**Optical Sensors Based on Mesoporous Polymers**

Ruslan Davletbaev, Alsu Akhmetshina, Askhat Gumerov and Ilsiya Davletbaeva

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/57383

#### **1. Introduction**

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46 Optical Sensors - New Developments and Practical Applications

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Porous materials are widely used in such industries as chemical, food industry, petrochemis‐ try, medicine, and environmental protection. Nanoporous materials (NM), characterized by a pore size of 1 to 100 nm, are a great alternative to non-porous materials due to the presence of a number of unique properties. Among NM are microporous materials (e.g. zeolites) and mesoporous materials (e.g., porous polymers, aluminum or silicates). According to IUPAC nomenclature, the definition of "microporous" corresponds to the pore size of 2 nm, the definition of "mesoporous" corresponds to 2-50 nm [1]. By chemical composition NM are divided into aluminum silicates, metals, oxides, silicates, consisting only of carbon and organic polymers. These materials are combined in a high surface area and porosity. NM are used in various fields of chemistry and technology, depending on their chemical composition, pore size and distribution, porosity value.

Promising areas in which mesoporous materials can be successfully used are transparent optical chemical sensors and test methods for the determination of various substances. Sensors and sensing elements on optically transparent polymeric substrates can be more convenient in some cases of analytical practice, as they allow to observe visually the color change [2]. To determine the low concentrations of elements preliminary sorption concentration and subsequent determination by chemical or physico-chemical methods are used. The optical transparency of the sorbent allows to carry out the analytical reactions on sorbent surface and on their basis to develop sorption-photometric and test methods for the determination of substances.

Complexation reactions of organic reagents and a tested ion are the basis of optical chemical sensors for metal cations, they are accompanied by color change of the reaction system. In absorption-based optical sensors, the molecules of organic chromophores are used as a sensing

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

layer of the substance (receptor), selectively interacting with the analyte [3]. In the case of an luminescence-based optical sensor, the molecules of organic luminophorsare receptors. The latter also became common as active medium in dye lasers.

Therefore, the search for new optically transparent materials with developed specific surface, chemical resistance and high physical and mechanical properties is an urgent task of polymer

Optical Sensors Based on Mesoporous Polymers

http://dx.doi.org/10.5772/57383

49

**2. Fundamentals of synthesis of mesoporous polymers based on aromatic**

Isocyanates are able to enter into chemical reactions leading to the formation of polymers with different structure. In most cases for the isocyanates, the reactions of nucleophilic addition of compounds containing mobile hydrogen atoms are typical. The reactions of isocyanates with diols and diamines are the most important from a practical point of view. In the presence of catalysts (tertiary amines, alkoxides and carboxylates of quaternary ammonium base, etc.) isocyanates go through dimerization and trimerization to form uretidinedions and isocyanu‐ rates [10]. The possibility of catalytic homopolymerization of isocyanates by anionic mecha‐ nism is also known. As a rule, polyisocyanates of amide nature are polyaddition products. However, in the literature [11] there is information about the formation of polyisocyanate links of acetal nature (O-polyisocyanates). The possibility of transformations in different directions is due to the ambident nature of the anion at the end of the growing chain (Figure 1). These works give the information about obtaining the polyisocyanate links of acetal structure by copolymerization of ethylene oxide and aromatic isocyanates using IR spectroscopy and chemical degradation of the polymer. Typical for these polymers, N = C bond appears in the

In [12,13] it was established that the open chain analogs of crown ethers, which are block copolymers of ethylene oxide and propylene oxide containing terminal and potassium alcoholate groups, are effective initiators of opening the isocyanate groups along the thermo‐ dynamically more stable carbonyl group. It was assumed that the capture of the metal cation by polyester fragment acting as a linear podand promotes the preferential localization of the negative charge on the oxygen atom of the growing chain in anionic polymerization. It was also shown that such polyaddition occurs only when 2,4-tolylene diisocyanate is used

involving isocyanate groups of more active para-position to the reaction process.

**Figure 1.** Ambident nature of the anionic center in the reaction of 2,4-toluene diisocyanate with alcoholates

chemistry and materials science

IR spectra in the region of 1670-1680 cm-1.

**isocyanates**

Indicator papers, indicator tablets, powders, solutions in vials may be used as a substrate. There is a number of substrates, such as cellulose, ion-exchange resins, superfine silicas, polyvinyl chloride membranes, etc. where the complexing reagent can be immobilized. The main requirements to the material of the reagent carrier are optical transparency, high sorption rates, ease of synthesis, processability, inertness to the reactants, stability in acidic and alkaline media, high sensitivity to analytes [4].

The natural cellulose polymer is the most widely used as a matrix [5]. A solid carrier is soaked in the reagent solution and then dried. The process can be one-stage or multistage.

The methods of applying reagents to the polyurethane foams are worked out. These methods are based on pre-plasticization of polyurethane foam tablets and the subsequent treatment by small volume of acetone solution of an analytical reagent [6].

Silica gel is a porous, granular silica form synthetically produced from solutions of sodium silicate or silicon tetrachloride, or substituted chlorosilanes / orthosilicates. The active surface of silica gel with a large surface area is of great importance in the adsorption and ion exchange. The modification of the silica gel surface for the analytical reactions is carried out in two separate ways, viz. the organic functionalization, where the modifier is an organic group; and inorganic functionalization where a group fixed on the surface can be an organometallic composite or metal oxide [7].

One of the interesting areas of research is gelatin cured gel applied to the substrate made of a transparent polymer. Gelatin is a polydisperse mixture of polypeptides prepared by alkaline or acid hydrolysis of collagen. Biopolymer has high hydrophilicity, transparency in the visible spectrum, ability to form gels at any weight ratio of water-gelatin. These properties make the systems with immobilized reagents on the basis of such polymer especially attractive for analytical reactions involving water as a solvent [8].

In recent years modified ion-exchange materials prepared by sorption, or chemical grafting of organic reagents using conventional ion exchangers, have found wide application [9]. Such ion exchangers are used for the selective and group concentration of elements, but the most valuable thing is that owing to them, an appropriate element can be simultaneously concen‐ trated and determined quantitatively.

There are a number of shortcomings for the known substrates which limit the prevalence of these materials as the base of optical chemical sensors. Cellulose is non-transparent, has low resistance to aggressive environments, weak physical and mechanical properties. Polyur‐ ethane foams are also non-transparent and characterized by high desorption of the chromo‐ phores from the pores of the material. Gelatin gels have low physical and mechanical properties. Ion-exchange sorbents are characterized by the complexity of synthesis and low kinetic characteristics of sorption.

Therefore, the search for new optically transparent materials with developed specific surface, chemical resistance and high physical and mechanical properties is an urgent task of polymer chemistry and materials science

layer of the substance (receptor), selectively interacting with the analyte [3]. In the case of an luminescence-based optical sensor, the molecules of organic luminophorsare receptors. The

Indicator papers, indicator tablets, powders, solutions in vials may be used as a substrate. There is a number of substrates, such as cellulose, ion-exchange resins, superfine silicas, polyvinyl chloride membranes, etc. where the complexing reagent can be immobilized. The main requirements to the material of the reagent carrier are optical transparency, high sorption rates, ease of synthesis, processability, inertness to the reactants, stability in acidic and alkaline

The natural cellulose polymer is the most widely used as a matrix [5]. A solid carrier is soaked

The methods of applying reagents to the polyurethane foams are worked out. These methods are based on pre-plasticization of polyurethane foam tablets and the subsequent treatment by

Silica gel is a porous, granular silica form synthetically produced from solutions of sodium silicate or silicon tetrachloride, or substituted chlorosilanes / orthosilicates. The active surface of silica gel with a large surface area is of great importance in the adsorption and ion exchange. The modification of the silica gel surface for the analytical reactions is carried out in two separate ways, viz. the organic functionalization, where the modifier is an organic group; and inorganic functionalization where a group fixed on the surface can be an organometallic

One of the interesting areas of research is gelatin cured gel applied to the substrate made of a transparent polymer. Gelatin is a polydisperse mixture of polypeptides prepared by alkaline or acid hydrolysis of collagen. Biopolymer has high hydrophilicity, transparency in the visible spectrum, ability to form gels at any weight ratio of water-gelatin. These properties make the systems with immobilized reagents on the basis of such polymer especially attractive for

In recent years modified ion-exchange materials prepared by sorption, or chemical grafting of organic reagents using conventional ion exchangers, have found wide application [9]. Such ion exchangers are used for the selective and group concentration of elements, but the most valuable thing is that owing to them, an appropriate element can be simultaneously concen‐

There are a number of shortcomings for the known substrates which limit the prevalence of these materials as the base of optical chemical sensors. Cellulose is non-transparent, has low resistance to aggressive environments, weak physical and mechanical properties. Polyur‐ ethane foams are also non-transparent and characterized by high desorption of the chromo‐ phores from the pores of the material. Gelatin gels have low physical and mechanical properties. Ion-exchange sorbents are characterized by the complexity of synthesis and low

in the reagent solution and then dried. The process can be one-stage or multistage.

latter also became common as active medium in dye lasers.

48 Optical Sensors - New Developments and Practical Applications

small volume of acetone solution of an analytical reagent [6].

analytical reactions involving water as a solvent [8].

trated and determined quantitatively.

kinetic characteristics of sorption.

media, high sensitivity to analytes [4].

composite or metal oxide [7].
