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Applied Materials Today 9 (2017) 419–433

Contents lists available at ScienceDirect

Applied Materials Today

j ourna l h o mepage: www.elsev ier .com/ locate /apmt

eview

pplications of conducting polymer composites to electrochemical sensors: review

alenahalli Halappa Naveen1, Nanjanagudu Ganesh Gurudatt1, Yoon-Bo Shim ∗

epartment of Chemistry, Pusan National University, Busan 46241, Republic of Korea

r t i c l e i n f o

rticle history:eceived 7 August 2017eceived in revised form 30 August 2017ccepted 1 September 2017

his publication is dedicated to Prof. Josephang on the occasion of his 70th birthday.

a b s t r a c t

Conducting polymers (CPs) constitute an important class of organic functional materials. Their uniquephysical and electrical properties and being inexpensive, easy to synthesize, and suitable matrices forbiomolecule immobilization have enabled them to find a wide range of use, including playing roles insupercapacitors, batteries, electrochromic devices, solar cells, sensors, and biomedical applications. Toenhance the performance of the CPs, various composite materials have been synthesized with carbon-based materials, metals or metal oxides, etc. This review comprehensively discusses CPs and their

eywords:onducting polymerompositelectrode materiallectrocatalyst

composites, followed by their applications. The purpose of this review is to summarize the electrochem-ical sensors that are based on CPs and their composites and to address future aspects of electrochemicalsensing materials.

© 2017 Elsevier Ltd. All rights reserved.

lectrochemical sensor

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4192. Electrochemical properties of conducting polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .420

2.1. Composite materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4212.2. Conducting polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

3. Application to electrochemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4223.1. Electrochemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4223.2. Polyaniline and composite sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4233.3. Poly(diaminonaphthalene) and composite sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4243.4. Polypyrrole and composite sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4253.5. Polythiophene and composite sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4263.6. Polyterthiophene and composite sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4273.7. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

. Introduction platinum electrode in dilute sulfuric acid, was reported by Lethebyin 1862 [1]. These polymers are electrically conductive, and they

Organic �-conjugated polymers have been receiving muchttention since the first electrochemical preparation and charac-erization of polyaniline, formed by the oxidation of aniline on a

∗ Corresponding author.E-mail address: ybshim@pusan.ac.kr (Y.-B. Shim).

1 Authors contributed equally to this work.

ttp://dx.doi.org/10.1016/j.apmt.2017.09.001352-9407/© 2017 Elsevier Ltd. All rights reserved.

are thus called conducting polymers (CPs) despite having beenconsidered insulators before the major breakthrough of creatingelectrically conductive polymers on polyacetylene by treatmentwith oxidizing or reducing agents [2–4]. CPs are a new class of

functional materials [5], and they have received a great deal ofattention in various fields due to their light weight, workability,resistance to corrosion, low cost, and excellent electrical, mechani-cal, optical, and conducting properties [6,7]. The conductivity of CPs

420 M.H. Naveen et al. / Applied Materials Today 9 (2017) 419–433

erizati

w1[c(pbSwoitTcdoddtahfi

2

tCdt

Fig. 1. Electrochemical polym

as first observed in polyacetylene, whose conductivity increased0 orders of magnitude when it was oxidized by iodine vapor2,8,9]. The instability of polyacetylene in air has led to the dis-overy of various new types of CPs [10], including polyanilinePANI), polypyrrole (PPy), polyazulene, polyfluorene, polythio-hene, polyterthiophene, and polyaminonaphthalenes, for use inoth fundamental research and industrial applications [11,12].ince Heeger, MacDiarmid, and Shirakawa were jointly awardedith the Nobel Prize in Chemistry in 2000 for their pioneering work

n CPs, CPs have received much attention from both academic andndustrial communities. CPs generally exhibit semi-conductivity inheir pristine state but can be made more conductive by doping.he term “doping” means that the oxidation and reduction of a �-onjugated system generates p- and n-doped CPs, respectively, andoping can be controlled by chemical and electrochemical meth-ds. The oxidized or reduced states of CPs can exist as cation radical,ication, anion radical, or dianion species [6,9]. CPs are semicon-uctive in nature and have low conductivity. Thus, it is importanto increase their conductivity, especially for use in electrochemicalpplications. To overcome this drawback of CPs, composites of CPsave been intensively studied so that they can be applied in variouselds.

. Electrochemical properties of conducting polymers

Unlike conventional organic polymers, CPs have a high elec-

ron affinity and redox activity. The general physical properties ofPs depend on the size and length of the CPs, and they are alsoescribed in terms of molecular weight [13]. The chemical proper-ies of polymers mainly depend on the polymer chains, and they

on mechanism of polyaniline.

are insoluble in aqueous solution. CPs are usually prepared by oxi-dizing the monomer using oxidizing agents or applying potentialor current via electrodes [6,12,14,15]. The original mechanism ofPPy formation, first suggested by Diaz et al. in 1981 [16], prop-agated the polymerization by continuously growing chains viaside-to-side coupling of monomeric radical cations into oligomerradicals. As an example shown in Fig. 1, electrochemical growthof PANI involves an initial formation of oligomers, followed bynucleation and polymer chain growth [14]. The doping of CPscan be done electrochemically, which means that the oxidationof CPs generates a p-doped state and reduction of CPs gener-ates an n-doped state. The conductivity of the CPs can also becontrolled by these redox reactions. Electrochemical preparationmethods can control the properties, morphology, conductivity, andthickness of the polymers [2,12,17]. The structures of the differ-ent CPs are presented in Fig. 2. In the early stages of this field,the electrochemistry of CPs was intensively studied by Park’s andShim’s groups, who investigated the autocatalytic growth mech-anism of PANI and its growth kinetics on the electrode surface[14,15,18–21]. In addition, the degradation mechanisms of elec-trochemically prepared PANI, PPy, and polythiophene were alsoinvestigated [22,23]. During the past decades, a number of reportshave described applications of CPs. Both monomers and CPs canbe functionalized with different materials to tailor their proper-ties. The addition of substituents not only improves functions butalso improves the chemical and physical properties of the mainpolymer chain, including increasing its electrical and mechani-cal properties. To improve the properties of CPs, CP composites

may be prepared from highly conductive nanomaterials and cat-alysts.

M.H. Naveen et al. / Applied Materials Today 9 (2017) 419–433 421

s and

2

gpdlldrstfsCbabsctisu

Fig. 2. Conducting polymer

.1. Composite materials

Carbon-based materials, such as carbon nanotubes (CNTs),raphene, carbon dots, and porous carbon are considered to beromising materials in several applications [24–27]. The rapidevelopment of carbon materials occurred because of their excel-

ent conductivity, high chemical stability, mechanical strength, andarge surface area [28,29]. All of these properties can be utilized toevelop new types of electrochemical sensors. Using carbon mate-ials alone has a significant drawback, especially when used inensor devices: they lack selectivity for target molecules. Forma-ion of carbon composites and functionalization is therefore vitalor use in specific applications. Their use in electrochemical sen-ors specifically requires selectivity, and making composites withPs is a most promising way to improve their performance. Car-on materials are used in composites because they can be treateds inorganic or organic reagents, depending on their inherent car-on structure. Otherwise, metal or metal oxide nanoparticles (NPs),uch as Au, Pt, Pd, Ni, Cu, TiO2, MnO2, ZnO, and Fe3O4, are anotherlass of materials [30] that exhibit unique properties compared to

heir bulk counterparts. NPs have been exploited in many fields,ncluding their use in biomedical research, optoelectronics, cataly-is, sustainable energy, and electrochemical devices. In particular,se of metal/metal oxide NPs as electrode materials results in

their composite materials.

high performance relative to bulk metal electrodes [31,32]. Forexample, AuNPs display excellent activity and have been a basematerial for electrochemical sensing electrodes [33–35]. A num-ber of approaches have been used to synthesize NPs, including wetchemical synthesis, monolayer formation, dendrimer-mediatedassembly, electrochemical deposition and many more [30,36].Metal NPs possess high activity and high biocompatible surfacearea and can be functionalized with secondary materials, includingcarbon materials, enzymes, and CPs. The preparation of compos-ites of CPs and metal NPs has been reported [37]. CompositingCPs and metal NPs can prevent agglomeration and restacking ofmetal NPs by steric hindrance and electrostatic interactions. More-over, in electrochemical devices, these composites can increasethe electron transport rates between the electrolyte and currentcollectors or electrode materials. In the field of bioanalytical chem-istry, biologically active materials, mainly enzymes, DNA, proteins,antibodies, and antigens, have played important roles. Wang’sgroup reviewed that many enzymatic reactions are involved in alarge number of biosensor devices [38]. For these biomoleculesto be applied, they must be attached to or functionalized with

a matrix that is chemically inert and does not change the prop-erties of biomolecules. Use of a polymer matrix as the hostmaterial for the incorporation of biomolecules is a promisingpathway.

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22 M.H. Naveen et al. / Applied M

.2. Conducting polymer composites

The unique properties originating in CP-carbon compositesake them promising candidates, particularly in electrochemical

evices [39–41]. Several methods to synthesize new CP compositeaterials have been reported and involve using template-oriented

ynthesis, vapor polymerization, chemical functionalization, in situeneration of CP composites, etc., Beside these methods, electro-hemical techniques provide a convenient approach to fabricateP composites, and they can easily control the morphology, thick-ess, chemical state, and conductivity of the composites. Differentypes of materials and composites are presented in Fig. 2. Com-osites of CPs utilize CPs as a primary component and at leastne secondary component, which may be an organic, inorganic oriological species. These materials include metal ions, NPs, nano-tructures of metal and metal oxides, carbon materials, molecularpecies, i.e., metallophthalocyanines, and biologically active com-onents such as enzymes, proteins, antibodies, and antigens. Theoal of preparing a new composite material is to observe new dis-inct properties that are not observed in its individual components42–45]. Thus, using a synergetic strategy to generate compos-te materials is vital. CP nanocomposites demonstrate interfacialdhesion between CPs and the secondary component, and the com-osition ratio will affect the properties of the nanocomposites.omposites of CPs displayed enhanced properties compared to theere CPs [46]. These composites exhibit outstanding performance

n energy storage [47], as catalysts [48], in fuel cells, and in biomed-cal [49–53] and many more applications [54]. During the past fewecades, CP composites have been used in the fabrication of biosen-ors [52,55–58]. CPs are a promising base material that is suitableor the incorporation and functionalization of biomolecules [59].he most common immobilization methods are electrochemicalntrapment and physical adsorption within the CPs and attach-ent to the CPs by affinity interactions or covalent bonds. Physical

dsorption involves electrostatic interactions between the CPs andiomolecules because the CPs and biomolecules have distinct sur-ace charge properties based on their functional groups. However,ther interactions, such as hydrophobic and van der Waals forces,re also involved in the physical adsorption process, especially inhe adsorption of antibodies and proteins [60]. The adsorption pro-ess was studied in biosensors by using glucose oxidase on PPy61]. The main advantage of the adsorption technique is that itoes not require functionalized monomers. However, because ofelatively weak forces involved in the adsorption of biomolecules,he biomolecules may leach out from the interface of the elec-rode during long experiments [62]. The entrapment of enzymesy electrochemical techniques was later proposed to increase theinding efficiency of the biomolecules; as an example, glucosexidase was entrapped in PPy films via an electropolymerizationrocess to obtain a matrix of glucose oxidase and PPy [63,64]. Thisethod, compared to the physical adsorption process, resulted

n a direct and prolonged immobilization. However, the enzymesr biomolecules were partially exposed and buried within theolymer films, which may have resulted in less efficient targetccessibility. The main drawback of this process is that it is moreuitable for only water-soluble monomers and is not useful foronomers that are soluble in non-aqueous solvents. Thus, strongly

ttaching enzymes on CPs via covalent bonding were proposed.his approach commonly uses N-hydroxysuccinimide/1-ethyl-3-3-dimethyl-aminopropyl) carbodiimide (NHS/EDC) chemistry toouple carboxylic acids (–COOH) to amine (–NH2) groups. Mostmportantly, the coupling reaction can be performed in aqueous

olutions, which can preserve biomolecules [65]. These types ofensors displayed excellent stability during long measurementsue to the covalent attachment of the enzymes or proteins onhe stable electrode surface [66–68]. During the past few decades,

als Today 9 (2017) 419–433

CPs and their composites have been used in the fabrication ofbiosensors [52,56,58,69]. In addition, covalent functionalization ofCPs with biomolecules has been used to study drug delivery andbioimaging applications [70–74].

3. Application to electrochemical sensors

3.1. Electrochemical sensors

Electrochemical sensors convert chemical reactions of the tar-get species on electrodes into electrical signals that exhibit changesin current, potential, and conductivity [75]. These sensors canbe divided into voltammetric/amperometric, potentiometric, andimpedimetric sensors as illustrated by Wang, who has greatly con-tributed to the field of electrochemical sensors and biosensors[76,77]. Electrochemical methods offer very low detection limits,high selectivity, and require only a very small volume of sample toobtain the signals. Measurements of turbid samples of whole blood,fat globules, red blood cells, and hemoglobin are possible usingelectrochemical techniques, whereas spectrophotometric mea-surements of these systems usually present interference effects[78,79]. Electrochemical devices consist of a system of either two orthree electrodes. A typical three-electrode cell consists of a workingelectrode that is made of platinum, gold, and/or carbon; a ref-erence electrode that is usually a silver-silver chloride electrode(Ag/AgCl); and a platinum wire or foil is used as a counter/auxiliaryelectrode. The working principle of a voltammetric sensor is tomeasure the current while applying a potential to the working elec-trode relative to a reference electrode. The response current is theresult of the electrochemical reaction that occurs at the electrodesurface and the electrode/electrolyte interface layer. The electro-chemical reaction is limited by the rate of mass transport of thespecies to the electrode [80]. Voltammetric methods include linearsweep voltammetry, cyclic voltammetry, hydrodynamic voltam-metry, differential pulse voltammetry, square-wave voltammetry,polarography, and stripping voltammetry. These techniques havea wide range of applications, a broad dynamic range and a lowdetection limit. In amperometry, a constant potential is applied tothe working electrode and the resulting current is measured withrespect to time. The difference between amperometry and voltam-metry is that amperometry lacks a potential sweep, instead usinga potential step. The resulting current at a given potential is pro-portional to the concentration of the electroactive species in thesample, and the current is given by Cottrell’s equation. Amper-ometric sensors have more sensitive and selectivity because theoxidation or reduction potential used for detection is characteris-tic of the analyte species [81,82]. Potentiometric sensors measurethe potential difference between the working electrode and thereference electrode when no significant current is flowing [80].The measured potential is mainly used to determine ion activityin an electrochemical cell. In potentiometric measurements, thepotential difference can be calculated by using the Nernst equation,where Ecell signifies the observed cell potential at zero current. Thisvalue is also referred to as the electromotive force, or EMF. The low-est detection limits of potentiometric devices are currently oftenrealized with ion-selective electrodes (ISE) [77,83]. The impedanceof a generic electrical component is calculated by dividing the ACpotential by the AC current in electrochemical impedance spec-troscopy, i.e., dividing an incremental change in voltage by theresulting change in current. The experimental approach is to per-turb the cell with a small-magnitude alternating potential and to

observe the way in which the system reacts to the perturbation inthe steady state. Impedance sensors can be divided into two types,depending on the presence or absence of specific recognition ele-ments. The function of the first type of this sensor is based on an

M.H. Naveen et al. / Applied Materials Today 9 (2017) 419–433 423

F th catS l hydrC

i(ecatc

3

itiddthlPad

ig. 3. (a) Schematic of the MIP-conducting polymer hybrid electrode imprinted wiociety. (b) Schematic of the PtNPs and enzymes-loaded 3D porous PANI hydrogehemical Society.

mpedance change caused by the binding of targets to receptorsantibodies and nucleic acids) that have been immobilized onto thelectrode; the other type of sensor’s response is based on surfacehanges caused by adsorption and desorption of target species as

result of growth [78]. The discussion on the construction of elec-rochemical sensors in this review will be limited to CPs and CPomposites.

.2. Polyaniline and composite sensors

PANI has emerged as one of the most promising CP materialsn many applications. Composites of the polymer were used inhe fabrication of many devices for different applications includ-ng batteries, biofuel cells, and thermoelectric and energy storageevices [84–89]. In addition, a chemical sensor for the detection ofissolved oxygen was reported based on the PANI-modified elec-rode [90]. The most promising application of these composites,owever, was found in electrochemical applications. In particu-

ar, the fabrication of electrochemical sensors based on PANI andANI composites has become a popular approach in many electro-nalytical studies and offers high sensitivity and selectivity toetect target molecules. The detection of glucose was reported

echol. Adapted with permission from Ref. [90]. Copyright 2009 American Chemicalide electrode. Adapted with permission from Ref. [91]. Copyright 2013 American

using composites of PANI and NiCo2O4 NPs. This non-enzymaticglucose sensor exhibited high electrocatalytic performance in glu-cose analysis. The catalytic active sites in NiCo2O4 with conductivePANI played a synergetic role in the oxidation of glucose andexhibited good sensitivity and a lower detection limit [91]. Aselective dopamine sensor was developed by the electrochemicaldeposition of poly(3,4-ethylenedioxythiophene)-graphene oxide(PEDOT-GO) nanocomposite followed by electrochemical reduc-tion of nanocomposite [92]. As shown in Fig. 3a, a dual polymersystem of catalytic molecularly imprinted polymers on a PANI layerwas used as a recognition layer for the electrochemical detectionof catechol [93]. Despite the unique properties of PANI compos-ites, there are some limitations to their use in biosensors underphysiological conditions. For example, PANI is inactive at neu-tral pH. Composites of PANI with metal NPs and carbon materialswere developed that showed remarkable electrochemical activ-ity in many applications. For example, recently, highly sensitiveglucose sensor was developed based on a composite PtNP/PANI

hydrogel electrode. The nanostructured PtNPs and the PANI aero-gel on which they sit have a synergetic effect between them thatresults in the immobilization of a high density of GOx, which aidsefficient oxidation of glucose. This sensor probe showed high sensi-

4 ateri

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24 M.H. Naveen et al. / Applied M

ivity and a very fast amperometric response for glucose detection94]. As shown in Fig. 3b, a similar PtNP/PANI hydrogel-basedlectrode was used to detect human metabolites, and the per-ormance of the sensor probe was excellent [95]. Recently, anmpedimetric immunosensor based on a AuNP-functionalized PANInd multiwalled carbon nanotube (MWCNT) composite electrodeor 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide detection waseveloped [96]. The sensor probe was prepared by electropoly-erization of aniline and 3-aminobenzoic acid in the presence ofuNPs onto a MWCNT-coated screen-printed electrode. An anti-ody was covalently attached to the sensor probe to improve theensitivity of antigen detection. The developed impedimetric sen-or showed a wide linearity range, high stability, and a detectionimit of 0.3 ppb 2,4-D. Molybdenum disulfide (MoS2) is a layeredransition metal dichalcogenide in which the metal Mo layers areandwiched between two sulfur layers and stacked together byeak van der Waals interactions. MoS2 constitutes a graphene-likeanomaterial; however, the limitation of MoS2 is its low electroniconductivity. MoS2 was combined with PANI via the facile oxi-ation of aniline monomers on the MoS2 matrix. This compositeaterial served as an excellent conductive skeleton with a high

lectrocatalytic surface area, and it also supported a direct pathor electron transfer. The hybrid PANI-MoS2 material exhibited

superior electrochemical performance to pure PANI and MoS2;ence, this material was used for the electrochemical detection ofNA molecules [97]. Metal electrodes are very stable and can besed in many electrochemical applications; however, in biologicalpplications, the metallic surfaces may cause undesirable poison-ng effects as well as nonspecific catalysis. In the fabrication ofiosensors, one of the major issues faced by researchers is the foul-

ng of electrode surfaces. To overcome this obstacle, polyethylenelycol (PEG) was grafted onto PANI nanofibers [98]. This compos-te material was conductive and possessed a large surface area,

hile showing excellent antifouling activity in single protein solu-ions and complex human serum samples. The antifouling effectas attained by making the surface hydrophilic, thereby avoid-

ng interactions with nonspecific proteins in the complex biologicalamples. The researchers used this composite to fabricate an elec-rochemical biosensor to detect the breast cancer susceptibilityene BRCA1. Furthermore, gene sensing is a very powerful toolor the diagnosis of a wide variety of diseases, from common viralnfections to cardiovascular diseases and cancer. For the purpose ofetecting nucleic acids in biological samples, many CP compositesere prepared in lieu of metal or metal-oxide electrode materials.

or instance, a new method was developed for the sensitive detec-ion of nucleic acids using a surface with PANI deposited upon it99]. In this case, the polymer was deposited by enzyme-catalyzedormation and template-guided deposition in which a reporternzyme catalyzed the deposition of PANI onto the analyzed nucleiccid molecules by forming an intramolecular complex. The opticalnd electrical properties of the polymers thus formed are alteredfter the interaction with the nucleic acids and can be quantitativelyeasured. One of the more recent projects on this topic involves the

se of graphene-based PANI arrays for the electrochemical detec-ion of DNA, which focuses on improving the sensitivity of detectiony controlling the texture of the substrate [100].

.3. Poly(diaminonaphthalene) and composite sensors

One of the earliest preparations of poly(2,3-iaminonaphthalene) (DAN) and poly(1,8-DAN) throughlectrooxidative polymerization was reported in 1989 [101].

he polymer films were formed by potential-sweep electroly-is in electrolytic solutions. Following that, the preparation of-aminonaphthalene polymer from its monomers by using ahemical oxidative polymerization was reported in 1993 [102].

als Today 9 (2017) 419–433

They synthesized the polymers in high yields using H2O2 in thepresence of a Fe catalyst. One of the early reports of the use ofaminonaphthalene polymers for the electroreduction of oxygenwas published in 1991 [103]. The authors formed a film of poly(2,3-DAN) on the electrode surface and used it for electrochemicalcatalysis. In 2002, a copper complex of poly(1,5-DAN) was formedand, in combination with poly(terthiophene carboxylic acid), thiscomplex was used for the electrochemical reduction of oxygen[104]. Similar to this result, poly(1,8-DAN) and a cobalt complexon poly(1,8-DAN)-coated electrodes were also used for the elec-trochemical reduction of oxygen [105]. In 2000, a humidity sensorwas developed by using poly(1,5-DAN) [106]. Since polymers suchas PANI and PPy have relatively high conductivities in air, it isdifficult to detect a change in humidity by measuring conductivitychanges. Poly(1,5-DAN), on the other hand, exhibits electricalresistance that is proportional to the amount of absorbed water;it was therefore used in combination with doped carbon in thefabrication of a humidity sensor. The doped carbon contributedbetter sensing ability and higher stability to the polymer film. Apartfrom being used as a sensing material, poly(1,5-DAN) has also beenused in many different applications. Selenium (Se) is an importantantioxidant that is found in trace amounts in the human body;however, higher concentrations are toxic to humans as well asother organisms, and its detection is thus important. A strategy todetect Se(IV) ions by using poly(1,8-DAN)-modified gold electrodeswas developed [107]. Se(IV) specifically interacts with aromatico-diamines to form derivatives of piaselenole. Dopamine is one ofthe most important neurotransmitters; being electrochemicallyactive opens opportunities for its easy detection and thereby thediagnosis of many related diseases. In this regard, aminonaphtha-lene composites have been used as electrode materials to enhancethe electrode’s detection ability. In 2009, poly(1,5-DAN) wascombined with Cibacron blue to form a composite film that acts aselectrode material for the selective detection of dopamine [108].The sulfonate groups in the composite film help repel negativelycharged interfering species, thus facilitating the specific detectionof dopamine. Shim et al. entrapped reactive blue 4 dye in poly(1,5-DAN) to form a composite film layer on an electrode surface for thesimultaneous detection of dopamine and acetaminophen [109]. Tofurther this methodology, in 2015, Mir et al. combined poly(1,5-DAN) with GO to form a composite layer on AuNPs that had beenelectrochemically deposited on a carbon electrode surface. Thiselectrode surface attained a high signal enhancement and showedthe lowest detection limit for dopamine (5 nM) [110]. In additionto neurotransmitters, the DAN polymer has also been used as anelectrode material in the detection of glucose, H2O2, and otherenvironmentally and biologically important molecules. For exam-ple, in 2010, HRP was immobilized on poly(1,8-DAN) on a goldNP-modified electrode to detect H2O2 [111]. To simultaneouslydetect glucose, ascorbic acid, and dopamine, poly(1,5-DAN) withincorporated nickel NPs was prepared [112]. The polymer matrixshowed a strong electronic interaction with the metallic NPs,which resulted in enhanced conductive properties in the polymerfilm. To enhance the sensitivity of the sensing surface, Tamburriet al. grew a polymer film of poly(1,8-DAN) on platinum plates inthe presence of single-walled nanotubes to form a composite layer[113]. A series of films were prepared by using untreated and KOH-,HNO3-, and HNO3/H2SO4-treated nanotubes. The charge transportin the composite films was strongly enhanced by the insertionof nanotubes. The current density was enhanced as much as 140times greater than that of the pure poly(1,8-DAN) films, indicatinga possibility for significant signal amplification. To further this

improvement, in 2012, poly(1,5-DAN) was functionalized withMWCNTs and then used to form a composite film on an electrodesurface [114]. The composite film, in comparison with the polymersurface, showed an enhanced signal and analyte peaks that were

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M.H. Naveen et al. / Applied M

ore clearly separated in the voltammetric response. This materialas then used for the simultaneous detection of ascorbic acid andric acid, as well as the electrochemical detection of H2O2 whenorseradish peroxidase was immobilized.

.4. Polypyrrole and composite sensors

PPy is one of the most promising CPs and is formed by chemicalr electrochemical polymerization of pyrrole. Diaz et al. first pro-osed the polymerization mechanism of PPy [1]. Then, numeroustudies were performed in which polymer-coated electrodes werenvestigated. PPy and its composites with metal and metal oxidePs, MWCNTs, graphene, enzymes, and electron-transfer medi-tors have been studied as electrochemical electrode materials,articularly for electrochemical biosensor applications. Due to theeneficial and unique chemical, physical, and electronic proper-ies of PPy, this polymer has been used in electrochemical sensing

aterials. Wang et al. reported a new protocol for the electro-hemical detection of nucleic acids using PPy coated electrode inow injection analysis [115]. AuNP-PPy/Prussian blue nanocom-osite sensor was reportedly used for the detection of H2O2. Theomposite was prepared by the autopolymerization of pyrrole intoPy and the simultaneous reduction of AuCl4− to elemental Au;t the same time, Prussian blue was produced with the elemen-al Au and served as a catalyst. This H2O2 sensor probe exhibitedigh sensitivity, fast responses, and a good stability, which resulted

rom the PPy, and achieved a detection limit of 8.3 × 10−10 M [116].lectrochemical copolymerization of PPy on mesoporous Pt using

boron-doped diamond electrode for the sensitive and selectiveetection of H2O2 was developed. Synergic effects between thePrPt and the PPy copolymer exhibited a sensitive, selective, and

ccurate quantitation of H2O2. This high sensitivity may have orig-nated from the mesoporous nature of the PtNPs [117]. Anotheresearcher also developed a glucose sensor based on electrochem-cally deposited AuNPs that were covered by PPy [118]. A DNAensor was developed using a PPy-AuNP composite electrode inhich AuNPs were electrodeposited on a PPy layer to generateanocomposite films and increase the conductivity of the electrode.he DNA detection was performed by immobilizing the captureNA strand on the AuNPs, and the hybridization assay was per-

ormed by exposing the modified electrode to the target and thenhe HRP-modified probe strand. A measurable current was obtainedrom the reduction of benzoquinone, which was observed duringhe hydroquinone reduction that mediates electron transfer to theRP-labeled probe strand. This study demonstrated the detectionf DNA concentrations at the pM level [119]. The fabrication oflectrochemical biosensors based on graphene-PPy composites hasecome a promising approach in electroanalytical research aim-

ng for various analytical targets and using different strategies.ecently, a combination of graphene and overoxidized PPy compos-

tes was reported to simultaneously detect adenine and guanine.he sensor probe was prepared by the electrodeposition of PPy andO together by using a potential cycling method that was followedy the electrochemical reduction of GO. The obtained PPy-grapheneas then oxidized at +1.8 V, resulting in an oxidized surface of a

Py-graphene (PPyox-gra) composite layer. The oxidized form ofPy is permeable to cationic species and repels anionic and neutralpecies, which helps to avoid interference and confers selectiv-ty to the sensor probe. The PPyox-gra sensor probe was used touantify adenine and guanine and reached low-detection limits of.02 and 0.01 �M, respectively [120]. Mao et al. developed a sen-or probe that used a PPy-GO composite with poly(ionic liquids)

PILs). PILs/PPy/GO nanosheets containing sensor probes displayed

steady and sensitive detection of dopamine in the presence ofscorbic acid. The presence of PILs on the sensor probe played aey role in separating the oxidation peaks of the DA and AA [121].

als Today 9 (2017) 419–433 425

Tiwari et al. developed a genosensor by using a combination ofAuNP, graphene, and PPy materials. At first, GO-AuNP was preparedby a one-pot solution method and coated on an indium tin oxide(ITO) electrode. PPy was then electropolymerized on the GO-AuNPlayer. The authors observed that the changes in the PPy morphol-ogy increased with the scan rate. This sensor probe used methyleneblue (MB) as a redox indicator to monitor the electrochemicalsignals, and this probe shows a response time of 60 s and highsensitivity (1 × 10−15 M) [122]. In the development of enzymaticbiosensors, immobilization of enzymes on polymer films is vital.There are many ways to achieve the immobilization of enzymes:adsorption, entrapment, covalent binding, etc. Among these meth-ods, covalent bonding is the most promising because of the covalentattachment between the polymer film and the enzymes [123]. Thismethod was successfully adopted to develop glucose, aptamer,DNA sensors, and direct electron-transfer process of cytochrome cand other proteins for use in biosensor applications. An enzymaticglucose sensor was prepared using PPy nanowires GOx that wascovalently attached to PPyCOOH/PPy composite nanowires. Thissensor probe showed not only high sensitivity with a wide lineardynamic range and a low detection limit but also exhibited highstability and a specificity for glucose [124]. Another glucose sensorwas built by using a biocompatible chitosan-GOx complex immo-bilized on a PPy/Nafion/MWCNT nanocomposite electrode. Thechitosan-glucose oxidase was encapsulated inside a nanohybridPPy/Nafion/MWCNT film. The high surface area of the nanohybridfilms enhances GOx loading on the electrode surface, which givesthe sensor probe high sensitivity and the ability to detect 5 �Mglucose [125]. The authors found that the presence of Nafion in thecomposite enhanced the dispersion of the MWCNTs and increasedthe stability of the electrode. Another sensor probe, based on aZnO-PPy composite, was developed for the analysis of xanthine.The xanthine oxidase enzyme, immobilized on a ZnO-PPy com-posite electrode surface, showed good electrocatalytic activity forthe oxidation of xanthine [126]. Shim et al. synthesized a [2,5-di-(2-thienyl)-1H-pyrrole-1-(p-benzoic acid)] (DPB) monomer touse in the development of an aptamer sensor for the detectionof kanamycin. DPB monomers were self-assembled with AuNPsbefore electropolymerization on the electrode surface. An aptamerwas immobilized on the electrode via covalent bond formationbetween the –NH2 groups of the aptamer and the –COOH groups ofthe poly-DPB. The sensor response exhibited a pair of redox peaks atapproximately 0.26 and 0.08 V (vs. Ag/AgCl) when kanamycin wascaptured by the aptamer-immobilized probe. The sensor showedgood selectivity for kanamycin, good long-term stability (up to twomonths), and a detection limit of 9.4 ± 0.4 nM [127]. Wang et al.developed a neural microelectrode array by using direct electrode-position of a PPy-gra (PG) nanocomposite and used it to detectdopamine secretion from pheochromocytoma cells. The PG com-posites that had been deposited on the neural microelectrode arraydisplayed excellent selectivity for dopamine in the presence ofascorbic acid. The authors claim that the electrocatalytic graphenestructure, being highly negatively charged, is effective in attractingDA cations to the surface of PG nanocomposites and rejecting AAanions [128]. The amperometric results also suggest that ascorbicacid does not greatly interfere with the detection of dopamine. Thesensor probe was used to detect dopamine, which was releasedfrom the PC12 by stimulation with K+ ions. Mndek et al. developeda DNA sensor by using a MWCNT-PPy composite that had beenfunctionalized with a redox PAMAM dendrimer. The nanomaterialhas been demonstrated as a molecular transducer in the electro-chemical detection of DNA. As shown in Fig. 4, the MWCNT-PPy

nanocomposite was formed by electropolymerization of pyrroleon MWCNTs, and the covalent attachment of PAMAM was per-formed by an electrooxidation method. Ferrocenyl groups werethen attached, and an electrochemical signal from ferrocene was

426 M.H. Naveen et al. / Applied Materials Today 9 (2017) 419–433

F , PPy,C

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ig. 4. (a) Schematic representation of DNA biosensor preparation using MWCNTsopyright 2015 American Chemical Society.

sed to detect DNA hybridization. The sensor probe could sensehe DNA of the rpoB gene of Mycobacterium tuberculosis in real PCRamples [129]. Another aptasensor using a PPy-PAMAM dendrimeromposite was developed to directly detect ochratoxin A (OTA). Theptasensor had a detection limit of 2 ng L-1 OTA, which is below theTA concentration that is allowed in food by European regulations

130]. In addition, a PPy composite was used to develop an aptasen-or to detect prostate cancer biomarkers. The antifouling effect isne of the key characteristics of these types of sensors. Jolly et al.eveloped a sensor probe that has enhanced antifouling proper-ies by using a PPy-PEG composite electrode. The sensor probe wasonstructed by attaching PPy-PEG on a gold electrode surface andovalently attaching a N�,N�-bis(carboxymethyl)-l-lysine hydrateANTA) molecule that was associated with a Cu2+ ion complex. Thenhanced electrochemical properties of the PPy-PEG-ANTA/Cu2+

ensor probe originated from PPy and the redox-active copper com-lex. Polyhistidine-modified aptamers were immobilized by theu2+ redox complex onto the PPy-PEG adduct. The variation in theopper redox signal when �-methylacyl-CoA racemase (AMACR;504S) was detected was monitored by square wave voltammetrySWV). The sensor probe, composed of hybrid materials, displayednhanced antifouling properties and detected AMACR at the fem-omolar level (0.15 fM) [131]. A biotin-doped PPy dual responsivemmune sensor was developed by combined colorimetric recogni-ion and an electrochemical response to effect the ultrasensitiveetection of multiple tumor markers. The nanorough surface of

he PPy immune sensor, combined with multiple HRP-labeled andntibody-attached NPs, offers great advantages over smooth sur-ace sensor probes. The sensor probe demonstrated a high level ofnalytical and clinical performance [132].

PAMAM G4 dendrimers, and ferrocene. Adapted with permission from Ref. [126].

3.5. Polythiophene and composite sensors

Thiophene-based organic CPs have been known to have com-mendable conductive properties. Being semiconducting materialsthemselves, when conjugated with metal NPs, graphene, and car-bon materials [133], they show exceptional stability and enhancedconductivity. They can also be modified with specific functionalgroups that allow them to be chemically attached to transducermolecules in a biosensor, making them a very reliable group of sub-strate materials [134]. Thiophene-based CP composites have beenused in the fabrication of memory devices [135], electrochromicdevices [136] semiconductors [137], and thin-film transistors[138]. In addition to these applications, thiophene-based compositematerials have been used in the fabrication of many electrochem-ical sensors, including those that detect neurotransmitters [139],gases [140], glucose [141,142], and many other analytes. In the pro-cess of attaining high sensitivity and resolution in electrochemicalapplications, a layer of PPy was sandwiched between two lay-ers of PEDOT, and the surface was coated with AuNPs; the AuNPcoating slightly increased the response [143]. This work is one ofvery few in which different polymer layers have been conjugatedtogether to obtain high sensitivity. To detect analytes in vivo, stabil-ity and biocompatibility are as important as sensitivity, and muchresearch has been focused on achieving results in this regard. Fig. 5shows an example of this approach when a Nafion and PEDOTconjugate was electropolymerized on carbon-fiber microelectrodes

[144]. This method resulted in a highly stable and controllable sur-face for the selective and sensitive detection of dopamine in vivo.A novel human dopamine receptor (hDRD1) was conjugated witha CP nanofiber (NF) to form a multidimensional membrane for

M.H. Naveen et al. / Applied Materials Today 9 (2017) 419–433 427

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ig. 5. Biocompatible PEDOT:Nafion composite electrode for selective detection ofhemical Society.

he selective detection of dopamine [145]. To accomplish thiseat, multidimensional carboxylated PEDOT (MCPEDOT) NFs withanorods were used as a transistor in a liquid-ion-gated field-effectransistor-based biosensor. E. coli cells that expressed hDRD1 weremmobilized on MCPEDOT-NFs. The final membrane exhibited aapid, real-time response within 2 s and a high sensitivity of 100 fM.olythiophene materials have been proven to be one of the besturfaces for the stable immobilization of antibody molecules in aensor. Thiophene-based immunosensors are in development forhe detection of various diseases. The carboxylic acid-containing CPrecursors were self-assembled on the surfaces of AuNPs, and theunctional groups on the polymer precursor were used as a platformor the construction of a bioconjugate to sensitively and selectivelyetect human, non-small-cell lung cancer [146]. A polymer com-osite with GO was formed, and the GO was then reduced to form

more conductive surface with a high surface area [147]. Duringhis process, the AuNPs simultaneously formed on the materialy NaBH4 reduction. This process resulted in a highly conduct-

ng surface, the polythiophene made the electrode very stable,nd functional groups for the immobilization of the antibodiesnabled the sensitive detection of carcinoembryonic antigen (CEA).o achieve a more selective sensor surface, hyaluronic acid wasoped on to PEDOT [148]. This material has a highly porous struc-ure, is strongly hydrophobic, and has a large number of carboxyliccid functional groups. These functional groups were very useful inmmobilizing CEA antibodies to form a highly sensitive, specificallyinding sensor surface whose detection limit was 0.3 pg/mL. Theommercially available PEDOT:PSS was electrospun into nanofibersnd eventually formed into conducting paper [149]. This conduct-ng paper-based biosensor was demonstrated to sense CEA as anlternative for the conventional indium tin oxide, gold and glassyarbon electrodes.

Many different combinations of materials with the thiophene-ased polymers have been examined to be used as glucoseiosensors. For instance, immobilized GOx on a polymerized, newlyynthesized thiophene conducting monomer to achieve robustovalent binding between the biomolecule and the immobiliza-ion platform. The amine groups of the enzyme and the aldehyderoups of the polymer (which had been functionalized on the

ewly synthesized monomer) bound each other with the helpf glutaraldehyde. This conjugation between the polymer-coatedraphite electrode and the GOx enzyme resulted in a sensor with

low-detection limit of 2.29 �M for glucose molecules [150]. On

transmitters. Adapted with permission from Ref. [141]. Copyright 2015 American

the other hand, PEDOT:PSS and poly(vinyl alcohol) were com-bined to make a conducting matrix and use it as a substrate forthe immobilization of FAD-GDH. This conjugated material resultedin high enzymatic activity [151]. Three-dimensional nanoporousPEDOT was electrochemically prepared using a hard-templatemethod [152]. Since the nanostructure possessed a large surfacearea and is highly stable and conductive, the CuNPs were elec-trodeposited deeper into the PEDOT nanostructure. The preparedporous CuNP/PEDOT nanocomposite was excellent at electrocat-alyzing the oxidation of glucose, owing to its unique structure inwhich the electrodeposited CuNPs provided many active sites forthe oxidation of glucose and the 3D porous structure can facilitatethe diffusion of glucose molecules and accelerate electron trans-fer. In vivo examination of glucose has proven to be a more reliableand accurate assessment of diabetes than in vitro analysis of patientblood samples. Since CPs are non-toxic organic molecules, many ofthe polythiophene conjugates are used in the in vivo devices. Onesuch approach was taken in 2014: glucose-specific enzymes wereimmobilized on a PEDOT CP on the microneedle ‘Smart Patch’ sen-sor platform for painless and continuous intradermal sensing [153].Apart from being abnormally controlled in diabetes, glucose andits extracellular metabolism can also be indicative of many brain-related disorders, including the formation of tumors in the brain.As shown in Fig. 6, to monitor changes in the extracellular con-centration of glucose in the brain, Yang et al. composited PEDOTnanofibers with GOx and electrodeposited them on electrospunpoly(l-lactide) on platinum microelectrodes [154].

3.6. Polyterthiophene and composite sensors

Terthiophene polymers are another class of CPs, and theirunique properties have allowed them to perform excellently whencompared to other CPs. When a terthiophene monomeric unit isfunctionalized at its 3′-position, rapid polymerization is enabled,made possible by the reduced steric by the lack of a bulky func-tional group around the polymerization reaction site [155]. Anearly study of the electrochemical applications of terthiophenepolymers was made by Shim et al. In particular, their applica-tion was to develop electrochemical biosensors, chemical sensors,

and electrocatalytic composite materials [156–160]. For example,Shim et al. developed a portable DNA sensor by electropolymeriz-ing 5,2:5,2-terthiophene-3-carboxylic acid (TTCA) monomers on aGC electrode and utilizing it to directly detect DNA hybridization

428 M.H. Naveen et al. / Applied Materials Today 9 (2017) 419–433

Fig. 6. Schematic of fabrication process of GOx-incorporated PEDOT on a microelectrode array: (a) Pt microelectrode array. (b, c) Electrodeposition of GOx-incorporatedPEDOT film (PEDOT F-GOx). (c) Electrospinning of PLLA nanofibers on the microelectrode array. (d, f) Electrodeposition of PEDOT around the PLLA nanofibers to form GOx-incorporated PEDOT nanofibers (PEDOT NF-GOx). (g) Schematic of GOx entrapment within the PEDOT structure; corresponding microscope images (h–m). Copyright 2014Wiley. Used with permission from [151].

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161]. A heavy metal ion sensor was fabricated using compos-tes of 3,4-diamino-2,2:5,2-terthiophene (DATT) polymer and EDTAn the presence of a catalyst. Carboxylic acid groups of EDTA

ere covalently immobilized on the amine-functionalized poly-ATT layer, resulting in a stable and reusable electrode surface

hat could detect various heavy metal ions simultaneously [162].hey were also tested as biomimetic membranes [163] and to

etect superoxide in vivo using a microbiosensor [164]. A sensitivelectrochemical assay was developed to detect DNA and proteinsy covalently functionalizing hydrazine on the surface of poly-TCA. The detection signal was amplified by a polyTTCA/dendrimer

ensor construction and detection principle.

assembly that was loaded with AuNPs targeting DNA and proteinslinked with hydrazine labels [165]. The direct electron transfer ofcytochrome c (Cyt c) was also studied on a polyTTCA layer elec-trode. Cyt c that had been covalently bonded to a CP layer wasused in bioelectronic devices [166]. The applicability of these cytc-immobilized membranes as biosensors was tested in the deter-mination of nitric oxide (NO) [167] and NADH [168]. Furthermore,

a rapid method to detect heavy metal ions was developed andemployed a disposable sensor that had been modified with GO-doped DATT. The metal ions could selectively complex with thenitrogen atoms of the DATT ligand, supported by large amount

M.H. Naveen et al. / Applied Materials Today 9 (2017) 419–433 429

AuNi@

osptaawspAoecoafriwfgTmg3fawitaTpswwocccsSTt

Fig. 8. Preparation of the dealloyed-

f negatively charged GO. Compared with square-wave anodictripping voltammetry (SWASV) method, chronocoulometry (CC)erformed better, having a wider dynamic range and faster analysisime (<1 s). Additionally, the proposed method could be efficientlypplied to various water samples in a few seconds [169]. A benzoiccid group bearing 2,2′:5′,5′′-terthiophene-3′-benzoic acid (TTBA)as synthesized and used in various biosensors and chemosen-

ors, including those that detected glucose [170], H2O2 [171,172],hthalate esters [173], and cancer cells [174,175] [Fig. 7]. Recently,u and Ni, as a bimetallic alloy dendrite, were electrodepositedn the polyterthiophene CP. As illustrated in Fig. 8, this materialxhibited improved long-term stability and an increased electro-hemically active area, which was used for the sensitive detectionf low concentrations of H2O2 released from living cells, as wells for monitoring the electrochemical oxygen reduction reactionrom energy conversion [176]. Above all, a novel nanostructu-ed AuNP-encapsulated CP was developed for the detection ofnducible NO synthase (iNOS). The TTBA monomer and AuNPs

ere self-assembled and then electropolymerized on the GC sur-ace. Then, iNOS antibody was immobilized on the benzoic acidroups of a nanostructured polymer layer via covalent bonding.his immunosensor exhibited high stability and a good perfor-ance compared to other commercial methods [34]. Another

roup synthesized a new functionalized terthiophene monomer,-((2′:2′′,5′′:2′′′-terthiophene)-3′′-yl) acrylic acid (TAA), and used itor the analysis of a DNA assay [177]. Additionally, an acrylic acidnd methylhydroxy group-functionalized terthiophene monomeras synthesized from the same group, and it was electropolymer-

zed into CPs to act as a label-free DNA sensor [178]. Additionally,he gene-sensing ability of polyTAA was estimated in its oxidizednd reduced states with and without a Fe(CN)6

3−/4− redox probe.he target-specific oligonucleotide sequence was attached to theorous polyTAA surface via covalent bonding and showed a goodensitivity toward its complementary target in its oxidized stateith the redox probe [179]. Among these sensors, terthiophene CPsere successfully used to develop glucose sensors. In general, most

f glucose sensors are enzymatic sensors and they used enzymes asatalytic materials for the specific oxidation of glucose, which basi-ally hinders the effectiveness of the sensor in two ways. First, theost of the sensors is considerably expensive because of the neces-

ity of obtaining the pure form of the enzyme for sensor fabrication.econd, the lifetime and the stability of the electrodes are limited.he proteins are only stable for a short period of time and needo be properly maintained to get optimum results. To avoid these

pTBA electrode for H2O2 detection.

disadvantages, many researchers have formulated non-enzymaticsensor surfaces [38,180,181]. One such approach is the formationof molecularly imprinted polymer (MIP) surfaces; in 2017, Kimet al. fabricated a potentiometric non-enzymatic glucose sensorusing glucose-imprinted polyterthiophene surfaces [182]. Thesesurfaces were formed on AuNPs that had been electrodepositedon SPCE, where an MIP containing acrylamide and aminophenylboronic acid (as a host molecule to glucose) was immobilized on thecarboxylic acid-functionalized polyterthiophene CP. This sensorsystem’s detection limit was 1.9 × 10−7 M, with excellent selectiv-ity for glucose over other saccharides in finger-prick human bloodsamples. The thiophene-based CPs have also been proven to beexcellent mediators, increasing the stability of electrode surfaceswhen they are conjugated with the catalytic materials. Recently,a sensitive and selective NO sensor was fabricated using a zinc-dithiooxamide framework derived from porous ZnO NPs and apolyTTBA-rGO composite. The sensor probe displayed an outstand-ing electrocatalytic performance and was applied to the detectionof NO, which is released from normal and cancer cell lines. Theporous ZnO was immobilized on the polymer-rGO composite layer,which enhanced the surface area and ion-electron transport rate,resulting in efficient catalytic NO reduction [183].

3.7. Others

Even though the conventional polymers and their compositeshave been extensively used, some of the less-known polymerssuch as polyazulene, polyindole, and polycarbazole, have also beenutilized in different electrochemical sensors. In 2008, a label-freeDNA hybridization detection was reported using poly(indole-5-carboxylic acid) conducting polymer. The polymer was formedon the glassy carbon electrode and was used for complimentary,non-complementary, and single nucleotide mismatch detection[184]. In 2016, the ZnO functionalized poly(indole-5-carboxylicacid) nanocomposite was also tested for DNA detection [185]. Simi-larly, poly(indole-6-carboxylic acid) was also used for the detectionof DNA hybridization [186]. The poly(indole-6-carboxylic acid) wasdoped on to the MWCNT to form a nanostructure composite for thelabel-free femtomolar detection of hepatitis B virus related DNA[187]. Apart from the DNA detection, the indole-5-carboxylic acid

was also used to fabricate sensors for monitoring pesticides by con-jugating it with iron oxide nanoparticles. The polymer/iron oxidewas bioconjugated with acetylcholinesterase on the glassy car-bon electrode to obtain sensitive detection of pesticides malathion

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30 M.H. Naveen et al. / Applied M

nd chlorpyrifos [188]. The tetracyanoquinodimethane-conjugatedoly(indole-6-carboxylic acid) was used for the selective oxi-ation of dopamine [189]. For the oxidation of formic acid aold-composited polyindole was synthesized, where Au clustersere embedded in the cages of polymer flakes to form an interfacialu-polyindole composite [190].

Along with polyindole, polyazulene and its composites werelso studied for different applications. At early stage, electro-hemical growth and spectroscopic properties of polyazuleneere studied using impedance and in situ spectroelectrochem-

cal techniques at platinum electrodes [191]. Recently, azulenend thiophene acetic acid copolymer was formed on the electrodeurface to fabricate a sensor for the detection of H2O2 [192]. Innother report, p-type polyazulene was composited with n-typeolybenzimidazobenophenenthroline-poly(ethylene oxide) (BBL-EO) to form a donor–acceptor layer. This composite layer was usedor the electrochemical and spectrochemical studies [193]. Theolyazulene layer was electropolymerized to obtain a hydrophobicolid-state electrode for the selective identification of potassiumons. Polarization of polyazulene solid contact was implementeds a convenient approach to address the issues of irreproducibletandard potential of solid-state potassium ion-selective electrodes194]. Polyazulene and poly(3-[(E)-2-azulene-1yl)vinyl]thiopheneere prepared by electrochemical methods in various config-rations on to the platinum substrate [195]. Both monolayer-nd bilayer-modified electrodes with these materials were ana-yzed for their electrochemical performance as sensing probe

aterials for the detection of dopamine and 4-nitrophenol usingquare wave voltammetry. Some of the other polymers, suchs poly-ophenylenediamine, were conjugated on a single-walledarbon nanohorn. The polymer was molecularly imprinted topecifically recognize kanamycin molecules and the electrodeas used for its electrochemical detection [196]. Similarly,

unctional dicarbazole conductive polymer was electrochemi-ally polymerized and characterized for different electrochemicalpplications [197].

. Conclusions

We have summarized the preparation, functionalization, andecent advancements in the application of conducting poly-er conjugates in the fabrication of biosensors. The publishedorks suggest that among the conducting polymers, conjugates

f polyaniline, polypyrrole, and polythiophenes show promisingharacteristics for future applications. Furthermore, the poly-erthiophene composites have demonstrated a reasonably hightability and electrical conductivity compared to the other polymeromposites. Along with the conventional single polymer systems,any researchers are currently combining two or more polymersith a catalytic nanomaterial to form a highly sensitive conjugatedaterial that generates very sensitive signals when used for any

ype of sensing application. These nanomaterials with multipleunctional groups can provide the most suitable platform for thettachment of different recognition elements. These studies openhe way for a new class of materials, in which the unique char-cteristics of various materials can be combined to form a singleultifunctional material.

cknowledgement

This work was supported by the National Research Foundationf Korea (NRF) grant funded by the Korea government (MSIP) (No.015R1A2A1A13027762).

als Today 9 (2017) 419–433

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168] K.S. Lee, M.S. Won, H.B. Noh, Y.B. Shim, Triggering the redox reaction ofcytochrome c on a biomimetic layer and elimination of interferences forNADH detection, Biomaterials 31 (2010) 7827–7835.

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