Conducting Polymers and Metal-Organic Frameworks as Advanced Materials for Development of Nanosensors

Conducting Polymers and Metal-Organic Frameworks as Advanced Materials for Development of Nanosensors

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks as Advanced Materials for Development of Nanosensors MOONDEEP CHAUHAN • SANJEEV KUMAR BHAR...

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CHAPTER 3

Conducting Polymers and Metal-Organic Frameworks as Advanced Materials for Development of Nanosensors MOONDEEP CHAUHAN • SANJEEV KUMAR BHARDWAJ • GAURAV BHANJANA • RAJEEV KUMAR • NEERAJ DILBAGHI • SANDEEP KUMAR • GANGA RAM CHAUDHARY

INTRODUCTION TO CONDUCTING POLYMERS Nanotechnology has evolved as the utmost fascinating field and influenced numerous areas of material science, with the ability to deliver advanced structures having enhanced capabilities. At nanoscale dimensions, the physical, chemical, and biological properties of materials are profoundly different and much more exciting than their bulk counterparts. The introduction of novel properties in nanostructured materials makes them promising for many scientific and technologic applications, including sensors. In the recent years, the demand for improved physical, chemical, and biological recognition systems for use in environmental monitoring, healthcare sector, industrial safety control, etc. has immensely amplified. Nanotechnology-enabled sensors offer several benefits in terms of, for example, enhanced sensitivity and selectivity, limits of detection (LODs), and power requirement (Kalantar-zadeh and Fry, 2008). Conducting polymers (CPs) (also termed as intrinsic CPs or organic semiconductors) are an important class of functional materials. CPs have gained much attention since the discovery of polyacetylene (PA, in 1976), the conductivity of which was found greatly increased by several

orders of magnitude by simple halogen doping (I2 vapor). CPs display some significant advantage over both conventional organic polymers (in terms of strength, plasticity, flexibility, toughness, elasticity, etc.) and semiconductors (in terms of electric conductivity). Moreover, ease of synthesis, possibility of postsynthetic modifications, low density, and tunable optical and conducting properties have allowed CPs to emerge as commercially viable materials for the development of many types of sensors.

COMMON CLASSES OF CONDUCTING POLYMERS The backbone in CPs is made of highly p-conjugated sp2 hybridized chains, which are responsible for the phenomenon of charge delocalization in them. CPs generally exhibit low conductivity (w1010e105 S cm1) in their pure (pristine) state, but this property is effectively enhanced (w1e104 S cm1) by suitable doping of other species in them (Naveen et al., 2017). The term “doping” can be explained as a process that induces oxidation and reduction of a conjugated chain to generate positive (p-doping) or negative (n-doping) types of charge carriers. Subsequently, CPs become electrically conducting through the

Advances in Nanosensors for Biological and Environmental Analysis. https://doi.org/10.1016/B978-0-12-817456-2.00003-6 Copyright © 2019 Elsevier Inc. All rights reserved.

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Advances in Nanosensors for Biological and Environmental Analysis the monomers are oxidized electrochemically without the use of any oxidizing agent. Owing to their simplicity and reproducibility, electrochemical synthesis methods are comparatively straightforward and the structure and properties of the CPs can be controlled by manipulating the experimental parameters such as reaction potential, temperature, monomer concentration, and type of electrolyte. Electrochemical oxidation polymerization is useful to obtain high-quality thin films of CPs in various morphologic states (Li et al., 2012). Other than chemical and electrochemical methods, photopolymerization (or photoinitiation) is another technique employed for the synthesis of CPs, wherein the monomers are exposed to radiation such as ultraviolet (UV) light, visible light, or laser. The generated radicals or holes then drive the process of polymerization. Generally, this method can be categorized into direct and photosensitizer-mediated polymerization (Nguyen and Yoon, 2016). Direct photopolymerization involves the decomposition of the monomers into radicals by direct absorption of the energy radiation, whereas photosensitizermediated polymerization is based on the use of external photosensitizers (such as photocatalysts)

movement of charge carriers such as polarons and bipolarons or solitons along the conjugated polymer chain segments. Common classes of CPs that have been widely explored for research and industrial applications comprise PA, polyaniline (PANI), polythiophene (PTh), polyfluorene, polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT), etc., and their derivatives. The structures of some of the widely used CPs are depicted in Fig. 3.1. CPs are generally synthesized by chemical and electrochemical oxidation polymerization methods. The chemical oxidation processes involve the use of chemical oxidizing agents (such as ammonium persulfate, ferric chloride, copper sulfate) to polymerize monomer units. The charged monomers then couple to form polymeric chains. Mainly two types of synthesis mechanisms have been reported: (1) condensation polymerization, which proceeds via loss of small molecules, and (2) addition polymerization, in which the unsaturated monomers are added stepwise to the growing polymer chain. The chemical polymerization processes generally allow low-cost and large-scale synthesis of CPs. Such approaches have been used to synthesize many different types of CPs. In the electrochemical synthesis methods,

NH n n

Polyacetylene (PA)

Polyaniline (PANI)

S

O

O n

Poly (ethylenedioxythiophene) (PEDOT) S

n Polythiophene (PTh)

H N

n Polypyrrole (PPy)

FIG. 3.1 Structures of some important conducting polymers (Khatoon and Ahmad, 2017). (Reprinted with permission from Elsevier.)

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks that support the required transfer of energy from the radiation source to the monomer units.

SYNTHESIS OF NANOSTRUCTURED CONDUCTING POLYMERS A suitable control over the structure and morphology to nanometer dimensions could bring significant improvements in the properties and performance of CPs. For instance, owing to their high specific surface areas, the nanostructured CPebased sensors can permit enhanced interaction of analytes over their surface. As a result, such systems are characterized with improved response time and detection sensitivity (Li et al., 2009). Hence, many researchers have devoted their efforts to synthesize nanostructured CPs with controlled size and morphology. In general, the synthetic routes for nanostructured CPs can be broadly classified into two distinct approaches: template-free and external templatee based approaches.

Template-Assisted Synthetic Route Template-assisted synthetic route is a very commonly employed method for synthesizing CP nanostructures. Different types of templates ranging from insoluble hard templates (such as track-etched membranes) to soluble soft templates (such as surfactants) have been used. A posttreatment step is generally needed to remove the template and obtain pure CP nanostructures. Hard templates usually need harsh conditions for their removal. Relatively soft templates such as bulky dopant acids or surfactants are easier to be removed by simple washing with a suitable solvent. Hard templates Hard template method is a very general and conventional synthetic route for the production of monodispersed CP nanostructures inside the pores of a membrane with precise size (Martin et al., 1994). In the recent years, different types of porous hard templates such as anodic aluminum oxide (AAO), zeolites, ion-track-etched (polyester or polycarbonate) membranes, and glass membranes have been used for the synthesis

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of various nanostructures of CPs, including nanotubes, nanofibers, and nanowires. A typical synthesis approach mainly involves the inclusion of precursors into the pores of alumina or polycarbonate membrane and subsequently converting them into desired material (Fig. 3.2). For this purpose, AAO membranes can be prepared by electrochemical oxidation of aluminum metal. They consist of a dense array of cylindrical nanopores usually organized in a regular hexagonal symmetry. A wide range of pores with varied dimensions can be produced. Generally, the method involves the infiltration of monomers inside the pores of a membrane and then executing the process of polymerization (chemical or electrochemical). Note that the monomers can be loaded inside the pores of a membrane via different approaches, such as vapor phase deposition, negative pressure, liquid phase injection, or merely by immersion of the template into the monomer solution. Finally, the fabricated CP nanostructure can be separated from the template either by dissolving the template or by fabricating the nanostructures in an appropriate organic solvent. The AAO template method has been extended to synthesize nanotubes, nanorods, nanobelts, and thimblelike structure of another CP, i.e., PEDOT (Han and Foulger, 2005). Different morphologic structures of PEDOT were obtained by controlling the oxidant concentration and reaction temperature. For instance, tubelike structures with thin wall were formed at a low oxidant concentration. The wall thickness of those tubes increased (from 20 to 80 nm) by increasing the monomer concentration and temperature. At times, the required removal of template after the synthesis of CPs may face some challenges. The postsynthetic treatment may damage the formed nanostructures, thus leading to disorder or undesirable aggregation. In particular, use of harsh solvents such as highly concentrated NaOH, phosphoric acid, or organic solvents is not recommended when the final CP products are intended for biological applications. Furthermore, sometimes it is difficult to produce hard templates with uniform pore size distribution, which can complicate the scalability of the process.

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FIG. 3.2 Schematic illustration of a hard templateeassisted fabrication of conducting polymer (CP) nanostructures (Xia et al., 2010). (Adapted with permission from Elsevier.)

Some inorganic (such as metal oxide nanostructures) and biological (such as DNA) templates can also be used to fabricate CP nanostructures. A “nanofiber seeding” method has been used for the synthesis of PANI nanofibers (Zhang et al., 2004). In such approaches, the final morphologic structure of CPs depends largely on the dimension of the colloidal nanoparticles (NPs) used as the template. This approach is useful to produce nanostructured CPs in a relatively large quantity and with homogeneous size distribution. Similarly, synthesis of octahedral PANI micro/nanostructures has been reported, with the aid of cuprous oxide (Cu2O) crystals as template in the presence of phosphoric acid as dopant and ammonium persulfate (APS) as the oxidizing reagent (Zhang et al., 2005). Biological nanostructures, such as viruses, ferritin, DNA, with well-defined and uniform structures, also aid in the synthesis of nanostructured CPs with controlled morphologic structures and surface properties. Among the commonly known plant viruses, the tobacco mosaic virus (TMV) particle has a distinctive tubelike structure

in nanoscale dimension. The surface properties of TMV can withstand chemical or genetic modifications without disturbing the structure of the virus. Therefore the exterior surface of the TMV capsid has been proposed for the growth of nanostructured CPs. TMV has been used as a template in the fabrication of PANI and PPy nanowires (Niu et al., 2007).

Soft templates For the fabrication of CP nanostructures, the soft template method makes use of molecules that can be self-assembled into ordered molecular structures. The method involves self-assembly of monomers in the presence of suitable external agents. The external agent is usually required to be removed after the completion of reaction. The polymerization mechanism is driven by noncovalent interactions, such as hydrogen bonding, pep stacking, van der Waals forces, electrostatic interactions. Various soft templates such as surfactants, bulky dopant acids, micelles, aniline oligomers, soap bubbles, liquid crystals, and structure-directing molecules have been reported

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks for use in the synthesis of diverse categories of CP nanostructures. Like other template methods, postsynthetic treatment is usually needed to eliminate the surfactant so that pure CP nanostructures can be obtained (Fig. 3.3). Surfactants can also lead to the formation of reverse micelles, which have been widely used as nanoreactors to support the synthesis of various nanostructure materials. Various precursors can be inserted into the nanometer-sized aqueous domains restrained inside the reversed micelles acting as nanoreactors (Fig. 3.4). Sodium bis(2ethylhexyl)sulfosuccinate with hydrophobic tail groups (bulky) and hydrophilic head groups is one of the most frequently used anionic surfactants for the formation of reverse micelles. This surfactant has been used for the synthesis of PPy nanotubes. The reaction first involved a coordination interaction between an aqueous salt (FeCl3) solution and the surfactant in a nonpolar solvent (hexane) to form reverse micelles. Thereafter, pyrrole monomers were introduced into the reaction solution. Fe3þ ions present on the surface of

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the micelles (adsorbed to the head groups of the surfactant used) caused the chemical oxidation polymerization to yield PPy nanotubes. This polymerization reaction ended within a few hours. The obtained colored product was rinsed with excessive ethanol to eliminate the template (surfactant) (Jang and Yoon, 2003). Similar methods have also been reported for other CP nanostructures such as PEDOT nanorods and nanotubes.

Template-Free Synthetic Route In an accidental discovery, template-free formation of PANI microtubes was realized upon the addition of b-naphthalene sulfonic acid (b-NSA) as dopant during the oxidative polymerization reaction. Such template-free synthetic techniques omit the requirement of any insoluble template as well as postsynthetic template removal steps. The dopant or dopant/monomer self-assembled supermolecules behave as micelles like the soft template and allows the synthesis of nanostructured CPs. b-NSA has been widely used for

FIG. 3.3 Mechanism of the micelles acting as soft templates for the fabrication of conducting polymer (CP) nanostructures (Jang et al., 2004). (Reprinted with permission from RSC.)

FIG. 3.4 Mechanism of application of reverse micelles as soft templates for the synthesis of different conducting polymer (CP) nanostructures (Jang and yoon, 2003; Raghavan et al., 2017). (Adapted with permission from RSC and Elsevier.)

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fabrication of different anisotropic CPs nanostructures. Likewise, different sulfonic acids with varied molecular sizes (methanesulfonic acid, p-methylbenzenesulfonic acid, a-naphthalenesulfonic acid, 1,5-naphthalenedisulfonic acid, and 2,4dinitronaphol-7-sulfonate acid) can enable the synthesis of PANI microtubules in different structural shapes, such as granular and regular tubular. Note that PANI can be synthesized with almost unchanged conductivity irrespective of the doped type of sulfonic acids (Huang and Wan, 1999). The synthesis of nanofibers/nanotubes of CPs (such as PANI) has also been reported through the self-assembly process without the use of any protonic acid. For instance, the oxidation of aniline monomer by APS first results in the formation of reactive aniline cation radicals. Further addition of APS leads the radical to combine and form dimer cation radicals, which then aggregate into micelles of different morphologic structures, such as spheres, cylinders, and interconnected networks. Thus such micelles, in a way, act as a template for the fabrication of PANI nanostructures. Interfacial polymerization process has also been suggested to synthesize CP nanostructures. This process takes place in nonmiscible organic-water biphasic system. When the polymerization reaction is performed only at the interface of the immiscible solutions, the growth of polymers can be restricted in two-dimensional (2D) space. The template-free interfacial polymerization method is advantageous because pure CP nanostructures, such as ultrathin nanowires and nanofibers, with low dimensionality and narrow size distribution can be fabricated without the need for any further processing for template removal. The synthetic conditions are easily scalable, are reproducible, and can be accomplished with a wide range of dopants, solvents, monomer concentrations, and reaction temperatures. The interfacial process has been proposed to synthesize PANI nanofibers with uniform diameters (30e50 nm) (Huang et al., 2003). Apart from the above-discussed template-free methods, various other techniques such as

dip-pen nanolithography, electrospinning, mechanical stretching procedure, soft lithography or embossing, molecular combing method, whisker method have also been reported to produce CP micro/nanostructures.

SENSOR APPLICATIONS OF CONDUCTING POLYMER NANOSTRUCTURES The term “sensor” has been taken from the Latin word “sentire,” which means “to perceive.” A sensor is an analytical device that can respond to some stimuli, such as mechanical motion, heat, light, sound, by producing a functionally related output (electric signal). The signal is then measured by an observer or with an instrument. For practical applications, a sensor should be sensitive to the target analyte but insensitive to the distortions, noise, and other associated analytes. Various environmental influences, such as humidity, temperature, vibrations, can have negative impacts on the activity of the sensors. Therefore these influences must be addressed while designing any sensor. Depending on the application, a sensing device can have different requirements. However, irrespective of the associated application, sensors should provide low-cost, reliable, precise, and stable detection of the analytes (Kalantar-zadeh and Fry, 2008). Sensors can further be classified as physical sensors, chemical sensors, and biosensors. Physical sensors are generally used to analyze physical quantities such as distance, temperature, mass, pressure, whereas chemical sensors provide the detection and measurement of specific chemical substances. Biosensor is a subdivision of chemical sensors and it encompasses a biological recognition element (e.g., enzymes, antibodies, nucleic acids) linked to a transducer. Owing to the presence of specific recognition element, biosensors provide the desired selectivity while identifying the analyte of interest. On the basis of signal transduction, sensors can be categorized as electrochemical, optical, electric, mass, magnetic, and thermometric sensors. A large number of sensors

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks have been developed around electrochemical platforms owing to their remarkable sensitivity, simplicity of construction, and cost-effectiveness. Furthermore, electrochemical sensors can be divided into potentiometric, amperometric, and conductometric devices depending on the mode of measurement. The electronic conductivity of CPs is driven by their redox state (doping level), which can also be reversibly controlled by doping/dedoping. Thus CPs are good contenders for chemical and biological sensors, as their interactions with analytes may easily lead to measurable changes in parameters such as resistance, current, or potential. Subsequently, the analyte concentration can be correlated with the extent of change in these electronic parameters. Owing to their active and functional surfaces, CPs also support robust immobilization of receptors (e.g., biorecognition molecules) for developing selective electrodes for diverse categories of analytes. At the nanoscale, CPs possess distinctive and intriguing properties by virtue of their large surface areas. This effect can be understood from an example that showed that a PPy nanowiree based sensor for toxic gas and volatile organic compounds, fabricated with different-sized particles (20, 60, and 100 nm), would offer improved detection sensitivities with decreasing particle size (Kwon et al., 2010). Regardless of analytes, PPy NPs with the least diameter (20 nm) provided highest sensitivity. This behavior was attributed to the better conductivity of the smaller sized PPy NPs (20 nm [101 S cm1] > 60 nm [100 S cm1] > 100 nm [101 S cm1]). Likewise, a PEDOT nanorodebased sensor was reported to have better sensitivity toward NH3 and HCl vapors than conventional PEDOT films. Smaller PEDOT particles had a greater surface area and they also allowed rapid diffusion of the analyte into and out of the polymer (Jang et al., 2005). The sensitivity and selectivity of the nanostructured CPebased sensors can further be improved by complexing them with a second nanocomponent, such as carbon-based nanomaterials, metal NPs, metal oxide NPs, or biological materials.

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Conducting PolymereBased Electrochemical Chemosensors Electrochemical sensing is one of the foremost applications of CPs. This technique and the related devices have attracted the interest of the scientific community mostly because of some vital features, such as low cost, accuracy, and high sensitivity. Various CP nanostructures, including PANI, PPy, PTh, PEDOT, etc. and their derivatives, have been widely reported for the construction of electrochemical sensors for a variety of analytes. PANI is one of the most widely explored CPs owing to its low cost, ease of synthesis, and relatively high chemical stability and sensitivity. In an interesting example, the fabrication of nanostructured PANI mesh has been reported by employing a copper mesh as a hard template (Cai et al., 2018). This high-surface-area CP was then used for detection of ammonia, with high signal-to-noise ratio and good sensitivity features. The interfacial polymerization method has also been reported for the synthesis of PANI nanofiber sensors and their subsequent application as a sensing material for the detection of various analytes (Virji et al., 2004). Owing to their large surface area, porosity, and smaller diameters, PANI nanofibers allow enhanced diffusion of molecules and dopants into them, thus facilitating the development of improved sensors in comparison to conventional thin films. PPy exhibits high electric conductivity along with good chemical stability against environmental conditions. The presence of holes is the major reason behind the electric conductivity of PPy CPs. The surface charge of PPy can be altered by varying the dopant materials during its synthesis. PPy thin films (such as those developed on glass substrates by spin coating) have been demonstrated to have selective and reproducible gassensing properties. For instance, their interaction with an oxidizing gas (e.g., NO2) may result in an increase in the hole density. As a result, enhancement in the film conductivity can be taken as a measure of the gas concentration (Navale et al., 2014). CPs can also combine with many other materials to form composites that can then be used

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to develop even more sensitive sensor platforms. The CP-based composites possess the properties of individual components and may attain new properties with synergistic effects (El Rhazi et al., 2018).

Conducting polymeremetal nanoparticle composites Many works have been reported to fabricate new nanocomposites based on CPs and metal NPs for use in sensing applications. Particularly, metal NPs such as Au, Pt, Pd, Ag, Cu, Ni, and Bi have been integrated into CPs. The use of gold nanoparticles (AuNPs) exhibited improved conductivity, excellent electrocatalytic ability, ease of self-assembly, and biocompatibility. Prepared by a direct reduction of AuNPs over the PANI surface, the PANI/AuNP nanocomposite has been used for the detection of H2O2 (Hung et al., 2010). As a template and dopant, AuNPs have also offered the fabrication of highly stable, sensitive, and selective three-dimensional (3D) porous PEDOT, which was then used for the development of a sensor for nitrites (Lin et al., 2016). Conducting polymerecarbon nanocomposites Carbon nanomaterials such as graphene oxide (GO), carbon nanofibers, and carbon nanotubes (CNTs) play a very promising role in several interesting applications owing to their immense structural and functional functions, such as high aspect ratio, mechanical strength, and electric properties. GO, containing carboxyl group, can behave as a good doping agent for the chemical as well as electrochemical polymerization of CPs. The composites of GO with PPy and PEDOT have been reported for use in sensing applications, such as the detection of dopamine (DA) (Zhuang et al., 2011) and quercetin (Sun et al., 2013), with improved detection limits. A composite of poly((2,5-dithienyl-3,4-(1,8-naphthylene) cyclopentadienone)-co-4,7-bis(3-hexylthiophen2-yl)benzo[c][1,2,5]thiadiazole (poly(DTCPAco-BHTBT)) and carbon black has been developed for chemiresistive detection of several volatile organic compounds, i.e., acetone, toluene, carbon tetrachloride, and cylcohexane

(Mallya et al., 2014). Likewise, the graphenePANI composite film was found efficient for the voltammetric detection of 4-aminophenol (Fan et al., 2011). The combination of PANI and graphene was useful providing an effective microenvironment for the electrochemical reaction, which significantly enhanced the voltammetric response. This sensor provided low detection limit, excellent sensitivity, and long-term stability to highlight the practical significance of the CP-graphene compositeebased electrochemical sensors. Owing to their high electric conductivity, at times, the CP-graphene compositeebased electrochemical electrodes may encounter the problem of large background currents. This issue can lead to a decrease in the sensor sensitivity. However, this problem can be addressed by adopting appropriate conditions during the synthesis of CP-graphene composites. For instance, some researchers have reported the electrochemical coating of porous PPy(oxidized)-graphene composite on a glassy carbon electrode (GCE). This modified electrode was then used for the quantitative detection of adenine and guanine (Gao et al., 2014). The use of overoxidized PPy helped in facilitating the design of a porous electrode. Consequently, better electron transfer rates could result in an excellent electrocatalytic capability of the sensor toward the oxidation of adenine and guanine. Similar to the use of CP-graphene composites, CP-CNT composites have been proposed for the wide range of electrochemical sensing applications. A PPy(oxidized)-SWCNT composite (where SWCNT stands for single-walled CNT) has been reported for developing efficient and selective electrochemical sensors for nitrite, ascorbic acid, DA, and uric acid (Li et al., 2007). PANI nanofiber-SWCNTebased composites could also provide rapid (2 min) and sensitive detection of hydrochloride and ammonia vapors. The role of the addition of SWCNT in this sensor can be understood from the fact that a similar sensor based on PANI alone would take more than 15 min to provide the same response. The mixing of an appropriate amount of SWCNT could help in obtaining a composite material with enhanced and tunable conductivity (e.g., ranging from 104

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks to 102 S cm1). Moreover, such a composite also possesses a large number of redox sites at the junction, leading to an extended conjugated network (Liao et al., 2011).

Conducting polymeremetal oxide composites Composites containing CPs with metal oxides (Fe2O3, ZnO, CeO2, In2O3, WO3 SnO2, Bi2O3, TiO2, ZrO2, NiO, MoO3, etc.) have been produced for enhancing their overall sensing capabilities. For instance, Ram et al. (2005) fabricated the CP/metal oxide (i.e., CP/SnO2 and CP/TiO2) ultrathin films for sensing CO gas. Sadek et al. (2006) have prepared the PANI nanofiber/In2O3 NP composite by chemical polymerization of PANI nanofibers with In2O3. A fast response and rapid recovery times with better repeatability have been achieved with the abovementioned fabricated nanocomposite for sensing H2, NO2, and CO gases at room temperature. Ahmad et al. (2018) synthesized a PANIgraphene/nickel oxide nanocomposite by depositing nickel oxide (NiO) NPs on graphene sheet, followed by polymerization of aniline monomers on the graphene/nickel oxide surface. Incorporation of graphene/nickel oxide in PANI was found to enhance the amplitude of conductivity change, which was reported to be 99 times more than that of pure PANI. NiO NPs were observed to act as a bridge between PANI and graphene through which charge carriers can move from PANI to graphene and thereby enhancing the electric conductivity. The synthesized nanocomposite showed fast response with good recovery time, which were attributed to the large surface area of the material with accessible active sites. Moreover, the PANI-graphene/NiO composite displayed good selectivity toward NH3. Conducting polymeremetal-organic framework composites Metal-organic frameworks (MOFs), built from the coordination of an organic ligand and a metallic species, have emerged as materials of choice for various applications, including sensing of important biological and environmental parameters. MOFs are characterized by large surface area and

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accessible and tunable porosity. Therefore their combination with CPs has been suggested to deliver novel composites that can be explored for the development of electrochemical sensors. For example, PANI has been introduced in a UiO-66-NH2 MOF matrix and the resulting UiO66-NH2-PANI composite was used for the differential pulse stripping voltammetryeassisted high-performance, sensitive, broad, and reproducible detection of cadmium ions in aqueous solutions (Wang et al., 2017). The presence of PANI in the composite provided electric conductivity to the composite, while the MOF played a significant role in the efficient and rapid diffusion of the analyte.

Conducting PolymereBased Electrochemical Biosensors Clark and Lyons, in 1962, demonstrated the first example of electrochemical biosensors by integrating an enzyme to an amperometric oxygen electrode. This biosensor was used for the direct detection of glucose. With continuous development, biosensors have now become valuable devices for rapid and portable analysis in diverse sectors such as medical diagnosis, food industry, healthcare etc. CPs have also contributed a lot to the development of various important and commercially available electrochemical biosensors. They have been modified with different categories of specific biorecognition molecules, such as oligonucleotides, aptamers, antibodies, and enzymes, via covalent attachment, co-entrapment, physical adsorption, cross-linking, and affinity interactions. Each technique developed for interfacing the CPs with biomolecules has its own merits, demerits, and applications. In general, immobilization processes should be simple, accurate, and efficient and should not damage the activity of the recognition probe. For instance, physical adsorption is a very simple, facile, and high-yielding method; however, owing to the involvement of comparatively weaker forces, the attached bioreceptor can get desorbed and can be leached out from the support matrix over time (Ekanayake et al., 2007). The covalent bindingebased immobilization of biomolecules over CPs provides effective and more robust

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biointerfacing but it requires the material surface to possess a suitable reactive group (functional group, e.g., eNH2, eCOOH, eSH). Co-entrapment is another common way of immobilization in which biomolecules are mixed with the monomer units before starting the synthesis of the CPs. As enzymes possess a negative charge at high pH conditions, they can be linked to the cationic backbone of the CP films via the entrapment methods. Another strategy for the interfacing of CPs with biomolecules involves the use of reagents, e.g., glutaraldehyde, that can function as a cross-linker. In biosensors, the recognition signals are transformed into electric signals. The capture of the analyte molecule by the CP-biomolecule interface causes proportional deviations in the electric (or optoelectronic) properties of the surface. The electric behavior of the CP biosensor is then studied by recording its cyclic voltammogram, amperometric signal, or surface impedance (electrochemical impedance spectroscopy [EIS]). The oxidation/reduction potential of the CPs is a critical property to define the final sensitivity of the sensor. For instance, at lower potentials, some polymers (such as PPy) are highly electroactive and EIS is a more suitable technique to monitor the sensor-analyte interactions. The application of CPs in the development of electrochemical biosensors has allowed for the highly sensitive detection of various analytes, such as DA, glucose, pesticides, heavy metals, pesticides.

METAL-ORGANIC FRAMEWORKS MOFs, also referred to as porous coordination networks or porous coordination polymers, are the hybrid porous crystalline materials formed by the coordination bonding between inorganic metal ions and organic linkers. The desired geometries of MOFs can be controlled by the deliberate combinations of metal and an organic linker. For instance, the pore size of MOFs can be tuned using suitable multidentate ligands of varying lengths. Judicious selection of the coordination geometry of metal center(s) can contribute toward the

formation of one-dimensional, 2D, or 3D networks with repeating coordination entities (Fig. 3.5) (Dybtsev et al., 2017).

Synthesis and Properties of Metal-Organic Frameworks MOFs can be synthesized by different techniques, namely, solvothermal, microwave, layer-by-layer, ionothermal, sonochemical, diffusion, mechanochemical, ultrasonic, electrochemical, templateassisted, spray-drying, and high-throughput techniques. The solvothermal and hydrothermal routes are the most widely applicable approaches wherein the precursors, i.e., metal ions and organic linkers, are dissolved in a suitable solvent and then reacted at high temperatures. Various popular MOFs such as HKUST-1, MIL-100, MIL53, MOF-74, MIL-101, MOF-5, and UiO-66 have been synthesized using the solvothermal and hydrothermal methods. The microwave-assisted synthesis is used for growing MOFs with high efficiency and relatively smaller particle sizes. This method involves the use of microwave radiation for the generation of energy. The examples of MOFs synthesized with microwave radiation include nanosized Cr-MIL-101, MIL-140, and UiO-66. The electrochemical synthesis procedure has been reported for the growth of MOF layers of HKUST-1, MIL-100, Tb-BTC, and UiO-66. However, electrochemical synthesis of MOFs can only be achieved in electrode-specific conditions. MOFs with various topologies and functionalities have been synthesized by the careful selection of inorganic metal moiety and organic struts. The organic units often termed as linkers or ligands can include carboxylates, sulfonates, phosphonates, and even heterocyclic compounds such as cyclodextrin. The linkers can further be classified as ditopic, tritopic, tetratopic, or multitopic, depending on the presence of reactive end groups. Some commonly reported MOFs have been synthesized with the use of linkers such as benzene1,4-dicarboxylic acid (BDC or terephthalic acid), benzene-1,3,5-tricarboxylic acid (BTC or trimesic acid), and biphenyl-4,40 -dicarboxylic acid. The metal ions or clusters (e.g., Cu2þ, Zn2þ, Ln3þ, Al3þ, and Cd2þ) form the inorganic units of the

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FIG. 3.5 Assembly of typical metal-organic frameworks (zero, one, two, and three dimensional) (Gangu et al., 2016). (Reprinted with permission from Elsevier.)

MOFs and their coordination environment defines the geometry of the secondary building unit (SBU). Various types of SBU geometries with a different number of points of expansion, such as octahedron (six points), square paddle wheel (four points), triangle (three points), and trigonal prism (six points), can be formed, as exemplified in Fig. 3.6 (Dybtsev et al., 2017). MOFs are known to possess unique and excellent physical and chemical properties. These extraordinary properties include high crystallinity, high porosity, high surface to volume area (e.g., up to 8000 m2/g1), low density (e.g., 0.4 g/cm3), functionality of the internal surface, good thermal/mechanical stability, stable luminescence, tunable bandgap, tunable pore sizes (e.g., up to 5 nm), and achievable electronic properties. These unique properties are often accounted for by their molecular-scale porous characters. The availability of a wide range of combinations of metal ions and organic ligands is further useful to synthesize applicationspecific MOFs (Furukawa et al., 2013).

FIG. 3.6 Different types of secondary building units in metal-organic frameworks (Dybtsev et al., 2017). (Reprinted with permission from Elsevier.)

Applications of Metal-Organic Frameworks in the Development of Nanosensors for Environmental and Biological Analysis Owing to their unique properties, MOFs have been employed in a variety of applications, such as separation of gases and other small analytes, adsorptive storage of gases (H2, CO2, CH4, and O2), catalysis, photovoltaic devices, sensing of small molecules, biomedical imaging, drug delivery, and encapsulation of molecules. In the recent years, MOFs have been recognized as powerful

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materials for the development of many types of chemical and biochemical sensors. They can be used both as an individual sensory material and in combination with specific recognition molecules. Their sensing applications have been reported for a diversity of compounds ranging from small molecules to aromatic compounds, volatile gases, vapors, and biomolecules. The sensitivity offered by MOF-based detection relies heavily on the signal transduction mechanism employed during sensing. Nonetheless, most of the MOF-based sensing methods show high loading capacities of analytes because of the availability of large volume-to-surface ratio and favorable dynamics of analyte transport inside the MOF pores. Owing to their many advantageous features, such as high sensitivity, reusability, and signal stability, the MOF-based sensors are considered as the next-generation detection devices. The poor aqueous solubility and stability of many of the MOFs was a bottleneck in the early development of sensors; however, this issue has been sorted to a great deal. Various examples of water-stable, dispersible, and nanoscale MOFs have been proposed to allow their successful deployment in the realization of useful sensing systems (Xu et al., 2011). Luminescent MOFs (or LMOFs) have attracted most of the attention in the field of MOF-based sensors. Such optical sensors have enabled the sensing of many analytes with very low detection limits. LMOFs can even be used in their powdered form and thus may avoid the need for any film fabrication as required in some other techniques. For most of the reported cases, LMOFs were used without the need for postsynthetic treatments. Hence, they could retain their original sorptive properties while also retaining molecular specificities. Traditionally, the application of MOFs in electrochemical sensors was restricted because of their insulating nature. In the recent years, researchers have found ways of introducing conductivity in MOFs, which has allowed their applications in the development of electrochemical sensors. The introduction of conductivity in MOFs has been achieved via different approaches such as by using modified ligands, by doping MOFs with NPs, or

by mixing MOFs with other conducting elements. The electroactive MOFs have been suggested as potent alternatives to other conducting nanomaterials in the area of chemical and biochemical sensors. Apart from the sensors based on optical and electrochemical signals, MOFs have been reported in other formats also, which include quartz crystal microbalance (QCM), microelectromechanical systems, surface acoustic wave sensors, surface plasmon resonance, and microcantilevers (MCLs). An effective sensory material needs to fulfill certain key characteristics. For example, it should interact with the target analyte preferably with specificity in the presence of interferents and the response should ideally be reversible. MOFs are considered very useful for sensing applications because of their high surface area, pore size distribution, and high chemical/thermal stability. Depending on the constituents, the properties of MOFs can show significant changes upon their interaction with analytes. Thus the measurements of changes in mass, luminescence, conductance, surface area, etc. allow the use of MOFs as sensory materials. Over the past decade, several categories of MOFs have been demonstrated for the sensing of analytes such as small molecules, gases, metal ions, anions. The MOFderived sensors can be majorly classified into three different types based on the mode of signal transduction: optical, mechanochemical, and electrochemical sensors.

Metal-organic frameworkebased optical sensors These types of sensors function by measuring the optical properties of a material upon its intended binding with the analyte. The optical sensors provide high sensitivity and rapid results in real-time conditions without extensive sample preparation. The optical emission properties of LMOFs have been investigated for a number of sensing applications. The origin of luminescence in MOFs can be associated with several types of phenomena, such as linker-based luminescence (ligand-localized emission), ligand-to-metal charge transfer, metal-to-ligand charge transfer, metal-based emission, antenna effects, adsorbate-based emission,

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks sensitization, excimer/exciplex emission, and surface functionalization. The use of conjugated organic linkers is very popular to design the LMOFs. These linkers can absorb light in the visible or UV regions to provide a linker-based fluorescence. However, this kind of material fluorescence can be lost when transitionmetal ions are used for the synthesis of MOFs, as they show quenching capabilities. The use of orbital metal ions is preferred to synthesize LMOFs having an origin of luminescence from the metal centers. The lanthanoid ionebased luminescence is dependent on the transitions that are forbidden by electric dipole selection rules. Eu3þ and Tb3þ are the most commonly used lumophores providing strong red and green visible luminescence, respectively. When lanthanoid ions are used in combination with light-absorbing organic ligands, it leads to even stronger luminescence due to the antenna effect wherein a direct energy transfer takes place from the more readily available excited state of the linker to the linked metal center. The photoluminescence (PL) properties of LMOFs have been explored for the sensing applications. The quenching of the LMOF’s optical intensity upon host-guest interaction is one of the most common approaches for signal transduction. The reason behind the occurrence of quenching is mainly overlapping between the electron donor and acceptor. The process of interaction of MOFs with analytes may also proceed via the change in the materials PL intensity as a result of a change in the redox potentials of the metal centers. The measurement of the PL intensity at a fixed wavelength can be used to prepare calibration curves between the signals and the analyte concentration. The performance of LMOF-based optical sensors is dependent on the type of analyte and the medium used for the sensing experiments. The LMOF-based optical sensors offer advantages such as the production of a measurable signal that is visible to the naked eyes, low detection limits (up to single molecule level), and the ability to employ powdered materials directly without the need for film fabrication (or

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other treatment). However, some limitations of LMOF-based sensors have also been experienced, for example, poor water stability, counterproductive porosity, and permanent quenching. Sensing of cations and anions. Several works have been reported in the literature regarding the sensing of metal ions using lanthanide and transition-metalebased LMOFs. For example, a Eu-based LMOF, [EuL(H2O)4] $ 2H2O (L ¼ 1, 4,8,11-tetraazacyclotetradecane-1,4,8,11-tetrapropionic acid) has been reported for the sensing of several cations, such as Cu2þ, Agþ, Zn2þ, Cd2þ, and Hg2þ (Wang et al., 2014). The interaction of metallic ions with the empty coordination sites of azacycle organic ligand caused a decrease in the fluorescence intensity of the LMOF. Interestingly, when Agþ ions were added to the MOF solution, the fluorescence spectrum of MOF changed considerably from multiple peaks to a single peak. Such a remarkable change in the fluorescence spectrum can be attributed to the increased rigidity and modified paramagnetic spin state of the MOF. In another report, a UiO66-based highly sensitive colorimetric sensor has been developed for simultaneous detection of Bi3þ, Pb2þ, Zn2þ, Hg2þ, and Cd2þ ions. The sensor aided in the rapid and naked-eye detection of metal ions up to 1010 M (Lei et al., 2014). The use of hetero-MOFs, based on lanthanide and manganese ions (e.g., [Eu(PDA)3Mn1.5 (H2O)3]3 $ 5H2O and [Tb(PDA)3 Mn1.5 (H2O)3]3 $ 5H2O (where PDA is pyridine-2,6-dicarboxylic acid)) has been reported for the sensing of Zn2þ (Zhao et al., 2016). The fluorescence intensity of these MOFs increased considerably in the presence of Zn2þ while no change was observed in the presence of other metallic ions (e.g., Mn2þ, Ca2þ, Mg2þ, Fe2þ, Co2þ, and Ni2þ). Similarly, another group of researchers reported the use of [Eu(PDA)3Fe1.5(H2O)3] $ 1.5H2O for the detection of Mg2þ ions (Fig. 3.7) (Zhao et al., 2009). A biligand containing a lanthanide-based MOF, Eu2(FMA)2(OX) (H2O)4 3 $ 4H2O (where FMA is fumarate and OX is oxalate), has been

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FIG. 3.7 Selectivity feature of a luminescent [Eu(PDA)3Fe1.5(H2O)3].1.5H2O metal-organic framework (MOF) (where PDA is pyridine-2,6-dicarboxylic acid) toward the detection of Mg2þ ions. Emission intensity of MOF (at 613 nm) upon the addition of different metal ions (black, control; red, 1 equiv; green, 2 equiv; blue, 3 equiv) (Zhao et al., 2009). (Reprinted with permission from RSC.)

reported for the quenching-based highly sensitive and selective detection of Cu2þ (Cui et al., 2013). Sensing of nitroaromatics. LMOFs have been popularly used for the PL quenchingebased detection of several nitroaromatic compounds. Highly selective detection of three nitroaromatic compounds, i.e., 2,4,6-trinitrotoluene (TNT), RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and 2,4,-dinitrophenol, was proposed using a Cd-based LMOF, [Cd(NDC)0.5(PCA]$Gx (where NDC is 2,6-naphthalenedicarboxylic acid, PCA is 4-pyridinecarboxylic acid, and G is guest molecule). The nanocomposites of Eu-MOF and AuNPs and quantum dots have been reported for the highly sensitive TNT sensing (Kaur et al., 2014). Sensing of solvent molecules. The guestdependent luminescence properties of LMOFs have been exploited for the sensing of a number of small solvent molecules, such as acetone, dimethylformamide (DMF), toluene,

nitrobenzene. A 2D Cu-based MOF, Cu6L6 $ 3(H2O)$(DMSO) (where DMSO is dimethyl sulfoxide), has been demonstrated for selective detection of different aromatic compounds (i.e., toluene, nitrobenzene, aniline, and o-, p-, and m-dimethylbenzene) based on the fluorescence quenching mechanism. Lanthanide MOFs, such as Eu(BTC) (H2O)3 $ 1.5H2O, Eu2 (m2-pzdc) (m4-pzdc)(m2-ox)(H2O)4] $ 8H2O, Tb(BTC)$ (DMF)$(H2O), and Yb(BPT)(H2O)3(DMF)1.5 (H2O)1.25 (where BPT is biphenyl-3,40 ,5tricarboxylate), have also been used for the recognition of various solvents, as their luminescence intensity varies largely with the type of solvent molecule (Hu et al., 2014). Ytterbiumbased MOFs, namely, [NH3(CH2)2NH3]0.5 [Yb(OBA)2(H2O)] (where OBA is 4,40 oxybis(benzoate)), (NH4)[Yb(OBA)2(H2O)2], Na[Yb(OBA)2]$0.4DMF $ 1.5H2O, and Na [Tb(OBA)2]$0.4DMF $ 1.5H2O, have been used to develop sensing assays for various solvent molecules (Lin et al., 2010). Fluorescence quenching was caused by the energy transfer

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks between the solvent and the ligand molecules; it decreased in the following order: methanol > H2O > butanol > ethanol. Sensing of biomolecules. The LMOFs have been proposed for the detection of several bioanalytes. For instance, the Zn- and Cd-based MOFs (Zn-BDC and [Cd(atc) (H2O)2]n) (where atc is 2-aminoterephthalic acid) showed PL quenching when they were used for the detection of bovine serum albumin (Kumar et al., 2014). An “OFF-ON”-type sensor was reported for the detection of DA using the AbtzCdI2-MOF (where Abtz is 1-(4-aminobenzyl)1,2,4-triazole) (Cheng et al., 2017). The fluorescence of the Abtz-CdI2-MOF was found quenched in the presence of MnO 4 ions while the addition of DA caused the release of MnO 4 and restored the original fluorescence. An LOD of 57 nM was achieved in the linear range of 0.25e50 mM. An iron-based MOF of the MIL series (MIL-53) has been used as a biomimetic catalyst to develop a “turn-off” colorimetric assay for the quantitative detection of ascorbic acid. This was done by measuring the decrease in absorption of MOF at 450 nm upon the addition of the varying concentrations of the analyte (Guo et al., 2016). MOFs have also been utilized for the optical detection of nucleic acids and proteins. For example, a copper-based MOF, Cu(H2dtoa) (where dtoa is dithiooxamide anion), has been reported for the detection of HIV-1 U5 sequence and thrombin molecules. The quantifiable concentrations of the target DNA molecules were in the range of 10e100 nM for HIV-1 U5 DNA and 5e100 nM for thrombin, with detection limits of 3.0 and 1.3 nM, respectively (Zhu et al., 2013). The lanthanide-based LMOFs have been proposed for the detection of a bacterial endospore biomarker, dipicolinic acid (DPA). For example, a Tb-based MOF has shown excellent selective and highly sensitive binding with the DPA molecules (Bhardwaj et al., 2016). The LMOFs have also been conjugated with bacteriophages for the detection of whole bacterial cells,

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such as Escherichia coli and Staphylococcus aureus. These analyses were based on the measurements of change in the PL intensity of MOFs in the presence of varying bacterial concentrations (Bhardwaj et al., 2017) (Fig. 3.8).

Metal-organic frameworkebased mechanical sensors Mechanical sensors can convert the mass or molecular adsorption changes into vibration frequency or mechanical energy signals. QCM, surface acoustic wave sensors, and MCLs are the examples of such sensors. These types of sensors provide high detection sensitivities (even up to femtogram levels in MCLs); however, a special coating (e.g., organic polymers) is required in many cases. Quartz crystal microbalance sensors. QCM is a piezoelectric biosensor in which the change in resonance frequency due to the increase in mass upon analyte binding on the sensor surface is recorded. The QCM-based sensors detect analytes by measuring small changes in the frequency of a resonant vibration propagating perpendicular to the surface of a quartz crystal. When compared with other mass-based detection methods, the QCM sensors show relatively low sensitivity (e.g., 1 ng detection limit). Nonetheless, they are straightforward to use and provide an effective means of recording adsorption isotherms and surface kinetics, as well as sensing analytes. The MOFs can be deposited on QCMs as thin films with relative ease because of the presence of hydroxylated surface reactive sites. Selfassembled layers of MOFs coated on the QCM electrodes have been reported for the water sorption studies. A high thermal stability of MOFs allows them to be baked at high temperatures for the removal of adsorbed water and the subsequent sensor regeneration. On QCM platforms, the use of MOFs with high surface area and large pore volume (e.g., Cu3(BTC)2) has attracted application as a humidity sensor. Detection of water vapors was demonstrated using the electrochemically synthesized films of

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FIG. 3.8 A schematic of the application of fluorescent metal-organic frameworks (MOFs) in the

detection of Staphylococcus aureus (Bhardwaj et al., 2017). PL, photoluminescence. (Reprinted with permission from ACS.)

Cu3(BTC)2 over the QCM surface. This sensor showed a water sorption capacity of about 25e30 wt% (Kreno et al., 2012).

cantilever/MOF interface, with minute changes in unit cell dimensions (Kitagawa and Matsuda, 2007).

Microcantilever-based biosensors. In the MCL-based sensor technology, the presence of the analyte(s) is detected in two modes: (1) by measuring the change in the cantilever oscillation frequency after the uptake of analyte mass (dynamic mode) and (2) by measuring the strain-induced bending (static deflection mode). The structural flexibility of MOFs allows greater sensor sensitivity in static mode because of the large tensile/compressive stresses obtained at the

Metal-organic frameworkebased electrochemical sensors Owing to their large surface area and tunable pore environments, MOFs bear many characteristics that should attract their use in the development of electrochemical sensors. However, very few electrically conducting MOFs have been reported in the literature till date. This is due to the following reasons: (1) metal centers typically used in the synthesis of MOFs turn into redox-

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks inactive forms and (2) organic linkers (e.g., carboxylates) also do not facilitate required levels of electron transfer between them and the metal centers. However, some approaches have been established in the recent past to impart conductivity in MOFs. These methods include the use of redox-active linkers, preference of second-/thirdrow transition metals or heterobimetallic structures, doping of the synthesized MOFs with guest molecules, and hybridization with other conducting species. Conducting MOFs thus obtained have attracted applications in electronic devices for advanced applications in sensing, electrocatalysis, photonics, and microelectronics. The MOF-based electrochemical sensors can be classified into three main types based on the parameter used for signal measurement, i.e., potentiometric, amperometric, and impedimetric. These sensors and their applications are briefly discussed in the following sections. Potentiometric sensors. Potentiometric sensors are based on the measurement of change in potential experienced at the working electrode surface due to the binding of the receptor with the specific analyte. There have been limited efforts to use MOFs as potentiometric sensors. In one of the examples, a UiO-66-NH2-based biosensor has been reported for the detection of alkyl phosphonate nerve agent stimulant (dimethyl methylphosphonate) to an analyte concentration down to 3 ppb (Stassen et al., 2016). In another report, ZIF-70 was used to design an electrocatalytic biosensor for the in vivo measurement of a neurochemical (dialysate glucose) in the brain of guinea pigs. The electrocatalyst (methylene green) and enzyme (glucose dehydrogenase) were co-immobilized over the ZIF-70 matrixecoated electrode surface. This above sensor showed high sensitivity within a linear range of 0.1e2 mM (Lian et al., 2017). The nanocomposites of multiwalled CNTs and MOF (MWCNTs-Cu3(BTC)2) have been reported for the electrochemical detection of trace levels of lead using differential pulse anodic stripping voltammetry. The sensor showed very

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low detection limit, i.e., 0.79 nM, within a concentration range of 1.0e50 nM (Wang et al., 2013). Amperometric biosensors. Amperometric sensors work on the principle of measurement of current at the surface of the working electrode with respect to the reference electrode after a certain potential is applied. The interaction of the analyte with the receptor molecule leads to either oxidation or reduction of the analyte species, thereby leading to the generation of current. MOF-based amperometric sensors have been reported for various analytes. In one such report, an amperometric sensor was developed using the thin films of a zirconium-based porphyrin MOF (MOF-525) grown on conducting glass substrates. This MOF-525-based sensor was used for the cyclic voltammetryebased detection of nitrites. The developed sensor showed a detection limit of 2.1 mM with a linear concentration range of 20e800 mM nitrites (Kung et al., 2015). Similarly, a Cu-MOFmodified carbon paste electrode (CPE) was investigated for the electrooxidation of nitrites. It was observed that when Au microspheres were coated over the MOF surface to form Au/CuMOF/CPE, the oxidation potential of nitrite decreased further because of the accelerated electron transfer rate (Yuan et al., 2016). A Co-MOF (Co(pbda)(4,4-bpy)$2H2O]nCo-MOF)modified GCE (where pbda is 3-(pyridine3-yloxy)benzene-1,2-dicarboxylic acid and bpy is bipyridine) has been employed for the amperometric detection of hydrogen peroxide. The intrinsic peroxidase activity of this MOF caused the electrocatalytic reduction of H2O2, which then formed the basis of the detection. The sensor showed a low detection limit (3.76 mM) and high sensitivity (83.10 mA/ mM1 cm2) within a linear range of 5 mMe9.0 mM (Yang et al., 2015). In one of the MOF-based conductometric sensor design, Cu3(BTC)2 thin films were first grown over a PANI surface, followed by the immobilization of antiatrazine antibodies

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FIG. 3.9 Schematic of the metal-organic framework (MOF)-based four-probe conductometric sensor for the detection of atrazine (Bhardwaj et al., 2015). BTC, benzene-1,3,5-tricarboxylic acid; BDC-PANI, benzene-1,4-dicarboxylic acid-polyaniline. (Reprinted with permission from ACS.)

(Bhardwaj et al., 2015) (Fig. 3.9). The original impedance of the impedimetric biosensor was found to increase proportionally with respect to the increasing concentration of the pesticide. Thus the sensor was able to detect atrazine over a wide analyte concentration range (0.01 nMe1 mM) with a low detection limit (0.01 nM). The sensor was also tested successfully for the analysis of real water samples, spiked with atrazine. Impedimetric sensors. In impedimetric sensors, the receptor-analyte interaction is associated with the change in impedance (Z) across the surface of the working electrode (Achmann et al., 2009). Some researchers have employed MOFs for the development of impedimetric sensors. The high sorption behavior of MOFs has enabled their usage for reversible sensing of gases, with good selectivity. The adsorption/ desorption of gas molecules on MOF surface changes the dielectric properties of MOFs (impedance), which is measured by the sensor. In one of the first of such studies, Fe-BTC MOF was demonstrated as a selective impedimetric sensor for the detection of hydrophilic gases, such as ethanol and methanol, or humidity. The sensitivity of the developed sensor was found to be higher toward water (0e2.5 vol%) than toward ethanol (0e18 vol%) and methanol

(1e35 vol%) at 120 C (Achmann et al., 2009). EIS has also been used for the detection of CO2 using CDMOF-2. The free primary hydroxyl groups of CDMOF-2 reacted with the gaseous CO2 to form alkyl carbonate functions, providing selectivity to the sensor (Gassensmith et al., 2014). The zeolitic imidazolate framework (ZIF-8) has been reported for the real-time selective detection of I2. The earlier mentioned MOF was capable of performing the analysis at 25 C within 720 s of response time (Small and Nenoff, 2017). MOFs have also been reported useful for the impedimetric biosensing of environmental pollutants such as atrazine.

CONCLUSIONS The application of CPs and MOFs has seen some revolutionary advancements in the recent past. CPs combine the properties of organic polymers (strength, plasticity, flexibility, toughness, elasticity, etc.) and semiconductors (electric conductivity). Ease of synthesis, fabrication and further modification, resistance to corrosion, low cost, low density, and excellent electric, mechanical, and optical properties present them as attractive and commercially viable materials for sensor fabrication. Control over their size at nanoscale level has led to many significant improvements

CHAPTER 3 Conducting Polymers and Metal-Organic Frameworks in the performance of related sensors. CPs are nowadays widely used for the fabrication of chemical and biological sensors. The excellent properties of MOFs, such as high surface area, tunable porosity, chemical and thermal stabilities, and functionality, have enabled their exploration in a vast array of applications. With respect to their applications in nanosensors, it can be envisaged from the literature that most of the researchers have focused on utilizing the optical properties of the MOFs. Nonetheless, MOFs have a wide potential in the development of other types of nanosensors also, including electrochemical sensors. Presently, the application of MOFs in nanosensors is an emerging area of research, and in the near future, such sensors are well expected to leave their mark on the commercial level as well.

ACKNOWLEDGMENTS GRC and RK would like to acknowledge the support of UGC, India under INDO-US 21st Century knowledge Initiative project (F.No. 194-2/ 2016 (IC)). SK thanks the HSCST, Govt. of Haryana, India for research grant vide letter No. HSCST/R&D/2018/2103 dated August 01, 2018.

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Advances in Nanosensors for Biological and Environmental Analysis

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FURTHER READING Gu, X., Su, H., 2012. Materials Focus 1, 97e111. Hatchett, D.W., Josowicz, M., 2008. Chemical Reviews 108, 746e769. Liu, B., Wu, W.P., Hou, L., Wang, Y.Y., 2014. Chemical Communications 50, 8731e8734. Parmar, B., Rachuri, Y., Bisht, K.K., Suresh, E., 2017. Inorganic Chemistry 56, 10939e10949. Rong, J., Oberbeck, F., Wang, X., Li, X., Oxsher, J., Niu, Z., Wang, Q., 2009. Journal of Materials Chemistry 19, 2841e2845. Stock, N., Biswas, S., 2011. Chemical Reviews 112, 933e969.