The Concept of 2D Solid Solvents: A New View on Functionalized Silica-Based Materials

: In the presented work, we would like to introduce a new concept of a 2D solid solvent. This is a material, capable of selective ion or molecules capturing thanks to its developed surface, which is treated as deposited on a substrate (usually spherical nano-silica or mesoporous silica) 2D bi- or multi-component layer. The last one consists of two main components—active anchoring units and passive spacers that are surface analogues of solute and solvent in an ordinary solution. Whereas silica substrate, anchoring units, and spacers are connected and act cooperatively for one final goal, we consider to describe them as one part. In our work, we will clarify a definition of solid solvents as well as show some examples of them and their usage.


Introduction
In the last decades, the design, preparation, and investigation of different classes of nanodevices are developing and are generally believed to be broadly used in everyday life. Moreover, nanostructured materials are already used in most types of novel techniques. Nevertheless, their synthesis and characterization in most cases have a rather "bulk" nature, forcing the use of not individual atoms or molecules, but some volume of material. The last characteristic is rather desirable to be eliminated at the time, when the ability for separation, investigation, and manipulation of individual ions and molecules opens up brand new opportunities for syntheses of new materials and nanodevices' fabrication. The last one can be achieved if we will be able to separate individual molecules and fix them somewhere in the space in a way, allowing the possibility to work on them.
For such purposes, we would like to introduce a new concept of materials, a 2D solid solvent-a nanostructured material functionalized with both anchoring active groups and passive spacer groups. Thanks to its 2D character, it is possible to easily bond desired molecules somewhere on its surface and, owing to the features of its structure, the distance between bonded molecules became easy to control and operate. Such a class of material can be extremely helpful for individual molecules' separation with further characterization as well as for syntheses of novel nano-materials.

The Concept of 2D Solid Solvents
We would like to take a look at processes occurring on the surface of functionalized silica-based materials as in the processes in a 2D solution. Firstly, it is worth noting that a matter can form a 2D phase with its own unique properties in the different interphases. Namely, in the liquid-gas interphase, it is possible to obtain a floating Langmuir monolayer, a 2D phase of a surfactant, which in addition can be in various states of the  matter-2D gas, liquid, or solid-depending on the applied surface pressure. Each of these states can be characterized via the distance between molecules and the degree of ordering. Besides, in the case of using multiple surfactants, it could be possible to obtain a film with a mixed composition, which, correspondingly, can be treated as a 2D mixture of solution. Such a monolayer can be transferred onto a solid substrate once or multiple times by vertically dipping a solid substrate through the monolayer, resulting in obtaining mono-or multilayer so-called Langmuir-Blodgett (LB) films-a 2D solid on the substrate. Similar results can be obtained via the other methods, like the SAM technique (selfassembled monolayer)-spontaneous assembly of amphiphile molecules on a solid substrate by soaking the last one in a corresponding solution-or the CVD technique (chemical vapor deposition)-the formation of a film on a solid substrate using reactive molecules from the gas phase [1].
Moreover, a comparable structure ( Figure 1) can be obtained for the surface of functionalized silica-based materials after a corresponding functionalization (grafting technique) or even after their primary synthesis (co-condensation). In the case of multiple bonded groups, the surface of a silica matrix is composed of different anchoring molecules, which, depending on the nature of used compounds, could be chemically active and suitable for further functionalization or chemically passive. In our work, we introduce a new concept-a 2D solid solvent-a material with a large specific surface area able to immobilize ions or molecules on its surface and composed of three main parts-inert solid substrate, anchored passive groups (spacers), and anchored active groups. For characterization of such a material, we suggest to use the following assumptions: Silica substrate, spacers, and anchoring units are interconnected; they act like one system and thus must be treated as one part-a 2D solid solvent; The functionalized silica surface is treated as a 2D phase-a deposited 2D solution; The functionalized silica surface is treated as if it is made only of anchoring units and spacers; Spacers are treated as analogues of a solid solvent, unreacted active anchoring units as analogues of free cites in interstitial solid solutions, and reacted active anchoring units as occupied cites.
The reaction of anchoring units is treated as analogous of the occupation of cites in the interstitial solid solutions-for clarity, we will call this reaction "activation".

Practical Examples
In everyday life, silica, presented in a form of ordinary sand, is well-known for its stability, including chemical stability, owing to the formation of its strong Si-O-Si bonds. Nevertheless, this first impression is particularly false, for the following reasons: Silica possesses polymorphism and can be presented in different crystalline forms as well as in numerous variations of amorphous silica; Properties of bulk silica and its surface layer, possessing Si-OH groups, are completely different.
In our investigations, we have chosen silica matrixes as a suitable substrat for its possibility of the creation of materials with a large specific area and their further confident surface modification techniques. As a suitable start point for a 2D solid solvent, we would like to suggest silica nanospheres and mesoporous silica (Figure 2), such as SBA-15 (Santa Barbara Amorphous-15) and MCM-41 (Mobil Composition of Matter No. 41), as easy-tosynthesize, commercially available matrices with a large specific surface area. In this chapter, we will show and barely discuss some examples of 2D solid solvents as well as discuss some possibilities of their use.

Activation of SBA-15 with Metal Ions with Further Formation of Quantum Dots
In [2], a technique was shown to prepare a SBA-15 matrix with a functionalized surface with two types of groups-active ones (propylcarboxylate/propylphosphonate) and spacer (trimethylsilane-) groups. Herein, active groups are able to bond metal cations, forming corresponding salts. As the SBA-15 matrix is porous, possessing nanochannels with a diameter of ~5 nm, one can obtain a material in which pores will be filled with the mentioned salts. The amount of salt inside of the pores can be precisely controlled, changing the concentration on the surface of an active group during the matrix synthesis. Such an initial functionalized porous material can be subjected to partial thermal decomposition, resulting in the creation of the internal nanocrystals in the pores, which act as nanoreactors.
In the case of activation of propylphosphonate groups with Cu 2+ ion and further heating at a temperature of 350 °C, the obtained copper pyrophosphate nanocrystals can be treated as a semiconducting quantum dots (SQDs) with the size below 5 nm, owing to the channels' geometry limitations.

Activation of Silica Nano-Spheres with Mn12-Based Single-Molecule Magnets
According to [3], the Mn12 single-molecule magnets (SMMs) could be attached to the surface of spherical silica, functionalized in a 2D solid solvent way. Controlling the concentration of active anchoring groups, one can control the distance between them [4] and, as a result, between bonded individual molecular Mn12-based magnets, allowing their separation with further direct microscopic observation ( Figure 3). Moreover, it also allows their investigations as separate molecules [5][6][7][8], including the analysis of such magnetic properties as the magnetic states of the individual single-molecule magnet and their mutual interactions.

Activation of Mesoporous Thin Films on the Pores' Bottoms
The proposed material can be presented not only in the form of powder, but also in the form of a thin film. For example, in [7], a 2D solid solvent-based material was obtained in a way such that, on a solid substrate, a thin mesoporous film with hexagonallyarranged vertically aligned pores of the size ~2 nm was synthesized with the pore bottoms, containing active propylcarboxylate/propylphosphonate groups and spacer trimethylsilyl groups (Figure 4). Such a structure allows us to bond a small amount of ions or molecules (up to individual ions or molecules) at pore bottoms, both fixing them as individual groups and separating the last ones among themselves with the silica walls. Conducting a similar process as in Section 3.1, partial thermal decomposition, or using SMM, as in Section 3.2, it seems to be possible to obtain an order array of separated nanoparticles or molecular magnets [9].

Conclusions
In this work, we have proposed a new concept of material-a 2D solid solvent. Its definition-a material with a large specific surface area with a possibility to capture, separate, and immobilize ions or molecules on its surface, composed of three main parts, inert solid substrate, anchored passive groups (spacers), and anchored active groupswas given. The main assumptions about its surface were done. Finally, three examples of its use for nanoparticles' synthesis, direct molecules' observation, and new functional material synthesis were shown and briefly discussed.