range of 1 : 200 to 1 : 2000 , preferably 1 : 1000 (w / w) , the ratio of the silica precursor to the templating agent is in a range of1:1 to 100:1, preferably 10:1 (w / w) , the ratio of the templating agent to the inorganic catalyst is in a range of 0.1:1 to 20:1, preferably 1.7:1 (w / w) , while the ratio of the silica precursor to the inorganic catalyst agent is in a range of 0.5:1 to 30:1, preferably 1.5:1 (v / v) .

9. Method according to any one of the preceding claims, wherein the templating agent is CTAB, the silica precursor is TEOS, and the inorganic catalytic agent is NaOH (2M) .

10. Method according to any one of the preceding claims , wherein the templating agent inside the pores is removed by acid solution extraction with reflux of the DFNPs suspended in a solution of EtOH and 37% hydrochloric acid (HC1) at 60°C for 24 hours.

11. Method according to any one of the preceding claims , wherein the temperature and pressure are modulated to obtain nanoparticles with different porosity characteristics.

12. Method according to Claim 11, wherein to realize MSN the temperature is maintained at 60 °C at atmospheric pressure in an open container, while to realize DFNP the temperature is increased to a value comprised between 150 and 200°C, preferably 150 °C in a time interval of 5 to 10 minutes, preferably about 5 minutes and maintained in this temperature interval for 5-10 minutes under moderate agitation at a pressure comprised between 2 and 5 bar.

13. Method according to any one of the preceding claims, characterized in that it is used to produce nanoparticles with surface area between 1000 and 1100 m2 / g.

14. Method for producing dendritic fibrous structure silica nanoparticles comprising: providing a solution comprising water, a templating agent, a silica precursor and an inorganic catalyst; heating the solution in a closed tube or vessel at a temperature between 150 °C and 200°C, preferably 150°C, at a pressure between 2 and 5 bar; removing the templating agent by calcination or by acid solution extraction or by heat treatment, for example at a temperature of 250°C, wherein the ratio of the templating agent to water is in a range of from 1:200 to 1:2000, preferably 1:1000 (w / w) , the ratio of the silica precursor to the templating agent is in a range of from 1:1 to 100:1, preferably 10:1 (w / w) , the ratio of the templating agent to the inorganic catalyst is in a range of from 0.1:1 to 20:1, preferably 1.7:1 (w / w) , while the ratio of the silica precursor to the inorganic catalyst agent is in a range of from 0.5:1 to 30:1, preferably 1.5:1 (v / v) .

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