Semiconducting Nanostructures and …various metal oxides nanostructures, nanocomposites and...

INTRODUCTION

Semiconductor nanomaterials haveattracted significant attention inresearch andapplications in areas including energy conversion,sensing, electronics, photonics, and biomedicine.The parameters suchas size, shape, and surfacecharacteristics of semiconducting nanomaterialsare significant to control the properties for differentapplications.With the development of nanoscienceand nanotechnology, one-dimensional (1D)semiconducting nanostructured materials includingnanotubes, nanorods, nanosheets, nanoballs, and

ORIENTAL JOURNAL OF CHEMISTRY

www.orientjchem.orgEst. 1984

An International Open Free Access, Peer Reviewed Research Journal

ISSN: 0970-020 XCODEN: OJCHEG

2013, Vol. 29, No. (3):Pg. 837-860

Semiconducting Nanostructures and Nanocompositesfor the Recognition of Toxic Chemicals (A Review)

SADIA AMEEN1, M. SHAHEER AKHTAR2 and HYUNG SHIK SHIN1

1Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of ChemicalEngineering, Chonbuk National University, Jeonju, 561-756, Republic of Korea.

2New& Renewable Energy Material Development Center (NewREC), Chonbuk National University,Jeonbuk, Republic of Korea.

DOI: http://dx.doi.org/10.13005/ojc/290302

(Received: June 15, 2013; Accepted: August 25, 2013)

ABSTRACT

Semiconducting metal oxide and conducting polymers based nanostructures possessunique morphological, optical and structural properties for the important applications in emergingtechnologies. On the other hand, the semiconducting nanocomposites offer the possibilities tocontrol the properties by varying the composition of the materials and the related parameters suchas morphology and an interface. This article provides a condensed overview of metal oxide andconducting polymer based semiconducting nanostructures and nanocompositesfor thedevelopment of chemical sensors. This work is a comprehensive review of current researchactivities that concentrate on chemical sensors based on metal oxides and conducting polymersnanostructures like nanotubes, nanorods, nanosheets and nanoballs. The experimental principle,design of sensing devices, sensing mechanism, and the significant sensing parameters arecomprehensively discussed.

Key words: Semicondcuting Nanaostructures, Nanocomposites and Toxic Chemicals.

other nano-structured materials have been widelyapplied to fabricate a variety of chemosensors. The1D nanomaterial provides unique physicalandchemical properties, large surface area/volumeratio,good conductivity, excellent electrocatalyticactivity and high mechanical strength.1Forsomespecific applications,it is highly desired to controland alter the properties of nanomaterials withgreater flexibility and possibility and thus,nanocomposites are the best alternatives to servethese purposes. The nano composites generallycontain more than one single component andtherefore, it is a simple approach to achieve the

838 AMEEN et al., Orient. J. Chem., Vol. 29(3), 837-860 (2013)

properties which are different from those of singlecomponent nanomaterials.

Conducting polymers like polypyrrole,polythiophene, polyindol, polyaniline and polyfuranfamilies are known as p-type semiconductors withunique electronic properties, low energy opticaltransitions, low ionization potential and high electronaffinity2 thus, widely used as sensitive materials forconductometric polymer sensors. The mainadvantages of conducting polymers are thepossibilityof working at room temperatures andsimplicity of technology that significantly simplifiesdesign of conducting polymers based devices andallows using them in portable instruments.Conducting polymers could be easily synthesizedthrough simple chemical or electrochemicalprocessesand their conductivities could be alteredby modifying the electronic structures through dopingor de-doping procedures.3Therefore, conductingpolymers could suitably work as an effective workingelectrode and might offer the fast response towardsthe detection of various harmful chemicals.

The sensor technology is popularly knownfor the detection of harmful chemicals. The

sensitivity, selectivity, and stability are the mostimportant aspect of investigation of a variety ofsensors. Upto now, efforts have been made bycontrolling the sensors structures,4-5sensorfabrication techniques6-7and surface modification8-

9to detect the toxic chemicals. Among severalsensors like fluorescence based chemicalsensors,10chemically modified electrode chemicalsensors11and chemilumine scence basedsensors,12the electrochemical method provides theadvantages of high sensitivity, wide linear range,economical, rapid response, portability and easeof operating procedure.13-14However, theelectrochemical method is still a challenge toenhance the electron transfer rate over the surfaceof working electrode for sensors. Therefore, themodifications of the electrodes with differentinorganic and organic nanomaterials could bepromising for the reliable and effective detection ofharmful chemicals by electrochemical and current-voltage (I-V) characteristics.

In this review, we have briefly discussedthe semiconducting metal oxides nanostructureslike TiO2, ZnO, conducting polymers and itsnanocomposites for the efficient detection of harmful

Fig. 1: Surface view (a,b) and cross sectional (c,d) FESEM images of vertically alignedZnO NRs at low and high magnifications. Inset shows the TEM image of ZnO NRs. Reprinted

with permission from [Ameen S., 2012], Talanta100 (2012) 377. ¸$ 2012, Elsevier Ltd

839AMEEN et al., Orient. J. Chem., Vol. 29(3), 837-860 (2013)

Fig. 2: XRD patterns (a) and UV-DRS spectrum(b) of vertically aligned ZnO NRs.Reprinted withpermission from [Ameen S., 2012], Talanta 100

(2012) 377. ©©©©© 2012, Elsevier Ltd.

Fig. 3: FTIR (a) and Raman scattering (b)spectrum of vertically aligned ZnO NRs.Reprinted with permission from [Ameen S.,2012], Talanta 100 (2012) 377. ©©©©© 2012, Elsevier Ltd

and toxic chemicals. The chapter includes the briefliterature surveys, properties and the latest researchadvancements/trends for the development ofvarious metal oxides nanostructures,nanocomposites and conducting polymer basednanomaterials as efficient electrode for detectingharmful chemical through the effectiveelectrochemical technique. The preparationmethods, morphologies, the physical and chemicalproperties of metal oxides, nanocomposites andconducting polymers have shown the significantimpacts on the optical, electrical, electronicproperties of the nanomaterials, andtheir performances for detecting the harmfulchemicals.

Semiconducting Metal oxide Nanostructures forChemosensorsModified Electrode of Vertically Aligned ZincOxide Nanorods for the Detection of HydrazineChemical Sensor

The inorganic metal oxide materials innanoscale have recently received a great deal ofinterest owing to their unique structures, electrical,and catalytic properties.15-16Zinc oxide (ZnO)nanomaterials have been extensively explored asthe most encouraging materials for many optical-electrical and the photovoltaic devices.17-19 Thenanocrystalline ZnO20with wide band gap of ~3.37eVexhibits high exciton, binding energy, low-costsynthesis, biocompatibility, better electrochemical


activities, non-toxicity, chemical and photochemicalstability and high-electron communicationfeatures.21-22Moreover, ZnO nanomaterials basedelectrodes have shown good electrochemicalactivities towards chemicals, biomolecules andgases owing to their high-electron transfercharacteristics with high electrochemical andphotochemical stability.23-24 Recently, one-dimensional (1D) ZnO nanostructures such asnanorods, nanobelts, nanowires and nanotubeshave been studied for fabricating variouselectrochemical and photo electrochemical devicesdue to their sufficiently high surface-to-volume ratioand good electrical characteristics.25-261D ZnOaligned nanostructures could be promising andeffective electrodes for sensing applicationstowards the detection of various chemicals andbiomaterials. So far, few literatures have reportedon the electrochemical detection of hydrazine andits derivatives using ZnO nanostructures basedmodified electrodes.27-28Ni et al., fabricatedhierarchical micro/nanoarchitecture ZnO basedelectrodes for hydrazine sensing with the sensitivity

of ~0.51µAµM-1cm-229.Fang et al., demonstrated theutilization of ZnO nanoflowers modified electrodefor the fabrication of hydrazine chemical sensor withthe sensitivity of ~3.49 µA µM-1cm-2.30A. Umar et al.,reported the electrochemical sensor for thedetection of hydrazine over the surface of goldelectrode modified with ZnO nanorods.31Ameenet al.,32reported a simple and less expensive lowtemperature hydrothermal method for the synthesisof highly uniform and vertically aligned ZnO NRson FTO glass substrates. The synthesized verticallyaligned ZnO NRs electrodes were directly appliedfor the effective detection of hydrazine throughsimple current (I)-voltage (V) characteristics. Thefabricated vertically aligned ZnO NRs thin filmmodified electrode showed the rapid detection ofhydrazine and exhibited the high sensitivity of~4.42446 x 10-5 A.mM-1.cm-2and detection limit of~515.7 µM with correlation coefficient (R) of~0.73297 and short response time (10 s).

The highly dense ZnO NRs could be seenin the surface view image at low magnification (Fig.

Fig. 4: (a) Typical cyclo voltammetry curve of vertically aligned ZnO NRs without and withhydrazine, (b) the I–V characteristics of verticallyalignedZnONRsmodified hydrazinechemicalsensorinhydrazineconcentrationrangeof0.3 mM to 0.03 Min10mlof0.1 MPBS

solutionand(c)calibrationcurveofcurrentversusconcentrationsof the fabricatedhydrazinesensor.Reprinted with permission from [Ameen S., 2012], Talanta 100 (2012) 377. ©©©©© 2012, Elsevier Ltd


1(a)). The high magnification image, as shown inFig. 1(b), displays the typical hexagonal structure ofZnO NRs with the average diameter of ~200-300nm. The cross-sectional view (Fig. 1(c,d)) revealsthe densely vertically aligned ZnO NRs on FTOsubstrates. The ZnO NRs possesses the averagediameter of ~200-300 nm and length of ~3-5 µm. Itclearly suggests that the thin layer of ZnO seedsignificantly results the uniform vertically alignedZnO NRs on the FTO substrate with the averagethickness of ~5 µm. In support, the vertically alignedZnO NRs are further characterized by the TEManalysis, as shown in the inset of Fig. 1(b). Thetypical ZnO hexagonal structure with the averagediameter of ~200-300 nm is seen, which is muchconsistent with the FESEM results.

Fig. 2(a) shows the XRD patterns ofvertically aligned ZnO NRs deposited on the FTOsubstrate. The grown ZnO NRs exhibit the crystalline

peaks of ZnO at 32.2o, 34.8o, 36.6o, 48.1o, 57.3o,63.4o and 68.6o which match well with JCPDS No.36-1451.33These patterns are attributed to thetypical wurtzite structure of ZnO crystals. However,the diffraction peaks at 26.8o, 34.2o, 51.9o, 55o,62oand 66.2oare associated with of the FTOsubstrate.34Noticeably, the diffraction peak (101)indicates the preferential orientation due to thevertical growth of NRs on the FTO substrate.35Fig.2(b) shows the UV-DRS spectra and derived bandenergy plot of vertically aligned ZnO NRs is shownas inset of Fig. 2(b). The ZnO NR obtains the broadintense absorption edge from ~400 nm to lowerwavelengths region, originating from a charge-transfer process from the valence band toconduction band of ZnO.36The band gap of verticallyaligned ZnO NRs is calculated as ~3.29eV whichis very close to the band gap of bulk ZnOnanomaterials.37

Fig. 5: FESEM images of TiO2 NT arrays at low (a) and high (b) magnification.Inset of (a) showscross section image of TiO2 NT arrays, TEM image(c), and HRTEM image (d) of TiO2 NT arrays.Inset of (c) shows SAED patternsof TiO2 NT arrays, (e) Raman spectrum and inset of (e) showsthe correspondingRaman mapping images in 390-460 cm-1, and (f) thecorresponding Raman

mapping images 550–650 cm-1 of TiO2NT arrays.Reprinted with permission from[Ameen S., 2013], Appl. Phys. Lett.103 (2013) 061602. © © © © © 2013,AIP Publishing LLC


The structural properties of verticallyaligned ZnO NRs are studied by analyzing the FTIRspectrum, as shown in Fig. 3(a). A very intense IRband at ~554 cm-1 is ascribed to the typical Zn–Ogroup of bulk ZnO,38suggesting that the synthesizedZnO NRs possess pure Zn–O groups. Theappearance of IR peaks at ~3404 and ~1560 cm-1

are originated from OH stretching mode and waterscissoring vibration or the carboxylate anionasymmetrical stretching respectively.39Moreover,the IR band at ~1087 and ~878cm-1 are assignedto Å1 stretching frequency and the bending vibrationof nitrate respectively. The investigation of thestructural disorder and the defects of the verticallyaligned ZnO NRs are elucidated by the Ramanscattering spectroscopy, as shown in Fig. 3(b). The

presence of strong peak at ~437cm-1 is ascribed toE2 mode of ZnO crystal which matches with Ramanpeak of bulk ZnO crystals.40The other two weakpeaks at ~331.2 and ~382.1cm-1 are due to thesecond order Raman spectrum arising from zone-boundary phonons 3E2H–E2L for wurtzite hexagonalZnO single crystals and A1T modes respectively.However, the broader peak at ~586cm-1

corresponds to E1 (LO) mode of ZnO associatedwith oxygen deficiency in ZnO nanomaterials. It isknown that the appearance of intense E2 mode isrepresented to the better optical and crystallineproperties of ZnO nanomaterials.41In our case, thesynthesized vertically aligned ZnO NRs possessthe high crystallinity of ZnO crystals with lessoxygen vacancies.

Fig. 6: (a) Typical amperometric plot and (b) linear plot of current versus concentration of phenylhydrazine of TiO2 NT arrays based chemical sensor. (c) The I-V characteristics and (d) the

calibration curve of current versus phenyl hydrazine concentration of TiO2 NT arrays electrodebased chemical sensor at different phenyl hydrazine concentrations (0.25 lM–0.10mM) in 10ml of0.1M PBS, and (e) schematic illustration of proposed mechanism of phenyl hydrazine chemical

sensors over the surface of TiO2 NT arrays based electrode.Reprinted with permission from[Ameen S., 2013], Appl. Phys. Lett. 103 (2013) 061602. © © © © © 2013, AIP Publishing LLC


The electrochemical response of verticallyaligned ZnO NRs electrode is studied to investigatethe electro catalytic activity of vertically aligned ZnONRs towards hydrazine. Fig. 4(a) shows the typicalcyclic voltammogram (CV) of vertically aligned ZnONRs electrode without and with hydrazine in 0.1 Mphosphate buffer (pH=7.0) at the scan rate of 100mV/s. The sensing response is generally detectedby oxidation process of hydrazine. The CV withvertically aligned ZnO NRs electrode displays that

the oxidation process begins from ~-0.38V andachieves the maximum anodic current of~0.124mAat -0.109 V. Only the anodic current occurs due tothe irreversible electrochemical response towardshydrazine. It is noticed that the anodic current ofelectrochemical system has significantly increasedwith the addition of hydrazine concentrations in PBS,while a negligible or very low current is observedwithout hydrazine. The high height and current ofanodic peak are usually attributed to a faster

Fig. 7: FESEM images at different magnifications (a–c) and TEM image (d) of layered PANI nanosheets.Reprinted with permission from [SeoH.K., 2013], Talanta,104 (2013) 219. © © © © © 2013, Elsevier Ltd

Fig. 8: Topographic (a) and three dimensional (b) AFM images of layered PANI nanosheets.Reprinted with permission from [Seo H.K., 2013], Talanta, 104 (2013) 219. ©©©©© 2013, Elsevier Ltd


electron-transfer reaction in the electrochemicalsystem via high electro catalytic behavior ofelectrode.42Herein, the vertically aligned ZnO NRselectrode exhibits the increased current with highheight of anodic peak in presence of hydrazine,confirming the involvement of high electron transferprocess via high electro catalytic activity ofelectrode.The current (I)–voltage (V) characteristicshave been measured for the fabricated hydrazinesensor with vertically alignedZnO NRs thin film asworking electrode to evaluate the sensingproperties. The fabricated hydrazine chemicalsensor based on vertically aligned ZnO NRs thinfilm as working electrode is illustrated in Fig. 4(b). Aseries of I-V characteristics has been tested withvarious concentrations of hydrazine ranging from0.3µM–0.03M in 0.1 M PBS for detecting thesensitivity of the fabricated chemical sensor. FromFig. 4(b), the presence of hydrazine in the fabricatedchemical sensor shows an immediate increase inthe current (~4.6 x 10-6 A) as compared to without

hydrazine based chemical sensor (~9.3 x 10-7 A).The substantial increment in current clearly revealsthat a good sensing response towards the detectionof hydrazine is associated with the vertically alignedZnO electrode which might result from its betterelectrocatalytic activity and fast electron exchange.Moreover, a series of the I-V characteristics isshown in Fig.4(b) which elucidates the sensingproperties of the fabricated hydrazine chemicalsensor. With the increase of hydrazineconcentrations (0.3µM–0.03M), the current hascontinuously increased, which might originate bythe generation of large number of ions and theincrease of ionic strength of the solution with theaddition of different concentration of hydrazine. Thecalibration curve of current versus concentration isplotted to analyze the sensitivity of the fabricatedhydrazine sensor, as shown in Fig. 4(c). The currentincreases with the increase of hydrazineconcentrationsupto~3 mM and then reaches to asaturation level as visible in the calibrated plot. This

Fig. 9:(a) XRD patterns, (b) line scanning elemental mapping image and (c) the correspondingpie bar graph of layered PANI nanosheets. Reprinted with permission from

[Seo H.K., 2013], Talanta, 104 (2013) 219. ©©©©© 2013, Elsevier Ltd


phenomenon comes out due to the unavailability offree active sites over vertically aligned ZnO NRselectrode for hydrazine adsorption at the higherconcentration of hydrazine (>0.03 M).Thecalibration curve is plotted to estimate the sensitivityof hydrazine by taking the slope and divided by anactive area of the electrode (0.5 cm2). The verticallyaligned ZnO NRs electrode based hydrazinechemical sensor shows high and the reproduciblesensitivity of ~4.42446 x 10-5 A.mM-1.cm-2with acorrelation coefficient (R) of ~0.73297, the detectionlimit of ~ 515.7 µM and a short response time (10s).The fabricated hydrazine sensor shows a goodlinearity in the range of 0.3µM-0.3mM. The stabilityof hydrazine chemical sensor was determined bymeasuring the I-V characteristics for threeconsecutive weeks. The fabricated hydrazine sensordid not show any significant decrease in the sensingparameters or properties, confirming the long termstability of the fabricated hydrazine sensor basedon vertically aligned ZnO NRs electrode.Enhancement of the sensing properties might dueto the unique vertically aligned morphology of ZnONRs containing subtle optical, electronic behaviors

and strong electrocatalytic activity. Therefore, thevertically aligned ZnO NRs electrode is a promisingworking electrode for the effective detection ofhydrazine.

Prospective Electrode of TiO2 Nanotube Arraysfor the of Sensing of Phenyl Hydrazine

The nanostructures of titania (TiO2) areone of most versatile metaloxides, exhibiting exoticinert surface and the optical properties. The tailoringof multidimensional TiO2 nanostructures toonedimensional (1D) is playing a significant role fordeterminingthe physiological and electricalproperties. Among various1D TiO2 nanostructures,TiO2 NTs generally exhibits the large surfacearea43outstanding charge transportproperties,44excellent electronic, mechanical, andchemical stability properties.45In particular, TiO2 NTswith highly uniform morphology and unique-orientated growth properties are promising for theapplications in gas sensors, biosensors,46dyesensitizedsolar cells,47hydrogen generation,48andsupercapacitors. Recently, TiO2 nano structures with

Fig. 10: Raman scattering spectrum (a) and its corresponding Raman mapping in1300-1400cm-1 (b) and in 1550-1600cm-1 (c) of layered PANI nanosheets. Reprintedwith permission from [Seo H.K., 2013], Talanta, 104 (2013) 219. ©©©©© 2013, Elsevier Ltd


high surface area are extensively utilized forthedetection of harmful chemicals through sensing.Kwonet et al., studied the enhanced ethanolsensing properties over the surface of TiO2 NTselectrode based sensors.49 Chen et al., fabricateda room-temperature hydrogen sensor with TiO2 NTarrays based electrode.50Ameen etal.,51synthesized TiO2 NT arrays on Ti foil substrate

by simple electrochemical anodicoxidation andutilized as the working electrode for the fabricationof a highly sensitive, reliable, and reproduciblechemical sensor for the detection of harmful phenylhydrazine chemical. The fabricated phenylhydrazine sensor showed high sensitivity of ~40.9µAmM-1cm-2 and the detection limit of~0.22 µM. Thegood compatibility, high uniformity, and the large

Fig. 11(a): Typical cyclo voltammetry curve of fabricated phenol chemical sensor based layeredPANI nanosheets electrode without and with phenol (20 mM) in10ml of 0.1M PBS solution and (b)CV sweep curves in phenol concentrations of 20 mM–0.32 mM in 10ml of 0.1M PBS solution.Reprinted with permission from [Seo H.K., 2013], Talanta, 104 (2013) 219. ©©©©© 2013, Elsevier Ltd

Fig. 12(a): Schematic illustration of the fabricated phenol chemical sensor, (b) I–V characteristicsof layered PANI nanosheets based phenol chemical sensor at different phenol concentrations

(20 mM–0.32mM) in10ml of 0.1 MPBS and (c) the calibration curve of current versus phenolconcentration of the fabricated chemical sensor.Reprinted with permission from

[Seo H.K., 2013], Talanta, 104 (2013) 219. ©©©©© 2013, Elsevier Ltd.


surface area have made TiO2 NT arrays electrodeaspromising chemosensor for the rapid, sensitive, andselective detection of phenyl hydrazine chemical.

The grown TiO2 on Ti substrate displayshighly ordered and self-assembled NT arrays, asshown in Fig. 5(a). At high magnification Fig. 5(b), auniform and closely packed TiO2 NT arrays are seen.

Moreover, the grown TiO2 NT arrays presentdistinguishable diameter distribution. The averagediameter and wall thickness of TiO2 NT arrays areobservedas 100±20 nm and 20±5 nm, respectively.The in set of Fig. 5(a) depicts the cross-sectionalFESEM image of grownTiO2 NT arrays whichexhibits the average length of~15µm. A hollowtubular morphology could be seen in the TEM image

Fig. 13: FESEM images at (a) low and (b) high resolution and (c) TEM image of PPynanobelts. Reprintedwith permission from [Ameen S., 2014], Appl. Catl. B: Environ.144 (2014) 665, ©©©©© 2014, Elsevier Ltd

Fig. 14: (a) Raman speactrum and (b) corresponding Raman mapping images in 950-1050 cm-1and(c) 1550-1600 cm-1of PPynanobelts Appl. Catl. B: Environ. 144 (2014) 665, ©©©©© 2014, Elsevier Ltd


of grown TiO2 NT arrays (Fig. 5(c)),which isconsistent with the FESEM results. The TiO2

NTarrays exhibit the average diameter and wallthickness of100±20 nm and 20±5 nm, respectively.The SAED patternof TiO2 NT arrays (inset of Fig.5(c)) shows polycrystallinephases in the anataseTiO2. The HRTEM image of TiO2 NT arrays (Fig. 5(d))shows well-resolved lattice fringes of crystallineTiO2

NT arrays with plane spacing of ~0.35 nm,whichcorresponds to anataseTiO2 (101). Theseobservations considerably deduce the goodcrystallinity of grown TiO2 NT arrays. The phasecomposition and structural propertiesof TiO2 NTarrays are investigated by Raman scatteringspectroscopy and the corresponding mapping, asshown in Figs. 5(e) and 5(f). The grown TiO2 NTarrays obtain three active Raman modes at~396.1,~518.2, and ~637.4 cm-1 which correspond to theactive Raman modes of anatase phase withsymmetries ofB1g(1), (B1g(2) + Ag(1)), and Eg3,respectively, and match withthe Raman modes ofanataseTiO2.

52-53Consequently, Raman spectrumdoes not exhibit any Raman mode at~445 cm-1,indicating that no rutile phase exits in the grownTiO2

NT arrays. The inset of Fig. 5(e) shows the Ramanmappingin the range of ~390–460 cm-1 and revealsthe larger dark part which corresponds to the peakat ~396.1 cm-1whereas Fig. 5(f) shows the Ramanmapping in the range of~550–650 cm-1 which

exhibits highly uniform surface, suggesting that thegrown TiO2 NT arrays are in good qualityofanataseTiO2 phase.

The steady state current-time oramperometricresponses of the system aremeasured to further investigate the electrocatalyticbehavior of TiO2 NT arrays electrode towards phenylhydrazine chemical at low concentration. Thesteady-state current is achieved in a PBS of pH7withthe successive addition of phenyl hydrazine(0-0.3 µM)using the peristaltic pump with respectto Ag/AgCl reference electrode. The typicalamperometric plot is shown inFig. 6(a). In thebeginning, the electrochemical experimentsperformed in PBS without phenyl hydrazine forstabilizing the background current. The successiveaddition of phenyl hydrazine has shown the linearincrease in current, exhibiting a linear relationshipbetween the currentand phenyl hydrazineconcentrations. Fig. 6(b) depicts the linear plot ofcurrent versusconcentration of phenyl hydrazinefor TiO2 NT arrays electrode which again confirmthe linear relationship between current andconcentration of phenyl hydrazine. Theseobservations infer that the TiO2 NT arrayselectrodeis highly effective catalyst to detect thesensing response of phenyl hydrazine at very lowconcentration which might attribute to the good

Fig. 15:1H NMR spectra of PPynanobelts electrode after the sensing measurements.Appl. Catl. B: Environ. 144 (2014) 665, ©©©©© 2014, Elsevier Ltd.


electrocatalytic, and direct electron transfer or fastelectron exchange behavior of TiO2 NT arrays.Thecurrent (I)-voltage (V) characteristics are measuredfor evaluating the sensing properties (sensitivity,detection limit, and correlation coefficient) of thefabricated phenyl hydrazine chemical sensor overTiO2 NT arrays electrode. The current response ismeasured from0.0–2.5V, and the time delaying andresponse times are 1.0 and 10 s, respectively. FromFig. 6(c), the current has continuously increasedwith the increase of the phenyl hydrazineconcentrations from 0.25µM–0.10 mM,suggestingthe good sensing response towards phenylhydrazine chemical by TiO2 NT arrays electrode.The enhancement in current might due to the betterelectrocatalytic behavior, generation of largenumber of ions, and the increase of ionic strengthof the solution with the addition of phenyl hydrazine.The sensing parameters such as sensitivity,detection limit, and correlation coefficient arecalculated by a calibration curve of current versusphenyl hydrazine concentration of the fabricatedphenyl hydrazine chemical sensor. Fig. 6(d)presents the plot of calibration current versus phenylhydrazine concentration which reveals that the

current increases linearly up to the phenylhydrazine concentration of ~1µM and after wardsachieve a saturation level in the calibrated plot. Thesaturation point occurs due to the less availabilityof free active sites on the surface of TiO2 NT arrayselectrode for phenyl hydrazine chemical at higherconcentration (>10 µM) in PBS. The fabricatedphenyl hydrazine chemical sensor based on TiO2

NT arrays electrode exhibits significantly high andreproducible sensitivity of ~40.9 µAmM-1cm-2and thedetection limit of ~0.22µM with correlationcoefficient(R) of ~0.98601 and short response timeof 10 s. Importantly, the fabricated phenyl hydrazinechemical sensorbased on TiO2 NT arrays electrodedisplays a good linearity in the range of 0.25 µM-1µM. The phenyl hydrazine sensing mechanismover the surface of TiO2 NT arrays electrode isexplained by using the surface-depletion model.54

Fig. 6(e) shows the illustration of sensing responseof phenyl hydrazine chemical over the surface ofTiO2 NT arrays electrode. Primarily, the phenylhydrazine is chemisorbedon the surface of TiO2 NTarrays. TiO2 NT arrays interact easily withatmospheric oxygen by transferring electrons fromthe conduction band to the adsorbed oxygen atoms

Fig. 16: FESEM images of (a) Gr and (b) PANI/Gr composites. TEM images of (c) Gr and(d) PANI/Gr composites. Reprinted with permission from [Ameen S., 2012],

Sens. Act. B: Chem.173 (2012) 177, ©©©©© 2012, Elsevier Ltd.


and presents in the form of O-, O2-, etc.,54 as shown

in Fig. 6(e). Second, these oxygenated speciesinteract withphenyl hydrazine and oxidizes phenylhydrazine into less harmful diazenyl benzene onthe surface of TiO2 NT arrays.Thus,TiO2 NT arrayselectrode provides suitable surface for the oxidationof phenyl hydrazine and determines the sensingresponses by increases the current values. Thereusability and reproducibility of the fabricatedphenyl hydrazine chemical sensor based on TiO2

NT arrays electrode were elucidated by measuringthe sensing responses with the I-Vcharacteristicsfor three consecutive weeks. The sensingparameters or properties showed the negligibledrops in the fabricated phenyl hydrazine chemicalsensor based on TiO2NT arrays electrode, whichdeduces the long term stability of the fabricatedphenyl hydrazine chemical sensor. Thus, TiO2 NTarrays with anatase phase and good crystal qualityare promising and effective working electrode forthedetection of phenyl hydrazine chemical or otherhazardous chemicals.

Conducting Polymers based Nanostructures forChemosensors

Conducting polymers are known as p-typesemiconductors and offer unique electronicproperties owing to their good electrical conductivity,low energy optical transitions, low ionizationpotential and high electron affinity.55These polymerscould be easily synthesized through simplechemical or electrochemical processes and theirconductivities could be altered by modifying theelectronic structures through doping or de-doping

procedures.56The good selectivity, wide linearrange, rapid response, portability and the roomtemperature working abilities are the basicrequirements for the efficient working of chemicalsensors.57The conducting polymers are widelyused as sensitive materials for chemosensors asthey are effective working electrode and might offerthe fast response towards the detection of variousharmful chemicals.58

The Efficient Electrode of Layered Polyaniline(PANI) Nanosheets for the Detection ofHazardous Phenol Chemical

A unique organic p-type semiconductorpolymer, polyaniline (PANI) is highly searchedpolymer due to its unique acid-base chemistry,stable electrical conduction, high-environmentalstability and ease of fabrication.59Importantly, PANIpossesses the typical conjugated bonds in thepolymer skeleton which could be responsible forthe charge conduction due to the generation ofpolarons or bipolarons.60The variable conductivitymakes PANI as promising material for the specificapplication of electronics, optoelectronic,electrochemical, electrochromic, photovoltaic andsensing devices.61Moreover, the presence of thereactive NH- groups in the polymer chain (PANI),positioned on either side by phenylene rings impartsthe chemical flexibility to the system and improvesthe processibility to a large extent. PANInanomaterial shows the versatility of nanostructuresin the form of nanofibers, nanorods, nanowires andnanoflakes with high surface/volume ratio and lowdiffusional resistance62. Various PANI

Fig. 17: AFM images of (a) Gr and (b) PANI/Gr composites. Reprinted with permissionfrom [Ameen S., 2012], Sens. Act. B: Chem. 173 (2012) 177, ©©©©© 2012, Elsevier Ltd


nanostructures display the improved optical,structural, electronic and electrical properties whichmight act as useful candidate for the application inelectrochemical, electrochromic, biosensors andchemical sensors devices.63-64 Recently, PANInanomaterials have gained a great attention in thefield of sensors including gas sensor, biosensor andchemical sensors.65 In context of PANI basedsensors, A. L. Kukla et al., prepared PANI thin filmsfor detecting ammonia.66Y. Bo et al., explained the

electrochemical DNA biosensor by PANI nanowiresmodified graphene electrode.67Recently, P. Kunzoet al., fabricated the hydrogen sensor based onunique oxygen plasma treated PANI thin film. Anamperometric phenol biosensor based on PANIelectrode was studied in the aspects of optical andelectrochemical properties68.J. Zhang et al.,designed the composite electrode of PANI-ionicliquid-carbon nanofiber for the fabrication of highlysensitive amperometric biosensors towardsphenols69.Ameen et al., reported the layered PANInanosheets based electrodes as an effectivechemical sensor towards the efficient detection ofphenol.70The fabricated phenol sensor based onlayered PANI nanosheets exhibited a highsensitivity of ~1485.3 µA.mM-1.cm-2and very lowdetection limit of ~4.43 µM with correlationcoefficient (R) of ~0.9981 and short response time(10 s).

Fig.7 shows the FESEM and TEM imagesof layered PANI nanosheets. At low resolution (Fig.7(a-b)), the synthesized PANI displays the uniformand compact layered sheets like morphology. Thelayered PANI nanosheets exhibit the averagethickness of several hundred nanometers, asshown in Fig. 7(c). The morphology of synthesizedPANI is further characterized by TEM analysis, asdepicted in Fig. 7(d). The layered morphology ofPANI nanosheets is clearly visible which is similarto the FESEM results. There is no deformation ofthe morphology of layered PANI nanosheets underthe high electron beam of TEM, suggesting thestability of the synthesized layered PANInanosheets.

Fig.8 shows the topographic and threedimensional (3D) AFM images of layered PANInanosheets. The layered morphology ofsynthesized PANI is visibly recorded in thetopographic mode, as shown in Fig. 8(a). The 3DAFM image (Fig. 8(b)) has confirmed the samelayered morphology, as detected in the topographicmode. The roughness of layered PANI nanosheetsis estimated from AFM images by taking the valueof the root mean roughness (R

rms). The layered PANInanosheets exhibit relatively the high roughnessof ~52.3 nm. It is known that the electrode materialswith large roughness factor display higherelectrochemical behavior or the electrocatalytic

Fig. 18: (a) The I–V characteristics of PANI/Grcomposite thin film modified hydrazine

chemical sensor in hydrazine concentrationsof 0.01 µM–0.01 M in 10 ml of 0.1 M PBS

solution and (b) calibration curve of currentversus concentrations of the fabricatedhydrazine sensor. Inset (a) shows typical

cyclovoltametry curve for PANI/Gr modifiedelectrode without and with hydrazine, and inset(b) the calibration curve of current in the lower

concentrations region.Reprinted withpermission from [Ameen S., 2012], Sens. Act.B: Chem. 173 (2012) 177, ©©©©© 2012, Elsevier Ltd.


activity. The high roughness of layered PANInanosheets might improve the electrochemicalbehavior towards the detection of phenol.

Fig. 9(a) shows the X-rays diffraction (XRD)patterns of layered PANI nanosheets. Typically, twodiffraction peaks at 19.3o and 25.1o are recorded,corresponding to the periodicity parallel andperpendicular to the polymer chain, respectively.These peaks are also assigned to emeraldinestructure of PANI. The recorded XRD patterns aresimilar to PANI sheets or matrix.71 The elementcomposition of the layered PANI nanosheets isanalyzed by taking the line scan element mappingthrough EDS. Fig. 9(a, b, c) shows the line scanelement mapping image and pie profile of theelements. The C and N elements are majorlydistributed in the line scan mapping however, thetraces of Cl and S elements are also detected, asseen in the corresponding pie bar graph shown inFig. 9(c). The uniform distribution of C and Nelements confirm the formation of layered PANInanosheets.

The structural properties are furthercharacterized by the Raman scatteringspectroscopy, as shown in Fig. 10(a). The Ramanband at ~1169 cm-1 is attributed to the C–Cstretching/C–H in plane bending of the synthesizedlayered PANI nanosheets.72 The observation ofRaman band at ~1371 cm-1 is assigned to thepresence of C~N•+ (where ‘~’ denotes anintermediate bond between a single and a double)stretching mode of the delocalized polaronic chargecarriers in PANI.73 Moreover, the Raman bands at~1494 and ~1587 cm”1 is associated to N–Hbending/C–H bending of benzenoid and C=C(Quinoid)/C–C (benzenoid) stretchingrespectively.74 The corresponding Raman mappingimages in the two ranges of ~1300-1400 cm-1 and~1550 - 1600 cm-1 are depicted in Fig. 10(b-c). TheRaman mapping in the range of 1300-1400 cm-1

(Fig. 10(b)) shows the scattere dlight blue color onthe major blue area of mapping, which mightpresent the C~N•+ stretching modes (Raman shiftat ~1371 cm-1)of delocalized polaronic chargecarriers in PANI backbone. However, uniformdistribution of the C=C (Quinoid)/C–C (benzenoid)stretching in layered PANI nanosheets are observedin the range of 1550-1600 cm-1, as shown in Fig.

10(c). Thus, the uniformly distributed C=C/C-Cbonding along with C~N•+bonding in Ramanmapping confirms the formation of high quality oflayered PANI nanosheets.

The electrocatalytic activity of layeredPANI nanosheets electrode towards the detectionof phenol is examined by the cyclic voltametry (CV)analysis. Fig. 11 shows the typical CV of layeredPANI nanosheets electrode without and with aseries of phenol concentrations (20 µM-0.32 mM)in 0.1 M phosphate buffer (pH=7.0) at the scanrate of 100 mVs-1. The layered PANI nanosheetselectrode shows the relatively low redox currentin the absence of phenol, as shown in Fig. 7(a).Importantly, the prominent oxidation peak with themaximum anodic current of ~1.1 x 10-4 A at ~0.37V is obtained with the addition of lowest phenolconcentration (20 µM), indicating the significantsensing response and high electrocatalytic activityover the surface of layered PANI nanosheets. Theweak reduction peak with low cathodic current of-4.67 x 10-5 A is also observed in the CV curve.The electrochemical behavior with prominentoxidation peak along with weak reduction peak isquite similar to the reported literatures75and thus,the following reaction could be proposed forphenol. A series of CV plots have been carried outwith various phenol concentrations ranging from20 µM - 0.32 mM in 0.1 M PBS to further examinethe electrochemical properties of layered PANInanosheets electrode. Fig. 11(b) shows the typicalquasi-reversible redox peaks to theelectrochemical reaction of layered PANInanosheets electrode towards the phenolchemical in PBS. The oxidation peak is graduallyincreased with the increase of phenolconcentration from 20 µM - 0.32 mM. It is reportedthat the high height of oxidation peak and the highanodic current are referred to the faster electron-transfer reaction in the electrochemical system viahigh electrocatalytic behavior of the workingelectrode.76The highest anodic current is obtainedat the highest concentration of phenol (0.32 mM),which is about 2 times larger than the lowestphenol concentration 20 µM. The considerableincreased of the anodic current demonstrates thatthe electrochemical activity of layered PANInanosheetselectrode is remarkably promoted forthe detection of phenol chemical and thus, confirms


the involvement of high electrons transfer processvia high electrocatalytic activity of the electrode.

The fabricated phenol chemical sensor isillustrated in Fig. 12(a), which is comprised oflayered PANI nanosheets electrode as workingelectrode and Pt wire as cathode electrode in PBS.The measurement of current(I) - voltage(V)characteristics (Fig. 12(a, b, c, )) is performed forevaluating the sensing properties such assensitivity, detection limit and correlation coefficientof layered PANI nanosheets electrode towardsphenol chemical. After the addition of phenol(20µM), the sudden increase in the current of ~67.3µA is observed by the fabricated phenol chemicalsensor however, a low current (~11.1 µA) isrecorded without phenol based chemical sensor. Aseries of the I-V characteristics have beenestablished to elucidate the sensing parameters ofthe fabricated phenol chemical sensor with layeredPANI nanosheets electrode, as shown in Fig. 12(b).It is seen that thecurrent has continuously increasedwith the increase of the phenol concentrations (~20µM - 0.32 mM), suggesting the good sensingresponse toward the phenol chemical by thelayered PANI nanosheets electrode based phenolchemical sensor. This phenomenon might originateby the generation of large number of ions and theincrease of ionic strength of the solution with theaddition of different concentration of phenol.Acalibration curve of current versus phenolconcentration (Fig. 12(c)) is plotted to calculate thesensitivity of the fabricated phenol chemical sensor.The calibrated current linearly increases up to theincrease of the phenol concentrations ~80 µMandthen attains a saturation level in the calibrated plot.The occurrence of the saturation point might due tothe unavailability of free active sites over the layeredPANI nanosheets electrode for phenol adsorptionat higher concentration (>80 µM).The fabricatedphenol chemical sensor with layered PANInanosheets electrode achieves high, andthereproducible sensitivity of ~1485.3 µA.mM-1.cm-2

and the detection limit of ~4.43 µM with correlationcoefficient (R) of ~0.9981 and short response time(10 s). A good linearity in the range of 20 µM - 80µM is detected by the fabricated phenol chemicalsensor with layered PANI nanosheets electrode.For reproducibility and the stability of fabricatedphenol chemical sensor, the sensing response by

the I-V characteristics is measured for threeconsecutive weeks. It is found that the fabricatedphenol chemical sensor did not show any significantdecrease in the sensing parameters or properties,which deduces the long term stability of thefabricated phenol sensor based on layered PANInanosheets electrode. Thus, the unique layeredmorphology of PANI nanosheets is promising andeffective as working electrode for the detection ofphenol chemical.

PolypyrroleNanobelts as Prospective Electrodefor the Direct Detection of Aliphatic Alcohols

Polypyrrole (PPy), a conducting polymer,is much explored material because it shows highelectrical conductivity, high stability in air andaqueous media and thus, an extremely usefulmaterial for actuators, electric devices and for theefficient detection of the harmful chemicals.77-78Fewliteratures are reported on the sensor performancesof PPy nanostructure based electrodes for thedetection of aliphatic alcohols S. J. Hong et al.,.,studied nitro vinyl substituted PPy as a uniquereaction-based chemosensor for cyanideanion.79C.W. Lin et al., prepared the compositeelectrode of PPy-poly vinyl alcohol (PVA) byelectrochemical method for the detection ofmethanol and ethanol vapor.80-81L. Jiang et al.,prepared the composite films of PPy-PVA by in situvapor state polymerization method anddemonstrated the methanol sensing behaviorbased on the thickness of PPy-PVA filmelectrodes.82Recently, M. Babaei et al., determinedthe residual methanol content in the biodieselsamples by developing new PPy-ClO4 electrodesvia electrodeposition on interdigital electrodes83.The roughness and morphology of the PPy greatlyinfluence the responses for the detection of harmfulchemicals84. Ameen et al.,85 synthesized the uniquePPy nanobelts a by the in-situ chemicalpolymerization of pyrrole monomer in presence offerric chloride as oxidant and methylene blue asreactive self-degraded template. The synthesizedPPy nanobelts were directly applied as workingelectrode for the efficient detection of aliphaticalcohols using simple current (I)-voltage (V)characteristics.

From FESEM images (Fig. 13(a,b)), thesynthesized PPy nano materials possess smooth


and the uniform belt like morphology. Each PPynanobelt presents the average thickness of ~100nmand width of ~400 nm, as shown in Fig. 13(a,b,c).The morphology of the synthesized PPy has beenfurther characterized by the TEM analysis (Fig. 13(c)).Similar morphology and the dimensions areobserved in TEM image, which is consistent withthe FESEM results. Interestingly, the morphology ofPPy nanobelts has not changed under high energyelectron beam, indicating the stability of PPynanobelts.

The Raman scattering spectroscopy hasbeen used to analyze the structural properties ofPPy nanobelts. Fig. 14(a) shows the Ramanspectrum of synthesized PPy nanobelts. The strongRaman band at ~1558 cm-1 corresponds to thecharacteristics C=C backbone stretching of PPy29.The Raman bandsat ~1317 cm-1 and ~1044 cm-1

are attributed to the ring-stretching mode and theC-H in-plane of PPy respectively.86 However, thepeaks at ~972 cm-1 confirms the ring deformation,associated with the radical cation (polaron) inPPy.87The detailed structure of PPynanobelts areanalyzed by the Raman mapping. Thecorresponding Raman mapping images in the tworanges of ~950-1050 cm-1 and ~1550-1600 cm-1 aredepicted in Fig. 5(b, c). In the Raman mapping, theuniform scattered light green color in the major darkgreen area is seen in the range of 950-1050 cm-1

(Fig. 14(b)), which is assigned to the ring deformationassociated with the radical cation (polaron) in PPy(Raman shift at ~972 cm-1).On the other hand, theRaman mapping in the range of 1550-1600 cm-1

exhibits the uniform distribution of C=C stretchingin PPy nanobelts, as shown in Fig. 14(c). Thus, theuniformly distributed C=C bonding along with ringdeformation, associated with the radical cation(polaron) in Raman mapping confirms the formationof high quality of PPy nanobelts.

The electrochemical Impedancespectroscopy (EIS) measurements have beenperformed for the fabricated aliphatic alcoholschemical sensors based on novel PPy nanobeltselectrode to explain the electrocatalytic activity ofthe electrodes. The EIS plots of the fabricatedaliphatic alcohols chemical sensors based on novelPPy nanobelts electrode using 0.1 M phosphatesulfate solution (PBS) with methanol, propanol and

butanol at similar concentration of 20 µM. All EISmeasurements are carried out at a frequency rangefrom 100 kHz-1Hz. The fabricated aliphatic alcoholschemical sensor displays two semicircles, in whichthe large semicircle in the high frequency region isattributed to the parallel combination of the chargetransfer resistance (RCT) of the electrochemicalreaction and the double layer capacitance (Cdl) atthe interface of the PPy electrode/PBSelectrolyte.88RCT of sensor device defines theelectron-transfer kinetics of the redox probe at theelectrode interface.88In general, the signal responsefor sensing device is determined by the values ofRCT at the interfaces of the PPy electrode anddifferent concentrations of alcohol in PBS.89Herein,all electrochemical alcohol chemical sensorspresent similar nature of EIS plots in 10 ml of PBS(0.1M) with different alcohols at the sameconcentration of 20 µM. A relatively low RCT value(~353 Ω) is obtained by the fabricated methanolchemical sensor based on PPynanobelts electrode,however the propanol and butanol chemicalsensors show the high RCT values of ~507 Ω and~597 Ω respectively. Generally, the low chargetransfer rate at the interface of electrode/electrolytein the electrochemical system is originated fromhigh RCT value.90This result suggests that thefabricated methanol chemical sensor based on PPynanobelts electrode presents the better chargetransfer rate and the electrocatalytic activity towardsthe methanol chemical, resulting to the high sensingresponse on the surface of PPy nanobelts electrode.Whereas, other aliphatic alcohols based chemicalsensors are quite inferior to methanol chemicalsensor.

The detailed sensing properties includingthe detection limit, linearity, correlation coefficientand sensitivity are extensively evaluated by the twoelectrodes I-V characteristics measurements wherePPy nanobelts electrodes are used as workingelectrode while the Pt wire is applied as cathode.All I-V characteristics are measured with the appliedvoltage ranging from 0-2.5 V. The typical fabricatedalcohol chemical sensor depicts the aliphaic alcoholsensing mechanism over the surface of PPynanobelts. The fabricated aliphatic alcohol chemicalsensor is performed in PBS with and withoutalcohols and the sensing behavior is simplyexplained by the I-V characteristics. It is noticed that


the drastic increase in current is observed after theaddition of aliphatic alcohols (20 µM) in all thefabricated chemical sensors as compared tochemical sensor without alcohol. The addition ofmethanol chemical (20 µM) in PBS displays thehighest current of ~60.4 µA while the lower currentsof ~58.7 µA and ~55.4 µA are obtained with theaddition of propanol and butanol chemicals in PBSrespectively. This gradual increase in the currentindicates the rapid sensing response of PPynanobelts electrode towards the detection ofmethanol, propanol and butanol chemicals, whichmight result from the better electrocatalytic orelectrochemical behavior and the fast electronexchange of PPy nanobelts electrode. The I-Vresponses of PPy nanobelts electrode with variousconcentration of aliphatic alcohols ranging from 20µM - 1 mM in 10 ml of 0.1 M PBS have beenmeasured to investigate the detailed sensingbehavior of PPynanobelt electrode. The I-Vcharacteristics of the fabricated methanol chemicalsensor with various concentrations of methanolchemical (20 µM-1 mM) in 0.1 M PBS solution ofpH 7. When PPy nanobelts based electrode isexposed to methanol, the current drasticallyincreases with the increase of the methanolconcentrations, showing the good sensingresponse to methanol chemical. From the typicalcalibration curve the fabricated methanol chemicalsensor based on PPy nanobelts electrode exhibitsthe reproducible, reliable and the highest sensitivityof ~205.64µA.mM-1.cm-2 with the linearity of 20µM -0.16mM, detection limit of ~6.92 µM and correlationcoefficient (R) of ~0.98271. However, the fabricatedpropanol and butanol chemical sensors based onPPy nanobelts electrode show the low sensitivitiesof ~190.76 and ~146.34µA.mM-1.cm-2and moderatedetection limits of ~13.7 and ~12.06µM respectively.All aliphatic alcohol sensors display the sameresponse time of 10 s. As compared to otheraliphatic alcohols, the highest current response andsensitivity have been observed for methanolchemical sensing which might suggest the highelectron mobility and electrochemical activity overthe surface of PPy nanobelts electrode, asdescribed in EIS results.

The 1H-NMR spectra of PPy nanobeltselectrode before and after the sensingmeasurements as shown in Fig. 15, are recorded

by 600 MHz FT-NMR spectrometer (JNM-ECA600)in DMSO solvent. The 1H-NMR spectra of PPynanobelt displays the peaks (a, b, c) in the range of7.0-8.5 ppm, which are assigned to proton of NHgroup and the aromatic protons on pyrrole ring. Afterthe sensing measurements, two NMR peaks areseen at 5.0-4.5 ppm, indicating the interaction ofaliphatic alcohols on PPy nanobelts surface. TheNMR peaks at 5.0-4.5 ppm correspond to theprotons of carbon, attached with OH group(alcoholic group). The existence of these peaksmight suggest that the aliphatic alcohol first interactwith the surface of PPy nanobelts through NH groupduring the sensing measurement, which isproposed in the illustrated mechanism.

Semiconducting Nano composites forChemosensorsModified Electrode of Polyaniline / GrapheneComposite Thin Film for HydrazineChemosensor

The flat carbon nanosheets of sp2-bondedcarbon atoms called graphene (Gr), has recentlyattracted viable attention due to itshigh electricalconductivity,91 large specific surface area(theoretically2630 m2/g),92 low manufacturing costand good mechanical properties. Importantly Grpossesses very high mobility of charge carriers(200,000 cm2 V-1 s-1) due to it’s the unique two-dimensional carbon structure93which explorevarious application in super capacitors, solar cells,hydrogen storage, batteries and light emittingdiodes94. Owing to its unique properties, it offers anideal two-dimensional catalyst support to anchormetal and semiconductor catalyst nano particlesfor versatile selective catalytic or sensingperformances95. Gr composites with metal oxide andorganic semiconductors show the fascinatingapplications in electrochemical sensors andbiosensors96. TheGr composites are usuallyproduced by incorporating Gr into othermaterialswhich could enhance the electrochemicalproperties bythe synergic effects. Polyaniline (PANI)is known organic semiconductor or conductingpolymer and is promising host material for variousin organic semiconductors and carbon materialsbecause of relatively high conductivity, excellentchemical and the electrochemical stability97.Moreover, it has been reported that the propertiesof PANI could be easily improved by acid dopants


and the blending with organic/inorganicsemiconducting nanomaterials98. Recently, thecomposites of PANI and Gr exhibit tremendousapplicability in many electronic, optical,electrochemical and biosensors99.The PANI/Grcomposites display significant electrical conductivityand electrochemical properties toward variouselectrochemical, electrochromic and bionsensingdevices.100

The surface of Gr thin film (Fig. 16(a-c))exhibits layered morphology like the crumpledwaves with the average thickness of severalhundred nanometers. The in situ electrochemicallydeposited PANI/Gr composites film (Fig. 16(a-c))manifests the mixed morphology of the layered Grcovered by PANI molecules obtainedby thepolymerization of aniline monomer in HCl. It is seenthat during the polymerization of aniline inelectrochemical process, PANI molecules haveuniformly covered the Gr sheets. Similarly, the TEMimages present the transparent layered sheetmorphology of Gr which is stable under the electronbeam, as shown in Fig. 16(a-c).On the other hand,the transparent morphology along with aggregatedclusters is observed in PANI/Gr composite, shownin Fig. 16(d).The transparent edges and black partsare ascribed to Gr sheet and PANI in PANI/Grcomposites respectively.

Fig. 17 shows the AFM images of Gr sheetand PANI/Gr composite thin films. Like FESEM andTEM results, it shows layered morphology of Grsheets (Fig. 17(a)) and aggregated morphologyisseen in PANI/Gr composite as shown in Fig.17(b).By theroughness analysis, the PANI/Gr compositethin film displays the high roughness (Rrms = 5.201nm) compared to the roughness of Grsheets (Rrms= 3.702 nm). The highly roughened surface isgenerally resulted from the enhanced growth andadhesion of thin film.101In our case, the highly roughsurface of PANI/Gr composite might attribute to theuniform deposition of PANI on Gr sheet. The in situelectrochemically modified electrode of PANI/Grcomposite thin film has been utilized as workingelectrode toe valuate the hydrazine sensingproperties.

To understand the detailed sensingperformances of the fabricated hydrazine chemical

sensor, a wide range of the I–V characteristics havebeen carried outwith hydrazine solution of differentconcentrations, ranging from0.01 µM to-0.01 M in10 ml of 0.1 M PBS solution (pH = 7), as shown inFig. 18(a).The I–V characteristics clearly reveal thatthe current gradually increases with the increase ofhydrazine concentration from 0.01 µM to 0.01 M.This behavior might explain the increase of ionicstrength of the solution with the addition of differentconcentration of hydrazine which might producemore number of ions.The sensing mechanism ofhydrazine is generally explained bytheconcentration of electrolyte solution and thenature of the electrodes. It could be demonstratedby determining of intermediate inthe oxidationprocess.102The cyclovoltametry (inset of Fig.18(a))has been carried out for the PANI/Gr modifiedelectrode with out and with 0.01 µM hydrazine in0.1 M PBS (pH = 7) at the scan rateof 100 mV/s todefine the oxidation process. It is reported that theelectrolyte solution with pH 7 shows the improvedelectrocatalytic oxidation of hydrazine.103From CVwith hydrazine using the PANI/Gr modified electrode,the oxidation process starts from-0.25 V and reachesmaximum anodic current of ~6.2 µA at 0.35 V.Nocathodic current is displayed which indicates theirreversible electrochemical response towardhydrazine. The PANI/Gr modified electrode withhydrazine exhibits the higher current than that ofwithout hydrazine system, indicating the effectiveoxidative detection of hydrazine. Fig. 18(b) showsthe calibration curve of current versus concentrationto elucidate the sensitivity of the fabricatedhydrazine sensor. From this calibration plot, it is seen(inset of Fig. 18(b)) that the current increases withthe increase of the concentrations up to 0.1 mMand after wards,a saturation level is reached, whichmight due to the unavailability of free active sitesover PANI/Gr composite modified electrode forhydrazine adsorption at the higher concentration ofhydrazine (>0.1 mM).104In general, the sensitivity ofhydrazineis calculated from the slope of calibrationplot and divided by active area of electrode (0.25cm2). The fabricated PANI/Gr composite basedhydrazine chemical sensor shows reasonably highand reproducible sensitivity of<“32.54×10-5 Acm-

2mM-1 in the lineardynamic range of 0.01 µM–0.1mM. Moreover, a good linearity of ~0.78578 and thedetection limit of~15.38 mM with a shortresponsetime (10 s) are estimated from the I–V characteristics


ofhydrazine chemical sensor. To elucidate thestability of hydrazine chemical sensor, the I–Vcharacteristics are measured for three consecutiveweeks and no significant decrease is observed inthe I–V properties of the fabricated PANI/Grcomposite based hydrazine chemical sensor,indicating the fabricated hydrazine sensor showslong term stability. Interestingly, the sensitivity valueand other parameters from I–V characteristics ofthe fabricated PANI/Gr composite based hydrazinechemical sensor are superior to other reportedhydrazine chemical sensor.105-106The enhancementof sensing properties might due to the attractivefeatures of Gr sheet and PANI layer containingsubtle electronic behaviors and strong adsorptivecapability.107Additionally, some electrostaticinteraction between Gr and hydrazine might occurthrough positively charged nitrogen atoms ofhydrazine which results to increase the sensingproperties.

CONCLUSIONS

This article provides a review on currentresearch status of chemical sensors based onvarious new types of nano structured materials suchas nanotubes, nanorodsand nanosheets. Thesemiconducting metal oxide, conducting polymersand nanocomposite have been discussed in terms

of their morphological, structural, crystalline, optical,and sensing parameters. The synthetic proceduresand morphology of metal oxide, conductingpolymers and nanocomposites considerably affectthe optical, electrical and electrochemicalproperties of thin film electrode. The tailoring ofmultidimensional nanostructures to 1D is playing asignificant role for determining the physiologicaland electrical properties. Particularly forchemosensors, 1D ZnO and TiO2 nanostructurescould be promising and effective electrodes forsensing applications towards the detection ofvarious toxic chemicals.The modifications of theelectrodes with different inorganic and organicnanomaterials could be promising for the reliableand effective detection of harmful chemicals byelectrochemical and current– voltage (I–V)characteristics. On the other hand, a unique organicp-type semiconductor polymer, polyaniline (PANI)is highly stable polymer with good electricalconduction, high-environmental stability and easeof fabrication and thus, highly promising fordetecting the toxic chemicals. Future research wouldbe focused on the materials preparations byexploring new and advanced techniques for theoptimization of electrode thickness and electricalconductivity of metal oxide, conducting polymersand nanocomposites for the fabrication of highperformance of chemosensors.

REFERENCES

1. Asefa T., Duncan C.T., and Sharma K.K.,Analyst 134:1980-1990 (2009).

2. PandeyR.K., and Lakshminarayanan V. Appl.Catal. B: Environ.125: 271-281 (2012).

3. MashatL.A., TranH.D., WlodarskiW.,KanerR.B., and ZadehK.K. Sens. Act. B:Chem.134: 826-831(2008).

4. Sapsford K.E., Ngundi M.M., Moore M.H,Lassman M.E., Shriver L.L.C., Taitt C.R., andLigler F.S., Sens. Actuators B113: 599-607(2006).

5. Chai K.T.C., Hammond P.A., and CummingD.R.S.,Sens. Actuators B 111-112:305-309(2005).

6. Hyodo T., Mori T., Kawahara A., Katsuki H.,Shimizu Y., and Egashira M., Sens. ActuatorsB77: 41-47(2001).

7. Thi´ebaud P., Beuret C., de Rooij N.F., andKoudelka-Hep M., Sens. Actuators B70: 51-56(2000).

8. Huang X.J., Sun Y.F., Wang L.C., Meng F.L.,and Liu J.H., Nanotechnology15: 1284-1288 (2004).

9. Chow E., Wong E.L.S., B¨ocking T., NguyenQ.T., Hibbert D.B., and Gooding J.J., Sens.Actuators B111–112: 540-548 (2005).

10. Bozkurt S.S., Merdivan E., and Benibol Y.,Microchim. Acta,168: 141-145(2010).

11. Umar A., Rahman M. M., and Hahn Y. B.,Electrochem.Commun.,11: 1353-1357(2009).

12. Jiang Z., Wang J., Meng L., Huang Y., andLiu L., Chem.Commun.,47: 6350-6352(2011).


13. Zhang H.J., Huang J.S., Hou H.Q., and YouT.Y., Electroanalysis21: 1869-1874 (2009).

14. Zheng L., and Song J.F., Talanta79: 319-326(2009).

15. Ameen S., Akhtar M.S., Kim Y.S., and ShinH.S., Appl. Catal., B 103: 136-142 (2011).

16. Ameen S., Akhtar M.S., Song M., and ShinH.S., ACS Appl. Mater. Interfaces4: 4405-4412 (2012).

17. Ameen S., Akhtar M.S., Kim Y.S., Yang O.B.,and Shin H.S., Electrochim.Acta56:1111-1116 (2011). (b) Jing Z., Wang J., Li F., Tan L.,Fu Y., and Li Q., J. Nanoeng. Nanomanuf.2:133-142 (2012).

18. Ameen S., Akhtar M.S., Seo H.K., Kim, Y.S.,and Shin H.S., Chem. Eng. J. 187:351-356(2012).

19. Ameen S., Akhtar M.S., Kim Y.S., Yang O.B.,and Shin H.S., Colloid Polym. Sci. 289:415-421 (2011).

20. Ameen S., Akhtar M.S., Kim Y.S., Yang O.B.,and Shin H.S., Microchim. Acta172:471-478(2011).

21. Umar A., Kim S.H., and Hahn Y.B., Curr. Appl.Phys. 8793-797 (2008).

22. Umar A., Rahman M.M., Vaseem M., andHahn Y.B., Electrochem. Commun.11:118-121(2009).

23. Baskoutas S., and Bester G., J. Phys. Chem.C115: 15862-15867 (2011).

24. Chrissanthopoulos A., Baskoutas S.,Bouropoulos N., Dracopoulos V.,Poulopoulos P., and Yannopoulos S.N.,Photonics Nanostruct.9: 132-139 (2011).

25. Gao Y.F., Nagai M., Chang T.C., and ShyueJ., J. Cryst. Growth Des.7: 2467-2471 (2007).

26. Galoppini E., Rochford J., Chen H.H., SarafG., Lu Y.C., Hagfeldt A., and Boschloo G., J.Phys. Chem. B110: 16159-16161(2006).

27. Dar G.N., Umar A., Zaidi S.A., Baskoutas S.,Kim, S.H., Abaker M., Al-Hajry A., and Al-Sayari S.A., Sci. Adv. Mater.3: 901-906(2011).

28. Ibrahim A.A., Dar G.N., Zaidi S.A., Umar A.,Abaker M., Bouzid H., and Baskoutas S.,Talanta93: 257-263(2012).

29. Ni Y., Zhu J., Zhang L, and Hong J.,Cryst. Eng.Comm. 12: 2213-2218 (2010).

30. Fang B., Zhang C.H., Zhang W., and WangG.F., Electrochim. Acta55:178-182(2009).

31. Umar A., Rahman M.M, and Hahn Y.B., J.Nanosci. Nanotechnol.9: 4686-4691(2009).

32. Ameen S., Akhtar M.S., and Shin H.S.,Talanta1003:77-383 (2012)

33. Al-Hajry A., Umar A., Hahn Y.B, and Kim D.H.,Superlatt. Microstruct.45: 529-534(2009).

34. Ameen S., Akhtar M.S., Ansari S.G., YangO.B., and Shin H.S., Superlatt. Microstruct.46:872-880(2009).

35. Laudise R.A., Kolb E.D., and Caporason A.J.,J. Am. Ceram. Soc.47: 9-12 (1964).

36. a) Sun J.H, Dong S.Y., Feng, J.L., Yin X.J.,and Zhao X.C., J. Mol. Catal. A: Chem.335:145-150(2011); (b) Poulopoulos P.,Baskoutas S., Pappas S.D., Garoufalis C.S.,Droulias S.A. ,Zamani A., and Kapaklis V, J.Phys. Chem. C115: 14839-14843(2011).

37. (a) Dhakshinamoorthy A., Visuvamithiran P.,Tharmaraj V., and Pitchumani K., Catal.Commun.16:15–19 (2011. (b) Abaker M.,Umar A., Baskoutas S., Dar G.N., Zaidi S.A.,Al-Sayari S.A., Al-Hajry A., Kim S.H., andHwang, S.W, J. Phys. D: Appl. Phys.44:425401-425405 (2011).

38. Ameen S., Akhtar M.S., and Shin H.S., Chem.Eng. J.195: 307-313(2012).

39. Silverstein R.M., Webster F.X., SpectrometricIdentification of Organic Compounds, JohnWiley & Sons, New York, NY, USA, 1998.

40. Roy C., Byrne S., McGlynn E., Mosnier J.P.,de Posada E., Mahony D.O., Lunney J.G.,Henry M.O., Ryan B, and Cafolla A.A., ThinSolid Films436: 273-276 (2003)

41. Serrano J., Manjon F.J., Romero A.H., WidulleF., Lauck R., and Cardona M., Phys. Rev. Lett.90: 055510-055514(2003).

42. Umar A., Rahman M.M., Kim S.H., and HahnY.B., Chem. Commun.166-168 (2008).

43. Sun L., Li J., Wang C. L., Li S. F., Lai Y. K.,Chen H. B., and Lin C. J., J. Hazard.Mater.171:1045-1050 (2009).

44. Perillo P.M., and Rodriguez D.F., Sens.Actuators B: Chem.171:639-643 (2012).

45. Xu H., Zhang Q., Zheng C.L., Yan W., andChu W., Appl. Surf. Sci.257: 8478-8480(2011).

46. Wang Q., Pan Y.Z. ,Huang S.S., Ren S.T., LiP., and Li J.J., Nanotechnology22: 025501(2011).

47. Shankar K., Mor G.K., Prakasam H.E. ,Yoriya


S., Paulose M., Varghese O.K., and GrimesC.A., Nanotechnology 18:065707 (2007).

48. Mohapatra S.K., Misra M., Mahajan V.K., andRaja K.S., J. Phys. Chem.C111: 8677-8685(2007).

49. Kwon Y., Kim H, Lee S., Chin I.J., Seong T.Y.,Lee W.I., and Lee C., Sens. ActuatorsB:Chem.173: 441-446 (2012).

50. Chen K.S., Xie K., Feng X.R., Wang S.F., HuR., Gu H.S., and Li Y., Int.J. HydrogenEnergy37: 13602-13609 (2012).

51. Ameen S., Akhtar M.S., Seo H.K., and ShinH.S., Appl. Phys. Lett.103: 061602-061605(2013)

52. Wang J., and Lin Z.Q., Chem. Mater.20: 1257-1261 (2008).

53. Zhao J.L., Wang X.H., Chen R.Z., and. Li L.T.,Solid State Commun.134: 705-710 (2005).

54. Feng P., Wan Q., and Wang T.H., Appl. Phys.Lett. 87: 213111-313113(2005).

55. Ameen S., Akhtar M.S., and Husain M., Sci.Adv. Mater., 2: 441-462(2010).

56. Mashat A., Tran H.D., Wlodarski W., KanerR.B., andZadeh K.K., Sens. Act. B: Chem.134:826-831 (2008).

57. Pirsa S., andAlizadeh N., Sens. Act. B:Chem.147: 461-466(2010)

58. Zhu C.L., Chen Y.J., Wang R.X., Wang L.J.,Cao M.S., and Shi X.L., Sens. Act. B:Chem.140: 185-189(2009).

59. Virji S., Huang J., Kaner R.B., andWeillerB.H., NanoLett. 4:491-496 (2004).

60. Lee K., Cho S., Park S.H., Heeger A.J., LeeC.W., and Lee S.H., Nature 44:165-68(2006).

61. Ameen S., Akhtar M.S., Kim Y.S., and ShinH.S., Chem. Eng. J. 181–182: 806-812(2012).

62. Manigandan S., Jain A., Majumder S.,Ganguly S., and Kargupta K., Sens. ActuatorsB:Chem.133:187-194(2008).

63. Ding S., Chao D., Zhang M., and Zhang W, J.Appl. Poly. Sci.10:73408-3412 (2008).

64. Alexander P., Nikolay O., Alexander K., andGalina S., Prog. Polym. Sci. 28: 1701-1753(2003).

65. Ameen, S. Akhtar M.S. and Shin H.S., Sens.Actuators B: Chem.173:177-183 (2012).

66. Kukla A., Shirshov L., Yu, M., and PiletskyS.A, Sens. Actuators B: Chem. 37:135-140

(1996).67. Bo Y., Yang H., Hu Y., Ya T., and Huang S.,

Electrochim. Acta56: 2676-2681 (2011)68. Wang P., Liu M., and Kan J., Sens. Actuators

B: Chem. 140:577-584 (2009).69. Zhang J, Lei J., Liu Y., Zhao, J., Ju H., Zhang

J., Lei J., Liu Y., Zhao J., and Ju H., Biosen.Bioelect.24:1858-1863 (2009).

70. Seo H.K., Ameen S., Akhtar M.S., and ShinH. S., Talanta104: 219-227 (2013)

71. (a) Dai Z., Wei G., Xinghu X., and HongyuanC., Chin. Sci. Bull. 51:19-24 (2006). (b) HasimS., Raghavendra S.C., Revanasiddappa M,Sajjan K.C., Lakshmi M., and Faisal M., Bull.Mater. Sci. 34: 1557-1561 (2011).

72. Rai A.K., Kumar S., and Rai A.,Vib.Spectrosc.42: 397-402 (2006).

73. Grzeszczuk M., and Szostak R., Solid StateIonics 157: 257-262(2003).

74. Ameen S., Akhtar M.S, Kim Y.S., and ShinH.S., Appl. Catal. B: Environ. 103:136-142(2011).

75. Del M., Sotomayor P.T., Tanak, A.A., andKubota L.T., J. Electroanal. Chem. 536:71-81 (2002).

76. Umar A., Rahman M.M., Kim S.H., and HahnY.B., Chem. Commun.166-168 (2008).

77. Ayd1n R., Dogan H.Ö., Köleli, F, Appl. Catal.B: Environ478:140-141 (2013). (b) Son W.I.,Hong J.M., and Kim B.S., Kor. J. Chem. Eng.22(2) 285-290(2005).

78. Hong S.J, Lee C.H., Tetrahedron Lett.53:3119-3122 (2012).

79. Enel M.S¸ and Nergiz C., Curr. Appl. Phy.12:1118-1124 (2012).

80. Lin C.W., Hwang B.J., and Lee C.R., Mater.Chem. Phys.55: 139-144 (1998).

81. Lin C.W., Hwang B.J., and Lee C.R., J. Appl.Polym. Sci. 73: 2079-2087 (1999).

82. Jiang L., Jun, H.K., Hoh Y.S., Lim J.O., LeeD.D., and Huh J.S., Sens. Actuators B:Chem.105:132-137 (2005).

83. Babaei M., and Alizadeh N., Sens. ActuatorsB Chem.183: 617-626 (2013).

84. Quadrifoglio F., and Crescenzi V., J. Coll. Inter.Sci.35: 447-459 (1971).

85. Ameen S., Akhtar M.S., Seo H.K., and ShinH.S., Appl. Cat. B:Environ. 144: 665-673(2014).

86. Duchet J., Legras R., and Demoustier-


Champagne S., Synth. Met.98: 113-122(1988).

87. Gongcalves A.B., Mangrich A.S., and ZarbinA.J.H., Synth. Met.114: 119-124 (2000).

88. Fiordiponti P., and Pistoia G., Electrochim.Acta34: 215-221 (1989).

89. Li C.M., Sun C.Q., Chen W., Pan L., Surf. Coat.Technol. 198: 474-477 (2005).

90. Vorotyntsev M.A., Badiali J.P, and Inzelt G., J.Electroanal. Chem. 472: 7-19 (1999).

91. Du X., Skachko I., Barker A, and Andrei E.Y.,Nature Nanotechnology3: 491–495 (2008).

92. Stoller M., DPark. S., Zhu Y, An J., and RuoffR.S, Nano Letters8: 3498-3502 (2008).

93. Park S., and Ruoff R.S., NatureNanotechnology4: 217-224 (2009).

94. Zhang K., Zhang L.L., Zhao X.S., and Wu J.Chem. Mat. 22: 1392-1401 (2010).

95. Li Y, Tang L., and Li J., Electrochem.Comm.11: 846-849 (2009).

96. Shao Y, Wang, J., Wu H., Liu J.I., Aksa A, andLin Y., Electroanalysis22: 1027-1036 (2010).

97. Ameen S., Akhtar M.S. , and Husain M., Sci.Adv. Mater.2: 441-462 (2010).

98. Ameen S., Akhtar M.S., Kim, Y.S., Yang O.B.,

and Shin H.S, Microchim. Acta172:471-478(2011).

99. Ding S., Chao D., Zhang M., and Zhang W., J.Appl. Pol. Sci. 107: 3408-3412 (2008).

100. Kuilla T, Bhadra S., Yao D., Kim N.H.., BoseS., and Lee J.H, Prog. Pol. Sci. 35:1350-1375(2010).

101. Dar M.A., Ansari S.G. ,Ansari Z.A., UmemotoH., Kim Y.S., Seo H.K., Kim G.S., Suh E., andShin H.S., I. J. Ref. Metals Hard Mater.24: 418-436 (2006).

102. Cao X., Wang B., and Su Q., J. Electroanal.Chem. 361: 211-214 (1993).

103. Perez E.F., Neto G.O, Tanaka A.A., andKubota L.T., Electroanalysis10: 111-115(1998).

104. Ansari S.G., Ansari Z.A., Seo H.K, Kim G.S.,Kim, Y.S., Khang G., and Shin H.S., Sens.Act.B: Chem.132: 265-271 (2008).

105. Jayasri D, and Narayanan, J.S.S., J. Haz.Mater.144: 348-354 (2007).

106. Zare H.R., and Nasirizadeh N., Electrochim.Acta52: 4153-4160 (2007).

107. Hirata M., Gotou T., Horiuchi S., Fujiwara M.,and Ohba M., Carbon 42:2929-2937 (2004).

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