本 科 毕 业 论 文
Advances in Mesoporous Silica Materials with Complex Morphology and Their Applications
学院名称
化学与制药工程学院
专业班级 应化（国际班）16-1
学生姓名
朱家琪
学 号
201604301384
导师姓名 王金桂
2020 年 5 月 28 日
齐鲁工业大学（山东省科学院）本科毕业论文 原创性声明
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Complex Morphology and Their Applications
复 杂 形 貌 的 介 孔 二 氧 化 硅 材 料 的 研 究 及 其 应 用
学 院 名 称 化学与制药工程学院
专 业 班 级 应化（国际班） 16-1
学 生 姓 名 朱家琪
学 号 201604301384
导 师 姓 名 王金桂
专业技术职务 副教授
Content
Abstract ... 1
摘 要 ... 2
Chapter 1 Introduction ... 3
1.1 The Development Mesoporous Materials ... 3
1.2 The Structure and Morphology of Mesoporous Silica ... 6
1.2.1 Rod ... 6
1.2.2 Helical Rod ... 7
1.2.3 Cube ... 8
1.2.4 Ellipsoid ... 9
1.2.5 Sphere ... 11
1.3 The Applications of Mesoporous Silica ... 12
Chapter 2 The Mesoporous Silica with Complex Morphology ... 13
2.1 Ideas from Natural World ... 13
2.1.1 Inspirations from Natural World ... 13
2.1.2 Scope of Bio-inspired ... 13
2.2 Mesoporous Silica with Complex Morphologies ... 14
2.2.1 Normal Multipods structured nanoparticles ... 14
2.2.2 Controllable Multi-site Nucleation of Porous Pods ... 16
2.2.3 Janus Mesoporous Silica Nanocomposites ... 17
2.2.4 Eccentric Single-hole Nanocages ... 20
2.2.5 One-dimensional Diblock and Triblock Nanocomposites ... 21
2.3 Scope of The Complex Morphologies of Mesoporous Silica ... 22
Chapter 3 The Applications of Complex Mesoporous Silica Nanocomposites ... 24
3.1 Introduction ... 24
3.2 Application of Complex Mesoporous Materials ... 24
3.2.1 Drug Delivery ... 24
3.2.2 Catalysis ... 25
3.2.3 Enhanced Nano-bio Interactions ... 25
3.3 Scope of the Application of Mesoporous Materials ... 26
3.3.1 Fluorescent Probes for Fe3+ Detection ... 26
3.3.2 Adsorbent for Benzene Sensing ... 26
3.3.3 Synergistic Therapy ... 26
3.3.4 Brief Review of Future Research ... 27
Bibliography... 28
Acknowledgments ... 31
Abstract
According to the regulations of the International Union of Pure and Applied Chemistry, the mesoporous nanoparticle is a material with a pore diameter of 2-50nm. Due to mesoporous the high specific surface area, regularly ordered pore structure and continuously adjustable pore size of mesoporous materials, they have been continuously concerned by researchers since the synthesis by Mobil R & D in the year of 1992, and have been applied to catalysis, adsorption, biomedicine and many other fields. By the way, the materials still enjoy a promising development prospect.
This thesis takes time as the main line and mainly summarizes the development history of mesoporous silica, ranging from its main morphologies, structures, synthesis approaches, and application and development of silica materials with complex morphology. The first chapter mainly summarizes the development of mesoporous materials and the synthesis methods of mesoporous silica materials with simple morphology. The second chapter mainly introduces the development history from the appearance of mesoporous silica materials with complex shapes, as well as the main complex shapes, synthesis methods, etc. In addition, some vivid illustrations are also added to this article to help readers quickly understand mesoporous silica materials with complex morphology. The third chapter is based on the former chapter, summarizing the application of the mesoporous silica materials with complex morphology mentioned in drug delivery, catalysis and enhancement of interaction between biomolecules.
Key words：mesoporous materials; mesoporous silica; complex morphology; applications.
摘 要
根据国际纯粹与应用化学联合会的规定，介孔材料是孔径介于2-50nm的一种材料。
介孔材料因具有高的比表面积、规则有序的孔隙结构以及连续可调的孔径等特点，从 其1992年被美孚研发公司合成以来，持续受到研究人员的关注，并且被应用到催化、
吸附、生物医药等诸多领域，并且拥有广泛的发展前景。
本论文主要以时间为主线，综述了介孔二氧化硅的发展史，其主要的形貌、结构、
合成方法，以及具有复杂形貌的二氧化硅材料的应用和发展。第一章主要总结了介孔 材料的发展，以及具有简单形貌的介孔二氧化硅材料的合成方法。第二章主要介绍了 从复杂形貌的介孔二氧化硅材料的出现至今的发展历程，以及主要的复杂形貌、合成 方法等，此外一些生动的插图也被添加到本文中，以帮助读者快速理解具有复杂形貌 的介孔二氧化硅材料。第三章是在第二章的基础上，总结所提到的具有复杂形貌的介 孔二氧化硅材料在药物运输、反应催化以及增强生物分子间作用力等方面的应用。
关键词：介孔材料 介孔二氧化硅 复杂形貌 应用
Chapter 1 Introduction
1.1 The Development Mesoporous Materials
According to the definitions of the International Union of Pure and Applied Chemistry, mesoporous nanoparticles belong to a class of porous materials with a pore size between 2-50nm. Mesoporous materials have the characteristics of extremely high specific surface area, regular and ordered pore structure, narrow pore size distribution, and continuously adjustable pore size. It is mesoporous materials that play a significant role in catalytic reactions, photochemistry and biological simulation. Thereby, mesoporous materials have attracted interest in various research fields such as physics, chemistry, biology and materials since its appearance. Among all mesoporous materials, mesoporous silica is the most representative. Therefore, in this thesis, the study of mesoporous silica materials, especially those with complex morphology will be used as an example to specifically explain their development history and applications.
Before the discovery of mesoporous materials, crystalline aluminosilicate zeolite molecular sieve with uniform pore distribution and pore diameter, which is less than 2 nm,
has been widely used in petroleum smelting and molecular adsorption separation. However, it has been found that in practical applications, zeolite molecular sieves are difficult to efficiently process some macromolecular substances, such as macromolecules in heavy fractions of base oil, because these macromolecules cannot pass through the small pore size of zeolite molecular sieve. At that time, in order to solve this problem, many attempts were made to obtain the zeolite molecular sieve materials with an increased pore size and an ordered meso-structure, and the macroporous materials that could be used at the same time had a wide pore size distribution, which was hard to ensure its ordered mesoscopic structure, making macroporous materials unsuitable for many applications. Therein, it was against the background that the mesoporous nanocomposites with ordered mesoscopic structure and pore diameters between 2-50 nm came into being.
An approach for synthesizing mesoporous solids from aluminosilicate gels was reported by Kresge et al [1] from Mobil Research and Development Corporation in the presence of surfactants, providing these materials uniform channels with regular arrays and dimensions which can be tailored by choosing surfactants, chemical reaction conditions and other auxiliary chemicals in the year of 1992. The most well-known representatives of this class ranges from the silica solids MCM-41 (with mesopores in a hexagonal arrangement), to
MCM-48 (with mesopores in a cubic arrangement,), and finally to MCM-50 (with a laminar structure,) which space groups are p6mm, Ia3¯d and p2 respectively, and the 3D structures are shown in figure 1 below. [2] The M41S family of mesoporous materials similar to microporous crystalline zeolites, which exhibited a very large specific surface area, ordered pores in a uniform order and continuously tunable pore diameters ranging from 3 to 10 nm are synthesized by them. Beck et al [3] from Mobil described the synthesis, characterization, and suggested formation mechanism of M41S silicate aluminosilicate mesoporous molecular sieve family, and MCM-41 as an example of this family were arranged in the shape of a hexagon, with uniform mesopores, and its size can be designed in a range between 5 and 10 nm. Therefore, their discovery really attracted a large number of researchers to engage in the study of mesoporous materials, and the fields including catalysis, drug loading and molecular adsorption separation.
Figure 1. Structures of mesoporous M41S materials: a) MCM-41 b) MCM-48 and c) MCM-50. [2]
In the year of 1998, an ordered hexagonal mesoporous silica structure (SBA-15) has been prepared by Zhao et al [4] using an amphiphilic triblock copolymer to guide the organization of polymeric silica species, and resulting in a uniform pore size of approximately 30 nm. By the way, it was regarded as another important milestone in mesoporous materials research. The thick silica walls of SBA-15 (p6mm), in particular, ranging from 3.1 to 6.4 nm are different from thinner walled MCM-41 structures which are synthesized with conventional cationic surfactants, providing SBA-15 with a greater hydrothermal stability. At present, polymer surfactants and other surfactants used for such macroporous materials are classified as soft templates. The advantage of using a soft template is that the mesoporous material can be prepared at a relatively low temperature, and the pore structure of the resulting material can be easily controlled by simply modifying the template molecules.
The templates, are classified as instance products, ranging from SBA-15 [5], SBA-1 [6]
and even to MCM-48 [6] in the presence hard templates Besides, these templates were first utilized to synthesize the ordered mesoporous molecular sieves with carbon framework by Joo et al [5] and Ryoo et al [6], which is considered an important breakthrough in the history of mesoporous materials. The structure of the synthesized carbon referred as CMK family was by the composition of ordered nano-porous carbon, which was originally formed inside the cylindrical nanotubes of the SBA-15 template and the images of transmission electron microscopy and schematic model were presented in figure 2 below. Compared with surfactants, nano-casting strategy had brought forward incredible possibilities in synthesizing novel mesostructured materials, and finally resulted in a large amount of ordered nanowire arrays. [7] Therefore, mesoporous carbon has received much attention because its capacity of accommodating a large number of guest atoms, molecules or particles in the pores and its high conductivity, making it be used as an electrode material in the areas of supercapacitors, chemical sensors and batteries with high performance.
Figure 2. Ordered porous carbon prepared by template synthesis method using ordered mesoporous silica SBA-15. a) TEM image viewed along the direction of the ordered porous carbon. b) Schematic model for the carbon structure. [5]
Che and her colleagues [8] reported the surfactant-templated synthesis of the ordered chiral mesoporous silica, in the shape of a twisted hexagonal rod which diameter is 130-180 nm, and length 1-6 mm and a general method for the structural analysis of these chiral mesoporous crystals. In their previous study, they discovered for preparing well-ordered mesoporous silicas which is based on the self-assembly of chiral anionic surfactants and the inorganic precursors. N-acyl-L-alanine is a chiral organic molecule that can produce a chiral nematic phase with a small amount of decanol. Furthermore, aminosilane or quaternized
aminosilane plays a significant role as a co-structure-directing reagent (CSDA), which can be used to prepare mesoporous materials with inherent chirality.
1.2 The Structure and Morphology of Mesoporous Silica
During the past decades, mesoporous silica materials with various structures and morphology were synthesized and has attracted attention of a large number of researchers and chemists. At this section, we are going to briefly talk about these structures and morphologies and summarize what had been done previously by the scientists in this field.
1.2.1 Rod
Several morphologies of MCM-41 silica particles were controllably synthesized with alkaline surroundings. In the 80℃, Cai et al [9] used NaOH and a surfactant cetyltrimethylammonium bromide, which is referred as CTAB, in a very low concentration, reacts with tetraethyl orthosilicate (TEOS) to produce MCM-41 silica with an average particle size of 110 nm, and a diameter from 0.3 to 0.6nm. This silica rod which length is 1 um was synthesized in ammonia water, and its size and morphology can be easily controlled by changing the amount of solvent. The representative transmission electron microscopy (TEM) image of rodlike MCM-41 synthesized by Cai and his colleges was shown in figure 3 below in a scale of 500 nm.
As one of the most intensively investigated mesoporous materials, SBA-15 was firstly prepared by Zhao [4] and his colleagues in the year of 1998. According to the report of Sayari et al [10] in five years later since Zhao’s first synthesis of SBA-15, the most popular morphology of it is around few tens of micrometers long bundles, achieved by short rodlike particles coupled together. These rodlike particles used to be considered as the contribution of salt presenting in the process of synthesizing, whereas, as the matter of fact, the appearance of such particles may be largely due to the synthesis being carried out in a static condition and without stirring. Indeed, previous work of other scientists already reported that the use of salt (NH4F) under stirring resulted in wheatlike fibers instead of SBA-15 rods.
Figure 3.The TEM image of Rodlike MCM-41. [9]
1.2.2 Helical Rod
As Yang et al [11] reported, the study of the formation mechanism of the helical structure and the synthesis of the helical materials have always attracted quite more attention of scientists in various areas. In addition, a synthesis method of spiral mesoporous materials with chiral channels in the presence of achiral surfactants was investigated by them.
Furthermore, they proposed a simple and purely interfacial interaction mechanism to interpret the spontaneous formation of helical mesoporous structures of silica materials.
Apart from what we have mentioned above, Yang et al also reported the synthesis approach of helical mesoporous materials in the presence of achiral surfactants, where the size and pitch of the helical rod can be further controlled by adding perfluoro carboxylic acid.
By the way, in the section of The Development of Mesoporous Materials，we already talked about that Che et al synthesized ordered chiral mesoporous in the presence of the surfactants which played a role as template. Moreover, the vivid images that Che presented (figure 4 below) in her article clearly demonstrated its helical rob structures and morphologies of this mesostructured materials.
Figure 4 SEM image and schematic arrangements of a structural model of chiral mesoporous silica. a) SEM image of sample; b) Schematic drawing; c) Cross-section; d) One of the chiral channels in the material. [8]
1.2.3 Cube
A novel approach was investigated by Fan Li et al [12, 13], mainly for the synthesizing mesoporous silica nano-cubes and their shapes and sizes are entirely determined by the colloidal crystal templates. The preparation process of the silica nano-cubes they had already reported is based on the silica skeleton formed in the presence of surfactant and polymer sphere template system. After that, the bimodal dispersion of silica nano-cubes and nano-spheres which shapes, and sizes can be controlled by the colloidal crystal system, were obtained through the decomposition of 3D ordered structure. To be more detailed, after their experiments, the products were calcinated under the temperature of 550 ℃ to get the template removed. It was interesting that, the structural transformation took place there, leaving a bimodal dispersion of silica nanoparticles. Among these compounds, the large particles with cubic shape were finally obtained after several times of centrifugations. By the way, the clear image obtained by transmission electron microscopy (TEM) of their shapes was presented in figure5 below.
Figure 5. a) TEM image of the silica particles b) A highly amplified view of the silica nanocubes obtained after centrifugation. [12]
1.2.4 Ellipsoid
As Shaodian Shen et al [14] reported on their article on the journal of Chem. Mater., these complex preparation procedures, high cost on experimental apparatus and non-ideal yield extremely restrict and limit the developments and applications of ellipsoidal particles synthesized by indirect methods. Their work was keen on developing a facile method to prepare ellipsoidal particles with relatively less expensive experimental equipment and higher yields than other scientists and researcher had done previously. Under this kind of circumstance and condition, Shaodian Shen et al introduced the synthesis of silica ellipsoids with hexagonal meso-structure by organic-inorganic co-assembly in the presence of potassium chloride and ethanol, which are regarded as co-solvents. In addition, the aspect ratio of the ellipsoid will be systematically tunable if the concentration of ethanol is well controlled. In fact, their investigation filled the gap of the synthesis method of anisotropic ellipsoidal nanoparticles.
In their reported experimental procedures, the templates utilized during synthesis of non-spherical particles were poly (ethylene oxide) and poly (propylene oxide). In the meantime, tetraethyl orthosilicate as the silica precursor with the assistance of co-solvents (potassium chloride and ethanol) where the pH value is less than 7 at room temperature. The product is in the shape of an ellipsoid, with a highly ordered 2D hexagonal mesoporous structure, and its pre channels is just parallel to the major axis of the ellipsoid. After calcination, the ellipsoidal mesoporous silica was observed by scanning electron microscopy (SEM) and its megascopic morphology was also presented in figure 6 below.
Figure 6. SEM image of the calcined ellipsoidal mesoporous silica. [14]
In the study of Nanjing Hao et al [15, 16], MCM-41 nanoparticles in the morphology of an ellipsoid were synthesized in the presence of surfactant-template cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzene sulfonate (SDBS). Unlike the work of Shaodian Shen et al, Hao and his colleagues had obtained a non-spherical mesoporous silica which pore channels are parallel along the minor axis. As shown vividly in figure 7 below, the MCM-41 nanomaterials possess a diameter around 100 nm and a length 200 nm or so.
Figure 7. (A)–(C) are TEM images of ellipsoid-like nanoparticles at different magnifications;
(D) is their schematic model with parallel channels along the minor axis. [15]
1.2.5 Sphere
Kazuhisa Yano and Yoshiaki Fukushima [17] reviewed that the Stöber method was first proposed in the year of 1968 by W. Stöber for the purpose of synthesizing non-porous monodisperse silica microspheres in micro scale in the presence of water, alcohol, ammonia and tetraalkoxysilane media. In their laboratory, Yano et al used tetramethoxysilane (TMOS) and n-alkyltrimethylammonium bromide that played a role as raw materials on the preparation of mesoporous silica microspheres. The obtained microspheres were synthesized under well controlled experimental conditions.
Apart from what we introduced above, Yano et al investigated the influence of polymers and their experimental variable was the presence or absence of polyethylene glycol (PEG).
In the end, they found that there were some differences appearing in the group with the addition of PEG. Small particles were generated on the surface of normal size mesoporous silica spheres while it did not occur in the group without PEG addition. By the way, the mesoporous silica spheres of this two groups were observed by scanning electron microscopy (SEM) and the images are presented in figure 8 below.
Figure 8. SEM micrographs of mesoporous silica spheres synthesized without (a) and with (b) PEG [17]
Nooney and co-workers [18] proposed a novel method for the preparation of spherical mesoporous silica materials in a tunable diameter widely ranging from 65 to 740 nm.
Through adjusting the amount of silicate source and the concentration of surfactant, the meso-structure silica spheres were obtained. As we mentioned above, the surfactant belongs to the category of soft templates, and the synthesizing procedure is so-called soft-templating
methods. The approach that Nooney et al investigated is regarded as an effective and easy- achieved procedure for the self-assemble synthesis of highly ordered mesoporous silica spheres with controlled size.
1.3 The Applications of Mesoporous Silica
In terms of rod-like mesoporous silica nanoparticles, they have an advantage in catalysis, separation, guest molecule encapsulation and internal surface modification, which will be restricted in the process of intraparticle diffusion, compared with mesoporous silica nanoparticles with other morphologies. [10]
Che and colleagues [8] reported the synthesis of mesoporous silica materials with helical rod morphology and introduced their promising applications in the areas of chiral catalysis and recognition. By the way, it is one of the reasons why mesostructured nanoparticles have attract more and more chemists and researchers to do research in this field.
The hard-templating method proposed by Li and co-researchers [13]for direct synthesis of silica nanoparticles with cubic morphology, provided a novel prospect for the applications relating to photonics, optoelectronics and so forth.
The ellipsoid-like mesoporous silica nanoparticles (MCM-41) with short channels parallel with the minor axis, synthesized by Hao and co-workers [15] by using a mixture of surfactants presented a high capacity of drug delivery as an ideal carries on nano scale in theranostics, a sustained release profile, and a promising catalytic performance assembled by silver nanoparticles.
The mesoporous silica with uniform spherical particles and well-ordered shell prepared by Yoon et al [19] is much more suitable for the applications ranging from catalysis and selective adsorption.
Chapter 2 The Mesoporous Silica with Complex Morphology
2.1 Ideas from Natural World
There are various features worth learning in the natural world, and these features can be applied to develop novel multi-functional materials. In recent decades, biomimetic technology has been fully developed in various fields, and microorganisms, viruses, proteins, plants and so forth have broaden the horizons of researchers and chemists, and the novel ideas for the preparation of versatile nano materials have been put forward.
2.1.1 Inspirations from Natural World
One very example popped on the journal of Nature Nanotechnology published by Nian Liu and co-workers [20]. They designed an anode of lithium battery anode in nano scale due to inspiration of the structure of pomegranate.
The biologist Ran Nathan and his colleagues [21] investigated the mechanism of seed dispersal in a long distance in the natural world. From the point view of a chemists or researcher focusing on the synthesis of mesoporous materials, the pods on the surface of mesoporous nanoparticles can also play a role in delivering just as what the pods of tribulus did.
In addition, a phage with a retractable tail can attach and penetrate the host cell membranes through its own tip which is in the shape of the spikes was introduced by Taylor et al [22]. Dai and colleagues [23] summarized the novel designs of nanoparticles according to the study of tumor microenvironment on therapeutic effectiveness. To begin with the hierarchical structures and functions of complex biological systems such as viral capsids, red blood cells and platelets, Hu et al [24] explored the biomimetic engineering of synthetic materials.
2.1.2 Scope of Bio-inspired
However, as Croissant and colleagues [25] reported in their article that was published on the journal of Advanced Materials in the year of 2015, how to design a complex structure and morphology through the simulations of natural objects is still a problem making the researchers annoyed at nano scale.
2.2 Mesoporous Silica with Complex Morphologies
By adjusting the surface dynamics to control the number of nucleation sites, a series of mesoporous nanoparticles with precise and controllable surface topology were formed. Zhao et al [26] described several surface topological structures, and several pods from one to four, appeared on the surface of these mesoporous nanomaterials enhancing the capacity of bacterial adhesion according to the investigation of them. As far as the following parts of this section is concerned, they will mainly center with these particles with complex morphologies.
2.2.1 Normal Multipods structured nanoparticles
Nowadays, the nanoparticles with multipods are mainly based on the materials ranging from dense crystals and polymers are well developed, and the following will concentrate on these materials and their brief introduction. Now, the dense crystals go first.
2.2.1.1 Dense Crystals
Cheng et al [27] reported their investigations relating to the synthesis of nanomaterials with metal such as Ag and Au attached, resulting in the final products with complex morphologies. As far as the schematic illustration presented below is concerned, the brief procedures of preparing multipods nanoparticles based on nanocrystals are described vividly.
By the way, the morphologies of these gold nanoparticles (referred as Au NPs) are tunable and controllable through adjusting the concentration of silver ions solution. For instance, the sharper Au/Ag nanoparticles will be synthesized if the concentration of silver ions solution is relatively high. Similarly, the morphologies will be quite different if the Ag+ solution is dilute.
Figure 9. Schematic image of the silver ion-mediated growth of gold nanocrystals with complex morphologies. [27]
2.2.1.2 Polymers
Elvin Blanco and co-workers [28] reviewed the addition of cationic polymers, ranging from poly (ethyleneimine) to poly (l-lysine), and even to the design of nanoparticles which led to the release of therapeutic drugs from the intimal compartment with high efficiency as well. It is the interaction between the cationic charge of the nanoparticles and the negative charge on the outer surface of inner membrane. In addition, they also reported that the anionic polymers were prone to neutralize protons and finally resulted in the so-called proton sponge effect.
The metal-organic framework, which is referred as MOF, is essentially a kind of porous polymers that enjoys a great popularity in recent years mainly due to its large internal surface areas, adjustable porosities, and so forth. Yayuan Liu et al [29] talked about the synthesizing procedure of yolk-shell nanoparticles with heterostructures based on this kind of polymer, and the flow chart of their preparing process was cited in figure 10 below, which will help the readers to understand that very well.
Figure 10. The illustration of synthesis of nanoparticles with complex morphology. [29]
Although mesoporous materials with complex morphology can be synthesized based on dense crystals and polymers, unfortunately these materials do not provide sufficient storage space for functional guest molecular loading and highly limit the further practical applications of these nanoparticles with multipods surface topologies. For this reason, a novel and promising synthesis method is urgently needed to be proposed and developed by later researchers and chemists.
2.2.2 Controllable Multi-site Nucleation of Porous Pods
As Teeraporn Suteewong et al [30] reported in their paper, the one-pot synthesizing approach for a kind of mesoporous silica nanomaterials which have the morphologies of both cube and hexagon inside one particle. These multi-compartment of mesoporous silica nanomaterials are made up of a core in the shape of a cubic mesoporous morphology and normally no less than four branches, where the mesopores with hexagonal cylinder morphology, growing outward from the apex of the cubic nucleus. At the same time, the growth degree of these class of nanomaterials is easily controlled through adjusting the amount or concertation of addition agents.
To be more detailed, most of multicompartment mesoporous silica nanomaterials prepared in a dilute ethyl acetate (EtOAc) had short branches and which diameters were no longer than their size of core. [30] Whereas, the nanomaterials synthesized under relatively high concentration EtOAc, normally had branches up to two micrometers and then turned to rods. In addition, the schematic illustrations of multicompartment mesoporous silica nanomaterials synthesized under 183 mM EtOAc were presented below in figure 11, which is cited from the article of Teeraporn Suteewong and colleagues.
Figure 11. Schematic images of multicompartment mesoporous silica nanomaterials in the 183 mM EtOAc circumstances with (a) one arm, (b) two arms, (c) three arms, (d) four arms, and (e) two arms merged into one. [30]
Jonas Croissant et al [31] published their paper on the journal of Advanced Materials concerning the one-pot two-step synthesis of crystal periodic mesoporous organosilica (PMO) nanoparticles with multipods, in which the design of complex structure and morphology were investigated. In detail, the procedures consist of two steps ranging from the condensation of benzene-based sphere-shaped PMO cores, to the condensation of ethylene-based rodlike PMO pods on the cores. First of all, the PMO nanospheres were obtained, and then these spheres turned to nanorods after the addition of 1,2-bis(triethoxysilyl)ethylene. The formation of final products was processed with a consistent stir, which is considered as a crucial parameter to control the morphology of the mixed nanoparticles.
2.2.3 Janus Mesoporous Silica Nanocomposites
Xiaomin Li and other researchers [32] successfully prepared an upconvertion nanoparticle (referred as UCNP) which outer layers are single-crystal-nanocubic periodic mesoporous organosilica (PMO) and has an ordered mesoporous structure in a novel approach.
The materials are known as asymmetric Janus nanocomposites which possess a quite uniform size approximately 300 nm and large surface area around 1290 m2·g-1. Additionally, the distinct pore sizes of their dual separated mesopores are 2.1 nm and 3.5-5.5 nm respectively, and these materials has an excellent capacity of multiple guests loading and delivery, which is of great significance to solve the problem of insufficient single storage space where other carriers can be used to load multiple drug species. The preparation of dual-compartment Janus mesoporous silica nanocomposites ( abbreviated as UCNP@SiO2@mSiO2&PMO) then would experience a post-treatment process at the temperature of 55 ℃ in the presence of ethanol for around four hours. Followed by the
removal of cetyltrimethylammonium bromide (CTAB) as surfactant templates, the mesostructured template/surfactant CTAB underwent the extraction from the mesopores by using NH4NO3 ethanol solution for three four-hour cycles.
By the way, in order to present the synthesis steps which Li et al applied during their study and investigation, the flow chart is cited and presented below in figure 12.
Figure 12. Flow chart of the synthesis procedure of dual-compartment Janus mesoporous silica nanocomposites UCNP@SiO2@mSiO2&PMO based on the anisotropic island nucleation and growth approach. [32]
Another kind of Janus nanoparticle in the shape of a dumbbell was investigated by Tianyu Yang and other scientists [33] for the catalytic reaction on oil-water interface. Yang et al proposed an unsophisticated wet-chemistry approach that can be used to synthesize mesoporous carbon–organosilica Janus nanoparticles with a wonderful dumbbell morphology in the presence of dual surfactants which are of importance on controlling porosity and energy of interface, which is demonstrated in the figure 13 below. It is this wet-chemistry approach that provided a platform where a mesoporous resorcinol formaldehyde (abbreviated as RF) sphere grew into an organosilica sphere solely in one step.
As for its catalytic performance, the Janus particle-derived catalysts can efficiently catalyze the biphasic hydrogenation reactions even without any stir. It is admitted that this kind of Janus particles demonstrate an innovative application in interface catalysis.
Furthermore, the synthesis procedure investigated by Yang, could interestingly be selectively load metal nanoparticles such as Pt to on the RF compartment. Subsequently, the RF compartments were taken place by carbon. By the way,this novel Janus catalyst can be
used for a typical another oil-water reaction such as Pt-catalyzed nitroarene reduction reaction.
Figure 13. Schematic illustration of the synthesis of the Janus nanoparticles. [33]
The dual mesoporous Fe3O4@mC&mSiO2 (mC is mesoporous Carbon and m SiO2 is mesoporous SiO2 for short ) Janus nanoparticles with independent compartments of hydrophobic carbon and hydrophilic silica in a scale of single particle have been firstly synthesized by Tiancong Zhao and colleagues [34] by using a novel approach based on the selective packaging of surface charge mediation. The obtained Janus nanoparticles consist of a mesoporous rod-shaped SiO2 with adjustable mesopores which length ranges from 50 to 400 nm, width around 2.7- 100 nm. By the way, the mesopore and diameter of mesoporous Fe3O4@mC magnetic nanosphere are 10 nm and 150 nm, respectively. Similarly, Fe3O4@mC&mSiO2 Janus nanoparticles are also a kind of biphasic interface catalyst like what Tianyu Yang et al investigated. Different from Yang, what Zhao studied is focusing on the excellent performances in biphasic reduction of 4-nitroanisole, and excitingly the conversion efficiency is up to 100 %. As well, the schematic illustration of the preparation procedure of this Janus nanoparticles was presented blow.
Figure 14. A) Synthesizing scheme of Fe3O4@mC&mSiO2. [34]
2.2.4 Eccentric Single-hole Nanocages
Xiaomin Li and other researchers [35] synthesized a class of asymmetric single-pore mesoporous silica nanocages with an eccentric hollow spheres in a novel approach known as the anisotropic encapsulation method. Besides, these kinds of nanoparticles were successfully synthesized from capsules in nano scale that was fabricated by the open pores on the surface of mesoporous shell and which uniform particle size ranged from 100 to 240 nm. As Li reported in their paper, this unique nanocarrier, the eccentric hollow cavity and big hole around 25 nm were observed, serving as a storage space and channel for large guest molecules. At the same time, the size of these uniform mesopores range from 2 to 10 nm and their high surface area is approximately 500 m2·g-1. By the way, their silica shells are able to effectively provide storage space for some small guest molecules. The obtained single-hole mesoporous nanocages can also be subsequently functionalized with upconversion nanoparticles (UCNPs).
To be more specific, the preparation procedure of the obtained single-hole mesoporous nanocages consist of three steps:anisotropic encapsulation, hydrothermal treatment and HF etching, and the flow chart was presented below in figure 15. During the first step, the template in the shape of a mesostructured, hexadecyltrimethylammonium bromide (abbreviated as CTAB) and the silica precursor, 1,2-bis(triethoxysilyl) ethane (BTEE), were used along with the addition of periodic mesoporous organosilica (PMO)resulting in the formation of the eccentric SiO2@PMOcore@shell nanocomposites. Subsequently, the nanocomposites underwent the twelve-hour hydrothermal treatment at 60 °C , and finally the etch of the dense SiO2 nanoparticles and the fabrication of the eccentric hollow PMO nanoparticles were achieved. In the last step, it was the hydrogen fluoride solution that was utilized to get hollow PMO nanoparticles etched. Furthermore, the transmission electron microscopy (TEM) micrographs show the evolution of the eccentric hollow PMO nanomaterials in figure 16 below.
Last but not the least, in the process of etching, the PMOs with open holes on the surface of their shells at the thinner side were obtained due to the eccentric morphology of the hollow PMO nanoparticles.
Figure 15. Preparing procedure for the asymmetric single-hole mesoporous nanocages. [35]
Figure16. The TEM images demonstrating the morphology evolution of the eccentric nanocomposites. [35]
2.2.5 One-dimensional Diblock and Triblock Nanocomposites
Li et al [36] synthesized other types of mesoporous nanocomposites with asymmetric diblock and triblock in anisotropic epitaxial growth under the induction of degradation-restructuring. Interestingly, the mesoporous silica nanocomposites possess asymmetric nanorods which length is tunable between 50 nm and 2 μm and the surface area is up to 1200 m2g-1. The uniform diblock mesoporous silica nanomaterials in one dimensional space, were prepared by highly ordered hexagonal mesoporous organosilica nanorod, and SiO2 which is closely connected in the shape of a nanosphere. As for the triblock mesoporous silica nanocomposites, they were synthesized by a mesostructured nanocube, or a nanosphere with radial mesopores, and even mesostructured nanorod in the shape of the hexagons through the anisotropic growth of mesopores. In the end, the match-like asymmetric Au-NR@SiO2&EPMO mesoporous nanorods were obtained along
with characteristics ranging from high surface area to special passages in 1D space, and even to functional asymmetry.
There are two crucial steps for the growth of the asymmetric nanorods. The first step processes concerning the essential silica seed cores. Besides, the second point for the fabrication of the mesoporous nanorods that are asymmetric, is the degradation and restructuring process of the silica-based seeds in nano scale. The degradation of the silica-based nano seeds is achieved through maintaining the initial dense SiO2 seeds in cetyltrimethylammonium bromide (CTAB) solution under the basic solution for a long time while the organosilane precursors were added in advance. For the purpose of brief understanding, the schematic flow chart of Li et al draw in their article was cited in the format of figure17 below.
Figure 17. Schematic illustration for the synthesis of asymmetric diblock nanocomposites under the degradation-restructuring induced anisotropic epitaxial growth. [36]
2.3 Scope of The Complex Morphologies of Mesoporous Silica
Recent years, the rapid advances of mesoporous silica nanomaterials with complex morphologies and structures witnessed their wide applications in various fields that closely related to human life. However,as Zhao and other scientists [26] mentioned in their recent article,the nucleation sites used for the assemble mesoporous pods are highly restricted to only one site for the formation of asymmetric dimer structure, and the distributions of the
island nucleation and growth of these mesoporous pods are arbitrary. It has got to be admitted that controlling the number and distribution of nucleation sites for the synthesis of multipods mesoporous nanoparticles with tunable surface topological structures precisely is still a great challenge.
Therefore, Zhao et al studied the novel approach of surface-kinetic meditated multisite nucleation towards precise adjustment of the nucleation number of periodic mesoporous organosilicas (referred as PMOs) on the surface of resorcinol-formaldehyde resin (abbreviated as RF). Subsequently, the Fe3O4@SiO2@RF&PMOs mesoporous nanocomposites with tetrapods was produced. Because of the multipods structure with strong bio-nano interactions and large surface area around 584 m2·g−1, the nanocomposites with complex morphology were applied for antibiotic loading. Furthermore, the performance of the complex nanoparticles with tetrapods on segregating bacterial is excellent and their inhibition ability is up to 90% which last for a long time period. The multipods are prepared from the Fe3O4@SiO2@RF core@shell nanosphere which diameter is approximately 260 nm as a center, and four PMO cubes in the size of 150 nm as tetrapods on the surface of RF shell.
Besides, the conversion procedure from Fe3O4 nanoparticle to Fe3O4@SiO2@RF&PMOs was illustrated below in a colored flow chart.
Figure 18. Flow chart from Fe3O4 nanoparticle to Fe3O4@SiO2@RF&PMOs. [26]
Chapter 3 The Applications of Complex Mesoporous Silica Nanocomposites
3.1 Introduction
According to the previous investigations on mesoporous silica, their applications are highly influenced by their structures and morphologies. That is to say, the mesoporous silica nanocomposites with various structures and morphologies have a potential on some unique applications, which are very depend on their diameters, pore sizes, and so forth.
Generally speaking, the applications of mesoporous silica range from catalytic and sorption applications, to semiconducting applications, to drug delivery and even to the imaging in biomedicine. The following sections will talk about the applications related to drug delivery, catalysis and the enhancement of nano-bio interaction of complex mesoporous silica nanocomposites.
3.2 Application of Complex Mesoporous Materials 3.2.1 Drug Delivery
As Teeraporn Suteewong et al [30] reported, they have successfully synthesized multicompartment mesoporous silica nanoparticles with cage-like cubic morphology and four hexagonal branches. Furthermore, their investigation suggests that the multicompartment mesoporous silica nanoparticles with complex morphology can be applied to in catalysis or drug delivery through adjusting the various pore environments.
Jonas Croissant and colleagues [25] investigated the synthesis of hybrid multipods periodic mesoporous organosilica (mp-PMO for short) nanoparticles with crystal structures solely in a one-pot two-step procedure. Besides, in their study, the mp-PMO could be applied for multidrug delivery, on the condition that selective drug adsorption could be adjusted. In the end, the investigation of mp-PMO with complex nanostructures subsequently enjoys a great popularity among researchers and chemists in various relevant domains.
Xiaomin Li et al [32] investigated the Janus mesoporous silica nanoparticles with dual-compartment, this kind of nanocomposites were further applied to
nano-biomedicine for dual-drugs controllable release which are triggered by heat and up-conversion luminescence.
3.2.2 Catalysis
Tianyu Yang and co-workers [33] successfully synthesized mesoporous Janus solid nanoparticles in the shape of a dumbbell, which can be used for the interface catalysis between oil and water. As the researchers expected, this Janus catalyst can assemble at biphasic interface, and subsequently stabilize the Pickering emulsion. The droplets, which sizes are ranging from 30 to120 mm, are clearly observed with optical microscopy and optical micrograph is presented below in figure 19. What astonished them most is that the morphologies and sizes of droplets are still retained four weeks later, which indicates the high stability of the Pickering emulsion.
Figure 19. Optical micrograph of the stabilized emulsion. [33]
The asymmetric nano catalysts obtained by Tiancong Zhao and other chemists [34]
show very good activity not only in oil-water reduction of 4-nitroanisole with an excellent conversion efficiency, but also in biphasic cascade reactions for the synthesis of cinnamic acid, with a turnover frequency up to 700 h-1. Beyond what mentioned above, other applications in various fields are still under investigation due to the high surface area, magnetic properties, and functionalization of this complex mesoporous silica nanomaterials.
3.2.3 Enhanced Nano-bio Interactions
Recently, Zhao and other researchers, what we already mentioned in last chapter 2.3, proposed a multi-pods mesoporous silica nanocomposites Fe3O4@SiO2@RF&PMOs with controllable surface topological structures, which can
be used to enhance the bacterial adhesion and inhibition. In their experiments, the performance of the prepared complex mesoporous nanocomposites on bacterial segregation and inhibition was excellent lasting for long time.
3.3 Scope of the Application of Mesoporous Materials
In this section, the lasted applications of the mesoporous nanocomposites are introduced in several areas in 2020, which will highly indicate the future development of mesoporous silica nanocomposites.
3.3.1 Fluorescent Probes for Fe3+ Detection
Yufei Dong et al [37] proposed a preparation approach firstly of ordered mesoporous silica (referred as OMS below) encapsulated carbon quantum dots, abbreviated as CQDs. The novel nanocomposites were planned to be used as CQDs@OMS fluorescent probes of the detection of ferric ions due to their quick response of Fe3+. Apart from that, the nanocomposites also have good advantages such as almost no toxicity, high fluorescence intensity and biocompatibility, and good water solubility in water phase.
3.3.2 Adsorbent for Benzene Sensing
Bralee Chayasombat and co-workers [38] investigated the application of mesoporous foam silica synthesized and meanwhile functionalized by hexamethyldisilazane, referred as HMDS resulting in a strong influence and effects on the surface adsorption capacity of benzene. After the functionalization process, the pore size and nature of mesoporous foam silica which the HMDS was successfully functionalized on, retained for a long time and the. By the way, the further analysis showed that the benzene adsorption is much higher than that without any functionalization and finally led to the usage of mesoporous foam silica being a sensing material for benzene detection.
3.3.3 Synergistic Therapy
Furthermore, Bo Yang and other researchers [39] also investigated the application of mesoporous silica nanoparticles (MSN for short) metal-phenolic networks (abbreviated as MPN) in the shape of a core-shell structure relating to combinatorial photothermal therapy and chemotherapy. During their investigation, they proposed the
synthesis of mesoporous silica nanoparticles with metal-phenolic networks in an approach of super assembling. The MSN@MPN system possessed a capacity of killing tumor cells effectively after the loading of anti-cancer drugs. The images of
MSN@MPN obtained by transmission electron microscopy were presented below in figure 20.
Figure 20. (a) and (b) are the TEM images of MSN@MPN nanoparticles.[39]
3.3.4 Brief Review of Future Research
According to what we mentioned above, the present applications of mesoporous silica nanocomposites are highly depending on the pore size, surface area, structures, morphologies and some other unique properties provided by some certain functional groups.
In addition, the versatility of mesoporous silica materials is attracting the attention of a growing number of researchers at present. The low-cost and facile synthesis procedure of mesoporous silica materials with multiple excellent properties has become the orientation of the scientific study in the following decades within this domain. However, the various demand for future mesoporous silica nanocomposites normally possess much more complex structures and morphologies resulting a contradiction among the synthesis approach, production cost, and expected performance and properties. Therefore, how to balance these crucial factors is of significance and importance, and it is indeed a challenge for the scientists in future.
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Acknowledgments
First of all, please allow me to express my sincere thanks to my mentor, Professor Wang Jingui. I am lucky to meet such a humble and knowledgeable teacher.
His enthusiasm for scientific research deeply affected me. It was him who helped me answer my doubts in scientific research, gave me a new understanding of scientific research, and completed this paper independently.
In addition, I would also like to thank Liu Xuefei, a graduate student. He provided many suggestions and advice about the composition of my bachelor thesis and told me how to look up and download literatures and papers. Furthermore, another graduate Shao Yuanchao also does me a favor in other ares.
As for my students, Li Geng, Xu Fulin and Wang Junmei, who are under the supervision of Prof. Wang, I have to admit that they really encouraged and inspired me when I was disappointed. My sincere appreciation belongs to them.
Besides, I herein express my gratitude to my parents for their support to my study and life.
