Elaborate Chemical Sensors Based on Graphene

Send Orders for Reprints to [email protected] Current Organic Chemistry, 2015, 19, 1117-1133

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Elaborate Chemical Sensors Based on Graphene/Conducting Polymer Hybrids Jaehyun Hur1, Sung-Hoon Park2,† and Joonwon Bae3* 1

Department of Chemical and Biological Engineering, Gachon University, Seongnam, Republic of Korea 461701; 2Materials Research Center, Samsung Electronic, Yong-In, Republic of Korea 446-712; 3Department of Applied Chemistry, Dongduk Women's University, Seoul, Republic of Korea 136-714 Abstract: Owing to their unique and indispensable physical, chemical, electrical/ electronic, and mechanical properties, graphene and conducting polymers have become attractive materials in all fields of science and technology. This mini-review provides a comprehensive summary on the cutting-edge sensor technologies based on graphene/conducting polymer hybrid materials. In this article, we briefly mention the preparations and characteristics of graphene and conducting polymers. In addition, the expected synergistic effects resulting from the combination of graphene and conducting polymers as sensing media are addressed. Subsequently, an extensive and succinct summary on the impressive chemical and biological sensors using graphene/conducting polymer hybrid materials will be covered. A short introduction to the emerging sensors using the elegant nanocomposites will also be described. Finally, perspectives and challenges of graphene/conducting polymer sensors are outlooked. It is obvious that this article is important for understanding the current sensor technologies and offers essential information to the researchers in diverse fields.

Keywords: Conducting polymer, graphene, hybrid, sensor. 1. INTRODUCTION Recently, a tremendous effort has been devoted to the research on graphene and graphene based materials. Graphene is a monolayer of graphite with a honeycomb structure in which carbons have a sp2 hybrid orbital. This two-dimensional material, graphene is dramatically different from other carbon allotropes such as carbon nanotubes and fullerenes [1-3]. Thus it exhibits interesting properties, for example, a high carrier mobility and capacity, a tunable band gap, an ambipolar electric field effect, and quantum Hall effect [4, 5]. In addition, it is natural that graphene has a great potential for energy storage/conversion, displays, and sensors [6-18]. In general, the fabrication methods for graphene are categorized into two strategies, solution process [19-23] and chemical vapor deposition (CVD) [24-29]. Because the CVD method has several advantages over solution based method, diverse CVD-grown graphene nanostructures have been produced [24-29]. Conducting polymers (CPs) such as polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh) and their derivatives are a family of polymers containing a large conjugation/resonance structure with many sp2 hybridized carbons [30]. It allows the intermolecular delocalized charge transport. Since their discovery in 1976, they have become important owing to their easy preparation, attractive features, and wide potential applications [31-33]. Various composites using CPs can utilize the inherent advantages of each component and can demonstrate improved performances owing to synergy effect [34]. Recently, CPs have been combined with graphene or graphene based nanostructures [35, 36]. In addition, elegant nanomaterials such as metal/semiconductor nanocrystals and biomolecules can be incorporated into the hybrid systems to improve resulting perform-

*Address correspondence to this author at the Department of Applied Chemistry, Dongduk Women's University, Seoul, Republic of Korea 136-714; Tel: +82-2-940-4506; E-mail: [email protected] † Current Address : Deparment of Mechanical Engineering, Soongsil University, Seoul, Republic of Korea 156-743. 1875-5348/15 $58.00+.00

ances [37, 38]. That is, Graphene/CP hybrid composites can provide a platform for application in advanced devices such as energy storage/conversion systems, displays, optoelectronics, and sensors. Among these emerging devices, sensors are practically important for end-user applications, diagnosis of diseases, and environmental monitoring [39-45]. It has been well known that graphene is one of the most efficient materials as a sensing medium [46]. Nonetheless, there has been a great research effort to improve the performance, reliability, durability, and processability of graphene sensors by adding a secondary material. It can be a lucrative and promising strategy to employ a class of CPs as a secondary component for elaborate sensor systems [30-32, 47-49]. Therefore, herein, it is meaningful to summarize the recent research activities on the Graphene/CPs hybrid sensors to provide essential information for future progress. First, the intrinsic properties of representative CPs are described shortly for better understanding. Then an extensive review on the Graphene/CPs sensors will be highlighted remarkably. In addition, the challenges that we face for the advancement of technology will also be covered briefly. It can be expected that this article can be of practical help for future research. 2. GENERAL REMARKS 2.1. Advantages of Graphene and Graphene Hybrids as Sensing Material Graphene has also been utilized for new chemical, electrical, and biological sensors thanks to its unique properties [6-9]. It is generally accepted that detection of numerous molecules that have a high oxidation/reduction potential is achievable due to the large electrochemical potential window of graphene [12]. In addition, the presence of defects and edges on graphene layer provides a high electron transfer rate, leading to an enhanced sensitivity [6]. Moreover, graphene can exhibit exceptionally high carrier mobility and density, low intrinsic noise, and facilitated charge transport. The bandgap of graphene can be altered by the incorporation of © 2015 Bentham Science Publishers

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Fig. (1). Chemical structures of some of the most representative conducting polymers.

dopants [9]. In particular, graphene can interact intimately with aromatic compounds. Finally, graphene is also biocompatible [12]. In particular, CVD-grown graphene has recently become impressive as a sensing material owing to optical transparency combined with above mentioned advantages. While graphene sheets accumulated in solution diminish the electron mobility due to the dissipation of electron flow into many layered graphene, CVD graphene produced under vacuum enables the generation of ultra-thin films to minimize the electrochemical resistance between graphene layers. Therefore, CVD graphene can be easily extended to wider substrates to produce flexible, transparent, and recyclable electronic devices, representatively sensors [12]. 2.2. General Properties of Conducting Polymers CPs are an attractive family of materials that combines the advantages of both polymers and metal/semiconductors. The presence of electrochemical activity enables them to be strong candidates for applications in catalysis, sensors, and electronics. CPs have fascinating characteristics owing to the presence of conjugated electron systems (Fig. 1) [30-32]. The oxidation level in CPs is significantly affected by overall doping/dedoping process. Therefore, most CPs have been synthesized by oxidative polymerization mechanism employing proper oxidizing agents. In general, metal salts for example, ferric chloride, ferric perchlorate, ferric nitrate, and copper chloride have been prevalently used to produce CPs. It was demonstrated by Jang et al. that the doping/dedoping could be controlled by the appropriate selection of fabrication methods [50, 51]. Accordingly, most sensor systems based on CPs mainly depend on the electrochemical detection mechanism [51-53]. Recently, CP nanomaterials have emerged as strong candidates for diverse elegant applications such as flexible electronics, energy devices, and ultrasensitive sensors. They must inherit the fascinating properties of their bulky components and also possess unique characteristics associated with high surface area and ultrasmall structures. This leads to enhanced interactions between incorporated guest molecules, which would be recognized as analytes. Also, the engineered nanostructure permits more rapid signal transduction and fast recovery compared with the bulky counterparts. Therefore, it can be inferred that the CP nanomaterials can be promising as sensing media [50, 51]. 2.3. Fabrication of Conducting Polymer Based Materials The development of synthetic routes to CP based materials is an interesting research subject in contemporary science and technology. The synthetic ways to produce CP based materials can be classified into three major categories such as hard-template, softtemplate, and template free synthesis. First, the hard template way is desirable for tailoring a precise morphology of resultant materi-

als, especially nanomaterials [54-56]. Exciting nanostructures such as core/shell, nanocapsules, and nanotubes can be fabricated using various well-known hard templates such as colloidal silica, membranes, and mesoporous media. An alternative approach has appeared to effectively produce CP based materials. This strategy encompasses a concept that use micelles from the surfactants composed of block copolymers and polymeric electrolytes as templates [57-59]. In this case, it has been reveal that the morphology of generated micelle is strongly dependent on the various parameters such as ionic character, concentration, and chain length of surfactant, critical micelle concentration, temperature, additive and cosurfactant, and solvent. At last, template-free path is very straightforward without employing templates. On the other hand, this method is limited to designated precursors. The first demonstration for this method was reported by Kaner and co-workers [60]. Jang et al. have showed the mass production of PANI nanorods by a dispersion polymerization [61]. They described that the dispersion polymerization was conducted with hydrochloric acid as a dopant and ammonium peroxydisulfate as an oxidizing agent. It was possible to produce PANI fibers with a high yield in gram scale (Fig. 2).

Fig. (2). Scanning electron microscope (SEM) image of polyaniline nanorods synthesized by dispersion polymerization. (Adapted from reference [61]).

It has become clear that these three common methodologies are suitable for the successful fabrication of micro and nanoscale CP based materials having tailored morphologies. In particular, nanostructured materials show very unique properties due to their high surface areas and small dimensions. By virtue of the high surface area, CP nanomaterials are capable of exhibiting enhanced performance via subtle interaction between sensing medium and analyte. The rapid adsorption/desorption behavior leads to fast response/recovery time even under ambient conditions. Conse-

Elaborate Chemical Sensors Based on Graphene/Conducting

Current Organic Chemistry, 2015, Vol. 19, No. 12

a

b

c

d

e

f

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Fig. (3). Typical shapes of CP nanomaterials. a) PPy nanoparticle, b) PANI nanofiber, c) PPy nanotubes, d) PANI nanofilm, e) nanocrystal on PPy nanofiber, f) nanocrystal on PPy nanotube.

electron donors (e. g. Na). Through this doping process, defects such as polaron, bipolaron, and soliton are introduced, which would be available as the charge carriers (Fig. 4). The way in which charges can be stabilized on the polymer chains and the nature of the charge transport are still unclear. The "doping" process is often referred by analogy with the doping of inorganic semiconductors, but this term is not correct because a redox reaction happens in a polymer and the insulating polymer is converted to an ionic complex. The doping reactions are usually summarized as CPneutral chains + n(A-)aq + mS
[CP+n An-Sm] + ne

(Eq 1)

CPneutral chains + n(C-)aq + ne + mS
[CP -n Cn-Sm]

(Eq 2)

-

Fig. (4). A schematic diagram of the formation of (b) polaron, (c) bipolaron, and (d) soliton pair on a conjugated polymer chain [62].

quently, there has been a great interest in the preparation and application of CPs based nanomaterials as supported by the rapid increase in patent and publication. To date, numerous CPs nanomaterials have been prepared by diverse strategies as summarized in Figure 3. Nanoparticles, nanofibers, and nanotubes can have been prepared by typical wet emulsion systems (Fig. 3a-c). In addition, multi-dimensional CP nanostructures can have been realized by combined approaches, for example, template method and electrospinning (Fig. 3d~f). These materials might be more lucrative as sensing media owing to the intriguing nanoscale architectures [50]. 2.4. Charge Transport in Conducting Polymers It is remarkable that the conjugated chemical structure with alternating single and double bonds is necessary for polymers to become intrinsically conducting. This is because the polymers must have not only charge carriers but also an orbital system that allows the charge carriers to move. Thus, polyacetylene has been widely studied as a typical conducting polymer. Because most polymers do not possess inherent charge carriers, the carriers must be provided by oxidation with electron acceptors (e. g. I2) or reduction with

+

where the aq, A , C , e, and S means aqueous, anion, cation, electron, and solvent, respectively. In addition, m and n are stoichiometric coefficients. In a word, CPs have a considerable overlap of delocalized pi-electrons along the polymer chains (Fig. 1). However, this is insufficient for electrical conductivity, for which a redox perturbation is compulsory. A large range of conductivity can be obtained upon doping with both n and p type dopants, as well as protonic acid. Charges introduced into the backbone polymers through doping are embedded in different states as shown in Figure 4. Therefore, the nature of charge in CPs is dependent on material and doping level. When a charge moves through an insulating crystal, it will be permanently surrounded by a lattice polarization. A polaron is a moving charge accompanying with the polarization field. In chemical terminology, a polaron is just a radical ion. When a secondary electron is added to the polaron, it is called a "bipolaron". In case a secondary electron is added to anywhere else on the polymer chain, it is denoted as a "soliton" [62]. At present, prediction of charge carrier mobility is practically impossible. Therefore, reliable experimental data are required to find out transport parameters and models. In addition, the question of whether polarons or bipolarons are the more stable components in CPs is still under extensive debate. 2.5. Conducting Polymers as Sensing Media As mentioned earlier, most sensor systems based on CPs depend on the electrochemical detection. The major response mechanism of CPs is composed of oxidation/reduction, swelling, confor-

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#PCN[VGU

 4GEQIPKVKQP

%QORWVGT

 6TCPUFWEVKQP

&37 

(

UQWTEGOGVGT

 UKIPCN

(

VXEVWUDWH EWTTGPV Fig. (5). Schematic illustration of an electrochemical sensor consisting of a conducting polymer film, electrodes, and substrate (T: transducer, E: electrode). The conducting polymer plays the role of transducer [53].

mational change, and charge transfer. From the viewpoint of sensor, the most striking features of CPs are [47, 50-53]. a. easy synthesis by electrochemical and chemical polymerization; b. high sensitivity and selectivity toward a broad range of analytes under room temperature; c. facilitated signal transport and electrical conductivity; d. possibility for convenient modification of polymer backbone structure; e. tunability of inherent properties of CPs; f. facile fabrication of devices by both wet and dry methods. These desirable properties allow the miniaturization and mass production of sensors. In general, CPs are really sensitive to their surrounding environments. Therefore, they have been particularly suitable for sensor transducers. At the same time, there are several dominant parameters affecting the performances of CP sensors such as sensitivity, selectivity, surface area, stability, surface properties. If these conditions are secured, a representative mechanism is the electrochemical sensing described in the following sections. Most CP sensors depend on the electrochemical detection such as amperomentric, potentiomentric, and conductometric mechanisms, as will be summarized in the Table 1-3. The electrochemical sensor senses the charge transport of CPs induced by exposure to some analytes. Subsequently, the change can be correlated associatively to the concentration of the analytes [52, 53]. 2.6. Role of the Conducting Polymers in Sensors Conducting polymers can play a highly critical role as a main component of diverse elegant sensors. They might serve as catalyst, redox mediator, on/off switch, and resistor, at the same time, act for molecular recognition and pre-concentration of analyte [63]. Even if conducting polymers are generally poor catalysts for chemical reactions, however, conducting polymers are generally known to reduce the overpotential for an electroactive analyte, thus enhancing the signal-to-noise ratio. Sometimes, conducting polymers are used as matrices for the immobilization of truly efficient catalysts. The above-mentioned overpotential is mainly derived from slow electron transfer between redox analytes and electrodes. That is, the presence of conducting polymer provides a threedimensional reaction layer for high redox reaction efficiency. The use of redox mediator is particularly effective for biological sensors.

Due to enormous changes in conductivity upon variations in the redox state, conducting polymers may act as chemically sensitive switches dependent on redox species. PPy, PANI, and poly(3methylthiophene) can show this behavior [63]. An electro-sensing performance is functionally enhanced if a material has an ability to pre-concentrate the analyte with selective molecular interactions. Most of the materials other than conducting polymers have exhibited somewhat limited properties. Thus, conducting polymers can overcome this limitation significantly. 2.7. Outline of this Article Judging from context deferred from the previous explanations, it is certain that the Graphene/CPs hybrids will be great materials for sensors. Therefore, in the following sections, we will discuss impressive research activities on Graphene/CP sensors concisely. Numerous sensors using Graphene/CP hybrids are covered depending on the type of CP. It would become clear that most of the sensors can be classified as chemical/electrochemical/biological sensors. Subsequently, miscellaneous sensors using Graphene/CP will also be addressed briefly. At last, challenges and outlook will be presented. 3. CHEMICAL SENSORS BASED ON GRAPHENE/CONDUCTING POLYMERS It has been considered during the last decades that reliable, fast, and accurate detection of analytes is critical for progress of technology. To date, an immense research effort has been devoted to the development of graphene sensors as reviewed by famous researchers [64-70]. In addition, most popular CPs have been introduced for performance improvements in the sensors. In this section, it is necessary and meaningful to discuss representative sensing mechanisms briefly. As we mentioned earlier, most CP based sensors rely on the electrochemical sensing mechanism. Simultaneously, field effect transistor (FET) type sensors have attracted much attention because they can deliver a high selectivity and enhanced signal transduction. Figure 5 illustrates a scheme for an electrochemical sensing system based on a conducting-polymer. The sensor system recognizes the charge transport in conducting polymers induced by exposure to analytes. The change can be monitored by the electronic components. The complete sensing cycle involves analyte recognition, signal transduction, and electrical layout [53]. On the other hand, Figure 6 describes a basic architecture of FET type sensor. It can offer interfacial events at molecular level, label-free recognition, simplicity, and low volume. This strategy

Elaborate Chemical Sensors Based on Graphene/Conducting

has been used for signal amplification to enhance the sensing capability. Under this configuration, the drain-to-source current was measured by varying the gate voltage (Vg) to investigate the electrical characteristics of the sensing material. Then, it is possible to determine the behavior (n or p type) of sensing media. It is remarkable that graphene might show an ambipolar behavior. Moreover, this structure provides better stability and durability under operation. When the CP nanomaterials are used as conductive channel of FET sensors, they can bypass the signal loss by lateral current shunting, then can provide improved performance through depletion/accumulation of charge carriers in bulk of the nanomaterials [51~53].

Fig. (6). Schematic diagram of an FET type sensor.

For instance, because PPy is a p-type semiconductor under a doped state, the negative Vg can give rise to an increase in the oxidation level of PPy chains. A variation in potential originated from PPy and analyte interactions can affect the source-drain current in a similar manner to the effect of applying Vg. Consequently, the successful employment of PPy as an electrochemical sensing medium can be confirmed by the apparent dependence of source-drain current on Vg. Finally, the chemiresistive sensor is dependent on the change in normalized electrical resistance calculated by (R-R0)/R0, where R and R0 denote the real time and initial resistance [52]. The sensitivity of the chemiresistive sensor upon exposure to analyte is defined as the normalized resistance change upon exposure to a chemical

Fig. (7). Polymerization mechanism of pyrrole. (Adapted from reference [50]).

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and inert gas for a fixed time. This system is relatively poorly reproducible and reliable. Thus it has been replaced by more accurate electrochemical or FET type sensors. 3.1. Graphene/Polypyrrole (PPy) Chemical and Biochemical Sensors It has become well-known that graphene sensors can detect most of common analytes [64-70]. Therefore, it can be inferred that the use of CPs is targeted to monitor specific chemicals/molecules as supported by extensive literatures. Above all, it is conspicuous that the molecular imprinting is an interesting and lucrative sensing method as described in the following sections. The most widely used conducting polymer, PPy is selected to start discussion. Table 1 summarizes the representative Graphene/PPy sensors and Figure 7 shows the chemical structure of PPy synthesized by electrochemical oxidation polymerization. The most popular type is an electrochemical sensor. Thus we have focused on the kind of analyte and sensing material. The detection of toxic gases (such as NH3 (ammonia) and NOx) and chemicals is still important for Graphene/PPy sensors [71-79]. It has become obvious that sensitivity, repeatability, and fast recovery are the most important issues associated with the gas sensors. Some of the cases are introduced in this section. A useful sensor was made using chemically reduced graphene oxide (RGO) in which graphene oxide (GO) was reduced by the introduction of pyrrole vapor on Au electrode [71]. It was reported that RGO obtained by pyrrole vapor exhibited a sensitive (more than 2.7 times at 50 ppm of NH3) response to NH3 than that of pristine RGO. They also claimed that the self-assembled sensor device had responsive repeatability to ammonia vapor and also demonstrated excellent reproducibility to ammonia gas. This strategy was applied to the realization of an ultrafast and sensitive sensor for ammonia gas. Hu et al. reported that the sensing performance of RGO reduced by pyrrole vapor showed a response time of 1.4 s and a detection limit of 1 ppb [72]. The RGO prepared with pyrrole vapor had advantages of easy preparation, scalability, low power consumption, and

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Table 1. A brief summary of Graphene/PPy sensors.

Material

Type

Target

Detection Limit

Ref.

RGO reduced by PPy

Resistive

NH3

50 ppb

[71]

RGO reduced by PPy

Resistive

NH3

1 ppb

[72]

Graphene/PPy

Resistive

NH3

Recovery time < 5 min

[73]

Graphene/PPy nanofiber

Resistive

Gas (NO2)

Graphene/PPy/chitosan

Electrochemical

Nitrite

Graphene/PPy

Resistive

Volatile Organic Compounds

Graphene/CdS/PPy

Photo-

(molecular Imprint)

electrochemical

RGO/PPy (molecular Imprint) GO/cyclodextrin/Au GO/PPy (molecular Imprint) Graphene/CNT/ Chitosan/Ag Graphene/Au/PPy (molecular Imprint) Graphene/PPy (molecular Imprint)

[74] 0.1
M As low as 0.06 g/g sample

[75] [76]

4-aminophenol

2.3x10-8 M

[77]

Electrochemical

Tetrabromo- bisphenol A

0.23 nM

[78]

Electrochemical

Chrysoidine

1.7x10-8 M

[79]

Electrochemical

Quercetin

4.8x10-8 M

[80]

Electrochemical

Neomycin

7.63x10-9 M

[81]

Electrochemical

levofloxacin

0.53
M

[82]

Electrochemical

Trimethoprim

1.3x10-7 M

[83]

Aflatoxin B-1

1 fM

[84]

Microcystin-LR

3.7x10-17 M

[85]

Graphene/PPy/Au/

Electrochemical

Ionic liquid

(Immunosensor)

Graphene/PPy/Au/

Electrochemical

Ionic liquid

(Immunosensor)

RGO/PPy/Au

Electrochemical (Biosensor)

Organophosphorus pesticides

0.5 nM

[86]

Electrochemical

Sildenafil

6.2 nM

[87

Electrochemical

Psychotropic drugs

Pico gram/mL

[88]]

Electrochemical

Butylated hydroxyanisole

7.63x10-8 M

[89

Electrochemical

Dopamine

1.0x10-11 M

[91]

Resistive

Dopamine

18.29 pM

[92]

Electrochemical

Dopamine

0.1
M

[93]

Electrochemical

Tryptamine

7.4x10-8 M

[94]

Graphene/Au/PPy

Electrochemical (biosensor)

H2O2

0.67 nM

[95]

RGO/PPy nanotube

FET

H2O2

100 pM

[96]

Graphene(enzyme)/PPy

Enzymatic

Glucose

3
M

[97]

RGO/PPy

Electrochemical

Hg2+ & Pb2+ ions

4 pM

[99]

RGO/PPy

Electrochemical

Metal ions

[100]

Graphene/PPy

Electrochemical

Humidity

[101]

RGO/PPy (Molecular Imprint) Graphene (NH2)/PPy (Molecular Imprint) Graphene/Prussian blue PPy (Molecular Imprint) CNT/PPy (Molecular Imprint) RGO/PPy/Au Graphene/PPy (over oxidized) Graphene(SO3)/CNT/ PPy (Molecular Imprint)

Elaborate Chemical Sensors Based on Graphene/Conducting

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&% H GNGEVTQFG

&G6 K

8KUKDNG .KIJV

0 , 3 +PVGTCEVKQP

9% 6CTIGV

Fig. (8). A proposed mechanism for photoelectrochemical sensing [77].

low cost. They also insisted that the sensor was ultrasensitive and had a short recovery time. It has become certain that the combination of PPy with graphitic materials such as graphene, RGO, and GO can produce a more sensitive and rapid sensor. Lee and coworkers prepared nanocomposites by in situ polymerization of pyrrole on the surface of graphitic materials [73]. A performance improvement is associated with the promoted electron charge transfer between PPy and analyte interface, and the efficient transfer of electrical resistance variation was attributed to the uniformly dispersed RGO in PPy matrix. The sensitivity was excellent even at 100 °C. While they did not provide an accurate detection limit, instead, they achieved an ultrafast recovery time of a couple of minutes. Hur’s group has prepared RGO/PPy nanofiber composite using a one-step redox reaction under UV radiation at ambient conditions [74]. It was interesting that GO was effectively reduced to RGO without any chemical agent during the formation of PPy. This nanocomposite sensor exhibited 4 times higher sensitivity to NO2 than that of pristine RGO. It was demonstrated that the detection of nitrite using a modified glassy carbon electrode coated with a Graphene/PPy/chitosan film [75]. The electron transfer behavior in this system was examined and it revealed that a linear response was obtained in the range of 0.5-722 M with a detection limit of 0.1 M for nitrite. On the other hand, volatile organic compounds (VOCs) can be a potential hazard to environment. Thus it is important to monitor VOCs accurately. In general, electrochemical sensors have been appropriate for detection of organic molecules. Zhang et al. designed a sensor based on PPy composite solid-phase microextraction (SPME) fiber [76]. PPy/beta-naphthalenesulfonic acid (PPy/beta-NSA) and Graphene/PPy composite SPME fiber coatings were involved. It could be inferred that the PPy composite SPME fiber coatings achieved an elevated extraction capacity and selectivity for the polar VOCs having conjugate structures (such as 4-heptanone, 4-heptanol, 4-nonanone, methyl 5-methylsalicylate, nonane, decanal, undecanal, and dodecanal) due to the inducing polar functional groups in the PPy. In particular, the average contents of nonane, decanal, undecanal, and dodecanal from coriander samples were found to be 0.79, 0.13, 0.06, and 0.21 g/g sample, respectively. The combination of graphene with PPy enabled the detection of a wide spectrum of molecules [77-79]. A sensitive photoelectrochemical sensor for 4-aminophenol was constructed on an electrode modified with CdS nanocrystal, graphene, and molecularly imprinted PPy [77]. It looks interesting to see a diagram of photoelectrochemical sensing mechanism, therefore, Figure 8 shows a proposed mechanism for photoelectrochemical sensor. They explained that adsorbed 4-aminophenol molecules were quickly oxidized on

the electrode by photogenerated holes from CdS under visible light irradiation, resulting in increased photocurrent response. It was obvious that graphene facilitated electron transfer at the CdS modified electrode. A response to 4-aminophenol on the modified electrode was enhanced owing to the binding of molecularly imprinted PPy with 4-aminophenol. The response was linear in the range of 5.0 x 10-8 mol/L to 3.5 x 10-6 mol/L, with a detection limit of 2.3 x 10-8 mol/L. A similar strategy was employed for tetrabromobisphenol A detection using a surface molecular imprinted sensor [78]. It was shown that the response was linear in the range of 0.5-4.5 nM with a detection limit of 2.3 nM. It was feasible to detect a dye molecule chrysoidine using an imprinted electrochemical sensor based on glassy carbon electrode modified with GO/betacyclodextrin/Au nanoparticles composites [79]. It has also been demonstrated that the molecularly imprinted electrochemical sensor was fabricated by electropolymerization using chrysoidine and pyrrole as template molecule and functional monomer, respectively. The signal detection was available in the range of 5.0 x 10-8 mol/L to 5.0 x 10-6 mol/L, with a detection limit of 1.7 x 10-8 mol/L. The precise and timely monitoring of biomolecules is very important for comfortable human life [80-90]. For this purpose, molecular imprint strategy has been widely used. The molecular imprinting has become a commonly accepted methodology for recognition of biological target molecules. Combined with template molecules, monomers can build multiple interactions, which are recorded through polymerization. After removal of template molecules, cavities remain which allow them to distinguish template molecules. Combined with the molecular imprinting, the preparation of molecularly imprinted polymers in electrochemical or biological sensors has considerable possibility for applications. One of the antiseptics, quercetin could be detected with a sensor using molecularly imprinted PPy film incorporated with GO [80]. The possible detection range was 6.0 x 10-7 mol/L to 1.5 x 10-5 mol/L, with a detection limit of 4.8 x 10-8 mol/L. The technique could be applied for the identification of rutin or morin having a similar structure. An antibiotic, neomycin derived from the Streptomyces fradiae could also be monitored by a sensor developed on Au electrode decorated with chitosan-Ag nanoparticles/Graphene/CNT [81]. It was remarkable that electropolymerization was conducted to produce molecularly imprinted polymers using neomycin as the template, and pyrrole as the monomer. The linear range was found to be 9.0 x 10-9 mol/L to 7.0 x 10-6 mol/L, with a detection limit of 7.6 x 10-9 mol/L. A strategy was developed for the levofloxacin sensor based on molecularly imprinted polymer incorporation with graphene-Au nanoparticles [82]. The molecularly imprinted PPy served as the recognition element. In particular, Graphene/Au dra-

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GMs

PR patterns 1. RIE

PL

2. PR removal 2

Flexible graphene film

PR/graphene film

1. PL 2. Cr/Au deposition 3. Lift-off

HIV-2 Ag CPPyNPs

Cr/Au

1. PL 2. DAN/ DMT-MM

1. DMT-MM 2. HIV-2 Ag

3. CPPyNPs 4. PR removal

GM nanobiohybrids

CPPyNP-GMs

GM microelectrodes

Fig. (9). Schematic illustration of the FET
type graphene hybrid immunosensor based on graphene with close
packed PPy nanoparticle on the flexible substrate (PL: photolithography, PR: photoresist, RIE: reactive-ion etching, GM: graphene micropattern). (Adapted from reference [90]).

Liguiid-io o on Liguid-ion gate dielectric

Vg

y CPPyNPs HIV-2 Ag

S

D Flexible substrate

Fig. (10). Schematic illustration of the graphene micropattern nanobiohybrids operated by liquid-ion gating (S: source, D: drain, and Vg: gate voltage). (Adapted from reference [90]).

matically elevated the electrooxidation of levofloxacin. The signal was proportional to the concentration of levofloxacin in the range of 1.0-100 M, with a detection limit of 0.53 M. The sensor was reliably reproducible. A synthetic antimicrobial agent, trimethoprim was analyzed by a sensor, developed by electropolymerization of pyrrole and molecularly imprinted polymer on glassy carbon electrode [83]. It was natural that the sensor displayed a high selectivity and sensitivity with a linear range of 1.0 x 10-6 mol/L to 1.0 x 10-4 mol/L showing a detection limit of 1.3 x 10-7 mol/L. An immunosensor for aflatoxin B-1 was realized. The sensor was composed of a Graphene/CP/Au nanoparticles/ ionic liquid composite film on Au electrode [84]. The CP promoted adhesion of antibody and charge transfer while the Au nanoparticles warranted a fast electron transfer. In addition, the ionic liquid offered a benign environment. The dynamic range was measured to be 3.2 fM – 0.32 pM with a detection limit of 1 fM. An indicator of eutrophication in water, microcystin could also be detected with a similar sensor [85]. It was impressive that electron transfer rate was estimated by modeling. The electrochemical response was linear between 1.0 x 10-16 mol/L to 8.0 x 10-15 mol/L with a detection limit of 3.7 x 10-17 mol/L. A new type of biological sensor for organophosphorus pesticides was obtained with a RGO/PPy/Au nanoparticle hybrid on electrode [86]. Acetylcholinesterase (AChE) was immobilized on the RGO/PPy/ Au nanoparticle hybrid. It showed a high electrical conductivity and electrocatalytic activity. The rapid and sensitive detection was demonstrated in the range of 1.0 nM to 5.0 M with a detection limit of 0.5 nM. It is also interesting to monitor a major component of sexual health product-sildenafil by emerging sensors [87]. A

molecularly imprinted polymer with p-phenylenediamine as a monomer was constructed on a RGO coated glassy carbon electrode. The detection limit was found 6.2 nM. An interesting disposable sensor array for psychotropic drug determination was constructed with a molecularly imprinted polymer (pyrrole as monomer) film on electrode [88]. Important experimental parameters controlling the performance of sensor array were investigated and thus optimized. The sensor showed detection limits down to 3.3 and 8.9 pg/mL for methcathinone and cathinone in a practical serum, respectively. This work provided a promising potential in clinical applications. A common antibiotic in foodstuffs, butylated hydroxyanisole was also identified with a molecular imprinting technique using pyrrole [89]. Recently, an interesting research on flexible HIV immunoassays was reported by Jang and co-workers [90]. Figure 9 illustrates the fabrication process for an HIV immunoassay based on graphene layer with close-packed PPy nanoparticle (CPPyNPs) arrays in a flexible system. A flexible graphene film had been obtained by CVD and subsequent dry-transfer. The graphene layer was patterned with uniform shapes and sizes by conventional photolithography (PL). Thermal evaporation was used to construct the source and drain electrodes followed by a lift-off process. The surface of exposed graphene was engineered to immobilize the PPy nanoparticle on the side plane of graphene via treatment with 1,5-diaminonaphthalene (DAN). HIV-2 Ag was then covalently attached to the PPy nanoparticle surface. The close-packed HIV-2 Ag-PPy nanoparticle arrays had rough surfaces, because of the attached HIV-2 Ag. The minimum detection level of this sensor was 1 pM, ten times higher than that of graphene/Ag hybrid. Figure 10 displays the schematic diagram of immunosensor. The intriguing properties of CPs endow graphene/CP a tremendous potential as sensors for physiological active substances such as hormones, vitamin, neurotransmitters, and enzymes [91-98]. In this article, we focus on sensors used for dopamine and hydrogen peroxide detection. Qian et al. studied the detection of dopamine using molecularly imprinted PPy coated carbon nanotubes [91] and RGO/Au nanoparticle [92] hybrids, in a respective research work. The PPy/CNT sensor showed the detection limit of 1.0 x 10-11 M in the linear range of 5.0 x 10-11 ~ 5.0 x 10-6 M [91]. On the other hand, RGO/Au nanoparticle sensor exhibited an extremely low detection limit of 18.29 pM due to the presence of Au nanoparticles [92]. Dopamine can be monitored in the presence

Elaborate Chemical Sensors Based on Graphene/Conducting

of ascorbic acid by overoxidized Graphene/PPy on a glassy carbon electrode [93]. It was impressive that the system showed two linear ranges with a transition point at 25.0
M. The detection limit was 0.1
M. Huang and colleagues reported the determination of tryptamine using a molecularly imprinted film at graphene/PPysulfonated/hyaluronic acid-multiwalled carbon nanotubes modified electrode [94]. Tryptamine is a monoamine alkaloid found in plants, fungi, and animals and is believe to play a role as neurotransmitter or neuromodulator. At the same time, it provides a backbone structure for a group of chemicals such as neurotransmitters and psychoactive drugs. It was remarkable that the selectivity was good even in the presence of interferents such as tyramine, dopamine, and tryptophan. A linear signal was observed in the range of 9.0 x 10-8 ~ 7.0 x 10-5 M with a detection limit of 9.0 x 10-8 M. Hydrogen peroxide-an important chemical for the prevention of cancer could have been examined by CPs incorporated sensors [95, 96]. A CP, 2,5-di-(2-thienyl)-1-pyrrole-1-(p-benzoic acid) was electropolymerized on a modified electrode after deposition of Au nanoparticles and RGO [95]. The resulting GRO/Au/functional PPy sensors demonstrated an enhanced signal transduction with a detection limit of 0.67 nM. Surprisingly, the sensor performance was retained even after 3 months storage at 4 °C. A field effect transistor type hydrogen peroxide sensor was reported by Jang's research group. They introduced a Graphene (RGO)/PPy nanotubes nanocomposite as a conductive channel [96]. In addition, the same research group has reported extensively the activities on the fabrication and use of conducting polymer nanostructures with tailored architectures. It was important to secure a reliable electrical contact between graphene and PPy nanotubes. They overcame this by employing an ionic liquid as gate. The detection limit was as low as 100 pM with a conspicuous reproducibility. The PPy/Graphene/glucose oxidase enzymatic biosensor was used for in vivo electrochemical glucose detection [97]. They demonstrated successfully the feasibility of conjugating glucose oxidase to graphene and used this as a platform for the electrochemical sensing of glucose with an excellent sensitivity of 3 M. They also revealed that PPy-graphene-glucose oxidase electrodes exhibited a better performance than pristine PPygraphene counterparts owing to the presence of enzyme. This is a good example of enzyme immobilization to graphene and graphene/CP hybrids. Kwon et al. has developed a novel sensor based on graphene derived from CP precursor. Few layer graphene was prepared from electropolymerized PPy precursor on Cu substrate [98]. In particular, the graphene was transferred onto a flexible substrate such as PET. The sensor employed antivascular endothelial growth factor (VEGF) RNA aptamer conjugated graphene as a practical sensing medium. The aptasensor displayed improved reusability, bendability, and durability with an exceptionally low detection limit of 100 fM. In addition, miscellaneous sensors have been realized [99-101]. Heavy metal ions such as Pb2+ [99] and Hg2+ [100] could be monitored quantitatively by RGO/PPy sensors with a low detection limit of several pM. A humidity sensor was reported using Graphene/PPy material [101]. The sensing capability was reliable in the relative humidity range of 12~90 % with an even higher sensitivity value (S=138). Finally, a Graphene/PPy hybrid was utilized as an actuator [102]. 3.2. Graphene/Polyaniline (PANI) Chemical and Biochemical Sensors Polyaniline is one of the most popular CPs due to their various electronic states as presented in Figure 11. As PANI is a major class

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of CPs, it has been exploited in various application systems. In particular, the electrically conducting nanocomposite of PANI and CNT has been used as an efficient sensing medium as reviewed previously [103]. In general, PANI has been synthesized by electrochemical processes. That is, in this case, the main challenge is to isolate PANI in the emeraldine salt (ES) form. It is generally known that PANI shows a relatively poor processability compared with other CPs [104].

Fig. (11). Reversible doping/dedoping of polyaniline. (Adapted from reference [50]).

On the other hand, the electrochemical properties and behaviors of Graphene (GRO)/PANI nanocomposites have been investigated extensively [105-110]. Because PANI has been considered as a strong candidate for electrode material in capacitors, the electrochemical performances of Graphene/PANI hybrids were also examined by numerous research groups [106-109]. The accurate revealing of charge/discharge pathways in Graphene/PANI nanocomposite was also pursued [107]. To develop water-dispersible PANI and PANI/graphene composite, poly(styrenesulfonic acid) was introduced [110]. It is reasonable that the general trends witnessed in the previous section are also eligible for Graphene/PANI sensors. Table 2 overviews the various sensors using Graphene/PANI based materials as sensing media. As seen in the Graphene/PPy sensors, the sensitive monitoring of toxic gases or chemicals is the most challenging task. It has been accessible to measure directly/indirectly the level of toxic gases/chemicals with the aid of PANI [111-122]. Al-Mashat et al. prepared graphene by a chemical synthetic route and ultrasonicated with a mixture of aniline monomer and initiator (ammonium persulfate) to form PANI [111]. They used this composite as a hydrogen sensor and compared the performance with that of pristine graphene and PANI. The sensor sensitivity was 16.57 % toward 1 % of hydrogen gas. The graphene nanosheet/PANI with an improved conductivity due to the presence of
-
conjugation was successfully employed into methane sensor [112]. It was conspicuous that the detection range of the sensor was enlarged by the addition of PANI. The suffocating ammonia (NH3) gas was detected by Graphene/PANI hybrid sensors [113-115]. Zhang and co-workers reported the sensitive detection of ammonia with RGO/PANI sensors [113, 114]. The sensor exhibited a response of 37 % at 50 ppm NH3. The sensing range was extended significantly by the introduction of PANI [115]. The linear signal observed in the range of 1-6400 ppm of ammonia. The detection limit was 1 ppm, much

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Table 2. A brief summary of Graphene/PANI sensors.

Material

Type

Target

Graphene/PANI

Resistive

H2

Resistive

CH4

RGO/PANI

Resistive

NH3

Ref 114 37.1%@50ppm

[113, 114]

Graphene/PANI

Conductometric

NH3

1 ppm

[115]

GO/PANI

Electrochemical

Methanol

ppm level

[116]

Graphene/PANI

Electrochemical

Toluene

100 ppm

[117]

Graphene/PANI

Electrochemical

Hydrazine

15.38 mM

118]

Electrochemical

Organic Pollutants

Graphene/PANI

Electrochemical

4-aminophenol

Graphene/Pd/PANI

Electrochemical

Hydroquinone & Catechol

RGO/PANI/Pd

Electrochemical

Bromate

1
M

[122]

Gonadotropin

0.286 pg/mL

[123]

Etodolac

10 ng/mL

[124]

Salbutamol

0.04 ng/mL

[125]

Oxytetracycline

2.3x10-6 mg/L

[126]

Electrochemical

Serotonin

11.7 nM

[127]

Electrochemical

Dobutamine

[128]

Gallic acid

[129]

Graphene(nanosheet)/ PANI

Graphene/cyclodextrin/ Poly(N-acetylaniline)

Graphene/PANI Graphene/PANI/Bi2O3

Electrochemical (Immunosensor) Electrochemical

Graphene/PANI/

Electrochemical

Poly(acrylic acid)/Au

(Immunosensor)

GO/PANI RGO/Au/PANI (Molecular Imprint) Graphene nanoribbon/ PANI RGO/TiO2/PANI

Graphene/CNT/PANI

Graphene/PANI/Au Graphene/PANI/ Ionic Liquid Graphene/PANI/Pt Graphene/PANI Graphene/PANI/Au/ Glucose oxidase

Electrochemical (Aptasensor)

Photoelectrochemical Electrochemical

1% H2

[111]

[112]

[119] 6.5x10-8 M

[120] [121]

1.7x10-7 M

[130]

H2O2

7.1
M

[131]

Electrochemical

H2O2

0.06
M

[132]

Electrochemical

H2O2 & Glucose

0.18
M glucose

[133]

Electrochemical (Enzymeless)

Electrochemical (Biosensor)

Glucose

[134]

Glucose Electrochemical

[135] (Whole blood)

Electrochemical

Nanowire

(Biosensor) Electrochemical (Biosensor)

DNA

3.25x10-13 M

[136]

DNA hybridization

3.2x10-14 M

[137]

2.11 pM

[138]

2.5x10-16 M

[139]

Graphene(sheet)/PANI/

Electrochemical

DNA

Au

(DNA sensor)

BCR/ABL fusion gene

RGO/PANI

16.57% to

Ref.

H2O2

(Biosensor)

Graphene/PANI

Graphene/PANI/DNA

Detection Limit

Electrochemical (DNA sensor)

DNA sequence

Elaborate Chemical Sensors Based on Graphene/Conducting

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)TCRJGPG 20##0+

%&

1127

4J$ #2

Fig. (12). Schematic representation for the dual-signalling electrochemical sensor [119].

smaller than that of PANI (ca. 10 ppm). A very dangerous vapor, methanol was also traced by a RGO/PANI nanocomposite utilizing a similar signal conduction mechanism [116]. They argued that the response time and reversibility of GO/PANI was much better than pure PANI, at the same time, more sensitive to methanol than to ethanol and propanol. The sensor could detect ppm level of methanol with electrochemical monitoring. A parallel theoretical study revealed that the sensing mechanism was partly dependent on the hydrogen bonding between sensor and methanol. A representative toxic aromatic chemical, toluene was also detected with a Graphene/PANI sensor [117]. It has been feasible to prepare a hydrazine sensor with RGO/PANI composites with a detection limit of 15 mM [118]. Zhu et al. invented a new dual-signaling electrochemical sensing method using a Graphene/beta-Cyclodextrin (CD)/Poly(N-acetylaniline) (PNAANI) film displaying host-gust interactions to distinguish organic pollutants such as rhodamineB (RhB) and 1-aminopyrene (1-AP) [119]. Figure 12 presents a conceptually interesting dual-signalling electrochemical sensor using host-guest interaction with cyclodextrin. In addition, 4-aminophenol was determined by a voltammetric method with a Graphene/PANI composite film [120]. The introduction of metal nanoparticles for example, Pd to the Graphene/PANI was used for the detection of chemicals such as hydroquinone [121], catechol [121], and bromate [122]. Wang et al. reported a very stable electrochemical sensor in which peak current attenuation was less than 3 % even after 200 cycles [121]. PANI is also eligible as the sensing material to physiological active substances [123-129]. As described earlier, the water dispersible Graphene/PANI/ poly(styrenesulfonic acid) was available as an ascorbic acid sensor [110]. One of the most important challenges that we face is the diagnosis of diseases such as cancer and chronic diseases [123-126]. Teixeira et al. demonstrated detection of a key diagnostic marker of pregnancy-gonadotropin [123]. The sensor was simply generated by adding PANI layer via electropolymerization on a graphene substrate. A linear dependency in signal was observed in the range of 0.001-50 ng/mL in real urine, with a detection limit of 0.286 pg/mL. A film type Graphene/PANI/Bi2O3 hybrid sensor was realized to perceive an antiinflammatory drug, etodolac [124]. This sensor was reported to be stable even in the presence of interferents. The limit of detection was 10 ng/mL. A laborious addition of several components was pursued to sense a component of drug-salbutamol [125]. Huang et al. incorporated Au, PANI, and poly(acrylic acid) to a graphene layer for detection of salbutamol. The response was linear between 0.08 - 1000 ng/mL, having a detection limit of 0.04 ng/mL. A

GO/PANI also recognized a kind of antibiotic-oxytetracycline with a limitation of 2.3 x 10-6 mg/L [126]. The addition of molecularly imprinted polymer/Au nanoparticle mixture to pre-existing RGO/PANI nanocomposite was a versatile approach to monitor serotonin [127]. Mixing of graphene nanoribbons with PANI could be a feasible method to sense dobutamine with an elevated sensitivity [128]. In general, graphene nanoribbons can have been obtained by the unzipping of carbon nanotubes. Addition of titania to Graphene/PANI appeared as an interesting way to a novel photochemical sensor for gallic acid [129]. The elucidation of mechanism is thought be noteworthy from the viewpoint of performance improvement. The primary chemicals generated from life activity such as hydrogen peroxide [130-133], glucose [133-135], and DNA [136-139] are still important targets of diverse emerging sensors using PANI. The enzymatically induced deposition of PANI on a Graphene/CNT/Au-Pt alloy electrode was a feasible pathway to an immunosensor toward hydrogen peroxide [130]. The sensor was revealed to be stable due to the presence of various components with a detection limit of 1.7 x 10-7 M. Aniline could be introduced as a dispersing and stabilizing agent for a RGO/Ag nanoparticle H2O2 sensor [131]. The use of aniline and Ag nanoparticles made it possible to prepare an enzymeless sensor. An ionic-liquid functionalized Graphene/PANI sensor was produced by a layer-by-layer assembly as H2O2 sensor [132]. Positively charged graphene sheets were functionalized by both ionic liquid and negatively charged sulfonated PANI. The ionic liquid provided more electric charges to the system thus elevated dispersibility. A graphene layer was deposited with both PANI and Pt nanoparticles to construct a Graphene/PANI/Pt nanocomposite sensor for monitoring H2O2, which would be also available as a glucose sensor [133]. This sensor interface can be a platform for diverse sensors. It has become clear that Graphene/PANI nanohybrids are useful for determination of glucose [134]. The originality of this work was the transfer of graphene/PANI hybrid material onto interdigitated array. The ability to precisely control each layer is thought to be a main reason for signal enhancement. A paper disk type sensor equipped with Graphene/PANI/Au nanoparticle/enzyme was used for the in situ detection of whole blood glucose [135]. Figure 13 describes an interesting glucose sensor using paper disk as substrate. To perform the sensing operation, screen-printed carbon electrode modified with Graphene/PANI/Au/glucose oxidase was covered by a paper disk impregnated with a sample. The electrochemical measurement was then carried out after addition of PBS solution. The sensor covered the full range of clinically relevant concentrations of glucose in whole blood. Synergistic combination of graphene and PANI

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Fig. (13). Schematic illustration of paper disk type sensor equipped with Graphene/PANI/Au sensing medium [135].

nanowire is a very promising strategy to monitor DNA [136]. The control over nanostructure in PANI appears lucrative to improve the electron transfer. Genes or gene sequence could have been determined by Graphene/PANI hybrids [137-139]. 3.3. Graphene/Thiophene Derivatives Hybrid Sensors Polythiophene (PTh) has been considered to be a very attractive material to most electronic devices such as solar cell, integrated circuit, transistors, and energy conversion systems due to its excellent properties. However, extensive use of PTh has been hindered by relative difficulty in synthesis, high cost, fragility to thermal annealing, and insufficient charge carrier transport [140]. Therefore, a couple of derivatives such as poly(3-hexylthiophene) (P3HT) and poly(3,4-ethylenedioxythiophene) (PEDOT) have attracted interests owing to the improved properties obtained through the modification of chemical structure. For example, P3HT was used as a p-type polymer on a piezoelectric semiconducting zinc oxide (ZnO) thin film to construct a nanogenerator [140]. In addition, phenyl-C-61-butyric add methyl ester (PCBM) was added to P3HT to improve carrier transport. Other issues concerning PEDOT (along with other PTh derivatives) are water-stability and adhesion. In order to improve those characteristics, a polyelectrolyte such as poly(styrene sulfonate) (PSS) was incorporated into the PEDOT backbone to obtain PEDOT:PSS [141]. An additional mixing of PEDOT: PSS with poly(vinylalcohol) or carbon nanostructures can generate composite materials with enhanced stability and adhesion [141]. A Film type PEDOT has relatively high electrical conductivity, flexibility, and transparency. However, the improvement for these properties is still required to be adopted for sensor applications. Especially, mixing with graphene has been considered as an ideal method for the purposes [142-146]. Seo et al. has reported the preparation of a Graphene/PEDOT composite film [142]. The use of graphene with PEDOT resulted in the improvement of electrical conductivity, transmittance, and mechanical strength. Simultaneous electrochemical redox polymerization of PEDOT and electrochemical reduction of GO on a glassy carbon electrode was reported to be a facile route to RGO/PEDOT biosensors [143]. This work is interesting because the authors measured the ascorbic acid composition in commercially available juices. A mechanically strong hydrogel composed of Graphene/PEDOT in which 3,4-ethylenedioxythiophene played a role of reducing agent to transform GO to graphene was reported [144]. Suh et al. suggested that vapor phase polymerized PEDOT on graphene can hold a great potential as a sensing medium [145]. This strategy is considered to be a type of electronic

nose which can show a very fast recovery time shorter than 100 s for most common volatile organic compounds. Table 3 includes the representative Graphene/PTh based chemical and biochemical sensors. A magnetic nanoparticle decorated RGO/PEDOT sensor was used for the identification of polar (ethanol, methanol, acetone, water) and nonpolar (chloroform, styrene, dichlorobenzene, toluene) volatile organic compounds at ppm level [146]. Also, graphene/PEDOT was suitable for monitoring of nitrite [147]. A wide linear range was observed from 0.3 to 600
M with a detection limit of 0.1
M. The tailoring of PEDOT nanostructures could generate a stable aqueous dispersion of PEDOT [148]. The noncovalent functionalization of PEDOT by graphene led to a sensor with stable dispersion in the range of 4
M - 2.48 mM with a detection limit of 1.2
M. Addition of nanographitic platelets into PEDOT:PSS matrix produced a nitroaromatic compound sensor [149]. The authors reported that the sensor could be operated under ambient conditions even after storage under laboratory conditions for several weeks. Graphene/PEDOT was employed as a sensing material toward catechol and hydroquinone [150, 151]. PEDOT combined with nitrogen doped graphene was developed to detect the two molecules with the range of linearity 1-10
M. The detection limit for catechol and hydroquinone was 0.26 and 0.18
M, respectively [150]. The acetoaminophen commonly called Tyrenol was monitored by a Graphene/PEDOT sensor [152, 153]. The incorporation of PEDOT nanotubes promoted signal transduction thus increased sensitivity [153]. As we have witnessed for PPy and PANI, PEDOT combined with graphene can trace the presence of physiological active substances such as 1-cystein [154], dopamine [155], and glucose [156]. Coating of Au nanoparticles was found to be efficient for the detection of 1-cystein [154]. 3.4. Sensors Using Graphene and Other Conducting Polymers Other CPs can also be advantageous as sensing subjects [157159]. But, relatively fewer researches have been conducted. A macroscopic thin film of graphene flakes functionalized with 1-Pyrenecarboxylic Acid was laminated on transparent polydimethylsiloxane (PDMS) membranes [157]. PCA allowed the non-covalent (
stacking) interaction that is absent in the pristine graphene. This material can be useful as an optical sensor. It was possible to impart electrical conductivity by adding conductive additives such as carbon black and carbon nanomaterials to a versatile insulator, polyurethane [158]. The synergistic hybridization of graphene and

Elaborate Chemical Sensors Based on Graphene/Conducting

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Table 3. Sensors based on graphene and polythiophene derivatives.

Material

Graphene/PEDOT

Graphene/PEDOT

Type Electrochemical (biosensor) Chemi-resistive (Electronic nose)

Target

Detection Limit

Ref

Ascorbic acid

2.0
M

143

VOCs

ppm

145

Graphene/PEDOT/Fe3O4

Chemi-resistive

Volatile Organic Comp.

ppm

146

Graphene/PEDOT

Electrochemical

Nitrite

0.1
M

147

GO/PEDOT nanorods

Electrochemical

Nitrite

1.2
M

148

Chemi-resistive

Nitroaromatics

Electrochemical

Hydroquinone & Catechol

0.18 & 0.26
M

150

GO/PEDOT film

Electrochemical

Hydroquinone & Catechol

1.6
M

151

GO/PEDOT

Electrochemical

Acetaminophen

0.57
M

152

RGO/PEDOT nanotube

Electrochemical

Acetaminophen

Electrochemical

1-cysteine

0.02
M

154

Electrochemical

Dopamine

39 nM

155

Electrochemical

Glucose

0.3
M

156

Graphitic platelets/ PEDOT:PSS Graphene(N-doped)/ PEDOT

Graphene(SO3)/PEDOT/ Au RGO/PEDOT Graphene/PEDOT:PSS/ Glucose oxidase

poly(beta-cyclodextrin) is a mild and novel way to fabricate a quercetin sensor [159].

149

153

mentioned advantages. Therefore, extensive research activities are still underway. It is clear that this article might be able to provide essential information for future research works.

3.5. Graphene/Conducting Polymer Peculiar Sensors Graphene/CPs hybrids are eligible as media to perceive diverse physical phenomena and chemical species [160-162]. Graphene/poly(vinyliedene fluoride) is an excellent strain gauge at even low level of 0.1 %, overwhelming conventional materials [160]. Graphene/CPs is also of practical importance as solid transducers in ion-selective electrodes [161, 162]. 4. CONCLUSION AND OUTLOOK In this article, we have addressed the general features such as synthetic route, structure, and charge transfer of most popular CPs. Importantly, the interesting sensors using PPy, PANI, and PEDOT combined with graphene were summarized concisely. It has become evident that there has been a great progress for technology dealing with graphene as a sensing material. In addition, the use of CPs as conducting additives has attracted a great interest to improve performances of pre-existing graphene sensors. Therefore, novel sensors showing high sensitivity and selectivity, stability, reproducibility, and mobility have been demonstrated extensively. Because the synthetic strategy and electric transfer mechanism is relatively monotonous, most sensors using Graphene/CPs are classified as chemical/electrochemical sensors. On the other hand, it is acceptable that molecular imprinting is regarded as one of major routes to new sensors. One of the most urgent challenges that the researchers face recently in terms of fabrication and application of CPs is to improve the processability and water-dispersability, retaining the above-

CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(NRF-2014R1A1A2054027). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2056302). REFERENCES [1]

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Current Organic Chemistry, 2015, Vol. 19, No. 12

Hernandez, R.; Riu, J.; Bobacka, J.; Valles, C.; Jimenez, P.; Benito, A.M.; Maser, W.K.; Xavier, R.F. Reduced graphene oxide films as solid transducers in potentiometric all-solid-state ion-selective electrodes. J. Phys. Chem. C, 2012, 116, 22570-22578.

Received: November 14, 2014

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Li, F.; Ye, J.; Zhou, M.; Gan, S.; Zhang, Q.; Han, D.; Niu, L. All-solid-state potassium-selective electrode using graphene as the solid contact. Analyst, 2012, 137, 618-623.

Revised: December 31, 2014

Accepted: March 08, 2015