Supramolecular self-assembly of the hottest nanocl

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Supramolecular self-assembly of nanoclusters

Abstract: in the application of nano materials, the assembly of nanoclusters or nanoparticles will be a very critical step. Supramolecular chemical assembly methods of nanoclusters can be divided into two categories, namely colloidal crystal method and template method. Colloidal crystal method is to use the self-assembly characteristics of colloidal solution to assemble nano clusters into superlattices, which can obtain two-dimensional or three-dimensional ordered superlattices. Template method uses the recognition between Nano clusters and assembly templates to drive the assembly of clusters. Applicable templates include solid films, monolayers, organic molecules, biomolecules, etc. Among them, the monolayer template is the most studied and the most mature one; The rigorous molecular recognition function between biomolecules makes it a very promising assembly template, and it is possible to realize the assembly between different nano clusters with biomolecular templates

key words: nanoclusters; Supramolecular; Self assembly; Colloidal crystal; Formwork; Monolayer; DNA

since the early 1980s, scientists have constantly found that ultrafine particles have special properties in one way or another. The combination of this kind of atoms has many names, such as nanoparticles, nanocrystals, quantum dots, etc., but it is usually called nanoclusters [1]. Nanotechnology is more and more widely used in various fields. The continuous emergence of new nano cluster synthesis methods opens the door of hope for the development and application of quantum dot lasers, single electron processors, high-density magnetic storage materials, high-performance color copying and other technologies [1, 2]. From nano clusters to applied devices, the assembly of clusters must be involved. The assembly of clusters is difficult to be completed by conventional physical or chemical methods, and the supramolecular self-assembly method will successfully solve this problem

in addition to maintaining the original characteristics of nano clusters, the superlattice assembled by supramolecular method also has some special properties due to the coupling between regularly arranged nano clusters, and the properties of superlattice can be modulated by changing the organic molecules between Nano clusters. Therefore, the nano cluster superlattice (1) checks whether the power line connected to the pipe ring stiffness tester is connected normally. It has very special optical, electrical and magnetic properties, and has great application prospects in the fields of optics, electronics, information storage and so on [3, 4]

the supramolecular chemical assembly methods of nano clusters reported in the current literature can be divided into two categories: one is to use the self-assembly characteristics of colloids to assemble clusters into colloidal crystals to obtain two-dimensional or three-dimensional nano cluster superlattices; The other is to use molecular recognition between nanoclusters and assembly templates to complete the assembly of nanoclusters

1. Colloidal crystal method

it is well known that colloids have the characteristics of self-assembly, and nano clusters are easy to disperse in solvents to form colloidal solutions. Therefore, as long as appropriate conditions are available, nano clusters can be assembled to form a regular arrangement. If the nanoclusters are "dissolved" in an appropriate organic solvent to form a colloidal solution, the superlattice of nanoclusters can be further assembled [3]. The conditions required for this self-assembly process [4] are: (1) hard sphere repulsion, (2) uniform particle size, (3) van der Waals force between particles, and (4) gradual destabilization of the system. Among them, conditions (1) and (3) are the inherent properties of the colloidal solution system of nano clusters, and condition (2) is mainly achieved through the control of the preparation conditions of nano clusters and the application of appropriate separation methods [5]. Therefore, in fact, the operable factor in the assembly process is mainly the control of the stability of the colloidal solution system. The important factors affecting the stability of the system include particle size, the properties of cluster coated molecules, the type of solvents and the "concentration" of nanoclusters

bawendi et al. [4] dissolved the CdSe nanoclusters coated with TOPO (trioctyl phosphine oxide) and top (trioctyl phosphine) in the mixed solvent of octane and octanol at a certain pressure and temperature, and then reduced the pressure to gradually volatilize the octane with low boiling point. Due to the low solubility of CdSe nanoclusters coated with TOPO and top in octanol, the colloidal crystals of the nanoclusters "precipitated" from the solution. Through high-resolution electron microscope analysis, the ordered arrangement range of the superlattice thus assembled can reach several microns (Fig. 1). Pileni et al. [6] found that if the colloidal solution of Ag2S nanoparticles is dropped onto a certain medium, the regularly arranged nanoparticle superlattice film can be obtained after the solvent volatilizes. The degree of order of the obtained superlattice film is very related to the drop rate of the colloidal solution. When adding it drop by drop (each drop will wait for the solvent to evaporate and then drop another drop), only a small area of the film can be obtained, while when adding a few drops at the same time and then waiting for the solvent to evaporate, the film area is much larger

Figure 1 high resolution electron microscope photograph of CdSe quantum dot superlattice assembled by colloidal crystal method (the size of quantum dot in the figure is 4.8nm)

(a) image of (101) plane arranged by FCC and characteristic electron diffraction pattern (b) image of (100) plane arranged by FCC and characteristic electron diffraction pattern

because the molecular recognition function is weak in the assembly process, this kind of assembly process is difficult to control, and the requirements for assembly conditions are very strict. However, three-dimensional superlattices can be assembled by colloidal solution, which is difficult to achieve by other methods

2. Template method

when using template method to assemble nanoparticles, due to the recognition between the selected assembly template and nanoparticles, the template has a guiding role in the assembly process, and the assembly process is more perfect. The selected template can be solid matrix, monolayer or multilayer film, organic molecule or biomolecule, etc

(1) solid polymer film template

van blaaderen et al. Used solid templates to complete the three-dimensional assembly of micron sized particles [7]. They used electron beam to make regularly arranged holes on the polymer film, and the depth and diameter of these holes were matched with the assembled particles. These polymer films were used as assembly templates to assemble the micron particles dispersed in the solution. Through the selection of appropriate mixed solvents and the adjustment of ionic strength, the particles were deposited layer by layer on the template to form a three-dimensional ordered structure. They call this method "colloidal epitaxy". The biggest difficulty in realizing the assembly of nanoparticles by colloidal epitaxy is the preparation of ordered nano pores. Common physical methods are difficult to meet the requirements, while chemical methods provide some possibilities. Spatz et al. [8] prepared gold nanoparticles in the reverse micelles formed by block copolymers. After the solvent volatilized, it was observed that the gold nanoparticles formed a very regular arrangement under the electron microscope. This gives us an enlightenment. Because block copolymers can form regularly distributed micro structures in solid films [9], they can be used as nano cluster assembly templates with potential application value

(2) monolayer template

the technology of self-assembled monolayers has been very mature today [10]. Due to the very regular structural arrangement of monolayers, they are very suitable for the assembly template of nano clusters. Alivisatos et al. [11] used molecules with active groups at two ends to assemble a monolayer with - SH at the exposed end on the metal surface, and assembled it on the monolayer through the interaction between CdS nanocrystals and - SH, thus obtaining a two-dimensional film of nanocrystals

kunitake [12] team assembled long-chain amine salts with metal complex anions as the corresponding ions into organic/inorganic multilayers, that is, organic/inorganic superlattices. In this system, metal compound nanoclusters can be further synthesized. In this way, the sandwich structure of organic molecular membranes inorganic nanoclusters will be obtained

Fendler has made important contributions in this field. His team has done a lot of in-depth work on the crystallization guided by monolayers and the formation of nanocrystalline films, and literature [13] has summarized it

(3) simple organic molecular template

as the template for nano cluster assembly, the organic molecules usually have coordination groups at both ends, and the nano clusters are assembled through their coordination with the metal ions on the surface of the nano clusters. The most commonly used compound is dithiol. Because the mercapto group has strong coordination ability, the assembly is very effective. Schiffrin team assembled gold nanoclusters with dithiol to obtain the structure of so-called "super clusters" [14]

(4) biomolecular templates

are often used as templates for the assembly of nanoclusters, usually DNA molecules or their fragments. Unlike simple organic molecular templates, the assembly process is not realized through the recognition between templates and nanoclusters, but through the molecular recognition between oligonucleotide molecules combined with nanoclusters and templates. At present, only the assembly of gold nanoparticles by this method has been reported in the literature [15]. They introduced a sulfhydryl group into the oligonucleotide molecule, combined the oligonucleotide molecule to the cluster through the coordination between the gold cluster and the sulfhydryl group, and then completed the assembly of the gold cluster by pairing the oligonucleotide with the base of the DNA molecule as a template [16] (method a); Alternatively, two gold nanoclusters are coated with oligonucleotides containing complementary base sequences respectively, and then they are mixed to obtain a two-dimensional or three-dimensional gold nanocluster assembly system [17] (method B). Figure 2 is the mechanism diagram of the two assembly methods, and figures 3a and 3b are the assembly results of the two methods. DNA has a more perfect and rigorous molecular recognition function, which makes the assembly process highly selective; Because of the thermal instability of biological molecules, when the assembled nanoclusters are heated to a certain temperature, the DNA molecules are destroyed and the nanoclusters will be redistributed. In addition, because the driving force of assembly comes from the molecular recognition of the molecules coated on the nanoclusters, it will be possible to use this method to realize the assembly of nanoclusters with different types and particle sizes, which has potential application value in the preparation of nano devices with special properties and requirements

method a method B

Figure 2 Schematic diagram of the mechanism of gold cluster assembly by biomolecular template method

Figure 3 transmission electron microscope photos of nano cluster assemblies obtained by DNA template method

(a) two cluster and three cluster assemblies obtained by method a; (B) The two-dimensional assembly obtained by method B

(5) hybrid template method

the assembly of nano clusters can also use some other methods, such as the method used by Andrews et al. [18] in the assembly of two-dimensional gold cluster superlattices: wrap the gold clusters with a layer of dodecyl mercaptan molecules, organize them on SiO2 matrix into monolayers, and then replace mercaptan with molecules with functional groups with strong binding force with gold at both ends, The two-dimensional superlattice of gold clusters is very stable by making the gold clusters closely combine with each other without destroying the structure of the original single-layer film

the research of nano assembly technology started soon and has achieved many very meaningful research results. At present, great progress has been made in the study of the properties of nano cluster assemblies. The single electron response Coulomb effect of metal matrix organic molecule nano particle junctions assembled by LB film technology has been proved [19]; The nonlinear optical properties of silver cluster films assembled by LB film technology have also obtained gratifying research results [20]; The most exciting thing is that the automatic control function unit composed of four quantum dots has been realized in experiments [21]. It is reasonable to believe that with the development of nano assembly technology

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