Brief History, Preparation Method, and Biological Application of Mesoporous Silica Molecular Sieves: A Narrative Review

It has been more than 30 years since the first ordered mesoporous silica molecular sieve (MCM-41) was reported, but the enthusiasm for exploiting mesoporous silica is still growing due to its superior properties, such as its controllable morphology, excellent hosting capability, easy functionalization, and good biocompatibility. In this narrative review, the brief history of the discovery of mesoporous silica and several important mesoporous silica families are summarized. The development of mesoporous silica microspheres with nanoscale dimensions, hollow mesoporous silica microspheres, and dendritic mesoporous silica nanospheres is also described. Meanwhile, common synthesis methods for traditional mesoporous silica, mesoporous silica microspheres, and hollow mesoporous silica microspheres are discussed. Then, we introduce the biological applications of mesoporous silica in fields such as drug delivery, bioimaging, and biosensing. We hope this review will help people to understand the history of the development of mesoporous silica molecular sieves and become familiar with their synthesis methods and applications in biology.


Introduction
Porous materials have been widely used for thousands of years. One of the most well-known materials in this category is activated carbon, which is commonly used as an adsorbent [1][2][3]. Over the past few decades, porous materials, especially nano-scale porous materials, have received widespread attention due to their large surface area, open structure, and so on [4][5][6][7]. Well-established nanoporous materials include mesoporous silica molecular sieves [8], zeolite molecular sieves [9], metal-organic frameworks [10], covalent organic frameworks [11], nanoporous carbon [12], etc. Among these, mesoporous silica molecular sieves have stood out in the field of biological applications because of their adjustable pore size, easy functionalization, high stability, and physiological inertia [13][14][15]. According to the IUPAC nomenclature rules, mesoporous materials usually refer to materials with a pore size between 2 and 50 nm. Moderate-sized apertures ensure that mesoporous silica molecular sieves have a large surface area and are easy to fill with various materials. Additionally, the non-toxic properties of silica have been approved by the FDA (Food and Drug Administration) and EFSA (European Food Safety Authority), and related materials are widely used in the cosmetics and food additive industries [16][17][18][19]. Thus, mesoporous silica molecular sieves have great application potential as a carrier in drug delivery [20], bioimaging [21], biosensing [22], materials, etc., and have become one of the most important materials in the field of biological applications.  [47] The ideal mesoporous silica for biological applications usually has a moderate pore size (less than 200 nm) and large surface area. With this in mind, researchers have focused Molecules 2023, 28,2013 3 of 17 their attention on synthesizing mesoporous silica microspheres with nanoscale dimensions. In 1997, Grun et al. [48] reported for the first time the synthesis of submicron spherical MCM-41 particles using a modified Stöber method, which involved adding a cationic surfactant as a template. Before long, Cai et al. [49] obtained spherical MCM-41 particles with an average size of 110 nm by reacting the extremely dilute solution of CTAB with TEOS in a sodium hydroxide medium at 353 K. Kobler et al. [50][51][52] prepared a series of MCM-like near-spherical silica particles with relatively uniform particle sizes between 40 and 150 nm via altering the incorporated functional group and the amount of TEA added in the synthesis. Although mesoporous silica has an abundant number of pores, there are continuous efforts to increase its volume. Some researchers have attempted to add hollow structures to the mesoporous silica to achieve this [53][54][55]. Zhu et al. [56] reported for the first time the synthesis of hollow mesoporous silica spheres via penetrating pore channels across the shells using PVP and CTAB as co-templates, which are uniform in size and have a high drug storage capacity. Another requirement for biological applications is that the material be degradable in vivo. To meet this requirement, Zhao's group [57] invented a novel monodispersed three-dimensional dendritic mesoporous silica nanosphere that not only had tunable pore size and thickness, but were also biodegradable in simulated body fluid. As shown in Figure 1, the dendritic mesoporous silica nanosphere had a highly uniform particle size and mesostructures, making it a very attractive material for various applications. The ideal mesoporous silica for biological applications usually has a moderate pore size (less than 200 nm) and large surface area. With this in mind, researchers have focused their attention on synthesizing mesoporous silica microspheres with nanoscale dimensions. In 1997, Grun et al. [48] reported for the first time the synthesis of submicron spherical MCM-41 particles using a modified Stöber method, which involved adding a cationic surfactant as a template. Before long, Cai et al. [49] obtained spherical MCM-41 particles with an average size of 110 nm by reacting the extremely dilute solution of CTAB with TEOS in a sodium hydroxide medium at 353 K. Kobler et al. [50][51][52] prepared a series of MCM-like near-spherical silica particles with relatively uniform particle sizes between 40 and 150 nm via altering the incorporated functional group and the amount of TEA added in the synthesis. Although mesoporous silica has an abundant number of pores, there are continuous efforts to increase its volume. Some researchers have attempted to add hollow structures to the mesoporous silica to achieve this [53][54][55]. Zhu et al. [56] reported for the first time the synthesis of hollow mesoporous silica spheres via penetrating pore channels across the shells using PVP and CTAB as co-templates, which are uniform in size and have a high drug storage capacity. Another requirement for biological applications is that the material be degradable in vivo. To meet this requirement, Zhao's group [57] invented a novel monodispersed three-dimensional dendritic mesoporous silica nanosphere that not only had tunable pore size and thickness, but were also biodegradable in simulated body fluid. As shown in Figure 1, the dendritic mesoporous silica nanosphere had a highly uniform particle size and mesostructures, making it a very attractive material for various applications. Figure 1. Synthesis process, mechanism of interfacial growth, and representative micrographs of the dendritic mesoporous silica nanospheres. Reproduced with permission from Ref. [57], Copyright 2014, American Chemical Society.

Preparation Methods
As noted earlier, the first ordered mesoporous silica (MCM-41) was synthesized by means of a liquid crystal templating method. Usually, a typical lyotropic liquid crystal template can be prepared by mixing water and surfactant. The synthesis of MCM-41 is based on a silica skeleton precipitated around a lyotropic liquid crystal template formed by mixing quaternary ammonium surfactant with water. In fact, the synthesis of mesoporous silica is usually carried out via a liquid crystal templating method. Template-assisted technology has long been the most utilized method for the synthesis of ordered mesoporous materials, and various surfactant molecules can be used as templates to direct the Figure 1. Synthesis process, mechanism of interfacial growth, and representative micrographs of the dendritic mesoporous silica nanospheres. Reproduced with permission from Ref. [57], Copyright 2014, American Chemical Society.

Preparation Methods
As noted earlier, the first ordered mesoporous silica (MCM-41) was synthesized by means of a liquid crystal templating method. Usually, a typical lyotropic liquid crystal template can be prepared by mixing water and surfactant. The synthesis of MCM-41 is based on a silica skeleton precipitated around a lyotropic liquid crystal template formed by mixing quaternary ammonium surfactant with water. In fact, the synthesis of mesoporous silica is usually carried out via a liquid crystal templating method. Template-assisted technology has long been the most utilized method for the synthesis of ordered mesoporous materials, and various surfactant molecules can be used as templates to direct the synthesis of mesoporous silica [58][59][60]. As the liquid crystal template is not a solid template, a method called "soft-matter templating" is often used, and at times the endotemplate method [61].
In general, the structure of the liquid crystal template strongly depends on the conditions in the solution, such as the pH, temperature, and the concentration [62].
The discovery of MCM-41 promoted research on mesoporous silica materials. Since then, efforts have been made to uncover the exact mechanism behind the formation of mesoporous silica. Narayan et al. [63] clearly demonstrated the formation mechanism of MCM-41 ( Figure 2). In addition to the liquid crystal templating mechanism, the synthesis of MCM-41 also involves a sol-gel process wherein the polymerization of silica is carried out around the micelles. Once the silica settles, the porosity can be obtained after the removal of the template from the mesostructured solid via calcination or solvent extraction. Most other types of mesoporous silica have similar synthesis processes. Thus, a typical synthesis process for mesoporous silica can be divided into three steps: (i) choosing an appropriate template and forming a liquid crystal template under certain conditions; (ii) adding the silicon source, waiting for the silica to settle completely around the surfactant micelles, and then performing an aging treatment; and (iii) finally, removing the template.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 18 synthesis of mesoporous silica [58][59][60]. As the liquid crystal template is not a solid template, a method called "soft-matter templating" is often used, and at times the endotemplate method [61]. In general, the structure of the liquid crystal template strongly depends on the conditions in the solution, such as the pH, temperature, and the concentration [62]. The discovery of MCM-41 promoted research on mesoporous silica materials. Since then, efforts have been made to uncover the exact mechanism behind the formation of mesoporous silica. Narayan et al. [63] clearly demonstrated the formation mechanism of MCM-41 ( Figure 2). In addition to the liquid crystal templating mechanism, the synthesis of MCM-41 also involves a sol-gel process wherein the polymerization of silica is carried out around the micelles. Once the silica settles, the porosity can be obtained after the removal of the template from the mesostructured solid via calcination or solvent extraction. Most other types of mesoporous silica have similar synthesis processes. Thus, a typical synthesis process for mesoporous silica can be divided into three steps: (i) choosing an appropriate template and forming a liquid crystal template under certain conditions; (ii) adding the silicon source, waiting for the silica to settle completely around the surfactant micelles, and then performing an aging treatment; and (iii) finally, removing the template. The original MCM-41 was synthesized in alkaline media and by means of a hydrothermal treatment. Generally, the synthesis is carried out under high temperature (≥100 °C) in an autoclave and requires a long crystallization time. Under certain conditions, the preparation of mesoporous silica can also be performed at room temperature [64][65][66]. In 1994, Huo et al. [67] reported room-temperature synthesis of mesoporous silica under acidic conditions for the first time. Compared to hydrothermal synthesis, the room temperature method is easy to carry out and does not require special equipment, which makes it more suitable for large-scale production. Although the room temperature method has many advantages, the crystallinity of mesoporous silica can be improved by increasing the temperature and pressure of the synthesis process within a certain range. Therefore, The original MCM-41 was synthesized in alkaline media and by means of a hydrothermal treatment. Generally, the synthesis is carried out under high temperature (≥100 • C) in an autoclave and requires a long crystallization time. Under certain conditions, the preparation of mesoporous silica can also be performed at room temperature [64][65][66]. In 1994, Huo et al. [67] reported room-temperature synthesis of mesoporous silica under acidic conditions for the first time. Compared to hydrothermal synthesis, the room temperature method is easy to carry out and does not require special equipment, which makes it more suitable for large-scale production. Although the room temperature method has many advantages, the crystallinity of mesoporous silica can be improved by increasing the temperature and pressure of the synthesis process within a certain range. Therefore, although it has been used for decades, the hydrothermal method remains the most efficient technique for the synthesis of mesoporous silica. Additionally, there are continuous efforts to prepare mesoporous silica with the high-temperature hydrothermal method in order to obtain mesoporous silica with unusual stability [68][69][70]. Xiao's group [71] reported, for the first time, the synthesis of the mesoporous silica JLU-20 in strong acidic media at high temperatures (190 • C) using a mix of fluorocarbon and Pluronic P123 surfactants. Compared with the mesoporous silica prepared with the traditional method, the condensation degree of JLU-20 is higher, indicating a greatly improved thermal stability. Additionally, to further improve the synthetic efficiency, a microwave-assisted technique has been applied to the preparation of mesoporous silica [72][73][74]. It has been demonstrated that the microwave-assisted hydrothermal process has the advantages of a shorter reaction time, lower energy consumption, and more uniform product morphology compared to the conventional autoclave heating method.
Nanoscale mesoporous silica microspheres have attracted great interest because of their great potential for biological applications. The combination of the classical Stöber method and template method has been proven to be a successful strategy for the synthesis of mesoporous silica microspheres [75,76]. The typical Stöber process usually involves the hydrolysis and condensation of tetraethylorthosilicate (TEOS) in an alcohol-water mixture using an ammonia catalyst. In this process, monodisperse spherical silica can be obtained via a seed growth mechanism driven by surface tension. When the surfactant template is present, the particle growth process is accompanied by a cooperative selfassembly process, leading to the formation of spherical mesoporous silica. Recently, Garrido et al. [77] (Figure 3) proposed a reproducible "one-pot" method for the preparation of spherical mesoporous silica that could be functionalized with inorganic species based on a combination of the Atrane and the Stöber methods. This method simultaneously ensures the spherical morphology and dispersed degree of heterogeneous elements, which make it a critical technique for the synthesis of elemental functionalized spherical mesoporous silica.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 18 although it has been used for decades, the hydrothermal method remains the most efficient technique for the synthesis of mesoporous silica. Additionally, there are continuous efforts to prepare mesoporous silica with the high-temperature hydrothermal method in order to obtain mesoporous silica with unusual stability [68][69][70]. Xiao's group [71] reported, for the first time, the synthesis of the mesoporous silica JLU-20 in strong acidic media at high temperatures (190 °C) using a mix of fluorocarbon and Pluronic P123 surfactants. Compared with the mesoporous silica prepared with the traditional method, the condensation degree of JLU-20 is higher, indicating a greatly improved thermal stability. Additionally, to further improve the synthetic efficiency, a microwave-assisted technique has been applied to the preparation of mesoporous silica [72][73][74]. It has been demonstrated that the microwave-assisted hydrothermal process has the advantages of a shorter reaction time, lower energy consumption, and more uniform product morphology compared to the conventional autoclave heating method. Nanoscale mesoporous silica microspheres have attracted great interest because of their great potential for biological applications. The combination of the classical Stöber method and template method has been proven to be a successful strategy for the synthesis of mesoporous silica microspheres [75,76]. The typical Stöber process usually involves the hydrolysis and condensation of tetraethylorthosilicate (TEOS) in an alcohol-water mixture using an ammonia catalyst. In this process, monodisperse spherical silica can be obtained via a seed growth mechanism driven by surface tension. When the surfactant template is present, the particle growth process is accompanied by a cooperative self-assembly process, leading to the formation of spherical mesoporous silica. Recently, Garrido et al. [77] (Figure 3) proposed a reproducible "one-pot" method for the preparation of spherical mesoporous silica that could be functionalized with inorganic species based on a combination of the Atrane and the Stöber methods. This method simultaneously ensures the spherical morphology and dispersed degree of heterogeneous elements, which make it a critical technique for the synthesis of elemental functionalized spherical mesoporous silica. Furthermore, the seed growth mechanism presented by Stöber has also been extended to prepare mesoporous silica microspheres under different conditions. For example, Pan et al. [78] reported the synthesis of mesoporous silica microspheres with uniform and tunable particle size by using hexadecyl trimethyl ammonium chloride as a template and triethanolamine (TEA) as an alkaline catalyst. The particle size of mesoporous silica microspheres can be tuned by changing the amount of TEA used. As shown in Figure 4, the obtained mesoporous silica microspheres are highly monodispersed and can be used for nuclear-targeted drug delivery. Furthermore, the seed growth mechanism presented by Stöber has also been extended to prepare mesoporous silica microspheres under different conditions. For example, Pan et al. [78] reported the synthesis of mesoporous silica microspheres with uniform and tunable particle size by using hexadecyl trimethyl ammonium chloride as a template and triethanolamine (TEA) as an alkaline catalyst. The particle size of mesoporous silica microspheres can be tuned by changing the amount of TEA used. As shown in Figure 4, the obtained mesoporous silica microspheres are highly monodispersed and can be used for nuclear-targeted drug delivery. In order to increase the loading capacity of these materials, efforts have been made to synthesize hollow mesoporous silica microspheres [79]. Soft templating methods such as the micelle templating method [80], microemulsion templating [81], etc., play an important role in the synthesis of hollow mesoporous silica microspheres. Han et al. [82] reported a series of mesoporous silica hollow spheres which were synthesized via a micelle and emulsion dual-templating route. The obtained particles had a diameter in the range of 80~220 nm, constant shell thickness of about 20 nm and constant mesopore size of about 4 nm. In particular, a hard templating method using polymer beads [83], silica colloids [84], or metal oxides [85] as the seed cores was developed to synthesize discrete, uniform, and monodisperse hollow mesoporous silica microspheres to meet the rigorous requirements of biological applications. For example, Wu et al. [86] reported a salt-assisted acid-etching strategy to prepare hollow mesoporous silica ( Figure 5). The solid SiO2 core templates were prepared first via the classical Stöber process, and then pre-mixed TEOS and octadecyltrimethoxysilane (C18TMS) were added to the reaction system to promote the growth of the mesoporous shell around the solid SiO2 core. A salt-assisted acid strategy was then used to etch the solid SiO2 core of the SiO2@mSiO2. Further removal of the surfactant C18TMS provides highly uniform hollow mesoporous silica nanovehicles (HMSVs). With a similar process, and the addition of 1,4-bis(triethoxysily) benzene, an organosilica-functionalized SiO2@moSiO2 and HMOVs were subsequently obtained. The as-synthesized HMOVs were found to have high cargo-loading capacity and pH-responsive drug-releasing behavior due to their hollow structure and phenylene-bridged framework. This seed growth and core-etching method based on a colloidal solution of hard templates has been proven to be an effective strategy for the synthesis of hollow mesoporous silica microspheres by many research works in the past decade. In order to increase the loading capacity of these materials, efforts have been made to synthesize hollow mesoporous silica microspheres [79]. Soft templating methods such as the micelle templating method [80], microemulsion templating [81], etc., play an important role in the synthesis of hollow mesoporous silica microspheres. Han et al. [82] reported a series of mesoporous silica hollow spheres which were synthesized via a micelle and emulsion dual-templating route. The obtained particles had a diameter in the range of 80~220 nm, constant shell thickness of about 20 nm and constant mesopore size of about 4 nm. In particular, a hard templating method using polymer beads [83], silica colloids [84], or metal oxides [85] as the seed cores was developed to synthesize discrete, uniform, and monodisperse hollow mesoporous silica microspheres to meet the rigorous requirements of biological applications. For example, Wu et al. [86] reported a salt-assisted acid-etching strategy to prepare hollow mesoporous silica ( Figure 5). The solid SiO 2 core templates were prepared first via the classical Stöber process, and then pre-mixed TEOS and octadecyltrimethoxysilane (C 18 TMS) were added to the reaction system to promote the growth of the mesoporous shell around the solid SiO 2 core. A salt-assisted acid strategy was then used to etch the solid SiO 2 core of the SiO 2 @mSiO 2 . Further removal of the surfactant C 18 TMS provides highly uniform hollow mesoporous silica nanovehicles (HMSVs). With a similar process, and the addition of 1,4-bis(triethoxysily) benzene, an organosilica-functionalized SiO 2 @moSiO 2 and HMOVs were subsequently obtained. The as-synthesized HMOVs were found to have high cargo-loading capacity and pH-responsive drug-releasing behavior due to their hollow structure and phenylene-bridged framework. This seed growth and core-etching method based on a colloidal solution of hard templates has been proven to be an effective strategy for the synthesis of hollow mesoporous silica microspheres by many research works in the past decade. There are also many other novel approaches that have been proposed for the direct generation of mesoporous silica, such as the aerosol-based procedure [87], the biphase stratification approach [88][89][90], the evaporation-induced aggregating assembly method [91,92], and so on.

Biological Applications
Due to the chemical stability of SiO2 matrices, the abundant pores in mesoporous silica are an ideal vessel for a variety of molecules. Furthermore, the excellent biocompatibility of SiO2 matrices further broadens mesoporous silica's application prospects in fields such as drug delivery, bioimaging, biosensing, and so on [93][94][95][96][97][98][99].
Vallet-Regi et al. [100] were pioneers in using mesoporous silica as a drug carrier. They firstly integrated ibuprofen into two MCM-41 materials with different pore sizes and subsequently examined the in vitro process of releasing the drug to a simulated body fluid. Since then, mesoporous silica has been widely used for drug delivery because of its high drug loading capacity, biocompatibility, and so on. Hollow mesoporous silica microspheres with a uniform particle size have become the most popular drug carrier [101][102][103][104]. Li et al. [105] (Figure 6) prepared mesoporous-silica-coated Au nanoparticles by employing gold nanoparticles (Au) as a core template and surfactant cetyltrimethyl ammonium bromide (CTAB) as the template directing mesoporous structure. After the CTAB and Au templates were removed via calcination and subsequently etching, hollow mesoporous silica nanoparticles were obtained and used to load doxorubicin hydrochloride (DOX), leading to a nanocapsule HMSN (Dox). It has been proven that the drug-releasing behavior of HMSNs (Dox) is consistent with the Fickian diffusion mechanism and can be tuned by adjusting the thickness of the mesoporous shell. There are also many other novel approaches that have been proposed for the direct generation of mesoporous silica, such as the aerosol-based procedure [87], the biphase stratification approach [88][89][90], the evaporation-induced aggregating assembly method [91,92], and so on.

Biological Applications
Due to the chemical stability of SiO 2 matrices, the abundant pores in mesoporous silica are an ideal vessel for a variety of molecules. Furthermore, the excellent biocompatibility of SiO 2 matrices further broadens mesoporous silica's application prospects in fields such as drug delivery, bioimaging, biosensing, and so on [93][94][95][96][97][98][99].
Vallet-Regi et al. [100] were pioneers in using mesoporous silica as a drug carrier. They firstly integrated ibuprofen into two MCM-41 materials with different pore sizes and subsequently examined the in vitro process of releasing the drug to a simulated body fluid. Since then, mesoporous silica has been widely used for drug delivery because of its high drug loading capacity, biocompatibility, and so on. Hollow mesoporous silica microspheres with a uniform particle size have become the most popular drug carrier [101][102][103][104]. Li et al. [105] (Figure 6) prepared mesoporous-silica-coated Au nanoparticles by employing gold nanoparticles (Au) as a core template and surfactant cetyltrimethyl ammonium bromide (CTAB) as the template directing mesoporous structure. After the CTAB and Au templates were removed via calcination and subsequently etching, hollow mesoporous silica nanoparticles were obtained and used to load doxorubicin hydrochloride (DOX), leading to a nanocapsule HMSN (Dox). It has been proven that the drug-releasing behavior of HMSNs (Dox) is consistent with the Fickian diffusion mechanism and can be tuned by adjusting the thickness of the mesoporous shell. To meet the ever-increasing demand for precise treatment, drug delivery systems that can target diseased tissues are urgently needed [106][107][108]. For the treatment of cancer, in particular, a targeted controlled-release drug carrier can lower dosage, prolong drug duration, and prevent damage to healthy tissue. Chen et al. [109] reported multi-functional hollow mesoporous silica nanoparticles with enhanced drug delivery and in vivo imaging properties. As shown in Figure 7, the drug deliverer was prepared using a generally applicable surface engineering technique, and it had a uniform particle size. By grafting a CD105-targeting antibody on the surface of the particles, a 3-fold tumor uptake on average was achieved in 4T1 murine breast-tumor-bearing mice compared with the non-targeted group, as observed by a quantitative PET imaging study via chelating radioactive 64 Cu to the particles and an in vivo NIRF imaging study via anchoring NIR dye on the particles. Furthermore, its hollow core confers on the drug carrier a 3~15-times-higher DOX loading capacity than that of carriers without a hollow structure, and its highest loading capacity was estimated to be 1129.2 mg/g. Their work showed that a diseasedtissue-targeted drug delivery system can be developed via the elaborated surface functionalization of mesoporous silica, suggesting that in vivo targeted cancer therapy can benefit from advances in mesoporous silica nanotechnology. To meet the ever-increasing demand for precise treatment, drug delivery systems that can target diseased tissues are urgently needed [106][107][108]. For the treatment of cancer, in particular, a targeted controlled-release drug carrier can lower dosage, prolong drug duration, and prevent damage to healthy tissue. Chen et al. [109] reported multi-functional hollow mesoporous silica nanoparticles with enhanced drug delivery and in vivo imaging properties. As shown in Figure 7, the drug deliverer was prepared using a generally applicable surface engineering technique, and it had a uniform particle size. By grafting a CD105-targeting antibody on the surface of the particles, a 3-fold tumor uptake on average was achieved in 4T1 murine breast-tumor-bearing mice compared with the nontargeted group, as observed by a quantitative PET imaging study via chelating radioactive 64 Cu to the particles and an in vivo NIRF imaging study via anchoring NIR dye on the particles. Furthermore, its hollow core confers on the drug carrier a 3~15-times-higher DOX loading capacity than that of carriers without a hollow structure, and its highest loading capacity was estimated to be 1129.2 mg/g. Their work showed that a diseased-tissuetargeted drug delivery system can be developed via the elaborated surface functionalization of mesoporous silica, suggesting that in vivo targeted cancer therapy can benefit from advances in mesoporous silica nanotechnology. To meet the ever-increasing demand for precise treatment, drug delivery systems that can target diseased tissues are urgently needed [106][107][108]. For the treatment of cancer, in particular, a targeted controlled-release drug carrier can lower dosage, prolong drug duration, and prevent damage to healthy tissue. Chen et al. [109] reported multi-functional hollow mesoporous silica nanoparticles with enhanced drug delivery and in vivo imaging properties. As shown in Figure 7, the drug deliverer was prepared using a generally applicable surface engineering technique, and it had a uniform particle size. By grafting a CD105-targeting antibody on the surface of the particles, a 3-fold tumor uptake on average was achieved in 4T1 murine breast-tumor-bearing mice compared with the non-targeted group, as observed by a quantitative PET imaging study via chelating radioactive 64 Cu to the particles and an in vivo NIRF imaging study via anchoring NIR dye on the particles. Furthermore, its hollow core confers on the drug carrier a 3~15-times-higher DOX loading capacity than that of carriers without a hollow structure, and its highest loading capacity was estimated to be 1129.2 mg/g. Their work showed that a diseasedtissue-targeted drug delivery system can be developed via the elaborated surface functionalization of mesoporous silica, suggesting that in vivo targeted cancer therapy can benefit from advances in mesoporous silica nanotechnology. Researchers engaged in biological or medical research usually aim to analyze specific regions of cells or organisms through visual and clear images. To that end, numerous works have determined the biological information of cells, tissues, and even organisms directly by means of a non-invasive optical imaging technique [110][111][112][113]. Many of these studies are based on mesoporous silica functionalized by fluorophores, upconversion nanoparticles, etc. [114]. Typically, many fluorophores can be covalently grafted to a silica matrix via organo-bridged silane [115], while fluorescent inorganic compounds can be easily encapsulated by silica shells [116]. Kim et al. [117] reported a fluorescent probe prepared by covalently immobilizing rhodamine 6G onto mesoporous silica via the silane bridge; the resulting rhodamine 6G silica particle (RSSP) can be used as a fluorescent probe for the detection of Fe (III) and applications in bioimaging. Recently, Wu et al. [118] ( Figure 8) prepared tri-dye-doped mesoporous silicon nanoparticles (MTP-MSNs) via the copolycondensation of dye-modified silane during the modified Stöber process. The resulting material can be used for multicolor cellular imaging basing on the FRET-based two-photon fluorescence mechanism. After the loading of DOX, the pores of MTP-MSNs were closed by AS1411 aptamer, which acted as both a pore-blocking agent and a targeting tumor agent. The resulting drug delivery system has a controlled release ability in vivo and was proven to be an ideal material for bioimaging and targeted therapy for specific cancers. Researchers engaged in biological or medical research usually aim to analyze specific regions of cells or organisms through visual and clear images. To that end, numerous works have determined the biological information of cells, tissues, and even organisms directly by means of a non-invasive optical imaging technique [110][111][112][113]. Many of these studies are based on mesoporous silica functionalized by fluorophores, upconversion nanoparticles, etc. [114]. Typically, many fluorophores can be covalently grafted to a silica matrix via organo-bridged silane [115], while fluorescent inorganic compounds can be easily encapsulated by silica shells [116]. Kim et al. [117] reported a fluorescent probe prepared by covalently immobilizing rhodamine 6G onto mesoporous silica via the silane bridge; the resulting rhodamine 6G silica particle (RSSP) can be used as a fluorescent probe for the detection of Fe (III) and applications in bioimaging. Recently, Wu et al. [118] ( Figure 8) prepared tri-dye-doped mesoporous silicon nanoparticles (MTP-MSNs) via the copolycondensation of dye-modified silane during the modified Stöber process. The resulting material can be used for multicolor cellular imaging basing on the FRET-based two-photon fluorescence mechanism. After the loading of DOX, the pores of MTP-MSNs were closed by AS1411 aptamer, which acted as both a pore-blocking agent and a targeting tumor agent. The resulting drug delivery system has a controlled release ability in vivo and was proven to be an ideal material for bioimaging and targeted therapy for specific cancers. In general, the integration of sensing imaging abilities with drug delivery is a common topic of research [119][120][121]. For example, Yan et al. [122] (Figure 9) prepared two novel drug delivery nanovehicles with bovine serum albumin-or myoglobin-gated as well as upconverted nanoparticle-embedded mesoporous silica. The resulting In general, the integration of sensing imaging abilities with drug delivery is a common topic of research [119][120][121]. For example, Yan et al. [122] (Figure 9) prepared two novel drug delivery nanovehicles with bovine serum albumin-or myoglobin-gated as well as upconverted nanoparticle-embedded mesoporous silica. The resulting UCNP@mSiO 2 could be used for the controlled release of the encapsulated DOX when activated by tumor microenvironment stimuli such as an acidic pH, glutathione, or H 2 O 2 . In tests on tumorbearing mice, these drug delivery nanovehicles showed a good inhibitory effect on tumor growth and negligible damage/toxicity to the normal tissues. Meanwhile, the DOX release behavior could be tracked in real time via monitoring the Förster resonance energy transfer between the UCNP cores and DOX.
UCNP@mSiO2 could be used for the controlled release of the encapsulated DOX when activated by tumor microenvironment stimuli such as an acidic pH, glutathione, or H2O2. In tests on tumor-bearing mice, these drug delivery nanovehicles showed a good inhibitory effect on tumor growth and negligible damage/toxicity to the normal tissues. Meanwhile, the DOX release behavior could be tracked in real time via monitoring the Förster resonance energy transfer between the UCNP cores and DOX. Recently, stimuli-responsive drug delivery nanovehicles based on mesoporous silica have been widely studied for their ability to control drug release from pores under various stimuli, such as magnetism, ultrasound, light, and so on [123][124][125]. Li et al. [126] ( Figure  10) synthesized a novel pH-and ultrasound-responsive DOX-releasing material based on polydopamine-coated mesoporous silica. The as-prepared DOX-loaded nanoparticles displayed a pH-related response induced by the degradation of polydopamine under acidic conditions, which triggers the release of encapsulated DOX from pores upon ultrasound exposure. Furthermore, it was observed that the nanoparticles could be used to inhibit the growth of tumor cells through the controlled release of DOX. Moreover, the nanoparticles possessed a photothermal effect due to NIR adsorption by the polydopamine layer, demonstrating their good potential for chemotherapy in cancer treatment. Recently, stimuli-responsive drug delivery nanovehicles based on mesoporous silica have been widely studied for their ability to control drug release from pores under various stimuli, such as magnetism, ultrasound, light, and so on [123][124][125]. Li et al. [126] (Figure 10) synthesized a novel pH-and ultrasound-responsive DOX-releasing material based on polydopamine-coated mesoporous silica. The as-prepared DOX-loaded nanoparticles displayed a pH-related response induced by the degradation of polydopamine under acidic conditions, which triggers the release of encapsulated DOX from pores upon ultrasound exposure. Furthermore, it was observed that the nanoparticles could be used to inhibit the growth of tumor cells through the controlled release of DOX. Moreover, the nanoparticles possessed a photothermal effect due to NIR adsorption by the polydopamine layer, demonstrating their good potential for chemotherapy in cancer treatment. Another important aspect in the biological applications of mesoporous silica relates to the development of novel hemostatic materials [127][128][129][130][131]. The high specific surface area of mesoporous silica enables it to rapidly absorb large amounts of water from the blood, causing a higher concentration of clotting factors and platelets and thus promoting hemostasis. For example, Zhang et al. [132] prepared a large-pore mesoporous silica particle that has a shorter blood clotting time in vitro than Celox™ granules. They further immo- Another important aspect in the biological applications of mesoporous silica relates to the development of novel hemostatic materials [127][128][129][130][131]. The high specific surface area of mesoporous silica enables it to rapidly absorb large amounts of water from the blood, causing a higher concentration of clotting factors and platelets and thus promoting hemostasis. For example, Zhang et al. [132] prepared a large-pore mesoporous silica particle that has a shorter blood clotting time in vitro than Celox™ granules. They further immobilized the mesoporous silica particle onto cotton fibers to form a mesoporous silicacotton composite, which leads to a shorter blood clotting time and less blood loss than that seen with raw mesoporous silica powder in mouse bleeding models. In addition, mesoporous silica can also be doped with antibacterial drugs to prevent infections. Chen et al. [133] loaded tannic acid (TA), silver nanoparticles, and calcium ions onto mesoporous silica nanoparticles (MSNs) to prepare a novel biomaterial (Ca-TA-MSN@Ag) with the dual functions of hemostatic performance and antibacterial activity ( Figure 11). The porosity of MSNs, TA molecules, and calcium ions could lead to a triple synergistic hemostatic effect, and so the time needed for Ca-TA-MSN@Ag to reach hemostasis is shorter than that for pure MSN. Furthermore, Ca-TA-MSN@Ag also showed excellent antibacterial activity due to having loaded Ag nanoparticles and TA molecules.

Conclusions
Since the discovery of mesoporous silica molecular sieves, their synthesis and application have attracted extensive attention from all over the world. This mini review briefly introduced the history of developmental and biological applications of mesoporous silica molecular sieves. Many significant families of mesoporous silica molecular sieves have been found, including M41s and SBA. The morphology of mesoporous silica has also developed from bulk materials to monodisperse nanoparticles, and some silica even has a hollow structure. Tuning the morphology of mesoporous silica can be achieved via the choice of molecular template or by adjusting the reaction parameters of the solution. By utilizing the well-known Stöber method, monodisperse mesoporous silica microspheres Figure 11. The potential (a) hemostatic and (b) antibacterial mechanism of Ca-TA-MSN@Ag. Reproduced with permission from Ref. [133], Copyright 2021, Elsevier.

Conclusions
Since the discovery of mesoporous silica molecular sieves, their synthesis and application have attracted extensive attention from all over the world. This mini review briefly introduced the history of developmental and biological applications of mesoporous silica molecular sieves. Many significant families of mesoporous silica molecular sieves have been found, including M41s and SBA. The morphology of mesoporous silica has also developed from bulk materials to monodisperse nanoparticles, and some silica even has a hollow structure. Tuning the morphology of mesoporous silica can be achieved via the choice of molecular template or by adjusting the reaction parameters of the solution. By utilizing the well-known Stöber method, monodisperse mesoporous silica microspheres can be obtained. Furthermore, the combination of the seed growth mechanism originating from the Stöber method with a templating method can be used to develop hollow mesoporous silica microspheres with a higher hosting capacity. Additionally, the soft templating methods based on micelle or microemulsion play an important role in the synthesis of hollow mesoporous silica microspheres.
The properties of mesoporous silica molecular sieves, including a high biocompatibility, excellent hosting capability, etc., make them a popular carrier in biological applications. The most important application of mesoporous silicas is as a drug carrier. Generally, uniform spherical and hollow structures improve the drug loading and delivery capacities of mesoporous silica. Meanwhile, the incorporation of targeting, controlled release, and stimuli-responsive functions makes drug-loaded mesoporous silica promising for precision therapy. In addition, the functionalization of drug-loaded mesoporous silica with optical units, such as fluorophores, upconversion nanoparticles, etc., enables us to track the progress of drugs through a non-invasive bio-imaging technique. In the future, novel mesoporous silica-based drug delivery systems that combine functions such as targeting, stimuli responses, controlled release, and optical imaging will be of great significance in the field of disease treatment.