Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives

*Ema Horak, Robert Vianello and Ivana Murković Steinberg*

#### **Abstract**

Benzimidazole derivatives are well-known biologically active substances, and therefore, they are mostly synthesised for therapeutic purposes. However, such heteroaromatic molecular systems own structure-related properties that enable a variety of applications, especially in optical science. Multifunctionality of the benzimidazole unit, such as electron accepting ability, *π*-bridging, chromogenic pH sensitivity/switching and metal-ion chelating properties, makes it an exceptional structural candidate for the design of optical chemical sensors and functional materials. Development of smart molecular sensors and novel (nano)materials is the emerging trend observed in materials and optical sensing science in general, in which the benzimidazole molecular systems strongly contribute and participate. In this chapter, we summarised recent advances in optical sensing (nano)materials that incorporate the benzimidazole structural moiety. Solid-state optical sensing systems, including self-assembled molecular materials based on benzimidazoles, are reviewed and discussed. In addition, immobilisation of benzimidazole derivatives onto or into various substrates and matrices, such as organic and inorganic polymers, bulk membranes and nanoparticles, utilising different chemical and physical methods, is presented and analysed.

**Keywords:** benzimidazole, functional materials, optical sensor, solid-state, absorbance, fluorescence, aggregation-induced emission

#### **1. Introduction**

Optical chemical sensors are widely applied in chemical science and technology, as well as in other disciplines such as biology, medicine and environmental science. They enable continuous monitoring of the target analytes and exhibit high sensitivity and fast response time. The biggest advantages of optical chemical sensors, in comparison to other sensing devices, are the economic production, ease of operation and the possibility of on-site application without reference devices, which are preferred in chemical and biological applications. Performance of every chemical sensor is primarily determined by the sensing chemistry that operates in the background, that is, the recognition unit—receptor. The receptor, the core of every optical chemical sensor, is the sensing molecule that selectively responses to the presence of the target analyte by changing the photophysical properties of the observed molecular system. Fluorescence techniques are

most commonly applied for the generation and transfer of the analytical signal, while providing high sensitivity and selectivity. Therefore, fluorescent sensing molecules are the most promising candidates for the chemical sensing. Their design often starts from the heterocyclic molecular skeleton, due to its excellent spectral properties and the ability to detect diverse analytes. Heterocyclic chromophores and fluorophores are the most investigated classes of optical sensing molecules; hence, the interest for benzimidazole as a structural block of novel molecular systems is constantly increasing. Although benzimidazole derivatives are primarily known as biologically and therapeutically active substances [1], such heteroaromatic molecules have structure-related properties that enable a variety of applications in optoelectronics and non-linear optics (NLO) [2, 3], photovoltaics [4, 5], sensing [6] and bioimaging [7, 8]. Indeed, multifunctionality of the benzimidazole unit, such as electron accepting ability, *π*-bridging, chromogenic pH sensitivity/switching and metal-ion chelating properties, makes it an exceptional structural candidate for the design of optical chemical sensors [9, 10]. From the chemical point of view, the benzimidazole ring possesses a high degree of stability. Benzimidazole, for example, is not affected by concentrated sulphuric acid and is quite resistant to reduction. Oxidation cleaves its benzene ring, yet only under vigorous conditions. The two imidazole nitrogens are different from one another in their nature, which makes the properties of the ring system diverse in character. The hydrogen attached to the nitrogen can easily tautomerise to the other nitrogen atom. With the p*Ka* values 5.3 and 12.3, benzimidazoles are weakly basic, being somewhat less basic than the imidazoles and sufficiently acidic to make them usually more soluble in polar environments and less soluble in organic solvents. Benzimidazole, for example, is soluble in hot water but difficultly soluble in ether and insoluble in benzene, all of which can be modified upon the substitution. The acid/base properties of benzimidazoles are due to the stabilisation of the charged ion by the resonance effect.

However, development of optical chemical sensors is much more complicated than designing a sensing molecule (recognition unit), since the process combines molecular recognition, material science and device implementation. Employing the sensing chemistry in a form of optical sensing material is perhaps the key step towards the ultimate goal, since its implementation can directly result in a functional sensor. Although there is a large number of fluorescent indicators and sensing molecules presented in literature, many of them lose their selectivity upon the implementation in functional devices, which makes the design of optical sensing materials a very challenging task [11, 12].

Recently, developments in optical sensing molecular systems that incorporate benzimidazole structural unit are reviewed and discussed [13]. As can be deduced from a given review, molecular sensors based on benzimidazole derivatives are mainly applied in solution, while materials for optical sensing are still rare, yet very promising. Development of novel (nano)materials and especially 'smart' molecular sensors, some of which include nanotubes, nanowires and nanoparticles, is the emerging trend observed in materials and optical sensing science. Although the growth of scientific interest in benzimidazole-based materials is evident in the last decade, such systems are indeed untapped potential in the field of optical chemical sensing.

In this chapter, we summarised the recent advances in optical solid state sensing systems and (nano)materials that incorporate the benzimidazole structural moiety. Immobilisation of benzimidazole derivatives in bulk membranes, polymers, sol-gel materials, as well as self-assembled (nano)materials for optical sensing are reviewed and discussed. Representative examples have been selected and commented in next sections, based on the type of applied material.

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*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

Polymers are the most commonly used support for optical chemical sensors [14]. They can be utilised to immobilise the sensing component, but can also directly participate in the sensing mechanism. In general, most commonly used polymers for analyte sensing are cellulose derivatives and hydrogels because of their excellent mechanical properties, stability at broad temperature and pH ranges, as well as high permeability towards water, ions and undissolved gases. In addition, polymers like polyurethane and pHEMA are biocompatible, a fact that enables new possibilities of their application.

The simplest form of the polymeric sensing material is the polymer membrane in which the chemosensor molecule is physically entrapped [11, 12]. Such dye-impregnated polymers are widely used in sensing chemistry, due to economic and simple methods of preparation. The choice of polymer depends on its permeability towards a specific analyte, its stability, availability and potential for immobilisation. Still, the development of such membranes is a challenging task because the polymer microenvironment has a strong effect on spectral characteristics of the immobilised sensing molecule, its acid-base equilibrium, selectivity towards the analyte and fluorescence lifetime. Ion-selective optodes are well-known examples of such optical sensing systems, where the bulk membranes are mostly formed from plasticised PVC [15]. An alternative for dye-impregnated polymers are the polymers with covalently attached fluorescent molecules. Stability of covalently bonded systems provides many advantages and can significantly improve analytical performance of chemical sensor. Another class of materials for optical sensing is the luminescent polymers. Design and synthesis of novel conjugated or coordination polymers is a constantly growing area of research, due to the enormous potential for the application of these

The polymer-based materials for optical sensing that incorporate benzimidazole unit are summarised in **Table 1**, while the representative examples have been

Polymer-based sensing materials incorporating physically entrapped benzimidazole-based receptors are very often used when relying on the electrochemical detection [16, 17]. However, examples utilising optical sensing techniques are not that common. For example, novel fluorescent sensors are developed by the immobilisation of benzimidazole-based ionophores in plasticised PVC, resulting in ionselective optode for mercury [18] and silver detection [19]. Presented ion-selective optodes are complex systems, where a number of parameters, including lipophilicity, polarity and microviscosity affect the heterogeneous ion-exchange equilibrium. The same sensing mechanism is presented for benzimidazole-based acrylonitrile derivatives [20] and Schiff bases [21], where novel colorimetric and fluorimetric sensing materials are applied for detecting the acidity changes. Moreover, immobilisation of this class of compounds into polymer matrices is demonstrated as a convenient way to overcome certain problems of organic fluorophores occurring in aqueous solution, such as hydrolysis of imino-bond or low quantum yields. For instance, a reversible spectroscopic response to pH is achieved because protonation of the immobilised benzimidazole Schiff bases occurs on the stable benzimidazole moiety (electron acceptor), while the imino bond of the Schiff base remains preserved [21]. At the same time, spectral properties of fluorescent sensing molecules are significantly altered due to the interactions between molecules in bulk, that is, in novel environment, where the molecular system becomes more rigid with partially disabled *cis-trans* isomerisation. Optical properties of developed materials

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

**2. Polymer-based sensing materials**

kinds of functional materials.

**2.1 Dye-impregnated polymers**

selected and discussed in next sections.

#### **2. Polymer-based sensing materials**

*Chemistry and Applications of Benzimidazole and its Derivatives*

tion of the charged ion by the resonance effect.

materials a very challenging task [11, 12].

most commonly applied for the generation and transfer of the analytical signal, while providing high sensitivity and selectivity. Therefore, fluorescent sensing molecules are the most promising candidates for the chemical sensing. Their design often starts from the heterocyclic molecular skeleton, due to its excellent spectral properties and the ability to detect diverse analytes. Heterocyclic chromophores and fluorophores are the most investigated classes of optical sensing molecules; hence, the interest for benzimidazole as a structural block of novel molecular systems is constantly increasing. Although benzimidazole derivatives are primarily known as biologically and therapeutically active substances [1], such heteroaromatic molecules have structure-related properties that enable a variety of applications in optoelectronics and non-linear optics (NLO) [2, 3], photovoltaics [4, 5], sensing [6] and bioimaging [7, 8]. Indeed, multifunctionality of the benzimidazole unit, such as electron accepting ability, *π*-bridging, chromogenic pH sensitivity/switching and metal-ion chelating properties, makes it an exceptional structural candidate for the design of optical chemical sensors [9, 10]. From the chemical point of view, the benzimidazole ring possesses a high degree of stability. Benzimidazole, for example, is not affected by concentrated sulphuric acid and is quite resistant to reduction. Oxidation cleaves its benzene ring, yet only under vigorous conditions. The two imidazole nitrogens are different from one another in their nature, which makes the properties of the ring system diverse in character. The hydrogen attached to the nitrogen can easily tautomerise to the other nitrogen atom. With the p*Ka* values 5.3 and 12.3, benzimidazoles are weakly basic, being somewhat less basic than the imidazoles and sufficiently acidic to make them usually more soluble in polar environments and less soluble in organic solvents. Benzimidazole, for example, is soluble in hot water but difficultly soluble in ether and insoluble in benzene, all of which can be modified upon the substitution. The acid/base properties of benzimidazoles are due to the stabilisa-

However, development of optical chemical sensors is much more complicated than designing a sensing molecule (recognition unit), since the process combines molecular recognition, material science and device implementation. Employing the sensing chemistry in a form of optical sensing material is perhaps the key step towards the ultimate goal, since its implementation can directly result in a functional sensor. Although there is a large number of fluorescent indicators and sensing molecules presented in literature, many of them lose their selectivity upon the implementation in functional devices, which makes the design of optical sensing

Recently, developments in optical sensing molecular systems that incorporate benzimidazole structural unit are reviewed and discussed [13]. As can be deduced from a given review, molecular sensors based on benzimidazole derivatives are mainly applied in solution, while materials for optical sensing are still rare, yet very promising. Development of novel (nano)materials and especially 'smart' molecular sensors, some of which include nanotubes, nanowires and nanoparticles, is the emerging trend observed in materials and optical sensing science. Although the growth of scientific interest in benzimidazole-based materials is evident in the last decade, such systems are indeed untapped potential in the field of optical chemical

In this chapter, we summarised the recent advances in optical solid state sensing systems and (nano)materials that incorporate the benzimidazole structural moiety. Immobilisation of benzimidazole derivatives in bulk membranes, polymers, sol-gel materials, as well as self-assembled (nano)materials for optical sensing are reviewed and discussed. Representative examples have been selected and com-

mented in next sections, based on the type of applied material.

**160**

sensing.

Polymers are the most commonly used support for optical chemical sensors [14]. They can be utilised to immobilise the sensing component, but can also directly participate in the sensing mechanism. In general, most commonly used polymers for analyte sensing are cellulose derivatives and hydrogels because of their excellent mechanical properties, stability at broad temperature and pH ranges, as well as high permeability towards water, ions and undissolved gases. In addition, polymers like polyurethane and pHEMA are biocompatible, a fact that enables new possibilities of their application.

The simplest form of the polymeric sensing material is the polymer membrane in which the chemosensor molecule is physically entrapped [11, 12]. Such dye-impregnated polymers are widely used in sensing chemistry, due to economic and simple methods of preparation. The choice of polymer depends on its permeability towards a specific analyte, its stability, availability and potential for immobilisation. Still, the development of such membranes is a challenging task because the polymer microenvironment has a strong effect on spectral characteristics of the immobilised sensing molecule, its acid-base equilibrium, selectivity towards the analyte and fluorescence lifetime. Ion-selective optodes are well-known examples of such optical sensing systems, where the bulk membranes are mostly formed from plasticised PVC [15].

An alternative for dye-impregnated polymers are the polymers with covalently attached fluorescent molecules. Stability of covalently bonded systems provides many advantages and can significantly improve analytical performance of chemical sensor. Another class of materials for optical sensing is the luminescent polymers. Design and synthesis of novel conjugated or coordination polymers is a constantly growing area of research, due to the enormous potential for the application of these kinds of functional materials.

The polymer-based materials for optical sensing that incorporate benzimidazole unit are summarised in **Table 1**, while the representative examples have been selected and discussed in next sections.

#### **2.1 Dye-impregnated polymers**

Polymer-based sensing materials incorporating physically entrapped benzimidazole-based receptors are very often used when relying on the electrochemical detection [16, 17]. However, examples utilising optical sensing techniques are not that common. For example, novel fluorescent sensors are developed by the immobilisation of benzimidazole-based ionophores in plasticised PVC, resulting in ionselective optode for mercury [18] and silver detection [19]. Presented ion-selective optodes are complex systems, where a number of parameters, including lipophilicity, polarity and microviscosity affect the heterogeneous ion-exchange equilibrium. The same sensing mechanism is presented for benzimidazole-based acrylonitrile derivatives [20] and Schiff bases [21], where novel colorimetric and fluorimetric sensing materials are applied for detecting the acidity changes. Moreover, immobilisation of this class of compounds into polymer matrices is demonstrated as a convenient way to overcome certain problems of organic fluorophores occurring in aqueous solution, such as hydrolysis of imino-bond or low quantum yields. For instance, a reversible spectroscopic response to pH is achieved because protonation of the immobilised benzimidazole Schiff bases occurs on the stable benzimidazole moiety (electron acceptor), while the imino bond of the Schiff base remains preserved [21]. At the same time, spectral properties of fluorescent sensing molecules are significantly altered due to the interactions between molecules in bulk, that is, in novel environment, where the molecular system becomes more rigid with partially disabled *cis-trans* isomerisation. Optical properties of developed materials

